18th March 2017
Direct Observation of Individual Hydrogen Atoms at Trapping Sites in a Ferritic Steel
Hydrogen embrittlement (HE) is one of the most devastating and unpredictable, yet least understood, mechanisms of failure experienced by engineering components. The presence of hydrogen leads to a severe degradation in mechanical properties and consequently a loss in structural integrity of a vast range of metals and alloys. Despite considerable research over decades, the precise mechanisms responsible for the embrittling process are still not understood. A key factor limiting our understanding is the difficulty in experimentally observing the distribution of hydrogen within a material, particularly at the atomic scale. Even ultra-high resolution electron microscopy cannot distinguish hydrogen atoms from the surrounding material in engineering materials. The Oxford Atom Probe group, working with colleagues at Sheffield, Zurich and Brisbane, report in Science the use of isotopic doping for the unambiguous 3D characterisation of individual hydrogen atoms within a ferritic steel. The research demonstrates the first direct atomic-scale observation of the precise manner in which a microstructural feature acts to trap hydrogen, in this case within the core of carbides. The techniques developed are not limited to steels, and may prove significant for other technologically relevant systems, such as nickel-based superalloys and titanium alloys where hydrogen can play an important role in degrading in-service performance.
22nd February 2017
Electron-phonon interactions from first principles
The electron-phonon interaction plays an important role in many properties of solids, from superconductivity to the electrical and optical properties of metals and semiconductors.
During the last two decades density-functional theory and many-body perturbation theory methods have made it possible to perform accurate and predictive calculations of the electron-phonon interaction. In this article Feliciano Giustino reviews the state of the art in ab initio theories of electron-phonon interactions in solids and their applications to problems of current interest in materials physics.
3rd February 2017
Environmentally-assisted grain boundary attack as a mechanism of embrittlement in a nickel-based superalloy
The loss of ductility of high strength polycrystalline superalloy in the mid-temperature regime is of huge importance in both aerospace propulsion and land based power generation applications. Recent systematic work by André Németh and his colleges from Oxford Materials in collaboration with Rolls Royce plc have given new mechanistic insight using the unique range of advanced characterisation techniques available in the department. The physical factors responsible for the ductility dip were established as embrittlement from internal intergranular oxidation along the g-grain boundaries, and in particular, at incoherent interfaces of the primary g’-precipitates with the matrix phase. These fail under local microstresses arising from the accumulation of dislocations during slip-assisted grain boundary sliding. At high temperatures, ductility is restored because the accumulation of dislocations at grain boundaries is no longer prevalent. The results are published open access in Acta Materialia.
3rd February 2017
Electrically tunable organic–inorganic hybrid polaritons with monolayer WS2
A collaboration between the Nanostructured Materials Group and the Photonic Nanomaterials Group has reported the controlled generation and electrical switching of polaritons using 2D materials and organic dyes. A polariton is a quantum superposition of an optical photon with an excitation in a material, and could hold the key to low power nonlinear optical devices (effectively allowing optical signals to interact with each other) due to interactions between the material excitations. Different types of polaritons exist based on different types of excitation, and bring different degrees of nonlinearity. In this recent paper in Nature Communications, two different types of excitation - a Wannier-Mott exciton in monolayer WS2 and an electronic transition in a dye molecule - are coupled simultaneously to the photon, and we show that application of an electric field to the WS2 substantially changes the fractions of these species in the superposition, effectively allowing switching between them. This switching will have the effect of changing the degree of nonlinearity in the polaritons, and can thus be used as a means to modulate a nonlinear device electrically.
21st December 2016
Sequential nucleation of phases in a 17-4PH steel
Guma Yeli, Maria Auger, Paul Bagot, George Smith and Michael Moody in the Atom probe tomography group of the Department of Materials in collaboration with Keith Wilford in Rolls Royce have studied the microstructure and mechanical properties of 17-4PH steel under thermal ageing at 480oC and 590oC. The findings have been reported recently in Acta Materialia. For the first time, the sequential nucleation and growth of Cu-rich precipitates (CRPs), Nb-rich precipitates, Cr-rich precipitates and Mn, Ni, Si (MNS)-rich phase has been characterized at the atomic scale. The precipitates that are initially nucleated have been found to act as favoured sites for heterogeneous nucleation of subsequent phases. MNS phase precipitates were transformed from Nb-rich precipitates. This study also works to correlate the microstructure to the observed mechanical properties. The contributions of precipitates on the precipitation hardening has been estimated and compared with the Vickers hardness results.
