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Oxidation kinetics of aluminum diboride
Journal of Solid State Chemistry 207 (2013) 163–169
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Oxidation kinetics of aluminum diboride Michael L. Whittaker a,n, H.Y. Sohn b, Raymond A. Cutler c a b c
Department of Materials Science and Engineering, University of Utah, 122S. Central Campus Drive, Salt Lake City, UT 84112, USA Department of Metallurgical Engineering, University of Utah, 135S 1460 E, Rm 00412, Salt Lake City, UT 84112, USA Ceramatec, Inc., 2425S. 900W., Salt Lake City, UT 84119, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 21 May 2013 Received in revised form 30 August 2013 Accepted 21 September 2013 Available online 26 September 2013
The oxidation characteristics of aluminum diboride (AlB2) and a physical mixture of its constituent elements (Alþ 2B) were studied in dry air and pure oxygen using thermal gravimetric analysis to obtain non-mechanistic kinetic parameters. Heating in air at a constant linear heating rate of 10 1C/min showed a marked difference between Al þ2B and AlB2 in the onset of oxidation and final conversion fraction, with AlB2 beginning to oxidize at higher temperatures but reaching nearly complete conversion by 1500 1C. Kinetic parameters were obtained in both air and oxygen using a model-free isothermal method at temperatures between 500 and 1000 1C. Activation energies were found to decrease, in general, with increasing conversion for AlB2 and Al þ2B in both air and oxygen. AlB2 exhibited O2-pressureindependent oxidation behavior at low conversions, while the activation energies of Al þ2B were higher in O2 than in air. Differences in the composition and morphology between oxidized Alþ 2B and AlB2 suggested that Al2O3–B2O3 interactions slowed Al þ2B oxidation by converting Al2O3 on aluminum particles into a Al4B2O9 shell, while the same Al4B2O9 developed a needle-like morphology in AlB2 that reduced oxygen diffusion distances and increased conversion. The model-free kinetic analysis was critical for interpreting the complex, multistep oxidation behavior for which a single mechanism could not be assigned. At low temperatures, moisture increased the oxidation rate of Alþ2B and AlB2, but both appear to be resistant to oxidation in cool, dry environments. & 2013 Elsevier Inc. All rights reserved.
Keywords: AlB2 Oxidation Kinetics Activation energy Energetics
1. Introduction Aluminum diboride (AlB2) is a promising energetic fuel additive because of its high volumetric heat of combustion and favorable oxidation characteristics [1]. It may also be an important reaction intermediate in lithium aluminum hydride/lithium borohydride hydrogen storage systems [2]. Therefore, it is critical to understand the oxidation kinetics of AlB2, in order to either exploit its rapid heat release as an energetic additive, or to prevent oxide formation and degradation during thermal cycling in hydrogen storage systems. The oxidation characteristics of both aluminum and boron powders have been well studied. As boron particles react with oxygen, its diffusion towards the unreacted particle core is retarded by the formation of a thickening, molten, vitreous boron oxide (B2O3) layer [3,4]. Because B2O3 melts at 450 1C, but does not boil until 2067 1C [5], boron oxidation is generally slow and incomplete. Aluminum oxidation kinetics are initially controlled by a thin (2–4 nm) amorphous layer, which limits Al3 þ diffusion from the melt. As this layer increases
n Corresponding author. Current address: Northwestern University, 2220 Campus Dr., Evanston, IL 60208, USA. Tel.: þ 1 801 347 6924. E-mail address: [email protected] (M.L. Whittaker).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.09.028
to 6–10 nm thickness, oxygen diffusion limits growth and crystallization proceeds through polymorphic transformations, which change growth rates [6,7]. In Al–B–O systems, the oxide interactions gain another level of complexity. Al2O3 and B2O3 will react to form 2Al2O3 B2O3 (Al4B2O9) at temperatures below 1035 1C, and decompose into 9Al2O3 2B2O3 (Al18B4O33) at higher temperatures [8,9]. Because B2O3 melts at 450 1C, transport at high temperatures is generally sufficient for the two oxides to come into contact and react. Solid Al4B2O9 acts to remove liquid B2O3 from particle surfaces in mixed systems, and may adopt a needle-like morphology [10], instead of a spherical shell. The oxidation kinetics of Al–B mixed powder systems or AlB2 have not been investigated to the authors' knowledge. The complex interactions between oxides, polymorphic phase transformations in Al2O3, reaction of unoxidized aluminum and boron, peritectic decomposition of AlB2 or Al4B2O9, vaporization of B2O3 and morphological changes make it exceedingly difficult to analyze this system with a single mechanistic model. Therefore, it is of interest to obtain kinetic parameters that account for all of the processes that occur as aluminum and boron co-oxidize and provide a quantitative description of oxidation rates and their temperature dependence unambiguously. A second purpose of this paper is to report on the resistance of AlB2 to oxidation in moist environments at low temperatures.
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extent of reaction, or conversion, α, for each temperature. The rate of the conversion can be written as [13]
2. Experimental procedure 2.1. Powder preparation 2
AlB2 powder with an average surface area of 2.0 m /g (d50 particle size of 5.7 μm) was purchased from ABCR (Karlsruhe, Germany). The powder (synthesized by H.C. Starck) had reported impurity concentrations of 1.9 wt% O, 0.2 wt% C, 0.1 wt% N and 0.1 wt% Fe, and also contained 6% unreacted aluminum [11]. This powder was compared to a stoichiometric physical mixture of boron (H.C. Starck amorphous boron 95%, 10.9 m2/g, 0.2 μm d50) and aluminum (Valimet H3 aluminum 99.99% 1.4 m2/g, 2.7 μm d50). Aluminum and boron powders were mixed in a stainless steel mill using spherical WC-Co media and hexane to distribute particles and prevent oxidation [11], resulting in a surface area of 6.2 m2/g and a d50 of 2.3 mm. X-ray diffraction patterns and scanning electron micrographs of representative Al þ2B and AlB2 samples are shown in Fig. 1.
