Review on Sol-Gel Derived Coatings:Process, Techniques and Optical Applications

J. Maler. Sci. Technol, Vol.18 No.3, 2002211Review on Sol-Gel Derived Coatings:Process, Techniques and Optical ApplicationsS.M.Atial.2, Jue WANG", Guangming WUlt. Jun SHENI) and Jianhua MAI)1) Pohl Institute of Solid State Physics, Tongji University, Shanghai 20092, China2) Phys. Dept, Faculty of Kducation, Kalfer El-Sheikh, EgyptTB3 A[Manuscripl reeived April 16, 2001, in revised form July 10. 2001]Sol-gel process is one of the simplest techniques to manufacture thin films. The quality of the prepared films dependson the parameters of the sol-gel process and the used technique for deposition. A great variety of the sol-gelderived films have been prepared for dfferent applications. We present a review on the sol-gel derived coatings. Thedescription of the process is irtroduced in details. Different sol-ge!l deposition techniques are mentioned. The opticalapplications of the sol-gel derived coatings are reviewed.KEY WORDS: Sol-gel process, Coating, Thin flm1. Introduction'sol' form by treatment with a suitable reagent, e.g. waterwith HCI for oxide ceramics; (ii) allowing/ inducing the solAs one of the simplest technique to manufacture thiuto change into a 'gel' by polycondensation; (iv) shaping thefilms, sol-gel method can provide almost any single- or multi-gel (or viscous sol) to the finally desired forms or shape suchcomponent oxide coating on glass or metalsle. Sol-gel de-as thin film, fiber, spheres or grains and (v) finally convert-rived coatings can be employed as optical, electronic, mag-ing (sintering) the shaped gel to the desired ceramic materialnetic: or coating with chemical functons3. Sol-gel derivedat temperatures (~500° C) generally much lower than thoseceramic fIms are widely used as a protected layer againstrequired in the conventional procedure of melting oxides to-corrosion and oxidation of stainless stel4l, Agol, Al6, andgether.Cul7I substrates.In order to form an inorganic network from a solution(pure or colloidal solution) a network-forming step is required.This step mainy depends on the structure of the sol and can2. Sol-Gel ChemistryThe earliest sol-gel derived material was reported by.is a mechanism that is based on the growth of molecules, lead-M. Ebelmen in 1845， at the Manufacture de Ceramiqucsing to macromolecules that then grow together to an infinitenetwork. This mechanism leads Lo the so-called polymerizedde Sevres in Francel%l. Sol-gel processescribes in gen-gels and is very common in the acid-catalyzed hydrolysis anderal, the formation of solid materials, mainly inorganic non-condensation of tetraalkyl silicales. The other type is basedmetallic materials from solutions. This can be a solution ofmonomeric, oligomeric, polymeric or colloidal precursors. Inon the aggregation of colloida!particles from a so-called colnature for example, minerals like agate or chalcedony haveerwise the whole procedure would end up in a precipitationbeen formed by a poly-condensation reaction from aqueousprocess with no sol phase to be identified. The network-siliccous solutions. Bcfore going through the description offorming step in these sols is the aggregation of particles tosol-gel processes, somne terms might be introduced!1ol. A col-loid is a suspension in which the dispersed phase is so smallan infnite network. In the case of acid-catialyzed silica from(1~1000 nm) that gravitational forces are negligible and in-alkoxides the polymerization process can be simply controlledLeractions are dominated by short-range forces, such as Vanby the limitation of water, which lcads to stable sols if enoughder Waals attraction and surfacc charges. A sol is a colloidalunhydrolyzed三SiOR groups can be maintained to keep thesuspension of solid particles in a liquid. Hydrolysis is a reaverage molecular weight small.Usually, alkoxides are used as starting compounds, thataction in which metal alkoxide (M-OR) react with water tois, organometallic substances of the form M(OR)n (M: metalform metalydroxide (M-OH). Condensation is a reaction+HO-M)Iof valencen, R: alkyl group CzH2+1)- By two groups ofwhich occurs when two metals bhydroxides (M-OHcombine to give a metal oxide species (M-O-M). The reac-reactions, hydrolysis and condensation, the alkoxides are con-tion forrus one molecule of water. A gel consists of spongeverted to three-dimensionally connected networks. An exam-like, three dimensional solid network whose pores are flledple is the formation of silica glass from TEOS (tetraethoxysi-with another substance (usually the original solvent of thelane).sol)l. When gels are prepared, the pore liquid mainly consistsof water and/or alcohol. The resultant wet gels are thereforeSi(OC2Hs)a + 4H2→Si(OH)4 + 4C2HsOH (hydrolysis)called aquagel, hydrogel, or alcogels. When the pore liquid isreplaced by air without decisively altering the network struc-Si(OH)4→SiO2 + 2H2O (condensation)ture or the volume of the gel body, aerogels are oblained (orOverall reactioncryogels when the liquid is frcezedrying). Gel point is themoment al which the network of linked oxide particles spansSi(OC2Hs)4 + 2H2O→SiO2 + 4C2HsOIIthe container holding the sol. At the gel point the sol beconesa mininum anount toan alcogel. The alcogel can be removed from its original COI-中国煤化工s.. More generally for antainer and can stand on its own,the following reactionshe sol gel process (Fig.1)iJ consists of: (i) preparingMYHCNMHGa homogeneous solution of easily purified precursor(s) gener-ally in an organic solvent miscible with water or the reagentM(OR), + nH2O→M(OFH)。