A STUDY OF THE KINETICS AND THERMODYNAMICS OF LASER IGNITION IN THE COMBUSTION SYNTHESIS OF GROUP IV-B CARBIDES FROM THE ELEMENTS

[摘要] The goal of this work was to develop an understanding of the mechanism of ignition in the combustion synthesis of group IV-B carbides from the elements. To this end kinetic and thermodynamic aspects of the ignition process were studied using a cw laser as the ignition source.In order to study the ignition process, several experimental techniques were incorporated. First, a continuously scanning quadrupole mass spectrometer was used to monitor the vapor species emitted from the laser heated region of the sample during the induction period and ignition. The results of these experiments were compared with results of equilibrium thermochemical calculations to obtain estimates of the degree to which thermodynamic equilibrium was attained during ignition and the effects of impurities on the product composition.Split-frame high speed photography and optical pyrometry were used to obtain kinetic and temperature data on sample heating prior to ignition and during the attainment of self-sustained combustion. From the high speed photography the rate of spread of thermal energy prior to ignition and the propagation of chemical reaction after ignition were measured. From the optical pyrometry data the temperature distribution within the sample was measured prior to ignition and during the attainment of self-sustained combustion. The results of the photography and pyrometry experiments were used to calculate the Arrhenius kinetic parameters associated with the attainment of self-sustained combustion. Also noted from the photography and pyrometry data were unusual ignition and combustion events.The product composition and surface structure were studied using scanning electron microscopy and photomicroscopy, respectively.The results indicated that substantial pre-ignition reaction occurred in the samples. The ignition mechanism seemed to proceed by the following sequence: (1) metal fusion followed by capillary flow through the sample pores thereby wetting the surfaces of the nonmetal particles; (2) nonmetal dissolution into the liquid metal until a critical concentration is reached; (3) product carbide precipitation. Calculations of the enthalpy and entropy of activation from the experimental data suggested step 2 was probably the rate limiting step in the reaction mechanism. In the laser heated region of the sample, continued dissolution of the nonmetal in the liquid product after step 2 was often observed to occur. Thus, the melting point of the product was lowered and a product of variable composition resulted.

[发布机构] Rice University

[效力级别] chemistry