Pyroelectric detectivity

Lithium-Niobate and Lithium-Tantalate Lithium-niobate (LN) and lithium-tantalate (LT) are uniaxial p3Toelectrics, having trigonal structure, with spontaneous polarization arising from asymmetrical displacement of lithium relative to the other ions. These materials Tc values are 1,210 °C and 620 °C, respectively. They are always produced commercially in single-crystal forms. Both are much used for surface acoustic wave devices (e.g., high-frequency filters), while LT is used for pyroelectric detection due to its large pyroelectric coefficient and low permittivity. 

Glass A.M. Investigation of the electrical properties of Sri iBa (Nb206 with special reference to pyroelectric detection. J. Appl. Phys. 1969 40 4699-4713 Hirano S., Kato K. Preparation of crystalline LiNbOs films with preferred orientation by hydrolysis of metal alkoxides. Adv. Ceram. Mater. 1988a 3 505-506 Hirano S., Kato K. Formation ofLiNbOs Films by hydrolysis ofmetal alkoxides. J. Non-Cryst. Solid 1988b 100 538-541... 

Figure 18 Scbenulk diagram of pyroelectric detection cell used for optical fflter absorptioo Iob 131].

A. M. Gian, J. S. PaicL i. W. Goodby, D. H. Obon. and J. M Geary, Pyroelectric detection with fTOCctk Uquid cryatah, J. AppL Pkys. 6th2m (1986). 

M. E Linen and A. M. Glaaa, Pyroelectric detection. PHiidpla mti Appbcmtkma tf Ftr-roelectries and ReUutd Mataiah, Qarendon Plesa. Oxford. 1977. p. 561. 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

The most commercially important application that takes advantage of the pyroelectric effect ia polycrystalline ceramics is iafrared detection, especially for wavelengths ia excess of 2.5 p.m. AppHcations range from radiometry and surveillance to thermal imaging, and pyroelectric materials work under ambient conditions, unlike photon detectors, which require cooling. 

Solid-state light detectors are best in the near IR and IR regions. Here also the spectral response depends on the nature of the material, i.e. the semiconductor. In the very far IR photons cannot be detected directly and the only answer is to convert light into heat. The pyroelectric detectors work on this principle. 

The most recent generation of NDIR analyzers have evolved to satisfy the frequently harsh industrial environments encountered. These analyzers utilize solid-state sensors for the detection of infrared radialion. Most frequently used sensors are lead selenide (PhSc). thermopiles, or pyroelectric detectors. The gas analyzers generally are configured as single-path instruments, dual-beam with a reference palh. or dual-channel with a reference filter. 

This expression is the basic description for the use of the pyroelectric effect in a host of sensor applications including the well known optical detection devices (82,83). A particularly useful way of describing this type of system is with an equivalent circuit where the pyroelectric current generator drives the pyroelectric impedance and the measuring amplifier circuit as shown in Figure 11. 

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. 

Note that most, but not all, detectors are made so that the direction of it is normal to the element electrode plane, i.e. p = 7r. In the discussion which follows, it will be assumed that this is the case and the term pyroelectric coefficient will be applied to p.) The pyroelectric charges can be detected as a current ip, flowing in an external circuit such that ... 

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. 

Pyroelectric materials are used mainly for the detection of infrared radiation. The elements for the detectors are typically thin slices of material (e.g. 1.0 x 1.0 x 0.1 mm) coated with conductive electrodes, one of which is a good absorber of the radiation. 

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. 

Strontium barium niobate is a single-crystal material with the tungsten bronze type of structure which is made by the Czochralski method but has yet to find a major use. It has relaxor characteristics of the type shown in Fig. 7.1 which give it a high pyroelectric coefficient and detectivity, but its high permittivity lowers the figure of merit l. ... 

All pyroelectric materials are piezoelectric and therefore develop electric charges in response to external stresses that may interfere with the response to radiation. This can largely be compensated for by the provision of a duplicate of the detecting element that is protected from the radiation by reflecting electrodes or masking, but which is equally exposed to air and mounting vibrations. The principle is illustrated in Fig. 7.7. The duplicate is connected in series with the detector and with its polarity opposed so that the piezoelectric outputs cancel. This results in a small reduction in sensitivity (< 3 dB) but compensation is an... 

Fig. 7.14 A thermal image produced by an infrared camera exploiting uncooled pyroelectric ceramic detection technology. (Courtesy of QinetiQ Limited.)...

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