Theses and Dissertations - Department of Metallurgical and Materials Engineering

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The recovery of the by-products of coal. A monograph describing the Semet-Solvay By-Products Plant and processes at Holt, Ala.
(University of Alabama Libraries, 1919) Glazner, John Frank; University of Alabama Tuscaloosa
A hundred years of development have passed since the streets of the city of London were first lighted with coal gas. This beginning of preparing coal gas by the distillation of cial has been expanded and developed into our modern great by-products recovery industries. The history of these years plainly shows how the proper utilization of by-products can revolutionize an industry. No attention was given to the recovery of by-products of coal as a factor in reducing the cost of production, when coal gas was produced first by the distillation of coal. Special laws were enacted in England at one time to prevent the contamination of streams and lakes with tar. We are now beginning to realize how wasteful our methods of coking coal have been and how necessary it is to conserve our supplies of fuel.
Designing, manufacturing, testing, and optimizing of micro-fuel cells
(University of Alabama Libraries, 2009) Lu, Yuhao; Reddy, R. G.; University of Alabama Tuscaloosa
Micro-fuel cells are considered as promising electrochemical power sources in portable electronic devices. Performance of micro-fuel cells are closely related to many factors, such as processes of fabrication, designs of flow fields, and operating conditions. In the present research, micro-proton exchange membrane fuel cells (PEMFCs) and micro-direct methanol fuel cells (DMFCs) were systematically investigated from the aspects of structure design, bipolar/end plates (BPs) fabrication, and fuel cells evaluation. In chapter 3, compared with conventional machining and rapid prototyping (RP) technology, microelectromechanical system (MEMS) technology was the practicable method to fabricate the BPs with channels of a few microns width. Experimental and modeling methods were employed to analyze performance of the micro-PEMFC in chapter 4. Contact resistance changed significantly the distribution of overpotential in the micro-PEMFC and decreased the current output. Small dimensions of the micro-channel drastically affected the species transport and resulted in a non-uniform current distribution along channel direction at low cell potential (high current). In chapters 5, four kinds of flow fields, mixed multichannel serpentine with wide channels, single channel serpentine, double channel serpentine, and mixed multichannel serpentine with narrow channels, were applied to micro-PEMFCs. Results suggested that the micro-PEMFC with good performance should use the flow field with a mixed multichannel design and long micro-channels. In chapter 6, the same flow fields were studied in the micro-DMFCs. Concentration and flow rates of methanol solution affected performance of micro-DMFCs. A micro-DMFC with the long and narrow channels needed to take long time to reach the stable stage when an electric load on it was changed. In chapter 7, a passive air-breathing micro-DMFC with low loading of catalysts was developed. Performance of the passive micro-DMFC became poor with the increase in concentration of methanol solution. Power densities of the passive micro-DMFC drastically depended on the current scanning rate. Finally, cobalt phthalocyanine was introduced to platinum catalyst system to improve and optimize the micro-DMFCs. After heat-treatment at 635 oC, CoPc-Pt/C demonstrated good electrocatalytic activity for oxygen reduction reaction (ORR) and high methanol tolerance. However, CoPc-Pt/C heated at 980 oC showed a good electrocatalytic activity for MOR in DMFCs.
The effect of copper on the eutectoid transformation in ductile iron
(University of Alabama Libraries, 2010) Samuel, Chris; Viswanathan, S.; University of Alabama Tuscaloosa
As a result of the shortage in the availability of suitable steel scrap, trace elements are unintentionally added to ductile iron from the scrap available for melting. The effect of some of these trace elements on graphite shape, the resulting microstructure, and the dimensional behavior of the cast component are not well understood. The lack of control of these trace elements leads to excessive scrap as well as additional heat treatment costs, especially when ferritic or fully pearlitic microstructures are required. This work focuses on the effect of one element, copper, that occurs as a trace element or is often deliberately added when pearlitic microstructures are desired. Ductile iron samples with copper levels ranging from 0 to 0.8 wt. % were investigated. Gleeble dilatometry was used to characterize phase transformations and microstructure development. The diffusion coefficient of carbon in ferrite in the presence of copper and silicon was measured using multicomponent solid-solid diffusion experiments. Copper appears to have little or no effect on the diffusion coefficient of carbon in ferrite. Interrupted solidification experiments are used to explain solidification and segregation in ductile iron, and a revised model of ductile iron solidification is presented. It is shown that the segregation of copper during solidification is key to the pearlite promoting effect of copper and is related to the decrease in the driving force for the diffusion of carbon through the ferrite shell.
