Pyroelectrics high permittivity

Strontium barium niobate is a single-crystal material with the tungsten bronze type of structure which is made by the Czochralski method but has yet to find a major use. It has relaxor characteristics of the type shown in Fig. 7.1 which give it a high pyroelectric coefficient and detectivity, but its high permittivity lowers the figure of merit l. ... 

Ferroelectric materials, especially polyciystalhne ceramics, are utihzed in various devices such as high-permittivity dielectrics, ferroelectric memories, pyroelectric sensors, piezoelectric transducers, electrooptic devices, and PTC (positive temperature coefficient of resistivity) components. 

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... 

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],... 

The compounds K5Nb3OFi8 and Rb5Nb3OFi8 display promising properties for their application in electronics and optics. The compounds can be used as piezoelectric and pyroelectric elements due to sufficient piezo- and pyroelectric coefficients coupled with very low dielectric permittivity. In addition, the materials can successfully be applied in optic and optoelectronic systems due to their wide transparency range. High transparency in the ultraviolet region enables use of the materials as multipliers of laser radiation frequencies up to the second, and even fourth optical harmonic generation. 

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. 

For practical purposes, the very small signals generated by pyroelectric elements must be amplified. The most widely used first stage consists of a field effect transistor (FET) which responds to electric potential rather than to charge. In this case, it is advantageous for the material to have a low permittivity to match the low input capacitance of the FET. Therefore the compositions with high... 

The pyroelectric coefficient p, is a useful parameter with which to compare different materials. If the thin film acts as a dielectric in a capacitor and an external resistance is connected between the electrodes, a pyroelectric current, I, flows in the circuit this can be expressed as I = pA(dT/dt) where dT/dt is the rate of change of temperature, and A is the cross-sectional area of the device. In a thermal imager many considerations, other than a high value of p, must be borne in mind,when designing a pyroelectric detector capable of resolving a temperature difference in the scene temperature of O.IK. For example, the figure of merit for a thermal imaging device requires the pyroelectric materials to have low values of permittivity. 

Experimental techniques are discussed for the characterisation of potentially useful thin film materials, including measurement of pyroelectric coefficient and dielectric data (permittivity and dielectric loss). It is noted that, when considering a complete thermal imaging system, it is not sufficient to consider material parameters in isolation, and that the combined features of LB films render them particularly suitable to high system performance. 

Lithium-Niobate and Lithium-Tantalate Lithium-niobate (LN) and lithium-tantalate (LT) are uniaxial p3Toelectrics, having trigonal structure, with spontaneous polarization arising from asymmetrical displacement of lithium relative to the other ions. These materials Tc values are 1,210 °C and 620 °C, respectively. They are always produced commercially in single-crystal forms. Both are much used for surface acoustic wave devices (e.g., high-frequency filters), while LT is used for pyroelectric detection due to its large pyroelectric coefficient and low permittivity. 

IPUDO (for the molecular structure see Tab. 9.2) is an example of a symmetrically substituted diacetylene. For the monomer as well as polymer crystals, non-centrosymmetrical orthorhombic crystal structures can be found already at room temperature [84]. The c-axis is the polar axis of the monomer crystal. IPUDO can only be polymerized by y radiation ( °Co). For 85% polymerized crystals pyroelectric properties were observed only in b direction. The permittivity of IPUDO is highly anisotropic, with maximum values of 8.5 for the... 

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