With the rapid progress of engineering technologies, the importance of specific dielectric properties of ceramic materials has been increasingly recognized in the microwave frequency in electrical applications. The dielectric properties of materials have been a subject of many theoretical and experimental investigations. Research on the designing of the dielectric materials with specific dielectric properties has been increasingly recognized in the microwave frequency in electrical applications.
The microwave window is a structure placed over an antenna that protects the antenna from its physical environments. This material is required low dielectric properties, high mechanical and thermal properties. The optimized dielectric properties are ideally microwave transparent. The ceramic materials exhibit excellent dielectric and mechanical properties for rapid developed condition of microwave windows.
The dielectric properties of ceramics depend on the microstructure of materials such as, elastic modulus and thermal conductivity. The previous literatures report that the relationship between the dielectric properties and the microstructure is an important issue to minimize the microwave absorption loss and to control the dielectric constant in the microwave region. The this study is to fabricate the controlled microstructure of model materials to analyze the dielectric properties.
The alumina, silica and silicon nitride were chosen as a model of monolithic windows material. The dielectric constant ($＼epsilon_r$) and the dielectric loss (tans) in the microwave frequencies were measured using the Hakki - Coleman dielectric resonator method. The dielectric constant is found to be roughly proportional to the volume fraction of pore and grain size but the size of the pore and pore interfaces do not affect the dielectric constant. The dielectric constant results of all specimens with different pore size are in accordance with Lichteneker's approximation. The dielectric loss increases according to porosity and grain size. The dielectric loss could be controlled by pore and grain size. From the results it can be revealed that eventhough pore is the most effective factor in the drastic reduction of dielectric constant in ceramics, the size control of pore is considered for its lower loss.
The alumina composites were fabricated to analyze the synergic role played by the size of the second phase and volume of loading on the dielectric properties of ceramic composites. The dielectric constant linearly changes with the amount of second phase. The results clearly evidence that the dielectric constant is independent of the shape of second phase. The loss in alumina composite may be explained as follows: (i) the conductivity of the composites, which depends on the second phase distribution as explained from percolation theory. (ii) the interface conductivity of composite.
The finite-element method was introduced to analyze dielectric properties of ceramic composite. The results indicate that the dielectric constant is independent of interface. But the Ioss tangent in the microwave region is found to vary as a function of second phase size. Results of alurnina/paraffin agreed with calculated values using a conductive interface model. The results of FEM could provide evidence to substantiate the interfacial effect of ceramic composite. It can be revealed that the interface of pore/alumina is conductive and leads to increase in the dielectric loss.
The Free-space measurement system was designed and setup to measure the dielectric properties at high temperature. It is particularly suitable for nondestructive and contactless dielectric measurement of ceramic materials. The performance of measurement system was evaluated by measuring dielectric properties of alumina and quartz up to 1,400℃. Future improvement in the system will enable characterize the dielectric properties of microwave windows materials at higher temperature.