Combustion of activated aluminum

Combustion and Flame 145 (2006) 464–480 www.elsevier.com/locate/combustflame

Combustion of activated aluminum Arno Hahma a,b,∗ , Alon Gany a , Karri Palovuori c a Faculty of Aerospace Engineering, Technion—Israel Institute of Technology, Technion City, 32000 Haifa, Israel b Department of Chemistry, University of Jyväskylä, FIN-40014 Jyväskylä, Finland c Laboratory of Energetic Systems, Tampere University of Technology, FIN-33720 Tampere, Finland

Received 29 July 2005; received in revised form 12 December 2005; accepted 9 January 2006 Available online 24 February 2006

Abstract Combustion of activated aluminum was studied by four different methods: microscopic imaging of the preignition process, digital imaging of the combustion process at pressures up to 64 bar in air, nitrogen, and carbon dioxide, TGA, and DSC. Activation by three fundamentally different methods was found effective in enhancing both the ignitability and the burn rate. The complex fluoride coating prevented agglomeration completely in all stages of combustion, while the nickel and cobalt coatings promoted agglomeration of aluminum oxide at combustion, but prevented the agglomeration of the aluminum metal before combustion. Nickel coating catalyzed aluminum nitride formation, accelerating burn rate more than other coatings in air and in nitrogen, while complex fluoride coating was most effective in carbon dioxide. Carbon coagulation in carbon dioxide quenched burning in many cases at higher pressures than 8 bar. The complex fluoride activation accelerated combustion in CO2 extremely effectively, but did not prevent carbon shell formation and subsequent quenching at high pressures. Ni coating negated the effects of carbon coagulation in CO2 , but enhanced the burn rate only slightly. Co coating reduced the carbon shell formation, but did not accelerate combustion in CO2 . Only the Ni coating applied in large amounts promoted combustion in nitrogen. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Activated aluminum; Propulsion; Metal combustion; Burn rate acceleration; Carbon coagulation; Agglomeration; Metal powder; Ignition

1. Background Aluminum is a widely used energetic additive to energetic materials and an additive to fuels for airbreathing propulsion. However, efficient ignition and burning of aluminum is a problem [1–8] due to the strong oxide layer naturally formed on aluminum. In addition, agglomeration of Al particles before and

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E-mail address: [email protected] (A. Hahma).

during the combustion process severely restricts the burn rate of aluminum particles, reducing combustion efficiency in rocket motors [9–18]. To improve burn efficiency and to ensure ignition, two routes can be taken: particle size reduction, particle surface activation, or both. By using finer particles the ignitability can be improved and the burn time of the aluminum particles can be shortened, but agglomeration [9–18] becomes stronger, unless submicrometer powders are used [19–23]. This in turn is prohibitively expensive and creates other problems, such as sensitivity, high percentage of oxide in the

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material leading to reduced energy density, and problems with chemical stability and shelf life. By using relatively coarse, activated aluminum powders, these problems can be avoided, yet the advantages of fast ignition and burn rate can be utilized.

2. Materials 2.1. Methods of activation and the aluminum powder Three types of activation were used: nickel coating, cobalt coating, and complex fluoride coating. The activation processes themselves are subject to another paper and are therefore described only briefly below. The effect of the metal coatings is based on intermetallic reaction between the aluminum and the metal coating, which provides a heat pulse at the melting point of the aluminum, aiding ignition. Nickel and cobalt are oxidized more easily than aluminum metal at elevated temperatures, providing additional energy to heat the particles above the melting point of aluminum, and the resulting metal oxide reacts with Al in a thermite reaction, generating an even more intense heat source. These extra heat sources contribute to the ignition of the aluminum particles. The resulting NiAl or CoAl alloy formed at the surface of the particle burns faster than aluminum alone due to nickel or cobalt oxide working as an oxygen carrier (oxidation by ambient oxygen, reduction back to the metal by aluminum) provided the combustion rate is not diffusion-limited. The coating can be applied using any electroless nickel or cobalt plate solution on aluminum powder and the thickness can be controlled by bath parameters and time [24–32]. Another possibility is to let aluminum itself reduce the metal from solutions of its salts [33]. Cobalt coating was applied similarly by substituting nickel for cobalt in the baths. Gas phase coating with metal carbonyls can be used to produce a pure metal coating not containing phosphorus or other remains of the reducing agents in the baths [34]. The complex fluoride activation is based on destruction or weakening of the oxide layer on the surface of the aluminum. Fluorides reduce the melting point of aluminum oxide by acting as solvents for aluminum oxide above the melting point of the fluoride, accelerating the transport of the gaseous oxidizer through the oxide layer. Coating with complex fluorides is carried out in aqueous solution by precipitating the complex fluorides generated in situ on the aluminum particles [35]. The aluminum powder studied in this work was coated with tripotassium hexafluoroaluminate, K3 AlF6 , using aqueous potassium hydrogen fluoride, KHF2 , as the source of fluoride and the aluminum

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particles themselves as the source of aluminum in the complex fluoride. The complex fluoride is formed in situ from the reaction of aluminum with KHF2 as follows: 6KHF2 (aq) + 2Al → 2K3 AlF6 ↓ + 3H2 ↑.

