135 Matching Results

Search Results

Advanced search parameters have been applied.

Pressure dependence of defect emissions and the appearance of pressure-induced deep centers in chalcopyrite alloys AgxCu1-xGaS2

Description: We present the pressure dependence of the defect emissions in the chalcopyrite alloy semiconductor Ag{sub x}Cu{sub 1-x}GaS{sub 2} for values of the alloy concentration x varying between 0 and 1. A large variation in the pressure coefficients of the different defect emissions with x was found. In one alloy concentration x=0.25 deep levels were found to appear under pressure. Plausible explanations of our results have been proposed.
Date: December 31, 2000
Creator: Choi, In-Hwan & Yu, Peter Y.
Partner: UNT Libraries Government Documents Department

Dymalloy: A composite substrate for high power density electronic components

Description: High power density electronic components such as fast microprocessors and power semiconductors must operate below the maximum rated device junction temperature to ensure reliability. function temperatures are determined by the amount of heat generated and the thermal resistance from junction to the ambient thermal environment. Two of the Largest contributions to this thermal resistance are the die attach interface and the package base. A decrease in these resistances can allow increased component packing density in MCMs, reduction of heat sink volume in tightly packed systems, enable the use of higher performance circuit components, and improve reliability. The substrate for high power density devices is the primary thermal link between the junctions and the heat sink. Present high power multichip modules and single chip packages use substrate materials such as silicon nitride or copper tungsten that have thermal conductivity in the range of 200 W/mK. We have developed Dymalloy, a copper-diamond composite, that has a thermal conductivity of 420 W/mK and an adjustable coefficient of thermal expansion, nominally 5.5 ppm/C at 25 C, compatible with silicon and gallium arsenide. Because of the matched coefficient of thermal expansion it is possible to use low thermal resistance hard die attach methods. Dymalloy is a composite material made using micron size Type I diamond powder that has a published thermal conductivity of 600 to 1000 W/mK in a metal matrix that has a thermal conductivity of 350 W/mK. The region of chemical bonding between the matrix material and diamond is limited to approximately 1000 A to maintain a high effective thermal conductivity for the composite. The material may be fabricated in near net shapes. Besides having exceptional thermal properties, the mechanical properties of this material also make it an attractive candidate as an electronic component substrate material.
Date: June 29, 1995
Creator: Kerns, J.A.; Colella, N.J.; Makowiecki, D. & Davidson, H.L.
Partner: UNT Libraries Government Documents Department

Bi-2212 and Bi-2223 wire development

Description: The results of innovative processing of Bi-2212 by isothermal melt processing and by controlled oxygen pressure cooling yield improved properties over the conventional routes. The addition of large grains of Ag has resulted in improved core/interface geometry and better performance in Bi-2212 and Bi-2223. A deformation processing study of Bi-2223 showed the effects of sheath material, relative core thickness, and reduction per pass on core/interface uniformity.
Date: December 1, 1995
Creator: Willis, J.O.; Ray, R.D. II & Holesinger, T.G.
Partner: UNT Libraries Government Documents Department

NON-IDEALITY IN H PERMEATION THROUGH PLATE MEMBRANES

Description: Under the commonly employed experimental conditions of a significant upstream concentration of H and c{sub H} {approx} 0 downstream, expressions are given for obtaining the concentration-independent D*{sub H} from the concentration dependent D{sub H} employing the known non-ideality. A procedure is given for determining the concentration profile for a given upstream concentration for an alloy where the non-ideality is known as a function of H concentration. For the Pd{sub 0.81}Ag{sub 0.19} alloy (423 K) the nonideality, f(r)<1 decreases the flux but for alloys where the non-ideality is in the opposite direction, f(r)>1, the flux will be greater which would be an advantage for the experimental purification of H{sub 2}.
Date: July 13, 2006
Creator: Shanahan, K; Ted B. Flanagan, T & D. Wang, D
Partner: UNT Libraries Government Documents Department

Self dressing resistance welding electrode. Technical progress report, first quarter 1992

Description: This grant was divided four tasks. Task one was to produce prototype electrodes and test to establish optimum weld parameters for a variety of applications. Task two was to expand the concept application--design it into a broader range of electrodes and produce prototype electrodes for test to establish the optimum electrode parameters. Task three was to develop a process to produce the refractory reinforced embodiment. Produce prototypes employing a variety of electrically conductive materials, test to verify performance improvement, and establish weld parameters. Task four was to write a final report. Progress made on each task is described.
Date: December 31, 1992
Creator: Prucher, B.
Partner: UNT Libraries Government Documents Department

