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Electrical polarity in a biological material produced by a change in temperature. Pyroelectricity is probably a basic physical property of all living organisms. First discovered in 1966 in tendon and bone, it has since been shown to exist in most animal and plant tissues and in individual cells. Pyroelectricity appears to play a fundamental part in the growth processes (morphogenesis) and in physiological functions (such as sensory perception) of organisms.
The elementary components (for example, molecules) of biological (as well as of nonbiological) pyroelectric structures have a permanent electric dipole moment, and are arranged so that all positive dipole ends point in one direction and all negative dipole ends in the opposite direction. This parallel alignment of elementary dipoles is termed spontaneous polarization because it occurs spontaneously without the action of external fields or forces. In this state of molecular order, the structure concerned has a permanent electric dipole moment on a microscopic and macroscopic level.
Spontaneous polarization is temperature-dependent; thus any change in temperature causes a change of the dipole moments, measurable as a change of electric charges at both ends of the polar axis. This is the pyroelectric effect. All pyroelectric structures are also piezoelectric, but the reverse is not true.
Prerequisites for the development of spontaneous polarization and pyroelectric activity in biological structures are (1) the presence of a permanent dipole moment in the molecules or molecular aggregates and (2) a molecular shape that favors a parallel alignment as much as possible (or at least does not impede it). Both these conditions are ideally fulfilled in bar- or board-shaped molecules with a permanent dipole moment along the longitudinal molecular axis. Several important organic substances have these molecular properties, and therefore behave pyroelectrically in biological structures. Examples include the epidermis of animals and plants, sensory receptors in animals, and tissues of the nervous and skeletal systems.
Living organisms are able to detect and discriminate between different stimuli in the environment, such as rapid changes of temperature, of illumination, and of hydrostatic and uniaxial pressure. These stimuli represent different forms of energy and are transduced, or converted, into the nearly uniform type of electrical signals whose voltage-time course frequently depends on (X = external stimulus, t = time). Such electrical signals have been recorded on cutaneous sensory receptors, on external nerve endings, on epidermal structures, and even on the cell wall of single-cell organisms. The mechanisms of detection and transduction in these biological systems, still little understood, may lie in the pyroelectric behavior of the structures. Pyroelectric (and thus piezoelectric) behavior has been proved to exist in most biological systems, which means that these systems should in principle be able to function as pyroelectric detectors and transducers.
The property of certain crystals to produce a state of electric polarity by a change of temperature. Certain dielectric (electrically nonconducting) crystals develop an electric polarization (dipole moment per unit volume) when they are subjected to a uniform temperature change. This pyroelectric effect occurs only in crystals which lack a center of symmetry and also have polar directions (that is, a polar axis). These conditions are fulfilled for 10 of the 32 crystal classes. Typical examples of pyroelectric crystals are tourmaline, lithium sulfate monohydrate, cane sugar, and ferroelectric barium titanate.
Pyroelectric crystals can be regarded as having a built-in or permanent electric polarization. When the crystal is held at constant temperature, this polarization does not manifest itself because it is compensated by free charge carriers that have reached the surface of the crystal by conduction through the crystal and from the surroundings. However, when the temperature of the crystal is raised or lowered, the permanent polarization changes, and this change manifests itself as pyroelectricity.
The magnitude of the pyroelectric effect depends upon whether the thermal expansion of the crystal is prevented by clamping or whether the crystal is mechanically unconstrained. In the clamped crystal, the primary pyroelectric effect is observed, whereas in the free crystal, a secondary pyroelectric effect is superposed upon the primary effect. The secondary effect may be regarded as the piezoelectric polarization arising from thermal expansion, and is generally much larger than the primary effect. See Piezoelectricity
Pyroelectrics have a broad spectrum of potential scientific and technical applications. The most developed is the detection of infrared radiation. In addition, pyroelectric detectors can be used to measure the power generated by a radiation source (in radiometry), or the temperature of a remote hot body (in pyrometry, with corrections due to deviations from the blackbody emission). See Pyrometer, Radiometry
An infrared image can be projected on a pyroelectric plate and transformed into a relief of polarization on the surface. Other potential applications of pyroelectricity include solar energy conversion, refrigeration, information storage, and solid-state science.
the phenomenon of the appearance of an electric field in certain crystals (pyroelectrics) upon heating or cooling. Pyroelectricity was known to and described by ancient Greek scientists. The nature of pyroelectricity was explained in 1756 by the Russian academician F. U. T. Epinus. Pyroelectricity was investigated by the English scientists, J. Canton and D. Brewster, and also by R. J. Haiiy and P. Curie.