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Low-Temperature Synthesis of Actinide Tetraborides by Solid-State Metathesis Reactions

Description: The synthesis of actinide tetraborides including uranium tetraboride (UB,), plutonium tetraboride (PUB,) and thorium tetraboride (ThB{sub 4}) by a solid-state metathesis reaction are demonstrated. The present method significantly lowers the temperature required to {approx_equal}850 C. As an example, when UCl{sub 4}, is reacted with an excess of MgB{sub 2}, at 850 C, crystalline UB, is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl{sub 3}, as the initial step in the reaction. The UB, product is purified by washing water and drying.
Date: December 14, 2004
Creator: Lupinetti, Anthony J.; Garcia, Eduardo & Abney, Kent D.
Partner: UNT Libraries Government Documents Department

Inelastic neutron studies of the low energy phonon excitations in the RENi{sub 2}B{sub 2}C superconductors (RE = Lu, Y, Ho, Er)

Description: The authors studied the low-energy phonon excitations for wavevectors close to the Fermi surface nesting vector {rvec {xi}}{sub m} {approx_equal} 0.55{rvec a}. They find that above T{sub c} the frequencies of the {Delta}{sub 4} [{zeta}00] lowest-lying optical and acoustic phonon modes decrease with decreasing temperature, for {rvec {xi}} close to {rvec {xi}}{sub m}, and there is a shift of intensity from the upper to the lower mode, an effect characteristic of coupled modes. From approximately 120K down to temperatures in the vicinity of T{sub c}, only a single unresolved peak is observed. Below T{sub c} the phonon spectra of the Y and Lu compounds change dramatically: they consist of a sharp peak at approximately 4.5 meV with a weak shoulder at the higher energy side. No such sharp peak was observed below T{sub c} in the Ho and Er compounds.
Date: December 31, 1997
Creator: Bullock, M.; Stassis, C.; Zarestky, J.; Goldman, A.; Canfield, P.; Shirane, G. et al.
Partner: UNT Libraries Government Documents Department

MAGNETIC STRUCTURE AND MAGNETIC IMAGING OF RE{sub 2}Fe{sub 14}B (RE=Nd,Pr) PERMANENT MAGNETS

Description: This chapter aims to review the magnetic structures observed in the RE{sub 2}Fe{sub 14}B (RE = Nd, Pr) system using various TEM magnetic imaging techniques. The authors focus on studies of die-upset Nd-based permanent magnets conducted mainly at Brookhaven National Laboratory in the past several years. Investigations on Nd-Fe-B sintered magnets and single crystals, as well as Pr-Fe-B die-upset magnets also will be covered. In Sec.2 and Sec.3 they review the microstructure, including grain alignment and secondary phases of the materials, and grain boundary structure and composition of the intergranular phase. Sec.4 is devoted to the domain structure, such as the width of domain and domain wall and domain-wall energy. Monte Carlo simulation of the effects of demagnetization fields will be presented in Sec.5. In-situ experiments on the dynamic behavior of domain reorientation as a function of temperature, pinning, grain boundary nucleation related to coercivity under various fields are described in Sec.6. Finally, in Sec.7 the correlation between microstructure and properties are discussed.
Date: April 20, 2000
Creator: Zhu, Y. & Volkov, V. V.
Partner: UNT Libraries Government Documents Department

{mu}{sup +}SR studies of magnetic properties of boron carbide superconductors

Description: Positive-muon spin relaxation ({mu}{sup +}SR) has been carried out in the recently-discovered rare-earth boron carbide superconductors RNi{sub 2}B{sub 2}C, R = Ho, Er and Tm. For R = Ho and Er zero-field {mu}{sup +}SR measurements showed a well-defined internal field below the Neel temperatures of 5.5 K coexisting with the superconducting state down to 0.1 K. The observed temperature dependence of the order parameter for Ho is consistent with a 2-dimensional Ising model. For R = Tm a spontaneous internal field appears above 30 K, whose magnitude saturates below about 3 K at a value corresponding to a rare earth moment much smaller than for Ho and Er. Transverse-field {micro}{sup {minus}}SR measurements in R = Tm showed a superconducting penetration depth {lambda}, = 1,200 {angstrom}. The temperature dependence of {lambda} is consistent with conventional s-wave pairing.
Date: June 8, 1994
Creator: Le , L.P.; Heffner, R.H. & Nieuwenhuys, G.J.
Partner: UNT Libraries Government Documents Department