(1)

The coating reaction expends some of the base material, which has been taken into account when calculating the percentage of the coating on the powder. The percentages have been calculated as mass per cent of the particle mass. For example, 1% F coating means 1% of the particle mass consists of fluorine. This criterion was selected since, especially with the metal-coated powders, the exact amount of matter precipitated on the particle surface cannot be accurately preadjusted, but the amount of metal added into the baths can be accurately measured. Thus, by precipitating all the metal added, the amount of metal is known exactly, while the amount of other material, such as metal hydroxides, phosphorus, oxide of the resulting metal, and organic residue, can vary from batch to batch. Work is still in progress to solve these problems with the metal-coated powders; however, the amount of foreign material on the surface does not affect the activity of the material, but it is only an inert, undesirable ballast. The specific surface area of the powder was not affected by any of the coating methods. Thus, no correction for increased or decreased surface area needs be taken into account in the following discussion. Carlfors Bruk A100-aluminum powder was selected as the base metal. This material consists of air-atomized, irregular-shaped, pure aluminum powder (80–100 µm), which has very good rheological properties and high packing density despite its irregular particle shape. The specific surface of the powder is 0.23 m2 /g, as measured by nitrogen adsorption (BET). The aluminum content of this powder is more than 99.5%, indicating there is less than 1% of aluminum oxide on its surface. The powder also has a relatively large particle size, easing optical microscopy and photography. The activation was expected to accelerate ignition and combustion rates to such an extent that determining them for finer powders was considered too difficult. Therefore, the coarsest possible aluminum powder that still can be ignited was selected as the reference material, which together with availability problems excluded spherical powders with large particle sizes. 2.2. Reactive gases Three gases were selected as the reactive medium, air, carbon dioxide, and nitrogen, obtained from commercially available cylinders. Ordinary, technical grade of 99.9% purity was used.

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The reactive gases were selected to represent the major species under realistic combustion conditions. Water vapor is also present in large quantities in flames, but it was not possible to carry out measurements in H2 O(g) at high pressures with the equipment available.

3. Equipment

used for preignition studies, i.e., agglomeration, since the microscope had too short a depth of focus to study combustion and also because smoke from the combustion would obscure the images. The microscope imaging allowed studies at atmospheric pressure only and the maximum magnification was diffractionlimited to 20× due to the small size of the CCD element compared to the 24 × 36 mm film frame normally used with the microscope.

3.1. Camera

3.3. Pressure vessel and teleobjective

The images of the combustion were recorded with a Redlake Imaging SC2000 high-speed CCD camera, which was connected to a microscope while studying the preignition processes and to a teleobjective while studying the combustion characteristics of the samples. The camera has a maximum frame rate of 2 kHz, but the practical maximum frame rate was limited to 1 kHz, because too small a resolution was available at 2 kHz. The rate of 2 kHz was used only to measure combustion times of individual particles at the highest pressures, when the particles burned faster than within one frame or 1 ms.

The combustion processes were filmed with a teleobjective (focal length 125 mm, lens diameter 40 mm) through a window in a pressure vessel (Fig. 2). The vessel allowed a maximum of 150 bar pressure; up to 64 bar was used in this study. In the vessel, the chromium–nickel band was placed between two electrical terminals and the camera was aimed at the sample from the side to prevent obscuration by the smoke. Maximum optical magnification was 15×, which did not allow for much smaller powder particles than used in this study. In addition to the sample bulk combustion, the focus depth of this system—especially at small apertures (high F-stop value)—even allowed observation of individual burning particles without losing focus. Particle combustion times were obtained from such individual particles detected in almost every run.

3.2. Microscope Microscopic images were recorded through a Nikon stereomicroscope, which has an attachment for a film camera. The camera connection was modified to adapt the high-speed digital camera instead. The sample was placed on an electrically heated chromium–nickel band, which was installed into a thermally and electrically insulating crucible under the microscope (Fig. 1). This equipment was mainly

3.4. Thermoanalysis Thermogravimetry and DSC were carried out with Netzsch STA 449C equipment, which is capable of performing TGA and DSC simultaneously.

Fig. 1. Measurement cell for preignition and agglomeration studies with the lid removed showing the crucible of machinable ceramics, the Cr–Ni band, and the thermoelement. The microscope objective is visible above the cell. The protective gas feed tube through the crucible wall from behind is not visible in the picture.

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Fig. 2. The pressure vessel arrangement for combusting the metal powders in reactive gases under pressure. The camera with the teleobjective is on the left with a focusable spotlight on top. On the right, the pressure vessel with windows, a pressure display, and a control computer are shown.

4. Experimental 4.1. Microscope The sample (20 to 40 mg) was placed on the chromium–nickel band (cross section 0.15 by 3.0 mm), the glass lid on the crucible was closed, and gas flow (100 to 150 ml/min) was started. The band was heated as rapidly as possible to 1200 ◦ C and the metal powder was ignited, unless a protective gas was used to prevent ignition. This setup was selected to simulate actual conditions in combustion as closely as possible, i.e., to heat up to ignition point as rapidly as possible. The temperature was limited by the Cr– Ni band material and was selected to be as high as possible within this limit in order to get into the temperature range of typical flames. Argon was used as the protective gas to prevent ignition and the crucible was flushed with the gas flow for 1 min or more before the experiment. The gas was fed through a capillary tube mounted through the crucible wall (not visible in Fig. 1) and it exited by leaking out of the crucible from joints, i.e., between the glass lid and the crucible body. Because the joints had small clearances, ambient air could not enter while the gas was flowing. The camera and the heating were started at the same time and continued until the sample ignited or maximum time of image recording was reached. No shutter or aperture was used to restrict the passage of light to the camera, as there was too little light available from the phenomenon itself, necessitating external lighting with a focused halogen lamp. As a drawback of this setup, the focal depth of the cam-

era was extremely short due to absence of an aperture. The resulting film files were edited afterward and only the phenomenon itself was saved, removing uninteresting, dark parts. 4.2. Pressure vessel The sample was placed on the Cr–Ni band (cross section 0.10 by 2.0 mm) attached to the terminals in the pressure vessel lid. The lid was carefully attached to the vessel and the bolts were closed. The camera was focused on the sample and the objective aperture was adjusted based on experience from previous experiments. Often the amount of light was excessive or insufficient due to the very limited dynamics of the CCD camera, which necessitated repeated experiments to catch the phenomenon and experience was needed in order to predict the right settings for shutter speed and aperture at each pressure and gas. The heating and image recording were started simultaneously and the experiment was continued for 8 s, which is the maximum time of recording at 1 kHz for the camera system. Afterward, the recordings were examined and the actual combustion phenomenon was saved to a video file and compressed to save storage space. With this process, the lightness of the recordings was normalized to the same level (when possible within the bounds of the color depth in the videoclip) to be able to distinguish more accurately when the sample ignited and to measure the time of combustion. The time of combustion was used to determine both the bulk and particle burn rates. The horizontal field of view in the recordings was 2.0 ± 0.05 mm and

