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In search of novel magnetocaloric materials, a number of rare earth-based phases were designed, synthesized and investigated. These compounds were prepared by arc-melting or sintering, followed by annealing at high temperature to obtain phase-pure materials. Single crystal and powder X-ray diffraction were employed for phase identification, purity assessment, structure solution and refinement. Energy dispersive X-ray spectroscopy (EDS) was used to determine sample compositions. A Quantum Design SQUID magnetometer equipped with an alternating current (ac) transport controller (model 7100) was employed to measure magnetic data and evaluate magnetocaloric properties. The crystal structure and physical properties were analyzed via electronic structure calculations. In this thesis work, the RE5Ga3 and RECo2 (RE = rare earth) materials were chosen as a starting point for structural modifications. Specifically, substitution of Co for Ga (and vice versa) or rare earth replacement was used to design new materials. In total, four families were investigated: Ho5Ga3-xCox (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1), Er5Ga3-x(Fe/Co)x (x = 0, 0.4), RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er), and Gd(Co1-xGax)2 (x = 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1). The samples were prepared by arc-melting, wrapped in Ta foil, sealed in evacuated silica tubes and annealed at specific temperatures. The Ho5Ga3-xCox and Er5Ga3-x(Fe/Co)x systems features a Mn5Si3-type-to-Cr5B3-type structural transformation, driven by geometric factors. On the other hand, the structural transformation in the RE(Co0.667Ga0.333)2 and Gd(Co1-xGax)2 systems appears to be controlled by the valence electron count (VEC). The RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er) phases adopt a hexagonal MgZn2-type structure (P63/mmc). Structural and magnetic properties of the MgZn2-type RE(Co0.667Ga0.333)2 materials were investigated via single crystal and powder X-ray diffraction, powder neutron diffraction (PND), and magnetic measurements. In addition to the hexagonal MgZn2-type structure, four other structures were discovered in the Gd(Co1-xGax)2 system: cubic MgCu2- (Fd3 ̅m), orthorhombic MgSrSi- (Pnma), orthorhombic CeCu2- (Imma), and hexagonal AlB2-type structure (P6/mmm). When Ga content increases, the structure moves from a “condensed cluster-based arrangement” to a “3D Network” to a “2D Network”. Meanwhile, coordination number (CN) of Co or Ga atoms changes from 6 to 4, and then to 3. Magnetic properties of many of the RE-based phases were evaluated via temperature- and field-dependent magnetization measurement. Materials exhibited a sharp ferromagnetic transition and their MCE in terms of the isothermal magnetic entropy change, was explored. / Thesis / Doctor of Philosophy (PhD)

Silicon oxide and silicon oxynitride thin films with in-situ cerium (Ce) doping were deposited using electron-cyclotron-resonance plasma enhanced chemical-vapor deposition (ECR-PECVD) on p-type silicon substrates. Oxygen was gradually substituted by nitrogen to produce SiOxNy thin films with different layer compositions. Refractive indices extracted from variable-angle spectroscopic ellipsometry (VASE) measurements classified the thin films into two main groups, SiOx and SiOxNy. The thin film composition was studied by Rutherford Backscattering Spectrometry (RBS), verifying the gradual increase in nitrogen content. Photoluminescence (PL) spectra of samples were obtained using a 375 nm laser diode as an excitation source. All samples were subjected to post-deposition annealing treatment for 1 hour at different temperatures varying from 800 to 1200 °C in both 95% N2 and 5% H2 and pure N2 gas environment, to investigate the effect of hydrogen passivation on the PL irradiance. Samples subjected to annealing yielded considerably stronger blue/white PL emission than as-deposited ones, due to the formation of Ce-containing clusters at a temperature of 1200 °C. Optimum layer composition and annealing condition to produce SiOxNy thin films with maximized Ce3+ excitation efficiency were determined. Besides, the effect of hydrogen fluoride (HF) etching on PL irradiance was studied, showing that an HF (1%) etching duration of 90 s yields the highest PL irradiance. Electrical measurements were carried out for all Ce doped samples as preliminary work for light-emitting device fabrication. ITO and Al are coated as electrodes on the front side of the thin films and backside of the substrates, respectively, using a radio frequency (RF) magnetron sputtering system. I-V measurements were performed to investigate the carrier injection properties and the dominating mechanism of carrier conduction was determined. / Thesis / Master of Applied Science (MASc)

