Loss mechanisms in piezoelectrics: how to measure different losses separately

Losses in piezoelectrics are considered in general to have three different mechanisms: dielectric, mechanical, and piezoelectric losses. This paper deals with the phenomenology of losses first, then how to measure these losses separately in experiments. We found that heat generation at off-resonance is caused mainly by dielectric loss tan delta' (i.e., P-E hysteresis loss), not by mechanical loss, and that a significant decrease in mechanical Qm with an increase of vibration level was observed in resonant piezoelectric ceramic devices, which is due to an increase in the extensive dielectric loss, not in the extensive mechanical loss. We propose the usage of the antiresonance mode rather than the conventional resonance mode, particularly for high power applications because the mechanical quality factor QB at an antiresonance frequency is larger than QA at a resonance frequency.     

Interaction between non-parallel dislocations in piezoelectrics

The total interaction force F¹² between two crossing (non-intersecting) straight dislocations is found and analyzed for the three types of piezoelectric media of unrestricted anisotropy: an unbounded body, an infinite plate and a half-infinite body. In the latter two cases the dislocations are supposed to be parallel with the surfaces, which are in turn implied to be mechanically free of tractions and electrically closed (metalized). The found force F¹² is orthogonal to the parallel planes, P and Q, containing the crossing dislocations. In an unbounded medium the value F¹² proves to be independent of the distance between P and Q. On the other hand, it depends on directions of the dislocations and on their Burgers vectors: the force F¹² may be either attractive or repulsive. In a plate the interaction becomes sensitive to dislocation positions y^(1,2) with respect to the surfaces. Only in the situations, when dislocations are much closer to each other than to the both surfaces, their interaction may be approximately described by the solution for an unbounded medium. Otherwise, corrections arising from the image forces due to the plate surfaces become essential. The dislocation in the vicinity of a surface strongly acts on its counterpart only until the latter situates even closer to the same surface than the first one. When the second dislocation leaves this narrow zone, the interaction force on it abruptly decreases to a very small level. With an increase in the thickness of the plate, this behavior becomes more and more pronounced. In a half-infinite medium the interaction between the dislocations is exactly described by a Heaviside step-like dependence F¹²∝H(y⁽¹⁾-y⁽²⁾) valid for any y^(1,2). It is shown that we deal here with an analog of the plane capacitor effect.     

A penny-shaped crack in piezoelectrics: resolved

This paper reconsiders the problem of a penny-shaped crack in a piezoelectric medium under uniform mechanical as well as electric loadings applied at infinity (mode I). The analysis is based on the general solution of three-dimensional piezoelasticity, which is represented by four quasi-harmonic displacement functions. It is shown that these harmonics can be expressed in terms of two potentials of a simple layer. By using the previous results in potential theory, exact solution of the mode I crack is obtained. In particular, the solution is complete and in terms of elementary functions.     

High performance single crystal piezoelectrics: applications and issues

Relaxor-based single crystal piezoelectrics began to live up to their promise of providing unprecedented increase of bandwidth, sensitivity and high energy density for advanced piezoelectric transducers and actuator applications. Large size crystals are now available, at reduced cost, for device fabrication and performance tests. The newly gained insights from related investigations opened new opportunities for developing novel piezoelectrics.     

宽功率范围超声功率计的设计研究

设计了一种结合机械天平、辐射压力法和智能控制的大功率超声功率计，介绍该功率计的设计原理，对其误差进行分析，实验证实该功率计具有较好的稳定性、较高的测量精度和较大的测量范围。     

Modeling 1-3 composite piezoelectrics: thickness-mode oscillations

A simple physical model of 1-3 composite piezoelectrics is advanced for the material properties that are relevant to thickness-mode oscillations. This model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer are derived. A number of examples illustrate the implications of using piezocomposites in medical ultrasonic imaging transducers. While most material properties of the composite roughly interpolate between their values for pure polymer and pure ceramic, the composite's thickness-mode electromechanical coupling can exceed that of the component ceramic. This enhanced electromechanical coupling stems from partially freeing the lateral clamping of the ceramic in the composite structure. Their higher coupling and lower acoustic impedance recommend composites for medical ultrasonic imaging transducers. The model also reveals that the composite's material properties cannot be optimized simultaneously; tradeoffs must be made. Of most significance is the tradeoff between the desired lower acoustic impedance and the undesired smaller electromechanical coupling that occurs as the volume fraction of piezoceramic is reduced.     

