Stability of morphotropic (110) oriented 0.65PMN-0.35PT single crystals

Electromechanical properties of (1-x)Pb (Mg1/3Nb2/3)O3-xPbTiO3 (PMN-PT) single crystals with x = 0.35 were investigated as a function of different external disturbances. The polarization dependence on the electromechanical properties was first studied in order to determine the best polarization path. The correlation with X-ray measured phase ratio is presented and shows that the maximum of electromechanical properties may be correlated with a minimum rhombohedral/tetragonal phase ratio. Temperature, stress, electric field, and time (aging) stability was studied in order to determine performance-limiting factors of these materials. The rhombohedral/tetragonal phase transition is observed on temperature (80 degrees C), inducing a decrease of the electromechanical coupling factor (from 85% to 50%); but the whole properties are recovered while returning to room temperature. Stress measurement shows a large depoling of sample for stresses above 30 MPa. The PMN-PT single crystals were found to be surprisingly stable during aging, except for mechanical and dielectric losses. The same tendency was found on alternating current (AC) electric field dependence.     

Interpretation of T(m) and T* in relaxor ferroelectric 0.93Pb(Zn₁/₃Nb₂/₃)O₃-0.07PbTiO₃

The dielectric property of relaxor ferroelectric 0.93Pb(Zn?/?Nb?/?)O?-0.07PbTiO? single crystals (PZN-0.07PT) is studied to investigate the ferroelectric phase transition. The temperature dependence of the susceptibility of the central peak in the light scattering shows a very similar behavior to the real part of the dielectric constant, indicating that the diffuse peak in the temperature dependence of the dielectric constant is induced by relaxation of the polarization. We suggest that the origin of the relaxation above T(m) is the non-180° polarization flipping of a small polar nanoregion (PNR). Additionally, the behavior of the PNRs is interpreted through both the Brillouin spectra and high-energy synchrotron-radiation powder diffraction patterns. Our results reveal that T*~ 499K is the temperature below which the growth rate of the dynamic polarization fluctuation is suppressed by the additional random electric field induced by the generation of the static PNRs.     

2D Intrinsic Ferromagnetic MnP Single Crystals

2D intrinsic ferromagnetic materials are highly anticipated in spintronic devices due to their coveted 2D limited magnetism. However, 2D non‐layered intrinsic ferromagnets have received sporadic attention, which is largely attributed to the fact that their synthesis is still a great challenge. Significantly, manganese phosphide (MnP) is a promising non‐layered intrinsic ferromagnet with excellent properties. Herein, high‐quality 2D MnP single crystals formed over liquid metal tin (Sn) is demonstrated through a facile chemical vapor deposition technique. The introduction of liquid metal Sn provides a fertile ground for the growth of 2D MnP single crystals. Interestingly, 2D MnP single crystals maintain their intrinsic ferromagnetism and exhibit a Curie temperature above room temperature. The research enriches the diversity of 2D intrinsic ferromagnetic materials, opening up opportunities for further exploration of their unique properties and rich applications.     

The dielectric properties of nylon 6,6/aramid fibre microcomposites in the presence of transcrystallinity

Dielectric spectroscopy was applied in the present work for the first time to polymeric composite materials containing transcrystallinity, wherein the dielectric properties of pure nylon 6,6 and of aramid fibre-reinforced nylon 6,6 microcomposites were examined over wide frequency and temperature ranges. The temperature behaviour of the dielectric losses of the materials indicated three polarization processes, related to either local or collective molecular mechanisms of motion. In addition, interfacial polarization of the Maxwell–Wagner–Sillars type was observed. The dielectric response was found to be sensitive to the presence of transcrystallinity in the microcomposites. It was found that the activation energy of the α, β and γ relaxations exhibits typical variations in the presence of reinforcement and transcrystallinity. The dielectric strength, calculated from fitting of the relaxation spectra to the Havriljak–Negami empirical term for all the relaxation processes, was found to be very sensitive to the morphology of the systems. The specific values at 1 MHz of the dielectric constant, dielectric loss and dielectric loss tangent as a function of temperature for the transcrystalline layer were retrieved from the composites data using the rule-of-mixtures. A comparison was conducted between the values of the transcrystalline layer and those of the bulk matrix to determine the effect of the transcrystalline layer on the dielectric properties.     

