Quartz glass, thermal conductivity

Irradiation by fast neutrons causes a densification of vitreous silica that reaches a maximum value of 2.26 g/cm3, ie, an increase of approximately 3%, after a dose of 1 x 1020 neutrons per square centimeter. Doses of up to 2 x 1020 n/cm2 do not further affect this density value (190). Quartz, tridymite, and cristobalite attain the same density after heavy neutron irradiation, which means a density decrease of 14.7% for quartz and 0.26% for cristobalite (191). The resulting glass-like material is the same in each case, and shows no x-ray diffraction pattern but has identical density, thermal expansion (192), and elastic properties (193). Other properties are also affected, ie, the heat capacity is lower than that of vitreous silica (194), the thermal conductivity increases by a factor of two (195), and the refractive index, increases to 1.4690 (196). The new phase is called amorphous silica M, after metamict, a word used to designate mineral disordered by radiation in the geological past (197). 

The transmission of heat is favored by the presence of ordered crystalline lattices and covalently bonded atoms. Thus graphite, quartz, and diamond are good thermal conductors, while less-ordered forms of quartz such as glass have lower thermal conductivities. Table 7.3 contains a brief listing of thermal conductivities for a number of materials. Most polymeric materials have X values between 10 and 10° W m- K"1. 

Heat must also flow through the walls of a device. DEP utilises electric field inhomogeneities which means that the fields (and heat production) within the liquid tend to be quite localised. The external heat flows can occur over much wider areas so it is sometimes possible to use even poorly conducting materials such as ordinary glasses and quartz. In critical cases, silicon, which has a high thermal conductivity (150 W/m s °C), is used. 

As a result, cuvettes for Raman spectroscopy should be carefully selected. They may, due to their impurities, add a background to the spectrum of the sample. In addition, all cuvette materials produce their own Raman spectra, which have to be considered, when the Raman spectra of the sample are evaluated. Fig. 3.5-17 a shows a Raman spectrum of a typical optical glass BK7, Fig. 3.5-17 b that of quartz glass suprasil, and Fig. 3.5-17 c of sapphire. Suprasil is a synthetic quartz which does not normally contain impurities. Therefore, Suprasil of ESR quality is highly recommended as Raman cuvette material. Also, sapphire is a good cuvette material, as it is very hard, inert, has a good thermal conductance, and shows only weak but sharp Raman lines (Porto and Krishnan, 1967). It is used for the production of the universal Raman cell (Schrader, 1987). The sharp Raman lines of sapphire observed in the spectra of the sample may be subtracted from the spectrum or used as internal standard for quantitative analyses (Mattioli et al, 1991). 

FIGURE 8,4 Thermal conductivity of quartz and glassy silica as a function of temperature [7]. The quartz thermal conductivity exhibits a T behavior at low temperature, a peak at about 10 K, then reduction at higher temperatures. This is typical of a crystalline solid. For amorphous glass the thermal conductivity increases as T2 plateaus between 1 to 10 K and then increases monotonically with temperature. Also plotted are the predictions of the Cahill-Pohl and Einstein models. The Cahill-Pohl model provides accurate predictions for temperatures higher than 50 K but cannot predict the low temperature behavior. The Einstein model predictions are much lower than the measured values. 

It has been proposed that the thermal conductivity of wet beads of granular material be estimated as a function of material content and the thermal conductivity of each of the three phases [114]. The results of the method were validated in a small number of materials such as crushed marble, slate, glass, and quartz sand. 

In a typical microfluidic setting, the most power absorbing material is the liquid sample in the microchannels. The common materials for microfluidic chips are glass, quartz, and thermal plastics such as polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), and polycarbonate, which usually have very small absorption as compared to liquid materials. The absorption of the liquid medium increases with its ionic content which increases the conductivity and with the operating frequency. Take water as an example. 

The following polymeric materials have been used as ablative materials for specific applications phenolics, phenyl silanes, nitrile phenolics, nitrile rubber, silicones, epoxy polyamide, and novolac epoxies. Fillers and reinforcements are used in ablative formulations to improve performance and reduce thermal conductivity. Common ablative fillers are glass, siUca, and quartz cloth carbon and graphite cloth microbubbles (phenolic, silica, and glass) and asbestos fiber. 

It is believed that the upper operable temperature of these insulations is at least 1000°F. This is primarily dictated by limitations of the materials of construction. There are good reasons, however, to expect considerable extensions on this limit if, for example, stainless steel is substituted for aluminum and quartz fibers are used in place of glass. Figure 4gives the calculated apparent thermal conductivity of Linde insulation SI-4atboundary temperatures other than ambient and liquid oxygen (-297°F). 

The thermal treatment in this technique is similar to that applied in TGA (vide supra). However, the solid-state reaction may be carried out in a quartz glass oven or in a stainless-steel device. The gases evolved, e. g., HCl or H2O, are determined by mass spectrometry (MS) or with a thermal conductivity detector (TCD). A suitable device including MS is seen in Fig. 1 and is described in more detail elsewhere [29]. 

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