Lowering the Temperature of Liquid Nitrogen

Lowering the Temperature of Liquid Nitrogen


Typical Phase Diagram

Liquid nitrogen (LN) is widely used in experiments where cryogenic temperatures are needed. The temperature of LN at 1 atm is 77.3 K. Some experiments might require temperatures slightly below this limit.

In the phase diagram we are on the equilibrium line between the liquid and the gaseous phase. To push this equilibrium to lower temperatures, toward the triple point we need to lower the pressure. The equilibrium temperature at the triple point is 63.1 K.

One way of doing this would be to connect a vacuum pump to the LN reservoir and pump on the nitrogen gas. This, however, requires an airtight seal of the reservoir and might introduce unwanted vibration into the system due to the mechanical vacuum pump operation.

Another way of  reaching the triple point is to bubble helium gas through the liquid nitrogen.[1] The bubbles will contain both He and N gas in equilibrium with liquid N. The nitrogen partial pressure will be a fraction of the total pressure in the bubbles. As liquid nitrogen vaporizes into the bubble, the necessary heat is drawn from the liquid, cooling it further, until the triple point at 63.1 K is reached.

With this method, the LN reservoir can be easily refilled as no airtight seal is necessary.

[1] Xu, Jansch, and Yates, Jr., J. Vac. Sci. Technol. A 11, 726 (1993)

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