Thermal conductivity cryogenics

Gaseous helium is commonly used as the working fluid ia closed-cycle cryogenic refrigerators because of chemical iaertness, nearly ideal behavior at all but the lowest temperatures, high heat capacity per unit mass, low viscosity, and high thermal conductivity. 

Detector cooling often is accompHshed by providing good thermal conductivity to a suitable cryogen (2). The most readily available coolants are sohd carbon dioxide [124-38-9] [124-38-9] at 195 K, Hquid nitrogen, N2, at 77 K, andhquid hehum. He, at 4.2 K (see Carbon dioxide Helium group ... 

The apparent thermal conductivity of powder insulation at cryogenic temperatures is generally obtained from... 

Data on the thermal conductivity for a variety of foams used at cryogenic temperatures have been presented by Kropschot Cryogenic Technology, R. W. Vance, ed., Wiloy, New York, 1963, p. 239). Of aJl the foams, polyurethane and polystiyene have received the widest use at low temperatures. The major disadvantage of foams is that they tend to crack upon repeated thermal cychng and lose their insulation value. 

In Table 3.5, the low-temperature thermal conductivity k of some materials of cryogenic interest is reported together with an analytic fit. 

The low-temperature thermal conductivity of different materials may differ by many orders of magnitude (see Fig. 3.16). Moreover, the thermal conductivity of a single material, as we have seen, may heavily change because of impurities or defects (see Section 11.4). In cryogenic applications, the choice of a material obviously depends not only on its thermal conductivity but also on other characteristics of the material, such as the specific heat, the thermal contraction and the electrical and mechanical properties [1], For a good thermal conductivity, Cu, Ag and A1 (above IK) are the best metals. Anyway, they all are quite soft especially if annealed. In case of high-purity aluminium [2] and copper (see Section 11.4.3), the thermal conductivities are k 10 T [W/cm K] and k T [W/cm K], respectively. 

As we shall see, it is impossible to carry out measurements of thermal conductivity in a very extended range of temperature on the same sample (with the same g factor) and with the same cryogenic apparatus. Hence the measurements were performed in two overlapping ranges, as reported in the following sections. 

Cryogen Name Vapor Pressure, MPa Gas Density, g/1 Liquid/Gas Expansion Ratio Heat Capacity Cp, J/(kg K) Heat Capacity Cv, J/(kg K) Thermal Conductivity x 10-2 w/(m-K) Viscosity Pa-sec x 105 (cP) Solubility in Water, 0°C, v/v... 

K is called the thermal conductivity, which for air at STP is. 023 W/(m K). One way to reduce this energy flow is to decrease the pressure. Cryogenic liquids (such as liquid nitrogen, which boils at 77K) are commonly stored in Dewar flasks, which are double-walled containers with an evacuated region between the walls. 

Adequate predictions of thermal conductivity for pure metals can be made by means of the Wiedemann-Franz law, which states that the ratio of the thermal conductivity to the product of the electrical conductivity and the absolute temperature is a constant. High-purity aluminum and copper exhibit peaks in thermal conductivity between 20 and 50 K, but these peaks are rapidly suppressed with increased impurity levels and cold work of the metal. The aluminum alloys Inconel, Monel, and stainless steel show a steady decrease in thermal conductivity with a decrease in temperature. This behavior makes these structural materials useful in any cryogenic service that requires low thermal conductivity over an extended temperature range. 

All cryogenic liquids except hydrogen and helium have thermal conductivities that increase as the temperature is... 

Since heat transfer through these insulations can occur by several different mechanisms, the apparent thermal conductivity of an insulation that incorporates all of these heat-transfer possibilities offers the best means of comparing these difference types. Table III provides a listing of some accepted k values for popular insulations used in cryogenic storage and transfer systems. 

The major disadvantage of foams is not their relatively high thermal conductivity compared with that of other insulations, but rather their poor thermal behavior. When applied to cryogenic systems, they tend to crack on repeated cycling and lose their insulation value. 

For cryogenic temperature measurements, furnaces consist of thermally conductive jackets filled with liquid nitrogen (boiling point 77.35 K) or liquid helium (boiling point 4.215 K). The heat dissipation from resistance heating elements competes with the cooling effects of these fluids to permit stable temperature control down to near absolute zero [10]. 

Discontinuities in the lattice such as vacancies, impurities, or grain boundaries also act to scatter phonon propagation, hence a lower thermal conductivity is expected in solids containing these defects at cryogenic temperatures. Whichever mecha-... 

The adhesive s thermal conductivity is important in minimizing transient stresses during cooling. This is why thinner bond thickness and adhesives or sealants with higher levels of thermal conductivity generally have better cryogenic properties. 

The chemical and physical properties of each of these window materials vary widely. For example, polyimide is flexible, semitransparent, and chemically inert, but it has an upper working temperature of 673 K (for information about the properties of Kapton see http //www2.dupont. com/Kapton/en US / assets / downloads / pdf/ summaryofprop.pdf). Beryllium is stiff, has a low density, high thermal conductivity, and a moderate coefficient of thermal expansion it can be machined and is very stable mechanically and thermally. It also retains useful properties at both elevated and cryogenic temperatures. However, it does require a few safety-related handling requirements that are well documented (for detailed environmental safety and health information about beryllium see http //www.brushwellman.com). Nonetheless, as is stated in the Brush Wellman literature (for detailed environmental safety and health information about beryllium see http //www.brushwellman.com), "handling beryllium in solid form poses no special health risk."... 

Fig. 1-7 Apparent thermal conductivities of typical cryogenic insulation material (a) multilayer insulations (b) opacified powders (c) glass fibers (cf) powders (e) foams, powders, and fibers, according to Ref. 1. [1 Btu in/h ft2 ° F = 144 mW/m °C]...

All cryogenic liquids except hydrogen and helium have thermal conductivities that increase as the temperature is decreased. For these two exceptions, the thermal conductivity decreases with a decrease in temperature. The kinetic theory of gases correctly predicts the decrease in thermal conductivity of all gases when the temperature is lowered. 

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