3,458 research outputs found
Substitutional solution of silicon in cementite: A first-principles study
No full textCementite precipitation from austenite in steels can be suppressed by alloying with silicon. There are, however, no validated thermodynamic data to enable phase equilibria to be estimated when silicon is present in cementite. The formation energies of Fe3C, (Fe11SiFe4c)C-4 and (Fe11SiFe8d)C-4 have therefore been estimated using first-principles calculations based on the total energy all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation to density functional theory. The ground state properties such as lattice constants and bulk moduli have also been calculated. The calculations show that (Fe11SiFe4c)C-4 and (Fe11SiFe8d)C-4 have about 52.06 kJ mol(-1) and 37.17 kJ mol(-1) greater formation energy, respectively, than Fe3C. The formation energy for hypothetical cementite Si3C has also been calculated to be about 256 kJ mol(-1). Silicon substitution significantly reduces the magnetic moments at the Fe(4c) site for both (Fe11SiFe4c)C-4 and (Fe11SiFe8d)C-4, irrespective of the Si substitution sites. The calculated electronic structures indicate that the magnetic moment reduction at the Fe(4c) site by the Si substitution at 4c site is indirect through the neighboring carbon atom, whereas at the 8d site it is direct. (C) 2008 Elsevier B.V. All rights reserved.X114645sciescopu
epsilon-Carbide in alloy steels: First-principles assessment
No full textThere is now a large number of sophisticated steels which rely on silicon as an alloying addition with the purpose of avoiding the precipitation of cementite. However, there is also evidence that the silicon can enhance the formation of epsilon-carbide; the mechanism of this effect is not understood and the absence of appropriate thermodynamic data makes it impossible to conduct calculations. We report here some ab initio calculations which throw light on both of these issues and suggest novel experiments. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.X11sciescopu
Gasification of glucose in Supercritical water
No full textGasification of 0.6 M glucose in supercritical water was investigated at a temperature range from 480 to 750 degreesC and 28 MPa with a reactor residence time of 10-50 s. The yield of hydrogen among gaseous products increased very sharply with increasing temperature above 660 degreesC. On the other hand, the yield of carbon monoxide decreased with temperature, most probably due to the role of a water-gas shift reaction. Carbon gasification efficiency reached 100% at 700 degreesC. A simplified model was proposed for the reaction pathways related to hydrogen production. The rates for glucose conversion and COD degradation were obtained by assuming pseudo-first-order kinetics
Photon Collection from Wavelength-scale Photonic Crystal Light Emitters (Plenary speaker)(Invited)
No full textFirst-principles investigation of magnetism and electronic structures of substitutional 3d transition-metal impurities in bcc Fe
No full textThe magnetic and electronic structures of 3d impurity atoms from Sc to Zn in ferromagnetic body-centered-cubic iron are investigated using the all-electron full-potential linearized augmented plane-wave method based on the generalized gradient approximation (GGA). We found that, in general, the GGA results are closer to the experimental values than those of the local spin density approximation. The calculated formation enthalpy data indicate the importance of a systematic study on the ternary Fe-C-X systems rather than the binary Fe-X systems in steel design. The lattice parameters are optimized and the conditions for spin polarization at the impurity sites are discussed in terms of the local Stoner model. Our calculations, which are consistent with previous work, imply that the local spin polarizations at Sc, Ti, V, Cu, and Zn are induced by the host Fe atoms. The early transition-metal atoms couple antiferromagnetically, while the late transition-metal atoms couple ferromagnetically to the host Fe atoms. The calculated total magnetization (M) of bcc Fe is reduced by impurity elements from Sc to Cr as a result of the antiferromagnetic interaction, with the opposite effect for solutes which couple ferromagnetically. The changes in M are attributed to nearest neighbor interactions, mostly between the impurity and host atoms. The atom averaged magnetic moment is shown to follow generally the well-known Slater-Pauling curve, but our results do not follow the linearity of the Slater-Pauling curve. We attribute this discrepancy to the weak ferromagnetic nature of bcc Fe. The calculated Fermi contact hyperfine fields follow the trend of the local magnetic moments. The effect of spin-orbit coupling is found not to be significant although it comes into prominence at locations far from the impurity sites.open11sciescopu
First-principles prediction of spin-density-reflection symmetry driven magnetic transition of CsCl-type FeSe
No full textBased on results of density functional theory (DFT) calculations with the local spin density approximation (LSDA) and the generalized gradient approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic (AFM) states over a wide range of lattice constants. At 3.12 angstrom in the GGA, the equilibrium state is found to be AFM with a local Fe magnetic moment of +/- 2.69 mu(B). A metastable FM state with Fe and Se local magnetic moments of 2.00 and -0.032 mu(B), respectively, lies 171.7 meV above the AFM state. Its equilibrium lattice constant is similar to 2% smaller than that of the AFM state, implying that when the system undergoes a phase transition from the AFM state to the FM one, the transition is accompanied by volume contraction. Such an AFM-FM transition is attributed to spin-density z-reflection symmetry; the symmetry driven AFM-FM transition is not altered by spin-orbit coupling. The relative stability of different magnetic phases is discussed in terms of the local density of states. We find that CsCl-type FeSe is mechanically stable, but the magnetic states are expected to be brittle. (C) 2010 Elsevier B.V. All rights reserved.X1156sciescopu
Electronic structure and volume effect on thermoelectric transport in p-type Bi and Sb tellurides
No full textThermoelectric transport properties (Seebeck coefficient, S, and electrical conductivity, sigma) of p-type Bi and Sb tellurides are investigated using a first-principles all-electron density-functional approach. We demonstrate that the carrier concentration, band gap, and lattice constants have an important influence on the temperature behavior of S and that the volume expansion by 5.5% in Sb(2)Te(3) results in an increase in S by 33 mu V/K at 300 K. We argue that in addition to the electronic structure characteristics, the volume also affects the value of S and hence should be considered as an origin of the experimental observations that S can be enhanced by doping Sb(2)Te(3) with Bi (which has a larger ionic size) in Sb sites or by the deposition of thick Bi(2)Te(3) layers alternating with thinner Sb(2)Te(3) layers in a superlattice, Bi(2)Te(3)/Sb(2)Te(3). We show that the optimal carrier concentration for the best power factor of Bi(2)Te(3) and Sb(2)Te(3) is approximately 10(19) cm(-3).open114343sciescopu
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