A. Schematic diagram of powder laser directed energy deposition technology;

B. Nitinol phase diagram;

C-H. As an example, columnar, tubular and honeycomb structures were obtained;

F. Comparison of behavior of materials under isothermal loading;

G. Comparison of the behavior of various materials under adiabatic loading;

H. Comparison of hysteresis and efficiency of various materials

On November 29, Beijing time, "Science" published online a co-authored paper entitled "Anti-fatigue and High-performance Elastothermal Refrigeration Materials Made of Additive Manufacturing" by Associate Professor Qian Suxin of Xi'an Jiaotong University School of Energy and Department of Materials Science and Engineering.

Elastothermal refrigeration is a new solid-state refrigeration technology, which uses Ni Ti shape memory alloy to undergo reversible phase transformation under axial tension, compression and torsion. Compared with traditional vapor compression refrigerants, Ni Ti shape memory alloy and other elastothermal refrigerants have no greenhouse gas effect and have the advantage of environmental protection. In addition, the energy density of the elastothermal effect is significant, ranking first in 17 non vapor compression refrigeration technologies evaluated by the U.S. Department of energy in 2014. At present, the fatigue life of the bomb thermal mass is the primary factor restricting its engineering application, and the efficiency of the bomb thermal mass and the bomb thermal refrigerator still needs to be improved.

Therefore, the University of Maryland, Amsterdam laboratories, Colorado University of mining, Xi'an Jiaotong University and Iowa State University cooperated to focus on the two key issues of improving efficiency and fatigue life. Using powder laser directed energy deposition technology, Ni Ti alloy with nano composite structure was prepared, which can be directly formed into columnar, tubular, honeycomb and other materials for bomb heating Structure of the cold regenerator. Different from the traditional shape memory alloy, the results show that the alloy has quasi linear stress-strain response, hysteresis independent of loading rate and significantly reduced phase transformation hysteresis. The advanced in-situ loading synchronous X-ray diffraction and electron microscopy show that the reason for the low phase transition hysteresis is that the interface dislocation generated by the nano Ni3Ti and NiTi boundaries can become the nucleation point of the phase transition process, effectively reduce the potential barrier to be overcome in the phase transition process, and reduce the friction loss of the phase interface. The above characteristics can improve the fatigue life, so that the elastothermal effect does not change significantly after a million loading and unloading cycles.

In addition, the author Qian Suxin and so on have also carried out theoretical analysis and Research on the thermodynamic characteristics of nitinol alloy made from additive and its application in the thermoelastic refrigeration device. The hysteresis characteristic of Ni Ti alloy, which is independent of the loading rate, makes it possible to use Brayton regenerative refrigeration cycle based on adiabatic phase change effectively without sacrificing the operating frequency and cooling power. At the same time, the quasi linear stress-strain characteristics improve the importance of the design of unloading power recovery in the elastothermal refrigerator. Considering that the unloading power recovery technology in the existing elasto thermal refrigeration devices has been mature, considering that the unloading power can be fully recovered, taking GB / T7725 air conditioning standard working condition as an example, the thermal perfection of the samples of nitinol alloy made by the additive is 60%, which is close to 4 times of the thermal perfection of the traditional melting and casting nitinol alloy materials.

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