Strength and flexibility of bulk high-{Tc} superconductors

Description: Strength, fracture toughness, and elastic modulus data have been gathered for bulk high-temperature superconductors, commercial 99.9% Ag, and a 1.2 at.% Mg/Ag alloy. These data have been used to calculate fracture strains for bulk conductors. The calculations indicate that the superconducting cores of clad tapes should begin to fracture at strains below 0.2%. In addition, residual strains in Ag-clad (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes have been measured by neutron diffraction. An explanation is offered for why many tapes appear to be able to tolerate large strains before exhibiting a reduction in current transport.
Date: August 1, 1996
Creator: Goretta, K.C.; Jiang, M.; Kupperman, D.S.; Lanagan, M.T.; Singh, J.P.; Vasanthamohan, N. et al.
Partner: UNT Libraries Government Documents Department

Nonmagnetic crystal-electric-field ground state in the heavy-fermion compound PrInAg{sub 2}

Description: The authors have performed inelastic neutron scattering measurements that confirm that the crystal-electric-field split ground state in the heavy-fermion compound PrInAg{sub 2} is a nonmagnetic, non-Kramers doublet. This implies that a quadrupolar Kondo interaction is responsible for the enhanced thermodynamic properties observed at low temperatures. They also observe anomalous broadening of the inelastic peaks and suggest two possible causes for this broadening.
Date: June 18, 1998
Creator: Kelley, T.M.; Beyermann, W.P.; Robinson, R.A.; Nakotte, H.; Canfield, P.C. & Trouw, F.
Partner: UNT Libraries Government Documents Department

Rapid fabrication of materials using directed light fabrication

Description: Directed light fabrication (DLF) is a rapid fabrication process that fuses gas delivered metal powders within a focal zone of a laser beam to produce fully dense, near-net shape, 3-dimensional metal components from a computer generated solid model. Computer controls dictate the metal deposition pathways, and no preforms or molds are required to generate complex sample geometries. The focal zone of the laser beam is programmed to move along or across a part cross-section, and coupled with a multi-axis sample stage, produces the desired part. By maintaining a constant molten puddle within the focal zone, a continuous liquid/solid interface is possible while achieving constant cooling rates that can be varied between 10 to 10{sup 4} K s{sup -1} and solidification growth rates (that scale with the beam velocity) ranging up to 10{sup 2} m s{sup -1}. The DLF technique offers unique advantages over conventional thermomechanical processes in that many labor and equipment intensive steps can be avoided. Moreover, owing to the flexibility in power distributions of lasers, a variety of materials can be processed, ranging from aluminum alloys to rhenium, and including intermetallics such as Mo{sub 5}Si{sub 3}. As a result, the rapid fabrication of conventional and advanced materials are possible.
Date: October 1, 1997
Creator: Thoma, D.J.; Lewis, G.K.; Milewski, J.O.; Chen, K.C. & Nemec, R.B.
Partner: UNT Libraries Government Documents Department

THE ELECTRONIC STRUCTURE OF AG/CU(100) SURFACE ALLOYS STUDIES BY AUGER-PHOTOELECTRON COINCIDENCE SPECTROSCOPY.

Description: We have measured the Ag and Pd M{sub 5}VV Auger spectrum in coincidence with Ag and Pd 4d{sub 5/2} photoelectrons for the Ag/Cu(100) and Pd/Cu(100) systems, respectively, as a function of admetal coverage. These systems form surface alloys (i.e. random substitutional alloys in the first atomic layer) for impurity concentrations in the 0.1 monolayer range. For these systems, the centroid of the impurity 4d levels is expected to shift away from the Fermi level by {approx}1 eV [Ruban et al., Journal of Molecular Catalysis. A 115 (1997) 421], an effect that should be easily seen in coincidence core-valence-valence Auger spectra. We find that the impurity Auger spectra of both systems shift in a manner that is consistent with d-band moving away from EF. However, the shift for Pd is considerably smaller than expected, and a shift almost absent for Ag. The disagreement between theory and experiment is most likely caused by the neglect of lattice relaxations in the calculations.
Date: October 8, 2001
Creator: Arena, D. A.; Bartynski, R. A. & Hulbert, S. L.
Partner: UNT Libraries Government Documents Department

