Titanates ferroelectric properties

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates ate Hsted in Table 15. The most important apphcations of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba2Ti04, barium orthotitanate, titanates do not contain discrete TiO ions but ate mixed oxides. Ba2Ti04 has the P-K SO stmcture in which distorted tetrahedral TiO ions occur. 

The dielectric constant of barium titanate, along [001] is about 200 and along [100] it is 4000 at room temperature.3 The spontaneous polarization at room temperature is 26 X 10-6 C./cm.2, and the value of the coercive field has been found to vary from 500 to 2000 volts/cm. The crystal structure of barium titanate at room temperature can be represented by a tetragonal unit cell with size of a0 = 3.992 A., and c0 = 4.036 A., but the symmetry becomes cubic above 120°C., at which temperature the crystals no longer exhibit ferroelectric properties. 

Ferroelectric behaviour is limited to certain materials and to particular temperature ranges for a given material. As shown for barium titanate in Section 2.7.3, Fig. 2.40(c), they have a Curie point Tc, i.e. a temperature at which the spontaneous polarization falls to zero and above which the properties change to those of a paraelectric (i.e. a normal dielectric). A few ferroelectrics, notably Rochelle Salt (sodium potassium tartrate tetrahydrate (NaKC406.4H20)) which was the material in which ferroelectric behaviour was first recognized by J. Yalasek in 1920, also have lower transitions below which ferroelectric properties disappear. 

Barium titanate (BaTiOj), a perovskite-type electro-ceramic material, has been extensively studied and utilized due to its dielectric and ferroelectric properties. The wide applications of barium titanates include multiplayer capacitors in electronic circuits, nonlinear resistors, thermal switches, passive memory storage devices, and transducers. In addition, barium titanate can be used for chemical sensors due to its surface sensivity to gas adsorption. 

An extensive series of phases of general formula Ba Ti5,0 +2y are formed from the reaction of Ti02 with barium oxide. The simplest, BaTiOs, is known as barium titanate. These materials are of interest because of their ferroelectric properties, which result from the differences in the relative sizes of the Ba andTi ions. The Ti ions are located between six Ba ions, which occupy octahedral positions. However, the Ti ions are small compared to the Ba and so are free to move within their octahedral Ba holes . 

As concerns the piezoelectric layer, the first choice often goes to lead zirconate titanate (PZT) because of its outstanding piezoelectric, pyroelectric and ferroelectric properties. Nickel ferrite (NF) is not widely employed for the synthesis of the multilayered composites owing to a strong reduction of its magnetization in the lower grain size limit. However, a very thin NF layer can help to attain entirely different properties and, hence, this material has been chosen as a sandwiched layer in the present work. 

Although correlation between parameters is a function of the data structure and has nothing to do with deficiencies in the model, it has implications for both the choice of the model and the design of the experiment. EVANS described his experiences with the determination of the crystal structure of tetragonal barium titanate (BaTiOa). The problem was ample in that it involved only three atomic positional parameters (one for Ti and two for 0), plus nine thermal parameters. There was considerable interest in the details of the structure because of the ferroelectric properties of the material. The proposed model was essentially a simple cubic arrangement of atoms, but with Ti displaced slightly from the center of an octahedron. By ordinary x-ray standards, this distortion (which was expected to be on the order of 0.15 A) could be measured with a standard error of 0.01-0.02 A if... 

Conducting polymers also can be utilized to form core-shell structures with high dielectric constant particles. Fang et al. used PANl to encapsulate barium titanate via in situ oxidative polymerization. They examined the influence of the fraction of BaTiOs particles on the ER behavior, and found that the PANl/ BaTiOs compo-sites-based ERFs exhibit a better ER effect than does pure PANl, which result might be due to the unique ferroelectric properties as well as the high dielectric constant of BaTiOs nanoparticles. 

