Study Of The Ca(Li_1/3Nb_2/3)O_3-δ-based Ceramics Sintered At Low Temperature

On the basis of research and analysis on a number of both domestic and foreign studies on microwave dielectric ceramics, this paper has systematically investigated the composition, microstructure, and dielectrics properties of Ca(Li_1/3Nb_2/3)O_3-δ ceramics, and their mutual relationship. Discussion is made on the principal influence mechanism of dielectric constant, dielectric loss, resonance frequency temperature coefficient. Feasible modification method and related mechanism is proposed for dielectric constant control, dielectric loss reducing, and resonance frequency temperature coefficient adjustment. Un-doped Ca(Li_1/3Nb_2/3)O_3-δ ceramics is of typical oxygen vacancy- type perovskite structure oxide, with the increase of B site Zr⁴⁺ substitution, it was transformed gradually into over the oxidized type.The influence of B site Zr⁴⁺ substitution on the lattice parameter, the tolerance factor, the degree of B-site 1:2 ordering and the ionic polarizability and the relationship between the crystal structure and dielectric properties of Ca (Li_1/3Nb_2/3)_1-xZr_3x]O_3-δ (0.0≤x≤0.1) ceramics was investigated. A increase of the ionic polarizability in Ca[(Li_1/3Nb_2/3) 1-xZr3x]O_3-δ ceramics caused by B site Zr⁴⁺ substitution.Therefore, theεr value of specimens increased, but the degree of B-site 1:2 ordering and the Qf value decreased. TheÏ„f value increased from ?16.3 to ?7.3 ppm /oC with increasing Zr ⁴⁺content from 2.0 to 10.0mol%. The influence of multiple substitution of Zr⁴⁺ and Ti⁴⁺ on the ceramic structure and dielectric properties of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15-xTi_x]O_³⁺δ (0.0≤x≤0.15)ceramics was studied. The substitution of Ti⁴⁺ at B site caused the increase of dielectric constant is increased. At first the Qf value is increased from 13100 GHz to 16570GHz as Ti⁴⁺ content increased from 0 to 10mol%, then it is decreased to 16020GHz at Ti⁴⁺ content of 15mol%. The reason is the substitution of Zr⁴⁺ to Li+ at B-site generated redundant electronic, lowered the resistance of ceramics, and leaded to an increase of conductivity loss, while the substitution of Ti⁴⁺ to Nb5+ generated hole, the electronic concentration was decreased, and conductivity loss decreases, but with increasing Ti⁴⁺ content to a certain degree, conductivity loss increased again due to the hole concentration increased. With a increase of Ti⁴⁺ content , theÏ„f value moved to the negative direction. At the same time, effect of preparation processing on dielectric properties of Ca(Li_1/3Nb_2/3)O_3-δ-based ceramics was discussed.The effect of A site Ba²⁺, Sr²⁺ and Nd³⁺ substitution on the crystal structure and dielectric properties of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ ceramics was investigated. The proper substitution of Ba²⁺ and Sr²⁺ can improve the temperature coefficient of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ system, which was resulted from the decrease of the B-site bond valence of the ABO3 perovskite structure. At x=2.5mol%, good dielectric properties were obtained in (Ca_1-xBa_x)[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ (0.0≤x≤0.2) ceramics:εr=34.3, Qf =13400Ghz andÏ„f = 2.1 ppm/oC; the dielectric properties wereεr=32.5, Qf =13500GHz andÏ„f = -2.4 ppm/oC for (Ca_1-xSr_x) [(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ (x=5mol%). The proper substitution of Nd³⁺ can enhanced the degree of B-site 1:2 ordering. The Qf value of the specimens increased rapidly. Excessive substitution made the degree of B-site 1:2 ordering.Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ ceramics were selected as the research objects, and ZnO-B₂O₃-SiO2 (ZBS), ZnO-B₂O₃-Na2O (ZBN) and B₂O₃ were regarded as sintering aids. The effect of single aid or multiple aids on the sintering behavior, microstructure and dielectric properties of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ ceramics was discussed, and the low temperature sintering mechanism was revealed. The results of research show that ZBS, ZBN and B₂O₃ can also lower the sintering temperature. The effect of B₂O₃ aid was best. When the B₂O₃ of l.0wt% were added, the optimum microwave dielectric properties:εr =33.1,Qf =13700GHz andÏ„f =- 6.8×10^-6/℃were obtained at the sintering temperature of 990oC. Furthermore. The specimens with 2.0wt% B₂O₃ sintered at 940oC for 4h showed good microwave dielectric properties ofεr=31.4 ,Qf =8700GHz andÏ„f = - 5.2×10^-6/℃. Multiple aids can reduced the sintering temperature more effectively. Sintered at 950℃, the dielectric properties of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15]O_³⁺δ with 3.0wt%ZBN +2.0wt% B₂O₃ wereε_r=31.2, Qf=10530Ghz andÏ„_f = ?5.1ppm/℃.The influence of electric charge and structure unbalance substitution of Zr⁴⁺ at B site on the original defect model in Ca(Li_1/3Nb_2/3)O_3-δ ceramics was studied. The result shows that the electron concentration rises with a increase of the partial pressure of oxygen and dopants concentration, the oxygen vacancy concentration decreased, and Ca(Li_1/3Nb_2/3) O_3-δ compounds changed from oxygen vacancy-type into oxygen ionic Surplus-type compounds. At the same time, the defect chemistry of Ca[(Li_1/3Nb_2/3)_0.95Zr_0.15-xTi_x]O_3+δ (0.0≤x≤0.15) ceramics was discussed. The study shows that the decrease of electron concentration with a increase of Ti⁴⁺ content, and the increase of electron concentration with increasing Nd³⁺ content. The influence mechanism of point defect electronic conductivity on the dielectric loss of microwave dielectric ceramics was discussed.