Dielectric behaviour of strontium tartrate single crystals

Strontium tartrate trihydrate (STT) crystals have been grown in silica hydrogel. Various polarization mechanisms such as atomic polarization of lattice, orientational polarization of dipoles and space charge polarization in the grown crystals have been understood using results of the measurements of dielectric constant (έ′) and dielectric loss (tan δ) as functions of frequency and temperature. Ion core type polarization is seen in the temperature range 75–180°C, and above 180°C, there is interfacial polarization for relatively lower frequency range. One observes dielectric dispersion at lower frequency presumably due to domain wall relaxation.     

Defect control for polarization switching in BiFeO₃ single crystals

BiFeO? (BFO) single crystals were grown and the effects of Zn and Mn co-doping on the polarization and leakage current properties were investigated at 25 °C for establishing materials design based on defect chemistry. Although Zn doping or Mn doping led to a deterioration in the properties, Zn-Mn co-doping led to a large remanent polarization (36 μC/ cm2), a low coercive field (19 kV/cm), and a relatively low leakage current density (~10?? A/cm2). It is proposed that defect dipoles composed of Zn2+ and Mn?+ act as effective nucleation sites for ferroelectric domains during polarization switching in BFO crystals.     

Thermal hysteresis in the dielectric properties of composites based on transition metal oxide and sulfide nanoparticles stabilized in a low-density polyethylene matrix

We have studied the temperature dependence of the complex dielectric permittivity of composites based on a low-density polyethylene matrix containing dispersed nanoparticles of zinc oxide and manganese oxide and an analogous composite with nanoparticles of cadmium sulfide. Laws governing the electric conduction and dielectric polarization as functions of the temperature and the size of nanoparticles are established and a possible mechanism responsible for the appearance of thermal hysteresis of the dielectric properties in the materials studied is proposed.     

Electrolytic growth and properties of single crystals of ammonium hydrogen tartrate

The growth of orthorhombic disphenoidal and long needle habit single crystals of ammonium hydrogen tartrate (AHT) in silica gels under the influence of an externally applied uniform field is described. The properties resulting from different growth parameters are reported. Variation in the electrical conductivities of AHT is illustrated. Two distinct electrical conductivity processes are reported. The polarization effect contributing to the dielectric constants is studied. The mechanism of dielectric behaviour which is different in lower and higher temperature ranges is discussed.     

Polarization filters with an autocloned symmetric structure

The process of fabricating photonic crystals comprised of alternately stacked high- and low-index dielectric materials on periodic substrates to form zigzag films is called the autocloning technique. In this study, we have fabricated TiO2/SiO2 two-dimensional polarization filters by using electron beam gun evaporation with ion-beam-assisted deposition. The shape of the zigzag structure is preserved, and the total thickness is 8?μm. The symmetric structural design can be utilized as an antireflection coating applied to reduce ripples and achieve a 200?nm working wavelength range.     

铁电体陶瓷的晶体结构及介电特性

ＢａＴｉＯ３基铁电体材料的晶体结构和介电特性有密切联系。这类材料有三个铁电相转变。通过离子的置换可以改变材料的介电特性，并可使三个相变温度发生变化。材料的介电损失一般认为由畴界移动和电传导两个机制造成的。材料中异类离子的存在对这些过程有明显的影响。结构中发生正离子的有序化对降低介电损失有重要作用。铅基Ｐｅｒｏｖｓｋｉｔｅ晶体具有无序的八面体堆垛结构和微小的畴区特征，因而这种驰豫型铁电体材料有漫散相变现象和较高的介电率。     

Dielectric and Ferroelectric Properties of Complex Perovskite Ceramics Under Compressive Stress