Modeling 1-3 composite piezoelectrics: hydrostatic response

A simple physical model of 1-3 composite piezoelectrics that was advanced for the material properties relevant to thickness-mode oscillations is extended to address the hydrostatic response. The model is valid when the lateral spatial scale of the composite is sufficiently fine that the composite can be treated as an effective homogeneous medium. Expressions are derived for the composite's material parameters in terms of the volume fraction of piezoelectric ceramic and the properties of the constituent piezoelectric ceramic and passive polymer. The results are similar to those derived by Haun and Newnham (1983, 1986) using a parallel-series connectivity model. The model is illustrated by analyzing composites made from conventional PZT5 and anisotropic modified lead titanate piezoelectric ceramics. For PZT5, the composite structure enhances its hydrostatic charge coefficient, hydrostatic voltage coefficient, hydrophone figure of merit, and hydrostatic coupling coefficient, while three of these quantities fall short of their pure ceramic values in the modified lead titanate composites. The shortfall is due to an enhanced composite that arises from lateral stress on the polymer being transferred to a longitudinal stress along the ceramic rods by the Poisson effect in the polymer, thus producing a charge through the ceramic's d(33).     

Probabilistic inverse problem and system uncertainties for damage detection in piezoelectrics

This paper provides a probabilistic formulation to design a monitoring setup for damage detection in piezoelectric plates, solving a model-based identification inverse problem (IP). The IP algorithm consists on the minimization of a cost functional defined as the quadratic-difference between experimental and trial measurements simulated by the finite element method. The motivation of this work comes from the necessity for a more rational design criteria applied to damage monitoring of piezoelectric materials. In addition, it is very important for the solving of the inverse problem to take into account the random nature of the system to be solved in order to obtain accurate and reliable solutions. In this direction, two investigations are considered. For the first, the experimental measurements are simulated combining a finite element and a Monte Carlo analysis, both validated with already published results. Then, an uncertainty analysis is used to obtain the statistical distribution of the simulated experimental measurements, while a sensitivity analysis is employed to find out the influence of the uncertainties in the model parameters related to the measurement noise. Upon the study of the measurements, they are used as the input for the damage identification IP which produces the location and extension of a defect inside a piezoelectric plate. For the second investigation, a probabilistic IP approach is developed to determine the statistical distribution and sensitivities of the IP solutions. This novel approach combines the Monte Carlo and the IP algorithm, considering the trial measurements as random. In conclusion, the analysis demonstrates that in order to improve the quality of the damage characterization, only a few material parameters have to be controlled at the experimental stage. It is important to note that this is not an experimental study, however, it can be considered as a first step to design a rational damage identification experimental device, controlling the variables that increase the noise level and decrease the accuracy of the IP solution.     

Towards the computation of electrically permeable cracks in piezoelectrics

The focus of this paper is not the well-known and examined behavior of electrically impermeable and fully permeable cracks but the analysis of limited permeable cracks, i.e. the influence of a dielectric medium inside the crack. The boundary conditions of the impermeable or the fully permeable crack can be considered as simple approximations representing upper and lower bounds for the electrical energy penetrating the crack. In this paper, the accuracy of a known theoretical approach “capacitor analogy” for analyzing limited permeable cracks in piezoelectric ceramics is verified by means of numerical methods and analytical estimations. Different crack configurations in 2D and 3D are analyzed. The cracks are subjected to combined electrical and mechanical loads, which lead to a mixed Mode loading in Mode-I and Mode-IV. The influence of errors committed by the capacitor analogy approaching the crack tip is investigated using piezoelectric crack weight functions and special finite element techniques meshing the medium inside the crack. The numerical results of stress intensity factors, obtained by the crack tip element method which is valid for loaded crack faces, are presented. Finally, the theory is applied to the evaluation of a fracture experiment with a DCB specimen.     

