Fangyu CaoDepartment of Mechanical Engineering,

University of Maryland,

College Park, MD 20742

Paul KalinowskiATEC, Inc.,

College Park, MD 20742

John LawlerATEC, Inc.,

College Park, MD 20742

e-mail: j.lawler@atec-ahx.com

Hak Seung LeeDepartment of Mechanical Engineering,

University of Maryland,

College Park, MD 20742

Bao Yang1

Department of Mechanical Engineering,

University of Maryland,

College Park, MD 20742

e-mail: baoyang@umd.edu

Synthesis and Heat TransferPerformance of Phase ChangeMicrocapsule EnhancedThermal FluidsPolyalphaolefins (PAOs) are widely implemented for electronics cooling, but suffer froma low thermal conductivity of about 0.14 W/mK. However, adding thermally conductive,phase-change-material (PCM) particles to a PAO can significantly improve the fluidthermal properties. In this paper, PCM microcapsules and silver-coated PCM microcap-sules were synthesized using the emulsion polymerization method and the thermal per-formance of PCM fluids was studied in a microchannel heat sink and compared with thatof the pure PAO. A test loop was designed and fabricated to evaluate the synthesizedPCM fluids and it was found that fluid with uncoated PCM microcapsules has a 36%higher heat transfer coefficient than that of the pure PAO. Additionally, the heat transfercoefficient of PCM fluids with silver-coated PCM microcapsules was also 27% higherthan that of pure PAO, but lower than that of fluids with uncoated PCM microcapsules.The thermal resistance of the uncoated PCM fluid was about 20% lower than that of thepure PAO fluid at the same pumping power, despite the PCM fluid’s higher viscosity.Pumping tests were run for several hours and showed no evidence of particle accumula-tion or settling within the heat transfer loop. [DOI: 10.1115/1.4030234]

Keywords: phase change materials (PCM), thermal conductivity, heat transfer fluids

1 Introduction

Interest in engineered suspensions of micro-/nanoparticles inliquids has increased in recent years, particularly due to the poten-tial for higher fluid thermal conductivities, resulting in smallerand more compact heat exchangers [1–8]. Several researchers[9–13] have experimentally investigated the convective heat trans-fer of aqueous metal oxide or nitride nanoparticles in circulartubes and results show enhancement of the heat transfer whennanoparticles are added to the cooling fluid. For example, Cheinand Chuang [14] studied copper oxide/water mixtures in a micro-channel heat sink and their experimental results indicated higherenergy absorption of thermal fluids compared with pure water atlow flow rates. However, no heat transfer enhancement wasobserved at higher flow rates.

Microencapsulated PCMs, i.e., PCMs encapsulated in a micron-sized shell, can mitigate PCM leakage and potential undesirableinteractions between the PCM and the base fluid.[3,4,15–22] Raoet al. [23] and Dammel and Stephan [24] conducted experimentalstudies on the convective heat transfer characteristics of microen-capsulated PCM particles suspended in water in minichannels. Atlow particle concentrations (below 5%), higher heat transfer wasfound at all flow rates. Higher heat transfer coefficients wereachievable with higher particle concentration, however only atlow flow rates. The results also showed that the maximumenhancement occurred if the inlet temperature of the fluids wasslightly below the theoretical melting temperature of the PCM.The main enhancement provided by PCM is a significant increasein thermal storage density due to the latent heat of melting [17].However, thermal conductivity of PCMs is relatively low. Forexample, the PCM wax and PAO fluid have comparable thermalconductivities of around 0.14 W/mK. Low thermal conductivityrestricts heat transfer rate as well as accessibility to thermal

energy stored away from heat transfer interfaces. Therefore, there isa need for microencapsulated PCMs with high thermal conductivityin addition to high heat capacity.

A combination of these two advantages to enhance the heattransfer properties of a heat transfer fluid is rarely reported. In ourprevious research, we developed a comprehensive process tomicroencapsulate paraffin wax PCMs in melamine–formaldehyderesin shell for high heat capacity and suppressed supercooling[22]. In this study, thermal conductivity of the PCM microcap-sules is further enhanced with silver coating on the surface ofmelamine–formaldehyde resin shell. The as-produced PCMmicrocapsules, with and without a silver coating, are dispersed inPAO to make phase changeable, thermal conductive fluids, andtheir convective heat transfer characteristics are investigated in aheat transfer loop.

