Article pubs.acs.org/EF
Boron as Fuel for Ceramic Thermites Marc Comet,* Fabien Schnell, Vincent Pichot, Julien Mory, Benedikt Risse, and Denis Spitzer NS3E, UMR 3208 CNRS/ISL/UdS, French-German Research Institute of Saint-Louis (ISL), BP 70034, 68301 Saint-Louis Cedex, France ABSTRACT: Boron submicrometer particles (22.3 m2/g) were used as fuel to prepare energetic compositions with bismuth(III) oxide (2.7 m2/g) and copper(II) oxide (10.7 m2/g), leading to unconventional submicrometer-thermites being made solely of ceramic compounds. The morphology of the mixtures was studied according to their boron content: (i) by comparing their calculated apparent density with their experimental one and (ii) by a photometric technique based on the analysis of the gray levels of the samples. The analysis of the early step of the reaction, corresponding to the preignition exotherm observed in DSC experiments, has shown that boron is first reduced by Bi2O3 leading to a molten bismuth layer at the surface of boron particles. The dissolution of Bi2O3 in this metallic coating favors the reaction. Conversely, the B2O3 formed by boron oxidation further reacts with Bi2O3, forming a glass like layer that limits the diffusion of Bi2O3. The reaction is decelerated by this phenomenon and requires a higher temperature to reach completion (second exotherm). This mechanism accounts for the incomplete oxidation of boron in Bi2O3/B compositions. The nature of the crystallized phases present in the combustion residues was identified by X-ray diffraction (XRD) and correlated to the evolution of combustion heat measured by calorimetry. For both compositions (Bi2O3/B and CuO/B), the combustion heat remains at a high level over a wide composition range. This result was explained by the formation of different boron oxides (B2O3, B7O). Boron-based thermites are relatively insensitive to friction and impact but possess extremely low sensitivity levels to electrostatic discharge and ignite quite easily in contact with an open flame. The investigation of the ejection rate and the reactive power has shown that boron-based nanothermites are 2 orders of magnitude less reactive than their aluminum counterparts. Because of their moderate reactivity, boron-based submicrometer thermites are promising candidate materials for ignition devices and for specific propulsion applications.
1. INTRODUCTION The reduction of metallic oxides by aluminum was demonstrated as early as 1865 by the Russian chemist Beketov. Experimental details on the preparation, the ignition, and the use of “thermit” compositions for welding metallic pieces were reported by Goldschmidt in several patents.1,2 Interestingly, Goldschmidt’s research was mainly focused on the use of metal fuels like aluminum, magnesium, and calcium for reducing metallic oxides or sulfides. The traditional sense of the word “thermite” originates from this pioneering work and has remained unchanged for one century. Recent research dealing with the preparation of thermites from submicrometer-sized particles leads to extend the classical meaning of this term to pyrotechnic mixtures containing significant amounts of metal-based compounds as reactive species. For instance, the energetic compositions prepared from metallic salts such as potassium permanganate enclosed in iron oxide3 or silver iodate4 can be considered as thermites. The term “thermite” also includes combustible substances made from a metallic oxide mixed with a nonmetal, such as red phosphorus.5 From a chemical standpoint, boron is a ceramic material and belongs to the metalloid family. The high oxidation heat of boron (≈58 kJ/g) predestines its use in pyrotechnic compositions. Binary compositions with potassium nitrate, which are often called BNP in military reports, have strong combusting properties that can be used in optopyrotechnic ignition systems.6 Their relative insensitivity to mechanical stress originates from the superficial layer of boric acid covering the boron particles, limiting the interactions between the © XXXX American Chemical Society
reactive phases B/KNO3 and making the ignition difficult to perform. An in-depth analysis of the research published on nanothermites has shown that most of the metastable interstitial composites (MICs)7 are prepared using aluminum nanoparticles as fuel. Although the few boron-based compositions mentioned by Grubelich et al. have promising thermochemical properties,8 their experimental properties have been little studied in open literature. The relatively low toxicity of boron oxides accounts for the growing interest in the use of boron as fuel in energetic compositions. For instance, Sabatini et al.9 have recently used mixtures of micrometer-sized amorphous boron (B, 16.82 μm) and boron carbide (B4C, 7.80 μm) with potassium nitrate to prepare barium-free green light emitters. The burn time and the luminous intensity can be tuned through the B/B4C ratio, and the characteristics of these new compositions outperform those of traditional bariumbased pyrotechnics. Nakamura et al.10 have previously prepared and characterized mixtures from boron and cupric oxide particles with typical micrometric sizes, with low calculated surface area approximating 2.5 m2/g and 0.14−1.9 m2/g for B and CuO powders, respectively. The predominance of aluminum is not only due to the wide availability of commercial nanopowders prepared by plasma explosion,11 evaporation,12 or Temiset process13 but also to the outstanding performances of MICs prepared from this metal.14 Received: January 22, 2014 Revised: March 25, 2014
