M. L. Zhao

Dielectric dispersion of CaCu3Ti4O12 ceramics at high temperatures

CaCu3Ti4O12 ceramics has been prepared by the conventional solid-state reaction method under different sintering conditions. Dielectric properties have been investigated in the temperature range of 25–350°C within the frequency range of 40Hz–10MHz. It has been found that both the dielectric constants and the grain sizes increase with the sintering time. A relaxation has been observed in the frequency range of 100Hz–100kHz, apart from the one already known in the frequency range higher than 100kHz. High-temperature dielectric dispersion shows one large low-frequency response and two Debye-type relaxations. Activation energy values of these two relaxations are 0.084 and 0.678eV, respectively. An equivalent circuit model is suggested, which well explains the dielectric dispersion.

Dielectric relaxor properties of K0.5Bi0.5TiO3 ferroelectrics prepared by sol–gel method

Lead-free potassium–bismuth titanate, K0.5Bi0.5TiO3 (KBT), ferroelectric ceramics were fabricated from the natural sintering of powders prepared by the thermal decomposition of adequate precursor solutions. Their crystal structure was determined by x-ray diffraction, and the temperature dependence of dielectric constants were measured. The results show that KBT may be a kind of order–disorder relaxor ferroelectric with a first-order phase transition below the temperature of dielectric constant maximum.

High permittivity and low dielectric loss in ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12

Ceramics with the nominal compositions of CaCu3−xLa2x∕3Ti4O12 were prepared by the conventional solid-state reaction technique. It was found that these ceramics have the properties of rather high dielectric permittivity and sufficiently low dielectric loss. In particular, CaCu2.9La0.2∕3Ti4O12 ceramics sintered at 1050°C for 20h showed the high dielectric permittivity of 7500 with weak frequency dependence below 1MHz, the low dielectric loss less than 0.05 in the wide frequency range of 120Hz–200kHz (with the minimum 0.012 at 20kHz), and the small temperature coefficient about ±15ppm∕°C between −80 and 125°C. From the analysis, it was suggested that CaTiO3 secondary phase due to Cu deficiency and La doping plays the important roles in the observed excellent dielectric properties in CaCu3−xLa2x∕3Ti4O12 ceramics. The results indicated that they are a very promising class of high-permittivity materials for practical applications.

A study of the vibrational and thermoelectric properties of silicon type I and II clathrates

First principles calculations are employed to investigate both type I and II silicon clathrates. The phonon and infrared spectra of both types of clathrate are obtained. We find a localized vibrational mode of Si46 whose frequency is 375.0 cm−1, where the vibration in the mode localizes in the hexagonal chain. The heat capacity of both clathrates is the same as that of the diamond phase Si (d-Si). When the temperature is lower than 100 K, the Debye temperatures of the clathrates are higher than that of d-Si; however, the Debye temperatures of both clathrates at high temperature (>100 K) are lower than that of the d-Si. The mean free paths (λ) and thermal conductivities (κ) of the clathrates are larger than those of d-Si at low temperature. The Seebeck coefficients (S) of the clathrates are higher than that of d-Si in the temperature interval 300–1000 K; however, both clathrates exhibit a lower value of σ/τ when compared to the d-Si.

Transient electro-osmotic flow of generalized Maxwell fluids in a straight pipe of circular cross section

The transient electro-osmotic flow of a generalized Maxwell fluid with fractional derivative in a narrow capillary tube is examined. With the help of an integral transform method, analytical expressions are derived for the electric potential and transient velocity profile by solving the linearized Poisson-Boltzmann equation and the Navier-Stokes equation. It was shown that the distribution and establishment of the velocity consists of two parts, the steady part and the unsteady one. The effects of relaxation time, fractional derivative parameter, and the Debye-Hückel parameter on the generation of flow are shown graphically and analyzed numerically. The velocity overshoot and oscillation are observed and discussed.

Piezoelectricity and local structural distortions in (Na0.5Bi0.5)1−xSrxTiO3-Bi12TiO20 flexoelectric-type polar ceramics

We have previously described sintered Na0.5Bi0.5TiO3-Bi12TiO20 composites as flexoelectric-type polar ceramics because they have a net macroscopic flexoelectric polarization. Here, we report on the universal existence of the macroscopic flexoelectric polarization in the (Na0.5Bi0.5)1−xSrxTiO3-Bi12TiO20 system, in which enhanced piezoelectricity is observed. By combining Raman spectroscopy and x-ray photoelectron spectroscopy techniques, we have identified the local flexoelectric polarization as distorted BiO5 polyhedra and TiO6 octahedra in the SrTiO3-Bi12TiO20 ceramic. The macroscopic polarization may be due to the partial alignment of these distorted units located within the grain boundary amorphous phases. Bi12TiO20 could have an important role in these flexoelectric-type polar ceramics.

Vibrational and thermal properties of small diameter silicon nanowires

We present the results of vibrational and thermal properties for small diameter silicon nanowires (Si-NWs) from first principles calculations. Phonon spectrums of the Si-NWs are obtained based on the density functional perturbation theory. We found that heat-carrying acoustic branches exhibit “bending,” which results from the strong interaction between acoustic and no-zero-frequency flexural modes. The bending of acoustic branches implies that the phonon group velocity (V=dω/dq) of Si-NWs is less than that of corresponding bulk silicon. Therefore, a lower lattice thermal conductivity of Si-NWs can be caused by the bending of acoustic phonon. In comparison with bulk silicon, optical branches of Si-NWs exhibit “blueshift,” which is due to the high frequency vibration of silicon atoms at the edge of Si-NWs. From the obtained phonon spectrums, specific heat is calculated. The specific heat of Si-NWs is also lower than that of bulk silicon crystal. The reduction in the specific heat is due to the small magnitu...

Interface-induced piezoelectricity in an unpoled Na0.5Bi0.5TiO3-based composite ceramic

Composite ceramics containing Bi12TiO20 and Na0.5Bi0.5TiO3 grains were fabricated by an ordinary sintering technique. An anomalous piezoelectric response consistent with triclinic symmetry was found in unpoled samples and could be detected up to at least 765 °C. This piezoelectric effect appears concurrently with macroscopic cellular interfaces in as-sintered ceramics, suggesting that it may originate from an interface effect. Since these kinds of composites also have near-zero temperature coefficients of the resonance frequencies over the temperature range from 400 to 520 °C, they have potential as high-temperature piezoelectric materials.

Two methods to solve a fractional single phase moving boundary problem

A moving boundary problem of a melting problem is considered in this study. A mathematical model using the Caputo fractional derivative heat equation is proposed in the paper. Since moving boundary problems are difficult to solve for the exact solution, two methods are presented to approximate the evolution of the temperature. To simplify the computation, a similarity variable is adopted in order to reduce the partial differential equations to ordinary ones.

Piezoelectric properties and structural phase transitions of naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal

Electrical properties of a naturally polarized (Na0.75K0.25Bi)0.5TiO3 crystal have been investigated over a broad range of temperatures. Noticeable piezoelectric resonance signals have been observed up to 540°C. Based on the piezoelectric measurement, we suggest a R3c-P4bm-Pm3¯m sequence of structural phase transitions in this crystal. Moreover, at temperatures above 520°C, mobile sodium ions in the crystal are proposed to contribute to the electrical conduction; they have an activation energy of 2.06eV. This migration of Na+ at high temperatures can explain the naturally generated piezoelectricity in (Na0.75K0.25Bi)0.5TiO3 crystal.