Effect of Humidity on the Thermal Properties of Aluminum Nanopowders with Different Surface Coatings

Abstract

To investigate the effect of surface coating materials on the humidity stability of aluminum (Al) nanopowders, three kinds of core–shell structure Al nanopowders with an Al₂O₃ passivation coating, carbon coating, and plasticizer dioctyl sebacate (DOS) coating were prepared through laser-induction complex heating method. After one year’s storage at 95% relative humidity, their thermal properties were tested through differential scanning calorimeter (DSC) and thermal gravimeter (TG) analysis. The results show that the thermal properties of Al₂O₃-passivated Al nanopowders are entirely lost under high humidity because the Al₂O₃ passivation coating is very sensitive to moisture. The thermal properties of carbon-coated Al nanopowders are not well-protected under a high humidity due to the uneven thickness and structural defects of carbon coatings. However, the thermal enthalpy of DOS-coated Al nanopowders remains at 3.56 KJ/g under high humidity, which indicates that an organic DOS coating with a hydrophobic nature has an excellent protective effect on the thermal properties of the Al nanopowders. Given the good forming performance of organic DOS coatings and other components of propellants, DOS-coated Al nanopowders are a kind of energetic material with potential application value.

1. Introduction

Aluminum (Al) nanopowders are a new metal fuel for energetic materials (such as propellants, explosives, and pyrotechnics), and they have attracted significant interest because of their unique characteristics that cannot be obtained in the conventional Al powders [1]. As a substitute for the conventional Al powders, the incorporation of Al nanopowders in composite solid propellants has been shown to increase the burning rate [2,3] and specific impulse of these propellants [4], and decrease their signal characteristics, etc. [5].

However, the large specific surface area, which gives Al nanopowders a high reactivity, also means that they can easily interact with the surrounding air and water vapor, resulting in aging problems [6]. An Al₂O₃ surface coating formed by the passivation of Al nanoparticles protected Al from further oxidation by oxygen, but it did not protect it from the attack of moisture [7,8]. Alex (a nanosized Al powder produced by the electro-explosion process) at room temperature in a saturated water atmosphere lost 100% of its energy within 13 days, but conventional Al powders lost only 10% of their power under the same conditions [4]. A similar aging phenomenon was also observed for Als (a nanosized Al powder produced by plasma explosion), which lost 100% of its energy within 74 days at room temperature in a humid atmosphere (relative humidity > 90%) [9]. Our previous results [10] also indicate that the thermal properties of Al nanopowders produced by a laser-induction complex heating method were profoundly affected by the ambient humidity. Moreover, the aging of the Al nanopowders is accelerated when the relative humidity of the environment is higher. The aged Al nanopowders have a negative effect on the performance of energetic materials [7], so it is very important to research a suitable surface coating for improving the humidity stability of Al nanopowders.

A strategy developed to protect the thermal properties of Al nanopowders from humidity aging is to encapsulate Al nanoparticles by using non-oxide coating materials. Some non-oxide protective coatings were proposed to solve this problem, such as aluminum diboride (AlB₂) coatings [11], transition metal coatings [12], nickel coatings [13], perfluoroalkyl carboxylic acid coatings [14], stearic acid coatings [15], carboxylic acid coatings [16], glycidyl azide polymer coatings [17], aluminum iodate hexahydrate coatings [18], acetylacetone coatings [19], polydopamine coatings [20], perfluoroalkyl acid coatings [21], hydroxyl-terminated polybutadiene coatings [22], carbon nanotubes coatings [23], oleic acid coatings [24], perfluorinated oligomer coatings [25], perfluoroalkylsilane coatings [26], and tetraamine copper nitrate coatings [27]. These inorganic and organic coatings protected the Al nanopowders against oxidation, but durable protection from humidity aging is difficult to achieve. In our previous work, carbon coatings [28] and hydroxyl-terminated polybutadiene (HTPB) coatings [29] were also used to encapsulate Al nanoparticles, and carbon-coated Al nanopowders and HTPB-coated Al nanopowders have better thermal properties than Al₂O₃-passivated Al nanopowders. In particular, HTPB is an organic binder component of composite solid propellant. HTPB coatings can also improve the processing compatibility of Al nanopowders with other components of propellant.

Herein, dioctyl sebacate (DOS, C₂₆H₅₀O₄) as an organic plasticizer component of composite solid propellant was used to encapsulate Al nanoparticles. To assess the protective effects of different surface coatings, the humidity stability of three Al nanopowders with an Al₂O₃ coating, carbon coating and plasticizer DOS coating were studied, and their aging mechanisms were explored.

