Fused silica thermal properties

Vycor has a thermal coefficient of expansion of 7.5 x 10 7 Acm/cm/°C and can be fused directly to fused silica. This property has provided an excellent technique... 

In addition to providing highly selective separations, there are a multitude of other desired characteristics that a gas chromatographic stationary phase should possess. These properties include high viscosity, low surface tension allowing for wetting of the fused silica capillary wall, high thermal stability, and low vapor pressure at elevated temperatures. The stationary phase solvent should also not exhibit unusual mass transfer behavior. 

Gas chromatographic methods have been successfully used for the determination of penicillin molecules bearing neutral side-chains in milk and tissues (95, 97), but cannot be used for amphoteric -lactams. Gas chromatography of penicillin residues is further complicated by the necessity for derivatization with diazomethane. This derivatization step is particularly important because it not only leads to formation of the volatile penicillin methyl esters but also improves their chromatographic properties (thermal stability and decreased polarity). Using a fused-silica capillary column in connection with a thermionic nitrogen-selective detector, excellent separation and sensitivity figures were obtained. 

The thermal conductivity (in W m 1 K-1) of PDMS (0.15) appears to be sufficient, although it is lower than PC (0.16), PET (0.2), glass (0.7-1.0), fused silica (1.38), and silicon (124) [159,246]. Since the channels in the plastic chip are usually narrow (i.e., with high surface-to-volume ratio), the heat dissipation properties of the plastic (e.g., acrylic) channel compared favorably with that of a fused silica capillary (75 pm i.d.) [186]. 

Epoxy polymers (including epoxy novolacs) have been designed to meet most of these requirements and are almost universally used in such encap-sulant applications. Epoxy polymers exhibit superior adhesion that in many cases eliminates the need for a barrier or junction coating. They have a low coefScient of thermal expansion low shrinkage and low injection velocity, which means that low transfer or injection pressures can be used. These polymers also possess excellent mechanical properties coupled with low moisture and gas permeability. Above all, they are cheap and readily available. Other transfer-molding materials used to a limited extent include silicones, phenolic materials, and even polyesters. Most molding formulations are highly filled (70-75%) with materials such as quartz, fused silica, short... 

Properties Rupture modulus up to 50,000 psi, d 2.5, thermal shock resistance 900C, highest continuous-use temperature 700C. Glass ceramics lie between borosilicate glasses and fused silica in high-temper-ature capability. 

Properties Colorless crystals or white powder odorless and tasteless. D 2.2-2.6 thermal conductivity about half that of glass, mp 1710C, bp 2230C, high dielectric constant, high heat and shock resistance. Insoluble in water and acids except hydrogen fluoride soluble in molten alkai when finely divided and amorphous. Combines chemically with most metallic oxides melts to a glass with lowest known coefficient of expansion (fused silica). Noncombustible. 

Fused silica is by far the primary optical material for lens manufacturing. Its mechanical and thermal properties are well known, and its grinding and polishing infrastructure are well established. It is relatively inexpensive and has good index homogeneity over large areas. Lens-grade fused silica, however, is an expensive material. The temperature-dependent properties of fused silica are summarized in Table 13.7. 

Thermal expansion is a most important property in practice. It is used in most everyday thermometers. The shattering of ordinary glass on being cooled rapidly is due to the thermal contraction of the outer layers, and is prevented in special glasses such as Pyrex glass or fused silica, which have low thermal expansion. The thermal expansion of components in electronic devices is important, and the difference in thermal expansion of materials in a construction can lead to grave difficulties. The coincidence of the thermal expansion of steel and concrete at normal temperatures allows the use of steel-reinforced concrete in buildings. 

Amorphous or fused silica has long been recognized as a very valuable material of construction, especially for use at elevated temperatures, because it has great strength and a coefficient of expansion so low as to make it resistant to thermal shock. On the other hand, lack of practicable means of fabrication has impeded its widespread use even in view of its outstanding properties. 

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