Contraction at low temperatures

Tlie minimum found by XRD is confirmed. The lower values are due to the thermal contraction at low temperatures, (g) 6-parameter vs. x. The lines are guides to the eye. After data of Hewat et al. (1991). 

The unit in our initial tests incorporated no modifications except opening some clearances between mating parts at room temperature to avoid bind from differential contraction at low temperatures. As our test experience increased, some changes in hydraulic balance were made. 

Thermal Contraction. Thermal contraction at low temperatures is determined by the push-rod method measuring the elongation outside a cryostat, direct measurement on the sample within a cryostat by capacitive strain gauges, and reflective or interferometric methods. The last method involves cryogenic problems (see Fig. 12). A laser beam is directed onto a tubular specimen fixed within a cryostat. A mirror and a transparent mirror are placed on the bottom and top of the specimen, respectively. Both reflected beams are focused on an... 

Plastics expand at high temperatures and contract at low temperatures to different amounts, depending on their coefficients of thermal expansion. After bonding, part movement is restricted, and the two materials must expand or contract to the same extent. If the plastics have different coefficients of thermal expansion, stresses are produced, which lower the strength of the joint. Coefficients of thermal expansion can be decreased by adding fillers or reinforcements or by... 

Quite often problems arise when instruments for normal seiwice are subjected to low temperature use. Since some metals become brittle at low temperatures, the instrument hteraUy falls apart. Elastomeric gaskets and seals contract faster with decreasing temperatures than the surrounding metal parts, and the seal often is lost. Even hermetically sealed instruments can develop pin holes or small cracks to permit ciyogenic liqmds to enter these cases with time. Warming the instrument causes the trapped hquid to vaporize, sometimes generating excessive gas pressure and failure of the case. 

Fig. 7-20 Examples at low temperatures of ther-mal contraction in unfilled plastics and steel. With RPs using TS plastics change can be significantly reduced or even at zero (using graphite, etc.).

The use of 0(3P) atoms produced by microwave irradiation of He-Oa mixtures has shown that alkenes react with atomic oxygen in solution or neat to give predominantly epoxides. Unlike reactions in the gas phase, at low temperature these produce useful product yields and distributions. Similar yields suggest that epoxide formation and 1,2-H/1,2-C shifts/ring contractions compete. 

Peak positions. Shifts in the positions of the peaks in an experimental powder XRD pattern may arise due to a number of instrumental factors. Furthermore, comparison of powder XRD patterns recorded at different temperatures may show differences in appearance (particularly in regions with significant peak overlap) as a result of anisotropic thermal expansion/contraction. This issue is particularly relevant when an experimental powder XRD pattern recorded at ambient temperature is compared with simulated powder XRD patterns for known crystal structures determined from single-crystal XRD data at low temperature. 

Not much is known about the properties of 1,2,5-thiadiazines. However, there is one outstanding property the contraction of the 1,2,5-thiadiazine ring, which is shown both by monocyclic 1,2,5-thiadiazines and 2,1,4-benzothiadiazines. For instance, 1,2,5-thiadiazine 187, which is stable at low temperatures, tends to revert to 1,2,5-thiadiazolium cyanide 185 on standing at room temperature for several days (79TL1281). In this contraction, the ring sulfur atom is retained. Usually the sulfur atom is extruded to give imidazole derivatives, e.g., 1,2,5-thiadiazine 184 (R = i-Pr), when heated, yields thiazolidinethione 194 (49JOC946). In the 2,1,4-... 

The Mg-octahedral sheet is larger than the tetrahedral sheet (b-axis) and is forced to contract (Radoslovich, 1963b). Substitution of the larger Fe2+ would presumably increase the strain beyond tolerable limits. Such adjustments would be less readily expected in minerals formed at low temperatures. Although substitution of A1 and Fe3+ would tend to decrease the size of the octahedral sheet, the net positive charge of the sheet is increased (0.10—0.50), decreasing the anion-anion repulsion. Vacancies in the octahedral sheet generally increase as octahedral R3+increases, but this seldom... 

As a matter of fact, 42 is readily accessible, starting with the acyloin condensation of succinic esters in the presence of trimethylsilylchloride to provide 1,2-disiloxycyclo-butene 161 in high yields 93). Bromination of 161 in pentane at low temperature, led to the 1,2-cyclobutanedione 162, which underwent acid or base induced ring contraction to 1-hydroxycyclopropanecarboxylic acid 4294). More conveniently, 42 was prepared in a one-pot reaction by first adding bromine in CH2C12 at —10 °C and then ice-water to 161 the hydroxyacid 42 was obtained by continuous extraction in 94 % yield, Eq. (52) 95-96). 

Fukushi K, Nagai M. Kamata Y, Kadotani K (1984) Mechanical properties of low thermal contraction GFRP. In Hartwig G, Evans D (eds) Nonmetallic materials and composites at low temperature, vol 3. Plenum, New York, p 187... 

The formation of cholest-14-ene derivatives by hydrochlorination-dehydrochlorination of A8(14)-olefins (282) is accompanied at low temperature by the formation of 17a-cholestane derivatives, which become the main products at —60 to —78 °c.229 230 It is suggested230 that an acid-catalysed contraction of ring C leads to a... 

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