Dielectric and ferroelectric properties of NBT-BT systems

ABSTRACT

The (1-x)Na_0.5Bi_0.5TiO₃- xBaTiO₃ ceramics (x = 0.1, 0.135 and 0.17) were fabricated by a conventional solid phase sintering process. The bulk density of the obtained samples exceeded 95% of the theoretical relative density as determined by Archimedes method. Dielectric and ferroelectric measurements of these ceramics were performed. Measurements of the ferroelectric properties show that, above the depolarization temperature T_d, the shape of the hysteresis loops approaches that of linear dielectrics. The dielectric study results correlate with the hysteresis loops measurements. The relaxor-like behavior of the investigated materials was revealed.     

铁电体电源中介质电阻特性及对输出电荷的影响

爆炸驱动铁电体脉冲电源利用铁电陶瓷在冲击压力作用下去极化释放电荷而产生电流,可以作为脉冲功率源的初始电源,也可直接驱动高阻抗负载产生脉冲高电压。通常情况下,铁电陶瓷可以看作理想的绝缘体,但在数GPa冲击波压力作用下,铁电陶瓷电阻率可能会明显下降并形成漏电导,使部分去极化释放电荷在铁电陶瓷内部流失,导致铁电陶瓷剩余极化电荷输出效率下降。以PZT95/5铁电陶瓷作为初始储能介质,以爆炸冲击波加载PZT95/5铁电陶瓷释放电荷对脉冲电容器充电,充电结束后电容器电压维持期间检测到明显的反向电流,根据铁电陶瓷输出电流和工作电压,得到冲击波作用过程中铁电陶瓷的瞬态电阻率曲线,并分析了电阻率下降对输出电荷的影响。进一步研究表明,冲击压力在铁电陶瓷边侧产生的稀疏波是引起电荷输出效率降低的主要因素,而铁电陶瓷电阻率下降对电荷输出效率的影响很小。     

晶粒尺寸及衬底应力对铁电薄膜特性的影响

考虑衬底应力、畴壁运动和畴结构变化, 建立了修正的Landau-Devonshire热力学模型, 计算了生长在不同衬底上的含有纳米晶粒的PbZr_0.4Ti_0.6O₃(PZT)薄膜的电滞回线, 研究了矫顽场、剩余极化强度和相对介电常数对晶粒尺寸以及薄膜厚度的依赖关系. 结果表明, 矫顽场和相对介电常数对晶粒尺寸的依赖关系呈类抛物线状;衬底压应力使矫顽场和剩余极化强度增大, 使相对介电常数减小;随着厚度增加, 矫顽场先缓慢增加, 到200 nm 关键词： 铁电体 晶粒尺寸 衬底应力 薄膜厚度     

Electret effect in heterogeneous systems comprising low-dimensional particles with electrically active surface

Experimental and theoretical investigations of the electret effect in low-dimensional micas with different degrees of dispersion and adsorption are carried out at temperatures in the range 20–140°C. The results obtained allow the electric activity of the mica particles comprised in these systems to be determined and the efficiency of application of low-dimensional micas for fillers of composite materials to be estimated.     

Enhancement of dielectric and ferroelectric properties in ferroelectric superlattices

I studied theoretically the enhancement of remanent polarization and dielectric permittivity of interfacial-coupled ferroelectric superlattices based on the Landau–Ginzburg theory. Our model adopts the Landau–Khalatnikov equation to describe hysteresis behavior and takes the time-dependent space-charge-limited conductivity into account to investigate the ferroelectric and dielectric properties of ferroelectric superlattices. The results are in good agreement with recent experimental observations on the enhancement of remanent polarization and permittivity of BaTiO₃/SrTiO₃ superlattices and heterolayered Pb(Zr,Ti)O₃ thin films.     

Low-dimensional systems: Quantum size effects and electronic properties of semiconductor microcrystallites (zero-dimensional systems) and some quasi-two-dimensional systems

