bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nucleation & Growth of Precipitates in Transformations Driven by Diffusion
Authors: Clavaguera N. , Clavaguera-Mora M.T. 
Published: April 2011

ABSTRACTA theoretical analysis of the transformation kinetics which accounts for nuclei, either prequenched or created homogeneously, and whose growth are controlled by diffusion is presented. The change in growth habit intervening during the transformation is analysed in terms of the evolution of the free energy difference between the precipitate and the matrix at the interface, ΔG1. In the Avrami formalism, this quantity accounts for the competition between interface and diffusion controlled growth whereas the nucleation events are driven by the free energy difference between the precipitate and the bulk matrix. Competition and selection of precipitate phases in highly undercooled melts using the CALPHAD approach for the evaluation of the free energies and the changes in diffusivity with concentration are analysed. Experimental vs. calculated data are discussed in some rapidly solidified metallic systems.
Precipitation Kinetics in an Air-Cooled Aluminum Alloy: A Comparison of Scanning and Isothermal Calorimetry Measurement Methods
Authors: Smith George W. 
Published: April 2011

ABSTRACTThe purpose of this work is to study precipitation kinetics in air-cooled aluminum alloy 339 by both differential scanning calorimetry (DSC) and differential isothermal calorimetry (DIC) and thereby establish the equivalence of the two methods. To accomplish this, precipitation time constants, τ and activation energies, Eact, were determined by both techniques. Kissinger analysis of the DSC data yielded Eact and τ values. Using DIC we measured precipitation time constants, Arrhenius plots of which gave Eact. Activation energies and τ values from both methods agree provided DSC temperature scan rates are slow compared to the instrumental lag time. Thus, DSC and DIC have been shown to be equivalent (at least for this test case).
Underpotential – Overpotential Phase Transformation Phenomena in Copper Deposition Processes on Glassy Carbon
Authors: Arzhanova T. A. 
Published: April 2011

ABSTRACTThe initial stages of copper electrodeposition on glassy carbon were studied from a solution of 1 M CuSO4 in 0.5 M H2SO4 by the methods of potentiostatic current transients and pulse experiments. The experimental data were analyzed according to the theory of potentiostatic current transients for three-dimensional multiple nucleation with diffusion controlled growth. The theory was found to be in good agreement with experimental data for instantaneous copper nucleation in the cathodic overpotential range η = −0.18 – −0.40 V. It is shown that in case of progressive nucleation at η ≤ −0.16 V, this theory can be used only for trend analysis of copper nucleation process parameters. An important influence of the initial state of glassy carbon surface on the nucleation rate was found determined by the underpotential. It is shown that the dependence of nucleation rate on the underpotential is inadequate and is determined by cathodic overpotential.
Characterization of Composite Ceramic High Level Waste Forms
Authors: Frank S. M. , Bateman K. J. , DiSanto T. , Johnson S. G. , Moschetti T. L. , Noy M. H. , O'Holleran T. P. 
Published: April 2011

ABSTRACTArgonne National Laboratory has developed a composite ceramic waste form for the disposition of high level radioactive waste produced during electrometallurgical conditioning of spent nuclear fuel. The electrorefiner LiCl/KCl eutectic salt, containing fission products and transuranics in the chloride form, is contacted with a zeolite material which removes the fission products from the salt. After salt contact, the zeolite is mixed with a glass binder. The zeolite/glass mixture is then hot isostatic pressed (HIPed) to produce the composite ceramic waste form. The ceramic waste form provides a durable medium that is well suited to incorporate fission products and transuranics in the chloride form. Presented are preliminary results of the process qualification and characterization studies, which include chemical and physical measurements and product durability testing, of the ceramic waste form.
Phase Formation in Zr/Fe Multilayers During Kr Ion Irradiation
Authors: Motta A. T. , Paesano A. , Birtcher R. C. , Ryan E. A. , Bruckmann M. E. , Teixeira S. R. , Amaral L. 
Published: April 2011

