Multifunctional Sol-Gel Nanocomposite Coatings for Aerospace, … · 2019-07-17 · Sol-gel...

Multifunctional Sol-Gel NanocompositeCoatings for Aerospace, Energy,and Strategic Applications: Challengesand Perspectives

R. Subasri and K. R. C. Soma Raju

ContentsIntroduction to Sol-Gel-Derived Nanocomposite Coatings and Their Key Features . . . . . . . . . . . 3Applications of Sol-Gel Coatings for Aerospace Sector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Chrome-Free Conversion Coatings on Aluminum Alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Ultrahydrophobic/Superhydrophobic/Icephobic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Scratch-Resistant Coatings on Transparent Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Antibacterial Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Applications for Energy Sector with Focus on Solar-Selective Coatings for Use in Heat . . . . . 12Collection Element of Solar Thermal Power Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12About Concentrated Solar Power Plant (CSPP) and Parabolic Trough Collector (PTC) . . . 12Heat Collection Element (HCE) and Role of Solar-Selective Coatings (SSC) . . . . . . . . . . . . . . 13Sol-Gel-Derived SSC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Applications for Defense and Strategic Sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Sensors for Detecting Chemical/Biological Agents and Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . 19Radiation Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Functional Coatings for Ceramic Radomes/IR Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

AbstractSol-gel technique is a wet chemical synthesis procedure involving the hydrolysisof either a fully hydrolyzable metal/silicon alkoxide or an organically modifiedsilane followed by condensation and polymerization reactions. Through thismethod, ceramics, glasses, and hybrid nanocomposite materials of high purityand homogeneity can be produced than when obtained through conventionalprocesses that involve high-temperature treatment conditions. Sol-gel-derivedhybrid nanocomposite coatings combine the interesting properties such as

R. Subasri (*) · K. R. C. S. RajuCentre for Sol-Gel Coatings, International Advanced Research Centre for Powder Metallurgyand New Materials (ARCI), Hyderabad, Indiae-mail: [email protected]; [email protected]

© Springer Nature Switzerland AG 2019Y. Mahajan, R. Johnson (eds.), Handbook of Advanced Ceramics and Composites,https://doi.org/10.1007/978-3-319-73255-8_49-1

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flexibility, hardness, etc. drawn from an organic polymer and an inorganic glassand hence are of great interest for aerospace, energy, and defense applications,due to their distinct advantages. Varied functionalities like corrosion protection,antireflection, scratch resistance, antibacterial, water/oil repellant, erosion resis-tant, and antistatic are possible to be obtained using this technique. Sol-gelnanocomposite films on appropriate substrates are also capable of being used assensors for detecting chemical/biological warfare agents as well as for sensingionizing radiation in the environment. Despite many advantages of this technique,there are still certain challenges that need to be circumvented in order to fullyharness the potential of the coatings derived from this process. This chaptermainly focuses on the potential applications of sol-gel nanocomposite coatingsfor aerospace, energy, and strategic sectors, where challenges in using them forapplications and future perspectives on how they can be mitigated are discussed.

KeywordsSol-gel nanocomposite coatings · Chrome-free · Corrosion resistant ·Nanocontainers · Self-healing · Solar selective · Antimicrobial, scratch resistant ·(Ultra)hydrophobic · Chemical/biological sensor

List of Abbreviationsγ-MAPTS γ-trimethoxysilylpropylmethacrylateAPTMS 3-trimethoxysilylpropylamineAR AntireflectiveATMOS bis[3-(trimethoxysilyl)-propyl]amineCEST Carboxyethylsilanetriol sodium saltCFU Colony formation unitsCNT Carbon nanotubeCSP Concentrated solar powerCSPP Concentrated Solar Power PlantDienTMOS (3-trimethoxysilylpropyl)diethylenetriamineDNA Deoxyribonucleic acidDNI Direct normal irradianceenTMOS Bis [3-(trimethoxysilyl)-propyl]ethylenediamineFIB Focused ion beamHCE Heat Collection ElementHMVF High metal volume fractionIR InfraredLMVF Low metal volume fractionMPTES 3-mercapto-propyltriethoxysilaneMTES TriethoxymethylsilaneMTMS TrimethoxymethylsilaneNO Nitrogen monoxideOD Optical density

2 R. Subasri and K. R. C. S. Raju

PC PolycarbonatePDA polydiacetylenePDMS Poly-dimethylsiloxanePMPS PolymethylphenylsiloxanePTC Parabolic Trough CollectorPV PhotovoltaicPVDF Polyvinylidene fluoridePVP Poly-vinylpyrrolidoneRH Relative humidityROS Reactive oxygen speciesRT Room temperatureSNR Signal-to-noise ratioSSC Solar-Selective CoatingsTEOS TetraethoxysilaneTMOS TetramethoxysilaneUV UltravioletVTES Vinyltriethoxysilane

Introduction to Sol-Gel-Derived Nanocomposite Coatingsand Their Key Features

Sol-gel process is defined by its characteristic hydrolysis and condensation reactionsof metal/silicon alkoxides to form metal/silicon oxide network colloidal particlesdispersed in a continuous liquid phase called sol, which is followed by gelation ofthe sol as a result of further progress of hydrolysis and condensation reactions,leading to the growth of the particles and evaporation of liquid phase and leavingsolid as a major phase [1]. Sol-gel process can lead to different products, namely,powders, coatings, fibers, and aerogels, depending on how the liquid phase isremoved from the sol. When fully hydrolyzable alkoxides are used as startingmaterials, pure inorganic sols are obtained from which pure inorganic coatings canbe derived. Pure inorganic sol-gel coatings have one major drawback regardingcoating thickness, wherein thickness lower than one micrometer can be obtainedin a single coating deposition step. Organic-inorganic hybrid coatings can beobtained by two methods: (1) hydrolysis and condensation of organically modifiedsilanes in conjunction with silicon/metal alkoxides or (2) through introductionof polymers into the metal/silicon oxide network. In case of the former, the organicmoieties in the organically modified silanes could either be polymerizable ornon-polymerizable. In this case, the organic and inorganic networks are bondedeither through covalent or ionocovalent bonding. In the case of the organic-inorganichybrid coatings, the inorganic moiety gives good mechanical properties, and theorganic moiety provides flexibility, compatibility with organic primers/paints [2],and possibility of achieving higher coating thickness at a lower curing temperature(<100 �C), as compared to 400–800 �C for curing the pure inorganic coatings.

Multifunctional Sol-Gel Nanocomposite Coatings for Aerospace, Energy, and. . . 3

Organic-inorganic hybrid sol-gel coatings also are compatible with various kindsof corrosion inhibitors, pigments, etc. [3, 4]. Figure 1 shows a schematic of thedifferent products that a sol-gel processing can lead to. The most commonly usedorganically modified precursors are methyltriethoxysilane, methyltrimethoxysilane,vinyltriethoxysilane, phenyltrimethoxysilane, 3-aminopropyltrimethoxysilane,3-methacryloxypropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, etc.Hybrid nanocomposite coatings can be obtained by physical dispersion of nano-particles into the hybrid sol-gel matrix or by synthesizing the sol with suitableprecursors so as to generate required metal oxide network nanoparticles in situ inthe hybrid sol-gel matrix. Sol-gel-derived pure inorganic and organic-inorganichybrid nanocomposite coatings have been explored for various applications, likecorrosion mitigation (barrier and self-healing type), antireflective, solar selective,solar control, antibacterial, self-cleaning/photocatalytic, easy-to-clean, and scratchand abrasion resistances, which have potential applications in aerospace, energy, andstrategic sectors.