Dielectric and ferroelectric properties of complex perovskite PZT-PZN ceramic system were investigated under the influence of the compressive stress. The results showed that the dielectric properties, i.e. dielectric constant ( εr ) and dielectric loss ( tan δ), and the ferroelectric characteristics, i.e. the area of the ferroelectric hysteresis loops, the saturation polarization ( P(sat) ), and the remnant polarization (Pr) changed significantly with increasing compressive stress. These changes depended strongly on the ceramic compositions. The experimental results on the dielectric properties could be explained by both intrinsic and extrinsic domain-related mechanisms involving domain wall motions, as well as the de-aging phenomenon. The stress-induced domain wall motion suppression and non-180° ferroelectric domain switching processes were responsible for the changes observed in the ferroelectric parameters. In addition,a significant decrease in those parameters after a cycle of stress was observed and attributed to the stress induced decrease in switchable part of spontaneous polarization. This study clearly show that the applied stress had significant influence on the electrical properties of complex perovskite ceramics.     

Low temperature microwave characterisation of lithium fluoride at different frequencies

Precise knowledge of dielectric properties of materials is required to implement the material in high frequency devices and circuits. At microwave frequencies complex permittivity (dielectric constant and loss tangent) are the two mandatory parameters prior to any design. We have identified Lithium Fluoride as a potential candidate, which can be used in conjunction with superconducting and non-superconducting parts of several microwave communication devices. Even though dielectric constant of LiF is known at room temperature there only limited data presented at cryogenic temperatures. We have used a dielectric post resonator for the microwave characterisation of the rod shaped LiF crystal. In this paper, we have reported the dielectric constant (perpendicular component of the real part of complex permittivity) and loss tangent of two LiF crystals as a function of temperature (15–290 K) at frequencies of 8 and 16.5 GHz. We have also studied and reported the temperature coefficient of frequency and permittivity. The concept of using temperature coefficient of frequency as a standard is proved to be wrong in this paper. Microwave properties of other Fluorides are also compared with the LiF crystal.     

Electromechanical coupling properties of [001], [011] and [111] poled Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 single crystals

The electric field induced “butterfly” curves and polarization loops, and the stress induced strain and polarization responses of [001], [011] and [111] oriented Pb(Mg_1/3Nb_2/3)O₃-0.32PbTiO₃ (PMN-0.32 PT) relaxor ferroelectric single crystals have been systematically investigated by experiment study. The focus is on the effect of constant compressive bias stress on the electromechanical coupling behavior along three crystallographic directions of PMN-0.32 PT single crystals. Dependence of the coercive field, remnant polarization, dielectric constant, and piezoelectric coefficient on the bias stress has been quantified for PMN-0.32 PT single crystals oriented in three different directions. Obtained results show that the large piezoelectric responses under zero compressive stress in [001] and [011] orientation are dominated by intrinsic crystal lattice while the engineered domain structure has a relatively minor effect. It is found that observed responses under stress cycle for [001] oriented crystals are due to polarization rotation and phase transformations. However, those for [011] and [111] oriented crystals are due to domain switching. The “butterfly” curves and polarization loops driven by electric field under different bias compression are described by two non-180° domain switching.     

Influence of annealing on dielectric and polarization behavior of PVDF thick films

The polyvinylidene fluoride (PVDF) thick film has been fabricated by a solution casting method. The fabricated film is subjected to annealing at 50, 90, 100, 110 and 130?°C for 5 h. The effect of annealing on structural, crystalline, dielectric and polarization behavior is investigated. The β-phase PVDF is found to coexist with α-phase for annealing temperature upto 100?°C, after that β-phase is converted to α-phase. The film annealed at 100?°C, exhibits enhanced permittivity, reduced tangent loss and enhanced polarization. The dielectric permittivity and tangent loss of film annealed at 100?°C are ~11 and ~0.025 respectively for the frequency range of 10³–10⁵ Hz. The saturation polarization for this film is ~1.27 µC/cm². The enhanced dielectric permittivity and polarization for the film annealed at 100?°C might be attributed to increase in crystalline α and β-phase interface as well as crystalline amorphous interface. The thick film of PVDF with improved dielectric and polarization behavior could be useful for high power electronics application.     

