H. A. Calderon

Interlayer-Expanded Molybdenum Disulfide Nanocomposites for Electrochemical Magnesium Storage

Mg rechargeable batteries (MgRBs) represent a safe and high-energy battery technology but suffer from the lack of suitable cathode materials due to the slow solid-state diffusion of the highly polarizing divalent Mg ion. Previous methods improve performance at the cost of incompatibility with anode/electrolyte and drastic decrease in volumetric energy density. Herein we report interlayer expansion as a general and effective atomic-level lattice engineering approach to transform inactive intercalation hosts into efficient Mg storage materials without introducing adverse side effects. As a proof-of-concept we have combined theory, synthesis, electrochemical measurement, and kinetic analysis to improve Mg diffusion behavior in MoS2, which is a poor Mg transporting material in its pristine form. First-principles simulations suggest that expanded interlayer spacing allows for fast Mg diffusion because of weakened Mg-host interactions. Experimentally, the expansion was realized by inserting a controlled amount of poly(ethylene oxide) into the lattice of MoS2 to increase the interlayer distance from 0.62 nm to up to 1.45 nm. The expansion boosts Mg diffusivity by 2 orders of magnitude, effectively enabling the otherwise barely active MoS2 to approach its theoretical storage capacity as well as to achieve one of the highest rate capabilities among Mg-intercalation materials. The interlayer expansion approach can be leveraged to a wide range of host materials for the storage of various ions, leading to novel intercalation chemistry and opening up new opportunities for the development of advanced materials for next-generation energy storage.

Mechanical properties of nanocrystalline Ti-Al-X alloys

Abstract Nanocrystalline alloys have been produced by means of mechanical milling and spark plasma sintering. Two types of materials have been obtained, TiAl–X and Al 3 Ti–X alloys, X represents Cr, Mn or Fe. Sintered TiAl–X alloys have a two-phase microstructure consisting of the γ-TiAl phase and the α 2 phase, this last one with a globular morphology. Their average grain size varies between 100 and 150 nm. The Al 3 Ti–X alloys are constituted by a single phase with an L1 2 structure and an average grain size of about 30 nm. Compression tests are used to evaluate the mechanical properties of these materials at temperatures ranging from 298 to 773 K. Very high flow stresses are found for the TiAl–X alloys, with maximum values of approximately 3 GPa. Surface traces develop during deformation at room temperature of these materials. Microscopic observation reveals dislocation activity in the larger grains. The nanocrystalline Al 3 Ti–X alloys show no ductility at room temperature and a rather high fracture strength of about 2.5 GPa. Deformation of heat-treated Al 3 Ti–X alloys (larger grain sizes) produces plastic deformation with dislocation activity and lower flow stresses (∼1 GPa).

Production and Characterization of (Al, Fe)-C (Graphite or Fullerene) Composites Prepared by Mechanical Alloying

Abstract (Al, Fe)-Cgraphite and (Al, Fe)-Cfullerene composites have been prepared by mechanical alloying using ball milling of powders. Consolidation has been achieved by a spark plasma sintering technique (SPS). Results of XRD and TEM indicate that pure fullerene withstands milling. SEM results show homogeneous powders after milling but with different morphologies depending on the specific system. Milling produces a fine mixture of Al or Fe and graphite or fullerene. SPS produces a dense material with a nanocrystalline structure. The sintered samples have a metallic matrix (Al or Fe) with a fine dispersion of AI4C3 in the case of Al-C( graphiteorfullerene), Fe3,C in the case of Fe-C(graphite), and fullerene in the case of Fe-C (fullerene), Hardness measurements show that higher values are obtained in the Al-C(fullerene), and Fe-C(graphite ) specimens.

Atomic resolution phase contrast imaging and in-line holography using variable voltage and dose rate.

The TEAM 0.5 electron microscope is employed to demonstrate atomic resolution phase contrast imaging and focal series reconstruction with acceleration voltages between 20 and 300 kV and a variable dose rate. A monochromator with an energy spread of ≤0.1 eV is used for dose variation by a factor of 1,000 and to provide a beam-limiting aperture. The sub-Ångstrøm performance of the instrument remains uncompromised. Using samples obtained from silicon wafers by chemical etching, the [200] atom dumbbell distance of 1.36 Å can be resolved in single images and reconstructed exit wave functions at 300, 80, and 50 kV. At 20 kV, atomic resolution <2 Å is readily available but limited by residual lens aberrations at large scattering angles. Exit wave functions reconstructed from images recorded under low dose rate conditions show sharper atom peaks as compared to high dose rate. The observed dose rate dependence of the signal is explained by a reduction of beam-induced atom displacements. If a combined sample and instrument instability is considered, the experimental image contrast can be matched quantitatively to simulations. The described development allows for atomic resolution transmission electron microscopy of interfaces between soft and hard materials over a wide range of voltages and electron doses.

