Network-modifying ions

The network-modifying ions (commonly alkali and alkaline-earth ions) are ionically bound to the network although the field strength and diameter of the alkali ions allow them some mobility. 

The action of water and acids During attack the alkali and alkaline earth network-modifying ions are exchanged by H" or HjO" from acid solution. 

The properties of high quality vitreous silica that determine its uses include high chemical resistance, low coefficient of thermal expansion (5.5 x 10-7 /° C), high thermal shock resistance, high electrical resistivity, and high optical transmission, especially in the ultraviolet. Bulk vitreous silica is difficult to work because of the absence of network-modifying ions present in common glass formulations. An extensive review of the properties and structure of vitreous silica is available (72). 

By comparing mobilities obtained from tracer measurements with those from electrical conductivity measurements for network-modifying ions with known transference numbers, one can calculate correlation factors /, These are of the order of 1 /3 to 1 /2. Conclusions, drawn from the magnitude of these correlation factors, concerning the mechanism of diffusion... 

Network-modifying Ion. One of the ions in a glass which, according to Zachariasen s theory (see under glass), do not participate in the network. They must have a rather large radius and a low valency, e.g. the alkali metals and the alkaline earths. 

In Fig. 2-45, the degrees of polymerization of (PO ) units are shown as the number of linked tetrahedra. Therefore, different structural units result. These units are produced by adding network-modifier ions to the phosphate glasses. First, however, it must be ensured that the addition of alkali or alkaline-earth... 

By increasing the amount of network-modifier ions, a glass containing a network was modified into an invert glass, composed of diphospate or orthophospate units, containing more than 50 mol% network modifier ions. The phosphate structural units of the glass that became increasingly smaller... 

Vibration Losses. These losses occur by a resonance phenomena involving both the network-forming ions and the network-modifying ions. As shown in Fig. 2.37 (curve 3), vibration losses take place over a broad frequency range because of the variation in both the mass and the location of the different ions in the glass network. The resonant frequency of this loss mechanism is given by/ j = where A is a constant relating the dis-... 

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

ZnO displays similar redox and alloying chemistry to the tin oxides on Li insertion [353]. Therefore, it may be an interesting network modifier for tin oxides. Also, ZnSnOs was proposed as a new anode material for lithium-ion batteries [354]. It was prepared as the amorphous product by pyrolysis of ZnSn(OH)6. The reversible capacity of the ZnSn03 electrode was found to be more than 0.8 Ah/g. Zhao and Cao [356] studied antimony-zinc alloy as a potential material for such batteries. Also, zinc-graphite composite was investigated [357] as a candidate for an electrode in lithium-ion batteries. Zinc parhcles were deposited mainly onto graphite surfaces. Also, zinc-polyaniline batteries were developed [358]. The authors examined the parameters that affect the life cycle of such batteries. They found that Zn passivahon is the main factor of the life cycle of zinc-polyaniline batteries. In recent times [359], zinc-poly(anihne-co-o-aminophenol) rechargeable battery was also studied. Other types of batteries based on zinc were of some interest [360]. 

Figure 45 shows the fluorescent lifetime as a function of the ionic radius of the network modifier. At 30 per cent modifier, all glasses exhibit shorter lifetimes with increasing radius of the ion. This is interpreted by Gallagher et al. to be associated with increased europium-oxygen bonding. At 15 per cent modifier the trend is in the opposite direction and is similar to that... 

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