Pyroelectricity, discussion

Sources and detectors Specific discussions of sources and detectors have been covered elsewhere in this article. The issues here are more service and performance related. Most sources have a finite lifetime, and are service replaceable items. They also generate heat, which must be successfully dissipated to prevent localized heating problems. Detectors are of similar concern. For most applications, where the interferometer is operated at low speeds, without any undesirable vibrational/mechanical problems, the traditional lithium tantalate or DTGS detectors are used. These pyroelectric devices operate nominally at room temperature and do not require supplemental cooling to function, and are linear over three or four decades. 

Significant advances have occurred in microfabricated ion sensitive and Pd gated field effect devices and fiber optic, chemically rsnsitive elements. These elements are beginning to find their way into commercial development. Recent advances in these devices are discussed and compared. Pyroelectric sensor devices developed here are reviewed. A discussion of the utility of these devices is presented. 

This book is specifically addressed to the properties of polar oxides as well as to their chac-terization and imaging techniques. The dielectric, optic, piezoelectric, pyroelectric behavior of this class of materials is discussed. Emphasis is placed on novel methods in the field of electrical and optical investigations, scanning probe microcoscopy (spm) techniques and advanced X-ray analysis. The book starts with tutorial reviews, and arrives at up-to-date results about polar oxides. Therefore, it not only stimulates and further motivates young scientists but is of considerable interest for the members of our community. 

Note that most, but not all, detectors are made so that the direction of it is normal to the element electrode plane, i.e. p = 7r. In the discussion which follows, it will be assumed that this is the case and the term pyroelectric coefficient will be applied to p.) The pyroelectric charges can be detected as a current ip, flowing in an external circuit such that ... 

The section above considered the physical properties that are important in determining the performance of a pyroelectric detector, measurement methods used to obtain the physical parameters which determine pyroelectric performance are worthy of critical discussion, as many techniques are reported in the literature. 

The following discussion separates pyroelectric materials into 3 groups intrinsic pyroelectrics which are operated well below Tc, dielectric bolometer materials which are operated close to Tc, but with an electrical bias applied and ferroelectric thin films. 

There are many different types of pyroelectric, including single crystals, polymers, ceramics and thin films and several reviews [2,3,28,29] have considered the properties of many pyroelectric materials in detail, so the discussion here will be confined to a brief review of pyroelectric ceramics and thin films. 

The unique dielectric properties and polymorphism of PVDF are the source of its high piezoelectric and pyroelectric activity.75 The relationship between ferroelectric behavior, which includes piezoelectric and pyroelectric phenomena and other electrical properties of the polymorphs of polyvinylidene fluoride, is discussed in Reference 76. 

Therefore the pyroelectric coefficient is a vector but, because in practical applications the electrodes that collect the pyrocharges are positioned normal to the polar axis, the quantities are usually treated as scalars, and this is done in the following discussion. 

Commercial products based on copolymers of ethylene and TEE are made by addition copolymerization initiated by free radicals [89]. Small amounts (1 to 10 mol%) of modifying comonomers are added to eliminate a rapid embrittlement of the product at exposure to elevated temperatures. Examples of the modifying comonomers are perfluorobutylethylene, hexafluoropropylene, perfluorovinyl ether, andhexafluor-oisobutylene [90]. Additional information on the methods to prepare ETEE copolymers are discussed in [88]. ETFE resins are essentially alternating copolymers [90], and in the molecular formula they are isomeric with PVDF with a head-to-head, tail-to-tail structure. However, in many important physical properties, the modified ETFE copolymers are superior to PVDF with the exception of the latter s remarkable piezoelectric and pyroelectric characteristics. 

The solids discussed in the remainder of this chapter have one thing in common They exhibit various polar effects, such as piezoelectricity, pyroelectricity, and ferroelectricity. Piezoelectric crystals are those that become electrically polarized or undergo a change in polarization when subjected to a stress, as shown in Fig. 15.12c to /. The application of a compressive stress results in the flow of charge in one direction in the measuring circuit and in the opposite direction for tensile stresses. Conversely, the application of an electric field will stretch or compress the crystal depending on the orientation of the applied field to the polarization in the crystal. 

The neutral beam density in the interaction region can be determined from a measurement of the energy deposited by the fast neutral beam into a pyroelectric crystal whose response is first calibrated by a well-characterized ion beam, as discussed by Wetzel et al. (1987). As an alternative, the well-established Kr or Ar absolute ionization cross sections (known to better than 5%) can be used to calibrate the pyroelectric crystal. The calibrated detector is then used to determine the flux of the neutral target beam in absolute terms. This procedure avoids the frequent and prolonged exposure of the delicate pyroelectric crystal to fairly intense ion beams (Freund et al., 1990 Tamovsky and Becker, 1992). 

G. Meslin said that potassium dichromate is paramagnetic. He found that the magnetic susceptibility of the powder is 0-13 x 10 mass unit and G. Quincke gave for a soln. of the dichromate between 18° and 20°, 0-76 Xl0 mass unit. J. Forrest measured the variations in the parallel and transverse components of the magnetization of the crystals. P. Weiss and P. Collet found the paramagnetism of a sohi. of the dichromate is constant between 14° and 50° and the subject was discussed by P. Weiss, L. A. Welo and A. Baudisch, and S. Berkman and H. Zocher. W. G. Hankel and H. Lindenberg found that the crystals exhibit pyroelectricity in that when warmed the (OOl)-face is usually positively electrified, and the (OOf)-face negatively electrified—the reverse behaviour is rare. 

The type of detector used in an FT-IR spectrometer is highly dependent upon the bandwidth (i.e. the spectral frequencies), the modulation rate of the interferometer, and the intensity of the radiant flux. Several types of detectors are used in the infrared regions photoconductive, photovoltaic, bolometers, pyroelectric and Golay cells. A detailed discussion of detectors may be found elsewhere.12 In general, the photovoltaic and photoconductive detectors can be used in the near- and mid-infrared regions as rapid response, high sensitivity detectors. Usually the bandwidths are limited and will not cover the total ran passed by the beamsplitter. Examples of such detectors are given in Table I. As can be seen from the... 

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. 

In the discussions which follow, attention will be directed toward two approaches which have found the greatest utility in infrared systems, namely, bolometers and the pyroelectric effect. Others will be discussed briefly. A list of thermal effects is included in Table 2.4. 

The principal noise sources are temperature (3.9), Johnson and amplifier noise. Temperature noise sources have been discussed in detail by Logan (Logan and Moore [3.9], Logan [3.10]) who has shown that in addition to the radiative conduction, conduction and convection into the ambient gas in the encapsulation and lateral conduction into the surrounds of the element are important. With present materials, however, the Johnson noise under most circumstances is more important. If we assume that the principal contribution to r in Fig. 3.10 comes from the dielectric loss of the pyroelectric material, then the appropriate expressions for the Johnson noise limited noise equivalent power (see (3.16)) is... 

There exists a wide variety of approaches to the use of charge transfer devices in infrared focal planes. We shall discuss five high packing density, high quantum efficiency, approaches appropriate for series-parallel scan 1) IR sensitive CCD, 2) ctirect injection hybrid, 3) direct injection extrinsic silicon, 4) accumulation mode extrinsic silicon, and 5) infrared sensitive CID with silicon CCD signal processing. The reader is referred to a review article by Steckl et al. for a comprehensive discussion of a number of other approaches not discussed here which include indirect injection pyroelectric detectors and Schottky barrier photoemissive injection [6.1]. Three approaches in our list of five do not require... 

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