This review is concerned with quantum confinement effects in low-dimensional semiconductor systems. The emphasis is on the optical properties, including luminescence, of nanometre-sized microcrystallites, also referred to as zerodimensional systems. There is some discussion on certain of the two-dimensional systems, such as thin films and layer structures. The increase in energy of excitation peaks (blue shift) as the radius R of a microcrystallite is reduced is treated theoretically, and experimental data when they are available are used to assess the reliability of the different models that have been used. These experiments normally make use of microcrystallites dispersed in a large-bandgap matrix such as glass, rocksalt, polymers, zeolites or liquids. Exciton binding energies E b are larger than for bulk semiconductors, and oscillator strengths are higher for the microcrystallites. The regimes of direct interest are as follows. Firstly there is the so-called weak-confinement regime where R is greater than the bulk exciton Bohr radius a B . Experimentally, semiconductors such as CuCl with a B , 7 Å, are suitable for study in this case. Secondly there is the moderate-confinement regime, where R , a B , and a h < R < a e , a h and a e being the hole and electron Bohr radii, respectively. Finally there is the strong-confinement regime, with R < a B , and R < a h , a e . For this case we are concerned with a ladder of discrete energy levels, as in molecular systems, rather than energy bands. The electrons and holes are treated as independent particles, and for excited states we refer to electron-hole pairs rather than excitons. Suitable materials for investigation in this regime are the II-VI semiconductors, and also GaAs and Ge, for which a B is relatively large. Although a number of different theoretical models have been used, none can be described as completely first-principles calculations, and there is room for improvement on this aspect. However, useful expressions have been developed by Brus and by Lippens and Lannoo, giving the energy of excited states as a function of R , in terms of the bulk energy gap, kinetic energy, Coulomb energy and correlation energy. Other phenomena discussed are firstly biexciton formation by the use of high intensity laser beams and secondly nonlinear optical effects. Strong nonlinearities and short decay times for excited states have been predicted, and the models developed cover both the resonant and the non-resonant cases. The possibility of using microcrystallites embedded at reasonable concentrations in a glass matrix in the field of optical communications and optical switching is also considered.     

The role of kinetic effects in voltammetry studies of intercalation systems

The role of kinetic effects in voltammetry studies of intercalation systems is examined. These effects disguise features arising from the thermodynamic properties, such as from coexisting phases. It is shown that a quantitative analysis of the phase diagram of the intercalation system is nevertheless possible. The results are applied to Li_xVS₂.     

The effects of uniaxial stress distribution on the ferroelectric properties of thin films with first-order phase transition

Based on the Landau theory, the first-order phase transition properties of ferroelectric thin films have been studied by taking into account uniaxial stress distribution effects. The stress is supposed to decrease from interface to surface exponentially according to the experimental results. It is shown that tensile stress decreases the polarization and the Curie temperature while compressive stress increases the polarization and the Curie temperature. A stress-driven phase transition is found at the critical stress. Our prediction is compared with the available experiment results.     

Finite size effects in stochastically driven systems use of the Poisson representation

A method is presented to take into account finite size effects in a system under the influence of external noise. Inclusion of external noise in a master equation results in an effective master equation in which new transitions among states are possible. The steady state properties of a chemical systems are calculated using the Poisson representation of the master equation.     

Size effects in ferroelectric nanocones

Ferroelectric size effects in cone-shaped nanoparticles (nanocones) are investigated for the first time. The Euler-Lagrange equations are solved using the direct variational technique. An approximate analytical expression is derived for the dependence of the ferroelectric phase transition temperature on the nanocone size. It is shown that the transition temperature for nanocones can be 2.5 times higher than the transition temperature for bulk materials.     

Finite size effects in epidemic spreading: the problem of overpopulated systems

In this paper we analyze the impact of network size on the dynamics of epidemic spreading. In particular, we investigate the pace of infection in overpopulated systems. In order to do that, we design a model for epidemic spreading on a finite complex network with a restriction to at most one contamination per time step, which can serve as a model for sexually transmitted diseases spreading in some student communes. Because of the highly discrete character of the process, the analysis cannot use the continuous approximation widely exploited for most models. Using a discrete approach, we investigate the epidemic threshold and the quasi-stationary distribution. The main results are two theorems about the mixing time for the process: it scales like the logarithm of the network size and it is proportional to the inverse of the distance from the epidemic threshold.     

Influence of lateral size on dielectric properties of ferroelectric thin films with structure transition zones

By taking into account structural transition zones near the lateral and thickness direction edges, this paper uses a modified transverse Ising model to study dielectric properties of a finite size ferroelectric thin film in the framework of the mean-field approximation. The results indicate that the influence of the lateral size on the dielectric susceptibility cannot be neglected and lateral structural transition zones could be a crucial factor that improves the mean susceptibility of the fixed size film.     

