Passivation of the surface of aluminum nanopowders by protective coatings of the different chemical origin

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1 Applied Surface Science 253 (2007) Passivation of the surface of aluminum nanopowders by protective coatings of the different chemical origin Young-Soon Kwon a, Alexander A. Gromov b, *, Julia I. Strokova b a Research Center for Machine Parts and Materials Processing, School of Materials and Metallurgical Engineering, University of Ulsan, San-29, Mugeo-2 Dong, Nam-Ku, Ulsan , Republic of Korea b Chemical Technology Department, Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk , Russia Received 26 January 2006; received in revised form 25 December 2006; accepted 25 December 2006 Available online 12 January 2007 Abstract The results of investigation and analysis of electro-exploded aluminum nanopowders, whose surface were passivated with the following substances: liquids nitrocellulose (NC), oleic acid (C 17 H 33 COOH) and stearic acid (C 17 H 35 COOH), suspended in kerosene and ethanol, fluoropolymer; solids boron and nickel; gases N 2,CO 2 and air (for a comparison) are discussed. The surface protection for the aluminum nanopowders by coatings of different chemical origins leads to the some advantages of the powders properties for an application in energetic systems, e.g. solid propellants and green propellants (Al H 2 O). Aluminum nanopowders with a protected surface showed the increased stability to oxidation in air during the storage period and higher reactivity by heating. The TEM-visual diagram of the formation and stabilization of the coatings on the particles has been proposed on the basis of experimental results. The kinetics of the interaction of aluminum nanopowders with air has been discussed. The recommendations concerning an efficiency of the protective non-al 2 O 3 layers on aluminum nanoparticles were proposed. # 2007 Elsevier B.V. All rights reserved. Keywords: Aluminum; Nanopowder; Surface passivation; Oxidation; Stability; Coating; DTA DSC TG; TEM 1. Introduction Recently, aluminum nanopowders (ANPs) produced by the electrical explosion of wires (EEW) method became the deeply commercialized industrial product [1]. The EEW ANPs are widely studied as the promising components for energetic systems of different types [2]. The most important problem of the wide application of ANPs is the dependence of their properties on the production conditions. There are a lot of parameters, which affect on the physical and chemical properties of the EEW ANPs: characteristics of electric circuit of powder production (entered energy and rate of energy entering into a wire by explosion, voltage, capacity and Abbreviations: ALEX, aluminum explosive (TM); EEW, electrical explosion of wires; ANP, aluminum nanopowder; S sp, area of the specific surface (m2/g); a s, mean-surface particle diameter (nm); TEM, transmission electron microscopy; C Al, metal aluminum content (wt. %); TG, thermogravimetry; DSC, differential scanning calorimetry; UDP, ultra dispersed powder; EDS, energy dispersive X-ray spectrometry; XRD, X-ray diffraction * Corresponding author. Tel.: ; fax: address: a_a_gromov@yahoo.com (A.A. Gromov). inductivity of electric circuit, explosion frequency), a gas atmosphere and its pressure during the explosion (Ar, Ar + H 2, N 2, N 2 +CO 2, etc.), a wire composition and diameter, passivation conditions (gas, solid or liquid passivation reagents, their concentration, solvent used, etc.) [3]. It is rather difficult to vary electric characteristics within a wide range of values because they are optimal depending on a type of machine for nanopowders production. The most propagated machine used in the USA and Russia is UDP-4 (ultra dispersed powder) and its analogues (Fig. 1). After production in the EEW machine all metallic nanopowders (Al, Cu, Fe, etc.) trend to self-ignition under contact with air and, thus, the passivation of the particles surface is required. In fact, the majority of commercially available ANPs (a s = nm) passivated by oxide layers consist of amorphous or crystalline g-al 2 O 3 [4]. Such process of the oxide layer formation on a fresh surface of the spherical metal nanoparticles under slow surface oxidation in air should be called passivation-up-to-self-saturation (Eq. (1) and (2)): 2Al ðsþ þ3=2o 2 ðgþ ¼Al 2 O 3 ðsþ (1) 2Al ðsþ þ3h 2 O ðgþ ¼Al 2 O 3 ðsþ þ3h 2 ðsolvedþ (2) /$ see front matter # 2007 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 Y.