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High Critical Current Densities in Nb3Sn Films with Engineered Microstructures--Artifical Pinning Microstructures

Description: Films with layers of Nb, Cu, and Sn have been fabricated to simulate a Nb{sub 3}Sn bronze-type process. These Nb{sub 3}Sn films have produced critical current densities greater than 1 x 10{sup 6} A/cm{sup 2} at 4.2 K and 7.5 T. Niobium films doped with Y, Sc, Dy, Al{sub 2}O{sub 3}, and Ti have been deposited with e-beam co-evaporation onto 75 mm diameter Si wafers with a 100 nm SiO{sub 2} buffer layer. The Nb layer was followed by a layer of Cu and a layer of Sn to complete the bronze-type process. The films with the highest J{sub c} had about 8 vol. % Sc and about 18 vol. % Al{sub 2}O{sub 3}. Characterization of the microstructure by TEM shows that these high J{sub c} films contained high density of inclusions about 5 nm in size and that the grain size of the Nb{sub 3}Sn is about 20-25 nm for samples heat treated at 700 C for up to eight hours.
Date: July 1, 1997
Creator: Dietderich, D.R.; Kelman, M.; Litty, J.R. & Scanlan, R.M.
Partner: UNT Libraries Government Documents Department


Description: Results are reported concerning studies of hydraulic cyclones (hydroclones) for application to particle size classification of ThO/sub 2/. Tests were run with a 0.50-in. diam. hydroclone. It was shown that a single pass through this hydroclone would reduce the material greater than 10 mu from 12 wt.% to less than 1% while the mean diameter was reduced from 1.7 to 0.9 mu . When the overflow (fine) fraction was again pumped through the hydroclone, the overflow solids from the second pass were 98 wt.% less than 2 mu in diameter. The mean diameter was reduced from 0.9 mu to about 0.7 mu by the second pass. A yield of 5 to 7 wt.% of solids (to the overflow streamn) and a ratio of overflow to feed volume of 0.25-0.30 were observed during the hydroclone tests. Although this low yield is undesirable, recommendations are made that would increase the yield sufficiently to make desirable the incorporation of hydroclones in the ThO/sub 2/ production flow sheet. (auth)
Date: February 1, 1958
Creator: Bennett, L.L. & Thomas, D.G.
Partner: UNT Libraries Government Documents Department

Effect of Grain Size on the Acoustic Emission Generated During Plastic Deformation of Copper

Description: Acoustic emission signals from polycrystalline Al 1100 samples during plastic deformation were analyzed with respect to the strain rate and grain size. A kinematic model is proposed to account for the observed behavior. An experimental acoustic emission parameter, equivalent to the average energy of the acoustic events, correlates satisfactorily with the computed energy of moving dislocations during the deformation process. Both energies attain a maximum value for a certain grain size and are directly dependent on the strain rate.
Date: May 1, 1980
Creator: Baram, J. & Rosen, M.
Partner: UNT Libraries Government Documents Department

Evolution of 2D Potts Model Grain Microstructures from an Initial Hillert Size Distribution

Description: Grain growth experiments and simulations exhibit self-similar grain size distributions quite different from that derived via a mean field approach by Hillert [ 1]. To test whether this discrepancy is due to insufficient anneal times, two different two-dimensional grain structures with realistic topologies and Hillert grain size distributions are generated and subjected to grain growth via the Monte Carlo Potts Model (MCPM). In both cases, the observed self-similar grain size distributions deviate from the initial Hillert form and conform instead to that observed in MCPM grain growth simulations that start from a random microstructure. This suggests that the Hillert grain size distribution is not an attractor.
Date: October 19, 1998
Creator: Battaile, C.C. & E.A., Holm
Partner: UNT Libraries Government Documents Department

Processing Variables of Alumina Slips and Their Effects on the Density and Grain Size of the Sintered Sample

Description: High densities and small grain size of alumina ceramic bodies provide high strength and better mechanical properties than lower density and larger grain size bodies. The final sintered density and grain size of slip-cast, alumina samples depends greatly on the processing of the slip and the alumina powder, as well as the sintering schedule. There were many different variables explored that include initial powder particle size, slurry solids percent, amount and type of dispersant used, amount and type of binder used, and sintering schedule. Although the experimentation is not complete, to this point the sample with the highest density and smallest grain size has been a SM8/Nano mixture with Darvan C as the dispersant and Polyvinyl Alcohol (PVA) as the binder, with a solids loading of 70 wt% and a 1500 C for 2 hours sintering schedule. The resultant density was 98.81% of theoretical and the average grain size was approximately 2.5 {micro}m.
Date: January 1, 2002
Creator: Rowley, R. & Chu, H.
Partner: UNT Libraries Government Documents Department


