On November 29th, Beijing time, "Science" published an online collaborative paper "Fatigue-resistant High-performance Elastic Thermal Refrigeration Materials Made by Additive Manufacturing" with Associate Professor Qian Suxin from Xi'an Jiaotong University School of Energy and Department of Materials Science and Engineering at the University of Maryland.
Elastothermal refrigeration is a new solid-state refrigeration technology that utilizes a nickel-titanium shape memory alloy to undergo a reversible phase change under axial tension, compression, and torsion, and utilizes this phase-change latent heat refrigeration. Compared with traditional vapor compression refrigeration working fluids, elastic thermal refrigeration working fluids such as nickel-titanium shape memory alloy do not have any greenhouse gas effect, and have the advantage of environmental protection. In addition, the energy density of the elastothermal effect is significant, ranking first in the 17 non-vapor compression refrigeration technologies evaluated by the US Department of Energy in 2014. At present, the fatigue life of elastic thermal working medium is the primary factor restricting its engineering application, and the efficiency of elastic thermal working medium and elastic thermal refrigerator still needs to be improved.
To this end, the University of Maryland, Arms Laboratory, Colorado School of Mines, Xi’an Jiaotong University, and Iowa State University collaborated to focus on the two key issues of improving efficiency and fatigue life. Using powder laser directed energy deposition technology, we prepared Nano-composite nickel-titanium alloy materials can be directly shaped into columnar, tubular, honeycomb, etc. structures that can be used for elastic heat refrigeration regenerators. Different from the shape memory alloy of traditional melting casting process, the experiment found that the additive-made nickel-titanium alloy has a quasi-linear stress-strain response, hysteresis characteristics independent of loading rate, and significantly reduced phase transition hysteresis. The advanced in-situ synchronous X-ray diffraction and electron microscopy characterization show that the cause of the low phase transition hysteresis is that the interface dislocation generated at the nano-scale Ni3Ti and NiTi grain boundaries can become the nucleation point of the phase transition process, and the effective reduction of the phase transition process needs to be overcome Potential barrier and reduce the friction loss at the phase interface. The above characteristics can improve the fatigue life, so that the elasto-thermal effect remains unchanged after millions of loading and unloading cycles.
In addition, the author Qian Suxin also carried out theoretical analysis and research on the thermodynamic characteristics of additive manufacturing of nickel-titanium alloy and its application in elastic thermal refrigeration devices. The additive-made hysteresis characteristics of nickel-titanium alloy independent of loading rate make it possible to effectively use the Brayton regenerative refrigeration cycle based on adiabatic phase transition, without the need to run Stirling based on isothermal phase transition at the expense of operating frequency and cooling power cycle. At the same time, the quasi-linear stress-strain characteristics increase the importance of the unloading work recovery design in the elastic heat refrigerator. Considering that the unloading work recovery technology in existing elastic thermal refrigeration devices is mature, under the condition that the unloading work can be fully recovered, taking the standard working conditions of GB / T7725 air conditioner as an example, the thermal perfection of the additive manufacturing nickel-titanium alloy sample reaches 60%, which is close to 4 times the thermal perfection of traditional fused cast nickel-titanium alloy materials.