Fee Phase Research Articles

Enhanced Structural and Magnetic Ordering of FePt/Mn-Oxide Bilayers by Ion-Beam Bombardment and Annealing

Structural and magnetic properties of FePt thin films were affected strongly by capped MnOx layers prepared by ion-beam bom-bardment and post-annealing. As-deposited FePt/MnOx bilayer exhibited a magnetically soft fee phase, and it turned to an ordered fct FePt phase with large coercivity (~8 kOe) after annealing at 550°C. Increasing the %02/Ar in capped MnOx layer during de position resulted in smaller ordered FePt grains separated by grain boundaries of MnOx. We found that the superlattice (001) peak is broadened considerably with larger amount of MnOx incorporated into FePt, likely due to the hindered formation of hard phase. Our results indicate that FePt/MnOx films deposited with lower %02/Ar, the oxygen atoms may occupy the interstitial positions in the FePt lattice to induce a local strain thus enhancing the FePt ordering. Further increased %02/Ar in capped MnOx layer, the excess oxygen atoms act a diffusion barrier effectively to inhibit the FePt grain growth and ordering.

Bulk aluminum at high pressure: A first-principles study

The behavior of metals at high pressure is of great importance to the fields of shock physics, geophysics, astrophysics, and nuclear materials. We study here bulk crystalline aluminum from first principles at pressures up to 2500 GPa - soon within reach of laser-based experimental facilities. Our simulations use density-functional theory and density-functional perturbation theory in the local-density and generalized-gradient approximations. Notably, the two different exchange-correlation functionals predict very similar results for the fcc -> hcp, fcc -> bcc, and hcp -> bcc transition pressures, around 175, 275, and 380GPa, respectively. In addition, our results indicate that core overlaps become noticeable only beyond pressures of 1200 GPa. From the phonon dispersions of the fee phase at increasing pressure, we predict a softening of the lowest transverse acoustic vibrational mode along the [110] direction, which corresponds to a Born instability of the fee phase around 725 GPa.

New Fabrication Process for ${\hbox{Nb}}_{3}{\hbox{Sn}}$ Conductors Through Diffusion Reaction Between Nb and Ag-Sn Alloys

We investigated the superconducting properties of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn formed through the diffusion reaction between Nb and Ag-Sn alloys, including Ag-9at%Sn ductile fee phase and Ag-(12-16)at%Sn ductile zeta phase alloys, and Ag-24at%Sn brittle epsiv phase alloy. We fabricated single-core and multi-core composite wires by the rod-in tube and powder-in-tube processes. The composite wires were heat treated at 650-900degC to form Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filaments. With increase of Sn content in the Ag-Sn alloy cores, the superconducting properties of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn composite wires were improved. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and Bc <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (4.2 K) values for the Nb/Ag-Sn wires are similar to those for the Nb/Cu-Sn. However the Ic values are relatively small, due to their very thin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn layers below 0.4 mum. Ag is apparently not effective to increase the formation rate of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn layer. By using the 200- and 40000-core wires, T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> and B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> (4.2 K) values were increased by 0.5-1 K and 2-5 T, respectively for these multifllamentary wires, while from 10 to 500 times larger Ic (4.2 K) were obtained for the wires. The typical values for Tc and Bc2 (4.2 K) are 16.5-17.7 K and 16-21 T, respectively. The addition of 5at%Cu to Ag-Sn alloys improves their superconducting properties.

Preparation and study of the critical-mass-free plutonium ceramics with neutron poisons Hf, Gd and Li

Powder sintering was used to produce homogeneous type oxide ceramics of Pu with Hf, Gd and Li6. In all the ceramics, there is the number of neutron poison (Hf, Gd and Li) atoms per plutonium atom needed, according to the physical calculation, for them to be free of critical mass. PuO2 stabilizes high-temperature modifications of cubic HfO2or hexagonalGd2O3i however, at the ratio given by the physical calculation, the plutonium is insufficient for their full stabilization. Addition of yttrium oxide as an additive stabilizing the fee phase of HfO2 resulted in cubic solid solution (Pu,Hf,Y)O2-x. Pu/Li/Hf and Pu/Li/Si ceramics produced by sintering of PuO2 and compound Li2HfO3 or 6Li4SiO4 powders is characterized with presence of two phases. The method of differential thermal analysis demonstrated the phase stability of (Pu-Hf, Pu-Gd, Pu-Li-Hf) oxide ceramics in the 20-1500°C temperature range. Ceramic (Pu/Li/Si) has several endothermal effects. Tests in boiling water solutions of various composition suggest that the specimens of Pu, Hf oxides and ternary oxides (Pu, Hf, Y) O2 are less stable in weakly acidic media than in weakly alkaline medium and distilled water. The obtained results were used as a basis to estimate the assumed solid solution region boundaries for binary Hf, Pu and ternary Hf, Pu,Y oxides on the side of HfO2.