Description: Fluidized calcination involves the injection of an atomized feed solution containing dissolved solids into a bed of fluidized partioles at elevated temperatures suitable for drying and calcining. The study was conducted in a threeinch diameter fluidized column using aluminum oxide as bed material and aqueous aluminum nitrate solution as feed. Products were removed at regular intervals to maintain a constant bed weight. Particle growth was traced by adding radioactive aluminum oxide seeds of a given size to the starting bed and following their progress as they grew into successively larger sieve fractions. The effects on the growth rate of operating variables and physical properties of the feed were studied, including fluidizing air velocity, atomizing air rate, column temperature, feed concentration, feed rate, and viscosity and surface tension of the feed. For each product using screen analysis and gammacounting data a volume-surface mean diameter of the seedcontaining particles was calculated. Upon statistical analysis a linear relationship between the mean diameter of seed-containing particles and time exhibited very strong correlation, substantiating the hypothesis that particle growth was proportional to its surface area. From this linear relationship the over-all growth constant, equal to the slope, was obtained. Attrition effect of the atomizing air was found statistically to be non-significant. Normal growth far outweighed attrition and for steady-state operation other methods to produce seeds, such as jet or target attrition must be employed to balance normal growth. (auth)
Date: June 1, 1960
Creator: Lee, B. S.
Partner: UNT Libraries Government Documents Department


Description: Tests were performed on a thoria slurry flowing in a pipe to determine the magnitude of the possible error involved in the sampling process. Evidence indicates that a correct sample is obtained by withdrawing the sample isokinetically (i.e., by facing the sampler into the flow and adjusting the sampler velocity to match the ambient velocity) provided that the sampler is larger than some minimura diameter that is dependent on the mean eddy length and/ or the mean particle size. (auth)
Date: April 21, 1960
Creator: Wichner, R P
Partner: UNT Libraries Government Documents Department

Nb3Sn Artificial Pinning Microstructures

Description: Extension of the APC approach to Nb{sub 3}Sn requires that a second phae be incorporated into the Nb{sub 3}Sn layer. The second phase would increase pinning strength by either reducing the grain size or by the second phase pinning the flux itwelf. The following criteria for elements to be candidates for the APC approach are: (1) they must form intermetallic compounds with Cu or Sn and (2) they must have negligible solubility in Cu and Nb or they must be strong oxide formers. many of the rare earth elements satisfy these criteria. To circumvent the large strains required to produce wires with a fine distribution of the second phase, film deposition techniques have been used. Critical current densities for Nb films doped with Ti and Y are about 4,000 A/mm{sup 2} at 6T and 4.2 K.
Date: December 12, 1996
Creator: Dietderich, D.R. & Scanlan, R.M.
Partner: UNT Libraries Government Documents Department

Influence of Iron Oxide Particles on the Strength of Ball-Milled Iron

Description: Detailed microstructural and mechanical property studies of ball-milled iron, in the powder and consolidated states, are reviewed and assessed. The analyses cover three and one-half orders of magnitude of grain size (from 6 nm to 20 mm) and focus on the influence of oxide particles on the strength. The study includes the early work of Koch and Yang, Kimura and Takaki and continues with the more recent work of Umemoto et al and Belyakov, Sakai et al. It is shown that the major contributors to strength are the nanooxide particles. These particles are created by adiabatic shear banding during ball-milling leading to a bimodal distribution of particles. The predicted strength from particles, {sigma}{sub p}, is given by {sigma}{sub p} = B {center_dot} (D*{sub S}){sup -1/2} where D*{sub S} is the surface-to-surface interparticle spacing, and B = 395 MPa {center_dot} {micro}m{sup -1/2}. A model is proposed that accounts for the influence of the bimodal particle size distribution on strength.
Date: December 7, 2005
Creator: Lesuer, D R; Syn, C K & Sherby, O D
Partner: UNT Libraries Government Documents Department

A viscoplastic micromechanical model for the yield strength of nanocrystalline materials

