Network former

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. 

Inorganic networks, especially glasslike gnes can be characterized by the ratio of network forming (e.g. [siO ] ) to network modifying (e.g. =Si-0 Na ) units. Network formers (e.g. SiO, B 03, A 2°3 T 2 are (in opposition to the majority of organic units) three-dimensional crosslinking units. 

Organic Network Formers. As indicated above, an additional organic network can be built up by organic polymer synthesis within an inorganic network. The basic principles are shown in Equations 3 to 5 with a vinyl, methyl methacrylate and epoxide polymerization ... 

Figure 2. Adsorption isotherms of CC>2 on different network modified adsorbents 70, network former Si02 to network modifier (am) ratio (molar) 30 70 50, 50 50 10, 90 10 0, 100% Si02 ...

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

Fig. 12.7 Some examples of network formers and network modifiers used for the synthesis of ormosils.

R = ratio of total number of oxygen ions to total number of network formers. 

Transition metal Modern raw material Colouring ion Colour in tetrahedral coordination (network former) Colour in octahedral coordination (network modifier)... 

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. 

The increasing addition of a modifier to a given network former leads to the progressive breaking of all oxygen bridges as in (4.2). 

The case of boron as a network former cation is somewhat specific in that this element has no available d orbitals. However, a p orbital is available when the boron has a coordination number of 3, which allows stabilisation of an electronic doublet of the oxygen or sulphur introduced by the modifier. This oxygen or sulphur giving up a doublet to another boron atom increases the cross-linking by the formation of two BO4 tetrahedra. In hybridisation terms, the boron is altered from the sp configuration to the sp configuration. The coordination change of boron has been especially well observed by NMR (Bray and O Keefe, 1963 Muller-Warmuth and Eckert, 1982). 

Although the environment of the network former cation is relatively well known, that of the modifier cation is much less so, due to the lack of appropriate spectroscopic techniques. The absence of direct experimental data has given rise to the coexistence in the literature of very different hypotheses ranging from models based on a totally random distribution of ionic bonds to those based on zones rich in modifier cations which alternate with less rich zones (Greaves, 1985). 

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... 

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. 

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