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Guthrie M.,ESS AB
Journal of Physics Condensed Matter | Year: 2015

The ability to manipulate structure and properties using pressure has been well known for many centuries. Diffraction provides the unique ability to observe these structural changes in fine detail on lengthscales spanning atomic to nanometre dimensions. Amongst the broad suite of diffraction tools available today, neutrons provide unique capabilities of fundamental importance. However, to date, the growth of neutron diffraction under extremes of pressure has been limited by the weakness of available sources. In recent years, substantial government investments have led to the construction of a new generation of neutron sources while existing facilities have been revitalized by upgrades. The timely convergence of these bright facilities with new pressure-cell technologies suggests that the field of high-pressure (HP) neutron science is on the cusp of substantial growth. Here, the history of HP neutron research is examined with the hope of gleaning an accurate prediction of where some of these revolutionary capabilities will lead in the near future. In particular, a dramatic expansion of current pressure-temperature range is likely, with corresponding increased scope for extreme-conditions science with neutron diffraction. This increase in coverage will be matched with improvements in data quality. Furthermore, we can also expect broad new capabilities beyond diffraction, including in neutron imaging, small angle scattering and inelastic spectroscopy. © 2015 IOP Publishing Ltd.

Du Plessis H.E.,Sasol Limited | De Villiers J.P.R.,University of Pretoria | Kruger G.J.,University of Johannesburg | Steuwer A.,ESS AB | And 2 more authors.
Journal of Synchrotron Radiation | Year: 2011

Fischer-Tropsch (FT) synthesis is an important process in the manufacturing of hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The reduced iron catalyst reacts with carbon monoxide and hydrogen to form bulk Fe5C2 Hägg carbide (χ-HC) during FT synthesis. Arguably, χ-HC is the predominant catalyst phase present in the working iron catalyst. Deactivation of the working catalyst can be due to oxidation of χ-HC to iron oxide, a step-wise decarburization to cementite (θ-Fe3C), carbon formation or sintering with accompanying loss of catalytic performance. It is therefore critical to determine the precise crystal structure of χ-HC for the understanding of the synthesis process and for comparison with the first-principles ab initio modelling. Here the results of high-resolution synchrotron X-ray powder diffraction data are reported. The atomic arrangement of χ-HC was confirmed by Rietveld refinement and subsequent real-space modelling of the pair distribution function (PDF) obtained from direct Fourier transformation. The Rietveld and PDF results of χ-HC correspond well with that of a pseudo-monoclinic phase of space group P1-[a = 11.5661 (6) Å, b = 4.5709 (1) Å, c = 5.0611 (2) Å, α = 89.990 (5)°, β = 97.753 (4)°, γ = 90.195 (4)°], where the Fe atoms are located in three distorted prismatic trigonal and one octahedral arrangement around the central C atoms. The Fe atoms are distorted from the prismatic trigonal arrangement in the monoclinic structure by the change in C atom location in the structure. © 2011 International Union of Crystallography.

Altenkirch J.,University of Manchester | Altenkirch J.,Laue Langevin Institute | Steuwer A.,ESS AB | Steuwer A.,Nelson Mandela Metropolitan University | Withers P.J.,Laue Langevin Institute
Science and Technology of Welding and Joining | Year: 2010

This paper examines the effect of three friction stir welding process parameters on the residual stresses, hardness and distortion for butt welded aluminium-lithium AA2199 alloy, a novel, low density high strength alloy with potential in the aerospace sector. A systematic set of nine trial welds is examined at different tool rotation and traverse speeds as well as tool downforces. The tensile residual stresses (∼50% of parent material yield strength) and the hardness drop in the weld line varied little with any of the friction stir welding process parameters. However, their breadth increased with rotation speed and downforce and decreased with increasing translation speed, which is consistent with increasing heat input. Weld distortion took the form of a saddle with the longitudinal bending distortion correlating strongly with the width of the tensile zone consistent with it being driven by the magnitude of the tensile buckling forces in the weld region. © 2010 Maney Publishing.

Tchelidze L.,ESS AB | Stovall J.,Rutherford Appleton Laboratory
IPAC 2013: Proceedings of the 4th International Particle Accelerator Conference | Year: 2013

High power hadron linear accelerators are designed based on 1 W/m loss limit criteria. The loss limit originates from the hands-on-maintenance allowance of accelerators and limits average dose rate level to less than 0.1 - 1 mSv/h (limited access time) at 30 - 40 cm from a machine after 100 days of continuous operation and 4 hours of down time. However, machine activation and thus beam loss limit depends on incident particle energy and 1 W/m is only a good approximation for energies 100 - 200 MeV and higher (in H-/H+ accelerators). At lower energies though, one could allow much higher than 1 W/m without excess activation. A careful analysis of energy dependent loss limits was performed for proton linear accelerators as part of the study for the European Spallation Source (ESS) linac (linear accelerator), for energy range 5 MeV - 80 MeV. ESS linac is to be built in Lund, Sweden and will deliver 5 MW proton beam to the target. MARS code was used for calculations and beam loss limits were derived as a function of energy.

Chatterji T.,Laue Langevin Institute | Ouladdiaf B.,Laue Langevin Institute | Henry P.F.,ESS AB | Bhattacharya D.,Indian Central Glass and Ceramic Research Institute
Journal of Physics Condensed Matter | Year: 2012

We have investigated magnetoelastic effects in multiferroic YMnO 3 below the antiferromagnetic phase transition, T N70K, using neutron powder diffraction. The a lattice parameter of the hexagonal unit cell of YMnO 3 decreases normally above T N, but decreases anomalously below T N, whereas the c lattice parameter increases with decreasing temperature and then increases anomalously below T N. The unit cell volume also undergoes an anomalous contraction below T N. By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equation, we determined the lattice strains Δa, Δc and ΔV due to the magnetoelastic effects as a function of temperature. We have also determined the temperature variation of the ordered magnetic moment of the Mn ion by fitting the measured Bragg intensities of the nuclear and magnetic reflections with the known crystal and magnetic structure models and have established that the lattice strain due to the magnetoelastic effect in YMnO 3 couples with the square of the ordered magnetic moment or the square of the order parameter of the antiferromagnetic phase transition. © 2012 IOP Publishing Ltd.