2.2. Thermal gravimetric analysis Thermal gravimetric analysis (TGA) was used to analyze the oxidation behavior of AlB2, Alþ 2B, aluminum and boron. Isothermal studies were performed on 25 mg samples of Alþ 2B or AlB2 powder using a Netzsch STA 409 in N2–20%O2 (referred to as air) or industrial grade O2 flowing at a rate of 25 millimol cm 2 s 1. For isothermal studies, samples were brought to temperature between 500 and 1000 1C as quickly as possible ( 75 1C/min) and held at temperature for 5–7 h. Linear heating rate studies were conducted from room temperature to 1500 1C at a rate of 10 1C/min. Analysis of isothermal data was conducted using the modelfree formalism of Kujirai and Akahira [12]. This method was very useful for obtaining quantitative and comparable kinetic data for Al þ2B and AlB2 without assigning specific reaction mechanisms. The benefit of this approach is that it can provide activation energies and kinetic parameters for complex processes with multiple, interdependent and simultaneous phase transformations, diffusion, chemical reactions, and other activated processes. A series of isothermal reactions were performed, and the ratio of the weight gain at a time tα to the maximum weight gain gave the
dα ¼ kðTÞf ðαÞ dt
ð1Þ
where f(α) is the function that describes the oxidation process, and k(T), the reaction rate constant, displays Arrhenius behavior as E kðTÞ ¼ Aexp ð2Þ RT where E is the activation energy, R is the gas constant, T is absolute temperature, and A as the pre-exponential factor. Activation energies for specific conversions, Eα, can be found from plots of ln(1/tα) vs. 1/T since tα ¼
1 Aα exp ½ Eα =RT
ð3Þ
where Aα is the pre-exponential factor at a given conversion. These activation energies give a non-mechanistic activation energy for the global process at a specific value of conversion [14]. Kinetic data were then compared with X-ray diffraction (XRD) patterns and scanning electron micrsoscope (SEM) images to form a general qualitative description of oxidation in these systems.
2.3. Moisture sensitivity The moisture sensitivity of the uncoated AlB2 powder was measured at 20 or 40 1C for relative humidities of 10, 75, or 90%. Three silanes and an amine (see Table 1) were used to coat the AlB2 powder in an attempt to improve its oxidation resistance. The silane treatments were prepared by making a constantly stirred solution of 95 vol% methanol–5 vol% distilled water, adjusting the pH to 4.5–5.5 with acetic acid, adding 35 g of AlB2 powder to 100 cc of solution, and finally adding 2 g of the silane solution. The powders in solution were stirred for 30 min at 500 rpm, filtered, washed with methanol, rinsed with acetone, and dried at 110 1C for 15 min. The amine solution was made by adding 2.15 g of octadecylamine (Aldrich 305391) to 500 cc of hexane and heating to get into solution. The AlB2 powder (35 g) was stirred for two hours and then filtered, rinsed with hexane, and dried at 110 1C for 15 min.
Fig. 1. XRD pattern of (a) Al þ2B, showing mainly aluminum peaks, and (b) AlB2, with only aluminum boride peaks. Inset: SEM backscattered electron images of Al þ 2B mixture and AlB2 powder agglomerate. Markers are 10 μm.
Table 1 Kinetic parameters of oxidation in air and oxygen. Material
Conversion Range (Δα)
ln A air (s 1)
ln A O2 (s 1)
E air (kJ/mol)
E O2 (kJ/mol)
Alþ 2B AlB2
0.15–0.50 0.15–0.50
11 83αþ 58
43α þ42 38
77 778αþ 576
241α þ 262 396
M.L. Whittaker et al. / Journal of Solid State Chemistry 207 (2013) 163–169
Saturated salt solutions were used to establish constant humidity environments [15]. A relative humidity chamber at E10% was made by adding KOH (Alfa Aesar 13451) to deionized water to form a saturated solution in the bottom of a bell jar. Relative humidity chambers at 75% and 90% were prepared using NaCl (Sigma Adrich S9886) and KNO3 (Spectrum P1345), respectively. The bell jars were equilibrated at temperature inside convection ovens (Yamato DKN 400). Powders were weighed (Shimadzu AUW 2200) before starting the tests and at periodic intervals during the test. Accelerated testing at 60 or 80 1C in 75% relative humidity was performed for uncoated and coated AlB2 powders in comparison to B, Al, and Al þ2B. All powders were dried at 110 1C for 24 h before insertion into the humidity chambers.
3. Results and discussion 3.1. Constant heating rate oxidation in air The oxidation characteristics of boron, aluminum, Al þ2B and AlB2 in air can be seen in Fig. 2 as the powders were heated in air at a constant heating rate (10 1C/min). It is clear that the boron powder did not completely oxidize by 1500 1C under these conditions. This highlights the kinetic limitation of boron discussed previously, where the viscous B2O3 skin retards the oxidation of the core of the particle. At E500 1C the boron began to rapidly oxidize, corresponding to oxidation of the surface of the fine boron particles. As the conversion approached 0.5 ( E800 1C), oxidation slowed as a result of the growing oxide layer. Slow
Fig. 2. Conversion (α) of different fuels (B, Al, Al þ2B, and AlB2) to their oxides when heated in air at 10 1C/min. AlB2 formation delays onset of oxidation, but oxidizes rapidly above 750 1C to reach over 97% conversion compared to just 78% for the Al þ 2B mixture.