Thermodynamic Modeling of the Ho-Pd System

Thermodynamic Modeling of the Ho-Pd System

Morphology and Orientation Relationships of Ge Precipitates in Ag-Ge Alloys

Abstract Ag-Ge forms a simple binary eutectic with only two solid phases - a Ag-rich fee phase and a Ge-rich diamond-cubic phase, both with very limited solid solubility for the other element. Because both phases are cubic and there are no intermediate compounds, precipitation of Ge in an Al matrix might be expected to be crystallographically and morphologically simple. However, previous investigations of similar alloys (Al-Ge and Al-Si) have shown that precipitation in such apparently simple eutectic systems can be surprisingly complex [1,2]. For example, it was found that Ge precipitation in Al-Ge alloys depends sensitively on the availability of lattice vacancies and that at least five different orientation relationships and even more types of morphologies can form during aging after a quench. In the course of that work it was discovered that twinning is a key factor in the development of precipitate morphologies. To understand the role of twinning in precipitation this study investigates the characteristics of precipitation in Al-Ge alloys.

Structural Phase Transitions of Endohedral Metallofullerene La@C82 Studied by Single Crystal X-Ray Diffraction

Recently we have succeeded to synthesize solvent-free single crystals of endohedral metallofullerene La@C82 with the size of 0.5×0.2×0.2mm3 for the first time. The low temperature (T) x-ray diffraction measurements (20K–300K) have elucidated successive phase transitions and the existence of four phases, (I) the face-centered cubic (fee) structure (300K–180K), (II) the rhombohedral structure (180K-140K), (III) the triclinic structure (140K–20K) and (IV) the simple cubic (sc) structure (132K-20K). The rhombohedral and triclinic phases are characterized by the C82-cage orientational order, while the sc phase is characterized by the molecular electric dipole order. In the fee phase, both are not in order. As for the two low-est-T phases, the crystal remains in the triclinic phase for slow cooling below 132K. On the other hand with rapid cooling, the crystal undergoes the phase transition to the sc phase at 115K, which is stable up to 132K. The sc phase is more stable than the triclinic phase, contrary to the usual cases.

New optical media based on Ge4Sb1-xBixTe5

Ge4SbTe5 is known for its applicability as an optical media for phase change recording. One major problem of using this material is its slow crystallization rate, thus the erasability is not satisfactory. In this study, we explored the possibility of bismuth addition to enhance the crystallization rate. Ge4Sb1-xBixTe5 films were prepared by rf magnetron sputtering onto glass or Si substrate. Thermal analysis was done by DSC measurements, crystalline phase after annealing was examined by X-ray diffraction, and optical properties were taken by using an optical spectrometer. It was found that the Bi modified 415 maintains an fee phase up to x = 0.5. The crystallization temperature was lowered from 227 (x = 0) to 202 °C (x = 0.5), and the activation energy for crystallization is greatly lowered from 4.03 to 2.70 eV. However, the optical contrast is slightly lowered. All these factors denote that the 5 at% Bi added 415 film might be a better candidate than the un-doped ones.

Phonon contributions to the entropies ofhP24and fccCo3V

Inelastic neutron-scattering spectra and neutron-diffraction patterns were measured on the alloy Co3V at temperatures from 1073-1513 K, where the hP24 (ordered hexagonal) and fee structures are the equilibrium states of the alloy. Phonon density of states (DOS) curves were calculated from the inelastic-scattering spectra, allowing estimates of the vibrational entropy in the harmonic and quasiharmonic approximations. The vibrational entropy of the hP24-fcc phase transition at 1323 K was found to be 0.07k(B)/atom. The anharmonic contributions to the entropy over a temperature range of 100 K were comparable to the vibrational entropy of this phase transition. The anharmonic softening of the phonon DOS was only slightly larger for the hP24 than the fee phase, however, so the anharmonic effects contribute only slightly to the difference in entropy of the two phases. The simple Gruneisen approximation was inadequate for predicting the thermal softening of the phonon DOS.

