Dennis E. Brown

A Transforming Metal Nanocomposite with Large Elastic Strain, Low Modulus, and High Strength

S-T-R-E-T-C-H Me Most metals show elastic strain limits well below 1%, beyond which permanent plastic deformation occurs. Metal nanowires can be elastically stretched to much higher strains, on the order of 4 to 7%. However, when placed inside a metal matrix to form a composite, these nanowires can no longer be stretched to the same extent, even when the nanowires are well distributed and show good bonding with the matrix. Hao et al. (p. 1191; see the Perspective by Zhou) used a shape memory alloy as the matrix material to produce a much better (more elastic) composite. The use of a shape-memory metal alloy as a matrix better exploits the inherent elastic properties of niobium nanowires. [Also see Perspective by Zhou] Freestanding nanowires have ultrahigh elastic strain limits (4 to 7%) and yield strengths, but exploiting their intrinsic mechanical properties in bulk composites has proven to be difficult. We exploited the intrinsic mechanical properties of nanowires in a phase-transforming matrix based on the concept of elastic and transformation strain matching. By engineering the microstructure and residual stress to couple the true elasticity of Nb nanowires with the pseudoelasticity of a NiTi shape-memory alloy, we developed an in situ composite that possesses a large quasi-linear elastic strain of over 6%, a low Youngs modulus of ~28 gigapascals, and a high yield strength of ~1.65 gigapascals. Our elastic strain-matching approach allows the exceptional mechanical properties of nanowires to be exploited in bulk materials.

Measuring velocity of sound with nuclear resonant inelastic x-ray scattering

Nuclear resonant inelastic x-ray scattering is used to measure the projected partial phonon density of states of materials. A relationship is derived between the low-energy part of this frequency distribution function and the sound velocity of materials. Our derivation is valid for harmonic solids with Debye-like low-frequency dynamics. This method of sound velocity determination is applied to elemental, composite, and impurity samples which are representative of a wide variety of both crystalline and noncrystalline materials. Advantages and limitations of this method are elucidated.

Direct evidence on magnetic-field-induced phase transition in a NiCoMnIn ferromagnetic shape memory alloy under a stress field

The magnetoelasticity and magnetoplasticity behaviors of a Ni–Co–Mn–In ferromagnetic shape memory alloy (FSMA) induced by the reverse phase transformation interplayed under multiple (temperature, magnetic, and stress) fields were captured directly by high-energy synchrotron x-ray diffraction technique. The experiments showed the direct experimental evidence of that a stress (∼50MPa) applied to this material made a complete recovery of the original orientations of the martensite variants, showing a full shape memory effect. This finding offers the in-depth understanding the fundamental properties and applications of the Ni–Co–Mn–In FSMA with the magnetic-field-induced reverse transformation.

The ultrahigh mechanical energy-absorption capability evidenced in a high-strength NbTi/NiTi nanocomposite

A nanocomposite composed of NbTi nanowires uniformly embedded in NiTi matrices was fabricated, which exhibits an ultrahigh mechanical-damping capability. The absorption energy measured under an applied 8% strain is up to 54 MJ/m3, which is over three times higher than that (∼16 MJ/m3) found in the well-known Ni-Ti alloys. In-situ synchrotron x-ray diffraction reveals that a redistribution of stress between the nanowires and matrices was evidenced from an abrupt change in residual lattice strains. The ultrahigh mechanical-damping property is attributed to a combination of the strong interaction of nanowires and matrices and the plastic deformation occurring in NbTi nanowires during deformation causing large energy dissipation.

A smart strategy to fabricate Ru nanoparticle inserted porous carbon nanofibers as highly efficient levulinic acid hydrogenation catalysts

Herein, we first put forward a smart strategy to in situ fabricate Ru nanoparticle (NP) inserted porous carbon nanofibers by one-pot conversion of Ru-functionalized metal organic framework fibers. Such fiber precursors are skillfully constructed by cooperative assembly of different proportional RuCl3 and Zn(Ac)2·2H2O along with trimesic acid (H3BTC) in the presence of N,N-dimethylformamide. The following high-temperature pyrolysis affords uniform and evenly dispersed Ru NPs (ca. 12–16 nm), which are firmly inserted into the hierarchically porous carbon nanofibers formed simultaneously. The resulting Ru-carbon nanofiber (Ru-CNF) catalysts prove to be active towards the liquid-phase hydrogenation of levulinic acid (LA) to γ-valerolactone (GVL), a biomass-derived platform molecule with wide applications in the preparation of renewable chemicals and liquid transportation fuels. The optimal GVL yield of 96.0% is obtained, corresponding to a high activity of 9.56 molLA h−1 gRu−1, 18 times that of using the commercial Ru/C catalyst. Moreover, the Ru-CNF catalyst is extremely stable, and can be cycled up to 7 times without significant loss of reactivity. Our strategy demonstrated here reveals new possibilities to make proficient metal catalysts, and provides a general way to fabricate metal–carbon nanofiber composites available for other applications.

