Growth and characterization of lead free piezoelectric single crystals

Abstract

A significant portion of the progress made in the past decades in the development of piezoelectric sensors and transducers is mixed with the progress made in the field of material science, and in particular with piezoelectric materials. This is primarily due to the excellent piezoelectric, dielectric, electrostrictive and ferroelectric properties, among others, found in piezoelectric materials. Secondly, we highlight the relative ease of producing piezoelectric materials in polycrystalline form, that is, in the form of ceramics and thin films, and more recently in the form of volumetric single crystals (large crystals). In 1997, the feasibility of producing (large) volumetric piezoelectric single crystals of the (1-x) [Pb(Mg1/3, Nb2/3) O3]-(x) PbTiO3 family was disclosed. This viability was achieved by using the Bridgman vertical crystal growth technique and the simultaneous control and manipulation of the molten solution. The physical properties of these single crystals were significantly higher than those obtained in ceramic materials. As an example, single crystals have electromechanical coupling factors, k33, on the order of 93%, dielectric constant, K3, 30,000 and piezoelectric coefficient, d33, greater than 2300 pC/N. It was only from the use of these single crystals that industries began to produce a new and superior generation of ultrasonic piezoelectric transducers for medical imaging, as described above. Although lead-based perovskites have excellent electrical properties, in recent years several restrictions have been imposed by government agencies in several countries regarding the continuity of production and use of Pb-containing materials. Thus, the search for new compositions of piezoceramics with similar or even superior physical properties of Pb perovskites have become one of the area's major scientific and technological challenges. Among the most promising compositions that could replace leaded piezoelectric materials are certainly those based on the Sodium Potassium Niobate system [(Na, K)NbO3 - KNN]. KNN features a perovskite-like crystalline structure, such as PZT, and a rich phase diagram with various morphotropic phase contours (CFM). This is very important because it represents the possibility of maximizing the electrical properties of the KNN, especially the piezoelectric ones, in the vicinity of its CFM, such as the PZT. Therefore, motivated by this scientific and technological relevance, this project proposes the growth and characterization of physical properties of (Na,K)NbO3 based volumetric piezoelectric single crystals. Thus, the execution of this project will establish favorable conditions both for conducting fundamental studies and for the application of these materials in the technological sector. The main objective of this project is the growth and structural and electrical characterization of volumetric (Na,K)NbO3 (KNN) based monocrystalline piezoelectric materials produced by the vertical Bridgman technique. Among the main goals and activities to be developed in this project we highlight: 1) implementation of the vertical Bridgman technique; 2) study of thermal properties (eg. melting temperature, refractoriness and degree of incongruity) of the post-precursors by thermal analysis (DSC/DTA); 3) growth of volumetric KNN piezoelectric monocrystals, without doping, free of secondary phases; 4) growth of KNN volumetric piezoelectric single crystals, doped with "hardening" and "softening" di-elements, for the production of high and low quality factor piezoelectric crystals, respectively; 5) characterization of structural, piezoelectric and dielectric properties of the materials produced. (AU)