THE ANALYSIS OF REFRACTORY BORIDES, CARBIDES, NITRIDES, AND SILICIDES

Description: Methods are presented for the analysis of 41 refractory materials. An evaluation of the accuracy and the precision of these techniques are also given The materials studied are the borides of hafnium, molybdenum, niobiumL rhenium, tantalum, thorium, titanium, tungsten, uranium vanadium, and zirconium; the carbides of hafnium molybdenum, miobium, silicon, tantalum, thorium, titanium, tungsten, uranium, vanadium, and zircomium; the nitrides of boron, hafnium, niobium, silicon, tantalum, titanium, uranium, and zirconium; the silicides of molybdenum, rhenium, tantalum, titanium, tungsten, vanadium, and zirconium; and mixed carbides of uranium with hafnium, niobium, tantalum, or zirconium. (auth)
Date: March 1, 1959
Creator: Kriege, O.H.
Partner: UNT Libraries Government Documents Department

Crystallographic texture determinations from inverse susceptibility measurements

Description: Determination of the quantitative relationship between crystallographic texture and magnetic properties in advanced permanent magnets may be hampered by complex microstructures, which complicate methods that rely on diffraction, or by interparticulate interactions, which adversely affect methods based on magnetic remanence measurements. To this end, new techniques in the determination of texture of bulk permanent magnets are being explored to overcome these inherent experimental difficulties. The analysis of inverse paramagnetic susceptibility measurements constitutes a new method to investigate crystallographic texture. Such measurements also provide Curie temperature data, which is sensitive to chemical changes that may have occurred in the magnetic phase during processing. The mathematical formalism underlying the analysis of inverse susceptibility measurements is outlined, and is used to evaluate magnetic measurements taken from a series of Nd{sub 2}Fe{sub 14}B magnets that have been processed by different means, and thus contain different degrees of texture. While this method does provide qualitative information concerning the relative crystallographic alignment of magnet samples, it needs calibration to obtain an explicit value for a texture order parameter.
Date: October 1, 1996
Creator: Lewis, L.H. & Welch, D.O.
Partner: UNT Libraries Government Documents Department

{open_quotes}Exchange-spring{close_quotes} Nd-Fe-B alloys: Investigations into reversal mechanisms and their temperature dependence

Description: In order to investigate factors affecting coercivity a series of two-phase Nd{sub 2}Fe{sub 14}B-based nanocomposite alloys with different excess iron concentrations were produced by melt-spinning methods. The constituent grain size was estimated by diffraction methods to be in the range of 150 {Angstrom} - 500 {Angstrom}, and room-temperature demagnetization curves verify that the alloys studied exhibit a modest remanence enhancement. Isothermal remanence magnetization (IRM) and dc-demagnetization (DCD) measurements performed at temperatures in the range 275 K {le} T {le} 350 K illustrate that the coercivity and irreversible magnetization develops in a bimodal, incoherent manner from a demagnetized state but upon demagnetization from a saturated state the system evinces collective, exchange-coupled behavior as illustrated by the reversible magnetization M{sub rev}. The temperature dependencies and values of the irreversible susceptibility X{sub irr} (DCD) suggest that a moderating phase with a magnetic anisotropy intermediate to the two constituent main phases may be present in the alloys.
Date: October 1, 1996
Creator: Lewis, L.H.; Welch, D.O. & Panchanathan, V.
Partner: UNT Libraries Government Documents Department

MAGNETIZATION REVERSAL IN MELT-QUENCHED NdFeB.

Description: Melt-quenched NdFeB is an important modern permanent magnet material. However there still remains doubt as to the magnetization reversal mechanism which controls coercivity in material prepared by this processing route. To investigate this problem a new technique based on measurements of reversible magnetization along recoil curves has been used. The technique identifies the presence of free domain walls during magnetic reversal. For this study samples of isotropic (MQI), hot pressed (MQII) and die upset (MQIII) melt-quenched NdFeB were examined. The results indicate that in MQI free domain walls are not present during reversal and the reversal mechanism is most likely incoherent rotation of some form. Free domain walls are also not present during reversal in the majority of grains of MQII, even though initial magnetization measurements indicate that the grain size is large enough to support them. In MQIII free domain walls are present during reversal. These results are attributed to the reduced domain wall nucleation field in MQIII compared with MQII and the increased dipolar interactions in MQIII.
Date: April 5, 1999
Creator: CREW,D.C.
Partner: UNT Libraries Government Documents Department

