Synthesis, Analysis, and Characterization of Aluminum Nanoparticles Coated with 2,2,4-Trimethylpentane

Abstract

In this study, to solve the problem of low activity of aluminum nanoparticles in combustion, aluminum nanoparticles were coated with 2,2,4-trimethylpentane (C₈H₁₈-Al), enabling the deactivation of aluminum nanoparticles to be effectively inhibited. The morphological characteristics, particle size distribution, chemical state, and thermal properties of C₈H₁₈-Al were characterized via SEM, TEM, DLS, XPS, and TG-DSC. The stability and energy performance of C₈H₁₈-Al were studied based on the national standard test method. The results showed that C₈H₁₈-Al had a typical shell–core structure with a smooth surface and good sphericity. The particle size was normally distributed, and the content of active aluminum nanoparticles was high (85.45%), with good thermal stability and a fast energy release rate (about four times that of ordinary nano aluminum particles). The results demonstrated that an in situ C₈H₁₈ coating is beneficial for the preparation of structurally stable aluminum nanoparticle composites with good performance.

1. Introduction

Exploring the structure and properties of metal nanoparticles is imperative in research areas such as materials synthesis, catalysis, and combustion applications [1]. For aluminum nanoparticles, due to their high surface activity and high specific surface area properties [2], problems such as the deactivation of aluminum nanoparticles leading to reduced active aluminum nanoparticle content have created challenges in the combustion community. Therefore, in order to maintain the high activity of aluminum nanoparticles in practical applications, the surfaces of aluminum nanoparticles need to be treated. The main methods currently used to maintain the activity of aluminum nano powders are inert gas protection [3], natural passivation [4], and surface coating [5]. Among them, the surface coating method can effectively solve the problems of agglomeration and easy oxidation by forming a nanoscale core–shell structure and changing the surface functionalization properties of nano aluminum nanoparticles, which can effectively improve the combustion of aluminum nanoparticles [6].

Most coatings on the surfaces of aluminum nanoparticles are inorganic, including carbon [7,8], metal and oxide [4,9,10], and nonmetallic coatings [11]. Yi et al. [7] investigated the structural stability and thermal decomposition mechanisms of nanocarbon-material-coated aluminum powder particles, Al@C, and found that the core–shell structure of Al@C can provide effective protection for Al surface activity. Kou et al. [9] found that Fe/Al composites prepared via the one-step reduction method have a large specific surface area. The coated Fe has the ability to effectively reduce alumina, thus increasing the exothermic rate of Al nanoparticles. Sung et al. [11] prepared AlN particles via the plasma technique with high dispersion and kept the particle size smaller than that of the original Al particles. However, lowering of the onset oxidation temperature and the poor compatibility of the components due to the inorganic coating of the aluminum nanoparticles led to reduced applicability. These factors limit the applications of aluminum nano powders in combustion.

In addition, coating materials are generally made from organic materials such as polymers [5,12], ether [13,14,15], and propellant components [16,17]. Kim et al. [12] prepared PTFE/Al using PTFE-coated aluminum nanoparticles with higher thermogravimetric enthalpy (4.80 kJg⁻¹) than that of pure Al powder (0.88 kJg⁻¹). A PTFE coating can also be used as an oxide layer to protect Al nanoparticles. Sun et al. [14] studied ether-coated Al nanoparticles using ReaxFF molecular dynamics and confirmed the presence of the ether coating through TEM of ECANPs with a thickness of 4.6 nm and concluded that the ether coating can effectively prevent the oxidation reaction of Al nanoparticles. Wang et al. [16] prepared Al/CuO/NC/AP composites via the electrospray method and found that ammonium perchlorate could improve the performance of Al/CuO nano alumina thermite.

There are other organic coatings that also serve to protect aluminum nanoparticles. To reduce the ignition temperature of Al nanoparticles, Zhang et al. [17] prepared Al@PFHP nanoparticles with a uniform structure and high sphericity with a lower ignition point (140 °C lower) and longer burning time (about 1.5 times more) compared to the original Al nanoparticles. Li et al. [18] comparatively studied the flame propagation behavior and minimum explosion concentration of AL and SA-Al and found that a stearic acid coating was able to suppress the explosion of aluminum nanoparticles. Organic materials are gradually attracting the interest of researchers as an ideal protective material because of their structural stability, insensitivity to water and air, and compatibility. 2,2,4-Trimethylpentane (C₈H₁₈) [19,20] is stable and non-aggregating and is often applied as a standard for testing the anti-explosive properties of gasoline. Additionally, the high percentage of C₈H₁₈ in the fuel mixture is considered to be able to reduce engine detonation [21].