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the average particle radius was 40 µm; the radius distribution was not known. These values divided by the observed time of combustion gave the average burn rate over the time interval in question. The burn rate in bulk was determined over five or more parallel runs and the average over these is reported here. The error limits for burn rates in bulk were determined as root mean square of the maximum error of the recording time and the measured distance. The effect of the particle size on the error limits was ignored, since the particle size is of the same order of magnitude as or smaller than the measurement error of the distance. The reported particle burn rate is an average over all single-particle burn rates, which could be determined for each sample in the five parallel runs. The number of datapoints in this case varied from 12 up to more than 50, depending on how many individual particles were seen in the recordings. Error limits were determined from the maximum error of the time of combustion. The particle size distribution was not known; however, its effect on the error limits for the particle burn rates is small compared to the uncertainties in the combustion time at high burn rates imposed by the finite frame rate of the camera. At low burn rates, the effect of the particle size deviation is more appreciable and the error limits were therefore multiplied by 2 for rates lower than 8 mm/s.

5. Results 5.1. Preignition and microscopic studies The preignition studies showed similar behavior for all types of activated aluminum: no agglomeration at all was detected. This is in agreement with previous observations [36,37] with nickel-coated aluminum powders, and we extend this observation to other types of activation as well. Untreated aluminum showed a strong tendency to agglomerate to large droplets, when the powder was heated to 1200 ◦ C, while only the shape of the particles changed with the coated materials due to surface tension of the molten aluminum. Even prolonged heating and temperature cycling did not cause any agglomeration of any sample other than the untreated material (Figs. 3a–3d). The loss of focus in Fig. 3 could not be prevented, since the dimensions of the aluminum droplet were larger than the focus depth of the microscope. However, the phenomenon is very clearly distinguishable from the supplementary recordings for this paper. After the experiments the Cr–Ni band was intact, confirming that it did not take any part in the agglomeration or combustion of the powder placed on it. The reactive or protective gas atmosphere had no effect on the agglomeration behavior of the samples either.

Fig. 3. Complex fluoride-coated aluminum before (a) and after (b) the preignition experiment and heating to 1200 ◦ C in air. No agglomeration was seen, only strong oxidation of the metal, as can be seen from reduction of gloss in (b) as well as from the thermoanalytical results. The nickel- and cobalt-coated powders behaved similarly. As a reference, untreated powder before and after heating is shown in the frames (c) and (d), respectively. One large and numerous small agglomerates are visible in the frame (d). The loss of focus is due to the small focus depth of the microscope; focusing on the particles and agglomerates simultaneously was not possible. The supplementary recordings about these experiments are much more comprehensive.

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5.2. Thermoanalysis 5.2.1. General The materials were investigated with DSC and TGA to measure their reactivity against three gases, air, carbon dioxide, and nitrogen. DSC provides more information about reactions, especially with the metal-coated powders, which do not cause a change in the sample mass if only intermetallic reactions are taking place. All the metal-coated powders demonstrated exothermic reactions near the melting point of aluminum, confirming the intermetallic reaction taking place, but not limited to it. The experiment conditions for both analyses were as follows: starting temperature, room temperature; heating rate, 20 ◦ C/min, and a 10-min isotherm at the end of the run to complete reactions; final temperature, 1100 ◦ C; reactive gas flow rate, 20 ml/min; sample amount, 5 ± 1 mg, evenly spread across the pan bottom (single-particle layer); pan material, Al2 O3 . The amount of the sample had to be kept to a minimum to avoid cumulative heating and premature ignition of the sample and to have as good as possible heat transfer to the pan and to the instrument. All the materials reacted to some degree with all gases, even untreated aluminum, but the activated powders were substantially more reactive than untreated material, as expected. Interestingly, there are two sharp weight increases in the TGA in air, one at the melting point of aluminum and another about 50 to 70 ◦ C later observed for the activated powders. The latter is probably due to thermal expansion breaking the oxide shell, causing fresh aluminum to break out and oxidize. As the coating materials weaken the oxide, this happens earlier and more easily than with untreated aluminum. Any intermediate compounds between the coating materials and aluminum metal do not show on TGA, since no matter is transported to or from the particle in intermediate reactions, while the reactions do appear on the DSC, provided energy exchange is taking place, e.g., intermetallic reactions. The complex fluorides have already been reacted with aluminum (Eq. (1)) and cannot form any intermediates at all, which is consistent with the DSC results; there is no extra energy exchange compared to untreated aluminum before the oxidation of the aluminum itself starts. The metal coatings can form intermetallic compounds or act as oxidizers to aluminum, if the metal coating itself has been oxidized beforehand, but these reactions do not affect sample mass either. As a result, intermetallic reactions are only visible on the DSC, where they occur earlier than the weight increases observed in the TGA (compare Fig. 4 with Fig. 5, Fig. 6 with Fig. 7, and Fig. 8 with Fig. 9).

