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Melting of Ni nanowires with and without oxide capping / Z.F. Zhou in Acta materialia, Vol. 58 N° 8 (Mai 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 8 (Mai 2010) . - pp. 3059–3067 Titre : Melting of Ni nanowires with and without oxide capping Type de document : texte imprimé Auteurs : Z.F. Zhou, Auteur ; Y.C. Zhou, Auteur ; Y. Pan, Auteur Année de publication : 2011 Article en page(s) : pp. 3059–3067 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Nickel  Nanocrystalline  materials  Melting  point  Surface  Grain  boundaries Résumé : The effects of wire diameter and grain size on the melting point of Ni nanowires with and without oxide capping were systematically examined. It was found that, for uncapped Ni nanowires, the melting point drops monotonically with grain size and also with the nanowire diameter; for oxide-capped Ni nanowires, the melting point rises when the nanowire diameter is decreased. Findings indicate that: (i) the reduced cohesive energy of under-coordinated atoms at the surface skin or grain boundaries and the fraction of such under-coordinated atoms dominate the undercooling of pure Ni nanowires; (ii) bond nature alteration by oxidation strengthens the surface bonds and hence results in the observed overheating; (iii) the bonds in the core interior retain their bulk nature, making contribution to neither the observed undercooling nor overheating. All these findings are very consistent with the bond–order–length–strength theoretical predictions. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410000546 [article] Melting of Ni nanowires with and without oxide capping [texte imprimé] / Z.F. Zhou, Auteur ; Y.C. Zhou, Auteur ; Y. Pan, Auteur . - 2011 . - pp. 3059–3067. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 8 (Mai 2010) . - pp. 3059–3067 Mots-clés : Nickel  Nanocrystalline  materials  Melting  point  Surface  Grain  boundaries Résumé : The effects of wire diameter and grain size on the melting point of Ni nanowires with and without oxide capping were systematically examined. It was found that, for uncapped Ni nanowires, the melting point drops monotonically with grain size and also with the nanowire diameter; for oxide-capped Ni nanowires, the melting point rises when the nanowire diameter is decreased. Findings indicate that: (i) the reduced cohesive energy of under-coordinated atoms at the surface skin or grain boundaries and the fraction of such under-coordinated atoms dominate the undercooling of pure Ni nanowires; (ii) bond nature alteration by oxidation strengthens the surface bonds and hence results in the observed overheating; (iii) the bonds in the core interior retain their bulk nature, making contribution to neither the observed undercooling nor overheating. All these findings are very consistent with the bond–order–length–strength theoretical predictions. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410000546

The effect of oxygen on transitional Marangoni flow in laser spot welding / C.X. Zhao in Acta materialia, Vol. 58 N° 19 (Novembre 2010) 

Public ISBD [article]  in Acta materialia > Vol. 58 N° 19 (Novembre 2010) . - pp. 6345–6357 Titre : The effect of oxygen on transitional Marangoni flow in laser spot welding Type de document : texte imprimé Auteurs : C.X. Zhao, Auteur ; C. Kwakernaak, Auteur ; Y. Pan, Auteur Année de publication : 2011 Article en page(s) : pp. 6345–6357 Note générale : Métallurgie Langues : Anglais (eng) Mots-clés : Weld  pool  Instability  Flow  motions  Laser  spot  welding  Visualization Résumé : Owing to its significant influence in heat and mass transfer, liquid metal flow during stationary laser welding has been studied for different concentrations of oxygen in the surrounding environment. Two predominant types of surface flow motion are observed: an inward flow, corresponding to a positive surface tension temperature gradient, and an outward flow, corresponding to a negative gradient. Time-dependent changes in oxygen concentration at the surface are observed to flip the surface tension temperature gradient from negative to positive under appropriate shielding conditions. The oxygen concentration affects not only the flow motion, but also the laser absorption coefficient, which increases with increasing environmental oxygen concentration. It is shown here that the evolution of dissolved oxygen in the weld pool as a function of temperature can have a profound influence on the fluid flow and hence on energy transport. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410005008 [article] The effect of oxygen on transitional Marangoni flow in laser spot welding [texte imprimé] / C.X. Zhao, Auteur ; C. Kwakernaak, Auteur ; Y. Pan, Auteur . - 2011 . - pp. 6345–6357. Métallurgie Langues : Anglais (eng) in Acta materialia > Vol. 58 N° 19 (Novembre 2010) . - pp. 6345–6357 Mots-clés : Weld  pool  Instability  Flow  motions  Laser  spot  welding  Visualization Résumé : Owing to its significant influence in heat and mass transfer, liquid metal flow during stationary laser welding has been studied for different concentrations of oxygen in the surrounding environment. Two predominant types of surface flow motion are observed: an inward flow, corresponding to a positive surface tension temperature gradient, and an outward flow, corresponding to a negative gradient. Time-dependent changes in oxygen concentration at the surface are observed to flip the surface tension temperature gradient from negative to positive under appropriate shielding conditions. The oxygen concentration affects not only the flow motion, but also the laser absorption coefficient, which increases with increasing environmental oxygen concentration. It is shown here that the evolution of dissolved oxygen in the weld pool as a function of temperature can have a profound influence on the fluid flow and hence on energy transport. DEWEY : 669 ISSN : 1359-6454 En ligne : http://www.sciencedirect.com/science/article/pii/S1359645410005008