Pyroelectric coefficient/dielectric

Ferroelectric materials are capable of being polarized in the presence of an electric field. They may exhibit considerable anomalies in one or more of their physical properties, including piezoelectric and pyroelectric coefficients, dielectric constant, and optoelectronic constant. In the latter case, the transmission of light through the material is affected by the electric field, which produces changes in refractive index and optical absorption coefficient. Varying the applied field changes the phase modulation. 

Dielectric constant, e33/e9 frequency - 103 Hz frequency -106 Hz Dielectric losses, tan S (along OZ) frequency -103 Hz frequency -106 Hz Pyroelectric coefficient p l(f9 C cni2 -K1... 

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. 

Dielectric constants, dissipation factors, and pyroelectric coefficients of the alternating LB films comnosed of Ba salts of Dhenvlovrazine derivatives. 

Equations (6) and (7) express these relationships. are the elastic compliance constants OC are the linear thermal expansion coefficients 4 and d jj,are the direct and converse piezoelectric strain coefficients, respectively Pk are the pyroelectric coefficients and X are the dielectric susceptibility constants. The superscript a on Pk, Pk, and %ki indicates that these quantities are defined under the conditions of constant stress. If is taken to be the independent variable, then O and are the dependent quantities ... 

The class of ferroelectric materials have a lot of useful properties. High dielectric coefficients over a wide temperature and frequency range are used as dielectrics in integrated or in smd (surface mounted device) capacitors. The large piezoelectric effect is applied in a variety of electromechanical sensors, actuators and transducers. Infrared sensors need a high pyroelectric coefficient which is available with this class of materials. Tunable thermistor properties in semiconducting ferroelectrics are used in ptcr (positive temperature coefficient... 

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. 

Although pyroelectric coefficients for LB films tend to be smaller than for most other materials, the values of p/er obtained compare favourably. Also, in contrast, dielectric loss appears to be low over a wide frequency range. Due to the unique deposition method involving individual monolayers, it is possible to obtain films with the exact optimum thickness. To date, it has been found that LB films are not significantly piezoelectric. 

The development of pyroelectric thermal imaging devices is dependent upon the screening of suitable materials. The materials,which are most likely to find application, will have large values of pyroelectric coefficient,and small permittivity and dielectric loss. The ability to produce thin films of optimum thickness will also be significant. 

Equation (2.37) shows that large signals are obtained from pyroelectric materials which have a large ratio of pyroelectric coefficient to dielectric constant. 

Total pyroelectric effect for linear dielectrics at the room temperature is just slightly temperature dependertt. Pyroelectric coefficient decreases with decreasing temperature and reaches very small value close to the 0 K temperature. In certain... 

This paragraph gives the necessary conventions and abbreviation for the tensor components of elastic, piezoelectric, pyroelectric and dielectric properties and for the thermal expansion coefficient (Tables A. 1 and A.2). Tensor and matrix notation is adopted according to Nye (1957) material tensors tables according to Sirotin and Shaskolskaya (1982). 

From an applications standpoint, measurement of other properties, namely dielectric constant and dielectric loss, again measured using an impedance analyzer, and Tc, which may be determined from k versus T measurements or by temperature dependent X-ray diffraction, allows for calculation of material and device figures of merit. These parameters are important in determining the suitability of a material for a particular application. Recently, pyroelectric coefficients and device performance figure of merit have also been determined for solution-derived ferroelectric thin films. ... 

Curie temperature f dielectric constant P pyroelectric coefficient C0volumetric specific heat /(thermal... 

Methods for Measuring the Pyroelectric Coefficient of Composites The most usual way to measure the pyroelectric coefficient is by the coavenlkmal quasi-static method [48,154,155]. Since tte polarizalioo P measure is surface charge density o QIA (Q denotes electric charge and A is the surface of the dielectric film), Eq. (25) can be rewritten as follows ... 

Pyroelectric polymers have additional features as pyroelectric devices, such as large surface area, thin films, low thermal diffusion, small dielectric constant, flexible for conformity to curved surface and low cost, though their pyroelectric coefficients are generally smaller than other materials. Pyroelectric devices using polymer elements with these features may have a large number of applications which ate not necessarily restricted to IR radiation detector. 

El.polarization, P, Elec, field, E, Dielectric Susceptibility Xp=dP/dE Pyroelectric Coefficient pp dP/dT Thermal dielec. Susceptibility dE/dT=-pp/xp... 

Similar approach can be applied to the systems, which exhibits electric field, , paired with electric polarization, P, adding thus the term (P + pp T) dE ) for the pyroelectric coefficient of dielectric polarization, Pp. Mechanical deformation, e, paired with mechanical strain-tension, ct, contributes the coefficient for mechano-elastic properties,, as (e + cr T) do. Chemical potential, p, paired with mole number, N, adds the terni (N + K r T) dp ) for the coefficient of chemical activity, /( t. 

Pyroelectric devices convert changing incident thermal radiation to an electrical output, and are now much used in intruder detectors, thermal imaging systems etc. Conventionally, ceramics have been used in such applications however, considering the desirable properties of large pyroelectric coefficient, high volume resistivity, low dielectric constant and loss, and low specific heat, it can be seen that, apart from the rather low pyroelectric coefficient, polymeric materials are superior to ceramics in several respects. 

Slow heating rates favor the interaction between the film and the substrate. Thick substrate-film interfaces are obtained in these films, damaging the electrical properties of the film (Table 27.3). Dielectric permittivities k), remanent ferroelectric polarizations (P, pyroelectric coefficients (y), and larger coercive fields (Ec) measured in the films annealed with slow heating rates are lower than those measmed in films annealed with rapid heating rates. 

Thermal annealing for ceramic conversion Film thickness (A) Thickness of the film-substrate interface (A) Dielectric permittivity (k) Remanent polarization iP pC crn ) Coercive field ( kV cm" ) Pyroelectric coefficient, after poling (y, C cm K- )... 

---