Mechanisms of Electro-Mechanical Coupling in Polycrystalline Piezoelectric Ceramic Materials
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MECHANISMS OF ELECTRO-MECHANICAL COUPLING IN POLYCRYSTALLINE PIEZOELECTRIC CERAMIC MATERIALS A Thesis By Mohammad Jahangir Hossain Submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy School of Materials Science and Engineering Faculty of Science University of New South Wales April 2016 PLEASE TYPE THE UNIVERSITY OF NEW SOUTH WALES Thesis/Dissertation Sheet Surname or Family name: Hossain First name: Mohammad Jahangir Other name/s: Abbreviation for degree as given in the University calendar: PhD School: School of Materials Science and Engineering Faculty: Science Title: Mechanisms of electro-mechanical coupling in polycrystalline piezoelectric ceramic materials Abstract 350 words maximum: (PLEASE TYPE) Piezoelectric ceramics have extensive applications in electronic industries as sensor and actuator device materials. Most of the currently used piezoelectric ceramics in these electronic devices are based on PbZrxTi1-xO3 (PZT). Due to international consent towards removing toxic substances from the electronic and electrical equipment and biological harmful effects of Pb to the environment, it is necessary to find lead-free compositions with comparable properties to those of PZT. Promising lead-free piezoceramics are mainly based on solid solutions incorporating either Bi1/2Na1/2TiO3 (BNT) or NaxK1-xNbO3. Although lead-free compositions of piezoelectric materials have been reported with enhanced properties, no single lead-free piezoelectric material has been identified for the replacement of PZT over the wide operating conditions that it is currently used for. To improve the electro-mechanical properties of these materials further will require deep knowledge about the underlying structural origin of electric-field-induced strains. A sample cell has been developed, capable of measuring the structural variations of piezoceramics under applied electric field using low-energy X-ray scattering techniques, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The electro-mechanical coupling mechanisms in PZT and BNT-6.25BaTiO3 (BNT-6.25BT) have been studied using in situ low- energy (12.4 keV) and in situ high-energy (73 keV) synchrotron XRD. The results show that for both systems the intrinsic lattice strains and extrinsic non-180 domain switching strains are larger at the surface than in the bulk. The structure property relationships in a series of (100-x)BNT-xBT (BNT-xBT) solid solutions with the BT content ranging from 5 mol% to 8 mol% in 0.25 mol% steps have been studied using in situ high-energy synchrotron XRD under unipolar stress and bipolar electric field. During application of both stress and electric field, lower BT content samples (x < 5.75) tended to transform to rhombohedral symmetry, while higher BT content (x > 7) tended to go tetragonal. Compositions between these tended to transform to mixed phase symmetry. The results show that the stress and electric-field-induced phase transformation mechanisms are highly analogous. 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Table of contents Table of contents ................................................................................................................ i Abstract ............................................................................................................................. v List of publication and presentation ............................................................................... viii Acknowledgements .......................................................................................................... ix List of figures .................................................................................................................... x List of tables .................................................................................................................... xv 1 Introduction ............................................................................................................... 1 2 Literature review ....................................................................................................... 7 2.1 Fundamental of electro-mechanical coupling .................................................... 7 2.1.1 Dielectrics and polarisation mechanisms .................................................... 7 2.1.2 Piezoelectricity ............................................................................................ 9 2.1.3 Ferroelectricity .......................................................................................... 11 2.1.4 Relaxor ferroelectricity ............................................................................. 16 2.2 Lead-free piezoelectric ceramics ...................................................................... 18 2.2.1 Bismuth sodium titanate ............................................................................ 18 2.2.2 Barium titanate .......................................................................................... 20 2.2.3 BNT-BT solid solution .............................................................................. 20 2.3 Diffraction methods for studying piezoelectric ceramics ................................. 21 2.3.1 Considerations for in situ measurements .................................................. 24 3 Experimental procedure .......................................................................................... 26 i 3.1 Fabrication method of ceramic materials ......................................................... 26 3.1.1 Sample preparation for low-energy XRD ................................................. 27 3.1.2 Sample preparation for high-energy XRD under electric field ................. 29 3.1.3 Sample preparation for high-energy XRD under stress ............................ 30 3.2 Experimental methods ...................................................................................... 32