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Review ArticlePhase Change Material (PCM) Microcapsules for ThermalEnergy Storage

Guangjian Peng 12 Guijing Dou 1 Yahao Hu 1 Yiheng Sun 1 and Zhitong Chen 3

1College of Mechanical Engineering Zhejiang University of Technology Hangzhou 310014 China2Key Laboratory of EampM Zhejiang University of Technology Ministry of Education amp Zhejiang ProvinceHangzhou 310014 China3Department of Mechanical and Aerospace Engineering e George Washington University Washington DC 20052 USA

Correspondence should be addressed to Zhitong Chen zhitongchengwuedu

Received 25 November 2019 Accepted 23 December 2019 Published 12 January 2020

Guest Editor Yanguang Zhou

Copyright copy 2020 Guangjian Peng et al is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Phase change materials (PCMs) are gaining increasing attention and becoming popular in the thermal energy storage eldMicrocapsules enhance thermal and mechanical performance of PCMs used in thermal energy storage by increasing the heattransfer area and preventing the leakage of meltingmaterials Nowadays a large number of studies about PCMmicrocapsules havebeen published to elaborate their benets in energy systems In this paper a comprehensive review has been carried out on PCMmicrocapsules for thermal energy storage Five aspects have been discussed in this review classication of PCMs encapsulationshell materials microencapsulation techniques PCM microcapsulesrsquo characterizations and thermal applications is reviewaims to help the researchers from various elds better understand PCM microcapsules and provide critical guidance for utilizingthis technology for future thermal energy storage

1 Introduction

e fast technological and economic development for theprogression of societies worldwide induces a quickly in-creasing energy demand e most consumption of energycomes from fossils fuels which are limited and drive severeenvironmental pollutions and climate changes [1] ereforethe eective utilization of energy becomes a major issuewhich has compelled the trend to be shifted towards theutilization of sustainable and renewable energy resources [2]Renewable energy sources such as solar energy are abundantlong-term available accessible and environmentally friendlywhich make them alternative to fossil fuels However theintermittency of many renewable energy sources signicantlyreduces the energy conversion eciency and becomes themajor constraint of developing the relevant power generationtechnologies [3]ermal energy storage (TES) is a promisingsolution for this issue and therefore undergoes rapid devel-opment e implementation of TES enhances the overalleciency and the dispatchability of power generation

applications with renewable sources [4ndash6] TES which utilizesthe change of the internal energy within the storage mediacan be classied into thermochemical- sensible- or latentheat storage [7] Compared to sensible heat latent heatstorage is amore ecientmethod and provides amuch higherenergy density with a smaller temperature dierence betweenstoring and releasing heat [8 9]

Phase change materials (PCMs) also called latent heatstorage materials can storerelease a large amount of energythrough forming and breaking molecular bonds [10ndash12]Traditional composite PCMs appear loose and diuse to thesurface gradually [13 14] In addition PCMs have a limitedthermal conductivity and suer the leakage issue for meltedstorage materials [15] To overcome these problemsmicroencapsulated PCMs have been developed [16ndash18]PCM microencapsulation is a process of coating individualPCM droplet or particle with a continuous lm to producePCM microcapsules [19 20] PCM microcapsules containtwo main parts a PCM as the core and a polymer or aninorganic shell as the PCM container Currently a few

HindawiAdvances in Polymer TechnologyVolume 2020 Article ID 9490873 20 pageshttpsdoiorg10115520209490873

review articles on PCM microcapsules are available Most ofthe review articles published previously on PCM micro-capsules are related to their synthesis and applications Tothe best of the authorsrsquo knowledge previously reportedliterature has not adequately encompassed the character-ization and applications of PCM microcapsules iscomprehensive review attempts to summarize the recentdevelopments in PCM microcapsules and offers differentaspects of PCM microcapsules for thermal energy storageapplications A brief introduction on PCMs (organic in-organic and eutectic) and encapsulation shell materials(organic inorganic and organic-inorganic hybrid materials)was presented And themicroencapsulationmethods such asphysical chemical and physical-chemical processes wereintroduced e characterization of PCM microcapsulessuch as thermal conductivity thermal stability mechanicalstrength and chemical properties was discussed and ana-lyzed Finally the practical applications of PCM micro-capsules for thermal energy storage and other industrialprocesses were reported

2 Phase Change Materials (PCMs)

PCMs have a high heat of fusion in general and can storerelease a large amount of energy during meltingsolidifyingprocesses [21 22] PCMs have been considered as storagemedia with a wide range of applications including cooling offood products spacecraft thermal systems textiles buildingsolar systems and waste heat recovery system [23 24] Forexample PCMs can decrease the electricity consumption viareducing fluctuations in air temperature and shift coolingloads to the off-peak period in the building PCMs can beclassified into subcategories based on the solid-liquid solid-solid solid-gas and liquid-gas phase transformation andvice versa [25] For TES systems solid-gas phase transfor-mation and liquid-gas phase transformation are impracticalbecause of large volume and high pressure of the system[26] For example the water-steam system is not com-mercially viable for large-scale TES In the case of solid-solidPCMs their heat of phase transition is much less than that ofsolid-liquid PCMs [27] us solid-liquid PCMs are idealcandidates for TES systems e solid-liquid PCMs havefavorable phase equilibrium high density minor volumechanges and low vapor pressure at the operation temper-ature during phase transition In addition solid-liquidPCMs also exhibit little or no subcooling during freezingmeltingfreezing at same temperature and phase segregationand sufficient crystallization rate ere are many factorsinfluencing effectiveness and applications of solid-liquidPCMs heat capacity thermal conductivity latent heatphase transition temperature and so on

PCMs possess long-term chemical stability are no firehazard are nontoxic and noncorrosive do not undergodegradation after long-term thermal cycles are nonexplosivecompounds and have compatibility with other materialsand good chemical properties capable of completing re-versible freezingmelting cycle [28] ey can be classifiedinto three main groups as shown in Figure 1 organic PCMsinorganic PCMs and eutectic PCMs [29 30] Organic PCMs

are further described as paraffin and nonparaffin materials(fatty acids alcohols and glycols) [31 32] e paraffin oneof the by-products of petroleum refinery consists of carbonand hydrogen atoms joined with the general formulaCnH2n+2 where n is the number of carbon (C) atoms (if1le nle 4 the material is gas if 5le nle 17 the material isliquid and if nge 18 the material is solid) [33] Fatty acids arecarboxylic acids with long hydrocarbon chain of carbon andhydrogen atoms with a general formula CH3(CH2)2nCOOH(10le nle 30) [34] Organic PCMs are abundant and com-mercially available at reasonable cost However utilizingPCMs in traditional manner has several limitations such asliquid leakage during melting state and low thermal con-ductivity [35 36] Inorganic PCMs contain salt hydratescompounds and metals Salt hydrates can be considered asalloys of inorganic salts and water forming a typical crys-talline solid with the general formula M middot nH2O (where M isthe salt component andn is the molecular number) eysuffer from several problems (such as tendency to super-cooling segregation not melting congruently high volumechanges corrosion and phase separation over repeatedphase change cycles) which limit their applications in TESfacilities [37ndash39] Inorganic compounds are generallyregarded as being unsuitable for applications in TES becausethey own relatively small latent heat capacity and areharmful to the environment and human health [40 41]Metals promising inorganic PCMs diminish some of saltproblems but they are only suitable for high-temperatureapplications (4370K) [42ndash44] Eutectic PCMs can be definedas a minimum melting composition of two or more com-ponents each of which freezes and melts congruently toform a mixture of the componentsrsquo crystals in progress ofcrystallization [11 28] Eutectic PCMs can be adjusted bymixing inorganic-inorganic organic-organic or a combi-nation of the two PCMs at a desired ratio for specific ap-plications ey also possess high thermal conductivity anddensity without segregation and supercooling while theirspecific heat capacity and latent heat are much lower thanthose of paraffinsalt hydrates [45 46]

On the other hand PCMs can also be categorizedaccording to their phase transition temperatures low-temperature PCMs (melting point lt220degC) intermediate-temperature PCMs (220degC le melting point le420degC) andhigh-temperature PCMs (melting point gt420degC) [47]Generally low-temperature PCMs are paraffin fatty acidspolymeric materials sugar alcohols polyalcohol and so on[48] Melting points of most organic compounds are below80degC thus organic PCMs fall under the category of low-temperature PCMs However most of the inorganic-inor-ganic eutectic PCMs fall under the category of intermediate-temperature PCMs High-temperature PCMs includingnitrates metal carbonates sulfates fluorides chlorides andso on can be widely applied for high-temperature TESrequiring temperature gt500degC [49]

3 Encapsulation Shell Materials

To effectively reduce the leakage of PCMs during the solid-liquid phase transition and avoid the reaction of the PCMs

2 Advances in Polymer Technology

with the surrounding environment microencapsulation ofthe PCMs has been widely used in recent years Microen-capsulation technology can also provide high thermal cy-cling stability relatively constant volume and large heattransfer area for PCM-based thermal storage [50] Shellmaterials play an important role in the morphology me-chanical properties and thermal properties of the producedmicrocapsules [51] and can be classified according to thechemical nature into three categories of organic inorganicand organic-inorganic hybrid materials [22] as shown inFigure 2

31 Organic Shells Organic shell materials generally consistof the natural and synthetic polymeric materials whichpossess good sealing properties good structural flexibilityand excellent resistance to the volume change associated withrepeated phase transformations of PCMs [52] Commonlyused organic shell materials include melamine formaldehyde(MF) resin [53ndash56] urea formaldehyde (UF) resin [57ndash59]and acrylic resin [60ndash65] MF resin has the advantages of lowcost good chemical compatibility and thermal stability [54]Mohaddes et al successfully used MF as the shell material toencapsulate n-eicosane and applied such microcapsules totextiles [66] DSC results show that the melting and crys-tallization latent heats of the MF-based microcapsules are1666 Jg and 1624 Jg respectively Fabrics doped with thistype of microcapsules exhibit a lower value of thermal delayefficiency and higher thermoregulation capacity

Among the group of acrylic resins the copolymers ofmethacrylate possess nontoxicity easy preparation goodthermal stability and chemical resistance [52] Alkan et alreported that n-eicosane microencapsulated with poly-methylmethacrylate (PMMA) shell had good thermal sta-bility [67] It is a three-step degradation process duringthermogravimetric analysis (TGA) tests and the phasechange temperature remained mostly unchanged after 5000-cycle DSC tests Ma et al used poly(methylmethacrylate-co-divinylbenzene) (P(MMA-co-DVB)) copolymer as the shellmaterial to successfully encapsulate the binary core mate-rials namely butyl stearate and paraffin [68] e preparedmicrocapsules exhibited compact surface and regularspherical shape with a relatively uniform size of 5ndash10 μm

Moreover phase change temperature for such microcap-sules can be adjusted by controlling the ratio of butyl stearateto paraffin Wang et al prepared capric acidUF micro-capsules by adding various contents of graphene oxide (GO)to investigate the effects of GO on the thermal properties ofmicrocapsules [57] It was found that the microcapsules with06 GO had the highest enthalpy of 10960 Jg and en-capsulation ratio of 607 ermal conductivity was alsogreatly improved by adding more of GO Additionally themicrocapsules with GO exhibited smoother surfaces thanthose without GO

32 Inorganic Shells Due to the flammability low thermalconductivity and poor mechanical properties of the poly-meric shell materials [69] the application of microcapsuleswith organic shells has limitations in some situations In-stead inorganic materials have been gradually employed asshell materials for microcapsule preparation in recent yearsCompared with organic materials inorganic shells generallyhave higher rigidity higher mechanical strength and betterthermal conductivity [52] Silica (SiO2) [70ndash72] zinc oxide(ZnO) [73 74] titanium dioxide (TiO2) [75ndash77] and cal-cium carbonate (CaCO3) [78ndash80] are generally utilized asinorganic shell materials

e advantages of high thermal conductivity fire re-sistance and easy preparation lead silica to be one of the

Phase change materials

Organic Inorganic Eutectic

Paraffins

Metallic

compoundsOrganic-

inorganic

Inorganic-

inorganicMetal

alloyEsters

Fatty acids

Saltshydrates

Organic-organic

SaltsAlcohols

Figure 1 Classification of phase change materials (PCMs)

Shellmaterials

Organic

Silica (SiO2)Zinc oxide (ZnO)

Titanium dioxide (TiO2)

PMMA-SiO2PMMA-TiO2

PMF-SiC

InorganicOrganic-inorganic

hybrid

MF resinUF resin

Acrylic resin

Figure 2 Shell materials of microencapsulation for PCMs

Advances in Polymer Technology 3

most commonly used shell materials Silica is oftenemployed to microencapsulate organic paraffin waxes[78 81] inorganic hydrated salts [71 82ndash84] and fatty acids[85 86] Liang et al used silica as the shell material and n-octadecane as the core material to prepare nanocapsulesthrough interfacial hydrolysis and polycondensation oftetraethoxysilane (TEOS) in miniemulsion [87]e thermalconductivity of such nanocapsules was measured to be above04Wmminus 1Kminus 1 e melting enthalpy and encapsulation ratioreached 1095 Jg and 515 respectively After 500 thermalcycles the enthalpy of the nanocapsules remained almostunchanged and no leakage was observed

e synthesis of the silica shell usually needs tetrae-thoxysilane (TEOS) as a silica precursor However thehydrolysis and polycondensation of TEOS could cause thesilica shell to be not compact enough and have a relativelyweak mechanical strength [88] Compared with silica shellsCaCO3 shells exhibit higher rigidity and better compactnessYu et al utilized CaCO3 shells to encapsulate n-octadecanevia a self-assembly method [88]e obtained microcapsulesshowed spherical shape with a uniform diameter of around5 μm and had good thermal conductivity thermal stabilityanti-osmosis properties and serving durability