Investigation of the effect of hafnium and chromium additions on the microstructures and short-term oxidation properties of DC magnetron sputtered β-nial bond coats deposited on Ni-based superalloys
(University of Alabama Libraries, 2010) Bestor, Michael Alan; Weaver, Mark Lovell; University of Alabama Tuscaloosa
Thermal barrier coatings play a major role in protecting turbine blades from extreme operating environments and extending service lifetimes. Reactive elements (e.g. Zr, Hf, Y, Si, etc.) have been shown to enhance the oxidation performance of such coating systems when added in appropriate amounts to overlay bond coats. This study investigated the influences of processing parameters along with Hf and Cr additions on the short-term oxidation performance of β-NiAl based coatings deposited via direct current magnetron sputtering onto CMSX-4 and René N5 substrates. Sputtering parameters were optimized to yield a zone T microstructure. The results indicate that the coatings are deposited as a solid solution and precipitation with the RE-doped coatings occur following annealing at 1000°C for times up to four hours. Precipitates form heterogeneously within the coatings with larger precipitates forming at grain boundaries and smaller ones forming within the grains at prior dislocation lines. Small incorporations of Cr into the NiAl-1Hf coating increased the average grain size and precipitate size. Transmission electron microscopy and atom probe tomography confirmed that the chemistry of the precipitates is mostly β'-Ni2AlHf accompanied by HfC and α-Cr. The results from the isothermal oxidation studies at 1050°C indicated that increasing the preoxidation annealing time from two to four hours decreased the mass gains observed with the specimens up to 100 hours. However, significant oxidation at the coating/substrate interface was discovered and the amount of oxidation increased with Hf content and preoxidation annealing time. This oxidation is thought to be caused by the large number of pinhole defects with the iii zone T microstructure and large grain boundary volume. Increased Hf concentrations were also found at the coating/substrate interface and this has been shown to lead to dramatic internal oxidation. The NiAlCrHf samples contain larger grain sizes and precipitates and a thinner TGO than the NiAl-Hf coatings. This combined with the lower mass gains during isothermal oxidation indicate that the NiAlCrHf coatings rapidly form a thin, protective α-Al2O3 layer that limits additional transport of oxygen to the bond coat. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl-Hf coatings.
Microstructural formations and phase transformation pathways in tantalum carbides
(University of Alabama Libraries, 2010) Morris, Robert Allen; Thompson, Gregory B.; University of Alabama Tuscaloosa
Transition metal carbides have a large assortment of applications because of their high hardness, chemical resistance, and high melting temperatures. Tantalum carbide (TaC) and its sub-stoichiometric Ta2C and Ta4C3 phases have emerged as candidate materials for ultra-high temperature structural applications. A consequence of the high melting temperature is the limiting methods to fabricate near-net shape, near full density tantalum carbides. In general, hot-isostatic pressing (HIP) and/or arc melting/vacuum plasma spraying (VPS) of powders are the viable means of manufacturing. In HIP'ing, the phase formation is through solid-state reactions whereas arc melting/VPS involves rapid solidification. Additionally, the precipitation of multiple phases generates various orientation relationships that influence the grain morphology. Depending on carbon content, the grains were equiaxed, equiaxed with a cross-hatch pattern of thin laths of secondary phases, to acicular grains. The microstructures were quantified through a series of different 2D and 3D analytical techniques. To understand how these microstructures developed, a series of XTa:(1-X)C (0.5
Fundamental studies on electrochemical production of dendrite-free aluminum and titanium-aluminum alloys
(University of Alabama Libraries, 2010) Pradhan, Debabrata; Reddy, R. G.; University of Alabama Tuscaloosa
A novel dendrite-free electrorefining of aluminum scrap was investigated by using AlCl_3 -1-Ethyl-3-methyl-imidazolium chloride (EMIC) ionic liquid electrolyte. Electrodeposition of aluminum were conducted on copper/aluminum cathodes at voltage of 1.5 V, temperatures (50-110ºC), stirring rate (0-120 rpm), molar ratio (MR) of AlCl_3 :EMIC (1.25-2.0) and electrode surface modification (modified/unmodified). The study was focused to investigate the effect of process variables on deposit morphology, cathode current density and their role in production of dendrite-free aluminum. The deposits were characterized using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Modified electrodes and stirring rate (60 rpm) eliminate dendritic deposition by reducing cathode overpotential below critical overpotential (-0.54 V) for dendrite formation. Pure aluminum (>99%) was deposited with current efficiency of 84-99%. Chronoamperometry study was conducted using AlCl_3 -EMIC and AlCl_3 -1-Butyl-3-methyl-imidazolium chloride (BMIC) (MR = 1.65:1) at 90ºC to understand the mechanism of aluminum electrodeposition and find out diffusion parameter of electroactive species Al_2 C_7 ^- . It was concluded that electrodeposition of aluminum is a diffusion controlled instantaneous nucleation process and diffusion coefficient of Al_3 C_7 ^- was found to be 5.2-6.9 × 10-11 m^2 /s and 2.2 × 10-11 m^2 /s for AlCl_3 -EMIC and AlCl_3 -BMIC, respectively. A novel production route of Ti-Al alloys was investigated using AlCl_3 -BMIC-TiCl_4 (MR = 2:1:0.019) and AlCl_3 -BMIC (MR = 2:1) electrolytes at constant voltages of 1.5-3.0 V and temperatures (70-125°C). Ti sheet was used as anode and cathode. Characterization of electrodeposited Ti-Al alloys was carried out using SEM, EDS, XRD and inductively coupled plasma-optical emission spectrometer (ICP-OES). Effect of voltage and temperature on cathode current density, current efficiency, composition and morphology of Ti-Al alloys were determined. The Ti-Al alloys containing about 13-27 atom % Ti were produced using both electrolytes. The current efficiency of AlCl_3 -BMIC was varies between 79-87%. But lower current efficiency (25-38%) was obtained for AlCl_3 -BMIC-TiCl_4 electrolytes due to the formation of TiCl_3 passive layer on the electrodes. To increase the productivity, constant current method (160-210 A/m^2 ) was implemented. This fundamental study on low temperature production of dendrite-free aluminum and Al-Ti alloys is not only efficient but also opens a novel route in aluminum and titanium process metallurgy.
Perpendicular magnetic anisotropy materials for reduced current switching devices
(University of Alabama Libraries, 2010) Tadisina, Zeenath Reddy; Gupta, Subhadra; University of Alabama Tuscaloosa
Recently, spin-transfer switching of magnetic tunnel junctions (MTJ's) has become a very active area of research. It is theoretically postulated that using perpendicular magnetic anisotropy materials will substantially reduce the critical current density for switching, resulting in lower energy devices, while keeping the thermal stability high. A range of perpendicular anisotropy material systems, including (i) multilayers, (ii) crystalline alloys, and (iii) amorphous alloys have been intensively studied in this dissertation. The surface and bulk anisotropy, damping parameter, and structural properties of these material systems have been investigated. Magnetic tunnel junctions based on some of these perpendicular material schemes have been fabricated, and their transport properties have been measured and related to the anisotropy. We have found several promising approaches to magnetic tunnel junctions utilized in spin-torque transfer random access memory (STT-RAM).
Quantitative microanalysis techniques for magnetic nanostructures
(University of Alabama Libraries, 2010) Henry, Karen Torres; Thompson, Gregory B.; University of Alabama Tuscaloosa
Microanalysis techniques are used to characterize magnetic nanostructures. To advance these materials in many applications, it is necessary to understand the microstructure. For example, subtle compositional fluctuations within a nanostructure can significantly influence the material properties. In this work, microanalysis techniques have been used to quantify composition, volume fraction, and long-range order parameter in magnetic nanostructures. A methodology for determining an optimal voxel dimension range was developed using a model system for the experimentally collected atom probe tomography data. The influence of voxel dimension on volume fraction and composition of chemically partitioned phases is examined. Atom probe tomography is used to understand the influence of Pt enrichment at grain boundaries in the A1 to L10 polymorphic phase transformation. The Pt enrichment at grain boundaries in atom probe tomography analysis provides experimental verification of modeling predictions of Pt surface segregation. It is also observed that upon phase transformation to L10, the Pt grain boundary enrichment decreased. Field ion microscopy and atom probe tomography is used to evaluate the field evaporation behavior of (001) planes in ordered FePt. Both experimental and simulation results have shown that the difference in evaporation field between the two components of the alloy contributed to the trajectory aberrations near the (002) pole and zone axes. The model system shows that chemical order within a structure introduces aberrations in the reconstruction of the atomic planes limiting the spatial and chemical fidelity of characterizing such structures using current reconstruction methodologies. Finally, a comparison of the experimental results to simulations is used to assess the viability of electron diffraction in the quantification of S in FePt thin films and nanoparticles. A multislice approach was used to simulate CBED patterns of FePt films with various thicknesses, compositions, orientations, and S values. In general, electron diffraction provides a technique to determine order parameter of small volumes with the implementation of multislice simulations.
