Effect of SiC weight percentage on tribological properties of Al-SiC metal matrix composites under acid environment

In this paper, Al-SiC MMCs reinforced with different weight percentages of SiC content (5 wt.%, 7.5 wt.% and 10 wt.%) are fabricated through the liquid stir casting method. The effect of weight percentage of SiC on friction and wear properties of Al-SiC MMC is investigated. Tribological tests are conducted under acid environment in a pin-on-disk tribotester by varying the design parameters (applied normal load and sliding speed) while the duration of each experiment is kept constant for 30 minutes. It is seen from the result that wear increases with increase in applied load and sliding speed but friction coefficient shows a decreasing trend with increase in load. The addition of SiC reinforcement increases the wear resistance of the metal matrix composite. The scanning electron microscope (SEM) and energy dispersive X-Ray (EDX) technique are used to analyse the wear mechanism of worn surface. From the microstructure study, it is seen that adhesive, abrasive and corrosive wear mechanisms are present for removal of material from the Al-SiC MMCs

Electrochemical Generation of High-Valent Oxo-Manganese Complexes Featuring an Anionic N5 Ligand and Their Role in O―O Bond Formation

Generation of high-valent oxomanganese complexes through controlled removal of protons and electrons from low-valent congeners is a crucial step toward the synthesis of functional analogues of the native oxygen evolving complex (OEC). In-depth studies of the water oxidation activity of such biomimetic compounds help to understand the mechanism of O―O bond formation presumably occurring at the last step of the photosynthetic cycle. Scarce reports of reactive high-valent oxomanganese complexes underscores the impetus for the present work, wherein we report the electrochemical generation of the non-heme oxomanganese(IV) species, [(dpaq)MnIV(O)]+ (2), through a proton-coupled electron transfer (PCET) process from the hydroxomanganese complex [(dpaq)MnIII(OH)]ClO4 (1). Controlled potential spectroelectrochemical studies of 1 in wet acetonitrile at 1.45 V vs. NHE revealed quantitative formation of 2 within 10 min. The high-valent oxomanganese(IV) transient exhibited remarkable stability and could be reverted to the starting complex (1) by switching the potential to 0.25 V vs. NHE. The formation of 2 via PCET oxidation of 1 demonstrates an alternate pathway for the generation of the oxomanganese(IV) transient (2) without the requirement of redox-inactive metal ions or acid additives as proposed earlier. Theoretical studies predict that one-electron oxidation of [(dpaq)MnIV(O)]+ (2) forms a manganese(V)-oxo (3) species, which can be oxidized further by one-electron to a formally manganese(VI)-oxo transient (4). Theoretical analyses suggest that the first oxidation event (2 to 3) takes place at the metal-based d-orbital whereas, in the second oxidation process (3 to 4), the electron eliminates from an orbital composed of equitable contribution from metal and ligand, leaving a single electron in the quinoline-dominated orbital in the doublet ground spin state of the manganese(VI)-oxo species (4). This mixed metal- ligand (quinoline)-based oxidation is proposed to generate a formally Mn(VI) species (4), a non-heme analogue of the species ‘compound I’, formed in the catalytic cycle of cytochrome P-450. We propose that the highly electrophilic species 4 catches water during cyclic voltammetry experiments and results in O―O bond formation leading to electrocatalytic oxidation of water to hydrogen peroxide