Crystalline metal oxides such as ZnO and TiO2 havemultifunctional properties including catalytic photochem-ical and antibacterial characteristics ey are often used asshell materials to obtain PCM microcapsules with someinteresting features Li et al employed ZnO as the shellmaterial and n-eicosane as the core material to synthesizemultifunctional microcapsules with latent heat storage andphotocatalytic and antibacterial properties [73] e thermalperformance of the microcapsules depends on the ratio of n-eicosane to Zn(CH3COO)22H2O In the study of Liu et alTiO2 shells were used to encapsulate n-eicosane via inter-facial polycondensation followed by impregnation of ZnO[89] e obtained microcapsules possessed both thermalstorage and photocatalytic capabilities e melting tem-perature and corresponding latent heat were 4176degC and18827 Jg respectively

33 Organic-Inorganic Hybrid Shells In order to overcomethe disadvantages and combine the advantages of organicand inorganic materials researchers have been turning toutilize organic-inorganic hybrid shells to microencapsulatePCMs In organic-inorganic hybrid shells inorganic ma-terials can enhance mechanical rigidity thermal stabilityand thermal conductivity whilst organic materials offerstructural flexibility [52] Shells formed from polymers suchas PMMA and PMF and doped with SiO2 or TiO2 are widelyused to encapsulate PCMs [90ndash93]

Wang et al synthesized n-octadecane microcapsuleswith PMMA-silica hybrid shells through photocurablePickering emulsion polymerization [94] e producedmicrocapsules had good morphology and had particlesranging in the size from 5 μm to 15 μm When the weightratio of MMA to n-octadecane is 1 1 the prepared mi-crocapsules exhibited the highest encapsulation efficiency of6255 Zhao et al successfully prepared bifunctional

microcapsules by using n-octadecane as the core and PMMAdoped with TiO2 as the hybrid shell [95] It was found thatincreasing TiO2 could improve the thermal conductivity ofmicrocapsules but led to lower enthalpy and encapsulationefficiency e initial degradation temperature of the mi-crocapsules with 6 TiO2 reached 2284degC which demon-strated that the microcapsules had good thermal stabilityMultifunctional microcapsules consisting of n-octadecanecores and poly(melamine formaldehyde)silicon carbide(PMFSiC) hybrid shells were synthesized via in situ poly-merization by Wang et al [91] e microcapsules displayedregular-spherical morphology Contrasted to the micro-capsules without SiC heat transfer rate of the microcapsuleswith 7 SiC had a significant enhancement and thermalconductivity improved by 6034

4 Microencapsulation of PCMs

Microencapsulation is the utilization of a film-formingmaterial to coat a solid or liquid and form 1ndash1000 μmparticles which are called microcapsules Microencapsula-tion methods can be classified into three categoriesaccording to the synthesis mechanism physical methodschemical methods and physical-chemical methods [15]Detailed classification of these methods is listed in Figure 3

41 PhysicalMethods In physical methods the formation ofmicrocapsule shells only involves physical processes such asdrying dehydration and adhesion e commonly usedphysical methods for encapsulating PCMs are spray-dryingand solvent evaporation e process of spray-drying in-cludes (1) preparing oil-water emulsion containing PCMsand shell materials (2) spraying the prepared oil-wateremulsion in a drying chamber by using an atomizer (3)drying the sprayed droplets through drying gas stream at asuitable temperature and (4) separating the solid particlesby cyclone and filter [15] Borreguero et al synthesizedmicrocapsules with a paraffin RubithermregRT27 core andpolyethylene EVA shell with and without carbon nanofibers(CNFs) via spray drying [96] With the addition of CNFsboth the mechanical strength and thermal conductivity ofmicrocapsules were enhanced and the heat storage capacitywas maintained DSC tests also showed that the micro-capsules still possessed good thermal stability after the 3000-thermal chargedischarge cycles Hawlader et al utilizedspray-drying method to microencapsulate paraffin withgelatin and gum arabic [19] e obtained microcapsulespossess spherical shape and uniform size e heat storageand release capacity of the microcapsules prepared at thecore-to-shell ratio of 2 1 reached 21644 Jg and 221217 Jgrespectively

e basic steps of a solvent evaporation method are asfollows (1) preparing polymer solution by dissolving shellmaterials in a volatile solvent (2) adding PCMs to solutionto form OW emulsion (3) forming shells on the droplets byevaporating the solvent (4) obtaining microcapsulesthrough filtration and drying Lin et al used the solventevaporation method to encapsulate myristic acid (MA) with


ethyl cellulose (EC) [97] e melting and solidifyingtemperatures were 5332degC and 4444degC with melting andsolidifying enthalpies of 12261 Jg and 10424 Jg respec-tively Wang et al utilized solvent evaporation to synthesizemicrocapsules by using sodium phosphate dodecahydrate(DSP) as the core and poly(methyl methacrylate) (PMMA)as the shell [98] e optimal parameters found throughsystematic analyses for preparing high-performance mi-crocapsules are as follows 80degCndash90degC for the synthesistemperature 240min for the reaction time and 900 rpm forthe stirring rate during microencapsulation process eprepared microcapsules had an energy storage capacity of1429 Jg at the endothermic peak temperature of 515degC

42 Chemical Methods Chemical microencapsulationmethods utilize polymerization or a condensation process ofmonomers oligomers or prepolymers as raw materials toform shells at an oil-water interface e chemical methodsmainly include in situ polymerization interfacial poly-merization suspension polymerization and emulsion po-lymerization e differences between these fourpolymerization methods are shown in Figure 4

In situ polymerization forms a shell on the surface of thedroplet by polymerization of the prepolymers which areformed by prepolymerization of the monomers (seeFigure 4(a)) e general steps of the in situ polymerizationprocess are as follows [99] (1) preparing the OW emulsionby adding PCMs to aqueous solution of surfactant (2)forming a prepolymer solution (3) adding the prepolymersolution to the OW emulsion followed by adjusting to theappropriate reaction conditions and (4) synthesizing themicrocapsule Decanoic acid was successfully micro-encapsulated using poly(urea formaldehyde) (PUF) poly(-melamine formaldehyde) (PMF) and poly(melamine ureaformaldehyde) (PMUF) via in situ polymerization byKonuklu et al [100] e microcapsules coated by PUF

exhibited higher heat storage capacity but weaker me-chanical strength and lower heat resistance while the mi-crocapsules with PMF shells had higher thermal stability butsmaller thermal energy storage capacity Compared to thePUF- and PMF-coated microcapsules the PMUF-encap-sulated microcapsules possessed perfect thermal stability inthat no leakage was found at 95degC Zhang et al synthesizeddual-functional microcapsules consisting of n-eicosanecores and ZrO2 shells via in situ polycondensation [101]ese 15ndash2 μm spherical microcapsules showed the char-acteristics of thermal energy storage and photo-luminescence Additionally the synthesized microcapsulespossessed good thermal reliability with the thermal propertyremaining almost unchanged after 100 thermal cycles Suet al microencapsulated dodecanol with methanol-modifiedmelamine formaldehyde (MMF) prepolymer by in situpolymerization [102] It was found that with increasingstirring rates the average diameter of microcapsules sharplydecreased and the encapsulation efficiency increased ehighest encapsulation efficiency of microcapsules reachedwas 974 In another study Su et al used MMF as the shellmaterial to encapsulate paraffin via in situ polymerization[103]

In interfacial polymerization as shown in Figure 4(b)two reactive monomers are dissolved separately in the oilphase and the aqueous phase and polymerization thenoccurs at the oil-water interface under the action of aninitiator e general steps of interfacial polymerization areas follows (1) forming an OW emulsion containing PCMsand hydrophobic monomer (2) adding the hydrophilicmonomer to initiate polymerization under suitable condi-tions (3) obtaining microcapsules through filtering wash-ing and drying is method is usually used in thepreparation of organic shell materials such as polyurea andpolyurethane [15] e microencapsulation of butyl stearate(BS) and paraffin as binary core materials using polyureapolyurethane as the shell material was successfully carriedout via the interfacial polymerization method by Ma et al[104] e phase change temperature of microcapsules wasadjusted by changing the ratio of the two core materials eTGA results showed that the obtained microcapsulesdecomposed in three steps above 190degC implying the mi-crocapsules possessed good thermal stability Lu et al usedpolyurethane to form a cross-linked network shell to en-capsulate the butyl stearate core via interfacial polymeri-zation [105] Siddhan et al microencapsulated n-octadecaneusing toluene-24-diisocyanate (TDI) as a oil-solublemonomer and diethylene triamine (DETA) as a water-sol-uble monomer through interfacial polymerization [106]erelevant results suggested that the core content and en-capsulation efficiency of the microcapsules reached as highas 70 and 92 respectively when the ratio of core tomonomer was 37 and the ratio of PCM to solvent was 6

In the suspension polymerization process disperseddroplets containing PCMs monomers and initiators aresuspended in continuous aqueous phase by using surfactantsand mechanical stirring Free radicals of the oil-solubleinitiator are then released into the emulsion system toinitiate polymerization of the monomers under suitable

Physical

Spray drying Economialeasily scaled up

Low costlow yield

Prepolymerizationuniform morphology

Simple processhigh monomer activity

Cotrollable reaction heatoil-soluble initiator

Smaller particle sizewater-soluble initiator

Controllable particle sizeagglomeration

Suitable for inorganicshells

Solventevaporation

In situpolymerization

Interfacialpolymerization

Suspensionpolymerization

Emulsionpolymerization

Coacervation

Sol-gel method

Chemical

Mic

roen

caps

ulat

ion

Physical-chemical

Figure 3 e classification of microencapsulation methods forPCMs


stirring rate and temperature [15] as shown in Figure 4(c)Wang et al employed suspension polymerization to suc-cessfully microencapsulate n-octadecane using thermo-chromic pigmentPMMA shells with five different pigmentMMA ratios ranging from 0 14 43 71 and 143wt[107] e microcapsules without pigment were found toachieve the highest melting and crystallization enthalpies of14916 Jg and 15255 Jg respectively Sanchez-Silva et alutilized different suspension stabilizers to microencapsulateRubithermregRT31 with polystyrene by suspension poly-merization [108] e DSC results showed that the micro-capsules had the lowest (757 Jg) and highest (1353 Jg)thermal storage capacity when PVP and gum arabic wereused as suspension stabilizers Tang et al used suspensionpolymerization to microencapsulate n-octadecane with n-octadecyl methacrylate (ODMA)-methacrylic acid (MAA)copolymer [109] e prepared microcapsules exhibitedspherical shape with an average diameter of around 160 μme microcapsules achieved the highest phase change en-thalpy of 93 Jg when the ratio of monomers to n-octadecanewas 2 1

In emulsion polymerization as shown in Figure 4(d) thedispersed phase consisting of PCMs and monomers is firstlysuspended in continuous phase as discrete droplets with theaid of vigorous agitation and surfactants Water-solutioninitiators are then added to trigger polymerization [110]is method is often employed to polymerize organic ma-terials such as PMMA and polystyrene to formmicrocapsule

shells Sahan et al utilized emulsion polymerization toencapsulate stearic acid (SA) cores using poly(methylmethacrylate) (PMMA) and four other PMMA-hybrid shellmaterials [111] e microcapsules had a mean diameter of110ndash360 μm and thickness of 17ndash60 μm For all microcap-sules the heat storage capacity was below 80 Jg and thedegradation temperature was above 290degC Sarı et al suc-cessfully used PMMA shells to microencapsulate paraffineutectic mixtures (PEM) containing four different contentsvia emulsion polymerization [112] Spherical microcapsuleswith a particle size of 116ndash642 μm were obtained emelting temperature for the microcapsules with the highestPEM content varied from 20degC to 36degC and the heat storagecapacity reached 169 Jg Sarı et al also prepared polysty-rene- (PS-) coated microcapsules containing capric lauricand myristic acids by using emulsion polymerization [113]ese microcapsules could melt and freeze in a wide tem-perature range of 22degCndash48degC and 19degCndash49degC respectivelye corresponding latent heats for melting and freezing werelocated in the range of 87ndash98 Jg and 84ndash96 Jg respectivelyAfter 5000 thermal cycle tests thermal properties of thesemicrocapsules remained almost unchanged demonstratingthat they had a good thermal reliability

43 Physical-Chemical Methods Physical-chemical methodis the combination of physical and chemical processesPhysical processes such as phase separation heating and

(a)

(b)

(c)

(d)

Oil-soluble initiatorWater-soluble initiator

Monomer AMonomer B

Figure 4 Schematic diagrams of various chemical microencapsulation methods (a) in situ polymerization (b) interfacial polymerization(c) suspension polymerization and (d) emulsion polymerization


cooling are combined with chemical processes such as hy-drolysis cross-linking and condensation to achieve micro-encapsulation e most representative and commonly usedphysical-chemical methods are coacervation and the sol-gelmethod

e coacervation method can be further divided intosingle coacervation and complex coacervation For prepa-ration of microcapsules single coacervation requires onlyone type of shell material while complex coacervation re-quires two kinds of opposite-charged shell materials Typ-ically the microcapsules prepared using complexcoacervation have better morphology more uniform sizeand better stability e main steps in complex coacervationprocesses are as follows [114] (1) PCMs are dispersed in anaqueous polymer solution to form the emulsion (2) a secondaqueous polymer solution with opposite charges is addedleading the shell material to deposit on the surface of thedroplet by electrostatic attraction and (3) cross-linkingdesolation or thermal treatment is employed to obtain stablemicrocapsules

