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Abstract: While Ti-Nb binary alloys have been extensively studied over the years, there are discrepancies in the literature relating to the β-α″ martensitic transformation that indicate an incomplete understanding. In particular, there are inconsistencies regarding the phase constitution of as-quenched material and the ability for α″ phase to be thermally-induced. To resolve these issues, the β-α″ phase transformation is studied in a Ti-24Nb (at.%) alloy subjected to two different water quench conditions. It is demonstrated that only the fastest quench rates (≳500˚C/s) result in the formation of α″ phase, and these materials exhibit a reversible thermally-induced β-α″ transformation at low temperatures. In contrast, slightly slower water quench procedures lead to a β+ω microstructure, and these samples do not exhibit a thermally-induced β-α″ transformation, even when cooled to -168˚C. Despite this, room temperature mechanical testing results indicate that α″ can be induced by stresses as low as 200 MPa. To explain these results, it is proposed that quenching stresses play a role in the competition between α″ and athermal ω phases at rapid quench rates. The present results are then reconciled with the body of literature and a broader understanding of phase stability in metastable β-titanium alloys.

Open Access

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Abstract: Organic solar cells (OSCs), also known as organic photovoltaics (OPVs), are an emerging solar cell technology composed of carbon-based, organic molecules, which convert energy from the sun into electricity. Key for their performance is the microstructure of the light-absorbing organic bulk heterojunction. To study this, organic solar films composed of both fullerene C60 as electron acceptor and different mole percentages of di-[4-(N,N-di-p-tolyl-amino)-phenyl]-cyclohexane (TAPC) as electron donor were evaporated in vacuum in different mixing ratios (5, 50 and 95 mol%) on an ITO-coated glass substrate held at room temperature and at 110 °C. The microstructure of the C60: TAPC heterojunction was studied by grazing incidence wide angle X-ray scattering to understand the effect of substrate heating. By increasing the substrate temperature from ambient to 110 °C, it was found that no significant change was observed in the crystal size for the C60: TAPC concentrations investigated in this study. In addition to the variation done in the substrate temperature, the variation of the mole percent of the donor (TAPC) was studied to conclude the effect of both the substrate temperature and the donor concentration on the microstructure of the OSC films. Bragg peaks were attributed to C60 in the pure C60 sample and in the blend with low donor mole percentage (5%), but the C60 peaks became nondiscernible when the donor mole percentage was increased to 50% and above, showing that TAPC interrupted the formation of C60 crystals.

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Abstract: n this work the suitability of nanostructured ferrite materials with the general formula AFe2O4 (where A is a divalent cation) for photocatalytic applications is investigated. Spinel ferrite MgFe2O4 nanoparticles and macroporous CaFe2O4 sponge structures were produced by microwave-assisted syntheses in high-boiling organic solvents and subsequent calcination in air. The elemental composition of the products was monitored by energy dispersive X-ray spectroscopy and the synthesis procedures were optimized to ensure an ideal stoichiometry of the products. Phase purity of the products was confirmed by calcination studies combined with diffraction experiments and by a wide variety of spectroscopic techniques. The morphology of the ferrite materials is characterized by electron microscopy, gas physisorption and mercury intrusion porosimetry. Regarding the electronic band structure of ferrites, a vast dissent is found in published literature. This is addressed by a thorough characterization of the electronic structure using photoelectrochemical measurements, X-ray based spectroscopic techniques, and by a detailed interpretation of their optical absorption spectra. The determined band positions suggest that CaFe2O4 is suitable for photocatalytic hydrogen evolution under visible light, while MgFe2O4 is not. Nevertheless, both phases remain inactive in hydrogen evolution test reactions as well as other photocatalytic experiments. X-ray based spectroscopy suggests that the presence of a transition metal with d5 electronic configuration causes a strong discrepancy between the fundamental electronic band gap and the one determined by optical spectroscopy. The Fe3+ crystal field orbitals involved in the ligand-to-metal charge transfer excitations that are responsible for the absorption of visible light are highly localized at the Fe3+ centers. The weak orbital overlap causes a low mobility of excited charge carriers explaining the inactivity in photocatalysis. Additional to the optical and photocatalytic properties, the magnetism of the synthesized materials is investigated by Mössbauer spectroscopy and SQUID magnetometry. While CaFe2O4 exhibits antiferromagnetic behavior, the MgFe2O4 nanoparticles exhibit a tunable magnetization, that depends on crystallite size and cation inversion and is therefore adjustable by post-synthetic calcination. First attempts towards the synthesis of magnetic NiFe2O4 and MnFe2O4 nanoparticles were made, to extend the scope of magnetic nanoparticles that can be synthesized via the microwave-assisted reaction. Attempting to combine the optical and magnetic characteristics of ferrites with other chemical functionalities in a composite material, phase-pure MgFe2O4 nanoparticles were immobilized on functionalized, ordered-mesoporous SiO2 and organosilica host networks.

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Abstract: CeOs 4 Sb 12 (COS) and PrOs 4 Sb 12 (POS) are two representative compounds that provide the ideal vantage point to systematically study the physics of multi- f -electron systems. COS with Ce 4 f 1 , and POS with Pr 4 f 2 configurations show distinct properties of Kondo insulating and heavy fermion superconductivity, respectively. We unveiled the underlying microscopic origin by angle-resolved photoemission spectroscopy studies. Their eV-scale band structure matches well, representing the common characters of conduction electrons in R Os 4 Sb 12 systems ( R = rare   earth ). However, f electrons interact differently with conduction electrons in COS and POS. Strong hybridization between conduction electrons and f electrons is observed in COS with band dependent hybridization gaps, and the development of a Kondo insulating state is directly revealed. Although the ground state of POS is a singlet, finite but incoherent hybridization exists, which can be explained by the Kondo scattering with the thermally excited triplet crystalline electric field state. Our results help us to understand the intriguing properties in COS and POS, and provide a clean demonstration of the microscopic differences in heavy fermion systems with 4 f 1 and 4 f 2 configurations.

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Abstract: Understanding how catalytic asymmetric reactions with racemic starting materials can operate would enable new enantioselective cross-coupling reactions that give chiral products. Here we propose a catalytic cycle for the highly enantioselective Rh(I)-catalysed Suzuki–Miyaura coupling of boronic acids and racemic allyl halides. Natural abundance 13C kinetic isotope effects provide quantitative information about the transition-state structures of two key elementary steps in the catalytic cycle, transmetallation and oxidative addition. Experiments with configurationally stable, deuterium-labelled substrates revealed that oxidative addition can happen via syn- or anti-pathways, which control diastereoselectivity. Density functional theory calculations attribute the extremely high enantioselectivity to reductive elimination from a common Rh complex formed from both allyl halide enantiomers. Our conclusions are supported by analysis of the reaction kinetics. These insights into the sequence of bond-forming steps and their transition-state structures will contribute to our understanding of asymmetric Rh–allyl chemistry and enable the discovery and application of asymmetric reactions with racemic substrates.