Therefore, to avoid the oxidation reaction of aluminum nanoparticles easily in air, thereby reducing the energy release, in this study, 2,2,4-trimethylpentane was used for the in situ coating of aluminum nanoparticles, and morphological characterization and performance tests of the prepared coated samples were performed. The coating effects and properties of the 2,2,4-trimethylpentane coating material on aluminum nanoparticles were also investigated. As expected, 2,2,4-trimethylpentane was able to form a stable protective film on the aluminum nanoparticles indicated, thus effectively maintaining the high activity of aluminum nanoparticles and solving the problem of the limited application of aluminum nanoparticles in the combustion community.

2. Experiment

2.1. Materials

2,2,4-Trimethylpentane (C₈H₁₈) was provided by China Sichuan Hongbo New Material Co., Ltd. (Mianyang, China). Aluminum wire (0.2 mm diameter, 99.5% purity) was supplied by China Shijiazhuang Zhongli Zinc Industry Co., Ltd. (Shijiazhuang, China). Ethanol and ethyl acetate (analytical purity) were supplied by Beijing Tongguang Fine Chemical Co., Ltd. (Beijing, China). Chemicals other than aluminum wire were directly used for in situ coating experiments without further treatment.

2.2. Fabrication of C₈H₁₈-Al Nanoparticles

C₈H₁₈-Al nanoparticles were prepared via the in situ coating of aluminum nanoparticles prepared via the electro-explosive by wire (EEW) method with 2,2,4-trimethylpentane. Aluminum vapor was generated by the separation explosion of aluminum wires (0.2 mm) in an explosion chamber with a transient high voltage (6.5 kv) supplied by the pulsed power generator. The distance between electrodes was 24 mm. Since the explosion chamber was filled with an inert gas (argon), the aluminum vapor collided with the gaseous medium and cooled rapidly, condensing into ultra-fine metal powder (aluminum nanoparticles). Subsequently, the prepared pure aluminum nanoparticles were collected by the separation unit-buffer tank-powder collection unit and stored hermetically to avoid oxidation. To implement the in situ encapsulation of aluminum nanoparticles, 1 g of collected aluminum nanoparticles was taken and dispersed in a solution of ethyl acetate with a concentration of 10% ethanol. C₈H₁₈ was added while stirring the solution until the aluminum nanoparticles were uniformly distributed in the solution. Then, the solution was placed in a fume hood until the volume evaporated completely. Finally, the black powder product coated with C₈H₁₈ aluminum nanoparticles was screened. The products were collected and stored in a vacuum-tight environment for further morphological characterization and performance testing.

3. Characterization and Performance

3.1. Equipment

The morphological structure of the C₈H₁₈-Al was analyzed via scanning electron microscopy (S4800 cold field emission SEM, Hitachi, Japan) and transmission electron microscopy (Tecnai G2 F20 FEM, FEI, Hillsboro, OR, USA). The particle size distribution of the sample was obtained from dynamic light scattering (Nanotrac Flex DLS, Microtrac Inc., Norcross, GA, USA). The chemical states of the constituent elements on the surface of the C₈H₁₈-Al were measured via X-ray photoelectron spectroscopy (Thermoescalab 250Xi XPS, Thermo Electron Corporation, Waltham, MA, USA). Analyses of the slow heating process of the samples were carried out via thermogravimetric analysis and differential scanning calorimetry (STA229F3 TG-DSC, Netszch, Germany). The energy performance of C₈H₁₈-Al was characterized by a microcomputer automatic calorimeter (TRHW-7000C, Hebi Tianrun Technology Co., Ltd., Hebi, China).

