Special durability tests have been made on optical glas durability were developed and listed in the glass catalogues. Quantifying such chemical attack is extremely difficult and has been the subject of international research. 2.5 ELECTRICAL PROPERTIES The data used for the electrical characterisation of glasses [ 3, 18, 25] include the volume and surface resistivity the dielectric constant the dielectric loss and the dielec tric strength. As may be seen in Table 8, fused silica has a high volume resistivity of 1032 cm, but the addition of other oxides decreases the resistivity. Organic glass has values similar to inorganic multicomponent glass. Generally, the electrical resistivity, or its reciprocal value the conductivity, of multicomponent glasses depends on the chemi cal composition, on the temperature, and to some extent also on the atmospheric condi tions. Inorganic glass is an ionic conductor. The transport of alkali ions is easier than that of alkaline earth ions. The mobility decreases as the ionic radius increases. The glasses are insulators at low temperatures and increase their conductivity as the tem- perature is raised because of the greater ease with which the ions can move when ther mal energy has weakened the binding forces to the silica network. In the temperature range of 25-1200oC, the resistivity may vary between 10Q2 cm and 1 Q2 cm. at normal temperatures and in a humid atmosphere, the volume conductivity is surpassed by the much higher surface conductivity. The adsorbed H2O, which forms an electro- lyte with the dissolved alkali, is responsible for this phenomenon. The reciprocal of surface conductivity, surface resistivity, is usually defined in Q2 cm Glass has a special position among solid dielectrics because of its extended range of dielectric constants. Organic glass has very low values, and high lead silicate glass shows the highest values. The dielectric constant of most types of glass decreases as the frequency of the applied field increases. For each frequency, the dielectric constant increases with increasing temperature, but the increase is less at high frequencies. The lost in a dielectric due measured by the angle 3 between the current and the charging potential. The value tan 8 depends on frequency and temperature The dielectric strength of a glass is the voltage required to puncture it, usually ex ressed in V per cm. Unfortunately, it is not a true physical constant of a material because it is dependent on the thickness of the glass. Generally, the dielectric strength decreases with increasing sample thickness. The values listed in Table 8 are measured at glass thickness'between 0 1 and 0.25 mm. At lower temperatures, the magnitude of the dielectric strength is relatively insensitive to temperature. However, after a higher temperature limit has been exceeded, the glass becomes progressively weaker. The a heating effect gth when the frequency of the applied voltage is higher may be due to
17 Special durability tests have been made on optical glass. Qualitative gradings of durability were developed and listed in the glass catalogues. Quantifying such chemical attack is extremely difficult and has been the subject of international research. 2.5 ELECTRICAL PROPERTIES The data used for the electrical characterisation of glasses [1,3,18;25] include the volume and surface resistivitv, the dielectric constant, the dielectric loss and the dielectric strength. As may be seen in Table 8, fused silica has a high volume resistivity of 10 ~9 f2 cm, but the addition of other oxides decreases the resistivity. Organic glass has values similar to inorganic multicomponent glass. Generally, the electrical resistivity, or its reciprocal value the conductivity, of multicomponent glasses depends on the chemical composition, on the temperature, and to some extent also on the atmospheric conditions. Inorganic glass is an ionic conductor. The transport of alkali ions is easier than that of alkaline earth ions. The mobility decreases as the ionic radius increases. The glasses are insulators at low temperatures and increase their conductivity as the temperature is raised because of the greater ease with which the ions can move when thermal energy has weakened the binding forces to the silica network. In the temperature range of 25 - 1200~ the resistivity may vary between 1019 ~'2 cm and 1 f2 cm. At normal temperatures and in a humid atmosphere, the volume conductivity is surpassed by the much higher surface conductivity. The adsorbed H20, which forms an electrolyte with the dissolved alkali, is responsible for this phenomenon. The reciprocal of surface conductivity, surface resistivity, is usually defined in f) cm 2. Glass has a special position among solid dielectrics because of its extended range of dielectric constants. Organic glass has very low values, and high lead silicate glass shows the highest values. The dielectric constant of most types of glass decreases as the frequency of the applied field increases. For each frequency, the dielectric constant increases with increasing temperature, but the increase is less at high frequencies. The energy lost in a dielectric due to electrical conduction losses (the dielectric loss) is measured by the angle 3 between the current and the charging potential. The value tan 5 depends on frequency and temperature. The dielectric strength of a glass is the voltage required to puncture it, usually expressed in V per cm. Unfortunately, it is not a true physical constant of a material because it is dependent on the thickness of the glass. Generally, the dielectric strength decreases with increasing sample thickness. The values listed in Table 8 are measured at glass thickness' between 0.1 and 0.25 mm. At lower temperatures, the magnitude of the dielectric strength is relatively insensitive to temperature. However, after a higher temperature limit has been exceeded, the glass becomes progressively weaker. The decrease in strength when the frequency of the applied voltage is higher may be due to a heating effect
TaBle 8 ELECTRICAL PROPERTIES Material Electrical resistivity Dielectric loss Dielectric strength (Q2 Eat10Hzad29°Cg8 at 1 MHz25°Ccm) Fused silica 385 2x105 54x10 100-103 6.4 36x105 Soda lime borosilicate glass 000 5.1 26x103 48x10° 250° 102-10425° 0.8-3.1x10° Lead silicate glass 584 10 0 10210525° 8x10 <1x Acrylic <lx10 Poly 1x103
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