De-alloying and stress-corrosion cracking. Final report

Description: This research program has had two major areas of focus that are related: (1) alloy corrosion and (2) the role of selective dissolution in the stress corrosion cracking of alloy systems. These interrelated issues were examined using model systems such as Ag-Au and Cu-Au by conventional electrochemical techniques, in situ scanning tunneling microscopy (STM), in situ small angle neutron scattering (SANS), ultrahigh speed digital photography of fracture events, and computer simulations. The STM and SANS work were specifically aimed at addressing a roughening transition known to occur in alloy systems undergoing corrosion at electrochemical potentials greater than the so-called critical potential. Analytical models of de-alloying processes including the roughening transition were developed that specifically include curvature effects that are important in alloy corrosion processes. Stress-corrosion experiments were performed on the same model systems using rapid optical and electrochemical techniques on 50 {micro}m--250 {micro}m thick sheets and small diameter wires. The primary goal of this work was to develop a fundamental understanding of the corrosion and electrochemistry of alloys and the stress-corrosion cracking processes these alloys undergo. Computer simulations and analytical work identified surface stress and an important parameter in environmentally assisted fracture. The major results of the research on this program since the summer of 1993 are briefly summarized.
Date: September 1, 1998
Creator: Sieradzki, K.
Partner: UNT Libraries Government Documents Department

Age hardening in rapidly solidified and hot isostatically pressed beryllium-aluminum-silver alloys

Description: Three different alloys of beryllium, aluminum and silver were processed to powder by centrifugal atomization in a helium atmosphere. Alloy compositions were, by weight, 50% Be, 47.5% Al, 2.5% Ag, 50% Be, 47% Al, 3% Ag, and 50% Be, 46% Al, 4% Ag. Due to the low solubility of both aluminum and silver in beryllium, the silver was concentrated in the aluminum phase, which appeared to separate from the beryllium in the liquid phase. A fine, continuous composite beryllium-aluminum microstructure was formed, which did not significantly change after hot isostatically pressing at 550 C for one hour at 30,000 psi argon pressure. Samples of HIP material were solution treated at 550 C for one hour, followed by a water quench. Aging temperatures were 150, 175, 200 and 225 C for times ranging from one half hour to 65 hours. Hardness measurements were made using a diamond pyramid indenter with a load of 1 kg. Results indicate that peak hardness was reached in 36--40 hours at 175 C and 12--16 hours at 200 C aging temperature, relatively independent of alloy composition.
Date: July 1, 1995
Creator: Carter, D.H.; McGeorge, A.C.; Jacobson, L.A. & Stanek, P.W.
Partner: UNT Libraries Government Documents Department

Landau-Ginzburg model of interphase boundaries in CsCl-type ferroelastics due to M{sup -}{sub 5} mode instability: LaAg{sub 1-x}In{sub x}

Description: We have constructed a Landau-Ginzburg model of ferroelastic domain walls in the tetragonal phase of CsCl-type intermetallic compounds. This improper ferroelastic phase transition is driven by the condensation of a degenerate zone-edge phononmode of M{sup -}{sub 5} symmetry and can be described by a six-component order parameter. Analytic and numerical kink-type soliton solutions for the order parameter profile and the strain distribution are obtained for three different interphase boundaries: a twin boundary and two antiphase boundaries. The stability, merging and splitting of various domain types are also studied. Specifically, a symmetry-allowed product phase of I4/mmm (D{sup 17}{sub 4h}) symmetry has been experimentally observed in pseudobinary rare earth alloys of composition RAg{sub 1-I} In{sub I} (R=La, Ce, Pr) and in related systems (YCu, LaCd).
Date: July 1, 1995
Creator: Hatch, D.M.; Saxena, A. & Barsch, G.R.
Partner: UNT Libraries Government Documents Department

Capillary flow solder wettability test

Description: A test procedure was developed to assess the capillary flow wettability of solders inside of a confined geometry. The test geometry was comprised of two parallel plates with a controlled gap of constant thickness (0.008 cm, 0.018 cm, 0.025 cm, and 0.038 cm). Capillary flow was assessed by: (1) the meniscus or capillary rise of the solder within the gap, (2) the extent of void formation in the gap, and (3) the time-dependence of the risen solder film. Tests were performed with the lead-free solders.
Date: January 1, 1996
Creator: Vianco, P.T. & Rejent, J.A.
Partner: UNT Libraries Government Documents Department