Fig. 1 Schematic showing different products of a sol-gel reaction


Applications of Sol-Gel Coatings for Aerospace Sector

Chrome-Free Conversion Coatings on Aluminum Alloys

Aluminum alloys are economical, one of the few selected from the familyof lightweight materials with high strength-to-weight ratio and hence are predomi-nantly used in aerospace applications as structural materials [5]. Their resistanceto uniform corrosion is well-known due to the thin passivating oxide layer formedon the surface. However, the passive layer is deteriorated in chloride- or sulfate-containing electrolytes, which results in pitting corrosion. Prevention of corrosionthrough deposition of paints is the most convenient method. The barrier protectiondepends on the dense structure and thickness of the coating system. Since directapplication of paints on metal substrates does not yield good adhesion, conversioncoatings along with primers are commonly used prior to application of paints.Conversion coatings have two roles to play, namely, (a) improving the adhesionof primer with the substrate and (b) rendering a self-healing (or self-repairing) effectwhen there is damage in the paint and primer layer that extends down to thesubstrate. Hexavalent chromium-based conversion coating is known to be the bestself-healing coating with unmatched performance. However, chromate conversioncoatings have been globally banned due to their carcinogenic effect [6, 7].Consequently, worldwide researchers are seriously investigating various materialsto identify an effective self-healing alternative to replace hexavalent chrome[8–10]. Pure inorganic and organic-inorganic hybrid coatings derived from thesol-gel route have mainly been investigated for this purpose. In order to enhancethe corrosion resistance, corrosion inhibitors are added into the sol-gel coatingmatrix or paint. Direct addition of corrosion inhibitors to the sol-gel matrix forprolonged corrosion protection was reported to be deleterious to the barrier proper-ties of the coating [11, 12]. More recent methods of achieving long-lasting corrosionprotection are (a) by incorporation of corrosion inhibitors packaged inside “smart”nanocontainers which are dispersed in a sol-gel matrix to obtain self-healing coat-ings and (b) by employing barrier coatings as top coat, which are also capable ofrendering superhydrophobic effect [13].

Certain coatings are called self-healing coatings as they have the ability ofautomatically repairing any damage such as scratch caused by the external force,thereby preventing corrosion of the underlying substrate without human interven-tion. These are particularly interesting from an application point of view [14]. Theconcept of using self-healing coatings for corrosion protection of metals originatedfrom self-healing polymers, where any defect/scratch in the polymer could beautonomically healed by incorporating containers with monomers and catalystsinto the polymer [15]. Whenever there is a scratch/crack in the polymer material,the monomer and catalyst are released from their respective containers and poly-merized to heal the crack. However, a majority of these materials have seriouschemical and mechanical limitations, hindering their use as coatings for practicalapplications. Appropriate modifications to the chemistry of the self-healingmaterials make them amenable to be incorporated as a part of the coating matrix to


yield self-healing coatings, which are highly stable to species present in the envi-ronment. The encapsulation provides a means to contain other chemical agents alsothat can provide multi-functionalities like corrosion inhibition, antimicrobial activ-ity, self-cleaning property, etc. in addition to self-healing effect. Recent focus hasbeen on using inorganic systems as encapsulation materials. Carbon nanotubes(CNTs), clay nanotubes, mesoporous silica, and layered clays have been used asnanocontainers to encapsulate the corrosion inhibitors [16–27].

Ultrahydrophobic/Superhydrophobic/Icephobic Coatings

A surface is called “superhydrophobic” if it exhibits nearly zero wetting.Superhydrophobic surface can be obtained by creating an appropriate optimumroughness that has micro- and nanostructural hierarchical features and simulta-neously passivating with a coating that has low-surface energy. Superhydrophobicsurfaces possess self-cleaning property since dust does not adhere to them. Therehave been several reports on development of superhydrophobic coatings for self-cleaning applications [28–31]. Figures 2 and 3 depict the wetting behavior ofa hydrophobic and superhydrophobic surface, respectively, as per models proposedby Young, Wenzel, and Cassie-Baxter. Superhydrophobic surfaces have beenreported to be generated by top-down techniques like lithography, template-based

Liquid

Vapor

SolidgSL gSV

q

gLVFig. 2 A schematicrepresentation of water dropon a surface in equilibriumstate, as presented by Young[36]. (Reproduced withpermission from Taylor andFrancis)

a b

q'q'

Fig. 3 (a) Wenzel model and (b) Cassie-Baxter model [36]. (Reproduced with permission fromTaylor and Francis)


synthesis, and atmospheric RF plasma deposition of surfaces with CFx nanoparticleswithout the use of any roughness generation and vacuum systems. Bottom-upapproaches like self-assembly/self-organization where nanostructures can be manip-ulated into ordered arrays to impart a functionality to the material, chemicalvapor deposition, electrochemical deposition, and chemical bath deposition havealso been reported to yield superhydrophobic surfaces. A combination of both theseapproaches can also be employed. Wet chemical technique like sol-gel depositionis known to be versatile and cost-effective in creating a superhydrophobic surface.Superhydrophobic surfaces also can be potential candidates for exhibiting icephobicproperties. Icing on critical structures such as hydroelectric power lines, high-tensioninsulators, wind turbines, aircrafts, ship hulls, highways, and automobile wind-shields is a serious problem that calls for immediate mitigation, since it poses systemoperational issues and safety concerns. Commonly employed strategies to mitigateice buildup are based on chemical, mechanical, or thermal deicing methods, and theyhave several shortcomings. Normally, freezing point depressants such as NaCl, KCl,CaCl2, and MgCl2 are used as deicing agents for highways, and ethylene glycol andpropylene glycol, alone or in combination with sodium formate, calcium magnesiumacetate, sodium acetate, and urea, are used as deicing fluids for aircraft wings andrunways. Both systems are time-consuming and expensive. Moreover, the deicingfluids are toxic and not eco-friendly. Alternatively, ice on the wing surface can beremoved by mechanical means such as simple scraping or by introducing vigorousvibration to the body. But such methods may likely cause damage to the surfaces,thereby reducing their service life. Though heating the affected surface is found to bean effective method to melt ice, it is not efficient and economical as large quantity ofenergy is required. It can be seen that none of the aforementioned techniques preventformation of ice and its accumulation over a period of time. These methods are usedonly after accumulation of ice over the affected parts. Therefore, the best practicewould be a preventive method, i.e., to make a surface to which ice would not adhereto at all. Such a surface is termed as an “icephobic” surface [32–34]. It has beenreported that there is an electrostatic interaction between ice and surface ofmaterial on which ice forms, the resulting energy is substantially higher than theintermolecular chemical bond energy and van der Waals forces of attraction. Use ofcoating materials with a very low dielectric constant on the ice-forming surfacewould help in reducing the electrostatic interactions at the ice-dielectric interface,thereby aiding in considerably reducing the adhesion strength of ice [35]. Ice maynot adhere to an engineered superhydrophobic surface as water droplet does notwet it. Hence, superhydrophobic surfaces are also expected to be icephobic.Although there are several studies reported on sol-gel-derived hydrophobic surfaces,there are only few reports on superhydrophobic coatings as icephobic coatings[36–40]. Few reports have investigated on the use of metal oxide nanoparticlessuch as ZnO, TiO2, etc. and polymers such as Teflon on etched metal surfaces forproducing icephobic surfaces [36, 37]. It was shown that coatings that exhibitedsuperhydrophobicity also exhibited reduced ice adhesion.