A
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The comparison of the Dth./BBT value with the apparent density, determined experimentally by weighing of a given volume of the composition (Dexp./BBT), was used to elucidate the evolution of the morphology of BBT compositions according to their boron content. The photometric analysis of the BBT color according to their boron content was carried out by photographs of pellets, in a studio equipped with an illuminating device releasing a light with stable intensity. The BBT discs with a diameter of 4 mm prepared by the compression of powders have a smooth surface, limiting local shadow effects that may affect the accuracy of the measurement in loose powders. The variation of the greyscale gives valuable information on the way the oxide and the boron particles are arranged in the BBT compositions. The comparison of the values measured by this technique requires to work under the same experimental conditions, that is, camera, lens, and light intensity, for all the samples analyzed. 2.3. Calorimetry Experiments. The heat of combustion was measured with a C 2000 basic calorimeter from IKA Company, using a C 62 calorimetric bomb. The calibration was performed by the combustion of 2.14 g of a high explosive, the 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), commonly used as reference for this purpose (Qexp./RDX = 5749 J/g). The BBT were tested as loose powders to facilitate their inflammation and their full combustion. The resistive heating of a metal wire is used to ignite a propellant grain (GBTU 125) which subsequently falls in the BBT powder, initiating the combustion. Differential scanning calorimetry (DSC) curves were measured on a Q1000 apparatus from TA Instruments under a nitrogen flow of 50 mL/min. The experiments were carried out on 3.0−3.5 mg samples with heating rates of 4 K/min in M 20 sealed crucibles from Swissi. A refrigerated cooling system (RCS 90) was used to perform specific characterizations on BB samples. One-off experiments were performed at the maximum temperature allowed by the RCS 90 system (550 °C), but the highest operating temperature (450 °C) for series of experiments was fixed in order to avoid the degradation of the metallic coating of M 20 crucibles. 2.4. Sensitivity Determination. The friction sensitivity of BBT was measured with a Julius-Peters apparatus. Friction force is applied by a moving porcelain peg to the sample (10 μL) deposited on a static porcelain plate. The press intensity is determined by the relative position of weights suspended from a lever. The measuring range extends from 4.9 to 353.2 N. The sensitivity to impact was measured with a fall-hammer apparatus. The tested sample (40 μL) is put between two metallic cylinders, which are inserted in a guide ring. The energy released by the impact is fixed through the height of fall (0.1 to 1 m) and the weight of the hammer (1 and 5 kg); it ranges from 1 to 49 J. The electrostatic spark sensitivity was determined with an ESD 2008 tester from OZM Research. The BBT sample is positioned between two electrodes with a distance of 1 mm. The energy of the electrostatic discharge can be adjusted through the capacitance and the voltage, from 1.4 × 10−4 to 10 J. 2.5. Observation of BBT Combustion. The ESD 2008 tester was also used to ignite BBT loose powder samples in combustion tests. The reaction was observed by a Phantom V1610 ultra high speed camera. This experiment provides the reaction duration (τr in s) and the fraction of the initial sample
The peculiar reactivity of aluminum nanoparticles originates from their core−shell morphology. According to Levitas and his co-workers, the pressure exerted by the molten aluminum core on the alumina shell upon high heating rates (>106 K/s) makes it rupture and produces the dispersion of aluminum droplets with nanometric size at 100−250 m/s.15,16 For submicrometer clusters of boron nanoparticles, the mechanism is somewhat different. According to Young et al., the ignition of such particles first involves the evaporation of B2O3, which is then followed by the combustion of the “clean” boron core.17 As the melting point of boron (MP/B = 2349 K) is higher than the boiling point of diboron trioxide (BP/B2O3 = 2133 K), the formation of a melt dispersion mechanism similar to the one observed with Al nanoparticles is impossible. For this reason, the reaction of B-based nanothermites (BBT) is expected to propagate slower than the one of their Al-based counterparts (ABT). By studying several calorimetric techniques of two Bbased submicrometer-sized thermites (Bi2O3/B and CuO/B), valuable information on their reaction mechanisms were found. The combustion of BBT loose powders initiated by electrostatic discharge was observed by high speed video in order to determine their reactive power and their ejection rate.