Description: In this paper we present a micromechanical approach based on Fast Fourier Transforms to study the role played by dislocation glide and grain boundary (GB) accommodation in the determination of the plastic behavior of nanostructured materials. For this, we construct unit cells representing self-similar polycrystals with different grain sizes in the nanometer range and use local constitutive equations for slip and GB accommodation. We study the effect of grain size, strain rate and pressure on the local and effective behavior of nanostructured fcc materials with parameters obtained from experiments and atomistic simulations. Predictions of a previous qualitative pressure-sensitive model for the effective yield strength behind a shock front are substantially improved by considering strain partition between slip and GB activity. Under quasiestatic conditions, assuming diffusion-controlled mechanisms at GB, the model predicts a strain-rate sensitivity increase in nanocrystalline samples with respect to the same coarse-grained material of the same order as in recently published experiments.
Date: March 14, 2006
Creator: Lebensohn, R; Bringa, E & Caro, A
Partner: UNT Libraries Government Documents Department


Description: The processing-structure-property relationship is investigated for electrodeposited foils of the gold-copper alloy system. A model is presented that relates the deposition process parameters to the nanocrystalline grain size. An activation energy of 1.52 eV {center_dot} atom{sup -1} for growth is determined for a long pulse (>10 msec) mode, and is 0.16 eV {center_dot} atom{sup -1} for short pulses (<5 msec). The affect of nanocrystalline grain size on the mechanical properties is assessed using indentation measurements. A Hall-Petch type variation of the Vickers microhardness with nanocrystalline grain size (>6 nm) is observed for Au-Cu samples with 1-12 wt.% Cu as tested in cross-section. The hardness increases three-fold from a rule-of-mixtures value <1 GPa to a maximum of 2.9 GPa.
Date: February 25, 2005
Creator: Jankowski, A F; Saw, C K; Harper, J F; Vallier, R F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

Final Report LDRD 04-ERD-021

Description: In this project, we performed experiments and simulations to establish constitutive models for plastic behavior and to determine the deformation mechanism of nanocrystalline materials at different grain sizes (<100 nm) and high strain rates (>10{sup 6}/s). The experiments used both laser-induced shocks and isentropic compression to investigate, for the first time, the high-strain-rate deformation of nanocrystalline Ni. Samples were characterized using transmission electron microscopy, nanoindentation, profilometry, and x-ray diffraction before and after loading. We validated constitutive models using both atomistic molecular dynamics and continuum simulations performed at the boundary of their current computational possibilities to match experimental scales.
Date: February 23, 2007
Creator: Bringa, E
Partner: UNT Libraries Government Documents Department

Do grain boundaries in nanophase metals slide?

Description: Nanophase metallic materials show a maximum in strength as grain size decreases to the nano scale, indicating a break down of the Hall-Petch relation. Grain boundary sliding, as a possible accommodation mechanisms, is often the picture that explain computer simulations results and real experiments. In a recent paper, Bringa et al. Science 309, 1838 (2005), we report on the observation of an ultra-hard behavior in nanophase Cu under shock loading, explained in terms of a reduction of grain boundary sliding under the influence of the shock pressure. In this work we perform a detailed study of the effects of hydrostatic pressure on nanophase Cu plasticity and find that it can be understood in terms of pressure dependent grain boundary sliding controlled by a Mohr-Coulomb law.
Date: October 27, 2006
Creator: Bringa, E M; Leveugle, E & Caro, A
Partner: UNT Libraries Government Documents Department

Characterization of Densified Fully-Stabilized Nanometric Zirconia by Positron Annihilation Spectroscopy

Description: Fully-stabilized nanometric zirconia samples with varying degrees of porosity and grain sizes were analyzed using the coincidence Doppler broadening mode of the positron annihilation spectroscopy (PAS). A decrease in the low momentum fraction was observed and coincided with a decrease in porosity. In addition to pores, it is proposed that defects in the negatively charges grain boundary space region act as positron trapping centers; their effectiveness decreases with an increase in grain size. It is shown that PAS is sensitive to small grain size differences within the nanometric regime in these oxide materials.
Date: April 5, 2005
Creator: Garay, J E; Glade, S C; Asoka-Kumar, P; Anselmi-Tamburini, U & Munir, Z A
Partner: UNT Libraries Government Documents Department

Chem-Prep PZT 95/5 for neutron generator applications : development of laboratory-scale powder processing operations.