Hawlader et al adopted complex coacervation to en-capsulate paraffin cores with gelatin and acacia [19] eauthors reported that the melting and solidifying enthalpiesof the microcapsules could reach as high as 23978 Jg and23405 Jg respectively when the homogenizing time was10min the amount of used cross-linking agent was 6ndash8mland the ratio of core to shell was 2 1 Similarly three types ofcore materials ie n-hexadecane n-octadecane and n-nonadecane were microencapsulated with natural andbiodegradable polymers in the form of a gumArabic-gelatinmixture through complex coacervation by Onder et al [115]e microcapsules containing n-hexadecane and n-octa-decane were prepared at the dispersed content of 80 in theemulsion and showed good enthalpies of 1447 Jg and1658 Jg respectively For microcapsules with n-non-adecane obtained at the dispersed content of 60 in theemulsion the enthalpy value was only 575 Jg

e sol-gel method is an economical and mild process tosynthesize PCM microcapsules e general steps in mi-crocapsule preparation are as follows (1) the reactive ma-terials involving PCMs precursor solvent and emulsifierare uniformly dispersed in a continuous phase to form acolloidal solution by hydrolysis reaction (2) through con-densation polymerization of monomers a gel system with athree-dimensional network structure is formed and (3) themicrocapsules are formed after drying sintering and curingprocesses [114] Sol-gel methods are usually used to syn-thesize inorganic shells such as SiO2 and TiO2 shells Lat-ibari et al successfully fabricated nanocapsules containingpalmitic acid (PA) as the core and SiO2 as the shell via thesol-gel method by adjusting the pH value of the solution[116] e results indicated that the capsules obtained at thepH value of 11 115 and 12 had an average particle size of1837 nm 4664 nm and 7225 nm respectively and thecorresponding melting latent heats were 16816 kJkg17216 kJkg and 18091 kJkg respectively Cao et almicroencapsulated paraffin with TiO2 shells through sol-gelprocess and reported that the sample with a microencap-sulation ratio of 855 had melting and solidifying latent

heats of 1611 kJkg (at the melting temperature of 588degC)and 1446 kJkg (at the solidifying temperature of 565degC)respectively [117]

5 Characterization of PCM Microcapsules

e physical thermal chemical and mechanical propertiesof PCM microcapsules are significantly affected by the rawmaterials and synthesis processes during microencapsula-tion Since these properties are of great importance to ap-plications of PCM microcapsules there is an increasingdemand for accurate quantification and characterization ofthe physical thermal chemical and mechanical properties

51 Physical Characterization Traditionally the size dis-tribution of microcapsules is analyzed by two methodsparticle size analyzer (PSA) and observation statistics ForPSA microcapsules are first dispersed in a suitable mediumand a laser particle size analyzer is then employed to detectthe microcapsulesrsquo size distribution Abadi et al used adynamic light scattering analysis apparatus from MalvernInstruments Co UK to obtain microcapsules size distri-bution [122] e analysis results showed that the sizedistribution curve of all the microcapsule samples presenteda single peak and approximated a normal distribution curvee average size was found to increase from 145 nm to200 nm with the increasing amount of core materials (n-hexadecane) For observation statistics scanning electronmicroscopy (SEM) and optical microscopy (OM) were firstused to take images of a number of microcapsules randomlyand the size distribution was then statistically determined bymeasuring the diameter of each microcapsule Sun et alcombined SEM micrographs with Image-Pro analysissoftware to analyze the particle size distribution of micro-capsules [118] By measuring more than 200 microcapsulesthe particle size was found to range from 10 μm to 74 μmwith a mean diameter of 4277 μmWang et al measured andanalyzed the diameters of 300 microcapsules from the SEMand OM images to obtain an average diameter of 12166 μm[119]

In the microencapsulation process of PCMs not allPCMs are encapsulated by shells Productive ratio which isdefined as the ratio of the mass of microcapsules to the totalmass of core and shell materials is usually used to evaluatethe conversion rate of raw materials is parameter isdominated by many factors including core-shell ratioemulsifier concentration and agitation rate Xu et al pre-pared paraffinPMMA microcapsules using a redox initi-ator [61] When the concentration of paraffin was less than70 the productive ratio of microcapsules was higher than95 Roy et al found that the productive ratio was irrelevantto the content of the crosslink agent glutaraldehyde (GTA)[120] However increasing the biopolymer ratio of chitosan(CH) to type-B gelatin (GB) from 13 to 15 or lowering thecoreshell ratio from 375 to 250 could lead to a higherproductive ratio

Encapsulation efficiency which is defined as the ratio ofthe mass of core to the mass of shell (or microcapsule)


reflects the core contents in microcapsules However it isdifficult to directly measure the mass of the core and shell fora single microcapsule erefore the encapsulation effi-ciency is usually evaluated by calculating the ratio of thelatent heat of PCM microcapsules to that of pure PCMsusing Ref [99 101 121] as shown in equations (1) and (2)

encapsulation efficiency ΔHmicrocapsules

ΔHPCMstimes 100 (1)

or

encapsulation efficiency ΔHmmicrocapsules + ΔHcmicrocapsules

ΔHmPCMs + ΔHcPCMs

times 100

(2)

where ΔHmicrocapsules is the latent heat of microcapsulescontaining PCMs and ΔHPCMs is the latent heat of purePCMs ΔHmmicrocapsules and ΔHcmicrocapsules are the latentheat of melting and the crystallization for microcapsulesrespectively ΔHmPCMs and ΔHcPCMs are the latent heats ofmelting and crystallization for pure PCMs respectivelyEquations (1) and (2) have a slight difference when super-cooling phenomena occur According to equation (1) Zhanget al acquired a highest encapsulated paraffin content of895 [121] Zhang et al prepared microcapsules containingn-eicosane core and ZrO2 shell [101] It was found that theencapsulation ratio could be improved from 5243 to6198 by changing the mass ratio of core to shell from 1 1to 2 3

Optical microscopy (OM) scanning electron micros-copy (SEM) and transmission electron microscopy (TEM)are the most commonly-used techniques to characterize themorphology of microcapsules Since microencapsulation ofPCMs is sensitive to the raw materials additives andsynthesis conditions the obtained microcapsules can havevarious smoothness sizes and shapes Figure 5(a) shows theSEM image of microcapsules having smooth and compactsurfaces However as shown in Figure 5(b) the OM iseconomic and easy to be operated but has relatively lowmagnification compared with the other two techniquesCompared with OM SEM has higher resolution to accu-rately measure the thickness of microcapsule shells (seeFigure 5(c)) OM can provide the true color images ofmicrocapsules compared with SEM and TEM e OMimages of microcapsules observed under natural light shownin Figure 5(d) indicated that the n-octadecanePMMAmicrocapsules were transparent

52 ermal Characterization In addition to high stabilityPCM microcapsules gain increasing popularity due to theirhigh thermal storage capacity Differential scanning calo-rimetry (DSC) is used to analyze the thermal properties ofpure PCMs and microcapsules A DSC thermogram iscomposed of endothermic and exothermic curves whichcontain the critical information for the thermal propertiessuch as the onset and peak temperature of the melting and

crystallization process or enthalpy on DSC cooling andheating runs Since shell materials have lower latent heatstorage capacity the overall heat capacity for PCM micro-capsules is significantly lower than pure PCMs eimplementation of thermal insulation over the shell geo-metric confinement or absence of nucleation agents givesrise to the encounter of supercooling a shift of onset phasechange temperature Supercooling will lead a longer thermaldischarge time and casts significant effects on the overallthermal performance of PCM microcapsules [122] Tosuppress this phenomenon Wu et al added 1-tetradecanoland paraffin into the oil phase of n-octadecanepolyureamicrocapsules as nucleation agents [123] However thesupercooling degree further increased as 1-tetradecanolincreased from 83wt to 125 wt and paraffin encap-sulated into core materials was helpful for promoting theformation of triclinic crystal l in n-octadecane is studyindicated that paraffin had a positive impact on the crys-tallization of n-octadecane Chen et al successfully sup-pressed supercooling of n-octadecanemelamineformaldehyde microcapsules by adding alkylated grapheneinto core materials [124] is is because the added alkylatedgraphene which advanced the crystallization of the corematerial to a certain degree enhanced heterogeneous nu-cleation and thermal transfer

PCM microcapsules are usually exposed to extremecircumstances especially high temperatures which cancause severe thermal degradations ermal stability is usedto describe the resistance to thermal decomposition ofPCMs microcapsules ermogravimetric analysis (TGA) isused to test thermal stability by increasing the temperatureat a very stable step with a small interval and measuring theweight loss of the sample simultaneously e TGA curveshows the varying of weight ratio with temperaturechanging Jiang et al investigated the thermal properties ofPCM microcapsules based on paraffin wax core andP(MMA-co-MA) shell with nanoalumina (nano-Al2O3)[125] Core material paraffin degraded at the onset tem-perature of 15988degC and remained about 2063 wtmass at200degC Almost no char was maintained at 230degC P(MMA-co-MA) shells also presented a one-step degradation with arapid weight loss rate at 3423degC and decomposed com-pletely at around 450degC When PCMs were encapsulated inshells they showed a three-step degradation e first stagewas related to the evaporation of the leakage core materialsout of the broken shell starting at around 150degC e secondstage was attributed to PCMs from the inside of micro-capsules completely evaporating at around 230degC Finally ataround 350degC the P(MMA-co-MA) shell underwent py-rolysis and the products decomposed thoroughly e in-crease in the onset temperature of the second stageindicated the protective effect of shells which enhanced thethermal stability of core materials

ermal reliability is qualitatively analyzed by subjectingPCM microcapsules to an extremely large number of re-peated cycles of phase change and after multiple cycles ofmelting and crystallization change of thermal storage ca-pacity (by DSC) thermal stability (by TGA) chemicalcharacterization (by FTIR (Fourier-transform infrared


spectroscopy)) and so on was detected Zhang et al con-ducted a 100-loop scan of phase-change circulations [101]All the curves in the DSC thermograms had a surprising highcoincidence with the first loop e melting peaks thecrystallization peaks and the onset points maintained stablyafter 100 loops and the peak temperatures only showedminor deflection within 05degC Liu et al investigated mi-crocapsulesrsquo reliability with dodecanol core and melamineformaldehyde (MF) resin shell [126]ey obtained the DSCcurves of MEPCMGO CNT after 25 50 75 and 100 phasechange cycles which presented little difference from the firstSimilar conclusions were obtained by other works [67 127]Comparison between the two spectra found that the fre-quency values of characteristic peaks did not change whichindicated that the chemical structure of microcapsules wasnot affected by thermal cycling

Under the steady-state condition the thermal conduc-tivity of a material is defined as the heat flow through a unitcross-sectional area under a unit temperature gradientHigher thermal conductivity can provide faster heat transferrate which is of great importance to many applications eefficiency of energy storage and release can be improved byincreasing thermal conductivity Due to the small size ofmicrocapsules it is difficult to directly measure the thermalconductivity of single microcapsules e theory was pro-posed to calculate the conductivity of single microcapsulesby [128]

1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)

where (dcdp)3 ρsρs + yρc kp kc and kw represent thethermal conductivities of the single microcapsule corematerial and shell material respectively dp is the diameter ofthe microcapsule particle while dc is the diameter of thecore However the performance of multiple microcapsulesrsquothermal conductivity is more critical in actual applicationsus the thermal conductivity is often analyzed in bulkmicrocapsules It can be approximately performed by put-ting microcapsules between two plates with heat flowingthrough the sample Establishing an axial temperaturegradient the temperature difference is measured based onthe heat output from the heat flow transducer at thermalequilibrium e equation becomes [129]

θ Aχh

T1 minus T2( 1113857 (4)

where θ is the heat χ is thermal conductivity A is the surfacearea h is the thickness of the sample and T1 and T2 are thetemperatures of two surfaces To improve the thermal con-ductivity of microcapsules researchers usually add extramaterials with high thermal conductivity such as graphitecarbon nanotubes and nanometals directly to the core or shellof the microcapsules Li et al improved the compatibility ofcarbon nanotubes with core materials by grafting stearyl

(a) (b)

(c) (d)

Figure 5 Images of microcapsules (a) surfaces smooth and compact with defects (b) rough and porous (c) shell thickness and (d) regularspherical structure


alcohol (SA) onto carbon nanotubes and then coating PCMswith melamine resin [130] It was found that the addition ofcarbon nanotubes improved the performance of microcap-sules and the thermal conductivity of microcapsules of carbonnanotubes increased by 792Wang et al employed the samestrategy to obtain the thermal conductivity of paraffincalciumcarbonate microcapsules with different high-thermal-con-ductivity fillers [131] With different concentrations of flakegraphite (FG) expanded graphite (EG) and graphite nano-sheets (GNS) the results revealed a significant improvement ofthermal conductivity When phase change composites con-tained 20wt GNS they could form a stable and densethermal conductivity network with a thermal conductivity of2581Wmminus 1Kminus 1 which is 70 times that of pure paraffin waxJiang et al [83] prepared microcapsules with paraffin as aphase change material and polymethyl methacrylate as a wallmaterial and then embedded nano-Al2O3 on the wall material[125] Microcapsules with 16 monomer mass fraction ofnano-Al2O3 had the best performance and the enthalpy andthermal conductivity were 9341 Jgminus 1 and 031Wmminus 1Kminus 1 re-spectively Sarier et al separately coated n-octadecane and n-hexadecane core materials with urea formaldehyde resin andthen added reduced nanosilver particles to the wall material toprepare PCMs microcapsules [132] e microcapsulesmodified by the nanosilver particles had higher thermalconductivity than the pure urea formaldehyde resin wallmaterial microcapsules e microcapsules modified by dif-ferent methods exhibit better thermal conductivity and im-proved the thermal regulation performance

