Pyroelectric current

For pyroelectric measurements, we used two A1 electrodes on both sides of the alternating LB film as shown in Figure 14. The electric current generated on linearly heating the LB film was measured by a picoammeter in the temperature range from -30° to 60 °C. The pyroelectric coefficient p is calculated from the observed pyroelectric current/ by... 

Figure 27 shows the pyroelectric current trace and the temperature cycle around 20 °C applied to the pyroelectric measurement of the alternating film consisting of the barium salt of Compound II and stearic acid. As the temperature increased, positive current flowed through the circuit to reach an almost constant value. When the temperature started to decrease, the current immediately changed its flow direction to negative until it reached an almost constant value. This square wave pattern of the current flow did not vary during continuous one-hour measurement. Similar current... 

Figure 27. Pyroelectric current trace and temperature cycle around -20 °C for the alternating LB film of Ba salts of Compound II and stearic acid.

When the film is short-circuited and heated to high temperatures at which the molecules attain a sufficiently high mobility, a current is observed in the external circuit. This phenomenon is called pyroelectric effect, thermally stimulated current, or, when the film has been polarized by a static field prior to measurement, depolarization current. The conventional definition of pyroelectricity is the temperature dependence of spontaneous polarization Ps, and the pyroelectric constant is defined as dPJdd (6 = temperature). In this review, however, the term will be used in a broader definition than usual. The pyroelectric current results from the motion of true charge and/or polarization charge in the film. Since the piezoelectricity of a polymer film is in some cases caused by these charges, the relation between piezoelectricity and pyroelectricity is an important clue to the origin of piezoelectricity. 

Figure 20 indicates the pyroelectric current in calender-rolled rigid poly (vinyl chloride) (PVQ (Furukawa and others, 1968). In Fig. 20, results are shown for fifteen specimens cut from a sheet of PVC film, with the polarity map as Fig. 21. The map shows a heterogeneity of polarity.

The pyroelectric current is expected from the depolarization of spontaneous polarization (i.e., motion of polarization charge) as well as from the motion of true charge. The polarity of the pyroelectric current is, however, inverted between the two, as is shown schematically in Fig. 22. It is difficult to assume a polarization charge in non-polar or weakly polar polymers such as polyethylene (PE), polypropylene (PP), and... 

Fig. 20. Pyroelectric current of 15 specimens of poly(vinyl chloride) film cut out from a calender-rolled sheet as indicated in Fig. 21. Electrode area = 1 x 1 cm2 film thickness = 0.2 mm heating rate=6 K/min. Reproduced from Furukawa and others [J. Appl. Polymer Sci. 12,2675 (1968)] by permission of John Wiley Sons,...

Fig. 22. Relation between polarities of poling field and pyroelectric current of polarized films a dipole orientation, b ion separation, and c homocharge injection...

The pyroelectricity or depolarization current is closely related to the piezoelectricity of Groups (B) and (C), because the pyroelectric current is caused by the thermal activation of charges in the film. The application of polypeptide film to an acoustic transducer was proposed by Fukada, Tamura, and Yamamoto (1968). 

This expression is the basic description for the use of the pyroelectric effect in a host of sensor applications including the well known optical detection devices (82,83). A particularly useful way of describing this type of system is with an equivalent circuit where the pyroelectric current generator drives the pyroelectric impedance and the measuring amplifier circuit as shown in Figure 11. 

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 20. Simultaneous measurements of the transient current response of a Pd-MOSCAP (i ) and the pyroelectric current from the pyroelectric enthalpiemeter (i ) illustrated in Figure 18. Upper two curves are at room tempe ature and with 0.17. Hj in and the lower two are for 17. 1 in N .

Thus, from Equation (11.2), the pyroelectric current ip, generated per watt of input power (the current responsivity Rf) is ... 

The voltage responsivity of the detector shown in Figure 11.1 is simply derived from the pyroelectric current ip and the electrical admittance Y presented to it. Ignoring for the moment the ac conductance of the pyroelectric element ... 

The pyroelectric coefiicient (p) is a useful parameter with which to compare different materials. If the thin film acts as a dielectric in a capacitor, and an external resistance is connected between the electrodes, then a pyroelectric current (I) flows in the circuit. This situation can be expressed as... 

Figure 7 Pyroelectric current of amorphous PZT (52/48) film on gold. Film was heated at 350°C for 1 h, and the thickness was 3000 A. (After Xu et al. [58].)...

The pyroelectric coefficient p, is a useful parameter with which to compare different materials. If the thin film acts as a dielectric in a capacitor and an external resistance is connected between the electrodes, a pyroelectric current, I, flows in the circuit this can be expressed as I = pA(dT/dt) where dT/dt is the rate of change of temperature, and A is the cross-sectional area of the device. In a thermal imager many considerations, other than a high value of p, must be borne in mind,when designing a pyroelectric detector capable of resolving a temperature difference in the scene temperature of O.IK. For example, the figure of merit for a thermal imaging device requires the pyroelectric materials to have low values of permittivity. 

The pyroelectric effect is well documented, and has been reported for a wide range of inorganic and organic materials [1]. One of the most promising applications of pyroelectricity is in the area of infra-red or thermal imaging. Thermal information from a scene is derived by measuring the pyroelectric currents from elements in an array onto which the scene has been projected. In order to optimise the performance of thermal imaging devices, it is necessary to be able to characterise pyroelectric materials,both in terms of their intrinsic properties,and their performance in m el systems. 

A pyroelectric material is one which possesses a temperature-dependent, spontaneous electric polarisation. If such a material experiences a change in temperature, a charge is develop across the material in response to the change in electric dipole, 6P if an electrical circuit is completed, then a short-circuit current results which is termed the pyroelectric current (figure 1). This can be expressed as ... 

Values of pyroelectric coefficient are calculated from equation (l),by measuring the pyroelectric current for a known controlled temperature profile. A computer-controlled system has been developed, to force the temperature of a sample under test, and log the corresponding pyroelectric current. This system is shown schematically in figure 2. Samples are mounted in an earthed vacuum chamber. Evacuation of the sample chamber removes a significant proportion of absorbed water, which is otherwise responsible for spurious, thermally induced currents. The temperature of the sample is controlled,in both heating and cooling, by a Peltier thermoelectric heat pump. 

Pyroelectric currents are proportional to the time derivative of the temperature and, in case of... 

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