Influence of Gas Type on the Thermal Efficiency of Microwave Plasmas for the Sintering of Metal Powders

Microwave plasmas have enormous potential as a rapid and energy efficient sintering technology. This paper evaluates the influence of both plasma atmosphere and metal powder type on the sintering temperatures achieved and the properties of the sintered powder metal compacts. The sintering is carried out using a 2.45 GHz microwave-plasma process called rapid discharge sintering (RDS). The sintering of three types of metal powder are evaluated in this study: nickel (Ni), copper (Cu) and 316L stainless steel (SS). An in-depth study of the effects of the plasma processing parameters on the sintered powder compacts are investigated. These parameters are correlated with the mechanical performance of the sintered compacts to help understand the effect of the plasma heating process. The substrate materials are sintered in four different gas discharges, namely hydrogen, nitrogen oxygen and argon. Thermocouple, pyrometer and emission spectroscopy measurements were taken to determine the substrate and the discharge temperatures. The morphology and structure were examined using scanning electron microscopy and X-ray diffraction. The density and hardness of the sintered compacts were correlated with the plasma processing conditions. As expected higher densities were obtained with powders with lower sintering temperatures i.e. nickel and copper when compared with stainless steel. Under the power input and pressure conditions used, the highest substrate temperature attained was 1,100°C for Cu powder sintered in a nitrogen atmosphere. In contrast under the same processing conditions but in an argon plasma, the temperature achieved with SS was only 500°C. The effect of the plasma gas type on the sintered powder compact chemistry was also monitored, both hydrogen and nitrogen yielded a reducing effect for the metal in contrast with the oxidising effect observed in an oxygen plasma.     

Co-universal C*-algebras associated to generalised graphs

We introduce P-graphs, which are generalisations of directed graphs in which paths have a degree in a semigroup P rather than a length in ?. We focus on semigroups P arising as part of a quasi-lattice ordered group (G, P) in the sense of Nica, and on P-graphs which are finitely aligned in the sense of Raeburn and Sims. We show that each finitely aligned P-graph admits a C*-algebra C*_min (Λ) which is co-universal for partialisometric representations of Λ which admit a coaction of G compatible with the P-valued length function. We also characterise when a homomorphism induced by the co-universal property is injective. Our results combined with those of Spielberg show that every Kirchberg algebra is Morita equivalent to C*_min (Λ) for some (?²* ?)-graph Λ.     

Complex Spherical Designs and Codes

Real spherical designs and real and complex projective designs have been shown by Delsarte, Goethals, and Seidel to give rise to association schemes when the strength of the design is high compared to its degree as a code. In contrast, designs on the complex unit sphere remain relatively uninvestigated, despite their importance in numerous applications. In this paper, we develop the notion of a complex spherical design and show how many such designs carry the structure of an association scheme. In contrast with the real spherical designs and the real and complex projective designs, these association schemes are nonsymmetric.     

A MnIIMnIII‐Peroxide Complex Capable of Aldehyde Deformylation

Ribonucleotide reductases (RNRs) are essential enzymes required for DNA synthesis. In class Ib Mn₂ RNRs superoxide (O₂^.?) was postulated to react with the Mn^II₂ core to yield a Mn^IIMn^III‐peroxide moiety. The reactivity of complex 1 ([Mn^II₂(O₂CCH₃)₂(BPMP)](ClO₄), where HBPMP=2,6‐bis{[(bis(2‐pyridylmethyl)amino]methyl}‐4‐methylphenol) towards O₂^.? was investigated at ?90 °C, generating a metastable species, 2 . The electronic absorption spectrum of 2 displayed features (λ_max=440, 590 nm) characteristic of a Mn^IIMn^III‐peroxide species, representing just the second example of such. Electron paramagnetic resonance and X‐ray absorption spectroscopies, and mass spectrometry supported the formulation of 2 as a Mn^IIMn^III‐peroxide complex. Unlike all other previously reported Mn₂‐peroxides, which were unreactive, 2 proved to be a capable oxidant in aldehyde deformylation. Our studies provide insight into the mechanism of O₂‐activation in Class Ib Mn₂ RNRs, and the highly reactive intermediates in their catalytic cycle.     

LIGHT-INDUCED PERTURBATION OF AROMATIC RESIDUES IN BOVINE RHODOPSIN AND BACTERIORHODOPSIN*

Light-induced changes in the UV absorption spectrum of bovine rod outer segment membranes were measured by conventional difference spectroscopy and by flash photolysis methods. Different thermal intermediates of rhodopsin (lumirhodopsin, metarhodopsin I, metarhodopsin II, and meta-rhodopsin III) have absorption spectra in the ultraviolet which differ from the rhodopsin spectrum and from each other. The spectra associated with metarhodopsin I, metarhodopsin II, and metarhodopsin III are characteristic of perturbation of a small number of tyr. and/or trp residues, most likely one trp residue. These aromatic residues are in the neighborhood of the retinal Schiff base and undergo coordinated changes of interaction with retinal during the bleaching sequence. At the metarhodopsin II stage, the magnitude of the UV spectral changes is consistent with the exposure of a previously shielded trp residue to an aqueous environment. The present results are consistent with previous spectral studies which limit the extent of light-induced conformational changes to regions of the protein in the neighborhood of the retinal Schiff base. An analogous study was made on light-adapted purple membranes of Halobacterium halobium. The UV absorption spectrum associated with the deprotonated Schiff base intermediate of the trans-bacteriorhodopsin cycle is indicative, in part, of aromatic residue perturbation. However, significant changes in the secondary and tertiary structures of the bacterio-rhodopsin protein characteristic of a delocalized conformational change are unlikely at this intermediate stage.     

Dual Families of Interacting Particle Systems on Graphs

A simple condition for IPS (Interacting Particle Systems) with nearest neighbor interactions to be self-dual is given. It follows that any IPS with the contact transition and no spontaneous birth is self-dual. It is shown that families of IPS exist in which every IPS is dual to every other, and such that for every pair of IPS, one is a thinning of the other. Further, all such IPS have the same form for an equilibrium distribution when expressed in terms of survival probabilities. Convergence results from a wide class of initial infinite measures follow.