Calculation of the coordinate dependences of the effective pyroelectric coefficient under conditions of rectangular heat flux modulation with the use of digital methods for signal processing

A method is proposed for determining the coordinate dependences of the effective pyroelectric coefficient with the use of digital methods for signal processing. An expression is derived for calculating the pyroelectric coefficient under conditions of heat flux modulation with rectangular pulses. The proposed method makes it possible to determine the polarization state of bulk ferroelectric materials.     

Pyroelectric properties of PZT-type ferroelectric ceramics in the morphotropic phase-transition region

The temperature changes in the pyroelectric coefficient of the PZT-type materials synthesized in the morphotropic phase-transition region have been dynamically investigated under different polarization conditions. The temperature ranges of thermodynamic instability of the domain and phase structures after the “high-voltage” polarization have been revealed. The samples undergoing a phase transition into the ferroelectric state in an electric field exhibit enhanced thermal stability of domain structures.     

Thermally stimulated processes in samarium-modified lead titanate ferroelectric ceramics

The thermally stimulated processes in a samarium-modified lead titanate ferroelectric system are analyzed from the thermally stimulated depolarization discharge current. The discharge due to the space charge injected during the poling process, the pyroelectric response and a conduction process related to oxygen vacancies are evaluated considering a theoretical decomposition by using a numerical method. The pyroelectric response is separated from other components to evaluate the polarization behavior and some pyroelectric parameters. High remanent polarization, pyroelectric coefficient and merit figure values are obtained at room temperature.     

Effect of temperature on the polarization distribution in BTS functional gradient ceramics

The effect of the chemical composition gradient in barium titanate-stannate (BTS) piezoelectric ceramic on the depth distribution of the effective pyroelectric coefficient was investigated. A correlation between the coordinate dependence of polarization in the sample and the polarization temperature is demonstrated.     

电场中三层铁电超晶格的物理性质研究

基于微分算符技术的相关有效场理论研究用伊辛模型描述的铁电三层超晶格的物理性质。讨论了电场对电极矩、电极化率、热电系数等物理性质的影响。     

Electrical domain morphologies in compositionally graded ferroelectric films

We present a nonlinear thermodynamic formalism coupled with an electrostatic analysis of uniaxial n-layered compositionally graded heteroepitaxial ferroelectric films and extend this formalism to continuously graded ferroelectric films. We show that the domain morphology and its subsequent evolution in the presence of an electric field are determined by the spontaneous polarisation of the film induced through the compositional grading. The results for compositionally graded epitaxial (001) (Ba,Sr)TiO(3) and (001) Pb(Zr,Ti)O(3) films on (001)SrTiO(3) demonstrate that, while the domain morphologies in these two films are different in appearance, the dielectric displacement and the dielectric permittivity of such graded ferroelectric films exhibit a strong nonlinear behaviour which results in a high dielectric tunability. These findings indicate that it is possible to design specific domain structures that will yield desirable dielectric properties by controlling the strength of the compositional grading in the films.     

A new method for determining the coordinate dependences of the pyroelectric current in ferroelectric materials

A simplified method is proposed for determining the distribution of the effective pyroelectric coefficient over the depth of the sample. This method does not require a complex mathematical analysis involving solving ill-posed problems and makes it possible to determine the polarization state in the bulk of massive ferroelectric materials. The proposed method is tested with samples of the PZT commercial piezoceramic material.     

Ferroelectric properties of triglycine sulfate crystals with a nonuniform distribution of chromium impurities

The specific features of the ferroelectric behavior of triglycine sulfate crystals with a nonuniform distribution of chromium impurities are analyzed. It is demonstrated that the dielectric hysteresis loops of the samples exhibit large shifts both along the polarization and electric field directions. The results obtained are well explained within the phenomenological approach with due regard for the gradient term in the expansion of the free energy. It is established that the unipolarity coefficient and the pyroelectric signal of nonuniform crystals remain virtually unchanged in multiple heating-cooling cycles.     

The influence of structural and electrical parameters of the pyroelectric phase on the pyroelectric properties of a polymer-pyroelectric ceramic composite

The influence of structural and electrical parameters of the pyroelectric phase on the pyroelectric properties of composites based on poly(vinylidene fluoride), vinylidene fluoride-tetrafluoroethylene copolymer (F-42), and pyroelectric ceramics of the lead zirconate titanate family with different structures is investigated. It is revealed that the pyroelectric coefficient of the composite is determined by the reorientation polarization and the mobility of domains in the pyroelectric ceramic phase, which depend on the homogeneous parameter of the spontaneous strain of the perovskite cell.     

