Curie temperature ferroelectric

The behavior of PTC materials is very different from NTC materials. Commercial PTC devices rely on the changes associated with the ferroelectric Curie temperature (0c). Typical PTC behavior is shown in Figure 30.19. In regions AB and CD the material is showing NTC behavior. But at the Curie temperature (0c) there is a large positive change in p. 

KTaO (LB Number 1A-5). KTa03 is cubic at aU temperatures, and its dielectric constant becomes very large at low temperatures without a phase transition (Fig. 4.5-14). It is generally believed that this behavior is related to the zero-point lattice vibrations. Replacement of Nb by Ta generally lowers drastically the ferroelectric Curie temperature, as seen by comparing Fig. 4.5-14 with Fig. 4.5-12. (This effect is well demonstrated later in Figs. 4.5-39 and 4.5-40). 

Hydrothermal BaTiOs powders, particularly very fine powders (less than — 100 nm) prepared at lower temperatures, show some structural characteristics that are not observed for coarser powders prepared by solid-state reaction at higher temperatures. X-ray diffraction reveals a cubic structure that is normally observed only at temperatures above the ferroelectric Curie temperature of 125-130°C. The possible causes for the apparent cubic and nonferroelectric structure are not clear and have been discussed in detail elsewhere (74). They include the idea of a critical size for ferroelectricity and, particularly for powders prepared by precipitation from solution, the presence of a high concentration of point defects due to hydroyxl groups in the structure. 

Briefly explain why the ferroelectric behavior of BaTi03 ceases above its ferroelectric Curie temperature. 

With X-ray fluorescence stoichiometry can be determined to 0.01-0.1 % when suitable standards are available. Precise lattice parameters and pycnometric density determinations have been used to determine deviations from stoichiometry or nonstoichiometry in Bertholide-type compounds. Other instrumental techniques such as nuclear magnetic resonance and comparison of ferroelectric Curie temperatures have also found applications. ... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Barium titanate [12047-27-7] has five crystaUine modifications. Of these, the tetragonal form is the most important. The stmcture is based on corner-linked oxygen octahedra, within which are located the Ti" " ions. These can be moved from their central positions either spontaneously or in an apphed electric field. Each TiO octahedron may then be regarded as an electric dipole. If dipoles within a local region, ie, a domain, are oriented parallel to one another and the orientation of all the dipoles within a domain can be changed by the appHcation of an electric field, the material is said to be ferroelectric. At ca 130°C, the Curie temperature, the barium titanate stmcture changes to cubic. The dipoles now behave independentiy, and the material is paraelectric (see Ferroelectrics). 

The ferroelectricity usually disappears above a certain transition temperature (often called a Curie temperature) above which the crystal is said to be paraelectric this is because thermal motion has destroyed the ferroelectric order. Occasionally the crystal melts or decomposes before the paraelectric state is reached. There are thus some analogies to ferromagnetic and paramagnetic compounds though it should be noted that there is no iron in ferroelectric compounds. Some typical examples, together with their transition temperatures and spontaneous permanent electric polarization P, are given in the Table. 

Tantalum and niobium are added, in the form of carbides, to cemented carbide compositions used in the production of cutting tools. Pure oxides are widely used in the optical industiy as additives and deposits, and in organic synthesis processes as catalysts and promoters [12, 13]. Binary and more complex oxide compounds based on tantalum and niobium form a huge family of ferroelectric materials that have high Curie temperatures, high dielectric permittivity, and piezoelectric, pyroelectric and non-linear optical properties [14-17]. Compounds of this class are used in the production of energy transformers, quantum electronics, piezoelectrics, acoustics, and so on. Two of... 

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

Crystals with one of the ten polar point-group symmetries (Ci, C2, Cs, C2V, C4, C4V, C3, C3v, C(, Cgv) are called polar crystals. They display spontaneous polarization and form a family of ferroelectric materials. The main properties of ferroelectric materials include relatively high dielectric permittivity, ferroelectric-paraelectric phase transition that occurs at a certain temperature called the Curie temperature, piezoelectric effect, pyroelectric effect, nonlinear optic property - the ability to multiply frequencies, ferroelectric hysteresis loop, and electrostrictive, electro-optic and other properties [16, 388],... 

In particular cases, oxyfluoride compounds with island-type crystal structures, such as K3NbOF6, K3TaOF6, K3Nb02F4 and K3Ta02F4, display ferroelectric-ferroelastic properties, with Curie temperatures of 283, 310, 420, 465°K, respectively [150, 191]. 

The function of I2g> (T) in the vicinity of the phase transition to centrosymmetric conditions usually has a linear character. Such behavior corresponds to ferroelectrics that undergo type II phase transitions and for which the SHG signal, l2Curie temperature is described by the Curie - Weiss Equation ... 

Above a specific temperature, the Curie temperature, a ferroelectric substance becomes paraelectric since the thermal vibrations counteract the orientation of the dipoles. The coordinated orientation of the dipoles taking place during the ferroelectric polarization is a cooperative phenomenon. This behavior is similar to that of ferromagnetic substances, which is the reason for its name the effect has to do nothing with iron (it is also called seignette or rochelle electricity). 

Above a temperature called the Curie temperature, Tc, ferroelectric behavior is lost, and the material is said to be in the paraelectric state in which it resembles a normal insulator. 

As a ferroelectric material, each piezoelectric ceramic is characterized by a Curie point or Curie temperature, T (Jaffe et al., 1971). Above this temperature, the ferroclcctricity is lost. An irreversible degradation of the... 

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