**4. Conclusion**

78 Sintering of Ceramics – New Emerging Techniques

a b

**10** μ**m 10** μ**m** 

Crystal face

 Fig. 10. Anisotropic grain microstructure for sample sintered at a) 800 °C and b) 825 °C.

grain boundaries that ultimately control the properties of the alumina compact.

There are some investigations on the anisotropic grain growth as well as the presence of faceting grain of alumina interface in the presence of glass (Simpson and Carter, 1990). The faceting of alumina interface in the presence of a glass is a factor affecting the grain boundary movement during sintering. The geometry and movements of facet during liquid phase sintering process are expected to influence directly the final microstructure of the

For a faceted grain in a liquid matrix, the dihedral angle is not uniquely defined because of the torque on a facet, contrary to the case of a rounded grain. When two faceted grains are in contact with a certain angle of crystallographic orientation in a liquid matrix, the shape of the contact boundary in equilibrium must be that of the minimum interfacial energy. These abnormal grains originate from heterogeneities in the chemical composition, particle size and packing density. In the LTCC process, the substrate also plays an important role in the development of microstructure during sintering since it is a co-firing process where the conductor and the substrate are sintered simultaneously. So, the nuclei of abnormal grains may be formed during different stages of sintering. From XRD peak observation for samples sintered at 800 °C and 825 °C, a few phases including CaSiO3 and Ca3(SiO4)O were present suggesting the nucleation rate is not zero. The results found was consistent with the work of Rohrer (2001) which noted that at lower temperature, the anisotropy grain of the surface energy is expected to be higher. The wollastonite phase (CaSiO3) becomes a liquid phase before it decomposes. The liquid phase exists preferentially, filling spaces in small pores and particle contact. The capillary force that is generated by the liquid phase can drag the nearby particle close together. The distribution of the liquid phase may not be uniform because the amount of liquid is small. According to Kang et al, (1991), liquid phase, even in a small amount forms due to the presence of impurities, grains are faceted and abnormal grain growth occurs. Such heterogeneities may trigger the formation of nuclei of abnormal grain. Because the nuclei are small, they have a greater chance to grow at their own pace until they encounter other abnormal grains (Chen and Tuan, 2000). However, when a liquid phase forms due to a high concentration of impurities, the grain shape is faceted and abnormal grain growth As a conclusion, the performance of the laminated sample strongly depends on the starting raw materials and processing method; good technical skills and handling procedures are required to minimize the defect/issue occurring in the LTCC multilayer process. As with other advanced electronic ceramics, the quality of raw materials will have a marked effect on the final properties of a ceramic product. The raw material selection includes characterization of purity, particle size distribution, surface area, consistency and cost. The processing of a ceramic material must be optimized with respect to microstructural characteristics, because only by this way we can produce ceramics with properties that approach the level of intrinsic dielectric properties. The chosen raw material is the first key to controlling the microstructure and making a consistent product.

The present study is able to explain the effect of the different sintering temperatures on the whole substrate properties and printed thick-film. The microstructural evolution at various sintering temperatures was carefully tracked. The sintering temperature therefore plays a vital role in forming microstructures, porosity and shrinkages of the as-prepared materials. This is more complex than the simple sintering of metal grains. In order to understanding better the microstructure evolution, the study of samples fired should be carried out at the different stages of the sintering process. This work will be very helpful to predict how the microstructural factors and material properties of the LTCC system would affect the microwave-frequency performance of relevant devices. It should be mentioned that the results obtained from this work is not always be consistent with observations from other works.