Fracture toughness of commercial hard magnets

Description: In some demanding magnetic applications such as electric vehicle drive motors in which high rpm and small air gaps are necessary, the Nd{sub 2}Fe{sub 14}B magnets need to be used as a structural element. Any improvement in their fracture resistance will reduce the failure rates during commercial use. For current uses, an improvement in the toughness would allow greater machinability and easier handling. As a prelude to a program to improve the fracture toughness of Nd{sub 2}Fe{sub 14}B magnets, the toughness of a series of commercial materials was measured to determine if a reproducible and distinguishable measurement could be made.
Date: April 1, 1996
Creator: Horton, J.A.; Wright, J.L. & Herchenroeder, J.W.
Partner: UNT Libraries Government Documents Department

Microstructural characterization of high energy product Nd-Fe-B rapidly solidified ribbons

Description: The bonded Nd-Fe-B market has experienced the fastest growth of any permanent magnet market. Rapidly solidified Nd-Fe-B forms the basis for this bonded magnet industry. Rapid solidification is carried out by melt spinning, producing a highly stable and magnetically hard microstructure. This study focuses on a microstructural analysis of this melt spun ribbon using transmission and scanning electron microscopy (TEM and SEM), atom probe field ion microscopy (APFIM) and Auger electron spectroscopy (AES). The ribbons showed a uniform grain size and shape distribution through the thickness of the melt spun ribbon. Grain sizes ranged from 20 to 40 nm. AES showed neodymium enrichment on a fracture surface and corresponding iron and boron depletion suggesting that the eutectic 70Nd-30Fe phase is present with a thickness of approximately 1 nm. Atom probe composition analysis of grain interiors gave results very close to the nominal composition and some preliminary evidence of a grain boundary phase.
Date: November 1, 1998
Creator: Horton, J.A.; Miller, M.K.; Heatherly, L.; Jones, J.W.; Russell, K.F. & Panchanathan, V.
Partner: UNT Libraries Government Documents Department

PROCESSING, MICROSTRUCTURE AND CREEP BEHAVIOR OF MO-SI-B-BASED INTERMETALLIC ALLOYS FOR VERY HIGH TEMPERATURE STRUCTURAL APPLICATIONS

Description: This research project is concerned with developing a fundamental understanding of the effects of processing and microstructure on the creep behavior of refractory intermetallic alloys based on the Mo-Si-B system. During this year, the microstructure, bend strength and compressive creep behavior of a Mo-3Si-1B (in wt.%) alloy were studied. The microstructure of this alloy was three-phase, being composed of {alpha}-Mo, Mo{sub 3}Si and T2-Mo{sub 5}SiB{sub 2} phases. The elastic limit strength of the alloy remained quite high until 1200 C with a value of 800MPa, but dropped rapidly thereafter to a value of 220 MPa at 1400 C. Results of compressive creep tests at 1200 C showed that the creep rates were quite high and varied nearly linearly with stress between 250 and 500 MPa, which suggests that diffusional mechanisms dominate the creep process. Microstructural observations of post-crept samples indicated the presence of many voids in the {alpha}-Mo grains and few cracks in the intermetallic particles and along their interfaces with the {alpha}-Mo matrix. These results and presented and discussed.
Date: February 8, 2005
Creator: Vasudevan, Vijay K.
Partner: UNT Libraries Government Documents Department

FIELD DEPENDENCE OF THE SPIN REORIENTATION TEMPERATURE IN MICRO AND NANOCRYSTALLINE FORMS OF Nd{sub 2}Fe{sub 14}B.

Description: Insight into the anisotropy behavior of Nd{sub 2}Fe{sub 14}B may be obtained by measurements of the spin reorientation temperature T{sub S} where the overall magnetocrystalline anisotropy changes to allow the magnetic moment to relax from an easy axis to an easy cone configuration. DC magnetization measurements made at various applied fields on sintered and nanocrystalline forms of Nd{sub 2}Fe{sub 14}B indicate a T{sub S} that remains constant for the sintered sample but is strongly field-dependent for the nanocrystalline forms of the material. Specifically, T{sub S} decreases with decreasing applied fields of strengths 5 T, 1 T and 0.01 T. A simple model that minimizes the total energy of the system leads to the conclusion that the spin reorientation temperature is insensitive to applied field. Therefore it is concluded that the apparent decrease in the system's spin reorientation temperatures with decrease in measuring field can be attributed to the nanoscale structure of the system and a difference in the anisotropy constants compared to their bulk values.
Date: August 18, 2002
Creator: LEWIS,L.H. & HARLAND,C.L.
Partner: UNT Libraries Government Documents Department