+ nROH (hydrolysis)used in the next step; (i) converting the solution to theM(OH)。→MOn/2 + (n/2)}H2O (condensation)↑To whom correspondence should be adressedM(OR)。+ (n/2)H2O→MOn/2 + nROH (overal1)E-mail: gmingwuQonline.sh.cn，Assoc. Prof, Ph.D212J. Mater. Sci. Technol., Vol.18 No.3, 2002. Ihin flnmg>Fbx F:rC JCrysaleSohatrs HCetamn-ma. PuwderPerurur :Melt (at lower-Olasstmparatutaey凹凹Fig.1 Steps in the sol-gel process for ceramic malerials (af-ppingWel: layer fomrtinSolvent evapuraltionter R.C.Mehrorall)yFig.2 Dip coating process: dipping the substrale in the s0-lution, formation of the wet layer, and gelation of the.nore complicated, especially because the hydrolysis occurs inlayer by solveat evaporationsuccessive steps. In the case of silicon-ethoxide this meansSi(OC2Hs)a + H2O→Si(OH)(OC2Hs)3 + C2HgOHDeposited fimSi(OH) (OC2Hs)3 + H2O→Si(OH)2 (OC2Hx)2 + CzHIsOIIGelaticnSi(OH)2(OCzHs)2 + H2O→Si(OH)3(OC2Hs) + C2HsOHSi(OH);(OC2I1) + H2O→si(OH)4 + CzHsOHAgregptonrurt hermore, the condensation reactions already start beforewraralcohol1 he hydrolysis is compleled. Thus not only reaction such as_Wie Film thicknes(OH)zSi0H + HOSi(OH)3→(OH)s Si-O-Si (OH)3+ H2ODiluted SolFunalion ofire proceeding, but also reaction such asfilm is hundcredby surface lenston一Subtrale(OH),(OR),SiOH + ROSi(OH),(OR)w→Fig.3 Gelation process during dip coating process obtained(OH)(OR),Si -0 - Si(OH),(OR). + ROHby evaporation of the solvent and subsequent desta-bilization of the solwhere xt+y=o+w=3. It is clear that siloxane bonds (Si-O-Si),which coustitute the basic structural units of silicate glasses,Sol-gel derived coating techniques are summarized below.ure formed by sol-gel condensation. The reactions in the sol-gel processes depend on many parameters, for examples:3.1 Dip coating techniques●Compositions and concentrations of alkoxides and sol-Dip coating techniqucs can be described as a process'ent used,where the substrate to be coated is immersed in a liquid●Amount of added water,and then withdrawn with a well-defned withdrawal speed uD-Catalyst used (type concentration),der controlled temperature and atmospheric conditions. TheFurther additives: cheating substances such as dike-schematics of the dip coating process are shown in Fig.2.1ones or desiccating controlling chemical additives (DCCA)In the dip coating process, the atmosphere controls the:uch as NH2CHO,evaporation of the solvent and subsequent destabilization of●Sequence in which the components are added,the sols by solvent evaporation leads to a gelation process●Time schedule of mixing, for example aging (pre-and the formation of a transparent film due to tbe small par-lydrolysis) of components or intermediates tbat react slowly,ticle size in the sols (nanometer range). The gelation process●Further conditions of mixing (e.g.. mixing eficiency,during dip coating process is shown in Fig.3.The resulting fiIm has to be densified by thermal treat-surface-to-volume ratio, ultrasonic agitation, atmosphere),ment and the densification temperature depends on the com-●Temperature.position.The coating thickness is defined mainly by the withdrawal3. Sol-Gel Derived Coatingsspeed, the solid content and the viscosity of the liquid. Thereare six forces acting on the coating during withdrawingl2a: (1)To get a sol-gel derived coatings on a transparent sub-viscous drag upward on the liquid by moving substratcs; (2)strate with a high optical quality, the coating process shouldforce on gravity; (3) resultant force of surface tension in thele carried out in a very clean room, the coating liquid hasconcavely shapedneniscus;(4) inrtial force of the boundary1o be filtered and the substrate has to be cleaned properly.F; (5) surface tension'The sol gel method of fabricating thin films offers potentialgradien中国煤化工oining presur (inidvantages over traditional techniques as shown below:portantYHCNM H Gate speed are high1. Low temperature processingWh |2. Easy coating of large surfaceenough to lower the curvature of the gravitational meniscus,3. Small thicknessthe deposition flm thickness, h, is that which balances the4. High optical qualityviscous drag (anUo/d) and gravity force (pgh)4s, the thick-ness is given by5. High purity. ..J. Mater. Sci. Technol, Vol.18 No.3, 2002213[ Criergaes 一- Dsrsnreruen