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International Journal of Energetic Materials and Chemical Propulsion
ESCI SJR: 0.28 SNIP: 0.421 CiteScore™: 0.9

ISSN Print: 2150-766X
ISSN Online: 2150-7678

International Journal of Energetic Materials and Chemical Propulsion

DOI: 10.1615/IntJEnergeticMaterialsChemProp.v2.i1-6.50
pages 113-132

RECENT STUDIES OF THE KINETICS OF SOLID BORON GASIFICATION BY B2O3(g) AND THEIR CHEMICAL PROPULSION IMPLICATIONS

Alessandro Gomez
Yale University
Daniel E. Rosner
High Temperature Chemical Reaction Engineering Laboratory Yale University, New Haven CT 06520-2159 Y.S.
Roni Zvuloni
High Temperature Chemical Reaction Engineering Laboratory Yale University, New Haven CT 06520-2159 Y.S.

ABSTRACT

Interfacial kinetics are likely to play a rate-limiting role in the combustion of small boron particles and yet little is known about boron gasification by B2O3(g), which may be important because of its formation from O2 in the boundary layer near the gasifying surface. For these reasons, intrinsic kinetics of the high temperature B2O3(g)/B(s) reaction were investigated utilizing newly designed flow reactor techniques, together with a 'real-time' (boron) element detection technique based on microwave-induced plasma emission spectroscopy. Known amounts of the gaseous reactant B2O3 were generated using a resistively heated Knudsen effusion source operating in a high velocity argon gas background. Reaction rate measurements on a joule-heated boron filament covered the surface temperature interval 1330-2050 K at BB2O3 partial pressures between 6·10−3 and 6·10−2 Pa. Results revealed remarkably high reaction probabilities over a broad temperature range (ca. 1400-2000 K) with a maximum close to unity, at ca. 1950 K, much higher than that for the O2(g)/B(s) reaction and comparable to that previously observed (locally) for O-atom attack of boron. Transition conditions for 'passivation' of the B2O3(g)/B(s) reaction were identified experimentally and found to be in qualitative agreement with quasi-equilibrium model predictions, which also indicate that the dominant product species in the "active" regime were (BO)2 over the intermediate temperature regime, ca. 1050-1500 K, and BO in the high temperature regime, ca. 1500-2200 K. Mechanistic implications of these results include a high sticking (O-atom deposition) probability for B2O3 on solid boron, but with a fall-off above ca. 2000 K, causing e to drop precipitously despite the stability of BO(g) at these temperatures. These kinetic data are used to discuss the expected sequence of rate-controlling processes for the combustion of individual B(s) particles in air under typical ramjet conditions. While most previous boron particle combustion and extinction laboratory experiments have been performed in the regime of gas-phase diffusion control, under conditions of actual ramjet interest the gas/solid kinetics for the efficient B2O3(g)/B(s) reaction and the slower O2(g)/B(s) reaction, as well as non-continuum transport effects, become rate-limiting.


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