Glasses chemical constants

Source Kodak Technical Literature (ACCUMAX 2000) for polyester film Tables of Physical and Chemical Constants, Longman, London, 1973, p. 254 H. J. I ischbcck and K. H. Fischbeck, Formulas, Facts and Constants, Spiinger-Verlag, Berlin, 1987, for glass data. 

Glass-ceramics contain an amorphous phase and a crystalline phase minimum. Their attractive properties are chemical durability, mechanical strength, lower thermal expansion coefficient compared to glasses, dielectric constant, and electromagnetic radiation transmittance. Because of these properties, they find applications in electronics, military, vacuum technology, households. 

Continuous recuperative furnaces employing metallic recuperators (heat exchangers) have been in use since the 1940s. Operation of these furnaces is simplified and the combustion process is more precisely controlled no reversal of air flow causes temperature variations. The recuperator metal must be caretiiUy selected because of chemical attack at high temperature. Recuperative furnaces are often used in the production of textile fiber glass because they maintain a constant temperature. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Glocker and Frohnmayer determined the characteristic constant c for nine elements (Reference 2, Table 4) ranging in atomic numbers from 42 (molybdenum) to 90 (thorium). They proved that identical results could be obtained with the sample in the primary (polychromatic) or in the diffracted (monochromatic) beam. The method was applied with good results to the determination of barium in glass of antimony in a silicate of hafnium in the mineral alvite and of molybdenum, antimony, barium, and lanthanum in a solution of their salts—for example, 5.45% barium was found on 90-minute exposure by the x-ray method for a glass that yielded 5.8% on being analyzed chemically. 

Borst, C. L., Gill, W. N., and Gutmann, R. J., Chemical-Mechanical Polishing of Low Dielectric Constant Polymers and Organosilicate Glasses, Boston Kluwer Academic Publishers, 2002, pp. 1-5. 

During chemical and physical transformations, any of the four variables in the ideal gas equation P, V, n, T) may change, and any of them may remain constant. The experiments carried out by Robert Boyle are a good example. Boyle worked with a fixed amount of air trapped in a glass tube, so the number of moles of gas remained the same during his experiments. In other words, n was held constant. Boyle also worked at only one temperature, so T remained constant. Example applies the ideal gas equation to this situation. 

The coefficient of linear thermal expansion is almost constant, for most types of glass, for temperatures up to 400-600°C. The actual value depends on the chemical constitution of the glass. It then increases rapidly above a certain temperature, often called the... 

Shell Chemical Company), exhibits a maximum at 300 nm, corresponding to that of the model chromophore anisole. The fluorescence intensity decreases monotonically with increasing concentration of 2,4-dihydroxybenzophenone (DHB) and, furthermore, decreases with time on continued excitation (274 nm) in the spectrophotometer. The fluorescence loss with time may be resolved into two exponential decays. Initially, a relatively rapid fluorescence loss is observed within 20 sec, followed by a slower loss. Loss constants for the initial (k ) and secondary (kj) exponential decays for 1.5 ym films (on glass slides) containing varying concentrations of DHB are provided in Table I (entries 1-3). The initial loss constants are seen to decrease more markedly with increasing DHB concentration than the secondary constants. 

The variation of the Chin-Gilman parameter with bonding type means that the mechanism underlying hardness numbers varies. As a result, this author has found that it is necessary to consider the work done by an applied shear stress during the shearing of a bond. This depends on the crystal structure, the direction of shear, and the chemical bond type. At constant crystal structure, it depends on the atomic (molecular volume). In the case of glasses, it depends on the average size of the disorder mesh. 

An important advance in the understanding of the chemical behaviour of glasses in aqueous solution was made in 1977, when Paul (1977) published a theoretical model for the various processes based on the calculation of the standard free energy (AG ) and equilibrium constants for the reactions of the components with water. This model successfully predicted many of the empirically derived phenomena described above, such as the increased durability resulting from the addition of small amounts of CaO to the glass, and forms the basis for our current understanding of the kinetic and thermodynamic behaviour of glass in aqueous media. 

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