High current and high power superconducting rectifiers

Results on three experimental superconducting rectifiers are reported. Two of them are 1 kA low frequency flux pumps, one thermally and magnetically switched. The third is a low-current high-frequency magnetically switched rectifier which can use the mains directly.     

Toughening mechanisms in high impact polystyrene

In situ scanning electron microscope crack propagation experiments have been performed on a number of polystyrene and high impact polystyrene blends so that dynamic observations can be made of the mechanisms of failure. Brittle fracture is observed in low rubber phase volume systems, whereas high rubber phase volume systems exhibit a ductile tearing mode of fracture. As the rubber phase volume is increased there is an increased density of crazes, which leads to a reduction in width of material between them. The subsequent failure of the crazes leaves bridging ligaments. Under increasing load these fail in a manner dependent on their thickness such that there is a brittle-ductile transition at a ligament thickness around 3m. We argue that this alteration in mechanism could be caused by either the loss of the triaxial stress state or the reduced probability of extrinsic flaws being found in the smaller ligaments, resulting in inhibition of crazing. The stress required for failure at the crack tip consequently increases from that for craze formation to the yield stress of the dense polymer. This in turn allows a larger crazed deformation zone (already increased due to the stress relief effects of crazing) to form, hence a further toughness increase.     

Criteria of electromechanical fracture of piezoelectrics initiated by electrode edges

A criterion of the fracture of an electroelastic body initiated by concentration of the electric field intensity on the edges of electrodes is proposed. The relations of fracture mechanics as applied to the electric breakdown of a dielectric are used on the basis of electromechanical analogies. A relation was obtained, from which follows that in the case of electromechanical fracture of an electroelastic dielectric, the surface energy consists of mechanical and electrical components, each of which in the particular case contains the criterion of fracture of an elastic medium and criterion of electric breakdown of a dielectric.Translated from Problemy Prochnosti, No. 7, pp. 42–46, July, 1994.     

功率超声换能器电声效率及辐射声功率的测量

在高频电功率计方法的基础上,本文提出了一种能够直接测量大功率压电超声换能器在实用状态下的辐射超声功率以及电声效率的新方法。与传统的高频电功率计法相比,本方法避免了介电以及机械损耗功率的测试,简化了测试步骤。     

An electroacoustic wave in a gap between piezoelectrics undergoing oscillatory displacement

The variation of electroacoustic-wave velocity in a vacuum gap of piezoelectric crystals under the action of homogeneous oscillatory displacement of one of the crystals along the gap boundary is considered.     

基于声功率的液体中大功率超声场的评价方法

为解决超声清洗中超声设备性能的评价,以及工程应用中超声强度的选取和控制问题,提出了一种根据声功率大小来测定声场强度的评价方法。制作了一个专门的超声清洗系统,并用瓦特计法测定清洗液中得到的声功率,同时用一个探头从清洗液中提取信号;通过测量清洗液中的声功率与探头输出信号电压间的关系,用数字形式来显示超声场强度。用此法对不同清洗槽进行测量,结果表明,该方法可以作为工程中大功率超声场的一种评价方法。     

Deformation mechanisms in oriented high density polyethylene

The deformation behaviour of single crystal texture HDPE has been examined in tension as a function of the orientation of the tensile axis with respect to the chain direction. Over most of the orientation range examined, it was found that slip processes parallel to the chain direction were the dominant modes of deformation. Fibrillar slip becomes relatively more important than chain slip as the strain increases and as ₀, the initial value of the angle between the tensile axis and the chain direction, decreases. Lamellar slip was only observed over a limited range of orientations due to the high initiation stress required for the process. At low ₀ values, lamellar separation accounted for a substantial part of the applied strain. Stress-induced martensitic transformation, which was observed in samples with ₀>26°, cannot account for an important fraction of the strain although the resolved shear stress required for the activation of the martensitic transformations is of the same order of magnitude as that required for chain slip.     