53 Chemical Characterization e chemical structure andcomposition of PCMs shell partial additives and micro-capsules are analyzed by Fourier transform infrared (FTIR)spectroscopy e infrared spectrum belongs to the ab-sorption spectrum which is obtained by the absorption ofinfrared light with a specific wavelength when the com-pound molecules vibrate Zhang et al studied microcapsulescontaining an n-eicosane core and ZrO2 shell [101] esemicrocapsule samples were prepared with different core-shell ratios and some of them were prepared with the ad-dition of NaF It showed a series of similar characteristicabsorption bands ree intensive absorption peaks wereobserved at the wavelength of 2959 cmminus 1 2919 cmminus 1 and2853 cmminus 1 respectively due to the alkyl C-H stretchingvibrations of methyl and methylene groups Moreover therewere two absorption bands at 1472 cmminus 1 and 1379 cmminus 1which are caused by the C-H stretching vibration ofmethylene bridges e infrared spectra also exhibited anabsorption peak at 722 cmminus 1 in accordance with the in-planerocking vibration of methylene groups ere was a broadabsorption peak centered at 3437 cmminus 1 which could beassigned to the stretching vibration of the O-H bond ofadsorptive water Moreover it can provide the existence ofsome specific elements or bands For example through FTIRspectra Zhang et al found the characteristic absorption at474 cmminus 1 due to the ZrO stretching vibration [91] con-firming they successfully prepared the microcapsules withzirconia shells [133]

X-ray diffraction (XRD) is another technique to effi-ciently identify the specific materials in samples If the shellmaterial is inorganic XRD is more suitable to recognize itscomposition [15] Each crystal material has its specificstructure which can be used not only to recognize thechemical composition but also to distinguish the state of theexistence Xu et al obtained XRD patterns that helped todetermine the crystalline structures of microcapsule shells[134] Moreover the diffraction peaks in the XRD patternspresented the MnO2SiO2 hierarchical shell which wereconsistent with those reported for MnO2 compound pat-terns which confirmed that the microcapsules they syn-thesized were in interfacial polycondensation and ananoflake-like MnO2 layer was fabricated onto the surface ofSiO2 shell through template-directed self-assembly In ad-dition this study also identified an amorphous nature of theSiO2 shell

54 Mechanical Characterization e characterization ofthe mechanical properties of microcapsules can be dividedinto detection of a single microcapsule and detection ofmultiple microcapsules at the same time e mechanicalproperties of a single microcapsule are mainly characterizedby atomic force microscopy (AFM) micronano indentationdetection and microcompression All these methods havenearly the same detection theory by applying a small load ona single microcapsule that can be monitored and controlledin real time researchers acquire the load deformation curveor failure characteristics of the microcapsule en themechanical properties of the tested samples can be analyzede elastic modulus and other mechanical parameters of thesingle microcapsule can be determined by this method andone can recognize the strength characteristics of differentmicrocapsules by comparing the rupture loads of separatemicrocapsules AFM is an analytical technique which can beused to study the surface structure of solid materials in-cluding insulators In an AFM test one end of the micro-cantilever that is sensitive to weak force is fixed while theother end has a tiny tip in light contact with the surface of thesample Because of the extremely weak repulsive force be-tween the tip and the sample surface atom the micro-cantilever with the tip of the needle will undulate in adirection perpendicular to the surface of the sample Basedon this theory the microcapsules can be loaded-tested by amicrocantilever Giro-Paloma et al analyzed the forceneeded to break phase change slurry microcapsules andevaluated the effective Youngrsquos modulus by AFM [135]MicronalregDS 5007X and PCS28 a laboratory-made samplewere used to characterize e results showed that the meaneffective Youngrsquos modulus of MicronalregDS 5007 X andPCS28 was 200MPa and 43MPa at 23degC respectively eneeded forces to break the microcapsules were 11 μN and25 μN

AFM analysis of the mechanical properties of micro-capsules is relatively simple but there are shortcomingsFirstly there can be a significant error in calculating theapplied load using the displacement of the microcantileverWhen the tip of the cantilever beam is not in contact with the


surface of the sample the cantilever beam may be bent dueto the attraction between the atoms It is difficult to rec-ognize the contact zero point In addition when a load isapplied the measured value may be deviated due to thedeformation of the cantilever beam as shown in Figure 6(a)Secondly during operation it is necessary to ensure that thetip is pressed directly above the microcapsules and that nooffset occurs Otherwise the cantilever beam will be sub-jected to additional bending moments and the expectedcapsule failure mode will not be obtained irdly since themicrocapsules are deformed during the pressing process themeasured displacement is not equal to the tip indentationdepth

e micronanoindentation method applies a load to themicrocapsules by using different shapes of indenters toobtain a load displacement curve and analyze mechanicalproperties of the microcapsules Compared to AFM analysisthe nanoindentation method is superior in that its load canbe detected directly by the inductor on the pressin device asshown in Figure 6(b) However similar problems exist innanoindentation it is hard to ensure that the indenter ispressed from the top of the microcapsules and the mi-crocapsules will deform during the pressing process whichaffects the accuracy of the measurement results Yang et alprepared microcapsules with nano CaCO3organic com-posite shells by in situ polymerization [136] Youngrsquosmodulus values of the microcapsules were in a range of275ndash346GPa using micro-nanoindentation Lee et alfound that the modulus and hardness of hardener-loadedmicrocapsules both increased with size with Youngrsquosmodulus and hardness of the microcapsules measured 3 to 5times by nanoindentation [137] Microcompression alsoknown as a micromanipulation system uses a cylindrical flathead (larger than the microcapsule size) to uniaxiallycompress a single microcapsule placed on a glass slide asshown in Figure 6(c) By detecting the shape variation oc-curring when the indenter is in contact with the micro-capsule the force change of the microcapsule during thecompression test can be measured until the microcapsulebroke e microcapsule compression load-displacementcurve is obtained which is used to analyze the mechanicalproperties of individual microcapsules e advantage of themicrocompression method is that the mechanical propertiesof the microcapsules under large deformation can be de-termined and not only the elastic deformation and plasticdeformation of the microcapsules but also the damage loadand deformation of the microcapsules can be accuratelydetermined However the accuracy of the micro-compression test results greatly depends mainly on theresolution of the test instrument e instrument can onlyperform compression tests on a single microcapsule at a timeand cannot achieve continuous compression tests It is alsonecessary to use relevant theoretical models to determine themechanical parameters such as the elastic modulus of themicrocapsule wall material

Sun et al used micromanipulation to test the mechanicalproperties of microcapsules made of two different wallmaterials [96] including MF resin and UF resin [138] MFand UF microcapsules showed that the yield point of

deformation was 19plusmn 1 and 17plusmn 1 and deformation atbursting was 68plusmn 1 and 35plusmn 1 respectively e forceneeded to break the microcapsule and deformation atbursting for both microcapsules increased proportionallywith their diameter Zhang tested bursting force of single drymicrocapsules in two samples different in size and wallthickness by this micromanipulation [139] e burstingforce of the microcapsules in one sample ranged from 50 to220 μN and the diameter from 13 to 70 μm while thebursting force in the other was from 20 to 175 μN and thediameter from 07 to 37 μm

ere are mainly two ways to test the mechanicalproperties of multiple microcapsules e first method is toplace multiple microcapsules between two plates as shown inFigure 6(d) By applying pressure to the plate to break themicrocapsules the load displacement curve is detected inthis process e mechanical properties of the microcapsuleare then obtained We can also observe the deformation andfailure of the microcapsule in different stages by loading themicrocapsules in multiple steps e second way is tomeasure the mechanical properties of microcapsules by theshear test e microcapsules are dissolved in a solvent andbroken by means of oscillation and centrifugation etcenthe quality of the microcapsule after the destructive test isobtained By comparing with the original microcapsule thebroken ratio is obtained which is used tomeasure the overallmechanical properties of themicrocapsule In comparison tothe detection of a single microcapsule the detection ofmultiple microcapsules can only obtain qualitative resultsand accurate mechanical parameters cannot be obtainede detection results have relatively large errors In contrastthe advantage of multiple microcapsule detection is that it issimple to operate and low in cost By depositing multiplemicrocapsules between two plates Su et al observed thesurface morphological structure change after compressing[140] When the mass ratio of the core and shell material was3 1 the yield stress of microcapsules was about 11times 105 PaWhen the compression was increased beyond this value themicrocapsules showed plastic behaviors Moreover it wasfound that the mechanical intensity of double-shell mi-crocapsules was better than that of single shelled ones Liet al mixed microcapsules with alcohol and centrifuged[130]e precipitate was then washed by distilled water anddried e mechanical strength of microcapsules wascharacterized by using the breakage rate which is calculatedusing the following equation

breakage rate () initialmass minus mass after centrifugation

initialmass

times 100

(5)

the breakage rates of microcapsules at 6000 rpm for 20minand 8000 rpm for 10min were 115 and 156 respectivelyHowever the breakage rate of microcapsules having carbonnanotubes grafted with stearyl alcohol (CNTs-SA) at thesame speed for the same time was 0 e breakage rate at8000 rpm for 20min was 398 for microcapsules and 215


for microcapsulesCNTs-SA which indicated that the me-chanical property of microcapsulesCNTs-SA was betterthan that of microcapsules without modication is wasmainly attributed to the modied CNTs which showedstronger mechanical properties when compared withpolymers

6 Applications of PCM Microcapsules

PCM microcapsules expand the application elds of thePCMs due to their unique properties such as (1) chemicaland thermal stabilization (2) higher amount of energeticchanges and (3) suitable solid-to-liquid phase transitionPCMmicrocapsules provide a reliable way for mixing liquidand solid PCMs with polymer and other structural materialswhich reduce toxicity and protect core material from en-vironmental inyenuence Figure 7 shows potential applicationsof PCM microcapsules and we will give more details in thissection

61 Textile e application of PCM microcapsules in thetextile industries is an old topic but draws continuallygrowing attention by the researchers For example PCMmicrocapsules are used for protection from extreme coldweather outdoor wear such as snowsuits trousers earwarmers boots and gloves [15 141] to provide extraprotection from extreme cold weather PCM microcap-sules can be coated on the surface of fabric or embeddedwithin the ber to enhance their thermal storage capac-ities (by 25ndash45 times compared to the reference fabricber for particular temperature intervals) [115 141] Sarier

et al indicated that the PCM microcapsules with silvernanoparticles had high thermal stability high thermalstorage capacities good durability and improved thermalconductivity erefore this technology is promising fortextile industry applications such as automotive and ag-riculture textiles sportswearprotective clothing and

hd

(a)

F

(b)

F

(c)

F

(d)

Figure 6 Schematic diagram of microcapsule mechanical properties testing (a) atomic force microscopic analysis (b) micro-nano-indentation detection (c) micromanipulation system and (d) multiple microcapsules between two plates

PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar

Figure 7 Potential applications of PCMs microcapsules in textileslurry building foams solarelectrical-to-energy and biomedicine


medical textiles [132] Nejman et al found that themodified fabric obtained by the printing method had thelargest enthalpy value and the lowest gas permeabilitywhile the padding method had the smallest enthalpy valueand the highest air permeability [142] Scacchetti et alinvestigated the thermal and antimicrobial properties ofcotton with silver zeolites functionalized via a chitosan-zeolite composite and microcapsules of PCMs [143] eyrecommended to use chitosan zeolite for the production oftextiles for superior antibacterial and thermoregulatingproperties PCM microcapsules as additives improve thethermal comfort and flame-retardant property of thetextiles the results showed that PCM microcapsules weredistributed onto textile substrates homogeneously and wasdurable with repeated washings [144 145] In additionthermoregulating properties of the fabrics with PCMmicrocapsules were supported through thermal historymeasurement results

62 Slurry Another interesting application of PCM mi-crocapsules is in the slurry industry PCM microcapsulesslurry with high latent heat has been widely used in the fieldsof cooling and heating because of its high heat rate as anenhanced heat transfer fluid (HTF) and thermal storagemedium (TSM) PCM microcapsules slurry has all thebenefits of PCM suspension without coalescing problemdue to the shells preventing the contact between PCMdroplets [146] ere are many research and review paperson characterization of thermalrheological properties forPCM microcapsules [147ndash150] ey used heat transfercoefficient Nusselt number and wall temperature to di-rectly and indirectly reflect the heat transfer properties ofPCM microcapsules slurry Song et al investigated laminarheat transfer of PCM microcapsules slurry and demon-strated that the heat transfer coefficient increased with in-creasing Reynolds number and volume concentration ofmicrocapsules [151] Roberts et al compared the heattransfer performance of metal-coated PCM microcapsulesslurry with nonmetal-coated PCMmicrocapsules slurry withsame PCM content [152] ey observed a further 10increase in heat transfer coefficient and PCM microcapsulesinducing pressure drop in slurry Zhang and Niu studied thethermal storage properties of PCM microcapsules slurrystorage device and stratified water storage tank [153] eyshowed that PCM microcapsules slurry had higher thermalstorage capacity Xu et al proposed the PCM microcapsuleswith Cu-Cu2OCNTs (carbon nanotubes) as the shell andtheir dispersed slurry for direct absorption solar collectors[154] ey pointed out that the high heat storage capabilityand excellent photothermal conversion performance ofPCMsCu-Cu2OCNTs microcapsule slurry made it as oneof the most potential heat transfer fluids for direct ab-sorption solar collector

63 Building In building applications PCM microcapsulescan be embedded into concrete mixes wall boards cementmortar gypsum plaster sandwich panels and slabs to meetthe energy demand of the building for cooling heating air

conditioning ventilation domestic hot water systems andlighting [155] In fact the concrete is the major constructionmaterial in building and the embedment of PCM micro-capsules into concrete enhances the thermal and acousticinsulation of walls Cabeza et al investigated a new concretewith PCM microcapsules for superior thermal properties[156] ey showed that the concrete wall containing PCMmicrocapsules had smoother fluctuations of temperatureand thermal inertia indicating this technology can providepromising energy saving for buildings In addition addingPCM microcapsules to concrete was found to significantlyincrease the overall mechanical resistance and stiffness[157 158] In addition the existence of PCM microcapsuleshad no effect on drying shrinkage of cementitious com-posites and the thermal deformation coefficient of PCMmicrocapsules was similar to the shell materials [122] Suet al studied nano-silicon dioxide hydrosol as the surfactantfor preparation of PCM microcapsules for thermal energystorage in buildings [159] ey pointed out that nano-sil-icon dioxide hydrosol could be used as a surfactant and forimproving the shell integritycore material content of PCMmicrocapsules which had an additional benefit for thermalconductivity enhancement Essid et al studied compressivestrength and hygric properties of concretes by incorporatingPCM microcapsules [160] ey indicated that it was suf-ficiently safe to use the mixture of concrete and PCM mi-crocapsules as structural material though its compressivestrength is lower and porosity is higher than the pureconcrete PCM microcapsules can also be embedded intowall boards cement mortar gypsum plaster sandwichpanels and slabs in buildings to reduce the power demand inboth residential and commercial building sectors [161ndash166]