ABSTRACTA detailed study has been conducted of the effect of Kr ion irradiation on phase formation in Zr-Fe metallic multilayers, using the Intermediate Voltage Electron Microscopy (IVEM) at Argonne National Laboratory. Metallic multilayers were prepared with different overall compositions (near 50–50 and Fe-rich), and with different wavelengths (repetition thicknesses). These samples were irradiated with 300 keV Kr ions at various temperatures to investigate the final products, as well as the kinetics of phase formation. For the shorter wavelength samples, the final product was in all cases an amorphous Zr-Fe phase, in combination with Fe, while specially for the larger wavelength samples, in the Fe-rich samples the intermetallic compounds ZrFe2 and Zr3Fe were formed in addition to the amorphous phase. The dose to full reaction decreases with temperature, and with wavelength in a manner consistent with a diffusion-controlled reaction.
Role of Interstitial and Interstitial-Impurity Interaction on Irradiation-Induced Segregation in Austenitic Steels
Authors: Nastar M. , Bellon P. , Martin G. , Ruste J. 
Published: April 2011

ABSTRACTSegregation under irradiation in austenitic steels is due to a permanent flux of vacancies and interstitials produced by irradiation towards sinks like surfaces and interfaces. A model based on a mean field lattice rate theory is proposed where kinetics and thermodynamics are treated in a mutually consistent way. For a Fe-Ni-Cr ternary alloy, the 15 parameters defining the jump frequencies of vacancies were fitted on equilibrium properties including ordering energies and tracer diffusion experiments with no use of segregation data. Measurements of RIS by Auger Electron Spectroscopy (AES) were used in the last step of the fitting procedure in order to choose the best set of the 27 interstitial jump frequencies. This fitting procedure strongly supports the idea that the interstitials are contributing to RIS in Fe-Cr-Ni alloys. We also simulate the trapping of interstitials by an impurity model and reproduce the total inhibition of RIS by this impurity as observed experimentally [1].
Response of Zircon to Electron and Ne+ Irradiation
Authors: Devanathan R. , Weber W. J. , Boatner L. A. 
Published: April 2011

ABSTRACTZircon (ZrSiO4) is an actinide host phase in vitreous ceramic nuclear waste forms and a potential host phase for the disposition of excess weapons plutonium. In the present work, the effects of 800 and 900 keV electron, and 1 MeV Ne+ irradiations on the structure of single crystals of ZrSiO4 have been investigated. The microstructural evolution during the irradiations was studied in situ using a high-voltage electron microscope interfaced to an ion accelerator at Argonne National Laboratory. The results indicate that electron irradiation at 15 K cannot amorphize ZrSiO4 even at fluences an order of magnitude higher than that required for amorphization by 1.5 MeV Kr + ions. However, the material is readily amorphized by I MeV Ne+ irradiation at 15 K. The temperature dependence of this amorphization is discussed in light of previous studies of radiation Zdamage in ZrSiO4.
Non-Equilibrium Formation Of Silicon Nitride During Both Ball Milling And Ion Bombardment
Authors: Li Z. L. , Williams J. S. , Llewellyn D. J. , Wong-Leung J. , Giersig M. , Chivers D. J. 
Published: April 2011

ABSTRACTPhase evolution during ball milling of Si in NH3 gas and during subsequent annealing has been studied and compared with nitride formation during ion bombardment of Si. X-ray diffraction, differential thermal analysis, Rutherford backscattering and channeling, combustion analysis and transmission electron microscopy have been used as analytical techniques. Results have shown that an amorphous SixNy(Fe) phase forms during milling which transforms into α-Si3N4 and FeSi2 on annealing. During ion bombardment, slightly N-rich Si3N4 is formed but it is mostly crystalline at temperatures between 150 and 450°C.
Densification and Crystallization Kinetics of Cordierite Glass-Ceramic Coatings on Rigid Substrates
Authors: Calata Jesus Noel , Wen Sihua , Lu Guo-Quan 
Published: April 2011