However, recent studies show that the icephobic property is greatly influenced byboth surface morphology and its intrinsic surface free energy [38, 39] which is


evident from Fig. 4. Though both superhydrophobic coatings exhibited higher watercontact angle at room temperature, they could not exhibit similar behavior at sub-zerotemperatures. It has also been shown that as compared to a non-icephobic coating,the icing temperature of icephobic coatings can be lowered by up to 6.9� [39].Nucleation temperature lowering is influenced by three favorable conditions,namely, the surface roughness, Cassie wetting mode, and low-surface free energy.

It needs to be mentioned here that though there are few studies on the applica-bility of superhydrophobic coatings as icephobic coatings, there are not many studieson standardization of the test procedures. Hence, immediate priority would be toformulate a standard test procedure. This is one of the challenges that need to beaddressed before commercialization of such coatings could take place.

Scratch-Resistant Coatings on Transparent Plastics

Sol-gel compositions are synthesized based on coating property requirement andcomposition of the substrate on which the coatings are to be applied. Coatings onmetals and other inorganic materials like glass can be heat treated at higher temper-atures, but plastics have to be heat treated at lower than their glass transitiontemperatures. Sol-gel science has evolved over the years from pure inorganiccoatings to successful development of present day organic-inorganic hybrid coat-ings. Curing temperatures range between 50 �C to a maximum of 500 �C arereported. Organically modified silanes are used for the synthesis of sols for hybridcoatings [41–57]. Organic groups of such silanes could be non-polymerizable, suchas aryl or alkyl groups which do not participate in cross-linking, or they could bepolymerizable like the vinyl, epoxy, or acryl groups that participate in cross-linkingof precursors. Even in hybrid coatings, though the same sol is coated on metals andplastics, the coating properties largely differ based on the curing conditions. As canbe seen from the following Fig. 5, present authors have observed that a silica-zirconia coating has exhibited a pencil scratch hardness of more than 8H andan adhesion strength of 5B on steel substrates, while the same compositionresults in a pencil scratch hardness of 4H with an adhesion strength of 3B onpolycarbonate (PC).

Adhesion of the coating to the substrate also depends on the precursors used forthe sol synthesis. Even organic catalysts are used for the sol synthesis if coatings are

Fig. 4 Wetting behaviors ofnon-icephobic and icephobiccoatings at room temperatureand at �10

�C. (Reprinted

(adapted) with permissionfrom [39]. Copyright (2014)American Chemical Society)


meant for plastic substrates. It was reported that an adhesion-promoting zirconiumpropoxide was used for sol synthesis [58], which promoted zirconia-type bonds andacted as a catalyst to open 3-glycidoxypropyltrimethoxysilane epoxy rings resultingin more polymerized organic domains. This has reportedly increased plasticity of thecoating and hence resistance to scratch loads. Recent investigations show that evenwith low-temperature heat treatment at 110 �C for 3 h, zirconia nanofluids provideexcellent adhesion and a scratch hardness of 5H on PC while maintaining an opticaltransmission of 96%. Furthermore, an optimum amount of hydrolyzing agent, i.e.,water [59], can promote silica network formation resulting in high hardness andsignificant scratch resistance.

Many practical applications require multifunctional properties which a singleprecursor cannot meet, and hence, composite coatings are explored. Combinationof two or more metal alkoxide precursors results in highly cross-linked networkmaterials from Al2O3, TiO2, SiO2, ZrO2, etc. blending best properties of each of theconstituent of the total composition. In situ prepared nanoparticulate oxide networksof such nanocomposite hard coatings will exhibit improved scratch and abrasionresistance which is beyond the typical values exhibited by coatings obtained throughsingle precursor coatings. Alternatively, composite coatings can also be obtainedfrom a sol that is prepared from hydrolysis and condensation of organicallymodified alkoxysilane which results in simple silica network, and additionallyone or two components can be chosen from pre-prepared nanoparticles of ZnO,SiC, Al2O3, AlOOH, TiO2, SiO2, Y2O3, ZrO2, CeO2, and SnO2, iron oxides, andTa2O5 as per the functional requirement of the coating. However, SiO2 particlesand/or precursors that can result in SiO2 network with suitable organic ligandsare particularly preferred for many applications. Alkoxysilanes such as tetra-methoxysilane (TMOS), tetraethoxysilane (TEOS), trimethoxymethylsilane(MTMS), triethoxymethylsilane (MTES), 3-trimethoxysilylpropylamine (APTMS),and γ-trimethoxysilylpropylmethacrylate (γ-MAPTS) are some of precursors

0

1

2

3

4

5

6

7

8

9

Steels PCsubstrate

Scratch hardness, HAdhesion, B

Fig. 5 Effect of curingconditions on the mechanicalproperties of silica-zirconiacoating. (unpublished work)


prominently used for the synthesis of sols used to deposit hard coatings onplastics [60, 61].

Hard poly(methylsiloxane)s and colloidal silica coating formulations introducedby industrial majors like General Electric Co., Dupont Co., and Lucite have beenin use for several decades now in Europe and the USA to improve scratch andabrasion resistance of polycarbonate head lamps and side windows of automobiles.Excellent scratch and abrasion resistance while maintaining high transparency atlow processing temperature are the main features responsible for successful indus-trialization of the above products. As per 1997 industrial statistics, 64% of ophthal-mic lenses used in the USA are made of CR39 monomer, and 47% of them arecoated with hard sol-gel materials. Vinyltriethoxysilane (VTES)- and 3-mercapto-propyltriethoxysilane (MPTES)-derived UV curable compositions were used for theabove applications. A joint venture company with a name Exatec® was floated byGeneral Electric Plastics and Bayer AG with a sole aim of replacing automotiveglass with coated polycarbonate sheet [62]. PC is transparent to microwave, andhence microwave heating was also evaluated as a rapid curing method to selectivelycure only the coating applied on PC while maintaining excellent hardness [63, 64].

