Skip to Main Content
Mobile Menu

Master’s & Doctoral Defenses

upcoming defensesThe Public presentation portion of a defense is open to everyone and is an especially valuable opportunity for graduate students to experience the process firsthand.

Note: All information is provided by the academic units.

Anthony Hobza

Title: Effects of the Internal Magnetic Field on the Magneto-Mechanics of Magnetic Shape Memory Alloys
Program: Doctor of Philosophy in Materials Science and Engineering
Advisor: Dr. Peter Müllner, Materials Science and Engineering
Committee: Dr. Lan Li, Materials Science and Engineering, Dr. Charles B. Hanna, Physics, and Dr. Carlos Garcia-Cervera, Materials Science and Engineering
Date: July 26, 2017
Time: 3:30 p.m.
Location: Micron Engineering Complex, Room 106

Read Anthony Hobza's Abstract Here

Magnetic shape memory alloys (MSMAs) such as Ni-Mn-Ga deform under mechanical stress or in a magnetic field with strain of up to 12% at frequencies in the kHz range. This means MSMAs can deform dynamically in a variable magnetic field at frequencies orders of magnitude higher than typical shape memory alloys or pneumatic motors. Coupling of magnetic and mechanical properties allows applications not only in actuation but also in strain sensing and energy harvesting.

Ni-Mn-Ga has a non-cubic crystal lattice with large magnetocrystalline anisotropy energy; the magnetic moments align in a preferred lattice direction. Twin boundaries separate domains with different orientations of the crystal lattice and axis of easy magnetization. An external magnetic field perpendicular to the east axis results in a stress. Application of stress magnetically or mechanically moves twin boundaries. Twin boundaries move to grow the energetically preferred domain, resulting in a macroscopic shape change and/or larger magnetization.

The contribution of the internal field energy varies widely due to sample geometry and twin microstructure. Typically, the effects of the internal field are ignored or oversimplified when studying the magneto-mechanics of bulk single crystals of Ni-Mn-Ga. The effects of the internal energy and dependence on twin microstructure will be explored experimentally and with numerical methods for three sets of experiments. In the first set of experiments, we consider the torque exerted by a magnetic field on a Ni Mn Ga bar. We find that the torque substantially depends on the specific arrangement of twin domains. In the second series of experiments, we explore using Ni-Mn-Ga as a strain transducer. We find a non-linear, hysteretic strain-voltage relationship. The third set of experiments considers the transformation of mechanical work into electrical energy. For all three cases, the experiments cannot be explained by a simple approach where the total magnetization of the sample is given by the fraction of each twin domain. The results depend on twin microstructure which modulates the internal magnetic field.

This study demonstrates a large dependence of the torque and magnetic energies on the twin microstructure. Thus, the design of MSMA-based actuators and sensors must include design of the twin microstructure and a strategy to maintain this twin microstructure over the duration of the operation of the device.

Andrew Poley

Title: Deriving Landscape-Scale Vegetation Cover and Above Ground Biomass in Semi-Arid Ecosystems Using Imaging Spectroscopy
Program: Master of Science in Geophysics
Advisor: Dr. Nancy Glenn, Geosciences
Committee: Dr. Jennifer Forbey, Biological Sciences,  Dr. Dylan Mikesell, Geosciences, and Dr. Jennifer Pierce, Geosciences
Date: July 18, 2017
Time: 10:00 a.m.
Location: Multipurpose Classroom Building – Room 203

Read Andrew Poley's Abstract Here

Recent environmental disturbances in semi-arid ecosystems caused by climate changes have highlighted the need to monitor current and future vegetation conditions across the landscape. Imaging spectroscopy and remote sensing provides the necessary information to derive vegetation characteristics at high-spatial resolutions over large areas. The work of this thesis is divided into two sections focused on imaging spectroscopy (remote sensing) to classify vegetation, estimate vegetation cover, and approximate above-ground biomass in semi-arid ecosystems.

The first half of this thesis assesses the ability of imaging spectroscopy to derive vegetation classes and their respective cover across large environmental gradients and ecotones often associated with semi-arid ecosystems. Optimal endmember selection and endmember bundling are coupled with classification and spectral unmixing techniques to derive species of vegetation and vegetation abundancies across Reynolds Creek Experimental Watershed (RCEW) in Southwest Idaho at high spatial resolution (1 m). Results validated using field data indicated classification of aspen, Douglas fir, juniper, and riparian classes had an overall accuracy of 57.9% and a kappa coefficient of 0.43. Plant functional type classification, consisting of deciduous and evergreen trees, had an overall accuracy of 84.4% and a kappa coefficient of 0.68. Shrub, grass, and soil cover were predicted with an overall accuracy of 67.4% and kappa coefficient of 0.53. I conclude that high-spatial resolution of vegetation communities in semi-arid ecosystems across large environmental gradients can be achieved using imaging spectroscopy with high accuracy.

The second half of this thesis focuses of monitoring the changes of above-ground biomass (AGB) from the 2015 Soda Fire, which burned portions of southwest Idaho and southeastern Oregon. Classifications derived in the first study are used to estimate AGB loss within a portion of RCEW, and these estimates are used to compare to gross estimates made over the full extent of the Soda Fire. There was an AGB loss of 241M kg within RCEW and approximately 3.6B kg lost over the full extent of the Soda Fire. Additionally, a post-fire analysis was performed to provide insight into the amount of AGB that returned to both RCEW and the full extent of the Soda Fire. An estimated 2,100 – 208,000 kg of AGB had returned to the burned portion of RCEW one-year post fire, and approximately 3.2M kg of AGB had returned over the full extent of the Soda Fire. These AGB loss and re-growth estimates can be used be researchers to monitor carbon fluxes caused by wildfires in semi-arid ecosystems.