-S. Kwon et al. / Applied Surface Science 253 (2007) Fig. 1. EEW facility for the metal nanopowders production UDP-4G: (1) explosion chamber; (2) powder filter; (3) powder collector; (4) high-voltage circuit. The structure of oxide layers for Al nanoparticles produced by EEW was studied in [5]: the layer of amorphous Al 2 O 3 grows up to the certain thickness (5 7 nm) and than crystallizes with the g-al 2 O 3 formation in the equilibrium conditions. The hypothesis of the formation of the electric double layer with the additional capacity [6] on the metal-oxide interface inside the particles has been proposed for the explanation of the relatively high metal content (85 90 wt. %) for EEW ANPs passivated in air [7]. But the experimental approaches for the achievement of higher metal content in Al particles as well as the chemical analysis of the processes occurred with a particle passivation by different substances should be developed [8,9]. There is almost no information in the majority of papers on ANPs behavior about their stability under storage. The results of non-oxide layers applied on Al nanoparticles passivation were discussed in the work [10], but metal content was significantly decreased after such treating, probably, because of the non-developed passivation technique. The efforts of the ANPs obtained with higher metal content by the application of the non-oxide passivation coatings are discussed in this work. The Al nanopowders were prepared by EEW method, collected and after that passivated before the reaction with air with application of the different substances on their surface: liquids nitrocellulose (NC), oleic acid (C 17 H 33 COOH) and stearic acid (C 17 H 35 COOH), suspended in kerosene and ethanol, fluoropolymer; solids amorphous boron, nickel; gases N 2,CO 2 and air (for comparison). This technique of particles coating before their contact with air was selected because the coating of preliminary air-passivated particles is useless: the already formed oxide layer on particles of powder (10 15 wt.% of g-al 2 O 3 for the particles of 100 nm in diameter) cannot be removed or substituted by any kind of after-passivation treatment. The properties of produced powders with non-oxide coating films were comprehensively studied by TEM, XRD, DSC-TG, microcalorimetry and chemical analyses. Experimental data on the structure and the thermal destruction of non-oxide coated ANPs have been accumulated and analyzed. 2. Experiment ANPs were produced in argon, nitrogen or (N 2 +CO 2 ) atmosphere by using the EEW facilities developed by the High Voltage Institute and Institute of High Current Electronics, Tomsk, Russia, which were reported elsewhere [2,7]. The list of samples studied within this work and their specific surface area (S sp ) determined by BET method as well as metal aluminum content (C Al ) measured after passivation by volumetric analysis [11], and mean-volume particle diameter (a v ) are shown in Table 1. Sample 1 (commercial powder ALEX [12], Table 1) with widely studied characteristics has been taken for comparison. Samples 2 and 3 were obtained from the composite wires Al Ni and Al B, respectively [13]. The Ar gas media in the explosion chamber was added to 10 vol.% of H 2 for the production of sample 4, and N 2 gas and (N 2 +CO 2 ) gas mixture were used for samples 5 and 6. Air-, CO 2 - and N 2 -passivation (samples 2 6) was carried out for 72 h at the T =30 2 8C and P = 1.1 atm. in the medium of (Ar vol.% air) [5]. Samples 7 11 were passivated by organic substances in solvents before ANP contact with air: wt.% stearic acid (C 18 H 36 O 2 ) solution in ethanol (C 2 H 6 O); Table 1 Properties of aluminum nanopowders No. Sample code Initial wire composition Gas media in explosive chamber Passivation condition S sp (BET) (m 2 /g) C Al (wt.%) 1 ALEX Al Ar Air Al (Ni) Al (Ni) Ar Air Al (B) Al (B) Ar Air Al (Ar + H 2 ) Al Ar + 10 vol. H 2 Air Al (N 2 /CO 2 ) a Al N 2 CO n/a 6 Al (N 2 +CO 2 /N 2 ) a Al N 2 +CO 2 N n/a 7 Al (St Ac) ethanol Al Ar Stearic acid in ethanol Al (St Ac) kerosene Al Ar Stearic acid in kerosene Al (Ol Ac) ethanol Al Ar Oleic acid in ethanol Al (F) Al Ar Fluoro-polymer Al (NC) Al Ar Nitrocellulose in ethanol a Data was taken from Ref. [16]. C Al (wt.% 12 month aged)

