Please give a lot of attention and participation from the members of the Mechanical Engineering department.

▣ 주   제: Superelasticity of ThCr2Si2-Structured Intermetallic Compounds: Making and Breaking Bonds in the Solid State

▣ 연   사: Seok-Woo Lee 교수

▣ 소   속: University of Connecticut

▣ 일   시: 2024. 7. 9.(Tue) 11:00

▣ 장   소: 제4공학관 D602호

▣ 초   청: 강건욱 교수

▣ 초   록

Elastic strain limit, the measure of the maximum elastic deformability, of most crystalline solids is less than one percent because plastic deformation or fracture usually occurs at very small strain. To obtain a large elastic strain limit, a crystalline material needs to undergo a reversible structural transition.

In this presentation, a new class of superelastic materials, ThCr2Si2-structured intermetallic compounds (CaFe2As2, (CaK)Fe4As4, LaRu2P2, SrNi2P2), which undergo a unique reversible structural transition, lattice collapse-expansion, will be introduced. Under uni-axial compression along c-axis, Si-Si type bonds are formed, leading to the lattice collapse with more than 10% decrease in length. Once the applied stress is relaxed, thermal vibration breaks Si-Si type bonds, and the lattice expands back to the original length. This making and breaking bond process can induce the giant elastic strain limit up to 17%. If the temperature is low enough (below 40K), Si-Si type bonds can be maintained even without applied stress, leading to cryogenic shape memory effect. It is also possible to induce the thermal actuation if the residual compressive stress is developed by adding nanoscale precipitation. Some ThCr2Si2-structured intermetallic compounds are high temperature superconductor, and their superconductivity can be switched on and off through lattice collapse-expansion process. SrNi2P2 is special because it exhibits superelasticity under both compression and tension because only the fraction of P atoms in SrNi2P2 are bonded at the stress-free state. C-axis compression forms a chemical bond between unbonded P atoms (2/3 of P atoms) and causes lattice collapse while c-axis tension breaks a chemical bond between bonded P atoms (1/3 of P atoms) and causes lattice expansion. As a result, SrNi2P2 exhibits tension-compression asymmetry in mechanical response, and this asymmetry leads to the elastocaloric effect comparable with conventional shape memory alloys such as Nitinol.