F[ Sulation !Nazle ]Sustrate、Movable hoiderTo furmarewwwwwwwwwwwwwwwWWwFig.6 Schematic diagran of equiprent for spray depositiontechniqueFig.4 Schematic diagram of angle dependent dip coatingthe deposited film become thin by the centrifugal draining,and tend to be uniform due to the balance between the cen-trifugal force which drives fow radially outward and viscousforce (friction) which acts radially inward. Even with nhon-planar substrates very hornogeneous coating thickness can beobtained.The quality of the coating depends on the rheological par-邕rameters (rheology is a complex paramneter and depends onparticle shape, temperature, solvent, concentration and par-ticle interaction) of the coating liquid. Another inportant中干吊中parameter is the Reynolds number of the surrounding atmo-sphere.If the rotation velocity is in a range that the atmspberic friction leads to high Reynolds numbers (turbulence),Fig.5 Schematic diagram of the stages of the spin coatingdisturbances in the optical quality are observed.There are several attermpts to describe the, thickness ofprocess: deposition of the sol, spin up, spin of andthe 6lm obtained by spin coating. Meyethoferl1?! describedgelation by solvent evaporationthe dependence of the final tbickness of a spin coated layer onthe processing and materials parameters like angular velocity,d=a(nU)viscosity and solveat evaporation rate by the semiempirical.pg1formulawhere c1 is a constant about 0.8 for Newtonian liquids.d=((-2)(2)When the substrate speed and viscosity are low (often thecase for sol-gel deposition), this balance is modulated by thewith Pa is mass of volatile solvent per unit volume, Pn。isinitial value of ph is final thickness, η is viscosity, w is an-ratio of viscous drag to liquid-vapor surface tension, 7Yzv,cording to the relationship derived by Landau and Levich!41gular speed, m is evaporation rate of the solvent, m has to bedetermined empirically.Scriven!9l described the thickness of an itially uniformnmU.)3/3d = 0.94-flm during spin of by0(p.9)/2doThe interesting part of dip coating processes is that by choos-d()= 1+ 4pwl-1/3)1ing an appropriate viscosity the thickness of the coating canbe varied with high precision from 20 nm up to 50 nmwhere do is the initial thickness, t is time, ρ is the density,while maintaining high optical quality. More recently, anand w is the angular velocity. Lai and Chen(18.191 indepen-angle-depending dip coating process has been developed15,161.dently determined the thicknesses of fiIms obtained by spinwhere the thickness of the coating is controiled by the anglecoating, using diferent angular speeds, and their results couldbetween the substrate and the liquid surface as shown in Fig.4.be ftted very well with the equationDip coating processes are used for plate glass and bulbs.d=A.BDip coating processes have also been developed for curvedsurfaces like eyeglass lenses, mainly to employ scratch resis-where A and B are constants to be deternined emprcally.tant coatings for plastic substrates. For bottles, a variation ofthe dip coating process has been developed by revolving the3.3 Spray coating tecbniquesbottle during the withdrawal process.Spray coating techniques are widely used in industry fororganic lacquers. For coating irregularly shaped glass forms3.2 Spin coating processlike pressed glass parts, lamps or container glass (cold endIn the spin coating process, the substrate spins around an coat i中国煤化工- ,aration of optical coat-axis that should be perpendicular to the coating area. Theings I- advantages compared tospin- on process has been developed for the so-called spin-onthe H0 far realized processingglasses in microelectronics and substrates with a rotationalspedYHC N M H G° times lstet the wastesymmetry, e.y. optical leases or eyeglass lenses. The stagesof coating sols is much smaller, coating sols with rather shortof the spin. coating process are shown schematically in Figpot lives can be used and the coating step is suitable for 8The spin coating process is carried out in four stages: de-tablishing an in-line process. Figure 6 shows the schematicposition of the sol, spin up, spin of and gelation by solventdiagram of normal spray technique. Philips has developed aevaporation. The spin coating difers from dip-coating in that214J. Mater. Sci. Technol, Vol.18 No.3, 2002The process can be basically described as fllwing: acbuck holds the substrate in an upside down state using vac-uum, a tubular dispense unit can gently be moved under thesolSubstratesurface of the substrate using a driving unit on guide rails,The dispense unit consists of a circulating solution wave merg-ing from a slot or a microporous cylinder, a solution reservoircollecting the excess of fuid and a punping system to en-Coatingsure a continuous solution feed during the deposition travel.There is no physical contact between the tube applicator and.the subsPtrate, but a close interval permitting the