ABSTRACTWe report our findings on the sintering kinetics of slip-cast glass-ceramic coatings on rigid substrates at temperatures between 900°C and 1000°C; the glass-ceramic crystallizes into cordierite at sintering temperatures above 900°C. Changes in coating thickness during sintering were monitored in situ using an optical setup, and the crystallization of the coatings was determined by x-ray diffraction. The coatings went through a rapid shrinkage due to initial densification, followed by an expansion stage caused by the transformation of the glass into a crystalline phase with a lower density. The rate and extent of the expansion were more noticeable at sintering temperatures above 900°C. We also found that wetting between the glass-ceramic and the substrate was important to the sintering kinetics and sintered microstructure of the coatings. For coatings formed on silicon substrate, growth of large pores was observed at the glass-ceramic/silicon interface. Using wafer-curvature measurement technique, we found a significant tensile stress build-up in the coatings during the initial stage of sintering. We believe that the combination of poor wetting and the tensile stress build-up led to the observed desintering phenomenon at the glass-ceramic/silicon interface.
Influence of Mo on the Epitaxial Crystallization of Silicon
Authors: Thoma S. , Lindner J. K. N. , Stritzker B. 
Published: April 2011

ABSTRACTThe influence of Mo atoms on the solid phase epitaxial crystallization of amorphous silicon layers on (100) and (111) Si substrates has been studied by RBS/channeling and cross-sectional TEM. For this purpose, Mo doped amorphous surface layers were produced by low temperature 180 keV Mo+ ion implantation with different doses. Mo is observed to cause enhanced crystallization rates for both (100) and (111) substrates, compared to literature data on pure amorphous silicon. Similar to pure Si, recrystallization in <100> directions is much faster than in <111> directions, where two different velocities are found. For (111) substrates, the formation of thick, uniformly twinned layers is observed. Annealing for several hours at 550° C does not lead to detectable changes of the Mo depth distribution, but for high doses the formation of hexagonal MoSi2 precipitates is observed.
Metastable Phase Formation and Microstructural Evolution During Self-Propagating Reactions in Ai/Ni and Ai/Monel Multilayers
Authors: Van Heerden D. , Gavens A. J. , Jayaraman S. , Weihs T. P. 
Published: April 2011

ABSTRACTThe quenched microstructures of self-propagating reactions in Al/Ni and Al/Monel foils are examined. The quenched reaction fronts in both films are shown to be approximately 30μm long and exhibit a continuous evolution from an as-deposited to a fully reacted microstructure. In both multilayers the first phase to form is isostructural with Al9Co2, and nucleates on the Al side of the layer interface. Growth of the intermetallic occurs in a two step process. First the grains grow laterally along the interface until they impinge on each other, followed by growth normal to the interface into the Al layer. It is shown that planarity of this interface during the latter growth stage is determined by the width of the intermetallic grains immediately prior to their growth normal to the interface into the Al layer. On the basis of these observations it is argued that in modeling these reactions diffusion through the intermetallic during the reaction must be considered, and that atomic diffusion may be considerably more three- dimensional in practice than the current models suggest.
Modeling Self-Propagating Exothermic Reactions in Multilayer Systems
Authors: Jayaraman S. , Mann A. B. , Knio O. M. , Heerden D. Van , Bao G. , Weihs T. P. 
Published: April 2011

ABSTRACTSelf-propagating reactions in free-standing multilayer foils provide a unique opportunity to study very rapid, diffusion-based transformations in non-equilibrium material systems. To fully understand the coupling between mass and thermal diffusion controlling these reactions and to optimize the commercial use of reactive foils, we have undertaken analytical and numerical modeling. Our analytical model predicts an increase in the reaction velocities with decreasing bilayer thickness down to a critical bilayer thickness and a reversal in this trend below the critical thickness. Predicting reaction characteristics such as the flame thermal width, the reaction zone width and the effect of variations in material properties with temperature has proven analytically intractable. To overcome these limitations, we have also used numerical methods to determine the composition and temperature profiles ahead of the reaction front for different multilayer periods and premixing. The results are compared with experimental values where possible.
Effects of Finite Rate Phase Transformation Kinetics on The Steady-State Solidification Front Propagation Speed in Undercooled Pure Liquids
Authors: Norris J. A. , Kassoy D. R. 
Published: April 2011

ABSTRACTThis novel approach to modeling the steady-state solidification of undercooled pure liquids is based upon first principles. Continuum equations are used to describe a volumetrically averaged, coexisting mixture of solid and liquid in the thin phase transformation zone between regions of pure liquid and pure solid. These equations are coupled with a dynamic equilibrium based rate law that describes temperature dependent phase transformation kinetics. The time scale associated with finite rate phase transformation is found to depend on a solidification activation energy, thermal energy, and the enthalpy of fusion. The model leads naturally to an eigenvalue problem for the solidification front propagation speed. In addition, the variation of the volumetrically averaged liquid fraction across the solidification zone is predicted.
Phase Transformation as a Function of Particle Size in Nanocrystalline Zirconia
Authors: Chraska Tomas , King Alexander H. , Berndt Christopher C. , Karthikeyan J. 
Published: April 2011