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diffusion through the oxide layer suppressed rapid oxidation up to 1500 1C. In combustion systems the first process happens almost instantaneously, the second process is referred to as the ignition delay and the burning of particles without an oxide layer was not observed. Boron did not fully oxidize at 1500 1C, and its rate of oxidation was significantly reduced around 800 1C, corresponding to the thickening of molten B2O3 commonly observed. The weight gain of aluminum also aligned well with previous studies [16] in which TGA showed increasing oxidation rates at E500, 800, and 1200 1C. In combination with a relatively small particle size, this periodically accelerating reaction rate allowed aluminum to reach full conversion to Al2O3 by 1500 1C. Al oxidation initiated at about the same temperature as B, but slowed between 600 and 750 1C before dramatically increasing to around 1000 1C. It slowed again between 1000 and 1150 1C and then rapidly increased from 1150 to 1500 1C, where all of the initial aluminum had oxidized. SEM images confirmed the retention in shape of aluminum particles heated to 750 1C (not shown). This phenomenon is significant because the core–shell morphology of aluminum at temperatures above 660 1C resulted in much different behavior than would be observed from bulk aluminum or larger particles. The physical mixture of aluminum and boron showed weight gain dissimilar to either of its constituents. Weight gain increased nearly linearly from around 600 1C to a maximum conversion of 0.78 near 1250 1C. A rule of mixtures calculation of the expected conversion of Al þ2B gave α¼0.67, indicating that the addition of aluminum to boron increased its extent of reaction over the expected value for powders that oxidized independently and that interactions between reaction products were occurring. An increase in the extent of oxidation was observed to an even larger degree in AlB2, where significant weight gain did not occur below 750 1C, but nearly full conversion was reached by 1250 1C. X-ray diffraction patterns of Alþ 2B and AlB2 heated to a temperature of 1250 1C at 10 1C/min are given in Fig. 3. Because B2O3 is amorphous it was not detected by XRD, but was certainly present in all reaction products based on the initial stoichiometry of the system. The main oxidation product in both systems was Al4B2O9, although it is likely that Al18B4O33 was present at higher temperatures but transformed into Al2O3 and Al4B2O9 upon cooling. Because liquid B2O3 was the rate-limiting factor for the oxidation of boron, the removal of B2O3 was critical for increasing reaction kinetics. In both systems, B2O3 was essentially removed from the system through the formation of an aluminum borate via co-oxidizing aluminum. In Al þ2B, liquid B2O3 was free to diffuse towards aluminum particles, and could react with Al2O3 on the surface of these particles to form a solid compound, more closely resembling the oxide of aluminum than boron. In AlB2, the borate may have been the first-formed oxidation product, or else it formed rapidly after aluminum and boron reacted with oxygen. Both systems provide inherent mechanisms for the increase in
Fig. 3. XRD scans of (a) Al þ 2B and (b) AlB2 after heat treatment at 1250 1C in air. SEM inserts show Al4B2O9 formation in both materials, but aluminum borate formation from the boride shows extensive needle-like growth compared to oxidized elemental mixture. Markers are 10 μm.
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oxidation of boron through removal of liquid B2O3 and thus a reduction in the barrier to diffusion of oxidizing species. The SEM image insets in Fig. 3 suggest that the key difference in conversion between Al þ2B and AlB2 was the morphology of Al4B2O9, which formed needles on the surface of AlB2 particles that reduced diffusion distances to the particle core at the base of the growing needles. While oxidized Al þ2B appeared to contain short needle-like structures, they were much more pronounced in the AlB2 sample because aluminum and boron oxidized concomitantly and could immediately react to give Al4B2O9.
3.2. Isothermal oxidation in air and oxygen Figs. 4 and 5 show the model-free isothermal kinetic analysis for Al þ2B and AlB2, respectively, in air. The temperature vs. time curves in Figs. 4a and 5a show that the samples reached their hold temperature within approximately 10 min and were maintained at that temperature for over five hours. From Figs. 4 and 5b it is clear that little oxidation occurred during the ramps. Figs. 4 and 5c give plots of ln(1/t) vs. 1000/T for specific values of conversion, from which the activation energy was found from the slope of each line
Fig. 4. Isothermal kinetic analysis for Al þ 2B powder in air. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
Fig. 5. Isothermal kinetic data for AlB2 in air. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
M.L. Whittaker et al. / Journal of Solid State Chemistry 207 (2013) 163–169
multiplied by the gas constant. The Eα from Figs. 4 and 5c are plotted vs. α in Figs. 4 and 5d. The differences in the general reaction behavior between Al þ2B and AlB2 observed in the constant heating rate study were reflected in the activation energies and activation energy trends found in the isothermal studies. Calculations were restricted to a range of conversion values between α¼0.15–0.5 because at low conversions oxidation during non-isothermal conditions may have contributed to the weight gain, and at high conversions the rate of
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weight gain became too low. The activation energies for Alþ 2B dropped rapidly from 355 kJ/mol at α¼ 0.15, and approached an average value of 80 kJ/mol. For AlB2 they decreased nearly linearly from 475 kJ/mol at α¼ 0.15 to 190 kJ/mol α¼ 0.5. The same analysis was performed for Al þ2B and AlB2 oxidized in pure oxygen as shown in Figs. 6 and 7, respectively. The higher oxygen partial pressure led to Eα values between 223–128 kJ/mol for Al þ2B and 512–372 kJ/mol for AlB2. Interestingly, the Eα for Alþ 2B decreased nearly linearly with increasing α, while those of
Fig. 6. Isothermal kinetic data for Al þ2B in oxygen. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
Fig. 7. Isothermal kinetic data for AlB2 in oxygen. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
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AlB2 appeared to level out at an average of 396 kJ/mol, suggesting that the oxidation mechanisms (or relative rates of dominant mechanisms) differ between O2 and air. For AlB2, the Eα in the range α¼0.15–0.3 were very similar in both air and O2. There was also a characteristic bending of all curves in both plots (Figs. 5 and 7c) in this conversion range, suggesting an oxidation mechanism that was nearly independent of the partial pressure of oxygen or temperature in the initial stages of AlB2 conversion. No such trend was observed for Alþ2B. Comparing Figs. 4 and 6b , there was no significant difference between the extent of reaction at 620 1C in air and 600 1C in oxygen. Based on the previous observations of reaction behavior in this system, it is very likely that the increased oxygen partial pressure slowed down the oxidation of aluminum by reacting with exposed aluminum surfaces more readily and therefore closing cracks in the oxide more quickly. Kinetic parameters Eα and Aα (or their apparent trends) for AlB2 and Alþ2B are summarized in Table 1. It is unclear whether the relatively slow reaction rates observed in this study correlate to oxidation kinetics in propellant or detonation systems where reactions occur on 4–10 orders of magnitude faster timescales. The reduction in the activation energy of AlB2 through the formation of aluminum borate needles on the surface of oxidizing particles as the reaction progresses would certainly be a useful mechanism for utilizing the high thermodynamic potential of boron with more favorable reaction kinetics, and would suggest that reacting aluminum and boron to form AlB2 provides a benefit over the physical mixture. It is known that aluminum borate needles form during rapid thermite reactions between B2O3 and aluminum [10], but only in situ testing and characterization of AlB2 as an additive in energetic mixtures will confirm whether this mechanism is active with oxidants which allow faster time scales.
