Grain boundary (GB) studies in nano-and micro-crystalline materials

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2011
Dr. Javier E. Garay, Graduate Advisor

Polycrystalline  materials  are  composed  of  grains  and  grain  boundaries.    The  total  volume  of  occupied  grain boundaries in polycrystalline material depends on the grain size. When grain size decreases the volume fraction of grain boundaries increases. For example, when grain size is 10 nm grain boundary volume fraction is ~ 25%. In polycrystalline materials, different properties (mechanical, electrical, optical, magnetic)   are affected by the size of their grains and by the atomic structure of their grain boundaries.  Nanocrystalline materials have unique properties compared to coarse grain counterpart because of the presence of more grain boundaries. Increased understanding of the role of grain boundaries play in nanocrystalline materials promotes the tuning of materials properties.

In order to study the grain boundaries in different materials, fully dense bulk materials are processed using Current Activated  Pressure  Assisted  Densification  (CAPAD)  technique.  CAPAD  is  a  unique  technique  for  materials processing. It offers faster processing of nanoscale materials compared to traditional sintering technique. Joule heating and pressure are used to densify the  materials in CAPAD system. X-ray analysis, Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) are used to characterize the materials.

There are three different parts in this dissertation: (1) Affect of grain size on grain boundary curvature on different materials; for example, nano and micro crystalline aluminum (metallic bond), silicon (covalent bond) and iron oxide  (ionic  bond);  (2)  Grain  boundary  geometry  analysis  of  nanocrystalline materials  and  (3)  Grain  size dependent electrical and optical property investigation.

In the first part of the dissertation, the effect of grain size on the grain boundary curvature is investigated. Several different types of materials were chosen, such as, micro and nano crystalline aluminum (Al), silicon (Si) and iron oxide  (Fe₂O₃).  It  is  found  that  the  slope  of  grain  boundary  curvature  is  grain  size  and  material  dependent. Nanocrystalline materials have higher grain boundary curvature compare to microcrystalline materials.

In the second part of the dissertation, grain boundary geometry of iron oxide is investigated by Electron Back  Scattered Diffraction (EBSD) method. Coincident Site lattice (CSL) value of 5 and 19 was dominating in iron oxide system.

Finally,  both  nano  and  micro  crystalline  optically  transparent  strontium  titanate  are  processed.  Grain  size           dependent relative permittivity, dielectric loss and optical transparency are investigated. Relative permittivity of densified strontium titanate measured at room temperature at 1 MHz shows very high and it is in between 180 to 268. Relative permittivity increases by increasing grain size. By performing TEM-EDS point analysis, it is found that in addition to grain size space charge potential at grain boundaries gives higher relative permittivity.