Solution sol-gel synthesis and phase evolution studies of cordierite xerogels, aerogels and thin films

August 1994

Materials Letters 20 ( 1994) 355-362

Solution sol-gel synthesis and phase evolution studies of cordierite xerogels, aerogels and thin films P.N. Kumta ‘, R.E. Hackenberg b, P. McMichael a, W.C. Johnson ’ aDepartment ofMaterials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890, USA b Department ofMaterials Science and Engineering, University ofpittsburgh, Pittsburgh, PA 15261, USA ’ Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903-2442, USA Received 15 April 1994; in final form 3 May 1994; accepted 11May 1994

Abstract A sol-gel process employing silicon alkoxide, chelated aluminum set-butoxide, and magnesium acetate as starting precursors, was used to synthesize cordierite xerogels, aerogels and thin films. The xerogels were prepared using normal drying conditions, while supercritical drying conditions were employed to synthesize the aerogels. Thin films were grown by spin coating polymerized sols on ( 100) silicon substrates. The aerogel and xerogel powders as well as thin films were studied for phase evolution and phase stability using X-ray diffraction. All three forms indicated the formation of the p-cordierite phase at 900°C. In addition, the aerogels and thin films showed evolution of other phases. The initiation of the p+a cordierite transformation was observed in the temperature range 1000-l 100°C for both the xerogels and thin films, while p-cordierite obtained from the aerogels showed a much higher stability and transformed to a-cordierite only at 1200°C.

1. Introduction Cordierite ( 2Mg0.2A1203.5Si02) has gained considerable importance in recent years in electronic packaging because of its low dielectric constant ( z 5 at 1 MHz) and good thermal expansion match to silicon. These properties, coupled with the recent technological breakthroughs in its processing, have made it a popular substrate material in comparison to A1203 for high-speed microelectronic packaging [ 1,2 1. Considerable work has been published regarding conventional solid-state and non-conventional, lowtemperature synthesis and processing of this material [ 3-9 1. Several difficulties were initially encountered in processing cordierite due to its high melting point ( > 1600°C). As a result, sintering aids had to be employed. The formation of excessive amounts of liquid phase during sintering caused significant devia-

tions in the thermal expansion coefficient, and also resulted in a considerable amount of porosity due to gas entrapment and bubble formation [ lo- 12 1. Furthermore, solid-state reactions often lead to the formation of undesired secondary phases which tend to remain as trace impurities even after prolonged hightemperature treatments. With the advent of low-temperature chemical synthesis techniques, in particular the sol-gel process, these problems have been gradually surmounted. The sol-gel process has become a very attractive synthesis technique for processing a large number of technologically important glasses, glass-ceramics and crystalline ceramics. This is mainly because of its ability to generate stoichiometric materials of high purity with good control over particle size [ 13-26 1. Recently, the technique has also become very popular for the synthesis of thin films for optical and elec-

0167-571x/94/%01.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO167-577x(94)00123-5

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tronic applications, primarily because of the simplicity ofthe process. The process also offers the flexibility to generate products of different morphology by employing suitable catalysts to control the kinetics of the reaction. Additionally, by employing different drying techniques, it is also possible to extract the solvent products of the gelation reactions to produce aerogels with significant variations in the pore size and structure. This has a tremendous impact on the surface area of the gels and on their potential catalytic and optical properties [ 27-301. The sol-gel technique has traditionally relied on the use of metal-organic compounds (metal alkoxides) which are extremely moisture sensitive due to their strong hydrolytic tendencies. This proved a major barrier to the synthesis of homogeneous gels, particularly in the case of multicomponent materials, due to the preferential hydrolysis exhibited by the alkoxides of the more oxophilic elements. These hurdles have been overcome recently by the use of complexing agents which stabilize the metal centers of the metal alkoxides, thereby lowering their afftnity towards hydrolysis. Thus, ceramics uniform in composition and multi~omponent glasses of the desired stoichiomet~ can now be synthesized. In addition, line particles synthesized by the technique contribute to significant reductions in sintering temperatures. All these attributes make the sol-gel process attractive in the area of microelectronic packaging [ lo]. Different modifications of the sol-gel process for the synthesis of cordierite have been advanced. They involve different starting reagents and subsequent investigations have focused primarily on the synthesis of the final cordierite phase and the evaluation of its dielectric properties [31-351. Similarly, the use of chelating agents and their effect on the chemical reactions and chemical structure of the gels have also been studied by Babonneau et al. [ 36 ] and Selvaraj et al. [ 371. All these studies have, however, concentrated on relating the chemical nature of the starting materials and the chemical structure of the precursors to the gel network and, consequently, to the formation of the cordierite phase. Recently, Werckmann et al. [ 381 have used electron microscopy to investigate precipitate formation and to establish the level of compositional inhomogeneity in the gel. However, almost no studies have been conducted on the use of a single sol-gel process to synthesize xero-

