**4. Summary**

92 Sintering of Ceramics – New Emerging Techniques

sample. The conventional-sintered sample shows ecliptic spherical shape, whereas the microwave-sintered sample shows an orientational strip-like microstructure. The growth of grains in microwave-sintered BTO is obvious orientational, which is very different from the familiar conventional-sintered BTO. The XRD (Fig.6) analysis indicates that the perovskite structure of microwave-sintered BTO is tetragonal, whereas the conventional-sintered sample is cubic. The reason for this abnormal phenomenon can perhaps be attributed to the polarisation of BTO as a typical piezoelectric material in the microwave field, and also to the

Fig. 6. XRD patterns of doped BTO samples: (A) conventional sintering; (B) microwave

**3.3 The study of insulator materials for the microwave sintering of oxide thermistor** 

CoMnNiO series NTC and BaTiO3-based PTC ceramics, having good dielectric coupling characteristic, can more easily couple with microwaves, therefore, generating heat to achieve sintering. However, due to the problems of thermal cracks during the microwave sintering and especially in the rapid cooling procedure, the production rate of oxide thermistor

Chang et al. have successfully prepared MgA12O4-LaCrO3 insulator materials which mix MgA12O4 spinel materials with superior thermal properties and LaCrO3 perovskites materials with high efficiency absorption of microwave[20].The design of this kind insulating system solves the problems of thermal cracks often occur in the microwave sintering of many oxide ceramics, such as CoMnNiO series NTC ceramics and BaTiO3-based PTC ceramics. Additionally, the MgA12O4-LaCrO3 insulator materials can produce a homogeneous heating for the samples in the microwave sintering, and grain crack-free and dense ceramic samples. Adopting MgAl2O4-LaCrO3 susceptor in the temperature range of

enhancement of grain diffusion by the microwave-sintered.

sintering[19].

**ceramics** 

ceramics by microwave sintering is very low.

The fundamentals, applications of microwave sintering to thermistor ceramics and MgA12O4-LaCrO3 insulator materials for the microwave sintering of oxide thermistor ceramics are reviewed in this chapter. The advantages of microwave sintering against conventional sintering methods for thermistor ceramics are summarized as follows:(1) Microwave sintering consumes much lower energy than conventional sintering; (2) Higher heating rates can be attained and thus the sintering time reduces by using microwave sintering;(3) Generally higher density and better grain distribution can be achieved through microwave sintering. What's more, microwave sintering can enhance the densification rate and hence improve the uniformity and consistency of thermistors to a greater extent than the conventional sintering.

In the past year and half, significant developments and advances have taken place in the field of microwave sintering of ceramics. In spite of these successes, microwave processing has yet to see wide spread application in the ceramic industry. Additionally, more research must be conducted to have a full understanding of the process. It can be predicted that there is a great future for microwave sintering for the successful commercialization for specialty ceramics.
