Pyroelectric films

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. 

The extent of piezoelectric activity depends upon the poling time, field strength, and temperature. The dipoles are aligned relative to the direction of the field during the poling process. The constants that characterize the activity of piezoelectric and pyroelectric films have been listed in Table 14.14. The axes are defined as length 1, width 2, and thickness 3. Two numeric subscripts are used to respectively denote the axis of polarization (applied electrical field) and the axis of mechanical strain or applied stress. 

At a depth equal to thermal diffusion (2IC/uiC) below the IR absorbing layer, the thermal amplitude will be reduced to l/e of its value at the absorbing layer. If the thickness of the pyroelectric film is greater than this length for the dropping period, the... 

Polymer Ferroelectrics. In 1969, it was found that strong piezoelectric effects could be induced in the polymer poly(vinyhdene fluoride) (known as PVD2 or PVDF) by apphcation of an electric field (103). Pyroelectricity, with pyroelectric figures of merit comparable to crystalline pyroelectric detectors (104,105) of PVF2 films polarized this way, was discovered two year later (106.)... 

The FT-IR technique using reflection-absorption ( RA ) and transmission spectra to quantitatively evaluate the molecular orientation in LB films is outlined. Its application to some LB films are demonstrated. In particular, the temperature dependence of the structure and molecular orientation in alternate LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid (and of their barium salts) are described in relation to its pyroelectricity. Pyroelectricity of noncentrosymmetric LB films of phenylpyrazine derivatives itself is represented, too. Raman techniques applicable to structure evaluation of pyroelectric LB films are also described. 

RA and transmission techniques [3], and applied it to the studies of LB films of cadmium stearate [3], azobenzene-containing long-chain fatty acids and their barium salts [4], dipalmitoylphosphatidylcholine (DPPC) [5], and polyion complexes [6]. Furthermore, we explored the relationship between the molecular orientation evaluated by this method and pyroelectricity in alternating (noncentrosymmetric) Y-type LB films consisting of a phenylpyrazine-containing long-chain fatty acid and deuterated stearic acid and of their barium salts [7]. 

The first term represents the temperature dependence of the film density, being directly related to the thermal expansion coefficient of the film, and also indirectly related to that of the substrate. Generally, the thermal expansion coefficient of the organic substance is much larger than that of the inorganic substance like glass. In the case of the alternate LB film of fatty acid/alkylamine or fatty acid/alkylaniline, the pyroelectric coefficient p decreased as the thermal coefficient of the substrate increased [18]. 

RELATIONSHIP BETWEEN MOLECULAR ORIENTATION AND PYROELECTRICITY IN ALTERNATING LB FILMS... 

It has been known that alternating LB films consisting of two different amphiphiles have noncentrosymmetric structures, and therefore, are expected to provide piezoelectric, pyroelectric and nonlinear optical properties. Therefore, we prepared alternating LB films consisting of 5-(p-dodecyloxyphenyl)pyrazine-2-carboxylic acid (DOPC)... 

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 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... 

Now, we consider the effect of the number of unit PS bilayers n of the alternating S(PS)n-Ba and S(PS)n films on their pyroelectricity and mole-cular orientation at room temperature [7]. In Figure 21, the pyroelectric coefficients for both alternating films at 30 °C are plotted against the n value. The coefficients of the S(PS)n films apparently decrease with the increase in the n value. The value at n = 9 (1.1 p.Cm2K ) is half of that (2.1 pCm K1) at n = 3. However, the coefficients of the S(PS)n-Ba films are almost constant (ca. 1.3 a.Cm 2K ) within the experimental error. 

Figure 22 is a plot of the orientation angle y of the hydrocarbon chain axis of DOPC in the S(PS)n film and of DOPC-Ba in the S(PS)n-Ba film as a function of then n value. For the S(PS)n films, the y values increase with increasing n value. But, for the S(PS)n-Ba films, the y values are almost independent of the n value. These results correspond well with those for the pyroelectric coefficients, and indicate that if the molecular orientation is unchanged, the pyroelectricity is also unchanged, but if the molecules are more inclined, the pyroelectricity is decreased. The increase in inclination... 

The alternating LB films with more highly oriented constituent molecules give rise to larger pyroelectricity. 

If the molecular orientations are in the same order, the acid alternating film has larger pyroelectricity than the barium salt film. This may be due to a weaker intermolecular interaction in the former film than in the latter film. 

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