HRXRD study of the theoretical densities of novel reactive sintered boride candidate neutron shielding materials

Reactive Sintered Borides (RSBs) are novel borocarbide materials derived from FeCr-based cemented tungsten (FeCr-cWCs) show considerable promise as compact radiation armour for proposed spherical tokamak,[1],[2],[3],[4],[5]. Six candidate compositions (four RSBs, two cWCs) were evaluated by high-resolution X-ray diffraction (XRD), inductively coupled plasma (ICP), energy dispersive X-ray analysis (EDX) and scanning electron microscopy (SEM) to determine the atomic composition, phase presence, and theoretical density. RSB compositions were evaluated with initial boron contents equivalent to 25 at% 30 at%. All RSB compositions showed delamination and carbon enrichment in the bulk relative to the surface, consistent with non-optimal binder removal and insufficient sintering time. Phase abundance within RSBs derived from powder XRD was dominated by iron tungsten borides (FeWB/FeW2B2), tungsten borides (W2B5/WB) and iron borides. The most optimal RSB composition (B5T522W) with respect to physical properties and highest ρ/ρtheo had ρtheo = 12.59 ± 0.01 g cm-3 for ρ/ρtheo = 99.3% and had the weigh-in and post-sintered W : B : Fe abundance closest to 1 : 1 : 1. This work indicates that despite their novelty, RSB materials can be optimized and in principle be processed using existing cWC processing routes

New Superconducting and Semiconducting Fe-B Compounds Predicted with an Ab Initio Evolutionary Search

New candidate ground states at 1:4, 1:2, and 1:1 compositions are identified in the well-known Fe-B system via a combination of ab initio high-throughput and evolutionary searches. We show that the proposed oP12-FeB2 stabilizes by a break up of 2D boron layers into 1D chains while oP10-FeB4 stabilizes by a distortion of a 3D boron network. The uniqueness of these configurations gives rise to a set of remarkable properties: oP12-FeB2 is expected to be the first semiconducting metal diboride and oP10-FeB4 is shown to have the potential for phonon-mediated superconductivity with a Tc of 15-20 K.Comment: 7 pages, 6 figure

MgB2 conductors for dc and ac applications

The paper presents discussion on up to date results on MgB2 conductors from the point of view of their future dc and ac applications. Basic physical parameters of MgB2 compound relevant to conductors are introduced. Different conductor preparation methods and conductor architectures are presented and attainable critical current densities discussed. Some numerical results on critical currents and ac losses of future multifilamentary MgB2 conductors with magnetic cladding of their filaments are given. Recently observed anomalous decrease of ac susceptibility at 50 K in copper clad Powder-in-tube, PIT, MgB2 wires is presented.Comment: Paper presented at EUCAS'01 conference, Copenhagen, 26-30 August 200

Computational study of boron nitride nanotube synthesis: how catalyst morphology stabilizes the boron nitride bond

In an attempt to understand why catalytic methods for the growth of boron nitride nanotubes work much worse than for their carbon counterparts, we use first-principles calculations to study the energetics of elemental reactions forming N2, B2 and BN molecules on an iron catalyst. We observe that in the case of these small molecules, the catalytic activity is hindered by the formation of B2 on the iron surface. We also observe that the local morphology of a step edge present in our nanoparticle model stabilizes the boron nitride molecule with respect to B2 due to the ability of the step edge to offer sites with different coordination simultaneously for nitrogen and boron. Our results emphasize the importance of atomic steps for a high yield chemical vapor deposition growth of BN nanotubes and may outline new directions for improving the efficiency of the method.Comment: submitted to physical review

Interstitial compounds as fuel cell catalysts - Their preparative techniques and electrochemical testing

Preparation and electrochemical testing methods for fuel cell catalysts using interstitial compound

Comparative experimental and Density Functional Theory (DFT) study of the physical properties of MgB2 and AlB2

In present study, we report an inter-comparison of various physical and electronic properties of MgB2 and AlB2. Interestingly, the sign of S(T) is +ve for MgB2 the same is -ve for AlB2. This is consistent our band structure plots. We fitted the experimental specific heat of MgB2 to Debye Einstein model and estimated the value of Debye temperature (theta) and Sommerfeld constant (gamma) for electronic specific heat. Further, from gamma the electronic density of states (DOS) at Fermi level N(EF) is calculated. From the ratio of experimental N (EF) and the one being calculated from DFT, we obtained value of Lembda to be 1.84, thus placing MgB2 in the strong coupling BCS category. The electronic specific heat of MgB2 is also fitted below Tc using pi-model and found that it is a two gap superconductor. The calculated values of two gaps are in good agreement with earlier reports. Our results clearly demonstrate that the superconductivity of MgB2 is due to very large phonon contribution from its stretched lattice. The same two effects are obviously missing in AlB2 and hence it is not superconducting. DFT calculations demonstrated that for MgB2 the majority of states come from Sigma and Pi 2p states of boron on the other hand Sigma band at Fermi level for AlB2 is absent. This leads to a weak electron phonon coupling and also to hole deficiency as Pi bands are known to be of electron type and hence obviously the AlB2 is not superconducting. The DFT calculations are consistent with the measured physical properties of the studied borides, i.e., MgB2 and AlB2Comment: 16 pages Text + Figs: comments/suggestions welcome ([email protected])/www.freewebs.com/vpsawana

X-ray photoelectron spectroscopy study of radiofrequency-sputtered refractory compound steel interfaces

Radiofrequency sputtering was used to deposit Mo2C, Mo2B5, and MoSi2 coatings on 440C steel substrates. Both sputter etched and preoxidized substrates were used, and the films were deposited with and without a substrate bias of -300 V. The composition of the coatings was measured as a function of depth by X-ray photoelectron spectroscopy combined with argon ion etching. In the interfacial region there was evidence that bias produced a graded interface in Mo2B5 but not in Mo2C. Oxides of iron and of all film constituents except carbon were presented in all cases but the iron oxide concentration was higher and the layer thicker on the preoxidized substrates. The film and iron oxides were mixed in the MoSi2 and Mo2C films but layered in the Mo2B5 film. The presence of mixed oxides correlates with enhanced film adhesion