Ultra-high Speed Neutron Diffraction Studies of Combustion Synthesis

Full text: Combustion synthesis is the process whereby a self-sustaining reaction rapidly converts solid starting materials into the desired product. It has great potential for the sustainable and economic synthesis of advanced materials because the heat of formation of the product provides the bulk of the energy requirement. Since the combustion temperature of up to 3000K may be achieved on a sub-second time frame, refinement and control of the process is very difficult and time-consuming using standard laboratory equipment. Very high speed in-situ diffraction studies are the only way in which actual reaction pathways and kinetics can be revealed. In-situ neutron diffraction at time resolutions in the range 380-900 milliseconds recorded on the instrument D20 at the Institut Laue-Langevin have been used to capture the reaction mechanisms during the furnace ignited combustion synthesis of a range of MnAXn-1 phases (M-early transition metal, A-group IIIA or IVA element, X-either C or N), intermetallic compounds and intermetallic compound-refractory carbide composites. The diffraction patterns reveal a hitherto unsuspected complexity. Some reactions occur completely in the solid state and others involve a transient liquid phase(s). In all but binary mixtures, transient solid phases that may persist for as little as a few seconds are observed. The data and subsequent Rietveld refinements allow some/all of the following to be determined: i) The sequence of phases and reaction mechanism ii) The crystal structure, atomic substitutions etc of the phases present iii) The 'trigger' mechanism for the self-sustaining reaction iv) The reaction kinetics (via quantitative phase analysis using the scale factors) v) The activation energy for growth of the product phase (via a non-isothermal Avrami kinetic equation) vi) The average sample temperature (via thermal expansion/contraction of known phases) vii) The formation enthalpy of the product phase (via excursions in the sample temperature determined at (v) above). The complexity of the problem and power of the method will be illustrated with examples from each of the types of system studied. (authors)