Pyroelectric ceramics

Pyroelectricity is a property inherent in crystals with unique polar axes that are, consequently, without a center of symmetry. However, as opposed to piezoelectricity not all symmetry groups (point groups) lacking a center of symmetry are pyroelectric, whereas all pyroelectrics are also piezoelectrics. Most pyroelectrics belong to the hnear dielectrics as the polarization is linearly dependent on the electric field. However, some-for example, KDP and Seignette salt-are nonhnear pyroelectrics-that is, they are ferroelectrics. 

An individual volume element of the tourmaline crystal shows, even in the absence of an electric field, a spontaneous electric polarization owing to the polar axis. This polarization creates apparent surface charges at opposite ends of the polar axis that, however, are compensated by an adsorbed water film (and the 

Studies are currently in progress to develop functional transducers to transform thermal energy in electric energy with a power output of several W cm working volume, and an efficiency of 20%. 

The inverse efFect-that is, the dependence of temperature on an external electric field (AT = q-E where q is the electrocaloric coefficient)-is used today in devices for electrostatic cooling to achieve temperatures close to the absolute temperature, as required for experiments conducted in the Large Hadron Collider. 

Pyroelectrics are materials that possess a unique polar axis and are spontaneously polarized in the absence of an electric field. Of the 20 crystal classes that display piezoelectricity, 10 exhibit pyroelectricity. Since pyroelectric ceramics are a subset of piezoelectric materials, they are also piezoelectric in nature. The polarization exhibited by pyroelectric materials is also a function of temperature, and the change in polarization with temperature may be expressed by 

Since pyroelectric ceramics are also piezoelectric, a temperature change also induces a change in the polarization due to the secondary pyroelectric effect, which is described by the product of the thermal expansion strain times the piezoelectric coupling coefficient. While this secondary effect can be large in polymers due to their large thermal expansion coefficients, in ceramics, it is typically small compared with the (first-order) pyroelectric effect. 

The pyroelectric properties of ceramics may be measured using a current amplifier and a heating source to heat or cool the sample at a controlled rate. With this approach, the measured current, /, is approximately equal to the pyroelectric current, expressed by 

The pyroelectric coefficient may then be calculated from the measured current flow and the rate of temperature change. 

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.  

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Pyroelectrics. Pyroelectric ceramics are materials that possess a uoique polar axis and are spontaneously polarized ia the abseace of an electric field. Pyroelectrics are also a subset of piezoelectric materials. Ten of the 20 crystal classes of materials that display the piezoelectric effect also possess a unique polar axis, and thus exhibit pyroelectricity. In addition to the iaduced charge resultiag from the direct pyroelectric effect, a change ia temperature also iaduces a surface charge (polarizatioa) from the piezoelectric aature of the material, and the strain resultiag from thermal expansioa. 

The main categories of electrical/optical ceramics are as follows phosphors for TV, radar and oscilloscope screens voltage-dependent and thermally sensitive resistors dielectrics, including ferroelectrics piezoelectric materials, again including ferroelectrics pyroelectric ceramics electro-optic ceramics and magnetic ceramics. 

Pyroelectric Ceramics and Thin Films tion, Properties and Selection... 

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. 

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. 

Fig. 7.14 A thermal image produced by an infrared camera exploiting uncooled pyroelectric ceramic detection technology. (Courtesy of QinetiQ Limited.)...

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]... 

Ferroelectric composites are alternatives to standard piezoelectric and pyroelectric ceramics such as lead zirconate titanate (PZT) and BaHOs (BT). They combine the strong ferroelectric and dielectric properties of ceramics with the easy processing and good mechanical properties of polymers. Dispersion of micrometer-sized ferroelectric particles in an electrically passive epoxy matrix was first published by Furukawa et al. [1976] and later extended to ferroelectric matrices such as poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-co-3-fluoroethylene) (PVDF-TrFE) [Hsiang et al., 2001 Hilczer et al., 2002 Gimenes et al., 2004 Lam et al., 2005 Beloti et al., 2006]. However, the necessity of miniaturization of electronic components and... 

Pyroelectric ceramics can be used to detect any radiation that produces a change in the temperature of the crystal, but are generally used for IR detection. Because of their extreme sensitivity a rise in temperature of less than one-thousandth of a degree can be detected. This property finds application in devices such as intruder alarms, thermal imaging, and geographic mapping. 

Because of its high piezoelectric constants, PZT is the most widely used pyroelectric ceramic in... 

Pyroelectric sensors utilize a wide range of material forms crystals, bulk ceramics, thick layers, and thin films. Noteworthy among new compositions with improved performance is the class of relaxor pyroelectrics. Many novel pyroelectric sensors utilize thin and thick films. Bulk pyroelectric ceramics for room temperature, and pyroelectric sensors, are widely available. New compositions with higher permittivity are available now. 

Pyro-dad. Trade-name. A process for depositing multilayer coatings of metals and ceramics. (Aremco Products Inc. USA). Pyroelectric Materials. Pyroelectrics develop an observable spontaneous electric moment only when heated. Cf ferroelectrics. Pyroelectric ceramics include some niobate-zirconate systems, barium titanate modified with zirconia and hafnia, and lanthanum-modified lead... 

Grouiez B, Parvitte B, Joly I, Courtois D, Zeninari V (2008) Comparison of a quantum cascade laser used in both CW and pulsed modes application to the study of SO lines around 9 fm. Appl Phys B 90 177-186 GuggUla P, Batra AK, Currie JR, Aggarwal MD, Alim MA, Lai RB (2006) Pyroelectric ceramics for infrared detection applications. Mater Lett 60 1937-1942... 

Therefore a composite oon i of highly piezo- and pyroelectric ceramic material combined with a polymer would be the ideal rcpiacemem to obtain the properties of both... 

PZFNTU = doped lead zirconate, a typical pyroelectric ceramic [37]. 

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