2 Experiment

2.1 Synthesis and Property Characterization. The micro-capsules, consisting of paraffin wax in resin shell, were synthe-sized using the well-developed in situ polymerization method, asdetailed in previous publications [15,22]. Typically, to produceprepolymer for the in situ polymerization, melamine and formal-dehyde are added into distilled water with adjusted pH at around8.5 and kept at 70 �C. Meanwhile, paraffin wax is added into waterwith surfactant and adjusted pH at around 4.0, then mechanicallystirred at 60 �C to make an oil-in-water emulsion. Theas-produced prepolymer solution is then added to the emulsion, inwhich melamine-formaldehyde shell is formed and condensed onthe surface of the paraffin-wax droplets. The final fluid is filteredand the solid white product is washed with distilled water andacetone, and then dried at 60 �C overnight.

To enhance the thermal conductivity of the microcapsules, athin layer of silver was coated on the surface of the PCM micro-capsule particles. Coating of a silver layer on surface ofmicroPCMs was performed in an aqueous solution based onso-called silver mirror reaction [25–27]. Typically, Tollens’

1Corresponding author.Manuscript received April 29, 2014; final manuscript received November 23,

2014; published online May 14, 2015. Assoc. Editor: L. Q. Wang.

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reagent is produced by adding ammonia aqueous solution drop-wise into a silver nitrite aqueous solution while stirring, until thefluid turns back to a clear and transparent solution. PCM micro-capsules are then added into this solution while stirring. Sufficientamount of reducer such as glucose or formaldehyde solution isadded into the suspension dropwise to produce a silver layer onthe surface of the microcapsules. The mixture is filtered after reac-tion, and the black-to-gray powder is then washed with water andacetone and dried for future use.

By controlling the surface properties of the uncoated microcap-sules, silver can be coated uniformly (in sample PCM_Ag1 andPCM_Ag3) or roughly (in sample PCM_Ag2), which affects theirheat transfer properties. Silver layers are smooth coating on theoriginal microcapsules. To create a rough silver layer on themicrocapsules, the melamine–formaldehyde resin surface needs tobe activated by stirring in diluted tin chloride solution for 10 minand then washed twice with distilled water [25,26]. After that, thesame process as described above was performed, using surface-activated microcapsules instead of the original ones. The experi-mental results are summarized in Table 1.

The structure of the silver-coated PCM microcapsules wasexamined with energy-dispersive spectra (EDS) using a scanningelectronic microscope (SEM, Hitachi SU-70), as shown in Fig. 1.The phase transition processes and latent heat capacity of thePCM microcapsules and their dispersions in PAO fluids weremeasured using the differential scanning calorimetry (DSC, TA-Q100). The weight and volume percentage of silver in the core-multi-shell particles could be determined by comparing the latentheat of microcapsules before and after silver-coating.

The thermal conductivity of the pure PAO and the dispersionsof PCM microcapsules in PAO were measured at room tempera-ture using a 3x-wire method [4,28,29] and the dynamic viscosityof the pure PAO and PCM microcapsules fluids was measuredusing a commercial viscometer (Brookfield DV-I Prime) at roomtemperature. It is interesting that the PCM-Ag microcapsules withsmooth silver coating are significantly more thermal conductivethan the rough ones, even though the silver fraction of the later(3.4 vol. %, in PCM-Ag2) is apparently larger than that of thesmooth coated PCM-Ag3 microcapsules (2.4 vol. %). Meanwhile,PCM-Ag1 and PCM-Ag2 with rough silver coating elevate viscos-ity of heat transfer fluids more significantly than PCM-Ag3. Thisindicates that the rough coating of silver does not form a continu-ous Ag layer but discrete silver clusters on the surface ofmicroPCMs, which is less effective in enhanced thermal conduc-tivity. As a result, it can be expected that the PCM-Ag3 fluidwith smooth silver coating layer performs better as a heat transferfluid than that of the other two PCM-Ag samples with roughsilver-coating.

2.2 Heat Transfer Measurements. A heat transfer loop wasdesigned and built specifically for evaluating the heat transfer per-formance of the PCM fluids in a liquid-cooled cold plate, and aschematic of the loop is shown in Fig. 2. The hold-up volume ofthe entire system is about 80 ml and the individual components inthe heat transfer loop include: a reservoir tank, a centrifugalpump, a Coriolis flow meter, two heat exchangers, and a micro-channel cold plate (the test section). The reservoir tank consists ofa modified 120 ml polyethylene bottle, which insures the system is

full of liquid and allows the fluid level to be monitored. A stirreris used to make sure the stable and uniform distribution of themicrocapsules in fluids. A centrifugal pump circulates the fluidthrough the entire system and the flow rate is measured using aflow and density meter and a transmitter-display. The coolant ispreheated before entering the test section and cooled upon leavingthe test section using two heat exchangers, which consist of twocopper cold plates with internal criss-crossed fins brazed together.The temperatures of the working fluids (50/50 aqueous ethyleneglycol) in the secondary loops of these two heat exchangers arecontrolled by two thermal baths. The test section consists of aMicrocool CP-101 microchannel cold plate and a 10� 10 mmceramic heater soldered to the bottom. The temperature of theheater was monitored using two T-type thermocouples attached tothe bottom of the cold plate. Two digital multimeters were used tomeasure the voltage and current being supplied to the heater wiresand the power input is adjusted using a variable transformer. The

