The Magnetocaloric and Elastocaloric Effects in Magnetic Shape Memory Alloys
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The present work focuses on the microstructural and magneto-thermo-mechanical characterization of off-stoichiometric meta-magnetic shape memory alloys (MMSMAs) with the aim of identifying key material parameters to optimize their giant magneto- (MCE) and elastocaloric (ECE) effects. Several alloy compositions of NiFeGa, Ni(Co)MnIn, and Ni(Co)MnSn have been studied to quantify their solid state energy conversion performance, in particular the conversion of magnetic and mechanical energy into thermal energy, and to reveal how this performance is influenced by microstructural features tailored using carefully selected heat treatments. To identify how heat treatments influence the energy conversion performance of the selected alloys, a previously established thermodynamic framework, which defines the refrigerant capacity (RC) of non-shape memory alloys (SMAs), was employed with the thermodynamic relations describing first order martensitic phase transitions. Applying the RC framework to MMSMAs demonstrated that most of the key materials parameters relating to the martensitic transition can be combined to predict the magnetic field, or mechanical stress, needed to complete the transformation across a specific operating temperature range; reducing these driving force requirements was the primary focus of the experimental part of this work. Magnetometry, calorimetry, and specialized compression tests were performed on the heat treated SMAs. An experimental apparatus, called the magneto-thermo-mechanical characterization (MaTMeCh) device, was designed, constructed, and implemented to reduce the magnetic field requirements needed for a complete field-induced martensitic transformation by simultaneous application of external stress and magnetic field. This device allows simultaneous control of temperature, magnetic field, and stress while measuring magnetization, strain, stress, and temperature. With the MaTMeCh device, uniaxial compressive stresses up to 200MPa, magnetic fields up to 9T, and temperatures between -100°C and 80°C can be applied to a compression specimen. Using the MaTMeCh device, cyclic stress-assisted magnetic field induced transitions demonstrated the full thermal energy conversion capabilities of cost effective meta-magnetic shape memory alloys. Prior to magneto-thermo-mechanical characterization, numerous SMA compositions were fabricated and heat treated to promote grain growth and a specific degree of long range crystallographic order. Studies, herein, indicated that minimizing the microstructural grain constraint, by producing a large grain size to thickness (GS/t) ratio in polycrystalline SMA ribbons, ultimately reduced the magnetic field levels needed to completely harness the SMA’s magnetocaloric effect. Additionally, B2 crystallographic ordering in NiCoMnIn single crystals was found to offer a more efficient magnetic to thermal energy conversion efficiency than the typical L21 ordered alloys. Larger caloric effects were measured in the alloys exhibiting a higher martensitic transformation temperature, i.e. the B2 ordered alloys. As such, B2 ordered single crystals were characterized with the MaTMeCh device, thus lending the ability to measure their maximal magnetocaloric performance.
Bruno, Nickolaus Mark (2015). The Magnetocaloric and Elastocaloric Effects in Magnetic Shape Memory Alloys. Doctoral dissertation, Texas A & M University. Available electronically from