Laser ignition of aluminum and boron based powder systems

Powders of various metals and boron are widely used in mixed fuel compositions to increase the combustion temperature and specific impulse of rocket engines. The article presents the results of an experimental study of the oxidation and ignition in air of ultrafine aluminum powders $\mathrm{Alex}$, amorphous boron and microsized aluminum powders $\mu\mathrm{Al}$, aluminum borides $\mathrm{AlB}_2$ and $\mathrm{AlB}_{12}$. Metal and boron powders were heated and ignited by a cw $\mathrm{CO}_2$ laser in the heat flux density range $65$–$190$ W/cm$^2$. Based on thermal analysis data, it was found that the powder reactivity parameters are arranged in the following sequence (in descending order of activity): $\mathrm{Alex}\to\mathrm{B}\to\mathrm{AlB}_{12}\to\mathrm{AlB}_2\to\mu\mathrm{Al}$. During the oxidation of amorphous boron and aluminum dodecaboride $\mathrm{AlB}_{12}$, the total specific heat release and the rate of mass change have maximum values. $\mathrm{Alex}$, boron and $\mathrm{AlB}_{12}$ powders ignite more easily in air under the action of an external radiant source. Power exponent $n$ as a function of the ignition delay time $t_{ign}$ on the heat flux density $t_{ign}(q)=Aq^{-n}$ for $\mu\mathrm{Al}$ powders, $\mathrm{AlB}_2$ and $\mathrm{AlB}_{12}$ are approximately the same and equal to $\approx2.0$, for ultrafine $\mathrm{Alex}$ and boron powders it is lower and amounts to $n=1.5$ and $1.0$, respectively.