Ram Devanathan

Recent developments in proton exchange membranes for fuel cells

Proton exchange membranes (PEMs) that operate at temperatures above 120 °C are needed to avoid catalyst poisoning, enhance electrochemical reactions, simplify the design and reduce the cost of fuel cells. This review summarizes developments in PEMs over the last five years. In order to design new membranes for elevated temperature operation, one must understand the chemistry, morphology and dynamics of protons and water molecules in existing membranes. The integration of experiment with modelling and simulation can shed light on the hierarchical structure of the membrane and dynamical processes associated with molecular transport. Based on such a fundamental understanding, membranes can be modified by controlling the polymer chemistry and architecture or adding inorganic fillers that can retain water under low relative humidity conditions. The development of anhydrous membranes based on phosphoric acid doped polymers, ionic liquid-infused polymer gels and solid acids can enable fuel cell operation above 150 °C. Considerable work remains to be done to identify proton transport mechanisms in novel membranes and evaluate membrane durability under real world operating conditions.

Modeling and Simulation of Nuclear Fuel Materials

We review the state of modeling and simulation of nuclear fuels with emphasis on the most widely used nuclear fuel, UO2. The hierarchical scheme presented represents a science-based approach to modeling nuclear fuels by progressively passing information in several stages from electronic structure calculations to continuum level simulations. Such an approach is essential to overcome the challenges posed by radioactive materials handling, experimental limitations in modeling extreme conditions and accident scenarios, and the small time and distance scales of fundamental processes. When used in conjunction with experimental validation, this multiscale modeling scheme can provide valuable guidance to development of fuel for advanced reactors to meet rising global energy demand.

Molecular Dynamics Simulations Reveal that Water Diffusion between Graphene Oxide Layers is Slow.

Membranes made of stacked layers of graphene oxide (GO) hold the tantalizing promise of revolutionizing desalination and water filtration if selective transport of molecules can be controlled. We present the findings of an integrated study that combines experiment and molecular dynamics simulation of water intercalated between GO layers. We simulated a range of hydration levels from 1 wt.% to 23.3 wt.% water. The interlayer spacing increased upon hydration from 0.8 nm to 1.1 nm. We also synthesized GO membranes that showed an increase in layer spacing from about 0.7 nm to 0.8 nm and an increase in mass of about 15% on hydration. Water diffusion through GO layers is an order of magnitude slower than that in bulk water, because of strong hydrogen bonded interactions. Most of the water molecules are bound to OH groups even at the highest hydration level. We observed large water clusters that could span graphitic regions, oxidized regions and holes that have been experimentally observed in GO. Slow interlayer diffusion can be consistent with experimentally observed water transport in GO if holes lead to a shorter path length than previously assumed and sorption serves as a key rate-limiting step.

Insights into the radiation response of pyrochlores from calculations of threshold displacement events

We have used molecular-dynamics simulations to examine the displacement threshold energy (Ed) surface for cations and anions in Gd2Ti2O7 and Gd2Zr2O7 pyrochlores. In both pyrochlores, the Ed surface is highly anisotropic and it requires less energy to displace anions than cations. Both anion and cation Ed values are higher in the titanate compared to the zirconate. Titanium displacement energies are in excess of 170eV for all directions examined, because cation exchange is less energetically favorable in Gd2Ti2O7 compared to Gd2Zr2O7. These high-energy Ti displacements result in the formation of defect clusters that may prevent efficient defect recovery. This provides an explanation for the difference in susceptibility to amorphization between titanate and zirconate pyrochlores.

Molecular dynamics simulation of energetic uranium recoil damage in zircon

Defect production and amorphisation due to energetic uranium recoils in zircon (ZrSiO4), which is a promising ceramic nuclear waste form, is studied using molecular dynamics (MD) simulations with a partial charge model. An algorithm that distinguishes between undamaged crystal, crystalline defects and amorphous regions is used to develop a fundamental understanding of the primary damage state. The amorphous cascade core is separated from the surrounding crystal by a defect-rich region. Small, chemically inhomogeneous amorphous clusters are also produced around the core. The amorphous regions consist of under-coordinated Zr and polymerised Si leading to amorphisation and phase separation on a nanometer scale into Zr- and Si-rich regions. This separation could play an important role in the experimentally observed formation of nanoscale ZrO2 in ZrSiO4 irradiated at elevated temperatures.