creation ofa spontancous meniscus and, at the time a larminar regimeduring deposition is achieved, a coating is generated on thesurface with good uniformity. The substrate being fixed, thetravel motion of dispense unit is controlled under high ac-curacy to obtain a constant and laminar speed. To ensurea two material variety stacking, the machine operates withFig.7 Schematic diagram of the flow-coating processtwo separate disp:nse units, including two diferent lines andpumping circuits for both low and high index solutions. Allparts in contact with chemical solutions are made of inert ma-terials: stainless steel or tefon. The surrounding conditionsare also under control to prevent a contamination of coatings(clean room) and in addition tu smooth the evaporation ofsolvent during the layer drying (larminar airflow associated toan evaporation mask).Shese KauileThe coating thickness can be controlled by the depositionPerie ptieShe sakerate vd following equationd=kxvgF'ig.8 Schematic diagram of the capillary coating processwhere a is the exponent and k is empirical factor, dependingon the viscosity, surface tension and density of the fuid used.(after Flochl22I)3.6 Roll coating:ombined spin and spray process for functional sol-gel coat-Roll coating is a process whereby liquid flows into a nar-l1gs on T V screenslurow gap between two rotating cylinders, the surfaces of whichProcesses sinilar to spray coating， where very finemove either in the same direction (forward) or opposite di-droplets are produced (atomizers) like the pyrosol-process,rections (reverse). Some of the liquid passes through the gapr general lead to very homogeneous coatings on hard sub-and splits downstream into two thin films, each coating onestrates, but the coating malerial docs not hit the surface inof the rolls. 'This tccbnique is extensively used in the paint-the form of liquid droplets but in the form of dried small par-ing, photographic and tape-recording industries for coveringtucles in the nanometer range. Due to the high rcactivity ofa large surface area with one or several uniform layers.these particles when reaching the hot surface, a continuous4. Optical Applications of Sol-Gel Coating Filmsglass 6lm can be forned.A great variety of the sol gel derived coating films have9.4 Flow coating processesbeen prepared for different applicatious. Somne of the sol-gel .In the flow coating process the liquid coating system iscoating fIms are in practical use. Most of such filns are usedgoured over the substrate to be coated as shown schematicallyfor optical purposes. Many optical and electronic films are. Fig.7. The coating thickness depends on the angle of incli-promising for the use in electronic, optoelectronic, and pho-sation of the substrate, the coating liquid viscosity and thetonic devices and many efforts are being made to develop; alvent evaporation rate. Flow coating processes at presentthem. In this section the optical applications of the sol-gelsre used for outitting of automotive glazing from polycar-derived coating are summarized.: onate with hard coating but also can be used for float glass: ) employ functional coatings. As a variation of this process,4.1 Optical waveguides:he spinning of the substrate after coating may be belpful inOptical waveguides are fundamental to the technology oforder to obtain more homogencous coatings. If no spinningintegrated opticsl241. Sol-gel syntbesis of planar waveguidescrocess is employed, the coating thickness increases from thehas received much attention in the past decade because of: p to the bottorn of the substrate.the ease with which films can be made by spin and dip coat-ing. Also, the refractive index of the film is easily, Lailored1.5 Capillary coatingby altering the chemical composition of the solution[25l. TheSpray and spin coating processes are characterized by thesol-gel process also offers a promising alternative to conven-iuct that the coating material cannot be brought all onto thetional techniques for the fabrication of waveguides incorporat-substrate. So, in spray coating processes more than 100%ing surface-relief structures. There are many sol-gel derivedoverspray is obtained, and similar amounts are wasted withplanardielectricdes based on Si dioxide26~29, Tidioxide[28-31waveguiaes.basea.onand organically moditied ceramics, which aresoin coating. Dip and flow coating processcs mainly dependon the shelf life of the coating material and in optical dip中国煤化工inorganic function-cating only 10 to 20 percent of the coating liquid actuallyalities a35]. Figure 9 showscan be used for the fabrication of coatings. To overcome thesea schemiSsurements at He-Neproblems, the so-called capillaryninar flowcoatingaser wTMHC N M H Gensity of the lightcess has been developed by Flochl2scattered out of the waveguide is measured by a detector as: smbines the high optical quality of the dip coating processa function of the distance x.vith the