Papers by Keyword: Complex Perovskite

Abstract: Ba(Co_1/3(Nb_1-xW_5x/6)_2/3)O₃ (0<x≤0.05) (BCWN) microwave dielectric ceramics have been prepared by solid-state reaction method. The microstructure and microwave dielectric properties of BCWN ceramics were investigated systematically in this paper. The results show that the sintering temperature decreases with the substitution of W⁶⁺. The Ba(Co_1/3(Nb_1-xW_5x/6)_2/3)O₃ solid solution is formed with the small substitution of W⁶⁺ (x≤0.01). With the increase of x, the impurity phase BaWO₄ occurrs and the grains become less homogeneous. Combined with the analysis of X-Ray Diffraction, the cation ordering degree on B-site increases in the sample of x=0.01, which leads to the decrease of the dielectric loss. However, the second phase BaWO₄ inhibits the formation of the ordered structure with the reduction of sinterability. Due to the formation of BaWO₄, the values of ε_r and τ_f of all samples decrease subsequently. The optimized microwave dielectric properties of Ba(Co_1/3(Nb_1-xW_5x/6)_2/3)O₃ ceramics were obtained in the samples of x=0.01: ε_r=33.8, Q×f =110761 GHz, τ_f = -17 ppm/°C.

Abstract: Origins of high Q are considered on intrinsic as high symmetry, ordering structure and high density crystal structure. It was concluded that the high symmetry brings high Q instead of ordering comparing some cases as follows: As if ordering ratio of Ba(Zn1/3Ta2/3)O3 (BZT) is high of about 80%, Q values are distributed from low to high Q. Disordered BZT ceramics with high density obtained for short sintering time by spark plasma sintering (SPS) showed high Q. Ba(Zn1/3Nb2/3)O3 (BZN) with order-disorder transition showed high Q at disorder form sintered over the transition temperature. And, the disordered BZN with high Q annealed at lower temperature changed to order structure without improvement of Q.

Abstract: The effect of V2O5 as additives of different content on the densification, microstructure and dielectric properties of Ba(Mg1/3Nb2/3)O3 (BMN) was investigated. The sintering temperature of the V2O5-doped BMN samples were lowered down to 1250 oC with 0.5wt% V2O5 compared to pure BMN ceramics where the sintering temperature was 1500 oC at least. The εr and τf was not influenced much, while the Q value was affected by the sintering temperature and the V2O5 additives. The Q value decreased with decrease of the sintering temperature and the V2O5 addition increasing due to the bulk density and the second phase. Good microwave dielectric properties of Q·f=42100GHz, εr =31.7 and temperature coefficient of resonance frequency (τf) =22.7ppm/ oC were obtained with 0.25 wt% V2O5 sintered at 1350 oC for 4h.

Abstract: Ba3(Ca1.18Nb1.82)O9-d (BCN18) powder was synthesized using a wet chemical method from mixtures of all water-soluble compounds including Ba, Ca and Nb-citrate. It has been found that NH4NO3 in the initial solutions plays an important role in controlling the enthalpy of low temperature combustion process as well as the gel decomposition temperature. Further steps include evaporating, drying and calcinating. The obtained gels were characterized by TG-DSC, and the powder was characterized with XRD, TEM and BET. The experimental results have indicated that the heating temperature was only 800°C for synthesizing the powder and the average particle size was only about 40-50 nm. Furthermore it was found that a pure BCN18 phase with complexperovskite structure was formed at 800°C, which was about 800°C lower than that of the traditional solid-reaction method. So it is more practical and more superior to the traditional solid-reaction method and the present wet-chemical method in alcohol salt system reported in literature.

Abstract: The electronic structure of Ba(Mg1/3Nb2/3)O3 as a typical example of Ba(B¢1/3B¢¢2/3)O3 complex perovskites was investigated using the quantum chemical SCF- Xa-SW method. The calculations indicated that ionic bond strength plays the most important role in the structure stability of Ba(Mg1/3Nb2/3)O3 and BaTiO3. It was found that the bonding strength between oxygen and cations of B-site in Ba(Mg1/3Nb2/3)O3 is greater than that in BaTiO3, and the unbalance force in Ba(Mg1/3Nb2/3)O3 leads to the tendency of B-site ion arranging orderly. The more stable ordered structure of BMN phase can contribute to their better dielectric properties. The calculated results agree with the experimental observations, so it is helpful to designing and synthesizing the microwave dielectric ceramic with high Q-factor.

Abstract: Ba(Me1/3Nb2/3)O3 (Me=Zn, Co, Ni and Mg) ceramics were prepared using the conventional mixed oxide route; additives included Al2O3, Ga2O3, SiO2, WO3, B2O3 and V2O5. Powders were mixed, milled for 18h, calcined at 1100°C, remilled pressed into pellets at 100 MPa, sintered in air at temperatures in the range 1350-1550°C and then cooled at 360C h–1 to 5°C h–1. Products were characterised in terms of phase analysis (X-ray diffraction), microstructure (SEM and TEM) and electrical properties (relative permittivity, εr, dielectric Q value and temperature coefficient of resonant frequency,τf). The Q values of the Ba(Me1/3Nb2/3)O3 ceramics depend on the degree of cation ordering and the additives. Slow cooling leads to 1:2 ordering of the B sites and enhanced dielectric Q values. For samples cooled at 5°C h–1 after sintering the Qxf values are in the range 28000 to 98000 GHz, and are in the sequence Ba(Ni1/3Nb2/3)O3, Ba(Co1/3Nb2/3)O3, Ba(Mg1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3. Additions of BaO-4WO3 or V2O5 yield higher Qxf values than Al2O3. Highly ordered Ba(Zn1/3Nb2/3)O3 has a relative permittivity of 39.4, but most other Ba(Me1/3Nb2/3)O3 ceramics exhibit εr of 31-32. The temperature coefficient of resonant frequency, τf, varies from –18 ppm/°C (Ba(Ni1/3Nb2/3)O3) to +32 ppm/°C (Ba(Mg1/3Nb2/3)O3); the sintering additives (Al2O3 and BaO-4WO3) change τf by typically 10-16 ppm/°C.

Abstract: Ba3(Ca1.18Nb1.82)O9-δ (BCN18) is a novel complex perovskite structure proton conductor. Nanocrystalline BCN18 was synthesized by means of sol-gel method. The solid oxide electrolyte was prepared by using this powder, and its sintering behavior was studied. SEM and XRD were performed to demonstrate its sintering properties. Its electrical conductivity was systematically investigated at intermediate temperature (400～600°C).