2. EXPERIMENTAL SECTION 2.1. BBT Preparation. Submicrometer-sized powders of Bi2O3 (2.7 m2/g) and CuO (10.7 m2/g) were purchased from Sigma-Aldrich with batch references MKBB9188 and MKBH9047 V, respectively. Amorphous boron particles of type 1 (22.3 m2/g) were provided by the company Chemetall. The powders were used as received to prepare BNT compositions. The mixing of boron and oxide particles was performed in 50 mL of acetonitrile used as dispersing phase. The homogenization was carried out by stirring the medium for 15 min with an ultrasound bath. Acetonitrile was distilled in a rotary evaporator operating at 90 rpm. Subsequently, the BBT samples were dried by maintaining the temperature at 353 K and reducing the pressure below 0.1 kPa during 30 min. BBT compositions were prepared in 5 g batches, according to the compositions given in the mixing Table 1. Table 1. Mixing Table Used to Formulate BBT Compositions Bi2O3/B (wt %) CuO/B (wt %)
98/2; 95/5; 90/10; 80/20; 70/30; 50/50; 35/65 98/2; 95/5; 90/10; 85/15; 80/20; 70/30; 60/40; 50/50
For reasons of clarity, the BBT compositions will be designated as follows thereafter: BBx and CBx for Bi2O3/B and CuO/B respectively. The “x” suffix will indicate the percentage by weight of boron in the BBT composition. 2.2. Morphological Characterization. The morphology of BBT samples was observed by scanning electron microscopy (Zeiss DSM 982 Gemini SEM). The apparent density of metallic oxide (DLP/Oxide) and boron (DLP/B) powders was measured by weighing the amount filled in the 40 μL calibrated volume of a specifically designed spatula. The theoretical value of the apparent density (Dth./BBT) was calculated from these experimental values and the boron weight content (μB) according to eq 1: 1 Dth./BBT = 1 − μ μ B + DB D LP/Oxide
LP/B
(1)
B
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Figure 1. Observation by scanning electron microscopy at the same magnitude of: the BBT precursors Bi2O3 (A), CuO (B) and boron (C) and the thermite materials BB20 (D) and CB10 (E).
the proportions of the components. Recent research in the field of metastable interstitial composites showed that particle distribution18 and porosity19 significantly influence the reactivity of Al-based nanothermites. The individual components of BBT were observed by scanning electron microscopy (Figure 1A-C). Bismuth oxide particles are prolate spheroids with a shape factor close to one. For this reason, the mean diameter (ΦBi2O3 ≈ 250 nm) can be approximated from their BET surface, using a spherical model. Bi2O3 particles have a smooth surface and superficially interact with each other. Copper oxide and amorphous boron samples are made of small, strongly aggregated elementary particles, with average diameters of 90 and 120 nm, respectively. In BB20 composition, boron particle clusters are located between Bi2O3 particles and do not cover their surface (Figure 1D). In other words, boron aggregates are either not fully disintegrated in the mixing process or do not have a good affinity for bismuth oxide particles. In the case of CB10 composition, CuO and boron clusters are observed in the center and at the bottom of the micrograph (Figure 1E), respectively. However, the presence of B (respectively CuO) particles in CuO (respectively B) clusters cannot be answered by SEM observations. The evolution of the apparent density of the BBT compositions provides further information on their micro-
mass (m0 in g), which is ejected by the reaction (γe). The ejection rate (ER) is expressed according to eq 2: m0γe ER = τr (2) The reactive power (Pr in W/g) is calculated from the heat of combustion (Qexp in J/g) according to eq 3: PR =
Q exp τr
(3)
The determination of these physical values has allowed the comparison between the reactivity of BBT studied here (Bi2O3/B and CuO/B) and their aluminum-based counterparts (Bi2O3/Al and CuO/Al).