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The third and largest weight increase in TGA originates from normal oxidation or nitridation of aluminum when the diffusivity through the oxide shell increases as a function of temperature. This can be seen very clearly in the DSC as well, where the exotherms at the same temperatures are so large they must originate from oxidation/nitridation of the base metal. All reactions are stronger with the activated materials compared to the untreated aluminum, since the activation accelerates diffusion through the oxide shell. The diffusion is particularly accelerated at temperatures above 800 ◦ C, when the activating materials, especially the complex fluorides, melt and dissolve the aluminum oxide, greatly improving the transport of oxygen into the underlying aluminum metal by diffusion and convection. The mechanism with the metal-coated powders is not known exactly, but it probably depends on oxygen transport by the metal oxide (M + O2 → Mx Oy , Mx Oy + Al → M + Al2 O3 , M + O2 → Mx Oy , . . . ). In DSC, it is worth noting that the rate of reactions is faster the less oxidizing the atmosphere is. Thus, the highest heat output rate is obtained in nitrogen, not in air. This demonstrates that the reactions are indeed diffusion-limited by the oxide film and also shows the effects of activation. The less oxide film there is to remove, the more effective the activation and the faster the base metal can react, even though the oxidizing species is less reactive than oxygen. No actual ignition or combustion is seen with any sample, since ignition was deliberately prevented by reducing the amount of sample enough not to obtain vertical, steep signals, which would reduce the accuracy and reliability of the results. The weight decrease of the metal-coated powders before the melting point of aluminum is due to organic residue at the surface originating from the coating baths and to high phosphorus content in the coating metal. The coating bath for cobalt and nickel contained sodium hypophosphite as a reducing agent, which generates a plating with from several percent up to 15% of phosphorus content in the metal by mass. Phosphorus and its oxides may evaporate at elevated temperatures, giving rise to a weight decrease, which can amount to 1.5% at maximum with 10% Nicoated powder. In addition, the bath also contained relatively large amounts of citric acid as a complexing agent and buffer, which caused citrates to precipitate on the metal surface, and most of the weight decrease was caused by citrates. All this was detected only after carrying out the thermoanalyses and subsequent qualitative analysis of the Ni- and Co-coated powders, confirming the unexpected citrate precipitation. Unfortunately, it was not possible to repeat the experi-

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Fig. 4. TGA of ordinary and activated aluminum in air. A 5 ± 1 mg sample was used in each experiment. A mass 188% of the original corresponds to complete reaction to aluminum oxide.

Fig. 5. DSC of ordinary and activated aluminum in air. Sample mass was 5 ± 1 mg.

ments with “clean” metal powders, since the citrate precipitation problem has not yet been solved. Under nitrogen, the weight loss is less than in air or CO2 , since the citrates are pyrolyzed, while oxidation is taking place in air and hydrolysis under CO2 , leading to a larger weight loss. Any thermite or other intermediate reactions at temperatures lower than the melting point of the particle are unlikely, due to the very low diffusion rate of solid materials. As thermite or other reactions between the coating and the base metal are gasless, the weight decrease cannot originate from such.

5.2.2. Air Fluoride-coated powder. The fluoride-coated powder started reacting at even lower temperatures than the metal-coated powders (tendency toward exothermic direction; weight increase in the TGA), but showed less reactivity at higher temperatures in air. It is quite surprising that the fluoride coating is effective at a temperature much lower than the melting point of the complex fluoride (above 800 ◦ C) and that the activation is of the same order of magnitude as the one produced by the metal coatings, even though the coating itself does not provide any extra heat to the particle like the metal coatings do.

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Fig. 6. TGA of ordinary and activated aluminum in carbon dioxide. Sample mass was 5 ± 1 mg. A mass 188% of the original corresponds to a complete reaction to aluminum oxide.

Fig. 7. DSC of ordinary and activated aluminum in carbon dioxide. Sample mass was 5 ± 1 mg.

Metal-coated powders. Only the Co-coated and the 10% Ni-coated powders were able to fully overcome the endotherm originating from aluminum melting and the heat flow turned exothermic in air, while under CO2 and N2 this effect is absent for all powders and only a smaller endotherm is seen, which also indicates exothermic reactions taking place. Hence, the difference from untreated material measured in CO2 and N2 represents the intermetallic reaction enthalpy, while the much larger difference in air must originate from oxidation of the aluminum and, to a lesser degree, that of the coating metal in addition to the intermetallic reactions. This can be seen from the TGA measurements, where there is a jump of weight in air

at the melting point of aluminum, while there is no such weight increase in the other gases. In the case of Co coating, oxidation of the coating material alone does not explain the large exotherm and weight increase (see Fig. 4) occurring at the melting point of Al, since only a small amount of the coating was present (in the case of Co, 1% by mass of the Al). This amount would give rise to a weight increase of only 0.2%, while TGA shows an increase of about 5%. Hence, the base metal must have been oxidized as well. It is also interesting to note that this happens already before the melting point of aluminum. In the case of 10% Ni coating the weight increase agrees with the theoretical weight increase of 2.2%

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Fig. 8. TGA of ordinary and activated aluminum in nitrogen. Sample mass was 5 ± 1 mg. A mass 152% of the original corresponds to a complete reaction to aluminum nitride.

Fig. 9. DSC of ordinary and activated aluminum in nitrogen. Sample mass was 5 ± 1 mg.

from the Ni → NiO reaction, and the above conclusion about Al oxidation at the melting point cannot be made with certainty. However, the shape of the exotherm peak with Ni-coated material matches that of the melting endotherm, indicating a connection to aluminum rather than to the coating material, i.e., aluminum oxidation rather than oxidation of the coating only. Naturally, both can happen simultaneously and the observed weight increase corresponding to nickel oxidation only may be only a coincidence, but this is unlikely. With Co-coated material the oxidation of Al is evident from the 25-fold weight increase compared with how much the Co → CoO reaction gives rise to. In addition, the larger the exotherm at this point, the later the next exotherm appears. This agrees with the

idea of aluminum oxidation at the melting point, since a thicker oxide shell thus formed would confine the particle longer, before molten aluminum breaks out and additional oxidation takes place at higher temperatures. 5.2.3. Carbon dioxide and nitrogen Fluoride-coated powder. The fluoride-coated powder was especially active in CO2 and N2 , showing much stronger exotherms in the DSC and a steeper weight increase in the TGA than the metal-coated powders at high temperatures. The effect at the melting point in these gases was comparable to that for the metal-coated powders. This is also contrary to expectations, since the complex fluoride does not produce

