Materials interface properties bring both opportunities and challenges for materials design. Especially for applications at extreme conditions, the metastable nature of interface inevitably leads to the big question of structural stability. In this thesis, the nanopore surface in mesoporous MCM-41 and the grain boundaries in nanocrystalline yttria-stabilized zirconia (YSZ) were selected as the two presentative interface types. They were studied with atomistic simulation methods to help understand and improve their structure stability at different conditions, and other related properties for relevant applications.
For MCM-41, its thermostability and pore structure transformation mechanisms subjected to temperatures from 300 K up to 2885 K was studied by the combination of molecular dynamics (MD) and Monte Carlo simulations. Silica was experimentally characterized to inform the models and enable prediction of changes in gas adsorption/separation properties. MD simulations suggest that the pore closure process is activated by a collective diffusion of matrix atoms into the porous region, accompanied by bond reformation at the surface. Degradation is kinetically limited, such that complete pore closure is postponed at high heating rates. Applying the Kissinger equation, a strong correlation between the simulated pore collapse temperatures and the experimental values was found, which implies an activation energy of 416 ± 17 kJ/mol for pore closure. MC simulations give the adsorption and selectivity for thermally treated MCM-41, for N 2, Ar, Kr and Xe at room temperature within the 1–10000 kPa pressure range. Relative to pristine MCM-41, it was observed that increased surface roughness due to decreasing pore size amplifies the difference of the absolute adsorption amount differently for different adsorbate molecules. In particular, it was found that adsorption of strongly-interacting molecules can be enhanced in the low-pressure region while adsorption of weakly-interacting molecules is inhibited. This then results in higher selectivity in binary mixture adsorption in mesoporous silica.
For YSZ, the stabilization effect of La3+ doping on the grain boundary structure of YSZ was studied using MC simulation. It reveals the segregation of La3+ at eight tilt grain boundary (GB) structures and predicted an average grain boundary (GB) energy decrease of 0.25 J/m 2, which is close to experimental values reported in the literature. Cation stabilization was found to be the main reason for the GB energy decrease, and energy fluctuations near the grain boundary are smoothed out with La 3+ segregation. Then the segregation effect on materials ionic conductivity was studied with MD simulation. Both dynamic and energetic analysis on Σ13 (510)/ GB structure revealed La3+ doping hinders O 2– diffusion in the GB region, where the diffusion coefficient monotonically decreases with increasing La3+ doping concentration. The effect was attributed to the increase in the site-dependent migration barriers for O2– hopping caused by segregated La3+, which also leads to anisotropic diffusion at the GB.
|Commitee:||Castro, Ricardo H.R., Stroeve, Pieter|
|School:||University of California, Davis|
|Department:||Materials Science and Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Subjects:||Engineering, Materials science|
|Keywords:||Atomistic simulation, Ceramics, Dopant segregation, Gas adsorption, Grain boundries, Porous materials|
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