Thermodynamics and Reaction Kinetics During Mechanochemical Synthesis and Environmental Testing of Lanthanide and Actinide Refractory Materials

High energy ball milling is a broad family of well-known techniques for materials processing that includes mechanochemical synthesis, in which the milled materials react during milling. The latter may involve components reacting in the solid, liquid, or gas phases, and the operative kinetics have hardly been studied. Several manufacturers of high energy ball mills have recently made available instrumented milling vessel lids. These lids are able to monitor the temperature of the lid and the gas pressure within the vessel in situ during milling experiments. This ability is a significant improvement over the ad hoc instrumentation setups sometimes discussed in literature, and enables a level of precision in investigating milling processes not available previously. This dissertation describes work done investigating the mechanochemical synthesis and compound formation in the Ce-Si, Ce-S, U-Si, and Dy-N systems, including the thermodynamic and kinetics processes that govern mechanochemical synthesis of these materials. Particular attention is devoted to two distinct mechanochemical phenomena. The first process is a fast reaction that occurs after a characteristic milling time, termed mechanically-induced self-propagating reaction (MSR). This behavior is sometimes observed during milling of the elemental constituents of compounds with a high enthalpy of formation. The second process is compound formation during milling of elemental metals in reactive gasses such as oxygen, nitrogen, or hydrogen. Mechanochemical synthesis experiments have been conducted while monitoring the temperature and pressure of the milling vessel in situ to gain insight into the reaction kinetics. A new approach to analyzing this data in conjunction with simple physical models of the milling mechanics has been developed. The effects of compound formation on the system geometry and energy transfer were assessed by experiment. The kinetics model explicitly considers milling energy and energy transfer to form chemically active surface site generation, and should be applicable to many systems involving gas reactions with ductile solids. Furthermore, the model allows for the incorporation of thermally activated kinetics if appropriate experimental data is available. The results discussed in this dissertation are drawn from four research papers submitted or accepted by peer-reviewed journals. Several unique phenomena are observed due to a recently developed capability for in situ monitoring of temperature and pressure during milling. As such, this dissertation represents several significant steps forward in the understanding of the processes occurring during mechanochemical synthesis

Size, Surface Structure, and Doping Effects on Ferromagnetism in SnO\u3csub\u3e2\u3c/sub\u3e

The effects of crystallite size, surface structure, and dopants on the magnetic properties of semiconducting oxides are highly controversial. In this work, Fe:SnO2 nanoparticles were prepared by four wet-chemical methods, with Fe concentration varying from 0% to 20%. Analysis confirmed pure single-phase cassiterite with a crystallite size of 2.6 ± 0.1 nm that decreased with increasing. Fe% doped substitutionally as Fe3+. Pure SnO2 showed highly reproducible weak magnetization that varied significantly with synthesis method. Interestingly, doping SnO2 with Fe \u3c 2.5% produced enhanced magnetic moments in all syntheses; the maximum of 1.6 × 10−4 µB/Fe ion at 0.1% Fe doping was much larger than the 2.6 × 10−6 µB/Fe ion of pure Fe oxide nanoparticles synthesized under similar conditions. At Fe ≥ 2.5%, the magnetic moment was significantly reduced. This work shows that (1) pure SnO2 can produce an intrinsic ferromagnetic behavior that varies with differences in surface structure, (2) very low Fe doping results in high magnetic moments, (3) higher Fe doping reduces magnetic moment and destroys ferromagnetism, and (4) there is an interesting correlation between changes in magnetic moment, bandgap, and lattice parameters. These results support the possibility that the observed ferromagnetism in SnO2 might be influenced by modification of the electronic structure by dopant, size, and surface structure

Role of Oxygen Defects on the Magnetic Properties of Ultra-Small Sn\u3csub\u3e1−x\u3c/sub\u3eFe\u3csub\u3ex\u3c/sub\u3eO\u3csub\u3e2\u3c/sub\u3e Nanoparticles

Although the role of oxygen defects in the magnetism of metal oxide semiconductors has been widely discussed, it is been difficult to directly measure the oxygen defect concentration of samples to verify this. This work demonstrates a direct correlation between the photocatalytic activity of Sn1−xFexO2 nanoparticles and their magnetic properties. For this, a series of ~2.6 nm sized, well characterized, single-phase Sn1−xFexO2 crystallites with x = 0−0.20 were synthesized using tin acetate, urea, and appropriate amounts of iron acetate. X-ray photoelectron spectroscopy confirmed the concentration and 3+ oxidation state of the doped Fe ions. The maximum magnetic moment/Fe ion, μ, of 1.6 × 10−4 μB observed for the 0.1% Fe doped sample is smaller than the expected spin-only contribution from either high or low spin Fe3+ ions, and μ decreases with increasing Fe concentration. This behavior cannot be explained by the existing models of magnetic exchange. Photocatalytic studies of pure and Fe-doped SnO2 were used to understand the roles of doped Fe3+ ions and of the oxygen vacancies and defects. The photocatalytic rate constant k also showed an increase when SnO2 nanoparticles were doped with low concentrations of Fe3+, reaching a maximum at 0.1% Fe, followed by a rapid decrease of k for further increase in Fe%. Fe doping presumably increases the concentration of oxygen vacancies, and both Fe3+ ions and oxygen vacancies act as electron acceptors to reduce e−-h+ recombination and promote transfer of electrons (and/or holes) to the nanoparticle surface, where they participate in redox reactions. This electron transfer from the Fe3+ ions to local defect density of states at the nanoparticle surface could develop a magnetic moment at the surface states and leads to spontaneous ferromagnetic ordering of the surface shell under favorable conditions. However, at higher doping levels, the same Fe3+ ions might act as recombination centers causing a decrease of both k and magnetic moment μ

