Pyroelectric applications detectors

Pyroelectric infrared detectors are inferior in detectivity by one or two orders of magnitude compared with photoconductors such as cadmium mercury telluride, as shown in Fig. 7.15. However, such materials require temperatures of 200 K for efficient operation and generally respond to rather narrow bands at the infrared wavelengths. Pyroelectric devices can discriminate temperature differences of 0.1 K but find many useful applications in which the discrimination is limited to about 0.5 K. They have the great practical advantage of operating at normal ambient temperatures. 

What is the objection to using PZT compositions of the type developed for piezoelectric applications in a pyroelectric infrared detector What are the properties needed and how have they been achieved ... 

The ferroelectricity, combined with excellent mechanical properties and the ease of fabrication, have rendered PVF2 a versatile material for piezoelectric and pyroelectric applications (transducer, stems, ultrasonics, loudspeakers, microphones, finger-press switches, ultrared detectors). Furthermore, PVF2 can be found in semiconductor applications and in the electrical-electronic market (plenum cables, aircraft wiring, computer... 

In the following, the operating performances and tyidcal structures of pyroelectric single-detector, one- and two-dimensiooal array detectors are discussed, with special attention to PVDF, P(VDF/iyFE>, P(VDCN/VAc). and polyurca polymers, in S kms II. m. and rV. In Sections V and VI, other applications of pyroelect devices using polymer clement arc introduced. 

Lead titanate (PbTiOs, PT) is a significant component material in electronics such as capacitors, ultrasonic transducers, thermistors, and optoelectronics. It is also a promising material for pyroelectric infrared detector applications because of its large pyroelectric coefficient and relatively low permittivity [88]. 

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. 

Sources and detectors Specific discussions of sources and detectors have been covered elsewhere in this article. The issues here are more service and performance related. Most sources have a finite lifetime, and are service replaceable items. They also generate heat, which must be successfully dissipated to prevent localized heating problems. Detectors are of similar concern. For most applications, where the interferometer is operated at low speeds, without any undesirable vibrational/mechanical problems, the traditional lithium tantalate or DTGS detectors are used. These pyroelectric devices operate nominally at room temperature and do not require supplemental cooling to function, and are linear over three or four decades. 

In ferroelectrics the major contributor to tan 3 is domain wall movement which diminishes as the amplitude of the applied field diminishes the value applicable to pyroelectric detectors will be that for very small fields. The permittivity is also very sensitive to bias field strength, as is its temperature coefficient. The properties of some ferroelectrics - the relaxors - are also frequency dependent. It is important, therefore, to ensure that when assessing the suitability of a ferroelectric for a particular application on the basis of measured properties that the measurements have been made using values of the parameters (frequency, field strength etc.) appropriate to the application. This is not always done. 

The OMA technology has been recently applied to infra-red spectroscopy ( 18) as well. A pyroelectric vidicon (utilizing a triglycine sulfate as the sensor element) has been used as a thermal rather than a photon multichannel detector. Simultaneous spectral detection, in the 1-30 um spectral region, was accomplished that has proven to be particularly useful for IR pulse laser applications, Fig.12,... 

IR radiation is emitted from the electrically modulated light source. The analytically relevant spectral range is transmitted through an interference filter, the sample chamber, and the membrane. This radiation is focused on a thermal detector (Dl), pyroelectrical or thermopile. The reflected radiation from the filter is used as a reference (D2). A comparison of the ATR-, the fiber-, and the transmission-method. Secs. 6.5.2.1, 6.5.4.2, and 6.5.4.4, shows that the ATR method is most versatile for all applications and that the transmission method allows the lowest limit of detection for gases (Hadziladzaru, 1994). The properties of the ATR method by employing wavelength selection with tunable interference filters has been studied by Lebioda (1994). 

Chemical and physical processing techniques for ferroelectric thin films have undergone explosive advancement in the past few years (see Ref. 1, for example). The use of PZT (PbZri- cTi c03) family ferroelectrics in the nonvolatile and dynamic random access memory applications present potentially large markets [2]. Other thin-film devices based on a wide variety of ferroelectrics have also been explored. These include multilayer thin-film capacitors [3], piezoelectric or electroacoustic transducer and piezoelectric actuators [4-6], piezoelectric ultrasonic micromotors [7], high-frequency surface acoustic devices [8,9], pyroelectric intrared (IR) detectors [10-12], ferroelectric/photoconduc-tive displays [13], electrooptic waveguide devices or optical modulators [14], and ferroelectric gate and metal/insulator/semiconductor transistor (MIST) devices [15,16]. 

Chapter V of this Handbook is entirely devoted to the potential applications of the side chain liquid crystal polymers, but it is interesting to mention here the main areas where FLCPs could play a role. As far as we know, the following applications can be considered for SmC LCPs nonlinear optics, pyroelectric detectors, and display devices. 

Piezoelectricity links the fields of electricity and acoustics. Piezoelectric materials are key components in acoustic transducers such as microphones, loudspeakers, transmitters, burglar alarms and submarine detectors. The Curie brothers [7] in 1880 first observed the phenomenon in quartz crystals. Langevin [8] in 1916 first reported the application of piezoelectrics to acoustics. He used piezoelectric quartz crystals in an ultrasonic sending and detection system - a forerunner to present day sonar systems. Subsequently, other materials with piezoelectric properties were discovered. These included the crystal Rochelle salt [9], the ceramics lead barium titanate/zirconate (pzt) and barium titanate [10] and the polymer poly(vinylidene fluoride) [11]. Other polymers such as nylon 11 [12], poly(vinyl chloride) [13] and poly (vinyl fluoride) [14] exhibit piezoelectric behavior, but to a much smaller extent. Strain constants characterize the piezoelectric response. These relate a vector quantity, the electrical field, to a tensor quantity, the mechanical stress (or strain). In this convention, the film orientation direction is denoted by 1, the width by 2 and the thickness by 3. Thus, the piezoelectric strain constant dl3 refers to a polymer film held in the orientation direction with the electrical field applied parallel to the thickness or 3 direction. The requirements for observing piezoelectricity in materials are a non-symmetric unit cell and a net dipole movement in the structure. There are 32-point groups, but only 30 of these have non-symmetric unit cells and are therefore capable of exhibiting piezoelectricity. Further, only 10 out of these twenty point groups exhibit both piezoelectricity and pyroelectricity. The piezoelectric strain constant, d, is related to the piezoelectric stress coefficient, g, by... 

Ca, Ba, Sr ) into PT ceramics is reported to enhance the pyroelectric properties. Substitution of these ions results in the reduction of lattice anisotropy leading to hard and dense ceramics with high mechanical strength. From the standpoint of thermal responsivity, the performance of a pyroelectric detector is enhanced when it is in the form of a thin film. In recent device applications, it has become increasingly necessary to consider the improvement of these films in then-performance in addition to the large component area and connection to many circuit elements. Recently, calcium- and lanthanum-doped lead... 

---