Cylindrical interface cracks in 1-3 piezocomposites

1-3 Piezocomposites are made by embedding piezoelectric fibers/rods in polymer matrix materials. Fiber–matrix interface fracture can affect the performance of piezocomposites. In this paper, axisymmetric interfacial cracks in piezocomposites are studied by considering an idealized model of a single piezoelectric fiber in a matrix material. The displacement discontinuity method is used to formulate the Mode I and II crack problems. The fundamental solutions required for DDM are derived explicitly by using the electroelastic field equations and Fourier integral transforms. The dependence of Mode I and II stress intensity factors of single and multiple interface cracks on fiber and matrix material properties, crack length and distance between cracks are investigated.     

Effective electroelastic moduli of 3-3(0-3) piezocomposites

In a high volume fraction particle-loaded piezocomposite, some ferroelectric particles appear to be in contact, as in a 3-3 connectivity material; others are isolated in the polymer matrix, as in a pure 0-3 connectivity material. Such a material can be considered as a composite of composites characterized by a 3-3(0-3) connectivity. This paper follows two others that have described a matrix method to calculate all of the effective parameters of pure 0-3 and 3-3 connectivity piezocomposites. These previous models are used to obtain the effective properties of a 3-3(0-3) composite. A proportion of 0-3 connectivity in the composite is introduced, and the effective properties are studied as a function of this proportion. Experimental results compared with these predictions show that the model allows an evaluation of the proportion of 0-3 connectivity through the analysis of the electromechanical performance of the composite samples.     

A coupled analytical model for hydrostatic response of 1-3 piezocomposites

This study presents a fully coupled analysis of a unit cell of a 1-3 piezocomposite under hydrostatic loading. The governing equations for coupled axisymmetric electroelastic field of a transversely isotropic piezoelectric medium and a transversely isotropic elastic medium are used. A reduced form of the analytical general solutions expressed in terms of series of modified Bessel functions of the first and second kind are used. The solution of the boundary-value problem corresponding to a unit cell is presented. The effective properties of a 1-3 piezocomposite are obtained for different fiber volume fractions, polymer and piezoceramic properties, and fiber aspect ratios. Comparisons with previously reported simplified and uncoupled models are made.     

Theoretical and experimental investigations of lateral modes in 1-3piezocomposites

Materials with a periodic microstructure show resonances caused by the elastic wave Bragg diffraction. This paper presents a simple approach to describe these resonances (called lateral resonances) in 1-3 piezoelectric composite materials which have a 2-D periodicity. Our model is based on the analysis of the propagation of transverse waves in a 2-D periodic medium of infinite thickness and takes into account the periodic and interfacial boundary conditions. This model predicts the displacement field vectors and frequencies of lateral resonances from which the phase velocity of lateral waves is determined. The theoretical and experimental variations of this velocity versus the ceramic rod width to pitch ratio are compared. It is shown that the first lateral mode frequency is maximum when the ceramic volume fraction is around 0.65. Theoretical predictions of the mechanical displacement at the composite surface are compared with measurements obtained by an interferometric laser technique. A good agreement is observed, showing that lateral waves are mainly vertically polarized     

Evaluation of piezocomposites for ultrasonic transducerapplications influence of the unit cell dimensions and the properties ofconstituents on the performance of 2-2 piezocomposites

A theoretical model on piezoceramic polymer composites with laminar periodic structure is presented. A salient feature of this model is that it can treat explicitly how the unit cell dimensions and other material properties influence the performance of an ultrasonic transducer made of 2-2 piezocomposites. The model predicts that there exist a series of modes associated with the periodic structure of a composite, which is beyond the stop-band edge resonance prediction. One of the main concerns in designing a composite transducer is how the surface vibration profile changes with frequency and how this is influenced by the aspect ratio of the ceramic plate. It was predicted that as long as the thickness resonance is below the first lateral mode frequency, there is always a frequency f₁ which is near the thickness resonance and at which the polymer and ceramic vibrate in unison. The effect of aspect ratio is to change the position of f₁ with respect to the thickness resonance frequency and the bandwidth in which polymer and ceramic have nearly the same vibration amplitude and phase. It is also predicted that, when operated in a fluid medium such as water, there will be a resonance mode which has a frequency determined by the velocity of the fluid medium and the unit cell length d and is associated with the oscillation of the fluid. The behavior of a composite plate as an acoustic transmitter and receiver and the influence of the aspect ratio of the ceramic plate on them are also investigated     

