Paul J. Schilling

Extended x‐ray absorption fine structure of metastable bcc and fcc phases in mechanically alloyed Fe–Cu

The extended x‐ray absorption fine structure (EXAFS) technique has been employed to monitor the formation of thermodynamically unstable solid solutions induced by mechanical alloying in the positive heat of mixing Fe–Cu system. Local structural changes are clearly observed, with Cu taking on bcc coordination in the bcc (Fe80Cu20) solid solution, and Fe taking on fcc coordination in the fcc (Fe50Cu50) solid solution. Atomic level alloying is demonstrated through curve fitting to ab initio simulations. A difference is observed in the behavior of bcc and fcc ball milled samples, with a large reduction in first shell coordination number observed in bcc nanocrystalline structures (pure Fe and Fe80Cu20), but not in the fcc nanocrystalline structures (Fe50Cu50). This is believed to be due to the inability of the fcc structure to retain a high defect density.

Solid-state alloying in nanostructured binary systems with positive heat of mixing

Abstract While many binary systems exhibit a positive heat mixing that precludes intermixing in conventional bulk diffusion couples, it is possible to alloy in solid state some of these bulk immiscible elements in nanostructures. Molecular dynamics simulations demonstrate that in low-dimensional systems such as surfaces and in sub-nanometer layered superlattice structures, excess enthalpic and entropic energy contributions can provide a driving force for spontaneous intermixing to form substitutional solid solution alloys. Such driving forces diminish, however, in coarser nanophase binary mixtures when domain sizes reach beyond ≈1 nm. In this case, true alloying on the atomic level can be achieved by employing an external forcing mechanism such as severe mechanical deformation. In addition to single-phase alloys, we demonstrate, using X-ray absorption near-edge structure (XANES) analysis, a novel two-phase coexistence controlled by kinetically imposed polymorphic constraints. Using a phenomenological model, possible mechanisms responsible for driven alloying are discussed with reference to several previous proposals in the literature.

Two-phase coexistence region in mechanically alloyed Cu–Fe: an X-ray absorption near-edge structure study

Abstract A new technique using the X-ray absorption near-edge structure (XANES) has been developed to simultaneously determine the compositions and phase fractions of the two coexisting (f.c.c. and b.c.c.) supersaturated solid solutions in mechanically alloyed Cu 100− x Fe x ( x =50–80) formed by ball milling under cryogenic conditions. The XANES analysis indicates that throughout this region the two coexistent phases have almost identical composition, consistent with that of the overall mixture. This represents a novel two-phase coexistence representing a region of overlap in solubility rather than the normal miscibility gap. A mechanism is proposed to explain the nature of this two-phase region in terms of extended solubility induced by ball milling, the free energies of the two phases including defect enthalpies, and nucleation and growth barriers for the b.c.c.–f.c.c. (and f.c.c.–b.c.c.) transformation.

X-ray absorption fine structure study of the atomic and electronic structure of molybdenum disulfide intercalation compounds with transition metals

Abstract The local structures of ‘host’ and ‘guest’ layers of MoS2 intercalated with M(OH)2 (MMn, Co and Ni) prepared via interaction of single-layer MoS2 dispersions and solutions of M2+ salts were studied by X-ray absorption spectroscopy. According to M K-edge extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) results, the electronic structure and atomic environment of the M atoms in the intercalates are similar to that of the crystalline hydroxides M(OH)2. In the Ni intercalate, Mo K-edge EXAFS revealed a structural change of the ‘host’ MoS2 layers similar to that reported for water dispersions of MoS2 single layers. S K-edge XANES data indicate that the change is associated with increased electron density on the S atoms in the matrix. SO42− and Mo″− (4

Microstructure, strength, and reaction products of ground granulated blast-furnace slag activated by highly concentrated NaOH solution

Ground granulated blast-furnace slag was reacted in 5 M (pH 14.7) and 1.5 M (pH 14.2) NaOH solutions at a water/slag ratio of ∼0.4, and characterized by unconfined compressive strength testing, scanning electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. The reacted material consisted of a dense layered matrix interspersed with unreacted glass particles and regions of reaction products with higher porosity. CSH(I) and (C, M) 4 AH 13 were identified by x-ray diffraction. The C-S-H (calcium silicate hydrate) phase is proposed to consist mainly of structurally imperfect layers of tobermorite, interleaved with layers of (C, M) 4 AH 13 . Other cations, most significantly Na + , are incorporated into the structure. Use of the highly concentrated solution (5 M) produced a higher degree of reaction and, consequently, higher compressive strength (38 MPa after 28 days for 5 M solution vs 21 MPa for 1.5 M).

