Structures And Microwave Dielectric Characteristics Of Ca(B’_1/2Nb_1/2)O₃ -CaTiO₃（B’=Ga,Al）Ceramics

Complex perovskites constitute important subgroup of microwave dielectric ceramics in virtue of the excellent and tunable microwave dielectric properties. In this thesis, systematic studies of Ca(B’0.5Nb0.5)O3-CaTiO3 (B’=Ga, Al) ceramics that combine dielectric properties with B-site 1:1 ordering/disordering and octahedral tilting are conducted for an improved understanding of structure-property relations in A(B’0.5B’0.5)O3 complex perovskites.The crystal structure of Ca[(Ga0.5Nb0.5)1-xTix]O3 is indexed to be orthorhombic in space group Pbnm. Antiphase (b-) tilting, inphase (c+) tilting and antiparallel A-site cation displacement are detected in the perovskites, whereas the ionic size difference between Ga3+ and Nb5+ is too small for Ca[(Ga0.5Nb0.5)1-xTix]O3 to be able to form stable 1:1 ordered structrue. The tolerance factor (t) increases as Ti-substitution augments, thus the sbability of the perovskite structure is improved. With Ti-substitution in B-site, the dielectric constant (sr) increases as a result of increased microscope polarizability and enhanced cation rattling. The relationship between τf and εr is approcimately linear. The acceptable Qf value can be maintained despite the increased intrinsic dielectric losses as Ti-substituion increases.The improved Qf value is achieved by Al-substitution for Ga in Ca[(Ga0.5Nb0.5)0.5Ti0.5]O3 ceramics. The t of Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 increases from 0.958 to 0.968 as Al-substitution augments. It is well recognized that t= 0.985 and t= 0.965 are the onset of antiphase and inphase octahedral tilting in perovskites, respectively. However, the unexpected inphase tilting occurs in Ca[(AlxGa0.5-xNb0.5)0.5Ti0.5]O3 perovskite when t is larger than 0.965. Thus, the t which determines the onset of inphase tilting in Ca based A(B’0.5B’0.5)O3 complex perovskites may shift to a value greater than 0.965 according to the present work. Moreover, both b- and c+ tilting weaken as t increases due to Al substitution in B-sites. Owing to the Al-substitution which is accompanied by the subdued octahedral tilting, the ετ remains at around 50, the τf value which is affected by both ετ and octahedral tilting could be adjusted to near zero. In addition, the lattice defect which is caused by Ga-volatilization decreases with Al-substitution for Ga. Meanwhile, improved structural symmetry and atomic packing are resulted from the subdued octahedral tilting, which contributes to the smaller vibrational anisotropy and reduced anharmonicity, thus the Qf value is improved.Low-loss temperature-stable microwave dielectric ceramics with medium ετ are obtained in the present work:Ca[(Ga0.5Nb0.5)1-xTix]O3 (x=0.47)εr=51.6,Qf= 34,100GHz,τf=-0.3 ppm/℃；Ca[(AlxGa1-xNb0.5)0.5Ti0.5]O3(x=0.2-0.5)εr=49.2-53.4, Qf=35,900-40,000 GHz,τf=0.6-3.5ppm/℃。...