advantage that all the coating liquid can be ex-4.2 Anti-electing (AR) fimsFloited. Figure 8 shows the schematic of the capillary coatingAnti-reflecting films applied on glasses prevent the loss ofErocess.J. Mater. Sci. Technol, Vol.18 No.3, 2002215highly iV absorbent. Blue colored GeO2-V2Os coating filmshave been made from Ge and V alkoxidest7l. The coatingfilms have been heated at 500°C in reducing atmnosphere toreduce the V5+ ion to V + ions.DeecorRutileHeNe| prismn4.5 Electrooptic and nonlinear optic filmsNonlinear optic (NLO) fIms are the films in which the op-tical prop;$ of the material are dependent on the intensity.05-08 unNonlincar optic films lave many applications in the field ofx→integrated optics. The most requirement for the application isthat the films must be transparent. Fortunately, the sol-gel-derived fiIms can meet this requirement in most cases. Glassfims are of course transparent. Crystalline films also renainFig.9 Schematic of the set up used for attenuation measure-transparent since single crystal films are possibly obtained ornents at He-Ne laser wavelength (after M.Bahtata)crystallite size is usually much smaller than the wavelength oflights. Accordingly, sol gel method has a great potential forlights and the occurrence of stray lights in optical systemspreparation of electroptic and nonlinear optic thin fIms withand provide clear vision of pictures through a glass sheet orNLO organic dyes and inorganic crystals are the chicef can-windows. AR coatings are one of the early applications ofdidate materials. NLO organic dyes have advantages for usesol-gel methods's6l. For the case of single layer AR coating,in intcgrated optics including. ease of fabrication, lower di-the following condition is required, ne = (no x n)}/2, whereelectric constants, higher bandwidths, low temperature pro-m is the refractive index of the film, ng is the refractive in-cessing, faster response timcs, and higher electro-optic (EO)dex of the substrate, and n。is the refractive index of theand NLO responses with increased concentration[48). Unfor-surrounding mediun. To achieve this condition mr should betunately, NLO organics lack the optical, thermal, and me-very small in the order 1.3. Fukawa et al. (37] developed anti-chanical properties to be used alone, and must be incorpo-refecting coating by single layer with n equals 1.3 using therated within a host (e.g., polymers, glasses, polycerams).so-Bermetilod. Single layer AR coatings are only effective atone λ; but using several layers to increase the bandwidth overwhich low rflection losses can be obtainedl38. A multilayer4.6 Electrocbromic fImsanti-reflecting coating consisting of SiO2 and TiO2 layers hasAn electrochromic fiIm is colored when the electric fieldbeen applied to sheet glass9, which makes it pogsible to seeis applied and the color fades on reversing the field. There-the picture covered by the glass sheets very clearly. An alter-fore, electrochromic devices can be applied to optical displaynative method for obtaining broad band AR coatings uses aand windows controling the light intensity. Amorphous WO3single coating with graded porosity, which results in a gradualfilms are an important candidate for electrochromic displays.change in n through the film, thereby minimizing reflectionIn an electrochromic cell, both sides of a WO3 flm contacledlosses. Mukherjee and Lowdermilk(40] developed a techniquewith anelectrolyte have electrodes. The electric field appliedin which a Na2O-B2O3-SiO2 film is applied on glass by sol gelbetween the two plane electrodes causes the followingloringprocessing, the glass fIm in phase separated and etched withreaction accompanied by incorporation of electrons and ionshydrofuoric acid (HF) or ammonium fAuoride (NHa)F. Thisinto the f6lm:etching dissolves the B2O3-Na2O phase and siO2 skeletonsto a less extent, producing silica flm of high porosity. TheWO3+re- + xM+→Mx WO3porous silica film thus prepared has a refractive index gra-where M+ reresents H+ or Li+ and Mz WO3 is colored tung-dient in such a way that the refractive index becomes smallsten bronze. The color is blue for HxWOg. The use of H+towards the free surface of the flm.ions gives fast responding element and the use of Li+ giveshemically inactive and stable element. There are many4.3 Refecting filmsa chemicalnyDrepared byv the soL-gel method,Coating of sheet glass with relflecting flm scrves in cuttingelectrochpomic. flms were prepared by tbe sol-gel methodthe thermal radiation of sunligbt coming into room of build-and IrO2161. WO3, TiO2, MoO. and Nb2O3 show cathodicing, lowering the power required for cooling, the room dur-coloration, IrO2 and NiO show anodic coloration while Vz2Osing summer time. In2O3-SnO2[41], VO2-SiO242] and ZrO2]23|shows cathodic and anodic coloration. An electrochromicfilms were prepared for this purpose by sol gel process. Thesmart window, glass structure, ilustrated in Fig.10, consistscutting