Reaction kinetics and phase segregation in the 3 NiO + 2 Al → 3 Ni + Al2O3 thermite system

Dominique Vrel1,3, Patrick Langlois1, Ellen M. Heian2,3, Nikhil Karnatak2,3, Sylvain Dubois2,3, Marie-France Beaufort2,3

1- Laboratoire d’Ingénierie des Matériaux et des Hautes Pressions, UPR1311-CNRS, 99 avenue J.-B. Clément, 93430 Villetaneuse –France2- Laboratoire de Métallurgie Physique, UMR6630 CNRS/Université de Poitiers, Bat SP2MI, BP 30179, Bd Marie et Pierre Curie, 86962 Futuroscope cedex, France

3- Groupe Français d’Autocombustion, GDR2391SC CNRS, J.-C. Niepce, LRRS, BP 47870, 21078 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 sputtering20% diluent: 1 cm/s, mo1ten products40% diluent: 0.23 cm/s, solid state products 50% diluent: unstable propagation

Time Resolved X Ray Diffraction

LURE H10 beamline, = 0.173561 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 phasesbut 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 dueto 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.