Modeling (2-2) piezocomposites partially sliced in the polymer phase

Different attempts have been suggested to improve the efficiency of piezocomposites. Here, a new design which consists of subdicing the passive part of (2-2) composites is presented. A theoretical model based on the series-parallel connections of elastic sections has been developed. It is shown that the cuts increase the electromechanical coupling coefficient. The model shows that, at high ceramic percent, the resonance frequency decreases when cutting the composite. At low ceramic percent the behavior is the opposite. Samples of (2-2) piezocomposites with low, medium, and high ceramic percent have been made, comparing predictions with experimental results. The agreement is satisfactory. With this composite structure, another design parameter is added to the search for better piezocomposite performance.     

Electric field-induced strain behavior in lithium- and copper-added potassium sodium niobate piezoceramics and 1-3 piezocomposites

Potassium sodium niobate (KNN)-based leadfree materials were prepared and their field-induced strain behaviors were investigated. Ceramic lead-free piezoelectric materials were prepared in bulk and fiber forms with 1 mol% CuO-added potassium sodium niobate K0.5Na0.5NbO3 and x = 7 mol% lithium-modified (K(0.5-x/2)Na(0.5-x/2)Li(x))NbO(3) compositions. Fibers were drawn using a novel alginate gelation technique. Piezocomposites were prepared from these fibers with 1-3 connectivity and an epoxy matrix. A fully recoverable electrostrain of up to approximately 0.11% was observed in the CuO-added sample, whereas the Li-modified sample yielded up to 0.10% at 50 kV/cm electric field. A strain value of up to approximately 0.03% at 50 kV/cm electric field was obtained for piezocomposites prepared from lithium-modified fibers. The high-field converse piezoelectric coefficient was calculated from the strain-electric field (x-E) graph for all samples. Strain characteristics of the bulk and piezocomposite samples were analyzed based on the variation of strain with respect to square of the polarization (x-P2) to determine the electrostrictive contribution to the strain.     

Anisotropy of the electromechanical properties and a high piezoelectric sensitivity of the 1-1 type ferroelectric piezocomposites

The electromechanical properties of ferroelectric piezocomposites of the 1-1 and 1-0-1 types were studied for the first time. The effect of the geometric factor on the piezocoefficients, electromechanical coupling coefficients, detection parameters, and piezosensitivity was studied. Exhibiting a nonmonotonic concentration dependence of these parameters, the materials studied have no analogs among piezocomposites with the connectivities of other types.     

Averaging of the physicomechanical characteristics of piezocomposites (plane deformation)

The mechanical, electric, and piezoelectric characteristics of fibrous piezocomposites with doubly periodic system of fibers are averaged within the framework of a model of regular structures under the conditions of plane deformation. It is shown that, for the general case of stacking of fibers with arbitrary cross section, the problem is reduced to the determination of certain functionals specified by a system of linear algebraic and singular integral equations of the corresponding boundary-value problem of electroelasticity for the analyzed structure.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 2, pp. 67–72, March–April, 2004.     

Electro-transport and structure of 1-2-3 HTSC single crystals with different plane defects topologies

In this work we study the structural and resistivity parameters of the 1-2-3 system HTSC single crystals by substituting yttrium to holmium and with a low (5%) aluminum doping. It was determined that in the case of non-stoichiometric samples, the structure of crystals, directly after the solution-melt growth, is close to tetragonal. In the process of additional oxygen saturation we observed a ferroelastic transition to orthorhombic phase, accompanied by the formation of a domain twin structure. The twin boundaries and the aluminum impurities are efficient scattering centers of normal carriers. At aluminum concentration y ≤ 0.5 the tweed structure is not formed.     

pH 值对共沉淀制备的LiCo1/3Ni1/3Mn1/3O2结构与电化学性能的影响

用在不同pH值下制备的共沉淀前驱体合成了LiCo1/3Ni1/3Mn1/3O2材料.用XRD、IR、ESEM、交流阻抗、充放电测试仪测试了pH值对材料结构和电化学性能的影响.结果表明在pH=11条件下合成的材料的层状结构特征和结晶度较好;pH值对材料颗粒的大小没有显著的影响,所有材料的粒径均在0.3～0.7μm 范围内;pH=11时合成的材料的放电比容量最高(达163.48mAh/g),循环性能最好,可能是由于此条件下合成的材料在电池充放电过程中电荷传递速率较快所致.     