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any extra heat, as the fluorine is already attached to an aluminum atom in the complex fluoride (Eq. (1)). The complex fluoride cannot act as an oxygen/nitrogen carrier either. Therefore, the reactivity at higher temperatures was expected to be weaker than that of the metal-coated powders, but it was not. These findings indicate that the fluoride coating has to catalyze reactions with carbon dioxide and nitrogen. The improved reactivity in carbon dioxide depends on the acidity of CO2 , which promotes corrosion of aluminum, especially if fluorides are present, releasing HF, which attacks aluminum and aluminum oxide efficiently. As the HF or other active fluoride species are set free at the aluminum surface, they can react with unreacted aluminum immediately, explaining the greatly increased activity of fluoride-coated powders in carbon dioxide. Under nitrogen, the increased activity is due to more effective oxide shell destruction by the complex fluoride compared with the metal coatings. As the nitride does not form a solid layer on the particles, the reactions are not inhibited as in air, where the aluminum oxide eventually seals the core of the particle from further oxidation, unless excessive amounts of fluoride is used. The fluoride-coated material reacted almost completely in nitrogen and carbon dioxide, while it did not react more than to about 40% of the theoretical extent in air. Also, the difference in the reactivity between metal- and fluoride-coated powders is much larger in CO2 and N2 than in air. On the other hand, metalcoated powders generally burned faster and ignited more easily in the combustion experiments, except for carbon dioxide, in which the bulk burn rate of complex fluoride-coated powder was significantly higher than that of any other powder. Metal-coated powders. The metal-coated powders show increased reactivity in CO2 and N2 , but not to the extent of the fluoride-coated powder. With the metal-coated powders, galvanic corrosion might be responsible for the increased activity—there is always water present at the surface of the aluminum due to the polarity of aluminum oxide and the aqueous coating conditions, and even trace amounts can promote galvanic corrosion if an acidic substance such as carbon dioxide is present. However, the galvanic corrosion by water would be much weaker than the attack by HF under the same conditions, and less activation is seen than for the complex fluoride coatings. The metal-coated powders were slightly more active in N2 than in air. The same explanation as with the fluoride-coated powders is valid here; i.e., aluminum nitride does not protect aluminum from further reactions. The metal coatings cause nitridation at lower temperatures than the fluoride, since Ni and Co can act as nitrogen carriers while the complex

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fluoride cannot. At high temperatures, however, the solvent effect of the fluoride becomes more significant and nitridation is more complete than that for the metal-coated powders, as metals cannot act as solvents for aluminum oxide. Consequently, all activated powders showed somewhat increased reactivity in nitrogen compared to air. Untreated powder. Untreated aluminum did not react substantially under any of the conditions used. Under CO2 and N2 it reacted even more weakly than in air, as expected. 5.3. Combustion properties The results from the combustion experiments are summarized below according to the reactive gases. Each material is discussed for each gas under which the material was combusted. Untreated material could not be ignited in any experiments. Thus, no data on it were obtained. Igniting untreated A100 powder was attempted in several ways, such as prolonged heating, using activated aluminum as an ignitor, mixing oxidizers to the aluminum at one end of the sample mass, and changing the atmosphere in the pressure vessel to pure oxygen. Even zirconium and magnesium powders as ignitors failed to promote the combustion of the untreated aluminum powder for any pressure or gas that were tried, including pure oxygen at high pressure. 5.4. Combustion in air Different coatings caused quite different effects on the combustion behavior in air. In general, all coatings improved the ignition and combustion processes, but the metal coatings also caused agglomeration of the oxide, sometimes even inhibiting complete combustion. 5.4.1. Complex fluoride coating in air Fluoride coating generated well-behaved and predictable combustion characteristics. The burn rate was linearly dependent on pressure and no agglomeration was detected at any pressure, neither that of the metal nor that of the oxide. The ignition and burn rates were both less than those of the metal-coated powders at low pressures and ignition temperature was also somewhat higher, between 800 and 1000 ◦ C instead of 700 ◦ C with the metal-coated powders (determined from the TGA and DSC). At high pressures, the fluoride-coated powder burned at a comparable rate due to oxide agglomeration, slowing down the combustion of the metal-coated powders, but not the combustion of the fluoride-coated material. The combustion was diffusion-limited, since the combustion rate was linearly dependent on pressure.

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Fig. 10. Burn rate of activated aluminum in bulk (ignition front velocity) as a function of pressure in air.

Fig. 11. Average particle burn rate of activated aluminum as a function of pressure in air. The point at 64 bar pressure for 10% Ni has been removed, since the observed rate was limited by the camera frame rate, not actual combustion rate.

Linear dependency indicates diffusion limitation, since the metal-particle combustion complies with the shrinking core model with Knudsen diffusion conditions through the thickening oxide lobe [38]. The time of combustion for a single particle under these conditions is tc =

ρr 2 RT . 6DMyO2 p

(2)

The average combustion rate is inversely proportional to the time of combustion: 6DMyO2 p r vav = = (3) = const × p. tc ρrRT In the above, ρ = density of the particle (kg/m3 ), r = particle radius (m), R = universal gas con-

stant (8.3143 J/(mol K)), T = absolute temperature (K), D = diffusivity constant through the oxide lobe (m2 /s), M = molar mass of oxidizing gas (O2 , kg/mol), yO2 = molar fraction of oxygen in the oxidizing gas, p = pressure of the oxidizing gas (Pa), and vav average burn front velocity (m/s), i.e., burning rate. If diffusivity through the oxide lobe is the rate-determining step, the burn rate is a linear function of pressure as seen from Eq. (3). The results are presented in Figs. 10 and 11 and in Table 1. 5.4.2. Nickel and cobalt coatings in air The nickel coating was more effective, while the cobalt coating led to less vigorous reactions of the same type, with linear dependency of the pressure

