CC BY
88iSHS 2019
Moscow, Russia
NEW MATERIALS PREPARED BY ARRESTED REACTIVE MILLING AND MECHANISMS OF THEIR IGNITION AND COMBUSTION
E. L. Dreizin*"^, M. Schoenitz", K. L. Chintersingh", M. Mursalat",
S. K. Valluri", and D. Hastings"
aNew Jersey Institute of Technology, Newark, NJ, 07102 USA bTomsk State University, Tomsk, 634050 Russia
*e-mail: dreizin@njit.edu
DOI: 10.24411/9999-0014A-2019-10033
Arrested reactive milling (ARM) uses high-energy ball milling to prepare nanocomposite powders of reactive materials [1]. Starting components capable of highly exothermic reactions are combined in each powder particle; the particle sizes are typically varied in the range of 1-100 |im. Metal-metal, metal-metalloid, and metal-metal oxide composites were prepared previously and have demonstrated attractive ignition and combustion characteristics [2]. This talk will discuss several new materials prepared by ARM.
Previously, it was shown that mechanically alloyed AlTi powders burn faster than pure Al [3]; however, it was difficult to adjust the particle sizes of the milled powders to match those of a fine aluminum powder commonly used in energetic formulations. In this work, aluminum-rich AlTi composite powders with controllable particle size distributions were prepared by staged mechanical milling using liquid polar and nonpolar organic process control agents (PCAs). In the first milling stage, when a non-polar liquid, hexane, served as PCA, a composite powder was formed. The powder particle sizes were reduced in the second stage, using acetonitrile, a polar PCA. The prepared finely dispersed particles had lower onset temperatures for intermetallic formation reactions and lower ignition temperatures compared with the powder prepared in one stage using hexane as PCA. Combustion rates for all composites were greater than for reference aluminum, while net energy release remained comparable. The results suggest that staged milling with PCAs of different polarity is a viable method to control particle size distributions and ignition behavior in mechanically milled, metal-based reactive composite powders while maintaining the energy content. Particle size control is critical for adopting such materials in practical systems.
It was found recently that high energy milling can modify boron powders to improve their combustion characteristics. In particular, boron doped with iron was prepared and shown to have improved ignition kinetics and burn rates [4, 5]. Similar boron-based powders doped with other metals, including Co, Ni, Zr, and Hf, were prepared. Additionally, boron doped with iron prepared by ARM was compared to a similar compound, where iron coating was applied to boron powders using precipitation of iron pentacarbonyl, Fe(CO)5. For all prepared materials, thermo-analytical measurements were performed to quantify their oxidation behavior. Ignition temperatures were measured using a heated filament ignition experiment and particles were burned in air and in the products of a hydrocarbon flame. The results describing oxidation, ignition, and combustion of these materials will be presented and discussed. It is observed that iron is the most attractive dopant improving overall ignition and combustion characteristics of boron.
The most common application of ARM was to prepare thermite compositions, such as AlMoO3 [6-8], AlCuO [9-11], etc. Recently, analogous reactive materials were prepared where instead of metal oxides, metal fluorides served as oxidizers for metals or metalloids [12, 13]. Both Al- and B-based composites with CoF2, BiF3, and NiF2 as oxidizers were prepared, and their ignition and combustion behaviors were characterized. It was observed
XV International Symposium on Self-Propagating High-Temperature Synthesis
that composites containing metal fluoride oxidizers were readily ignited thermally; however, unlike similar thermites, they were insensitive to initiation by electrostatic discharge. This could be due to a high ionic conductivity of the fluorides, reducing the heat release caused by the discharge's current. In combustion, metal fluoride containing reactive powders release substantial amounts of gas products, which is expected to be beneficial for propellants and explosives. The burn rates of the prepared aluminum and boron-based composites in air are enhanced compared to those of elemental aluminum and boron powders, respectively. In the flames with CO, CO2, and H2O as main oxidizing species, the burn rates of the metal-metal fluoride composites are comparable to those of elemental metals.