of solar energy by reflection is more advantageous thanof two transparent conducting (TC) layers sandwiching anthe sun shielding-glass window, which absorbs sunlight.electrochronmic layer (EC) and counter electrode (CE) layer,which are separated by an ion-conducting (IC) layer. The4.4 Optical absorption and coloringthree inside layers essntially comprise an electro-optically ac-TiO2-SiO2 coating films are used for UV shielding:)tive battery. The EC layer is a mixed electron-ion conductor.since they have a strong optical absorption in the UV regionThe EC layer is cathodically colored, i.e, by double injectionof the spectrum. Sheet glasses are colored with thin coat-of electrons and charge compensating Lit. The layer trans-ing films containing coloring agents such as transition metalforms from a transparent“bleached” electronic insulator inelements4YYamamatoo[45~ made SiO2-RmOn (R=Cr, Mn,the delithiated state to a“colored" electronically conductingFe, Co, Ni and Cu) coating films froun tetraethoxysilane -metalmetal in the lithiated state. The CE (counter electrode) layernitrate-water- acid solutions. Most metal nitrates are solu-should also be a mixed electron- ion conductor,ble in water. In this method very high concentrations up to中国煤化i during deuble eltu30~50 mol fraction of metal oxides could be dissulved in the. transparent state duringstarting solution and that calculated from the extinction caMYHCNMHGefficient of the coluring eleunent assuming that the element isdissolved in gel or oxide matrix as ion. It is then suggested4.7 Photochromic filmsthat the strong color is caused by colloidal particles of theA photochromic flm is colored and their light absorbingoxide of the coloring element. Transparent, yellow. coloredproperties are changed when they are optically excited andcoating flm of the TiOz.CeO2 systeml*6) can be obtained bythe induced coloration can remain even after the excitationsol-gel coating using Ti isopropoxide and Ce chloride as start-source has been removed. Photochromic coloration involves啊亨数据s. This film has a bright yellow color and is also216J. Mater. Sci. Technol, Vol.18 No.3, 2002Organic polymer-cotinringscl flmTC_CH↓C●TC_Depi 12mStbstrale2)↓←- Ptterned gtl 0lmF'ig.10 An electrochromic smart window glass structureEOH GPIS] S(OR.JCH.CHCH,SIOR[ ARNO[ EiOH ]Fig.12 Schematic diagram. of the micropalterning process(after A.Matsudal6l)H,NCHCH,NHl IROJ-SICHLCH.CHthe glass composition for example, TiO2-SiO2 and SrO2-SiO2systems. A.Matsuda, et al.f5! fabricated pregrooves for optical memory disks in the sol-gel derived SIO2 flm on glassThermal trcaimtesubst:s of 130 mm in diameter, using a fine patterning pro-cess. The patterning is carried out under vacuum by pressing中一a stamper against the SiO2 gel flm containing poly(ethylene” aCglycol), PEG, and then the gel film is pre-heat treated with→AgCIbeing pressed by the stamper. After the gel film becomeshard, the stamper is removed and the glass disk substratewith patterned gel f6lm is finally beat-treated at 350°C toFig.11 Schematic diagram of the synthesis of photochromicdecompose PEG and to be densified.Ormocers (after H.Schmidtrol)N.Yamada, et al.166prepared aicropaernedhybridfilm on Si wafers from diethoxydimethylsilane (DEDMS) andihe trapping of electrons at appropriate sites within the mateTi(OC2Hs)4 modifed with ethyl acetoacetate (EAcAc). Gelrlal, which promotcs light absorption. There are mainly threeflms prepared on Si wafers were exposed on UV-light throughm1ost important material requirements for a photochromic sys-a mask. Irradiated flms were immersed in the mixed solution: :m which consists of photochromic dye and matrix material:of 2 ethoxyethanol and 0.5 mol/L HCI in a volumetric rationigh (acceptable) photochromic response to UV iradiation,of 1:1. They subsequently were rinsed in pure water. Whileaigh or very low fading rate of the colored form for switch-the unirradiated area was soluble in the mixed solution, theg or memory devices, respectively, and high thermal andirradiated region showed markedly lower solubility. The rigidphotochemical stability. L.Hou et al.!o2l have tailored pho~structure resulting from photopolymerization seemedto de。