Manoj K. Kolel-Veetil

Organometallic Routes into the Nanorealms of Binary Fe-Si Phases

The Fe-Si binary system provides several iron silicides that have varied and exceptional material properties with applications in the electronic industry. The well known Fe-Si binary silicides are Fe3Si, Fe5Si3, FeSi, α-FeSi2 and β-FeSi2. While the iron-rich silicides Fe3Si and Fe5Si3 are known to be room temperature ferromagnets, the stoichiometric FeSi is the only known transition metal Kondo insulator. Furthermore, Fe5Si3 has also been demonstrated to exhibit giant magnetoresistance (GMR). The silicon-rich β-FeSi2 is a direct band gap material usable in light emitting diode (LED) applications. Typically, these silicides are synthesized by traditional solid-state reactions or by ion beam-induced mixing (IBM) of alternating metal and silicon layers. Alternatively, the utilization of organometallic compounds with reactive transition metal (Fe)-carbon bonds has opened various routes for the preparation of these silicides and the silicon-stabilized bcc- and fcc-Fe phases contained in the Fe-Si binary phase diagram. The unique interfacial interactions of carbon with the Fe and Si components have resulted in the preferential formation of nanoscale versions of these materials. This review will discuss such reactions.

The effects of concentration dilution of cross-linkable diacetylenes on the plasticity of poly(m-carborane-disiloxane-diacetylene)s

Previous studies on poly(m-carborane-disiloxane-diacetylene)s established that the representative polymer 1, having the carborane, disiloxane and diacetylene moieties in a 1∶2∶1 ratio in its repeating monomeric unit, exhibited plastic characteristics at ambient temperature. In this study, the diacetylene-diluted variants of the above system were synthesized and thermally cured to determine their elastomeric properties. The resultant diacetylene-diluted polymer variants 4a, 4b and 4c, where the corresponding ratios of the constituents were 2∶3∶1, 4∶5∶1 and 9∶10∶1, were determined upon curing to possess Tg values of 56 °C, 45 °C and 35 °C, respectively. Even though the diacetylene dilution resulted in the lowering of the Tg values of the new systems, they were still found to possess predominantly plastic characteristics at ambient temperature.

Substitution of silicon within the rhombohedral boron carbide (B4C) crystal lattice through high-energy ball-milling

Boron carbide (B4C) is a ceramic with a structure composed of B12 or B11C icosahedra bonded to each other and to three (C and/or B)-atom chains. Despite its excellent hardness, B4C fails catastrophically under shock loading, but substituting other elements into lattice sites may change and possibly improve its mechanical properties. Density functional theory calculations of elemental inclusions in the most abundant polytypes of boron carbide, B12-CCC, B12-CBC, and B11Cp-CBC, predict that the preferential substitution site for metallic elements (Be, Mg and Al) is the chain center atom and that for non-metallic elements (N, P and S) it is generally the chain end atom of the three-atom chain in B4Cs rhombohedral crystal lattice. However, Si, a semi-metal, seems to prefer the chain center in B12-CCC and icosahedral polar sites in both B12-CBC and B11Cp-CBC. As a first step to testing the feasibility of elemental substitutions experimentally, Si atoms were incorporated into B4C at low temperatures (∼200–400 °C) by high-energy ball-milling. High-resolution transmission electron microscopy showed that the Si atoms were uniformly dispersed in the product, and the magnitude of the lattice expansion and Rietveld analysis of the X-ray diffraction data were analyzed to determine the likely sites of Si substitution in B4C. Further corroborative evidence was obtained from electron spin resonance spectroscopy, magic-angle spinning nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy characterizations of the samples. Thus, a simple, top-down approach to manipulating the chemistry of B4C is presented with potential for generating materials with tailored properties for a broad range of applications.

Formation and Stability of Metastable Tungsten Carbide Nanoparticles

The low temperature transformation pathways of tungsten carbide formation in the nanoscale regime were investigated using a reactive carbon molecule and tungsten. WC core/W shell nanoparticles produced by the decomposition of metastable W2C were discovered using TEM. XRD studies revealed both the elemental and carbide phases of tungsten. It was observed that the metastable W2C phase can be stabilized at RT by carbon encapsulation. These findings open new avenues to access core-shell morphologies of refractory carbides and to stabilize W2C nanoparticles at RT.

Direct formulation of nanocrystalline silicon carbide/nitride solid ceramics

We developed a new in situ reaction method to synthesize SiC and Si3N4 ceramic solids from meltable precursor compositions into shaped ceramic composites with nanocrystalline grains. The process uses Si powder and 1,2,4,5-tetrakis(phenylethynyl)benzene, which readily react above 1400xa0°C to form the SiC and Si3N4 crystallites in the presence of argon and nitrogen, respectively. X-ray diffraction analysis, Raman spectroscopy and density measurements indicated the formation of near stoichiometric SiC and Si3N4 within the shaped solid. Further characterization of electrical conductivity and oxidative stability of the prepared ceramics analyzed the influence of nanoscale features on intrinsic properties of resulting composites. The hardness and elastic modulus values for the synthesized SiC determined by nanoindentation varied in the range of 25–46xa0GPa and 300–440xa0GPa.Graphical Abstract

Anomalous behavior of the structural relaxation dispersion function of a carborane-containing siloxane

Broadband dielectric spectroscopic investigations of a vinyl-terminated carboranylenesiloxane, VCS, were performed at ambient and elevated pressures. At a constant structural relaxation time, results show that the structural relaxation dispersion function of VCS narrows with both increasing pressure and temperature. This narrowing is substantial in the case of pressurization and, consequently, the breakdown of the temperature-pressure superposition rule is observed. The interpretation of this breakdown is presented.

High-performance nanomaterials formed by rigid yet extensible cyclic β-peptide polymers

Organisms have evolved biomaterials with an extraordinary convergence of high mechanical strength, toughness, and elasticity. In contrast, synthetic materials excel in stiffness or extensibility,xa0and a combination of the two is necessary to exceed the performance of natural biomaterials. We bridge this materials property gap through the side-chain-to-side-chain polymerization of cyclic β-peptide rings. Due to their strong dipole moments, the rings self-assemble into rigid nanorods, stabilized by hydrogen bonds. Displayed amines serve as functionalization sites, or, if protonated, force the polymer to adopt an unfolded conformation. This molecular design enhances the processability and extensibility of the biopolymer. Molecular dynamics simulations predict stick-slip deformations dissipate energy at large strains, thereby, yielding toughness values greater than natural silks. Moreover, the synthesis route can be adapted to alter the dimensions and displayed chemistries of nanomaterials with mechanical properties that rival nature.Synthetic materials tend to excel in either stiffness or extensibility, whereasxa0a combination of the two is necessary to exceed the performance of natural biomaterials. Here the authors present a bioinspired polymer consisting of cyclic β-peptide rings that is capable of transitioning between rigid and unfolded conformations on demand.

Pressure- and Additive-Mediated Sintering of B4C at Relatively Low Temperatures

A significant improvement in sinterability of B4C was achieved at a relatively low temperature by applying high pressure (2xa0GPa) and adding a small amount (5xa0wtxa0pct) of Ni. The sintered B4C and Ni powder mixture showed improved hardness in the range of 21 to 32xa0GPa and improved modulus as compared to the sintered B4C powder without additive. This is mostly attributed to the formation of Ni4B3, as characterized by Reitveld refinement method and transmission electron microscopy (TEM), which enhances the bonding between B4C particles. These results provide a new avenue toward the development of sintering of B4C at relatively low temperatures (<0.5Tm of B4C).