J X Zhang

RAMAN SCATTERING INVESTIGATION OF THE PHASE TRANSITION IN NANOCRYSTALLINE (PB, CA, LA)TIO3

Nanocrystalline (Pb0.95−xCa0.05Lax)Ti1−x/4O3 (PCLT) powders were prepared by the sol–gel technique. Doping-induced or grain size-induced structure phase transition was investigated by using Raman scattering and x-ray diffraction for the PCLT system. Experimental results show that the soft phonon mode E(TO1) displays a decrease in frequency but an increase in linewidth with increasing concentration of La. A discontinuous behavior of the phonon energy is associated with a phase transition from a ferroelectric tetragonal to a paraelectric cubic phase, and occurs at a morphotropic phase boundary of x≈0.30 for grain sizes being 20 nm. The dependence of Raman phonon modes on the size of particles has been analyzed for nanocrystalline (Pb0.88Ca0.04La0.08)Ti0.98O3 with grain sizes of 10–60 nm and a grain size-induced phase transition is observed for sizes below 14 nm.

An investigation of the migration of oxygen deficiencies in the superconductor YBa2Cu3O7δ

The temperature and oscillation frequency dependences of internal friction and shear modulus of superconducting YBa2Cu3O7- delta specimens are measured using a multifunction internal-friction torsion pendulum to study the degree of ordering and the migration of oxygen deficiency sites in the superconductor YBa2Cu3O7- delta . It is found that relaxation is most likely to occur in units where one oxygen site is surrounded by four barium ions rather than along the O-Cu-O chain. This indicates that an internal-friction study provides a powerful way of investigating the relaxation process of oxygen atoms as well as the degree of ordering of oxygen deficiencies in relation to superconductivity features.

Anelastic relaxation of oxygen vacancies and high-Tc superconductivity of YBa2Cu3O7-δ

It is suggested that the variation of the internal friction (IF) characteristics of a single-phase YBCO specimen resulting from an increase of oxygen deficiency δ is associated either with an order diffusion process between layers, or with an order/disorder diffusion process in the Cu(2) plane. The experimental results are explained in the light of these oxygen-vacancy migration processes. The essential connections between high-Tc superconductivity, electric behaviour and oxygen-vacancy ordering are discussed.

Thermal hysteresis scaling for first-order phase transitions

We study the effects of time-dependent thermal cycling on the first-order phase transition in cubic Ising systems with four-spin interactions by means of Monte Carlo simulations. The thermal hysteresis or the energy dissipation Q of a thermal cycle can be scaled with respect to the linear heating or cooling rate R: . We find the exponent b is independent of the interaction strength. in an SC lattice while in an FCC lattice and a compressible Ising lattice respectively. These simulation results are also compared with the kinetic Ising model and N-vector model in Langevin dynamics.

Hysteresis scaling of the field-driven first-order phase transition in the Ising model

Dynamical phase transitions in the Ising model on hypercubic lattices are considered. Under a linearly swept magnetic field, the hysteresis loop that characterizes the field-driven first-order phase transition is studied carefully. Using the Glauber dynamics, we find that, in the mean-field approximation, the energy dissipation of this phase transition or the hysteresis loop area A of the M-H curve can be scaled with respect to the sweep rate h of magnetic field in the form with a = 2 and b = 2/3. However, b varies when fluctuations and spin correlations are taken into account. Monte Carlo simulation is used to obtain the scaling relation for A in two-, three- and four-dimensional Ising models and we obtain the exponents and respectively. These exponents are obviously different from those obtained by scaling A as for any temperatures in Ising models under a sinusoidal field. Finally we point out that, in the concept of universality, field-driven first-order phase transitions in the Ising model in different dimensions belong to different universal classes due to the spin fluctuation and correlation below the Curie temperature.

