Crystalline phases of sol-gel ZrO 2 in the ZrO 2 -SiO 2 system: Differential thermal analysis and electron microscopy st
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T. Lopez and R. Gomez Depart, of Chemistry, Universidad Autonoma Metropolitana-Iztapalapa, A.P. 55-534, 09340, Mexico D.F., Mexico (Received 22 April 1994; accepted 22 February 1995)
Zirconium oxide dispersed in silica was synthesized by the cogelation of tetraethoxysilane and zirconium acetate. For the gelation, acid and basic media were used. The final zirconia concentration was around 6.7 wt. %. Differential thermal analysis, thermogravimetric analysis, as well as electron microscope techniques were used for the characterization of the samples. It was found that the stabilization of the crystalline phases depends on the preparation conditions and thermal treatments. Tetragonal and monoclinic ZrO 2 phases were identified in basic preparations. However, in acidic samples only low-temperature tetragonal phase was identified.
I. INTRODUCTION Zirconia-mixed oxides are interesting materials due to their applications in ceramic toughening or in catalytic processes. Their applications have to do, in most cases, with the crystalline phases present in the powder. Hence, the experimental parameters that control the formation of the crystalline phases have been the subject of important studies of several laboratories.1"3 Alternative methods for the preparation of mixed oxides is then an important subject to be studied. Sol-gel processes have been shown to be a good alternative method for the synthesis of promising homogeneous and reactive solids. 45 The formation of ZrO 2 -SiO 2 , ZrO 2 -TiO 2 , or ZrO 2 -Al 2 O 3 materials from gels has been successfully achieved. 67 However, intensive characterization of the crystalline phase developed during the heat treatments has not been done. In the present work, phases transformations of ZrO 2 have been followed by electron microscopy studies in samples prepared by the cogelation of tetraethoxysilane and zirconium acetate. Additionally, differential thermal analysis and thermogravimetric techniques were used to identify the temperatures at which the phase transformations occur. The samples were prepared at pH 3 and pH 9, and the characterization was done in fresh and calcined samples. II. EXPERIMENTAL A. Sample preparations The preparation of ZrO 2 -SiO 2 glasses has been achieved by the cogelation of tetraethoxysilane (TEOS) and zirconium acetate. Acid (sample A) and basic (sample B) media were used for the gelation of TEOS. J. Mater. Res., Vol. 10, No. 6, Jun 1995 http://journals.cambridge.org
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The general procedure is as follows: A mixture of 36.8 ml of tetraethoxysilane (Aldrich 99.9%), 10 ml of HC1 acid (Baker 36% vol.) or NH 4 0H (Baker 35% NH 3 in H 2 O), and 50 ml of ethanol (Baker 99.9%) was refluxed and stirred. An aqueous solution of zirconium acetate (Baker 99.9%) was added dropwise to the solution. The reaction was complete in 5 h, after which the gel was formed. The final amount of zirconium oxide on silica was 6.7 wt. %. After gelation the solids were dried at 110 °C (fresh sample) and then calcined for 4 h at 400, 700, and 1000 °C. B. Chara
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