International consortium develops 3D printable alloy for more eco-friendly refrigerators

An international materials research project has yielded a 3D printable metal alloy that can make cooling systems, like those used in refrigerators, more eco-friendly and efficient.

Made from a combination of nickel and titanium, the material is a type of shape memory alloy that can repeatedly transform to pump heat out of a system. The challenge with such material has historically been that the alloy fails after a small number of cycles. Applying 3D printing, Hou et al. have been able to tune the microstructure of parts so that they can be cycled a million times while still retaining cooling capabilities. The method is termed solid-state elastocaloric cooling, and has the potential to transform the multibillion-dollar refrigeration and HVAC sector.

Refrigerators and the environment 

The current standard in cooling technology is vapor compression cooling. Invented in the 1800s as a means of producing ice, vapor compression is the system used in most air conditioning units and industrial and commercial refrigerators. The method utilizes a condensed liquid refrigerant. The cold mixture, after being subjected to a pressure reduction, is routed through a coil or tubes in an evaporator. A fan then circulates warm air over this coil/tubes, cooling the air in the process.

Diagram demonstrating the vapor compression refrigeration cycle. Image via the Heat Pump Association (HPA)
Diagram demonstrating the vapor compression refrigeration cycle. Image via the Heat Pump Association (HPA)

Though a reliable method of refrigeration, vapor compression cooling has its disadvantages. Many refrigerant liquids still used in the process damage the Earth’s ozone layer. Alternatives liquids also contribute to global warming as they remain in the atmosphere for years after usage. Having no negative ecological impacts ammonia can be used, however this compound is toxic, and incompatible with common copper pipes.

To help eradicate the damaging effects of refrigerant liquids, researchers have started seeking alternative means of cooling. Caloric cooling is one possible alternative. Realizing the potential of caloric materials, the U.S. Department of Energy (DoE), Iowa State and Ames Laboratory launched the CaloriCool consortium to develop such energy-conversion materials for industrial adoption. This most recent research builds on the shared knowledge of this group.

A million-cycle material

In caloric cooling there are three types of technology, magnetocaloric, electrocaloric and elastocaloric, each referring to the source of pressure applied to a material this being magnetic, stress, and electric fields, respectively. All of the methods are entirely vapor-free.

The new nickel-titanium alloy is an example of an elastocaloric material, therefore responding to pressure. It was created by the team by mixing metal powders in an L-DED process, producing nanocomposites within printed objects that improved its mechanical integrity. When subjected to pressure, the material far-exceeded the performance of other elastocaloric materials.

“The key to this innovation that is fundamental, but not often discussed, is that materials fatigue – they wear out,” commented UMD’s Professor Ichiro Takeuchi. “This is a problem when people expect their refrigerators to last for a decade, or longer. So, we addressed the problem in our study.”

According to the experimental findings, cycle testing of the material proved to enhance “the materials efficiency by a factor of four to seven—and repeatable elastocaloric performance over 1 million cycles.”

Structure of the elastocaloric nickel-titanium alloy. Image via Science magazine.
Structure of the3D printed elastocaloric nickel-titanium alloy. Image via Science magazine.

The full paper discussing the discovery, titled “Fatigue-resistant high-performance elastocaloric materials made by additive manufacturing” is published in Science magazine. The paper is co-authored by Huilong Hou, Emrah Simsek, Tao Ma, Nathan S. Johnson, Suxin Qian, Cheikh Cissé, Drew Stasak, Naila Al Hasan, Lin Zhou, Yunho Hwang, Reinhard Radermacher, Valery I. Levitas, Matthew J. Kramer, Mohsen Asle Zaeem, Aaron P. Stebner, Ryan T. Ott, Jun Cui and Ichiro Takeuchi.

An 18-people-strong team, the consortium of researchers are working across the Alliance for the Development of Additive Processing Technologies (ADAPT), at Colorado School of Mines, the University of Maryland (UMD), Ames Laboratory, Iowa, Xi’an Jiaotong University, China, and Iowa State University.

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Featured image shows a refrigerator. Photo by Pexels from Pixabay