Pyroelectricity, insulators

Bauer. Transparent pyroelectric sensors and organic field-effect transistors with fluorinated polymers steps towards organic infrared detectors. Dielectrics and Electrical Insulation, IEEE Transactions on, 13(5) 1087-1092, 2006. 

Pyroelectric transducers are constructed from single crystalline wafers of pyroelectric materials, which are insulators (dielectric materials) with very special thermal and electrical properties. Triglycine sulfate (NH7CH2COOH), H. SOt (usually dcutcraicd or with a fraction of the glycines replaced with alanine), is the most important pyroelectric material used for IR-detcction systems. 

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]. 

Pyroelectric materials change their electric polarization as a function of temperature. These materials may be insulators (dielectrics), ferroelectric materials, or semiconductors. A dielectric placed in an electrostatic field becomes polarized with the magnitude of the induced polarization depending on the dielectric constant. The induced polarization generally disappears when the field is removed. Pyroelectric materials, however, stay polarized and the polarization is temperature dependent. 

The principle of the Thomson polymer IR sensor is represented by Fig. 14 and Fig. 15. The P(VF2 TrFE) copolymer constitues the pyroelectric layer (5-10 pm), the insulation layer is a polyimide (10 pm). The upper electrode is also the infrared absorber and is made in aluminium or gold black. The lower level is the CCD level which constitues the second electrode. The polarisation of the polymer is realized by application of 100 V/pm between the two electrodes of the pyroelectric capacitor. 

Insulating perovskites are generally described as dielectrics (a term used to characterise a polarisable insulator), piezoelectrics, pyroelectrics and ferroelec-trics. These names (which are general and not confined to perovskites) describe the response of the material, which is always an electric polarisation, to an applied external stimulus. In a dielectric, for example, the stimulus is an applied electric field, and the response is an electric polarisation of the material. Both the response and the stimulus must be described with respect to the crystal structure of the material, as vectors. Thus, for a dielectric, the response of the material, the electric polarisation, needs to be characterised by a vector P, and the stimulus, the applied electric field, needs to be specified as a vector E, both described with respect to the crystal structure of the perovskite. In a cubic crystal P is proportional and parallel to E, but for most crystals this is not true and the relationship between these two vectors needs to be described in tensor notation. 

Hilczer, B., Kulek. J., (1998) The Effect of Dielectric Heterogeneity on Pyroelectric Response of PVDF, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 5, No. 1, pp. 45-50, ISSN 1070-9878. 

The characteristic of an insulating material that exhibits noncentrosymmetric crystal organization. In the strictest sense, a distinct characteristic of ferroelectric materials is the presence of hysteresis. All ferroelectric materials are both piezoelectric and pyroelectric. PZT is a common ferroelectric material. 

A laser flash technique has been used to determine the diffusivity of pyroelectric polymers such as polyvinylidene fluoride [83], whereas hot-wire techniques have been used to determine the thermal diffusivity of high-density polyethylene, low-density polyethylene propylene, and polystyrene [83], Dos Santos and coworkers [84] utilized the laser flash technique to study the effect of recycling on the thermal properties of selected polymers. Thermal diffusivity expresses how fast heat propagates across a bulk material, and thermal conductivity determines the woiking temperature levels of a material. Hence, it is possible to assert that those properties are important if a polymer is used as an insulator, and also if it is used in applications in which heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications (including food wrapping) were selected for this study. 

A somewhat different approach with a pyroelectric detector chip was su ested by Schreiter et al. (2006). The advantages of such a lead-zirconate-titanate-based detector array are the low heat capacity and high thermal insulation of the sensor. The surface of the sensor was coated with specific reagents to detect different chemicals. As an example, the coating with poly(methylsiloxane) was chosen to detect heptane with a detection limit of 10 ppm. With a bacterial surface layer and small Pt clusters, it was possible to study the catalytic oxidation of hydrogen in the range of 0.5-3.5 vol%. 

Meunier M, Quirke N et al (2001) Molecular modeling of electron traps in polymer insulators chemical defects and impurities. J Chem Phys 115 276-288 Mopsik FI, Broadhurst MG (1975) Molecular dipole electrets. J Appl Phys 46(10) 4204 Neugschwandtner GS, Schwoediauer R et al (2001) Piezo-and pyroelectricity of a polymer-foam space-charge electret. J Appl Phys 89 4503-4511... 

Sessler GM, Das-Gupta DK et al (1992) Piezo and pyroelectricity in electrets caused by charges, dipoles or both IEEE Trans Electr Insul 27 872-897 Sessler GM, Yang GM et al (1997) Electret properties of cycloolefin copolymers. In Aiuiual report, conference on electrical insulation and dielectric phenomena, IEEE service center, Piscataway, pp 467-470... 

T. Furukawa, Piezoelectricity and pyroelectricity in polymers, IEEE Trans. Elect. Insul. 24 375 (1989). 

C Muralidhar and P. K. C Pillai, Pyroelectric behaviour in barium tilanalclpolyvinylidene flnonde (PVDF) compoaite, Proc. Sih Ira. Syrup. Electrets. Heidelbetg. 1963. pp. 863-870. R. E. Newnhan, Compoaite Electfaceramics. Ferroelectrtcs 68 1 (1966X ). Wotak. Dielectric behaviour of 03-lype piezoelectric oompositca. IEEE Trims. Elect InsuL 28 116 (1993). 

Cross-talk measurements were compared between loaded and unloaded sensors. Levels of noise of 15 mV were noted on the unloaded sensors, while adjacent loaded sensors had noise of 7.S V. Thermal insulation was required to prevent the pyroelectric effect from overwhelming the piezoelectric effect utilized during the measurement. 

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