The gel structure in the ZrO
2-SiO
2 system, produced by the sol-gel process and the ultrasonic spray pyrolysis method using a solution of ZrOCl
2·8 H
2O-Si(OC
2H
5)
4-C
2H
5OH-H
2O system (mother solution), was investigated in relation to the phase crystallized by heating, from the standing point of the surface and the interface of ZrO
2 and SiO
2. The siloxane polymer of compound system changed from linear to three dimensional type with an increase in the molar ratio 'r' of added H
2O. In this case, the crystallization temperature of SiO
2 increased and the transformation rate of T-to M-ZrO
2 decreased, due to the mutual 'block effect' of SiO
2 and ZrO
2. Without the block effect, T-ZrO
2 was converted utterly to M-ZrO
2. The strain energy caused by the crystallization of SiO
2 at the interface preserved the metastable T-ZrO
2. The morphology such as the dispersive state of two oxide components should be considered for the study on the transformation of ZrO
2. The spherical ZrO
2-SiO
2 system particles were successfully produced by the ultrasonic spray pyrolysis method from the mother solution which had been preliminarily hydrolyzed.
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