schromic SO-Ormocer materials with the thermal and pho-crease the solubility. But using the diference in solubility,t schemical stabilitics being comparable to, and both the pbo-fine patterning of the bhybrid film was demonstraled.tochromic response and color fading rate much better thanhe SO-PMMA materials. Mennig et a.Gsy investigated pho-4.9 Fluorescent films: schromic in sol-gel thin flms of Na aluminoborosilicate con-Sillica gel doped with optically-active organic dye: sining nanosized crystallites of AgCl. AgCl was formed bymoleculessuchasEu,U,Er,andNdhe been used in the:ne exposure of the film to HCI vapor. Figure 11 shows adevelopment of fat-panel displayslo7]. For the application to+:hematic diagram of the synthesis of AgCI containing Orflat-panel displays, microspheres which Auorescence red, blue,:1ocer films. On exposure to UV light, the coatings turnedand green are desirable. In Table 1, the fuorescence peakbrown-violet. To bleach the coatings back to high trans-wavelengths for selected organic dye molecules and inorganicparency, a thermal treatment of about 400C is needed.ions doped into silica gel microspheres are presented. In prin-ciple, color display screens might be fabricated from either48 Films for micropatterningorganic or inorganic dopants.The luminescent solar concentrator (LSC) has a coatingMicropatterning is used for preparing optical waveguidescontaining fuorescent dye molecules that absorb solar radia-)r memory disc. Sputtering and lithography are the mainnethods for micro patterning. Tohge et al.84] developed ation and reemit it in such a way that the emission is trappednovel sol-gel micropatterning process in which SiO2-TiOz solin the fIm by total reflection and concentrated at the edges ofgel coating flm is mechanically stamped with a stamper. Forthe collector, which constitute photovoltaic cells as shown inustance, grooves of width less than one micron can be pre-Fig.13. A LSC has the advantage that both direct and diffusepared. Figure 12 shows the schematic diagram of the mi-lights are collected, tracking the sun is not necessary and thecropatterning process. The process consists of:atching of the(i) coating of gel flm on a glass substrateconcentr中国煤化工tivity of the photo-voltaic cal crysalization in(i) patterning of the gel film by pressing a stamperorganicYHC N M H Gm Ti butoxide using(ii) drying and heating.T'he most important point in the process is to control the25 mL of water in 70 mL of toluene'l. In the final stage ahardness of gel film just prior to stamping. This is achievedcold aqueous solution of TiCI4 (0.2 mol/L) was poured ontoby adding a relevant arnount of organic substances such asthe flm and kept for 10 h before sintering at 450°C. The flmssolyethylene glycol (PEG) to a sol. The chemical durabilitywere used for solar cell. Dye -sensitized solar cells made of the+ipecially to moisture is pronouncedly improved by choosingRudye -adsorbed mesoporous HyCOM films as photoanodesJ. Mater. Sci. Technol, Vol.18 No.3, 2002217Table 1 Fluorescence peak wavelength for selected organic dye molecules and inorgaric ionsdoped into sol-gel derived silica microspheres (excitation wavelength of 365 nm) (afterEJ.A Pope67)Dopant speciesPeak wavelength/ nmEmission colorOrganic:7-amino-4-methylcoumarin440bluePolypheny! vinylene (PPV)67royal blueCounarin 314T80tiel blueFluorescein sodium slat525reenRhodamine-6G560orangeRhodamine-B625deep redInorganic:Europium450Uranium (6+)520greenSamarium (3+)603pinkish redEuropium (3+)615red{4] K.Miyazawa, K.Ssuzuki and M.Y.Wey: J. Am. Ceram. Soc,] A.Morales and A.Duran: J. Sol-Gel Sci. & Technol, 1997, 8,Sun light;] K.Kato: J. Mater. Sci, 1993, 27, 4033.i P.Inpocenzi, M.Guglielmi, M.Gobbin and P.Colombo: J. Eu-rop. Ceram. Soc., 1992, 10, 431.[8] Alain C.Pierre: Introduction to Sol-Gel Processing, KluwerPhotovoltaicAcademic Publishers, London, 1998, P.[9 ] Helmut Schmidt: in Chemistry, Spectroscopy and Applicationof Sol-Gel Glasses, ed. R.Reisfeld, Springer- Verlag, Berlin,FluorescentUndoped1992, 119.dye filmsubstrate[10] C.Jefrey Brinker and George W .Scherer: Sol-Gel Science, Aca-demic press INC., New York, 1990, 1.[11] R.C.Mehrotra: in Chemistry, Spectroscopy and Application ofSol-Gel Glasses, ed. R.Reisfeld, Springer-Verlag, Berlin, 1992,Fig.13 Flat plate luminescent solar concentrator with thinfilm[12] C.J.Brinker, AJHurd, P.R.Schunk, G.C.Prye and C.S.Ashley:J. Non-Cryst. Solids, 1992, 147/148, 424.achieved better photo-energy conversion eficiency as com- [13] LEScriven: in Better Ceramics Through Chemistry l eds.CJ.Brinker, D.E.Clark and D.R.Ulrich, Mater. Res.. Soc.pared to those prepared using commercially available DegussaSymp. Proc, Vol. 121 (Materials Research Society, Pitts-P25 films.burgh, 1988, 717.[14] L.D.Landau and B.G.Levich: Acta Physiochim. URSS, 1942,5. Conclusion17, 42.[15] H.Dislich: J. Non-Cryst. Solids, 1997, 80, 115.The sol gel process is one of simplest method for prepar-[16] N.J.Arfsten, A.Ebrle, J.Oto and A.Reich: J. Sol-Gel Sci. &Technol. 