In situ growth stresses in iron-platinum and iron-copper alloy
(University of Alabama Libraries, 2010) Fu, Bianzhu; Thompson, Gregory B.; University of Alabama Tuscaloosa
Intrinsic thin film stress is evitable with the thin film deposition process and plays an important role in tuning the physical properties of thin films. In this thesis, the in situ and post growth stress evolution of the Fe-Pt and Fe-Cu alloy system was studied and correlated to the microstructure evolutions. At ambient temperature and constant deposition pressure, the growth stresses of both the Fe-Pt and Fe-Cu alloy were found to be dependent on the compositions and affected by their growth rates. The final intrinsic stress states after growth could be tuned to be either tensile, zero or compressive depending upon composition and deposition rate for similar grain sizes. This is due to the preferential segregation of one species (the more mobile element) to the grain boundaries. At elevated growth temperatures, the Fe-Pt alloy forms ordered phase while the Fe-Cu alloy forms phase separation. The magnitude of the compressive stress state is reduced as the Fe54Pt46 thin film orders in situ during growth. The compressive stress relaxation rate is increased with increasing substrate temperature or order parameter. This compressive stress reduction has been rationalized as a reduction of adatom mobility on the surface as Fe and Pt occupy specific lattice sites for L10 on each grain. The ordered nature of the grains contributes to additional chemical energy at the boundary which, upon ceasing deposition, significantly increases the stress relaxation rate. In contrary, the growth compressive stress of the Fe51Cu49 alloys in the continuous growth regime is increased with substrate temperature. This has been rationalized as the migration of adatoms to thermodynamically preferred surfaces during growth.
Tantalum clustering in the A1 to L1o iron-platinum-tantalum chemical ordering phase transformation
(University of Alabama Libraries, 2010) Means, Diondra; Thompson, Gregory B.; University of Alabama Tuscaloosa
A series of Fe₅₂Pt₄₈, Fe₅₂․₃Pt₄₆․₃Ta₁․₄ and Fe₅₂Pt₄₀․₇Ta₇․₃ thin films were sputter- deposited and subsequently annealed at 550°C and 750°C for 30 min. The as-deposited films adopted the A1 phase. The addition of Ta resulted in the change in the {111} to {200} fiber texture for these films. This has been explained in terms of the competition between surface energy and strain energy. The Fe₅₂Pt₄₈ and Fe₅₂․₃Pt₄₆․₃Ta₁․₄ films ordered into L1₀ at 550°C but the Fe₅₂Pt₄₀․₇Ta₇․₃ did not. Upon annealing at 750°C all three films phase transformed to the L1₀ phase. As the Ta concentration increased, the grain size was refined and hindered grain growth. Atom probe tomography revealed nano-scale clustering upon annealing for films that contained Ta. For the lower 1.4 at.% Ta concentration film, these clusters were predominately within the grains. For the 7.3 at.% Ta film, the clusters formed predominately in the grain boundaries. The formation of clusters appears to be a necessary initial step to allow the short range A1 to L1₀ ordering processes in FePt to occur.
An analysis of the grain refinement of magnesium by zirconium
(University of Alabama Libraries, 2010) Saha, Partha; Viswanathan, S.; University of Alabama Tuscaloosa
A Design of Experiments (DOE) approach was used to conduct a systematic study of the grain refinement of magnesium by zirconium; variables included the amount of zirconium, the pouring temperature, and the settling time prior to casting. Samples were poured into a special "hockey puck" mold designed to reproduce the conditions in permanent mold casting. Optical and scanning electron microscopy (SEM) was utilized to measure the grain size in the final microstructure. Sample dissolution followed by SEM was used to characterize zirconium particle size and morphology both in the master alloy and grain refined samples, while an AccuSizer 770 Photozone/Light Obscuration instrument was used to measure total particle size distributions in the master alloy and grain refined samples. Transmission Electron Microscopy (TEM) was used to identify particles that likely act as suitable heterogeneous nucleation sites for grain refinement. The TEM results show that a range of particle sizes are likely substrates and that only zirconium particles which are faceted are likely nucleation sites. It is apparent that only 1 to 3% of the total particles serve as nucleation sites, but a comparison of the grain density vs. faceted particle density shows close agreement. Equal Channel Angular Extrusion (ECAE) processing of the magnesium-15wt% zirconium master alloy to increase the number of faceted particles resulted in improved grain refinement efficacy. This work suggests that there is a tremendous potential to engineer a more efficient grain refiner.