3. RESULTS AND DISCUSSION 3.1. Morphology of BBT Mixtures. The reactivity and the sensitivity levels of submicrometer-sized energetic compositions strongly depend on their morphology, especially on the particle size and the degree of homogeneity. The knowledge of the way the fuel and oxidizer particles are arranged in the composition is an efficient tool to understand their reaction mechanism. Traditional pyrotechnic does not take into account the morphologic aspects and rather focuses on the chemistry and C
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attributed to the “dilution” of CuO/B clusters in the excess boron powder. The results obtained by analyzing the evolution of apparent densities not only are in good agreement with the observations made by SEM but also provide information on the morphology of the samples, which could not be obtained by simple microscopy. Bismuth oxide has a bright yellow color; the addition of small amounts of brown boron powder (0 < μB < 10 wt %) linearly darkens the material (Figure 3). The lightening effect observed
structure through a macroscopic analysis. To the best of our knowledge, this technique has never been reported before, and it seems highly relevant to characterize the morphology of energetic compositions containing submicrometer or nanometric particles. This method is based on the low apparent density of loose powders containing ultrafine particles. The density variations from the free powder to nonporous bulk material can be correlated to the morphologic changes occurring within the material. The theoretical value of apparent density is calculated from eq 3 and is represented in relation to the boron content for each type of composition (Figure 2). The dashed lines
Figure 3. Evolution of the grayscale of pelletized BB and CB materials according to their boron content.
when boron content is increased from 10 to 20 wt % is attributed to the spacing of Bi2O3 particles by boron particle clusters, which changes the interaction of the light with the sample. The maximal amplitude of the brightness peak is observed for 20 wt % of boron. The subsequent decrease in the gray level value is attributed to the formation of a continuous layer of boron particle clusters around oxide particles (μB > 35 wt %). The color of fuel-rich BB compositions logically shifts toward the one of pure boron. Copper oxide nanopowder has a black color that corresponds to a low gray level value (Figure 3). The addition of boron first makes the sample brighter (0 < μB < 10 wt %). A pronounced darkening of CB sample is observed when boron content is increased from 10 to 30 wt %. This effect is attributed to the particle agglomeration, which was highlighted by the density analysis technique (Figure 2). The coating of CuO/B aggregates by boron particle clusters occurs when boron content is between 30 and 50 wt %. 3.2. Calorimetric Properties of BBT Compositions. The decomposition of BBT mixtures was studied by differential scanning calorimetry (Figure 4). The first exothermic peak observed on the BB20 decomposition curve (220−309 °C) is attributed to a side reaction. The boric acid layer covering boron nanoparticles partially dehydrates upon heating and decomposes into metaboric acid (HBO2), which is stable in the temperature realm where the reaction is observed. As the melting point of HBO2 (236 °C) is very close to the onset temperature, it can be assumed that the exothermic signal is due to the acid−base reaction between the molten metaboric acid and the bismuth oxide, which has a basic nature. The exothermic peak observed at higher temperature (374−409 °C) is attributed to the first step of the thermite reaction. It corresponds to the preignition exotherm observed by Davies et al.,20 who have reported a higher onset temperature (440 °C), which can be traced back to the micrometric size of boron powder and to the use of a 50 K/min heating rate, which shifts
Figure 2. Comparison of the theoretical apparent density with the value measured experimentally, in the case of BB (A) and CB (B) compositions.
correspond to the measured apparent density. For BB compositions, this value is systematically smaller than the theoretical one. It indicates that boron particle clusters do not cover the surface of oxide particles, rather inserting themselves between the latter, making the BB density lower when the boron ratio increases (Figure 2A). The addition of boron (μB < 35 wt %) to copper oxide results in an apparent density that is higher than the one expected (Figure 2B). This effect highlights the good affinity between B and CuO particles, which come together when the acetonitrile used to mix the CB is evaporated, leading to a denser material. As the dimensions of particle clusters are similar for B and CuO samples, it can be assumed that the initial aggregates are partly disintegrated by the ultrasonic treatment applied during the CB preparation and that they rearrange in a more compact structure. When the boron content exceeds 35 wt %, the apparent density of CB mixtures becomes lower than it should be. This phenomenon is D
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Figure 4. Typical DSC curves obtained with BB20 and CB10 compositions, which are the most energetic of the BB and the CB series.
