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Graduate Defense: Andrew Armstrong

July 30 @ 8:00 am - 10:00 am MDT

Dissertation Information

Title: Material Design, Processing, and Engineering Requirements for Magnetic Shape Memory Devices

Program: Doctor of Philosophy in Materials Science and Engineering

Advisor: Dr. Peter Müllner, Materials Science and Engineering

Committee Members: Dr. Paul Simmonds, Materials Science and Engineering, Dr. Joseph Guarino, Mechanical and Biomedical Engineering, and Dr. Ladislav Straka, Materials Science and Engineering

Abstract

For magnetic shape memory (MSM) alloys, a magnetic field stimulates a shape change. We use the shape change to build devices such as micro-actuators, sensors, and microfluidic pumps. Currently, devices suffer from material and magnetic driver shortcomings, making the devices bulky, unreliable, unrepeatable, and prone to rapid failure. Here we address these shortcomings. To increase the operating temperature of the MSM material, we alloyed Fe and Cu to Ni-Mn-Ga. We showed that the element-specific contribution to the valence electron density as parameter systematically determines the effect of each element on the variation of the martensite transformation temperature of the 10M phase. To stabilize the material, we developed a micro-shotpeening process that adds stresses to the material surface, thereby inducing a fine twin microstructure. The treatment allowed nearly full magnetic-field-induced strain, and extended fatigue life of the material from only one thousand cycles in the electropolished state to more than one million cycles in the peened state. We measured the effect of the peening process on material actuation when in MSM pump configuration. In the polished state, the deformation was stochastic, with a sharp-featured, faceted shrinkage. In the treated state, the deformation was smooth and repeatably swept along the surface akin to a wave.

To actuate the MSM micropump without electromotor, we developed a linear electromagnetic actuation device and evaluated its effectiveness in the switching mechanism of the material. By compressing the magnetic field between opposing coils, we generated a strong magnetic field, which caused a localized region to switch at selected poles. In the next iteration of the drive, we inserted the MSM sample between two linear pole arrangements of high pitch density to approximate a moving vertical field. The incremental stepping of the vertical field between poles caused translation of the switched region. The results of this dissertation demonstrate the suitability of MSM alloys for high-precision, persistent, and reliable actuators such as micropumps.