BASIC PRINCIPLES OF OPERATION · 47 Figure 1.32. Schematic of pyroelectric sensor layout. (Reprinted with permission from Schreiter et al. 2006. Copyright 2006 Elsevier.) One example of a pyroelectric sensor is shown in Figure 1.32. The array is formed by a thin-film capacitor deposited on a supporting SiO 2 /Si 3 N 4 membrane. The dielectric is made of a thin film of ferroelectric lead zirconate titanate (PZT). To provide sufficiently high thermal isolation, the underlying silicon is removed by bulk micromachining. To enable elevated working temperatures, a local low-power thin-film heater arranged on bulk-micromachined membranes and integrable with the pyroelectric sen- sor structure was developed and tested. The functionalization of active elements of the pyroelectric de- tector array was accomplished by coating them with a chemically sensitive layer of polydimethylsiloxan (PDMS) for the detection of VOCs or with Pt-cluster arrays grown on bacterial surface layers (S-layers) serving as catalyst for hydrogen oxidation. Another commonly used pyroelectric material is lithium tan- talate (LiTaO 3 ). For use in sensors, the material is doped with lanthanum traces. Polyvinylidene fluoride (PVDF) is a synthetic organic pyroelectric polymer. 15. THERMAL CONDUCTIVITY SENSORS Thermal conduction is one form of thermal energy transfer from one object to another. Physical con- tact between two bodies is required for heat conduction. Kinetic energy is transferred to a cooler body from a warmer body by thermally agitating its particles. As a result, the cooler body gains heat while the warmer body loses heat. For instance, heat passage through a rod is governed by a law similar to Ohm's law; the heat flow rate is proportional to the thermal gradient across the material (dT/dx) and its cross-sectional area (A), or H = dQ dT = - kA dT dx (1.35) where k is called the thermal conductivity. The minus sign indicates that heat flows in the direction of temperature decrease. A good thermal conductor has a high value of thermal conductivity, whereas thermal insulators have low values of k.