ABSTRACTBulk zirconia undergoes a pressure-induced transformation from a (low pressure) monoclinic phase to a high pressure tetragonal phase, at around 3GPa (above 900K). We have studied the structures of zirconia nanoparticles formed by plasma-spraying an organo-metallic precursor. Inspection of the particles in the TEM reveals that they adopt one of two distinct crystal structures, depending upon their size. The smallest particles have the tetragonal structure, while larger ones are monoclinic. Interpolation of the data reveals a critical size above which the monoclinic structure is stable. Upon annealing, the zirconia particles coarsen and the small tetragonal particles transform to the monoclinic structure at about the critical size. Coarsening under these conditions produces irregular particle size distributions. We estimate that the surface-stress induced internal pressure in the tetragonal nanoparticles can be as high as 5 GPa and the corresponding surface stress is about 6N/m.
Oxidation Dynamics of Nanophase Aluminum Clusters: A Molecular Dynamics Study
Authors: Ogata Shuji , Campbell Timothy J. , Tsuruta Kenji , Nakano Aiichiro , Kalia Rajiv K. , Vashishta Priya , Loong Chun-K. 
Published: April 2011

ABSTRACTOxidation of an aluminum nanocluster (252,158 atoms) of radius 100Å placed in gaseous oxygen (530,727 atoms) is investigated by performing molecular-dynamics simulations on parallel computers. The simulation takes into account the effect of charge transfer between Al and 0 based on the electronegativity equalization principles. We find that the oxidation starts at the surface of the cluster and the oxide layer grows to a thickness of ∼28Å. Evolutions of local temperature and densities of Al and 0 are investigated. The surface oxide melts because of the high temperature resulting from the release of energy associated with Al-O bondings. Amorphous surface-oxides are obtained by quenching the cluster. Vibrational density-of-states for the surface oxide is analyzed through comparisons with those for crystalline Al, Al nanocluster, and α-Al2O3
Phase Transitions in Lead(II) Fluoride Upon Milling
Authors: DÉnÉ Georges , Madamba M. Cecilia 
Published: April 2011

ABSTRACTPbF2 is known to exist under two different polymorphic structures. Orthorhombic α-PbF 2 is stable at ambient temperature. It has the PbC12 structure. Cubic β-PbF2 is obtained by heating α-PbF2. It does not transform back to α-PbF2 on cooling, and it seems to be infinitely stable in the metastable state under ambient conditions. β-PbF2 crystallizes in the fluorite (CaF2) type. Owing to the large number of potential interstitial sites, many F- Frenkel defects can be formed, which make β-PbF2 the highest performance fluoride ion conductor among binary fluorides. In this work,
both phases of PbF2 have been ball milled. Milling α-PbF2 results in a partial transformation to microcrystalline β-PbF2. The energy required for obtaining the high temperature phase is probably provided in the mechanical form. Milling β-PbF2 leads to partial amorphization and formation of α-PbF2. In this case, milling transforms the high temperature polymorph to the low temperature form, by providing the energy required to overcome the activation energy that keeps PbF2 trapped in the high temperature β-form after cooling.
Growth of Single Crystal Yig Fibers by the Laser Heated Pedestal Growth Method
Authors: Lim HanJin , DeMattei R. C. , Feigelson R. S. 
Published: April 2011

