Physical properties at low temperatures

Since most of the physical properties at low temperatures are modified by the crystal field, knowledge of the crystal-field parameters is rather important. Until recently these parameters have been derived from integral effects like specific heat and magnetic susceptibility. Direct information about the crystal-field-split energy levels can, however, be obtained by spectroscopic methods. For metallic compounds neutron spectroscopy plays a key role. We have made an attempt to collect the presently known data in table 33.6. We abstained from making a... 

The following chapters discuss the history, development and physical properties of low-temperature ionic substances but in this section it is usefiil to discuss the differences that arise in changing from a molecular to an ionic environment and the implications that this will have for electrodeposition processes occurring at an electrode surfaces. 

Most commercial fluorocarbon elastomers have brittle points between -25°C (-13°F) and -40°C (-40°F). The low-temperature flexibility depends on the chemical structure of the polymer and cannot be improved markedly by compounding. The use of plasticizers may help somewhat, but at a cost of reduced heat stability and worsened aging. Peroxide-curable polymers may be blended with fluorosilicones, but such blends exhibit considerably lower high-temperature stability and solvent resistance and are considerably more expensive than the pure fluorocarbon polymer. Viton GLT is a product with a low brittle point of -51°C (-59°F) [48]. Tecnoflon for containing a stable fluorinated amide plasticizer reportedly exhibits improved low-temperature hardness, brittle point, and compression set without sacrificing physical properties [66]. Low-temperature characteristics of selected fluorocarbon elastomers are listed in Table 5.13 [9]. 

Another space-specific issue is the embrittlement or stiffening of an otherwise flexible adhesive at the low temperature extremes of space. Of the many polymer types, silicones best maintain their flexibility and physical properties at operating temperatures as low as —80 °C, or even lower for some formulations, thus are widely used in satellites and space vehicles. 

Mechanical Toughness Superior antistick and low frictional properties, high resistance to impact and tearing, and useful physical properties at cryogenic temperatures. 

Of particular interest are austenitic stainless steels, which are the most commonly used in cryogenic engineering. These steels are subdivided into structurally stable and metastable, depending on their nickel concentration. The metastable alloys have a martensitic phase transition at low temperatures, which significantly affects practically all their physical properties. The low-temperature mechanical properties of typical austenitic stainless steels are given in Table II. 

Fama L, Rojas AM, Goyanes S, Gerschenson L (2005) Mechanical properties of tapioca-starch edible films containing sorbates. LWT 38 631-639 Fama L, Flores SK, Gerschenson L, Goyanes S (2006) Physical characterization of cassava starch biofilms with special reference to dynamic mechanical properties at low temperatures. Catbohydr Polym 66 8-15... 

Mech nic lProperties. Extensive Hsts of the physical properties of FEP copolymers are given in References 58—63. Mechanical properties are shown in Table 3. Most of the important properties of FEP are similar to those of PTFE the main difference is the lower continuous service temperature of 204°C of FEP compared to that of 260°C of PTFE. The flexibiUty at low temperatures and the low coefficients of friction and stabiUty at high temperatures are relatively independent of fabrication conditions. Unlike PTFE, FEP resins do not exhibit a marked change in volume at room temperature, because they do not have a first-order transition at 19°C. They ate usehil above —267°C and are highly flexible above —79°C (64). 

Unsaturated resias based on 1,4-cyclohexanedimethanol are useful ia gel coats and ia laminating and molding resias where advantage is taken of the properties of very low water absorption and resistance to boiling water (6). Thermal stabiHty is imparted to molding resias, both thermoplastic (71,72) and thermoset (73—76), enabling retention of physical and electrical properties at elevated temperatures (77). Additionally, resistance to chemical and environmental exposure is characteristic of products made from these resias (78). 

Thin films (qv) of lithium metal are opaque to visible light but are transparent to uv radiation. Lithium is the hardest of all the alkaH metals and has a Mohs scale hardness of 0.6. Its ductiHty is about the same as that of lead. Lithium has a bcc crystalline stmcture which is stable from about —195 to — 180°C. Two allotropic transformations exist at low temperatures bcc to fee at — 133°C and bcc to hexagonal close-packed at — 199°C (36). Physical properties of lithium are Hsted ia Table 3. 

Physical Properties. Sodium metabisulfite (sodium pyrosulfite, sodium bisulfite (a misnomer)), Na2S20, is a white granular or powdered salt (specific gravity 1.48) and is storable when kept dry and protected from air. In the presence of traces of water it develops an odor of sulfur dioxide and in moist air it decomposes with loss of part of its SO2 content and by oxidation to sodium sulfate. Dry sodium metabisulfite is more stable to oxidation than dry sodium sulfite. At low temperatures, sodium metabisulfite forms hydrates with 6 and 7 moles of water. The solubiHty of sodium metabisulfite in water is 39.5 wt % at 20°C, 41.6 wt % at 40°C, and 44.6 wt % at 60°C (340). Sodium metabisulfite is fairly soluble in glycerol and slightly soluble in alcohol. 

Ethylene oxide is a colorless gas that condenses at low temperatures into a mobile Hquid. It is miscible in all proportions with water, alcohol, ether, and most organic solvents. Its vapors are flammable and explosive. The physical properties of ethylene oxide are summarized in Tables 1—7. 

Vulcanised rubbers possess a range of very desirable properties such as resilience, resistance to oils, greases and ozone, flexibility at low temperatures and resistance to many acids and bases. However, they require careful (slow) processing and they consume considerable amounts of energy to facilitate moulding and vulcanisation. These disadvantages led to the development of thermoplastic rubbers (elastomers). These are materials which exhibit the desirable physical characteristics of rubber but with the ease of processing of thermoplastics. 

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