The 4K pulse tube cryocooler developed by researchers at the University of Chinese Academy of Sciences. Image: Liubiao Chen/University of Chinese Academy of Sciences.Cryocoolers are ultracold refrigeration units used in surgery and drug development, semiconductor fabrication, and spacecraft. They can come as tubes or pumps, and in tabletop sizes or larger refrigerator systems.
The regenerative heat exchanger, or regenerator, is a core component of cryocoolers. But at temperatures below 10K (-441.67°F), its performance drops precipitously, with maximum regenerator losses of more than 50%.
Now, in a paper in Applied Physics Letters, researchers at the University of Chinese Academy of Sciences report using superactivated carbon particles as an alternative regenerator material to enhance cooling capability at temperatures as low as 4K.
In most cryocoolers, a compressor drives room temperature gas through the regenerator, which soaks up heat from the compression as the gas expands and cools. The oscillating ultracold gas then absorbs the heat trapped in the regenerator, and the process repeats.
Nitrogen is the most commonly used gas in cryocoolers. But for applications requiring temperatures below 10K, such as space telescope instruments and magnetic resonance imaging systems, helium is used, because it has the lowest boiling point of any gas and produces the coldest attainable temperatures.
However, helium's high specific heat (the amount of heat transfer needed to change the temperature of a substance) results in large temperature fluctuations during the compression and expansion cycle at low temperatures, which seriously affects cooling efficiency.
To address this problem, the researchers replaced the regenerator's conventional rare-earth metals with activated carbon, which is carbon treated with carbon dioxide or superheated steam at high temperatures. This creates a matrix of micron-size pores that increases the carbon's surface area, allowing the regenerator to hold more helium at low temperatures and thus remove more heat.
The researchers used a 4K Gifford-McMahon cryocooler to test the helium adsorption capacity of superactivated carbon particles with a porosity of 0.65 over a temperature range of 3–10K.
They found when they filled the regenerator with 5.6% of superactivated carbon particles with diameters between 50µm and 100µm, the resulting no-load temperature of 3.6K was the same as obtained using precious metals. At 4K, however, cooling capacity increased by more than 30%.
The researchers confirmed this improved performance by placing coconut-shell activated carbon into an experimental pulse tube and using a thermodynamic calculation model. "In addition to providing increased cooling capacity, the activated carbon can serve as a low-cost alternative to precious metals and could also benefit low-temperature detectors that are sensitive to magnetism," said researcher Liubiao Chen.
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.