Enhanced ferroelectric,piezoelectric and dielectric properties of (1-x)CaBi2Nb2O9-xBaZr0.2Ti0.8O3 high-temperature piezoelectric composite ceramics

Among Aurivillius layer-structured materials, CaBi₂Nb₂O₉ is a best potential candidate for ultrahigh-temperature applications because of its highest Curie temperature of about 940 °C. In this paper, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ composite ceramics were prepared by conventional solid-state sintering method. The dielectric results show that the introduction of BaZr_0.2Ti_0.8O₃ not only increases the permittivity of the material, but also reduces its dielectric loss. The optimum electrical properties were obtained in the x = 0.01 sample with piezoelectric coefficient (d₃₃) of 15.1 pC/N and high ferroelectric remnant polarization (P_r) of 9.9 μC/cm². Furthermore, the composite samples show good thermal depoling performance, the d₃₃ of the x = 0.01 sample is 13.8 pC/N, which is about 91% of the initial value after depoling at 800 °C. Therefore, (1-x)CaBi₂Nb₂O₉-xBaZr_0.2Ti_0.8O₃ is one of the candidates for high temperature piezoelectric materials.     

Mechanical properties and defect chemistry

A brief overview of changes in mechanical properties of solids driven by the chemistry of point defects is given. Two major types of effects are considered: direct effects caused by non-interacting point defects and collective effects induced by interacting point defects. The first group includes (1) changes in the linear dimensions of a solid in response to a change in defect concentration and (2) stress induced due to an inhomogeneous distribution of point defects, a so-called chemical stress. The second group includes (1) defect order–disorder transitions accompanied by self-strain and (2) deviations from linear elastic behavior due to the dissociation/association of point defects. All of the above become important if the concentration of point defects is very high (above 10²¹ cm^− 3). These effects may lead to significant anomalies in mechanical properties: spontaneous stress and strain and changes in elastic constants. These effects may significantly affect the application of materials with a large concentration of point defects.     

Love waves in functionally graded piezoelectric materials by stiffness matrix method

A numerical matrix method relative to the propagation of ultrasonic guided waves in functionally graded piezoelectric heterostructure is given in order to make a comparative study with the respective performances of analytical methods proposed in literature. The preliminary obtained results show a good agreement, however numerical approach has the advantage of conceptual simplicity and flexibility brought about by the stiffness matrix method. The propagation behaviour of Love waves in a functionally graded piezoelectric material (FGPM) is investigated in this article. It involves a thin FGPM layer bonded perfectly to an elastic substrate. The inhomogeneous FGPM heterostructure has been stratified along the depth direction, hence each state can be considered as homogeneous and the ordinary differential equation method is applied. The obtained solutions are used to study the effect of an exponential gradient applied to physical properties. Such numerical approach allows applying different gradient variation for mechanical and electrical properties. For this case, the obtained results reveal opposite effects. The dispersive curves and phase velocities of the Love wave propagation in the layered piezoelectric film are obtained for electrical open and short cases on the free surface, respectively. The effect of gradient coefficients on coupled electromechanical factor, on the stress fields, the electrical potential and the mechanical displacement are discussed, respectively. Illustration is achieved on the well known heterostructure PZT-5H/SiO₂, the obtained results are especially useful in the design of high-performance acoustic surface devices and accurately prediction of the Love wave propagation behaviour.     

Electromechanical properties of 2-2 cement based piezoelectric composite

A 2-2 type cement based piezoelectric composites were fabricated by dice-and-fill technique. The influences of lead magnesium niobate-lead zirconate-lead titanate (PMN) ceramic volume fraction on the electromechanical properties of the composite were investigated. The results indicate that the planar electromechanical coupling factor K_p of the composite is hardly influenced by the PMN volume fraction, which fluctuates between 35% and 37%, while both the thickness electromechanical coupling factor K_t and the mechanical quality factor Q_m exhibit the trend of increase. When PMN volume fraction is 69.2%, K_t could reach to 49.8%, which is larger than that of the PMN ceramic (K_t = 46%).When PMN volume fraction is 41.7%, Q_m is 4.5, which is also much less than that of the PMN ceramic (Q_m = 70). This means that the composite has wider frequency band and higher sensitivity to be used as transducers. With the increase of PMN volume fraction, the acoustic impedance of the composite also exhibits the trend of increase.     

Exact second order formalism for the study of electro-acoustic properties in piezoelectric structures under an initial mechanical stress

In this study we develop the exact second order formalism of piezoelectric structures under an external mechanical stress. Indeed, previous models are approximated since they consist in deriving all the equations in the natural coordinate system (corresponding to the pre-stress free case). Hence, our exact formalism proposes to obtain the whole of equations in the current coordinate system (which is the coordinate system after the pre-deformation). Then, this exact formalism is used to derive the modified Christoffel equations and the modified KLM model. Finally, we quantify the correction with the approximate formalism on several transfer functions and electro-mechanical parameters for a non hysteretic material (lithium niobate). In conclusion, we show that for this material, significant corrections are obtained when studying the plane wave velocities and the electrical input impedance (about 4%), whereas other parameters such as coupling coefficient and impulse response are less influenced by the choice of coordinate systems (corrections less than 0.5%).     

