Suppressing hydrogen evolution by aqueous silicon power dispersions

Abstract

Silicon dispersions in water are used to produce pyrotechnic time delay compositions employed in mine detonators. The delay elements are manufactured by pressing the pyrotechnic composition into aluminium tubes. The automated filling and pressing process requires powders with good free-flow behaviour. Spray drying of water-based slurries is an appropriate method for obtaining such free-flowing granules as it creates almost perfectly spherical particle agglomerates. In addition to the acceptable flow properties, this process provides well-mixed compositions at desired particle size distributions. However, a potential hazard situation arises when water reacts dissociatively with silicon to form SiO2 and hydrogen gas according to Si + 2H2O _ SiO2 + 2H2↑. The propensity of the silicon to react with water and to release hazardous hydrogen gas must thus be suppressed. To this end, the following methods were investigated as a means of diminishing the rate of hydrogen evolution: (i) controlling the slurry pH; (ii) adding organic corrosion inhibitors; (iii) controlled silicon air oxidation before slurrying; and (iv) adding suitable metal ions to provide an additional cathodic reaction to that of water. The effect of organic surface modifications and medium pH on the rate of corrosion of silicon was studied at ambient temperature. It was found that the rate of hydrogen evolution increased with increasing pH. Silanes proved to be more effective silicon corrosion inhibitors than alcohols, with vinyl tris(2-methoxyethoxy) silane producing the best results from the silanes investigated. Differential thermal analysis (DTA) studies were performed using a near-stoichiometric amount of lead chromate as oxidant. Comparable combustion behaviour was observed when both the fuel and the oxidant powders were either uncoated or silane modified. Mixtures of neat oxidant with silane-coated silicon showed poor burn behaviour and this was attributed to poor particle- particle mixing due to the mismatch in surface energies. The controlled silicon air oxidation results showed that the best hydrogen evolution inhibition was attained upon formation of a SiO2 passivating layer at 350 °C. However, Fourier transform infrared (FTIR) data also suggest that some inhibition was imparted below 350 °C and this is due mainly to the removal of silicon surface hydroxyl groups rather than an increase in the SiO2 thickness. DTA studies performed using a nearstoichiometric amount of lead chromate revealed that although heat treatment at higher temperatures provides better passivation; it reduces the reactivity of the silicon in pyrotechnic compositions. The ignition temperature increases while the energy output decreases. Water oxidises silicon via an electrochemical reaction that produces hydrogen gas. The last approach considered in this study was the introduction of a competing cathodic reaction as a means of suppressing the liberation of hydrogen. It was found that the addition of metal ions with a higher reduction potential than hydrogen ions, e.g. copper (II) ions, reduced the amount of hydrogen liberated. In the presence of copper ions the reaction with water featured three distinct stages. During the initial stage, copper is deposited on the silicon and a rapid drop in solution pH is observed. Most of the hydrogen evolved during a second active stage, with the pH showing a slight upward drift. Finally, in the third stage, hydrogen evolution stopped as the silicon surface became passive. The reduction in the total hydrogen evolved was attributed to copper deposits reducing the active surface area available for the oxidation of silicon and to the presence of copper which facilitates accelerated passivation of the uncoated silicon surface. The nature of the anions present affected both the amount of copper deposited on the silicon and the amount of hydrogen released. DTA studies showed that exposure of silicon to copper metal salt solutions also decreases the reactivity of the silicon fuel in pyrotechnic compositions. Copyright