国外透波材料高温电性能研究进展

综述了数十年来国外透波材料的发展,并结合实际例子对影响透波材料高温电性能的因素进行了分析.认为影响材料高温电性能的主要因素是结构/微结构、成分/微成分、物态物相变化等材料的本征因素.在此基础上,对热透波材料的研究前景进行展望.     

无机、有机及其复合电介质的纳米尺寸效应及研究进展

当电介质内的不均匀区域达到纳米级时,纳米尺度效应对介电性能的影响便不容忽视。分别以PMN-PT弛豫铁电单晶、ZnO压敏陶瓷、纳米SiO2/环氧树脂复合材料为例,通过实验证明了在无机铁电单晶、半导体陶瓷以及无机纳米/有机复合材料中存在的介电性能的纳米尺寸效应。     

Dielectric properties depending on frequency in organic light-emitting diodes

Dielectric properties of organic light-emitting diodes were investigated using ITO/Alq₃/Al device. In spite of various advantages in organic light-emitting diodes, a fundamental study on physical properties is not yet sufficient. Dielectric properties are used for studying fundamental physical properties of materials through a frequency-dependent response. We have investigated magnitude and phase of impedance, electrical conductivity, and the dielectric loss depending on a bias-voltage variation using ITO/Alq₃(60 nm)/Al device. The device shows a frequency-dependent response such that a major contribution is resistive below time constant and capacitive above time constant. Also, the device shows a voltage-dependent electrical conductivity in low-frequency region. A bulk resistance rapidly decreases as the frequency increases above 1 MHz. The dielectric loss shows that there appears an interfacial polarization in low-frequency region, and an orientational polarization in high-frequency region.     

Getting effective permittivity and permeability equal to −1 in 1D dielectric photonic crystals

J.B. Pendry (Phys. Rev. Lett., 86 3966 (2000)) mentioned the possibility of making perfect lenses using a slab of left-handed material with relative permeability, permittivity and optical index equal to ?1. This kind of metamaterial has been made in the microwaves domain, using metal and dielectric materials. On the other hand, it has been shown that lenses made using 2D dielectric photonic crystals can generate similar imaging properties, but until now, the image contains only a small part of the incident light. The paper shows, using a very simple analytical model, that 1D dielectric photonic crystals can generate left-handed materials with relative permeability, permittivity and optical index rigorously equal to ?1. Of course, such photonic crystals cannot be used to make perfect lenses, but this conclusion leads to the conjecture that 2D or 3D dielectric photonic crystals could be used in the visible region to realize superlenses.     

Radiation-thermal effects in single crystal corundum

Radiation-thermal effects were studied in corundum single crystals exposed to an ionizing radiation. The laws of variation of the electrical properties of a target material under the action of these factors were established. In particular, it is established that the dose dependence of the electric conductivity of irradiated samples is described by a curve with extremum. The radiation-induced phenomena observed are explained within the framework of existing models. The temperature dependence of the dielectric loss tangent of irradiated samples changes in a complicated manner (in particular, a maximum appears in this curve). There are two temperature regions in which the loss tangent exhibits special features. The activation energies have been determined and a hypothesis about the nature of the observed dielectric absorption is formulated.     

多铁性BaTiO3/La2/3Sr1/3MnO3复合薄膜的制备及其强磁电效应的研究

使用脉冲激光沉积技术,在(001)取向的LaAlO3(LAO)单晶基片上外延生长了BaTiO3/La2/3Sr1/3MnO3(BTO/LSMO)双层复合薄膜.电学和磁学性能的研究显示复合薄膜具有较低的相对介电常数(εr=263),优良的铁电和铁磁性能以及高于室温的铁磁居里温度(Tc=317 K).复合薄膜的磁电电压系数(αE)为176 mV/A,高于同类结构磁电系统一个数量级,相应的界面耦合系数k值为0.68,表明铁磁层和铁电层界面之间存在较大程度的耦合.