| In this thesis, we explore the growth and properties of electronic ceramics used in microwave resonator and energy storage. Their applications require high dielectric constants, which do not vary significantly with temperature and loss dissipation loss.; Single-phase Ba(Cd1/3 Ta2/3)O3 powder is produced using conventional solid state reaction methods. Ab-initio electronic structure calculations show that the covalent nature of the directional d-electron bonding in these high-Z oxides plays an important role in producing a more rigid lattice with higher melting points and enhanced phonon energies and consequently resulting materials with a high dielectric constant and a low microwave loss for Ba(Cd1/3Ta2/3) O3 and Ba(Zn1/3 Ta2/3)O3 ceramics.; Ba(Cd1/3Ta 2/3)O3 samples with high sintering density and excellent microwave properties are made with boron oxide as sintering aid at 1200--1350°C, corresponding to temperatures 300°C lower than samples prepared without a sintering aid. XRD combined with High Resolution Electron Microscopy (HREM) indicates that Ba( Cd1/3Ta2/3)O 3 ceramics prepared with boron oxide have a well-ordered hexagonal structure. Transmission Electron Microscope (TEM) results indicate that the improvement in densification contributes to the liquid sintering mechanism for boron concentrations exceeding 0.5wt%. Annealing treatment and high boron concentrations are also found to improve the microwave properties. For example, Ba( Cd1/3Ta2/3)O 3 doped with 0.5,vt%o B2O 3 ceramics annealed at 1250°C for 40 hours has a dielectric constant (epsilonr) and temperature coefficient of resonant frequency (tauf) of 32 and 80 +/- 15 ppm/°C respectively and a loss tangent (Q) of < 2 x 10-5 at 2 GHz.; Ceramic injection molding methods were subsequently developed to fabricate the microwave devices. A high sintering density (∼ 94%) sample with epsilon r (∼ 30), tauf (0.1 ppm/°C), and loss tangent (< 1.7 x 10-5) at 2 GHz was achieved using a high temperature 1680°C and 48h sintering process.; Doping Sc into Ba0.7 Sr0.3TiO3 ceramics changes its crystal structure from tetragonal to rhombohedral structure and significantly reduces the dielectric constant, of Ba0.7 Sr0.3TiO3. In contrast, BaTiO3 and Ba0.7Sr 0.3TiO3 with V-doping maintain the tetragonal crystal structures. Leakage current in these materials can be reduced by doping with vanadium. The leakage current is also strongly affected by point defects induced by neutron damage or annealing treatment for undoped Ba0.7Sr0.3TiO 3 ceramics. |
