Pyroelectric coefficients

Fig. 95 shows the change in cell parameters, density and Curie temperature for ceramics with initial compositions of Li(Tai xMgx)03.3xF3x (where 0 < x < 0.2) versus x value. It should also be mentioned that the pyroelectric coefficient for x = 0.05 was found to be 4.0 nC cm 2 K l. 

Dielectric constant, e33/e9 frequency - 103 Hz frequency -106 Hz Dielectric losses, tan S (along OZ) frequency -103 Hz frequency -106 Hz Pyroelectric coefficient p l(f9 C cni2 -K1... 

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. 112. Pyroelectric coefficients of RbsNb3OFi8 and KsNbsOFw crystals in the temperature range around room/ambient temperature. Reproduced from [443], A. 1. Agulyansky, J. Ravez, R. Von Der Mtihll, A. Simon, Ferroelectrics 158 (1994) 139, Copyright 1994, with permission of Taylor Francis, Inc., http //www.routledge-ny.com.

At high temperatures, the module of the pyroelectric constants of both compounds increases more significantly and reaches an extremum at about 480K. Fig. 113 shows the temperature dependence of the pyroelectric coefficients. This phenomenon could be related to the change in dilatation mechanism that was observed while investigating the temperature dependence of the lattice parameters (see Fig. 102). 

In general terms, the pyroelectric coefficient of a free sample consists of three components. The first, called the real coefficient, depends on the derivative of spontaneous polarization with respect to the temperature. The second is derived from the temperature dilatation and can be calculated based on mechanical parameters. The third coefficient is related to the piezoelectric effect and results from the temperature gradient that exists along the polar axis of the ciystal. 

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. 

The hiding of the pyroelectric coefficients seems to be correlated to the maximum c parameter, which in turn corresponds to the transition temperature. The shift along the Oz direction, Az, of the niobium atoms, which are located within the octahedrons, is responsible for the compound s polar properties. When c is at its maximum, this shift is enhanced and leads apparently to maximum spontaneous polarization P The value of Ps increases in the temperature range of 300 to 490K and then decreases at temperatures above 490°K. 

The compounds K5Nb3OFi8 and Rb5Nb3OFi8 display promising properties for their application in electronics and optics. The compounds can be used as piezoelectric and pyroelectric elements due to sufficient piezo- and pyroelectric coefficients coupled with very low dielectric permittivity. In addition, the materials can successfully be applied in optic and optoelectronic systems due to their wide transparency range. High transparency in the ultraviolet region enables use of the materials as multipliers of laser radiation frequencies up to the second, and even fourth optical harmonic generation. 

Ferroelectric materials are capable of being polarized in the presence of an electric field. They may exhibit considerable anomalies in one or more of their physical properties, including piezoelectric and pyroelectric coefficients, dielectric constant, and optoelectronic constant. In the latter case, the transmission of light through the material is affected by the electric field, which produces changes in refractive index and optical absorption coefficient. Varying the applied field changes the phase modulation. 

Here, A is the area of the electrode, dT/df the rate of the temperature change, and p=dPldT is called the pyroelectric coefficient. 

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]. 

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... 

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... 

Dielectric constants, dissipation factors, and pyroelectric coefficients of the alternating LB films comnosed of Ba salts of Dhenvlovrazine derivatives. 

Pyroelectricity of the film is highly dependent on the range of temperature applied to the sample film. The LB film of Compound I showed dull pyroelectric response in the high-temperature range. The signs of pyroelectric coefficients were changed at a temperature characteristic of the film. 

Equations (6) and (7) express these relationships. are the elastic compliance constants OC are the linear thermal expansion coefficients 4 and d jj,are the direct and converse piezoelectric strain coefficients, respectively Pk are the pyroelectric coefficients and X are the dielectric susceptibility constants. The superscript a on Pk, Pk, and %ki indicates that these quantities are defined under the conditions of constant stress. If is taken to be the independent variable, then O and are the dependent quantities ... 

The pyroelectric coefficient at constant strain, p is expressed by the polarization model, using the quasi-harmonic approximation, as... 

The only contribution to primary pyroelectricity is the change in dipole oscillations with temperature at fixed lattice constants (strain). The calculated values for the primary and secondary pyroelectric coefficients are plotted in Figure... 

Thermal expansion coefficients and the primary and secondary pyroelectric coefficients of animal bone. Nature 224, 798 (1969). 

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