Study of Basic Oxidation and Combustion Characteristics of Aluminum

Article pubs.acs.org/EF

Study of Basic Oxidation and Combustion Characteristics of Aluminum Nanoparticles under Enginelike Conditions Charalambos Mandilas,† George Karagiannakis,*,† Athanasios G. Konstandopoulos,*,†,‡ Carlo Beatrice,§ Maurizio Lazzaro,§ Gabriele Di Blasio,§ Santiago Molina,∥ José V. Pastor,∥ and Antonio Gil∥ †

Aerosol & Particle Technology Laboratory (APTL), Chemical Process & Energy Resources Institute, Center for Research & TechnologyHellas (CERTH/CPERI), P. O. Box 361, 57001 Thermi-Thessaloniki, Greece ‡ Department of Chemical Engineering, Aristotle University, P. O. Box 1517, 54006 Thessaloniki, Greece § Istituto MotoriCNR, Viale Marconi, 81056 Naples, Italy ∥ CMTMotores Térmicos, Universitat Politècnica de València, Camino de Vera s/n, 46022 Valencia, Spain S Supporting Information *

ABSTRACT: The prominent aim of this investigation was the examination of the in-principle feasibility of aluminum combustion under internal combustion engine (ICE)-like conditions. This study was performed in the framework of recent consideration of metallic nanoparticles as alternative fuels for ICE engines. Aluminum nanoparticles of different morphologies and sizes were studied with respect to their fundamental oxidation characteristics via thermogravimetric analysis under various nitrogen−oxygen environments and by spark-ignition of Al nanopowder “strips” under controlled airflow at ambient pressure. The ICE-like tests included measurements performed in two different arrangements; namely, a shock-tube setup and a constantvolume combustion vessel. A set of engine tests was also performed in a single-cylinder compression-ignition engine with a customized, single-shot aerosol injection system. Burned powder samples were, in all cases, examined via in situ and ex situ techniques for the identification of products and their morphologies. The results largely verified that combustion of aluminum particles in an engine environment is indeed feasible. Nonetheless, prominent differences, in terms of the products formed and their morphologies/structures, were identified among the various oxidation/combustion techniques employed.

1. INTRODUCTION Aluminum is a well-known solid fuel for rocket propellant, and its combustion under relevant conditions has been studied by several research groups.1−3 Selection of aluminum fine powder as energetic material for rocket engines is due to its high energy content (33 MJ/kg), combined with its low theoretical density and the fact that it is the most abundant metal in nature, constituting approximately 8.2% (mass basis) of earth’s crust. Nanoaluminum powder, in particular, has recently attracted great attention, because of its high burning rate and relatively low ignition time and temperature. The overall reaction of stoichiometric aluminum oxidation with air is provided below:

Additional prominent advantage of aluminum combustion is the fact that it can be emission-free as, contrary to fossil fuels combustion, it derives no CO2, CO, or unburned hydrocarbons. On the other hand, the current dominant production process of the metal (Hall−Héroult process) is associated with both direct (sacrificial carbon anodes) and indirect (high power consumption generated from fossil fuels) CO2 emissions. There has been much research regarding replacement of carbon anodic electrodes with inert ones during past decades and it is believed that in the mid-to-long term a viable solution will emerge. If this is combined with the fact that, inevitably, utilization of electricity generated via renewable means will eventually become a necessity, it can be deduced that future environment-friendly Al metal production is feasible. In addition, one will need to consider that oxidized Al nanofuel, as derived from reaction 1, can be collected and recycled back to the metal. Moreover, recent advances in nanotechnology4,5 have already allowed mass production of metallic nanoparticles via established methods, and this is expected to be intensified in the near future. The mechanism of aluminum oxidation is complex, involving several individual steps concerning formation of intermediate Al−O species of varying stoichiometry as well as phase changes of reactant metal and product oxidized species. Such a

2Al + 1.5O2 + 5.64N2 → Al 2O3 + 5.64N2 ; ΔH = −837.85 kJ/mol (298 K)

(1)

Additionally, the presence of nitrogen can also lead to the nitridation of metal, according to the following reaction: 2Al + N2 → 2AlN;

