Laser ablation of a terfenol-D (Tb₀.₃Dy₀.₇Fe₁.₉₂) microparticle aerosol and subsequent supersonic nanoparticle impaction for magnetostrictive thick films
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This dissertation describes using microparticles of the (giant) magnetostrictive material Terfenol-D (Tb0.3Dy0.7Fe1.92) in the Laser Ablation of Microparticle (LAM) aerosols process for the generation of nanoparticles and their subsequent supersonic impaction to form nanostructured magnetostrictive thick films. Solid Terfenol-D was ground to a powder having diameters from 0.3 to 3 µm. This microparticle powder was then aerosolized and ablated by a KrF ultraviolet, pulsed laser in a continuously flowing aerosol process. The nanoparticles formed from the ablation were then accelerated through a supersonic nozzle into vacuum where they impacted onto a substrate at room temperature forming a film. The nanoparticles were amorphous, as shown by x-ray diffraction analysis of the deposited films and by Transmission Electron Microscopy of individual particles, and had a size distribution typical of the LAM process: 3 to 20 nm in diameter with a mean size less than 10 nm. The deposited films were characterized using the cantilever method to determine magnetostriction and elastic modulus. Values of magnetostriction were on the order of 15 ppm for LAM deposited films. The films were porous, due to their granular nature, reducing the elastic modulus to about 15 GPa. The reduced magnetostriction (1/30 that of comparable thin films) was due to oxidation. Spectroscopic analysis of the ablation plasma provided data in determining the source of the oxidation. Calculations showed that the extent of oxidation in the films was dependent on the microparticle feedstock size. For typical aerosol densities used in the LAM process, calculations showed that material made from microparticles having a diameter larger than 3 µm was not significantly affected by background gas impurities or by an oxide shell on the microparticles, whereas 0.3 µm diameter microparticles resulted in completely oxidized nanoparticles and hence films that were completely oxidized. From the behavior of the deposited films, the aerosolized microparticles had a mixture of diameters in between these two cases.