Finally, new spherical powders were prepared by ARM using thermites and other reactive material compositions when milling was performed in presence of two immiscible fluids serving as PCA. Similar spherical powders were prepared milling individual powders of elemental metals and metal oxides. Spherical powders with dimensions varied from tens to hundreds of |im were obtained with AlCuO thermites as well as with pure Al, B, Ti, AlB composites and with other materials. The spherical powder particles were filled to a relatively high density, although certain porosity remained. In experiments, it was observed that the sizes of formed spheres decreased at longer milling times. The filling density of the spherical powder particles increased respectively. Although the mechanism of formation of these spherical powders is not understood, it is hypothesized that such powders were formed from Pickering emulsions produced by the PCA components interacting with the suspended powder. Spherical powders formed only for specific milling conditions and after certain milling times. Enhanced flowability of spherical powders makes them easy to handle, mix with other components, and attractive for additive manufacturing.
For all prepared powders, a suite of experimental techniques illustrated in Fig. 1 is used to characterize their behavior. These techniques are supplemented by detailed thermo-analytical measurements and by characterizing particle sizes and shapes for the prepared powders.
prepared by ARM: A. Powder ignition using an electrically heated filament; B. Powder ignition by electrostatic discharge; C. Powder ignition by a CO2 laser beam and combustion in air; D. Powder combustion in products of a gaseous flame; E. Constant volume explosion experiment.
ISHS 2019 Moscow, Russia
1. E.L. Dreizin, M. Schoenitz, Nano-composite energetic powders prepared by arrested reactive milling, US Patent 7.524.355, 2009.
2. E.L. Dreizin, M. Schoenitz, Mechanochemically prepared reactive and energetic materials: a review, J. Mater. Sci, 2017, vol. 52, pp. 11789-11809.
3. Y. Shoshin, E.L. Dreizin, Laminar lifted flame speed measurements for aerosols of metals and mechanical alloys, AIAA Journal, 2004, vol. 42, pp. 1416-1426.
4. K.L. Chintersingh, M. Schoenitz, E.L. Dreizin, Combustion of boron and boron-iron composite particles in different oxidizers, Combust. Flame, 2018, vol. 192, pp. 44-58.
5. K.L. Chintersingh, M. Schoenitz, E.L. Dreizin, Boron doped with iron: Preparation and combustion in air, Combust. Flame, 2019, pp. 286-295.
6. D. Stamatis, E.L. Dreizin, K. Higa, Thermal initiation of Al-MoO3 nanocomposite materials prepared by different methods, J. Propul. Power, 2011, vol. 27, pp. 1079-1087.
7. S.M. Umbrajkar, S. Seshadri, M. Schoenitz, V.K. Hoffmann, E.L. Dreizin, Aluminum-rich Al-MoO3 nanocomposite powders prepared by arrested reactive milling, J. Propul. Power,
2008, vol. 24, pp. 192-198.
8. R.A. Williams, M. Schoenitz, A. Ermoline, E.L. Dreizin, Low-temperature exothermic reactions in fully-dense Al/MoO3 nanocomposite powders, Thermochim. Acta, 2014, vol. 594, pp. 1-10.
9. A. Ermoline, D. Stamatis, E.L. Dreizin, Low-temperature exothermic reactions in fully dense Al-CuO nanocomposite powders, Thermochim. Acta, 2012, vol. 527, pp. 52-58.
10. D. Stamatis, Z. Jiang, V.K. Hoffmann, M. Schoenitz, E.L. Dreizin, Fully dense, aluminum-rich Al-CuO nanocomposite powders for energetic formulations, Combust. Sci. Technol.,
2009, vol. 181, pp. 97-116.
11. S.M. Umbrajkar, M. Schoenitz, E.L. Dreizin, Exothermic reactions in Al-CuO nanocomposites, Thermochim. Acta, 2006, vol. 451, pp. 34-43.
12. S.K. Valluri, I. Monk, M. Schoenitz, E. Dreizin, Fuel-rich aluminum-metal fluoride thermites, Int. J. Energ. Mater. Chem. Propul., 2017, vol. 16, pp. 81-101.
13. S.K. Valluri, M. Schoenitz, E.L. Dreizin, Boron-Metal fluoride reactive composites, MRS Fall Meeting, MRS, Boston, MA, 2017.