2. Experiment

2.1. Materials Preparations

Three kinds of Al nanopowders with different surface coatings used in this study were synthesized using laser-induction complex heating technology (the schematic diagram is shown in Figure 1), which is based on the characteristic that the absorption rate of the laser increases with the increase in temperature. Firstly, the high-frequency induction power was activated to melt the pure bulk Al (99.6%, CAS No: 7429-90-5) in the crucible, and the induction current was continuously increased to further increase the melt temperature. Secondly, high-density CO₂ laser irradiation was introduced into the Al liquid, and the absorption of liquid metal to laser energy gave rise to the rapid evaporation of Al atoms. Finally, the evaporated Al atoms collided with argon (99.9999%, CAS No: 7440-37-1) molecules, which eventually condensed into small particles and lost their kinetic energy.

Al₂O₃-passivated Al nanopowders were obtained by introducing small doses of oxygen (99.7%, CAS No: 132259-10-0) before exposure to air. To obtain carbon-coated Al nanopowders, a mixture of argon and methane (CH₄, 99.999%, CAS No: 74-82-8) was introduced into the reaction chamber, with the objective of causing the thermal decomposition of methane [28]. Carbon atoms were produced by the pyrolysis of methane and deposited on the surface of Al nanoparticles through physical adsorption. The preparation process of plasticizer DOS-coated Al nanopowders is similar to that of binder HTPB-coated Al nanopowders [29]. The cleaned lid containing the cooling water system was evenly daubed using a certain amount of the propellant organic composition DOS (99%, CAS No: 122-62-3). The vaporized Al atoms collided with argon gas and penetrated into the organic coating.

2.2. Materials Characterizations and Measurements

The humidity stability of Al nanopowders with different surface coatings was tested using a constant temperature and humidity test chamber. Three prepared Al nanopowders with different surface coatings were placed in three lipless weighing bottles, respectively. A temperature of 20 °C and relative humidity of 95% were set, and all powders were stored and aged for one year before analysis. The phases of the original and aged Al nanopowders with different surface coatings were determined with X-ray diffraction (XRD, Bruker D8 Advance, Bruker, Billerica, MA, USA). The thermal properties of the fresh Al nanopowders and aged Al nanopowders with different surface coatings were measured using differential scanning calorimeter and thermal gravimeter (DSC-TG, NETZSCH STA 449 F5) in dry oxygen with a heating rate of 10 °C/min. The morphology and particle size of the fresh Al nanopowders were characterized using transmission electron microscopy (TEM, FEI Tecnai G2 F20, FEI, Hillsboro, OR, USA). The internal structure of the aged Al nanopowders with different surface coatings were characterized using high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010, JEOL, Kyoto, Japan).

3. Results and Discussion

Figure 2 shows the XRD patterns of the original and aged Al nanopowders with different surface coatings. In Figure 2a, the diffraction peaks of the original Al₂O₃-passivated Al nanopowders at 38.5°, 44.7°, 65.1°, 78.2°, and 82.4° belonged to the (111), (200), (220), (311), and (222) planes, which can be indexed to face-centered-cubic Al (JCPDS No. 04-0787) [3,5]. Compared with the original sample, the aged Al₂O₃-passivated Al nanopowders at 95% RH showed obvious hydrolysis, and the appearance of the aged sample changed from black to white. The diffraction peaks corresponding to bayerite Al(OH)₃ (JCPDS No. 01-077-0117) were observed (as shown in Figure 2b), and no diffraction peaks of Al nanopowders were found, which indicates that Al₂O₃-passivated Al nanopowders were hydrolyzed entirely at 95% RH. In addition, the four weak peaks marked with an asterisk in Figure 2b can be indexed to hexagonal Al₂O₃.3H₂O (JCPDS No. 08-0096), which is caused by moisture adsorption on the surface of the Al₂O₃ coating. For carbon-coated Al nanopowders (as shown in Figure 2c), the aging sample shows the same diffraction peaks of Al and carbon as the original sample, but the appearance of the aged sample changed from black to dark grey. No peak corresponding to the Al(OH)₃ phases was detected in the aged carbon-coated Al nanopowders. However, some weak diffraction peaks belonging to kappa-Al₂O₃ (JCPDS No. 04-0878) also appeared, which suggests that the aged carbon-coated Al nanopowders were oxidized to some extent. For DOS-coated Al nanopowders (as shown in Figure 2d), the aging sample and original sample show almost identical XRD patterns, and the color of the aged sample at 95% RH shows little change compared with the original sample, indicating that there is no hydrolytic phenomenon in the DOS-coated Al nanopowders. The broadened diffraction peak of DOS-coated Al nanopowders at about 20° suggests the presence of an amorphous substance.