Pyro- and Piezoelectric Properties The electric field application on a ferroelectric nanoceramic/polymer composite creates a macroscopic polarization in the sample, responsible for the piezo- and pyroelectricity of the composite. It is possible to induce ferroelectric behavior in an inert matrix [Huang et al., 2004] or to improve the piezo-and pyroelectricity of polymers. Lam and Chan [2005] studied the influence of lead magnesium niobate-lead titanate (PMN-PT) particles on the ferroelectric properties of a PVDF-TrFE matrix. The piezoelectric and pyroelectric coefficients were measured in the electrical field direction. The Curie point of PVDF-TrFE and PMN-PT is around 105 and 120°C, respectively. Different polarization procedures are possible. As the signs of piezoelectric coefficients of ceramic and copolymer are opposite, the poling conditions modify the piezoelectric properties of the sample. In all cases, the increase in the longitudinal piezoelectric strain coefficient, 33, with ceramic phase poled) at < / = 0.4, the piezoelectric coefficient increases up to 15 pC/N. The decrease in da for parallel polarization is due primarily to the increase in piezoelectric activity of the ceramic phase with the volume fraction of PMN-PT. The maximum piezoelectric coefficient was obtained for antiparallel polarization, and at < / = 0.4 of PMN-PT, it reached 30pC/N. 

The importance of perovskites became apparent with the discovery of the valuable dielectric and ferroelectric properties of barium titanate, BaTiOj, in the 1940s. This material was rapidly employed in electronics in the form of capacitors and transducers. In the decades that followed, attempts to improve the material properties of BaTiOj lead to intensive research on the structure - property relations of a large number of nominally ionic ceramic perovskite-related phases with overall compositions ABOj, with a result that vast numbers of new phases were synthesised. 

Transparent ferroelectric single crystals are traditionally used for electro-optic applications. Since the optical transparency was first discovered in lead-lanthanum-zirconate-titanate (PLZT), ferroelectric ceramics have been investigated in great depth such that, today, their characteristics allow them to compete with single crystals for certain electro-optic applications. The electro-optic properties of PLZT compositions are intimately related to their ferroelectric properties. Variations in ferroelectric polarization with an electric field, such as in a hysteresis loop, also affect the optical properties of the material. 

Ferroelectric properties of lead zirconate titanate imder radial load. Appl. Phys. 

Barium titanate is a crystalline ceramic compound with outstanding diaelectric, piezoelectric, and ferroelectric properties. It is used in capacitors and as a piezoelectric transducer. 

Barium Titanate. BaTi03, m.p. 1618°C. Made by heating a mixture of barium carbonate and titania at 1300-1350°C. Because of its high dielectric constant (1350-1600 at 1 MHz and 25 C) and its piezoelectric and ferroelectric properties, it finds use in electronic components its Curie temperature is 120-140°C. The properties can be altered by the... 

Perovskite. CaTi03 m.p. 1915°C sp. gr. 4.10. This mineral gives its name to a group of compounds of similar structure, these forming the basis of the titanate, stannate and zirconate dielectrics. Extensive solid solution is possible so that the potential range of compositions and ferroelectric properties is very great. R. S. Roth (7. Res. Nat. Bur. Stand., 58,75,... 

Kay, H.F. and Vousden, P. (1949) Symmetry changes in barium titanate at low temperature and their relation to its ferroelectric properties. Philos. Mag.,... 

BARIUM TITANATE. BaTiOj M.p. >1500°C. Pure form undergoes abrupt phase change from tetragonal to cubic at 130°C, the Curie temperature. Barium titanate is usually produced by the solid-state reaction of barium carbonate and titanium dioxide. Has widespread use in the electronics industry because of its high dielectric constant, and piezoelectric and ferroelectric properties. The high dielectric constant of BaTiOj and the ease with which its electrical properties can be modified by combination with other materials make it exceptionally suitable for miniature capacitors. 

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Barium titanate is usually produced by the soHd-state reaction of barium carbonate and titanium dioxide. Dielectric and pie2oelectric properties of BaTiO can be affected by stoichiometry, micro stmcture, and additive ions that can enter into soHd solution. In the perovskite lattice, substitutions of Pb ", Sr ", Ca ", and Cd " can be made for part of the barium ions, maintaining the ferroelectric characteristics. Similarly, the TP" ion can partially be replaced with Sn +, Zr +, Ce +, and Th +. The possibihties for forming solution alloys in all these stmctures offer a range of compositions, which present a... 

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