the decomposition temperature to higher values. The mechanism of the Bi2O3/B reaction was studied by analyzing the preignition exotherm, which corresponds to the early step of the thermite reaction. The reaction completion, corresponding to the broad final exotherm is reached at higher temperature (447−527 °C). In the case of CB10 mixture, a side reaction occurs at lower temperatures (83−211 °C). Copper oxide not only has a more marked basic nature than bismuth oxide, but also possesses hydroxyl surface functional groups that can be removed as “chemical water” upon heating. The reaction between the surface species of CuO and the boric acid accounts for the exothermic signal observed. The redox reaction between CuO and boron starts when the temperature reaches 400 °C and significantly accelerates above 500 °C. The onset temperature is lower than the one that can be calculated from the results reported by Nakamura et al. (516 °C), which indicates that the use of submicrometer particles seems to lower the decomposition temperature. However, the lower onset temperature could also be the result of the hermetically closed DSC crucible, which would be in good agreement with the decrease of the ignition temperature observed by these authors when they performed the reaction under argon pressure. The side reaction of BB and CB mixtures probably leads to bismuth and copper complex borate salts. In the case of BB compositions, the reaction occurs at relatively high temperature and seems to be compatible with potential pyrotechnic applications. Conversely, the poor stability of CB mixtures at low temperature precludes their use for this purpose. The first step of the reaction of BB mixtures has been investigated in details through consecutive thermal cycles performed with a single sample by differential scanning calorimetry (Figure 5). The DSC curve presented in Figure 5A, shows that the acid−base reaction between HBO2 and Bi2O3 is an irreversible process, which has no incidence on the subsequent thermite reaction. The sharp endothermic signal related to bismuth melting (269.4−272.6 °C) is only observed when the reaction corresponding to the preignition exotherm (372−414 °C), has previously happened (Figure 5B). Bismuth melting is therefore the signature of the thermite reaction. Interestingly, the bismuth solidification is not observed on the cooling branch (2) of the DSC curve, in the temperature domain where it should normally appear. It seems to be shifted to lower temperatures, in the form of several lowenergy exothermic signals in the temperature zone extending
Figure 5. Study of the reaction mechanism using DSC cycles, with 4 K/min heating and cooling rates, performed on BB20 (A) and BB30 (B) compositions.
from 106.9 to 125.3 °C and from 154 to 174.8 °C. The sum of the energies released is 1.5 to 2.0 times smaller than the energy absorbed thereafter by the bismuth melting, when the sample is heated again on the branch (3) of Figure 5B. It seems important to note here that the exothermic signals formed during the cooling step do not originate from the condensation of the water coming from the dehydration reactions of Bi2O3/B surfaces. Several reasons can be given to support this assertion: first, the energy released would correspond to a high water content (about 6 wt %), which is not compatible with the low water proportion measured by thermogravimetric analysis on pure bismuth oxide (0.34 wt %) and boron (1.2 wt %) nanoparticles. Second, in the presence of molten bismuth at 450 °C, the water would instantaneously react to give bismuth oxides. The solidification of bismuth in a temperature range below its classical melting temperature indicates that the metal is formed at the nanoscale by the reaction, probably in the form of boron particles enclosing shells. The melting point depression and the decrease of melting heat were experimentally evidenced by Sun et al. on aluminum nanoparticles.21 The difference between the solidification heat and the melting energy is attributed to the coalescence of bismuth nanostructures in macroscopic droplets during the final heating step. The existence of a single endothermic signal at 269.5 °C for the bismuth melting on branch (3) demonstrates that the total bismuth fraction in the residues has been transformed into bulk metal. The preignition reaction duration of BB mixtures, calculated from the width of the exothermic peak assigned to thermite decomposition, takes approximately 10 min. The condensed phase mobility of reactive species occurring above Tamman’s temperature has been reported by Jian et al.22 as a determining mechanism in thermite initiation. According to these authors, E