3.2. Morphology Analysis

The prepared C₈H₁₈-Al samples were uniformly coated with conductive resin and sprayed with gold to obtain the surface samples for SEM measurement. The different magnifications of the C₈H₁₈-Al samples are shown in Figure 1a (magnification 25.0K) and Figure 1b (magnification 100K) in order to highlight the grain sizes and compare them. The test results of the C₈H₁₈-Al samples showed that the C₈H₁₈-Al nanoparticles were spherical in shape with regular forms and relatively smooth-looking surface textures. Most of the nanoparticles were less than 200 nm in size, but a few larger particles were still present in the field of view due to interparticle aggregation and the stacking of clusters.

The imaging principle is basically the same for both SEM and TEM. SEM is able to observe only the surface morphology of the sample, while the electron beam of TEM is able to penetrate a certain thickness (about 50 nm) of the sample. Meanwhile, due to the different densities of the nanoscale aluminum particles and the shell structure of C₈H₁₈, the photographic results exhibited different gray levels. The milled C₈H₁₈-Al nanoparticle samples were dispersed in an anhydrous ethanol solution, sonicated, and then dropped on the microgrid and dried to produce samples for the TEM test. The TEM test results for the C₈H₁₈-Al nanoparticles are shown in Figure 2a. Here, it can be seen that brief ultrasonic oscillation alleviated the agglomeration phenomenon of C₈H₁₈-Al nanoparticles. The particles were intact in shape with smooth edges, indicating the structural stability of the in situ C₈H₁₈ coating.

To gain insight into the interfacial information between the C₈H₁₈ coating and the Al nanoparticles, HRTEM imaging of the C₈H₁₈-Al nanoparticles was performed in order to visually measure the thickness of the coating. The C₈H₁₈-Al nanoparticles clearly presented a typical core–shell structure with well-defined boundaries. The shell–core interface structure is controlled by the C₈H₁₈ shell structure, the shell is dense and complete, there is no diffraction stripe at the inner side of the interface, and the cladding thickness is about 3.12 nm. A thin oxide layer (1.07 nm) was also visible, and no obvious crystal structure was seen in the field of view, indicating an amorphous alumina structure. The core of C₈H₁₈-Al nanoparticles has a distinct lattice stripe with a measured lattice spacing of 0.233 nm, which corresponds to the (111) face of the face-centered cubic Al [21]. Moreover, the C₈H₁₈-Al nanoparticles prepared by this method can effectively reduce the thickness of the oxide layer [16].

3.3. Particle Size Analysis

It is necessary to know the particle size distribution of C₈H₁₈-Al samples because smaller aluminum nanoparticles are theoretically more conducive to a positive combustion reaction [22]. Dynamic light scattering (DLS) has been widely used as an inexpensive and fast particle size determination method for metal particle size measurement [23]. We dispersed 1.0 mg of the C₈H₁₈-Al sample in 150 mL of ethanol and shook it via ultrasound before testing to ensure the precision of the measurement results. The particle size distribution of the C₈H₁₈-Al particles was analyzed using statistical methods, as shown in Figure 3. The median particle size D50 of C₈H₁₈-Al nanoparticles was greater than 200 nm, while D95 was equal to 310 nm, which is generally consistent with the conclusions obtained from the SEM observations. The particle size distribution of the samples showed an overall normal distribution and more concentrated particle size distribution. This result indicated that the overall homogeneity of the samples with in situ coating of nano Al powder with C₈H₁₈ performed better.

3.4. Active Aluminum Content Analysis

Nano aluminum powder theoretically has higher reactivity than micron aluminum powder, but the active aluminum content decreases dramatically with decreasing particle size because nano aluminum powder offers easy oxidation. It is necessary to understand the effect of the level of active aluminum content on combustion performance [24]. In order to characterize the content of active aluminum of the coated samples, the content of active aluminum was obtained by calculating the volume of hydrogen gas released from the reaction of active aluminum with sodium hydroxide according to the gas volume method of YS/T 617.1-2007 [18] using Formula (1); the specific experimental methods and equipment are not described here. Simultaneously, to illustrate how effective the C₈H₁₈-coated nano Al powder was, the results were compared with two sizes and types of Al powder (ordinary nano Al and micro Al), as shown in

Table 1. Testing results of active Al content.

