Local-rules based topological modeling of tetrahedral ceramic network structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1998.Includes bibliographical references (p. 147-156).by Caroline Esther Jesurum.Ph.D

Intermediate-range ordering and topochemical heterogeneity in binary and ternary silicate glasses

The fact that glasses can be synthetically produced and engineered allows plenty of opportunity to control their structure. Even though it may sound simple, the glass formation is controlled by physical-chemical principles and any change in the composition, temperature, cooling rate, processing type, etc. impacts the final material - and consequently its properties. While glasses are considered non-crystalline solids due to the absence of longrange periodicity, they show a regular construction, defined by the short-range and intermediate-range order. Their characteristics are described by the topology, which denotes the basic geometrical arrangement of the structural units and allocation of the atoms. At macroscale, for a glass of the same composition, the structure and properties reveal to be homogeneous, independently of the processing, temperature or precursor material. However, at microscale, the same glass may show a different picture, revealing a topological heterogeneity of a few nanometers. Due to technological limitations, the main difficulty is to directly access this region. There is a consensus that the topological heterogeneity, however, manifests as a significant peak at very low temperatures (about 5 K) or low-frequencies (about 1THz or 33 cm 1) by collective vibrational modes. Since the main model for estimating the phonon contribution to the specific heat in a crystal, the Debye model, does not predict any peak at low temperature and there are no models to describe these manifestations in vitreous materials, usually it is considered an anomaly. This anomalous peak has been called Boson peak. Even though it remains as one of the major debated and unsolved problems of condensed-matter physics, intense investigations in these almost 50 years brought an enormous knowledge about most of its characteristics. In order to access the intermediate-range order and the topochemical heterogeneity of selected binary and ternary glass network formers made by reactive powder sintering process, investigations of the vibrational density of states in the region of the Boson peak has been conducted. Foremost, this study describes that the feature of the Boson peak is governed by topological heterogeneity as well as topochemical heterogeneity. Together with other characterization methods, this has been shown as a powerful descriptive route to understand glass functionality and glass structure in a more extended perspective. Even though it is important for the wide relevance of fundamental knowledge of glasses, this is notably important for high-technological glasses and in which bottom-up strategies are necessary to design new glass compositions with straightforward applications

Molecular Dynamics Simulations of the Structures of Europium Containing Silicate and Cerium Containing Aluminophosphate Glasses

Rare earth ion doped glasses find applications in optical and photonic devices such as optical windows, laser, and optical amplifiers, and as model systems for immobilization of nuclear waste. Macroscopic properties of these materials, such as luminescence efficiency and phase stability, depend strongly on the atomic structure of these glasses. In this thesis, I have studied the atomic level structure of rare earth doped silicate and aluminophosphate glasses by using molecular dynamics simulations. Extensive comparisons with experimental diffraction and NMR data were made to validate the structure models. Insights on the local environments of rare earth ions and their clustering behaviors and their dependence on glass compositions have been obtained. In this thesis, MD simulations have been used to investigate the structure of Eu2O3-doped silica and sodium silicate glasses to understand the glass composition effect on the rare earth ions local environment and their clustering behaviors in the glass matrix, for compositions with low rare earth oxide concentration (~1mol%). It was found that Eu–O distances and coordination numbers were different in silica (2.19-2.22 Å and 4.6-4.8) from those in sodium silicate (2.32 Å and 5.8). High tendencies of Eu clustering and short Eu-Eu distances in the range 3.40-3.90 Å were observed in pure silica glasses as compared to those of silicate glasses with much better dispersed Eu3+ ions and lower probability to form clusters. The results show Eu3+ clustering behavior dependence on the system size and suggest for low doping levels, over 12,000 atoms to obtain statistical meaningful results on the local environment and clustering for rigid silica-based glasses. The structures of four cerium aluminophosphate glasses have also been studied using MD simulations for systems of about 13,000 atoms to investigate aluminum and cerium ion environment and their distribution. P5+ and Al3+ local structures were found stable while those of Ce3+ and Ce4+ ions, through their coordination numbers and bond lengths, are glass composition-dependence. Cerium clusters were found in the high cerium glasses.P5+ coordination numbers around cerium revealed the preference of phosphorus ions in the second coordination shell. Total structure factors from MD simulations and experimental diffraction results show a general agreement from comparison for all the cerium aluminophosphate glasses and with compositional changes up to 25 Å-1. Aluminum enters the phosphate glass network mainly as AlO4 and AlO5 polyhedra only connected through corner sharing to PO4 tetrahedra identified by Q11(1 AlOx), Q12(2 AlOx), Q21(1 AlOx), and Q22(2 AlOx) species

