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Abstract
Of the three principal uncooled detection mechanisms, the pyroelectric is of the most recent origin. It was originally proposed as an infrared detector by Chynoweth in 1956, and reduced to early practice by Cooper, Putley, and others. Tompsett proposed its use in an uncooled thermal imaging camera tube, which was the first practical application. Uncooled pyroelectric array technology was first developed by Hopper in the U.S. and by Putley, Whatmore, and Porter in the U.K. All of the early pyroelectric arrays were of hybrid construction; the emphasis is now on monolithic construction with its superior thermal isolation. The pyroelectric effect is found in materials such as, but not limited to, ferroelectrics. This discussion emphasizes ferroelectrics; that is the only class that has been widely exploited. Ferroelectrics, such as barium strontium titanate (Ba x Sr 1−x TiO 3 ), strontium barium niobate (Sr 1−x Ba x Nb 2 O ), lithium tantalate (LiTaO 3 ), and lead titanate (PbTiO 3 ), exhibit internal spontaneous electric polarization, which can be measured as a voltage at electrodes placed on opposite faces of a sample of the ferroelectric material. At a constant temperature of the sample, this internal polarization is neutralized by mobile charges on the surface of the sample; thus there is no voltage difference measured by the electrodes. If the temperature is suddenly changed to a new value, the internal polarization will change, resulting in a measurable voltage difference, which will then be neutralized again by a changed distribution of surface charge. Thus pyroelectric detectors based on ferroelectric materials have no dc signal. The surface temperature must be modulated in time. This is usually accomplished by means of a radiation chopper, but it also can be achieved by constantly moving the infrared image across the material, such as has been employed with pyroelectric vidicon thermal imaging camera tubes. This chapter deals with the classic pyroelectric effect, which does not employ electrical bias. However, hybrid pyroelectric array technology usually employs an electrical bias to enhance the signal, see Hanson. These hybrid pyroelectric arrays are operated at a temperature just below the Curie temperature, above which the material loses its ferroelectric properties. Just below it, the temperature dependence of the internal polarization has its greatest value.
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CHAPTER 5
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