3 5560 Y.-S. Kwon et al. / Applied Surface Science 253 (2007) Table 2 Elemental and phase composition of aluminum nanopowders surface No. Sample code Weight content of elements (%) (EDS) a Phase composition (XRD) b O Al 1 ALEX Al ( ) 2 Al (Ni) % + 4% Ni Al ( ) 3 Al (B) Al ( ) 4 Al (Ar + H 2 ) Al ( ) 5 Al (N 2 /CO 2 ) Al ( ), g-al 2 O 3 ( ) 6 Al (N 2 +CO 2 /N 2 ) Al ( ), g-al 2 O 3 ( ) 7 Al (St Ac) ethanol Al( ), traces of Al 4 C 3 ( ) 8 Al (St Ac) kerosene Al 9 Al (Ol Ac) ethanol Al ( ), traces of Al 4 C 3 ( ) 10 Al (F) Al ( ) 11 Al (NC) n/a Al ( ), g-al 2 O 3 ( ) a At least three measurements were made for the O and Al wt. content in each sample. The powder surface of approximately 20 mm 20 mm was analyzed. The example of the EDS pattern and corresponding SEM image is shown on Fig. 6. b Phase identification was made under JCPDS database PCPDFWIN v In brackets JCPDS card number wt.% stearic acid solution in kerosene; wt.% oleic acid (C 18 H 34 O 2 ) solution in ethanol; wt.% fluoropolymer solution; wt.% nitrocellulose solution in ethanol. Ethanol and kerosene were selected as the solvents because they do not interact with ANPs. The solution for passivation was added to the fresh powder immediately after production and the powder suspension was mechanically stirred for 2 h. The temperature was maintained at C in order to avoid self-heating of the powder. A residual solvent was evaporated from ANPs by the vacuum treatment at the room temperature. After the passivation procedures all powders were stored in an open-to-atmosphere-box for 2 months in order to simulate the industrial conditions. The samples were also aged in air of 70% RH at the room temperature for 12 months after the first step of analysis (Table 1). Particles morphology and compositions (Table 2) were tested by TEM-EDS (Philips CM 200 FEG) and XRD (Rigaku MAX-B ) with CuK a radiation. DSC TG (Universal 2.4 F TA Instruments) was used for testing of ANPs non-isothermal oxidation (Table 3, Fig. 2). ANPs isothermal oxidation was studied by microcalorimetry (Fig. 3). 3. Experimental results The most convenient etalon for ANPs with non-oxide protective coatings discussed in this paper is, obviously, the oxide-passivated powders, i.e. ANPs passivated by air. Their thermal properties are well-known and widely discussed [2,12], that is why all properties of the experimental samples of EEW ANPs powders was compared with the commercially produced powder ALEX. With an exception of samples 4 and 8, all experimental powders had higher values of the specific surface area than ALEX. The effect of non-oxide passivation appeared stronger for the metal content. The organic-passivated samples showed wt. % of the metal aluminum. The reduction of C Al was maximal for ANP passivated by oleic acid down to 45 wt. %. Hence, the particles coating by liquid organic reagents led to the considerable reduction of the metal content in the powder (Table 1). Additionally, material coating permanently reacted with Al: the reduction in C Al after 12 months of ageing was higher for samples 7 11 than for gas- and solid-passivated powders. The results of EDS and XRD study of ANPs are presented in Table 2. All powders contained more than 10 wt. % of oxygen Table 3 Reactivity parameters of aluminum nanopowders (m = 7.5 mg) under non-isothermal heating in air (see Fig. 1) No. Sample code T ox peak (8C) Weight of coating (gases) (%) +Dm (up to 660 8C) (%) +Dm (up to C) (%) DHox Al up to 660 8C (J/g) 1 ALEX Al (Ni) Al (B) Al (Ar + H 2 ) Al (N 2 /CO 2 ) Al (N 2 +CO 2 /N 2 ) Al (St Ac) ethanol Al (St Ac) kerosene Al (Ol Ac) ethanol Al (F) Al (NC) C Al (%): metal content in the samples (Table 1); +Dm: mass increasing by oxidation. a aðal! Al 2 O 3 Þ¼ þdm C Al 0:89 100%. a a ( C) (%)

4 Y.-S. Kwon et al. / Applied Surface Science 253 (2007) Fig. 2. DCS and TG curves of aluminum nanopowders in air (numbers of samples correspond to Table 3): heating rate 10 K/min, etalon a-al 2 O 3. Fig. 3. Curves of isothermal heating of aluminum nanopowders: (a) T = 100 8C, (b) T = 150 8C, and (c) heat release at 150 8C. (g-al 2 O 3 according XRD data) on the particle surface. Traces of aluminum carbide were found by XRD for samples 7 and 9, but carbon was not determined by EDS, because carbon films were used as object slides for the samples. Boron was not found in the sample 3, probably, because of its low molecular weight and, hence, EDS method has low sensitivity for this element. Nitrogen was not found in samples 5 and 6 by EDS study. The crystalline g-al 2 O 3 (5 9 wt. %) was found for samples Al (NC), Al (N 2 /CO 2 ) and Al (N 2 +CO 2 /N 2 ). The intensive ANP oxidation in air began below the melting point of aluminum (660 8C). The results of DSC TG analysis of ANP samples in air are represented in Table 3. The nonisothermal heating of samples was