Low temperature mullite formation from sol-gel precursors by hot pressing
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The mullite phase has been synthesized by subjecting a suitably chosen precursor gel to different pressure and temperature conditions. At ambient pressure mullite phase grows at a temperature of 1400 °C when the gel is heated for 2 h. At a pressure of 24 bar, on the other hand, similar growth occurs at a temperature of 750 °C for 5 min.
The synthesis of mullite (3A12O3 • 2SiO 2 ) has attracted considerable attention in recent years because of its technological importance. 19 The sol-gel method has been exploited extensively to prepare high purity mullite powders. For acid-catalyzed gels formation of mullite has been reported at around 980 °C.10 By the sol-gel method mullite powders have been produced at temperatures ranging from 850 °C to 1350 °C. n We have been able to synthesize mullite from an acidcatalyzed gel at a temperature lower than that reported so far by using a hot-pressing technique. The details are reported briefly in this paper. A sol is prepared using AR grade A1(NO3)3 • 9H 2 O and Si(OC 2 H 5 ) 4 of appropriate amounts so that the target composition is 2.6A12O3 • 1.0SiO2 (mole%). Required amounts of distilled water, ethyl alcohol, and a few drops of concentrated nitric acid are added to Si(OC 2 H 5 ) 4 , keeping the pH to a value —2.5. The resultant solution is stirred for 2 h. Another solution of A1(NO3)3 • 9H 2 O in distilled water is prepared. The two solutions made as above are mixed and stirred for 1 h. The sol is now subjected to gelation at ~ 6 0 °C for a period of 24 h. The alcogel is then dried at 200 °C for 5 h. The dried gel powder is ground under acetone in a mortar. The powder is taken in a graphite mold that is mounted in a DSP 25ATS Sintering Press (manufactured by M/s. Dr. Fritsch Sondermaschinen, Germany). The pressing is done in an evacuated chamber with a pressure of 0.26 bar at temperatures varying from 500 °C to 750 °C for a few minutes with an applied pressure of 24 bar. We have also subjected the dried gel powder to heat treatments under ambient pressure at temperatures varying from 800 °C to 1400 °C for several hours. The crystalline phases in various samples have been identified by the x-ray diffractograms obtained from a Phillips 1710 powder diffractometer using CuK a radiation. The microstructure of the different samples has been studied by a JEM 200 CX transmission electron microscope operated at a voltage of 160 kV. Figure 1 shows the x-ray diffractograms of typical samples prepared under different pressure and temperature conditions. From the figure it is evident that no 2474
http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 10, Oct 1994
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crystalline phase (alumina or mullite) is formed up to a temperature of 800 °C when the heat treatment is carried out under ambient pressure. After a 2 h treatment at 1400 °C a substantial amount of mullite along with
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2 G (deg) FIG. 1. X-ray diffractograms of gels heat-treated at different temperatur
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