Lu Deng

Structure of International Simple Glass and properties of passivating layer formed in circumneutral pH conditions

Knowing the structure of a material is necessary to understand its evolution under various influences; here, the alteration by water of a reference glass of nuclear interest, called International Simple Glass (ISG), is studied. The ISG atomic structure has not yet been thoroughly characterized. Short- and medium-range order in this six-oxide glass was investigated by molecular dynamics (MD) methods. Combining the simulated data with experimental observations acquired from both pristine and altered ISG provided new insight into the formation of surface layers and passivation of the underlying glass. In the tested conditions of 90 °C, silica-saturated solution, and pH90°C 7, the passivating layer partly inherits the structure of the pristine glass network despite the release of mobile elements (Na, B, and some Ca), with a reorganization of the silicate network following B release. The layer appears to minimize its internal energy by relaxing strain accumulated during glass quenching. The resulting passivated glass shows a strong resistance to hydrolysis. The nanopores of this hydrated material, displaying a mean pore size of ∼1 nm, are filled with various water species. Water speciation determination inside the nanopores is therefore an achievement for future water dynamic study in the passivated glass.Nuclear glass: developing a new layer of understandingA combination of experiments and simulations has provided insight into the structural changes of a glass of nuclear interest in solution. Borosilicate glasses are used as containment matrices for the highly radioactive waste that results from spent-nuclear-fuel reprocessing. They are stored in deep geological repositories and while this is seen as a relatively long-term solution, how they respond to their environment is of obvious importance. A team, led by Stéphane Gin at the CEA, Marcoule, France, have now studied the changes that a reference glass known as International Simple Glass (ISG) undergoes when exposed to water. Their simulations and analytical experiments provide insight into the structure of pristine ISG while also observing the generation of a passivating layer, which partly inherits the structure of the pristine glass but is less reactive with water.

Effects of system size and cooling rate on the structure and properties of sodium borosilicate glasses from molecular dynamics simulations

Borosilicate glasses form an important glass forming system in both glass science and technologies. The structure and property changes of borosilicate glasses as a function of thermal history in terms of cooling rate during glass formation and simulation system sizes used in classical molecular dynamics (MD) simulation were investigated with recently developed composition dependent partial charge potentials. Short and medium range structural features such as boron coordination, Si and B Qn distributions, and ring size distributions were analyzed to elucidate the effects of cooling rate and simulation system size on these structure features and selected glass properties such as glass transition temperature, vibration density of states, and mechanical properties. Neutron structure factors, neutron broadened pair distribution functions, and vibrational density of states were calculated and compared with results from experiments as well as ab initio calculations to validate the structure models. The results clearly indicate that both cooling rate and system size play an important role on the structures of these glasses, mainly by affecting the 3B and 4B distributions and consequently properties of the glasses. It was also found that different structure features and properties converge at different sizes or cooling rates; thus convergence tests are needed in simulations of the borosilicate glasses depending on the targeted properties. The results also shed light on the complex thermal history dependence on structure and properties in borosilicate glasses and the protocols in MD simulations of these and other glass materials.

Structural role of ZrO 2 and its impact on properties of boroaluminosilicate nuclear waste glasses