溶胶-凝胶法制备Pb(Zr1?xTix)O3复合梯度薄摸以及介电与热释电特性

"在Pt/Ti/SiO2/Si基片上用溶胶-凝胶法生长制备了PZT(Pb(Zr1-xTix)O3)复合梯度铁电薄膜． 薄膜最终结构由6层组成,"向上"梯度薄膜在Pt底电极上的第一层从PbZrO3开始,顶层是PZT(50/50),即第一层是PbZrO3,第二层PZT90/10 (10%Ti),第三层是PZT80/20,第四层PZT70/30,第五层PZT60/40,第六层PZT50/50．每一层与此相反的是"向下"梯度PZT薄膜．用X射线衍射、俄歇电子能谱和阻抗分析来研究梯度薄膜的结构与介电特性．600     

Theoretical studies on the pyroelectric properties of two component composite ferroelectric thin film

Using a Ginzburg–Landau–Devonshire type model, the pyroelectric property and hysteresis loop are studied for a composite ferroelectric thin film that contains an insertion of a different type of ferroelectric material in parallel connection with the host ferroelectric film material. If these two types of ferroelectric materials are polarized along the same direction, there are two peaks occur in the pyroelectric coefficient vs temperature curve. If the two ferroelectric materials are polarized in opposite directions, one positive and one negative coefficient peaks occur in the temperature dependence of pyroelectric coefficient curve and the hysteresis loop appears to be a quasi-double loop. The effect of the external electric field on the pyroelectric coefficient is also investigated.     

Ferroelectric square lattice described by the transverse Ising model with long-range interactions

A ferroelectric square lattice described by the transverse Ising model is studied by taking into account the long-range interactions. The size dependence of the Curie temperature as well as the polarization of the lattice is studied. Dielectric peaks and pyroelectric peaks are found in the edge area of the lattice which vary in position with the interaction range. It is found that the interaction range has a strong influence on the ferroelectric properties of the lattice.     

Effect of thermal phenomena on a second-order phase transition in the Landau-Ginzburg model

The influence of electrocaloric and pyroelectric effects on a phase transition in a ferroelectric material has been studied. The difference in the parameters of the Landau model for isothermal and adiabatic processes has been indicated. The temperature dependence of the spontaneous polarization is described by a special function (the probability integral) that results in the disappearance of the second-order phase transition.     

Pseudo-spin model of compositionally graded ferroelectrics

Recently, much attention has been focused on the study of compositionally graded ferroelectric films (CGFF). We devote this article to the microscopic theory of CGFF. Our theory is based on the transverse Ising pseudo-spin model. However the Hamiltonian used in recent papers does not contain the linear term in pseudo-spins. We show that it is necessary to modify the Ising Hamiltonian for homogenous material by addition of linear term in pseudo-spin. This term corresponds to the energy of the equivalent external field. Due to this term polarization is also included in the paraelectric phase.     

Long—Range Effects on the Pyroelectric Coefficient of Ferroelectric Superlattice

Long-range effects on the pyroelectric coefficient of a ferroelectric superlattice consisting of two different ferroelectric materials are investigated based on the transverse Ising model.The effects of the interfacial coupling and the thickness of one period on the pyroelectric coefficient of the ferroelectric superlattics are studied by taking into account the long-range interaction.It is found that with the increase of the strength of the long-range interaction,the pyroelectric coefficient decreases when the temperature is lower than the phase transition temperature;the mumber of the pyroelectric peaks decreases gradually and the phase transition temperature increases,It is also found that with the decrease of the interfacial coupling and the thickness of one period.the phase transition temperature and the number of the pyroelectric peaks decrease.     

Pyroelectric effect and spontaneous polarization in high-temperature ferroelectric LaBGeO5 (LBGO)

The temperature dependence of the pyroelectric coefficient of the new high-temperature ferroelectric LBGO in a single-domain state is determined. These data are used to reconstruct and describe the temperature dependence of the spontaneous polarization within the 50-to 540°C range. The possible existence of a second phase transition in this crystal is discussed.     