Thermal Sprayed Titanium Anode for Cathodic Protection of Reinforced Concrete Bridges

Stable operation of cobalt catalyzed thermal sprayed titanium anodes for cathodic protection (CP) of bridge reinforcing steel was maintained in accelerated tests for a period equivalent to 23 years service at Oregon Department of Transportation (Oregon DOT) bridge CP conditions with no evidence that operation would degrade with further aging. The cobalt catalyst dispersed into the concrete near the anodeconcrete interface with electrochemical aging to produce a more diffuse anode reaction zone. The titanium anode had a porous heterogeneous structure composed of α-titanium containing interstitial oxygen and nitrogen, and a fee phase thought to be Ti(O,N). Splat cooling rates were 10 to 150 K/s, and microstructures were produced by equilibrium processes at the splat solidification front. Nitrogen gas atomization during thermal spraying produced a coating with more uniform composition, less cracking, and lower resistivity than using air atomization.

Observation of ion-induced alloy phase formation in an immiscible YTa system

Amorphization was achieved by room temperature 200 keV xenon-ion mixing in an immiscible YTa system, which has a positive heat of formation exceeding +40 kJ/mol. In addition, three new metastable crystalline phases, i.e., an fee phase (fee-I) in Y-rich films, an hep and another fee phase (fee-II) in Ta-rich films, were observed at different ion mixing stages. To give a possible explanation to these anomalous alloying behaviors, a Gibbs-free-energy diagram of the system was calculated based on Miedema's model of heat of formation and a method suggested by Alonso. The free energy of the YTa multilayered films containing excess interfacial free energy was also estimated and added into the diagram. It was found that the excess interfacial free energy stored in the multilayered films could increase the initial energetic state of the multilayers and therefore played an important role in ion-induced alloying in this immiscible system.

Llo Phase Formation in CoPt Thin Films

ABSTRACTThe high magnetic anisotropy and high coercivity of equiatomic CoPt thin films make them attractive as potential materials for magnetic recording applications. Magnetic coercivity (Hc) over 10 kOe has been measured in films in which the as-deposited fee phase has been partially transformed to the atomically ordered Ll0 phase. Very high Hc has been related to high volume fraction and small size of the Ll0 precipitates. A better understanding of the Ll0 phase formation and quantification of volume fraction is critical to optimizing the magnetic properties of this material.As we have previously reported, an increase in Hc was observed with an increase in Ll0 volume fraction in 10 nm thick, equiatomic CoPt films. In our current investigation we have observed that, at anneal temperatures far from the order/disorder transition temperature, e.g. at T = 0.6 Tc, numerous, very fine Ll0 precipitates are seen, some of which cluster at the parent phase grain boundaries. At T = 0.85 Tc, very few, larger Ll0 regions were seen. As precipitates of the ordered phase grow to impingement, antiphase boundaries (APB) are formed. This is consistent with a heterogeneous nucleation and growth mechanism for the formation of the Ll0 phase throughout the temperature range studies.

Notiz: Sulphate Rotator Phases in Polarised Light

Abstract Li2SO4 has a well defined high-conducting fee phase in the temperature range 577-860 °C, and LiNaSO4 a bcc phase in the range of 518-614 °C. Both salts were studied in polarised light during sequences going from melt, through the cubic phase into the "room temperature" phase, and back to the cubic phase. The two cubic sulphate phases are compared with organic plastic rotator phases, especially regarding the formation of vapour snakes.

Evolution of a metastable FCC solid solution during sputter deposition of TiAlB alloys

The microstructure evolution of a nominally αTi7Al7B alloy synthesized by sputter deposition has been examined. A highly supersaturated metastable single phase solid solution with a face centered cubic structure was identified as being the only phase present in the as-sputtered material. In contrast, Ti7Al and Ti7B alloys sputtered under the same conditions were also single phase but exhibited the expected hexagonal structure of α-Ti. The selection of the fee phase is based solely on kinetics, as the thermodynamically favored phase is always α (hcp). The reasons for the kinetic bias, however, are not clear. Annealing of the Ti7Al7B sputtered film at 900 °C for 1 h yielded precipitation of nano-scale TiB dispersoids in a single phase α-Ti matrix. Preliminary hardness and microstructural stability results of the dispersion unveiled promising potential for use of the physical vapor deposition technique to deposit TiB strengthened Ti-matrices on fibers in high-temperature metal matrix composites.

