Viscosity silicate glasses

Alkali cellulose, 4 716 Alkali earth metal nitrides, 17 206-207 Alkali flame-ionization detector (AFID), gas chromatography, 6 381 Alkali-gravity-viscosity (AGV) charts, for silicate glasses, 22 462, 463 Alkali halide disk method, 14 229 Alkali-immobile compounds, dye release from, 19 288... 

Compomers contain no water, but rather are mainly formulated from the same components as conventional composite resins. Typically this means macromonomers, such as bis-glycidyl ether dimethacrylate (bisGMA) or its derivatives and/or urethane dimethacrylate, blended with viscosity-reducing diluents, such as triethylene glycol dimethacrylate (TEGDMA). These polymer systems are filled with non-reactive inorganic powders, for example, quartz or a silicate glass [271]. 

The basic structural units of silicate glasses are tetrahedra with Si+4 in the center bonded covalently to O-2 at each corner. In pure silica all comer oxygen ions are shared by two tetrahedra (Figure 15.6). The result is a covalently bonded glass with a very high viscosity. As other oxides are added, not all of the oxygen ions share two comers. This lowers the viscosity. 

The viscosity of silicate glasses drops with decreasing O/Si ratios. The data from H. J. L. Trap and J. M. Stevels, Glastech Ber. 6 (1959) V131, are for equal molar additions of Na20, K20, CaO, SrO, and BaO. 

Oxynitride glasses are silicate or alumino-silicate glasses in which oxygen atoms in the glass network are partially replaced by nitrogen atoms. As nitrogen increases, glass transition temperature, elastic modulus, viscosity and hardness increase while thermal expansion coefficient decreases. 

When creating a glass, one has to watch all the properties. This is because as you increase one material to obtain certain desirable properties, other properties are likely to change which may or may not be desirable. For example, by adding PbO to silicate glass, there is an increase in viscosity while at the same time there is a decrease in electrical resistivity.9... 

Zijlstra, A. L. The viscosity of some silicate glasses in connection with thermal history. Phys. Chem. Glasses to be published (1963). 

Note The remainder of the composit consisted of 80 wt% filler consisting of barium- aluminum-boron silicate glass powder, silicon dioxide-zirconium dioxide, and ytterbium fluoride. For dental composites lower viscosities are preferred because of their flowability. 

Dandk and LiCko (1981) verified their apparatus in a very broad temperature range, from room temperature up to 1600°C. They measured the density and viscosity of glycerine, boron oxide, sodium tetraborate, and sodium-calcium silicate glass with the composition 17 mass % Na20, 10% CaO, 73% Si02. The results of measurement are in agreement with the literature data regarding the precision of the apparatus used. 

The galvanic reactions will take place at a much faster rate in the low viscosity phase of sulphates in boiler deposit than in highly viscous silicate glass. However, rapid reactions at the tube surface/deposit interface may not be necessary or appropriate for development of a strong bond between the ash deposit and boiler... 

For a length L = 0.2 m, a density p = 750kgm and g = 9.8 ms , the stress a = 1.5 kPa. To avoid significant stretching, the tensile strain rate must be less than 0.2 s , which means that the tensile viscosity must exceed 7500Nsm. When these two conditions are imposed on a typical shear flow curve in Fig. 5.18, it is clear that the melt must be highly non-Newtonian. A similar process cannot be used for Newtonian silicate glasses... 

As indicated by the above discussion, it is important to know the temperature dependence of viscosity. It has become standard industrial practice to use an empirical equation, known as the Vt el-Fuklier-Taiiimann (VFT) equation to describe this temperature dependence. For silicate glasses, the VFT equation often fits the temperature dependence over ten orders of magnitude in viscosity. The VFT equation involves three empirical constants (t/, C and Tj,) and can be written... 

The determination of viscosity from the couple exerted on concentric cylinders is used in many viscometers. For silicate glasses, a wide range of techniques are used for measuring viscosity. For low viscosities (1 Pa s to 1 MPa s), a rotating spindle technique, based on the above principle, is often used. For higher viscosities, techniques based on fiber elongation, compression of a cylinder and be im-bending are utilized. 

HGURE 21.4 (a) Viscosity as a function of temperature (T) for several silicate glasses units of ii are dPa s. (b) Viscosity as a function of temperature (1/7) for the main glass-forming oxides. Notice the effect that a small amount of water has on the viscosity of silica glass. 

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