27 Al MAS-NMR spectra of alumina aerogels
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27 Al
MAS-NMR spectra of alumina aerogels
Yasuyuki Mizushima,a) Makoto Hori,b) and Minoru Sasakic) Colloid Research Institute, 350-1 Ogura, Yahatahigashi-ku, Kitakyushu 805, Japan (Received 8 September 1992; accepted 30 April 1993)
The 27A1 MAS-NMR spectra of alumina aerogels were determined. The spectra showed different profiles according to the supercritical methods used during drying. The alumina aerogel supercritically dried in a CO 2 extractor (80 °C, 15.7 MPa) displayed a pentahedral-coordinated aluminum structure. On the other hand, the alumina aerogel supercritically dried in an autoclave (270 °C, 26.5 MPa) revealed no pentahedralcoordinated aluminum structure. The absence of pentahedral-coordinated aluminum was one factor for the lower temperature of transformation from the y to the 0 phase. Monolithic alumina aerogels were prepared by two different supercritical drying methods. It was found that alumina aerogels developed different properties in the crystal phase transformation and different specific surface areas during the heating process.1 To form the aerogels, aluminum sec-butoxide (Al(OBusec)3) was first reacted with ethyl acetoacetate (etac). Ethanol was then added and the mixture was hydrolyzed with water diluted with ethanol (Al(OBu sec ) 3 : etac: H 2 O: ethanol = 1 : 1 : 3 : 1 2 ) . The alumina sol was maintained at 60 °C for one week for gelation and aging purposes. One supercritical drying method consisted of alumina wet gel being dried under supercritical conditions of a mixture of carbon dioxide and ethanol in a CO 2 extractor at 80 °C, 15.7 MPa. In the other, alumina wet gel was dried under supercritical conditions of ethanol alone in an autoclave at 270 °C, 26.5 MPa. The boehmite phase was recognized in the alumina aerogel prepared in the autoclave, while the alumina aerogel formed in the CO 2 extractor was amorphous. The phase transformation from the y to the 0 alumina phase in the alumina aerogel prepared in the autoclave occurred at a lower temperature than that formed in the CO 2 extractor.1 This difference in the phase transformation may be due to differences in the alumina microstructure of the aerogels at the different temperatures. To investigate the alumina aerogel microstructures, the spectra of aerogels fired at temperatures of 300 °C and 600 °C for 5 h were determined using an 27A1 MAS-NMR spectrometer (Doty; 400 MHz, Varian Instruments Ltd.). A1(H2O)63+
"'Superconductivity Research Laboratory, c/o Japan Fine Ceramics Center, 4-1 Mutsuno 2-Chome, Atsuta-ku, Nagoya 456, Japan. b) Present address: Kurosaki Refractories Co., Ltd., 1-1, Higashihama, Yawata-nishi, Kitakyushu 806, Japan. c) Present address: R & D Laboratories-I, Central R & D Bureau, Nippon Steel Corp., 1618 Ida, Nakahara-ku, Kawasaki 211, Japan. J. Mater. Res., Vol. 8, No. 9, Sep 1993
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was used as an external reference. A zirconia rotor was employed, having a variable spin rate. Figure 1 shows the 27A1 MAS-NMR spectra of the alumina aerogels fired at 300 °C
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