gels, aerogels and thin films with the aim of studying the effect of the gel structure on phase evolution. The present study examines the use of a simple sol-gel process, employing chelated aluminum see-butoxide and magnesium acetate, to synthesize cordierite aerogels, xerogels, and thin films on silicon and study the phase evolution and phase transformation characteristics.

2. Experimental procedure The objective of the work was to synthesize aerogels, xerogels and thin films of cordierite using a simple sol-gel process in~o~orating complexed Al-alkoxides. Chelated aluminum alkoxide, aluminum di(set-butoxide) ester acetoacetic chelate (Al( OC4H9)2 ( C6H903), “Al-chelate”), tetraethyl orthosilicate ( Si(OC2H5)4, “TEOS”) and magnesium acetate tetrahydrate (Mg(CH&00)2*4H20) obtained commercially (Aldrich Chemical Company, Inc., Milwaukee, Wisconsin, 53233) were used as the starting materials to synthesize the cordierite gels. Stoichiomet~~ amounts ( Mg : Al : Si : HZ0 = 2 : 4: 5 : 20) of the abovementioned precursors were dissolved in methanol. The process used was very similar to that published in the literature by Hogan and Risbud [ 391. A flow sheet of the synthesis scheme is shown in Fig. 1. Specifically, TEOS was added to methanol (5 :50 = 1: 10 molar ratio) in a three-necked flask fitted with a condenser. A few drops of 1.0 N HCI were added to initiate hydrolysis of TEOS. The mixture was then refluxed for 3 h at 60-65°C under constant stirring and then cooled to room temperature. At this stage, a premixed solution of Al-chelate and methanol (4: 40= 1: 10 molar ratio) was added and the entire mixture was allowed to react for 12 h under constant stirring conditions. Finally, a solution of magnesium acetate in deionized water (DI) (2: 20~ 1: 10 molar ratio) was added and mixed to generate a clear sol. The sol was then covered with a parafilm wrap and allowed to gel. Gelation occurred in 5- 10 h after which the gels were dried in air. The aerogels were synthesized using the identical sol-gel process as described above. Sols prepared following identical procedures were poured into ordinary glass test tubes (X 10 mm diameter) and then loaded into an autoclave. Argon gas was used at a

P.N. Kumta et al. /Materials Letters 20 (1994) 355-362

TEOS

HCl

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a ramp-up rate of 2”C/min and a ramp-down rate of 4”C/min were employed. The powders were then analyzed using X-ray diffraction (XRD) employing Cu Ku radiation (Rigaku 0/e diffractometer, Tokyo, Japan). The aerogels were subjected to a similar heat treatment as the gels, except that heat treatments at 300 and 700°C were not necessary due to the extraction of the organics during the previous supercritical drying of the gels. The aerogels were then analyzed using X-ray diffraction. The thin films were also subjected to heat treatments at 700°C for 1 h and 900°C for 3 h followed by heat treatments at 1000, 1100 and 1200°C for 5 h using similar ramp rates as described above for the bulk gels.

