Compositional and microstructural evolution during annealing of Terfenol-D nanoparticulate films
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Michael F. Becker Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78731; and Department of Electrical and Computer Engineering, University of Texas at Austin, Austin, Texas 78731
John W. Keto Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78731; and Department of Physics, University of Texas at Austin, Austin, Texas 78731
Desiderio Kovarb) Materials Science and Engineering Program, University of Texas at Austin, Austin, Texas 78731; and Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78731 (Received 24 February 2011; accepted 19 July 2011)
Although highly magnetostrictive thin films of Terfenol-D have been produced by a variety of methods, high-quality thick films have proved to be far more challenging to produce. To date, thick film processes have resulted in nanoparticulate films that contain significant porosity that reduces stiffness and results in oxidation and poor magnetostrictive performance. With the goal of understanding microstructural and compositional factors that affect performance, nanoparticulate Terfenol-D thick films were produced by laser ablation of microparticle aerosols combined with supersonic impaction. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, x-ray photon spectroscopy, and magnetic measurements were performed on nanoparticles and on films as-deposited and after annealing in vacuum or in a reducing atmosphere. These measurements show that segregation occurs during oxidation of the films, prior to annealing, and results in films with poor magnetostriction. The segregation persists during annealing with no visible changes to the morphology or density of the nanoparticulate films exposed to temperatures as high as 800 °C. These results suggest that oxidation and segregation must be avoided to produce highly magnetostrictive thick films.
I. INTRODUCTION
Highly magnetostrictive thin films ( , 5 lm) of Terfenol-D have been successfully produced by sputtering,1 ion beam deposition,2 and Pulsed Laser Deposition (PLD).3 Thicker magnetostrictive films (5–100 lm) would be useful in many devices such as micropumps, microvalves, and microactuators. However, the few methods that have been reported produce particulate thick films with pore space between the active particles that are either unfilled4 or filled with an inactive polymer binder.5 Since magnetostriction is severely degraded by oxidation that occurs in open pores spaces,6,7 the use of such particulate films as actuators requires a viable approach for further densifying the films during a post-process annealing. Higher film density has the added advantage of increasing the displacement
and/or force capability for thick film actuators since the displacement/force capability increases with Young’s modulus of the films, which is in turn strongly dependent on relative density.8,9 In this article, microstructure, crystallization, composition, and magnetic properties are studied before and after annealing of nanop
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