Tourmaline, polarizer

Some piezoelectric crystals are electrically polarised in the absence of mechanical stress one example is gem-quality tourmaline crystals. Normally, this effect is unnoticed because the crystal does not act as the source of an electric field. Although there should be a surface charge, this is rapidly neutralised by charged particles from the environment and from the crystal itself. However, the polarisation decreases with increasing temperature and this can be used to reveal the polar nature of the crystal. If tourmaline is heated its polarisation decreases and it loses some of its surface charges. On rapid cooling it has a net polarisation and will attract small electrically charged particles such as ash. Such crystals are known as pyroelectric, and ferroelectric crystals are a special subclass of pyroelectric crystals. 

Faye, G. H., Manning, P. G. Nickel, E. H. (1968) The polarized optical absorption spectra of tourmaline, cordierite, chloritoid, and vivianite ferrous-ferric electronic interaction as a source of pleochroism. Amer. Mineral., 53, 1174—201. 

Grum-Grzhimailo, S. V. (1956) Tourmaline, their examination in polarized ultraviolet light. Trudy Inst. Krist., Akad. Nauk. SSSR, 12,79-84. 

Although laboratory polarimeters generaUy use Nicol prisms as polarizers and analyzers, dichroic crystals (such as tourmaline) or dichroic sheet polarizers (such as Polaroid) may be used in the construction of special apparatus.f... 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Hexahydrate, plate-like crystals of pale olive-green color by transmitted light and of a brilliant green to reddish green by reflected light. The crystals polarize light S times as much as tourmaline. Loses its water at 100° and becomes red. Almost insol in water sol in about 1000 parts boiling... 

When subjected to a temperature change, certain crystals become electrically-polarized. Tourmaline, a silicate of boron and aluminum, symmetry 3m, is the best-known example. On heating, such a crystal becomes negatively charged on one side and positively charged on the other. Pyroelectricity is a tensor of rank 1 (vector). 

The semiprecious gemstone tourmaline, with an approximate formula CaLi2Al7(OH)4-(B03)3Si60i8, has a pyroelectric coefficient, 7r of 4 X 10 C m K-. The unique polar axis is the crystallographic c axis. What is the change in polarisation caused by a change of temperature of 100 °C ... 

Pyroelectric materials exhibit a spontaneous polarization that is a strong function of temperature because the dipole moments vary as the crystal expands or contracts. This was observed in the mineral tourmaline in the seventeenth century. Pyroelectricity occurs in organic crystals such as triglycine sulfate (TGS), ceramics such as... 

Mg, or (A1 + Li). The crystals are trigonal, elongated, and variably coloured, the two ends of the crystals often having different colours. Tourmaline is used as a gemstone and because of its doubie refraction and piezoeiectric properties is aiso used in polarizers and some pressure gauges. 

Fig. 13.1 Examples of nrai-polar, piezoelectric and pyroelectric crystals calcite (a), ZnS (b) and tourmalme (c). An arrow shows the direction of the polar axis in tourmaline...

Pyroelectrics. In a crystal belonging to polar classes there is only one polar axis with a symmetry of the polar vector. These crystals are also piezoelectric, but, in addihon, manifest spontaneous polarization and all other polar properties. Such crystals are called pyroelectrics. An example is tourmaline having symmetry and shown in Fig. 13.1c. Pyroelectric crystals are also used in techniques as piezoelectrics and also as detectors of infrared light or a heat how. There are many organic pyroelectric crystals, e.g., p-nitroaniline, one of the best generators of the optical second harmonic. 

The first paper on piezoelectricity by Jacques and Pierre Curie was presented to the Societe mineralogique de France at the session on the 8th of April 1880 and later to the Academic des Sciences at the session on the 24th of August 1880. Pierre and Jacques Curie at first discovered the direct piezoelectric effect in crystals of tourmaline. They noticed that as result of pressure applied along a particular direction, electrical charges proportional to the pressure and of opposite polarities appear on opposite crystal surfaces. They called this effect polar-electricity . Later, they discovered a similar effect in quartz and other crystals, which have no centre of symmetry. At that time Pierre Curie was only 21 years old. His notes published in Comptes Rendus in 1880 and 1881 are a real gem, as they deal with all essential phenomena (surface charge is proportional to pressure and does not depend on the thickness of the crystal etc.). 

The first attempt to establish an atomic theory of piezoelectricity is corrsidered to be the work of Lord Kelvin. Using Debye s theory of electrical polarization Schrodinger attempted to determine the order of magnitude of the piezoelectric constants of tourmaline and quartz. However the first to succeed was Bom in 1920 in his book Lattice-dynamical theory . An atomic model for the qualitative explanation of piezoelectric polarization of quartz was discovered by the method of X-ray analysis by Bragg and Gibbs in 1925. 

An individual volume element of the tourmaline crystal shows, even in the absence of an electric field, a spontaneous electric polarization owing to the polar axis. This polarization creates apparent surface charges at opposite ends of the polar axis that, however, are compensated by an adsorbed water film (and the... 

A piezoelectric sensor is a device that can convert mechanical stress into an electrical charge, and vice versa. An electric polarization occurs in a fixed direction when the piezoelectric crystal is deformed. The polarization causes an electrical potential difference over the crystal. Natural piezoelectric materials are quartz and tourmaline, and synthetic polymers such as polyvinyUdene fluoride (PVDF) exhibit piezoelectricity several times greater than quartz. Because the effect is reversible, which means that the electrical stimuli can lead to mechanical deformations, the piezoelectric effect is also useful to create some actuators in smart clothing. 

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