Structural Cryogenic Supports

As discussed in previous sections, it is important to minimize the heat leak into the cryogenic section of the ASU. This is accomplished by containing the equipment in an insulated enclosure termed a coldbox. The coldbox is a structural steel frame which is paneled with metal sheeting and provides structural support for the equipment and also containment for the insulation. There are essentially two different types of insulation material used in ASUs. The first and most popular is perlite. The perlite coldboxes are insulated in the field with perlite blown in from the top of the coldbox. The second insulation material is rock wool. Rock wool has to be packed manually. 

Unequivocal evidence for the existence of the 7-norbornyl cation, however, was subsequently provided by Sunko and coworkers by using the cryogenic matrix isolation technique. They codeposited 7-chloronorbornane 146 and a large excess of SbFj at -263°C then slowly warmed it to -173°C. The IR spectrum was in very good agreement with the calculated frequencies [MP4(sdq,fc)/6-31G //MP2(full)6-31G + ZPE level] for the 7-norbornyl cation 80 (Scheme 5.11). Further warming gave the 2-norbornyl cation 126 at -73°C. Tlie possibility of the formation of a complex of 146 with Sbf. was ruled out when similar experiments with both 146 and 147 resulted in identical spectra. Recent ab initio calculations support the nonclassical bent structure of the cation. 

An overall view of the basic tensile-test apparatus is shown in Fig. 1. To eliminate any interaction between the 12-T magnet and the surrounding room-temperature structure, the entire load frame of the tester is fabricated from nonmagnetic stainless steel. The crosshead, shown in the fully raised position in Fig. 1, is supported by two 5.3-m-long, 100-mm-diameter columns. It is raised and lowered by two hydraulic pistons and can be hydraulically clamped to the columns at any position. The column height was made sufficient to allow the cryogenic structure to be raised above the cryostat, the cryostat to be removed, and the structure then lowered back down for the sample to be changed. 

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