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Tamman’s temperature (TT) is “half the melting temperature -expressed in Kelvin- of oxidizers”. The TT values for bismuth oxide (272 °C), boron (901 °C) and boron trioxide (89 °C) were calculated, based on this definition. The progressive decomposition is attributed to the migration of Bi2O3 from the oxide particle to boron. The reaction gives liquid bismuth and solid boron trioxide. By capillary forces the metal encloses the boron particles, while the lighter B2O3 produced by the reaction rises to the surface of molten bismuth, and reacts with Bi2O3 excess to give glass compounds.23 The energy absorbed by the melting of the bismuth formed by the BB reaction is determined from the DSC curves by the integration of the endothermic peak. The amount of metal present in the combustion residues is calculated by dividing this value by the melting enthalpy of bismuth (ΔHm(Bi) = 51.816 J/g). The amount of B2O3 produced by the reaction and the part of unreacted Bi2O3 are finally deduced from this value. The evolution of the composition residues according to the boron content of BB mixtures has several interesting aspects (Figure 6A). First, boron is always in excess regardless
species, namely Bi2O3 and B2O3. The mean thickness of molten bismuth layer on the surface of boron particles was calculated from their specific area (≈ 22.3 m2/g) and depends on the boron content in the BB (Figure 6B). It can be noticed that the amount of bismuth produced by the reaction increases with the metal film thickness, which leads to assume that molten Bi probably dissolves bismuth oxide favoring its diffusion to the boron surface. The curves representing the variation of Bi2O3 and B2O3 intersect at a boron content of 17.7 wt % which is in the composition range where the bismuth formation is inhibited. The intersection corresponds to a Bi2O3/B2O3 equimolar mixture. According to the Bi2O3−B2O3 phase diagram reported by Hoch,24 the stable phases that should be observed are (Bi2O3)2:(B2O3) and (Bi2O3)3:(B2O3)5 in a 1:2 molar ratio. In the relatively mild experimental conditions used for DSC experiments, the formation of the metastable bismuth orthoborate (BiBO 3) seems more likely.23 The glassy compounds formed by the reaction between Bi2O3 and B2O3 inhibit the thermite reaction by a combined physical and chemical process. First, they merge at the surface of molten bismuth and form a shell that limits the diffusion of unreacted Bi2O3 to the boron core through molten Bi. Second, the Bi2O3 combined with the B2O3 excess can no longer react with boron. The use of a boron proportion higher than 20 wt % in BB mixtures makes no sense, as it does not lead to an optimized thermite reaction but only to the formation of the different (Bi 2O 3) n(B2 O3 )m phases, which are found in the B2 O3 prevalence zone of the Bi2O3−B2O3 phase diagram. 3.3. Sensitivity of BBT Compositions. In view of their sensitivity thresholds (Table 1), BB and CB mixtures can be considered as relatively insensitive to friction. Their ignition levels are markedly higher than those of their Al-based counterparts (360 >360 128 112
>49.6 >49.6 47.1 >49.6
0.14 49.6 >49.6 >49.6 >49.6
733.22 20.36 20. The formation of partially oxidized boron species in the reduction process of metallic oxides by amorphous boron is a documented phenomenon, which has been used, for instance, by Okada et al.26 to prepare hexaboron monoxide (B6O). It is not possible to obtain further information on the nature of boron compounds present in the reaction products of BBT, due to the amorphous nature of these materials. The main crystallized phases produced by the combustion were identified from JCPDS cards. Bismuth (01-085-1329) is the main reaction product in BB residues, although small amounts of unreacted Bi2O3 (01-074-2351) were also found. The decomposition of CB mixtures always gives copper (00-004-0836), which is mixed with cuprous oxide (Cu2O; 00-005-0667) in the residues of CB containing less than 30 wt % of boron. Cuprous oxide comes from the thermal decomposition of CuO; its presence shows that the thermite reaction is incomplete in spite of boron excess. The partial combustion of BBT is attributed to the coating of the unreacted species by the molten phases produced by the reaction. This mechanism is in good agreement with the macroscopic texture of the combustion residues. The combustion residues of BB5 contain millimeter beads of bismuth. The size of these metallic spheres becomes submillimeter when the boron content is increased to 10 wt %. The compositions having a higher content of boron give powdery residues whose texture is reminiscent of pure boron. The reaction of CB mixtures containing 5 to 20 wt % of boron gives aggregated nodules with reddish color, which strongly stick by a welding effect on the stainless steel walls of the combustion chamber. In the presence of a boron excess (≥30 wt %), the residues look very similar to those formed by the combustion of the BB mixtures with high boron content. The residues produced by the reaction of fuel rich BBT are
