Pyroelectric substance

The first term represents the temperature dependence of the film density, being directly related to the thermal expansion coefficient of the film, and also indirectly related to that of the substrate. Generally, the thermal expansion coefficient of the organic substance is much larger than that of the inorganic substance like glass. In the case of the alternate LB film of fatty acid/alkylamine or fatty acid/alkylaniline, the pyroelectric coefficient p decreased as the thermal coefficient of the substrate increased [18]. 

When this polymer is processed in a particular way, it becomes piezoelectric and pyroelectric. A piezoelectric substance produces an electric current when it is physically deformed or alternatively undergoes a deformation caused by the application of a current. A pyroelectric material is one that develops an electrical potential in response to a change in its temperature. 

In Franklin s time there were four known sources of electricity frictional, atmospheric (lightning), pyroelectricity (due to heating or cooling of certain substances) and animal electricity, as from the torpedo fish. All these sources yield what is somewhat inaccurately described as "static electricity. Since as we now know, the phenomena observed in connection with "static electricity are due to relatively small electric charges at high intensities it is not surprising that only scattered observations were made that indicated that electricity could be associated with chemical action. 

The chemists from Orsay have synthesised chiral compound p-decyloxybenzy-lidene-p -amino-2methylbutylcinnamate (DOBAMBC), Fig. 13.3. Indeed, in the temperature range 95-117°C, this substance showed a linear electro-optical effect characteristic of a pyroelectric phase. The effect was observed in thick home-otropically oriented layers. Due to chiral stmeture of DOBAMBC molecules, the SmC phase had a spiral structure with the helical axis perpendicular to the limiting glasses. Fig. 13.4a. Under a microscope the preparation showed a conoscopic cross typical of a uniaxial phase, and, upon application of the in-plane electric field E, ... 

As we have seen, locally the smectic C layers are polar, belonging to pyroelectric class C2. Macroscopically SmC either forms a helical structore or does not. So, we can discuss a structure without helicity. In a sense, the formation of a helix is equivalent to formation of ferroelectric domains which would reduce overall macroscopic polarisation. Thus we can consider the (1) (very important) and (2) (additional) requirements fulfilled. As to the phase transition (3), we know that in the smectic A phase, even chiral, there is no polar axis, therefore that phase can be considered as a paraelectric phase. The two-component order parameter of the A -C transition is the same as the parameter of the A-C transition in an achiral substance, namely 9exp (i(p), where we recognise the tilt 9 and azimuth (p angles. The spontaneous... 

Marcasite [Named after Arabic or Moorish for pyrite and similar substances] (ICSD 26756 and PDF 37-475) FeS, M= 119.979 46.55 wt.% Fe 53.45 wt.% S (Sulfides and sulfosalts) Coordinence Fe(6) Orthorhombic a s 444.3 pm b = 542.3 pm c= 338.76 pm C18, oP6 (Z=2) S.G. Pnnm P.G. 222 Marcasite type Isotropic R = 48.9-55.5% 6-6.5 (HV 941- 1288) 4900 Habit tabular, cickscomb aggregate, faces curved. Color white green. Luster metallic. Diaphaneity opaque. Fracture conchoidal. Cleavage 101. Twinning 101. Streak black. Pyroelectric. Occurrence sedimentary, magmatic, metamorphic, and hydrothermal... 

For solids, the pyroelectric effect and piezoelectric effect are characteristic for a wider range of crystallographic (symmetry) classes, rather than ferroelectricity itself. The critical point for a substance to be considered ferroelectric is the reorientation of dipoles, i.e., macroscopically, the change of the polarization sign under... 

FIGURE 7.32. (1) The pyroelectric coefficient and (2) the spontaneous polarization as functions of temperature for substance (7.viii). 

A related effect is pyroelectricity, which is the generation of voltage by the heating or cooling of a crystal. Because substances tend to expand when heated and contract when cooled, changes in temperature of polar crystals tend to change the overall dipole moment and voltage of the crystal. 

Figure 5. Response of polar dielectrics (containing local permanent dipoles) to an applied electric field from top to bottom paraelectric, ferroelectric, ferrielectric, antiferroelectric, and helielectric (helical anti-ferroelectric). A pyroelectric in the strict sense hardly responds to a field at all. A paraelectric, antiferro-electric, or helieletric phase shows normal, i.e., linear dielectric behavior and has only one stable, i.e., equilibrium, state for E=0. A ferroelectric as well as a ferrielectric (a subclass of ferroelectric) phase shows the peculiarity of two stable states. These states are polarized in opposite directions ( P) in the absence of an applied field ( =0). The property in a material of having two stable states is called bistability. A single substance may exhibit several of these phases, and temperature changes will provoke observable phase transitions between phases with different polar characteristics.

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