Antibacterial Coatings

Bacteria are one of the first and most widely available families of microorganisms onearth. While some of them are useful, some of them are harmful for the survival ofhuman life. Even today, health and life expectancy of human being is largelydependent on controlling breeding and spreading of these bacteria. Microorganismsare found everywhere right from simple office desk, computer keyboard, mobile,money purse, pen, spectacles, hearing aids, dress material, socks, railings of build-ings, vehicle doors, handles, steering wheels and passenger seats, food packagingto toilet seats, and much more. Such a long list implies how easily a person can beaffected while using any one of them. Moreover, keeping surgical devices, operationtheaters, and labs in sterile condition is one of the toughest tasks as they are breedinggrounds for some of the strains of bacteria and a greatest threat to the patients withpoor immune system. Some of the naturally available materials such as silver,copper, titanium oxide, ZnO, natamycin, quaternary amine compounds, phospho-nium salts, and natural substances like tea tree oil and chitosan [65–67] haveexcellent antibacterial property. As presented in Fig. 6, one of such studies showsthat cobalt doping reduces the band gap energy of ZnO and hence exhibits a betterphotocatalytic activity under visible light irradiation.

Escherichia coli and Staphylococcus aureus bacteria were used to assess thebactericidal efficiency of the ZnO films doped with different concentrations ofCo. The improvement in antibacterial activity of Co-doped ZnO films was attributedto the high charge separation efficiency and reactive oxygen species (ROS) gener-ation ability of the films which in turn enhanced their photocatalytic degradationproperty. C0 to C3 in the figure refer to the concentration of Co from 0 to 15 wt% insteps of 5 wt%. Among other materials, silver is highly toxic to bacteria but do not


pose any serious trouble to human life. Hence it has been a highly sought aftermaterial for antibacterial effect. Nanopowder, rods, and tubes are the most efficientforms of silver due to large surface to volume ratio and, hence, are easy to releasesilver ions for antibacterial functionality. Silver ions destruct bacterial cell wall,cause degradation of its plasma membrane, and prevent bacterial reproductionthrough binding to its DNA base. A low concentration (2.5 wt%) of silver isshown to be sufficient for the required antibacterial activity [68]. Organic-inorganichybrid sol-gel coatings are flexible, hard, solvent resistant, long-lasting, and eco-nomical, possess high toughness, and easy to formulate, apply, and cure. Hybridsol-gel coatings such as SiO2, Al2O3, TiO2, and ZrO2 and their combinations arebeing extensively used as a host material for nanosilver particles to apply as coatingon surfaces of interest. However, the sol composition is designed based on thesubstrate to be coated, as some of the substrates such as metals and inorganicmaterials can be heat treated at higher temperatures (�500 �C), while some of thematerials like paper, leather, and plastics have to be heat treated at lower tempera-tures (�50 �C). Radiation curing is used quite often for the substrates that aretemperature sensitive. Mechanical properties such as hardness and scratch andabrasion resistance of the coatings are strongly influenced by the above factorsand can be tailor-made to meet the end requirements. Coatings can be applied byspin, dip, spray, pour, brush, and sponge techniques. Coatings are tested in Staph-ylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa cultures.The presence of Staphylococcus aureus is found in human respiratory tract and theskin and is responsible for skin infections, respiratory diseases, and food poisoning.Escherichia coli is commonly found in the lower intestine of humans and otheranimals. They are responsible for food contamination and poisoning. Pseudomonasaeruginosa is found in soil, water, skin flora, and man-made environments. It is saidto be the toughest strain that can survive in any harsh environment. It is considered tobe inactive for healthy people but can be dangerous to the people with compromisedimmune system.

Fig. 6 Bacterial inhibition ofCo-doped ZnO thin filmstested against E. coli andS. aureus bacteria whilemonitoring the optical density(OD) at 600 nm [67]


Coated samples are tested in incubators with 90% RH and 37 �C for 24 h. Colonyformation units (CFU/ml) are first counted on the control as well as test sample overwhich bacterial growth has been initiated. Subsequently, cell reduction can becomputed using this formula: Cell reduction% = {1-test sample CFU per ml/controlsample CFU per ml) � 100}. Test specimen is judged to be antibacterial if theestimated cell reduction is found to be more than 99%. Treatment of textile fiberswith nanosilver-containing sol-gel hybrid coatings opens new possibilities for theimprovement of additional functionalities such as antimicrobial property for odor-free dress material. These coatings can also be used for drug delivery devices,implants, water purification, human tissue, and antifouling applications. A recentstudy shows that zinc-encapsulated silica nanoparticles produced through sol-gelprocess exhibit efficient antimicrobial activity and lower toxicity than other afore-mentioned materials paving its potential for the development of effective antimicro-bial nanoparticle agents [69].

Applications for Energy Sector with Focus on Solar-SelectiveCoatings for Use in Heat

Collection Element of Solar Thermal Power Plants

The increasing need for energy around the globe has motivated the scientists todevelop technologies that helped in the utilization of renewable energy resourcessuch as geothermal, wind, and solar radiation for generation of power. Out of thethree sources, solar energy is copious and is available in all regions of the globeand, hence, has attracted as a major alternative energy resource. Direct normalirradiance (DNI) is a parameter used to assess a particular region whether suitablefor solar power generation. DNI is defined as the flux of radiant energy, i.e., powerreceived by a surface of unit area which is always kept perpendicular to the sun raysthat are coming in a straight line from the direction of sun at its current position inthe sky [70]. A worldwide mapping of global estimate of long-term yearly DNI isdepicted in Fig. 7. Regions starting from yellow color shade moving toward brownand pink indicate the areas well suited for concentrated solar power (CSP) technol-ogy, since such areas experience more than 1500 kWh/m2 of long-term average DNIper year.

About Concentrated Solar Power Plant (CSPP) and Parabolic TroughCollector (PTC)

Semiconducting materials absorb photons from light and emit electrons. An electriccurrent results when such free electrons are collected and transferred through wires.A photovoltaic (PV) device is made based on this principle, which converts sunlightdirectly into electricity through quantum conversion, whereas thermal conversionhelps in solar thermal device to convert solar radiation into heat and subsequently


converts it into electricity using a typical thermal power plant or engine. Conversionefficiency of solar thermal energy units that use conventional molten salt as heattransfer fluid is reported to be 39.9%, and more recent results show that theefficiency can be further enhanced to 45.4% with new heat transfer fluid and asuper critical Rankine cycle used with a heliostat-based design. As against, a typicalphotovoltaic unit has a conversion efficiency of 20–27% only [71–73]. Concentratedsolar power plant (CSPP) systems use multiples of mirrors to reflect a large amountof solar radiation and concentrate onto a small area as in a heliostat design. Parabolictrough collector (PTC) is one of the most commonly used devices for CSPP. PTCuses a parabolic trough mirror to reflect the light as a concentrated beam ontoa metallic-selective absorber tube placed at its focal point. A thermic fluid runningthrough the tube then gets heated up to the operating temperature, which depends onPTC design. As shown in Fig. 8, heat from the thermic fluid is then transferred towater through a separate heat exchanger to generate steam and is subsequently usedto run a turbine for power generation. Solar thermal collectors are categorized intolow-, intermediate-, and high-temperature collectors based on type of concentrationsuch as no concentration flat plate, medium-concentration, and high-concentrationdesigns with operating temperatures reported to be < 200 �C, 150–500 �C, and>500 �C, respectively, for different applications.

