Thermal conductivity quartz

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. 

Catalytic activity for the selective oxidation of H2S was tested by a continuous flow reaction in a fixed-bed quartz tube reactor with 0.5 inch inside diameter. Gaseous H2S, O2, H2, CO, CO2 and N2 were used without further purification. Water vapor (H2O) was introduced by passing N2 through a saturator. Reaction test was conducted at a pressure of 101 kPa and in the temperature range of 150 to 300 °C on a 0.6 gram catalyst sample. Gas flow rates were controlled by a mass flow controller (Brooks, 5850 TR) and the gas compositions were analyzed by an on-line gas chromotograph equipped with a chromosil 310 coliunn and a thermal conductivity detector. 

DiGuilio and Teja have developed a two-wire technique to obtain absolute values of thermal conductivity. Quartz capillaries filled with liquid gallium served as insulated hot wires. They measured the thermal conductivity of the NaN0j-KN03 eutectic in the range 525-590 K with 1% accuracy. Radiation by the fluid was also accounted for. 

Data from inter alia Sundbcrg 1991), Banks Robins (2002). Halliday Resnick (1978). Italics show recommended values cited by Eskilson el at. (2000). Note that thermal conductivity increases with quartz content, and that most rock materials have a specific heal capacity of around 800 J/kg/K, or slightly over 2 MJ/m /K (compare with copper at only 386 J/kg/K). 

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. 

The higher thermal conductivity of inorganic fillers increases the thermal conductivity of filled polymers. Nevertheless, a sharp decrease in thermal conductivity around the melting temperature of crystalline polymers can still be seen with filled materials. The effect of filler on thermal conductivity for PE-LD is shown in Fig. 2.5 [22], This figure shows the effect of fiber orientation as well as the effect of quartz powder on the thermal conductivity of low density polyethylene. 

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. 

Temperature programmed reduction (TPR) was performed using an equipment described in detail elsewhere [12]. Approximately 100 mg of catalyst was loaded in the quartz reactor tube and was heated at a rate of 0.167 K/s in a flow of 0,5 cm3(STP)/s of 66% hydrogen in argon. Hydrogen consumption was detected with a thermal conductivity detector (TCD). In order to prevent preliminary reduction of the catalyst, samples containing palladium were cooled to about 223 K during the time required to stabilize the detector. 

The most extensive test which has been made of this conduction model for thermal explosion is to be found in the work of Vanp e on the explosion of CH2O + O2 mixtures. He used a calibrated thread of 10 per cent Rh-Pt alloy of 20 m diameter (jacketed by a 50-m quartz sleeve) suspended at the center of a cylindrical vessel to measure directly his reaction temperature during the induction periods preceding explosion. By Uvsing He and Ar as additives and vessels of different diameters he was able to verify the dependence of the critical explosion limits on vessel size and on thermal conductivity of the gas mixture. In addition, he was able to check the maximum predicted temperature at the center of the vessel just prior to explosion and also the value of 8c = 2 [Eq. (XIV.3.12)], the critical explosion parameter for cylindrical vessels. Finally, with a high-speed camera, he was able to show directly that the explosions in this system do start at the center, the hottest region, " and propagate to the walls. 

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