My group conducts research in materials for nuclear energy, catalysis, adsorption, and other energy-related applications. Particular focus is given to simulation of materials for plasma-facing environments, including nuclear fusion reactors; transport and radiation damage in materials, including simulations of nuclear fission reactor fuels; catalytic and adsorption processes in zeolites and other porous materials; characterization of materials; and modeling and simulation of porous materials.
Nuclear fusion is the source of all energy we utilize today. Unfortunately, the best reactors to produce energy from nuclear fusion are, well, a little bulky. The nearest one to us, for example, has a mass of almost 2 × 1030 kg and produces 400 YW (4 × 1026 W) of power (you might know it as the Sun). Clearly, this gravitationally confined environment is not feasible on Earth, so we have had to get more clever.
Instead, humans have envisioned inertial confinement reactors, in which high-power lasers focus onto a very small sample, instantaneously compressing it and starting fusion reactions. Inertial confinement is the basis of the National Ignition Facility in California (pictured at left). Unfortunately, this type of reactor (like all others) has many hurdles to overcome as well.
The third option devised to date is magnetically confined fusion, which is the basis for ITER, a test reactor currently under construction near Cadarache, France. ITER is the culmination of around 50 years of plasma physics research, and is expected to be the first terrestrial fusion reactor to produce more power than is required to heat the plasma for longer than a few seconds. However, the conditions of the plasma in ITER and future magnetic confinement devices are becoming increasingly hostile to the materials from which the device is constructed. These materials challenges may come to dominate research in fusion during the coming years, as we strive to achieve the dream of a terrestrial nuclear fusion reactor.
Recent work in plasma-facing materials has revealed that helium, the product of nuclear fusion, creates many unexpected and/or detrimental phenomena when implanted in metals due to helium plasma exposure. Helium bubbles, in particular, are possible sources of increased hydrogen (specifically tritium) retention, as well as plasma contamination by the divertor material (tungsten). Our work comes in at the microscopic level: we examine the transport processes involved in hydrogen, helium, lithium, and beryllium transport inside the divertor and first wall of the reactor, as well as smaller plasma devices intended as model systems for ITER. Specifically, we wish to determine the interplay between the various reactor materials as they sputter and redeposit on other surfaces, changing the surface and diffusion properties within the metal.
While nuclear fusion holds great promise, it is, at present, impractical as a source of energy for our everyday use. Nuclear fission, on the other hand, provides approximately 20% of electrical energy in the United States and represents approximately 8% of our total energy use annually. Nuclear reactors have the distinct advantage that, once built, they are extremely cheap to operate, and they have excellent safety records: in the USA, accidents at nuclear power facilities have claimed just three lives—ever. In fact, fission power causes fewer deaths per unit of electricity than all other power sources, and most of those deaths are those of miners.
Nuclear reactors, much like cell phone data plans, are generally divided into “generations,” such as “Generation III reactors.” A big part of the progress in nuclear energy is our improved understanding of nuclear reactor physics, particularly how the reactor behaves during disruptions from steady-state in terms of power output and radiation production. Nuclear reactor materials, which include fuel, cladding, and structural materials, are susceptible to damage both from neutrons and the resulting hydrogen they decay into, as well as fission gas production (xenon, krypton) from the fission process and radioactive decay of fission products to produce high-energy electrons (beta particles) and helium nuclei (alpha particles).
My group’s work in this area, supported by a Faculty Development Grant from the Nuclear Regulatory Commission, uses the tools of computational materials science and applied physics to model heat and mass transfer processes in nuclear materials, particularly radiation effects and thermal transient phenomena. We also study the thermodynamics and materials physics of alternative nuclear fuels such as metallic uranium—molybdenum alloys. Our work is in tandem with the Missouri University Research Reactor (MURR), the largest academic research reactor in the country.
Zeolites and Similar Porous and Catalytic Materials
Zeolites are crystalline aluminosilicates that revolutionized the petrochemicals industry when they were introduced into the cracking and refining processes decades ago. The key to this revolution was the small pores zeolites possess: the constrained environment of a zeolite pore either prevents large molecules (e.g., asphalt-range hydrocarbons) from forming, or prevents their diffusion out of the zeolite before they react again.
Zeolite-catalyzed processes typically involve acid sites, another prominent feature of zeolite chemistry: the aluminum atoms, which have trivalent chemistry, create a net negative charge when in the tetravalent sites normally occupied by silicon atoms. This negative formal charge creates a Lewis base site near each aluminum atom. If the compensating cation is hydrogen, the zeolite has net Brønsted acidity. Most zeolites, especially low-aluminum zeolites, are strong acids, making them fairly potent acid catalysts capable of doing size-selective chemistry.
My group’s primary research in zeolites is in their spectroscopic properties. Specifically, we want to be able to predict—quickly and accurately—the infrared spectrum of a zeolite using nothing but computational tools. Sounds simple, right? Except that the vibrations are tightly coupled, the crystals are huge (as simulations go, at least), and the models we have…well…stink. As such, we proceed along all of these fronts, doing DFT-based calculations of periodic structures as a baseline, comparing those to simulations of finite, molecule-like portions of zeolite crystals, and comparing both sets of calculations to faster, though typically less-accurate, models of zeolite vibration.
Another avenue of research in my group is zeolites that have been modified to act as Brønsted bases. Our goal is to find the right combination of crystal structure and Si/Al ratio, combined with treatment procedure and catalytic reaction conditions, which will make nitrided zeolite catalysis a reality. This research is directed toward oxygenated molecules, which are often upgraded by way of base-catalyzed reactions.
A typical starting point for these materials is treatment of a zeolite with high-temperature ammonia or other amine-generating agents. This process substitutes some of the oxygen for NH groups, which act as Brønsted (and Lewis) bases. Our challenge as we move forward is to find a viable combination of catalyst stability, reactant/product species, and reaction pathway characteristics that make alkaline zeolite catalysis a reality.