Pyroelectric determination

The high-frequency dielectric constant is determined by the effects of electronic polarization. An accurate estimate of this property lends confidence to the modeling of the electronic polarization contribution in the piezoelectric and pyroelectric responses. The constant strain dielectric constants (k, dimensionless) are computed from the normal modes of the crystal (see Table 11.1). Comparison of the zero- and high-frequency dielectric constants indicates that electronic polarization accounts for 94% of the total dielectric response. Our calculated value for k (experimental value of 1.85 estimated from the index of refraction of the P-phase of PVDF. ... 

As for the physical properties of crystals, some account of crystal morphology and optics has been given in Chapters II and III, where, however, these subjects were developed only as far as was necessary for identification purposes. For structure determination further consideration of both these subjects, as well as others such as the magnetic, pyroelectric, and piezo-electric properties of crystals, is desirable this will be found in Chapter VIII. 

A close correlation between the polarities of piezoelectricity and pyroelectricity was found for PVC and poly (vinylidene fluoride) (PVDF) films (Nakamura and Wada, 1971). However, it must be emphasized that the polarity of piezoelectricity is determined not only by the polarity of the charge distribution but also by that of heterogeneous strain. The origin of heterogeneous strain in the elongation of film may derive from heterogeneity in the structure of the film. 

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. 

There are a wide variety of measurement systems available that will determine capacitance and loss. These include capacitance bridges (manual and auto-balance), impedance analysers, network analysers etc. It is important for the user to consider the frequency range over which the pyroelectric devices are to be used, as this will largely determine the selection of the instrument to use. As most pyroelectric detectors are used in the range 0.1 to 100 Hz, the instrument should ideally permit measurement over this frequency range. There are very few commercial capacitance and impedance analysers that will work below 20 Hz, and many low cost units... 

An example of quasi CW THz detection [86] uses a THz wave parametric oscillator (TPO) consisting of a Q-switched Nd YAG laser and parametric oscillator [87,88], In this technique, MgO LiNb3 is employed as a non-linear material to generate CW THz. Silicon prisms couple the THz radiation from the non-linear crystal where it is detected using a pyroelectric detector. THz images are collected at discrete THz frequencies and then spectroscopically analyzed using a component spatial pattern analysis method to determine sample composition. 

All signal detectors are required to detect the signal against a background of noise . Therefore, the signal-to-noise ratio must be optimized or, put another way, for maximum sensitivity the noise has to be minimized. The sensitivity of any detector is determined by the noise level in the amplified output signal. In the case of a pyroelectric detector and its associated circuitry, the principal sources of noise are Johnson noise, amplifier noise and thermal fluctuations. 

Structure of [5]-heIiphos obtained by XRD. The structures of the pyroelectric 4,8,12-trioxa-12c-phospha-4,8,12,12c-tetrahydrodibenzo[cd,mn]pyrene (61), of oligo(thioarylene)cyclophosphonites 62, of the Diels-Alder cyclo-adducts 63, 64, and of a rotational isomer of a l,2-diphenyl-3,4-diphosphinidene-cyclobutene (65) have been determined by X-ray crystallographic analysis. 

The neutral beam density in the interaction region can be determined from a measurement of the energy deposited by the fast neutral beam into a pyroelectric crystal whose response is first calibrated by a well-characterized ion beam, as discussed by Wetzel et al. (1987). As an alternative, the well-established Kr or Ar absolute ionization cross sections (known to better than 5%) can be used to calibrate the pyroelectric crystal. The calibrated detector is then used to determine the flux of the neutral target beam in absolute terms. This procedure avoids the frequent and prolonged exposure of the delicate pyroelectric crystal to fairly intense ion beams (Freund et al., 1990 Tamovsky and Becker, 1992). 

Figure 21.3. Binding energies per atom, ebm, for neutral silicon cluster size distributions determined with the pyroelectric calorimeter. The half-width of the size distributions is about 50 atoms for smaller cluster sizes and about 250 atoms for larger cluster sizes. The open circle represents the bulk value of the cohesive energy of silicon. ...

These ferroelectric copolymers are also pyroelectric. That means that a capacitor formed with the polymer will get a charge proportional to the temperature increase of the material. The measurement of the voltage of the capacitor or of the charge on the electrodes allows the temperature rise of the capacitor and thus the incident IR flux to be determined. 

These two factors are significant in determining the signal to noise ratio of the pyroelectric capacitor - multiplexor couple. Among the different materials only the copolymer is directly compatible with the semiconductor fabrication process. The cr olymer also shows a low thermal diffusion and the best merit factor. 

In the Raman and i.r. spectra of crystalline methyl 3,6-dideoxy-/3-D-ri o-hexo-pyranoside measured at low temperatures, four O-H bands were observed in the O-H stretching region. These were correlated with four distinct hydrogen bonds determined from X-ray data, and deuterium-isotope dilution methods were used to assist the analysis. An interesting study of the far-i.r. spectrum of a pyroelectric sucrose single crystal has been carried out, and the temperature dependence was correlated with polarization of the low-frequency mode (49.5 cm" ) along the binary axis. ... 

While the pyroelectric, dielectric, and thermal properties of pyroelectric materials are clearly fundamental in determining the basic performance of a device, there are many other properties to be considered when making a selection of an appropriate material. These are the following Electrical resistivity. The circuit in Fig. 2 shows a resistor Rq connected across the active element. This serves three functions. 

Note that besides the primary pyroeffect contribution, determined by polarization temperature variations, there can be the contributions from secondary and ternary pyroelectric effects. Latter effects are due to thermal expansion and inhomogeneous... 

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