Microstructures and oxidation behavior of sputter-deposited overlay coatings based on â-NiAl
(University of Alabama Libraries, 2011) Alfano, Joel; Weaver, Mark Lovell; University of Alabama Tuscaloosa
Nickel-based superalloy components in the hot sections of commercial gas turbine engines are often protected by aluminide coatings due to their ability to function in oxidative and corrosive environments. However, the microstructures of these coated systems are metastable and change in service due to interactions with the environment and interdiffusion with the underlying substrate. The extent of these changes depends critically upon coating microstructure, chemistry, and the environment that the coated component operates in. This thesis highlights the influences of chemical composition, post-deposition annealing, and isothermal oxidation at 1050°C on the microstructures and properties of NiAl-Zr and NiAl-Cr-Zr overlay bond coatings. In particular, the results indicated that in slightly Ni-rich NiAl-based coatings, coating/substrate interdiffusion and Al-depletion within the coating could be inhibited by increasing the Zr content from 0.3 at.% to 1.0 at.% Zr. However, subsequent additions of 5 at.% Cr to coatings containing 1 at.% Zr, resulted in interdiffusion and Al-depletion levels more similar to low Zr or Zr-free coatings. Results are discussed relative to conventional coating systems.
Processing and characterization of tantalum-hafnium carbides
(University of Alabama Libraries, 2011) Schulz, Bradford Christopher; Thompson, Gregory B.; University of Alabama Tuscaloosa
A series of (TaC)100-x(HfC)x specimens were manufactured by two different processing techniques and studied to determine the effect of hafnium content on the tantalum carbide microstructure. The first set of (TaC)100-x(HfC)x specimens, where X is 0.3, 3.0, 16.5, and 19.8 at.% HfC, were fabricated by the vacuum plasma spraying process (VPS) with subsequent sintering and hot isostatic pressing (HIPing) to homogenize the microstructure. The second set of specimens of (TaC)100-x(HfC)x, where X is 5.8, 10.7, 17.6, and 25.0 at.% HfC, were fabricated from arc melted powder blends (AMPP) of these compositions with subsequent hot isostatic pressing and spark plasma sintering (SPS). It was found that as HfC content increased, the grain size was reduced, the porosity fraction increased, and volume fraction of TaC, Ta2C and Ta4C3 changed. The reduction of grain size with increasing HfC content has been explained by the system being driven further into a compositionally lower melting temperature phase field upon solidification of either the powders in the VPS plasma plum or the initial state of the arc melted powders for AMPP-SPS. This increase in liquid fraction caused greater under-cooling and the formation of more nucleation sites that lead to a finer grain size. The changing volume fraction of (TaC)100-x(HfC)x and sub-stoichiometric tantalum carbide phases has been explained through the unequal loss of constituent species during processing. The addition of HfC content improved the micro hardness values as tested by Knoop indentation at room temperature. It was observed that the micro hardness values increased with respect to increasing HfC content. The addition of HfC content was found to improve the oxidation resistance at 1000oC. The oxide scale was composed primarily of orthogonal and hexagonal Ta2O5 and monoclinic HfO2 phases.