Phase-stress partition and stress-induced martensitic transformation in NbTi/NiTi nanocomposite

The phase-stress partition and stress-induced martensitic transformation in a NbTi/NiTi nanocomposite were investigated by employing in situ synchrotron x-ray diffraction during tensile cycling. The phase-stress partition behavior in the nanocomposite is significantly different from that previously reported in the metal-matrix composites. Beyond the initial elastic deformation, the stress carried by the NbTi nanowires increased significantly with increasing macroscopic strain, while the stress taken by the NiTi matrix decreased gradually. We also found that the stress-induced martensitic transformation of the NiTi matrix still proceeded even though the matrix carried decreasing stress rather than constant or increasing stress well known in binary NiTi alloys.

Neutron scattering studies of the ferroelectric distortion and spin dynamics in the type-1 multiferroic perovskite Sr 0.56 Ba 0.44 MnO 3

The magnetic order, spin dynamics, and crystal structure of the multiferroic Sr0.56Ba0.44MnO3 have been investigated using neutron and x-ray scattering. Ferroelectricity develops at TC=305 K with a polarization of 4.2 microC/cm2 associated with the displacements of the Mn ions, while the Mn4+ spins order below TN = 200 K into a simple G-type commensurate magnetic structure. Below TN the ferroelectric order decreases dramatically demonstrating that the two order parameters are strongly coupled. The ground state spin dynamics are characterized by a spin gap of 4.6(5) meV and the magnon density of states peaking at 43 meV. Detailed spin wave simulations with a gap and isotropic exchange of J=4.8(2) meV describe the excitation spectrum well. Above TN strong spin correlations coexist with robust ferroelectric order.

Diverse effects of two-dimensional and step flow growth mode induced microstructures on the magnetic anisotropies of SrRuO3 thin films

Geometrical anisotropy axes of diverse SrRuO3 (SRO) films grown by random and directional two-dimensional and step flow modes are determined and their characteristic angular magnetizations are understood in terms of growth mode induced structural effects. Two-dimensional SRO films possess single-crystal-like structural qualities. Angular magnetization measurements show sharp minima and indicate the films’ easy axis to be in the [310] direction. In contrast, examination of step flow SRO films shows the presence of degenerate multiple in-plane domains and the anisotropy axis in a direction close to [110] even though directional surface steps are clearly visible.

Versatile nickel–tungsten bimetallics/carbon nanofiber catalysts for direct conversion of cellulose to ethylene glycol

We herein propose a novel synthetic methodology for a series of nickel–tungsten bimetallics/carbon nanofiber catalysts (Ni, 0.37–2.08 wt%; W, 0.01–0.06 wt%) in situ fabricated by pyrolysis (950 °C) of Ni, W and Zn-containing metal organic framework (Ni0.6−x–Wx–ZnBTC, x = 0–0.6) fibers. The resulting catalysts (Ni0.6−x–Wx/CNF) have uniform particles (ca. 68 nm), evenly dispersed onto the hierarchically porous carbon nanofibers formed simultaneously. All of the Ni0.6−x–Wx/CNF catalysts prove to be highly active towards direct conversion of cellulose to ethylene glycol (EG). A large productivity ranging from 15.3 to 70.8 molEG h−1 gW−1 is shown, two orders of magnitude higher than those by using other W-based catalysts reported.

A Novel Stretchable Coaxial NiTi‐Sheath/Cu‐Core Composite with High Strength and High Conductivity

Stretchable conductors have attracted broad attention recently because they play a key role in the development of stretchable electronics such as fl exible displays, stretchable circuits, functional electronic eyes, dielectric elastomeric actuators, and so on. [ 1–5 ] The major challenge towards stretchable conductors is the development of stretchable electrical wiring that is both conductive and stretchable. [ 6 ] To our knowledge, two strategies have been employed to achieve stretchable conductors: one is to fabricate wavy or net-shaped conductive structures by releasing a pre-strained rubber substrate with conductive materials lying on it, [ 7–11 ] and the other is to disperse the conductive material in a rubber matrix. [ 12–14 ] However, compared to common metal conductors, such as Cu, Ag, and Al, with good electrical conductivity ( ∼ 10 7 S m − 1 ), controllability, stability, and high strength, [ 15 ]