Browsing by Author "Thompson, G. B."
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Item Compositional Dependent Thin Film Stress States(2010-08-23) Thompson, G. B.; Fu, B.; University of Alabama TuscaloosaThis paper addresses in situ stress evolution of two-component FexPt1−x, where x spanned 0 to 1, alloy thin films. The stresses of the high-temperature, quenched-in, solid solution phase was determined by in situ wafer curvature measurements during ambient temperature growth. The measured stresses were shown to be compositional dependent and spanned both compressive and tensile stress states. Under specific growth conditions, a “zero-stress” state could be achieved. The alloy stress states did not show any significant stress recovery upon ceasing the deposition, i.e. the stress state during growth was retained in the film. X-ray diffraction, transmission electron microscopy, and atom probe tomography were used to characterize the microstructures of each thin film. The evolution of the stress state with composition is described in terms of a chemical potential term for preferential segregation of one species in the alloy to the grain boundaries.Item Compositional Evolution During the Synthesis of FePt Nanoparticles(2008-09-25) Thompson, G. B.; Srivastava, Chandan; Nikles, David E.; University of Alabama TuscaloosaA series of FePt nanoparticles was synthesized by the thermal decomposition of iron pentacarbonyl and reduction in platinum acetylacetonate in phenyl ether solvent. A range of precursor molar ratios of 2, 1.5, and 1 between iron pentacarbonyl and platinum acetylacetonate was studied. After 30 min of reflux, the synthesis method produced a wide distribution in composition and size for the nanoparticles. Given 200 min of reflux, it was observed that the particle-to-particle composition and size narrowed, and the atomic ratio of Fe to Pt, for the majority of nanoparticles, approached the initial precursor molar ratios except for the molar ratio of 1. It is speculated that the compositional variability may be a result of the slow kinetics of iron pentacarbonyl’s decomposition in the reaction.Item Formation of FePt Nanoparticles by Organometallic Synthesis(2007-05-23) Bagaria, H. G.; Johnson, D. T.; Srivastava, C.; Thompson, G. B.; Shamsuzzoha, M.; Nikles, D. E.; University of Alabama TuscaloosaOur interest in determining the mechanism of FePt nanoparticle formation has led to this study of the evolution of particle size and composition during synthesis. FePt nanoparticles were prepared by the simultaneous reduction of platinum acetylacetonate and thermal decomposition of iron pentacarbonyl. During the course of the reaction, samples were removed and the particle structure, size, and composition were determined using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy–energy dispersive x-ray spectrometry. Early in the reaction the particles were Pt rich (greater than 95at.% Pt) and as the reaction proceeded the Fe content increased to the target of 50%. The particle diameter increased from 3.1to4.6nm during the reaction. Energy dispersive x-ray spectrometry measurements of individual particle compositions using a high resolution TEM showed a broad distribution of particle compositions with a standard deviation greater than 15% of the average composition.Item The hidden structure dependence of the chemical life of dislocations(American Association for the Advancement of Science, 2021) Zhou, X.; Mianroodi, J. R.; da Silva, A. Kwiatkowski; Koenig, T.; Thompson, G. B.; Shanthraj, P.; Ponge, D.; Gault, B.; Svendsen, B.; Raabe, D.; Max Planck Society; RWTH Aachen University; University of Alabama Tuscaloosa; University of Manchester; Imperial College LondonDislocations are one-dimensional defects in crystals, enabling their deformation, mechanical response, and transport properties. Less well known is their influence on material chemistry. The severe lattice distortion at these defects drives solute segregation to them, resulting in strong, localized spatial variations in chemistry that determine microstructure and material behavior. Recent advances in atomic-scale characterization methods have made it possible to quantitatively resolve defect types and segregation chemistry. As shown here for a Pt-Au model alloy, we observe a wide range of defect-specific solute (Au) decoration patterns of much greater variety and complexity than expected from the Cottrell cloud picture. The solute decoration of the dislocations can be up to half an order of magnitude higher than expected from classical theory, and the differences are determined by their structure, mutual alignment, and distortion field. This opens up pathways to use dislocations for the compositional and structural nanoscale design of advanced materials.Item In Situ Transmission Electron Microscopy of Ion Irradiated Fe-Pt Alloy Thin Films(2006-12-21) Thompson, G. B.; Morgan, N. W.; Birtcher, R. C.; University of Alabama TuscaloosaWe report the microstructural evolution during irradiation of FePt and FePt 25at.% thin films sputter deposited onto electron transparent silicon monoxide substrates. The films were studied in situ for 500keV Kr+ irradiation up to a fluence of 1015ions/cm2 or 4displacements∕atom (dpa). Upon irradiation to approximately 1dpa, the initial disconnected granular morphology became continuous. In particular, for FePt, accelerated grain growth was observed once the continuous morphology was achieved during ambient temperature irradiation. No atomistic (chemical) ordering from the as-deposited A1 phase into either the L10 FePt or L12 Fe3Pt phases was observed during ambient temperature irradiation. After irradiation, the specimens were then in situ annealed. The intermetallic ordering temperature, compared to that of an unirradiated film, was lowered by ≈200°C for FePt 25at.