Abstract
Microwave sintering behavior of functional ceramics (BMT, BaTiO3 and ZnO) and refractory ceramics (Al2O3, ZrO2 and SiC), using either 24 GHz millimeter-wave or 2.45 GHz microwave as sources, were examined. The BMT, Al2O3 and ZrO2 materials show low microwave absorption characteristics, whereas the SiC, BaTiO3 and ZnO materials exhibit high microwave absorption properties. They are all well-behaved in microwave sintering process, since the heating rate slows down at high temperature due to thermal conduction loss from the surface of the samples. Using SiC materials as susceptors to absorb the microwave can heat up the surface of the samples and greatly improve the temperature uniformity for the samples in microwave sintering process. The microwave absorption efficiency (h=2pf·e¢·tan d) were evaluated from their microwave heating profiles, using a simplified thermal model. The e¢·tan d for Al2O3 materials thus derived is in accord with the values reported in the literature and the heating profiles simulated for ZnO materials is in good agreement with the experimental results, illustrating the feasibility of the analyzing methodology. The larger microwave absorption efficiency at 24 GHz renders the application of millimeter-wave source more efficient in microwave sintering process than the 2.45 GHz microwave source. Fast firing of Bi2O3-based ZnO varistor materials, which includes zero minutes soaking at 1100℃ with 120℃/min heating and 145℃/min cooling rate, was made possible by using millimeter-wave sintering (mS) or microwave sintering (mS) technique. The overall sintering time of these processes is less than 18 minutes. By contrast, the conventional sintering (cS) process needs higher sintering temperature (1200℃), longer soaking time (60 min) and slower ramping rate (30℃/min) to obtain ZnO materials with the same marvelous nonlinear properties as those prepared by mS-process. The overall sintering time of the cS process is 528 min. In this study, we have also densify PZT、MLCC and ZrO2 ceramics using 24 GHz millimeter-wave and 2.45 GHz microwave, then identified these materials’ densification behavior and properties. We have proof that microwave can improve materials’ densification, in addition to the increase in materials’ properties