MICROSTRUCTURE-PROPERTY RELATIONSHIPS OF HETEROGENEOUS ELECTROCERAMICS

Based on an improved effective-medium theory and a proper treatment of ac response of grain boundary barrier layer, the theoretical relationships describing ac electrical properties of heteroge-neous electroceramics are discussed in terms of the theoretical relation-ships, and some important results consistent -with experimental results are obtained.

Thermoelectric Properties of Misfit Layered Bismuth-Based Rhodium Oxides, (Bi,Pb)₂Sr₂Rh₂O_y

Rhodium oxides, including a misfitlayered structure with alternate stacking of a rock salttype layer and a hexagonal RhO2 layer, are expected to have good thermoelectric properties. Among them, the thermoelectric properties (electrical conductivity (σ), Seebeck coefficient (S), Figure of merit (ZT) and calculated thermal conductivity (κ) by S, σ, ZT, and absolute temperature (T)) of bismuth-based rhodium oxides ((Bi1-x,Pbx) 2Sr2Rh2Oy, x = 0 and 0.02, hereafter BSR and BPSR, respectively) were investigated. In comparison with Bi2Sr2Co2Oy (BSC) at 700°C, S and κ enhanced (increased S, 110 (BSR) and 105 μV K-1 (BPSR) from 85 μV K-1 (BSC) and decreased κ, 0.32 (BSR) and 0.50 W m-1 K-1 (BPSR) from 1.75 W m-1 K-1 (BSC)), whereas σ decreased (15 (BSR) and 31 S cm-1 (BPSR) from 70 S cm-1 (BSC)). BPSR reached the highest ZT value of 0.067 at 700°C, compared to those of 0.056 (BSR) and 0.027 (BSC).

Study of Structural and Electrical Properties of a New Type of Complex Tungsten Bronze Electroceramics;Li₂Pb₂Y₂W₂Ti₄V₄O₃₀

A polycrystalline ceramic, a new type of complex tungsten bronze type structure, having a general formula Li2Pb2Y2W2Ti4V4O30 has been prepared relatively at low temperature using a mixedoxide technique after optimizing the calcination conditions on the basis of thermal analysis. The material has been characterized by different experimental techniques. The formation of the material under the reported conditions has been confirmed by an X-ray diffraction technique. A preliminary structural analysis of the material showed the formation of single phase compound in an orthorhombic crystal structure at room temperature. Studies of dielectric properties (εr, tanδ ) of the above compound as a function of temperature at different frequencies exhibit a ferroelectric phase transition of diffuse type. The electrical properties of the material have been studied using ac impedance spectroscopy technique. Detailed studies of impedance and related parameters exhibit that the electrical properties of the material are strongly dependent on temperature, and bear a good correlation with its microstructure. The temperature dependence of electrical relaxation phenomenon in the material has been observed. The bulk resistance, evaluated from complex impedance spectra, is found to decrease with rise in temperature, exhibiting a typical negative temperature co-efficient of resistance (NTCR) – type behavior similar to that of semiconductors. A small contribution of grain boundary effect was also observed. The complex electric modulus analysis indicates the possibility of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The ac conductivity spectra exhibit a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law.

Enhanced Dielectric Response in Mg-doped CaCu_3Ti_4O_(12) Ceramics

CaCu3Ti4O12 ceramics substituted by Mg for Ca were prepared by the solid state reaction method. The crystal structures, microstructures, and dielectric properties of the Ca1-xMgxCu3Ti4O12 ceramics were investigated. At lower doping concentrations, the substitution of Mg for Ca caused a decreased lattice constant, while at higher doping concentrations, some of the Mg dopants started to replace Ti and resulted in an increased lattice constant, and some could also replace Cu due to the similar ion radius between Mg and Cu ions. Mg doping was found to promote the grain growth of Ca1-xMgxCu3Ti4O12 ceramics during sintering. Grain boundary resistance of the Ca1-xMgxCu3Ti4O12 ceramics was found to be increased by Mg doping. Enhanced dielectric properties was observed in the Ca1-xMgxCu3Ti4O12 ceramics with x=0.05 for the frequency range from i kHz to 20 kHz. For other doping concentrations, the dielectric losses of Ca1-xMgxCu3Ti4O12 ceramics were generally lowered.

SYNTHESIS AND MICROWAVE DIELECTRIC PROPERTIES OF Li_(2)MgSiO_(4)CERAMICS PREPARED USING CITRATE GEL ROUTE

The comparison of the low temperature sintering and the microwave dielectric properties of Li_(2)MgSiO_(4)(LMS)ceramics prepared by citrate gel(CG)route and solid state(SS)ceramic route are discussed in this paper.The LMS prepared using CG route sintered at 1175℃/2h hasε_(r)=5.3 and tanδ=1×10^(-3)at 9 GHz.The si ntering temperature of LMS is lowered to 950℃with the addition of 5 wt%lithium magnesium zinc borosilicate glass and hasε_(r)=5.6 and tanδ=2×10^(-3)at 9GHz.The amount of glass required to lower the sintering temperature of ceramics prepared using CG are slightly higher than that of Ss ceramic route.The LMS ceramics prepared using SS ceramic route shows excellent microwave dielectric properties with low sintering tem-perature compared to CG route.

SYNTHESIS AND PHASE ANALYSIS OF A NEW FERROELECTRIC(K_(0.5)Na_(0.5))NbO_(3)-Bi_(0.5)K_(0.5)TiO_(3)-BaTiO_(3)TERNARY SOLID SOLUTION

In order to develop new lead-free pieozoelectric/ferroelectric materials,the y(K_(0.5)Na_(0.5))NbO_(3)-(1-y)[(1-x)Bi_(0.5)K_(0.5)TiO_(3-x)BaTiO_(3)]solid solution system is synthesized and the partial ternary phase diagram in the KNN-rich region with compositions of y=0.98 to y=0.96 is established.All the compositions studied show a gradual phase change from a tetragonal to an orthorhombic structure as the BT component increases.A mixture of tetragonal and orthor-hombic phases is found in the composition range of x=0.25 to 0.60 and y=0.97,and x=0.50 to 0.75 and y=0.96,indicating a ternary morphotropic phase boundary region.The piezoelectric and ferroelectric properties of the 0.97(KNN)-0.03[0.60BKT-0.40BT]ceramics are character-ized.The increase in remnant polarization as compared with pure KNN shows improved ferroe-lectricity by forming the ternary solid solution.