Helium Temperatures Below

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

The second observation is that the EB-curable adhesive resins show no change in adhesive properties from liquid helium temperatures up to temperatures Just below their service temperatures. This can be seen for the adhesives, EB2000 and 1 1L. At this time there is no explanation for this behavior. 

Superconductivity has been known since 1911, and superconducting systems based on various metal alloys (e.g., NbTi and Nb3Sn) are currently used as magnets and in electronics. These materials exhibit superconductivity only at temperatures below 23 K and require cooling by liquid helium. The discovery of ceramics that exhibit superconductivity at temperatures up to 120 K, the so-called high-temperature superconductors, has sparked a tremendous amount of scientific activity and commercial interest around the world. 

Giauque s technique still is used in contemporary low-temperature research. A sample containing paramagnetic ions (Fe, for example) is bathed in liquid helium under reduced pressure to chill it to a temperature below 4.2 K. The sample is held in a strong magnetic field, which aligns the magnetic... 

The recoil-free fraction /a of transition metal complexes or proteins in frozen solution can be as small as 0.1-0.3, when measured just below the melting point, but the /-factor increases strongly when the temperature is lowered to fiquid nitrogen temperatures (77 K), and at fiquid helium temperatures (4.2 K) it may reach values of 0.7-0.9 [35]. This makes a substantial difference to the acquisition time of the spectra because of the square dependency on the signal (3.1). 

The phenomenon of superconductivity was discovered at the beginning of the twentieth century by the Dutch physicist H. Kamerlingh Onnes, during the first attempts to liquefy helium (which at atmospheric pressure boils at 4.2 K). After refining the technique of helium liquefaction, in 1911, Onnes attempted to measure the electrical resistance of metals at these extraordinary low temperatures, and realized that at 4 K the resistance of mercury, as well as that of other metals indicated in Figure 1, became too low to be measured. This change in electrical property became the indication of the new superconductive physical state. The temperature below which materials become superconducting is defined as the critical temperature, Tc. 

Dr. Erickson For those interested in coordination chemistry, certain other transition metal atoms are suitable for Mossbauer spectroscopy. One in particular is ruthenium which is just below iron in the Periodic Table. It is a difficult isotope to work with since it requires helium temperatures almost exclusively. I don t know whether it is possible to work at nitrogen temperatures or not, but Kistner at Brookhaven has examined various ruthenium compounds from the 2-j- to the 8+ oxidation states with interesting results. These are not published yet, but at least his work offers the possibility of going down one element below the other in the Periodic Table to study chemical effects. Osmium, which is below ruthenium, can also be Mossbauered. Some sort of systematic study like this involving elements in the various transition series would be extremely interesting. 

Alternatively, the composition of this mixture may be estimated by gas chromatographic analysis of a sample in hexane solution. A 6.2 mm X 1.5 m 2% SE-30 column at temperatures of 70-150° and a helium flow rate of 25 mL/min gives excellent separations. Using a temperature program of 8°/min and an initial temperature of 70°, the retention time of W(CO)6 is 4 minutes and that of W(CO)5(CS) is 10 minutes. The peak areas for these compounds are taken to be indicative of their relative molar quantities. Small amounts of pure W(CO)5(CS) may be collected from the gas chromatograph using oven and detector temperatures below 70°. 

The original studies (McDonald et cd., 1965) that reported the formation of these species were limited by the available technology to temperatures above 77 K. At those temperatures, the minority S = species are the only ones detectable by EPR, but at temperatures below 25 K, attained by the use of liquid helium as a refrigerant, other species can be observed (Salerno and Siedow, 1979 Rich et al., 1978). These species have much faster relaxation rates because they... 

For the light molecules He and H2 at low temperatures (below about 50°C.) the classical theory of transport phenomena cannot be applied because of the importance of quantum effects. The Chapman-Enskog theory has been extended to take into account quantum effects independently by Uehling and Uhlenbeck (Ul, U2) and by Massey and Mohr (M7). The theory for mixtures was developed by Hellund and Uehling (H3). It is possible to distinguish between two kinds of quantum effects— diffraction effects and statistics effects the latter are not important until one reaches temperatures below about 1°K. Recently Cohen, Offerhaus, and de Boer (C4) made calculations of the self-diffusion, binary-diffusion, and thermal-diffusion coefficients of the isotopes of helium. As yet no experimental measurements of these properties are available. 

Tunneling junctions are most often produced in a crossed stripe geometry so that 4-terminal measurements of their current-voltage characteristics can be made. Electrical contacts are made to the films ( often with miniature brass "C" clamps ), the samples are mounted in a Dewar insert, and cooled to liquid helium temperatures ( 4.2 ° K or below ). 

Liquid helium presents an interesting case leading to further understanding of the third law. When liquid 4He, the abundant isotope of helium, is cooled at pressures of < 25 bar, a second-order transition takes place at approximately 2 K to form liquid Hell. On further cooling Hell remains liquid to the lowest observed temperature at 10 5 K. Hell does become solid at pressures greater than about 25 bar. The slope of the equilibrium line between liquid and solid helium apparently becomes zero at temperatures below approximately 1 K. Thus, dP/dT becomes zero for these temperatures and therefore AS, the difference between the molar entropies of liquid Hell and solid helium, is zero because AV remains finite. We may assume that liquid Hell remains liquid as 0 K is approached at pressures below 25 bar. Then, if the value of the entropy function for sol 4 helium becomes zero at 0 K, so must the value for liquid Hell. Liquid 3He apparently does not have the second-order transition, but like 4He it appears to remain liquid as the temperature is lowered at pressures of less than approximately 30 bar. The slope of the equilibrium line between solid and liquid 3He appears to become zero as the temperature approaches 0 K. If, then, the slope is zero at 0 K, the value of the entropy function of liquid 3He is zero at 0 K if we assume that the entropy of solid 3He is zero at 0 K. Helium is the only known substance that apparently remains liquid as absolute zero is approached under appropriate pressures. Here we have evidence that the third law is applicable to liquid helium and is not restricted to crystalline phases. 

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