Table 1 Properties of PCM samples dispersed in PAO with the PCM weight percentage of 20%

Sample Shell material Latent heat (J/g) Ag (Vol. %) Ag shell thickness (nm) Thermal conductivity (W/mK) Viscosity (cP)

Pure PAO N/A 0 0 NA 0.140 7.3microPCM Polymer 42.6 0 NA 0.152 11.8PCM_Ag1 Polymer rough Ag 33.0 2.2% 73 0.153 13.1PCM_Ag2 Polymer rough Ag 29.0 3.4% 117 0.239 13.6PCM_Ag3 Polymer smooth Ag 32.0 2.4% 82 0.251 11.2

Fig. 1 (a) TEM images of the polymer shell of PCM-microcapsules and SEM image of (b) PCM-microcapsulesbefore silver coating, (c) with smooth silver coating, and (d)with rough silver coating. EDS analysis of (e) silver and (g) car-bon in the smooth-coated microcapsule and (f) (h) that inrough-coated microcapsules are also shown.

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cold plate is incorporated into a milled Teflon block to reduceheat losses. Four K-type thermocouples were placed in the systemto measure the bulk temperature of the coolant after each heatexchanger and the test section. All thermocouples were calibratedin a thermal bath and the maximum deviation was 6 0.2 �C.

3 Results and Discussion

3.1 Pressure Drop Results. The microchannel cold plate wastested with pure PAO, PAO suspensions containing uncoatedPCM microcapsules, and PAO suspensions containing PCMmicrocapsules with a smooth silver coating (A3). Flow rates from

Fig. 2 Schematic diagram (left) of the experimental setup for heat transfer measurement and aphotograph of the microchannel (right)

Fig. 3 (a) Pressure drop in the cold plate at different inlettemperatures and (b) friction factor comparison: PAO, PCM,and PCM-Ag3

Fig. 4 (a) Heat transfer coefficient and (b) Nusselt number atdifferent inlet temperatures

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60 ml/min to 220 ml/min, heater power input from 30 W to 70 W,and inlet temperatures from 20 �C to 35 �C were tested. Addition-ally, pressure drop tests were conducted without heat input at fourdifferent fluid inlet temperatures. The results are depicted inFig. 3(a) and as expected for this range of Reynolds number, thepressure drop increases linearly with flow rate. It was also foundthat the fluids with PCM particles have a higher viscosity thanpure PAO.

Figure 3(b) compares the friction factor, f ¼ ðDP � Dh � A2f =

q � L � _V2Þ, obtained for each of the three different fluids (PAO,PCM, and PCM-Ag3) in the experiments. All results were alsocompared with the Darcy friction factor for a rectangular ductbased on the channel width/height ratio [30], f ¼ ð84:9=ReÞ. Itcan be seen that the results for the three fluids are in good agree-ment with the numerical model, given the uncertainties in the vis-cosities of the fluids, the dimensions of the microchannel, and theincreased pressure drop in the entrance and exit of the cold plateand the developing regions of the microchannel.

3.2 Heat Transfer Results. The trials with heat input wereconducted over a range of flow rates and heater powers and theaverage heat transfer coefficients were calculated for each power

input using the equation h ¼ ðQ=Asuf=Tsuf � TinÞ. The experimen-tal results are plotted in Figs. 4(a) and 4(b). For the pure PAOtests, the heat transfer coefficient increases as the temperature isincreased, due to a slight increase in the fluid’s heat capacity withtemperature. However, the absorption of heat by the melting ofthe PCM results is governed by a much more complex relationshipbetween the heat transfer coefficient, the flow rate, the inlet tem-perature, and the heat input. The heat transfer coefficients forthe PCM fluids increase as the inlet temperature is increasedfrom 20 �C to 30 �C, but then decreases at higher temperatures.The amount of PCM that melts in the cold plate depends on theinlet temperature, flow rate, and heat input and the greater theamount of PCM that melts in the cold plate, the higher the heattransfer coefficient becomes. However, at inlet temperaturesabove 30 �C, a portion of the PCM has melted before entering thetest section, decreasing the amount of PCM available for phasechange in the cold plate, thus resulting in decreased heat transfercoefficients.