Heat Collection Element (HCE) and Role of Solar-Selective Coatings(SSC)

In order to utilize solar radiation effectively, there should be an absorber material thatcan absorb this radiation and transmit the same to convert the radiation to do usefulwork. However, the material that absorbs such energy also gets heated and starts

Fig. 7 Global long-term average estimate of DNI. (Source: public domain DNI Solar Map © 2016Solargis)


radiating the energy back at wavelengths of 1500 nm or more. Hence, for effectiveutilization of energy, the absorbed energy should not be radiated back, which meansthe same material, while absorbing the solar radiation over the wavelengthrange 300–1500 nm, should also simultaneously act as a mirror for the infraredradiation. This property is termed as low emittance. No naturally available materialhas such contrasting properties, and hence, surface engineering is the only solutionfor obtaining high-absorbing and low-emitting surface, which is termed as solar-selective property or spectral selectivity. This implies that ideally, the absorptionover the active solar wavelength range should be 100%, while the IR emissionshould be zero.

Figure 9 shows a heat collector element (HCE) which is a tubular radiationabsorber device that contains a metallic tube encased in a borosilicate cover glasstube. HCE is the most essential element of PTC responsible for the efficientconversion of flux of radiant energy to thermal energy. The annular space of theabsorber tube and borosilicate glass tube is most commonly evacuated to minimizeheat loss due to convection [74]. The absorber tube is deposited with a solar-selective coating (SSC) for enhanced efficiency in converting solar radiationto thermal energy. There are six types of SSCs, namely:

(a) Intrinsic: modified transition metals and semiconductors such as metallic W,HfC, and V2O5.

(b) Semiconductor-metal tandems: semiconductors such as Si, Ge, and PbS witha band gap of 0.5 eV to 1.26 eV absorb short wavelength radiation withunderlying metal substrate providing necessary low emittance.

Fig. 8 Schematic representation of the CSPP-based PTC design. (Source: public domain)


(c) Multilayer absorbers: multilayer interference stacks where metals such as Mo,Ag, Cu, etc. are used as absorber materials, while Al2O3, SiO2, and CeO2 wereused as dielectric layers stable up to 400 �C.

(d) Metal-dielectric composite coatings: a highly absorbing cermet material wheremetal nanoparticles such as Ni, Cu, Ag, V, Cr, or Mo dispersed in a SiO2 orAl2O3 dielectric composite are used as absorber material on a polished highlyreflective metal surface.

(e) Textured surfaces: optical trapping of solar radiation is possible with the use oftextured oxide coating on a highly reflective polished mirror like substratesurface by multiple reflections among the pores and dendrite-like structures.

(f) Selective solar-transmitting coatings on a black body absorber: doped semicon-ductor such as SnO2:F, SnO2:Sb, or ZnO:Al which when deposited on anabsorber shall result in selectively solar-transmitting coating [75].

Hexavalent chrome-based electrolytic coating was one of the earliest successfulcandidate materials for SSC. But it is no longer being considered as such, due to itstoxic and carcinogenic nature. Vacuum-based vapor deposition techniques were used

Vacuum between glass and metal tubes

Antireflective coating

Borosilicate cover glass tube

Metal receiver tube

Solar selective coating

Bellow

b

a

Antireflective coated glass envelope tube

Glass-to-metal seal

BellowsVacuum between the envelope tube and the absorber tube

Absorber coating on metal tube

Fig. 9 (a) Heat collectionelement. (Source: publicdomain) and (b) its schematicrepresentation


to develop various compositions that could be used as semiconductor-metal tan-dems, multilayer absorbers, and metal-dielectric composites as next-generation SSC.Most efficient vapor-deposited SSCs are multilayered coatings. One of the metalcoatings such as Ni, Cr, Cu, or Ag is used as first layer on substrate to reflect backinfrared (IR) radiation. A nanocermet-based absorber layer is applied as a secondlayer, and an antireflective (AR) top coat is applied as a third layer [76].

Surface plasmon resonance effect is described as the collective oscillation of theconduction band electrons of the metal nanoparticles manifested due to the interac-tion of their conduction electrons with incident photons. These nanoparticles aredistributed in dielectric matrix of nanocermet coatings and are said to be responsiblefor exhibiting high absorbance in the visible wavelength range of solar spectrum.One of the nanoscale metal particles of Ni, Cu, Co, Mo, W, Pt, etc. dispersed in NiO,Cr2O3, or Al2O3 matrix and deposited routinely by vapor-based processes is beingused as cermet absorbers. Multilayered composites such as TiNxOy, TiC/TiNxOy/AlN, etc. are also proven to be successful candidate materials for SSC applications.SiO2 nanocoatings are applied as antireflective coatings over the absorber multi-layers. In a new high-temperature tandem absorber, non-oxide multilayered TiAlN/TiAlON/Si3N4 coating stack was also proposed, where first and second layerswere used as absorber layers, while the third Si3N4 was used as an antireflectivelayer [77, 78]. The absorbance values of such multilayer solar-selective stacksranged between 90% and 97% with emittances in the range 0.04–0.14. Thoughthese coatings exhibit very good solar-selective properties, vacuum-based coatinglines need to be set up for their production and, hence, are quite expensive.

Sol-Gel-Derived SSC

Nanocomposite sol-gel-derived SSCs are eco-friendly; adherent to many substratematerials such as metals, glass, and plastics; and amenable to large-scale automationand, hence, seem to be promising candidates for use as SSCs [79]. In such a case,instead of deposition of the metal IR reflective coating, ground and polished sub-strates themselves were used in conjunction with sol-gel compositions to generatethe SSCs. Carbon nanoparticles in ZnO and NiO matrix [80] and CuCoMnOx andCuFeMnO4 spinels [81] are some of the novel compositions proposed in the recentyears.

Graded metal-dielectric composites that meet the dual purpose of acting asradiation absorbers and IR reflective layers are some of the successful candidatesfor SSCs. Some of the published literature further used double cermet layer films,where the first layer on substrate was a cermet layer with high metal volume fraction(HMVF) in a dielectric matrix followed by a second cermet layer with low metalvolume fraction (LMVF) in a dielectric matrix [82–84]. Metals such Cu, Ni, Ag, andAu incorporated in sol-gel dielectric matrix of TiO2, Al2O3, etc. have also beenshown to be promising for solar thermal applications. A cross section of sol-gel-derived five-layered Ag-TiO2/TiO2/SiO2/TiO2/SiO2 coating [85] when preparedusing focused ion beam (FIB) and observed through a high-resolution microscope


is shown in Fig. 10. Figure 10 a shows the cross section of the as-cut sample usingFIB, and Fig. 10b depicts the cross section with the same coating stack observedunder an SEM where sample was prepared using a conventional technique.Figure 11 depicts the SEM image of the surface of a five-layered Ag-TiO2/TiO2/SiO2/TiO2/SiO2 coating, showing the presence of Ag nanoparticles dispersed in theamorphous sol-gel matrix.

