Glass network formers

The glassy systems mentioned in Figs. 4.1(h) and 4.2 show that quite complex chemical compositions have been prepared in the glassy state. Up to three basic constituents are present in all ionically conducting glasses network formers, network modifiers and ionic salts, in different proportions. 

Borates, through their ability to act as glass network formers, can act as excellent char formers and drip suppressants in fire retardant applications. In many cases this involves processing into polymeric materials, leading to specific requirements for thermal stability and particle size. Most common borate materials, however, exhibit relatively low dehydration temperatures and may be unsuitable for use in many polymer systems. Zinc borates are often used because they have unusually high dehydration onset temperatures and can be produced as small particle size powders. 

The influence of the glass network former in phosphate, borate and germanate glasses and of the network modifier on the charge transfer spectra is described in ref. (64), and the values of Eu3+ and of the 4/ -> 5d transition of Tb3+ are presented in Table 3. As can be seen from this table the two quantities are influenced in the same manner by the network former of the glass. 

Change of glass network former Change of glass network modifier Eu3+ (charge transfer) Tb3+ (4/- 5 d) ... 

A linear relation is observed between Pauling electronegativity of the glass network former cations, P, B and Ge, and the h values of the matrix. However, the slope of the straight line depends on the probe ion (Fig. 9). This can be explained as follows while the... 

The multiphonon emission rates for the silicate glass in fig. 35.7 are much faster than for the crystals. Studies of rare earths in glass demonstrate that multiphonon relaxation is due predominantly to the highest frequency vibrational modes which are associated with the glass network former (Reisfeld, 1975). In both crystalline and amorphous rare earth hosts, materials having low vibrational frequencies generally have more fluorescing levels and hence more possibilities for laser action. 

The composition of optical glasses has an effect on their durability. The main effect is from the type of network former that the glass contains. There are three main glass network formers—silicon, boron, and phosphorus. 

Most glass-ceramics have low dielectric constants, typically 6—7 at 1 MHz and 20°C. Glass-ceramics comprised primarily of network formers can have dielectric constants as low as 4, with even lower values (K < 3) possible in microporous glass-ceramics (13). On the other hand, very high dielectric constants (over 1000) can be obtained from relatively depolymerized glasses with crystals of high dielectric constant, such as lead or alkaline earth titanate (11,14). 

Addition of an alkali metal oxide as a "network modifier to the "network former causes pH sensitivity, i.e., small amounts of alkali metal induce superficial gel layer formation as a merely local chemical attack and so with limited alkali error larger amounts will result in more pronounced dissolving properties of the glass up to complete dissolution, e.g., water-glass with large amounts of sodium oxide. Simultaneous addition of an alkaline earth metal oxide, however, diminishes the dissolution rate. Substitution of lithium for sodium in pH-sensitive glass markedly reduces the alkali error. 

Non-glass-forming (very low amount of network formers)... 

For a simple glass made of a network former and a network modifier, M2X, a large increase in ionic conductivity with alkali content results in a large increase of AGmjx. mainly from entropic origin. In the case of... 

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