Structure And Microwave Dielectric Properties Of Complex Perovskite Ceramics

Ba-and Ca-based complex perovskite ceramics have important applications in microwave communication systems because of their good microwave dielectric properties. In this thesis, effects of A site, B site substitution and A/B sites co-substitution on the structures, microstructures and microwave dielectric properties were systematically investigated for complex perovskite ceramics together with the structure/property modifications.High Qf values were obtained in Ca_1-xNd_2x/3TiO₃ ceramics especially at x=0.39. This high Qf was due to the proper correlation length maintaining the charge neutrality caused by the combination of A-site cation vacancies and B-site oxygen vacancies. With increasing content of A-site cation vacancy, the tolerance factor decreased to enhance the oxygen octahedral tilting and suppress the cation rattling, and as a result, both the dielectric constant (ε_r) and temperature coefficient of resonant frequency decreased (tf).The crystal structure of Ca[(Mg_1/3Ta_2/3)_1-xTi_x]O₃ (CMTT) ceramics changed from monoclinc to orthohombic by introducing of Ti4+, which gradually destroyed the B site 1:2 cation ordering. However, the 1:2 cation ordering could maintain in micro area until x=0.2 according to the SAED patterns, and this indicated that the cation ordering change in the present system fitted the space charge model. The similar ionic radius between Ti⁴⁺and Ta⁵⁺should be the key facotr on determining the ordering change model. The Qf value decreased as the cation ordering was weakened, while theî–¸ increased as the cation rattling was enhaced by introducing Ti⁴⁺with small ionic radius. The increased dielectric constant and the weakened oxygen octahedra tilting caused tf to increase.Based on Ca[(Mg_1/3Ta_2/3)o.9Ti_0.1]O₃ ceramics, the domain structures of Ca-based complex perovskite ceramics were comprehensively investigated by using SAED and NBD. There were two kinds of domains in these ceramics, the antiphse domain and the ferroelastic domain. The antiphase domain was caused by the dislocation of neighbour ions which were order arranged. As some fundamental perovskite crystal planes could cross the the antiphase domain boundaries, the energy of the boundaries would not be too high. While the ferroelastic domains were in fact groups of twin domains with the same or distinct orientations. Generally, the twin domains could be reflective or rotative. The refection planes of the reflective twin domains were always normal to [001]_c or [110]_c direction, while the rotative domains could be 90°rotation along the [001]_c zone axis or 120°rotation along the [111]_c zone axis. In-phase and antiphase tilting of oxygen octahedra, antiparallel dispalcement of A site cation and B site cation ordering could all cause the formation of ferroelastic domains.A/B site co-substituted (Ca_1-0.39xNd_0.26x)[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (CNMNT) and (Ca_1-0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1-xTix]O₃(CLMTT) ceramics were prepared and evaluated. As the same as in CMTT ceramics, the crystal structures of these two systems changed from monoclinic to orthorhombic as the cation ordering being destroyed. The Qf value sharply decreased because of the loss of cation ordering and introducing of A site cation vacancy. Introducing Ti⁴⁺increased theε_r, while introducing A-site cation vacancy enhaced the oxygen octahedra tilting, and they also caused the unusual variation of tf. However, these two systems had differences as indicated by the SAED patterns, CNMNT ceramics tended to form 1:2 cation ordering in micro area while CLMTT ceramics tend to form 1:1 ordering.Raman spectra analysis revealed that the three systems had different types of short range cation ordering, which could be determined by different Ag modes around 800cm^-1. This was caused by the differences of A-O hybridization in these systems. Enhanced A-O bond made the ceramics tend to form 1:1 short range cation ordering while the weakened A-O bond lead to 1:2 short range ordering. Both kinds of short range cation ordering were suggested to be appreciably beneficial to the Qf value.For the modified Ba(Zn_1/3Nb_2/3)O₃ ceramics with Mg²⁺-substitution for Zn2+, Mg²⁺could perfectly entered the Zn²⁺site. The sintered Ba[(Zn_1-xMg_x)_1/3Nb_2/3]O₃ ceramics could be well ordered since Mg₂₊-substitution increased the order-disorder transition temperatures. Long time annealing at temperatures below the transition temperature could further enhance the cation ordering, and the better effects were observed at the vicinity of the transition temperature. However, Mg²⁺-substitution rarely affected the degree of cation ordering as the cation ordering degree varied little with compositions. However, the Qf values varied significantly by Mg²⁺-substitution, and this should be affected by the sizes and distribution of ordering domains. The homogeneous domain structure with small domain sizes yielded the high Qf state, while inhomogeneous domain structure with large domain sizes resulted in the low Qf state. Furthermore, homogeneous domain structure with domain size around 40nm should be critical for high Qf value.Some good combinations of microwave dielectric properties were obtained: Ba[(Zn_1-xMg_x)_1/3Nb_2/3]O₃ (x=0.4):ε_r=36, Qf=94,400GHz, tf=25.8ppm/℃; Ca[(Mg_1/3Ta_2/3)_1-xTi_x]O₃ (x=0.45):ε_r=45.1, Qf=34,800GHz, tf=17.4ppm/℃, Ca_1-xNd_2x/3TiO₃ (x=0.39):ε_r=103, Qf=15,340GHz,tf=247ppm/℃.â…¦...