Pyroelectric heat sensors

Detectors for IR radiation relate to the source, heat sensors being suitable. Older instruments used thermocouple detectors. Pyroelectric detectors, which change electrical properties when exposed to IR radiation, esf>ecially doped triglycine sulfate, are often used. 

Some liquid-crystalline materials contain smectic phases which give ferroelectric and pyroelectric properties and are proposed for use in optical shutters, displays and heat sensors, as has been discussed by Simmonds in Chapter 7. 

The general conclusion to be drawn from these studies is that the use of small pyroelectric elements as heat flow sensors in chemical investigations holds some promise. The early stage of the studies makes it difficult to assess the extent of their utility. New adsorber materials are an essential requirement if these structures are to fulfil their promise. 

Biosensors based on the heat produced by enzyme/substrate reactions have traditionally used microcalorimeters (1), thermistors (2), and Peltier or other macro devices <3,A) The area has been reviewed by Guilbault (5). The size, response time, and thermal mass of these detectors suggests that thermally responsive microsensors need to be explored. The ideal sensor would be inexpensive, and require simple, low cost support electronics. A fiber optic based sensor (Part A), and a pyroelectric polymer film based sensor (Part B) are described below. 

Thermoelectric-, pyroelectric-, and thermoconductivity-based devices are other representatives of thermometric gas sensors (Korotcenkov 2011). In particular the thermal conductivity technique for detecting gas is suitable for the measurement of high (vol. %) concentrations of binary gas mixes. The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment (see Fig. 1.14). If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. The higher their thermal conductivity, the lower the concentration which can be measured (Table 1.14). Power loss of a single filament thermistor by heat conduction via the ambient gas can be expressed as... 

A somewhat different approach with a pyroelectric detector chip was su ested by Schreiter et al. (2006). The advantages of such a lead-zirconate-titanate-based detector array are the low heat capacity and high thermal insulation of the sensor. The surface of the sensor was coated with specific reagents to detect different chemicals. As an example, the coating with poly(methylsiloxane) was chosen to detect heptane with a detection limit of 10 ppm. With a bacterial surface layer and small Pt clusters, it was possible to study the catalytic oxidation of hydrogen in the range of 0.5-3.5 vol%. 

PVDF exhibits pyroelectric as well as piezoelectric properties [44], a feature that has made the material useful for infrared sensing in motion detectors and thermal cameras. This attribute has been used recently in a thermally based method to measure ultrasonic power [45]. A 52 pm thick film of PVDF 60 mm in diameter was used as the sensor, which was backed with a rubber based material that generated heat due to ultrasonic absorption. Powers up to 1 W over a frequency range of 1-3.5 MHz were measured in this preliminary study. 

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