Zintl cluster chemistry in the alkali-metal-gallium systems

Description: Previous research into the alkali-metal-gallium systems has revealed a large variety of networked gallium deltahedra. The clusters are analogues to borane clusters and follow the same electronic requirements of 2n+2 skeletal electrons for closo-deltahedra. This work has focused on compounds that do not follow the typical electron counting rules. The first isolated gallium cluster was found in Cs{sub 8}Ga{sub 11}. The geometry of the Ga{sub 11}{sup 7{minus}} unit is not deltahedral but can be described as a penta-capped trigonal prism. The reduction of the charge from a closo-Ga{sub 11}{sup 13{minus}} to Ga{sub 11}{sup 7{minus}} is believed to be the driving force of the distortion. The compound is paramagnetic because of an extra electron but incorporation of a halide atom into the structure captures the unpaired electron and forms a diamagnetic compound. A second isolated cluster has been found in Na{sub 10}Ga{sub 10}Ni where the tetra-capped trigonal prismatic gallium is centered by nickel. Stabilization of the cluster occurs through Ni-Ga bonding. A simple two-dimensional network occurs in the binary K{sub 2}Ga{sub 3} Octahedra are connected through four waist atoms to form a layered structure with the potassium atoms sitting between the layers. Na{sub 30.5}Ga{sub 60{minus}x}Ag{sub x} is nonstoichiometric and needs only a small amount of silver to form (x {approximately} 2--6). The structure is composed of three different clusters which are interconnected to form a three-dimensional structure. The RbGa{sub 3{minus}x}Au{sub x} system is also nonstoichiometric with a three-dimensional structure composed of Ga{sub 8} dodecahedra and four-bonded gallium atoms. Unlike Na{sub 30.5}Ga{sub 60{minus}x}Ag{sub x}, the RbGa{sub 3} binary is also stable. The binary is formally a Zintl phase but the ternary is not. Some chemistry in the alkali-metal-indium system also has been explored. A new potassium-indium binary is discussed but the structure has not been completely characterized.
Date: March 27, 1998
Creator: Henning, R.
Partner: UNT Libraries Government Documents Department

Mechanical properties of Pb-free solder alloys on thick film hybrid microcircuits

Description: The technology drivers of the electronics industry continue to be systems miniaturization and reliability, in addition to addressing a variety of important environmental issues. Although the Sn-Pb eutectic alloy is widely used as a joining material in the electronics industry, it has drawn environmental concern due to its Pb content. The solder acts both as an electrical and mechanical connection within the different packaging levels in an electronic device. New Pb-free solders are being developed at Sandia National Laboratories. The alloys are based on the Sn-Ag alloy, having Bi and Au additions. Prototype hybrid microcircuit (HMC) test vehicles have been assembled to evaluate Pb-free solders for Au-Pt-Pd thick film soldering. The test components consist of a variety of dummy chip capacitors and leadless ceramic chip carriers (LCCC`s). The mechanical properties of the joints were evaluated. The reflow profiles and the solid state intermetallic formation reaction will also be presented. Improved solder joint manufacturability and increased fatigue resistance solder alloys are the goals of these materials.
Date: March 10, 1998
Creator: Hernandez, C.L.; Vianco, P.T.; Rejent, J.A. & Hosking, F.M.
Partner: UNT Libraries Government Documents Department

Processing and fabrication of high-{Tc} superconductors for electric power applications

Description: Recent developments in the powder-in-tube fabrication of (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes include identification of high current transport regions of the superconductor core, optimization of conductor design and processing to take advantage of these high current regions, optimization of superconductor powders and heat treatments, and incorporation of flux pinning defects into the superconductor grains. These developments are briefly discussed and their implications are assessed.
Date: September 1997
Creator: Balachandran, U.; Iyer, A. N.; Goretta, K. C. & Haldar, P.
Partner: UNT Libraries Government Documents Department

Development of materials related to the 60T and 100T magnets

Description: In the past year, the effort in materials science related to the 60T and 100T magnets at Los Alamos has been concentrated in three areas: (a) development of a fabrication route for Cu-Ag wire in collaboration with Handy and Harman and IGC and (b) investigation of the mechanical properties of a variety of potential high strength high conductivity materials (c) selection of the reinforcement materials for the coils and development of a fabrication route for these materials. The selection of the conductors and reinforcement materials is based on their mechanical properties and electrical properties at cryogenic temperature ({minus} 196 C). The authors have taken the approach of trying to relate the properties both to design requirements and to the service life of magnet. Thus, they have given some consideration both to the role of the internal stresses developed during the fabrication on the elastic-plastic transition and on the mechanical and thermal stability of heavily drawn wires. The feasibility of the fabrication route and the cost of manufacturing the materials must also be considered. They have emphasized the need to develop a fabrication route capable of producing the conductors with homogeneous mechanical and electrical properties and with a cross-section of 8.6 mm x 5.2 mm and 146 m in length or longer for a 100T magnet.
Date: December 31, 1997
Creator: Han, K. & Embury, J.D.
Partner: UNT Libraries Government Documents Department

