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The application of helium-3


The combined properties of he-3 have attracted the great favor of researchers in the field of cryogenic physics and cryogenic engineering, among which the most striking is that he-3 plays a unique role in obtaining the low-temperature environment below 1K, and this temperature range is an important field of modern high-tech scientific research such as basic physics. He-3 has the properties of low boiling point, low density, high specific heat capacity and high thermal conductivity, which make it a very special refrigerant in cryogenic engineering, especially at the extreme low temperature near absolute zero.


In 1956, G.K. Walters and W.M. Fairbanks found that at temperatures below 0.87 K, the 3He and 4He mixtures were completely different phases, with the lighter rich 3He phase floating at the top and the heavier rich 4He phase sinking at the bottom. The rich 3He phase, also known as concentrated phase, is almost pure 3He at less than 0.3k. The rich 4He phase is called the dilution phase, which contains 6.4% of 3He. Even near absolute zero, there is still 6.4% of 3He dissolved in 4He. This characteristic is the basis of a dilution refrigerating machine that can continuously obtain the mill-open temperature.


In 1962, H. London and Peter Mendelssohn, among others, are again proposing practical technology plans for diluted refrigeration. The principle of dilution refrigeration is similar to that of transpiration refrigeration. At low temperature, 4He is in a superfluid state and a lazy liquid, while 3He is still a normal fluid and an active component. Therefore, if a container contains a mixture of 3he-4he, the 4he-rich phase in the bottom layer can be considered as the effect of only supporting or "mechanical vacuum" compared with the 3he-rich phase in the top layer. You just have to take some method to get rid of the 3He that's dissolved in the 4He phase, and the concentration of the 3He in the bottom 4He phase goes down, and the equilibrium between the two phases is going to be damaged, and the 3He atoms in the 3He phase will pass through the boundary layer and become separated into the 4He phase. From the interface, this is equivalent to transpiration of 3He, except that instead of transpiration of 3He molecules into the gas space, "transpiration" enters the superflow state of 4He in the liquid phase. This process is actually a continuous dilution of 3He, and if the dilution continues, the liquid is continuously cooled. Therefore, this refrigeration method is called dilution refrigeration.


Of course, there is still a big difference between 3he-4he dilution refrigeration and 3He transpiration refrigeration. As mentioned above, in the process of transpiration refrigeration, the steam pressure of 3He drops sharply as the temperature drops, so there is no gas to be pumped and the refrigeration process has to be terminated. The limiting temperature of 3He transpiration refrigeration is 0.25K. Dilution refrigeration is different. The content of 3He in the 4He phase is constant. No matter how low the temperature is, the pump can always maintain a constant 3He cycle quantity, so it can get a much lower temperature than 3He transpiration refrigeration


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