Plastic selenium

Polarized incident radiation is necessary also when anisotropic samples such as stretched polymers (Flournoy and Schaffers, 1966) are studied. Crystals may require immersion in order to assure the necessary optical contact. Plastic selenium has proved to be useful (Gottlieb, 1968). In this way the three different infrared spectra of an orthorhombic single crystal were obtained which are shown in Fig. 2.7-10 (cf. Sec. 2.7.6.4 Schrader et al., 1971). The orientation of the optical axis or axes with respect to the experimental frame is crucial. For the evaluation the field components have to be considered separately in order to interprete the response of the oriented transitions correctly. Addressing different components equivalently is achieved by rotating the sample together with the internal reflexion element with respect to the beam (Hobbs et al., 1983 Palm, 1994). 

Cobalt alloys, 7 219-226. See also Cobalt high temperature alloys for orthopedic devices, 3 728 selenium and metallurgy of, 22 98 Cobalt alloy T-800, 7 223 properties, 7 223t CobaltCII) aluminate, uses, 7 241t Cobalt aluminate blue spinel, formula and DCMA number, 7 348t Cobalt Aluminate Blue, pigment for plastics, 7 370t... 

The organoselenium compounds, their chemistry, and methods of preparation are discussed in the literature (3,4,7,11,12,14,59). Selenium-containing polymers are of interest because of their semiconducting and plastic properties. The biologically important compounds include selenoaminocarboxylic acids and derivatives, selenium-containing peptides, and selenium derivatives of pyrimidines, purines, cholines, steroids, coenzyme A, and other compounds (4,14). The biochemical and medical aspects and uses of oiganoselenium compounds have also been discussed (13,14,60—62). 

The crystalline state configuration of polymeric selenium is a helix in which all the rotational angles are of the same sign with a value of 78°. This is fairly close to the expected value of 90°.52 The barrier to rotation about the Se-Se bond is thought to be roughly the same as that about S-S bonds. No reliable experimental results are available on the statistical properties of these chains. Some values of the glass-transition temperature of polymeric selenium have been reported15 and these could at least provide a measure of the dynamic flexibility of the chains. However, these results are probably compromised by the presence of cyclic selenium molecules that act as plasticizer. 

SeFg can be prepared, stepwise directly from the elements (equation 31). A patent claims the formation of selenium haxafluoride from selenium and chlorine monofluoride, CIF, at temperatures above 200 °C under substantially anhydrous conditions. Below 200 °C, SeF4 is formed. The rotational movements of the SeFe molecules in the plastic and liquid phases have been studied by F NMR, IR, and Raman spectroscopy. 

The production of chemicals and pigments (coloring agents) is another major use of the selenium produced. The addition of selenium to paints, plastics, ceramics, and glazes adds any one of a number of possible colors, ranging from light orange to deep red. 

Normally, substrates are impermeable and deposits are held out on the surface. However, some years ago, workers Q) at Xerox observed, while depositing selenium onto flexible plastic substrates, that "good", continuous films were grown except under certain circumstances when the deposit took the form of particles. Examination of these deposits revealed that the particles were spherical, fairly uniform in size and, most remarkable, were organized into a monolayer array just beneath the surface of the substrate. Figure 2 shows a top view of a typical deposit as observed using the transmission electron microscope. There are usually the order of 109 particles per cm of deposit. 

Tellurium has the same structure as selenium, and it is probable that similar chain molecules exist in plastic sulphur. 

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