Pyroelectricity measurement techniques

MEASUREMENT TECHNIQUES IN PYROELECTRIC LANGMUIR-BLODGETT FILMS... 

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. 

Byer and Roundy (1972) Pyroelectric Coefficient Direct Measurement Technique and Application to NSEC Response Time Detector by R. L. Byer and C. B. Roundy, Eerrorelectrics, 3, 333-338, IEEE Trans. Ultrason., 1972, SU-19, 333-338. Accessed July 2014 at http //web.stanford.edu/ rlbyer/PDF AllPubs/ 1972Z20.pdf... 

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 section above considered the physical properties that are important in determining the performance of a pyroelectric detector, measurement methods used to obtain the physical parameters which determine pyroelectric performance are worthy of critical discussion, as many techniques are reported in the literature. 

Experimental techniques are discussed for the characterisation of potentially useful thin film materials, including measurement of pyroelectric coefficient and dielectric data (permittivity and dielectric loss). It is noted that, when considering a complete thermal imaging system, it is not sufficient to consider material parameters in isolation, and that the combined features of LB films render them particularly suitable to high system performance. 

L.M. Blinov, M. Ozaki and K. Yoshino, Flexoelectric polarization in nematic liquid crystals measured by a field onoff pyroelectric technique, JETP Lett. 69(3), 236-242, (1999). [Pisma Zh. Eksp. Tear. Fiz. 69(3), 220-225, (1999).]... 

The pyroelectric current i(T) at a temperature T can also be measured by uniformly heating or cooling the sample at a constant rate (generally, 1-4- °C/min) and the pyroelectric coefficient, p, can be calculated using the following expression (Byer-Roundy technique) ... 

Had we taken higher order terms of r into account, the divergence would disappear. The Curie-type order parameter relaxation has been studied on a typical nematic (5CB), see Fig. 6.16. The measurements have been made using a pulse pyroelectric technique [18]. As the nematic-isotropic transition in 5CB is weak first order, it clearly demonstrates some features of the softening the relaxation time of the orientational order parameter on the nematic side of the NI transition increases five times. 

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 ... 

F. 10.13 Setup for the measurements of the surface polarization by a pyroelectric technique short pulse of a Nd-YAG laser heats the polar surface layta- of a liquid crystal and the pyroelectric current is detected by an oscilloscope... 

The coefficient y is rotational viscosity of the director similar to coefficient yi for nematics. In fact, it does not include a factor of sin cp and, in the same temperature range, can be considerably larger than the viscosity ytp for the Gold-stone mode. This may be illustrated by Fig. 13.10 the temperature dependence of viscosities y and have been measured for a chiral mixture that shows the nematic, smectic A and smectic C phases [15]. The pyroelectric and electrooptic techniques were the most appropriate, respectively, for the measurements of ya and ytp describing the viscous relaxation of the amplitude and phase of the SmC order parameter. The result of measurements clearly shows that y is much larger than y and, in fact, corresponds to nematic viscosity yj. 

The absolute value of the Ps has been measured by the pyroelectric technique as explained in Section 11.3.1 but with an applied d.c. electric field, exceeding the AF-F transition threshold. The value of the observed polarization dramatically... 

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]... 

A typical example of the temperature behavior of the spontaneous polarization is shown in Fig. 7.32. The polarization was measured by the pyroelectric technique on the compound analogous to (7.iv), but containing a Cl-atom in the chiral center ... 

Thermoelectric-, pyroelectric-, and thermoconductivity-based devices are other representatives of thermometric gas sensors (Korotcenkov 2011). In particular the thermal conductivity technique for detecting gas is suitable for the measurement of high (vol. %) concentrations of binary gas mixes. The heated sensing element is exposed to the sample and the reference element is enclosed in a sealed compartment (see Fig. 1.14). If the thermal conductivity of the sample gas is higher than that of the reference, then the temperature of the sensing element decreases. The higher their thermal conductivity, the lower the concentration which can be measured (Table 1.14). Power loss of a single filament thermistor by heat conduction via the ambient gas can be expressed as... 

There are several methods for measurement of pyroelectric coefficients, but most can be grouped into two kinds. The objective in the first case is to directly measure the pyroelectric current and at the same time change the temperature. In the second method measurements of polarization or charge can be estimated either by the integration of pyroelectric current during the continuous heating of the sample or recording the hysteresis loop at various temperatures by the Sawyer-Tower technique. 

Eollowing are some techniques for measurement of pyroelectric coefficients. 

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. 

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