64 Foams Applying PCM microcapsules in foams canimprove thermal performances especially in the thermal-insulating ability Borreguero et al investigated rigid poly-urethane foams containing PCM microcapsules and indi-cated that thermal energy storage capacity of rigidpolyurethane foams was improved [167] Bonadies et alpresented hot storage and dimensional stability of poly(vinylalcohol)- (PVA-) based foams containing PCM microcap-sules [168] ey pointed out that microcapsules influencedthe amount of water uptake and the formation of crystallinedomain which affected the number of intra- and inter-molecular hydrogen bonds since several ndashOH groups ofPVA interact with microcapsule shells Li et al proposed anew strategy for enhanced latent heat energy storage withPCM microcapsules saturated in metal foam [169] Com-pared with the surface temperature of pristine PCM mod-ules with the thermal conductivity enhancement of metalfoam the surface temperature for the PCM microcapsulefoam and PCMfoam composite modules was reduced fromabout 90degC to 55degC and 45degC respectively Both leakage andlow thermal conductivity issues were solved with PCMmicrocapsulefoam composites Moreover enhanced ther-mal management with PCM microcapsule infiltrated incellular metal foam was considered [170] e low thermalconductivity of PCM and contact thermal resistance


between PCMsmicrocapsules resulted in remarkable surfacetemperature increase and huge temperature difference at thevicinity of heated surface for pure PCM microcapsules

65 Others Moreover PCM microcapsules still have otherpotential applications such as solar-to-thermal energystorage electrical-to-thermal energy storage and biomedi-cine [114] Zhang et al studied solar-driven PCM micro-capsules with efficient Ti4O7 nanoconverter for latent heatstorage [171] ey indicated that solar absorption capacityof the novel PCM microcapsules was calculated up to8828 and the photothermal storage efficiency of thePCMsSiO2Ti4O7 microcapsules was up to 8536 com-pared with 2414 for pure PCMs Zheng et al proposed ajoule heating system to reduce the convective heat trans-ferring from electrothermal system of the surrounding byinserting the highly conductive and stable PCMs micro-capsules [172] ey showed that the working temperaturecould be improved by 30 with loading of 5 PCMs mi-crocapsules even at lower voltage and ambient temperatureZhang et al developed the multifunctional PCMs micro-capsules for sterilization [173] ey discovered high anti-bacterial activity especially against E coli S aureus andBacillus subtilis and the antibacterial effectiveness of 2-hourcontacting PCM microcapsules was inhibited up to 646991 and 959 respectively

7 Conclusions and Outlook

Microencapsulation technology is employed to fabricatePCM microcapsules as new types of polymercompositematerials for thermal energy storage PCMs were classifiedinto three categories ie organic inorganic and eutecticmaterials And shell materials were also classified into threecategories ie organic inorganic and organic-inorganichybrid materials Available microencapsulation techniquesfor PCMs were reviewed and classified into three categoriessuch as physical chemical and physical-chemical processese selection of the best suited microcapsule techniquemainly depends on the specifications of PCMmicrocapsulesincluding materials of the coreshell microcapsule sizethickness of the shell mechanical behaviors and thermalproperties Additionally this comprehensive review paperexamined the technologies being used to characterize PCMmicrocapsules For example DSC is employed to analyzethermal properties FTIR is used to characterize the chemicalstructure and composition and AFM is utilized to measurethe mechanical properties respectively e thermalphysical chemical and mechanical properties of PCMmicrocapsules are heavily dependent on the raw materialsand synthesis processes during microencapsulation Finallythe applications of PCM microcapsules in textile slurrybuilding and foams were presented and explained in detailPCM microcapsules still have other potential applicationssuch as solar-to-thermal energy storage electrical-to-ther-mal energy storage and biomedicine e results are quitepromising for their future utilization in practice AlthoughPCM microcapsules may seem attractive thermal energy

storage materials there is still much to be explored andimproved in fabrication characterization and commercialutilization For example the encapsulation efficiency isunsatisfactory the contents of materials should be enhancedparticle sizes should be reduced and the crosslinking time istoo long Regarding characterization methods a standardmechanical characterization method for PCM microcap-sules is needed due to most of the PCM microcapsules nottested for leakage Last but not least a major obstacle to theindustrial application of PCMmicrocapsules is supercoolingthat should be solved in future

Conflicts of Interest

e authors declare that they have no conflicts of interest

Acknowledgments

e authors would like to gratefully acknowledge the sup-port from the National Natural Science Foundation of China(Grant nos 11772302 and 11402233)

References

[1] M S Dresselhaus and I L omas ldquoAlternative energytechnologiesrdquo Nature vol 414 no 6861 pp 332ndash337 2001

[2] Y Lin Y Jia G Alva and G Fang ldquoReview on thermalconductivity enhancement thermal properties and appli-cations of phase change materials in thermal energy storagerdquoRenewable and Sustainable Energy Reviews vol 82pp 2730ndash2742 2018

[3] M Waterson ldquoe characteristics of electricity storage re-newables and marketsrdquo Energy Policy vol 104 pp 466ndash4732017

[4] D Verdier A Ferriere Q Falcoz F Siros and R CouturierldquoExperimentation of a high temperature thermal energystorage prototype using phase change materials for thethermal protection of a pressurized air solar receiverrdquo EnergyProcedia vol 49 pp 1044ndash1053 2014

[5] M R Anisur M H Mahfuz M A Kibria R SaidurI H S C Metselaar and T M I Mahlia ldquoCurbing globalwarming with phase change materials for energy storagerdquoRenewable and Sustainable Energy Reviews vol 18 pp 23ndash30 2013

[6] I Sarbu and C Sebarchievici ldquoA comprehensive review ofthermal energy storagerdquo Sustainability vol 10 no 2 p 1912018

[7] M Karthikeyan and T Ramachandran ldquoReview of thermalenergy storage of micro- and nanoencapsulated phasechange materialsrdquo Materials Research Innovations vol 18no 7 pp 541ndash554 2014

[8] A Abhat ldquoLow temperature latent heat thermal energystorage heat storage materialsrdquo Solar Energy vol 30 no 4pp 313ndash332 1983

[9] S D Sharma and K Sagara ldquoLatent heat storage materialsand systems a reviewrdquo International Journal of Green En-ergy vol 2 no 1 pp 1ndash56 2005

[10] R Jacob and F Bruno ldquoReview on shell materials used in theencapsulation of phase change materials for high tempera-ture thermal energy storagerdquo Renewable and SustainableEnergy Reviews vol 48 pp 79ndash87 2015

[11] A Sharma V V Tyagi C R Chen and D Buddhi ldquoReviewon thermal energy storage with phase change materials and


applicationsrdquo Renewable and Sustainable Energy Reviewsvol 13 no 2 pp 318ndash345 2009

[12] B Zalba J M Marın L F Cabeza and H Mehling ldquoReviewon thermal energy storage with phase change materials heattransfer analysis and applicationsrdquo Applied ermal Engi-neering vol 23 no 3 pp 251ndash283 2003

[13] Y P Zhang K P Lin R Yang H F Di and Y JiangldquoPreparation thermal performance and application ofshape-stabilized PCM in energy efficient buildingsrdquo Energyand Buildings vol 38 no 10 pp 1262ndash1269 2006

[14] X Huang C Zhu Y Lin and G Fang ldquoermal propertiesand applications of microencapsulated PCM for thermalenergy storage a reviewrdquo Applied ermal Engineeringvol 147 pp 841ndash855 2019

[15] G Alva Y Lin L Liu and G Fang ldquoSynthesis charac-terization and applications of microencapsulated phasechangematerials in thermal energy storage a reviewrdquo Energyand Buildings vol 144 pp 276ndash294 2017

[16] P Schossig H-M Henning S Gschwander andT Haussmann ldquoMicro-encapsulated phase-changematerialsintegrated into construction materialsrdquo Solar Energy Ma-terials and Solar Cells vol 89 no 2-3 pp 297ndash306 2005

[17] X-x Zhang X-m Tao K-l Yick and X-c WangldquoStructure and thermal stability of microencapsulated phase-change materialsrdquo Colloid and Polymer Science vol 282no 4 pp 330ndash336 2004

[18] A Sarı C Alkan A Karaipekli and O Uzun ldquoMicro-encapsulated n-octacosane as phase change material forthermal energy storagerdquo Solar Energy vol 83 no 10pp 1757ndash1763 2009

[19] M Hawlader M Uddin and M M Khin ldquoMicro-encapsulated PCM thermal-energy storage systemrdquo AppliedEnergy vol 74 no 1-2 pp 195ndash202 2003

[20] V V Tyagi S C Kaushik S K Tyagi and T AkiyamaldquoDevelopment of phase change materials based micro-encapsulated technology for buildings a reviewrdquo Renewableand Sustainable Energy Reviews vol 15 no 2 pp 1373ndash13912011

[21] T Khadiran M Z Hussein Z Zainal and R Rusli ldquoAd-vanced energy storage materials for building applicationsand their thermal performance characterization a reviewrdquoRenewable and Sustainable Energy Reviews vol 57pp 916ndash928 2016

[22] F Agyenim N Hewitt P Eames and M Smyth ldquoA reviewof materials heat transfer and phase change problem for-mulation for latent heat thermal energy storage systems(LHTESS)rdquo Renewable and Sustainable Energy Reviewsvol 14 no 2 pp 615ndash628 2010

[23] F Tan and C Tso ldquoCooling of mobile electronic devicesusing phase change materialsrdquo Appliedermal Engineeringvol 24 no 2-3 pp 159ndash169 2004

[24] S Mondal ldquoPhase change materials for smart textilesndashanoverviewrdquo Applied ermal Engineering vol 28 no 11-12pp 1536ndash1550 2008

[25] K Pielichowska and K Pielichowski ldquoPhase change mate-rials for thermal energy storagerdquo Progress in Materials Sci-ence vol 65 pp 67ndash123 2014

[26] H Nazir M Batool F J Bolivar Osorio et al ldquoRecentdevelopments in phase change materials for energy storageapplications a reviewrdquo International Journal of Heat andMass Transfer vol 129 pp 491ndash523 2019

[27] D Kannan R Chellappa and W-M Chien ldquoermody-namic assessment of binary solid-state thermal storage

materialsrdquo Journal of Physics and Chemistry of Solids vol 66no 2ndash4 pp 235ndash240 2005

[28] W Su J Darkwa and G Kokogiannakis ldquoReview of solid-liquid phase change materials and their encapsulationtechnologiesrdquo Renewable and Sustainable Energy Reviewsvol 48 pp 373ndash391 2015

[29] M Kenisarin and K Mahkamov ldquoSolar energy storage usingphase change materialsrdquo Renewable and Sustainable EnergyReviews vol 11 no 9 pp 1913ndash1965 2007

[30] M M Farid A M Khudhair S A K Razack and S Al-Hallaj ldquoA review on phase change energy storage materialsand applicationsrdquo Energy Conversion and Managementvol 45 no 9-10 pp 1597ndash1615 2004

[31] A M Khudhair and M M Farid ldquoA review on energyconservation in building applications with thermal storageby latent heat using phase change materialsrdquo Energy Con-version and Management vol 45 no 2 pp 263ndash275 2004

[32] A Hasan and A A Sayigh ldquoSome fatty acids as phase-change thermal energy storage materialsrdquo Renewable Energyvol 4 no 1 pp 69ndash76 1994

[33] J Giro-Paloma M Martınez L F Cabeza andA I Fernandez ldquoTypes methods techniques and appli-cations for microencapsulated phase change materials(MPCM) a reviewrdquo Renewable and Sustainable EnergyReviews vol 53 pp 1059ndash1075 2016

[34] Y Yuan N ZhangW Tao X Cao and Y He ldquoFatty acids asphase change materials a reviewrdquo Renewable and Sustain-able Energy Reviews vol 29 pp 482ndash498 2014

[35] W Wang X Yang Y Fang and J Ding ldquoPreparation andperformance of form-stable polyethylene glycolsilicon di-oxide composites as solid-liquid phase change materialsrdquoApplied Energy vol 86 no 2 pp 170ndash174 2009

[36] H Ji D P Sellan M T Pettes et al ldquoEnhanced thermalconductivity of phase change materials with ultrathin-graphite foams for thermal energy storagerdquo Energy amp En-vironmental Science vol 7 no 3 pp 1185ndash1192 2014

[37] J P Da Cunha and P Eames ldquoermal energy storage forlow and medium temperature applications using phasechange materialsndasha reviewrdquo Applied Energy vol 177pp 227ndash238 2016

[38] S A Mohamed F A Al-Sulaiman N I Ibrahim et al ldquoAreview on current status and challenges of inorganic phasechange materials for thermal energy storage systemsrdquo Re-newable and Sustainable Energy Reviews vol 70 pp 1072ndash1089 2017

[39] N Xie J Luo Z Li et al ldquoSalt hydrateexpanded vermiculitecomposite as a form-stable phase change material forbuilding energy storagerdquo Solar Energy Materials and SolarCells vol 189 pp 33ndash42 2019

