| Ba(B’1/3B"2/3)O3 complex provskite ceramics are the important typical microwave dielectric materials. As the key materials for resonators and filters, they play an important role in the microwave telecommunication technologies. In the present thesis, effects of sintering aid, B-site substitution and post-densification annealing are applied to control the structure and microwave dielectric properties of the Ba(B’1/3Nb2/3)O3-based ceramics, especially the cations ordering, ordering domains and ion valence states are systematically investigated.The order-disorder transition temperature of Ba[(Zno.4Co0.6)1/3Nb2/3]O3 ceramics is determined by XRD and DTA methods. Addition of 0.25mol% B2O3 lowers the densification temperature, increases the ordering degree and hence the Qf value. With B2O3 content increasing to 0.5mol%, both of the ordering degree and Qf value decrease.B-site substitution by Ni and Co is carried out for Ba(Mg1/3Nb2/3)O3. Ni-substitution promotes the homogenous nucleation of ordering domains but does not favor domain growth, resulting in the decrease of domain size and ordering degree, and thereafter the Qf value decreases. With Ni-substitution increasing, the dielectric permittivity εr decreases slightly, while the temperature coefficient of resonant frequency τf shifts linearly from positive to negative. Ni volatilization results in the change of the ionic valence state which affects the cations ordering and microwave dielectric properties. Post-densification annealing significantly affects the cations ordering and Qf values of Ba[(Mg1-xNix)1/3Nb2/3]O3 ceramics, a proper annealing temperature is critical for obtaining the optimal Qf value. At the proper temperature, the annealing increases the ordering degree, further improving the Qf values.The variations in Raman spectra of Ba[(Mg1-xNix)1/3Nb2/3]O3 ceramics with Ni-substitution and post-densification annealing have been revealed. Applying the simple resonant model, the assignment of Raman modes for Ba(B’1/3B"2/3)O31:2 ordered structure has been determined. Ni-substitution obviously affects the vibration modes related to the Ba ions and O ions, but hardly affects those related to the Nb ions. After annealing, increase of intensity and blue-shift of location are observed for the Raman peaks. A certain correlation exists between the ordering degree and the intensity ratio of F2g(Ba) mode and A1g(O) mode, the ordered structure favors the F2g(Ba) mode more than the A1g(O) mode.The variations of structure as well as microwave dielectric properties in the Co-substituted Ba(Mg1/3Nb2/3)O3 (BMCN) ceramics are similar with Ba[(Mg1-xNix)1/3Nb2/3]O3 ceramics. Annealing at a proper temperature for BMCN ceramics increases the ordering degree obviously and results in a significant improvement for the Qf values. Especially, the influence of annealing atmospheres on Qf values of BMCN ceramics is deeply investigated. Annealing in N2 results in the highest Qf values. X-ray photoelectron spectra (XPS) reveal the variations of chemical valence state with annealing atmosphere and its effects on the Qf values. εr and τf have no obvious dependence on the annealing atmospheres.The order-disorder transition temperature of Ba[(Zn0.4Ni0.6)1/3Nb2/3]O3 ceramics is increased by Mg-substitution. With increasing Mg substitution, the ordering degree increases and contributes to the improvement of Qf values. Post-densification annealing further enhances the cations ordering and improves the Qf value for Ba[(Zn0.25Ni0.45Mg0.3)1/3Nb2/3]O3 ceramics.The effects of Zr-substitution on the structure as well as microwave dielectric properties of Ba(Ni1/3Nb2/3)O3 ceramics are systematically investigated. With increasing Zr-substitution, the B-site ions undergo a variation from 1:2 ordering to 1:1 ordering, and the 1:1 ordering degree first increases and then decreases. Post-densification annealing promotes the 1:2 cations ordering in the samples with 1%～2% Zr substitution, negating the the stabilization of the domain boundaries by Zr. Post-densification annealing hardly affects the 1:1 cations ordering in the samples with 5%～20% Zr substitution,1:1 cations ordering fits the space-charge model better. A small amount of Zr substitution makes a through improvement for the microwave dielectric properties, which are ascribed to the improvement of grain configuration and the large values of εr and τf in the end member BaZrO3.Excellent microwave dielectric properties have been achieved for the systems investigated:1) Ba[(Zn0.4Co0.6)1/3Nb2/3]O3+0.25mol%B203:εr=34,Qf=50,400 GHz,τf=5.5 ppm/℃;2) Ba[(Mg0.6Ni0.4)1/3Nb2/3]O3:εr=29.7, Qf=78,000 GHz, τf=9.8 ppm/℃;3) Ba[(Mg0.2Co0.8)1/3Nb2/3]O3:εr=31.7, Qf=76,900 GHz,τf=3.8 ppm/℃;4) Ba[(Zn0.25Ni0.45Mg0.3)1/3Nb2/3]O3:εr=31.9,Qf=56,500 GHz, zf=8.4ppm/℃;5) Ba[(Ni1/3Nb2/3)0.9Zr0.1]O3:εr=31.8, Qf=36,100 GHz, τf=7.8 ppm/℃. |
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