3.3. Moisture degradation The AlB2 powder was sensitive to moisture at low temperatures as shown by the data in Fig. 8. Thin silane or amine coatings (Table 2) could not be detected in the SEM but n-octadecyltrimethoxysilane eliminated any weight gain under these conditions and the amine coating was nearly as good. XRD of the uncoated AlB2 control sample after exposure to 90% relative humidity for 4 weeks at 40 1C showed that the material was unchanged. The low-humidity data suggests that simply storing the powder in closed, well-packaged containers with a desiccant is adequate for AlB2 powder. Weight change data at 60 or 80 1C in 75% relative humidity for the control (uncoated) and coated AlB2 powders in comparison to B, Al, and Al–B mixtures are shown in Fig. 9. The large weight gain for Al, which was orders of magnitude larger than for any other powder, is consistent with Al(OH)3 formation, which was confirmed by X-ray diffraction. The boron initially lost weight, presumably due to the formation of boric acid (H3BO3), which is soluble in water and has a high vapor pressure [17]. What was surprising was the low weight gain of the intimately mixed Alþ2B powder, which did not follow a rule of mixtures trend with regard to Al oxidation. This may be attributable to the interaction of boric acid with aluminum particles, lowering the pH at particle surfaces and suppressing Al(OH)3 formation. It is suspected that the milling step also provided additional passivation of Al. The AlB2 powder was much more resistant to degradation than fine Al powder, in accord with expectation. At 60 1C and 75% relative humidity, the silane (S1) coating provided the best protection (see Fig. 9a). Increased temperature accelerated the aluminum hydration and caused all coatings to show more than one weight gain at 75% humidity (see Fig. 9b). It is apparent that none of these coatings are impervious to moisture absorption, which is clearly activated by temperature. While the S1 and S2 silane coatings were an improvement over the uncoated control powder at 80 1C in 75% relative humidity, it is likely that hydrophobic polymeric coatings would have been a better choice for protecting the AlB2 powders from oxidation. These data suggest that AlB2 would only be stable against oxidation at the desired operation temperatures of hydrogen storage systems if the partial pressures of water vapor were low. Both AlB2 and Alþ2B mixtures could be used in energetic applications where oxidation could be controlled by Table 2 AlB2 surface treatments.
Fig. 8. Weight gain of AlB2 at 20 1C or 40 1C as a function of time in different relative humidities showing low-temperature oxidation.
Code Coating
Supplier
A S1 S2 S3
Aldrich 305391 Gelest SI06645 Gelest SI TB175.0 Shin Etsu KBM7103
Octadecylamine n-octadecyltrimethoxysilane Tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane 3,3,3 Trifluoropropyl trimethoxysilane
Fig. 9. Weight change for uncoated Al, B, Al þ 2B, and AlB2 in comparison to different silane and amine coatings (see Table 2) applied to AlB2. (a) 60 1C and (b) 80 1C.
M.L. Whittaker et al. / Journal of Solid State Chemistry 207 (2013) 163–169
vacuum packaging under inert gas, and it is possible that catalytic or energetically active hydrophobic coatings could be developed to increase performance and insensitivity. 4. Conclusions AlB2 is less sensitive to the onset of oxidation in dry air and oxygen than Al, B, or Alþ 2B, which is a desirable in energetic applications and as a reaction intermediate in hydrogen storage systems. The high conversion and rapid oxidation of AlB2 at higher temperatures is also attractive in combustion systems. The kinetics of oxidation for Alþ 2B and AlB2 in both dry air and dry oxygen were found to be dependent on the extent of reaction, and the activation energies calculated at incremental values of conversion generally decreased as the reaction progressed. The activation energies obtained by isothermal, isoconversional methods were not indicative of any particular mechanism, but corresponded to the global reaction pathway. The general decreasing trend was attributed to changing morphology and composition of the oxide products Al2O3 and B2O3, and their reaction to form Al4B2O9. AlB2 exhibited pressure-independent oxidation kinetics at low conversions, while oxidation of Al þ2B became more difficult in the pure O2 atmosphere. Based on these observations, AlB2 appears to be a viable energetic fuel and warrants testing under dynamic conditions. The low-temperature moisture sensitivity of AlB2 is an issue and degradation will occur unless the material is coated or stored under low-humidity conditions. The Al þ2B mixture was less susceptible than AlB2 despite having higher surface area, suggesting surface passivation as a result of milling or pH modification by boric acid improved degradation resistance.
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Acknowledgment This work was funded, in part, by ARDEC through DOTC-101001 ordnance technology base agreement 2010-646. Technical discussions with Paul Anderson, Marc Flinders, Lyle Miller, Gerald Stringfellow, and Anil Virkar were very helpful.