ΔH = − 317.98 kJ/mol (298 K) (2)

From an energetic point of view, reaction 2 is undesirable due to the substantially lower energy release as compared to (1). Based on the fact that reaction 1 is, in general, more thermodynamically favored, it is expected that oxidation will always proceed to a significantly higher degree than nitridation when air is used as the oxidizer. © 2014 American Chemical Society

Received: January 16, 2014 Revised: March 24, 2014 Published: April 21, 2014 3430

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2. EXPERIMENTAL SECTION

mechanism could differ, depending on the conditions of aluminum oxidation and the nature of the oxidizing environment. Huang et al.6 proposed a scheme of oxidation involving 11 different species and 12 individual reaction steps that was found to be valid for aluminum combustion under conditions similar to rocket propellant related applications. This scheme is reproduced below: Al + O2 ↔ AlO + O

(3)

Al + O ↔ AlO

(4)

AlO + O2 ↔ OAlO + O

(5)

Al 2O3 ↔ AlOAlO + O

(6)

Al 2O3 ↔ OAlO + AlO

(7)

AlOAlO ↔ AlO + AlO

(8)

AlOAlO ↔ Al + OAlO

(9)

AlOAlO ↔ AlOAl + O

(10)

OAlO ↔ AlO + O

(11)

AlOAl ↔ AlO + Al

(12)

Al ↔ Al(l)

(13)

Al 2O3 ↔ Al 2O3(l)

(14)

The aluminum fuels investigated in the current work included three different grades of commercial nanopowders. The most prominent difference among them was the APPS, which, based on the specifications provided from the manufacturers, varied in the range of 18−85 nm. The main physicochemical properties of the three nanopowders were determined by XRD, TEM/EDS analysis, and Brunauer−Emmett−Teller (BET) specific surface area measurements. A summary of these properties is included in Table 1. The aluminum

Table 1. Main Physicochemical Characteristics of Utilized Aluminum Nanopowders

nanopowder

estd av primary particle size/ manufacturer’s specification (nm)

av thickness of oxide layer (nm)

BET (m2/gr)

Al18

32/18

2−5

47.3

Al50 Al85

55/50 90/85

2−5 2−5

32.8 35.2

phases identified by XRD metallic Al, Al2O3 metallic Al metallic Al

nanopowders employed in the present study were obtained from NaBond Technologies Co. Ltd., READE Advanced Materials, and American Elements. On the basis of the APPS data provided by the manufacturers, the Al nanopowders are respectively referred to as Al18, Al50, and Al85 throughout this work. In all cases, their purity was 99.9+%. Representative TEM images of the three fresh aluminum nanopowders utilized are depicted in Figure 1. In general, they

Of great importance regarding the rates at which the above reactions occur is the average primary particle size (APPS) of the pulverized aluminum fuel. Past studies7 have shown that particle size governs the mechanism of combustion and, consequently, its efficiency. In general, the smaller the size of the average cluster or particle is, the faster the rate of combustion and the lower its activation energy. This has been attributed to the effects of surface area on the reactivity of a particle.7 The onset of oxidation and low temperature chemistry of Al or other metallic fine powders has been customarily studied via thermogravimetric analysis (TGA). For Al nanopowders in particular, TGA analyses have been published in several past studies.8,9 Bulk combustion of powder “strips” placed inside a container is another method that has been used to characterize the ignitibility of aluminum. An example is provided in a work by Hahma et al.,10 though this study solely involved micrometer-sized aluminum powder (APPS of 80 μm). Enginelike combustion of aluminum ultrafine powder has traditionally been explored with the use of shock-tube11−13 or constant-volume combustion vessel14−16 experiments. The current work addressed nanoaluminum combustion utilizing all aforementioned techniques (TGA, bulk powder combustion, shock-tube, and combustion vessel). In addition, mainly stemming from recent published works17 considering the possibility of metallic nanoparticles future utilization as zero-emission transportation fuels, the study ultimately included a set of real-engine experiments. The oxidation/ combustion behavior of different grades of aluminum powders and the main phenomena involved were compared using in situ and ex situ analysis (transmission electron microscopy/energy dispersive X-ray spectrometry (TEM/EDS) and X-ray diffraction (XRD)).

