Vapour phase growth and nanomechanical behaviour of VO2 microcrystals across phase transition (PhD)

Abstract

Phase change materials are playing an increasingly important role in small scale mechanical devices. My dissertation addressed the question of how temperature affects the mechanical behaviour of Vanadium Dioxide across the structural transition. I have also formulated various nanoindentation testing techniques to study the mechanical behaviour of various microscale structures. Considering the multitude of application of VO2 in micro electro mechanical devices, the immediate objective of this research project was to measure the elastic modulus and hardness of VO2 across the phase transition. Brief findings of consistent work done over last four years is given as follows: Coexisting phases in VO2 are identified using Raman, optical imaging and electrical measurements. Site specific in situ nanoindentation confirm the abrupt increase in elastic modulus (~ 17 GPa) and nanohardness (1 GPa) across the transition from monoclinic (insulator) to rutile (metal) phase. Electron back scattered diffraction (EBSD) analysis has been used to confirm the [100] orientation of monoclinic VO2 crystals selected for nanomechanical actuation measurement. The measured spontaneous strain across phase transition is found to be reversible and demonstrates that VO2 micro crystals can be used to make highly efficient thermal actuators. Bending behaviour study of VO2 microcantilever show higher stiffness for metallic phase of VO2. Spring constant of the microcantilever was found to be increased by a factor of 1.3 during the structural phase transition of VO2 from monoclinic to rutile structures. Growth of VO2 platelets has been successfully achieved for IR camouflage applications. Switchable friction behaviour has been demonstrated for two phases of VO2 and may have applications in gripping devices. Fracture toughness measurement is done using nanoindentation tests and high toughness value for metallic phase of VO2 is observed as compared to insulating phase. Overall experiments on single crystalline VO2 led to a mechanistic understanding of mechanical behaviour of VO2 suitable for micro-actuators, cantilevers, grasping and climbing robotic systems and IR modulation devices