Sample	Active Al Content (%)
C8H18-Al	85.45
Ordinary nano Al [25]	59.18
Micro Al [25]	90.73

. We observed that the content of the three active Al samples, from high to low, was micro Al > C₈H₁₈-Al > ordinary nano Al, which also indicated that the aluminum powder was inevitably oxidized during the preparation of C₈H₁₈-Al. The activity of aluminum powder coated with C₈H₁₈ was greater than 80%, which was much higher than the activity of ordinary aluminum powder (59.18%), and the active aluminum content increased by 26.27%, which indicated that the coating layer protected the active aluminum in the inner layer very well.

$${\mathrm{\omega }}_{active Al content}=\frac{0.00216\left(P_m-P_b-P_s\right)\times V}{\left(273+t\right)m_0}\times 100\%$$

(1)

where P_m is the mercury air pressure display value (kPa), P_b is the corrected value of the barometer display temperature (kPa), P_s is the saturation vapor pressure of water when the temperature is measured (kPa), m₀ is the mass of the sample (g), V is the volume of hydrogen gas produced (mL), 0.00216 is the conversion factor for converting hydrogen into active aluminum, and t is the temperature in the measuring tube at the time of measurement (°C)

3.5. X-ray Electron Spectroscopy (XPS) Analysis

X-ray photoelectron spectroscopy (XPS) is capable of qualitative and semi-quantitative analysis of the chemical state and electronic structure of elements on the surface of solid materials (<5 nm). Since the sample itself contains elemental carbon, the 2 mg powder sample was glued to an ordinary double-sided adhesive. The C₈H₁₈-Al nanoparticles were dried to avoid solvent influence on the analysis results, and characterization was performed. The total spectrum was analyzed via inverse fold product analysis using the XPS PEAK software which is Avantage (Thermo Scientific, Waltham, MA, USA). No elemental signals were detected besides the presence of C, O, and Al, which were labeled. The results are shown in Figure 4.

The Al2s spectrum shown in Figure 4a presents residual Al₂O₃ compounds at 120.2 eV [26] and is not described in detail here. Two chemical states, Al₂O₃ (74.3 eV) and singlet Al (72.0 eV), can be seen in Figure 4b [26], which proves the presence of a partial oxide layer of aluminum on the surface of the C₈H₁₈-Al sample during in situ cladding, as well as the sample processing measurements. The C1s envelope for the C₈H₁₈-Al control was deconvoluted into three peaks representing four types of carbon that correspond to C-C/C-H (284.8 eV), C-O (286.2 eV), and O=C-OR/O=C-OH (288.7 eV) [27,28]. This result indicates that part of the C₈H₁₈ was oxidized during the coating process, forming hydroxyl, carbonyl, and carboxyl functions. This coating then becomes a new oxide layer that is similar to aluminum oxide on the surface of aluminum nanoparticles. Based on the Al2p, C1s, and O1s spectra, we confirmed that the C₈H₁₈ coating was encapsulated in the periphery of the aluminum nanoparticles [9].

3.6. Thermal Analysis

TG-DSC studies were performed on C₈H₁₈-Al nanoparticles under an air atmosphere at a heating rate temperature of 20 °C/min from 23 to 1350 °C to study the conversion process, and the results are shown in Figure 5. The TG-DSC curves of C₈H₁₈-Al nanoparticles were analyzed with reference to the temperature dependence of C₈H₁₈ and its decomposition products [20] and the oxidation stage curves of micro-nano alumina powders [29,30]. According to the trend of the TG curve, the mass of C₈H₁₈-Al nanoparticles was divided into four stages with temperature, and the reasons for the changes were analyzed in conjunction with the DSC curves as follows:

• From 23 to 400 °C, the sample mass decreased by about 4.6%. Since amorphous alumina is hydrophilic [30], the increase in temperature leads to desorption of H₂O and CO₂ attached to the surface. At the same time, some of the amorphous carbon on the nanoparticle surface was oxidized into CO₂ spillover [31]. The thickening of the amorphous oxide layer Al₂O₃ of the shell structure occurred at this stage.
• The sample mass gradually increased by about 27.4% from 400 to 693 °C. The exothermic peak at 593 °C on the DSC curve was the first oxidative exothermic peak, and the integration yielded an exotherm of 2.98 kJ/g. A phase change peak caused by the melting of unreacted aluminum appeared at 647.5 °C. The oxidation onset temperature on the TG curve was 554.2 °C, at which point the amorphous Al₂O₃ started to convert into γ-Al₂O₃. The subsequent flat weight gain was due to the alumina shell hindering the contact of aluminum with oxygen to reduce the probability of oxidation reaction. In addition, as the temperature increased, the molten aluminum destroyed the oxide layer, and the eruption of reactive aluminum caused the internal alumina nanoparticles to begin oxidization. Meanwhile, this phase of C₈H₁₈ was oxidized and decomposed to produce a series of olefins (C₄H₈, CH₄, C₂H_2, and C₃H₆) [20].
• The temperature varied between 693 and 945 °C. The oxidation reaction of aluminum nanoparticles occurred predominantly at this stage [29], and the mass of the sample increased by 25.6%. The second exothermic peak on the DSC curve at 806.2 °C corresponded to the conversion of γ-Al₂O₃ to θ-Al₂O₃, which was calculated to be 2.72 kJ/g. The decomposition of C₈H₁₈ at this phase was basically complete [20].
• The temperature increased from 945 °C to the end of the ramp-up, by which time most of the Al nanoparticles were almost completely oxidized. The TG curve showed that there was still a mass change, which corresponded to a small exothermic peak at 1051.8 °C on the DSC curve, indicating that there was still an unreacted aluminum nanoparticle. After this point, the weight gain stopped, and θ-Al₂O₃ transformed into stably present α-Al₂O₃. The final C₈H₁₈-Al nanoparticles gained 67.3% in weight, with a total exothermic heat of 24.8 kJ/g.

A comparison with the measured results of different types of aluminum powder raw materials under similar test conditions (ordinary nano Al and micro Al) is shown in

Table 2. Thermal analysis data of three samples.

Sample	Onset Temperature (°C)	Weight Gain (∆m %)
C8H18-Al	554.2	67.3
Ordinary nano Al [25]	567.0	57.0
Micro Al [25,32]	555.7	81.0

. The order for the onset oxidation temperature from high to low was ordinary nano Al > micro Al > C₈H₁₈-Al, indicating that the C₈H₁₈-Al nanoparticles with higher reactivity and earlier reactions were able to accelerate the reaction rate of aluminum powder in the high-temperature gas phase environment. The order for the weight gain of nanoparticle samples from high to low was micron Al > C₈H₁₈-Al > ordinary nano Al. This result indicates that the active aluminum content of C₈H₁₈ nanoparticles may be between micron aluminum powder and normal nano aluminum powder, thereby confirming the results of the active aluminum content test.

3.7. Stability Performance

The thermal stability test of the C₈H₁₈-Al sample was carried out via the heating method at 75 °C with reference to part XIII of GJB5891.13-2006 [33]. The specimen vials containing 1.0 g of C₈H₁₈-Al nanoparticles were placed in an oven and heated continuously at 75 ± 2 °C for 48 h. A set heating temperature was used to evaluate the thermal stability of the sample by calculating the mass fraction reduction of the sample over a period of time according to Equation (2). The two sets of parallel tests were conducted on the C₈H₁₈-Al test sample, and the average value was taken for the two test cases with a difference of no more than 0.02% to obtain the weight loss mass fraction of the C₈H₁₈-Al sample: 0.55% ± 0.11%. The results indicated that the state of the coating layer of C₈H₁₈-Al nanoparticles was stable, and that the color of the samples did not change significantly before and after heating. Additionally, the samples had good thermal stability, which indicated that the coating layer could protect the nano aluminum powder well.

$$W=\frac{m_0-m_1}{m}\times 100\%$$

(2)

where W is the mass fraction of the sample reduction (%), m₀ is the total mass of the specimen vessel and specimen before heating (g), m₁ is the total mass of the specimen vessel and specimen after heating (g), and m is the mass of the sample weighed.

