Characterization and improvement of magnetostrictive composites

The recent discovery of giant magnetostrictive materials is enabling the design of new and improved actuators. Monolithic magnetostrictive materials, however, possess a low operating frequency (< 1kHz), due to eddy current losses. Magnetostrictive composites incorporating magnetostrictive particulate in an insulating matrix reduce eddy current losses enabling the use of magnetostrictive materials in high frequency applications (e.g SONAR transducers). This dissertation focuses on the characterization and improvement of magnetostrictive composites. The first part involves quasi-static and high frequency characterization of Terfenol-D composites while the second part involves the improvement of magnetostrictive composites using first nickel and then Gd5Si2Ge 2 as the magnetostrictive particulate. The purpose of the quasistatic testing was to evaluate the behavior of magnetostrictive composites under combined magnetic, thermal, and mechanical loading, and to determine fundamental properties used for design of sonar transducers that incorporate these materials. The purpose of the high frequency testing was to determine the dynamic response of magnetostrictive composites. Results indicate that composite properties, as pertaining to SONAR transducers, are comparable to monolithic Terfenol-D while reducing brittleness, providing higher operational frequencies (up to 100 kHz) and easier manufacturability. The second part of the dissertation involves studies to improve magnetostrictive composites. In the first study hollow and solid spherical nickel composites were manufactured in an attempt to prove the feasibility of using demagnetizing fields to align spherical particles along an easy axis. This is important because magnetostrictive composites using spherical Terfenol-D particles aligned along the easy axis (i.e. the [111] axis) would possess a larger magnetostriction than current Terfenol-D composites. Magnetostriction measurements of the nickel composites didn't conclusively show an alignment of nickel particles along the easy axis but they did demonstrate the feasibility of manufacturing magnetostrictive composites using hollow microspheres. In the second study a magnetostrictive composite containing Gd5Si2Ge2 particulate was manufactured and characterized. The reason for using Gd5Si2Ge 2 as the magnetostrictive particulate is that it possesses a magnetostriction an order of magnitude higher than Terfenol-D. Strain measurements of the composites were found to be significantly lower than those for bulk Gd5Si2Ge2, however, strains were still greater than those measured in Terfenol-D composites.