A. Hahma et al. / Combustion and Flame 145 (2006) 464–480 Table 1 Burn rate coefficients of activated Al in air (r = A + Bp) by least squares fit to the measured data Coating

Particles A (mm/s)

F 1% 1.2 Ni 1% −4.1 Ni 10% 0.0a Co 1% 14.0

Bulk B (mm/s/bar)

A (mm/s)

B (mm/s/bar)

1.6 1.5 3.1a 0.59

1.4 1.3b 2.7c 1.4c

0.14 0.55b 0.23c 0.44c

a Last point omitted. b First five points considered. c First four points considered.

up to 8–16 bars and a sudden drop of combustion rate thereafter due to oxide agglomeration. The difference in reactivity can be explained with the higher reaction enthalpy for the Ni–Al intermetallic reaction compared to that of the Co–Al reaction. Nevertheless, both coatings were very effective in promoting ignition of the samples and even accelerated the burn rate compared to the fluoride-coated aluminum. On the other hand, these coatings promoted a strong agglomeration of the resulting combustion products (aluminum oxide), starting at 8-bar air pressure and above, causing an abrupt drop in the burn rate at high pressures. This phenomenon can be traced to two reasons: reduction of the melting point of the aluminum oxide by the other metal oxide and the increased reaction rate, which overcomes heat losses and melts the oxides. With fluoride-coated aluminum, the reaction never reached high enough intensity to win the heat losses and the oxide was not melted or evaporated and no oxide agglomeration occurred. The increased heat generation rate is likely to be a more important factor, since the fluoride coating also reduces the melting point of aluminum oxide coating by acting as a solvent for Al2 O3 , yet no agglomeration was detected. The bulk burn rate coefficients of the metal-coated powders are significantly larger than those of the fluoride-coated ones, suggesting that the oxide agglomeration is indeed caused by the intensity of the reaction. Despite the agglomerates and the strongly reduced bulk burn rate above 4 bars, the metal-coated powders eventually burned as fast as or faster than the fluoride-coated one as particles when the pressure was increased enough. This can also be seen from the burn-rate equation parameters, where the pressure coefficient for the metal-coated powders is as high as or higher than that of the fluoride-coated one both in bulk and as particles. Results are presented in Table 1 and Figs. 10 and 11.

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5.5. Combustion in carbon dioxide Expectedly, the particle burn rates in carbon dioxide were lower than those in air. However, bulk burn rates of the metal-coated powders were often higher than in air, since the combustion was not vigorous enough to melt the oxide and cause agglomeration. The pressure limit for agglomeration was higher than in air and even then, agglomeration was always weaker. 5.5.1. Complex fluoride coating in carbon dioxide Fluoride-coated aluminum ignited as easily as in air, but burned much faster in bulk. Unfortunately, no particle burn data were obtained to confirm whether this powder would have burned faster in CO2 as particles, too. In addition, starting from 4 bars and up, the combustion was partly inhibited. This effect can be seen as a break on the burn rate vs pressure at each pressure starting from 4 bars (Fig. 12). Three possible reasons can be found for the above behavior: heat losses were higher than heat output because of lower heat of combustion and more effective cooling by CO2 compared to air due to the endothermic reactions CO2 → CO, C and due to increased radiation by the deposited carbon. Third, the carbon released from the carbon dioxide was deposited as a solid layer on the particles at high pressures, quenching combustion. Carbon-coated aluminum oxide particles with unburnt aluminum inside them were found in the remains after combustion experiments and the residue was pitch black instead of white. The deposited carbon was fluffy, sooty material at pressures under 4 bars and was not bound to the aluminum oxide residue, while at higher pressures, it was deposited as an increasingly thick and solid graphitelike layer on the particles. 5.5.2. Nickel and cobalt coatings in carbon dioxide The Ni- and Co-coated powders behaved similarly to powders in air, except for the agglomeration, which started at higher pressures and was weaker than in air. These powders also demonstrated carbon coagulation and subsequent quenching of the combustion starting from 4 bars, except the nickel-coated powders, which burned completely under all conditions tried. These powders show no break in the burn rate vs pressure (Fig. 12), until agglomeration occurs at 32 bars pressure. The 10% Ni-coated powder also caused almost as strong an oxide agglomeration as in air, only starting at higher pressure (32 bars instead of 8). The residue from the Ni-coated powders was not layered with carbon, but rather mixed with it, since the particles seemed thoroughly homogeneous under a microscope. The reason is probably the intensity of

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Fig. 12. Burn rate of activated aluminum in bulk (ignition front velocity) as a function of pressure in carbon dioxide.

Fig. 13. Average particle burn rate of activated aluminum as a function of pressure in carbon dioxide.

the reaction, which kept the entire particle molten during the combustion, allowing mixing of the deposited carbon. In addition, the nickel probably reacted with carbon to nickel carbide, preventing a solid graphite shell from forming and quenching reactions until the aluminum particle was exhausted. The bulk burn rate is lower for the higher nickel content material, but this can be addressed to stronger aluminum oxide agglomeration with the 10% powder; otherwise, increasing the nickel content increases burn rate, at least in air. In the other gases studied, a definite conclusion cannot be made, since there are not enough datapoints. The nickel-coated powders were also the only ones from which particle burn-rate data were obtained in CO2 , while the other powders did not generate any visible individual burning particles in this gas.

