Phase control of La2CuO4 in thin-film synthesis

The lanthanum copper oxide, La2CuO4, which is an end member of the prototype high-Tc superconductors (La,Sr)2CuO4 and (La,Ba)2CuO4, crystallizes in the "K2NiF4" structure in high-temperature bulk synthesis. The crystal chemistry, however, predicts that La2CuO4 is at the borderline of the K2NiF4 stability and that it can crystallize in the Nd2CuO4 structure at low synthesis temperatures. In this article we demonstrate that low-temperature thin-film synthesis actually crystallizes La2CuO4 in the Nd2CuO4 structure. We also show that the phase control of "K2NiF4"-type La2CuO4 versus "Nd2CuO4"-type La2CuO4 can be achieved by varying the synthesis temperature and using different substrates.Comment: 4 pages, 5 figures, submitted to PRB, revte

Selective Laser Sintering of Alumina-Boron Oxide Composites

The selection of an optimum composite system for selective laser sintering (SLS) is based on materials properties such as the melting point and the wettability between the components in the composite powder. The alumina-boron oxide composite system is attractive for SLS because the presence of the low melting component B203 (melting point 4500 C) can enhance sintering. A better wetting of solid alumina powder by molten boron oxide can also aid densification process. The alumina-boron oxide conlposite system has been investigated by SLS and selective laser reactive sintering (SLRS). The role of boron oxide content as a binder, laser power density, and secondary heat treatment on the microstructure and mechanical properties is discussed.Mechanical Engineerin

Phase Relations in the Li2O-V2O3-V2O5 System at 700 C: Correlations with Magnetic Defect Concentration in Heavy Fermion LiV2O4

The phase relations in the Li2O-V2O3-V2O5 ternary system at 700 C for compositions in equilibrium with LiV2O4 are reported. This study clarified the synthesis conditions under which low and high magnetic defect concentrations can be obtained within the spinel structure of LiV2O4. We confirmed that the LiV2O4 phase can be obtained containing low (0.006 mol%) to high (0.83 mol%) magnetic defect concentrations n{defect} and with consistently high magnetic defect spin S values between 3 and 6.5. The high n{defect} values were obtained in the LiV2O4 phase in equilibrium with V2O3, Li3VO4, or LiVO2 and the low values in the LiV2O4 phase in equilibrium with V3O5. A model is suggested to explain this correlation.Comment: 6 pages, 7 figures; Phys. Rev. B (accepted

Development of Nanocomposites for Solid Freeform Fabrication

Nanocomposites in which the constituents are mixed on a nanorneter scale can provide important advantages in the Selective Laser Sintering (SLS) and Selective Laser Reactive Sintering (SLRS) processes. The larger surface area and grain boundaries in the nanocolnposites compared to that in the conventional microcomposites are expected to enhance the solid state diffusion during laser irradiation as well as during any other subsequent processes. Our strategy is to design and develop nanocomposites in which one nanosize cOlnponent has a lower melting point than the other nanosize component, either of which can serve as the matrix phase. The nanoscale dispersion of the low melting component can aid the sintering process during SLS or SLRS. Nanocomposite powders of AI203-COOx, Ab03-NiO, A1203-CO and A1203-Ni have been synthesized by sol-gel processing and are evaluated by SLS.Mechanical Engineerin

Single-crystal growth and magnetic properties of the metallic molybdate pyrochlore Sm2Mo2O7

We have successfully grown cm3-size single crystals of the metallic-ferromagnet Sm2Mo2O7 by the floating-zone method using an infrared-red image furnace. The growth difficulties and the remedies found using a 2-mirror image furnace are discussed. Magnetization studies along the three crystalline axes of the compound are presented and discussed based on our recent proposal of an ordered spin-ice ground state for this compoun

Revised superconducting phase diagram of hole doped Nax_{x}(H3_{3}O)z_{z}CoO2⋅y_{2}\cdot yH2_{2}O

We have studied the superconducting phase diagram of \NaH\space as a function of electronic doping, characterizing our samples both in terms of Na content $x$ and the Co valence state. Our findings are consistent with a recent report that intercalation of \oxp\space ions into Na$_{x}$CoO$_{2}$, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal \Tc\space is achieved through a Co valence range of 3.24 - 3.35, while \Tc\space decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO$_{2}$ sheets as Co valence is varied into the optimal \Tc\space region is further evidence for this criticality.Comment: Paper updated on 29/10/2004, 4 pages, 4 figures. Physical Review Letters (in press

