Research On Micro Actuator Based On Vanadium Dioxide Structure Phase Change

Benefiting from the advance of science and technology,facilities become more compact and the devices scale down.Actuator is such a device which can outputs mechanical energies and motion with input of other forms of energies,thermal,electrical or optical energies.Micro-actuator that are mass-manufactured,compatible to aqueous environment,repeatable and of remote control is highly desired to manipulate and move tinny things like nano-device,biological tissues or medicine in many fields.Novel and advanced nano-materials open opportunity for fabricating the micro-actuator with new strategy of design.Vanadium dioxide（VO₂）is an electron-correlated transition metal oxide with unique metal-insulator transition（MIT）.Increasing the temperature up to 68℃or triggered by electrical or optical stimuli,the VO₂ undergoes insulator to metal phase transition as well as structural phase transition from monoclinic to rutile.The phase transition produces 1%size change and is also accompanied by abrupt changes in electric resistance and transmission/reflectivity of infrared light.In this thesis,I design and fabricate two different types of micro-actuators,Cr-VO₂ and SiO₂-Au-VO₂ with high quality single crystal VO₂ nanowires.Their actuation performances are systematically investigated.The research work includes:（1）Single crystal VO₂ nanowires are epitaxially and uniformly grown on r-cut sapphire substrate of 1.2 cm² using chemical vapor deposition（CVD）method.The nanowires are vertically aligned with the average diameter of～400 nm and the average length of 15μm.Phase transition of the nanowires is abrupt at 68°C.Patterning the nanowires is realized by pre-deposition of patterned Pt film of 20nm thick on sapphire substrate.The effects of substrate（Si,r-cut/c-cut sapphire）,vanadium source（VO₂,V₂O₅）and the location separation between the source powder and the substrate on the nanowire morphology are verified by parallel experiments.（2）Two bent bimorph structures,Cr and VO₂,SiO₂ and VO₂,are fabricated as,Cr-VO₂ and SiO₂-Au-VO₂ micro-actuators which output mechanical force through bending and relaxing of the bimorphs during phase transition of VO₂.Based on the first-order phase transition theory and the bending model of bimorph,the bent curvature as well as the actuation temperature are designed quantitatively by thickness of Cr layer.The electron beam effect for tailoring the Young’s modulus of amorphous SiO₂ and thus the curvature of SiO₂-Au-VO₂bimorph is experimentally identified.（3）The micro-actuator arrays are actuated by global heating,induction effect and laser projection.Laser projection is site-specific with a spot size of～2μm to activate micro-actuators locally for moving the desired object.Actuation of Cr-VO₂in a magnetic field by induction effect is remote and wireless.Moving silica micro-spheres in water with Cr-VO₂micro-actuator arrays is demonstrated.