铁电体中偶极子的滞后对剩余极化的影响

铁电体的剩余极化强度随温度降低而下降的特性引起了人们对铁电体存储数据失效的担心.运用铁电体的唯象理论和偶极子对交变电场的响应,提出了在电滞回线测量中偶极子的滞后冷冻效应模型,对极化的低温退化现象做了合理解释:温度下降导致吉布斯自由能势垒增大,致使偶极子对交变电场的响应时间延长.引入响应的滞后因子发现,极化强度随温度降低会出现峰值,在低温下降直至为零,可用偶极子的滞后与冻结效应描述.详细研究结果表明:因材料组份变化导致热力学参量的变化是重要因素:铁电-顺电相变中软模系数的增大会导致剩余极化峰移向高温;铁电性的增强,温度极化系数的增大和耐压强度或饱和电场的增强均会抑制滞后效应,从而使低温滞后效应移向低温.运用导出的公式数值模拟Ba Ti O_3/Bi Sc O_3复合陶瓷剩余极化强度的实验结果发现,Bi Sc O3含量的增加,使居里温度略有减小,但导致了软模系数较大幅度的增加,其结果是使偶极子的滞后效应发生在较高的温度.软模系数与铁电体的极化特性、铁电性、介电性和力学性均密切相关.研究结论表明:在低温下铁电体的铁电性没有失效,偶极子的低温冻结效应更有利于铁电体长久地保存数据.

Effect of hysteresis of dipole on remnant polarization in ferroelectrics

Decrease in remnant polarization at lower temperature, or low temperature degradation of polarization, in some ferroelectrics has attracted much attention. To investigate the mechanism of the decrease, phenomenological theory of ferroelectrics and the relevant mechanism of dipole in alternating electric field are used to develop a model of hysteresis-frozen effect of dipole in electric hysteresis loop measurement. Within the frame of Landau-Ginzburg-Devonshire theory, Ising model is used to derive the relationship among remnant polarization, coercive field, and saturated polarization strength. Then, two aspects are investigated: response of a dipole and thermodynamic properties of ferroelectric. Response of a dipole in an electric field is often described by relaxation time, on the assumption that Debye equation is satisfied. Potential barrier in the Debye equation is the Gibbs free energy barrier from one ferroelectric state, +P, to another ferroelectric state, -P. Increase in the Gibbs free energy barrier with temperature decreasing will prolong the relaxation time. As ferroelectrics can be taken as a capacitor, first order response function is used to introduce a hysteresis factor with measuring frequency and relaxation time into the expression of remnant polarization. In the aspect of thermodynamic properties of ferroelectric, the variation of compositions is a significant reason. In numerical simulation based on the derived formula the remnant polarization exhibits a frequency related peak, and shift of the peak depends on some other reasons: the increase of soft-mode coefficient in phase transition shifts the peak towards high temperature; the increases of coercive field, temperature-polarization coefficient (a concept defined in the present paper to indicate increase in polarization with increasing temperature) and saturated electric field shift the peak toward low temperature. Compared with the reported experimental results of BaTiO₃/BiScO₃ compound ceramics, the results show a good coincidence with numerical simulations. The parameter values of numerical simulation indicate that a large shift toward high temperature in peak of remnant polarization with increasing BiScO₃ composition ratio is due to the increase in soft-mode coefficient with only small decrease in the Curie temperature. The soft-mode coefficient and temperature-polarization coefficient are closely related to polarization characteristic, ferroelectric, dielectric and mechanical properties. Therefore, the decrease in remnant polarization at low temperatures, ascribed to the hysteresis of dipole to a constant measuring frequency, may have an influence on changes in various properties, but freezing effect of dipole at low temperature can help ferroelectrics to save data longer.