The cobalt-coated powder displayed behavior similar to that of the nickel-coated ones, but suffered from carbon shell formation to some extent, although much less than the fluoride-coated material. Results from the experiments in CO2 are displayed in Figs. 12 and 13 and Table 2. 5.6. Combustion in nitrogen In nitrogen, only the most active 10% nickelcoated powder maintained sustained combustion, probably due to catalytic effects of Ni. The added heat from intermetallic reactions alone cannot explain the reactivity, since the exotherm occurs at much lower temperatures than actual combustion, based on the thermoanalytical results. None of the other powders ignited, or reacted only partially, despite strong reac-

A. Hahma et al. / Combustion and Flame 145 (2006) 464–480 Table 2 Burn rate coefficients of activated Al in CO2 (r = A + Bp) by least squares fit to the measured data Coating

F 1% Ni 1% Ni 10% Co 1%

Particles

Bulk

A (mm/s)

B (mm/s/bar)

A (mm/s)

B (mm/s/bar)

2.1 1.9a

0.43 0.68b

2.6 1.2b 1.5 1.8b

0.36 0.28b 0.22b 0.33b

a Last point omitted. b First four points considered.

tion in TGA and DSC experiments. Also, increasing the nitrogen pressure very soon quenched even the 10% Ni-coated powder due to increased heat losses and only three measurement points were obtained, giving A = 0.83 mm/s and B = 0.36 mm/s/bar as the burn rate coefficients for 10% Ni-coated powder. No powder could be burned in nitrogen above 4 bars pressure. The only result obtained in nitrogen is presented in Fig. 14. When 0.2 bars partial pressure of oxygen was added (that is, nitrogen was added to air at normal pressure), all powders did burn and showed increasing burn rate as a function of pressure despite the constant (absolute) amount of oxygen. Quantitative measurements of the burn rates with added oxygen were not determined, but only point measurements were made to discover whether nitrogen can support Al combustion or if extra oxidizer is needed for less active powders. No particle burn rates were obtained in either nitrogen or nitrogen augmented with air, as no individual burning particles were detected.

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Nevertheless, the above experiments show that nitrogen took a major role in the reactions and the additional heat from reactions with oxygen kept the nitrogen–Al reaction going. Aluminum nitride was detected qualitatively after the experiments by adding a droplet of water to the residue, causing vigorous evolution of ammonia. Significant amounts of nitride were similarly detected even after experiments in air, especially if 10% nickel-coated powder was used. We conclude from this observation that nickel catalyzes nitride formation, which also explains the increased combustion rate in air. If the reactions are diffusion-limited, as seen in the case of complex fluoride-coated powders, then the burn rate under the same conditions should not be dependent on the coating. If, however, nitrogen also takes part in the reactions, the burn rate can increase, as nitrogen concentration in air is four times that of oxygen. An indication of this can be seen by observing the coefficients B for the particle burn rate in air, showing greatly enhanced burn rate for the 10% Ni-coated powder. The bulk burn rates are not relevant, since strong oxide agglomeration disturbed the measurements, and in CO2 the fluoride coating was more effective due to other catalytic effects.

6. Discussion The experiments demonstrated the effectiveness of aluminum activation by four fundamentally different methods: heat addition, destruction of oxide layer at elevated temperature, catalytic oxygen and nitrogen carrier effects, and enhanced corrosion at elevated temperatures in carbon dioxide.

Fig. 14. Burn rate of activated aluminum in bulk (ignition front velocity) as a function of pressure in nitrogen.

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The metal coating, adding heat at the beginning of the ignition process, usually proved more effective and more generally applicable, and it also included other effects, such as oxide melting point reduction, oxygen carrier effects, and some Al–N2 and Al–CO2 reaction catalysis, which can even accelerate combustion rates. The complex fluoride coating was most effective in carbon dioxide due to the acidity of that gas. The particle combustion rates are more informative as a measure of activation than combustion in bulk, which gives the propagation rate of the combustion wave rather than the mass burning rate of the particles. However, the bulk combustion rates are also presented, since they are a measure of agglomeration tendency, as can be seen in Figs. 10–14, in which agglomeration is displayed as a sharp reduction in bulk burn rate. The effects of catalysis or inhibition appear in both the bulk and the particle burn rates. The particle burn rates are decreasingly accurate the faster they are, since the camera image frequency limited time resolution to 0.5 ms. Also note that the fastest measurable combustion rate was 80 mm/s (40 µm/0.5 ms), limited by the frame rate of the camera. Thus, anything above that value could not be registered. Hence, in Fig. 11 the last point of 10% Nicoated aluminum is too low, since the actual burn rate exceeded the range of the equipment. This point was omitted from the burn rate coefficient calculations, but is shown in the figure for completeness. The tendency is still very clear, showing that the burn rate is linearly dependent on pressure, indicating diffusioncontrolled burn [41–44]; also see Eq. (3). In the case of nickel-coated powders, the combustion reactions also include nitride formation, which explains why the diffusion limit of the burn rate for these materials is higher than that of the fluoride-coated powders. Diffusion control indicates that the activation process, despite its effectiveness in aiding ignition and flame propagation, is less effective in promoting burn rate, since the rate is dependent on the oxidizing gas properties, even though heat generation at the surface of the particles has been found to affect burn rates as well [43]. However, the observation by Legrand is probably due to increased diffusion rate in the vicinity of the particle due to higher local temperature and not an intrinsic property of the material itself. This question could be verified by repeating the experiments in mixtures of helium and oxygen in the same concentration as in air, which should lead to faster particle burn rates, in analogy to those presented in Refs. [39,40]. Unfortunately, we did not have a possibility to test the idea within the scope of this work. Too strong an activation can also lead to negative results due to oxide agglomeration. Since the agglom-

