Passivation of the surface of aluminum nanopowders by protective ...

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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 ...
Applied Surface Science 253 (2007) 5558–5564 www.elsevier.com/locate/apsusc

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 680-749, Republic of Korea b Chemical Technology Department, Tomsk Polytechnic University, 30 Lenin Avenue, Tomsk 634050, 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 (C17H33COOH) and stearic acid (C17H35COOH), suspended in kerosene and ethanol, fluoropolymer; solids – boron and nickel; gases – N2, CO2 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–H2O). 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-Al2O3’’ 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; Ssp, area of the specific surface (m2/g); as, mean-surface particle diameter (nm); TEM, transmission electron microscopy; CAl, 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.: +7 3822 563169; fax: +7 3822 563435. E-mail address: [email protected] (A.A. Gromov). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.12.124

inductivity of electric circuit, explosion frequency), a gas atmosphere and its pressure during the explosion (Ar, Ar + H2, N2, N2 + CO2, 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 (as = 30–300 nm) passivated by oxide layers consist of amorphous or crystalline g-Al2O3 [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 þ 3=2O2 ðgÞ ¼ Al2 O3 ðsÞ

(1)

2Al ðsÞ þ 3H2 O ðgÞ ¼ Al2 O3 ðsÞ þ 3H2 ðsolvedÞ

(2)

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application of the different substances on their surface: liquids – nitrocellulose (NC), oleic acid (C17H33COOH) and stearic acid (C17H35COOH), suspended in kerosene and ethanol, fluoropolymer; solids – amorphous boron, nickel; gases – N2, CO2 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-Al2O3 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. 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 Al2O3 grows up to the certain thickness (5–7 nm) and than crystallizes with the g-Al2O3 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

2. Experiment ANPs were produced in argon, nitrogen or (N2 + CO2) 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 (Ssp) determined by BET method as well as metal aluminum content (CAl) measured after passivation by volumetric analysis [11], and mean-volume particle diameter (av) 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 H2 for the production of sample 4, and N2 gas and (N2 + CO2) gas mixture were used for samples 5 and 6. Air-, CO2- and N2-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 + 0.1 vol.% air) [5]. Samples 7–11 were passivated by organic substances in solvents before ANP contact with air:  7–0.1 wt.% stearic acid (C18H36O2) solution in ethanol (C2H6O);

Table 1 Properties of aluminum nanopowders No.

Sample code

Initial wire composition

Gas media in explosive chamber

Passivation condition

Ssp (BET) (m2/g)

CAl (wt.%)

CAl (wt.% 12 month aged)

1 2 3 4 5 6 7 8 9 10 11

ALEX Al (Ni) Al (B) Al (Ar + H2) Al (N2/CO2)a Al (N2 + CO2/N2) a Al (St Ac) ethanol Al (St Ac) kerosene Al (Ol Ac) ethanol Al (F) Al (NC)

Al Al (Ni) Al (B) Al Al Al Al Al Al Al Al

Ar Ar Ar Ar + 10 vol. H2 N2 N2 + CO2 Ar Ar Ar Ar Ar

Air Air Air Air CO2 N2 Stearic acid in ethanol Stearic acid in kerosene Oleic acid in ethanol Fluoro-polymer Nitrocellulose in ethanol

11.3 40.7 12.0 9.4 19.5 30.1 12.1 7.3 14.3 11.6 12.6

86 53 84 92 84 67 74 79 45 81 68

85 53 82 92 n/a n/a 70 59 43 81 58

a

Data was taken from Ref. [16].

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Table 2 Elemental and phase composition of aluminum nanopowders surface No.

1 2 3 4 5 6 7 8 9 10 11

Weight content of elements (%) (EDS)a

Sample code

ALEX Al (Ni) Al (B) Al (Ar + H2) Al (N2/CO2) Al (N2 + CO2/N2) Al (St Ac) ethanol Al (St Ac) kerosene Al (Ol Ac) ethanol Al (F) Al (NC)

O

Al

10  2 22  3 11  2 92 18  3 33  4 15  2 15  2 18  3 12  2

90  5 74  2% + 4% Ni 89  4 91  5 82  3 67  2 85  3 85  3 82  3 88  4

Phase composition (XRD)b

Al (04-0787) Al (04-0787) Al (04-0787) Al (04-0787) Al (04-0787), g-Al2O3 (47-1308) Al (04-0787), g-Al2O3 (47-1308) Al(04-0787), traces of Al4C3 (35-0799) Al Al (04-0787), traces of Al4C3 (35-0799) Al (04-0787) Al (04-0787), g-Al2O3 (47-1308)

n/a

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.1.30. In brackets – JCPDS card number.