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Oxidation kinetics of aluminum diboride Michael L. Whittaker a,n, H.Y. Sohn b, Raymond A. Cutler c a b c
Department of Materials Science and Engineering, University of Utah, 122S. Central Campus Drive, Salt Lake City, UT 84112, USA Department of Metallurgical Engineering, University of Utah, 135S 1460 E, Rm 00412, Salt Lake City, UT 84112, USA Ceramatec, Inc., 2425S. 900W., Salt Lake City, UT 84119, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 21 May 2013 Received in revised form 30 August 2013 Accepted 21 September 2013 Available online 26 September 2013
The oxidation characteristics of aluminum diboride (AlB2) and a physical mixture of its constituent elements (Alþ 2B) were studied in dry air and pure oxygen using thermal gravimetric analysis to obtain non-mechanistic kinetic parameters. Heating in air at a constant linear heating rate of 10 1C/min showed a marked difference between Al þ2B and AlB2 in the onset of oxidation and final conversion fraction, with AlB2 beginning to oxidize at higher temperatures but reaching nearly complete conversion by 1500 1C. Kinetic parameters were obtained in both air and oxygen using a model-free isothermal method at temperatures between 500 and 1000 1C. Activation energies were found to decrease, in general, with increasing conversion for AlB2 and Al þ2B in both air and oxygen. AlB2 exhibited O2-pressureindependent oxidation behavior at low conversions, while the activation energies of Al þ2B were higher in O2 than in air. Differences in the composition and morphology between oxidized Alþ 2B and AlB2 suggested that Al2O3–B2O3 interactions slowed Al þ2B oxidation by converting Al2O3 on aluminum particles into a Al4B2O9 shell, while the same Al4B2O9 developed a needle-like morphology in AlB2 that reduced oxygen diffusion distances and increased conversion. The model-free kinetic analysis was critical for interpreting the complex, multistep oxidation behavior for which a single mechanism could not be assigned. At low temperatures, moisture increased the oxidation rate of Alþ2B and AlB2, but both appear to be resistant to oxidation in cool, dry environments. & 2013 Elsevier Inc. All rights reserved.
Keywords: AlB2 Oxidation Kinetics Activation energy Energetics
1. Introduction Aluminum diboride (AlB2) is a promising energetic fuel additive because of its high volumetric heat of combustion and favorable oxidation characteristics [1]. It may also be an important reaction intermediate in lithium aluminum hydride/lithium borohydride hydrogen storage systems [2]. Therefore, it is critical to understand the oxidation kinetics of AlB2, in order to either exploit its rapid heat release as an energetic additive, or to prevent oxide formation and degradation during thermal cycling in hydrogen storage systems. The oxidation characteristics of both aluminum and boron powders have been well studied. As boron particles react with oxygen, its diffusion towards the unreacted particle core is retarded by the formation of a thickening, molten, vitreous boron oxide (B2O3) layer [3,4]. Because B2O3 melts at 450 1C, but does not boil until 2067 1C [5], boron oxidation is generally slow and incomplete. Aluminum oxidation kinetics are initially controlled by a thin (2–4 nm) amorphous layer, which limits Al3 þ diffusion from the melt. As this layer increases
n Corresponding author. Current address: Northwestern University, 2220 Campus Dr., Evanston, IL 60208, USA. Tel.: þ 1 801 347 6924. E-mail address: [email protected] (M.L. Whittaker).
0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.09.028
to 6–10 nm thickness, oxygen diffusion limits growth and crystallization proceeds through polymorphic transformations, which change growth rates [6,7]. In Al–B–O systems, the oxide interactions gain another level of complexity. Al2O3 and B2O3 will react to form 2Al2O3 B2O3 (Al4B2O9) at temperatures below 1035 1C, and decompose into 9Al2O3 2B2O3 (Al18B4O33) at higher temperatures [8,9]. Because B2O3 melts at 450 1C, transport at high temperatures is generally sufficient for the two oxides to come into contact and react. Solid Al4B2O9 acts to remove liquid B2O3 from particle surfaces in mixed systems, and may adopt a needle-like morphology [10], instead of a spherical shell. The oxidation kinetics of Al–B mixed powder systems or AlB2 have not been investigated to the authors' knowledge. The complex interactions between oxides, polymorphic phase transformations in Al2O3, reaction of unoxidized aluminum and boron, peritectic decomposition of AlB2 or Al4B2O9, vaporization of B2O3 and morphological changes make it exceedingly difficult to analyze this system with a single mechanistic model. Therefore, it is of interest to obtain kinetic parameters that account for all of the processes that occur as aluminum and boron co-oxidize and provide a quantitative description of oxidation rates and their temperature dependence unambiguously. A second purpose of this paper is to report on the resistance of AlB2 to oxidation in moist environments at low temperatures.
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extent of reaction, or conversion, α, for each temperature. The rate of the conversion can be written as [13]
2. Experimental procedure 2.1. Powder preparation 2
AlB2 powder with an average surface area of 2.0 m /g (d50 particle size of 5.7 μm) was purchased from ABCR (Karlsruhe, Germany). The powder (synthesized by H.C. Starck) had reported impurity concentrations of 1.9 wt% O, 0.2 wt% C, 0.1 wt% N and 0.1 wt% Fe, and also contained 6% unreacted aluminum [11]. This powder was compared to a stoichiometric physical mixture of boron (H.C. Starck amorphous boron 95%, 10.9 m2/g, 0.2 μm d50) and aluminum (Valimet H3 aluminum 99.99% 1.4 m2/g, 2.7 μm d50). Aluminum and boron powders were mixed in a stainless steel mill using spherical WC-Co media and hexane to distribute particles and prevent oxidation [11], resulting in a surface area of 6.2 m2/g and a d50 of 2.3 mm. X-ray diffraction patterns and scanning electron micrographs of representative Al þ2B and AlB2 samples are shown in Fig. 1.
