Effects of surface finish and mechanical training on Ni-Ti sheets for elastocaloric cooling

Abstract

Elastocaloric cooling has emerged as a promising alternative to vapor compression in recent years. Although the technology has the potential to be more efficient than current technologies, there are many technical challenges that must be overcome to realize devices with high performance and acceptable durability. We study the effects of surface finish and training techniques on dog bone shaped polycrystalline samples of NiTi. The fatigue life of several samples with four different surface finishes was measured and it was shown that a smooth surface, especially at the edges, greatly improved fatigue life. The effects of training both on the structure of the materials and the thermal response to an applied strain was studied. The load profile for the first few cycles was shown to change the thermal response to strain, the structure of the material at failure while the final structure of the material was weakly influenced by the surface finish.

Ultrahigh cyclability of a large elastocaloric effect in multiferroic phase-transforming materials

“…Meanwhile, the field-induced adiabatic temperature change T ad is an important parameter quantifying the caloric effect. Figure 5 demonstrates the comparison of the cyclability of elastocaloric effect and T ad of the alloys designed here with other elastocaloric materials in the polycrystalline form [4][5][6]16,[19][20][21][22][23]. As compared to other materials except Ti-Ni-based alloys, the present (Ni 51 Mn 33 In 14 [1].…”

Elastocaloric Cooling on the Miniature Scale: A Review on Materials and Device Engineering

“…Asaresult, carbide inclusions and voids acting as nucleation centres for cracksc an be avoided. [35] Mechanical-or electropolishing on the other hand smooths sharp edges.T his results in am ore homogenous stress distribution at the edges [38] and thus ah igher lifetime.F reestanding magnetrons putteredf ilms as shown in Figure 2a possess as harp edge,l eading in Ti-rich TiNiCuCo alloys to ar ather poor fatigue life due to the brittleness of these materials. Electropolishing (Figure2b) enables the same material to withstand 10 7 cycles due to negligible functional fatigue.…”

“…Mechanical‐ or electropolishing on the other hand smooths sharp edges. This results in a more homogenous stress distribution at the edges and thus a higher lifetime. Freestanding magnetron sputtered films as shown in Figure a possess a sharp edge, leading in Ti‐rich TiNiCuCo alloys to a rather poor fatigue life due to the brittleness of these materials.…”

NiTi‐Based Elastocaloric Cooling on the Macroscale: From Basic Concepts to Realization

“…Therefore, the wire fixtures are insulated allowing resistance and impedance measurement to observe the electric parameters to monitor material stabilization during training and for early detection of material failure. Engelbrecht et al . showed that the surface finish of NiTi sheets has a strong influence on their mean time to failure during cyclic tensile tests.…”

“…Af urther aspect to be investigated by using this test bench is in situ conditionm onitoring of the SMA wire.T herefore, the wire fixtures are insulated allowing resistance and impedance measurementt oo bserve the electric parameters to monitor material stabilization during training [114] and for early detection of material failure.E ngelbrecht et al [117] showed that the surface finish of NiTis heets has as trong influence on their mean time to failure during cyclic tensile tests.T his is ap rocess we want to monitor in future experimentsb yi mpedance and resistance measurements with the help of the electric skin-effect.T his effect occurs when applying ah igh-frequency alternating current on conductive material (in this case represented by the SMA) and causes ah igh current density at the surface of the SMA, which should make it possible to detects urfacec racks in the high-frequency impedance signal.…”