Heat flow, pyroelectric

It can be shown from Figure 11 and Equation 1 that the rise time of the signal will be controlled by the RC time constant of the circuit loading the pyroelectric current generator. The simplest case is where the heat flow into the pyroelectric is one dimensional as shown in Figure 12. Then the signal across the parallel load resistor of the pyroelect-ic element is... 

Figure 12 One dimensional heat flow in a pyroelectric microcalorimeter.

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. 

Many stodies had demonstrated that pyroelectric anemometers based on single-crystal LiTaOs have a very wide range and the promise of interesting response characteristics. The following example was well characterized by a conjugate gas-flow-heat-flow model. 

Thermometiic (calorimetric) Heat effects of a specific chemical reaction. Changes in temperature, heat flow, heat content Thermoelectric Pyroelectric Catalytic bead (pellistors) Thermal conductivity... 

Lateral flow of heat in the pyroelectric detector plate tends to even out the temperature differences, blurring the charge pattern and hence the final image. This process will depend on the thickness h of the plate and the thermal diffusivity A = Xjc of the plate material. Yidicon performance improves as both h and A are reduced in value, and a suitable figure of merit is... 

Fig. 2 Experimental setup for scanning pyroelectric microscopy (SPEM). As indicated, the sample may consist of two domains of opposite polarization. Local heating in either domain of polarization produces excess charges at both ends of the polar axis. This induces a current flow in the outer circuit. Opposite polarity is seen because of a change in the current direction.

FLCs possess a temperature-dependent spontaneous polarization and therefore they are pyroelectric because a change of temperature results in a flow of charge to and from the sur ces through the FLC sample. Pyroelectricity in FLCs was first observed in 1976 (92). The pyroelectric response can be measured by the dynamic Chynoweth technique (93j. In this method the FLC cell is periodically heated by a chopped light source. The AC pyroelectric current i( ) generated in the FLC cell can be detected with a lock-in amplifier and is given by... 

The major noise sources for a typical pyroelectric detector are the dielectric or Johnson noise, the amplifier current and voltage noise, and the thermal noise, caused by fluctuations in the power flow from the element to its heat sink. Each of these has an equivalent voltage generated at the amplifier input V y (given by equation (5.9)), and and Vj. respectively. These combine to give the total equivalent input noise according to the equation... 

---