2.2. Thermal gravimetric analysis Thermal gravimetric analysis (TGA) was used to analyze the oxidation behavior of AlB2, Alþ 2B, aluminum and boron. Isothermal studies were performed on 25 mg samples of Alþ 2B or AlB2 powder using a Netzsch STA 409 in N2–20%O2 (referred to as air) or industrial grade O2 flowing at a rate of 25 millimol cm 2 s 1. For isothermal studies, samples were brought to temperature between 500 and 1000 1C as quickly as possible ( 75 1C/min) and held at temperature for 5–7 h. Linear heating rate studies were conducted from room temperature to 1500 1C at a rate of 10 1C/min. Analysis of isothermal data was conducted using the modelfree formalism of Kujirai and Akahira [12]. This method was very useful for obtaining quantitative and comparable kinetic data for Al þ2B and AlB2 without assigning specific reaction mechanisms. The benefit of this approach is that it can provide activation energies and kinetic parameters for complex processes with multiple, interdependent and simultaneous phase transformations, diffusion, chemical reactions, and other activated processes. A series of isothermal reactions were performed, and the ratio of the weight gain at a time tα to the maximum weight gain gave the
dα ¼ kðTÞf ðαÞ dt
ð1Þ
where f(α) is the function that describes the oxidation process, and k(T), the reaction rate constant, displays Arrhenius behavior as E kðTÞ ¼ Aexp ð2Þ RT where E is the activation energy, R is the gas constant, T is absolute temperature, and A as the pre-exponential factor. Activation energies for specific conversions, Eα, can be found from plots of ln(1/tα) vs. 1/T since tα ¼
1 Aα exp ½ Eα =RT
ð3Þ
where Aα is the pre-exponential factor at a given conversion. These activation energies give a non-mechanistic activation energy for the global process at a specific value of conversion [14]. Kinetic data were then compared with X-ray diffraction (XRD) patterns and scanning electron micrsoscope (SEM) images to form a general qualitative description of oxidation in these systems.
2.3. Moisture sensitivity The moisture sensitivity of the uncoated AlB2 powder was measured at 20 or 40 1C for relative humidities of 10, 75, or 90%. Three silanes and an amine (see Table 1) were used to coat the AlB2 powder in an attempt to improve its oxidation resistance. The silane treatments were prepared by making a constantly stirred solution of 95 vol% methanol–5 vol% distilled water, adjusting the pH to 4.5–5.5 with acetic acid, adding 35 g of AlB2 powder to 100 cc of solution, and finally adding 2 g of the silane solution. The powders in solution were stirred for 30 min at 500 rpm, filtered, washed with methanol, rinsed with acetone, and dried at 110 1C for 15 min. The amine solution was made by adding 2.15 g of octadecylamine (Aldrich 305391) to 500 cc of hexane and heating to get into solution. The AlB2 powder (35 g) was stirred for two hours and then filtered, rinsed with hexane, and dried at 110 1C for 15 min.
Fig. 1. XRD pattern of (a) Al þ2B, showing mainly aluminum peaks, and (b) AlB2, with only aluminum boride peaks. Inset: SEM backscattered electron images of Al þ 2B mixture and AlB2 powder agglomerate. Markers are 10 μm.
Table 1 Kinetic parameters of oxidation in air and oxygen. Material
Conversion Range (Δα)
ln A air (s 1)
ln A O2 (s 1)
E air (kJ/mol)
E O2 (kJ/mol)
Alþ 2B AlB2
0.15–0.50 0.15–0.50
11 83αþ 58
43α þ42 38
77 778αþ 576
241α þ 262 396
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Saturated salt solutions were used to establish constant humidity environments [15]. A relative humidity chamber at E10% was made by adding KOH (Alfa Aesar 13451) to deionized water to form a saturated solution in the bottom of a bell jar. Relative humidity chambers at 75% and 90% were prepared using NaCl (Sigma Adrich S9886) and KNO3 (Spectrum P1345), respectively. The bell jars were equilibrated at temperature inside convection ovens (Yamato DKN 400). Powders were weighed (Shimadzu AUW 2200) before starting the tests and at periodic intervals during the test. Accelerated testing at 60 or 80 1C in 75% relative humidity was performed for uncoated and coated AlB2 powders in comparison to B, Al, and Al þ2B. All powders were dried at 110 1C for 24 h before insertion into the humidity chambers.
3. Results and discussion 3.1. Constant heating rate oxidation in air The oxidation characteristics of boron, aluminum, Al þ2B and AlB2 in air can be seen in Fig. 2 as the powders were heated in air at a constant heating rate (10 1C/min). It is clear that the boron powder did not completely oxidize by 1500 1C under these conditions. This highlights the kinetic limitation of boron discussed previously, where the viscous B2O3 skin retards the oxidation of the core of the particle. At E500 1C the boron began to rapidly oxidize, corresponding to oxidation of the surface of the fine boron particles. As the conversion approached 0.5 ( E800 1C), oxidation slowed as a result of the growing oxide layer. Slow
Fig. 2. Conversion (α) of different fuels (B, Al, Al þ2B, and AlB2) to their oxides when heated in air at 10 1C/min. AlB2 formation delays onset of oxidation, but oxidizes rapidly above 750 1C to reach over 97% conversion compared to just 78% for the Al þ 2B mixture.