Figure 1. TEM images of (a) Al18 nanopowder, (b) Al50 nanopowder, (c) Al85 nanopowder, and (d) high-resolution image showing the amorphous oxide layer of primary particles. consisted of dense, almost spherical primary particles that tended to form larger agglomerated structures. In all samples, some sintering can be observed and this is more prominent for the Al18 grade. As verified by EDS analysis, the rodlike structures, forming “‘necks’” among metallic primary particles, observed in this specific nanopowder, is attributed to the Al2O3 phase. Contrary to the other two grades, the presence of Al2O3 in Al18 is clearly detectable by XRD, as stated in Table 1, indicating that the oxide phase accounts for an appreciable amount (i.e., typically more than 5−10 wt % and for crystalline phases only). Metallic particles were found to be coated by an amorphous thin oxide shell (e.g., Figure 1d) of 2−5 nm in thickness, independently of the particle size. This layer is generally necessary for the passivation of the individual nanoparticles, and its formation constitutes a typical step during their production, in order to avoid undesired spontaneous ignition phenomena under ambient air. An approximation of the actual metal content of the three nanopowders studied was calculated by considering the APPS sizes measured via TEM imaging (sample images in Figure 1; APPS values in Table 1) 3431

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Figure 2. Schematic description of the spark-ignited self-propagating oxidation technique. under the assumption of perfectly spherical particles, an average oxide thickness of 3 nm, and the theoretical densities of Al and Al2O3. Additional relevant information as well as indicative primary particle size distribution per nanopowder employed are included in the Supporting Information. Note that, in the case of the Al18 powder, TEM analysis revealed a significant number of rodlike particles present in the sample examined (Figure 1a). Based on EDS measurements combined with XRD analysis, this rodlike morphology was attributed to the presence of a substantial amount of Al2O3 in the original sample. The size of these rodlike particles was not accounted for during estimation of APPS derived from TEM imaging because, by doing so, the assumption of a spherical structure would be violated. Such plain calculations revealed that the net metal contents for Al18, Al50, and Al85 were approximately 70%, 79%, and 87%, respectively. Based on the XRD and TEM/EDS findings reported above, it is reasonable to assume that, for the case of the Al18 grade, in addition to the formation of the amorphous protective oxide layer, the passivation process resulted in deeper oxidation of the sample. As a result a distinct crystalline oxide phase was formed, ultimately further reducing the Al content of the particular nanopowder to values on the order of 60% or even less. The TGA studies were conducted in a PerkinElmer Pyris-6 instrument under four different O2/N2 environments, with the O2 content ranging between 0% and 100%. The gaseous stream was continuously flowing over the tank containing the nanopowder, while temperature was increased from ambient conditions up to 1273 K at a rate of 5 K/min. Structural characterization of the product oxide powders was performed by XRD using a Siemens D-500 Kristalloflex X-ray powder diffractometer (Cu Kα radiation). Specific surface area (BET method) was measured with the aid of a nitrogen adsorption porosimeter (Micromeritics ASAP 2000). TEM was performed, using a JEOL, JEM 2011 microscope operating at an accelerated voltage of 200 kV. The microscope was also fitted with an Oxford Instruments INCAx-sight liquid nitrogen-cooled EDS detector with an ultrathin window for chemical analysis of the samples. The spark-ignited self-propagating “strip” oxidation technique was based on the main principles of the self-propagating high-temperature synthesis method.18 The experimental protocol was applied as follows: A quantity of 0.4−0.6 g from each Al nanopowder was placed in a molybdenum container/scaffold (dimensions, 46 mm × 20 mm × 11.5 mm). The powder formed a relatively thin strip-shaped layer, occupying the whole length of the container. A tungsten wire, connected with two welding equipment electrodes (Cemont Pratika 2160T MMA Welder) was dipped into the powder near one edge of the scaffold. At t = 0, the wire was spark-ignited by instantly applying voltage (230 V) from the welding device. Reaction was self-sustained thereafter. An air flow of 50 L/min (std) was flowing inside the chamber throughout the duration of the experiment. Figure 2 shows a simplified description of the spark-ignited “strip” oxidation method. An IR thermal camera (FLIR ThermaCAM SC2000) with a focal plane array detector type, a spectral range of 7.5−13 μm, an object temperature range up to 2000 °C, thermal sensitivity < 0.1 °C, and measurement accuracy of ±2% was used for the temperature measurements. The camera was mounted on a ZnSe window on the top of the reaction chamber. Its lens zoomed-in to record the

