Very Low Loss Ceramic Dielectric Resonator Materials
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Results on polycrystalline
alumina show that a Q of> 5X10 at 10 GHz and at room temperature are possible and Q's well in excess of I0W at 10 GHz and 77 K can be achieved. INTRODUCTION The requirement for miniature low loss microwave filters has led to the dielectric loading of cavity resonators to form dielectric resonators. The miniaturisation factor can be expressed as
D, where D is the diameter of the dielectric resonator, A, is the free space wavelength at the resonant frequency and e is the dielectric constant of the dielectric. There are three key parameters for the dielectric materials in filter applications: the dielectric constant (Fr), the quality factor (Q-=/tan5) and the temperature coefficient of resonant frequency ('r). The larger the dielectric constant, the smaller the filter. The higher the Q the lower the insertion loss and steeper the cut-off. A cf close to zero (< ± 10 ppm/K) is required for stability against ambient temperature change. Even though single crystals have a higher Q than the equivalent polycrystalline material they are not suitable for most applications due to their high cost. Some of the higher dielectric constant materials cannot be made as single crystals. The focus of this work has been to 189 Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
improve the properties of existing dielectrics through careful processing. Three materials have been studied here, Alumina (Fr = 10), Zro.875Sno.25Tio.87 50 4 (ZTS) (Er = 37) and Ba(Mgi1 3Ta2/3)O (BMT)(er = 24). Cryocoolers have provided another opportunity to improve filter performance as the dielectric loss of dielectrics generally decreases with temperature. The use of cooling also provides temperature stability which can be used to reduce the requirements for a very small tcf. The relaxing of this constraint broadens the materials options considerably and has made real the possibility of utilising cold dielectrics in filter assemblies. With sufficient cooling, high temperature superconductors can be used as the cavity housing and thus reduce the size still further. EXPERIMENTAL Samples of the dielectrics were produced by pressing appropriate powder in 13 mm diameter stainless steel dies at a pressure of 100 MPa. The resulting disks were sintered in air at between 1000 'C and 1600 'C for between 5 and 1800 minutes. Alumina discs were produced from commercial high purity alumina powders. BMT was obtained from the Murata Manufacturing Company and was also prepared in our laboratories by mixed oxide routes. ZTS was prepared in our laboratories by mixed oxide routes. The Q and was measured by a resonant cavity method using the TEo 01 mode. The sample is placed in a copper cavity on a 4 mm high low loss quartz spacer. The surface resistance of the copper has been calculated from the Q of the TEO,, resonance of the empty cavity to allow the results to be corrected for the loss due to the cavity walls[l]. The sintered samples were approximately 10 mm diameter, 4 mm thick discs. The TEo1 8 mode was examined using a HP8720
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