Prospects for Bright Field and Dark Field Electron Tomography on a Discrete Grid

Extended abstract of a paper presented at the Pre-Meeting Congress: Materials Research in an Aberration-Free Environment, at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, July 31 and August 1, 2004.

Experimental study of the thermal stability of austempered ductile irons

A nonisothermal annealing was applied to austempered Ni-Cu-Mo alloyed and unalloyed ductile irons to determine the thermal stability of the ausferritic structure. Differential thermal analysis (DTA) results were used to build the corresponding stability diagrams. The transformation starting temperature of the high carbon austenite was found to be strongly dependent on the austempering temperature, the heating rate, and the chemical composition of the iron. The Ni-Cu-Mo alloying elements and high austempering temperature increased the stability. The transformation of the austenite to ferrite and cementite is achieved via the precipitation of transition carbides identified as silico-carbides of triclinic structure.

Direct evidence that an apparent splitting pattern of gamma particles in Ni alloys is a stage of coalescence

High-resolution electron microscopy is used to determine the translation order domains of gamma particles in arrangements formed during Ostwald ripening in the alloy Ni-12at.%Al. Such arrangements have been often identified with the operation of a mechanism of particle splitting in the late stages of the coarsening process, that is when the elastic strain contribution becomes predominant. It is shown that the translation order domains are not identical in such a particle array. This leads to the conclusion that particle splitting is not necessarily the mechanism responsible for the formation of such characteristic particle arrangements. Instead, particle migration can also be considered to be the responsible mechanism for the formation of such a correlated gamma-particle spatial distribution observed in Ni-based alloys.

Fullerene-Metal Composites: Phase Transformations During Milling and Sintering

Composites of Fe-C60 and Al C60 produced by mechanical milling and sinterized by Spark Plasma Sintering are investigated with special attention to the mechanical properties of the products. The processing involves phase transformations of the fullerenes that are interesting to follow and characterize. This involves formation of tetragonal/rhombohedral diamond and carbides during sintering and milling. Transmission Electron Microscopy (TEM) and Raman Spectroscopy techniques are also used to confirm preliminary results of X Ray Diffraction (XRD) related to the formation of nanostructures i.e., grain size of the crystals during mechanical milling and after sintering, spatial distribution of phases and the different phases that are developed during processing.

Predicting the onset of rafting of γ′ precipitates by channel deformation in a Ni superalloy

The growth or shrinkage, normal to {001}, of the interfaces between the γ matrix and cuboidal γ′ precipitates is examined for a Ni-base superalloy, by considering the force acting on the interfaces. The force is produced by the precipitate coherency misfit and the stress produced by plastic deformation in channels of the γ matrix. A simple expression, which directly addresses the origin of the surface force, is given. The plastic deformation within the initially active γ matrix channels exerts the force to cause rafting. The subsequent activation of other types of channels also promotes the rafting in the same direction as the first active channels, when the plastic strain of the former channels increases. These issues are also discussed in terms of analysis based on those dislocations caused by the precipitate misfit and those produced by the plastic deformation.

High-resolution electron-microscopy analysis of splitting patterns in Ni alloys

The late stages of coarsening of coherent solid particles is strongly influenced by the reduction of elastic strain energy. This produces migration and alignment of particles as well as some other effects. In this investigation, the origin of the so-called splitting pattern arrangement of γ′ precipitate particles, an arrangement which has often been interpreted as being due to splitting of a larger particle into smaller ones, has been studied. The two-particle relationship as to whether they are in-phase or out-of-phase is examined by means of a translation order domain analysis of high-resolution electron-microscopy images along a zone axis parallel to [001]. Ni alloys have been used for the investigation including a binary Ni–Al alloy (producing different volume fractions) and two commercial multicomponent alloys with high volume fraction. About 72% of two-particle pairs forming the splitting configuration are in the out-of-phase relationship, indicating that adjacent pairs are randomly formed and that they are not formed by the splitting of a large particle. In addition, an elasticity analysis shows that the elastic interaction energy of two γ′ particles exhibits a minimum at a certain separation distance along ⟨ 100⟩.