The fluids containing the silver-coated PCM microcapsules hadlower heat transfer coefficients than the fluid containing PCM par-ticles with no metallic coating, which is surprising given thehigher thermal conductivity of the silver-coated PCM fluid. Thesilver-coated PCM microcapsules do have a lower latent heat thanthe pure PCM particles, thus it is suspected that this reduction inlatent heat is dominating the improvement in thermal conductiv-ity. As described above, the latent heat is responsible for alteringthe temperature gradients near the heat transfer surfaces, whichincreases the flow of heat into the fluid.

As shown in Fig. 4, the Nusselt numbers (Nu) of the PCM fluidsare higher than those of pure PAO and PCM-Ag3, due to thehigher heat transfer coefficients. The Nusselt numbers of PCM-Ag3 are lower than those of the pure PAO because of the higherthermal conductivity of the PCM-Ag3 fluid.

Using the equation below, the pumping power and the thermalresistance were also calculated in order to compare the thermalperformance of the pure PAO, PCM, and PCM-Ag3 fluids.

Pp ¼ _V � DP (1)

Rth ¼Tmax � Tin

Q(2)

The results are plotted in Fig. 5. It can be found that, as expected,the pumping power of the pure PAO decreases with increasing

Fig. 5 Thermal resistance at different inlet temperatures

Fig. 6 Extended duration pressure drop experiments

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inlet temperature, due to the fluid’s viscosity decreasing with tem-perature. At 30 �C, the thermal resistance of the PCM fluid isabout 20% lower than that of the PAO over the range of flow ratestested and the heat transfer coefficient of the PCM fluid is about36% higher than that of the PAO. However, as discussed, theadvantage of this large increase in heat transfer rate is reduced bythe increase in pumping power that is required to pump the PCMfluid. As can be seen in the figure, a similar result is found at othertemperatures.

Based on the above data, significant improvement on heat trans-fer performance is achieved with the use of PCM microcapsuleswith large latent heat capacity, in comparison to the pure PAO flu-ids. It appears that the addition of silver coating can potentiallylead to two competing effects: (1) increase fluid thermal conduc-tivity and (2) decrease the latent heat of the PCM microcapsules.Future research needs to be performed to find an optimum to bal-ance the thermal conductivity and latent heat capacity of PCM-Agmicrocapsules for heat transfer applications.

3.3 Extended Duration, Constant Flow Rate Experiments. Toinvestigate the stability of the PCM-enhanced fluids, experimentswere run for several hours without heat input. The tests were con-ducted with the PAO fluids containing silver coated PCM micro-capsules for 3 hr and the pressure drop versus time is plotted inFig. 6. The pressure drops were relatively constant over the dura-tion of these tests, which indicates that no accumulation of PCMparticles was occurring in the cold plate. The small pressure dropvariation can be attributed to fluctuations in the room temperatureand flow rate.

4 Conclusions

PCM fluids with uncoated and silver-coated microcapsuleswere synthesized and characterized, and their heat transfer per-formance was compared with that of pure PAO, which was thebase fluid for the PCM fluids. The PCM fluid with uncoated PCMmicrocapsules had a 36% higher heat transfer coefficient relativeto pure PAO over a range of flow rates and temperatures, whencompared at the same flow rate. The thermal resistance of thisPCM fluid at a given pumping power was about 20% lower thanthat of pure PAO. A silver-coated PCM-Ag3 fluid also showed anincreased heat transfer coefficient, which was greater than that ofpure PAO, but less than that of the uncoated PCM fluid. Thedecrease of heat transfer coefficient with silver coating is due tothe lower latent heat capacity of PCM-Ag3, even though its ther-mal conductivity is enhanced. Pumping tests conducted over sev-eral hours showed no effect of either particle accumulation orsettling within the heat transfer loop. From the results, it is clearthat this PCM fluid would provide improved cooling relative topure PAO in heat transfer applications.

Acknowledgment

The author Yang acknowledges the financial support fromNational Science Foundation (NSF) under Grant No. 1336778.The author Lawler acknowledges the financial support fromDepartment of Energy (DE-FG02-07ER86295).

Nomenclature

Af ¼ microchannel free areaAsuf ¼ heater surface areaDh ¼ hydraulic diameter of a heat sink microchannel

f ¼ Darcy friction factorL ¼ microchannel length

Nu ¼ Nusselt numberk ¼ thermal conductivity

Pp ¼ pumping powerQ ¼ heater power input

Rth ¼ thermal resistance

Re ¼ Reynolds’s numberTin ¼ fluid inlet

Tmax ¼ maximum heater temperatureTsuf ¼ average heater surface temperature

_V ¼ volumetric flow ratesDP ¼ pressure drop

q ¼ fluid density� ¼ kinematic viscosity� ¼ degree

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