Whatmore, R.W. “Pyroelectric Materials and Devices” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 54 Pyroelectric Materials and Devices 54.1Introduction 54.2Polar Dielectrics 54.3The Pyroelectric Effect 54.4Pyroelectric Materials and Their Selection 54.1 Introduction It was known over 2000 years ago that certain minerals such as tourmaline would attract small objects when heated. It was understood over 200 years ago that this attraction was a manifestation of the appearance of electrical charges on the surface as a consequence of the temperature change. This is called the pyroelectric effect and over the last 15 years has become the basis of a major worldwide industry manufacturing detectors of infrared radiation. These are exploited in such devices as “people detectors” for intruder alarms and energy conservation systems, fire and flame detectors, spectroscopic gas analyzers—especially looking for pollutants from car exhausts—and, more recently, devices for thermal imaging. Such thermal imagers can be used for night vision and, by exploiting the smoke-penetrating properties of long-wavelength infrared radiation, in devices to assist firefighters in smoke-filled spaces. The major advantages of the devices in comparison with the competing infrared detectors that exploit narrow bandgap semiconductors are that no cooling is necessary and that they are cheap and consume little power. The pyroelectric effect appears in any material which possesses a polar symmetry axis. This chapter describes the basic effect, gives a brief account of how it can be used in radiation detection, and discusses the criteria by which materials can be selected for use in this application, concluding with a comparison of the properties of several of the most commonly used materials. 54.2 Polar Dielectrics A polar material is one whose crystal structure contains a unique axis, along which an electric dipole moment will exist. There are 10 polar crystal classes: ?Triclinic 1 ? Monoclinic 2, m ?Tetragonal 4, 4mm ? Orthorhombic mm2 ?Hexagonal 6, 6mm ? Trigonal 3, 3m All crystals whose structures possess one of these symmetry groups will exhibit both pyroelectric and piezo- electric characteristics. In ferroelectrics, which are a subset of the set of pyroelectrics, the orientation of the polar axis can be changed by application of an electric field of sufficient magnitude. The original and final states of the crystal are symmetrically related. It is important to note that Roger W. Whatmore Cranfield University ? 2000 by CRC Press LLC 1. Not all polar materials are ferroelectric. 2. There is a set of point groups which lack a center of symmetry, without possessing a polar axis. The crystals belonging to these groups (222, – 4,422, — 42m, 32, – 6, – 6m2, 23, and — 43m)are piezoelectric without being pyroelectric. (432 is a noncentrosymmetric, nonpiezoelectric class.) Typical values of spontaneous polarizations (P s ) and Curie temperatures (T c ) for a range of ferroelectrics are given in Table 54.1. A very wide range of materials exhibit ferroelectric, and thus pyroelectric, behavior. These range from crystals, such as potassium dihydrogen phosphate and triglycine sulphate, to polymers, such as polyvinylidene fluoride, and liquid crystals such as DOBAMBC and ceramics, such as barium titanate and lead zirconate titanate. Most ferroelectrics exhibit a Curie temperature (T c ) at which the spontaneous plarization goes to zero. (A few ferroelectrics, such as the polymer polyvinylidene fluoride [PVDF] melt before this temperature is reached.) The fact that the orientation of the polar axis in ferroelectrics can be changed by the application of a field has a very important consequence for ceramic materials. If a polycrystalline body is made of a polar material, then the crystal axes will, in general, be randomly oriented. It cannot therefore show pyroelectricity. However, if an electric field greater than the coercive field (E c ) is applied to a ferroelectric ceramic, then the polar axes within the grains will tend to be reoriented so that they each give a component along the direction of the applied field. This process is called “poling.” The resulting ceramic is polar (with a point symmetry ￥m) and will show both piezoelectricity and pyroelectricity. 