3. Results and discussion

Fig. 1. Flow sheet showing the procedure used to synthesize cordierite aerogels, xerogels and thin films.

pressure of 1200- 1300 psi and the sample was slowly heated to 260°C at the rate of z l.S’C/min. At this point, the methanol and water mixture were supercritically extracted from the wet gels to obtain the aerogels. The sols used for the thin films were also prepared in the above manner. (00 1) silicon wafers were spin coated in a class 10 clean room. Six layers of the film were spin coated onto the substrate. Few drops of the sol were deposited on the wafer which was spin at 2000 rpm for 30 s. The wafers were heated to 400°C for 1 h to volatilize the organics between subsequent depositions. After all layers were deposited and heat treated to 400°C for 1 h, the samples were then heat treated in air to 700°C for 1 h and 900°C for 3 h using a ramp rate of 2”C/min and then cooled down at the rate of 4”Cfmin. Characterization The bulk gels were dried for several days and then crushed to a fine powder. Heat treatment was conducted in air initially at 300 and 700°C to remove the carbon while further heat treatments at 900, 1000, 1100 and 1200°C were also conducted to study the phase evolution. The samples were annealed for 5 h at each of these temperatures. In all heat treatments,

The objectives of the work were to synthesize xerogels, aerogels and thin films of cordierite using a single sol-gel approach employing identical precursors, in order to compare their phase evolution characteristics. Accordingly, the results of the work will be discussed in two sections. In section 3.1 the results of the phase evolution studies conducted on the xerogels and aerogels will be presented, while in section 3.2 experimental results obtained for the thin films will be discussed. 3.1. Phase evolution in bulk gels and aerogels Fig. 2 shows the X-ray diffraction plots obtained from the xerogels heat treated to 900, 1000, 1100 and 1200°C and annealed for 5 h at each of these temperatures. The xerogels exhibit phase evolution characteristics very similar to those reported in the literature for alkoxide derived gels [ lo,46 1. At 900°C the bulk xerogels show the formation of the intermediate u-cordierite phase without the presence of any secondary phases consistent with reports in the literature. The u-cordierite phase is a transitional phase having the stuffed P-quartz structure. On heating to lOOO”C,the decay of the u-phase and the evolution of the low-temperature a-cordierite phase is observed. Further heat treatments to 1100 and 1200°C result in the complete transformation of p-cordierite to form a-cordierite. Thus, the xerogels exhibit the

10

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Fig 2, XRD traces showing the phase evolution patbway foilowed by the xero&s heat treated at 900,1000, t 100 and 1200°C; ( x ) kcordierite and (I ) indialite.

Fig. 3. Photograph showing the as-prepared cordierite aerogets and as-d&d xerogels (oext to the scale). Note the traoslucenl nature of the aerogels in comparison to the transparent xerogets.

classical phase transformation and phase evolution pathways described in the literature. Similar studies were also conducted on the aerogels to observe the characteristics of phase evolution. Fig. 3 shows the bulk aerogel and xerogel pieces of cordierite. The aerogels were opaque in comparison to the xerogels which were translucent. The aerogels were also extremely light and fragile due to their highly porous nature. This is probably due to the network structure of the gels which is dependent on the aging time as well as on the kinetics of the gelation reaction. In the present study, the gels were supercritically dried within a day of gelation. The formation of a highly branched and condensed gel prior to supercritical extraction would help in the formation of a very fine pore structure resulting in a transparent aerogel which would also be mechanically stronger. Similar to the bulk xerogels, the aerogels were also heat treated to 900, 1000, I IO0 and 1200°C. Fig. 4 shows the XRD plots obtained for the aerogel samples. The sequence of phase evolution in the aerogels

shows significant differences in contrast to that seen in the case of the xerogels. As shown in Fig. 4, the formation of the p-cordierite phase at 900°C is seen very similar to p-cordierite formation in the xerogel. However, the XRD data obtained from the aerogels heated at 900 and 1000°C show additional peaks at 23.9” and 24.4” which were related to magnesium a~u~nosi~~ate f 3Mg0 AfzC13.6SiC12, MAS) phase. Similarly, the presence of the sapphirine and/or the spine1 can atso be detected at 1000°C. However, on further heat treatment at 11Oo”C,the MAS phase disappears, while y-cordierite and sapphirinelspinel remain as the only stable phases. Mullite is not observed in the aerogel and the p-cordierite is stable to higher temperatures. The pcordierite transforms only sluggishly to a-cordierite. It is only after prolonged heat treatment at 1200°C for approximately 10 h following prior treatment for 5 h at 900°C that the evolution of a-cordierite can be observed. However, there is also grow& of the sapphirine phase and the evolution of cristobalite similar to the case of the thin films indicating the effects

