Composition Design And Microwave Dielectric Properties Of α-CaSiO₃ Ceramics

With the rapid development of microwave communication technology in aerospace field, low dielectric microwave ceramics has become a research focus for recent 20 years, and the microwave dielectric materials with low sintering temperature(T < 1000°C), low dielectric constant(εr < 9), high quality factor(Q × f > 35000 GHz) and low τf(< ±5 ppm/°C) become more and more important. Based on the research status and problems(high sintering temperature and low Q × f) of low dielectric microwave ceramics, in this paper, α-Ca Si O3 ceramics were chosen as a study objective. The effect of composition design, preparation process, reverse phase transition and microstructure on the properties of α-Ca Si O3 ceramics were investigated.It was reported that the reverse phase transition appears from α-Ca Si O3 to β-Ca Si O3, and the mechanism of phase transition was analyzed by crystal structure and differential scanning calorimetry(DSC). The main research results are as follow:(1) The effects of calcined powders with different crystalline phases on the microwave dielectric properties of α-Ca Si O3 ceramics had been studied, and Ti O2 powder was added to adjust the τf value of α-Ca Si O3 ceramics. It shows that α-Ca Si O3 ceramics prepared with Cyclo-Ca Si O3 phase ceramic powder showed superior properties of εr = 6.92,Q × f = 26611 GHz,τf =-11.14 ppm/°C, and prepared with Pseudo-Ca Si O3 phase ceramic powder exhibited properties of εr = 6.95,Q × f = 18649 GHz,τf =-7.07 ppm/°C. Ti O2 can effectively adjust the τf value of α-Ca Si O3 ceramics, the addition of Ti O2 into α-Ca Si O3 ceramics resulted in the emergence of Ca Ti Si O5 phase which can inhibit the growth of α-Ca Si O3 grains by surrounding their boundaries. The α-Ca Si O3 ceramics containing 2wt% of Ti O2 sintered at 1300 °C showed microwave dielectric properties: εr = 7.86,Q × f = 16491 GHz,τf = 0.76 ppm/°C。(2) The influence of phase transition and internal stress on the microwave dielectric properties of α-Ca Si O3-Si O2 ceramics had been researched. The results show that the addition of Si O2 can inhibit the grain growth of α-Ca Si O3 and improve the internal stress, which inhibits phase transition from cyclo-Ca Si O3 phase to pseudo-Ca Si O3 phase and enhance the microwave dielectric properties obviously. The α-Ca Si O3 ceramics containing 6wt% of Si O2 sintered at 1350 °C showed dielectric properties: εr = 7.37, Q × f = 33,714 GHz and τf =-11.08 ppm/°C.(3) Al2O3 was selected as an additive to increase the quality factor of α-Ca Si O3 ceramics. It shows that the addition of Al2O3 into α-Ca Si O3 ceramics resulted in the emergence of Ca2Al2 Si O7 phase which can inhibit the growth of α-Ca Si O3 grains by surrounding their boundaries and improve the quality factor. The α-Ca Si O3 ceramics with 2wt% of Al2O3 sintered at 1250 °C showed superior properties: εr = 7.39,Q × f = 39523 GHz, τf =-13.28 ppm/°C. Moreover, the addition of 2.5wt% Ti O2 in α-Ca Si O3-2wt%Al2O3 ceramics could adjust the τf value near to zero( εr = 7.88,Q × f = 24412 GHz,τf =-0.52 ppm/°C).(4) The influences of B2O3 sintering aid on the microwave dielectric properties of α-Ca Si O3 ceramics and the mechanism of the reverse phase transition from α-Ca Si O3 to β-Ca Si O3 had been studied. It was found that with the addition of B2O3, the sintering temperature of α-Ca Si O3 ceramics is reduced to 1100 °C. The main crystal phase of ceramics transformed from α-Ca Si O3 to β-Ca Si O3 at 1100 °C, and then transformed from β-Ca Si O3 to α-Ca Si O3 at 1125 °C.The microwave dielectric properties of β-Ca Si O3 ceramics prepared by α-Ca Si O3 ceramic powder under phase reverse are: εr = 6.45,Q × f = 31914 GHz,τf =-35.36 ppm/°C, which is far more excellent than that of β-Ca Si O3 ceramics by β-Ca Si O3 ceramic powder.(5) The effects of Li2CO3 as sintering aid on the microwave dielectric properties of α-Ca Si O3-2wt%Al2O3 ceramics and the mechanism of the reverse phase transition had been studied. It shows that with the addition of Li2CO3, the sintering temperature of α-Ca Si O3-2wt%Al2O3 ceramics is reduced to 975 °C. The main crystal phase of the ceramics reversibly transformed from α-Ca Si O3 to β-Ca Si O3 when sintered at 800 °C. The microwave dielectric properties of β-Ca Si O3-Al2O3 ceramics prepared via the phase reversal are: εr = 6.86,Q × f = 47067 GHz,τf =-35.82 ppm /°C, which is much better than that of β-Ca Si O3 ceramics prepared by β-Ca Si O3 ceramic powder.(6) The microwave ceramics were obtained by low-temperature sintering aid control on α-Ca Si O3 reversed-phase transition to β-Ca Si O3 phase and Ca Ti O3 powder was added to adjust the τf value near to zero. Prepared using α-Ca Si O3 ceramic powder, β-Ca Si O3-Ca Ti O3 ceramics sintered at 975 °C have outstanding properties(εr = 8.07,Q × f = 40573 GHz,τf = 0.45 ppm/°C), which can be satisfied with the requirements of practical application.