Mechanical Property Cryogenic Temperatures

At present it is very difficult to estimate the synergetic effects of high mechanical loads, cryogenic temperatures and high fluences of nuclear radiation on polymer materials. In this review, therefore, the effects of each above-mentioned factor on the polymer properties will be separately introduced. 

Mechanical Properties. Table 2 shows the physical properties of Teflon PEA (22,23). At 20—25°C the mechanical properties of PEA, EEP, and PTEE are similar differences between PEA and EEP become significant as the temperature is increased. The latter should not be used above 200°C, whereas PEA can be used up to 260°C. Tests at Hquid nitrogen temperature indicate that PEA performs well in cryogenic appHcations (Table 3). 

The typical mechanical properties that qualify PCTFE as a unique engineering thermoplastic are provided ia Table 1 the cryogenic mechanical properties are recorded ia Table 2. Other unique aspects of PCTFE are resistance to cold flow due to high compressive strength, and low coefficient of thermal expansion over a wide temperature range. 

Properties. Table 1 hsts many of the physical, thermal, mechanical, and electrical properties of indium. The highly plastic nature of indium, which is its most notable feature, results from deformation from mechanical twinning. Indium retains this plasticity at cryogenic temperatures. Indium does not work-harden, can endure considerable deformation through compression, cold-welds easily, and has a distinctive cry on bending as does tin. 

Structural Properties at Low Temperatures It is most convenient to classify metals by their lattice symmetiy for low temperature mechanical properties considerations. The face-centered-cubic (fee) metals and their alloys are most often used in the construc tion of cryogenic equipment. Al, Cu Ni, their alloys, and the austenitic stainless steels of the 18-8 type are fee and do not exhibit an impact duc tile-to-brittle transition at low temperatures. As a general nile, the mechanical properties of these metals with the exception of 2024-T4 aluminum, improve as the temperature is reduced. Since annealing of these metals and alloys can affect both the ultimate and yield strengths, care must be exercised under these conditions. 

Ethylene trifluoroethylene (Tefzel) (ETFE) has good mechanical properties from cryogenic levels to 350°F (177°C). It has an upper continuous working temperature limit of 300°F (149°C). 

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. 

To date, most experiments with Au atomic contacts have been carried out at cryogenic temperatures or at room temperature in UHV, at ambient conditions in the gas phase, or in solution. Very few studies were reported in an electrochemical environment [205-208]. Electrochemical polarization offers the unique opportunity of tuning both the electrical and the mechanical properties of the respective atomic contacts by variation of the electrode potential. The electrodes could be charged and the local concentration of adsorbates at the atomic contacts can be varied in a rather controlled matter. 

The behaviour at low temperatures is good, depending on the mechanical constraints undergone. Mechanical properties vary little between room temperature and -70°C. Suitably designed parts are usable at cryogenic temperatures, -160°C for example. Notched Izod impact strengths are weak and parts must be designed to avoid stress concentrators. 

A torsional pendulum (Figure 5.80) is often used to determine dynamic properties. The lower end of the specimen is clamped rigidly and the upper clamp is attached to the inertia arm. By moving the masses of the inertia arm, the rotational momentum of inertia can be adjusted so as to obtain the required frequency of rotational oscillation. The dynamic shear modulus, G, can be measured in this manner. A related device is the dynamic mechanical analyzer (DMA), which is commonly used to evaluate the dynamic mechanical properties of polymers at temperatures down to cryogenic temperatures. 

The mechanical properties of PTFE at room temperature are similar to those of medium-density polyethylene, i.e., relatively soft with high elongation, and remaining useful over a wide range of temperatures, from cryogenic (just above absolute zero) to 260°C (500°F) its recommended upper use temperature.28 Stress-strain curves are strongly affected by the temperature however, even at 260°C (500°F) the tensile strength is about 6.5 MPa (942 psi).29... 

The available data on the mechanical properties of polymer materials at cryogenic temperatures have been reported for the last few decades mainly in close connection with space technology. 

An extensive compilation and evaluation of mechanical, electrical, and thermal properties of six commercially available polymers was performed by Reed et al. [14]. It was shown in their summarized data that polypyromellitim-ide (PPMI), which is obtained by the polycondensation between pyromellitic acid and aromatic diamine, exhibits excellent mechanical properties at both high and low temperatures and retains ductility even at cryogenic temperatures, as seen in Fig. 1. 

Halar ECTFE (ethylenechlorotrifluoroethylene) This material is an alternating copolymer of ethylene and chlorotrifluroethylene. This fluoropolymer withstands continuous exposure to extreme temperatures and maintains excellent mechanical properties across this entire range (from cryogenic temperatures to 180°C). It has excellent electrical properties and chemical resistance, having no known solvent at 121°C. It is also nonbuming and radiation-resistant. Its ease of processing affords a wide range of products. 

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."... 

---