Figure 12 shows the schematic and UV-Vis-NIR spectrum of one of themost successful sol-gel-derived solar-selective coating stacks where nanometer-thick alternating high- and low-refractive index TiO2 and SiO2 coatings wereused to enhance the visible light absorbance while maintaining lowerreflectance [85]. In another recent study, a high-absorbance solar-selective coatingwith a Cu-Co-Mn-O as first layer on substrate followed by Cu-Co-Mn-Si-O as

Fig. 10 (a) Micro image of a five-layered Ag-TiO2/TiO2/SiO2/TiO2/SiO2 coating absorber stack(a) as cut using FIB and (b) cross-sectional image after preparing the sample using a conventionaltechnique. (Unpublished work)

Fig 11 SEM image of thesurface of a five-layeredAg-TiO2/TiO2/SiO2/TiO2/SiO2 coating. (Uunpublishedwork)


second layer and finally Si-O as a third antireflecting three-layered stack wasproposed [81]. Single layer Ni-Al2O3 nanocomposite coatings on aluminum sub-strates were also reported to have exhibited an absorbance of 85% and emissivityof 0.05. However, when two layers of Ni-Al2O3 with a top antireflective layer ofSiO2 were deposited, the optical properties were found to improve the absorbancevalues of 93% and emissivity of 0.05 and were also stable in air up to300

�C. Recently, there has been an increased interest in SSCs based on Cu-Co-

Mn-O. It was shown that the absorptance of a single layer Cu-Co-Mn-O was 0.86,which could be enahanced to 0.94 when a Si-O layer was applied as a second layer.The absorption was further enhanced to 0.96 when the coating stack was designedas Cu-Co-Mn-O first layer, Cu-Co-Mn-Si-O second layer, and Si-O thirdlayer [81]. It was also clearly stated that the thickness of Cu-Co-Mn-Si layer was110 nm comprising agglomerates of nanocrystalline grains that are 5–20 nm indiameter. Table 1 summarizes the properties of some of the most successful sol-gelnanocomposite coating stacks as SSCs.

Sol-gel-derived solar-selective nanocomposite coatings can be quite promisingfor scale-up and for use in heat collection elements of solar thermal power plants.They have an immense potential to replace the toxic hexavalent chrome-basedcoatings as well as coatings generated using the cost-intensive vapor-based deposi-tion technology. However, ample care must be taken to control the thickness and

0

10

20

30

40

50

60

70

0 500 1000 1500 2000 2500Wave length [nm]

% R

efle

ctan

ce

Stainless steel substrate

Ag-TiO2

TiO2

SiO2

TiO2

SiO2a

b

Fig. 12 (a) Schematicrepresentation of the coatingstack adopted for multilayeredsolar-selective coatings onSS321 substrate and (b) thecorresponding UV-Vis-NIRspectrum where α= 95� 1%,ε = 0.14 [85]


metal volume fraction of the respective layers in order to obtain optimum opticalproperties. Accelerated testing and extensive field trials may be required to evaluatetheir durability under service conditions.

Applications for Defense and Strategic Sectors

Sensors for Detecting Chemical/Biological Agents and Radiation

Optical sensors are beginning to be applied for sensing, which offers higher resolu-tion, high signal-to-noise ratio (SNR), and thus large dynamic range for the param-eters under investigation. Fluorescence-based chemical sensors and biosensorsare being routinely used in chemical and biochemical analysis due to their highselectivity, sensitivity, simplicity, and fastness. Bio-organically doped sol-gelcompositions result in diverse properties leading to their potential use in manywide-ranging areas of material science such as optical materials, biocatalysts, elec-trochemistry, immunochemistry, chemical sensors, biosensors, etc. Physiologicalchanges of armed forces in the battlefield can be monitored with the help of sol-gel-derived biosensors. Sol-gel matrices are capable of being used as a very goodhost matrix to immobilize analyte-sensitive reagents. Analyte-sensitive reagents canbe added during sol synthesis such that the reagent molecules can be encapsulated inthe porous sol-gel coating structure and still permeate through the pores to sense theenvironment surrounding it. The composition of the sol-gel coating layer dependson the metal alkoxides selected for required functionality during sol synthesis.The advantage of producing the matrix through sol-gel technique is the possibilityto obtain homogeneous, pure matrices that can densify at low temperatures. Sol-gelcoatings can be used as optical transducer materials for monitoring chemical/bio-logical warfare agents such as radioactive material use or toxic gases. Here, thechange in color triggered by the reaction of the immobilized sensing materialmolecule in the sol-gel coating matrix strip with that of the surrounding atmospherecan be handy for the user in the affected area. A schematic of capsule-based pHsensor where a fluorescent dye is encapsulated in a polyelectrolyte membrane madeby layer-by-layer assembly and the response of the capsule-based pH sensor to localenvironment is depicted in Fig. 13 [86]. A similar concept has also been usedfor clinical diagnosis, food and drinking water quality assessment (pH and dissolved

Table 1 Most successful sol-gel solar-selective compositions

Sl. No Composition Substrate Absorptance (%) Emissivity Reference

1 Al2O3:Ni Aluminium 97 0.05 82

2 Cu-Mn-Si oxides Aluminium 95 0.035 81

3 Ag-TiO2/TiO2/SiO2 Stainless steel 95 0.15 85

4 Cu-Co-Mn-Si-O Stainless steel 96 0.12 81

5 Ni-Al2O3 Aluminum 93 0.05 84


oxygen monitoring), pollution monitoring, and its control for militaryapplications [86].

Sol-gel aerogel glasses have been successfully used for sensing wide-ranginganalytes such as color test for cations, pH indication, anions, and organic moleculesif a suitable analytical reagent is trapped in highly porous network and is exposed topore volume. For example, the use of dimethylglyoxime reagent helps in detectingNi+2 cation by turning a colorless glass into a red color. A red brownish glassprepared by co-trapping sodium rhodizonate and BaF2 turns colorless due tobleaching effect of SO4

2� inorganic anion. Phenolphthalein and thymolphthaleindoped glass can be used to detect protons for the use of pH indication. Even aninorganic ferrous salt can be trapped in sol-gel glass and used to detect organicanalyte such as orthophenanthrolin in a solution by changing a white glass toa characteristic red color. Sensor reaction times are as low as 1 s to as high as30 min. Major advantages of such sensors include stable and inert glass substrate andsimple preparation technique and can be easily prepared in various shapes as well asthin films [87]. Detection of toxic metal ions like cadmium, lead, or cyanide anionsin potable groundwater being supplied for military personnel employed in remotelocations is very essential. This is because the presence of cadmium and lead ina very low concentration <1 mg/l will have deleterious effect on animals andenvironment. They have potential to damage kidneys and reproductive systems.

Fig. 13 Schematic of (a) capsule-based pH sensor with the fluorescent dye encapsulated in apolyelectrolyte membrane made by layer-by-layer assembly along with SEM image showing thewires obtained via electrospinning and (b) response of the capsule-based pH sensor to localenvironment [86]


Anthryl tetra acid for cadmium detection and acetonitrile based for lead detectionsol-gel composition coatings can be used as selective sensors. Iron (III) porphyrinencompassed titanium carboxylate based sol-gel coating was proposed as a cyanidesensor. Oxalate oxidase encompassed sol-gel coating for oxalate sensing; porphyrin-bound dextran in sol-gel coatings was also developed for mercury sensors [88].Cobalt tetrakis porphyrin-doped sol-gel synthesized silica glass was proposed to beused for nitrogen monoxide (NO) sensing; other biomolecules such as myoglobin,cytochrome, and hemoproteins were demonstrated for sensing O2, NO, andCO. Here, the organically modified silica sol-gels are capable of showing changesin volume with respect to the interactions with solvents.

