Glasses, properties

The noncrystalline nature of glasses endows them with certain characteristics unique to them as compared to their crystalline counterparts. Once formed, the changes that occur in a glass upon further cooling are quite subtle and different from those that occur during other phase transitions such as solidification or crystallization. The change is not from disorder to order, but rather from disorder to disorder with less empty space. In this section, the implication of this statement on glass properties is discussed. 

In the range between the melting and glass transition temperatures, the material is usually referred to as a supercooled liquid. 

Given that (see Fig. 9.8) at the glass transition temperature, the specific volume Vs and entropy S are continuous, whereas the thermal expansivity a and heat capacity Cp are discontinuous, at first glance it is not unreasonable to characterize the transformation occurring at Tg as a second-order phase transformation. After all, recall that, by definition, second-order phase transitions require that the properties that depend on the first derivative of the free energy G such as 

Interestingly enough, if a glass-forming liquid were cooled slowly enough (at several times the age of the universe ) such that it follows the dotted line shown in Fig. 9.8Z) at a temperature T kau the entropy of the supercooled liquid would become lower than that of the crystal — a clearly untenable situation first pointed out by Kauzmann and referred to since as the Kauzmann paradox. This paradox is discussed in greater detail in Sec. 9.4.2. 

In a very real sense, Tg is a measure of the rigidity of the glass network in general, the addition of network modifiers tends to reduce Tg, while the addition of network formers increases it. This observation is so universal that experimentally one of the techniques of determining whether an oxide goes into the network or forms nonbridging oxygen is to follow the effect of its addition on T . 

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Glass-ceramics and phase-separated glasses. Properties,... 

N. P. Bansal and R. H. Doremus, Handbook of Glass Properties, Academic Press, Inc., New York, 1986. 

Another widely studied phenomenon in alkah borate glasses is the mixed alkah effect, the nonlinear change in glass properties when a second kind of alkah oxide is added into the single-alkali glass. Models have been suggested to explain the mixed alkah effect (144), but a universally accepted model has not been developed as of this writing. 

Caution is needed in applying the results of general chemical tests for glass durability across a wide spectrum of glass properties. Glass fibre strengths, for example, are sensitive to the physical, as well as the chemical, nature of the environment and should only be assessed by the direct strength measurements in conditions which closely approximate the final application situation . 

Mai.ow, G. 1982. The mechanisms for hydrothermal leaching of nuclear waste glasses properties and evaluation of surface layers. In Lutze, W. (ed)... 

O. V. Mazurin and co-workers, Handbook of Glass Data, Elsevier Science Publishing Co. Inc., New York, 1991. N. P. Bansal and R. H. Doremus, Handbook of Glass Properties, Academic Press, Inc., New York, 1986. 

Better glass products from special glass properties. 

Thus, the peculiarities of the epoxy glass properties may be due to some specific features of the free volume. For instance, it is possible to explain qualitatively all the unusual properties of epoxy glasses in terms of the size distribution of density fluctuation which may depend on cure conditions 15-20). At present, X-ray methods allow to directly measure the spatial distribution of density fluctuations in polymeric glasses 84). However, it is still unknown how this distribution correlates with properties. 

One organic solvent that is often used in EPR, but has proved problematic in the author s experience in terms of explosive thawing is neat 2-methyltetrahydroftiran (2-Me-THF). However, THF with as little as 2% v/v 2-Me-THF has the glassing properties of 2-Me-THF without its destructive properties. 

Sic-Oi bonding, however, has almost the same bond order as that in cluster (c), suggesting that the central region Sic-Oi bonding has less influence from embedding than the peripheral structure. This means that electronic states near the cluster center are little affected by both the cluster size and the embedding units. In other words, electronic states localized near the center are stable in silicate clusters, and thus suitable for e raction of glass properties. 

AA Owens Corning 1/16" parallel to flow. milled glass properties of RRIM... 

Enthalpy relaxation is one of the most widely studied in the context of both non-linearity and non-exponentiality of the measured glass properties. A convenient technique for these studies is scanning calorimetry. In simple cooling and heating experiments, heat capacity curves exhibit normal increase with characteristic hump of Cp above the glass transition as represented in Figure 9.08(A). The fictive temperature,... 

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