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diffusion through the oxide layer suppressed rapid oxidation up to 1500 1C. In combustion systems the first process happens almost instantaneously, the second process is referred to as the ignition delay and the burning of particles without an oxide layer was not observed. Boron did not fully oxidize at 1500 1C, and its rate of oxidation was significantly reduced around 800 1C, corresponding to the thickening of molten B2O3 commonly observed. The weight gain of aluminum also aligned well with previous studies [16] in which TGA showed increasing oxidation rates at E500, 800, and 1200 1C. In combination with a relatively small particle size, this periodically accelerating reaction rate allowed aluminum to reach full conversion to Al2O3 by 1500 1C. Al oxidation initiated at about the same temperature as B, but slowed between 600 and 750 1C before dramatically increasing to around 1000 1C. It slowed again between 1000 and 1150 1C and then rapidly increased from 1150 to 1500 1C, where all of the initial aluminum had oxidized. SEM images confirmed the retention in shape of aluminum particles heated to 750 1C (not shown). This phenomenon is significant because the core–shell morphology of aluminum at temperatures above 660 1C resulted in much different behavior than would be observed from bulk aluminum or larger particles. The physical mixture of aluminum and boron showed weight gain dissimilar to either of its constituents. Weight gain increased nearly linearly from around 600 1C to a maximum conversion of 0.78 near 1250 1C. A rule of mixtures calculation of the expected conversion of Al þ2B gave α¼0.67, indicating that the addition of aluminum to boron increased its extent of reaction over the expected value for powders that oxidized independently and that interactions between reaction products were occurring. An increase in the extent of oxidation was observed to an even larger degree in AlB2, where significant weight gain did not occur below 750 1C, but nearly full conversion was reached by 1250 1C. X-ray diffraction patterns of Alþ 2B and AlB2 heated to a temperature of 1250 1C at 10 1C/min are given in Fig. 3. Because B2O3 is amorphous it was not detected by XRD, but was certainly present in all reaction products based on the initial stoichiometry of the system. The main oxidation product in both systems was Al4B2O9, although it is likely that Al18B4O33 was present at higher temperatures but transformed into Al2O3 and Al4B2O9 upon cooling. Because liquid B2O3 was the rate-limiting factor for the oxidation of boron, the removal of B2O3 was critical for increasing reaction kinetics. In both systems, B2O3 was essentially removed from the system through the formation of an aluminum borate via co-oxidizing aluminum. In Al þ2B, liquid B2O3 was free to diffuse towards aluminum particles, and could react with Al2O3 on the surface of these particles to form a solid compound, more closely resembling the oxide of aluminum than boron. In AlB2, the borate may have been the first-formed oxidation product, or else it formed rapidly after aluminum and boron reacted with oxygen. Both systems provide inherent mechanisms for the increase in
Fig. 3. XRD scans of (a) Al þ 2B and (b) AlB2 after heat treatment at 1250 1C in air. SEM inserts show Al4B2O9 formation in both materials, but aluminum borate formation from the boride shows extensive needle-like growth compared to oxidized elemental mixture. Markers are 10 μm.
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oxidation of boron through removal of liquid B2O3 and thus a reduction in the barrier to diffusion of oxidizing species. The SEM image insets in Fig. 3 suggest that the key difference in conversion between Al þ2B and AlB2 was the morphology of Al4B2O9, which formed needles on the surface of AlB2 particles that reduced diffusion distances to the particle core at the base of the growing needles. While oxidized Al þ2B appeared to contain short needle-like structures, they were much more pronounced in the AlB2 sample because aluminum and boron oxidized concomitantly and could immediately react to give Al4B2O9.
3.2. Isothermal oxidation in air and oxygen Figs. 4 and 5 show the model-free isothermal kinetic analysis for Al þ2B and AlB2, respectively, in air. The temperature vs. time curves in Figs. 4a and 5a show that the samples reached their hold temperature within approximately 10 min and were maintained at that temperature for over five hours. From Figs. 4 and 5b it is clear that little oxidation occurred during the ramps. Figs. 4 and 5c give plots of ln(1/t) vs. 1000/T for specific values of conversion, from which the activation energy was found from the slope of each line
Fig. 4. Isothermal kinetic analysis for Al þ 2B powder in air. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
Fig. 5. Isothermal kinetic data for AlB2 in air. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
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multiplied by the gas constant. The Eα from Figs. 4 and 5c are plotted vs. α in Figs. 4 and 5d. The differences in the general reaction behavior between Al þ2B and AlB2 observed in the constant heating rate study were reflected in the activation energies and activation energy trends found in the isothermal studies. Calculations were restricted to a range of conversion values between α¼0.15–0.5 because at low conversions oxidation during non-isothermal conditions may have contributed to the weight gain, and at high conversions the rate of
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weight gain became too low. The activation energies for Alþ 2B dropped rapidly from 355 kJ/mol at α¼ 0.15, and approached an average value of 80 kJ/mol. For AlB2 they decreased nearly linearly from 475 kJ/mol at α¼ 0.15 to 190 kJ/mol α¼ 0.5. The same analysis was performed for Al þ2B and AlB2 oxidized in pure oxygen as shown in Figs. 6 and 7, respectively. The higher oxygen partial pressure led to Eα values between 223–128 kJ/mol for Al þ2B and 512–372 kJ/mol for AlB2. Interestingly, the Eα for Alþ 2B decreased nearly linearly with increasing α, while those of
Fig. 6. Isothermal kinetic data for Al þ2B in oxygen. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
Fig. 7. Isothermal kinetic data for AlB2 in oxygen. (a) Time–temperature plots, (b) conversion as a function of time, (c) Arrhenius plots used to calculate activation energies, and (d) activation energy as a function of conversion.