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1,,,,,,,,,,,,“,,,,,,,111”‘11

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50 60 angle,20

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Fig. 4. XRD traces showing the phases evolved on heat treating the aerogels at 900, 1000, 1100, 1200 and 1350°C; (X ) p-cordierite, ( + ) magnesium aluminosilicate, (*) sapphirine and/or spinel, (C) cristobalite and (I) indialite.

of the gel structure. Thus, the aerogels show remarkable stability for the u-cordierite phase. 3.2. Phase evolution behavior in thinjZms To compare the phase evolution characteristics of the aerogels and xerogels with those of the thin films, the deposited films were heat treated at identical temperatures. Fig. 5 shows the X-ray diffraction patterns obtained on the thin film samples after heat treatment at 900, 1000, 1100 and 1200°C. After heat treatment at 700°C the films were amorphous. However, at 900°C the formation of the u-cordierite phase can be seen [ 10 1. The presence of the phase was ascertained by matching the pattern to the JCPDS powder pattern for u-cordierite [40]. It should be noted that the (200) silicon peak occurs at 33.1”. There is also the presence of a magnesium alumino silicate ( 3Mg0.A1203.6Si02, MAS) phase identified by characteristic peaks at 23.87” and 24.42”. The peaks were matched to the JCPDS powder diffrac-

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,,,I,II,~I,IIIIII,,I,III.~,l

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_i

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angle,20 Fig. 5. XRD traces showing the phase evolution in the thin films heat treated at 900, 1000, 1100 and 1200°C; ( x ) p-cordierite, ( + ) magnesium aluminosilicate, (*) sapphirine and/or spinel, (C) cristobalite, (m) mullite, (I) indialite and (Si) silicon.

tion pattern, 14-346 [ 4 11. On further heat treatment at lOOo”C, there is a decrease in the amount of the MAS phase as indicated by the decreased intensities of the peaks at 23.87” and 24.42”. However, trace amounts of sapphirine and/or spine1 phase can also be seen. At 1 100°C the evolution of other phases including mullite (M), sapphirine ( 4Mg0.4A1203. 2Si02, S), cristobalite (C), and/or spine1 ( MgA1204, SP) becomes very prominent. It is very difficult to distinguish between the two phases of sapphirine and spine1 on the basis of the X-ray data, since the major peaks for the two phases occur at very similar values of 28. These phases were again identified by matching the resultant X-ray pattern to the JCPDS powder diffraction patterns for mullite, MgAl,O,, and sapphirine, respectively [ 42-44 1. At 1 lOO”C, as shown in Fig. 5, a-cordierite becomes prominent, while the MAS phase is completely eliminated. On further heat treatment to 1200°C u-cordierite gradually transforms. The u-a transformation is manifested by the gradual decay of

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the peak at 25.9”, characteristic of the stuffed P-quartz structure of p-cordierite, and the emergence of very small peaks at lO.l”, 27.2”, 28.42” and 36.1”, corresponding to the u-phase (JCPDS powder pattern, 13293 ) [ 45 ]. The transformation of p-cordierite to indialite (a-cordierite) at 1200°C is also accompanied by the growth of the spine1 and/or sapphirine phases. It should be noted that the complete transformation of the stuffed P-cordierite phase to indialite at 1200°C is also accompanied by the presence of cristobalite. Thus, it can be seen that the thin films deposited on the silicon substrates contain p-cordierite as the major phase which transforms to a-cordierite along with the formation of small amounts of several secondary phases. Indialite is a low-temperature metastable cordierite phase exhibiting the hexagonal structure. It is known to transform to the high-temperature orthorhombic cordierite phase around 1350°C. This sequence of formation of the p-cordierite and its transformation to the u-phase seen in the thin films is consistent with the classical phase evolution pathways exhibited by the alkoxide-derived bulk gels as shown by other studies reported in the literature [ 4648]. In the context of the present work, we surmise that the formation of the spine1 and/or sapphirine (SP, S), mullite (M), cristobalite, and the magnesium aluminosilicate phase (MAS) is a consequence of compositional inhomogeneities in the gel arising from the nature of the starting precursors as well as the aging characteristics of the gel. The gel inhomogeneity could be a consequence of using magnesium acetate as the source for magnesium. The addition of magnesium acetate in the prehydrolyzed sol containing Al and Si results in the acetate groups being located as discrete molecular units in the Al and Si alkoxy networks. Since magnesium is electropositive, an aqueous solution of magnesium acetate would tend to be very weakly acidic and, therefore, a large concentration of the element would remain as solvated acetate groups rather than solvated hydroxy species. Hence, it is very unlikely that magnesium would enter the gel structure by being bonded to Al and Si via oxygen bridging. This would imply that much of the reaction of magnesium with the other components would have to occur during the heat treatment of the gel itself. The kinetics of this reaction are once again dependent on the aging characteristics of the gel. The

