Curie point ferroelectric

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. 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

Figure 3.8 Anomalous temperature dependence of relative dielectric constant of ferroelectric crystals at the transition temperature (Curie point).

Structure of NaN02 below and above the CURIE point. Bottom domains in a ferroelectric crystal of NaN02... 

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... 

The structure of the ferroelectric, tri-glycine fluoberyllate [8, 9], has been determined, and participation of H-bonds in the polarization mechanism at the Curie point is described. Glycine silver nitrate is also ferroelectric [10], and a polarization mechanism, also involving H-bonds, is suggested. It is evident, from dielectric and structural... 

Finally, ferroelectricity is manifest in asymmetrical crystals producing domains of spontaneous polarization whose polar axis direction can be reversed in an electric field directed opposite the total dipole moment of the lattice. The two (or more) directions can coexist in a crystal as domain structures comprising millions of unit cells which contain the same electric orientation. The symmetry elements are temperature sensitive in ferroelectric materials [27]. At a particular temperature called the Curie Point the values of the piezoelectric coefficients reach particularly high values. Above the Curie Point the crystal transformation is to a less polar form and the ferroelectric nature disappears. 

CURIE POINT (or Curie Temperature). Ferromagnetic materials lose their permanent or spontaneous magnetization above a critical temperature (different for different substances). This critical temperature is called the Curie point. Similarly, ferroelectric materials lose their spontaneous polarization above a critical temperature. For some such materials, this lemperaLure is called the "upper Curie point." for there is also a "lower Curie point." below which the ferroelectric property disappears. See also Ferromagnetism. 

Barium titanate, BaTi03, is probably the most widely studied ferroelectric oxide. Extensive studies were conducted on this compound during World War II in the United States, England, Russia, and Japan, but the results were not revealed until after the war. Barium titanium(IV) oxide was found to be a ferroelectric up to a temperature of 120°C., which is its Curie point. Above 120°C., barium titanium(IV) oxide has the cubic perovskite structure, and below this temperature the oxygen and titanium ions are shifted and result in a tetragonal structure with the c axis approximately 1% longer than the a axis. Below 0°C., the symmetry of barium titanate becomes orthorhombic, and below —90°C., it becomes trigonal. 

The compounds have been of interest because of their physical properties. They are both photoconducting and ferroelectric. Antimony(III) iodide sulfide has a Curie point at 22°C.12... 

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. 

Many ferroelectrics possess very high permittivity values which vary considerably with both applied field strength and temperature. The permittivity reaches a peak at the Curie point and falls off" at higher temperatures in accordance with the Curie-Weiss law... 

PTC resistors could be classified as critical temperature resistors because, in the case of the most widely used type, the positive coefficient is associated with the ferroelectric Curie point. 

The grain size of a ferroelectric ceramic has a marked effect on the permittivity for the size range 1-50 /mi (see Fig. 2.48). Below about 1 /mi the permittivity falls with decreasing grain size. An important factor leading to this behaviour is the variation in the stress to which a grain is subjected as it cools through the Curie point. 

Ferroelectric materials above their Curie point behave electrostrictively and comparison of the electrostriction coefficient with d j2eP shows them to be of similar magnitude. This suggests that the large -coefficients shown by some ferroelectric materials are due to a combination of large electrostriction coefficients and large spontaneous polarization and permittivity values. 

As a ferroelectric perovskite in ceramic form cools through its Curie point it contracts isotropically since the orientations of its component crystals are random. However, the individual crystals will have a tendency to assume the anisotropic shapes required by the orientation of their crystal axes. This tendency will be counteracted by the isotropic contraction of the cavities they occupy. As a consequence a complex system of differently oriented domains that minimizes the elastic strain energy within the crystals will become established. 

In the majority of ferroelectric tungsten bronzes the polar axes are parallel to the A-site tunnels. PbNb206 is an exception below the Curie point it undergoes an orthorhombic distortion in the plane perpendicular to the A-site tunnels as indicated in Fig. 6.16. It possesses four possible polar directions and can be poled successfully in ceramic form. 

The contribution E(ds/dT) (Eq. (7.3)) can be made by all dielectrics, whether polar or not, but since the temperature coefficients of permittivity of ferroelectric materials are high, in their case the effect can be comparable in magnitude with the true pyroelectric effect. This is also the case above the Curie point and where, because of the absence of domains, the dielectric losses of ferroelectrics are reduced, which is important in some applications. However, the provision of a very stable biasing field is not always convenient. 

Ferroelectricity in partially deuteriated betaine arsenate has been investigated Ferroelectric betaine arsenate becomes antiferroelectric by deuteriation and the Curie point shifts monotonically with the D-content. Dielectric and caloric data of some crystals with different deuteriation degrees have been presented. 

Vinylidene fluoride-trifluoroethylene (VF2-F3E) copolymers exhibit a ferroelectric-paraelectric phase transition, the first such case found for a synthetic polymer. In this transition, the electric polarization and piezoelectric constant of the film disappear above the Curie point (Tcurie)- The temperature dependence of the dielectric constant, , obeys the so called Curie-Weiss law ... 

Pure BaTiOg is usually not employed for piezoelectric purposes. Further components, mostly PbTi03, are introduced lo control the Curie point and the piezoelectric properties. The dominating type of piezoelectrics is nowadays the so-called PZT ceramics based on a solid solution in the system PbZr03 —PbTiOj. The crystals of PbZr03 become ferroelectric on introduction of at least 3 % PbTiOs. ... 

This property is exhibited by ferroelectrics solely within a certain temperature range, up to the so-called Curie point, which occurs at 120 °C for BaTiOj and corresponds to the inversion temperature of the tetragonal to cubic structure. The sequence of BaTi03 inversions is as follows ... 

Tanaka et al. [110] studied the ferroelectric phase transition of a 52/48 random copolymer of PVDF and TrFE by pulsed proton NMR. Attention was focused on the dynamic properties of the phase transition near the Curie point. Two samples were studied one was isothermally crystallised at 125°C, and the other was rapidly quenched in liquid nitrogen from the melt. The... 

Exists in five cryst modifications. The tetragonal form (obtained by the wet process) appears to have the most desirable electric properties and is described here d 6.08. mp 1625. Curie point 120". Has ferroelectric and piezoelectric properties. Becomes parmanently polarized when exposed to high voltage direct current, provided the temperature is never allowed to rise above Curie pt. Has high dielectric properties which can be influenced by temp, voltage, and frequency. 

Orthorhombic crystals. Very freely sol in water. Has ferroelectric properties Curie point 47". Spontaneous polarization at room temp 2.2 X 10 coul/cm. Coercive field 220 v/cm. 

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