Mullite-dispersed silica ceramics were prepared through sol-gel processing by the use of tetraethoxy silane, aluminium nitrate and aluminium isopropoxide as the Si and Al sources where HCl and HN0(3) were the catalyst. Effect of the starting materials, solvents and catalysts was examined on the gelation time or temperature of mullite precipitation. Apparent activation energy of gelation ranged from 80 to 95kJ/mol. The presence of AI in the sols elongated the gelling time suggesting the formation of chelate bonds between AI and Si-OR or Si-OH bonds

Kawabata, Kouji

Miura, Yoshinari

Nanba, Tokuro

Osaka, Akiyoshi

Takada, Jun

Okayama University Digital Information Repository

Preparation of Mullite Dispersed Silica Ceramics through Sol-Gel Processing

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering, Okayama University,V01.26, No.2, pp.61 ~67, March 1992Preparation of Mullite Dispersed Silica Ceramicsthrough Sol-Gel ProcessingAkiyoshi OSAKA*, Kouji KAWABATA**, Tokuro NANBA*lun TAKADA* and Yoshinari MIURA*(Received January 7 , 1992)SYNOPSISMuIlite-dispersed silica ceramics were prepared through sol-gelprocessing by the use of tetraethoxy silane, aluminium nitrate and alu-minium isopropoxide as the Si and AI sources where Hel and HN03 werethe catalyst. Effect of the starting materials, solvents and catalysts wasexamined on the gelation time or temperature of mullite precipitation.Apparent activation energy of gelation ranged from 80 to 95k]/mol. Thepresence of AI in the sols elongated the gelling time suggesting the for-mation of chelate bonds between AI and Si-OR or Si-OH bonds.l.lN1RODUCTIONGlass and ceramics are conventionally prepared by melting or sintering the starting mate-rials at high temperatures. However, homogeneous glass cannot result when the melts experi-ence phase separation! or the system is so refractory that the batch melts only at very hightemperatures; ceramics will neither be obtained when they consist of crystalline phases thatare unstable or decomposed near the sintering temperatures. Sol-gel processing is advanta-geous because the glass or ceramics that cannot be available via the conventional methods areobtained at lower fIring temperatures by the use of solutions in which atomic level homogeneityis kept during preparation. 2 In this process hydrolysis and condensation reactions of organicand inorganic compounds takes place in the starting solutions resulting in gels that can becalcined to ceramic materials. Suppose gelation proceeds in a homogeneous mixture (a sol) of acomponent easy to become gelled and metallic ions that form highly ionic metal-oxygen bonds.The former component will construct a gel network, whose voids are occupied by the metal ions.***Department of Applied ChemistryNow at Industrial Technology Center of Okayama Prefecture6162 Akiyoshi OSAKA, Kouji KAWABATA, Tokuro NANBA, Jun TAKADA and Yoshinari MIURATEOS,DMFIIAl(OPr i)3 + SolventChelating agentRoomtemperature -i H2O, Acid catalyst IAl(N03)3- 9H20 + SolventRoom Cbelating agenttemperature..., TEOS, DMF I-l H2O, Acid catalyst IThe former component will construct a gel network, whose voids are occupied by the metal ions.The metal ions are held in the gel without being squeezed out by syneresis as gelling proceeds ifthey are strongly bonded to the gel matrix component. Subsequent proper heat treatment of thegel will lead to glass or glass ceramics in which ionic microcrystallites are homogeneously dis-persed Moreover, the control of the microcrystalline phase and their size or shape enables thepreparation of multi-functional ceramic materials that have high thermal stability or strength.For example, refractory coatings and fibers of high chemical or thermal resistivity are thusprepared.3 New such materialscan be developed only with basicunderstanding of the sol-gelprocess. In the present work wehave prepared silica ceramicsdispersed with mullite particles,and studied the effects of the fac-tors such as the raw materials,gelation temperature, solvent,and catalyst on