Neutron and resonant x-ray scattering studies of RNi{sub 2}B{sub 2}C (R = rare earth) single crystals

Description: This family of intermetallic compounds is ideal for the study of the interplay between superconductivity and magnetism since, in several of these compounds (Ho, Er, Tm, Dy), superconductivity coexists with magnetic ordering. The most important findings of the scattering studies are (a) in the Ho-compound, a complex magnetic structure characterized by two incommensurate wave vectors, {rvec k}{sub a} = 0.585 {rvec a}* and {rvec k}{sub c} = 0.915 {rvec c}*, exists in the vicinity of 5 K, where the almost reentrant behavior of this compound occurs; (b) an incommensurate magnetic structure with wave vector along {rvec a}*, close to the zone boundary, is observed in several of these compounds; and (c) pronounced soft-phonon behavior was observed for both the acoustic and first optical {Delta}{sub 4}[{xi}00] branches in the superconducting Lu and Ho compounds, a behavior characteristic of strongly coupled conventional superconductors. Furthermore, these phonon anomalies occur at wave vectors close to those of the incommensurate magnetically ordered structures observed in the magnetic compounds of this family. This observation suggests that both the magnetic ordering and phonon softening originate from common nesting features of the Fermi surfaces of these compounds. Band theoretical calculations are in qualitative agreement with these results.
Date: June 1, 1996
Creator: Stassis, C. & Goldman, A.I.
Partner: UNT Libraries Government Documents Department

Neutron scattering studies of the RENi{sub 2}B{sub 2}C (RE = Lu, Y, Ho, Er): Lattice dynamics

Description: The first chapter gives a brief overview of the system discussed in this dissertation. Chapters 2--5 and Appendix B of this dissertation consist of papers that are published, or have been submitted, which show experimental data regarding the phonon softening of LuNi{sub 2}B{sub 2}C. These papers have been removed and processed separately. Chapter 6 will contain a summary of the conclusions up to date. Appendix A will consist of a brief derivation of {chi}(q) which is talked about in the introduction of the dissertation. Appendix B will contain a Born-von Karman model fit to the experimental LuNi{sub 2}B{sub 2}C data and a comparison with experimental data. Appendix C will contain a brief summary of the work done on LuNi{sub 2}B{sub 2}C as well as a complete listing of experimental data taken on the crystals which may be needed later for theoretical models of this system. Appendix D will outline a brief introduction covering some of the field theory used in the theoretical work for this thesis.
Date: February 23, 1998
Creator: Bullock, M.
Partner: UNT Libraries Government Documents Department

Microstructure and thermal stability of transition metal nitrides and borides on GaN

Description: Microstructure and thermal stability of ZrN/ZrB2 bilayer deposited on GaN have been studied using transmission electron microscopy methods (TEM) and secondary ion mass spectrometry (SIMS). It has been demonstrated that annealing of the contact structure at 1100 C in N2 atmosphere does not lead to any observable metal/ semiconductor interaction. In contrast, a failure of the integrity of ZrN/ZrB2 metallization at 800 C, when the heat treatment is performed in O2 ambient has been observed.
Date: June 28, 2000
Creator: Jasinski, J.; Kaminska, E.; Piotrowska, A.; Barcz, A. & Zielinski, M.
Partner: UNT Libraries Government Documents Department

Processing, Microstructure and Creep Behavior of Mo-Si-B-Based Intermetallic Alloys for Very High Temperature Structural Applications

Description: This research project is concerned with developing a fundamental understanding of the effects of processing and microstructure on the creep behavior of refractory intermetallic alloys based on the Mo-Si-B system. In the first part of this project, the compression creep behavior of a Mo-8.9Si-7.71B (in at.%) alloy, at 1100 and 1200 C was studied, whereas in the second part of the project, the constant strain rate compression behavior at 1200, 1300 and 1400 C of a nominally Mo-20Si-10B (in at.%) alloy, processed such as to yield five different {alpha}-Mo volume fractions ranging from 5 to 46%, was studied. In order to determine the deformation and damage mechanisms and rationalize the creep/high temperature deformation data and parameters, the microstructure of both undeformed and deformed samples was characterized in detail using x-ray diffraction, scanning electron microscopy (SEM) with back scattered electron imaging (BSE) and energy dispersive x-ray spectroscopy (EDS), electron back scattered diffraction (EBSD)/orientation electron microscopy in the SEM and transmission electron microscopy (TEM). The microstructure of both alloys was three-phase, being composed of {alpha}-Mo, Mo{sub 3}Si and T2-Mo{sub 5}SiB{sub 2} phases. The values of stress exponents and activation energies, and their dependence on microstructure were determined. The data suggested the operation of both dislocation as well as diffusional mechanisms, depending on alloy, test temperature, stress level and microstructure. Microstructural observations of post-crept/deformed samples indicated the presence of many voids in the {alpha}-Mo grains and few cracks in the intermetallic particles and along their interfaces with the {alpha}-Mo matrix. TEM observations revealed the presence of recrystallized {alpha}-Mo grains and sub-grain boundaries composed of dislocation arrays within the grains (in Mo-8.9Si-7.71B) or fine sub-grains with a high density of b = 1/2<111> dislocations (in Mo-20Si-10B), which are consistent with the values of the respective stress exponents and activation energies that were obtained and ...
Date: March 31, 2008
Creator: Vasudevan, Vijay
Partner: UNT Libraries Government Documents Department