Metakaolin as a model system for understanding geopolymers

Geopolymers are a class of amorphous aluminosilicate materials that exhibit a range of properties depending on synthesis parameters. Determining the molecular interactions responsible for the different characteristics experimentally is hindered by the compositional variation of the source materials. Computational methods are thus used to provide atom level insights, with metakaolin used as a model system to represent the Al/Si geopolymer matrix. The formation of metakaolin through the thermal de-hydroxylation of kaolinite was simulated with molecular dynamics using an interatomic potential model identi_ed through testing of several models from the literature. The simulated metakaolin exhibited a 1:1 Al/Si ordering with a loss in periodicity due to the migration of aluminium ions through the structure. The change in the aluminium coordination as a function of de-hydroxylation results in a final structure composed of primarily 4-fold Al with up to 20% of the Al in a 5-fold coordination.A complex cavity network was identified and characterised in metakaolin and provided sites for the inclusion of sodium, potassium and calcium interstitial defect ions. The results showed that whilst ionic size controlled the final locations of the defect ions, ionic charge influenced the degree of interaction with the surrounding oxygen atoms and resulted in greater variations in the final defect site characteristics. Introducing hydroxyl groups into the structure caused the interactions of the defects with the aluminium to increase compared to silicon, demonstrating that the degree of source material hydration is as important as the type of metal cations present in the geopolymerisation reaction.A procedure for the generation of stable, partially hydrated metakaolin surfaces was developed and the resulting surfaces had a high degree of roughness that increased in the presence of water. The Al-terminated surfaces in metakaolin demonstrated the greatest level of interaction with water compared to Si, causing a surface puckering effect that resulted in a widening of the surface layers. The results indicate that water plays an important role, as the presence of water in the reaction mixture combined with high levels of structural disorder in the source materials increase their susceptibility to the caustic attack involved in geopolymerisation

Computer simulation and topological modeling of radiation effects in zircon

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2006.Includes bibliographical references.The purpose of this study is to understand on atomic level the structural response of zircon (ZrSiO4) to irradiation using molecular dynamics (MD) computer simulations, and to develop topological models that can describe these structural changes. Topological signatures, encoded using the concepts of primitive-rings and local clusters, were developed and used to differentiate crystalline and non-crystalline atoms in various zircon structures. Since primitive-rings and local clusters are general concepts applicable to all materials, and the algorithms to systematically identify them are well-established, topological signatures based on them are easy to implement and the method of topological signatures is applicable to all structures. The method of topological signatures is better than the Wigner-Seitz cell method, which depends on the original crystalline reference grid that is unusable in heavily damaged structures or regions; it is also better than those methods based only on local structures limited to first coordination shell, since one can decide whether or not to include ring contents of large rings into the topological signatures, effectively controlling the range of the topological signatures. The early-stage evolution of non-crystalline disorder and the subsequent recrystallization in zircon collision cascade simulations were successfully modeled by using the topological signatures to identify non-crystalline atoms. Simply using the number of displaced atoms was unable to correctly show the initial peak of structural damage followed by the subsequent annealing stage. Using the topological signatures, amorphization within a single collision cascade was observed in zircon.(cont.) In the radiation-induced amorphous zircon simulated in this study, the method of topological signatures was able to differentiate the amorphous region in the center of the simulation box and the crystalline region surrounding it. A few isolated remnant crystalline islands were identified in the amorphous region. About 5% of atoms in melted and melt-quenched structures were identified as crystalline atoms. Different amorphous zircon structures were found to be topologically different. Upon amorphization of zircon, the average ring size and the number of atoms in local cluster were found to increase. Larger average ring sizes were found in more pervasively amorphized structures. The radiation-induced amorphous structure was the least pervasively amorphized one, followed by the melt-quenched. The liquid-state amorphous structure was most pervasively amorphized and had the largest average ring size. Phase-separation of zircon into SiO2- and ZrO2-rich local regions was observed when zircon was amorphized in simulations, either thermally or by radiation. It was found in simulations using constant pressure ensembles that the zircon structure underwent abnormally huge volume swelling when it amorphized, which was attributed to the ion charges used in the potential model. Although the ion charges used in the originally chosen potential model were overall balanced, they were not balanced with regard to the phase decomposition products, and thus resulted in strong Coulombic repulsive force within locally SiO2- and ZrO2-rich regions when phase separation occurred. After the ion charges were re-balanced (and other potential parameters refitted), the volume expansion was found to be under control. The charge imbalance of SiO2 units was also found to produce unrealistically large fraction of 3-coordinated Si and shorter Si-O bond length.(cont.) The issue of charge-balance with regard to phase decomposition products applies to all complex ceramics that decompose into separate phases upon amorphization. Threshold displacement energies in zircon were systematically determined. Many special directions, such as those directed toward neighboring atoms or open spaces surrounding the PKA, were considered. Cascade detail was extensively examined, including PKA trajectory, cascade extent, time scale, thermal spike, recoil density, distribution of PKA energy among sub-lattices and number of displaced atoms. The crystallographic features of the zircon structure were found to have profound implications for collision cascades. It was found that energetic PKAs were always deflected into the open channel along the z direction. Their displacements along the longitudinal x direction were never greater than about 4 nm in our simulations. The estimation of the cascade extent assuming homogeneous media thus greatly over-predicts the PKA displacement along the longitudinal direction. The effects of PKA mass on collision cascade were studied by comparing the cascades caused by Zr and U PKAs. The U atoms were simply "super-mass" Zr atoms in this study: U-Zr, U-Si and U-O interactions were the same as Zr-Zr, Zr-Si and Zr-O interactions, respectively. It was found that heavier PKAs produced longer cascades, more structural damage, and higher temperature in thermal spike. U also traveled further along the longitudinal x direction because it was less prone to change of velocity direction. The depleted regions in the core of the cascades surrounded by a densified shell, which were found in simulations by Trachenko et al., were not found in our study. After extensive tests of recently published zircon potentials, it was found that three out of the five tested potentials yielded poor elastic constants and appear to be unfit for serious simulations. Published simulation results using these potentials should accordingly be viewed cautiously.by Yi Zhang.Ph.D