ABSTRACTYttrium iron garnet (YIG) single crystal fibers of nominal composition Y3Fe5O12were grown by the laser heated pedestal growth (LHPG) technique, a miniaturized floated-zone process. YIG which melts incongruently, was grown at a temperature below the peritectic decomposition temperature under self-adjusting conditions even though it has very narrow solidification region according to the Y2O3-Fe2O3phase diagram. YIG fibers in diameter ranges from 100 to 750 μm were grown at various growth rates and conditions, and analyzed by x-ray diffraction, electron microprobe, and IR-VIS spectroscopy. Infrared transparent YIG fibers were grown at rates below 12 mm/h in air. At these growth rates, yttrium orthoferrite and iron-oxide inclusions within the YIG fiber, which act as IR scattering centers, were significantly reduced. The transparency of the fibers was more dependent on the growth rate than the stability of the molten zone. Surface ridges containing an Fe-rich composition were observed at all growth rates. These were associated with molten zone instability.
Formation of a Metastable BCC Solid Solution and Decomposition to aC15 Laves Phase in Melt-Spun CrNb10Ti10
Authors: Chen Katherine C. , Kotula Paul G. , Chu Fuming , Thoma Dan J. 
Published: April 2011

ABSTRACTA metastable, disordered bcc phase has been formed from the melt in a Cr-rich alloy of the Nb-Cr-Ti system, where large volume fractions of the Laves phase would develop under equilibrium solidification conditions. X-ray diffraction (XRD) studies and lattice constant determinations confirm that the melt-spun ribbons contain a bcc phase beyond its terminal solid solution limits. Solidification pathways are proposed based upon metastable and equilibrium phase diagrams. Microstructures have been studied using optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cross-sectional TEM reveals tiny, dispersed Laves phase precipitates within the metastable bcc grains near the chill (wheel) side. Away from the wheel, the microstructure changes to a dendritic structure of the bcc andC15 Laves phase. Annealing of the melt-spun ribbons produces a fine, two-phase microstructure of the equilibrium phases. Microstructures from the melt-spun and arc-melted processing conditions are compared. The microstructural control afforded through this metastable processing route enables a methodology to tailor phase distributions for optimized toughness in Laves phase alloys.
Current Controlled Photoelectrochemical Etching of Gan Leaving Smooth Surfaces
Authors: Rotter T. , Uffmann D. , Ackermann J. , Aderhold J. , Stemmer J. , Graul J. 
Published: April 2011

AbstractWe have etched GaN grown by plasma source MBE in aqueous solutions of KOH in an electrochemical cell under HeCd laser illumination and additional current control.The etch rate was dramatically enhanced up to 8 μm/h by an applied current density of 6.4 mAcm-2. Photocurrent control leads to etched GaN surfaces exhibiting mirror-like appearance with uniform interference color. According to mechanical profilometry, they have a roughness of less than 3.5 nm after etching of several hundred nanometers, which is comparable to the roughness prior to etching. This etching process allows in situ control via photocurrent and induced yellow luminescence.
Pulsed Laser Etching of GaN and AIN Films
Authors: Chen H. , Vispute R. D. , Talyansky V. , Enck R. , Ogale S. B. , Dahmas T. , Choopun S. , Sharma R. P. , Venkatesan T. , Iliadis A. A. , Salamanca-Riba L. G. , Jones K. A. 
Published: April 2011

AbstractDue to limited success in wet etching of GaN and AIN, dry etching techniques have become more relevant for the processing of the GaN films. Here we demonstrate the results of an alternative dry etching process, namely, pulsed laser etching, for GaN and AIN. In this method, a KrF pulsed excimer laser (λ=248 nm, τ=30 ns) was used to etch epitaxial GaN and AIN films. The dependence of the etching characteristics on the laser energy density and the number of pulses has been studied. The etch depth showed a linear dependence on the number of pulses over a wide range of laser energy densities. The threshold intensity for GaN etching was determined to be 0.33 J/cm2. The etching rate was found to be a strong function of laser energy density. Above the threshold, the etch rate was found to be 300–1400 Å per pulse leading to etching rates of 0.1–1μm/sec depending upon the laser energy density and the pulse repetition rate. It is shown that the etching mechanism is based on laser induced absorption, decomposition and layer by layer removal of the GaN.
High Indium Content Ingan Films and Quantum Wells.
Authors: Van Der Stricht W. , Jacobs K. , Moerman I. , Demeester P. , Considine L. , Thrush E. J. , Crawley J. A , Ruterana P. 
Published: April 2011

AbstractInGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high Indium incorporation for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. TEM diffraction patterns have shown that the ternary InGaN layer can be chemically ordered. The In and Ga atoms occupy respectively the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P6 3mc symmetry group of the wurtzite GaN.