Potential new metallic refrigerant

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2010

When heated or cooled, shape-memory alloys transition from one crystalline phase to another.

KEY CONCEPTS
• Shape-memory alloys are potential refrigerants that can release heat, move into a different state and later absorb heat to revert back to their original state.
• Alloys based on a high content of titanium display good compatibility in transitioning from one phase to another and are potential refrigerants.
• A research program has been initiated to evaluate shape-memory alloys as possible refrigerants.

Regulatory drivers have been prompting the refrigeration industry to seek new refrigerants. As noted in a recent TLT article, the Montreal Protocol in 1987 called for the banning of chlorinated fluorocarbons (CFCs) because they were found to be ozone-depleting substances (1).

But the replacement refrigerants, hydrofluorocarbons (HFCs) also have negative characteristics related to their contribution to global warming. This factor has now led to the use of hydrofluorinated olefins (HFOs) such as HFO-1234yf that are being recommended for use in mobile air conditioners.

With the concern in using liquid refrigerants, other options are being explored. In a previous TLT article, a metal alloy based on manganese, iron, phosphorus and germanium was recommended as a refrigerant that can be used in a process called magnetic refrigeration (2).

A successful refrigerant is able to release heat, move into a different state and then at a later stage absorb the heat to revert back to its original state. Shape-memory alloys prepared from specific metals have potential to display this characteristic.

Ichiro Takeuchi, professor of materials science and engineering at the University of Maryland in College Park, Md., says, “A shape-memory alloy can undergo a transition from the austenite phase at a higher temperature to the martensite phase at a lower phase upon cooling. This process represents a move from one crystalline phase to another. The shape of the alloy can then be manipulated by expanding or twisting. Upon heating, the alloy will revert back into the austenite phase.”

One of the most popular shape-memory alloys is nitinol, which contains equal percentages of nickel and titanium. Takeuchi says, “Nitinol is easy to make, very ductile and its composition can be readily controlled. This alloy also undergoes the phase transition near room temperature, which makes it very adaptable for many applications.”

Originally, Takeuchi and his research associates initiated work to improve the properties of nitinol as a stent for treatment of artery blockages. Nitinol suffers from fatigue problems that can cause parts to fracture. This means nitinol has difficulty retaining its shape memory after a finite number of cycles.

The need to improve the performance characteristics of nitinol led Takeuchi and his research associates to conduct a study to determine how varying the composition of the alloy affects the shape memory characteristics.

PHASE COMPATIBILITY
Nitinol’s negative fatigue properties have been linked to the hysteresis that occurs during the transition between phases. A theory has been proposed that indicates hysteresis can be minimized if the geometric compatibility between the austenite and martensite phases is improved.

Takeuchi says, “Our objective was to reduce the hysteresis to such a small extent that the transformation between phases is effortless. This will hopefully minimize fatigue and prevent the shape-memory alloy from prematurely fracturing.”

Takeuchi and his research associates conducted a series of experiments to verify this theory. He says, “We used a combinatorial approach to evaluate a large number of different shape-memory alloys based on nickel, titanium and other elements in a short period of time.”

An ultrahigh vacuum magnetron co-sputtering system was used to prepare the alloys evaluated. Three sputtering guns were used to spray out each of the metals onto 400-micron-thick silicon wafers.

Different alloys were prepared by adjusting the power applied to each gun and the distance between the guns and the wafer. The experimental setup used to prepare the alloys is shown in Figure 1.


Figure 1. The apparatus shown was used to develop smart alloys that may be able to function as refrigerants. A testing program has been initiated to evaluate the smart alloys. (Courtesy of the University of Maryland)

During the transformation process, the differences between the crystalline austenite and crystalline martensite phases must be closely examined. In particular, any interface energy generated leads to hysteresis. Takeuchi says, “We are looking for shape-memory alloys that have perfect compatibility and minimum interface energy in moving back and forth between the austenite and martensite phases.”

After running many experiments, the researchers were able to identify a region with a higher level of titanium content where shape-memory alloy performance characteristics are optimized. Some of the alloys discovered have potential to function as a refrigerant. Takeuchi says, “We see potential for using a nitinol-based alloy that can maximize the absorption and release of latent heat as it moves between the austenite and martensite phases.”

A research program has now been established to evaluate a nitinol-based alloy as refrigerants. Takeuchi says, “We have the expectation that the smart metal alloy can be much more efficient than a conventional compressor system. This should lead to considerable energy savings.”

The researchers estimate that cooling efficiencies could reach as high as 175% of the existing technology, and carbon dioxide emissions could be reduced by 250 million metric tons per year if this technology replaces liquid refrigerants.

The researchers are preparing to build a 0.1-ton prototype for the testing. Takeuchi says, “Two parameters we will be assessing are the coefficient of performance of the refrigeration cycle and the durability of the smart metal alloy.” 

Further information can be obtained from a past article describing the work done to prepare smart metal alloys with minimum hysteresis (3) or by contacting Takeuchi at takeuchi@umd.edu. 

REFERENCES
1. Canter, N. (2009), “Refrigeration Lubricants: Transitioning to New Refrigerants,” TLT, 65 (12), pp. 30–39.
2. Canter, N. (2009), “Magnetic Refrigeration: Another Way to Cool,” TLT, 65 (5), pp. 12–13.
3. Cui, J., Chu, Y., Famodu, O., Furuya, Y., Simpers, J., James, R., Ludwig, A., Thienhaus, S., Wuttig, M., Zhang, Z. and Takeuchi, I. (2006), “Combinatorial Search of Thermoelastic Shape-Memory Alloys with Extremely Small Hysteresis Width,” Nature Materials, 5 (4), pp. 286–290.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.