Thus, sol-gel technology has a great potential to be used as gas sensors for bothtoxic and nontoxic analytes. Sensors for Cl2 gas are useful for use during wartime.The sol-gel technology makes it possible to make sensors for Cl2 gas. The weight(or swelling) of silicate matrices prepared using sol-gel method varies with chlorinecontent like NaCl/CaCl2 [89]. The enhanced swelling of these silicate matrices inhigh ionic strength can be explained by the development of significant osmoticpressure in the sol-gels. The movement of these ions along with their respectivehydration spheres results in the swelling of the sol-gels. This effect creates a criticalosmotic pressure within the framework that leads to swelling of the gel. The volumechange during swelling is proportional to the Cl2 gas content. Various silanes like bis[3-(trimethoxysilyl)-propyl]ethylenediamine (enTMOS), carboxyethylsilanetriol,sodium salt (CEST), (3-trimethoxysilylpropyl)diethylenetriamine (DienTMOS),and bis[3-(trimethoxysilyl)-propyl]amine (ATMOS) were investigated as silicatematrices for use in Cl2, pH, and salt sensors [89]. Figure 14 compares the responseof ATMOS and enTMOS sol-gels to NaCl and MgSO4. The response of these hybridsol-gels with respect to pH is shown in Table 2 [89].

Fig. 14 Response of ATMOS and enTMOS sol-gels to salt. (a) shows the response to NaCl, and(b) shows the response to MgSO4 [89]


Also, the organosilica sol-gels containing ionizable functional groups such as NHor COOH exhibited responses to variations in pH of the surrounding medium. Theionizable groups present in the materials lead to protonation/deprotonation as aconsequence of change in pH due to changes in the electrostatic interactions,which leads to volume changes that can be detected. In general, it is found that theorganosilica sol-gels containing amino groups undergo shrinkage at high pH, whileat low pH they exhibit swelling. Sol-gel coatings were also successfully usedas fiber-optic oxygen sensors with tunable oxygen sensitivity. Electro-spun fiberssynthesized through sol-gel process were used for pH sensing [90]. When dopedwith fluorescent molecules such as rhodamine6G and fluorescein, they can beused as laser dye. Customized sol-gel compositions can also be used for sounddetection [91]. Mesoporous silica films with an excellent heat-insulating propertyand the relatively low dielectric value play an important role in electronic andmagnetic devices; those with pore sizes of 5–50 nm are also of interest for applica-tions in photonics, optoelectronics, lightweight structural material, thermal insula-tion, and optical coating [91–94]. All such sensors may be useful to the armed forcesduring wartime and for rescue teams during natural calamities.

Sol-gel coating is the most suitable technology to generate sensors that mimicnatural biological system as many exotic material combinations can be synthesizedin near-theoretical purity and used in managing bioterrorism. Electronic nose thatworks based on sol-gel coatings can smell biochemical/poisonous warfare agentssuch as anthrax, sarin, and mustard gas in the real-time battlefield applications. Suchsensors are fabricated using functionally tunable nanoporous thin films deposited by

Table 2 Response of hybrid sol-gels with respect to pH. (Ref [89])

pH enTMOS+CEST, % mole fractionCEST

enTMOS+DienTMOS enTMOS ATMOS

32.0 26.0 22.0

Hydrophobic estimate

3.20 3.35 3.45 3.11 4.0 6.0

% Weight change of hybrid sol-gels

1.22 +19.8 +12.1 +18.6 +24.7 �0.14 +35.0

2.44 +18.0 +5.2 +10.45 +1.5 �4.5 +31.0

2.78 +8.6 �3.9 +5.5 �5.3 �6.0 +28.0

4.15 �7.0 �8.4 +0.03 �4.0 �6.6 +9.2

5.92 �19.0 �4.7 +3.6 �6.2 �6.5 +7.5

7.00 �3.5 �10.0 �0.25 �12.5 �7.6 +2.1

8.08 �37.0 �9.3 �5.7 �13.4 �8.5 �3.2

9.33 �34.0 �10.0 �2.8 �15.0 �9.0 �4.7

10.83 �14.4 �10.0 �1.4 �15.2 �9.1 �4.5

11.71 �45.4 �5.6 �0.63 �15.0 �8.2 �3.0

12.27 �37.4 �3.9 �0.55 �10.0 �5.7 �2.2

enTMOS, Bis[3-(trimethoxysilyl)-propyl]ethylenediamine; CEST, carboxyethylsilanetriol; sodiumsalt; DienTMOS, (3-trimethoxysilylpropyl)diethylenetriamine; ATMOS, bis[3-(trimethoxysilyl)-propyl]amine


sol-gel compositions and were subsequently chemically grafted with sulfonation andesterification (amino acid) reactions. Sensing material swell due to the analyte basedon diffusion into nanostructure of sol-gel coatings and extent of interaction betweenpolymeric sol-gel coating and analyte. It is said that such grafting techniquesimpart hydrophobicity and bioactivity, thereby behaving quite similar to biologicalnose [95, 96]. People living in remote areas especially armed forces and alliedservices posted at the frontiers/borders are forced to stock food items for longerperiods which quite often exceed or come close to the expiry of shelf life by the timethe item is consumed. One of the current reviews shows that a microencapsulationtechnology can be used for the development of functional coatings such as thermo-chromic materials for packaging. Food storage and its packaging can be one of theprominent fields where encapsulated thermochromic materials can be depositedin a nanostructured sol-gel matrix for food quality testing and as a shelf life indicator.VO2-doped SiO2, VO2- and W-doped VO2, and V1-x-yWxSiyO2 were reported to besome of the successful coating compositions experimented using sol-gel process.Another interesting application for VO2 coatings is that it can be used as energyefficient thermochromic smart windows that have potential to save 9.4% electricityconsumption for the shelters used to house defense personnel working in extremetemperature conditions. Color transition properties of polydiacetylene (PDA), poly-vinylpyrrolidone (PVP), and ZnO nanocomposites exhibit controlled reversiblethermochromic properties and hence have also been in use for chemical andbiosensors applications [97].

Radiation Sensors

Luminescence spectra of rare earth ions are quite informative, and in particularexcited states of Eu3+ have a strong dependence on dosage levels of gammaradiation. It was shown that the magnetic dipole transition 5D0–

7F1 of the europiumion presents a 900% enhancement for irradiation doses up to 400 Gy. Based on thischaracter, Eu3+-doped sol-gel synthesized silica glass was successfully developedas a detector for detecting gamma radiation that can be used as economic radiationdosimeter for nuclear installations, in nuclear warfare, and during natural disasters.Such behavior was shown to be due to the high local symmetry of europiumion-doped glass as against low symmetry found in glasses produced through con-ventional melting process [98]. The absorption spectra of irradiated Nd2O3-dopedsilicate glass magnetic dipole transition 5D0–

7F1 of Nd3+ ions were found to present

huge defects for irradiation doses up to 18 kGy. This shows that Nd3+ ions also havehuge potential for use as a radiation detector due to the reason that these ions exhibitenhanced absorption of gamma radiation, which reflects in color change due toirradiation that lasts for as high as 7 days [99].