Containerless melting and characterization of cast magnesium AZ31-B alloy at low superheat in the magnetic suspension melting process
(University of Alabama Libraries, 2011) Rimkus, Nathan Wayne; El-Kaddah, N.; University of Alabama Tuscaloosa
This study deals with containerless induction melting and characterization of cast magnesium AZ31-B alloy at low superheat via the Magnetic Suspension Melting (MSM) process. The operating conditions for melting and confinement of a 63.5mm molten column of the alloy with respect to current and frequency were found to range from 770-830 Ampere and 3552-3600 Hz, respectively. The macro/micro-structure, oxide entrapment, segregation of alloying elements, and the formation of intermetallic precipitates in MSM cast AZ31-B alloy in a unidirectional bottom chilled ceramic mold were investigated. For a baseline comparison, the alloy was also cast using conventional methods at a superheat of 60°C. Analysis of cooling curves showed that the cooling rate during solidification is essentially constant at about 1°C/s. The growth velocity, V, for MSM casting produced at low superheat is almost constant during solidification, around 1 mm/s, while the thermal gradient, G, decreases with increasing solid fraction from 2.07°C/mm at f=0, to 1.10°C/mm at f=0.5. In contrast, V for castings produced at high superheat is four times smaller than that for low superheat castings--the conditions that favor equiaxed dendritic solidification morphology. Metallographic examination of the MSM alloy cast at low superheat shows no evidence of oxide formation. It was also found that casting at this low superheat produced a fine globular grain structure compared to the equiaxed dendritic structure in conventionally cast alloys at high superheat. The average grain sizes for 5 and 8°C MSM produced castings were 83.96 and 94.81μm, respectively. The 60°C superheat castings were found to have a much larger average grain size of 334.42μm. Elemental segregation analysis was performed and showed the presence of primary-α Mg and secondary-α Al rich Mg phases, along with γMg_17 Al_12 and Al_8 Mn_5 intermetallic phases. For the globular structures, the intermetallic phases were found to form in the secondary-α phase along the grain boundaries. Comparatively, the dendritic entrapment of the secondary-α phase was found to lead to intermetallic phase formation in the matrix of the grains.
The role of copper on stress evolution and chemical ordering in iron-copper-platinum thin films
(University of Alabama Libraries, 2011) Hinson, Ross; Thompson, Gregory B.; University of Alabama Tuscaloosa
The chemical ordering of FePt requires an annealing temperature >500 °C. At these temperatures, grain growth attributed to thermal annealing lowers the areal density in computer hard drives. Multiple paths have been proposed to lower the ordering temperature in order to decrease the grain growth. Ternary additions show promise as a way to lower the ordering temperature and decrease the grain growth. Though multiple studies have been conducted on the incorporation of Cu into FePt, these studies have produced contradictory results on the ordering temperature. Currently, there is no literature on the in situ growth stresses during deposition of FePtCu as a function of ordering temperature.InsituFe_44.5 Pt_51 Cu_4.5 and Fe_50 Pt_50 (for comparison) thin films were produced in order to determine how stress and ordering are affected by the incorporation of Cu. The incorporation of Cu at the amounts studied did not change the stress behavior of FePt when compared to Fe_50 Pt_50 . The addition of Cu raised the ordering temperature and had little to no effect on the grain size as compared to the Fe_50 Pt_50 . This retardation of ordering has been explained by the placement of Cu at lattice sites which minimize the bond distance to Pt and maximize the bond distance to Fe.
High thermal energy storage density molten salts for parabolic trough solar power generation
(University of Alabama Libraries, 2011) Wang, Tao; Reddy, R. G.; University of Alabama Tuscaloosa
New alkali nitrate-nitrite systems were developed by using thermodynamic modeling and the eutectic points were predicted based on the change of Gibbs energy of fusion. Those systems with melting point lower than 130oC were selected for further analysis. The new compounds were synthesized and the melting point and heat capacity were determined using Differential Scanning Calorimetry (DSC). The experimentally determined melting points agree well with the predicted results of modeling. It was found that the lithium nitrate amount and heating rate have significant effects on the melting point value and the endothermic peaks. Heat capacity data as a function of temperature are fit to polynomial equation and thermodynamic properties like enthalpies, entropies and Gibbs energies of the systems as function of temperature are subsequently induced. The densities for the selected systems were experimentally determined and found in a very close range due to the similar composition. In liquid state, the density values decrease linearly as temperature increases with small slope. Moreover, addition of lithium nitrate generally decreases the density. On the basis of density, heat capacity and the melting point, thermal energy storage was calculated. Among all the new molten salt systems, LiNO3-NaNO3-KNO3-Mg(NO3)2-MgKN quinary system presents the largest thermal energy storage density as well as the gravimetric density values. Compared to the KNO3-NaNO3 binary solar salt, all the new molten salts present larger thermal energy storage as well as the gravimetric storage density values, which indicate the better thermal energy storage capacity for solar power generation systems.