Pyroelectric coefficient manipulation in doped TGS crystals

Pure and l-alanine doped Triglycine sulphate (TGS) crystals were grown in paraelectric phase (∼52 °C). Doped crystals show unequal growth rates along the ferroelectric axis. Pure TGS crystals show peculiar dielectric behavior in the ferroelectric phase, after crossing up and down the Curie point in two successive runs between room temperature and 80 °C. Much higher and unstable permittivity was found returning in the ferroelectric phase. At constant temperature (35 °C), permittivity follows a relaxation process, characterized by two relaxation times. l-Alanine doped TGS crystal shows more than one order of magnitude smaller permittivity and dielectric losses. Internal bias field of ∼1 kV/cm, induced by the dopant, made the crystal almost monodomain and pined polarization in one direction. Pyroelectric coefficient measurements were performed at constant heating rate of the samples, using a computer controlled He cryostat and Keithley 6517 electrometer. The temperature dependence of P⁺ polarization component, obtained by computer integration of the pyroelectric coefficient, was measured on a large temperature interval (−20/+80 °C). Pyroelectric coefficient of the doped samples was also measured by the same procedure, using a dc bias electric field, pointing in the opposite direction to the pined polarization. The polarization could be reversed, on the whole temperature range, by dc fields higher than bias or coercive field. Surprisingly, for the first time, the pyroelectric coefficient (p) was found constant on quite large temperature intervals. Doped TGS crystals show much smaller values of permittivity ?_r versus the pure one and consequently, get higher figure of merit M = p/?_r. The pyroelectric coefficient of this material can be tailored to become constant on a defined temperature range, under a dc field control. This characteristic makes this material valuable to be used as pyroelectric material for IR devices.     

Depolarization field and properties of thin ferroelectric films with inclusion of the electrode effect

The influence of metallic electrodes on the properties of thin ferroelectric films is considered in the framework of the Ginzburg-Landau phenomenological theory. The contribution of the electrodes with different screening lengths l _s of carriers in the electrode material is included in the free-energy functional. The critical temperature T _cl , the critical thickness of the film, and the critical screening length of the electrode at which the ferroelectric phase transforms into the paraelectric phase are calculated. The Euler-Lagrange equation for the polarization P is solved by the direct variational method. The results demonstrate that the film properties can be calculated by minimizing the free energy, which has a standard form but involves the coefficient of the term P². This coefficient depends not only on the temperature but also on the film thickness, the surface and correlation effects, and the electrode characteristics. The calculations of the polarization, the dielectric susceptibility, the pyroelectric coefficient, and the depolarization field show that the ferroelectric state of the film can be destroyed using electrodes from a material whose screening length exceeds a critical value. This means that the electrodes being in operation can induce a transition from the ferroelectric phase to the paraelectric phase. The quantitative criteria obtained indicate that the phase state and properties of thin ferroelectric films can be controlled by choosing the appropriate electrode material.     

Change of sign in the pyroelectric coefficient of KDP at 15.3 K

Low temperature measurements of the charge released (or absorbed) by prepolarized crystalline samples of ferroelectric potasium dihydrogen phosphate clearly indicate that just above 3 K the samples absorb charge, (i.e. the spontaneous polarization increases slightly). After passing through a maximum, the samples begin to release charge at 15.3 K, implying a change of sign in the pyroelectric coefficient, a behavior which had been only observed previously in a few non-ferroelectric or polycrystalline pyroelectric materials.     

Domain pattern in ferroelectric triglycine sulphate using pyroelectric effect

A method to study domain structure in ferroelectrics, using pyroelectric effect is described. Variation of pyroelectric signal from the surface of a triglycine sulphate crystal plate has been studied by scanning the surface of the crystal with a low wattage He-Ne laser beam. The integrated pyroelectric signal is due to two components, namely, (1) the primary component arising out of the change in spontaneous polarization with temperature and (2) the delayed component arising out of the possible polarization reversal. The component of an electric field along the ferroelectric axis due to thermal hemisphere within the crystal plate formed by the laser beam has been calculated and shown to exceed coercive field, making polarization reversal possible. The delayed pyroelectric signal is a measure of polarization reversal within the patch illuminated and its observed variation over the surface yields information of the domain structure.