To confirm whether DOS was coated on the surface of the Al nanoparticles, the Fourier transform infrared (FTIR) patterns of the DOS, DOS-coated Al nanopowders, and fresh Al nanopowders were tested and are shown in Figure 3. The absorption peaks of the two samples are essentially the same (Figure 3a), but the difference is that the absorption peaks of the DOS-coated Al nanopowder are weaker than those of the DOS, which confirms that DOS is present in the DOS-coated Al nanopowders. In addition, the peak of the fresh Al nanopowders at 955 cm⁻¹ corresponds to Al₂O₃ (Figure 3a), which indicates that the fresh Al nanopowders have an oxidation phenomenon and do not show the characteristic absorption peak of organic material. Therefore, this result also proves that organic DOS was indeed coated on the surface of Al nanoparticles and still retains its skeleton structure.

The DSC-TG results of the fresh Al nanopowders and aged Al nanopowders with different surface coatings at 95% RH are shown in Figure 4. The DSC curves (as shown in Figure 4a) of the fresh Al nanopowders, the aged carbon-coated Al nanopowders, and the aged DOS-coated Al nanopowders exhibit exothermic phenomena between ~400 °C and ~660 °C (Al melting point). The exothermic peaks at 551, 554, and 575 °C correspond to enthalpy changes of 3.92, 3.56, and 2.75 KJ/g, respectively. The variation in the exothermic peaks of the three Al nanopowders can be attributed to the nature of the different surface coating materials. For the fresh Al nanopowders, the passivation oxide layer on the surface of Al nanoparticles is very thin and has little effect on the exothermic peak temperature of Al nanopowders. For the aged carbon-coated Al nanopowders, the exothermic oxidation of the inner Al nanoparticles occurs only after the combustion of the carbon coating, which leads to a delay in the exothermic peak temperature of Al nanopowders compared with the fresh Al nanopowders. Similarly, the exothermic peak temperature of the aged DOS-coated Al nanopowders is delayed due to the thermal decomposition of the surface organic DOS coating. Correspondingly, the TG curves (as shown in Figure 4b) of the fresh Al nanopowders, the aged carbon-coated Al nanopowders, and the aged DOS-coated Al nanopowders exhibit a mass-gain-phenomenon between ~400 °C and ~660 °C. The mass gains of 31.7%, 23.9%, and 28.3% are observed, respectively. The heat release and mass gain can be attributed to the oxidation behavior of Al nanopowders, which is different from that of micron aluminum powder [18,19]. However, no exothermic peak and mass gain phenomena between ~400 °C and ~660 °C were observed in the DSC-TG curve of the aged Al₂O₃-passivated Al nanopowders, which indicates that no Al nanopowders exist in the powder, and this result is consistent with the XRD results (Figure 2b). The TG curves of the fresh Al nanopowders and the aged carbon-coated Al nanopowders below 150 °C show a weight loss of 4.7% and 6.5%, which is attributed to the release of absorbed gases and moisture on the surface of the nanopowders. For the DSC-TG curve of the aged DOS-coated Al nanopowders, no noticeable weight loss phenomenon below 150 °C is found. However, a minor exothermic peak between 150 °C and 400 °C is observed. The exothermic peak at 233 °C corresponds to the enthalpy change of 381 J/g and a significant weight loss of about 29.4%, which is due to the thermal decomposition of the surface organic DOS coatings [29]. For the DSC-TG curve of the aged Al₂O₃-passivated Al nanopowders below 400 °C, two minor endothermic peaks at 106 and 287 °C corresponding to enthalpy changes of 183 and 116 J/g were found, and corresponding weight losses of 18.7% and 23.3% were also observed. The two endotherms and weight loss of the aged Al₂O₃-passivated Al nanopowders are caused by the evaporation of adsorbed water and the thermal decomposition of aluminum hydroxide [6,9,30]. The DSC-TG analysis shows that the thermal properties of the Al₂O₃-passivated Al nanopowders are completely lost after being aged for one year at 95% RH, and the Al₂O₃ passivation coating has no protective effect on the inner Al nanopowders. In contrast, the carbon coating and the DOS coating all show a certain protective effect on the inner Al nanopowders. However, the aged DOS-coated Al nanopowders have a larger exothermic enthalpy change and a higher mass gain compared with the aged carbon-coated Al nanopowders, which indicates that the DOS coating has better protection on the thermal properties of Al nanopowders.