高温压电材料、器件与应用

作为重要的功能材料,压电材料已经在国民经济的多个领域里有着重要应用.随着现代工业的快速发展,特别是新能源、交通和国防工业的高速发展,功能材料的应用已经从常规使用转向极端环境下的服役.本文综述了具有高居里点的压电材料,包括钙钛矿型压电陶瓷、铋层状结构氧化物压电陶瓷、钨青铜结构压电陶瓷以及非铁电压电单晶等;介绍了其晶体结构特征和高温压电性能、最新研究进展,并列举了一系列的高温压电器件和应用,包括高温压电探测器、传感器、换能器和驱动器等.另外,本文总结了高温压电材料的热点研究问题,并展望了今后的发展方向.     

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ_B for 6.25% of Ni doping and 1.32 μ_B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy (V_Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy (V_N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy (V_Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V_Ga and formed a strong Ni–N bond. In this Ni–V_Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ_B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ_B.     

Sensor application-related defect chemistry and electromechanical properties of langasite

The operation of langasite (La₃Ga₅SiO₁₄) resonators as sensors at elevated temperature and controlled atmospheres is examined. This paper focuses on mapping the regimes of gas-insensitive operation of uncoated langasite resonators and the correlation to langasite's defect chemistry for temperatures up to 1000 °C. As a measure of sensitivity, the fundamental resonant mode at 5 MHz is estimated to be determined to within ± 4 Hz by network analysis for resonators operated in air at temperatures below 1000 °C. The calculated frequency shift induced by redox-related reactions in langasite only exceeds the limit of ± 4 Hz below p_O2 ≈ 10^− 17 bar at 1000 °C, below 10^− 24 bar at 800 °C and below 10^− 36 bar at 600 °C. Water vapor is found to shift the resonance frequency at higher oxygen partial pressures. In the hydrogen-containing atmospheres applied here, langasite can be regarded as a stable resonator material above oxygen partial pressures of about 10^− 13 and 10^− 20 bar at 800 and 600 °C, respectively.     

Thickness vibration of piezoelectric plates of 6mm crystals with tilted six-fold axis and two-layered thick electrodes

We perform a theoretical analysis of thickness vibrations in piezoelectric plates of crystals with 6 mm symmetry. The six-fold axis is tilted with respect to the plate surfaces. The major surfaces of the plate are covered with two layers of electrodes of different metals. The equations of linear piezoelectricity are used for the crystal plate. The electrodes are modeled by the equations of elasticity. Thickness vibrations frequencies and modes as well as impedance are calculated and examined.     

Synthesis of coryphantha elephantidens-like SnO2 nanospheres and their gas sensing properties

In this work, coryphantha elephantidens-like SnO₂ with porous structures were prepared successfully by a simple hydrothermal route, through adjusting the temperature of hydrotherm. Its morphology was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer–Emmett–Teller (BET). Compared to the regular nanospheres, the coryphantha elephantidens-like SnO₂ nanospheres had obviously higher gas-sensing response, owing to the special structure with large specific surface area (161.16 m² g^?1). It surprised us that the coryphantha elephantidens-like SnO₂ sensor could easily distinguish between ethanol and acetone, whose chemical property were similar. Moreover, it also exhibited wide measurement range, fast response speed (less than 10 s) and good repeatability at a low temperature (180 °C) to ethanol. The desirable specific surface area and pore volume were conducive to molecules adsorption and diffusion, which were believed to be the major cause of the improvement of gas sensing performance.     

Analysis of the energy extracted by a harvester based on a piezoelectric tile

In this paper, we analyze the maximum energy that can be extracted from a piezoelectric harvester subject to pulsed excitation, with an interface circuit composed by a standard bridge rectifier. We show that the optimal voltage of the DC load of the bridge rectifier is a fraction, comprised between 1/3 and ½, of the open-circuit voltage, depending on the piezoelectric losses and excitation time. A simple analytical model is provided, whose accuracy has been assessed against SPICE simulations. Furthermore, preliminary experimental tests carried out over a commercial piezoelectric tile confirm the validity of the proposed model.     

Electromechanical properties of lanthanum-doped lead hafnate titanate thin films for integrated piezoelectric MEMS applications

This paper focuses on the deposition and electromechanical characterization of lanthanum-doped lead hafnate titanate (PLHT) thin films as key material in piezoelectric microelectromechanical systems (pMEMS). PLHT (x/30/70) and PLHT(x/45/55) films with a thickness between 150 nm and 250 nm were deposited by chemical solution deposition (CSD). Thereby x varies between 0 and 10% La content. The electrical characterization shows that undoped (x=0) PLHT exhibit ferroelectric behavior similar to PZT of the same composition. La doping results in reduced ferroelectric properties and also affects the electromechanical properties. Measurements using a double beam laser interferometer yield a piezoelectric coefficient d ₃₃ of 60 pm/V, which stays constant with an increasing electric field. This leads to a linear displacement compared to undoped PLHT or conventional PZT films used for MEMS applications.     

