A systematic study of radiation enhanced diffusion; theoretical improvement and application

Han, Xiaochun

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https://hdl.handle.net/2142/50702 Copy

Description

Title

A systematic study of radiation enhanced diffusion; theoretical improvement and application

Author(s)

Han, Xiaochun

Issue Date

2014-09-16

Director of Research (if dissertation) or Advisor (if thesis)

Heuser, Brent J.

Doctoral Committee Chair(s)

Heuser, Brent J.

Committee Member(s)

• Stubbins, James F.
• Averback, Robert S.
• Zhang, Yang

Department of Study

Nuclear, Plasma, & Rad Engr

Discipline

Nuclear, Plasma, Radiolgc Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• New model
• Radiation enhanced diffusion
• Clustered defects
• Interstitial dominant diffusion

Abstract

A new rate theory model for the radiation enhanced diffusion (RED) has been developed. To improve the traditional mean-field theory, which assumes all defects are produced uniformly and continuously in the form of Frenkel pairs under irradiation, the new model considers more realistic defect production processes based on many experimental and theoretical findings. In the new model, both clustered and isolated defects are included and the importance of the contribution of interstitials to diffusion is emphasized. Applications of the new model to the experimental results in MgO, CeO$_2$ and UO$_2$ are generally successful. For the RED on the anion sublattice in MgO, experimental results have shown sink limited kinetics in the low temperature range and recombination limited kinetics in the high temperature range, which are the opposite of the predictions from the traditional RED model. Thus, a modified model was created by others to explain the new results quite successfully but with some deficiencies. Our new model corrects the deficiencies and reconstructs the sink limited kinetics with a linear dependence on radiation flux in the low temperature range, and recombination limited kinetics with a square root dependence on radiation flux in the high temperature range. Both analytical and numerical results agree with the experimental data. We find anion diffusion in MgO under irradiation is dominated by interstitials in the low temperature range, which is characterized by a low binding energy between small interstitial clusters. As temperature increases, in the intermediate temperature range, vacancies gradually become mobile and contribute to diffusion, which is characterized by a sharp increase in the diffusion coefficient. In the high temperature range, all defects are essentially freely-migrating and the new model returns to the traditional form. Defects are annealed by mutual recombination and an activation energy of half of the vacancy migration energy is found, which is a characteristic feature of the recombination limited kinetics. RED on the cation sublattice in MgO, CeO$_2$ and UO$_2$ have all shown low activation energies in the low temperature range, which is difficult to be explained by the traditional model. Our new model proposes an interstitial dominant diffusion mechanism with a weak bond between small interstitial clusters at low temperatures, and the results agree well with the experimental results.

Graduation Semester

2014-08

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http://hdl.handle.net/2142/50702

Copyright and License Information

Copyright 2014 Xiaochun Han

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