eration of the oxide severely limited the bulk burn rates, it may actually be more beneficial to prevent all kinds of agglomeration instead of striving for maximum ignitability and burn rate, at least if a high concentration of aluminum powder is present, which is the case especially in propulsion applications. If the particles ignite and burn very fast before they contact each other, oxide agglomeration has no significance as far as combustion is concerned, but can still have detrimental effects on heat transfer from the combustion products to the working fluid in, for instance, a rocket motor if large droplets of molten oxide are formed. At high pressures the least agglomerated powder often burned fastest (i.e., the fluoride-coated powder). On the other hand, this is not a problem in applications where aluminum powder is not used in bulk but in a mixture with other materials, preventing direct contact of aluminum particles with each other or at low pressures, where the oxide agglomeration does not become a problem. A typical example of such an application is a thermobaric explosive, where the metal powder is dispersed in air and where fast combustion at low pressures is desirable with a known ignition delay. Hence, selecting an ideal method of activation is heavily application-dependent. The DSC and TGA measurements show a clear correlation with the burn rates and ignitability observed. The steeper the slope of the TGA or the larger the exotherm in DSC, the higher the burn rate and the higher the burn rate vs pressure coefficient. Also, the lower the temperature of the first, strong exotherm the better the ignitability, i.e., bulk burn rate, ignoring the effects of agglomeration. Hence, thermoanalysis provides an easy and fast method of characterizing activated metals by the above criteria, e.g., starting temperature and slope of the exotherm. Thermoanalysis does not, however, provide any information about oxide agglomeration. As an example, the 10% nickel-coated aluminum is less reactive in TGA and DSC experiments in CO2 and N2 than fluoride-coated material, but nevertheless, the 10% Ni-coated powder was much easier to ignite even in carbon dioxide and nitrogen and was the only powder that provided particle burn rate data in all gases. This discrepancy can be traced back to the oxide agglomeration of the metal-coated powders and does not indicate a break of correlation between thermoanalysis and burn rate data. Carbon dioxide proved the most reactive gas in the thermoanalysis, and the highest bulk burn rates were observed in this gas as well. Unfortunately, particle burn rate data were not obtained for all powders in CO2 , possibly due to obscuration by free carbon (soot, smoke, and carbon coagulation) during the combustion process. The higher reactivity of

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carbon dioxide can be addressed to the acidic character of this gas combined with a catalytic effect of the activating materials in an acidic medium by enhanced corrosion of aluminum and aluminum oxide. At high pressures, CO2 caused solid carbon coagulation, which quenched the combustion before the particles had been completely consumed, unless a nickel coating was used. As a result, the activation efficiency is not only dependent on the application but also on the reactive gas atmosphere and pressure. Choosing a right combination of activation has many restricting boundary conditions discussed above, such as oxide agglomeration with strong activation by metal coatings, carbon coagulation at high pressures with fluoride coatings, energy losses due to nitride instead of oxide formation, and the fact that the activating materials are an inert ballast not providing energy to the combustions. These, together with the optimal amount of coating material, need to be balanced with the positive effects of the activation in order to benefit from using activated powders in an application.

7. Conclusions Agglomeration behavior and ignition and combustion properties of three types of activated aluminum powder were studied. The combustion properties were compared to the thermoanalysis (TGA and DSC) of the same materials. All types of activation completely prevented agglomeration before and at combustion, while nickel coating promoted agglomeration of the combustion residue due to increased heat output rate. The fluoride coating prevented agglomeration of the residue at all pressures and in all gases. Ignitability of all powders was greatly improved by the activation. Untreated material could not be ignited at all, while all activated powders ignited and burned in air and carbon dioxide, and powder coated with 10% of nickel even ignited and burned in nitrogen. The cobalt coating promoted exothermic reactions and oxidation of aluminum at the lowest temperature, much before the melting point of aluminum, followed by nickel and complex fluoride coatings, which promoted significant oxidation only at and after the melting point. Despite this, the nickel-coated powders demonstrated the best ignitability in the combustion experiments. The complex fluoride coating also promoted weakly exothermic reactions at and before the melting point of aluminum, but the main reaction only started at the melting point of the coating at higher temperature than the melting point of aluminum, leading to lower ignitability than the metalcoated powders demonstrated.

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The burn rate was increased by activation in all gases. The increase was particularly significant in carbon dioxide and less pronounced in air. In nitrogen, only one powder burned and no burn rate comparisons can be made. The fluoride coating increased the burn rate in carbon dioxide most effectively and the burn rate of this material in carbon dioxide even exceeded that of the same powder in air, while metal coatings were more effective in air. The increase in burn rate was addressed to catalytic effects (oxygen and nitrogen transport) by the activation, to accelerated diffusion rate through the oxide lobe of a burning particle and increased local temperature, also accelerating the diffusion. Catalytic effects by nickel were so strong that they promoted nitride formation even when the powder was combusted in air. Carbon coagulation and subsequent quenching of the combustion was observed in carbon dioxide at high pressures. The quenching by this coagulation was prevented by nickel coating, but not by complex fluoride coating. Results from thermoanalysis were found to correlate with the combustion data. The slope of the signal in the positive direction (heat flow or weight) was deemed the most important parameter and the onset temperature the second most important. Maximal slope and minimal onset temperature correlate with highest burn rate and best ignition properties. The final degree of reaction in a thermoanalytic experiment was not significant and did not correlate with combustion properties. Activation by complex fluoride and metal coatings was deemed a very effective way to regulate the combustion properties of aluminum in a wide range and, above all, it provides a way to completely prevent agglomeration of aluminum powder before, during, and after combustion. The activation allows the control of the ignition temperature and ignition delay. In addition, it also provides some control over the burn rate of aluminum powder. Acknowledgments This work was supported in part at the Technion— Israel Institute of Technology by a Fellowship of the Israel Council for Higher Education. Thanks are also due to Dr. Valery Rosenband for his help in carrying out the thermoanalyses. Supplementary material The online version of this article contains additional supplementary material. Please visit DOI: 10.1016/j.combustflame.2006. 01.003.

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