In situ sulfur K-edge X-ray absorption near edge structure of an embedded pyrite particle electrode in a non-aqueous Li+-based electrolyte solution

Abstract Changes in the composition of embedded pyrite (FeS 2 ) particle electrodes in 1 M LiClO 4 propylene carbonate solutions as a function of the applied potential have been examined in situ by S K-edge fluorescence X-ray absorption near edge structure (XANES), using a specially designed cell that minimizes attenuation of low energy X-rays. Pyrite electrodes that had been scanned from 3.0 V versus Li/Li + , i.e. close to the open circuit voltage, down to 1.0 V (fully discharged state, i.e. 4e − -reduction) and then half recharged (2e − -reoxidation) by scanning the potential in the positive direction up to 2.2 V versus Li/Li + , revealed features consistent with the presence of Li 2 FeS 2 , in agreement with in situ Fe K-edge results reported earlier by this research group. Moreover, only subtle changes were discerned between the in situ S K-edge XANES of the half-, and fully-recharged electrodes. This close resemblance may reflect similarities between the spectral signatures of Li 2 FeS 2 and Fe 1− x S (pyrrhotite), which is the main product of the discharge reaction. Evidence for the formation of elemental sulfur and Li 2 S, which are believed to be minor products of the reaction, was obtained from analysis of the first differential S XANES and selected difference spectra. The compositional variations of the embedded pyrite particles throughout the course of the electrochemical processes occur in the presence of a persistent sulfate coating.

Installation and operation of the LNLS double-crystal monochromator at CAMD

A new x‐ray beamline has been installed at CAMD utilizing a two‐crystal monochromator designed and built at LNLS. The beamline will operate in the 2–18 keV range using up to 4 mrad of dipole radiation from the CAMD storage ring. The monochromator maintains a fixed exit beam and fixed positions of the beam on the two crystals using mutually perpendicular elastic translations. With the ring operating at 1.5 GeV and 160 mA, Si(220) crystals will provide a flux of ∼3(109) photons/s/mrad at 8 keV, with an energy resolution ΔE<2 eV, to the experimental hutch. The beamline is equipped with an EXAFS endstation and will also be used for other x‐ray applications at CAMD. First results are presented.

The crystal structure of a macrocycle containing pyridine and piperazine subunits, and of a Cu(I) complex of its diprotonated cation

The 2:2 macrocycle C22H30N6, containing pyridine and piperazine subunits, crystallizes in monoclinic space group C2c with a = 17.614(3), b = 9.696(2), c = 12.662(2) A, β = 111.65(2)°, V = 2010(1) A3, Z = 4. The structure was refined to R = 0.036 for 1863 observed reflections. The molecule lies on an inversion center, and the piperazine subunits have the chair conformation. Surprisingly, this macrocycle becomes diprotonated, and the Cu(II) is reduced to Cu(I) upon reaction with CuCl2 in methanol. The resulting Cu(I) complex of the diprotonated macrocyclic dication, [CUCl(C22H32N6)]Cl2·2H2O, crystallizes in monoclinic space group P21n with a = 10.745(1), b = 13.728(2), c = 17.774(2) A , β = 98.08(1)°, V = 2596(1) A3, Z = 4. The structure was refined to R = 0.058 for 3989 observed reflections. The Cu atom coordinates approximately linearly to one of the pyridine N atoms and a chloro ligand with CuN distance 1.947(3) A, CuCl distance 2.133(1) A, and NCuCl angle 164.5(1)°. While the piperazine subunits maintain their chair conformations, one N of each is protonated and turned outward. The unprotonated piperazine N atoms form long interactions of 2.488(4) and 2.508(4) A, distance to the Cu atom.

X-ray absorption spectroscopy of iron-doped conducting polymers

Abstract X-ray absorption spectroscopy (XAS) measurements were performed at the Fe K edge to determine the iron local structure in chemically prepared polyaniline (PANI) and polypyrrole (PPy) samples prepared with FeCl3 as an oxidant. The samples were conditioned at different pHs by an acid–base treatment. In both as synthesized-doped polymers, the observed dispersed Fe atoms were predominantly coordinated to chlorine as Fe(III)Clx species, where x=6 for PANI and x=4 for PPy samples suggesting that the polymer–counteranion bonding is weaker than the iron–chelate one. For the PANI samples submitted to basic treatment, three different sets of Fe distances were found: five Fe–O at 2.00 A, three Fe–Fe at approximately 2.80 A and five Fe–Fe at 3.00 A and the near-edge spectra showed the presence of octahedrally coordinated Fe+3. These results strongly suggest the presence of small oxide/hydroxide aggregates. Similar data were obtained for PPy treated with NH4OH.

Investigation of Microcrack Growth in [0/90]s Composite Laminates

Techniques such as optical microscopy and X-radiography have provided useful information regarding damage in composite laminates, particular in therms of microcracking behavior in individual plies. This focuses on the investigation of microcracking and damage evolution in loaded composite laminates via X-ray computed microtomography. The main advantage in the use of such a technique is that damage within the composite can be assessed in three-dimensions without destruction of the composite. In this work, IM7/977–2, IM7/5555, and IM7/5276-1 coupons were uniaxially tested in a tensile substage, Graphs that convey microcracking density information as a function of applied load were created for [0/90/90/0] laminates. The three dimensional geometry and connectivity of microcracks and other damage in these samples were investigated through microtomographic reconstruction.Copyright