Chemical synthesis of magnetic Fe-B and Fe-Co-B particles and chains

With an objective to develop magnetic materials with high saturation magnetization for the Magnetically Assisted Chemical Separation (MACS) process the chemical synthesis of Fe-B and Fe-Co-B alloys by reducing iron and cobalt chloride solutions with potassium borohydride has been investigated systematically. The influence of the concentration of the reactants, applied magnetic field, reaction atmosphere, and method of mixing the reactants on the microstructure, particle size, composition and magnetic properties has been studied. Both M-B (M = Fe and Co) particles and elongated chains composed of nanometer size M-B particles have been obtained depending on the reaction conditions. The Fe-B samples exhibit saturation magnetization of M{sub S} of 120--190 emu/g, remanent magnetization M{sub r} of 10--22 emu/g, and coercive field H{sub c} of 400--900 Oe. A high M{sub S} value of 190 emu/g, which is close to the theoretical value of 218 emu/g for pure Fe, has been achieved particularly for samples with well-defined chain structures. Increasing the Co content in the Fe-Co-B alloys increases the boron content and thereby decreases the crystallinity and M{sub S} values although marginal increase in H{sub c} (1,250 Oe) and M{sub r} (36 emu/g) values could be made in some Fe-Co-B compositions. The chain structure with high M{sub S} may be attractive for other magnetic separation processes as well

Characterization of celotex and thermodynamic calculations of the formation of corrosion precursors on beryllium

The importance of safe and long-term storage of nuclear weapons components has drawn attention towards the characterization and understanding of the corrosion precursors formed on beryllium. A complete compositional and thermal characterization of celotex was carried out using energy dispersive spectroscopy, inductively coupled plasma-mass spectroscopic analysis, ion chromatography, and thermogravimetric analysis. Both fluoride and sulfate anions in addition to chloride as well as several metal ions are found to be present. The chloride present in celotex is lost at moderate temperatures 200 < T < 400 C, which may have important consequences on the formation of corrosion precursors. Equilibrium thermodynamic calculations on the stability of beryllium oxide in chlorinated environments were performed for a model system composed by beryllium, carbon, chlorine, hydrogen and oxygen. The calculations were aimed at describing the equilibrium, long-term behavior of a beryllium surface and its interaction with chlorine in a closed environment. The authors found that (i) carbon is essential for the formation of BeCl{sub 2}, and (ii) the chlorine-to-hydrogen ratio in chlorinated solvents is an important quantity since for solvents with chlorine-to-hydrogen ratio higher than 1, BeCl{sub 2} is not formed independently of the amount of solvent

Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4 detwinned crystals as a function of doping, temperature and magnetic field. Two magnetic modes were observed within the AF region of the phase diagram. The one at lower energies was identified with the spin-wave gap induced by the antisymmetric DM interaction and its anisotropic properties in magnetic field could be well explained using a canonical form of the spin Hamiltonian. A new finding was a magnetic field induced mode whose dynamics allowed us to discover a spin ordered state outside the AF order which was shown to persist in a 9 T field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these single magnon excitations we map out the Raman selection rules in magnetic fields and demonstrate that their temperature dependent spectral weight is peaked at the N\'eel temperature. The second aspect is related to phononic and magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We observed that around 1/8 Sr doping and independent of Nd concentration there exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers. The weak magnitude of existing charge/spin modulations in the Nd doped structure did not allow us to detect specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.Comment: 26 pages, 22 figure

Ultrahigh Surface Area Three-Dimensional Porous Graphitic Carbon from Conjugated Polymeric Molecular Framework

Porous graphitic carbon is essential for many applications such as energy storage devices, catalysts, and sorbents. However, current graphitic carbons are limited by low conductivity, low surface area, and ineffective pore structure. Here we report a scalable synthesis of porous graphitic carbons using a conjugated polymeric molecular framework as precursor. The multivalent cross-linker and rigid conjugated framework help to maintain micro- and mesoporous structures, while promoting graphitization during carbonization and chemical activation. The above unique design results in a class of highly graphitic carbons at temperature as low as 800 ??C with record-high surface area (4073 m2 g-1), large pore volume (2.26 cm-3), and hierarchical pore architecture. Such carbons simultaneously exhibit electrical conductivity &gt;3 times more than activated carbons, very high electrochemical activity at high mass loading, and high stability, as demonstrated by supercapacitors and lithium-sulfur batteries with excellent performance. Moreover, the synthesis can be readily tuned to make a broad range of graphitic carbons with desired structures and compositions for many applications.clos