Structural and electrical investigations of tungsten substituted barium strontium titanate ferroelectric ceramics

Abstract

Pyroelectric crystals possess the unusual characteristic of being permanently polarized within a given temperature range. Unlike the more general piezoelectric materials that produce a polarization under stress, the pyroelectric materials develop the polarization spontaneously and posses permanent dipoles. The polarization also changes with temperature, hence, the term pyroelectricity. A subgroup of the spontaneously polarized pyroelectric materials is a very special category of materials known as ferroelectric materials. Similar to pyroelectric materials, materials in this group possess spontaneous dipoles; however, unlike pyroelectrics, these dipoles are reversible by an electric field of suitable magnitude less than the dielectric breakdown of the material. Thus, the two conditions necessary in a material to be classified as a ferroelectric material are: 1. (i) the existence of spontaneous polarization and 2. (ii) the ability to reverse the direction of polarization. Ferroelectricity was discovered in Rochelle Salt (KNaC4H4O6.4H2O) in 1921 and then in polycrystalline Barium Titanate (BaTiO3) around the middle of 1940s. Thereafter, large number of new materials have been discovered which show ferroelectricity that leading to a significant number of industrial and commercial applications. The prominent properties of ferroelectric materials such as polarization hysteresis, large dielectric constant and remarkable piezoelectric, pyroelectric and electro-optical effects makes them useful for a number of electronic devices like small size high capacitance capacitors, piezoelectric sonar, pyroelectric security surveillance devices, medical diagnostic transducers, electrooptic light valves, ferroelectric memories, etc. Because of all these important applications, the field of ferroelectric materials is ever expanding. In the general group of ferroelectric materials, based on the basic structure, the following four types of ferroelectric materials are known: 1. (i) Tungsten-Bronze group 2. (ii) Perovskites group 3. (iii) Pyrochlore group 4. (iv) Bismuth Layer-Structured group. The material chosen in the present work is from pervoskite group and is detailed below. Barium Titanate (BaTiO3) Barium titanate is a well known ferroelectric material since 1940’s. It is also one of the oxygen octahedron group of ferroelectric materials. In this structure Ba2+ ions occupy corners of a cube, O2- ions occupy the centre of surface faces of the cube and at the centre of the cube small Ti4+ ion is located. Barium titanate (BaTiO3) has a paraelectric cubic phase above its curie temperature of about 120 ̊C. In the temperature range of 0 ̊C to 120 ̊C the ferroelectric tetragonal phase with c/a ratio ~1.01 is stable. Between -90 ̊C to 0 ̊C the ferroelectric orthorhombic phase is stable. On decreasing the temperature below - 90 ̊C phase transition from orthorhombic to ferroelectric rhombohedral phase leads to polarization along one of the [111] cubic directions. Among the ferroelectric materials, the perovskite materials like BaTiO3, are widely used as capacitors, ultrasonic transducers, dynamic random access memories (DRAMs), non-volatile ferroelectric random access memories, infrared sensors, pyroelectric infrared sensors etc. It shows positive temperature coefficient (PTC) of resistance. Due to PTCR properties, barium titanate is very often used as thermistors, e.g., in thermal switches. After an extensive literature surveys, in Journals like Materials Science, Ceramic International, Journal of European Ceramics Society, Ferroelectrics, Journal of Inorganic Materials, Solid State Communication, Physica B, etc. it was felt that although a lot of work has been reported for both A- as well as B- site substitutions in the Barium Titanate, however, extensive study of the changes in physical properties due to the systematic substitution of the tungsten in the barium (strontium) titanate materials is lacking, prompting this candidate to undertake the present work. In the present work, a systematic B-site substitution in barium (strontium) titanate was undertaken and their properties were investigated. The chemical formulas for the studied compositions are: Series 1: Series 2: Series 3: BaTi0.85W0.15O3 for optimization of synthesis parameters Ba(Ti1-xWx)O3 x = 0.0, 0.05, 0.15, 0.30 Ba0.8Sr0.2(Ti1-xWx)O3 x = 0.0, 0.05, 0.075, 0.1, 0.15, 0.30. All the samples were prepared by solid state reaction method at the optimized sintering conditions and studied for their structural, dielectric, ferroelectric, piezoelectric, dc conductivity and impedance properties. Also, Barium titanate and BaTi0.95W0.05O3 were synthesized by mechanical activation technique by using planetary ball-mill and its structural and electrical properties have been investigated.