The home air conditioner and refrigerator owe their success to vapor-compression technology. First, a compressor squeezes refrigerant vapor before it’s sent to a condenser. There the hot, pressurized gas cools and becomes a liquid. The liquid is then piped into an evaporator where it depressurizes and sucks up the surrounding heat. As the environmental temperature decreases, the liquid gains heat and evaporates before reentering the compressor and repeating the process.
Though inexpensive, the vapor-compression refrigeration cycle is neither the most efficient nor the most environmentally friendly process. Solid-state alternatives that apply a magnetic field, electric field, or stress to specially designed metal alloys don’t require greenhouse-gas-emitting refrigerants and have been gaining traction for decades. But the technology hasn’t quite achieved the temperature fluctuation necessary to compete with commercially available cooling technology (see the article by Ichiro Takeuchi and Karl Sandeman, Physics Today, December 2015, page 48). Now Daoyong Cong of the University of Science and Technology Beijing and a team of engineers, materials scientists, and physicists have designed a novel material whose temperature change could be big enough for large-scale refrigeration applications.
Metal alloys of nickel–titanium and nickel–manganese have already been studied as potential solid-state refrigerants. When those alloys are squeezed in one direction, the crystal’s unit cell distorts, as depicted in the image. As the alloy’s volume increases, so does its temperature, producing what’s known as an elastocaloric effect. To maximize such an effect, the researchers designed an alloy comprising Ni, Mn, and Ti and added a trace of boron for strength. Measurements during compressive loading and unloading of the sample yielded two important results: The alloy experiences a temperature change of 30 K, about 20% higher than any other elastocaloric material measured thus far, and the temperature swing is reversible.
At least one big wrinkle still needs to be ironed out. The material needs to undergo testing to determine if it can withstand the millions of stress cycles a commercially available unit would need to perform. Nevertheless, even though the elastocaloric effect may decrease if the material needs strengthening, the large temperature change already achieved affirms that it is a promising candidate for commercial solid-state refrigeration technology. (D. Cong et al., Phys. Rev. Lett. 122, 255703, 2019.)
