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Shape memory Nitinol alloys designed to cool refrigerators

February 04, 2016
Source: ASM International

Saarland University, Germany, reports that its engineers are using shape memory alloys to develop a new method of cooling in which heat and cold are transferred by a nickel-titanium alloy.

An extensive series of tests have yielded results that are now being exploited to develop a prototype cooling circuit that will further increase the efficiency of the process.

If a nickel-titanium wire or sheet is deformed or pulled in tension, the crystal lattice structure can change, creating strain within the material. This change in the crystal structure, known as a phase transition, causes the shape memory alloy to become hotter. If the stressed sample is allowed to relax after temperature equalization with the environment, it undergoes substantial cooling to a temperature about 20 Centigrade degrees below ambient temperature. ‘

"The basic idea was to remove heat from a space – like the interior of a refrigerator – by allowing a pre-stressed, super-elastic shape memory material to relax and thus cool significantly. The heat taken up in this process is then released externally to the surroundings. The SMA is then re-stressed in the surroundings, thereby raising its temperature, before the cycle begins again," explains Stefan Seelecke, Professor for Intelligent Material Systems at Saarland University.

In the experimental and modelling studies carried out so far, the researchers at Saarland University and the Center for Mechatronics and Automation Technology (ZeMA) in Saarbrücken have demonstrated that this type of cooling works, and that it can be used in practice. They used a model system to determine how to optimize the efficiency of the cooling process, examining such factors as how strongly the material has to be elongated or bent in order to achieve a certain cooling performance, or whether the process is more effective when carried out slowly or more rapidly. A thermal imaging camera was deployed to analyze precisely how the heating and cooling stages proceed.

The German Research Foundation, which has been funding the project for the last three years, has agreed to invest a further 500,000 euros. In total, the project has brought around 950,000 euros in funding to the region.


Subject Classifications

Industries and Applications | Industrial Design

Materials Properties and Performance | Thermal Properties

Metals and Alloys | Shape Memory Alloys

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