Pyroelectric measurements

Figure 13. Schematic illustration of pyroelectric measurement (A) static method (B) dynamic method [13].

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 14. Alternating LB films S(PS)n and the electric circuit for pyroelectric measurements [7].

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... 

The charge was measured as in the pyroelectric measurement above while the specimen was subjected to square step pressure cycles of amplitude 0-500 psi and duration 10 sec, applied at 1 minute intervals. It was found necessary to apply a correction to d to allow for the pyroelectric response of the sample. This response arose due to the adiabatic heating of the silicone oil on pressurisation. 

Pyroelectric measurements show the absence of ferroelectric properties, thereby indicting that the phase is not tilted. 

Fig. 4.5-84 (NH2CH2C00H)3 H2SO4. A versus T. Ps is parallel to the b axis. Gray circles, values determined by pyroelectric measurements. Triangles and brown circles. determined from hysteresis loop by different authors...

FIGURE 16 Spontaneous polarization, (nC/cm K) of a side-chain siloxane FLCP versus temperature (°C), determined by polarization reversal with repolarization frequency ( ) / = 0.5 and ( ) f 5 Hz. Continuous line determined from pyroelectric measurements [53j. 

In summary, chiral smectic-C phases lack a center of symmetry. Hence they can be used as materials for second-order nonlinear optics [120-124], and possess piezoelectric and pyroelectric properties. Pyroelectric measurements have been performed on LC polymers [125] as well as on LCEs [126-128]. Irradiation of an FLCE sample with light usually leads to a temperature increase resulting in a pyroelectric signal [129]. More interesting are systems in which dye molecules like azobenzenes lead to a shift of the phase transition temperature upon isomerization [19]. 

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. 

Fig. 4.5-75 ((NH4)3H)i c((ND4)3D)j (S04)2. Ps versus T. Parameter x. Gray circles (for Phase VI), determined by pyroelectric measurements. Brown circles (for phase VII), determined by hysteresis loop measurements...

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

Thermocouples, bolometers and pyroelectric and semiconductor detectors are also used. The first three are basically resistance thermometers. A semiconductor detector counts photons falling on it by measuring the change in conductivity due to electrons being excited from fhe valence band info fhe conduction band. 

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. 

Fig. 113. Pyroelectric coefficients of RbsNbjOFig and KsNbsOFis crystals measured in the high temperature range. Reproduced from [443], A. I. Agulyansky, J. Ravez, R Von Der Miihll, A. Simon, Ferroelectrics 158 (1994) 139, Copyright 1994, with permission of Taylor Francis, Inc., http //www. routledge-ny.com.

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... 

Using data obtained from thermocurrent, piezoelectric and dilatometric measurements, the first and second pyroelectric coefficients were calculated for the Rb5Nb3OFi8 crystal. Fig. 114 presents the results. 

Fig. 114. Pyroelectric coefficients of RbsNbjOFl8 crystal (free sample). 1 - Measured coefficient 2 - Second coefficient 3 - First coefficient.

Based on the results of the calculations, it should be mentioned that the first pyroelectric coefficient seems to be significantly higher than the measured value even at room temperature. This high value of the coefficient can be achieved by clamping (compressing) the sample. 

CB04. The spontaneous polarisation was measured by the pulse pyroelectric technique (Ps = 46 nC/cm ). The piezoelectric coefficient evaluated for CB04 was dsi = 1.6 pC/N. The estimation of the efficiency of the second harmonic generation for compound CB04 gives the value three times more than for quartz. 

The first clinical IRET used thermopile sensors to achieve non-contact temperature measurement in the ear. In 1991 a tympanic thermometer for home use was first introduced to the consumer market (Thermoscan HM 1). It utilized a pyroelectric sensor which requires the use of a suitable mechanical shutter or chopper mechanism, since it is only sensitive to temperature changes [3]. The main advantage of the pyroelectric sensor unit was its lower cost. However, prices for thermo-... 

In the most common LB films with the Y-type structure, the center of inversion exists, and hence they are not suitable for pyroelectric usages. On the other hand, since LB films with X- or Z-type structure have no center of symmetry, it is possible to construct the polar pyroelectric film with permanent dipoles pointing toward one direction. Similar structures can also be formed in hetero LB films with two different amphiphiles stacked altematingly. The first report on the pyroelectric LB film with X-or Z-type structure appeared in 1982 by Blinov et al. [12], It was followed by those of the alternate LB films by Smith et al. [13] and Christie et al. [14]. The polarized structure of the fabricated LB film can be checked by the surface potential measurements using the Kelvin probe [15], the Stark effect measurements [12], or the sign inversion of the induced current between heating and cooling processes. 

Figure 16 shows thermal stability of the pyroelectricity for the S(PS)9S4-Ba film [7]. After the current was measured on the heating process to 40 °C (the curve a), the sample was cooled to -30 °C, and then the current was measured again in the second heating process to 60 °C. This I-T curve (the curve b) is almost identical with the first one. However, the third and fourth I-T curves (the curves c and d) which were obtained after the repeated heating of th sample to 60 °C, show smaller negative currents than the previous ones. These results indicate that the heating of the sample... 

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