Superfluid helium

At even lower temperatures, some unusual properties of matter are displayed. Consequently, new experimental and theoretical methods are being created to explore and describe chemistry in these regimes. In order to account for zero-point energy effects and tunneling in simulations, Voth and coworkers developed a quantum molecular dynamics method that they applied to dynamics in solid hydrogen. In liquid helium, superfluidity is displayed in He below its lambda point phase transition at 2.17 K. In the superfluid state, helium s thermal conductivity dramatically increases to 1000 times that of copper, and its bulk viscosity drops effectively to zero. Apkarian and coworkers have recently demonstrated the disappearance of viscosity in superfluid helium on a molecular scale by monitoring the damped oscillations of a 10 A bubble as a function of temperature. These unique properties make superfluid helium an interesting host for chemical dynamics. 

Grebenev S, Toennies J P and Vilesov A F 1998 Superfluidity within a small helium-4 cluster the microscopic andronikashvili experiment Soienoe 279 2083... 

Jasny J, Sepiol J, Irngartinger T, Traber M, Renn A and Wild U P 1996 Fluorescence microscopy in superfluid helium single molecule imaging Rev. Sc/. Instrum. 67 1425-30... 

Obviously 9 =0 corresponds to the SmA phase. This transition is analogous to the nonnal-superfluid transition in liquid helium and the critical behaviour is described by the AT model. Further details can be found elsewhere [18, 19 and 20]. 

There is assumed to be no interaction between the superfluid and normal components, thus the superfluid component can diffuse very rapidly to a heat source where it absorbs energy by reverting to the normal state. It thereby produces the very high effective thermal conductivity observed in helium II. 

When the superfluid component flows through a capillary connecting two reservoirs, the concentration of the superfluid component in the source reservoir decreases, and that in the receiving reservoir increases. When both reservoirs are thermally isolated, the temperature of the source reservoir increases and that of the receiving reservoir decreases. This behavior is consistent with the postulated relationship between superfluid component concentration and temperature. The converse effect, which maybe thought of as the osmotic pressure of the superfluid component, also exists. If a reservoir of helium II held at constant temperature is coimected by a fine capillary to another reservoir held at a higher temperature, the helium II flows from the cooler reservoir to the warmer one. A popular demonstration of this effect is the fountain experiment (55). 

Superfluid helium can pass easily through openings so small that they caimot be detected by conventional leak detection methods. Such leaks, permeable only to helium II, are called supedeaks. They can be a source of fmstrating difficulties in the constmction of apparatus for use with helium II. 

Fig. 3. Phase diagram for helium-3 where A, B, and A1 represent the three superfluid phases and PCP is the polycritical poiat. The dashed lines iadicate the...

Helium Purification and Liquefaction. HeHum, which is the lowest-boiling gas, has only 1 degree K difference between its normal boiling point (4.2 K) and its critical temperature (5.2 K), and has no classical triple point (26,27). It exhibits a phase transition at its lambda line (miming from 2.18 K at 5.03 kPa (0.73 psia) to 1.76 K at 3.01 MPa (437 psia)) below which it exhibits superfluid properties (27). 

Liquid helium-4 can exist in two different liquid phases liquid helium I, the normal liquid, and liquid helium II, the superfluid, since under certain conditions the latter fluid ac4s as if it had no viscosity. The phase transition between the two hquid phases is identified as the lambda line and where this transition intersects the vapor-pressure curve is designated as the lambda point. Thus, there is no triple point for this fluia as for other fluids. In fact, sohd helium can only exist under a pressure of 2.5 MPa or more. 

We have remarked that a temperature of zero on the absolute temperature scale would correspond to the absence of all motion. The kinetic energy would become zero. Very interesting phenomena occur at temperatures near 0°K (the superconductivity of many metals and the superfluidity of liquid helium are two examples). Hence, scientists are extremely interested in methods of reaching temperatures as close to absolute zero as possible. Two low temperature coolants commonly used are liquid hydrogen (which boils at 20°K) and liquid helium (which boils at 4°K). Helium, under reduced pressure, boils at even lower temperatures and provides a means of reaching temperatures near 1°K. More exotic techniques have been developed to produce still lower temperatures (as low as 0.001°K) but even thermometry becomes a severe problem at such temperatures. 

