Thermal using pyroelectric devices

The use of bulk ferroelectrics in pyroelectric devices inevitably leads to a situation where the material must be cut, lapped and polished to make a thin, thermally-sensitive layer. If an array of detectors is required for thermal imaging, this must be metallized on both faces,... 

The use of bulk p)Toelectiics in pyroelectric devices inevitably leads to a situation where the material must be cut, lapped, and polished to make a thin, thermally sensitive layer. If an array of detectors is required for a thermal goal, this must be metallized on both faces, processed photolithographically, and bonded to a silicon read-out circuit to yield a complete hybrid array. Clearly, it would be desirable if the material could be deposited as a thin film, to remove the requirement for lapping and polishing, if possible directly onto a complete wafer of chips, where it could be processed to yield an array of thin. 

Detectors for IR radiation fall into two classes thermal detectors and photon-sensitive detectors. Thermal detectors include thermocouples, bolometers, thermistors, and pyroelectric devices. Thermal detectors tend to be slower in response than photon-sensitive saniconduc-tors. The most common types of detectors used in dispersive IR spectroscopy were bolometers, thermocouples, and thermistors, but faster detectors are required for FTIR. FTIR relies on pyroelectric and photon-sensitive semiconducting detectors. Table 4.5 summarizes the wavenumber ranges covered by commonly used detectors. 

Pyroelectric polymers have additional features as pyroelectric devices, such as large surface area, thin films, low thermal diffusion, small dielectric constant, flexible for conformity to curved surface and low cost, though their pyroelectric coefficients are generally smaller than other materials. Pyroelectric devices using polymer elements with these features may have a large number of applications which ate not necessarily restricted to IR radiation detector. 

Beause of simpUdty of (he PPES method, shown in Figures 15 and 16, sad high sensitivity, this method has already been applied in various fields such as theraaal wave scanning microscopy [28], thermal diffiisivity measurement 29, phase-transitkM diase study [30], and so oa. In all cases the pyroelectric device for ea design uses PVDF film. 

Pyroelectric devices are commonly used for the thermal detection infrared radiation or as temperature-sensitive sensors. In particular, more sophisticated detectors are able... 

Pyroelectric devices convert changing incident thermal radiation to an electrical output, and are now much used in intruder detectors, thermal imaging systems etc. Conventionally, ceramics have been used in such applications however, considering the desirable properties of large pyroelectric coefficient, high volume resistivity, low dielectric constant and loss, and low specific heat, it can be seen that, apart from the rather low pyroelectric coefficient, polymeric materials are superior to ceramics in several respects. 

In conclusion, it has been demonstrated that the pyroelectric properties of polar materials can be compared relatively simply through the measurement of a few key physical parameters (pyroelectric,dielectric and thermal coefficients) and the judicious use of appropriate figures-of-merit. It is essential that the dielectric properties are measured in the frequency range appropriate for device use, and this is typically in the range of a few to 100 Hz. The properties of many pyroelectric ceramics and thin films have been compared and it has been shown that good pyroelectric properties can be obtained from this films manufactured at relatively low temperatures, a fact that bodes well for their future applications in fully-integrated arrays. 

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. 

Pyroelectric ceramics can be used to detect any radiation that produces a change in the temperature of the crystal, but are generally used for IR detection. Because of their extreme sensitivity a rise in temperature of less than one-thousandth of a degree can be detected. This property finds application in devices such as intruder alarms, thermal imaging, and geographic mapping. 

Biosensors based on the heat produced by enzyme/substrate reactions have traditionally used microcalorimeters (1), thermistors (2), and Peltier or other macro devices <3,A) The area has been reviewed by Guilbault (5). The size, response time, and thermal mass of these detectors suggests that thermally responsive microsensors need to be explored. The ideal sensor would be inexpensive, and require simple, low cost support electronics. A fiber optic based sensor (Part A), and a pyroelectric polymer film based sensor (Part B) are described below. 

The pyroelectric coefficient p, is a useful parameter with which to compare different materials. If the thin film acts as a dielectric in a capacitor and an external resistance is connected between the electrodes, a pyroelectric current, I, flows in the circuit this can be expressed as I = pA(dT/dt) where dT/dt is the rate of change of temperature, and A is the cross-sectional area of the device. In a thermal imager many considerations, other than a high value of p, must be borne in mind,when designing a pyroelectric detector capable of resolving a temperature difference in the scene temperature of O.IK. For example, the figure of merit for a thermal imaging device requires the pyroelectric materials to have low values of permittivity. 

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