The influence of tetrahedral ordering on the microwave dielectric properties of Sr_(0.05)Ba_(0.95)Al_2Si_2O_8 and BaM_2M_2'O_8 (M = Al, Ga, M' = Si, Ge) ceramics

摘要

The feldspars Sr_(0.05)Ba_(0.95)Al_2Si_2O_8, BaAl_2Ge_2O_8 and BaGa_2Si_2O_8 with S.G. I2/c, and BaGa_2Ge_2O_8 with S.G. P2_1/a, were studied by means of crystal structural and microstructural analyses and dielectric measurements. All the investigated densely sintered single-phase feldspars exhibited a permittivity (epsilon) of 7-8 and a temperature coefficient of resonant frequency (tau_f) from -20 to -30 ppm/ deg C. In contrast to the s and tau_f the dielectric losses were found to be dependent on the annealing conditions. In Sr_(0.05)Ba_(0.95)Al_2Si_2O_8 the Q x f values increased from 42,500 to 92,600 GHz when the annealing time at 1400 deg C was increased from 1 to 162 h. Such a difference in the Qxf values as a result of various annealing conditions was attributed to different degrees of tetrahedral ordering. In contrast to aluminosilicate feldspars, Ge-containing feldspars can be sintered and ordered at low temperature. In BaAl_2Ge_2O_8 the Qxf values decreased when the sintering temperature exceeded the order-disorder I2/c reversible C2/m phase-transition temperature. The BaGa_2Si_2O_8 and BaGa_2Ge_2O_8 feldspars exhibited a rapid decrease of Qxf values when the annealing temperature approached the melting point. However, the BaAl_2Ge_2O_8 and BaGa_2Ge_2O_8 can regain their high Qxf values by annealing at 1000 deg C. The BaGa_2Ge_2O_8 stood out from the other investigated feldspars, with a sintering temperature of 1100 deg C, Qxf values of 100,000-150,000 GHz and a tau_f of -26 ppm/ deg C.