Pyroelectric figures of merit

Polymer Ferroelectrics. In 1969, it was found that strong piezoelectric effects could be induced in the polymer poly(vinyhdene fluoride) (known as PVD2 or PVDF) by apphcation of an electric field (103). Pyroelectricity, with pyroelectric figures of merit comparable to crystalline pyroelectric detectors (104,105) of PVF2 films polarized this way, was discovered two year later (106.)... 

This only contains parameters describing properties of the pyroelectric material, and is therefore a figure-of-merit, which can be used to compare different materials for their potential voltage responsivities. If CA Jp CE the voltage response is proportional to ... 

In conclusion, it has been demonstrated that the pyroelectric properties of polar materials can be compared relatively simply through the measurement of a few key physical parameters (pyroelectric,dielectric and thermal coefficients) and the judicious use of appropriate figures-of-merit. It is essential that the dielectric properties are measured in the frequency range appropriate for device use, and this is typically in the range of a few to 100 Hz. The properties of many pyroelectric ceramics and thin films have been compared and it has been shown that good pyroelectric properties can be obtained from this films manufactured at relatively low temperatures, a fact that bodes well for their future applications in fully-integrated arrays. 

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. ... 

Lateral flow of heat in the pyroelectric detector plate tends to even out the temperature differences, blurring the charge pattern and hence the final image. This process will depend on the thickness h of the plate and the thermal diffusivity A = Xjc of the plate material. Yidicon performance improves as both h and A are reduced in value, and a suitable figure of merit is... 

A pyroelectric ceramic has a tan <5 of 0.005 arising solely due to dielectric losses and er = 250 at 100 Hz. What is the minimum resistivity that the ceramic can have without incurring a 20% increase in tan S. What will be the consequent effect on the noise-related figure of merit of a 20% increase [Answers 7.2xl08nm 10% decrease]... 

However, the exploitation of pyroelectric LB films depends not only on the figures of merit listed in Table 4.1, but also on the ability to deposit such layers successfully on surfaces with low thermal mass. A substrate imposes two constraints on the pyroelectric response of a film. Not only does it provide an undesirable heat sink and reduce the excess temperature of... 

Table 4.1. Pyroelectric Coefficients and Figures of Merit for a Selection of Different Materials...

Comparison of P(VF2-TrFE) with polymer Xll [12] (Fig. 25), which is an FLCP having one of the best pyroelectric coefficients, is made in Table 7. It turns out that the figure of merit of polymer XII is an order of magnitude lower than that of P(VF2-TrFE). Three improvements are necessary to FLCPs to compete with P(VF2-TrFe), namely ... 

Table 7. Comparison of the pyroelectric coefficient and the figure of merit for copo(VF2-TrF) and polymer XII.

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. 

There are several other reasons why LB films are likely to be used in preference to other pyroelectric materials. The figure of merit referred to above, (p/e), is only a measure of the signal strength, and does not take into account the noise sources in a complete infrared detection system. If this is done, one discovers that it is essential to have low dielectric loss in the film over an extended frequency range. All ferroelectric liquid crystals are ruled out of contention on this basis, because they have high tan 5 values in the range IHz to lOkHz. Moreover, it is not possible to prepare very thin films in structures with low thermal mass and low thermal conductivity. 

Spun-cast polymers, such as polyvinylidenefluoride and its copolymers, show good values of pyroelectric coefficient, and favourable values of the figure of merit p/er. The major disadvantages of such materials are their large piezoelectric activity, and the necessity for electrical poling. In practical devices, piezoelectricity is a problem,because it can cause problems associated with microphony. 

The importance of low relative permittivity of pyroelectric materials to be used in thermal imaging devices has been stressed. However, the figure of merit p/CfOnly measures signal strength. Noise sources, in a practical system must also be considered, and, consequently, potential materials must also possess low dielectric loss. In cases where dielectric loss... 

The crossover frequency cOei = cr/a aotan 8 between resistor-type noise and dielectric noise is typically 0.1-10 Hz. At typical modulation frequencies of 30 Hz, the dielectric noise dominates. The figure of merit of dielectric and pyroelectric material parameters for an optimal detectivity can thus be formulated as... 

In summary, pyroelectric materials have found applications in a wide range of fields. For device applications and the convenience of application engineers, several figures of merit criteria are defined. Most of these can be calculated using the thermal, electrical, and optical characteristics of the materials available in the literature and various handbooks on the physical constants of materials. 

This highly stereoregular polymer displayed a very high relaxed dipole moment and poled films displayed pyroelectric behaviour with a pyroelectric response figure of merit superior to that of PVF2. By contrast, the Schrock initiator... 

Some LB films exhibit a temperature-dependent pyroelectric coefficient that makes them candidates for thermal imaging applications (1). Both X- and Z-type films and alternating Y-type films (Fig. 6) can be built with permanent polarization, whose magnitude is often temperature-dependent. Their pyroelectric coefficient p is small relative to standard semiconducting and ceramic materials, but so is their permittivity e this makes their ratio p/s, the figure of merit, comparable to that as for currently employed inorganic materials. The advantage of LB methods is that they can be used to prepare very thin, very uniform films. 

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. ... 

The pyroelectric coefficients of the polymer-ceramic composites are large oompared to polymers, and the relative permittivities are small compared to ceramics (64j. Therefore tte figures of merit are enhanced over conventional single-phase polymer materials. (In certain pyroelectric systems a useful figure of merit to p/c, where p to the pyroelectric coefficient and c is the electric permittivity [16]). 

Recently, the potential of LMM FLCs as pyroelectric infrared detectors has been investigated by Glass et al. [94], As expected, tbc pyroelectric coefficient reaches a maximum value just below the phase transition from the phase into the Sc phase. Despite the low Pg values of tbc FLC materials used, the figures of merit compare well with ferroelectric solid materials commonly used for infrared detection. It is to be expected that FLC polymers should have figures of merit of the same order of magnitude. The discussion will be continued and extended, going into more detaU. in Section VIII. 

To assess pyroelectric detector performance, it is necessary to ascertain its response to a changing source of radiation and compare this signal with the noise signals inherent in the detector and amplifier. The figures of merit used to compare pyroelectric materials are briefly explained below however, more detailed accounts can be found elsewhere [35,36]. 

Table 2 Pfoperties of rious Pyroelectric Detector Materials and Some Figure s of Merit (or Tlteir Detector Opcraiian... 

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