Recent studies funded by the US Department of energy have shown that coal and coal byproducts contain elevated contents of Rare Earth Elements (REEs), making them a potential resource for these critical materials. The approach employed in this research focused on the concentration and extraction of REEs from fine coal refuse derived from various preparation plants in the Appalachian coal basin of the United States. Initial efforts in this research focused on the identification and characterization of REEs in various fine coal refuse streams from nine distinct industrial preparation plants in Appalachia. The average REE content in these materials was determined to be approximately 200 ppm, but the REE content showed a strong correlation to the aluminum content, suggesting that the REEs are closely associated with the clay minerals present in the refuse. Given the relatively low REE concentrations, initial efforts sought to concentrate the REEs through decarbonization and dispersive liberation steps. In these tests, high-shear agitation in the presence of a polyelectrolyte, followed by sedimentation was able to isolate the REE-enriched fine clay particles from siliceous gangue minerals. Following the dispersive liberation step, all samples were found to have an REE content greater than 300 ppm, a benchmark used for many initial exploratory studies. In one case, the REE content was increased by more than 125%. Subsequent extraction tests initially utilized a direct ion-exchange leaching approach with ammonium sulfate as lixiviant. In all cases, the simple ion-exchange leaching process failed to recover significant quantities of rare earth elements, ultimately suggesting that the REEs in fine coal waste may be passivated or bound in a colloidal phase. To access this colloidal phase, several alternative approaches were evaluated, including leaching with alternative ion-exchange lixiviants, reductive leaching, gas-purged leaching, and others. The approach that showed the most promise was strong alkaline pretreatment, followed by ion-exchange leaching with ammonium sulfate at pH 4. A combination of strong alkali and high-temperatures treatment successfully liberated the REEs, converting them to a form amenable to ion-exchange leaching. The highest REE recovery achieved with this method was determined to be 39%. Lastly, bench-scale solvent extraction tests were used to further concentrate REEs in the leach solution and demonstrate that mixed rare earth concentrates can be successfully produced from fine coal refuse. / Master of Science / Since the introduction of personal electronics, rare earth elements (REEs) have become essential raw materials for modern life. They are used in many common household goods such as cell phones, computers, and flat screen TVs. They are also vital components in various industrial, medical, and military applications. Currently, the majority of the world's supply is obtained from China, which has raised concerns on the vulnerability of the supply chain and the potential impacts of supply disruption on clean energy technologies. In light of this risk, the US Department of Energy has classified a number of REEs as critical elements and has subsequently funded research to investigate ways to diversify the supply chain through alternative resources. The approach employed in this research seeks to extract and recover REEs from fine coal refuse. This industrial waste is a byproduct of the coal mining and beneficiation processes. Given the long legacy of coal mining in the Appalachian region, hundreds of millions of tons of fine waste are currently being stored in surface impoundments, and millions of tons of additional fine coal waste is being produced each year from active mining and beneficiation operations. By valorizing this waste material through REE recovery, mining companies will be incentivized to reprocess existing impoundments, ultimately promoting superior economic and environmental outcomes. Despite their name, rare earths are not "rare" from the standpoint of raw abundance; however, their scarcity is derived from the complexity of the extraction and separation processes. In China, the majority of the heavy rare earth elements are produced from ion-exchangeable clays. These clays have REEs weakly attached to the surface, so that they can be readily recovered by washing them with a salt solution that remove the positively charged rare earth ions from surface. The technical approach employed in this project sought to replicate this process for the clay materials found in fine coal refuse. Additional steps were needed to properly concentrate, activate, and extract the REEs; however, the end-to-end processing tests confirmed that mixed rare earth concentrates can be produced from fine coal wastes consisting primarily clay minerals.

Rare earth elements are the 14 lanthanides on the periodic table, plus yttrium and scandium. These elements play a critical role in modern-day technologies such as liquid-crystal displays, GPS systems, and fiber optic cables. A majority of the mining of these elements is from China; however, due to decreasing reserves a need for alternative processes for extracting and processing rare earth elements (REEs) is becoming increasingly important. Special focus has been placed upon the identification of REEs within coal refuse, but the phase designation and speciation is not fully understood. This investigation focuses on the characterization, speciation, and morphology of REEs within fine and coarse coal refuse. During this study, physical and chemical characterization was conducted on coal refuse samples to understand characteristics, which influence REE phase designation. Experimental methods were chosen to specifically evaluate REE content and speciation across four key characteristics: size distribution, density, seam location, and thermal decomposition. Characterization of the refuse material was conducted in two campaigns: (1) an exploratory campaign, which focused on size distribution, and physical imaging of REEs within fine refuse, and (2) a detailed campaign, which utilized sequential chemical extraction methods alongside calcination to understand the phases in which REEs are present in coarse refuse. The results show that REEs within fine coal refuse are smaller than ten microns and found with phosphorus. In general, as size decreased REE content increased, likely due to increased clay content. Further conclusion could not be drawn from simple microscopic analysis. Consequently, detailed chemical characterization was conducted to fully understand REE speciation. The tests showed that a majority of REEs within coarse refuse were within insoluble species. A calcination treatment was found to greatly increase the recovery of REEs from the metal oxide fraction, thus increasing the overall soluble species contained within the coarse refuse material. / Master of Science / Due to increasing global demand and limited reserves, alternative sources for rare earth elements (REEs) have become an increasingly important research topic. REEs are a vital component of many modern technologies, including GPS systems, fiber optic cables, and LCD screens. Current mining of REEs is primarily from Chinese reserves which are becoming increasing depleted and are not strictly regulated for environmental impact. Due to these challenges, other resources of REEs are of increasing importance. Prior research has found coal and associated byproducts to have concentrations of REEs that could be economically exploited, reducing the rate of depletion of REE resources worldwide. To develop more efficient and cost-effective processing methods, fundamental information on the mineral composition of REE-bearing materials is needed. With this information, engineers can develop better processes that can specifically target REE-containing minerals while maximizing economic and environmental outcomes. This research seeks to overcome this knowledge gap through advanced material characterization and well-controlled laboratory process testing of coal refuse. The results show that REEs typically congregate in specific material fractions (e.g. fine size, moderate density), and these materials can be readily transformed through simple heat treatment. This transformation greatly improves the processability and provides a pathway for the economic recovery of REEs from coal wastes. The further development and deployment of these technologies can have societal benefits such as: more jobs, reduced reliance on foreign sources, and environmental cleanup of current coal waste deposits.