The scanning-rate dependence of energy dissipation in first-order phase transition of solids

Abstract We propose that energy dissipation is an essential quantity in studying nonequilibrium first order phase transition driven by an external field, and the field scanning rate is a characteristic variable. The technique of differential scanning calorimetry (DSC) is applied to investigate the temperature scanning-rate dependence of energy dissipation ΔQ in first order structural phase transitions of RbNO3 powder, VO2 ceramics and Fe-16.5 wt% Mn alloy. We fit the experimental results to a general power law Δ Q = Δ Q 0 + A T n where ΔQ0 and A are two constants, and n is an exponent parameter.

Dynamic scaling of hysteresis in a linearly driven system

We investigate the scaling behaviour of hysteresis in three-dimensional continuum models in a linear driving mode. A non-conserving N-component vector Phi is the order parameter and its dynamics are specified by the time-dependent Ginzburg-Landau theory. The ( Phi 2)2 and ( Phi 2)3 models embody spatial fluctuations of Phi and their free-energy functionals have a ( Phi 2)2 and a ( Phi 2)3 interaction, respectively, with O(N) symmetry. On the other hand, the mean-field model ignores all spatial fluctuations of Phi . The area A of hysteresis loop, the energy dissipation per cycle, is taken as functions of the scanning rate R and temperature. For the ferromagnetic first-order phase transition (FOPT), a scaling relation A=aRn is determined where R is the field-sweeping rate, a is the coefficient and the exponent n approximately= 1/2 . We also discuss experimental evidence for real systems compatible with this result, which shows that the driving rate dependence of energy dissipation is important in non-equilibrium FOPT.

Growth and characterization of oriented Pb1−xCaxTiO3 thin films

Abstract Pb 1− x Ca x TiO 3 [PCT( x ), x =0∼20 mol%] crack-free thin films with various concentration of calcium were prepared on fused quartz, Si(100) and LaAiO 3 (100) substrates by a sol–gel technique. The perovskite phase of PCT films was formed at approximately 500°C, the films grown on fused quartz and Si(100) had a polycrystalline tetragonal structure, and the films grown on LaAiO 3 (100) substrate were highly c -axis oriented. The tetragonal factor ( c / a ) decreased with increasing Ca concentration. The optical transmission properties of the PCT films on fused quartz annealed at 500°C for 1 h with various thicknesses were measured in the wavelength range 190–900 nm. The thin films had good optical transmissitivity. The band gaps value decreased with increasing thin films thickness.

Synthesis and structure of nanocrystalline oxides based on PbTiO3 by sol-gel process

Abstract Nanocrystalline powders of PbTiO3, (Pb0.92La0.08)Ti0.98O3 and (Pb0.95 − x Ca0.05Lax)Ti 1 − x 4 O3 (0.05 ≤ x ≤ 0.25) have been prepared by a sol-gel method. Thermal analysis was used to analyze the thermal decomposition of the organic ligands of the dried gel. These powders were characterized by X-ray powder diffractometer, transmission electron microscopy and infrared spectroscopy. The effects of increasing substitution on the phase formation are discussed.

Internal friction study of transformation dynamics in BaTiO3 ceramics

Abstract The internal friction (IF) and relative change of modulus of a doped semiconducting polycrystalline BaTiO 3 sample were measured by an inverted torsion pendulum with forced oscillation as the sample was heated in vacuum from −150°C to 100°C. Two IF peaks, P 1 and P 2 , and two modulus, minima C 3v / C 2v and C 2v / C 4v , were observed corresponding to two phase transformations. All the IF peaks exhibit the characteristics of the first-order phase transformations. Each peak temperature is independent of frequency and the peak height increases with an increasing value of ( T /ω) n . A coupling coefficient of the phase interface to the oscillating stress is found to be α ( ω )= α ′ ω −1 . For the two peaks P 1 and P 2 in the doped BaTiO 3 specimen, the values of n and l have been determined to be n 1 = 0.225, n 2 = 0.190, l 1 = 0.101, l 2 = 0.204. Our study indicates that the IF arises from the motion of phase interfaces during the process of the first-order phase transformation. A dynamic interaction model is proposed to explain the special features of IF during the two phase transformations.