Tailoring of ceramic microstructures by electric field-activated grain growth
Abstract
The successful sintering of nanometric spinel powders to relative densities above 95% represents milestone #1. The agglomeration and intra-agglomerate strength of spinel powders represents a potential risk to reach the first milestone. Our previous studies have demonstrated that spinel powders can exhibit intra-agglomerate strength that hinders successful densification [81]. Should this problem arise we will pursue the synthesis of MgAl2O4 nanopowders using co-precipitation techniques previously established by Rufner and Castro [45]. Confirmation that the activation energy for grain growth is lowered by the applied field strength requires annealing at various temperatures between 700¡C and 1300¡C while maintaining an applied electric field strength of 1000V/cm. An Arrhenius analysis of the grain growth rate as a function of inverse temperature will allow for the determination of the respective activation energy for grain growth. Such series will be carried out for different applied field strengths above and below 1000 V/cm in order to evaluate the variation of activation energy. The objective of this series of experiments is to find an analytical expression of the activation energy in the presence of applied electric fields. Completion of this experimental series represents milestone #2. Identification of potentially occurring alternative grain growth mechanisms due to the applied electric field requires annealing of microstructures at constant temperature for different time intervals t2-t1=Delta_t and varying electric field strength. Analysis for changes in average grain size will allow determination of the associated grain growth exponent n and the associated pre-factor Kg0, which includes the activation energy. Measurements of grain sizes and grain size distributions using SEM and precession electron diffraction will be accommodated by an analysis of the local stress states surrounding grain boundary core structures. Nanodiffraction experiments will allow spatial mapping of stress states, and a subsequent correlation with the applied electric field. Successful determination of grain growth exponents that may or may not change in the presence of electric fields will represent milestone #3. One fundamental question is to what extent the applied electric field will interact with the space charge zone surrounding individual grain boundaries. EELS analysis can provide detailed information about the kind of defect structures and their distribution in the vicinity of grain boundary planes as a function of applied electric field strength during annealing. The successful confirmation of specific defect structures in MgAl2O4 and the correlation of their distribution with the applied field strength will represent milestone #4. Below a suitable temperature directional grain growth is expected since the applied electric field has the largest impact on grain boundaries that are perfectly aligned with the field direction. Dependent on the findings that lead to milestone #2, annealing experiments will be designed with increasing electric field strengths while minimizing simultaneous heating. The goal of this experimental approach is to identify annealing parameters for uni-directional grain growth, which will likely occur in the direction parallel to the electrical field. Accomplishing controlled uni-directional grain growth will mark milestone #5.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 11, 2018
- Source ID
- W911NF1610364
Entities
People
- Klaus van Benthem
Organizations
- Army Contracting Command
- United States Army
- University of California, Davis