Electric fatigue of (BaPb)TiO3 ferroelectric ceramics

The electric fatigue of (BaPb)TiO₃ ferroelectric ceramics was studied in present experiments. The results show that the resistance of the ferroelectric ceramics increases with the fatigue cycles, and the resistivity change increases with the Curie temperature. The SEM observations reveal that microcracks occurred inside the ceramics; electrode ions were transported into the ceramics; segregation of the ceramic element took place and sometimes new phases occurred near the electrode during electric fatigue.     

Development of ferroelectric oxides based resistive switching materials

Resistive random access memory (RRAM) is one of the most promising candidates that satisfies the requirements of new generation non-volatile memories, as a consequence of its high density, outstanding scalability, and low power consumption. The review is based on a summary of recent studies in ferroelectric oxides based resistive switching (RS) materials and devices. It highlights the various ferroelectric oxide materials with RS behaviour and the underlying mechanisms including filament-type and interface-type mechanism. In the end, the challenge in current RRAM for future high-density data storage applications is addressed.     

Cation ordering in complex oxides

Several recent papers have addressed the fundamental aspects of the stability and kinetics of ordering in complex oxides, and investigated systems where the properties are mediated by the degree of order. Cation ordering reactions have been shown to induce large alterations in the dielectric, ferroelectric, magnetic, and electronic response of many complex oxides. The majority of the cited publications focus on ion ordering in perovskite-related structures; however, additional examples include the ordering of structural slabs and formation of modulated intergrowth structures.     

有机铁电P(VDF-TrFE)薄膜极化疲劳特性

本文研究了有机铁电薄膜的极化疲劳过程.研究发现,随着疲劳过程的深入,薄膜铁电峰的峰位逐渐向高电压方向偏移,而薄膜的极化-电压滞洄线也逐渐变得圆滑,且剩余极化值随之下降.进一步的研究表明,只有双极性电压才能导致铁电疲劳的出现,而双极性电压的频率直接影响铁电疲劳速率:频率越高,疲劳越慢.文中对有机铁电疲劳机理做了初步探讨.     

Structural visualization of polarization fatigue in epitaxial ferroelectric oxide devices

Ferroelectric oxides, such as Pb(Zr,Ti)O(3), are useful for electronic and photonic devices because of their ability to retain two stable polarization states, which can form the basis for memory and logic circuitry. Requirements for long-term operation of practical devices such as non-volatile RAM (random access memory) include consistent polarization switching over many (more than 10(12)) cycles of the applied electric field, which represents a major challenge. As switching is largely controlled by the motion and pinning of domain walls, it is necessary to develop suitable tools that can directly probe the ferroelectric domain structures in operating devices-thin-film structures with electrical contacts. A recently developed synchrotron X-ray microdiffraction technique complements existing microscopic probes, and allows us to visualize directly the evolution of polarization domains in ferroelectric devices, through metal or oxide electrodes, and with submicrometre spatial resolution. The images reveal two regimes of fatigue, depending on the magnitude of the electric field pulses driving the device: a low-field regime in which fatigue can be reversed with higher electric field pulses, and a regime at very high electric fields in which there is a non-reversible crystallographic relaxation of the epitaxial ferroelectric film.     

Nanostructure patterns of piezoelectric and ferroelectric complex oxides with various shapes, obtained by natural lithography and pulsed laser deposition

A variety of patterns of well-ordered perovskite-type complex oxide nanostructures were obtained by pulsed laser deposition (PLD) on conductive single crystalline substrates through latex sphere monolayer and double-layer masks. Patterns obtained via deposition through hexagonally close packed (hcp) latex sphere monolayers include the usual arrays of hexagonally ordered nanoislands, hexagonal arrays of nanorings of much lower height coexisting with hexagonal nanoislands, hcp shell-like structures, and hexagonally interconnected nanocrown arrays. The formation of the nanopatterns depends on the ambient gas pressure during PLD, with well-separated nanoislands obtained at low pressures, and interconnected nanocrowns formed at high pressures. After annealing, the obtained BaTiO(3) and SrBi(2)Ta(2)O(9) nanopatterns were examined by piezoresponse force microscopy using conductive scanning probes. Under an external biasing electric field, polarization domain reversal was observed in the nanopatterns with deep submicron lateral dimensions. BaTiO(3) nanocrowns ～9?nm high are ferroelectric, while SrBi(2)Ta(2)O(9) nanorings of ～5?nm thickness show at least piezoelectric activity. In addition to the patterns of hexagonally ordered nanostructure arrays, a pattern of rhombohedrally ordered nanostructure arrays was also fabricated by deposition through latex sphere double layers.     

铁电薄膜电疲劳研究进展

铁电薄膜在交变电场下发生电疲劳现象是影响其商业应用的主要障碍，也是目前国内外研究的热点。本文从铁电薄膜电疲劳的影响因素、疲劳机理和消除疲劳的措施出发综述了近年来国内外在铁电薄膜电疲劳研究方面取得的进展。     

An accumulation damage model for fatigue fracture of ferroelectric ceramics

The accumulated plastic displacement criterion for crack propagation in traditional materials is extended to develop equations to predict the fatigue crack growth of ferroelectric ceramics subjected to combined electromechanical loads. The crack-line is perpendicular to the poling direction of the medium. An electric saturation zone and a stress saturation zone are assumed to develop at the crack tips when the medium is subjected to external electromechanical loads. This assumption makes it possible to obtain the accumulated plastic deformation in closed form. A fatigue crack growth law, which is a fourth-power function of the effective stress intensity factor, similar to the well-known Paris law, is derived. Graphical results for the effect of electric load on the effective crack tip stress intensity factor and crack growth rate are provided.     

