**2. Theoretical aspects of microwave sintering**

Microwave is the name given to electromagnetic radiation 1 m to 1 mm in wavelength that corresponds to a frequency of about 300MHz to 300 GHz. Microwave heating is a process in which the materials couple with microwaves, absorb the electromagnetic energy volumetrically, and transform into heat. This is different from conventional methods where heat is transferred between objects by the mechanisms of conduction, radiation and convection[4]. In conventional heating, the material's surface is first heated followed by the heat moving inward. This means that there is a temperature gradient from the surface to the inside. However, microwave heating generates heat within the material first and then heats the entire volume[5].

#### **2.1 The principle of microwave sintering**

Microwave sintering makes use of dielectric loss of materials in microwave electromagnetic fields to heat ceramic matrix to the sintering temperature fast. The microwave sintering

Microwave Sintering of Thermistor Ceramics 87

Jin et al.[12, 13] successfully calcined and sintered Mn0.43Ni0.9CuFe0.67O4 samples by microwaves. Fig.1 compares the XRD patterns of powders calcined by microwave treatment and conventional heating, respectively. As can be seen, phase analysis of conventionally calcined Mn0.43Ni0.9CuFe0.67O4 powders shows metallic oxide as the main crystalline phase and a very minor amount of spinel phase. These results indicate that oxides were still not fully reacted. However, for the microwave heating, the complete spinel phase has obtained when powders were calcined at 650°C. Consequently, the Microwave sintering leads to a lower calcination temperature than conventional methods. In the conventional sintering processes, extremely high sintering temperatures and long holding times (several hours) under air atmosphere are applied in the fabrication of thermistor products to achieve the highest density and minimum porosity. Fig.2 shows the SEM micrographs from the fracture surface of sintered samples. Although the same sintering temperature and holding time (1000°C, 40 min) were applied, the microstructures of conventional and the microwave sintered samples were different. It can be seen from the Fig.2(a) and 2(b) that the microwave-sintered samples for both nearsurface and interior regions are practically identical, showing intergranular fracture and uniform grain size. Additionally, most pores are located at the isolated grain boundaries. In contrast, as shown in Fig.2(c) and 2(d), the conventionally sintered samples show exaggerated grain growth and many isolated and closed pores. The exaggerated grain growth may indicate a very high mobility of grain boundaries[14]. The variation in densification parameter of the sintered samples as a function of holding time at 1000°C is shown in Fig.3. It can be observed that the green compacts are well densified when sintered at 1000°C for 40 min, showing a densification of above 0.82 and porosity of below 4.5%. Microwave-sintered samples exhibit enhanced densification for the same holding time compared with the conventionally sintered samples. That is because the densification is a thermally activated process and the activation energy for microwave densification is lower than the value for conventional sintering[15, 16]. Fig.4 compares the *B* constant and resistivity (*ρ*) of twenty components by microwave and conventional sintering. Microwave sintering can obtain the components with well uniformity of *B* value and resistivity, of which the *B*avg is 1930K (deviation of 1.47%) and resistivity *ρ*avg is 135Ωcm (deviation of 4.45%). However, the *B*avg is 1720K (deviation of 1.47%) and resistivity *ρ*avg is 78Ωcm (deviation of 25.34%) for the conventionally sintered components. Therefore, microwave sintering is more effective in promoting uniformity and

**3.1.2 Microwave sintering of NTC thermistor ceramics** 

consistency of components compared to conventional sintering.

There is an increasing need for thermistors with high-precision and exchange-type in various industrial and domestic applications, such as temperature measurements and controls. In order to obtain NTC thermistor with low *B* value (2100 K), high-precision, exchange-type and fine homogeneity, Mn0.43Ni0.90CuFe0.67O4 NTC thermistor material precursors were prepared by the polymerized complex method using microwave sintering and conventional sintering[17]. The obtained results indicated that the particles microwave calcined at 650°C were small and uniform. The ceramics mainly consisted of rich-Cu phase and poor-Cu phase. The homogeneity of components is significantly increased by microwave sintering. Moreover, the rate of finished products is increased to 85% from 30% compared with the one of conventional sintering. The sintering activity and electrical

effect mainly comes from the continuous changes of material polarization in alternating electromagnetic field. The continuous changes of dipoles in materials can produce strong vibration and friction, therefore, generating heat to achieve sintering[1].