The vacuum stability of the sample was tested using the pressure sensor method based on GJB772A-97 (501.2) [34]. Pretreatment of the C₈H₁₈-Al test sample was carried out with drying at 55 °C for 2 h before the experiment to avoid water molecules and alkane solvent molecules from affecting the test results. The vacuum stability test was conducted by weighing 1 ± 0.01 g of the pretreatment sample, and the average value was taken from three parallel tests. The gas yield of the experimental result for the C₈H₁₈-Al sample was 1.301 ± 0.132 mL/g, calculated according to Equation (3), which was less than the 2 mL/g specified in the national standard, indicating that the stability of C₈H₁₈-Al was sufficient:

$$V_s=2.69\times {10}^{-3}\frac{P_r}{T_a}\left(V_1-V_0\right)$$

(3)

where V_s is the volume of gas released from the specimen in the standard state (mL), 2.69 × 10⁻³ is the ratio of the temperature to pressure under standard conditions (K/Pa), P_r is the pressure of the gas released from the specimen (Pa), T_a is the experimental ambient temperature (K), V₁ is the sum of the reactor volume and the pressure measurement connection line volume (mL), and V₀ is the volume of the specimen (mL).

3.8. Energy Performance

In addition to analyzing the morphological characteristics and chemical state of the coating material, it is necessary to understand the effects of the combustion performance of the coating sample. The heat from the combustion of C₈H₁₈-Al nanoparticles was determined and compared using a fully automatic calorimeter under program control based on GJB 770B-2005 (703.1) [34]. To guarantee the accuracy of the test, each sample was tested three times in parallel, averaged, and compared with aluminum powder. The results are shown in

Table 3. Combustion heat of samples.

Sample	Combustion (MJ/kg)
C8H18-Al	26.41 ± 0.13
Ordinary nano Al [5]	19.69 ± 0.14
Micro Al [5,24]	28.40 ± 0.12

. The heat from the combustion of the samples was, from high to low, micro Al > C₈H₁₈-Al > ordinary nano Al, which agrees with the results of the active aluminum content test.

To further evaluate the energy generated by the combustion of C₈H₁₈-Al and the speed of the reaction, the differential pressure method was used to perform a confined exploder test to record the pressure generated during the combustion of C₈H₁₈-Al samples as a function of time. As shown in Figure 6, the maximum value of combustion pressure was 0.232 Mpa, and the maximum value of the pressure increase rate was 137 Mpa/s.

Table 4. Combustion performance comparison of samples.

Sample	P (Mpa)	(Mpa/s)
C8H18-Al	0.232	137
Ordinary nano Al [5]	0.238	34.1
Micro Al [5]	0.169	32.8

shows that the maximum pressure value for combustion of the C₈H₁₈-Al sample was higher than that of the micron aluminum powder and close to that of common nano aluminum powder. Nevertheless, the combustion rate of the C₈H₁₈-Al sample was about four times higher than that of the micron aluminum powder and normal nano aluminum powder. The results demonstrated that using C₈H₁₈ as a coating layer can enable aluminum powder to release energy rapidly. The test results for the heat of combustion were also verified.

4. Conclusions

In this paper, C₈H₁₈-Al nanoparticles were synthesized for the first time by the in situ coating of aluminum nanoparticles with 2,2,4-trimethylpentane. The morphological characteristics, particle size distribution, chemical state, and thermal properties of the C₈H₁₈-Al nanoparticles were analyzed and characterized. C₈H₁₈-Al nanoparticles have a typical spherical shell–core structure with uniformity, and the shell structure consists of a 3.12 nm thick coating and a 1.07 nm thick oxide layer. The particle size was normally distributed, and the median particle size was 219.3 nm. The active aluminum content was 85.45%, which was better than the result for normal aluminum nanoparticles but lower than that of micron-sized aluminum particles. The XPS results confirmed the presence of C₈H_18, which combined with alumina to form hydroxyl, carboxyl, and carbonyl functional groups on the surface of the aluminum nanoparticles. The presence of amorphous alumina in C₈H₁₈-Al affected the thermal properties and burning rate of C₈H₁₈-Al. Stability experiments confirmed that the C₈H₁₈ coating was not easily decomposed and could better protect the aluminum nanoparticles. The energy test results illustrated that C₈H₁₈ as a cladding layer can make aluminum nanoparticles release energy rapidly (about four times more rapidly than normal aluminum nanoparticles). In summary, the results showed that the in situ coating of aluminum nanoparticles with 2,2,4-trimethylpentane is an effective method to ensure the presence of active aluminum content.