Zeidler A.,University of Bath | Guthrie M.,Carnegie Institution of Washington | Guthrie M.,ESS AB | Salmon P.S.,University of Bath
High Pressure Research | Year: 2015

The room temperature structure of the alloy was measured by X-ray diffraction under compression at pressures up to. This alloy is used as a construction material in high pressure neutron-scattering research and has a mean coherent neutron scattering length of zero, that is, it is a so-called null-scattering alloy. A broad phase transition was observed from a hexagonal close-packed α-phase to a hexagonal ω-phase, which started at a pressure of and was completed by. The data for the α-phase were fitted by using a third-order Birch-Murnaghan equation of state, giving an isothermal bulk modulus and pressure derivative. The results will help to ensure that accurate structural information can be gained from in situ high pressure neutron diffraction work on amorphous and liquid materials where the alloy is used as a gasket material. © 2015 Taylor & Francis.

Tsirlin A.A.,Max Planck Institute for Chemical Physics of Solids | Abakumov A.M.,University of Antwerp | Ritter C.,Laue Langevin Institute | Henry P.F.,Helmholtz Center Berlin | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We present a comprehensive study of the crystal structure, magnetic structure, and microscopic magnetic model of (CuBr)LaNb 2O 7, the Br analog of the spin-gap quantum magnet (CuCl)LaNb 2O 7. Despite similar crystal structures and spin lattices, the magnetic behavior and even peculiarities of the atomic arrangement in the Cl and Br compounds are very different. The high-resolution x-ray and neutron data reveal a split position of Br atoms in (CuBr)LaNb 2O 7. This splitting originates from two possible configurations developed by [CuBr] zigzag ribbons. While the Br atoms are locally ordered in the ab plane, their arrangement along the c direction remains partially disordered. The predominant and energetically more favorable configuration features an additional doubling of the c lattice parameter that was not observed in (CuCl)LaNb 2O 7. (CuBr)LaNb 2O 7 undergoes long-range antiferromagnetic ordering at T N=32 K, which is nearly 70% of the leading exchange coupling J 4≃48 K. The Br compound does not show any experimental signatures of low-dimensional magnetism because the underlying spin lattice is three-dimensional. The coupling along the c direction is comparable to the couplings in the ab plane, even though the shortest Cu-Cu distance along c (11.69 Å) is three times larger than nearest-neighbor distances in the ab plane (3.55 Å). The stripe antiferromagnetic long-range order featuring columns of parallel spins in the ab plane and antiparallel spins along c is verified experimentally and confirmed by the microscopic analysis. © 2012 American Physical Society.

Mourigal M.,Laue Langevin Institute | Mourigal M.,Johns Hopkins University | Enderle M.,Laue Langevin Institute | Fak B.,Joseph Fourier University | And 8 more authors.
Physical Review Letters | Year: 2012

Polarized and unpolarized neutron scattering experiments on the frustrated ferromagnetic spin-1/2 chain LiCuVO 4 show that the phase transition at H Q of 8 T is driven by quadrupolar fluctuations and that dipolar correlations are short range with moments parallel to the applied magnetic field in the high-field phase. Heat-capacity measurements evidence a phase transition into this high-field phase, with an anomaly clearly different from that at low magnetic fields. Our experimental data are consistent with a picture where the ground state above H Q has a next-nearest neighbor bond-nematic order along the chains with a fluidlike coherence between weakly coupled chains. © 2012 American Physical Society.

Ainalem M.-L.,ESS AB | Nylander T.,Lund University
Soft Matter | Year: 2011

The control of DNA condensation, i.e. packaging or compaction, is essential for the living cell, but also important in many applications. One example is gene therapy that often utilises vehicles with the ability to condense DNA and thereby protect DNA against degradation, transport DNA across membranes (which act as barriers towards gene delivery), and regulate gene expression. This review discusses the ability of poly(amido amine) dendrimers to condense DNA molecules via attractive electrostatic interactions, which in turn leads to self-assembled structures with a rich variety of morphologies. The process of condensation is cooperative and kinetically controlled, and the structure of the aggregates strongly depends on the size and charge of the dendrimer, and the salt concentration of the aqueous solution. While globular aggregates are formed by large dendrimers, rods and toroids are formed by smaller sized dendrimers with lower total charge per molecule. The globular aggregates appear to be disordered, but the smaller dendrimers give rise to high-ordered packing of the DNA in ordered arrays according to a square or hexagonal unit cell. The high-ordered packing also indicates that the dendrimers deform while inducing the DNA to condense. © 2011 The Royal Society of Chemistry.

Tomas R.,CERN | Bach T.,CERN | Calaga R.,CERN | Langner A.,CERN | And 7 more authors.
Physical Review Special Topics - Accelerators and Beams | Year: 2012

The LHC is currently operating with a proton energy of 4 TeV and β * functions at the ATLAS and CMS interaction points of 0.6 m. This is close to the design value at 7 TeV (β *=0. 55m) and represented a challenge for various aspects of the machine operation. In particular, a huge effort was put into the optics commissioning and an unprecedented peak β beating of around 7% was achieved in a high energy hadron collider.

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