Helium is an interesting example of the application of the Third Law. At low temperatures, normal liquid helium converts to a superfluid with zero viscosity. This superfluid persists to 0 Kelvin without solidifying. Figure 4.12 shows how the entropy of He changes with temperature. The conversion from normal to superfluid occurs at what is known as the A transition temperature. Figure 4.12 indicates that at 0 Kelvin, superfluid He with zero viscosity has zero entropy, a condition that is hard to imagine.v... 

The phase diagram for helium is shown here, (a) What is the maximum temperature at which superfluid helium-II can exist (b) What is the minimum pressure at which solid helium can exist (c) What is the normal boiling point of helium-I ... 

The noble gases are all found naturally as unreactive monatomic a sc Helium has two liquid phases the lower-temperature liquid pha.<< xhibits superfluidity. 

The 1996 Nobel Prize in physics went to three researchers who studied liquid helium at a temperature of 0.002 K, discovering superfluid helium. A superfluid behaves completely unlike conventional liquids. Liquids normally are viscous because their molecules interact with one another to reduce fluid motion. Superfluid helium has zero viscosity, because all of its atoms move together like a single superatom. This collective behavior also causes superfluid liquid helium to conduct heat perfectly, so heating a sample at one particular spot results in an immediate and equal increase in temperature throughout the entire volume. A superfluid also flows extremely easily, so it can form a fountain, shown in the photo, in apparent defiance of gravity. 

In 1908, Kamerling-Onnes got the liquefaction of helium (discovered by Janssen e Lockyer during the solar eclipse of 18 August 1868). Kamerlingh-Onnes obtained in Leiden 60 cc of liquid helium extracted from several tons of monazite sable imported from India. Kamerlingh-Onnes himself discovered the X-transition and the superfluidity in 4He and in 1911 the superconductivity of Hg, a particularly pure substance at that time. In the race towards lower and lower temperatures, Kamerling-Onnes, pumping on liquid 4He, obtained 0.7K in 1926. 

Helium vapour pressure and latent heat of evaporation The latent heat of evaporation L and the vapour pressure />vap are fundamental parameters when using these two cryoliquids in the refrigeration process. Figure 2.5 shows L of 3He and 4He as a function of temperature. Note that L ( 20.9 J/g for 4He) is very small in comparison, for example, with that of hydrogen (445 J/g) or of nitrogen (200 J/g). Note also the minimum at 2.2 K in the graph for 4He, in correspondence with the superfluid transition. 

D. Vollhard, P. Woelfle The Superfluid Phases of Helium-3, Taylor Francis, London (1990)... 

There are two characteristics that make helium attractive for space applications the first is weight (about 0.125 kg/1) the second is its superfluidity. Helium becomes superfluid at T< 2.17K (p < 37.8 torr). Thanks to superfluiduty, helium forms a film that completely covers the walls of the container and guarantees a homogeneous cooling even if most of the liquid does not have a fixed position inside the container (no gravity). 

Superelectrons, 23 819-820 Superfibers, 24 624 Superficial filtering velocity, 26 710 Superficial velocity, 11 766 Superfilling, 9 774 Superfinishing stones, 1 19 Superflex catalytic cracking butylenes manufacture, 4 417 Superfluid helium, 17 353-354 Superfluid phases, of helium-3,17 354-355 Superfluids, 17 352-354 Superfractionation, 23 333-334 Superfund Amendments and... 

Fig. 3.3. Acoustic micrographs taken in superfluid helium at 0.2 K. (a) Bipolar transistor on a silicon integrated circuit. The aluminium lines making connections to the base and the emitter are 2 fan wide and 0.5 fan thick. Three images were taken at different heights, and superimposed with colour coding. The lens had a numerical aperture N.A. = 0.625 and a depth of focus less than 150 nm, / = 4.2 GHz. (b) Myxobac-terium, with different planes similarly colour-coded and superimposed, / = 8 GHz...

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