Description: Chemical synthesis methods are being developed as a future source of PZT 95/5 powder for neutron generator voltage bar applications. Laboratory-scale powder processes were established to produce PZT billets from these powders. The interactions between calcining temperature, sintering temperature, and pore former content were studied to identify the conditions necessary to produce PZT billets of the desired density and grain size. Several binder systems and pressing aids were evaluated for producing uniform sintered billets with low open porosity. The development of these processes supported the powder synthesis efforts and enabled comparisons between different chem-prep routes.
Date: December 1, 2003
Creator: Montoya, Ted V.; Moore, Roger Howard & Spindle, Thomas Lewis Jr.
Partner: UNT Libraries Government Documents Department

Activation Energy for Grain Growth in Aluminum Coatings

Description: To produce a specific grain size in metallic coatings requires precise control of the time at temperature during the deposition process. Aluminum coatings are deposited using electron-beam evaporation onto heated substrate surfaces. The grain size of the coating is determined upon examination of the microstructure in plan view and cross-section. Ideal grain growth is observed over the entire experimental range of temperature examined from 413 to 843 K. A transition in the activation energy for grain growth from 0.7 to 3.8 eV {center_dot} atom{sup -1} is observed as the temperature increases from <526 K to >588 K. The transition is indicative of the dominant mechanism for grain growth shifting with increasing temperature from grain boundary to lattice diffusion.
Date: October 14, 2004
Creator: Jankowski, A F; Ferreira, J L & Hayes, J P
Partner: UNT Libraries Government Documents Department

Effects of stabilization temperature on surface area and grain size of representative plutonium materials.

Description: Calcination at 400-1000C is used throughout the Department of Energy (DOE) complex to stabilize plutonium material for transportation and storage . The objectives of this stabilization are to remove moisture and other potentially water-producing phases, and to ensure that readsorption will not occur before material is placed in welded containers .Such moisture may threaten the integrity of containers through pressurization with radiolytically generated hydrogen. It is also considered valuable to reduce the fine (respirable) fraction of the material to mitigate potential impact of accidents.
Date: January 1, 2003
Creator: Boak, J. M. (Jeremy M.); Dale, D. J. (Deborah J.) & Eller, P. G. (Phillip Gary)
Partner: UNT Libraries Government Documents Department

Grain refinement in beryllium by equal channel angular extrusion.

Description: Ultrafine-grained Be is the material of choice for fabrication of the NIF target capsules. One method of producing ultrafine grains in metals is by imposing very large strains. Equal channel angular extrusion (ECAE) has been used to achieve these high strains. Previous work has shown that powder-source Be can be successfully processed by ECAE. Pure Be and Be-0.9 at% Cu alleys have been arc melted and cast into billets 5 mm in diameter by 30 mm in length. These billets were enclosed in cans fabricated from commercial purity Ni, with an electron-beam welded end plug. These cans were extruded at 425C in ECAE tooling with a 120' angle between the inlet and outlet channels. The billets were extruded up to 4 times. The microstructures of the powdersource Be and the arc-melted Be and Be-0.9 at% Cu materials will be presented, and the effects of the ECAE processing on the grain size will be discussed.
Date: January 1, 2003
Creator: Alexander, D. J. (David J.); Mauro, M. E. (Michael Ernest); Cooley, J. C. (Jason C.) & Dauelsberg, L. B. (Lawrence B.)
Partner: UNT Libraries Government Documents Department

Mechanical Behavior of Grain Boundary Engineered Copper

Description: A grain boundary engineered copper sample previously characterized by Electron Backscatter Diffraction (EBSD) has been selected for nanoindentation tests. Given the fact that grain boundaries have thicknesses in the order of 1 micron or less, it is essential to use nanomechanics to test the properties of individual grain boundaries. The Hysitron nanoindenter was selected over the MTS nanoindenter due to its superior optical capabilities that aid the selection and identification of the areas to be tested. An area of 2mm by 2mm with an average grain size of 50 microns has been selected for the study. Given the EBSD mapping, grains and grain boundaries with similar orientations are tested and the hardness and modulus are compared. These results will give a relationship between the mechanical properties and the engineered grain boundaries. This will provide for the first time a correlation between grain boundary orientation and the mechanical behavior of the sample at the nanoscale.
Date: August 8, 2006
Creator: Carter, S B & Hodge, A M
Partner: UNT Libraries Government Documents Department