Elsevier

Combustion and Flame

Volume 215, May 2020, Pages 78-85
Combustion and Flame

Effect of boron content in B·BiF3 and B·Bi composites on their ignition and combustion

https://doi.org/10.1016/j.combustflame.2020.01.026Get rights and content

Abstract

Composite powders combining boron with BiF3 and Bi in different amounts were prepared by high energy milling. Thermal analysis in an argon-oxygen mixture showed significant oxidation starting about 200 K lower than for pure boron. Selective oxidation of metallic Bi at low temperatures was observed. Composites containing either Bi or BiF3 ignited more readily than pure boron when heated by a CO2 laser beam. The composites containing BiF3 ignited more readily than boron when in contact with a hot wire. Burn times of particles aerosolized in air and ignited using the CO2 laser were measured as durations of the recorded emission pulses produced by burning particles. Statistical distributions of the measured burn times were correlated with the respective powder's particle size distributions. Compared to elemental boron, burn times of all prepared composites were shorter, including those containing only 10 wt.% of BiF3 or ca. 8 wt.% of Bi, and for most composites combustion temperatures were higher.

Introduction

Boron burning in oxygen releases more energy volumetrically than any other metal, making it an attractive fuel for ramjets and other applications [1,2]. Boron also has high gravimetric energy density [3]. However, it suffers from several drawbacks that limit practical applications. It typically ignites at a relatively high temperature of over ~1400 K [4,5]. Boron particles have long burn times and experience characteristic two-stage heterogeneous combustion [6], [7], [8], [9]. Like most metals, the predicted energy release of boron occurs after combustion products condense. However, condensed combustion products may not be desirable for many applications, leading to two-phase losses of specific impulse, abrasive damage to nozzles, etc. Recently, inclusion of fluoride oxidizers was found to improve the ignition [10,11] and combustion [12] properties of metals in aerobic conditions. In particle combustion studies of B·BiF3 and B·CoF2 composites (both with 50 wt.% boron), the composite particles burned faster in air than boron of comparable sizes.

However, the addition of a sizable mass fraction of oxidizers reduces the energy density of the fuel. To maintain thermodynamic appeal of boron combustion, the amount of fluoride additive should be minimized. In the current work, the amount of the fluoride oxidizer, BiF3 is varied systematically to consider the effect the B/BiF3 ratio on combustion of the composite material. Metallic Bi, a decomposition product of BiF3, is speculated to have a catalytic effect on B combustion, similar to the catalytic effect of transition metals [13], [14], [15]. To investigate this effect, composites with Bi were also prepared and characterized.

Section snippets

Material preparation

Fuel-rich boron-bismuth fluoride composites (referred henceforth as B·BiF3) and boron-bismuth composites (referred henceforth as B·Bi) were prepared by arrested reactive milling (ARM) [16] in a SPEX CertiPrep 8000 series shaker mill. Starting materials were B (SB95, SB Boron, 95%), BiF3 (Alfa Aesar, 99%), and Bi (−325 mesh, Alfa Aesar, 99.5%). Each hardened steel milling vial contained 5 g of powder. Vials were loaded under argon to prevent oxidation during milling. A small amount of hexane, ~

Particle morphology and composition

Backscattered electron images of the composites are shown in Fig. 1. Boron appears medium gray, and the additives, BiF3 or Bi, are bright. The carbon mounting tape appears as dark background. For all materials, particles generally retain the morphology of boron, comprising aggregates of boron primary particles. Both BiF3 and Bi are homogeneously distributed within the boron aggregates on a scale of several µm. On a finer scale, the additives are observed to decorate the surface of the boron

Thermally activated reactions

As seen in Fig. 2, the oxidation reaction observable by thermal analysis in all prepared composites occurs about 200 K below that observed for pure boron [17]. Qualitatively, all B·BiF3 composites exhibit similar oxidative behavior suggesting that only a small amount of BiF3 is sufficient to alter the mechanism of boron oxidation. As found earlier for such composites with 50 wt.% BiF3 [10], the presence of a metal fluoride accelerates decomposition of boric acid by a low-temperature

Conclusions

B·BiF3 and B·Bi composites were prepared by mechanical milling, ignited and burned in air. The presence of BiF3 or Bi reduces the temperature of stepwise oxidation of boron heated in an oxygenated environment. The effect of BiF3 is assigned to fluorination involving boric acid. Furthermore, formation of volatile BF3 and BOF on particle surface effectively heats the particle accelerating ignition. The effect of Bi is assigned to its selective oxidation and subsequent reduction by boron, making

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Office of Naval Research (ONR), Grant N00014-19-1-2048. Support and interest from Dr. Chad Stoltz of ONR are greatly appreciated.

References (28)

  • W. Zhou et al.

    Effect of fluorine on the combustion of “clean” surface boron particles

    Combust. Flame

    (1998)
  • A. Gany et al.

    Combustion studies of metallized fuels for solid-fuel ramjets

    J. Propul. Power

    (1986)
  • A. Maček et al.

    Combustion of boron particles at atmospheric pressure

    Combust. Sci. Technol.

    (1969)
  • B. Natan et al.

    Ignition and combustion of boron particles in the flowfield of a solid fuel ramjet

    J. Propul. Power

    (1991)
  • Cited by (30)

    View all citing articles on Scopus
    View full text