Addition of zirconia (ZrO2) to nuclear waste glasses, even in small amount, significantly affects physical properties such as chemical durability, density, viscosity, and glass transition temperature. Hence ZrO2 plays an important role in the development of nuclear waste glass compositions. It was found recently that addition of zirconia decreases the initial dissolution rate but increases long-term dissolution by changing the protective properties such as porosity of alteration layers. In this study, the International Simple Glass (ISG) with different amounts of ZrO2/SiO2 substitution was simulated using classical molecular dynamics (MD) simulations and recently developed composition-dependent potential sets. Local structural descriptors such as bond distances, bond angle distributions, and coordination numbers were systematically studied and compared with experimental values. Zr K-edge extended X-ray absorption fine structure spectra were also calculated and compared with experiment to resolve the local coordination environment around Zr. Medium-range structural information (e.g., Qn distribution, network connectivity, and ring-size distribution) showed that ZrO2/SiO2 substitution increases the overall network connectivity and the amount of smaller sized rings, which will decrease the initial dissolution rate by strengthening the glass network and limiting water diffusivity. Finally, diffusion and dynamic properties of the ions were studied and discussed to develop a better understanding of the chemical durability of these glasses.Nuclear glasses: Simulations of substitutionsMolecular dynamics simulations have been used to study how ZrO2/SiO2 substitution affects the structure and properties of International Simplified Glass (ISG). ISG is a model material used to understand the dissolution and corrosion mechanisms of borosilicate glasses, which are used to store radioactive waste. Adding ZrO2 to such nuclear waste glasses is known to significantly affect their properties, such as density, viscosity and importantly chemical durability, however understanding its effects on structure has been hindered by experimental difficulties. Now, Jincheng Du from the University of North Texas and co-workers have used molecular dynamics simulations to interrogate how the substitution of SiO2 with ZrO2 alters ISG structure. They found that it increases glass-network connectivity and the number of small ring structures, which will decrease initial dissolution rate by strengthening the glass network and reducing water diffusivity.

Mixed Network Former Effect on Structure, Physical Properties and Bioactivity of 45S5 Bioactive Glasses: An Integrated Experimental and Molecular Dynamics Simulation Study

Boron-containing bioactive glasses display a strong potential in various biomedical applications lately due to their controllable dissolution rates. In this paper, we prepared a series of B2O3/SiO2-substituded 45S5 bioactive glasses and performed in vitro biomineralization tests with both simulated body fluid and K2HPO4 solutions to evaluate the bioactivities of these glasses as a function of boron oxide to silica substitution. The samples were examined with scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectrometry after immersing them in the two solutions (simulated body fluid and K2HPO4) up to 3 weeks. It was found that introduction of boron oxide delayed the formation of hydroxyapatite, but all the glasses were shown to be bioactive. Molecular dynamics (MD) simulations were used to complement the experimental efforts to understand the structural changes due to boron oxide to silica substitution by using newly developed partial charge composition-dependent potentials. Local structures around the glass network formers, medium-range structural information, network connectivity, and self-diffusion coefficients of ions were elucidated from MD simulation. Relationships between boron content and glass properties such as structure, density, glass transition temperature, and in vitro bioactivity were discussed in light of both experimental and simulation results.

Molecular Dynamics-based Simulations of Bulk/Interfacial Structures and Diffusion Behaviors in Nuclear Waste Glasses

Article history: Received 5 June 2016 Received in revised form 21 September 2016 Accepted 22 September 2016 Available online 21 October 2016 A set of empirical potentials have been developed to enablemolecular dynamics simulations of oxide glasseswith themost common glass formers: silica, boron and aluminum oxides. Built upon the recent borosilicate potentials, this set of partial charge effective potentials features composition dependent variable atomic charges and pairwise short range interactions that ensure high computational efficiency. They can correctly reproduce the short range structure features of boroaluminosilicate glasses including [SiO4] tetrahedral network, aluminum coordination, and, importantly, the coordination change of boron as a function of composition. By using the newly developed potentials, a series of sodium boroaluminosilicate glasses were simulated and the structures analyzed in terms of bond distance, bond angle, and coordination number, which were compared with available theoretical, simulation and experimental results. Structural analysis such as polyhedral connectivity analysis, Q analysis, and ring size distribution were obtained to investigate themedium range structure features of these glasses. Furthermore,mechanical properties such as Youngs, shear and bulkmoduliwere calculated andwere found to be in good agreementwith experimental data. The vibrational density of stateswas also calculated and comparedwith previous MD and ab initio results. The results show B and B had distinctive spectra features and vibrational spectra were in good agreement with earlier ab initio studies.