the rate of gela-tion, microstructure of the ob-tained gels and thermal stabilityof the xerogels.Fig. 1. Procedure I and II of preparing precursor gels.2. EXPERIMENTALAluminium nitrate ennea-hydrate (Al(N03)3e9HP) and aluminium isopropoxide (Al(OPri )3)were the sources of aluminium while tetraethoxy silane (TEOS, Si(OEt)4) was that of silicon wherePri and Et denote CH(CH3)2 and C2Hs' respectively. At mixing the raw materials in preliminaryexperiments aluminium hydroxide was precipitated and the sol became opaque. With 0.8 to 1.4mol of HP2 or acetyl acetone (acac-H) per Al the solutions were found stabilized. In the follow-ing, therefore, 1 mol HP2 or acac-H per Al was added to the starting solutions. Chelates wereprobably formed between those additives and Al atoms. In order to fmd optimum conditionsunder which least pieces of cracks were produced in the gels after drying at 60°C, precursorgels were prepared under various conditions. The starting solutions also contained N,N-di-methyl formamide (DMF) as a drying control chemical agent (DCCN) which gave gels of betterquality.Fig. 1 indicates the experimental procedure. The mixing ratios of the raw materials, sol-vents and additives were tabulated in Table 1. In a few experiments Al solutions were added to aTEOS solution after the hydrolysis of TEOS proceeded to some extent,6 In the present study, how-ever, the starting solutions were prepared by mixing solutions of the Al sources with TEOS.7,8,9,10Al(OPri)3 could be dissolved in alcohols with the addition of acid. The sols were subject to gela-tion at 40 to 50°C and the gels were subsequently aged at 60°C for 24h. Thus obtained precursorgels were heated from the room temperature to 1300°C at a rate of lOCImin. In the course of thetreatment, they were held at 200, 400 and 600°C for SOh.MILl/ite Dispersed Sitica Ceramics through Sol-Gel ProcessingTable 1. (a) Starting materials and mixing ratios, and (b) cracking state of the xerogels(a) Starting materials63Raw materialsAl(NO)309HPAl(OPr\TEOS (Si(OEt)JSolventAlcohols (ROH)R=Me, Et, n-Pr, i-Pr,n-Bu, n-PenChelating agentAcetyl acetone(acac-H)H20 2Acid catalystHCl, HN03, H2S04HCOOH(0.OI,0.I,I,5N)(b) Cracking state of xerogels and optimum combination of the materialsCracking Chelating Acid, ROW DMF/Al source Solvent state* agent HP/(Al+Si) HP/(Al+Si) HP/(Al+Si)MeOH 0EtOH 0 HCl (IN)Al(NOJ3 n-PrOH 0 H20 2 HN03 2Procedure i-PrOH 0 (O.OIN)I n-BuOH 0 10n-PenOH lJ.MeOH lJ.EtOH lJ. HCl (5N)AI(OPr')3 n-PrOH 0 acac-H HN03 (5N) 2Procedure i-PrOH lJ.II n-BuOH 0 10n-PenOH lJ.* Cracking state of xerogels dried at 60°C0: No cracks; 0: few cracks; lJ.: cracked3. RESULTS3.1 Aged Gels and AdditivesTable 1 (b) indicates correlation of the cracked state of the gels with combinations of thesolvents or additives. The symbol 0 indicates the optimum combinations that gave crack-freedried gels. Better gels resulted from the iso-propyl alcohol solutions of aluminium nitrate andHPz as well as the n-butyl alcohol solutions of Al(OPri)3 and acetyl acetone. Hydrochloric acidand nitric acid were appropriate for the acid catalyst for the present systems though theAl(OPri)3 systems required more concentrated acid solutions for stable sols. Not only the pH of64 Akiyoshi OSAKA. Kouji KAWABATA. Tokuro NANBA. Jun TAKADA and Yoshinari MIURA80.0(-26.8)95.0(-32.4)Activation energy(kllmol)(In to )***89.0(-30.4)95.0n-BuOH (-32.8)Table 2. Gelation time and apparent activation energyGelation time, tgol (h)**Starting Acid:Hp Gelling temperature eC)Solutions {pH}* 40 45 50 55AI(N03)3 HCl (IN) {1.66} 60 38.3 24 14.6(Proced. I)HPz HN03(O.OlN) 65 33.3 19 12.3i-PrOH {U5}AI(OPr\ HCI (5N) {1.03} 45 24.7 18 8.8(Proced. II)acac-H HN03(5N) {050} 43 20.75 16.25 7.3TEOS'4HP' HCI 55.44EtOH·HX [11] HF 61.2------------------------------------0.lAI(NO,)3•0.95TEOS· No acid catalyst 89.26Hp·4EtOH [7]{pH}*: initial pH of the starting solution.** and ***: In tgol =In to + l:>.HIRT (eq. 1)300100..