Processing effects on the microstructure and ferroelectric properties of strontium bismuth tantalate thin films

Ferroelectric thin films of strontium bismuth tantalate compositions with Bi/Sr ratios of 2.75 and 2.50 have been produced by deposition of solutions derived from sol-gels followed by crystallization using rapid thermal processing (RTP) at 650 °C. Single-step and two-step processes have been used for the RTP crystallization of the films. Both the composition and the heating process used affect the grain size, grain shape and compositional heterogeneity of the films, acting on their ferroelectric switching behaviour and dielectric properties. Larger and more elongated grains are obtained by crystallization using a single-step process. These films exhibit less slanted hysteresis loops. The combination of the study of the pure-ferroelectric-switching loop and the lambda curves through the application of a Curie–Weiss-like model gives a deeper insight into the influence of film heterogeneity on its ferroelectric properties. The observation of net polarisation without poling in all the films is attributed to the effects of 180° ferroelectric domains. Received: 16 May 2001 / Accepted: 27 October 2001 / Published online: 20 March 2002     

Collective effects in the transport properties of two-dimensional electron systems

The results of the experimental study of the nontraditional manifestations of collective effects in the transport properties of two-dimensional electron systems are reported.     

The influence of defect site statistics on ferroelectric properties

Averaged Green's functions are determined for lattices containing point defects with given site-correlations by extending existing averaging procedures. The anharmonicity of ferroelectric crystals is treated in SCPA. The results show a strong influence of the correlations on ferroelectric properties. For comparison MD calculations were performed which show a good agreement with the analytic approximations.     

Dielectric losses in heterogeneous systems

Analytic formulas are derived for the effective complex dielectric constant of a matrix system with Maxwell-Wagner losses. An approximate solution is given for a system of dielectrics with a single relaxation time. It is shown that this approximation is applicable to systems containing a semiconductor with blocking electrodes. The effect of spread in the dimensions of the semiconductor component on the frequency dependence of losses in a matrix system is discussed.     

Specific features of the dielectric response of heterogeneous systems with a polar matrix containing electrically active inclusions

A theoretical model has been proposed which makes it possible to introduce the correction into the complex permittivity of electrically active condensed systems that contain a polar liquid matrix and low-dimensional solid particles with a developed electrically active surface. It has been established that there is an interfacial electrical interaction between surface charges of the solid component and polar molecules of the liquid matrix. The processes occurring during this interaction lead to the appearance of an intrinsic internal electric field in the system under investigation. The contribution from surface effects to the formation of the polarized state of the polar liquid medium has been investigated in terms of the proposed model. The possibility of controlling the processes of local change in the structure of the polar liquid component under the effect of the internal electric field by varying the electrically active specific surface area of the solid phase and the intrinsic dipole moment of molecules of the liquid matrix has been analyzed in the studied systems. The conditions providing for the appearance of a controlled potential gradient of the internal electric field have been determined for the dispersed systems under consideration. The parameter that makes it possible to evaluate the contribution from the interfacial electrical contact interaction to the dielectric response of the liquid component and the entire system has been introduced for the first time.     

The size of finite size effects in lattice gauge theories

If a quantum field is enclosed in a spatial box of finite volume, its mass spectrum depends on the box size L. For field theories in the continuum Lüscher has shown to all orders in perturbation theory that for large L this dependence is related to certain scattering amplitudes of the infinite volume theory. We derived the corresponding relations for lattice field theories. Assuming their validity for lattice gauge theory outside the perturbative region the magnitude of finite size effects on the spectrum is determined by a glueball coupling constant. This quantity is estimated by strong coupling methods.     

Transport properties of a ferroelectric tunnel junction in bilayer ferroelectric/manganite structures

This paper presents the results of an experimental investigation of current flow through a ferroelectric nanolayer in a series of bilayer ferroelectric/manganite structures prepared by metalorganic aerosol deposition on a MgO substrate. It has been shown that a variation in the thickness of the ferroelectric layer in a bilayer ferroelectric/manganite structure leads to a change in the metal-insulator phase transition temperature of the manganite layer and also affects the transport properties of the ferroelectric/manganite structure. Based on the analysis of the experimental results, it is demonstrated that the ferroelectric layer up to 4 nm thick exhibits polarization properties. In the theoretical analysis, it is assumed that electrical conduction of bilayer structures is provided by a dual mechanism combining the electron tunneling through a ferroelectric barrier and the ohmic current flow in a manganite layer. The possibility of retaining the mechanism of charge carrier tunneling in a bilayer ferroelectric/manganite structure through the ferroelectric layer up to 6 nm thick is discussed.