Structural evolution and growth mechanism of hierarchial heterostructures comprised of carbon nanotubes decorated with nanoparticles
(University of Alabama Libraries, 2011) Shi, Wenwu; Chopra, Nitin; University of Alabama Tuscaloosa
Novel hybrid materials composed of hydrogels and heterostructured 1-D nanostructures such as carbon nanotubes (CNTs) coated with nanoparticles are critical for development of multi-functional analytical systems and biological applications. In this thesis, CNT-nickel/nickel oxide (Ni/NiO) core/shell nanoparticle (CNC) heterostructures were prepared in a simple and single step synthetic approach. The high surface-to-volume ratio and aspect ratio of chemical vapor deposition (CVD)-grown CNTs (average diameter ~ 46±16.4 nm) allowed to uniformly coat with Ni/NiO core/shell nanoparticles (average diameter ~ 12±2 nm). The crystal structure, morphology, and phases of CNC heterostructures were characterized using high resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction (XRD). Single parameter controlled structural and morphological evolution of heterostructures was also evaluated. With the increase of reaction time, distribution density of nanoparticles on CNTs decreased and different shapes of nanoparticles also emerged. When reaction time extended to 15 hrs, due to the interaction between nickel and phosphine based stabilizers, phosphide nanoparticles on CNTs were also synthesized. Thermal stability of prepared heterostructures was evaluated in a N2-rich atmosphere. It was found that high temperature will result nanoparticles migration from CNTs to flat substrate. Meanwhile, decoration of nanoparticles effectively extended the stability range of CNTs from ~ 400 °C to temperatures greater than 600°C. Subsequently, as-produced CNC heterostructures were incorporated into poly vinyl alcohol (PVA) hydrogel. These CNC heterostructure-PVA hydrogels were rigorously characterized for their chemical functionality, morphology, and water absorbing capacity using spectroscopic (FTIR and UV-vis transmittance), SEM, and swelling/shrinking studies. CNC heterostructure-PVA hydrogel was utilized for separating and concentrating chemical species from a mixture. The approach also demonstrated that these hybrid materials can also selectively concentrate L-histidine or histidine-tagged green fluorescent protein (GFP) in the solution. Finally, a cycled magnetic field was applied to control the releasing speed of molecules loaded in the PVA hydrogel. Such selective and multi-component hydrogels can be very useful for developing advanced chemical and biological sensors.
Understanding the growth of graphene encapsulated noble metal nanoparticles: morphological and structural evolution studies, growth mechanisms, and characterization
(University of Alabama Libraries, 2011) Wu, Junchi; Chopra, Nitin; University of Alabama Tuscaloosa
The major goal of this work was to study the morphological evolution of noble metal nanoparticles such as gold, palladium, and platinum nanoparticles as a function of single-parameter variation in simple synthesis approach. As a next step, these nanoparticles were plasma oxidized to result in surface oxidized noble metal nanoparticles. These noble metal nanoparticles were further utilized for the growth of graphene shells in a chemical vapor deposition method resulting in graphene encapsulated noble metal nanoparticles. In regard to morphological evolution of noble metal nanoparticles, systematic studies were performed, where single growth parameter (temperature, metal salt concentration, surfactant type or concentration, seed amount, or growth duration) was varied while other parameters were kept constant. Gold nanoparticles were synthesized by both single-step method and seed-growth method while palladium and platinum nanoparticles were synthesized at high temperature by alcohol reduction. The size, shape, crystallinity, and sample heterogeneity for the nanoparticles were characterized by high-resolution transmission electron microscopy. Single parameter systematic studies allowed for fundamentally understanding the growth and evolution of noble metal nanoparticles. As temperature increased, nanoparticles size increased due to the decrease of absolute value of volume Gibbs free energy. With the existence of surfactant (e.g. hexadecyltrimethylammonium bromide), stabilizer molecules bind to nanoparticles surface with affinity to different facets. When synthesis temperature higher than boiling point of water, the annealing process resulted in rupture of surfactant from weak binding facets, and boosted anisotropic growth of nanoparticles. Subsequently, oxidation behavior of gold, palladium, and platinum were studied by X-ray photoelectron spectroscopy. Gold oxide, palladium oxide, and platinum oxide were found after plasma oxidation. Noble metal nanoparticles were plasma oxidized for 30 min, and then further utilized for chemical vapor deposition (CVD) of graphene shells. These graphene encapsulated noble metal nanoparticles were thoroughly characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Aggregation of noble metal nanoparticles was observed after graphene growth. Raman spectra showed D-, G-band for graphene encapsulated gold, palladium and platinum nanoparticles after CVD growth. Raman chemical mapping indicates large area growth of graphene encapsulated nanoparticles.