-....c:-10 •AI(N0S>3; HC'0 AI(N0S>3; HN03• AI(Opri)3; HCI0 AI(Opri)3; HN033 3.1 3.2 3.31000fT (K-1 )Fig. 2. Arrhenius type plots of gelation time (t8Ol) as a function of gelling temperature.The frequency factor and apparent activation energy are indicated in the rightmost column of Table 2.Mullite Dispersed Silica Ceramics through Sol·Gel Processing 6550013OO"C2000 1500 1000WAVENUMBER (em-i)Fig. 3. IR spectra of the gels strarting fromAl(N03)3 through procedure I.4000the starting solutions for the optimum combina-tions but the gelation time are indicated in Table2. Either sols from aluminium nitrate orAI(OPri)3 became gels in lO~lSh and 8~9h, respec-tively.zo3.2 Apparent Activation Energy for Gelation V5(I)Fig. 2 represent the plots of the gelation ~time (t gel) under the optimum conditions as a ~function of temperature. Table 2 shows the ap- -<parent activation energy for gelation t>H after ~an Arrhenius type equation (1).tgeJ = to exp (t>HlRT) ( I )The gelation time was shorter for the AI(OPrl )3systems under any acid catalyst, and the nitricacid catalysis shortened the gelation time. Theactivation energy for an aluminium source wassmaller for the HCI catalysis while the AI(OPri)3system gave a smaller activation energy underan acid catalyst. Table 2 also shows the activationenergies for the systems TEOS-H/)-HCI, HF afterColby et alP and TEOS-Al(Nq)3-HP after Bansal etal.7 The activation energy of the present systemswas greater than that (82~1l0 k]/moJ11 ) for the gelation of the TEOS systems and was almost aslarge as that for the system SAlp3e9SSi02 (89.2k]/moF). Comparing the tgel between the systemSAl20 3e9SSiO/ and the present lOAlp,e9QSi02 system indicates the larger AI content shortenedthe gelation time.3.3 Microstructure and Crystallization of the GelsFig. 3 represents IR spectra of the gels starting from aluminium nitrate. The as-dried xero-gel has a strong band near 3000 cm1 due either to O-H bonds of free water and silanol groups orto C-H bonds of organic groups. IR bands for Si-OR or Si-0-Si bonds are observed in the rangefrom 1000 to 1300cm1. The 1600cm1 band is due to free water and the c=o or C-N bonds of DMFcause a band near 1400cm-1. These two bands disappear when the gels are treated at 600°C. Aweaker band at 9S0cm-1 can be attributed to Si-0-AI bonds,12 indicating that the AI atoms are al-ready attached to the xerogel matrix. Formation of silica skeleton is completed after treating thegels at 1300°C since the 800cm-1 band corresponds to a symmetric stretching vibration of Si04tetrahedra found for silica glass and other silicates. The following X-ray diffraction (XRD) ex-periment confirms this result. Fig. 4 shows crystalline phases identified by the XRD patterns ofthe gels after the treatment at temperatures indicated. In the gels from aluminium nitrate mul-lite appears at 900°C while mullite is precipitated at 1100°C in the gels from AI(OPri),. Heatingthe gels at 1300°C results in cristobalite irrespective of the AI source. Table 3 summarizes the66 Akiyoshi OSAKA. Kouji KAW ABATA. Tokuro NANBA. Jun TAKADA and Yoshinari MIURATemperature(0C)1300120011001000900800Proced. I Proced. IIAI(N03)3, HCI AI(i-PrO)3, HCIi-PrOH n-BuOHCristobalite + Cristobalite +Mullite MulliteMulliteI- +AmorphousMullitet- +Amorphousf-AmorphousAmorphousdensity, crystalline phases and mulliteparticle size, determined by XRD, forthe gels treated at 1300'C. It is foundthat the gels turn into pore-free ce-ramics on heating at 1300'C since thedensity of the 1300'C gel (2.40g/cm3 ) isas large as that of the theoretical den-sity of muIIite dispersed silica ceramicsof composition lOAlP3e90Si02 • Table 3also showes that either of the AI sourcesproduce similar ceramics when thegels are treated above llOO°C.4. DISCUSSIONPrecipitation of mullite never occursFig. 