pH值对共沉淀制备的LiCo1／3Ni1／3Mn1／3Oz结构与电化学性能的影响

用在不同PH值下制备的共沉淀前驱体合成了LiCo1/3Ni1／3Mn1/3O2材料。用XRD、IR、ESEM、交流阻抗、充放电测试仪测试了PH值对材料结构和电化学性能的影响。结果表明在pH=11条件下合成的材料的层状结构特征和结晶度较好；pH值对材料颗粒的大小没有显著的影响，所有材料的粒径均在0．3～0.7μm范围内；pH=11时合成的材料的放电比容量最高（达163．48mAh／g），循环性能最好，可能是由于此条件下合成的材料在电池充放电过程中电荷传递速率较快所致。     

碳纤维经编织物定型参数及渗透特性

研究了真空辅助树脂注射(VARI)工艺中含定型剂织物定型参数对织物压缩特性、渗透特性及其复合材料力学性能的影响,确定织物最优的定型参数。采用DSC和IR分别对织物表面定型剂的熔融特性和组成进行了分析,测试了不同定型温度下的压缩、渗透特性及复合材料力学性能。结果表明:此定型剂为含有环氧基团的低熔点聚合物;低于100℃时,随着定型温度的升高,织物的压缩厚度和渗透率均下降;100~140℃时,随着定型温度的升高,织物的压缩厚度基本无变化,渗透率有所提高,定型温度对力学性能基本无影响。     

Polymeric encapsulation of 1-2-3 ceramic superconductors

YBa₂Cu₃O_7−x pellets have been found to degrade and lose superconducting properties when exposed to the normal atmosphere for long durations. Hence, coating the superconducting pellets with a polymer to protect it from moisture and CO₂ of the ambient atmosphere is highly desirable. In this paper the criteria of selection of a polymer and some preliminary studies on polymeric encapsulation of 1-2-3 pellets are reported. Fevitite epoxy and stycast have been found to show encouraging results.     

Crystal Growth by the TSFZ Method and Superconducting Properties of the Tetragonal 1-2-3 Type Cuprates

Single crystals of Ca _0.5 La _1.25 Ba _1.25 Cu ₃O _y and NdBaSrCu ₃ O _y with the dimensions of the order of 1×1×0.5 mm ³ have been grown by the travelling-solvent floating zone method. As-grown single-crystals are tetragonal and non-superconducting. Through the annealing in oxygen, single crystals of Ca _0.5 La _1.25 Ba _1.25 Cu ₃ O _y have become superconductors with T _c = 55 - 65 K, keeping the tetragonal structure. On the other hand, single crystals of NdBaSrCu ₃ O _y have changed from tetragonal to orthorhombic through the annealing in oxygen, though they have become superconductors with T _c = 55 K.     

2-2压电复合材料面板的压力放大作用

２－２压电复合材料应用普遍。材料中聚合物对压电陶瓷的应力作用，使性能得到施工改善。在复合材料两电极面上复盖薄面板，由于板的应力传递作用会使使接收国和辐射力产生放大效应，从而提高了接收灵敏度和发射响应。     

Intrinsic paramagnetic centers in 1-2-3 superconductors

The¹⁶⁹Tm enhanced NMR in TmBa₂Cu₃O_6+x (x=0.5, 0.6) at temperatures below 4.2K and the⁶³Cu(1) NQR in YBa₂Cu₃O_6.5 at temperatures above 4.2K are used to study properties of intrinsic paramagnetic centers incorporated into superconducting materials. The spin-lattice relaxation of thulium and copper nuclei reveals three types of paramagnetic centers to be present in oxygen-deficient 1-2-3 superconductors, those are (1) two-level ones with a spin S=1/2, localized outside CuO₂ bilayers, (2) singlet-ground-state paramagnetic centers with an integer spin S1 in CuO₂ bilayers, and (3) exchange copper-oxygen clusters with a half-integer spin S5/2, localized in a nearest neighborhood of CuO_x basal plane at boundaries of superconducting OrthoII microdomains.