maximum stress which can be applied with the fall hammer apparatus (49.6 J) with the exception of BB10 composition which reacts when the impact energy is higher than 47.1 J. According to the gray levels analysis (Figure 3), the BB10 thermite is in the composition domain where a morphological transition occurs. Boron particles which are at first “diluted” in Bi2O3 with minor incidence on the expansion of the oxide powder, progressively space Bi2O3 particles when boron content increases, leading to an unbroken succession of reactive domains, which could account for the sensitization observed in the case of the BB10 composition. The sensitivity levels of BBT mixtures (Table 2) are below the measuring threshold of the ESD tester (0.14 mJ), except for CB2 and CB5 compositions whose sensitivities surround the maximal value of the electrostatic discharge which can be generated by the human body (≈156 mJ). It can be noticed that the increase of the ESD thresholds of boron-lean CB compositions goes along with a marked reactivity collapse. The extreme sensitivity to ESD of all other BBT compositions is attributed to the insulating properties of these mixtures: the dissipation of the electrostatic energy induces a strong heating which takes place along the path followed by the discharge in the material. 3.4. Study of BBT Combustion. The explosion heat of BBT compositions was measured in a calorimetric bomb and represented according to their boron content (Figure 7). The reaction of CB compositions releases more energy than the one of their BB counterparts. The most energetic CB composition is obtained for a boron content of about 11.4 % by weight and has a 2.91 kJ/g explosion heat. This experimental value is very close to the theoretical value of 3.09 kJ/g reported by Fischer et al. for the CuO/B stoichiometric composition.8 According to
Figure 7. Evolution of the combustion heat of BBT compositions according to their boron content. G
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loose powders which are fixed in a monolithic state by the metallic superficial layer (Bi or Cu) which ensures the crosslinking of boron particles. This observation confirms that the abrupt reaction of BBT mixtures takes place on the surface of boron particles, which behave as the reactive substrate for this interfacial phenomenon. The proportion of boron necessary to reach the balanced stoichiometry in BB and CB mixtures, based on the formation of B7O, is equal to 32.8 and 48.8 wt %, respectively. The second peak on Qexp curves actually reaches its maximum amplitude for 21.3 and 31.6 wt % of boron, which means that the oxidation of boron does not give only B7O but rather a mixture of the latter with B2O3. The calculated B2O3:B7O ratio by weight is 40.5:59.5 for BB21.3 and 42.5:57.5 for CB31.6. The equilibrium is shifted toward the formation of B7O when the boron content in the BBT compositions is increased. The high sensitivity to electrostatic discharge of BBT compositions makes them easy to ignite with a low energy spark. In the combustion tests, the electrostatic energy may be high enough to initiate the abrupt decomposition, without dispersing the powdery sample by the mechanical effects of the discharge. The ignition of BBT was typically carried out with an 8 kV voltage and a capacitance of 5 nF corresponding to an energy of about 160 mJ. Under these conditions of initiation, the abrupt reaction of BBT occurs in only a few milliseconds. The observed phenomenon is 4 to 5 orders of magnitude faster than the one measured by DSC. The reaction velocity thus depends on the power used to initiate the material decomposition. The dual decomposition mode of BBT mixtures depending on the heating rate was also evidenced in the past on aluminum-based nanothermites27 and seems to be a feature of the reactive behavior of most energetic compositions. The observation of the BBT combustion by high speed video with the same recording parameters gives valuable qualitative information, such as the light intensity emitted by the reaction and the macroscopic morphology of the ejected matter. For CB materials, the CB10 composition is the more luminous; the droplets ejected by the reaction collide and gather by coalescence in the form of spheres with larger dimensions. The combustion of CB20 is less bright and produces droplets which do not rearrange into bigger objects. This observation confirms that CB20 reacts less exothermically than CB10 (Figure 7). The reaction of fuel-rich CB compositions gives aggregates with tree structure whose dimensions gradually increase with boron content. The reaction of BB5 and BB10 compositions not only gives numerous particles but also abundant fumes. The brightness passes through a maximum for BB10 material and then fades when the boron content rises. The decomposition of fuel-rich BB20 and BB30 mixtures gives mainly large aggregates with a tree structure, similar to those observed in the case of CB compositions. These results strengthen the hypothesis that boron plays the role of substrate for the combustion occurring at its surface. The interfacial reaction is responsible for the partial combustion of boron present in the composition. Furthermore, it strongly differs from the melt dispersion mechanism of aluminum nanoparticles13 and accounts for the moderate reactivity of BBT. In the abrupt decomposition mode, the metallic products (Bi or Cu) are vaporized by the heat released by the reaction, which ensures the convective propagation of the reaction. According to the diagram reported by Honig,28 bismuth has a higher vapor pressure than copper. This property favors the diffusion of the metallic vapor in the unreacted composition
and limits its premature condensation, increasing its activation effect and range. The reactivity of BB and CB compositions was compared by means of the physical quantities, namely the ejection rate and the reactive power (Figure 8), calculated from the eqs 2 and 3.
Figure 8. Graphical representation of the ejection rate (A) and reactive power (B).
