Pyroelectric element

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. 

For practical use of pyroelectric elements as infrared sensors and so forth, the induced voltage V is an important quantity. From the relations concerning surface charge. 

The origin of the pyroelectric effect, particularly in crystalline materials, is due to the relative motions of oppositely charged ions in the unit cell of the crystal as the temperature is varied. The phase transformation of the crystal from a ferroelectric state to a paraelectrlc state involves what is called a "soft phonon" mode (9 1). In effect, the excursions of the ions in the unit cell increase as the temperature of the material approaches the phase transition temperature or Curie temperature, T. The Curie temperature for the material used here, LiTaO, is 618 C (95). The properties of a large number of different pyroelectric materials is available through reference 87. For the types of studies envisaged here, it is preferable to use a pyroelectric material whose pyroelectric coefficient, p(T), is as weakly temperature dependent as possible. The reason for this is that if p(T) is independent of temperature, then the induced current in the associated electronic circuit will be independent of ambient temperature and will be a function only of the time rate of change of the pyroelectric element temperature. To see this, suppose p(T) is replaced by pQ. Then Equation U becomes... 

The general conclusion to be drawn from these studies is that the use of small pyroelectric elements as heat flow sensors in chemical investigations holds some promise. The early stage of the studies makes it difficult to assess the extent of their utility. New adsorber materials are an essential requirement if these structures are to fulfil their promise. 

The pyroelectric element is likely to be a specialty device, though it is far too soon to tell. Its primary advantage is that it can permit time integration of the species of interest and as a result can be used to detect extremely small concentrations. The most that can be said at the present is that more research is clearly needed for this structure. 

The voltage responsivity of the detector shown in Figure 11.1 is simply derived from the pyroelectric current ip and the electrical admittance Y presented to it. Ignoring for the moment the ac conductance of the pyroelectric element ... 

Figure 11.2 Voltage responsivity vs frequency for a typical small ceramic pyroelectric element.

For practical purposes, the very small signals generated by pyroelectric elements must be amplified. The most widely used first stage consists of a field effect transistor (FET) which responds to electric potential rather than to charge. In this case, it is advantageous for the material to have a low permittivity to match the low input capacitance of the FET. Therefore the compositions with high... 

The pyroelectric elements used in the devices described so far are commonly square plates with sides about a millimetre long and thicknesses around 30 /mi. Because entire scenes are focused onto the plates in thermal imaging, they have to be larger, typically squares of side about 1 cm the thicknesses are the same as for the simpler devices. 

To reduce expense, efforts are made to exploit integrated thin film technologies. For example, arrays have been produced via thin film deposition of the pyroelectric onto a sacrificial layer, e.g. a suitable metal or polysilicon, which is then selectively etched away. Thermal isolation of the pyroelectric element is achieved through engineering a gap between it and the ROIC silicon wafer. Yias in the supporting layer permit electrical connections to be made between the detector and the wafer via solder bonds. Imaging arrays have been produced in this way incorporating sputtered PST and sol-gel formed PZT films. 

Asymmetric two-site model. If the two sites are not equivalent they may be unequally populated in equilibrium under no field. This will in general give rise to a temperature-dependent polarization in zero field, that is, to a variety of pyroelectricity. A set of pyroelectric elements can be arrayed in a material so that the behaviour cancels, whereas they all contribute to the polarization response to an electric field. 

Various workers have developed models relating the thermal and electrical properties of a pyroelectric element [2] yielding an expression for voltage responsivity (amplitude of signal per unit input power) ... 

During recent years, the study of micro- and nanoscale fluids has shown significant opportunities for high detectivity of elementary devices with small size. For pyroelectric flow sensors, it is highly desirable to develop theoretical models, experimental methods for pyroelectric element preparation and sensor fabrication, and higher sensitivity with excellent mechanical properties. 

As was pointed in the previous section, the electric impedance of the pyroelectric element is very high. e.g.. 10" ft so that a high input-impedance amplifier b necessary to be endosed near the detector dement as shown Figure 2 and its output impedance should be low. A junction field-effect transistor (JFET) or an operathmal amplifier with a JFET input b usually used as the enclosed amplifier, because the JFET has a very high input impedance and generates only small voltage and current nobc. 

For room temperature operation, a most attractive thermal detector is the pyroelectric element. It is a small capacitor with a dielectric material that possesses a temperature sensitive dipole moment. So far, the most successful dielectric is triglycine phosphate (TGS), particularly if doped with L-alanine. Its Curie point is at 49 °C and, consequently, it must be operated below that temperature. (Above the Curie point, these dielectrics lose their pyroelectric properties.) Other suitable materials include lithium tantalate and strontium barium niobate. The voltage across a capacitor of charge Q is... 

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