Selected Publications
Uranium and thorium oxides have critical roles as fuels in existing nuclear power plants, as well as in proposed reactor concepts. The thermal conductivity of these materials determines their ability to transfer heat from the reactor core to the surrounding coolant. Additionally, these actinide compounds are of interest in condensed matter physics because of the 5f orbitals and unique electron interaction, coupling, and scattering events that can occur. Because of the radioactivity of thorium and uranium, thin film measurements of actinide materials are used to limit the amount of operator exposure, but standard thermal characterization methods are not well suited for thin films. This paper presents the process of depositing thin film UOx and ThOx samples of nm-μm thicknesses and the results of thermal property measurements. Thin films were deposited on silicon and glass substrates via dc-magnetron sputtering using an argon/oxygen mixture as the working gas. The thermal properties of the films were measured by the Thermal Conductivity Microscope (TCM). This uses one laser to generate thermal waves and a second laser to measure the magnitude and phases of the thermal waves to obtain the conductivity of materials. The results of the research show that the UOx film properties are lower than bulk values and that the role of the substrate has a considerable effect on determining the measured properties. Future work aims at improving the deposition process. Epitaxial film growth is planned. Additional understanding of thermal property measurements is targeted.
We have used spectroscopic ellipsometry to measure the optical constants of evaporated amorphous zinc arsenide (Zn3As2). A five parameter model using a Tauc-Lorentz oscillator was found to fit well each of six amorphous samples deposited on Si3N4/silicon, allowing the layer thicknesses and optical constants to be deduced. Layer thicknesses varied from 20 to 70 nm. The fitted value of the optical gap (Tauc gap) is 0.95 eV, close to the 1.0 eV band gap for crystalline bulk zinc arsenide. A single set of parameters from an ensemble Tauc-Lorentz model can be used to determine the thicknesses of amorphous Zn3As2 layers as long as the layers are \&\#x2273; 25 nm thick. Measured film thicknesses do not correlate with targeted thicknesses, likely due to low sticking coefficients of evaporated zinc arsenide.
To maintain high, broad-band reflectance, thin transparent fluoride layers, such as MgF2, are used to protect the of aluminum mirrors against oxidation since aluminum oxide absorbs short wavelength light. In this study, we present, for the first time, combined X-ray photoelectron spectroscopy (XPS) and ellipsometric (SE) studies of aluminum oxidation as a function of MgF2 over a range of layer thickness (0-6 nm). We also show for the first time, dynamic SE data which, with appropriate modeling, tracks the extent of oxide growth every few seconds over a period of several hours after the evaporated Al + MgF2 bilayer is removed from the deposition chamber, exposing it to the air. For each SE data set, because the optical constants of ultrathin metals films depend strongly on deposition conditions and their thickness, the optical constants for Al, as well as the Al and Al2O3 thicknesses, were fit. SE trends were confirmed by X-ray photoelectron spectroscopy. There is a chemical shift in the Al 2s electron emission peak toward higher binding energy as the metal oxidizes to Al+3. The extent of oxide growth can be modeled from the relative area of each peak once they are corrected for the attenuation through MgF2 layer. This generates an empirical formula: oxide thickness= k*log(t) +b, for the time-dependent aluminum-oxide thickness on aluminum surfaces protected by MgF2 as a function of MgF2 layer thickness. Here, k is a factor which depends only on MgF2 thickness, and decreases with increasing MgF2 thickness. The techniques developed can illuminate other protected mirror systems.
Aluminum enjoys broad band reflectivity and is widely used as an astronomical reflector. However, it oxides rapidly, and this oxide absorbs very short wavelength light, which limits the performance of aluminum mirrors. Accordingly, thin transparent layers, such as films of MgF2, are used to protect aluminum. In this study, we present an X-ray photoelectron pectroscopy (XPS) study of the chemical changes in MgF2 - protected aluminum that take place as it oxidizes (is exposed to the air). XPS reveals the rate of Al oxidation for different MgF2 thicknesses as determined from measurements obtained from 5 min to 8 months of air exposure. The degree of Al oxidation depends on the MgF2 over layer thickness.
While no solid barrier layer is transparent below ~103nm, simulations show that ~9.5nm LiF on 8.5nm MgF2 on Al could reflect some hydrogen Lyman lines better than a single fluoride layer does. Experiments are promising.