[40] N Zhang Y Yuan X Cao Y Du Z Zhang and Y GuildquoLatent heat thermal energy storage systems with solid-liquidphase change materials a reviewrdquo Advanced EngineeringMaterials vol 20 no 6 p 1700753 2018

[41] S E Kalnaeligs and B P Jelle ldquoPhase change materials andproducts for building applications a state-of-the-art reviewand future research opportunitiesrdquo Energy and Buildingsvol 94 pp 150ndash176 2015

[42] M M Kenisarin ldquoHigh-temperature phase change materialsfor thermal energy storagerdquo Renewable and SustainableEnergy Reviews vol 14 no 3 pp 955ndash970 2010

[43] M Liu W Saman and F Bruno ldquoReview on storage ma-terials and thermal performance enhancement techniquesfor high temperature phase change thermal storage systemsrdquo


Renewable and Sustainable Energy Reviews vol 16 no 4pp 2118ndash2132 2012

[44] H Ge H Li S Mei and J Liu ldquoLow melting point liquidmetal as a new class of phase change material an emergingfrontier in energy areardquo Renewable and Sustainable EnergyReviews vol 21 pp 331ndash346 2013

[45] L Wang and D Meng ldquoFatty acid eutecticpolymethylmethacrylate composite as form-stable phase change ma-terial for thermal energy storagerdquo Applied Energy vol 87no 8 pp 2660ndash2665 2010

[46] J-L Zeng Y-H Chen L Shu et al ldquoPreparation andthermal properties of exfoliated graphiteerythritolmannitoleutectic composite as form-stable phase change material forthermal energy storagerdquo Solar Energy Materials and SolarCells vol 178 pp 84ndash90 2018

[47] A Hoshi D R Mills A Bittar and T S Saitoh ldquoScreeningof high melting point phase change materials (PCM) in solarthermal concentrating technology based on CLFRrdquo SolarEnergy vol 79 no 3 pp 332ndash339 2005

[48] N R Jankowski and F P McCluskey ldquoA review of phasechange materials for vehicle component thermal bufferingrdquoApplied Energy vol 113 pp 1525ndash1561 2014

[49] B Xu P Li and C Chan ldquoApplication of phase changematerials for thermal energy storage in concentrated solarthermal power plants a review to recent developmentsrdquoApplied Energy vol 160 pp 286ndash307 2015

[50] R Al-Shannaq J Kurdi S Al-Muhtaseb and M FaridldquoInnovative method of metal coating of microcapsulescontaining phase change materialsrdquo Solar Energy vol 129pp 54ndash64 2016

[51] A Jamekhorshid S M Sadrameli andM Farid ldquoA review ofmicroencapsulation methods of phase change materials(PCMs) as a thermal energy storage (TES) mediumrdquo Re-newable and Sustainable Energy Reviews vol 31 pp 531ndash5422014

[52] M M Umair Y Zhang K Iqbal S Zhang and B TangldquoNovel strategies and supporting materials applied to shape-stabilize organic phase change materials for thermal energystorage-A reviewrdquo Applied Energy vol 235 pp 846ndash8732019

[53] D Yin L Ma J Liu and Q Zhang ldquoPickering emulsion anovel template for microencapsulated phase change mate-rials with polymer-silica hybrid shellrdquo Energy vol 64pp 575ndash581 2014

[54] Z Chen J Wang F Yu Z Zhang and X Gao ldquoPreparationand properties of graphene oxide-modified poly(melamine-formaldehyde) microcapsules containing phase changematerial n-dodecanol for thermal energy storagerdquo Journal ofMaterials Chemistry A vol 3 no 21 pp 11624ndash11630 2015

[55] R Huang W Li J Wang and X Zhang ldquoEffects of oil-soluble etherified melamine-formaldehyde prepolymers onin situ microencapsulation and macroencapsulation of n-dodecanolrdquo New Journal of Chemistry vol 41 no 17pp 9424ndash9437 2017

[56] J Su L Wang and L Ren ldquoFabrication and thermalproperties of microPCMs used melamine-formaldehyderesin as shell materialrdquo Journal of Applied Polymer Sciencevol 101 no 3 pp 1522ndash1528 2006

[57] X Wang Z Chen W Xu and X Wang ldquoCapric acid phasechange microcapsules modified with graphene oxide forenergy storagerdquo Journal of Materials Science vol 54 no 24pp 14834ndash14844 2019

[58] E N Brown M R Kessler N R Sottos and S R WhiteldquoIn situpoly(urea-formaldehyde) microencapsulation of

dicyclopentadienerdquo Journal of Microencapsulation vol 20no 6 pp 719ndash730 2003

[59] Y Zhao W Zhang L-p Liao S-j Wang andW-j Li ldquoSelf-healing coatings containing microcapsulerdquo Applied SurfaceScience vol 258 no 6 pp 1915ndash1918 2012

[60] Z Zhao X Zhou Q Tian X Wang W Li and D LiuldquoMicroencapsulation of triglycidyl isocyanurate by solventevaporation method for UV and thermal dual-cured coat-ingsrdquo Journal of Applied Polymer Science vol 131 no 212014

[61] D Xu and R Yang ldquoEfficient preparation and character-ization of paraffin-based microcapsules by emulsion poly-merizationrdquo Journal of Applied Polymer Science vol 136no 21 p 47552 2019

[62] J Huang T Wang P Zhu and J Xiao ldquoPreparationcharacterization and thermal properties of the microen-capsulation of a hydrated salt as phase change energy storagematerialsrdquo ermochimica Acta vol 557 pp 1ndash6 2013

[63] X Qiu W Li G Song X Chu and G Tang ldquoFabricationand characterization of microencapsulated n-octadecanewith different crosslinked methylmethacrylate-based poly-mer shellsrdquo Solar Energy Materials and Solar Cells vol 98pp 283ndash293 2012

[64] X Qiu G Song X Chu X Li and G Tang ldquoPreparationthermal properties and thermal reliabilities of micro-encapsulated n-octadecane with acrylic-based polymer shellsfor thermal energy storagerdquo ermochimica acta vol 551pp 136ndash144 2013

[65] S Ma G Song W Li P Fan and G Tang ldquoUV irradiation-initiated MMA polymerization to prepare microcapsulescontaining phase change paraffinrdquo Solar Energy Materialsand Solar Cells vol 94 no 10 pp 1643ndash1647 2010

[66] F Mohaddes S Islam R Shanks M Fergusson L Wangand R Padhye ldquoModification and evaluation of thermalproperties of melamine-formaldehyden-eicosane micro-capsules for thermo-regulation applicationsrdquo Appliedermal Engineering vol 71 no 1 pp 11ndash15 2014

[67] C Alkan A Sarı and A Karaipekli ldquoPreparation thermalproperties and thermal reliability of microencapsulated n-eicosane as novel phase change material for thermal energystoragerdquo Energy Conversion and Management vol 52 no 1pp 687ndash692 2011

[68] Y Ma X Chu W Li and G Tang ldquoPreparation andcharacterization of poly(methyl methacrylate-co-divinyl-benzene) microcapsules containing phase change tempera-ture adjustable binary core materialsrdquo Solar Energy vol 86no 7 pp 2056ndash2066 2012

[69] S Wu L Yuan A Gu Y Zhang and G Liang ldquoSynthesisand characterization of novel epoxy resins-filled microcap-sules with organicinorganic hybrid shell for the self-healingof high performance resinsrdquo Polymers for Advanced Tech-nologies vol 27 no 12 pp 1544ndash1556 2016

[70] H Zhang X Wang and D Wu ldquoSilica encapsulation of n-octadecane via sol-gel process a novel microencapsulatedphase-change material with enhanced thermal conductivityand performancerdquo Journal of colloid and interface sciencevol 343 no 1 pp 246ndash255 2010

[71] Z Liu Z Chen and F Yu ldquoPreparation and characterizationof microencapsulated phase change materials containinginorganic hydrated salt with silica shell for thermal energystoragerdquo Solar Energy Materials and Solar Cells vol 200p 110004 2019

[72] F He X Wang and D Wu ldquoNew approach for sol-gelsynthesis of microencapsulated n-octadecane phase change


material with silica wall using sodium silicate precursorrdquoEnergy vol 67 pp 223ndash233 2014

[73] F Li X Wang and D Wu ldquoFabrication of multifunctionalmicrocapsules containing n-eicosane core and zinc oxideshell for low-temperature energy storage photocatalysis andantibiosisrdquo Energy Conversion and Management vol 106pp 873ndash885 2015

[74] Y Bao Y Yan Y Chen J Ma W Zhang and C Liu ldquoFacilefabrication of BTAZnO microcapsules and their corrosionprotective application in waterborne polyacrylate coatingsrdquoProgress in Organic Coatings vol 136 p 105233 2019

[75] L Zhao H Wang J Luo Y Liu G Song and G TangldquoFabrication and properties of microencapsulated n-octa-decane with TiO2 shell as thermal energy storage materialsrdquoSolar Energy vol 127 pp 28ndash35 2016

[76] M Genc and Z Karagoz Genc ldquoMicroencapsulated myristicacid-fly ash with TiO2 shell as a novel phase change materialfor building applicationrdquo Journal of ermal Analysis andCalorimetry vol 131 no 3 pp 2373ndash2380 2018

[77] L Chai X Wang and D Wu ldquoDevelopment of bifunctionalmicroencapsulated phase change materials with crystallinetitanium dioxide shell for latent-heat storage and photo-catalytic effectivenessrdquo Applied Energy vol 138 pp 661ndash6742015

[78] Z Jiang W Yang F He et al ldquoMicroencapsulated paraffinphase-change material with calcium carbonate shell forthermal energy storage and solar-thermal conversionrdquoLangmuir vol 34 no 47 pp 14254ndash14264 2018

[79] T Wang S Wang L Geng and Y Fang ldquoEnhancement onthermal properties of paraffincalcium carbonate phasechangemicrocapsules with carbon networkrdquoApplied Energyvol 179 pp 601ndash608 2016

[80] T Wang S Wang R Luo C Zhu T Akiyama andZ Zhang ldquoMicroencapsulation of phase change materialswith binary cores and calcium carbonate shell for thermalenergy storagerdquo Applied Energy vol 171 pp 113ndash119 2016

[81] G Fang Z Chen and H Li ldquoSynthesis and properties ofmicroencapsulated paraffin composites with SiO2 shell asthermal energy storage materialsrdquo Chemical EngineeringJournal vol 163 no 1-2 pp 154ndash159 2010

[82] C Takai-Yamashita I Shinkai M Fuji and M S El Sal-mawy ldquoEffect of water soluble polymers on formation ofNa2SO4 contained SiO2 microcapsules by WO emulsion forlatent heat storagerdquo Advanced Powder Technology vol 27no 5 pp 2032ndash2038 2016

[83] J Zhang S S Wang S D Zhang et al ldquoIn situ synthesis andphase change properties of Na2SO4middot10H2OSiO2 solidnanobowls toward smart heat storagerdquo e Journal ofPhysical Chemistry C vol 115 no 41 pp 20061ndash20066 2011

[84] M Li W Wang Z Zhang et al ldquoMonodisperseNa2SO4middot10H2OSiO2 microparticles against supercoolingand phase separation during phase change for efficient en-ergy storagerdquo Industrial amp Engineering Chemistry Researchvol 56 no 12 pp 3297ndash3308 2017

[85] H Yuan H Bai X Lu et al ldquoSize controlled lauric acidsilicon dioxide nanocapsules for thermal energy storagerdquoSolar Energy Materials and Solar Cells vol 191 pp 243ndash2572019

[86] Y Lin C Zhu and G Fang ldquoSynthesis and properties ofmicroencapsulated stearic acidsilica composites with gra-phene oxide for improving thermal conductivity as novelsolar thermal storage materialsrdquo Solar Energy Materials andSolar Cells vol 189 pp 197ndash205 2019

[87] S Liang Q Li Y Zhu et al ldquoNanoencapsulation of n-octadecane phase change material with silica shell throughinterfacial hydrolysis and polycondensation in mini-emulsionrdquo Energy vol 93 pp 1684ndash1692 2015

[88] S Yu X Wang and D Wu ldquoMicroencapsulation of n-octadecane phase change material with calcium carbonateshell for enhancement of thermal conductivity and servingdurability synthesis microstructure and performanceevaluationrdquo Applied energy vol 114 pp 632ndash643 2014

[89] H Liu X Wang D Wu and S Ji ldquoFabrication and ap-plications of dual-responsive microencapsulated phasechange material with enhanced solar energy-storage andsolar photocatalytic effectivenessrdquo Solar Energy Materialsand Solar Cells vol 193 pp 184ndash197 2019

[90] R Palkovits H Althues A Rumplecker et al ldquoPolymeri-zation of wo microemulsions for the preparation oftransparent SiO2PMMA nanocompositesrdquo Langmuirvol 21 no 13 pp 6048ndash6053 2005

[91] X Wang C Li and T Zhao ldquoFabrication and character-ization of poly(melamine-formaldehyde)silicon carbidehybrid microencapsulated phase change materials with en-hanced thermal conductivity and light-heat performancerdquoSolar Energy Materials and Solar Cells vol 183 pp 82ndash912018

[92] C Li H Yu Y Song H Liang and X Yan ldquoPreparation andcharacterization of PMMATiO2 hybrid shell micro-encapsulated PCMs for thermal energy storagerdquo Energyvol 167 pp 1031ndash1039 2019

[93] A Zhao J An J Yang and E-H Yang ldquoMicroencapsulatedphase change materials with composite titania-polyurea(TiO2-PUA) shellrdquo Applied Energy vol 215 pp 468ndash4782018

[94] HWang L Zhao L Chen G Song and G Tang ldquoFacile andlow energy consumption synthesis of microencapsulatedphase change materials with hybrid shell for thermal energystoragerdquo Journal of Physics and Chemistry of Solids vol 111pp 207ndash213 2017