executed with a rate of heating 10 K/min. DSC and TG traces looked typically for all samples (Fig. 2): after the intensive oxidation before Al melting (660 8C) with a sharp mass gain, the second less intensive stage of complete oxidation up to C was followed. The temperature of oxidation peak (T ox peak ) was one of the parameter, which reflected on the powders reactivity. The maximum T ox peak was characteristic for Al (Ni) sample (Table 3). Probably, this was caused by the presence of refractory nickel (see Table 2) in the composition of the passivating layer on the particles surface. The T ox peak of samples passivated by air changed in the range of C and passivated by CO 2 and N 2 in the range of C and did not correlate with the dimensional characteristics of powders or the type of the passivating coating. Such ANPs behavior can be caused by the finest fraction of the powder, which strongly affected the total powder reactivity. The T ox peak was C lower for samples 5 6 than for ALEX. The weight of adsorbed gases did not exceed 4 wt. % for the dry ANP samples 1 6 except the sample Al (Ni). The mass of organic coatings (samples 7 11) on particles was 3 4 times more than for dry samples 1 6. It should be noted that the total value of the mass of material coating was even higher, because the oxidation processes of coating and interaction aluminumcoating occurred simultaneously with adsorption during heating. The important point of the study was DHox Al values (Table 3). The samples coated by boron and stearic acid in kerosene showed maximal DHox Al (6232 and 6282 J/g correspondingly). Probably, boron and residual kerosene put the additional heat to the aluminum sharp oxidation. The degree of conversion (a) for ANP samples up to C (last column, Table 3) was relatively high for all samples, except already oxidized nos. 6 (Table 2) and 11 interacting with NC.

5 5562 Y.-S. Kwon et al. / Applied Surface Science 253 (2007) The results of isothermal calorimetric study of the selected powders are presented on Fig. 3(a) (c). The samples were held for 60 days in air at constant temperature T = 100 8C(Fig. 3(a)) and 150 8C (Fig. 2(b)) correspondingly. The weight for the majority of samples sharply increased (maximal for the sample Al (Ni)) during the first 1 2 days and than it stabilized. Exceptional behavior for samples 7 and 9, i.e. mass decreasing, can be explained by evaporation of the residual solvent. However, for sample 8 the content of residual solvent kerosene was relatively small under isothermal heating, i.e. kerosene is not as volatile as the ethanol. The data of mass flow calorimetry showed that organic acids of passivated samples evolve more heat than the rest of samples (Fig. 3(c)). According to TEM results, all samples had agglomerates of particles (Fig. 4). The agglomeration was determined by the explosion process itself and aerodynamics of particles at the formation in a chamber. The necks between spherical particles formed (Fig. 4(a)) at the temperature slightly above the Al melting point, because the shape of Al liquid droplets was close to spherical at the moment when liquid jet of Al melt passed through them. Not smooth boundary between two spherical particles (Fig. 4(b)) obviously showed the liquid phase coalescence mechanism of particles agglomeration, which was an analog of the liquid-phase sintering of ceramic particles. The degree of agglomeration cannot be controlled at the stage of passivation, when particles have been already cooled, and did not discuss in this work. 4. Discussion 4.1. Gas- and solid-passivated ANPs (samples 1 6) The specific Al content for samples ALEX, Al (Ar + H 2 ) and Al (N 2 /CO 2 ) was the highest as compared to other powders (Table 1). At the same time, sample 4 (Al (Ar + H 2 )) had half of S sp as much as that of Al (N 2 /CO 2 ): the Al explosion in the mixture (Ar + H 2 ) conditioned the formation of powders less oxidized ones with larger particles (Tables 1 and 2). The Al content (C Al ) and S sp had correlation: the higher S sp the lower C Al especially for samples 6 and 2. Evidently, smaller particles were more oxidized. N 2 - and CO 2 -passivated samples contained Al 2 O 3 (Table 2). Hence, metastable (Al N) and (Al C) compounds formed on the particle surfaces during production and passivated in N 2 and CO 2 were oxidized during the following powder contact with air that is why T ox peak were also minimal for samples 5 and 6 (Table 3). The sample Al (Ni) consisted of both very fine fraction and large of particles. The 4wt.