Electrical properties and defect chemistry of anatase (TiO2)

The electrical properties of pure Anatase are investigated by impedance spectroscopy as function of temperature and oxygen partial pressure. The experimental results are fully interpreted by point defect chemistry. A transition from predominant Schottky to Frenkel cation disorder is observed when the temperature increases and/or the oxygen partial pressure decreases. The reduction enthalpies are near those obtained for Rutile in previous studies.     

Experimental investigation on the structural,dielectric, ferroelectric and piezoelectric properties of La doped ZnO nanoparticles and their application in dye-sensitized solar cells

Pure and lanthanum (La) doped ZnO nanorods were synthesized via co-precipitation method. The structure and morphology of as grown ZnO and La-ZnO nanoparticles were studied using transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) methods. The values of remnant polarization and coercive field were found to be 0.027 μC/cm² and 1.33 kV/cm, respectively, for as grown La-ZnO nanostructures. High Curie temperature (276 °C) for doped ZnO was observed in dielectric study. Piezoelectric coefficient at room temperature was found to be 101.30 pm/V. I-V characteristics were studied for both pure and doped ZnO nanoparticles. Photo-anodes of dye-sensitized solar cells (DSSCs) were made using ZnO and La-ZnO nanorods. The conversion efficiency and short circuit current density of La-ZnO nanorods based DSSC were 0.36% and 1.31 mA/cm², respectively, which were found to be largely enhanced when compared with that of pure ZnO based DSSC (0.20% and 0.94 mA/cm²).     

Energy trapping in high-frequency vibrations of piezoelectric plates with partial mass layers under lateral electric field excitation

We study coupled face-shear (FS) and thickness-twist (TT) motions of a piezoelectric plate of monoclinic crystals with mass layers on the central parts of the plate surfaces. The plate is driven by a lateral electric field. Mindlin’s first-order theory of piezoelectric plates is used. An analytical solution is obtained. Numerical results are presented for an AT-cut quartz plate, including the motional capacitance of the plate as a resonator and the vibration modes trapped under the mass layers in the central portion of the plate. The relationship between the dimension of the mass layers and the number of trapped modes is examined.     

PEG-20000 assisted hydrothermal synthesis of hierarchical ZnO flowers: Structure,growth and gas sensor properties

The hierarchically structured materials usually show improved physical and chemical properties. Here, using simple hydrothermal method and reagents of polyethylene glycol (PEG-20000), we synthesize the ZnO crystals and find that they exhibit hierarchical flower-like architectures assembled by nanorods. Further comparative studies demonstrate that PEG provides nucleation sites for the assembling of the nanorods, which play a critical role in producing such unique flower-like architectures. Consequently, the sensor made of ZnO nanoflowers is found to show good gas sensing performance to the hydrothion gas.     

Electromechanical and optical effects in photorefractive materials

From thermodynamic considerations, we derive a consistent set of equations coupling the steady-state photoinduced photorefractive space-charge field to mechanical deformations, dc permittivity, and refractive index changes. For the first time to our knowledge, these equations include piezoelectricity, electrostriction, the elasto-optic effect, the linear and quadratic electro-optic effects, as well as the influence of the volume forces and electric torques. We determine the conditions on the material parameters, for which volume forces and torques are of some importance on the optical properties. These equations resulting from a macroscopic approach are valid whatever the physical microscopic origins of the various effects. Non centrosymmetrical and centrosymmetrical materials are considered. The order of approximation is the lowest possible able to describe the mentioned effects in both types of materials. Received: 18 November 1998 / Revised version: 18 February 1999 / Published online: 12 April 1999     

Dielectric and piezoelectric properties of barium-modified Aurivillius-type Na0.5Bi4.5Ti4O15

In this study, monophasic Ba_x(Na_0.5Bi_0.5)_1−xBi₄Ti₄O₁₅ (x=0.03, 0.06, 0.09 and 0.12) ceramics fabricated from the powders synthesized via the solid-state reaction route exhibited relaxor behavior. X-ray diffraction analysis revealed that the barium-modified Na_0.5Bi_4.5Ti₄O₁₅ ceramics have a pure four-layer Aurivillius phase structure. Dielectric properties and phase transitions were studied and are explained in terms of lattice response of these ceramics. A shift in ferroelectric–paraelectric phase transition (T_c) to lower temperatures and a corresponding increase in permittivity peak with increasing concentration of Ba²⁺ are also observed. The decrease of orthorhombicity in the lattice structure by the larger Ba²⁺ ion incorporation, indicating an approach of a and b parameters, results in lower Curie temperature. The piezoelectric activity of Na_0.5Bi_4.5Ti₄O₁₅ (NBT) ceramics was significantly improved by the modification of barium. The Curie temperature T_c and piezoelectric coefficient d₃₃ for the composition with x=0.12 were found to be 635 °C and 21 pC/N, respectively. The relationship of polarization with lattice response is discussed.