Gaseous conductivity

Siace the pores ia an aerogel are comparable to, or smaller than, the mean free path of molecules at ambient conditions (about 70 nm), gaseous conduction of heat within them is iaefficient. Coupled with the fact that sohd conduction is suppressed due to the low density, a siUca aerogel has a typical thermal conductivity of 0.015 W/(m-K) without evacuation. This value is at least an order of magnitude lower than that of ordinary glass and considerably lower than that of CFC (chloro uorocarbon)-blown polyurethane foams (54). 

In free molecular flow, if gaseous conductance were not independent of the flow direction, a perpetual-motion machine could be constmcted by connecting two large volumes by a pair of identical ducts having a turbine in front of one of the ducts. A duct that has asymmetricaUy shaped grooves on its waU surface could alter the probabUity of molecular passage in such a way that for a tube of equal entrance and exit areas, the probabUity of passage would be made directional. 

Thermal-Conductivity Analyzer. The thermal-conductivity analy2er operates on the principle that the loss of heat from a hot wire by gaseous conduction to a surface at a lower temperature varies with the thermal conductivity of the gas, and is virtually independent of pressure between 1.3 kPa (10 mm Hg) and 101 kPa (1 atm). This technique is frequently used in continuous monitors for tritium in binary gas mixtures for immediate detection of process change. 

Electrical conductivity/electrical resistivity Dielectric constant and loss factor Oscillometry Gaseous conduction... 

F. A. Maxfield and R.R. Benedict, Theory of Gaseous Conduction, McGraw-Hill,... 

Most commercially relevant superinsulating Si02 aerogels have densities between 80 and 200 kg m. Their thermal conductivity values are dominated by conduction through the solid silica particle network at high densities and a combination of radiation and gaseous conduction through the air inside the pores at low densities. To produce the lowest conductivity materials, it is necessary to find an optimum between those two. It seems apparent that... 

The transfer of thermal energy occurs via three mechanisms. They are via solid conductivity, gaseous conductivity, and infrared radiation. Figure 6.14 shows a schematic presentation of various modes of thermal transport mechanisms in the GSA-SDS/FMWNT. In addition, the GSA-4SDS and GSA-SDS/FMWNT exhibit mechanical properties similar to that of polymeric foam, and thus thermal convection within the cells shall be considered as an additional component in deriving total thermal conductivity, which can be represented by Eq. 6.15. 

Note that by varying the gaseous conduction path length the heat input may be changed. In this way the wall and gaseous conduction may be distinguished from... 

The effect of test chamber wrapping and the presence of CO2 on the heat transfer to the calorimeter at 76°K is shown in Fig. 5. The curves are independent of pressure where radiation heat transfer is dominant, and then break upward to become a function of pressure when gaseous conduction is appreciable. The single-wrap insulation acts as a semifloating radiation shield, with a corresponding decrease in unit heat transfer. Figure 5 shows that the solid CO2 has no effect on the heat transfer and that the foil surface has not been blackened by solid CO2. 

It is of interest to note that the presence of wrapped insulation not only lowers the unit heat transfer but also allows a higher vacuum-space pressure before gaseous conduction starts to dominate. This effect allows a system to degas more H2 and thus lengthen the useful life of the line. 

Convection and gaseous conduction can be reduced to negligible values by evacuating the space between the cold chamber and the outside chamber wall. To maintain that vacuum, one must use materials that do not outgas and must keep the surfaces scrupulously clean. These issues and formulas for thermal conduction through gases are discussed in Chapter 13. 

At low pressures, the molecular path length is limited by the dewar dimensions, so the thermal conduction is proportional to the pressure. Figure 13.8 shows the heat conducted by the gas in the vacuum space of two hypothetical dewars as a function of pressure. Pressures below 10 or 10 " torr would be needed to make the gaseous conduction negligible compared to other sources of heat loads. 

---