Understanding the Fundamentals of Ionic Conductivity in Polymer Electrolytes

The rate of advancement for mobilized electronic technologies is outpacing the development of small efficient batteries. Lithium-ion batteries are currently the most widely used energy storage device for consumer electronics. Traditional lithium-ion batteries use a liquid electrolyte to separate the cathodes and anodes; however, conventional liquid electrolytes have inherent problems, such as consisting of flammable carbonate components, hazardous material, and have a significant cost/weight in the battery. In addition, the liquid electrolyte cannot prevent the growth of lithium dendrites during the charge/discharge cycle of the lithium-ion battery. These dendrites can connect the anode to the cathode of the battery cell through the liquid electrolyte separator, which will lead to high self-discharge currents through a low resistance path, igniting the flammable liquid electrolyte and causing fires or explosions. These problems have motivated the research community to resolve these related safety issues by using cheaper novel materials, such as polymerized electrolytes, to replace the traditional liquid electrolyte. Polymerized electrolytes can solve and alleviate some of the safety risks posed by liquid electrolytes. The main challenges regarding the use of polymerized electrolytes are an insufficient understanding of the ion transport mechanism and the inability to reach the desired industrial standard for conductivity of higher than 10-3 S/cm at ambient temperature. This dissertation presents the findings of experimental studies of several different polymerized electrolytes using broadband dielectric spectroscopy, Brillouin light scattering, differential scanning calorimetry, and rheology. Varying the mobile ion size with different chemical structures of polymerized electrolytes allowed extensive analysis. The study analyzed the charge and mass transport of several polymerized electrolytes and one monomeric precursor, which led to a proposed approach to estimate ionic diffusivities from the characteristic times of the conductivity relaxation and ion concentration without any adjustable parameters. Using the new and modified approach to estimate ionic diffusivities revealed that the charge transport is about ten times slower compared to that of ion diffusion, suggesting that a strong ion-ion correlation reduces ionic conductivity in polymerized electrolytes. Study of the activation energy of the ion diffusion shows a non-monotonous dependence on the mobile ion size, which indicates competition between coulombic and elastic forces controlling ion transport. This finding proposes that a simple qualitative model describing the activation energy for the ion diffusion would result in an increase in the dielectric constant of polymerized electrolytes can lead to a significant enhancement of conductivity of small ions (e.g., Li and Na)