title: Combustion front dynamics in the combustion synthesis of refractory metal carbides and di-borides using time-resolved X-ray diffraction.
abstract: A compact diffraction-reaction chamber, using a 2-inch photodiode array detector, has been employed to investigate the chemical dynamics at the combustion front of a selected series of refractory metal carbides and di-borides from their constituent element reactants as well as binary products from B4C as a reactant. These systems are denoted as (i) M + C --> MC; (ii) M + 2B --> MB2; and (iii) 3M + B4C --> 2MB2 + MC, where M = Ti, Zr, Nb, Hf or Ta. Time-resolved X-ray diffraction using intense synchrotron radiation at frame rates up to 10 frames s(-1) (or 100 ms frame(-1)) was employed. The combustion reactions were found to complete within 200-400 ms. In contrast to the Ta + C --> TaC combustion system studied earlier, in which a discernible intermediate sub-carbide phase was first formed, reacted further and disappeared to yield the final TaC product, no intermediate sub-carbide or sub-boride was detected in the current systems. Combustion for the Ti, Zr and Hf systems involved a liquid phase, in which the adiabatic temperatures Tad are well above the melting points of the respective reactant metals and have a typical combustion front velocity of 5-6 mm s(-1). The Nb and Ta systems have lower Tad, involving no liquid phase. These are truly solid combustion systems and have a lower combustion front velocity of 1-2 mm s(-1). The current study opens up a new avenue to chemical dynamics and macrokinetic investigations of high-temperature solid-state reactions.
authors.affiliation: Lawrence Livermore National Laboratory, University of California, Livermore, CA 94551, USA. exafs_materials@sbcglobal.net
authors.forename: Joe
authors.initials: J
authors.lastname: Wong
authors.forename: E M
authors.initials: EM
authors.lastname: Larson
authors.forename: P A
authors.initials: PA
authors.lastname: Waide
authors.forename: R
authors.initials: R
authors.lastname: Frahm
mesh_terms: D001896:Boron Compounds / Q000032:analysis / Q000737:chemistry*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006358:Hot Temperature; D007700:Kinetics; D008670:Metals / Q000032:analysis / Q000737:chemistry*; D013048:Specimen Handling / Q000295:instrumentation*; D013997:Time Factors; D014159:Transducers; D014961:X-Ray Diffraction / Q000295:instrumentation* / Q000379:methods
chemical_list: D001896:Boron Compounds; D008670:Metals