Biological strain/stress sensing power in plants and modelling mechanisms

This paper describes a cell based mechanosensing power and adapting modelling mechanisms found in plants. Biological structures consist of mechanical load carriers, which are strongly optimised in terms of mechanical strength and minimum mass. It is shown that bamboo structures are designed to have uniform stresses at all positions, applying various sizes of bamboo fibre at the optimal locations. As a significant biological design system, a biological sensing power for detecting the strain/stress induced by external loads can be accepted. The mechanosensing system can be regarded as that in which live cells detect the external mechanical strain/stress as biological electric signals generated by the motions of various ions throughout and/or into the channels of the cell membrane. The modelling mechanism is a cell based mechanosensor that depends on the kind of plant. Three types for detecting the mechanical strain/stress (tension sensitive, compression sensitive, and tension and compression sensitive types) are investigated. Changes in electrical signals at the local portion of a plant body due to the mechanical deformation of cell membranes, affect the modelling of a plant's tissue to fit external mechanical environments.     

高声速海底的反射损失和相移

高声速海底反射损失特性主要决定于海底折射率n0和海底损失参数ε,它的变化特性以x=sinθ/(√1-n20)为基本变量,大致可分为三个区域(1)近似线性区,(2)剧变区和(3)接近抛物线的缓变区.对远程声传播起决定贡献的线性区,反射损失LVr≈m(√εx);对垂直入射状态起主要作用的极近程声场,有LVr≈1n((m+n)/(m-n)),此时海底密度起主要作用.     

1kA高功率脉冲磁控溅射电源研制及试验研究

高功率脉冲磁控溅射(HPPMS)以其在真空镀膜上更大的优势而越来越受到重视,高压大电流电源是实现HPPMS的关键因素。本文研制了1000 A高功率脉冲磁控溅射电源,给出了电源框架图和主电路拓扑结构图。对脉冲部分采用仿真分析探索大模块IGBT的不均流因素,结果表明驱动一致性是影响均流的关键原因之一;分析了大电流时IGBT两端电压过冲问题,采用RCD吸收和续流回路能有效抑制电压过冲,使电压过冲在正常安全范围内。用所研制的电源进行等离子体负载实验,运行良好,为性能优异薄膜的制备奠定硬件基础。     

Study of friction and wear mechanisms at high sliding speed

The aim of this work is to propose an analysis of mechanisms inducing surface interaction by friction during high sliding speed. Specific devices including a ballistic setup were used to reproduce extreme sliding conditions combining high speed and high pressure. The titanium alloy/tantalum tribo-pair is chosen to investigate the frictional and material transfer mechanisms. The tangential force measurement is used to follow the evolution of the friction coefficient at a macroscopic scale. The evolution of the sliding surface was analyzed by confocal 3D microscope to evaluate material transfer and real contact surface area. Numerical modeling of micro-contact at the asperities scale is presented to illustrate the scenarii involved during friction. The energy needed to shear a junction is estimated and analyzed for several types of interaction. Different behaviors have been taken into account in order to investigate the global forces generated by the contact including strong and weak contacts. The analysis of energy is available to predict the global friction force in a large range of velocities. Correlations between experimental measurements and numerical predictions are used to validate the proposed approach. The results can be interpreted as following: (1) at lower velocity the main mechanism dominating the interaction between asperities becomes ploughing with large volume of plastic deformation (2) at higher velocity the main mechanism is shear localization requiring less energy and force for shearing the junctions.