Measurement of the pyroelectric coefficient

The pyroelectric coefficient can be measured in a variety of ways but a direct method is illustrated in Fig. 7.6. From Eq. (7.12) 

Pyroelectric materials respond to changes in the intensity of incident radiation and not to a temporally uniform intensity. Thus humans or animals moving across the field of view of a detector will produce a response as a result of the movement of their warm bodies which emit infrared radiation (a 10 /rm). To obtain a response from stationary objects requires the radiation from them to be periodically interrupted. This is usually achieved by a sector disc rotating in front of the detector and acting as a radiation chopper. 

All pyroelectric materials are piezoelectric and therefore develop electric charges in response to external stresses that may interfere with the response to radiation. This can largely be compensated for by the provision of a duplicate of the detecting element that is protected from the radiation by reflecting electrodes or masking, but which is equally exposed to air and mounting vibrations. The principle is illustrated in Fig. 7.7. The duplicate is connected in series with the detector and with its polarity opposed so that the piezoelectric outputs cancel. This results in a small reduction in sensitivity ( 3 dB) but compensation is an 

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Figure 22-11. Block diagram of the experimental set-up of the digital integration method, for measurement of the pyroelectric coefficient (Li et al., 1984, with permission).

Bergman and co-workers (2-4] were the first to report measurements of the pyroelectric coefficient of PVDF. Ibey reported values of —2.4 X 10 C/m K for biaxidly oriented films (films stretched in two perpendicular directioos in the plane of the film during manufacture) and about -0.5 X 10 CVm K for uniaxially oriented films. 

C Dias. M. Simon, R. Quad, and D. K. Das-Oupu, Measurement of the pyroelectric coefficient in composite using a temperature-modt ted excitatioa. 3. Pkys. D. AppL Pkys. 26 106(1993). 

Thermocurrent measurements were performed on crystals of RbsNbjOFig and K5Nb3OFi8 along the c direction [440, 443]. Fig. 112 shows the temperature dependences of the pyroelectric coefficients close to room temperature. 

The spontaneous polarization of FLCs can be measured by the pyroelectric technique using the temperature dependence of the pyroelectric coefficient 7 = dPsfdT [27], and by the conventional capacity Sawyer-Tower method [28] or by integrating the time dependence of the repolarization current ip (5 is the electrode area) [175, 176]... 

The sample preparation for a bulk pyroelectric measurement is very similar to what is required for a bulk piezoelectric measurement, namely a well-sintered ceramic disc that has been electrically poled. Determining the pyroelectric coefficient may be divided into two groups - the measurement of the pyroelectric current and the measurement of the charge. We will describe measurement techniques for both groups. In addition, the pyroelectric effect can be subdivided into primary and secondary effects. The primary effect is observed when the material is rigidly clamped under a constant strain to prevent any thermal expansion or contraction. Secondary effects occur when the material is permitted to deform, i.e. the material is under constant stress. Thermal expansion results in a strain that changes the spontaneous polarisation, attributable to the piezoelectric effect. Thus the secondary pyroelectric effect includes contributions caused by piezoelectricity. Exclusively measuring the pyroelectric coefficient under constant strain is experimentally very difficult. What is usually experimentally measured is the total pyroelectric effect exhibited by the material - the sum of the primary and secondary effects. 

The measured pyroelectric coefficient can be represented as the sum of the first coefficient (real pyroelectric coefficient - pt ) and the second coefficient, which depends on the piezoelectric constant (dtJ), the thermal... 

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... 

Amorphous LiNbOs films made by sol-gel processing were subjected to a series of characterizations [57]. It was found that an amorphous LiNbOs film obtained by heating the gel film at 100°C for 2 h showed P-E hysteresis with remnant polarization Pr = 10 pC/cm2 and coercive field Ec= 110 kV/cm. Electron diffraction of such film showed a diffuse ring pattern characteristic of an amorphous nature. These are shown in Fig. 6 in which the scale for E is 147 kV/cm division and that for P is 5.6 pC/cm2 division. Further measurement showed a pyroelectric coefficient of 8 pC/cm2 K at 28°C. Note that for singlecrystal LiNbOa, Pr = 50 pC/cm2 and the pyroelectric coefficient was reported to be 20 pC/cm2 K [1]. Further, a piezoelectric resonance was observed at similar frequency range for both amorphous and crystalline LiNbOa, characteristic of a ferroelectric material [57]. 

The pyroelectric coefficient distribution for 1,000 nm thick PZT film is reported in Fig. 2.19. One can see that it varies abruptly over the film thickness the profile is asymmetric due to the substrate presence on the only one surface. The plateau (indicating the corresponding bulk value) in the film central part is narrower than that for refraction index. The difference could be the result of higher sensitivity of pyroelectric properties to the surface influence (for instance due to the higher order pyroelectric effects contribution, see above) as well as due to different accuracy of n and O measurements. 

The surface polarization can be measured by different means. The most straightforward one is based on the pyroelectric technique [15]. To measure P one has to deal only with one surface of a cell with uniform director alignment, either planar or homeotropic at both interfaces. The main idea is to use a spatially dependent temperature increment in order to separate the contributions to the pyroelectric response coming only from the surface under study and not from the opposite one. By definition, the pyroelectric coefficient is y = dPIdT where P is macroscopic polarization of a liquid crystal and T is temperature. If we are interested only in the polarization originated from the orientational order we can subtract the isotropic contribution to y and calculate P in the nematic or SmA phases by integrating the pyroelectric coefficient, starting from a certain temperature T, in the isotropic phase ... 

The pyroelectric coefficient may then be calculated from the measured current flow and the rate of temperature change. 

The pyroelectric effect may be defined as the change in spontaneous polarisation, s, as a function of temperature. The symmetry requirements for pyroelectricity are far more restrictive compared with SHG and piezoelectricity. To exhibit a spontaneous polarisation, the material in question must crystallise in one of ten polar crystal classes (1, 2, 3, 4, 6, m, mm2, 3m, 4mm, or 6mm). Thus, polarity is required for pyroelectric behaviour. Determining the pyroelectric coefficient may be done two ways - either measuring the pyroelectric current or the pyroelectric charge. Both techniques will be described. 

In view of the pulse heating experimental results, Kepler and Anderson [82] suggested that a new effect, reversible i nges in crystallinity with temperature, could account for most, if not all, of the discrepancy between the measurements of secondary pyroelectricity and the total pyroelectric coefficient. 

Methods for Measuring the Pyroelectric Coefficient of Composites The most usual way to measure the pyroelectric coefficient is by the coavenlkmal quasi-static method [48,154,155]. Since tte polarizalioo P measure is surface charge density o QIA (Q denotes electric charge and A is the surface of the dielectric film), Eq. (25) can be rewritten as follows ... 

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