Structure of mechanically alloyed Ti-Al-Nb powders

Ti-Al-Nb ternary powder mixtures containing 24Al-11Nb, 25Al-25Nb, 37.5Al-12.5Nb, and 28.5Al-23.9Nb (at. pct) were mechanically alloyed in a SPEX 8000 mixer mill using a ball-to-powder weight ratio of 10:1. The structural evolution in these alloys was investigated by X-ray diffraction and transmission electron microscopy techniques. A solid solution of Al and Nb in Ti was formed at an early stage of milling, followed by the B2/body-centered cubic (bec) and amorphous phases at longer milling times. The stability of these phases and their transformation to other phases have been investigated by heat treating these powders at different temperatures. The B2/bcc phase transformed into an orthorhombic (O-Ti2AlNb) or a mixture of the orthorhombic (O) and hexagonal close-packed (α2-Ti3Al) phases, the proportion of phases being dependent on the powder composition. Milling beyond the amorphous phase formation resulted in the formation of an fee phase in all the powders, which appears to be TiN, formed as a result of contamination of the powder.

Magnetic and structural characterization of superconducting sodium-doped C60 prepared with the thermal decomposition of sodium azide

Abstract We have prepared Na4C60 samples by the different ways of the thermal decomposition of sodium azide NaN3. The superconducting (SC) samples were obtained by the decomposition under the dynamic vacuum, while the non-SC samples by the decomposition in the sealed tube. The SC sample was composed of two fee phases with a lattice constant a=14.207(5)A (phase A) and a=14.339(4)A (phase B). On the contrary, the non-SC sample had single fee phase which can be assigned to phase A. This result strongly suggests that phase B is the SC phase.

Pressure-induced cross-linking of C60

We show that fee C60 is converted to at least two different structures by high temperature and pressure. These new phases are stabilized by temperature-quenching. The synthesis condition of P=5GPa and 300°C<T<400°C produces an fee structure with a lattice constant of a0=13.6A which is 3.7% smaller than that of pristine fee phase. At P=5GPa and 500°C<T<800°C, a rhombohedral phase of a=9.77A and α=56.3° are stabilized. In these structures, the intermolecular distances are significantly smaller than the pristine phase. A cross-linking of C60 molecules is proposed based on the symmetry reduction shown by several spectroscopic data. These new phases are metastable and reverts to the conventional fee C60 by heating up to 300°C at ambient pressure.

Pressure Dependence of the Electrical Resistivity of C60Single Crystal

ABSTRACT The temperature dependences of the electrical resistivity of C60 single crystals under pressure up to 1 GPa are presented. The activation energy in the fee phase decreases with pressure, while it is insensitive to pressure in the sc phase. This indicates that a midgap state is influenced by intermolecular interaction.

The Initial Nucleation and Growth of Electrodeposits of Cobalt

SUMMARYThe kinetics of the early stages of the electrocrystallization of cobalt on a vitreous carbon electrode have been analyzed. The current time transients were found to have the same characteristics as those obtained for the deposition of nickel. The first electrocrystallization process is found to be the formation of a single monolayer of deposit on top of the vitreous carbon electrode. The next layer is formed via nucleation and three-dimensional growth. The analysis of the early stages of the current time transients revealed the possibility of codepositing two distinct crystalline forms of cobalt, e.g. the two allotropic fee and hcp structural phases. The formation of two forms of cobalt deposit is verified by the existence of two distinct anodic dissolution peaks when cobalt is dissolved by the application of an anodic potential sweep to the electrode.

Cd-In (cadmium-indium)

The assessed coefficients agree extremely well with existing experimental data. The liquid, fee, bet, and cph phases of the Cd-In system are satisfactorily represented by Redlich-Kister polynomial expressions to describe the Gex. G^In is best represented as a Wagner-Schottky-type, two sublattice phase where the defects are antistructure atoms, i.e., (Cd, In)o.75(In, Cd)o.25-With these descriptions, the resulting optimized coefficients for the Gex expressions reproduce the known experimental phase diagram within experimental error. There is also excellent agreement between the measured and calculated thermodynamic quantities.