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Scholarly Works (2 results)

Thesis
Peer Reviewed

Studies on Irradiation and Electric Potential Effects toward Mineral Atomic Structure and Chemical Reactivity

UCLA Electronic Theses and Dissertations (2019)

First, we focus on the effect of neutron irradiation on the durability of silicate and carbonate mineral aggregates. Comparisons of mineral dissolution rates, as a function of pH, temperature, or surface potential, were performed at pristine or Ar+ ion irradiated state. The experimental results were coupled with MD simulations of atomic scale alteration in the crystallographic structure of the mineral and any resulting changes in physical properties that result. In detail, albite (NaAlSi3O8), a 3D framework silicate, is compared with a less polymerized silicate (i.e., almandine) and carbonates (e.g. calcite, and dolomite), which are also often present in the mineral aggregates that compose concrete. When exposed to radiation, the crystal structures of minerals have possibilities to undergo significant alterations. These alterations may perhaps enhance its chemical durability, and thus degrade the infrastructure durability. This relatively higher enhancement in the dissolution rate of silicates compared to carbonates following irradiation has significant impacts on the durability of concrete containing them up on their exposure to radiation in nuclear power plant environments.

Second, the effect of electric potential to calcite dissolution kinetics is examined. The accelerated ion transportation by potential further enhances calcite dissolution, whereas the extent of induced dissolution depends on the pH, ionic strength, and temperature. Calcite dissolution rate is enhanced in acidic to neutral pH solution, but remains constant in alkaline pH. From this, it reveals that the potential-induced dissolution rates are governed by the rate-limiting step in the dissolution mechanism. In addition, by varying solution ionic strength, stronger ionic strength results in less dissolution rate enhancement due to the lower ion diffusivity. As from the examination of the temperature effect, the potential-induced dissolution rate enhancement shows that electric potential does not increase calcite dissolution rate significantly compared to temperature.

In sum, it can be concluded that the mineral chemical reactivity can be enhanced by introducing external stimulus, and the magnitude of the rate enhancement depends highly on the solid atomic structure and the solvent properties. The conclusion and proposed future perspective of these works can be helpful to enhance the durability of concrete infrastructures and the relevant engineering applications.

Cover page: Studies on Irradiation and Electric Potential Effects toward Mineral Atomic Structure and Chemical Reactivity

Article
Peer Reviewed

Experimental evidence of auxeticity in ion implanted single crystal calcite

UCLA Previously Published Works (2022)

We report initial experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite with Ar⁺ at room temperature using an ion energy of 400 keV and a dose of 1 × 10¹⁴ cm^-2. Lattice compression normal to the substrate surface was observed, which is an atypical result for ion implanted materials. The auxetic behavior is consistent with predictions that indicate auxeticity had been predicted along two crystallographic directions including [1010]. Materials with a positive Poisson's ratio experience lattice expansion normal to the substrate surface when ion implanted, whereas lattice contraction normal to the surface is evidence of auxetic behavior. Triple-axis X-ray diffraction measurements confirmed the auxetic strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate, as is typical with implanted single crystals. A symmetric (3030) ω:2θ line scan was used with X-ray dynamical diffraction simulations to model the strain profile and extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were performed to obtain a displacement-per-atom profile and implanted Ar⁺ concentration profile. It was found that the strain profile matches the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxeticity.

Cover page: Experimental evidence of auxeticity in ion implanted single crystal calcite