Ignition properties of metal powders depend strongly on particle concentration, owing to cooperative effects that have not yet been analytically treated. Further complications arise when the metal is finely subdivided. However, difficulties have been encountered with metals that form protective layers, particularly if metal and oxide are mutually soluble. This approach has been reasonably successful with metals that do not form a protective oxide layer. The ignition temperatures of metals have been linked with their low-temperature oxidation propenties. Ignition of metals is thought to be always more ยป preceded by slow oxidation that takes place on the metal surface. The presence of condensed-phase products also implies the predominant participation of heterogeneous reactions. Lack of reliable thermodynamic data required for equilibrium computations is one cause of difficulties encountered in the field. One consequence of this characteristic of metal flames is the upper limit on flame temperature imposed by the equilibrium between condensed-phase oxide and its vaporization and decomposition products. Primarily responsibie for the current lack of understanding is the formatlon of condensed-phase products. Compared with the present knowledge of combustion of conventionsl fuels, the burning of metals is as yet poorly understood.
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