Transition Metal Catalysts for Boron Ignition and Combustion

Boron is thermodynamically advantageous as a fuel for explosives and propellants because of its high volumetric and gravimetric heating values. However, there are limitations with its practical use due to long ignition delays because of the inhibiting oxide layer and long combustion times. Previous studies indicate that boron particles burn at temperatures below its boiling point, and combustion is limited by heterogeneous surface reactions with air. One proposed solution to improve the reaction rate without compromising energy density is to include transition metal (TM) additives that can accelerate surface oxidation by different mechanisms. Specifically, Bi leads to favorable redox reactions; Fe, Co, and Ni act as oxygen shuttle catalysts; and Zr and Hf lead to exothermic formation of respective borides. Boron composites were prepared by combining 95% pure commercial boron powders with 5 wt% of a selected TM additive powders (Ni, Co, Fe, Bi, Zr, or Hf) using a planetary mill and 20 mL of hexane as the process control agent. The samples were milled for 4 hours with the ball-to-powder (BPR) mass ratio of 10 and at a rotation speed of 350 RPM. Scanning electron microscopy was used to examine the powder morphology and elemental distribution on the particle surface. The samples were also characterized using low-angle laser scattering to obtain particle size distributions. Low-temperature thermogravimetric (TG) measurements in O₂/Ar environment up to 700 C were used to obtain oxidation kinetics of the prepared samples and to examine the role of the additives and the effect of milling on oxidation of boron. TG results show lower-temperature oxidation onset for all milled samples compared to starting boron. Samples containing Bi and Co have the greatest and fastest mass gain. To explore the effect of TM additives on ignition and combustion, the prepared powders were aerosolized, and single particles were ignited in room air using a 125-W CO₂ laser. The optical emissions of the burning particles were captured using photomultiplier tubes filtered at 700 and 800 nm to determine combustion temperatures and burn times as a function of the particle sizes. The kinetic model determined from low-temperature TG oxidation studies was used to estimate the particle combustion temperatures in air. The prediction was in agreement with experiment only for neat boron and B-Fe powders that burned at about 2500-3500 K. Selected samples of commercial boron, milled boron, and boron milled with Fe, Co, and Bi additives were also hand blended with 76 wt% potassium nitrate, KNO₃ to form pyrotechnic mixtures for larger scale ignition and combustion tests in room air. Approximately 10 mg of the B/KNO₃ powder samples were packed into an aluminum cavity with a 4.85 mm diameter and 1.41 mm depth. The powders were ignited using a 10-ms CO₂ laser pulse. The combustion event was recorded using a photodiode sensitive to emission with the range of wavelengths of 350-1100 nm, and using a high-speed monochromatic video camera equipped with a 0.7 – 4.5x Zoom Monocular lens. In addition a compact UV-VIS-NIR spectrometer (190-850 nm) set at 1000 ms integration time was used. The experiments determined the ignition delays, plume propagation rates, overall combustion times, and temperatures, for the ignited samples. Preliminary experimental data showed shorter ignition delays for B-Co composite, whereas B-Bi samples demonstrated shorter combustion times and faster plume propagation rates than other composites. Time-resolved spectra of the combustion events were processed assuming grey body emission and resulted in combustion temperatures above the boiling point of B₂O₃ and the melting point of boron. Commercial boron powders showed characteristic BO₂emission peaks and burned at ~3000 K, while some samples like B-Co showed no clear BO₂ emissions and burned at lower temperatures of 2400 K, indicating changes in the reaction mechanisms due to different TM additives.