Barium titanate BaTiOs

Barium titanate has been studied extensively since the end of World War II when it was identified independently in Russia, Japan and USA as a promising material with high permittivity for ceramic capacitors. Its ferroelectric activity is known since 1946 when it was discovered on ceramic samples. Since 1950 s also the single-crystals of desired size and quality are available. BaTiOs undergoes several phase transitions 

In the tetragonal phase the polar 4-fold axis (also spontaneous polarization vector) is oriented along one of the 100 c directions (6 variants) of the parent cubic phase. The tetragonal unit cell results from the elongation of the cubic unit cell in one of the 100 c directions. The other two cube edges are compressed (Fig. 7.15.). 

In the rhombohedral phase the polar 3-fold axis is oriented along one of the 

Barium titanate is also the first material with the ferroelectricity studied theoretically by Devonshire (1949). Theoretical description is based on the idea of the thermodynamic potentials known from the Landau-Ginzburg theory of phase transitions (Devonshire 1949, 1951). Barium titanate is mainly used in multilayer ceramic capacitors and in positive temperature coefficient (PTC) elements (Bhalla et al. 2000). 

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The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

Barium titanate, BaTiOs, is a ferroelectric material (see Chapter 9) widely used in capacitors because of its high dielectric constant. It was initially prepared by heating barium carbonate and titanium dioxide at high temperature. 

A wide array of ferroelectric, piezoelectric and pyroelectric materials have titanium, zirconium and zinc metal cations as part of their elemental composition Many electrical materials based on titanium oxide (titanates) and zirconium oxide (zirconates) are known to have structures based on perovskite-type oxide lattices Barium titanate, BaTiOs and a diverse compositional range of PZT materials (lead zirconate titanates, Pb Zr Tij-yOs) and PLZT materials (lead lanthanum zirconate titanates, PbxLai-xZryTii-yOs) are among these perovskite-type electrical materials. 

The inverse-micelle approach may also offer a generalized scheme for the preparation of monodisperse metal-oxide nanoparticles. The reported materials are ferroelectric oxides and, thus, stray from our emphasis on optically active semiconductor NQDs. Nevertheless, the method demonstrates an intriguing and useful approach the combination of sol-gel techniques with inverse-micelle nanoparticle synthesis (with OTO erafe-temperature nucleation and growth). Monodisperse barium titanate, BaTiOs, nanocrystals, with diameters controlled in the range from 6-12nm, were prepared. In addition, proof-of-principle preparations were successfully conducted for Ti02 and PbTiOs. Single-source alkoxide precmsors are used to ensure proper stoichiometry in the preparation of complex oxides (e.g. bimetallic oxides) and are commercially available for a variety of systems. The... 

Lattice dynamics in bulk perovskite oxide ferroelectrics has been investigated for several decades using neutron scattering [71-77], far infrared spectroscopy [78-83], and Raman scattering. Raman spectroscopy is one of the most powerful analytical techniques for studying the lattice vibrations and other elementary excitations in solids providing important information about the stmcture, composition, strain, defects, and phase transitions. This technique was successfully applied to many ferroelectric materials, such as bulk perovskite oxides barium titanate (BaTiOs), strontium titanate (SrTiOs), lead titanate (PbTiOs) [84-88], and others. 

Barium titanate, BaTiOs, finds application as a ceramic material, and is primarily of interest as a result of its electrical properties (see Chapter 60). 

The relative permittivity values normally encountered in crystals are rather small (Table 11.1). Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than those in normal dielectrics. For example, one crystallographic polymorph of barium titanate, BaTiOs. has a relative permittivity, e, of the order of 20 000 (more values are given in Table 11.2.). By analogy... 

As an example, consider barium titanate, BaTiOs. Above 120 °C the paraelectric form of BaTiOa has the cubic perovskite structure, with oq = 0.4018 nm (Figure 11.22a). The large Ba " cations are surrounded by 12 oxygen ions, and the medium-sized Ti" " " ions are situated at the centre of an octahedron... 

Cho, S. D., Lee, J. Y, and Paik, K. W., Effect of particle size on dielectric constant and leakage current of epoxy/barium titanate (BaTiOs) composite films for embedded capacities, in International Conference on Electronic Materials and Packaging, 2001, pp. 63-68. 

Electronic ceramics include barium titanate (BaTiOs), zinc oxide (ZnO), lead zirconate titanate [Pb(ZrJ ii ()03], aluminum nitride (AIN), and HTSCs. They are used in applications as diverse as capacitor dielectrics, varistors. 

Barium titanate (BaTiOs) was the first ceramic in which ferroelectric behavior was observed and is probably the most extensively investigated of all ferroelectrics. Its discovery made available ks up to two orders of magnitude greater than had been known before. This property was very soon utilized in capacitors and BaTiOs remains the basic capacitor dielectric in use today (although not in its pure form). There are several reasons why BaTiOs has been so widely studied ... 

NaKC4H40e 4H2O), monopotassium dihydrophosphate (KH2PO4), or barium titanate (BaTiOs). At sufficiently high temperatures ferroelectrics show normal dielectric behavior. However, below a certain critical temperamre (so called. Curie temperature), even a small electric field causes a large polarization, which is preserved even if the external field is switched off. This means that below the Curie point ferroelectric materials show spontaneous polarization. The phase transition at the Curie temperature is related to the change of the lattice symmetry of the sample. 

Ceramics. Barium titanate (BaTiOs) was discovered in 1943 independently from American, Japanese and Russian scientists and was thus the first polycrystalline ferroelectric ceramic with Perovskite structure (Fig. 7.23). 

Lead zirconate titanate (PZT), barium titanate (BaTiOs), lead titanate (PbTiOs), potassium niobate (KNbOa), lithium niobate (LiNbOs), lithium titanate (LiTaOs), sodium tungstate (Na2W03) and zinc oxide (ZnO) are some of the most typical piezoceramics. Of these, PZT is the most widely used due to its superior performance. However, the toxicity of lead has raised concerns over the use of PZT. A restriction on the amount of lead present has been placed and is focused at eliminating its use eventually. Nevertheless, PZT has no rival at present. 

One of the compounds that have drawn attraction of the investigators in the area of sol-gel powderless ceramics is barium titanate, BaTiOs. An initial report came from Frey and Payne (1995) who prepared and used barium and titanium methoxyethoxides under controlled atmosphere for synthesis. The modified alkoxides were hydrolyzed by addition of a solution of water in 2-methoxyethanol. The sol was cast, sealed and aged. Drying at above 50°C yielded monolithic gels sintering of the gels at 800-1300°C produced... 

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