Improving shape memory alloys for solid-state cooling

Shape memory alloys (SMAs) as elastocaloric materials have attracted interest in advancing solid-state cooling systems for applications such as electronic chip cooling or biomedical lab-on-a-chip systems. When stress is applied to these solid materials, their crystal structure changes, resulting in the release or absorption of large amounts of latent heat.

Once the phase transition is complete, the SMA returns to its original crystal structure and the process begins again. But as the number of cycles increases, the material breaks down, losing its ability to retain its initial crystalline shape, which severely diminishes its cooling capacity. Bumke et al. have developed an SMA film composed of titanium, nickel, copper and cobalt that can withstand more than 20 million cycles with negligible degradation.

Building on other SMA studies using sputtered TiNiCu films, the researchers suppressed functional fatigue by combining a small grain size with Ti₂Cu precipitates while deviating slightly from perfect crystallographic compatibility. This showed that a large compositional spread or range in the TiNiCu system is likely to have a high functional stability.

To increase the films’ cooling capacity, they added a layer of cobalt, leading to a temperature drop of 12.2 K compared to 9.1 K for the TiNiCu film. They also determined that their new thin films are ideal for cooling devices based on tension.

“Tension is favorable over compression in terms of heat transfer times and overall performance, but materials with high quality, no defects and high functional stability are needed, which is addressed by our material,” author Lars Bumke said.

Source: “Cu-rich Ti_52.8Ni_22.2Cu_22.5Co_2.5 shape memory alloy films with ultra-low fatigue for elastocaloric applications,” by Lars Bumke, Christiane Zamponi, Justin Jetter, and Eckhard Quandt, Journal of Applied Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0006301 .