temperature distribution across the powder container. The temperature of the combustion front was derived by postprocessing of the thermal imaging data acquired from the camera. The shock-tube tests were performed under synthetic air conditions, at a pressure of ca. 10 bar and temperatures of 900−1200 K. The apparatus used was of the conventional diaphragm-type, with a running section of 50 mm inner diameter and 6 m length. Helium was used as the driver gas. For reasons of illustration, a schematic of the basic features of the apparatus is shown in Figure 3; full description of

Figure 3. shock-tube measurement section and optical diagnostics setup. the equipment and data processing techniques used is however omitted here, as it can be found in a previous relevant study.19 Typical injection mass was on the order of a few milligrams of powder. The oxidation process of aluminum particles was characterized by means of two-color optical pyrometry. The thermal radiation signals were measured at wavelengths of λ1 = 800 nm and λ2 = 600 nm (band-pass interference filter; full width at half-maximum (fwhm) =10 nm). The constant-volume, high-temperature combustion vessel utilized is illustrated in Figure 4. Detailed information on the particular combustion vessel is provided in a previous study.19 Basic features of the vessel include the following: (a) adequate optical access for visualization of the injection/combustion process, (b) coil heater for initial temperature control, (c) refractory mortar (Greenlite 45 L) for 3432

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Figure 4. High-pressure, high-temperature constant-volume test rig setup.

Figure 5. First engine setup utilized for the studies of aluminum nanoparticles combustion (top) and nanoparticles injection system in the intake duct (bottom).

Table 2. Main Characteristics of the Engine Utilized for the Aluminum Nanoparticles Combustion Studies

insulation and consequent minimization of heat losses through the walls, and (d) batch operation, pressurized gas stream powder injection system. A shadowgraph arrangement was used for combustion imaging; images were recorded at a rate of 4 × 104 frames/s using a Phantom V12.0 high-speed camera. The engine setup is schematically shown in Figure 5 (picture on top). The main characteristics of the single-cylinder compressionignition ICE employed in the present study are reported in Table 2. The engine was instrumented with sensors for the required in-cylinder pressure and intake/exhaust line pressure/temperature measurements. Intake air flow was conditioned and measured by means of a volumetric flow meter. A simplified metallic powder feeding system was placed upstream of the intake valve runner (bottom picture of Figure 5). The feeding system was practically a nanopowder reservoir placed between a check unidirectional poppet valve and a solenoid valve, the latter connected with a low pressure (8 bar) nitrogen supply line. Each “shot” of powder was inducted inside the cylinder upon inlet valve opening and was of a quantity sufficient to perform “ideally” a stoichiometric combustion cycle. The exhaust line was equipped with a conventional diesel particulate filter system for the collection of combusted particles. It must be mentioned that combustion of metallic nanoparticles would naturally require the development of a properly customized ICE to efficiently eliminate undesired phenomena such as clogging, oil contamination, and wear that one would naturally expect to encounter upon combustion of solid fuels. Such issues will be addressed in future studies. However, given the basic-level proof-of-principle nature of the present work, a conventional engine of simple architecture, combined with single-shot nanopowder feeding/combustion, was chosen to serve the particular purpose.