54.3 The Pyroelectric Effect The pyroelectric effect is described by: P i = p i DT (54.1) where P i is the change in the coefficient of the polarization vector due to a change in temperature DT and p i is the pyroelectric coefficient, which is a vector. The effect and its applications have been extensively reviewed in Whatmore [1986]. The effect of a temperature change on a pyroelectric material is to cause a current, i p , to flow in an external circuit, such that i p = ApdT/dt (54.2) where A is the electroded area of the material, p the component of the pyroelectric coefficient normal to the electrodes, and dT/dt the rate of change of temperature with time. Pyroelectric devices detect changes in temperature in the sensitive material and as such are detectors of supplied energy. It can be seen that the pyroelectric current is proportional to the rate of change of the material with time and that in order to obtain a measurable signal, it is necessary to modulate the source of energy. As energy detectors, they are most frequently applied to the detection of incident electromagnetic energy, partic- ularly in the infrared wavebands. TABLE 54.1 Spontaneous Polarizations and Curie Temperatures for a Range of Ferroelectrics Material T c (k) P s (cm-2) T(k) KH 2 PO 4 (KDP) 123 0.053 96 Triglycine sulphate 322 0.028 293 Polyvinylidene fluoride (PVDF) >453 0.060 293 DOBAMBC (liquid crystal) 359 ~3 ′ 10 –5 354 PbTiO 3 763 0.760 293 BaTiO 3 393 0.260 296 ? 2000 by CRC Press LLC Typically, a pyroelectric detector element will consist of a thin chip of the pyroelectric material cut perpen- dicular to the polar axis of the material, electroded with a conducting material such as an evaporated metal and connected to a low-noise, high-input impedance amplifier—for example, a junction field-effect transistor (JFET) or metal-oxide gate transistor (MOSFET)—as shown in Fig. 54.1. In some devices, the radiation is absorbed directly in the element. In this case the front electrode will be a thin metal layer matched to the permittivity of free space with an electrical surface resistivity of 367 W/square. However, in most high-perfor- mance devices, the element is coated with a layer designed to absorb the radiation of interest. The element itself must be thin to minimize the thermal mass and, in most cases, well isolated thermally from its environ- ment. These measures are designed to increase the temperature change for a given amount of energy absorbed and thus the electrical signal generated. The necessary modulation of the radiation flux can be achieved either by deliberately only “looking” for moving objects or other radiation sources (e.g., flickering flames for a flame detector) or by interposing a mechanical radiation “chopper” such as a rotating blade. The voltage responsivity of a device such as this is defined as R v = V o /W, where V o is the output voltage and W is the input radiation power. For radiation sinusoidally modulated at a frequency w, R v is given by (54.3) where G T is the thermal conductance from the element to the environment, t T is the thermal time constant of the element, t E is the electrical time constant of the element, R G is the electrical resistance across the element, h is the emissivity of the element for the radiation being detected, and A is the sensitive area of the element. It is easy to show that the response of a pyroelectric device maximizes at a frequency equal to the inverse of the geometric mean of the two time constants and that above and below the two frequencies given by t T –1 and t E –1 , R v falls as w –1 . The consequence of this is that pyroelectric detectors have their sensitivities maximized by having fairly long electrical time constants (0.1 to 10 s) and that such detectors thus work best at low frequencies (0.1 to 100 Hz). However, if high sensitivity is not required, extremely large bandwidths with little sensitivity variation can be obtained by shortening these time constants (making R G and C E low and G T high). In this way, detectors have been made which give picosecond time responses for tracking fast laser pulses. There are several noise sources in a pyroelectric device. These are discussed in detail in Whatmore [1986]. In many cases of interest, the dominant noise source is the Johnson noise generated by the ac conductance in the capacitance of the detector element. This noise is given by DV j , where (54.4) FIGURE 54.1 Pyroelectric detector with FET amplifier. R RpA G v G TT E = ++ hw wt wt()()11 2212 2212// DVkT C CC j E EA = ì í ? ? ? ü y ? t ? >> - 4 12 12 tand w / / for ? 2000 by CRC Press LLC where k is Boltzmann’s constant, T is the absolute temperature, tand is the dielectric loss tangent of the detector material, C E is the electrical capacitance of the element, and C A is the input capacitance of the detector element amplifier. The input radiation power required to give an output equal to the noise at a given frequency in unity bandwidth is known as the noise equivalent power (NEP). This is given by NEP = V n /R v (54.5) where V n is the total RMS voltage noise from all sources. A performance figure of merit frequently used when discussing infrared detectors is the detectivity, usually designated as D*. This is given by D* = A 1/2 /NEP (54.6) Thus, the detectivity of a pyroelectric detector can be derived from Eqs. (54.3) to (54.6) and is given by (54.7) where c ￠ is the volume specific heat, e is the dielectric constant of the pyroelectric, and d is the thickness of the pyroelectric element. The roll-off in D* at high frequencies is thus 1/w 1/2 . Pyroelectric single-element IR detectors come in many different varieties. A typical commercial device will have the sensitive element made from a