Fabrication and testing of long length high-{Tc} composite conductors. Final report

Description: Presently some methods of HTS-conductors processing are under study in the authors laboratory. ``Powder-in-tube`` (PIT), ``Jelly-roll``, electrophorethis are among them. PIT process has developed predominantly both in a view of the achieved J{sub c} values Bi-2223 phase was used as a core material for these tapes. Since the main purpose of the task order was to enhance the development of long length high temperature superconductor tapes, the authors have considered reasonable to lay the perfection idea of the PIT process step by step or tape by tape. To realize it they have assumed, keeping stable the basic scheme of PIT process, to vary some technological parameters which are as follows: (1) type of initial powder; (2) sheath material; (3) tape construction (filaments number, cross section e.a.); and (4) processing regimes. This report covers the fabrication process and characteristics of the produced conductors.
Date: December 31, 1997
Creator: Fisher, L.M.
Partner: UNT Libraries Government Documents Department

Elevated temperature creep properties for selected active metal braze alloys

Description: Active metal braze alloys reduce the number of processes required for the joining of metal to ceramic components by eliminating the need for metallization and/or Ni plating of the ceramic surfaces. Titanium (Ti), V, and Zr are examples of active element additions which have been used successfully in such braze alloys. Since the braze alloy is expected to accommodate thermal expansion mismatch strains between the metal and ceramic materials, a knowledge of its elevated temperature mechanical properties is important. In particular, the issue of whether or not the creep strength of an active metal braze alloy is increased or decreased relative to its non-activated counterpart is important when designing new brazing processes and alloy systems. This paper presents a survey of high temperature mechanical properties for two pairs of conventional braze alloys and their active metal counterparts: (a) the conventional 72Ag-28Cu (Cusil) alloy, and the active braze alloy 62.2Ag- 36.2Cu-1.6Ti (Cusil ABA), and (b) the 82Au-18Ni (Nioro) alloy and the active braze alloy Mu-15.5M-0.75Mo-1.75V (Nioro ABA). For the case of the Cusil/Cusil ABA pair, the active metal addition contributes to solid solution strengthening of the braze alloy, resulting in a higher creep strength as compared to the non-active alloy. In the case of the Nioro/Nioro ABA pair, the Mo and V additions cause the active braze alloy to have a two-phase microstructure, which results in a reduced creep strength than the conventional braze alloy. The Garofalo sinh equation has been used to quantitatively describe the stress and temperature dependence of the deformation behavior. It will be observed that the effective stress exponent in the Garofalo sinh equation is a function of the instantaneous value of the stress argument.
Date: February 1, 1997
Creator: Stephens, J.J.
Partner: UNT Libraries Government Documents Department

Short-range order types in binary alloys: A reflection of coherent phase stability

Description: The short-range order (SRO) present in disordered solid solutions is classified according to three characteristic system-dependent energies: (1) formation enthalpies of ordered compounds, (2) enthalpies of mixing of disordered alloys, and (3) the energy of coherent phase separation, (the composition-weighted energy of the constituents each constrained to maintain a common lattice constant along an A/B interface). These energies are all compared against a common reference, the energy of incoherent phase separation (the composition-weighted energy of the constituents each at their own equilibrium volumes). Unlike long-range order (LRO), short-range order is determined by energetic competition between phases at a fixed composition, and hence only coherent phase-separated states are of relevance for SRO. The authors find five distinct SRO types, and show examples of each of these five types, including Cu-Au, Al-Mg, GaP-InP, Ni-Au, and Cu-Ag. The SRO is calculated from first-principles using the mixed-space cluster expansion approach combined with Monte Carlo simulations. Additionally, they examine the effect of inclusion of coherency strain in the calculation of SRO, and specifically examine the appropriate functional form for accurate SRO calculations.
Date: November 23, 1999
Creator: Wolverton, W.; Ozolins, V. & Zunger, Alex
Partner: UNT Libraries Government Documents Department