Development of Small-Volume, High-Precision, and Reliable Cryogenic Linear Actuators by Using Novel Intermetallic Compounds

Cryogenic actuator technologies are critical to achieve a high-precision mechanical motion of moving components of space instruments under extremely cold environments such as deep space or the shadow area of Moon. In order to make a technological breakthrough in cryogenic actuator technologies, it is necessary to discover, evaluate, and use advanced actuator materials that can outperform a conventional actuator material, such as shape memory alloys or piezo-electric materials. Therefore, we have investigated mechanical behaviors of ThCr2Si2-structured intermetallic compounds that has exhibited a strong potential as an excellent cryogenic actuator material. 
In order to measure mechanical properties at low temperature, we have built in-situ cryogenic micromechanical tester by combining cryostat, micromechanical testing stage, and scanning electron microscope. This state-of-the-art system allows us to measure force-displacement data of micrometer-size samples at 40~298 K in ultrahigh vacuum chamber, which provides the space-like environment. Micromechanical tests on various ThCr2Si2-structured intermetallic compounds confirmed the presence of a thermal actuation capability via making and breaking chemical bonds, which lead to the significant change in length of material. For instance, CaFe2As2 is able to exhibit ~5% of linear thermal actuation strain around 40 K. This solid-state bonding mechanism does not accumulate any mechanical damages during the repeated actuation cycles because this process is almost identical to simple bond stretching and contraction of linear elastic deformation. Thus, ThCr2Si2-structured intermetallic compounds are expected to exhibit the near-infinite fatigue resistance, which has never been observed in conventional actuator materials. In addition, for CaFe2As2, we have discovered the tunability of its thermal actuation temperature through microstructural control. Introduction of FeAs embryo structures via quenching from a high temperature (960oC) develops the compressive residual stress in CaFe2As2matrix and reduces the As-As distance. If the residual stress is high enough, it is possible to obtain thermal actuation even without applied stress because thermal contraction is sufficient enough to induce the As-As bond formation. This stress-free actuation allows the simpler design of cryogenic actuator system. 
We confirmed that other compounds, for instance, LaRu2P2, CaKFe4As4, and SrNi2P2, also exhibit the similar process of making and breaking chemical bonds. Recent computational studies predict the presence of over 2500 ThCr2Si2-structured intermetallic compounds. Therefore, there should be great opportunities to discover a group of new actuator materials. The study on these exciting material systems will eventually lead to material innovation that enables the breakthrough in cryogenic actuator technologies.