[95] J Zhao Y Yang Y Li et al ldquoMicroencapsulated phasechange materials with TiO2-doped PMMA shell for thermalenergy storage and UV-shieldingrdquo Solar Energy Materialsand Solar Cells vol 168 pp 62ndash68 2017

[96] A M Borreguero J L Valverde J F RodrıguezA H Barber J J Cubillo and M Carmona ldquoSynthesis andcharacterization of microcapsules containing Rubi-thermRT27 obtained by spray dryingrdquo Chemical EngineeringJournal vol 166 no 1 pp 384ndash390 2011

[97] Y Lin C Zhu G Alva and G Fang ldquoMicroencapsulationand thermal properties of myristic acid with ethyl celluloseshell for thermal energy storagerdquo Applied Energy vol 231pp 494ndash501 2018

[98] T Y Wang and J Huang ldquoSynthesis and characterization ofmicroencapsulated sodium phosphate dodecahydraterdquoJournal of Applied Polymer Science vol 130 no 3pp 1516ndash1523 2013

[99] H Zhang and X Wang ldquoFabrication and performances ofmicroencapsulated phase change materials based on n-octadecane core and resorcinol-modified melami-nendashformaldehyde shellrdquo Colloids and Surfaces A Physico-chemical and Engineering Aspects vol 332 no 2-3pp 129ndash138 2009

[100] Y Konuklu M Unal and H O Paksoy ldquoMicroencapsu-lation of caprylic acid with different wall materials as phasechange material for thermal energy storagerdquo Solar EnergyMaterials and Solar Cells vol 120 pp 536ndash542 2014


[101] Y Zhang X Wang and D Wu ldquoDesign and fabrication ofdual-functional microcapsules containing phase changematerial core and zirconium oxide shell with fluorescentcharacteristicsrdquo Solar Energy Materials and Solar Cellsvol 133 pp 56ndash68 2015

[102] J-F Su S-B Wang J-W Zhou et al ldquoFabrication andinterfacial morphologies of methanol-melamine-formalde-hyde (MMF) shell microPCMsepoxy compositesrdquo Colloidand Polymer Science vol 289 no 2 pp 169ndash177 2011

[103] J-F Su X-Y Wang S-B Wang Y-H Zhao and Z HuangldquoFabrication and properties of microencapsulated-paraffingypsum-matrix building materials for thermal energy stor-agerdquo Energy Conversion and Management vol 55 pp 101ndash107 2012

[104] Y Ma X Chu G Tang and Y Yao ldquoe effect of differentsoft segments on the formation and properties of binary coremicroencapsulated phase change materials with polyureapolyurethane double shellrdquo Journal of Colloid and InterfaceScience vol 392 pp 407ndash414 2013

[105] S Lu T Shen J Xing et al ldquoPreparation and character-ization of cross-linked polyurethane shell microencapsulatedphase change materials by interfacial polymerizationrdquoMaterials Letters vol 211 pp 36ndash39 2018

[106] P Siddhan M Jassal and A K Agrawal ldquoCore content andstability ofn-octadecane-containing polyurea micro-encapsules produced by interfacial polymerizationrdquo Journalof Applied Polymer Science vol 106 no 2 pp 786ndash792 2007

[107] H Wang J Luo Y Yang L Zhao G Song and G TangldquoFabrication and characterization of microcapsulated phasechange materials with an additional function of thermo-chromic performancerdquo Solar Energy vol 139 pp 591ndash5982016

[108] L Sanchez-Silva J F Rodrıguez and P Sanchez ldquoInfluenceof different suspension stabilizers on the preparation ofRubitherm RT31 microcapsulesrdquo Colloids and Surfaces APhysicochemical and Engineering Aspects vol 390 no 1ndash3pp 62ndash66 2011

[109] X Tang W Li X Zhang and H Shi ldquoFabrication andcharacterization of microencapsulated phase change mate-rial with low supercooling for thermal energy storagerdquoEnergy vol 68 pp 160ndash166 2014


[111] N Sahan D Nigon S C Mantell J H Davidson andH Paksoy ldquoEncapsulation of stearic acid with differentPMMA-hybrid shell materials for thermotropic materialsrdquoSolar Energy vol 184 pp 466ndash476 2019

[112] A Sarı C Alkan and C Bilgin ldquoMicronano encapsulationof some paraffin eutectic mixtures with poly(methyl meth-acrylate) shell preparation characterization and latent heatthermal energy storage propertiesrdquo Applied Energy vol 136pp 217ndash227 2014

[113] A Sarı C Alkan and A Altıntas ldquoPreparation charac-terization and latent heat thermal energy storage propertiesof micro-nanoencapsulated fatty acids by polystyrene shellrdquoApplied ermal Engineering vol 73 no 1 pp 1160ndash11682014

[114] A Arshad M Jabbal Y Yan and J Darkwa ldquoe micro-nano-PCMs for thermal energy storage systems a state of artreviewrdquo International Journal of Energy Research vol 43no 11 pp 5572ndash5620 2019

[115] E Onder N Sarier and E Cimen ldquoEncapsulation of phasechange materials by complex coacervation to improvethermal performances of woven fabricsrdquo ermochimicaActa vol 467 no 1-2 pp 63ndash72 2008

[116] S T Latibari M Mehrali M Mehrali T M I Mahlia andH S C Metselaar ldquoSynthesis characterization and thermalproperties of nanoencapsulated phase change materials viasolndashgel methodrdquo Energy vol 61 pp 664ndash672 2013

[117] L Cao F Tang and G Fang ldquoSynthesis and characterizationof microencapsulated paraffin with titanium dioxide shell asshape-stabilized thermal energy storage materials in build-ingsrdquo Energy and Buildings vol 72 pp 31ndash37 2014

[118] Y Sun RWang X Liu S Fang D Li and B Li ldquoDesign of anovel multilayer low-temperature protection compositebased on phase change microcapsulesrdquo Journal of AppliedPolymer Science vol 136 no 20 Article ID 47534 2019

[119] X Wang P Sun N Han and F Xing ldquoExperimental studyon mechanical properties and porosity of organic micro-capsules based self-healing cementitious compositerdquo Ma-terials vol 10 no 1 2017

[120] J C Roy S Giraud A Ferri R Mossotti J Guan andF Salaun ldquoInfluence of process parameters on microcapsuleformation from chitosan-type B gelatin complex coacer-vatesrdquo Carbohydrate Polymers vol 198 pp 281ndash293 2018

[121] J Zhang T Zhao Y Chai and L Wang ldquoPreparation andcharacterization of high content paraffin wax microcapsulesand micronanocapsules with poly methyl methacrylate shellby suspension-like polymerizationrdquo Chinese Journal ofChemistry vol 35 no 4 pp 497ndash506 2017

[122] H Peng D Zhang X Ling et al ldquon-Alkanes phase changematerials and their microencapsulation for thermal energystorage a critical reviewrdquo Energy amp Fuels vol 32 no 7pp 7262ndash7293 2018

[123] Q Wu D Zhao X Jiao et al ldquoPreparation properties andsupercooling prevention of phase change material n-octa-decane microcapsules with peppermint fragrance scentrdquoIndustrial amp Engineering Chemistry Research vol 54 no 33pp 8130ndash8136 2015

[124] D-Z Chen S-Y Qin G C P Tsui et al ldquoFabricationmorphology and thermal properties of octadecylamine-grafted graphene oxide-modified phase-change microcap-sules for thermal energy storagerdquo Composites Part B En-gineering vol 157 pp 239ndash247 2019

[125] X Jiang R Luo F Peng Y Fang T Akiyama and S WangldquoSynthesis characterization and thermal properties of par-affin microcapsules modified with nano-Al2O3rdquo Appliedenergy vol 137 pp 731ndash737 2015

[126] Z Liu Z Chen and F Yu ldquoEnhanced thermal conductivityof microencapsulated phase change materials based ongraphene oxide and carbon nanotube hybrid fillerrdquo SolarEnergy Materials and Solar Cells vol 192 pp 72ndash80 2019

[127] K Peng L Fu X Li J Ouyang and H Yang ldquoStearic acidmodified montmorillonite as emerging microcapsules forthermal energy storagerdquo Applied Clay Science vol 138pp 100ndash106 2017

[128] M I Hasan ldquoNumerical investigation of counter flowmicrochannel heat exchanger with MEPCM suspensionrdquoAppliedermal Engineering vol 31 no 6-7 pp 1068ndash10752011

[129] Y-D Guo J-F Su R Mu et al ldquoMicrostructure andproperties of self-assembly graphene microcapsules effect ofthe pH valuerdquo Nanomaterials vol 9 no 4 p 587 2019

[130] M Li M Chen and Z Wu ldquoEnhancement in thermalproperty and mechanical property of phase change


microcapsule with modified carbon nanotuberdquo AppliedEnergy vol 127 pp 166ndash171 2014

[131] T Wang Y Jiang J Huang and S Wang ldquoHigh thermalconductive paraffincalcium carbonate phase change mi-crocapsules based composites with different carbon net-workrdquo Applied Energy vol 218 pp 184ndash191 2018

[132] N Sarier E Onder and G Ukuser ldquoSilver incorporatedmicroencapsulation of n-hexadecane and n-octadecane ap-propriate for dynamic thermal management in textilesrdquoermochimica Acta vol 613 pp 17ndash27 2015

[133] Y Zhang X Wang and D Wu ldquoMicroencapsulation of n-dodecane into zirconia shell doped with rare earth Designand synthesis of bifunctional microcapsules for photo-luminescence enhancement and thermal energy storagerdquoEnergy vol 97 pp 113ndash126 2016

[134] Q Xu H Liu X Wang and D Wu ldquoSmart design andconstruction of nanoflake-like MnO2SiO2 hierarchical mi-crocapsules containing phase change material for in-situthermal management of supercapacitorsrdquo Energy Conversionand Management vol 164 pp 311ndash328 2018

[135] J Giro-Paloma C Barreneche M Martınez B SumigaA I Fernandez and L F Cabeza ldquoMechanical responseevaluation of microcapsules from different slurriesrdquo Re-newable Energy vol 85 pp 732ndash739 2016

[136] P Yang S Han J-F Su et al ldquoDesign of self-healing mi-crocapsules containing bituminous rejuvenator with nano-CaCO3organic composite shell mechanical propertiesthermal stability and compactabilityrdquo Polymer Compositesvol 39 no S3 pp E1441ndashE1451 2018

[137] J Lee M Zhang D Bhattacharyya Y C YuanK Jayaraman and Y W Mai ldquoMicromechanical behavior ofself-healing epoxy and hardener-loaded microcapsules bynanoindentationrdquoMaterials Letters vol 76 pp 62ndash65 2012

[138] G Sun and Z Zhang ldquoMechanical strength of microcapsulesmade of different wall materialsrdquo International Journal ofPharmaceutics vol 242 no 1-2 pp 307ndash311 2002

[139] Z Zhang ldquoMechanical strength of single microcapsulesdetermined by a novel micromanipulation techniquerdquoJournal of Microencapsulation vol 16 no 1 pp 117ndash1241999

[140] J Su L Ren and L Wang ldquoPreparation and mechanicalproperties of thermal energy storage microcapsulesrdquo Colloidand Polymer Science vol 284 no 2 pp 224ndash228 2005

[141] G Nelson ldquoApplication of microencapsulation in textilesrdquoInternational Journal of Pharmaceutics vol 242 no 1-2pp 55ndash62 2002

[142] A Nejman M Cieslak B Gajdzicki B Goetzendorf-Gra-bowska and A Karaszewska ldquoMethods of PCM micro-capsules application and the thermal properties of modifiedknitted fabricrdquo ermochimica Acta vol 589 pp 158ndash1632014

[143] F A P Scacchetti E Pinto and G M B Soares ldquoermaland antimicrobial evaluation of cotton functionalized with achitosan-zeolite composite and microcapsules of phase-change materialsrdquo Journal of Applied Polymer Sciencevol 135 no 15 p 46135 2018

[144] S Alay Aksoy C Alkan M S Tozum S Demirbag R AltunAnayurt and Y Ulcay ldquoPreparation and textile applicationof poly(methyl methacrylate-co-methacrylic acid)n-octa-decane and n-eicosane microcapsulesrdquo e Journal of theTextile Institute vol 108 no 1 pp 30ndash41 2017

[145] S Demirbag and S A Aksoy ldquoEncapsulation of phasechange materials by complex coacervation to improvethermal performances and flame retardant properties of the

cotton fabricsrdquo Fibers and Polymers vol 17 no 3pp 408ndash417 2016

[146] Z Qiu X Ma P Li X Zhao and A Wright ldquoMicro-en-capsulated phase change material (MPCM) slurries char-acterization and building applicationsrdquo Renewable andSustainable Energy Reviews vol 77 pp 246ndash262 2017

[147] M Kong J L Alvarado W Terrell Jr and C iesldquoPerformance characteristics of microencapsulated phasechange material slurry in a helically coiled tuberdquo Interna-tional Journal of Heat and Mass Transfer vol 101 pp 901ndash914 2016

[148] Z Qiu X Ma X Zhao P Li and S Ali ldquoExperimentalinvestigation of the energy performance of a novel micro-encapsulated phase change material (MPCM) slurry basedPVT systemrdquo Applied energy vol 165 pp 260ndash271 2016

[149] L Chen T Wang Y Zhao and X-R Zhang ldquoCharacter-ization of thermal and hydrodynamic properties formicroencapsulated phase change slurry (MPCS)rdquo EnergyConversion and Management vol 79 pp 317ndash333 2014

[150] R Zeng X Wang B Chen et al ldquoHeat transfer charac-teristics of microencapsulated phase change material slurryin laminar flow under constant heat fluxrdquo Applied Energyvol 86 no 12 pp 2661ndash2670 2009

[151] S SongW Shen JWang SWang and J Xu ldquoExperimentalstudy on laminar convective heat transfer of micro-encapsulated phase change material slurry using liquid metalwith low melting point as carrying fluidrdquo InternationalJournal of Heat and Mass Transfer vol 73 pp 21ndash28 2014

