Study On Microwave Dielectric Properties Of Ba(Mg_1/3Ta_2/3)O₃ Ceramics Doped With Nanoscaled Powder Additives

As the most outstanding representative of moderate ε_r microwave dielectricceramics and main materials used currently at high-frequency regions such as satellitecommunication systems etc, Ba(Mg_1/3Ta_2/3)O₃ (BMT) compounds have gotten closeattention in the field of microwave ceramics. In this dissertation, the mechanisms ofdielectric loss and temperature coefficient of resonant frequency ( Ï„_f) wereinvestigated from microscopic material parameters and, based on such mechanismsanalysis, the studies on modified BMT systems were conducted by using nanoscaledpowders additives. The goal of this work is to obtain novel modified BMT materialswith both good sinterability and excellent microwave dielectric properties.1. The theoretical studies on dielectric properties of BMT ceramics were analyzedand summarized: the dielectric loss can be classified as intrinsic loss and extrinsicloss, the former is determined by chemical composition and crystal structure, thelatter is closely related to material microstructure;temperature coefficient ofdielectric constant ( Ï„_ε) is found to be not only related with material composition,but also with crystal structure, which directly influences the temperaturedependence of material microscopic polarizabilities.2. A systematically study on modified BMT systems were conduced by dopingvarious metallic ions through conventional electronic ceramics technology. Theions parameters were correlated with material polar lattice vibrations throughclassical oscillator model, and the relation between materials properties and ionscharacteristics at different substituting sites were also investigated: dielectricconstant ( ε_r) and Ï„_f are mainly determined by polar phonons characteristics atlow frequency regions;the intrinsic loss is determined by anharmonic effects ofall polar phonons;doping different ions at different sites can lead to the changesof lattice phonons characteristics, thus improving dielectric properties.3. A new modified BMT system ceramics with both good sinterability and excellentmicrowave properties were prepared by doping Sn4+and Mn²⁺ in the form ofBaSO₃ (BS) nanoscaled powders and Mn²⁺ solutions, respectively. After dopedwith 1.5mol% Mn, the 0.85BMT-0.15BS system can be sintered at 1450℃ for 4h.The as-prepared sample shows a density of 7.33g/cm³;Ï„ f≈0ppm℃^-1;ε_r=24.9;Q× f= 1.85×10⁵GHz.4. BMT nanoscaled powders with high purity and good sintering activity wereeconomically synthesized at 800℃ through liquid-phase coating interfacialreaction and then were used as additives to improve the sinterability of BMTsystem. Uniform experimental design and multiple linear regressions were appliedto optimize the formula and preparation technology. When additive content is8.5wt%, the BMT materials sintered at 1550℃ for 4h show about 93.8% of thetheoretical density and good microwave dielectric properties: ε r=24.8;Ï„ f=6ppm℃-1;Q× f= 1 .24× 105GHz.