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AlB2 appeared to level out at an average of 396 kJ/mol, suggesting that the oxidation mechanisms (or relative rates of dominant mechanisms) differ between O2 and air. For AlB2, the Eα in the range α¼0.15–0.3 were very similar in both air and O2. There was also a characteristic bending of all curves in both plots (Figs. 5 and 7c) in this conversion range, suggesting an oxidation mechanism that was nearly independent of the partial pressure of oxygen or temperature in the initial stages of AlB2 conversion. No such trend was observed for Alþ2B. Comparing Figs. 4 and 6b , there was no significant difference between the extent of reaction at 620 1C in air and 600 1C in oxygen. Based on the previous observations of reaction behavior in this system, it is very likely that the increased oxygen partial pressure slowed down the oxidation of aluminum by reacting with exposed aluminum surfaces more readily and therefore closing cracks in the oxide more quickly. Kinetic parameters Eα and Aα (or their apparent trends) for AlB2 and Alþ2B are summarized in Table 1. It is unclear whether the relatively slow reaction rates observed in this study correlate to oxidation kinetics in propellant or detonation systems where reactions occur on 4–10 orders of magnitude faster timescales. The reduction in the activation energy of AlB2 through the formation of aluminum borate needles on the surface of oxidizing particles as the reaction progresses would certainly be a useful mechanism for utilizing the high thermodynamic potential of boron with more favorable reaction kinetics, and would suggest that reacting aluminum and boron to form AlB2 provides a benefit over the physical mixture. It is known that aluminum borate needles form during rapid thermite reactions between B2O3 and aluminum [10], but only in situ testing and characterization of AlB2 as an additive in energetic mixtures will confirm whether this mechanism is active with oxidants which allow faster time scales.
3.3. Moisture degradation The AlB2 powder was sensitive to moisture at low temperatures as shown by the data in Fig. 8. Thin silane or amine coatings (Table 2) could not be detected in the SEM but n-octadecyltrimethoxysilane eliminated any weight gain under these conditions and the amine coating was nearly as good. XRD of the uncoated AlB2 control sample after exposure to 90% relative humidity for 4 weeks at 40 1C showed that the material was unchanged. The low-humidity data suggests that simply storing the powder in closed, well-packaged containers with a desiccant is adequate for AlB2 powder. Weight change data at 60 or 80 1C in 75% relative humidity for the control (uncoated) and coated AlB2 powders in comparison to B, Al, and Al–B mixtures are shown in Fig. 9. The large weight gain for Al, which was orders of magnitude larger than for any other powder, is consistent with Al(OH)3 formation, which was confirmed by X-ray diffraction. The boron initially lost weight, presumably due to the formation of boric acid (H3BO3), which is soluble in water and has a high vapor pressure [17]. What was surprising was the low weight gain of the intimately mixed Alþ2B powder, which did not follow a rule of mixtures trend with regard to Al oxidation. This may be attributable to the interaction of boric acid with aluminum particles, lowering the pH at particle surfaces and suppressing Al(OH)3 formation. It is suspected that the milling step also provided additional passivation of Al. The AlB2 powder was much more resistant to degradation than fine Al powder, in accord with expectation. At 60 1C and 75% relative humidity, the silane (S1) coating provided the best protection (see Fig. 9a). Increased temperature accelerated the aluminum hydration and caused all coatings to show more than one weight gain at 75% humidity (see Fig. 9b). It is apparent that none of these coatings are impervious to moisture absorption, which is clearly activated by temperature. While the S1 and S2 silane coatings were an improvement over the uncoated control powder at 80 1C in 75% relative humidity, it is likely that hydrophobic polymeric coatings would have been a better choice for protecting the AlB2 powders from oxidation. These data suggest that AlB2 would only be stable against oxidation at the desired operation temperatures of hydrogen storage systems if the partial pressures of water vapor were low. Both AlB2 and Alþ2B mixtures could be used in energetic applications where oxidation could be controlled by Table 2 AlB2 surface treatments.
Fig. 8. Weight gain of AlB2 at 20 1C or 40 1C as a function of time in different relative humidities showing low-temperature oxidation.
Code Coating
Supplier
A S1 S2 S3
Aldrich 305391 Gelest SI06645 Gelest SI TB175.0 Shin Etsu KBM7103
Octadecylamine n-octadecyltrimethoxysilane Tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane 3,3,3 Trifluoropropyl trimethoxysilane
Fig. 9. Weight change for uncoated Al, B, Al þ 2B, and AlB2 in comparison to different silane and amine coatings (see Table 2) applied to AlB2. (a) 60 1C and (b) 80 1C.
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vacuum packaging under inert gas, and it is possible that catalytic or energetically active hydrophobic coatings could be developed to increase performance and insensitivity. 4. Conclusions AlB2 is less sensitive to the onset of oxidation in dry air and oxygen than Al, B, or Alþ 2B, which is a desirable in energetic applications and as a reaction intermediate in hydrogen storage systems. The high conversion and rapid oxidation of AlB2 at higher temperatures is also attractive in combustion systems. The kinetics of oxidation for Alþ 2B and AlB2 in both dry air and dry oxygen were found to be dependent on the extent of reaction, and the activation energies calculated at incremental values of conversion generally decreased as the reaction progressed. The activation energies obtained by isothermal, isoconversional methods were not indicative of any particular mechanism, but corresponded to the global reaction pathway. The general decreasing trend was attributed to changing morphology and composition of the oxide products Al2O3 and B2O3, and their reaction to form Al4B2O9. AlB2 exhibited pressure-independent oxidation kinetics at low conversions, while oxidation of Al þ2B became more difficult in the pure O2 atmosphere. Based on these observations, AlB2 appears to be a viable energetic fuel and warrants testing under dynamic conditions. The low-temperature moisture sensitivity of AlB2 is an issue and degradation will occur unless the material is coated or stored under low-humidity conditions. The Al þ2B mixture was less susceptible than AlB2 despite having higher surface area, suggesting surface passivation as a result of milling or pH modification by boric acid improved degradation resistance.
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Acknowledgment This work was funded, in part, by ARDEC through DOTC-101001 ordnance technology base agreement 2010-646. Technical discussions with Paul Anderson, Marc Flinders, Lyle Miller, Gerald Stringfellow, and Anil Virkar were very helpful.
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