Network-forming ions

The network-forming ions apart from Si4+ and B3+ are P5+, As3+, Ge4+, Sb3+ and usually also Al3+, Ti4+ and Sn4+. Among the basic oxides, which modify the network, apart from the alkali and alkaline earth oxides must be mentioned PbO because it greatly increases the refractive index (high polarizability) of the glass (crystal glass). Ions such as Fe2+, Co2+ and Ni2+ can appear both inside and outside the network, which influences the coloration of the glass. 

The results indicate that, for mold flux oxide compositions, the viscosity is dependent on the quantity of network forming oxides present, principally silica and alumina. This is demonstrated by the results of McCauley ( ) in Figure 1. In this case, it is the ratio of network forming ions to total anion concentration. However, as shown in Figure 2, the viscosity/reciprocal temperature relationship is not linear and cannot be adequately represented by the Arrhenius Equation over a wide temperature range. 

Figure 1. Viscosity at 1500°C as a function of network forming ions for the fluxes in Table I.

It is interesting to note that the diffusion coefficient of ions at 1000 C has approximately the same value as the oxygen diffusion coefficient. This means that AP" ions will function at least partly as network-forming ions in place of Si ions, and are at least partially built into the oxygen tetrahedra. Again, this experimental fact should be seen in conjunction with experimental results which show that, in crystalline silicates, the mobilities of oxygen ions and of silicon ions are of comparable magnitude (see section 6,2,2). 

Network-forming Ion. One of the ions in a glass that form the network in the glass structure as postulated by Zachariasen (see under glass). The ratio of the ionic radius of the network-forming ion to that of the oxygen ion must lie between 0.155 and 0.225 for triangular co-ordination, or between 0.225 and 0.414 for tetrahedral co-ordination such ions include B +, AP+, SP+ and P5+. 

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-... 

Modifiers in glass are compounds that tend to donate anions to the network, whereas the cations occupy "holes" in the disordered stmcture. These conditions cause the formation of nonbridging anions, or anions that are connected to only one network-forming cation, as shown in Figure 2. Modifier compounds usually contain cations with low charge-to-radius ratios (Z/r), such as alkali or alkaline-earth ions. 

A series of zinc diphosphonate complexes were synthesized in the presence of diamines of varying chain length. 1-hydroxyethylidenediphosphonate bridges the metal ions with the proto-nated diamine (ethyl-, butyl-, pentyl-, or hexylenediamine) filling in channels or residing between chains. All four structures are different with one-, two-, and three-dimensional networks formed. The coordination number (4-6) and geometry also varies.418... 

It is well known that the association-dissociation reaction goes in both directions and that there is always some finite fraction of ion pairs [28]. Counter ions that form ion pairs do not participate in creating osmotic pressure accounting for such counter ions is thus important for the correct determination of the dimensions of the network. 

The list of the new gels for which phase transitions are possible is supplemented in the paper by Amiya and Tanaka, who discovered discrete collapse for the most important representatives of biopolymers - chemically crosslinked networks formed by proteins, DNA and polysaccharides [45]. Thus, it was demonstrated that discrete collapse is a general property of weakly charged gels and that the most important factor, which is responsible for the occurrence of this phenomenon, is the osmotic pressure of the system of counter ions. 

The fluoride electrode is a typical example of an ion selective electrode. Its sensitive element is a crystal of lanthanum trifluoride that allows fluorine atoms to migrate into the network formed by lanthanum atoms (Fig. 18.3). Other electrodes use a mineral membrane obtained as agglomerates of crystalline powders (for measurement of Cl-, Br-, I , Pb++, Ag+ and CN ). Generally, the internal electrolyte can be eliminated (by dry contact). However, it is preferable to insert a polymer layer with a mixed-type conductivity to ensure the passage of electrons from the ionic conductivity membrane to the electronic conductivity electrode (Fig. 18.3). 

Octahedral Cr3+, V3+ and Mn3+ ions predominate in silicate glasses, while Fe3+ ions are mainly in network-forming (tetrahedral) sites. 

The primary reaction of any pozzolanic material is an attack on the SiOj or AljOj-SiOj framework by OH ions. It may be supposed that the OH ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached from the framework. It may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H, and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an amorphous material with and Na as the dominant cations, but the more abundant supply of Ca and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. 

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