Oxidic glass melts

In any event, a uniform second phase in a typically Inhomogeneous oxide glass melt is expected to yield a more uniform, i.e., stronger fiber than a non-uniform second phase, and the l/d ratio of the secondary domain may additionally also affect the fiber modulus. 

Reference electrodes to be used in oxidic glass melts are either based on the redox system oxygen/oxide or—in more rare cases—on systems metal/metal oxide [1]. Either of them must be constructed so that it yields a constant potential difference between the oxidic melt of interest and the metal parts of the electrode, i.e., it must represent a constant potential, the reference potential (difference) between the glass melt of interest and the metal contact of the electrode. In this chapter the first electrode type treated is the oxygen/oxide reference electrode, which is more often applied than the second type, the metal/metal oxide electrode, whose application is more complicated and thus usually less often applied than the oxygen/oxide reference electrode. 

The operation of potentiometric sensors is based on the measurements of concentration cell emf (see Chaps. 1 and 8), which makes it possible to extract the activity, concentration, or partial pressure of potential-determining species at the working electrode vs. RE. The WE potential may be established by a thermodynamic equilibrium or by a nonequdihrium steady state, whereas key requirements to the reference electrodes are related to their reversibility, stability, and, often, fast equilihratirMi on changing external conditions. The solid-state potentiometric sensors are used for a wide variety of technological applicatirms and probed species [2,3,5,15,18,86-91] their application for oxidic glass melts is addressed in Chap. 8. 

Due to the above requirements, typical optically-transparent materials, such as oxides (glass, quartz, alumina, zirconium oxide etc.) and halides (sodium chloride, lithium fluoride, calcium fluoride, potassium bromide, cesium bromide etc.) are usually unsuitable for use with fluoride melts. Therefore, no standard procedure exists at present for the spectral investigation of fluoride melts, and an original apparatus must be created especially for each particular case. 

A desirable glass melts at a reasonable temperature, is easy to work with, and yet is chemically inert. Such a glass can be prepared by adding a third component that has bonding characteristics intermediate between those of purely ionic sodium oxide and those of purely covalent silicon dioxide. Several different components are used, depending on the properties desired in the glass. 

Potash (composed of potassium oxide), also a flux, was mainly used as a glass modifier. It was generally introduced into the glass melt in the form of either pearl ash, composed of potassium carbonate, vegetable ash, one of the main constituents of which is potassium carbonate, or saltpeter, a mineral composed of potassium nitrate. 

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