Synthesis, Structural Characterization, and Transport Properties of Metastable Phases in the Mg-Sn System

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2016
Dr. Javier Garay Chairperson

Mg-Sn  alloys  have  many  uses  due  to  their  relative  elemental  abundance,  low  toxicity and  low  densities.  Some  applications  include  thermoelectric  energy  conversion,  hydrogen storage,  and  enhancement  of  structural  alloys.  The  thermoelectric  application  is  of  particular interest because Mg-Sn-Si based materials have shown promise in recent years.

It  is  well  accepted  that  materials  structure-property  relationships  are  at  the  root  of  the improvement of a device’s performance. Due to the interdependence of the transport properties, research  for  efficient  thermoelectric  conversion  has  focused  on  synthesizing  materials  with novel microstructures and compositions. Metastable structures present a unique opportunity in this  regard.  The  Mg-Sn  material  system  has  a  relatively  unknown  metastable  phase  which remains largely unstudied.

My  work  represents  one  of  the  first  studies  of  the  metastable  Mg-Sn  phase  in  a polycrystalline bulk form. I present a new synthesis route using a combination of high energy ball milling (powder synthesis) and current activated pressure assisted densification (CAPAD) (powder densification). This method allowed for the synthesis of the metastable trigonal phase at  600  °C  and  112  MPa,  significantly  lower  pressures  and  temperatures  than  have  been demonstrated  previously.  This  method  produces  samples  large  enough  for  the  first  neutron diffraction study allowing for Rietveld structural refinements to a high degree of accuracy.

Through careful control of the synthesis process, I have also studied and characterized the  densification  and  transformation  kinetics  for  the  trigonal  phase.  Analysis  of  the  real  time deformation data during CAPAD processing reveals that the transformation mechanisms can be isolated from the densification mechanisms. The transformation process is analyzed using the KJMA  model  modified  for  constant  heating  and  shows  an  activation  energy  of  74  kJ/mol. Transformation  under  isothermal  conditions  follows  a  second  order  rate  law  with  a  higher activation energy (448 kJ/mol) caused by differing degrees of transformation completion.

The amount of metastable phase can be  controlled by  varying the CAPAD processing parameters.  Incorporating  the  metastable  phase  into  the  microstructure  changes  the  thermal, electrical and Seebeck coefficient behavior. The first measurements of this trigonal phase show bipolar  diffusion  behavior  at  much  lower  temperatures  than  the  previously  reported  Mg-Sn based materials without the metastable trigonal phase.