Search

Presents a study on the defect structure of the dielectric ceramics using the positron annihilation technique. Experimental details; Results and discussion; Conclusion.

Ceramic samples with the nominal composition (1− x) BaTiO3+ x Ba3Ti2YO8.5 ( x=0−0.535) were prepared by the mixed oxide method. X-ray diffraction (XRD) analysis shows that the materials are of single phase with a cubic symmetry as x≤0.16. The compositions of the solid solutions ( x≤0.16) can be expressed equivalently as Ba(Ti1− yY y)O3−δ ( y≤0.122, y= x/(1+2 x)). For x>0.16, the materials are diphasic composites consisting of Ba(Ti1− yY y)O3 ( y=0.122) and Ba3Ti2YO8.5. The microstructure observation by scanning electron microscopy supports the XRD result. The dielectric behavior and phase transitions of the solid solutions ( x≤0.16) vary with different Y concentrations. The dielectric constant of the composites ( x>0.16) follows approximately the Lichteneker relation in a wide temperature range. [ABSTRACT FROM AUTHOR]

The crystalline structure, microstructure and dielectric properties of the (Sr1-1.5xBix)TiO3 (0 ≤ x ≤ 0.267) ceramics were studied. Cubic solid solutions were determined for x ≤ 0.2 at room temperature. However, lattice distortion was detected by Raman spectra. A dense microstructure with the grain sizes of 2-4 µm was obtained for (Sr1-1.5xBix)TiO3 (0 ≤ x ≤ 0.2) ceramics. The Bi concentration was examined and found to be in agreement with the nominal composition and overall uniformly distributed in the sample. Different from the observations in the earlier literature for other doped quantum paraelectrics, where only an induced dielectric anomaly was reported, there are three Bi induced dielectric modes A, B, and C in the Bi doped SrTiO3 samples. The occurrence of the "impurity modes" and the "ferroelectric relaxor mode" and their evolution are demonstrated as a function of Bi concentration. [ABSTRACT FROM AUTHOR]

The dielectric behavior and electrical conduction in Bi doped SrTiO3 annealed in both oxidizing (air and oxygen) and reducing atmospheres (nitrogen and “hydrogen+argon”) are studied in this letter. The dielectric constant and electrical conductivity are greatly enhanced in the sample annealed in a reducing atmosphere compared with that annealed in an oxidizing atmosphere. A Mott’s variable range hopping (VRH) conduction mechanism, σ=σ0 exp[-(T0/T)1/4], is observed in a strongly reduced sample, where a significant dielectric relaxation process is present. No direct correlation is found between the VRH conduction and the dielectric relaxation. The underlying physics of these results are briefly discussed. [ABSTRACT FROM AUTHOR]

The dielectric behavior and electrical conduction in Bi doped SrTiO3 annealed in both oxidizing (air and oxygen) and reducing atmospheres (nitrogen and “hydrogen+argon”) are studied in this letter. The dielectric constant and electrical conductivity are greatly enhanced in the sample annealed in a reducing atmosphere compared with that annealed in an oxidizing atmosphere. A Mott’s variable range hopping (VRH) conduction mechanism, σ=σ0 exp[-(T0/T)1/4], is observed in a strongly reduced sample, where a significant dielectric relaxation process is present. No direct correlation is found between the VRH conduction and the dielectric relaxation. The underlying physics of these results are briefly discussed. [ABSTRACT FROM AUTHOR]

The dielectric spectra of poly(vinylidene fluoride-trifluoroethylene) copolymers and poly(vinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene) terpolymers are studied in a wide temperature and frequency range with electric field. It is found that the dominant relaxation process displays a peculiar characteristic—a “dielectric relaxor” behavior, rather than a “ferroelectric relaxor” behavior reported in the current literature; furthermore, we show the experimental observation that the existence of nanometer scale crystals embedded in an amorphous matrix is the base of the dielectric relaxor behavior and high electroactive performance in these polymers. [ABSTRACT FROM AUTHOR]

The dc electric-field dependence of the dielectric properties of cadmium niobate Cd2Nb2O7 single crystals is studied, and characteristics of the dielectric behavior of Cd2Nb2O7 under dc bias are revealed. The temperature (Tm) where the dielectric relaxation mode occurs is independent of dc bias levels, while its intensity is greatly suppressed and finally eliminated with increasing dc bias from 0 to 15 kV/cm. The observed dielectric relaxation features of Cd2Nb2O7 can be attributed to “dielectric relaxor” behavior, rather than “ferroelectric relaxor” behavior previously designated in literature. The physical nature of the dielectric relaxor behavior is briefly discussed. [ABSTRACT FROM AUTHOR]

The dc electric-field dependence of the dielectric properties of cadmium niobate Cd2Nb2O7 single crystals is studied, and characteristics of the dielectric behavior of Cd2Nb2O7 under dc bias are revealed. The temperature (Tm) where the dielectric relaxation mode occurs is independent of dc bias levels, while its intensity is greatly suppressed and finally eliminated with increasing dc bias from 0 to 15 kV/cm. The observed dielectric relaxation features of Cd2Nb2O7 can be attributed to “dielectric relaxor” behavior, rather than “ferroelectric relaxor” behavior previously designated in literature. The physical nature of the dielectric relaxor behavior is briefly discussed. [ABSTRACT FROM AUTHOR]

The electrostrictive property and dielectric behavior of the stretched poly (vinylidene fluoride–trifluoroethylene) copolymers, denoted as P(VDF–TrFE), have been studied at cryogenic temperatures. Polarization hysteresis loops and high electrostrictive performance (strain ∼1%) are obtained at room temperature. An obvious dielectric relaxation process is observed and the relaxation rate follows the Vogel–Fulcher relation with T[sub VF]=160 K, U=0.18 eV, ν[sub 0]=5.7×10[sup 12] Hz. The dielectric relaxation behavior and high strain level decrease quickly with decreasing temperature, and below 160 K only a small strain level (∼0.1%) and an “intrinsic” dielectric constant sustains. The correlation of dielectric behavior and strain activity and the physical mechanism of the quick suppression of the strain with decreasing temperature are briefly discussed. The results indicate that a high dielectric constant is critical for obtaining high electrostrictive strains in the polymers at a reasonable electric field. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The direct-current (dc) electric-field dependence of dielectric constant around paraelectric-ferroelectric transition in Cd[sub 2]Nb[sub 2]O[sub 7] has been studied. The results show significant suppression of the dielectric constant by the application of dc bias. The electric-field dependence of the dielectric constant at the temperature around paraelectric-ferroelectric phase transition can be well described by the modified Devenshire relation including a cluster term, i.e., ε(E)=ε[sub 1]-ε[sub 2]E[sup 2]+ε[sub 3]E[sup 4]+(Px/ε[sub 0])[cosh(Ex)][sup -2]. The fit parameters indicate that the polar-cluster carries polarization P=∼0.5-2.5 mC/m[sup 2] with the cluster size of L=∼11-15 nm. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]