property

value/description

engine type displacement bore/stroke/conrod length compression ratio cooling system

single-cylinder Lombardini SP 15 LD 350 350 cm3 82/66/110 mm 20.3 air

3. RESULTS AND DISCUSSION 3.1. TGA Studies. The TGA experiments examined the effect of the ratio of oxygen/nitrogen and APPS on reaction kinetics. Figure 6a depicts the results obtained for the three nanopowders studied as well as for a standard micrometer-sized aluminum powder (CERAC, 44 μm, 99.5%) examined under air. Defining the slope of the rapid increase in weight gain as the “reactivity” of the powders, and then ranking them with respect to this parameter, the order was found to be coarse Al < Al18 < Al85 < Al50. Evidently, the very low weight gain observed for coarse Al (44 μm) highlights its significantly lower reactivity, as compared to that of the nanopowder grades. One would expect that due to its lower APPS and, consequently, significantly higher surface area, Al18 would encompass the highest reactivity. Such relationship between surface area and reactivity or burn rate of ultrafine Al particles has been reported in several past studies.7,9,15 The behavior observed during the current work was attributed to the fact that, according to the 3433

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for Al50 were 46% under air and 55% under pure O2. A possible reason for this could be attributed to the relatively high degree of agglomeration of the sample powders (Figure 1), which hindered access of air within the powder cluster. However, further justification for this discrepancy could be given by contemplating that the fast-oxidation phase at lower temperatures is likely to have caused local “hot-spots”, which led to partial melting of the metal. This was a possibility since, for all samples, the temperature at the onset of oxidation was close to the melting point of aluminum (933 K). It could be thus assumed that the transformation of partially liquefied metal into Al2O3 was accompanied by rapid solidification of the refractory oxide (melting point of 2345 K). This in turn caused the formation of relatively dense layers of oxide, thus inhibiting the immediate access of air into the bulk mass of the sample. Regarding the effect of oxygen content on Al50 reactivity (Figure 6b), a straightforward trend was identified. As expected, the increase of oxygen content from 2% to 100% caused an acceleration of the lower temperature fast-oxidation stage. Similarly, the weight increase attributed to oxidation was enhanced. In the case of pure N2 atmosphere, the TGA curve showed relatively slow weight increase at lower temperatures (800−1000 K), with its shape clearly resembling that of the 2% O2/98% N2 curve at T > 1000 K. Thus, it could be deduced that this second phase of weight increase (i.e., at T > 1000 K) in N2 containing atmospheres is predominantly attributed to the relatively slow nitridation reaction. 3.2. Spark-Ignited Bulk Combustion Studies. An overview of the main results obtained during spark-ignited, bulk combustion of the three aluminum nanopowders is provided in Table 3. Micrometer-sized coarse Al powder was

Figure 6. TGA analysis under air of the nanopowders studied and of one micrometer-sized aluminum powder (a) and the same analysis for Al50 nanoparticles under four different O2/N2 mixtures (b).

Table 3. Main Properties of Aluminum Nanopowders after Their Spark-Ignited Self Propagating Combustion

relevant analysis provided earlier in the Experimental Section, the XRD-detactable crystalline Al2O3 impurities in the Al18 fuel sample, together with the amorphous oxide layer (Figure 1d) accounted for at least 30% of the overall mass of the sample. For such a case, the maximum weight increase after full oxidation corresponds to a value of 35%, which is close to the terminal weight increase of 30% depicted in Figure 6a. Thus, the fact that this particular sample was, to some extent, already oxidized rendered it less reactive than the other two nanopowder grades. A further observation from the TGA results was that, in all cases, the nanopowders appeared to gain weight within two distinct temperature ranges: a rapid weight gain at the temperature range of 750−900 K and a second, slower weight gain region (>1000 K). Since aluminum oxidation is more thermodynamically favored, it is reasonable to assume that the first steep weight increase is due to metal oxidation. The second phase of weight gain is probably due to nitridation, presumably in combination with a second oxidation phase. Similar findings have been described by Eisenreich et al.9 Also common between the current study and the one of Eisenreich et al.,9 was the observation that the weight increase during the metal oxidation phase and the final weight change were noticeably lower than the respective theoretical values corresponding to full oxidation of the nanopowders tested. Accounting for the presence of the oxide layer in the original fuel particles, these theoretical values for Al50 and Al85 were estimated to be on the order of 65% and 75%, respectively. Based on Figure 6, the actual measured terminal weight increase for Al85 under air was 58% while the respective values

nanopowder

peak combustion temp recorded (K)

phases identified by XRDa

Al18

1827

Al50

1747

Al85

1642

Al2O3, AlN, Al Al2O3, AlN, Al Al, AlN, Al2O3

wt gain during TGA under air of oxidized sample (%) 2.9 6.5 11.0

a

Phases written in descending order with respect to the intensity of their characteristic peaks, as identified in their XRD patterns.