Unusual Crystallite Growth and Modification of Ferromagnetism Due to Aging in Pure and Doped Zno Nanoparticles

We report the unusual growth of pure and Fe-doped ZnO nanoparticles prepared by forced hydrolysis and the weakening of ferromagnetism due to aging in ambient conditions. More than four dozen nanoparticle samples in the size range of 4–20 nm were studied over 1 to 4 years. The as-prepared samples had significant changes in their crystallite sizes and magnetization as they aged in ambient conditions. Detailed studies using x ray diffraction and transmission electron microscopy (TEM) demonstrated that the crystallite size increased by as much as 1.4 times. Lattice parameters and strain also showed interesting changes. Magnetometry studies of Zn1−xFexO with x = 0–0.2 showed ferromagnetism at room temperature; however, keeping the samples in ambient conditions for one year resulted in modifications in the crystallite size and magnetization. For the Zn0.95Fe0.05O sample, the size changed from 7.9 nm to 9.0 nm, while the magnetization decreased from 1×10–3emu/g (memu/g) to 0.2 memu/g. Both magnetic and structural changes due to aging varied with the environment in which they were stored, indicating that these changes are related to the aging conditions

Magnetism of ZnO Nanoparticles: Dependence on Crystallite Size and Surfactant Coating

Many recent reports on magnetism in otherwise nonmagnetic oxides have demonstrated that nanoparticle size, surfactant coating, or doping with magnetic ions produces room-temperature ferromagnetism. Specifically, ZnO has been argued to be a room-temperature ferromagnet through all three of these methods in various experimental studies. For this reason, we have prepared a series of 1% Fe doped ZnO nanoparticle samples using a single forced hydrolysis co-precipitation synthesis method from the same precursors, while varying size (6 – 15 nm) and surface coating concentration to study the combined effects of these two parameters. Size was controlled by modifying the water concentration. Surfactant coating was adjusted by varying the concentration of poly acrylic acid (PAA) in solution. Samples were characterized by x-ray diffraction, transmission electron microscopy, x-ray photoelectron spectroscopy, optical absorptance spectroscopy, and magnetometry. No clear systematic effect on magnetization was observed as a function of surfactant coating, while evidence for a direct dependence of magnetization on the crystallite size is apparent

A Transdisciplinary Approach to Determining the Provenience of a Distorted, Pre-Columbian Skull Recovered in Rural Idaho

Transdisciplinary research involves cooperation, exchange of information, sharing of resources and integration of disciplines to achieve a common scientific goal. In this study, collaborators utilized tools and knowledge of materials science, anthropology, archaeology, geosciences and biology in an attempt to determine the provenience of skeletal remains of unknown origin. The exchange of ideas and skills along with the crossing of disciplines in this study sucessfully allowed the incorporation of expertise from many team members. This transdisciplinary approach to research provided a more comprehensive and detailed analysis than any one field alone could provide. An archaeological assessment of a human skull recovered in rural Idaho recognized cranial deformation and post-mortem application of a red pigment. A combination of scanning electron microscopy (SEM), x-ray fluorescence (XRF) and energy-dispersive x-ray spectroscopy (EDS) identified the major and trace elements present in the red post-mortem pigment as cinnabar and rare earth metals. Analysis via carbon and oxygen stable isotopes from teeth and bone to provided insight into the diet and habitat for distinct segments of the individual’s life, indicating a regional separation in early life versus late adulthood. Radiocarbon dating determined the approximate age of the skull to be between 600-700 years old and a forensic mtDNA assessmentcategorized a mitochondrial haplogroup for the remains as originating from the East African or Arabian Peninsula

Mechanochemical Synthesis of Cerium Monosulfide

Cerium sulfides were prepared by high-energy milling of cerium and sulfur. The reaction kinetics were observed by monitoring temperature and pressure in situ. For the first time, it is demonstrated that cerium monosulfide, which has historically been produced only at very high temperatures using both toxic and flammable gases, can be produced from a stoichiometric mixture of the elemental powders at near room temperature. It is shown that the monosulfide is produced through a mechanochemically induced self-propagating reaction of intermediate compounds

Mechanochemical Synthesis of Uranium Sesquisilicide

Uranium sesquisilicide (U3Si2) has been prepared by high energy ball milling of the elemental powders. Starting materials were combined in a planetary ball mill and milled with a 10:1 ball-to-powder charge for varying times between 0 and 24 hours. Temperature and pressure of the milling vial were monitored to gain insight into reaction kinetics. The development of USi3 as an intermediate phase is discussed. Starting materials and as-milled powders were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), demonstrating the viability of mechanochemical synthesis for U3Si2