16th December 2016
On the role of boron on improving ductility in a new polycrystalline superalloy
The mechanical behaviour of a prototype nickel-based superalloy at elevated temperature, with emphasis on the improvement in ductility conferred by boron has been reported in a recent open access publication in Acta Materialia by Paraskevas Kontis, Angus Wilkinson and Roger Reed. A miniaturised electro-thermal mechanical testing system and an in-situ SEM testing module with complementary HR-EBSD strain mapping analysis have been used. The improvement in ductility is observed to be greater at the lowest investigated strain rate, where the grain boundary character plays a significant role on the mechanical properties. The in-situ tests performed at 750 °C revealed directly the mechanism of ductility enhancement. The findings are supported further by high-resolution electron backscattered diffraction (HR-EBSD) strain mapping which confirms that the distribution of elastic strain and geometrically necessary dislocation (GND) content are influenced markedly by the addition of boron.
9th December 2016
Toward Lead-Free Perovskite Solar Cells
Since the first reports of solar cells with power conversion efficiencies around 10% in 2012, the science and technology of perovskite photovoltaics has been progressing at an unprecedented rate. The current certified record efficiency of 22.1% makes perovskites the first solution-processable technology to outperform multicrystalline and thin-film silicon. For this technology to be deployed on a large scale, one of the remaining challenges is the toxicity of lead. While lead is allowed in photovoltaic modules, it would be desirable to find alternatives which retained the unique optoelectronic properties of lead halide perovskites. In this manuscript, Feliciano Giustino from the Department of Materials and Henry J. Snaith from the Department of Physics offer their perspective on the latest developments in the materials science of lead-free perovskites and perovskite derivatives. The article was selected as ACS Energy Letters Editor's Choice and featured in the journal Cover.
8th December 2016
Laser writing of coherent colour centres in diamond
Members of the Photonic Nanomaterials Group with collaborators in the Department of Engineering Science have developed a new method for fabricating spin qubits for quantum computing, published this week in Nature Photonics. Using a single pulse from a femtosecond laser, they ‘write’ vacancies into a diamond containing a trace amount of nitrogen, and then anneal the diamond to produce nitrogen-vacancy (NV) complexes. The highly non-linear process of vacancy formation allows the vacancies to be positioned in the diamond crystal with a precision of around 100 nm, such that after annealing NV centres are formed within about 200 nm of the targeted position in the image plane. By controlling the laser pulse energy, single NV centres were generated with a high probability so that individual qubits could be addressed, and residual damage to the diamond lattice can be kept to a minimum, producing NV centres with coherent optical and spin properties required to build quantum computers. Not only does this method offer an attractive new way to make spin qubits in diamond for quantum technologies, it opens new possibilities for the engineering and study of single point defects in a range wide band gap materials.
Also see the NQIT website news.
25th November 2016
A synchrotron X-ray diffraction study of non-proportional strain-path effects
Researchers from Oxford Materials have been investigating fabrication methods that have the potential to significantly increase the ductility of metallic sheets. Such ductility gains are possible by subjecting ferritic steel to so-called non-proportional strain-paths. This effect was studied in a world-first experiment at the Diamond Light Source, using a bespoke biaxial testing mechanism to elucidate the micromechanical response in-situ during strain-paths known to give increased ductility. The high energy synchrotron diffraction data was used to measure lattice strain and texture development; shown to be strain-path sensitive. Replicating the ductility gains using crystal plasticity finite element modelling was also demonstrated, using the diffraction results to inform the governing dislocation-based constitutive laws.
This study by David Collins, Angus Wilkinson, Richard Todd was in collaboration with project partners at BMW-MINI, University of Bristol, Imperial College London and Diamond Light Source. The research is published open access in Acta Materialia.
15th November 2016
Resonant Optomechanics with a Vibrating Carbon Nanotube and a Radio-Frequency Cavity
Natalia Ares and co-workers in the Quantum electronic devices group have recented reported in Physical Review Letters a study of the optomechanical coupling between a carbon nanotube resonator and a radio-frequency tank circuit acting as a cavity. In this resonant regime, the vacuum optomechanical coupling is enhanced by the dc voltage coupling the cavity and the mechanical resonator. Using the cavity to detect the nanotube’s motion, they observed and simulated interference between mechanical and electrical oscillations. Measurement of the mechanical ring down showed that further improvements to the system could enable the measurement of mechanical motion at the quantum limit.