thin films are synthesized by spinning a sol that is, by and large, still undergoing condensation reactions. Hence, as opposed to the xerogel, which is air dried and then allowed to undergo complete hydrolysis and condensation, the thin films are synthesized from a sol that is largely unaged. This would imply larger diffusion distances for the magnesium ions in the case of the spin coated films which could be instrumental for the apparent gel inhomogeneities leading to the formation of small amounts of undesired phases. Recently, the effect of aging of the gel on the orientation of sol-gel films has been reported by Kushida et al. ]491. It is known that the orientation and texture of the crystalline phases evolving from the deposited film are largely affected by the substrate. In the present case no such influence of the substrate on the deposited film was seen due to the presence of the inherent amorphous oxide layer on the silicon. Differences in phase evolution behavior between the aerogels and thin films, including the presence of the sapphirine and/or spine& magnesium aluminosilicate and cristobalite phases, can be attributed to the aging characteristics of the gel as in the case of thin films. The pronounced stability of the p-cordierite phase, even at temperatures of 1100°C however can be related to the aerogel structure generated by the supercritical drying procedure. It is known that supercritical drying results in the complete loss of solvents from the gel clusters including the products of the hydrolysis and condensation reaction, thus generating porous channels [ 301. The pores collapse at high temperatures promoting diffusion of the species and allowing for the structural rearrangements responsible for the formation of a-cordierite. Hence, p-cordierite tends to remain stable at temperatures up to 1100°C for periods as long as 15 h as opposed to the xerogel and thin films where the ~-+a transformation is either complete or initiated at 1100°C. The phase evolution of cordierite in the thin films grown on ( 100) silicon substrates and aerogels differs significantly from the phase evolution pathway exhibited by the bulk xerogel powder, even though identical starting precursors and similar heat treatments were employed. Previous work by Roy et al. [50] illustrated the strong dependence of the gel structure on the starting precursors as well as the influence of the gel structure on the phase evolution in

P.N. Kumta et al. /Materials Letters 20 (1994) 355-362

the case of the xerogels. The present work clearly indicates that both the molecular structure of the precursor as well as the chemical and physical changes that occur during aging affect the phase transformation characteristics. More information on the structure of the gel and the changes occurring during the aging reaction are needed in order to provide definitive answers to the phase transformation variation seen in the three different cases.

4. Conclusions A simple sol-gel process employing complexed aluminum alkoxides and magnesium acetate was used to synthesize xerogels, aerogels and thin films on ( 100) silicon wafers. The sequence of phase formation includes the initial formation of the p-cordierite phase, followed by a gradual restructuring to form the a-cordierite phase in the case of bulk xerogels. In addition to the a-cordierite, thin films grown on ( 100) silicon show formation of small amounts of spine1 and sapphirine, mullite, and cristobalite as secondary phases at different temperatures. The aerogels also show presence of small amount of spine1 and/or sapphirine, magnesium aluminosilicate and cristobalite phases. The u-cordierite phase exhibits enhanced stability in the aerogels and begins to transform to indialite (a-cordierite) only at 1200°C.

Acknowledgement This work was supported by the National Science Foundation under Grants DMR-9204230 (REH and WCJ), DMR-9301014 (PNK) and CTS-9309073 (PNK). Acknowledgement is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society for support of this research (grant PRF 25507-G3). The authors thank M.A. Sriram for assistance on the SEM and Dr. Antonio Azevedo de la Costa for assistance in the use of the spin-coating equipment in the cleanroom facility. Finally, we thank Dr. Girish Harshe of Pennsylvania State University for assistance in the synthesis of the aerogels.

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