Accelerated sintering and phase transformation of TiO 2 in microwave radiation
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Accelerated sintering and phase transformation of TiO2 in microwave radiation Zhi-Peng Xie, Xu-Dong Fan, and Yong Huang State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China (Received 2 September 1997; accepted 30 March 1998)
The sintering process and phase transformation of submicrometer powder compacts of TiO2 were investigated using 2.45 GHz microwave radiation. The microwave-sintered samples were densified more rapidly and at much shorter time and lower temperature than that of the conventionally sintered samples. Also, an accelerated phase transformation from anatase to rutile was observed in microwave processing when the dwell time is cut down from 6 h in conventional to 30 min in microwave at 1000 ±C. The samples sintered in microwave show a refined microstructure and smaller grain sizes at high density near 98–99% theoretical. In addition, the reasons for rapidly heating behavior and enhanced sintering on TiO2 in microwave are discussed.
I. INTRODUCTION
Microwave processing of ceramic materials has been investigated over the last decade by various researchers, ranging from calcining, firing, sintering, and annealing for a wide variety of ceramics.1– 4 The growing interest in microwave processing over conventional processing methods is due to the fact that the electromagnetic wave interacts with ceramic materials leading to volumetric heating by dielectric loss, which will enhance the sintering process, decrease densification temperature, and improve product uniformity as well as microstructure and properties. Also, this unique heating method has the potential for energy saving and cost cutting when compared with conventional heating.5 –7 There has been considerable interest in microwave processing on oxide materials.8 –10 In the 1960s Tinga investigated how certain oxides heat in a microwave field and developed reaction cavities.11 Meek and his co-workers12,13 showed a significant difference between conventional thermal processing and microwave processing of oxide ceramics, such as Al2 O3 , ZrO2 , and their composites. They observed that high-purity, submicrometer powder compacts of pure Al2 O3 could be densified from an initial value of 50% to a final value of 95.7% theoretical by heating from room temperature to 1700 ±C using 6 GHz radiation, whereas conventional sintering of high-purity Al2 O3 powder compacts (50% initial density) required 20 h at 1600 ±C to achieve the same final density. Janney and Kimrey14 calculated the apparent activation energy for sintering Al2 O3 by the two processes and found that the apparent activation energy for microwave is about one-third that of conventional sintering. The large differences in the sintering rate were thought to be due to higher diffusion rates by the microwave field. Tian et al.15 showed ultra-fine grain J. Mater. Res., Vol. 13, No. 12, Dec 1998
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