M.S.

D.Phil. (Physics) / Please refer to full text to view abstract

As Terras Raras (TRs), particularmente a série dos lantanídios, são consideradas elementos estratégicos da tecnologia moderna devido ao seu papel essencial na catálise petroquímica, lasers, materiais luminescentes, e na fabricação de superimãs necessários para conversão de energia em carros elétricos e geradores eólicos. Atualmente, o comércio internacional de TRs é dominado pela China (> 90%). O Brasil apresenta um enorme potencial em termos de reservas de TRs, mas o país ainda carece de uma tecnologia competitiva para o processamento mineral, incluindo a separação e produção de elementos. Processos hidrometalúrgicos baseados na extração por solvente e troca iônica são normalmente empregados pelos produtores, mas os processos são bastante caros e poluentes, envolvendo centenas de reatores e colunas para superar as propriedades químicas muito similares dos elementos. Esta tese relata uma tecnologia alternativa verde chamada Nanohidrometalurgia Magnética (NHMM), que faz parte dos nossos esforços para introduzir a nanotecnologia na área mineral. A tecnologia é baseada em nanopartículas superparamagnéticas, previamente projetadas com um revestimento protetor e um agente complexante específico, como o DTPA, para sequestrar os elementos estratégicos da solução lixiviada. NHMM permite a captura e processamento de elementos metálicos diretamente a partir de solução aquosa, sem empregar solventes orgânicos. Depois de confinar o elemento com o uso de um ímã externo (Nd2Fe14B), ele pode ser facilmente separado da mistura e liberado pela aplicação de condições ácidas leves, enquanto as nanopartículas magnéticas retornam ao processo, para um novo procedimento em lote. Na escala laboratorial, todo o procedimento pode ser realizado no mesmo reator, cumprindo, além das instalações de processamento e recuperação, os requisitos mais importantes da Química Verde. Esta tese foi concentrada na extração e separação de elementos de lantanídios da monazita. Um estudo detalhado termodinâmico e cinético foi realizado para compreender o processo. O intrigante comportamento magnético das nanopartículas magnéticas revestidas de lantanídios também foi investigado, fornecendo um ensaio crítico de seu comportamento magnetoforético em solução aquosa. O comportamento magnetoforético das nanopartículas superparamagnéticas permitiu monitorar sua interação direta com os íons lantanídeos, ilustrando uma nova perspectiva no processo de separação. / Rare earths (RE), particularly the lanthanide series, are considered strategic elements in modern technology because of their essential role in petrochemical catalysis, lasers, luminescent materials, and in the fabrication of strong magnets required for energy conversion in electric cars and aeolic generators. Currently, the international RE commerce is dominated by China (> 90%). Brazil exhibits a huge potential in terms of RE reserves but the country still lacks a competitive technology for mineral processing, including the element separation and production. Hydrometallurgy processes based on solvent extraction and ionic exchange are normally employed by the producers, but the processes are rather expensive and polluting, involving hundred reactors and columns to overcome the challenge of dealing with elements exhibiting very similar chemical properties. This thesis reports an alternative green technology named Magnetic Nanohydrometallurgy (MNHM), reinforcing our attempts and efforts to introduce Nanotechnology in the mineral area. The MNHM technology is based on superparamagnetic nanoparticles, previously engineered with a protecting coating and a specific complexing agent, such as DTPA, for sequestering the strategic elements from the lixivia solution. MNHM allows the capture and processing of metal elements directly from aqueous solution, without employing organic solvents. After confining the element with the use of an external magnet (Nd2Fe14B), it can be easily separated from the mixture, and released by applying mild acidic conditions, while the magnetic nanoparticles return to the process, for a new batch procedure. At the laboratory scale, the entire procedure can be performed in the same reactor, fulfilling, in addition to the processing and recovery facilities, the most important requisites of Green Chemistry. This thesis was concentrated on the extraction and separation of lanthanide elements from monazite. A detailed thermodynamic and kinetic study was carried out to support the ideas, leading to a new separation technology, capable of obtaining and processing the lanthanides under green and sustainable conditions. The magnetophoretic behavior of the superparamagnetic nanoparticles allowed to monitor their direct interaction with the lanthanide ions, illustrating a new perspective in the separation process.

Graduation date: 1998

Thesis (Ph. D.)--Montana State University--Bozeman, 2003. / Typescript. Chairperson, Graduate Committee: Rufus L. Cone. Includes bibliographical references (p. 361-380).