Amorphization of silicon carbide by carbon displacement

We have used molecular dynamics simulations to examine the possibility of amorphizing silicon carbide (SiC) by exclusively displacing C atoms. At a defect generation corresponding to 0.2 displacements per atom, the enthalpy surpasses the level of melt-quenched SiC, the density decreases by about 15%, and the radial distribution function shows a lack of long-range order. Prior to amorphization, the surviving defects are mainly C Frenkel pairs (67%), but Si Frenkel pairs (18%) and antisite defects (15%) are also present. The results indicate that SiC can be amorphized by C sublattice displacements. Chemical short-range disorder, arising mainly from Frenkel pair production, plays a significant role in the amorphization.

Dynamic annealing of defects in irradiated zirconia-based ceramics

We have observed self-healing behavior in large scale molecular dynamics simulations of 30 keV Zr recoils in pure zirconia and 10 mole % yttria-stabilized zirconia. Our results reveal that dynamic annealing is highly effective during the first 5 ps of damage evolution, especially in the presence of oxygen structural vacancies introduced by aliovalent doping (Y3+ substitution for Zr4+). The presence of mobile oxygen vacancies results in near complete recovery of damage. Damage recovery on the cation sublattice is assisted by the anion sublattice recovery, which explains the remarkable radiation tolerance of stabilized zirconia. Ceramics engineered to heal themselves in this fashion hold great promise for use in high-radiation environments or for safely encapsulating high-level radioactive waste over geological time scales.

Electron-Hole Pairs Created by Photons and Intrinsic Properties in Detector Materials

A Monte Carlo (MC) code has been developed to simulate the interaction of gamma-rays with semiconductors and scintillators, and the subsequent energy partitioning of fast electrons. The results provide insights on the processes involved in the electron-hole pair yield and intrinsic variance through simulations of full electron energy cascades. The MC code has been applied to simulate the production of electron-hole pairs and to evaluate intrinsic resolution in a number of semiconductors. In addition, the MC code is also able to consider the spatial distribution of electron-hole pairs induced by photons and electrons in detector materials, and has been employed to obtain details of the spatial distribution of electron-hole pairs in Ge, as a benchmark case. The preliminary results show that the distribution of electron-hole pairs exhibit some important features; (a) the density of electron-hole pairs along the main electron track is very high and (b) most electron-hole pairs produced by interband transitions are distributed at the periphery of the cascade volume. The spatial distribution and density of thermalized electron-hole pairs along the primary and secondary tracks are important for large scale simulations of electron-hole pair transport.

High-energy radiation damage in zirconia: Modeling results

Zirconia is viewed as a material of exceptional resistance to amorphization by radiation damage, and consequently proposed as a candidate to immobilize nuclear waste and serve as an inert nuclear fuel matrix. Here, we perform molecular dynamics simulations of radiation damage in zirconia in the range of 0.1–0.5 MeV energies with account of electronic energy losses. We find that the lack of amorphizability co-exists with a large number of point defects and their clusters. These, importantly, are largely isolated from each other and therefore represent a dilute damage that does not result in the loss of long-range structural coherence and amorphization. We document the nature of these defects in detail, including their sizes, distribution, and morphology, and discuss practical implications of using zirconia in intense radiation environments.

Radiation response of FeTiO3, MgTiO3, and α-Al2O3

Abstract The radiation response of three rhombohedral oxides, namely, ilmenite (FeTiO3), geikielite (MgTiO3), and sapphire (α-Al2O3) has been examined by irradiating electron transparent samples with 1 MeV Kr+and 1.5 MeV Xe+ ions. The microstructural changes during irradiation were observed in situ in a high-voltage electron microscope using electron diffraction and microscopy. The irradiation conditions were designed to minimize beam heating and chemical effects due to the implanted ion. In all three materials, the critical temperature for amorphization was below 300 K indicating good room temperature resistance to amorphization by energetic beams. The resistance to amorphization was found to increase with the melting point of the oxide—sapphire was the most resistant and ilmenite the least resistant to amorphization.