%. No decrease in the ordering temperature was observed for irradiated FePt. The rate of FePt grain growth during annealing was very similar for both irradiated and unirradiated films over the 25–650°C temperature range investigated.Item L10 Ordering of FePt Thin Films using Sub-10 ms Laser Pulses(2009-03-12) Thompson, G. B.; Inaba, Y.; Kang, S.; Izatt, J. R.; Harrell, J. W.; Kubota, Y.; Klemmer, T. J.; University of Alabama TuscaloosaThe structural and magnetic properties of 10 nm FePt thin films annealed using a 1064 nm wavelength laser with 10, 7.5, 5.0, and 2.5 ms pulses have been examined. The A1 to L10 phase transformation was confirmed by x-ray diffraction (XRD). The maximum order parameter of 0.53 and coercivity of 5.36 kOe can be obtained with 10 ms pulse width laser annealing at a laser energy fluence of 10 J/cm2. The order parameter of the furnace annealed samples was approximately 1.0 suggesting that 10 ms is insufficient to obtain a fully ordered phase. The laser annealed grain size, as measured by in-plane XRD analysis, is 24 % smaller than that of furnace annealed sample for an equivalent order parameter demonstrating the merit of short time annealing.Item Lattice Expansion in Nanocrystalline Niobium Thin Films(2003-04-09) Banerjee, R.; Sperling, E. A.; Thompson, G. B.; Fraser, H. L.; Bose, S.; Ayyub, P.; University of Alabama TuscaloosaHigh-purity nanocrystalline niobium (Nb) thin films have been deposited using high-pressure magnetron sputter deposition. Increasing the pressure of the sputtering gas during deposition has systematically led to reduced crystallite sizes in these films. Based on x-ray and electron diffraction results, it is observed that the nanocrystalline Nb films exhibit a significantly large lattice expansion with reduction in crystallite size. There is however, no change in the bcc crystal structure on reduction in crystallite size to below 5 nm. The lattice expansion in nanocrystalline Nb has been simulated by employing a recently proposed model based on linear elasticity and by appropriately modifying it to incorporate a crystallite-size-dependent width of the grain boundary.Item Microstructures and Magnetic Alignment of L10 FePt Nanoparticles(2007-05-09) Kang, Shishou; Shi, Shifan; Jia, Zhiyong; Thompson, G. B.; Nikles, David. E.; Harrell, J. W.; University of Alabama TuscaloosaChemically ordered FePt nanoparticles were obtained by high temperature annealing a mixture of FePt particles with NaCl. After the NaCl was removed with de-ionized water, the transformed FePt nanoparticles were redispersed in cyclohexanone. X-ray diffraction patterns clearly show the L10 phase. Scherrer analysis indicates that the average particle size is about 8 nm, which is close to the transmission electron microscopy TEM statistical results. The coercivity ranges from 16 kOe to more than 34 kOe from room temperature down to 10 K. High resolution TEM images reveal that most of the FePt particles were fully transformed into the L10 phase, except for a small fraction of particles which were partially chemically ordered. Nano-energy dispersive spectroscopy measurements on the individual particles show that the composition of the fully transformed particles is close to 50/ 50, while the composition of the partially transformed particles is far from equiatomic. TEM images and electron diffraction patterns indicate c-axis alignment for a monolayer of L10 FePt particles formed by drying a dilute dispersion on copper grids under a magnetic field. For thick samples dried under a magnetic field, the degree of easy axis alignment is not as high as predicted due to strong interactions between particles.Item Selective Area Synthesis of Magnesium Oxide Nanowires(2007-11-28) Kim, G.; Martens, R. L.; Thompson, G. B.; Kim, B. C.; Gupta, A.; University of Alabama TuscaloosaSingle crystalline magnesium oxide MgO nanowires exhibiting a square cross section have been grown on 001-oriented MgO and Si substrates using the vapor-liquid-solid growth mechanism. While the nanowires grow vertically aligned on MgO, they display random orientations on the silicon substrate. For growth on MgO substrates, the selective placement and density of the nanowires can be controlled by using electron beam lithography for prepatterning the gold catalyst layer. The nanowire samples have been characterized using field-emission scanning electron microscopy and transmission electron microscopy. The described process for selective placement of the nanowires is attractive for their use as templates for coaxial coatings and also for their manipulation for potential device fabrication.Item Size Effect Ordering in [FePt]100-xCrx Nanoparticles(2006-03-02) Thompson, G. B.; Srivastava, C.; Harrell, J. W.; Nikles, D. E.; University of Alabama TuscaloosaA series of [FePt]100−xCrx nanoparticles (x=5, 10, and 16at.%) was chemically synthesized by two different techniques. In one method, the simultaneous chemical reduction of FeCl2∙4H2O, Pt-acetylacetonate, and Cr-acetylacetonate was used with 2, 4 hexadecanediol as the reducing agent and phenyl ether as the solvent. The as-prepared particles had a mean size of 1.5nm. In the second method, the simultaneous chemical reduction of Pt-acetylacetonate and Cr-acetylacetonate and the thermal reduction of Fe(CO)5 were used with adamantanecarboxylic acid as the reducing agent and hexadecylamine as the solvent. These as-prepared particles were 3.5nm in size. X-ray diffraction confirmed that the Cr formed a solid solution within the A1 FePt phase for both processes. Upon annealing, the Cr hindered sintered grain growth of FePt nanoparticle arrays. Consequently, we were able to use Cr as a means to tune the ordering temperature as a function of the size effect in FePt nanoparticles. The presence of Cr in the ordered FePt reduced the magnetic coercivity of the transformed nanoparticles.