4. Crystalline phases precipitated from the gels. in alumina-silica diphasic gels beforethey are heated above 1350°C7 becauseAI and Si atoms should travel longer distance to accomplish reformation of the bonds. In thepresent study we have observed muIIite in the gels heated at as low as 900°C. This suggests suchgreater structural modification is unnecessary because of monophasic nature of the presentgels. This can be attributed to the presence of Si-O-AI bonds in the gels confirmed by the IR bandnear 90Ocm-1• Moreover, the systems containing larger amount of AI became gel in shorter time.It is therefore considered that the formation of the Si-O-AI bonds is concerned with the apparentactivation energy of gelation. Colby et al. observedll that the activation energy for the gelationof TEOS under HCI and HF catalyst was almost the same while shorter gelation time was for theHCl They interpreted this result as showing that HCI is more active on hydrolysis of the Si-ORbonds resulting greater amount of silanol groups while HF is effective on condensation of theresulted Si-OH groups, and that the more silanol groups in the HCl-{'atalyzed sols enhance thefrequency for a group to encounter the other. ll In another words, the HCl catalysis has a largerTable 3. A few properties of the gel-derived mullite ceramics.Particle size of the precipitated mullite is shown.AI source Acid Density Precipitated Particle sizeand catalyst (glcm3) phase of muIlIiteprocedure (A)Al(N°3)) HCI 2.349 C+M 113Proced. I HNO) M 104AI(OPri)) HCl 2.380 C+M 109Proced. II HN03 2.294 M 100Mullite Dispersed Silica Ceramics thrmlgh Sol-Gel Processing 67to value of eq. (1) than the HF catalysis. The activation energy of Si-O-Si bond formation wasgiven as about 55k]/moF and that of hydrolysis of Si-OR bonds was 19.4k]lmol,11 which arefairly smaller than the present data (80~95 kllmol). Since the gelation reaction involves manyprocesses it is hard to give a strict meaning to thus obtained apparent activation energy, but wecan at least point out that the presence of AI ions in the sol retards the gelation. Further inter-pretation of the present results should wait for much more accumulation of experimental re-sults. However, we consider that some AI-bridged intermediates are formed by chelation be-tween AI and Si-OR or Si-OH bonds in the course of gelation and that the chelates suppress therate of hydrolysis and condensation of the Si-OR and Si-OH bonds. In the chelates the AI atomsare six-coordinated by oxygen atoms under the presence of water and pH lower than 6.55. CONa.uSlONUsing tetraethoxy silane, aluminium nitrate and aluminium isopropoxide as the Si and AIsources, mullite-dispersed silica ceramics were prepared through sol-gel processing. Variousalcohols were used for solvent while HCl and HN03 were the catalyst. Dimethyl formamide wasadded as a drying control agent. Optimum mixtures of these reagents were so determined as toresulting crack-free xerogels. Effect of the starting materials, solvents and catalyst was exam-ined on the gelation time or temperature of mullite precipitation. Apparent activation energyof gelation ranged from 80 to 95k]/mol and the presence of AI in the sols elongated the gellingtime.REFERENCES1 N.P. Bansal,]. Am. Ceram Soc., 71 (1988) 666-72.2 D.R. Uhlman et aI., "Better Ceramics through Chemistry," ed. by C.]. Brinker, D.E. Clark andD.R. Ulrich, Nocht-Holland, 1984, p. 59-70.3 S. Sakka, "Sol-Gel (Proceedings of the Winter School of Glasses and Glass Ceramics from Gels),"World Scientific, Singapore, 1989, p. 346-74.4 A.H. Boonstra et al.]. Non-Cryst. Solids, 109 (1989) 141-52.5 M. Nogami et aI.,]. Ceram. Soc. Japan, 98 (1990) 93-97.6 Y. Abe et al.,]. Non-Cryst. Solids, 108 (1989) 150-56.7 N. Bansal,]. Am. Ceram. Soc., 73 (1990) 2647-52.8 T. Tsuchiya et aI., Nippon Seramikkusu Kyokai Gakujutsu Ronbunshi, 97 (1989) 224-28.9 T. Lopez et al., ]. Non-Cryst. Solids, 108 (1989) 45-48.10 ].P. Boilot et ai, "Better Ceramics through Chemistry III," ed by C.]. Brinker, D.E. Clark andD.R. Ulrich, Materials Research Society, Pittsburgh, U.SA, 1988, p. 121-26.11 M.W. Colby et aI., ]. NonCryst. Solids, 99 (1988) 129-39.12 M. Nogami and Y. Moriya, Yogyo -Kyokai-Shi, 85 (1977) 59-65.

Preparation of Mullite Dispersed Silica Ceramics through Sol-Gel Processing

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