material of the type discussed in the next section, such as a piece of lithium tantalate crystal or a ferroelectric ceramic. The element size will be a few millimeters square. Typical performance figures at about 10 Hz would be a responsivity of a few hundred volts per watt of input radiation, a noise equivalent power of about 8 ′ 10 –9 W/Hz 1/2 , and a detectivity of about 2 ′ 10 8 cm Hz 1/2 W –1 for unity bandwidth. The detector can be fitted with a wide variety of windows, depending upon the wavelength of the radiation to be detected. As noted above, pyroelectric devices have also been used for thermal imaging. In this application, their main advantage when compared with photon detector materials such as mercury cadmium telluride (CMT) (which are more sensitive) is that they can be used at room temperature. All the photon detectors require cooling, typically to 77 K. A very successful device for pyroelectric thermal imaging is the pyroelectric vidicon which uses a thin plate of pyroelectric material contained in a vacuum tube. The thermal image is focused onto the surface of the material using a germanium lens. This causes the formation of a pattern of pyroelectric charges, which are “read” by means of an electron beam. Typical sensitivities for such devices are between 0.50 and 1 K temperature differences in the scene for an f/1 lens. This compares with <0.10 K for a cooled CMT detector- based imager. Recently, a solid-state approach to pyroelectric thermal imaging has been developed. In this, an array of many thousands of very small identical detectors, each between 50 and 100 mm square, depending on the array design, are linked to a silicon amplifier/multiplexer circuit which allows the signals from all the elements to be read onto a single output line. These devices have been primarily developed for thermal imaging applications and excellent sensitivities (close to those achieved by many cooled systems) have been demonstrated. 54.4 Pyroelectric Materials and Their Selection There are many different types of pyroelectrics and the selection of a material depends strongly upon the application. It is possible to formulate from the given equations a number of figures of merit which describe the contribution of the physical properties of a material to the performance of a device. For example, the current responsivity is proportional to F i : F i =p/c ￠ (54.8) D d kT p c o * ( ) ( tan ) =× ￠ × h dw4 1 12 12 12/// ee ? 2000 by CRC Press LLC The voltage response for a pyroelectric element feeding into a high-input impedance, unity gain amplifier (such as a source follower FET) as shown in Fig. 54.1 is proportional to F v : F v = p/c￠ee o (54.9) The detectivity is proportional to F D : F D = p/{c￠(ee o tand) 1/2 } (54.10) For the pyroelectric vidicon, thermal spreading of the pattern on the target is important and the relevant figure of merit is F vid : F vid = F v /K (54.11) where K is the thermal conductivity of the pyroelectric. It should be noted that the use of these merit figures must be tempered with a knowledge of the type of detector the material is to be used in. It is necessary, if possible, to match the capacitance of the detector to the input capacitance of the amplifier. Hence, low- permittivity materials are better suited to large-area detectors, and conversely arrays of small-area detectors are better served by materials with high permittivities. Table 54.2 lists the pyroelectric properties of several different materials, single crystals, ceramics, and poly- mers. It can be seen that triglycine sulphate (TGS) and its deuterated isomorph (DTGS) exhibit the highest value of F v and are frequently used for high-performance single-element detectors. These are the preferred materials for pyroelectric vidicon targets. However, they are water soluble, difficult to handle, and show poor long-term stability, both chemically and electrically, because of their low Curie temperatures. Furthermore, their dielectric loss is rather high, so that the F D figures are not so favorable. Lithium tantalate, on the other hand, is an oxide single-crystal material which possesses a relatively low value of F v but a very low loss so that F D is favorable. The material is very stable and is now widely used for single-element detectors. Its thermal conductivity is quite high so that it is not a good material for the pyroelectric vidicon. The ferroelectric polymers possess relatively low pyroelectric coefficients and low dielectric constants with high losses, so that their figures of merit are also quite low. Their low thermal conductivities make them quite favorable for use in the pyroelectric vidicon and the fact that they are commercially available in thin sections (down to 6 mm) at low cost, removing any requirement for expensive lapping and polishing, makes them attractive for some low-cost detectors. Their low permittivities make them particularly well suited to large-area detectors. The ceramic materials modified lead zirconate and modified