Preparation And Microwave Dielectric Properties Of Cadmium Tungstate Ceramics

With the rapid development of modern communications industry, especially in the mobile communication technology and global positioning system (GPS), small and light microwave devices have been paid much more attention. The low temperature co-firing technology (LTCC) has been developed rapidly in recent years, which can reduce the size of the device, realize the integration effectively has become a hot spot among researchers. LTCC technology requires the microwave dielectric ceramics should have lower sintering temperatures, proper dielectric constant (εr), low losses (tanδ) and near zero temperature coefficients (tf). However, a numbers of microwave materials that possess excellent microwave properties always require high sintering temperatures at present, and the high sintering temperatures have greatly limited its application. In this paper, we researched the microwave dielectric properties of CdWO4ceramics. The cadmium tungstate powders synthesized at low temperatures of200℃and its ceramics possessing high relative density were obtained. The microwave dielectric properties of the cadmium tungstate ceramics have been found and reported. By doping of nanoscale titanium dioxide, not only reduced the best sintering temperature of CdWO4ceramics from1025to900℃, but also adjusted its temperature coefficient to zero. The microwave properties of the composite ceramic meet the requirements of LTCC conditions, and it’s the ideal candidates materials for microwave device.(1) The CdWO4compound was synthesized by a traditional solid state method. The WO3&CdO were mixed according to chemical molar ratio1:1. The mixtures ball milled10hours, then calcined from room temperature to600℃. XRD showed that at the sintering temperature range from200to600℃the pure CdWO4powders were obtained and the cadmium tungstate with monoclinic structure belongs to space group of p2/c(13). The CdWO4ceramics were sintered at1075～1200℃. The results showed that ceramics with high relative density and grain size grew with the increasing of sintering temperatures. Its grain size was about18μm at1150℃, and at1200℃the surface of ceramic began to melt and the boundary for specimen was dimly. Under the sintering temperatures from950～1100℃, the ceramics showed excellent microwave properties. When the cadmium tungstate sintered at temperatures of1150℃, the best microwave dielectric properties was obtained: Q×f=41000GHz, εr=12.8, tf=-14ppm/℃. Compared with other tungstate, cadmium tungstate with lower synthesized temperature, low loss and low temperature coefficient is expected to become a new generation of microwave dielectric materials.(2) The cadmium tungstate prepared by using high energy ball mill method. The WO3&CdO mixtures of ball milling at different times were calcined under different temperatures, respectively. It was found that increasing of the ball-milling time was not only beneficial to pure phase formation of cadmium tungstate, but also can reduce the cadmium tungstate powder particle size effectively. Finally through high energy ball milling, the optimum condition for synthesis cadmium tungstate as follows:CdO&WO3mixtures ball milling30hours, then dried under at125℃. The average particle size about230nm of CdWO4powders was obtained finally. Those powders were sintered under at950～1100℃. The relative densities of ceramics were about92.5%, and the grain size increased with sintering temperature, the surface of ceramics began to melt at1100℃. The CdWO4ceramics sintering under1025℃possessed the optimum microwave dielectric properties:Q×f=27000GHz, εr=12.8, ty=19.3ppm/℃.(3) The TiO2powders with particle size about50nm were mixed with the CdWO4powders with particle size230nm prepared by high energy ball milling method. The mixtures were weighed according to the (1-x)CdWO4-xTiO2(x=0-0.22), then ball milled5h, and then the mixtures were sintered at different temperatures. The XRD, SEM and EDS showed that cadmium tungstate and titanium dioxide haven’t reacted during the sintering process; the two compounds can co-exist well. Addition of TiO2not only reduced the sintering temperature of cadmium tungstate ceramics effectively, but also adjusted the temperature coefficient to zero. The zero temperature coefficient of ceramic0.79CdWO4-0.21TiO2, sintered under900℃was obtained finally, and the microwave dielectric properties of0.79CdWO4-0.21TiO2ceramics:Q×f=15000GHz, εr=13.7, tf=0ppm/℃.