[152] N S Roberts R Al-Shannaq J Kurdi S A Al-Muhtaseband M M Farid ldquoEfficacy of using slurry of metal-coatedmicroencapsulated PCM for cooling in a micro-channel heatexchangerrdquoAppliedermal Engineering vol 122 pp 11ndash182017

[153] S Zhang and J Niu ldquoTwo performance indices of TESapparatus Comparison of MPCM slurry vs stratified waterstorage tankrdquo Energy and Buildings vol 127 pp 512ndash5202016

[154] B Xu C Chen J Zhou Z Ni and X Ma ldquoPreparation ofnovel microencapsulated phase change material with Cu-Cu2OCNTs as the shell and their dispersed slurry for directabsorption solar collectorsrdquo Solar Energy Materials and SolarCells vol 200 p 109980 2019

[155] Y Konuklu M Ostry H O Paksoy and P Charvat ldquoReviewon using microencapsulated phase change materials (PCM)in building applicationsrdquo Energy and Buildings vol 106pp 134ndash155 2015

[156] L F Cabeza C Castellon M Nogues M MedranoR Leppers and O Zubillaga ldquoUse of microencapsulatedPCM in concrete walls for energy savingsrdquo Energy andBuildings vol 39 no 2 pp 113ndash119 2007

[157] J Giro-Paloma R Al-Shannaq A Fernandez and M FaridldquoPreparation and characterization of microencapsulatedphase change materials for use in building applicationsrdquoMaterials vol 9 no 1 p 11 2015

[158] M Aguayo S Das A Maroli et al ldquoe influence ofmicroencapsulated phase change material (PCM) charac-teristics on the microstructure and strength of cementitiouscomposites experiments and finite element simulationsrdquoCement and Concrete Composites vol 73 pp 29ndash41 2016

[159] W Su J Darkwa and G Kokogiannakis ldquoNanosilicon di-oxide hydrosol as surfactant for preparation of micro-encapsulated phase change materials for thermal energystorage in buildingsrdquo International Journal of Low-CarbonTechnologies vol 13 no 4 pp 301ndash310 2018


[160] N Essid A Loulizi and J Neji ldquoCompressive strength andhygric properties of concretes incorporating micro-encapsulated phase change materialrdquo Construction andBuilding Materials vol 222 pp 254ndash262 2019

[161] C Castellon M Medrano J Roca et al ldquoEffect of micro-encapsulated phase change material in sandwich panelsrdquoRenewable Energy vol 35 no 10 pp 2370ndash2374 2010

[162] H Cui W Liao X Mi T Y Lo and D Chen ldquoStudy onfunctional and mechanical properties of cement mortar withgraphite-modified microencapsulated phase-change mate-rialsrdquo Energy and Buildings vol 105 pp 273ndash284 2015

[163] S R L Cunha V Alves J Aguiar and V M Ferreira ldquoUseof phase change materials microcapsules in aerial lime andgypsummortarsrdquo in Proceedings of the European Symposiumon Polymers in Sustainable Construction pp 315ndash322Europe 2011

[164] G Urgessa K-K Yun J Yeon and J H Yeon ldquoermalresponses of concrete slabs containing microencapsulatedlow-transition temperature phase change materials exposedto realistic climate conditionsrdquo Cement and ConcreteComposites vol 104 Article ID 103391 2019

[165] M Bahrar Z I Djamai M EL Mankibi A Si Larbi andM Salvia ldquoNumerical and experimental study on the use ofmicroencapsulated phase change materials (PCMs) in textilereinforced concrete panels for energy storagerdquo Sustainablecities and society vol 41 pp 455ndash468 2018

[166] S Ying Kong X Yang S Chandra Paul L Sing Wong andB SvijaW ldquoermal response of mortar panels with differentforms of macro-encapsulated phase change materials a finiteelement studyrdquo Energies vol 12 no 13 p 2636 2019

[167] AM Borreguero J F Rodrıguez J L Valverde T Peijs andM Carmona ldquoCharacterization of rigid polyurethane foamscontaining microencapsulted phase change materials mi-crocapsules type effectrdquo Journal of Applied Polymer Sciencevol 128 no 1 pp 582ndash590 2013

[168] I Bonadies A Izzo Renzi M Cocca M Avella C Carfagnaand P Persico ldquoHeat storage and dimensional stability ofpoly(vinyl alcohol) based foams containing micro-encapsulated phase change materialsrdquo Industrial amp Engi-neering Chemistry Research vol 54 no 38 pp 9342ndash93502015

[169] W Li R Hou H Wan P Liu G He and F Qin ldquoA newstrategy for enhanced latent heat energy storage withmicroencapsulated phase change material saturated in metalfoamrdquo Solar Energy Materials and Solar Cells vol 171pp 197ndash204 2017

[170] W Li H Wan H Lou Y Fu F Qin and G He ldquoEnhancedthermal management with microencapsulated phase changematerial particles infiltrated in cellular metal foamrdquo Energyvol 127 pp 671ndash679 2017

[171] Y Zhang X Li J Li C Ma L Guo and X Meng ldquoSolar-driven phase change microencapsulation with efficientTi4O7 nanoconverter for latent heat storagerdquo Nano Energyvol 53 pp 579ndash586 2018

[172] Z Zheng J Jin G-K Xu et al ldquoHighly stable and conductivemicrocapsules for enhancement of joule heating perfor-mancerdquo ACS Nano vol 10 no 4 pp 4695ndash4703 2016

[173] X Shchukin X Wang and D Wu ldquoDesign and synthesis ofmultifunctional microencapsulated phase change materialswith silversilica double-layered shell for thermal energystorage electrical conduction and antimicrobial effective-nessrdquo Energy vol 111 pp 498ndash512 2016


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High Energy PhysicsAdvances in

Hindawi Publishing Corporation httpwwwhindawicom Volume 2013Hindawiwwwhindawicom

The Scientific World Journal

Volume 2018

TribologyAdvances in



ChemistryAdvances in


Advances inPhysical Chemistry


BioMed Research InternationalMaterials

Journal of


Na

nom

ate

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ls


Journal ofNanomaterials

Submit your manuscripts atwwwhindawicom

review articles on PCM microcapsules are available Most ofthe review articles published previously on PCM micro-capsules are related to their synthesis and applications Tothe best of the authorsrsquo knowledge previously reportedliterature has not adequately encompassed the character-ization and applications of PCM microcapsules iscomprehensive review attempts to summarize the recentdevelopments in PCM microcapsules and offers differentaspects of PCM microcapsules for thermal energy storageapplications A brief introduction on PCMs (organic in-organic and eutectic) and encapsulation shell materials(organic inorganic and organic-inorganic hybrid materials)was presented And themicroencapsulationmethods such asphysical chemical and physical-chemical processes wereintroduced e characterization of PCM microcapsulessuch as thermal conductivity thermal stability mechanicalstrength and chemical properties was discussed and ana-lyzed Finally the practical applications of PCM micro-capsules for thermal energy storage and other industrialprocesses were reported

2 Phase Change Materials (PCMs)

PCMs have a high heat of fusion in general and can storerelease a large amount of energy during meltingsolidifyingprocesses [21 22] PCMs have been considered as storagemedia with a wide range of applications including cooling offood products spacecraft thermal systems textiles buildingsolar systems and waste heat recovery system [23 24] Forexample PCMs can decrease the electricity consumption viareducing fluctuations in air temperature and shift coolingloads to the off-peak period in the building PCMs can beclassified into subcategories based on the solid-liquid solid-solid solid-gas and liquid-gas phase transformation andvice versa [25] For TES systems solid-gas phase transfor-mation and liquid-gas phase transformation are impracticalbecause of large volume and high pressure of the system[26] For example the water-steam system is not com-mercially viable for large-scale TES In the case of solid-solidPCMs their heat of phase transition is much less than that ofsolid-liquid PCMs [27] us solid-liquid PCMs are idealcandidates for TES systems e solid-liquid PCMs havefavorable phase equilibrium high density minor volumechanges and low vapor pressure at the operation temper-ature during phase transition In addition solid-liquidPCMs also exhibit little or no subcooling during freezingmeltingfreezing at same temperature and phase segregationand sufficient crystallization rate ere are many factorsinfluencing effectiveness and applications of solid-liquidPCMs heat capacity thermal conductivity latent heatphase transition temperature and so on

PCMs possess long-term chemical stability are no firehazard are nontoxic and noncorrosive do not undergodegradation after long-term thermal cycles are nonexplosivecompounds and have compatibility with other materialsand good chemical properties capable of completing re-versible freezingmelting cycle [28] ey can be classifiedinto three main groups as shown in Figure 1 organic PCMsinorganic PCMs and eutectic PCMs [29 30] Organic PCMs

are further described as paraffin and nonparaffin materials(fatty acids alcohols and glycols) [31 32] e paraffin oneof the by-products of petroleum refinery consists of carbonand hydrogen atoms joined with the general formulaCnH2n+2 where n is the number of carbon (C) atoms (if1le nle 4 the material is gas if 5le nle 17 the material isliquid and if nge 18 the material is solid) [33] Fatty acids arecarboxylic acids with long hydrocarbon chain of carbon andhydrogen atoms with a general formula CH3(CH2)2nCOOH(10le nle 30) [34] Organic PCMs are abundant and com-mercially available at reasonable cost However utilizingPCMs in traditional manner has several limitations such asliquid leakage during melting state and low thermal con-ductivity [35 36] Inorganic PCMs contain salt hydratescompounds and metals Salt hydrates can be considered asalloys of inorganic salts and water forming a typical crys-talline solid with the general formula M middot nH2O (where M isthe salt component andn is the molecular number) eysuffer from several problems (such as tendency to super-cooling segregation not melting congruently high volumechanges corrosion and phase separation over repeatedphase change cycles) which limit their applications in TESfacilities [37ndash39] Inorganic compounds are generallyregarded as being unsuitable for applications in TES becausethey own relatively small latent heat capacity and areharmful to the environment and human health [40 41]Metals promising inorganic PCMs diminish some of saltproblems but they are only suitable for high-temperatureapplications (4370K) [42ndash44] Eutectic PCMs can be definedas a minimum melting composition of two or more com-ponents each of which freezes and melts congruently toform a mixture of the componentsrsquo crystals in progress ofcrystallization [11 28] Eutectic PCMs can be adjusted bymixing inorganic-inorganic organic-organic or a combi-nation of the two PCMs at a desired ratio for specific ap-plications ey also possess high thermal conductivity anddensity without segregation and supercooling while theirspecific heat capacity and latent heat are much lower thanthose of paraffinsalt hydrates [45 46]

On the other hand PCMs can also be categorizedaccording to their phase transition temperatures low-temperature PCMs (melting point lt220degC) intermediate-temperature PCMs (220degC le melting point le420degC) andhigh-temperature PCMs (melting point gt420degC) [47]Generally low-temperature PCMs are paraffin fatty acidspolymeric materials sugar alcohols polyalcohol and so on[48] Melting points of most organic compounds are below80degC thus organic PCMs fall under the category of low-temperature PCMs However most of the inorganic-inor-ganic eutectic PCMs fall under the category of intermediate-temperature PCMs High-temperature PCMs includingnitrates metal carbonates sulfates fluorides chlorides andso on can be widely applied for high-temperature TESrequiring temperature gt500degC [49]

3 Encapsulation Shell Materials

To effectively reduce the leakage of PCMs during the solid-liquid phase transition and avoid the reaction of the PCMs










Paraffins

Metallic

compoundsOrganic-

inorganic

Inorganic-

inorganicMetal

alloyEsters

Fatty acids

Saltshydrates

Organic-organic

SaltsAlcohols


Shellmaterials

Organic



PMMA-SiO2PMMA-TiO2

PMF-SiC


hybrid

MF resinUF resin

Acrylic resin




















Physical


Low costlow yield







Solventevaporation





Coacervation

Sol-gel method

Chemical

Mic

roen

caps

ulat

ion

Physical-chemical







(a)

(b)

(c)

(d)


Monomer AMonomer B

















or


ΔHmPCMs + ΔHcPCMs

times 100

(2)










1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls












Paraffins

Metallic

compoundsOrganic-

inorganic

Inorganic-

inorganicMetal

alloyEsters

Fatty acids

Saltshydrates

Organic-organic

SaltsAlcohols


Shellmaterials

Organic



PMMA-SiO2PMMA-TiO2

PMF-SiC


hybrid

MF resinUF resin

Acrylic resin




















Physical


Low costlow yield







Solventevaporation





Coacervation

Sol-gel method

Chemical

Mic

roen

caps

ulat

ion

Physical-chemical







(a)

(b)

(c)

(d)


Monomer AMonomer B

















or


ΔHmPCMs + ΔHcPCMs

times 100

(2)










1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





















Physical


Low costlow yield







Solventevaporation





Coacervation

Sol-gel method

Chemical

Mic

roen

caps

ulat

ion

Physical-chemical







(a)

(b)

(c)

(d)


Monomer AMonomer B

















or


ΔHmPCMs + ΔHcPCMs

times 100

(2)










1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls








(a)

(b)

(c)

(d)


Monomer AMonomer B

















or


ΔHmPCMs + ΔHcPCMs

times 100

(2)










1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls


















or


ΔHmPCMs + ΔHcPCMs

times 100

(2)










1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






1kpdp

1

kcdc+

dp minus dc

kwdpdc (3)


θ Aχh

T1 minus T2( 1113857 (4)


(a) (b)

(c) (d)















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls
















initialmass

times 100

(5)








hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls









hd

(a)

F

(b)

F

(c)

F

(d)


PCMs microcapsules

Textile

Medicine

Slurry

Foams

Building

Buildi

ElectricitySolar
















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls

















Acknowledgments


References






























































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





















































































































































































Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




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