Sensing shape recovery using conductivity noise in thin films of NiTi shape memory alloys
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1129-V13-01
Sensing shape recovery using conductivity noise in thin films of NiTi shape memory alloys U. Chandni1, M. V. Manjula1, Arindam Ghosh1, H. S. Vijaya2 and S. Mohan2 1
Department of Physics, Indian Institute of Science, Bangalore 560 012, India.
2
Department of Instrumentation, Indian Institute of Science, Bangalore 560 012, India.
ABSTRACT Low frequency fluctuations in the electrical resistivity, or noise, have been used as a sensitive tool to probe into the temperature driven martensite transition in dc magnetron sputtered thin films of nickel titanium shape-memory alloys. Even in the equilibrium or static case, the noise magnitude was more than nine orders of magnitude larger than conventional metallic thin films and had a characteristic dependence on temperature. We observe that the noise while the temperature is being ramped is far larger as compared to the equilibrium noise indicating the sensitivity of electrical resistivity to the nucleation and propagation of domains during the shape recovery. Further, the higher order statistics suggests the existence of long range correlations during the transition. This new characterization is based on the kinetics of disorder in the system and separate from existing techniques and can be integrated to many device applications of shape memory alloys for in-situ shape recovery sensing.
INTRODUCTION Near equiatomic nickel titanium (NiTi) shape memory alloys have gained considerable industrial focus owing to its varied applications including micro-actuators, positioners, micro-valves and so on [1]. In the thin film form, the displacive structural transformation from the high temperature cubic (B2:CsCl) austenite phase to the low temperature monoclinic (B19/19’) martensite phase is often influenced by external parameters such as the substrate, finite grain size, surface defects etc. The information content from the standard characterization schemes such as x-ray diffraction (XRD), differential scanning calorimetry (DSC), resistivity measurements, acoustic emission etc. is often limited and there is a need to have microscopically intuitive estimates of the temperature scales of martensite transformation that may be relevant to growing applications of NiTi thin films. Here, we report two different aspects of the conductivity noise measurements on dc-magnetron sputtered NiTi thin films [2]: (a) equilibrium resistance noise that arises from the coupling of electron transport to slow variations in the structural disorder, which was found to be sensitive to instantaneous bi-crystalline topology during the thermoelastic martensite transition; (b) the dynamic noise and higher order avalanche statistics while the system is being temperature-driven which serves as an excellent probe to the resistivity avalanches that correspond to the athermal martensitic phase transition.
EXPERIMENTAL DETAILS The samples were prepared by dc magnetron sputtering of a mosaic target, which consists of a patterned titanium disk of 76mm diameter and 0.8mm thickness laminated over a circul
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