1997, 8, 1099.ing the ceramic materials. It can be used for preparing thin [17] D.Meyerhofer:' J Appl Phys, 1978, 49, 3993.films, fbers, spheres, powders, aerogel, xerogels, and glasses. (181 J.HLai: Polymer Engineering and Science, 1979, 19, 1117.There are many parameters infuencing the sol-gel process, 191 B.T.Chen: Polymer Engineering and Science, 1983, 23, 399.such as compositions and concentrations of alkoxides and sol 20 J.Van Bommel: Class Research, 1997,7, 10.vent, catalyst, sequence in which the components are added,21 H.G.Floch, G.Bellevile and J.J.Priotton: Am. Ceram. Soc.and temperatures. There are many techniques to fabricate[22] F.elleville and H.G.Floch; SPIE, 1992, 1758, 40.thin films by the sol-gel process, such as dip coating process,， (23) lan M.Thomas: SPIE, 1994, 2288, 50.spin coating process, spray coating techniques, and capillary’ 24] B.Saleh and M.Teich: Fundamentals of Photonics, John Wileytechniques. A great variety of the sol-gel derived coating films& Sons, Inc. 1991have been prepared for dfferent aplications. As for the op- [25] G.W.Dale, H.H.Fox, BJJ. Zelinski and LWller-Brophy:tical application, the sal gel derived coatings have been usedProc. Mater. Res. Soc. Symp.. 1990, 180,as waveguides, anti-efecting films, reflecting films, coloring[26] R.Uirich and H.P.Weber: Applied Optics Il, 1972, 428.films, electrochromic films, photochromic fiImns, fuorescent27 W.Lukosc and K.T.Tiefenhaler: Optics Letters, 1983, B, 537.[28] L.Yang, S.Scott Saavedra, N.R.Armnstrong and J.Hayes: Analflms, and optical storage media.Chem, 1994, 66, 1254.G.Cranorand M.Cuglielmi: J Non-[29] L.Armnelao, P.Colombo, GCryst. Solids, 1992,138,198.P.Baudrg and M.A.Aegerter: J.Acknowledgement[30] La Sarra ER, Y:Charboullot,We are very grateful to the National Natural Science Founda-M.Bahtat, J.Mupier, L.Lou and J.Serughetti: SPIE, 1992,tion of China (No. 69978017 and 59802007) and Shanghai Edu-1758. 173.cation Committeve (No. JW99-TJ-03) for their help and financial32] H Schmidt: in Proc. Mater. Res. Soc. Symp, 1990, 180, 961.supports.331 HSchrnidtn1Krne. R.Kaseman and F.Tiefensee: SPIE, 1991,中国煤化工REFERENCESFYHC N M H Gd SRusci Appl Phys.(I I D.R.Uhlmann, T.Suratwala, K.Davidson, J.M.Boulton and [36] H.Schroeder, J: Phys. Thin F'ilms, 1969, 5, 87.G.Teowee: J. Non Crystalline solids, 1997, 218, 113.[37] M.Fukawa, T.Ikeda, T.Yoneda and K.Sato: Proc. 3rd-ICCG,[2] Massimo Guglielmi: J. Sol-Gel Sci. & Tecbnol, 1997, 8, 443.eds. HA.Meinema, S.I.M.A.Spee and M.A.Aegerter, 2000,[3 ] Sumio Sakka and Tosbinobu Yoko: in Chemistry, Spectroscopyand Application of Sol-Gel Glasses ed. R.Reisteld, Spring [38] P.K.Biswas, D.Kundu and D.Ganguli: J. Mater. Sci, Lett,Verlag, Berlin, 1992, 89.1989, 8, 1436.218J. Mater. Sci. Technol, Vol.18 No.3, 2002[39] R.B.Pettit, C.S.Ashley, S.T.Reed and C.J. Brinker: in Sol-Gel[57] K.Von Rottkay, T.Richardson， M.Rubin， J.Slack andTechnology for Thin Filtns, Fibers, Preforms, Electronics, SpeL.Kullman: Solid Stale lonics, 1998, 113/115, 425.cialty Shapes, ed. Lisa C.Klein, Noyes Publications, New Jer-58] R.Cinnsealach, G.Boschloo, S.N.Rao and D.Fitzmaurice: Sol.sey, 1988, 80.Energy Mater. Sol. Cells, 1999, 57, 107.(40] s.P.Mukherjee and W .H.Lowdermilk: J. Non-Cryst. Solids,59] M.Schmitt, S.Heusing, M.A.Aegerter， A.Pawlicka and1982, 489, 177.C.Avellianeda: Sol， Energy Mater. Sol. Cells, 1998, 54,|41] H.Haitjema in Proc. Solar Optical Materials, ed. byM.H.Hutchins, 1988, 69.:42] F.Hutter, H.Schmidt and H.Scholze: J. Nou-Cryst. Solids,60] M.Macek and B.Orel: Sol. Energy Mater. Sol. Cells, 1998,54, 121.1986, 82, 373.|43] H.Dislich and E.Hussamann: Thin Solid flms, 1981, 77, 129.[61] F.Michalak, L.Raulr and P.Alderert: SPIE, 1992, 1728, 278.44 P .Geotti-Bianchini, M.Guglielmi, P.Polato and G.D.Soraru: J. [62] L.Hou, B.Hofrann, M.Mennig aund H.Schunidl: in Sol-Gel Pra-Non-Cryst. Solids, 1984, 63, 251.duction, ed. H.Schmidt, Trans Tech Publications, Switzerland,5] Y.Yamamoto, K Makita, K.Kamiya and S.Sakka: J. Ceram.1998, 41.Soc. Jpn, 1983, 91, 222[63] M.Mennig, H.Krug， C.Fink- Straube, P.W .Oliveria and46] M.A Sainz, A.Duran and J.M.F.Navaro: J Non-Cryst Solids,1990, 121, 315.[64] N.Tohge, A.Matsuda, T.Minami, Y.Mat.suno, S.Katayama and.47] L.S.Hou and S.Sakka: J. Non-Cryst. Solids, 1989, 112, 424.Y.Ikeda: J. Non-Cryst. Solids, 1988, 100, 501.48] R.Lytel and G.Lipsomb: Mater. Res. Soc. Proc, 1990, 175，[65] A.Matsuda, Y.Matsuno, Y.Mitsuhashi, N.Tohge andT.Minami: in Sol-Gel Production, ed. H.Schmidt, Trans Tech。49] K.A.MacDonald, J.M.Bell, J.Barczynskxa and G.Voikel: SPIE,Publications, Switzerland, 1998, 111.0 C2017el..[66] N.Yamada, I.Yoshinaga and S.Katayama: in Sol-Gel Synthe0! D.C.Green, JM.Bell and C.B.Smith: SPIE, 1992, 1728, 26.sis and Processing, eds. S.Komarneni, S.Saka, P.P. Phule andRheisteld,M.Zayat, HMinti and A.Zastrow: Sol. EnergyR.M.Laine, The American Ceramic Society, Ohio, 1998, 235.a] L.H.M.Kring and W.Talen: Sol. Energy Matt. Sol. Cll, (67] Edward J.A.Pope: SPIE, 19949 288, 536.109MgKrings[68] R.Reisfeld:in Sol-Gel-Science and Technology, eds.T.Miki, K.Yoshimura,Y.TaM.Tawa,P.Jin andM.A.Aegerter, M.Jafelicci and E.D.Sou2a, World Scienific,S.Tanemura: SPIE, 1995, 2531, 1Singapore, 1989, 323.)4] F.H.Moser and N.R.Lynam: U. S. Patent, No.4959247, 1990.69] S.Kambe, K.Murakoshi, T.Kitamura, Y.Wada, S. Yanagida,55] P.Judenstein and J.Livage: SPIE, 1990, 1328, 344.H.Kominami and Y.Kera: Sol Energy Mater. Sol. Cells,56] M.Nabavi, s.Doeuff, C.Sanchez and J.Livage: Mater. Sci. Eng,2000, 61, 427.B3, 1989, 203.[70] H.Schumidt;: J. Non Cryst. Solids, 1994, 178, 302.中国煤化工MYHCNMHG