also tested; however, no ignition took place, thus confirming its low reactivity as identified in the aforementioned TGA studies. For the three nanosized grades, combustion temperatures peaked well above the melting point of the metal (933 K). The highest overall combustion temperature was recorded for the case of Al18 (1827 K), while peak temperatures for Al50 and Al85 were measured at 1747 and 1642 K, respectively. Note that these temperatures correspond to a mean value on the surface across the instantaneous combustion front. Therefore, the occurrence of spots or local areas with higher temperatures cannot be excluded. Prior to presentation of the results, it should be mentioned that potential imprecisions in obtained measurements, often associated with nanoparticles combustion monitoring with the aid of IR cameras, cannot be excluded. For example, the IR camera was operated under the assumption of gray body radiation, which in one hand may be a fair approximation for 3434

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Figure 7. Four sequential images, as recorded by the IR camera at 1 image/s rate, during spark-ignited self-propagating oxidation of Al50 nanopowder.

the case of an optically thick medium that is indeed the case here. However, due to such an assumption a certain degree of inaccuracy regarding calculated temperatures is to be expected. Moreover, during the particular experiments potential effects of secondary importance such as heat losses were considered negligible, and other factors such as particle packing and potential associated heat transfer differences among neighboring particles were not investigated to identify optimum formulations. In general, regarding temperature measurements presented here, one should consider them as indicative and focus on the relative differences among the nanopowders evaluated, as their combustion and relevant measurements performed referred to identical conditions. In any case, the most important findings of the particular oxidation approach are provided by combusted samples postanalysis performed that, as will be made evident later in the text, revealed clear differences among them with respect to both structural/ morphological characteristics and the reactions extent. The oxidized Al18 grade was found to have the lowest amount of unreacted metal. This was apparent by TGA performed for the burned samples, where the weight gain of Al18 was only 2.9%. Combusted samples of the remaining two powder grades had weight gains of 6.5% (Al50) and 11% (Al85). As already stated, the Al18 sample already contained appreciable amounts of Al2O3 in its original state. However, it can be legitimately concluded that Al50 was oxidized and nitridated to a larger degree than Al85, indicating that there was indeed a correlation between primary particle size and nanopowder reactivity. In all cases, AlN was present in XRDdetectable quantities demonstrating that the nitridation reaction was nonnegligible under the conditions employed. Depicted in Figure 7 are four sequential images (at a recording rate of 1 image/s), as captured by the IR thermal camera during combustion of the Al50 grade. These images are representative of the combustion process of all nanopowders tested. The first image shown was taken at t = 5 s after ignition. It can be seen that the reaction was intense and proceeded fast. In this particular case and on the basis of the analysis of all images recorded during oxidation, the maximum temperature was 1474 °C (1745 K) and occurred 11 s after ignition. Based on similar photographic evidence acquired for the three samples tested, it was found that the peak reaction temperatures were reached within 5−12 s from ignition. The selected TEM images of the combusted samples displayed in Figure 8, although referring to Al18 and Al50 grades only, are representative for Al85 too; i.e., the different morphologies/structures depicted were identified in all combusted samples. The identification of the Al-based phases

Figure 8. Indicative TEM images of oxidized nanopowders by sparkignited self-propagating oxidation: (a) Al18, (b) Al50, (c) highresolution image from oxidized Al50 sample, and (d) image showing aluminum nitrate formations.

corresponding to the various morphologies observed in the TEM images was facilitated by EDS analysis. The TEM images revealed relatively hollow oxidized particles of primary sizes that were generally in the range of 30−150 nm together with small particles (