%ofNi(Table 2), according to EDS, did not cover the surface of large Al particles completely, but deposited in a small oxidized fraction (TEM results). Thus, the explosion of Al Ni composite wires resulted in smaller particles formation (Table 1) than for Al without coating, but the size of such small particles was less than the border of stability about 30 nm for Al [14]. The residual non-oxidized Al particles completely reacted with air at the T < C: Ni, applied by such way, did not increase the stability of fine Al particles to oxidation in air. All powders contained 2 4 wt. % of gases Fig. 4. TEM-image of substructure of agglomerates of aluminum nanoparticles (sample 1), passivated by air: (a) necks between particles and (b) neck substructure. (Table 3), but Al (Ni) 9 wt.%. It was reflected high S sp for this sample. Additionally dissolved hydrogen evolved from Al particles can be a reason for the earlier oxidation onset (T 190 8C) for the sample Al (Ar + H 2 ) as compared to ALEX (Fig. 2). Boron-stabilized nano-al [15] was an attractive component for propellants, because boron-coated particles can increase powder combustion enthalpy. Aluminum content and S sp for this sample were nearly the same as for sample 1 (ALEX, Table 1), and the degree of conversion up to C was average (Table 3). Boron-coated powder had also the highest value of DH ox Al in air among dry samples 1 6. The certain

6 increasing of the weight for the sample Al (Ni) during the first day of isothermal ANPs oxidation in air at the T = 100 and T = 150 8C was caused, probably, by metallic Ni oxidation (Fig. 3(a) and (b)). For the other dry samples the mass gain during the first day by low-temperature isothermal oxidation was 1 3 wt.% corresponding to weak process Al oxidation by adsorbed water Liquid-passivated ANPs (samples 7 11) Stearic acid and oleic acid had the same effect on ANPs passivation they interacted with Al. In the case of oleic acid, interaction with metal was deeper C Al decreased to 45 wt.% in sample 8 and went on at sample ageing (Table 1). It is noticeable, that interaction of Al with stearic acid and oleic acid resulted in carbidization of particles surfaces (see Table 2). The NC coating acted as a strong oxidizer for Al: 12-months aged powder contained 10 wt.% of Al less than fresh one. Samples Al (F) and Al (NC) seemed to promise for using in combustion processes, but metal content for them was still low. Organic layers were not observed for majority of organic coated particles, which could protect Al particles from further oxidation. In the case of sample 9, the particles held more oxygen than other wet -passivated samples (Table 2). Moreover, according TEM, organic coated metal particles had two layers: an organic layer and an internal oxide layer (Fig. 5). The results of the isothermal oxidation test (Fig. 3a,b) showed the smooth mass loss for powders 7 and 9 (ethanol desorption), at the same time the heat releasing was maximal for the sample 9 (Fig. 3(c)). Y.-S. Kwon et al. / Applied Surface Science 253 (2007) Fig. 6. EDS pattern (a) and corresponding SEM image (b) of the sample 4 (see Table 2). 5. Conclusion Fig. 5. TEM-image of two-layer coating of organically passivated aluminum nanopowders. Eleven samples produced under different experimental conditions and stabilized by gas, liquid and solid coating films have been studied (Fig. 6). The advantage of non-al 2 O 3 coated (i.e. non-air passivated) aluminum nanoparticles was their expected higher combustion enthalpy than for Al 2 O 3 -passivated Al nanoparticles as well as better dimensional characteristics in the comparison with ALEX powder. The indirect reflection of the combustion enthalpy were the parameter DHox Al (J/g) and a (%) taken from the DSC and TG curves. The size of particles for the produced powders strongly correlated with the specific metal content in them. Assuming these four parameters (DH Al ox, a, S sp and C Al ), the most effective results was achieved in the samples 3 5, dry - passivated by boron, air and CO 2. Thus, the explosion of wires in nitrogen or mixture (argon + hydrogen) or high-melting and high-exothermic reagent application onto the initial wire led to the improvement in the powders energetic. The coating of particles by organic reagents was not profitable. For all organiccoated particles, the organic layer was penetrable to atmospheric oxygen under the storage and resulted in internal oxide layer formation (Fig. 5).

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