The maximum ejection rate is obtained for BB12.7 and CB15.5 compositions, but it is six times higher for the former than for the latter (Figure 8A). Similarly, the maximal reactive power of BB compositions is twice as high as that of CB mixtures (Figure 8B). From these data, it can be assumed that BB compositions are more reactive, despite their lower explosion heat (Figure 7). However, the CB materials retain their reactive power at a high level in a wide range of formulation (10−35 wt % of boron). This unusual behavior could originate from the formation of an intermetallic compound. Small amounts of boron (≈ 2 wt %) can dissolve in copper,29 but it seems doubtful that this dissolution process brings enough energy for causing the effect observed here. Moreover, no crystallized intermetallic species has been evidenced on the X-ray diffraction patterns performed on CB residues. The ejection rate and the reactive power of aluminum-based Bi2O3 and CuO nanothermites, prepared according to the theoretical stoichiometry, was investigated in similar experimental conditions. The reaction heats used in the calculation were determined experimentally by calorimetric experiments and were found to be 1.46 kJ/g and 3.10 kJ/g for Bi2O3/Al and CuO/Al, respectively. The Bi2O3/Al composition was found to be the most reactive with an ejection rate of 628 g/s and a reactive power of 29.2 MW/g. The less reactive CuO/Al composition still has an ejection rate of 113 g/s and a reactive power of 20.7 MW/g. Boron-based thermites have a moderate reactivity in comparison with those prepared from aluminum H
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about several references cited in this article. The authors would also like to thank the Direction Générale de l’Armement (DGA) from the French MoD and the French National Research Agency (ANR) for the funding of the SUPREMATIE project in the frame of which this research was performed.
nanopowders. It seems interesting to note that the classification of the relative reactivity of oxides Bi2O3 > CuO is the same for both fuels.
4. CONCLUSIONS Boron-based thermites Bi2O3/B (BB) and CuO/B (CB) are combustible ceramic materials that can easily be ignited by an open flame or electrostatic discharge but are relatively insensitivite to friction and impact. The morphology of BBT mixtures was investigated by analyzing the evolution of the apparent density of the compositions. This unconventional approach has shown that boron nanoparticles have a lower affinity for Bi2O3 than for CuO. A photometric technique, basing on the analysis of the greyscale of the pelletized compositions, has permitted to define three composition ranges corresponding to (i) isolated boron particles in the oxide powder, (ii) formation of a specific interaction B/oxide that changes the texture of the oxide powder, and finally, (iii) complete enclosing of oxide particles by the boron. The study by DSC of the preignition exotherm of BB mixtures has led to proposing a three-step mechanism accounting for the thermite reaction. First, the Bi2O3 migrates to the boron surface when it reaches its Tamman’s temperature. The second step is the redox reaction, forming molten bismuth and solid boron trioxide. The last step corresponds to the formation of glassy Bi2O3−B2O3 compounds at the surface of molten bismuth; this shell inhibits the thermite reaction by limiting the diffusion of the oxide to the boron core. The study of the explosion heat released by BBT compositions according to their boron content has shown that the reaction involves the formation of boron suboxide (B7O), shifting the stoichiometry to higher boron proportions. In the presence of excess boron, the reaction occurs at the surface of boron particles which behave as a reactive substrate. The abrupt combustion mode of BBT, which is initiated by a fast heating process, is 4 to 5 orders of magnitude quicker than the decomposition observed by DSC. The determination of the ejection rate and the reactive power has shown that BBT has a moderate reactivity compared to their aluminum-based counterparts and that BB compositions possess better performances than CB ones. By analogy with the conventional aluminothermy reactions, the term “borothermy” should be introduced to describe the specific reactivity of thermites prepared from boron. The reactivity of BBT makes them promising materials for specific propulsion applications as well as for the ignition of energetic materials with low thermal sensitivity.
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ABBREVIATIONS MICs = metastable interstitial composites BBT = boron-based thermite BB = Bi2O3/B composition CB = CuO/B composition. The abbreviations BBx and CBx refers to compositions containing x percent by weight of boron
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Yves Suma, Christian Jaenger, and Yannick Boehrer from the multimedia service, of the French−German research institute of Saint-Louis, for taking the photographs and high speed videos that have been used in this research. The authors would also like to warmly thank Dr. Trevor Griffiths from QinetiQ (UK) and Dr. Shingo Date from the National Defense Academy of Japan for having provided information I
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