Insulation low temperature

The structure of the insulating low-temperature phase is a monoclinic distortion of the corundum structure. Structure determination (Dernier and Marezio 1970) shows that the V-V distance increases from 2.697 to 2.745 A, which across shared octahedral edges increases from 2.882 to 2.907. The average V-O distance remains constant. The material is antiferromagnetic, with a moment of 1.2/ B on... 

It is also shipped by highway in tube trailers. Under DOT exemption, ethylene is also shipped liquefied at low temperatures in insulated cargo tanks on trucks and truck trailers, and in insulated, low-temperature tank cars of the DOT... 

The temperature of the metal-to-insulator transition in TTF—TCNQ is 53 K. For systems with increased interchain coupling, the transition temperature for the onset of metallic conduction increases roughly as the square of the interaction between the chains. This behavior is tme as long as the coupling between chains remains relatively weak. For compounds with strong interactions between stacks, the material loses its quasi-ID behavior. Thus, the Peieds distortion does not occur even at low temperatures, and the materials remain conductive. 

The thermal protection system of the space shutde is composed mainly of subliming or melting ablators that are used below their fusion or vaporization reaction temperatures (42). In addition to the carbon-carbon systems discussed above, a flexible reusable surface insulation composed of Nomex felt substrate, a Du Pont polyamide fiber material, is used on a large portion of the upper surface. High and low temperature reusable surface insulation composed of siHca-based low density tiles are used on the bottom surface of the vehicle, which sees a more severe reentry heating environment than does the upper surface of the vehicle (43). 

Because of Hquid helium s uniquely low temperature and small heat of vaporization, containers for its storage and transportation must be exceedingly weU insulated. Some containers are insulated with only a fairly thick layer of very efficient insulation, but containers with the least heat leak use an inexpensive sacrificial cryogenic Hquid, usually Hquid nitrogen, to shield thermally the Hquid helium contents. 

Electric Submersible Oil Well Pump Cable. These cables are rated up to 5 kV and are designed for highly corrosive oil wells that besides oil also contain brine and other harsh chemicals as well as gases under high pressure and high temperatures (6). Insulations can be based on polypropylene for low temperature wells or on ethylene—propylene mbber which is compounded with special ingredients in order to resist the environments of high temperature wells (Fig. 4). 

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

In the microelectronics industry, powdered metals and insulating materials that consist of noimoble metals and oxides are deposited by screen printing in order to form coatings with high resistivities and low temperature coefficients of resistance. This technique may be useful in depositing oxide—metal refractory coatings. 

Extruded Rigid Foa.m. In addition to low temperature thermal insulation, foamed PSs are used for insulation against ambient temperatures in the form of perimeter insulation and insulation under floors and in walls and roofs. The upside-down roof system has been patented (256), in which foamed plastic such as Styrofoam (Dow) plastic foam is appHed above the tar-paper vapor seal, thereby protecting the tar paper from extreme thermal stresses that cause cracking. The foam is covered with gravel or some other wear-resistant topping (see Roofing materials). 

TABLE 2-380 Thermal Conductivities of Insulating Materials at Low Temperatures (Grober) ... 

At very low temperatures with hquid air and similar substances, the tank may have double walls with the interspace evacuated. The weh-known Dewar flask is an example. Large tanks and even pipe hues are now built this way. An alternative is to use double walls without vacuum but with an insulating material in the interspace. Perlite and plastic foams are two insulating materials employed in this way. Sometimes both insulation and vacuum are used. 

Low-temperature tanks may be installed underground to take advantage of the insulating value of the earth. Frozen-earth storage is also used. The frozen earth forms the tank. Some installations using this technique have been unsuccessful because of excessive heat absorption. 

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. 

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