铁电薄膜极化反转电流的电场和温度特性研究

应用铁电薄膜极化反转的KA模型,讨论了极化反转电流随电场和温度的变化关系.结果表明,极化反转电流随电场的增加而增加,极化反转时间在低电场时成指数关系下降,高电场时幂律关系下降;在居里温度附近,极化反转电流随温度增加而减小,温度远低于居里温度时,随温度增加而增加.     

Metal organic chemical vapor deposition (MOCVD) of oxides and ferroelectric materials

The electrical properties of thin-film oxides span a tremendous range from insulating to superconducting. As a consequence, they are now finding an increasing number of applications in electronics and opto-electronics, both as stand-alone layers and for the provision of added functionality to electronic circuits, especially silicon. The modified precursor delivery technologies (e.g. aerosol or liquid delivery or injection techniques) coupled with improved precursors, and better engineering of the deposition kit, have transformed the prospects for the deposition of these materials by MOCVD. It is now possible to exploit the many traditional advantages of MOCVD, especially over other routes to thin-film oxide deposition. A wide range of oxides and particularly complex ferroelectric oxides are being produced on substrates of up to 200 mm in diameter. This paper highlights the progress and real advantages that MOCVD has for thin-film oxide deposition, including the design of improved precursors. Examples of the deposition of dielectric layers and lead-based ferroelectric layers for uncooled thermal imaging, and MEMS actuation applications are presented.     

Effects of porosity on electric fatigue behaviour in PLZT and PZT ferroelectric ceramics

Electric fatigue, namely the decay of the polarization and the consequent elastic strain with increased number of switching cycles under high a.c. field, severely limits the applications of ferroelectric and piezoelectric materials in high-strain electro-mechanical actuators and in thin films used in non-volatile memory devices. Electric fatigue tests have been conducted on lead zirconate titanate (PZT) and lanthanum-doped lead zirconate titanate (PLZT) ferroelectric ceramics. It was found that electric fatigue can be initiated by various factors, the porosity being one of them. Electric fatigue occurred in low-density(93%–97%) PLZT 7/65/35 ceramics after 10⁴ switching cycles, while the high-density (>99%) PLZT specimens of the same composition did not fatigue after 10⁹ switching cycles. It was also observed that for PZT ceramics, fatigue proceeded much more slowly in the samples with higher density (～98%) than those with lower densities (92%–96%). A tentative explanation for the origin of the fatigue mechanism associated with porosity is proposed.     

复合氧化物负热膨胀材料研究进展

在总结和分析负热膨胀材料的发展历史和近10年来的主要研究成果的基础上，简要介绍了几种具有异常的热膨胀行为的新材料的负热膨胀性能，并对负热膨胀机理作了探讨，总结了负热膨胀材料的结构特点，并对其应用前景和发展趋势进行了预测。     

Electric conductivity and oxygen permeability of modified cerium oxides

Electrical conductivities of samarium (Sm), terbium (Tb), praseodymium (Pr) and zirconium (Zr) doped ceria membranes were measured in T = 600–900°C and pO₂ = 10^–22–0.21 atm. Doping Sm and Pr in CeO₂ respectively enhances the ionic conductivity and electron-hole conductivity of ceria. Sm-Pr doped ceria exhibits both n-type (at lower pO₂) and p-type (at high pO₂) electronic and ionic conductivity in the temperature range studied. Adding Tb in the Sm doped ceria causes a reduction in ionic conductivity. Zr-Pr doped ceria is an n-type electronic conductor at low and p-type electronic conductor at high pO₂. The ionic conductivity of Zr-Pr doped ceria is lower than Sm doped ceria but higher than pure ceria. Oxygen permeation flux through the Zr-Pr doped ceria membrane, dominated by the slow oxygen ionic conduction, is similar to the ytttria doped bismuth oxide membrane, and smaller, but close to that of perovskite-type lanthanum cobaltite membrane.     

Environmental dynamic fatigue crack propagation in nylon 66

The introduction of plastics into the structural materials market necessitates careful study of their fatigue behaviour in different environmental conditions. Standard samples of nylon 66 were pre-treated in either air, water, or dilute sulphuric acid and then notched and examined for fatigue crack propagation in the various environments. Tensile dynamic fatigue tests under constant sinusoidal load amplitudes were carried out at different frequencies. Linear fracture mechanics concepts were found to be a useful tool when interpreting the results.     

Dependency of mechanochemical reactions forming complex oxides on the crystal structures of starting oxides

Mechanochemical reactions applicable to syntheses of perovskite-type compounds, LaMO₃, where M is trivalent metals, are taken as examples to investigate effects of crystal structures of starting materials, M ₂O₃, on the reactions. The investigated oxides M ₂O₃ are categorized into two groups: one is the corundum structure and the other the non-corundum structure. Under the experimental conditions, it has been found that the oxides with non-corundum structure react easily with La₂O₃ by their co-grinding at room temperature to form the corresponding compounds. On the contrary, none of such reactions can be occurred for the oxides with corundum structure. The difference has been discussed based on the grinding-induced phase transformation from loose packing of atoms to close packing of atoms. Based on the results, the extension to the synthesis of other compounds such as CrO₂ is presented.