To further investigate the reason for the humidity effect on the thermal properties of Al nanopowders with different surface coatings, the morphology and internal structure of the fresh Al nanopowders and the aged Al nanopowders with different surface coatings at 95% RH were investigated using TEM and HRTEM (shown in Figure 5). The fresh Al nanopowders show a spherical shape with an average particle size of 50 nm (Figure 5a). Three kinds of aged Al nanopowders with different surface coatings all show distinct core–shell structures (Figure 5b–d). No conspicuous lattice stripes of Al nanoparticles are found in the aged Al₂O₃-passivated Al nanopowders (Figure 5b), and the Al₂O₃·3H₂O shell and the Al(OH)₃ core are consistent with the XRD results (Figure 2b), suggesting that the inner Al nanoparticles were hydrolyzed entirely at 95% RH. By contrast, the aged carbon-coated Al nanopowders have apparent lattice stripes (Figure 2c), and the d-spacing of 0.23 nm corresponds to the d-value of (111) plane for Al (JCPDS card No. 44-0787). However, some amorphous structures were also found in the local area (see the white circle in Figure 5c), which suggests that some Al nanoparticles inside the aged carbon-coated Al nanopowders are oxidized, and this phenomenon is consistent with the XRD results (Figure 2c). No trace of oxidation is found inside the aged DOS-coated Al nanopowders, except the lattice stripes of Al nanoparticles (Figure 5d), suggesting that the Al nanoparticles encapsulated in the DOS coating are well-protected.

The different humidity stability of Al nanopowders with different surface coatings can be attributed to the properties and structures of the various coating materials. For the Al₂O₃-passivated Al nanopowders, the passive oxide film on Al nanoparticles is hydrophilic, and water molecules chemisorb on the available sites of the oxide surface in humid atmospheres. A relevant study shows that Al nanoparticles react much faster with H₂O than O₂ under moist conditions [30]. The hydroxide ions (OH⁻) in water belong to an anion defect and will be driven to the Al/Al₂O₃ interface under the action of anode potential. At the Al/Al₂O₃ interface, Al nanoparticles are oxidized to form hydration species, such as bayerite Al(OH)₃, and the protons are reduced to form H₂ by electrochemical reactions (2Al + 6H₂O → 2Al(OH)₃ + 3H₂). The result of hydrolysis is to accelerate the diffusion of hydroxide ions, promote the transfer of oxygen and water molecules, and further oxidize the inner Al nanoparticles. In the hydrolysis process, the Al(OH)₃ content increases with time, but the Al content decreases with time [6]. This process continues until the Al content disappears and its energy is completely lost. Therefore, the thermal property of the Al₂O₃-passivated Al nanopowders is deeply influenced by moisture and wholly lost after being aged for one year at 95% RH. For the carbon-coated Al nanopowders, the thickness of the carbon coating formed by the dissolution–diffusion–precipitation mechanism is not uniform due to the low solubility of carbon in Al. In Figure 5b, the carbon coating is only 2 nm thick at the thin position, while the thickness of the carbon coating is up to 4.5 nm at the thick part. Moreover, the onion-like graphite shell of carbon-coated metal nanoparticles contains a mass of structure defects (collapses, unevenness, dislocation, etc.) because of the severe curvature of the graphite atom layers [28]. Thus, the positions with thin thickness and structural defects of the carbon coating easily become the weak links in the oxidation of Al nanoparticles, and oxygen preferentially diffuses into the interior of the carbon-coated Al nanopowders to oxidize with aluminum through these weak links as oxidation channels. This inference is well-demonstrated by the presence of amorphous material at the position indicated by the white circle in Figure 5b. Although there was no hydrolysis of the carbon-coated Al nanopowders in the humidity environment, an oxidation reaction could not be prevented. Therefore, the thermal properties of the carbon-coated Al nanopowders were not well-protected. For DOS-coated Al nanopowders, organic DOS molecules are directly deposited on the surface of Al nanoparticles through the adsorption effect rather than being bonded through functional groups (such as COOH- [13,14] and OH- [22,29]). The organic DOS anchored on the surface of Al nanoparticles is hydrophobic, so the hydrophobic DOS coating as an effective “barrier” to hydrolysis can effectively restrain the adsorption of water molecules in humid atmospheres. Therefore, the organic DOS coating with hydrophobic groups plays a positive role in preventing Al nanopowders from reacting with moisture. Thus, the aging and humidity stability of DOS-coated Al nanopowders in humid atmospheres is significantly reduced and improved.

4. Conclusions

We prepared three kinds of Al nanopowders with different surface coatings (Al₂O₃ coating, carbon coating, and DOS coating) via laser-induction complex heating. The humidity stability of Al nanopowders is greatly affected by surface coating materials. The thermal properties of Al₂O₃-passivated Al nanopowders are wholly lost under a high humidity, because the Al₂O₃ coating is hydrophilic and very sensitive to moisture. The thermal property of carbon-coated Al nanopowders is not well-protected under high humidity due to the uneven thickness and structural defects of the carbon coating. However, the organic DOS coating with a hydrophobic nature has an excellent protective effect on the thermal property of the DOS-coated Al nanopowders. It is well-known that DOS is a commonly used plasticizer for composite solid propellants, so the processing compatibility of the DOS-coated Al nanopowders with other propellant components can also be improved.