Presentation on theme: "Reaction kinetics and phase segregation in the 3 NiO + 2 Al → 3 Ni + Al 2 O 3 thermite system Dominique Vrel 1,3, Patrick Langlois 1, Ellen M. Heian 2,3,"— Presentation transcript:
Reaction kinetics and phase segregation in the 3 NiO + 2 Al → 3 Ni + Al 2 O 3 thermite system Dominique Vrel 1,3, Patrick Langlois 1, Ellen M. Heian 2,3, Nikhil Karnatak 2,3, Sylvain Dubois 2,3, Marie-France Beaufort 2,3 1- Laboratoire d’Ingénierie des Matériaux et des Hautes Pressions, UPR1311-CNRS, 99 avenue J.-B. Clément, Villetaneuse –France 2- Laboratoire de Métallurgie Physique, UMR6630 CNRS/Université de Poitiers, Bat SP2MI, BP 30179, Bd Marie et Pierre Curie, Futuroscope cedex, France 3- Groupe Français d’Autocombustion, GDR2391SC CNRS, J.-C. Niepce, LRRS, BP 47870, Dijon cedex, France Introduction This reaction has been studied in order to produce liquid nickel, with the idea of using it as a sintering aid for SHS produced carbides The undiluted reactants have proven to be too exothermic for the reaction to be analyzed without diluents as it might yields to products at temperature close to their boiling point, which would be disastrous for the sample's densification Alumina was chosen as a diluent in order to avoid any contamination with by-products, as it already was one of the reaction's product. This study is part of a broader research on SHS products densification; results with a different stoichiometry should be published shortly Preparation of the samples Commercial powders thoroughly mixed in a Turbula homogenizer (24 h, 96 rpm) Diluent contents chosen as a function of total heat capacity 1 cm samples, 1.6 cm long, truncated diameter of 1 cm (flat surface for proper TRXRD analysis) Infrared analysis Temperature evolution along a line across the sample 0% diluent: 3.2 cm/s, intensive sputtering 20% diluent: 1 cm/s, mo1ten products 40% diluent: 0.23 cm/s, solid state products 50% diluent: unstable propagation Time Resolved X Ray Diffraction LURE H10 beamline, = nm, 25 diffraction patterns per second no distinct preheating (Al and alumina diffraction lines disappear simultaneously) no diffraction lines for about 1s (temperature exceeds 2030°C) nickel aluminides not present in end product may be present as transient phases but always very weak diffraction lines progressive right shift after reaction sample cools down and lattice parameter shrinks SEM observations and EDX maps 20% diluent nearly total segregation evidence of alumina melting porosity partially due to bubble formation sample lost its initial shape 40% diluent local segregation only evidence of Ni melting if there was alumina melting, it was not complete "normal" SHS-type residual porosity sample kept its initial shape with slight shrinking Conclusions diluent is very effective to control the reaction exothermicity reduced exothermicity induces the synthesis of (at least partially) solid state products nickel is always liquid with a diluent content of up to 40% No transient phase can clearly be identified; main products always are according to the desired stoichiometry with high diluent contents, reaction might be locally not complete with 50% diluent, reaction is unstable and rapidly comes to extinction Acknowledgments The authors would like to express their gratitude to M. Gailhanou and D. Thiaudière, from the LURE facility, for their help during the TRXRD experiments. Ellen M. Heian and Nikhil Karnatak wish to thank, respectively, the French Ministry of Research (Direction de la Recherche) and the Région Poitou-Charentes for financial support.