ANISOTROPY DETERMINATIONS IN EXCHANGE SPRING MAGNETS.

Description: Ferromagnetic nanocomposites, or ''exchange spring'' magnets, possess a nanoscaled microstructure that allows intergrain magnetic exchange forces to couple the constituent grains and alter the system's effective magnetic anisotropies. While the effects of the anisotropy alterations are clearly seen in macroscopic magnetic measurement, it is extremely difficult to determine the detailed effects of the system's exchange coupling, such as the interphase exchange length, the inherent domain wall widths or the effective anisotropies of the system. Clarification of these materials parameters may be obtained from the ''micromagnetic'' phenomenological model, where the assumption of magnetic reversal initiating in the magnetically-soft regions of the exchange-spring maqet is explicitly included. This approach differs from that typically applied by other researchers and allows a quantitative estimate of the effective anisotropies of an exchange spring system. Hysteresis loops measured on well-characterized nanocomposite alloys based on the composition Nd{sub 2}Fe{sub 14}B + {alpha}-Fe at temperatures above the spin reorientation temperature were analyzed within the framework of the micromagnetic phenomenological model. Preliminary results indicate that the effective anisotropy constant in the material is intermediate to that of bulk {alpha}-Fe and bulk Nd{sub 2}Fe{sub 14}B and increases with decreasing temperature. These results strongly support the idea that magnetic reversal in nanocomposite systems initiates in the lower-anisotropy regions of the system, and that the soft-phase regions become exchange-hardened by virtue of their proximity to the magnetically-hard regions.
Date: August 18, 2002
Creator: LEWIS,L.H. & HARLAND,C.L.
Partner: UNT Libraries Government Documents Department

A metallurgical approach toward alloying in rare earth permanent magnet systems

Description: TiC is added as an alloying agent to Nd{sub 2}Fe{sub l4}B (2-14-1) system to both alter the solidification behavior of the melt and to act as pinning sites to control grain growth. The addition of TIC to 2-14-1 results in a factor of three reduction of the quench rate required to produce amorphous material. In addition, the crystallization temperature of the glass is enhanced leading to enhanced nucleation and finer grain size during crystallization. TIC additions to the stochiometric melt affect the range of primary solidification of the 2-14-1 phase. When TIC is added to the limit of its liquid solubility, the primary solidification range is move further from the stochiometric composition to the Nd rich region.
Date: December 31, 1995
Creator: McCallum, R.W.; Willard, M.A.; Dennis, K.W.; Branagan, D.J. & Kramer, M.J.
Partner: UNT Libraries Government Documents Department

Vortex lattice structures in YNi{sub 2}B{sub 2}C

Description: The authors observe a flux lattice with square symmetry in the superconductor YNi{sub 2}B{sub 2}C when the applied field is parallel to the c-axis of the crystal. A square lattice observed previously in the isostructural magnetic analog ErNi{sub 2}B{sub 2}C was attributed to the interaction between magnetic order in that system and the flux lattice. Since the Y-based compound does not order magnetically, it is clear that the structure of the flux lattice is unrelated to magnetic order. In fact, they show that the flux lines have a square cross-section when the applied field is parallel to the c-axis of the crystal, since the measured penetration depth along the 100 crystal direction is larger than the penetration depth along the 110 by approximately 60%. This is the likely reason for the square symmetry of the lattice. Although they find considerable disorder in the arrangement of the flux lines at 2.5T, no melting of the vortex lattice was observed.
Date: December 1, 1997
Creator: Yethiraj, M.; Paul, D.M.; Tomy, C.V. & Forgan, E.M.
Partner: UNT Libraries Government Documents Department