Anomalous thermal and elastic properties of an epitaxial NiTi film exhibiting R-phase

The study focuses on NiTi, a Shape-Memory Alloy, because its phase transformations can be used for thermal management. The authors note that film-scale measurements are difficult, so they use transient grating spectroscopy (TGS) to characterize thermal diffusivity and elastic coefficients. They perform in-situ measurements while the film undergoes a thermally induced transformation sequence: austenite → R-phase → martensite → austenite. The results show a crossover in shear moduli between austenite and martensite, and a large (450%) change in thermal diffusivity between the R-phase and austenite. The authors report that there is no hysteresis between the R-phase and austenite, and they argue that the thermal diffusivity change is linked to an anomalous heat capacity of the R-phase. They also state that the temperature-dependent properties they measure can be used for simulating thermal management microsystems.