Thermal Explosion of Powder Mixtures: Numerical Approach

A numerical approach to investigate the heating and the ignition of powder mixtures by radiant energy is presented. The ignition study is based on the possibility of separating the initiation transient from the propagation process, by operating in thermal explosion mode.     

Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi₂, TiSi, Ti₅Si₄, and Ti₅Si₃) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.     

MSR Reduction of Hexachlorobenzene

This paper deals with the mechanochemical degradation of hexachlorobenzene over Li, Na, K, Mg, Ca, Ba, Ti, and W. Apart from the mixtures containing Ti and W, the other systems react in a combustive mode leading to the chlorides and the carbides of the inorganic substrates. Reaction stoichiometries and heats were experimentally determined. The activation time required to ignite the reaction decreases from Li to Mg and correlates with mechanical properties, such as the shear modulus and the mineralogical hardness, of the reducing agents. Physicochemical characteristics and electronic properties of the metallic substrates were then considered. A close correlation was observed when the ignition time of alkaline and alkaline earth metals was plotted as a function of the ionization energy. It was suggested that the reaction can spread out only when a given deformation mixing level and sufficient interfacial area have been reached due to milling. This condition depends on the reducing capability of the metallic substrate and on the mechanical qualities of the reacting blend.     

Conversion of 4-Methylpentan-2-ol over Powder Catalysts Prepared by Ball Milling

Amorphous powders prepared by mechanical alloying were tested as heterogeneous catalysts in the 4-methylpentan-2-ol conversion into its dehydration and dehydrogenation products. The reaction was performed both in a conventional fixed-bed flow microreactor and in a modified milling vial used as a mechanochemical batch reactor. In the thermal catalytic tests several amorphous alloys, CuM (M = Ti, Zr, Hf), PdZr, and CuPdZr were employed as catalysts. Pretreatment conditions and catalytic runs induced different structural evolution in the tested systems. A correspondence was observed between structural modifications and catalytic properties. The mechanochemical trials were performed over a-Cu₄₀Hf₆₀ powders. Conversion and selectivity results were found to depend on the impact energy. The determination of the milling dynamics parameters allowed evaluation of the mechanochemical activity and comparison of, on an absolute basis, the results of the catalytic trials performed by the two techniques.