Fabrication and characterization of graded magnetocrystalline anisotropy iron-nickel-platinum alloy thin films
(University of Alabama Libraries, 2011) Fu, Bianzhu; Thompson, Gregory B.; University of Alabama Tuscaloosa
The further increase of magnetic storage density is limited by superparamagnetism: the size of the magnetic grains has reached a scale (several nanometers) at which thermal fluctuations can erase the stored information. Attempts to increase the thermal stability by making the grains `magnetically harder' have failed because they have also made them impossible to write with available write fields. One approach to overcome this perplexing dilemma is to grade the uniaxial magnetocrystalline anisotropy, Ku, such that one end is `magnetically soft' to switch while the other end is `magnetically hard' to anchor the switch from intrinsic thermal stability issues. This change in Ku can be accomplished by changing the composition of the magnetic material along the magnetic easy-axis direction. However, there has been a lack of experimental studies on the fabrication of the gradients in the [001] orientation in FePt based structures coupled with isolated magnetic pillars to verify domain wall switching advantages in gradients. In this dissertation, a highly ordered [001] oriented FexNi0.48-xPt0.52 (0
Interface orientation dependent field evaporation behavior in multilayer thin films
(University of Alabama Libraries, 2011) Brons, Justin Glen; Thompson, Gregory B.; University of Alabama Tuscaloosa
In general, atom probe reconstruction algorithms assume a constant evaporation field across the surface of the specimen. In reality, chemical inhomogeneity modulates the evaporation field at the specimen surface, which introduces reconstruction artifacts and degrades the spatial resolution of the atom probe tomography (APT) technique. Multilayer thin films provide ideal specimen geometries to measure and quantify these artifacts. Thin films can be deposited with near atomic layer precision and can exhibit large planar surfaces with degrees of intermixing across the interfaces. Quantifying and rectifying interfacial compositional differences in atom probe data sets is critical, as such information can be used to understand the growth and intermixing of species in nanolaminate devices, such as giant magneto-resistance multilayers. A series of Fe/Ni and Ti/Nb multilayers featuring a bilayer repeat unit of equal thickness and repeat distance of approximately 4 nm have been sputter-deposited onto n-doped Si [001] substrates. The multilayers were focus ion beam (FIB) milled with an annular milling geometry into the required needle-shaped geometry for the APT analysis. Specimens were prepared with the film interfaces oriented with the chemical modulations for a given bilayer spacing parallel and perpendicular to the specimen apex to compare field evaporation behavior at these limiting geometries. For the Fe/Ni multilayers, the 4 nm bilayer films exhibited Fe intermixing within the Ni layers. The Electron Energy Loss Spectroscopy (EELS) based compositional profiles were used in order to cross-correlate across the two techniques using a chemical comparison. These profiles were acquired using aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Fe/Ni. In addition, little difference was observed between the parallel and perpendicular field evaporation limiting geometries. This has been contributed to Fe and Ni having a similar elemental evaporation field strength of 33 and 35 V/nm. The Fe/Ni results were compared to data obtained from a Ti/Nb multilayered thin film with a bilayer spacing of approximately 4 nm, prepared in similar parallel and perpendicular evaporation orientations as those discussed above. Compositional EELS profiles were collected using an aberration-corrected scanning transmission electron microscopy (STEM) with an approximate 100 pm electron probe. The slope of the compositional gradients between the layered interfaces showed very little differences between the EELS and atom probe data sets for Ti/Nb. In the perpendicular evaporation orientation, it was found that the layer thicknesses for both the Ti and Nb elemental layers were measured at values closer to the actual specimen, while the perpendicular orientation contained reconstruction artifacts that compressed the layer thicknesses. The EELS based compositional profiles were used in order to cross correlate across the two techniques using a chemical comparison. The compositional gradient across the interfaces was also closer to the true value in the parallel orientation, while the perpendicular orientation contained artifacts that altered the composition across the compressed interfaces. This study showed in the atom probe data there was upwards of 20 atomic % Nb was intermixed in the Ti layers. This finding was verified by the EELS compositional profiles.

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