Structural diversity and solution behavior of low-valent iridium complexes bearing 1,4-diazabutadiene ligands

Three types of 1,4-diazabutadiene stabilized low-valent iridium complexes, namely [IrCl(cod)(MesDAB)] (1), [IrCl(coe)(MesDAB)] (2) and [IrCl(MesDAB)₂] (3), have been prepared from 1,4-bis(2,4,6-trimethylphenyl)-1,4-diaza-1,3-butadiene (MesDAB) and [IrCl(cod)]₂ or [IrCl(coe)₂]₂, respectively. The complexes have been investigated by NMR spectroscopy and X-ray diffraction experiments. While tetra-coordinated 2 and penta-coordinated 3 maintain their solid state structure in solution, penta-coordinated 1 shows fluxional behavior. The crystal structures determined indicate strong π-backbonding towards the MesDAB ligand in all cases.     

Interaction of titanium diboride/boron nitride evaporation boats with aluminium

Hexagonal boron nitride/titanium diboride composites are widely used as evaporation boats for aluminium deposition to produce functional and decorative layers on different target materials. The lifetime of such a material is limited mainly by the interaction of the metal with the ceramic substrate, but the corrosion mechanism has still not yet been thoroughly investigated and understood. In this article the corrosion mechanism for the evaporation boats used was investigated using thermodynamic calculations, FESEM, EDX and XRD phase analysis. The analysis showed that hexagonal boron nitride (hBN), which is thermodynamically less stable than TiB₂, is passivated during the application process through the formation of AlN surface layers, whereas the thermodynamically more stable TiB₂ phase dissolves and Ti-rich components precipitate in cooler regions of the evaporation boats.     

Multtphase Materials with Lignin. III. Polyblends with Ethylene-Vinyl Acetate Copolymers

Abstract

Organosolv lignin (OSL) from red oak, and its corresponding hydroxypropyl lignin (HPL) derivative, were blended in the melt with polyethylene and with ethylene-vinyl acetate (EVA) copolymer, and injection molded. Lignin (derivative) content and vinyl acetate (VA) content both served as process variables. All mixtures produced immiscible blends, both by scanning electron microscopy and dynamic mechanical analysis. However, the compatibility of the two polymers increased with VA content. Both lignin products proved to be effective modulus builders, but. HPI, was more effective than OSL at VA contents above 20%. Modulus increased roughly in accordance with the rule of mixing. Inferior tensile strength properties were observed with the EVA copolymer if VA content was below 10%, and superior characteristics were noted in materials containing more than 25% VA and between 5 and 20% HPL.     

Sintering Mechanisms of Zirconium and Hafnium Carbides Doped with MoSi2

The microstructure of two pressureless-sintered ultra-high-temperature ceramics, namely ZrC+20 vol% MoSi₂ and HfC+20 vol% MoSi₂, was characterized by scanning and transmission electron microscopy. With regard to the ZrC–MoSi₂ system, Zr _x Si _y compounds and SiC were detected. In the HfC–MoSi₂ system, a mixed phase was detected at the triple points and identified as (Mo,Hf)₅Si₃. For both the systems investigated, the high wettability of the silicide-based phases on the matrix grains suggests that sintering is assisted by a liquid phase. This contribution reports for the first time on the sintering mechanisms of early transition metal carbides doped with MoSi₂ as a sinter additive, on the basis of the microstructural evolution observed upon sintering and in the light of phase diagrams and thermodynamical calculations.     

Experimental characterisation and constitutive modelling of RTM-6 resin under impact loading

The response to mechanical loading of the thermosetting resin system RTM-6 has been investigated experimentally as a function of strain rate and a constitutive model has been applied to describe the observed and quantified material behaviour. In order to determine strain rate effects and to draw conclusions about the hydrostatic stress dependency of the material, specimens were tested in compression and tension at strain rates from 10⁻³ to 10⁴ s⁻¹. A Standard screw-driven tensile machine was used for quasi-static testing, with an ‘in house’ hydraulic rig and Hopkinson bars for medium and high strain rates, respectively. At all rates appropriate photography and optical metrology have been used for direct strain measurement, observation of failure and validation of experimental procedures. In order to enable the experimental characterisation of this brittle material at very high rates in tension, a novel pulse shaping technique has been applied. With the help of this device, strain rates of up to 3800 s⁻¹ have been achieved while maintaining homogeneous deformation state until specimen fracture in the gauge section of the tensile specimens. The yield stress and initial modulus increased with increasing strain rate for both compression and tension, while the strain to failure decreased with strain rate in tension. An existing constitutive model, the Goldberg model has been extended in order to take into account the nonlinear strain rate dependence of the elastic modulus. The model has been validated against 3-point impact bending tests of prismatic RTM-6 beams.     

Square Foot Manufacturing: a new production concept for micro manufacturing

Today a large number of microstructures are already employed as separate components or as constituents of larger modules in a broad spectrum of production in medical technology, optics, biotechnology, mechatronics, fluidics, (micro)-forming and tool construction. Current research activities are directed towards the downscaling of manufacturing procedures or the formation of complex process chains for the manufacture of micro workpieces. The Square Foot Manufacturing concept represents one approach aimed at achieving significant technical, economic and ecological developments in the production of microstructures by means of machining techniques that can be applied to a spectrum of materials as broad as possible. This fabrication concept representing a refinement of existing desktop manufacturing concepts is currently being developed at the Institute of Production Engineering of Helmut-Schmidt-University/University of the Federal Armed Forces Hamburg. This paper is intended to provide an overview of the theoretical concept and the current state of its technical implementation.     

Paradigm change: small machine tools for small workpieces

To overcome current limitations in micro manufacturing a new approach instead of evolutionary small step progress has to take place. Applying small machine tools to manufacture small workpieces allows for a leap of improvements enabled by the small size itself. This miniaturization in comparison to simply down-scaling current concepts qualifies the use of materials and technologies commonly not found in machine tools. This contribution describes the aspects, methodology, qualifications as well as cause and effects pursued by the approach of small machine tools for small workpieces.