3 He- 4 He mixture cooling

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3 He- 4 He mix cooler

The 3 He- 4 He mixed cooling is a cooling method that takes advantage of the fact that when the isotopes 3 He and 4 He of the element helium are mixed, energy is absorbed and removed from the system in the form of heat. The process was developed in the early 1950s by Heinz London et al. suggested.

This cooling principle is used in commercially available mixture cryostats in order to achieve temperatures below 1  Kelvin , as required for basic research, e.g. B. in solid state physics or in low temperature physics are required.

principle

A helium mixture of the two isotopes 3 He and 4 He breaks down into two liquid phases below a temperature of about 1 Kelvin :

  • a concentrated phase, which consists almost exclusively of 3 He, and
  • a dilute phase consisting of about 6% 3 He and 94% 4 He.

The 3 He-rich concentrated phase floats on top of the dilute phase because of its lower density .

The 4 He of the dilute phase forms a quantum liquid with superfluid properties at temperatures below 2.2 K (lambda line) , i.e. H. the liquid no longer offers any frictional resistance . In this ideal liquid , the 3 He atoms move completely without friction - like atoms in empty space.

If a 3 He atom now passes from the concentrated phase over the phase boundary into the dilute phase, this mixing process corresponds to the transition between liquid and gaseous phase analogous to evaporative cooling , as it is e.g. B. is exploited in a conventional refrigerator. As in the case of evaporation, energy must also be expended in this transition via a phase interface. This energy is called the enthalpy of mixing and is extracted from the environment as heat; this is equivalent to cooling down.

Physical reasons for the enthalpy of mixing are:

  1. The 3 He atom is larger than the 4 He atom due to the stronger zero point oscillation , so that a 3 He atom can enter into more van der Waals bonds with the small 4 He atoms than with larger 3 He atoms.
  2. In addition, the energy states of the fermionic 3 He atoms must be occupied successively. In a mixture there is always less 3 He per volume than in a pure 3 He liquid. As a result, the Fermi energy in a mixture will always be lower than in a concentrated phase. In addition, the wave functions of the 3 He atoms are shielded by the 4 He atoms, so that sometimes several 3 He atoms can occupy the same (low) energy states despite the Pauli principle .

Cooling capacity

The cooling capacity of the system is proportional to the transfer rate of 3 He atoms across the phase interface (particles per unit of time). The cooling process works to arbitrarily low temperatures since the dilute phase of thermodynamic reasons always a non-zero proportion 3 includes He atoms: even at a temperature of 0 Kelvin ( absolute zero ) would be a 3 He- 4 He mixture never fully segregate, but an equilibrium would be established in which 6.5 percent of 3 He would remain dissolved in 4 He.

The difference in concentration of the 3 He in the two phases is maintained in the mixture cryostat by continuously pumping out the 3 He from the diluted phase. To play a 3 He- 4 to have He-mixing equilibrium, steady be 3 He atoms replenished from the concentrated phase.

Pumping off the 3 He requires a sufficiently high vapor pressure of the 3 He, which, however, decreases as the temperature drops. By a sufficiently wide 3 He vapor pressure for pumping to ensure effective, is in 3 He- 4 He dilution refrigerator-the dilute phase in the still is heated locally and thus maintained at a temperature of just under 1 Kelvin. In continuous operation, the 3 He gas is liquefied again on the other side of the cooling circuit in the condenser at around 1 Kelvin. The condenser also serves to pre-cool the cryostat. From here, the 3 He of the concentrated phase in the mixing chamber, in which the phase boundary between the concentrated and the dilute phase lies, is fed back via a heat exchanger using the countercurrent principle . This cycle causes continuous cooling.

Modern mixture cryostats achieve typical cooling capacities of around half a milliwatt and final temperatures of a few millikelvin. The final temperature of a system is practically limited by heat leaks and the efficiency of the heat exchangers.

swell

  1. ^ Matter and Methods at Low Temperatures, Frank Pobell, Springer Verlag, ISBN 978-3-540-46356-6 .