   

8–0.1 wt.% stearic acid solution in kerosene; 9–0.1 wt.% oleic acid (C18H34O2) solution in ethanol; 10–0.1 wt.% fluoropolymer solution; 11–0.1 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 30  5 8C 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 CuKa 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 45–81 wt. % of the metal aluminum. The reduction of CAl 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 CAl 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

Tox peak (8C)

Weight of coating (gases) (%)

+Dm (up to 660 8C) (%)

+Dm (up to 1400 8C) (%)

Al up to DHox 660 8C (J/g)

aa (500– 1400 8C) (%)

1 2 3 4 5 6 7 8 9 10 11

ALEX Al (Ni) Al (B) Al (Ar + H2) Al (N2/CO2) Al (N2 + CO2/N2) Al (St Ac) ethanol Al (St Ac) kerosene Al (Ol Ac) ethanol Al (F) Al (NC)

597 655 601 606 564 551 643 631 653 636 645

2 9 2 2 2 4 5 6 10 6 24

26 13 24 25 27 16 23 27 15 21 10

68 43 63 75 62 32 58 74 38 68 24

5465 5133 6232 4730 2546 2784 5997 6282 4875 5184 5947

89 88 84 92 83 54 88 100 95 94 40

CAl (%): metal content in the samples (Table 1); +Dm: mass increasing by oxidation. a aðAl ! Al2 O3 Þ ¼ C þDm 0:89  100%. Al

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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-Al2O3.

(g-Al2O3 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-Al2O3 (5–9 wt. %) was found for samples Al (NC), Al (N2/CO2) and Al (N2 + CO2/N2). 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 1400 8C was followed. The temperature of oxidation peak (Tox peak) was one of the parameter, which reflected on the powders reactivity. The maximum Tox 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’

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Fig. 3. Curves of isothermal heating of aluminum nanopowders: (a) T = 100 8C, (b) T = 150 8C, and (c) heat release at 150 8C.

surface. The Tox peak of samples passivated by air changed in the range of 597–655 8C and passivated by CO2 and N2 – in the range of 551–564 8C 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 Tox peak was 33–46 8C 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 Al heating. The important point of the study was DHox values (Table 3). The samples coated by boron and stearic acid in Al kerosene showed maximal DHox (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 1400 8C (last column, Table 3) was relatively high for all samples, except already oxidized nos. 6 (Table 2) and 11 interacting with NC.

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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 + H2) and Al (N2/CO2) was the highest as compared to other powders (Table 1). At the same time, sample 4 (Al (Ar + H2)) had half of Ssp as much as that of Al (N2/CO2): the Al explosion in the mixture (Ar + H2) conditioned the formation of powders less oxidized ones with larger particles (Tables 1 and 2). The Al content (CAl) and Ssp had correlation: the higher Ssp the lower CAl especially for samples 6 and 2. Evidently, smaller particles were more oxidized. N2- and CO2-passivated samples contained Al2O3 (Table 2). Hence, metastable (Al–N) and (Al–C) compounds formed on the particle surfaces during production and passivated in N2 and CO2 were oxidized during the following powder contact with air that is why Tox 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 4 wt. % of Ni (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 < 1000 8C: 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 Ssp 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 + H2) 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 Ssp for this sample were nearly the same as for sample 1 (ALEX, Table 1), and the degree of conversion up to 1400 8C was average (Table 3). Boron-coated powder had also the highest value of DHox Al in air among ‘‘dry’’ samples 1–6. The certain

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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. 4.2. 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 – CAl 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)). 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-Al2O3 coated (i.e. non-air passivated) aluminum nanoparticles was their expected higher combustion enthalpy than for Al2O3-passivated Al nanoparticles as well as better dimensional characteristics in the comparison with ALEX powder. The indirect reflection of the combustion enthalpy were the Al parameter DHox (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. Al Assuming these four parameters (DHox , a, Ssp and CAl), the most effective results was achieved in the samples 3–5, ‘‘dry’’passivated by boron, air and CO2. 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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