Mojtaba Biglar

Fracture Prediction of Piezoelectric Ceramic by the 2-D Boundary Element Analysis

Piezoelectric ceramics are widely used as sensors and multilayer actuators in many areas of industry [1], despite the absence of a fundamental understanding of their fracture behaviour [2]. Piezoelectricity is a linear effect that is related to the microscopic structure of the solid. Among piezoelectric materials, barium titanate is the most often used as a ceramic material. Barium titanate is a ferroelectric classic perovskite structure with wide-ranging material properties that depend on its composition [3, 4]. The perovskite structure ABO3 (Fig. 1) is the arrangement where the corner-sharing oxygen octahedra are linked together in a regular cubic array with smaller cations (Ti, Zr, Sn, Nb, etc.) occupying the central octahedral B-site, and larger cations (Pb, Ba, Sr, Ca, Na, etc.) filling the interstices between octahedra in the larger A-site [5]. Several studies have been carry out in order to study the influence of compositionally modified ceramic bodies and the sintering conditions on the properties of ceramic [6]. The ceramic material which is composed of the random orientation of these piezoelectric crystallites, is inactive, i.e., the effects from the individual crystals cancel each other and no discernable piezoelectricity is present [5]. The domains are the regions of equally oriented polarization vectors. To orient the domains the pooling method can be used. This technique consist in polarizing the ceramic through the application of a static electric field. Open image in new window Fig. 1 Perovskite structure

Application of the grain boundary formulation and image processing-based algorithm in micro-mechanical analysis of piezoelectric ceramic

The main subject of this paper is the micro-mechanical analysis of a piezoelectric ceramic. In micro-mechanical analyses, it is very important to have knowledge about the real and natural micro-structure of materials. Therefore, the barium titanate powder was prepared using the solid-state technique, and pellets and beams were manufactured by uniaxial and isostatic pressing. The boundary element method (BEM) is used in order to be combined with three different grain boundary formulations for investigation of micro-mechanics and crack nucleation and evaluation in piezoelectric ceramic. In order to develop a numerical programming algorithm, suitable models of polycrystalline aggregate have to be discretised for the BEM analysis. Hence, original comprehensive algorithms are designed on the basis of image processing methods. Several assumptions are made to model the grain boundary in micro-scale. In the first step, before having any cracks, the traction equilibrium and displacement compatibility are governing equations. When the onset micro-crack starts to initiate, one mixed-mode potential based cohesive law is applied to model grain boundaries and investigate the intergranular crack nucleation and evolution. Upon interface failure, a frictional law is utilised in order to study separation, sliding or sticking between micro-crack surfaces. Several numerical experiments on barium titanate polycrystalline aggregate are presented to show the effectiveness of image processing-based discretisation algorithms and grain boundary formulation in micro-mechanical analysis.

Application of BaTiO3 Perovskite Material for Piezoelectric Multilayer Actuators

In this paper, the results of the manufacturing of BaTiO3 material destined for use in stacked-disk multilayer actuator production are presented. SEM microstructures and electric properties of the fabricated pellets are presented and discussed. The dilatometric curve was executed using the high temperature dilatometer in order to determine at which temperature barium titanate pellets and beams should be sintered to receive full dense sinters. Finally, the problem of metal layer deposition on barium titanate ceramics during actuator fabrication is considered.

Micro-mechanical modelling of mechanical and electrical properties in homogeneous piezoelectric ceramic by using boundary integral formulations

Recent experiments on polycrystalline materials show that microcrystalline materials have a strong dependency ona grain size. In this study, mechanical and electrical properties of polycrystalline materials in micro level were studied by using averaging theorems. To completely understand the size-dependency of polycrystalline materials, an integral non-local approach that can predict the stress-strain relations for these materials was presented. In microcrystalline materials, crystalline and grain-boundary were considered as two separate phases. Mechanical properties of the crystalline phase were modelled using crystalline brittle material and is composed of randomly distributed and orientated single crystal anisotropic elastic grains. For microcrystalline materials, the surface-to-volume ratio of the grain boundaries is low enough to ignore its contribution to the elastic deformation. Therefore, the grain boundary phase was not considered in microcrystalline materials and mechanical properties of the crystalline phase were modelled using appropriate integral non-local approach. Finally, the constitutive equations for polycrystalline materials were implemented into a boundary integral equation and the results and some examples are provided for piezoelectric ceramic.

Synthesis of Barium Titanate Piezoelectric Ceramics for Multilayer Actuators (MLAs)

Abstract In this paper the characteristics of BaTiO3 ceramics synthesized by solid state method is presented. In order to receive the monophase ceramics the double activation and calcination were applied. A spray drier was used to granulate the powder of BaTiO3. Isostatic and uniaxial pressing were applied to manufacture the barium titanate pellets. The properties of fabricated BaTiO3 ceramics were determined at different stages of production. After the sintering phase, the hardness, the bending strength, the fracture toughness, and the coefficient of thermal expansion of barium titanate sinter were estimated. The BaTiO3 powder is characterized by spherical grains and the average size of 0.5 μm. The small value of the specific surface area of granulate ensured good properties of material mouldability and finally allowed to receive sinters of high density.