lead titanate are interesting in that they possess high pyroelectric coefficients with relatively high permittivities and low losses. The modified lead zirconate is a solid solution of lead zirconate with lead iron niobate and lead titanate, with small additions of uranium as a stabilizing dopant. The use of uranium in this material minimizes the dielectric constant and loss (thus maximizing F D ) while also permitting control over the electrical resistivity, allowing the gate bias resistor in TABLE 54.2Pyroelectric Properties of Selected Materials Material (Temperature) Pyroelectric Coefficient P 10 –4 cm –2 K –1 Dielectric Properties Volume-Specific Heat c￠ 10 6 Jm –3 K –1 Thermal Conductivity K 10 –7 m 2 s –1 F v m 2 C –1 F D 10 –5 Pa –1/2 F vid 10 6 sC –1 (1 kHz) e tand TGS (35°C) 5.5 55 0.025 2.6 3.3 0.43 6.1 1.3 DTGS (40°C) 5.5 43 0.020 2.4 3.3 0.60 8.3 1.8 PVDF polymer 0.27 12 0.015 2.43 0.62 0.10 0.88 1.6 LiTaO 3 crystal 2.3 47 0.005 3.2 13.0 0.17 4.9 0.13 Modified PZ ceramic 3.8 290 0.003 2.5 0.06 5.8 Modified PT ceramic 3.8 220 0.011 2.5 0.08 3.3 PZ = PbZrO 3 , PT = PbTiO 3 . ? 2000 by CRC Press LLC Fig. 54.1 to be designed into the sensor element. The modified lead titanate is doped with calcium titanate and lead cobalt tungstate. The use of hot pressing in ceramic manufacture permits the fabrication of very low porosity material, which can be lapped and polished to very thin sections (as low as 20 mm) while being mechanically strong enough to be placed on a mount which provides support only over a small area, permitting the fabrication of detectors with maximum sensitivity. While the F v values are relatively small in these materials, the F D values are as good as most of the single-crystal materials. They are very well suited to small-area detectors because their high dielectric constants enable the element capacitance to be matched to that of the amplifier. Pyroelectric ceramics are now finding use in a wide range of the infrared detector market, from low-cost intruder alarms to high-value imaging arrays. Recently, a new class of pyroelectric materials which use the effect in the region of T c have been developed [Whatmore, 1991]. In these materials a bias field must be applied to stabilize the effect, but F D values as high as 10 to 15 ′ 10 –5 Pa –1/2 have been recorded in such materials as barium strontium titanate or lead scandium tantalate, both perovskite ceramics. This mode of operation is usually called “dielectric bolometer.” Defining Terms Coercive field (E c ): The field required to invert a sufficient proportion of the polarization of a body of a poled ferroelectric such that the net measurable external dipole moment is zero. Curie temperature (T c ): The temperature at which the spontaneous polarization of a ferroelectric goes to zero. Ferroelectric: A polar dielectric in which the crystallographic orientation of the internal dipole moment can be changed by the application of an electric field. Paraelectric: The nonpolar phase into which the ferroelectric transforms above T c , frequently called the paraelectric phase. Piezoelectric: A material which possesses a noncentrosymmetric crystal structure which will generate charge on the application of a mechanical stress. As in the case of a pyroelectric, this can be detected as either a potential difference or as a charge flowing in an external circuit. Pyroelectric: A polar dielectric material in which the internal dipole moment is temperature dependent. This leads to a change in the charge balance at the surface of the material which can be detected as either a potential difference or as a charge flowing in an external circuit. Remanent polarization: The value to which the externally measured polarization of a ferroelectric body relaxes after it has been subjected to an electric field much greater than the coercive field, which is then removed. Saturation polarization: The value to which the externally measured electrical dipole moment of a ferro- electric body tends when subjected to an external electrical field greater than the coercive field. Spontaneous polarization: The value of the electrical dipole moment of a ferroelectric crystal due to the separation of positive and negative charges within the unit cell. Related Topic 55.1 Introduction References R. W. Whatmore, “Pyroelectric devices and materials,” Rep. Prog. Phys., vol. 49, pp. 1335–1386, 1986. R. W. Whatmore, “Pyroelectic ceramics and devices for thermal infrared detection and imaging,” Ferroelectrics, vol. 118, pp. 241–259, 1991. B. M. Kulwicki, A. Amin, H. R. Beratan, and C. M. Hanson, “Pyroelectric imaging”, Proc. 8th IEEE International Symposium on Applications of Ferroelectrics, p. 1–10, (IEEE Cat. No. 92CH3080-9), 1992. A. Hadni, “Applications of the pyroelectric effect,” J. Phys. E: Sci. Instrum., 14, 1233–1240, 1981. W.-S. Zhu, J. R. Izatt, and B. K. Deka, “Pyroelectric detection of submicrosecond laser pulses between 230 and 530 mm,” Appl. Opt., 28, 3647–3651, 1989. ? 2000 by CRC Press LLC