Functional Coatings for Ceramic Radomes/IR Windows

A structure that is meant for protecting the radar equipment such as its antennaand related hardware from aggressive weather, wind, rain, sand, and ice etc., duringits flight is referred as radome. It is made of a material which is transparentto electromagnetic signal that emanates or is received by the antenna. High-densitypolyethylene, polyurethane foam, PTFE-coated fabric, aramid, quartz, alumina,pyroceram, silica, silicon nitride, polyimide laminate, and fiber glass are some ofthe commonly used materials for radome fabrication. Radome performanceis affected due to many factors such as aerodynamic heating, static charge buildup,erosion due to rain, etc.; during projectile flight, absorption of water, and formationof water film on radome surface; and during its storage and throughout its service.With the advent of modern designs, the structural integrity of radome is necessaryover and above 350 �C.

The present practice to overcome aforementioned problems and to prolong thelife of radomes is to apply protective coatings. Aerodynamic heating is the heatwhich builds up on the surface of the body that is moving at very high speeds due toair friction. Hence, even optimally designed radome of hypersonic and reentryvehicles requires proper heat shield that is electrically transparent during operation.Hence, low dielectric constant materials are conventionally used as ablative coatingswhich sacrifice themselves by absorbing the energy and expending the massto achieve heat absorption either by blocking or dissipation of heat, therebyprotecting the hardware underneath the radome. Silicon resins such as poly-dimethylsiloxane (PDMS) and polymethylphenylsiloxane (PMPS) have Si-Obonds with high bond energy (443.7 kJ/mol) and about 51% high ionic characterresulting in excellent thermal and thermal oxidative stability and hence has becomean attractive candidate for use as ablative coatings [100]. When the projectile ismoving at very high speed, static electricity builds up on the surface of radome dueto friction with high-velocity air and particles within air. The buildup is so high that itcauses discharge resulting in interference with the electronic hardware of the radomeand damages it by generating pin holes on its surface. Conductive coatings areapplied on the outer surface of radome to impart antistatic property. Usually suchcoatings are very thin such that the transmission losses are negligible and maintainlow resistance to allow the static charge to discharge softly and minimize electricalinterference.

Highly reflective layers wherein TiO2-, ZnO-, and ZnS-loaded resins are appliedas thin coating on outer surface of radome are required to protect against high-energyband radiation inputs. SiO2-based coatings have also been suggested to be used asantireflective and antiflash coatings wherever necessary [101].

High-speed projectile radomes that are moving at speeds over 400 km/h facesevere erosion wear due to rain, dust, ice, and sand particles. during flight. Erosioninduces pitting, and its intensity varies based on the radome geometry, material, andits velocity leading to catastrophic failure of radome. Radome materials suchas Alumina and Pyroceram (trademark material) have excellent erosion resistanceand hence do not require any protective coating. However, materials such as glass


fabric-resin laminates, neoprene-coated glass fabric polyester, silicon nitride, andsteatite require protective coatings. There are different types of erosion-resistantcoatings such as neoprene, polyurethane, fluoroelastomer, and ceramic. Recently,there has been considerable effort by a group of researchers for the developmentof sol-gel coating recipes for protecting the surface against liquid impact erosion dueto the fact that the hybrid sol-gel coatings have the advantage of combined propertiesof both polymeric materials such as flexibility and low-temperature curabilityand that of ceramic materials such as good hardness and abrasion resistance. Hybridcoatings are said to have flexibility to absorb kinetic energy and avoid cracking dueto the impact of the raindrop and, hence, found to be quite promising. In a recentstudy, it was found that a simple silica sol was able to enhance rain erosion resistanceprovided the sol composition could be managed in such a way that the final coatingcontains more inorganic cross-linking. Moreover, the statistical experimental designshows that hydrolysis water content is the most important factor in enhancing theerosion resistance of the silica sol recipe that was optimized at 3/7 as the molar ratioof GPTMS/TEOS precursors with higher water content and 130 �C curing temper-ature for 90 min resulting in coatings that have 6H pencil scratch hardness [102].The above detailed investigative results substantiated the conclusions of a previoussimilar study with same set of precursors for sol preparation. It was found thatexternally added ZrO2 nanoparticles to SiO2 sol-gel coatings reinforced the coatingand exhibited enhanced rain erosion resistance, when more inorganic cross-linkingcould be obtained. The rain erosion resistance of such coatings was comparable tothat obtained for commercially available erosion protection coatings [103].

Water has a high dielectric constant and very high loss tangent for microwave/millimeter wave frequency. Hence, its ingress due to absorption of water or moistureby the porous or hygroscopic radome material fitted on a projectile during its storageperiod can degrade the critical sensor and guidance hardware housed in it causingreflection transmission and phase delay variations. This in turn will produce atten-uation and aberration. Silicone-based resins are suitable candidates for coatingapplication as they have good high-temperature capability. A recent investigationshows that CaO-B2O3-SiO2 sol composition coated on silicon nitride radome mate-rial exhibited increase in the temperature coefficient of dielectric constant with thecoating at 9.2 GHz frequency and found that the temperature stability of dielectricloss has improved due to coating [104]. Super hydrophobic sol-gel coating compo-sition based on polyvinylidene fluoride (PVDF) and SiO2 micro-sized particlescould effectively protect radomes from absorbing the moisture by physical sealingand due to water repelling behavior of the coating, thereby eliminating formation ofwater film on the radome surface. The average static water contact angle of thecoating was measured to be 157.5�, while its rolling angle was 9�. The coatingexhibited high thermal stability and mechanical durability [105].

As a summary, it can be proposed that a judiciously selected multifunctionalhybrid sol-gel coating composition could be a single layer solution that meets therequirement of being a good sealant to prevent water absorption, provide rain watererosion resistance and capable of efficiently protecting the radomes from severeenvironments during the flight of projectile.


Concluding Remarks

It can be seen from the above discussion that sol-gel coatings for corrosion protec-tion applications seem to be the most potential end use for aerospace applications.Introduction of superhydrophobicity/icephobicity or self-healing property in suchcorrosion protection coatings prolongs the corrosion resistance and adds substantialvalue to the protective coatings. Hard coatings are also equally promising for use onvarious substrates like transparent plastics, metals, and glass. The use of sol-gelcoatings for sensing applications also seem to be promising. Though successfulcommercialization of these coatings has been possible to a certain extent, the sol-geltechnology is still in the growth stage, and the high cost of raw materials willcontinue to be a determining factor for the widespread use of this coating technology.To sum up, there is a vast potential for use of sol-gel-derived/wet chemical-derivedcoatings for aerospace, energy, and strategic applications. Commercialization ofappropriate technologies calls for strong synergy between the know-how providerlike R&D institutes and the user industries.

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