Enhancement of thermal properties of Al/MoO3 thermite by electrostatic spraying

To improve the thermal properties of thermite safely and stably, electrostatic spraying was used to prepare the Al/MoO3 thermite. The Al/MoO3 thermites were detected and characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD), and thermal decomposition experiments were carried out by differential scanning calorimetry (DSC). The heat release of the Al/MoO3 thermite prepared by the electrostatic spray (1044 J g−1) is significantly higher than that of the thermite prepared by the ultrasonic (692 J g−1), which is due to more uniform dispersion between Al and MoO3. The initial reaction temperature and activation energy (Ea) of the former keep it steady. Electrostatic spray ensures the safety and stability of the Al/MoO3 thermite. This study provides a new idea for safely and stably improving the thermal properties of thermite by enhancing surface homogenization, which is of great significance for practical applications.


INTRODUCTION
Energetic materials (EMs) are a class of compounds or mixtures that contain a high amount of stored chemical energy. 1,2 These materials can release much heat violently during the reaction. The thermite stands out due to its excellent combustion efficiency, high energy-releasing rate, and high reaction exothermic enthalpy, which catches the attention of researchers. Thermite has been widely used in gas generator, 3 welding, 4 ammunition destruction 5 and high-energy additives in propellants. 6,7 The thermite composed of metal oxide and metal powder possesses unique chemical properties, which mainly is ascribed to the nature of its components. Typically, the thermite composed of Al and Fe 2 O 3 is used for railway welding. 8 In the selection of metal powder, Al has been used in various conventional explosives and propellants to increase energy and temperature due to the advantages of high unit density exotherm, high reactivity, and non-toxicity of reaction products. For metal oxides, there are so many choices. Wang 9 has successfully prepared Al/Fe 2 O 3 thermite by sol-gel method for exploring the thermite reaction rate. Wang 10 has synthesized Al/CuO thermite with core-shell structure through self-assembly method, and the result showed that it had a good exothermic performance. Song 11 has prepared Al/MnO 2 thermite as a destroyer. Experiments showed the Al/MnO 2 thermite generated a bright flame during the reaction, which could easily penetrate the steel target, indicating the thermite had a good cutting performance. Wolenski 12 has engineered the composition and morphology of Al/MoO 3 particles in thermite, which could promote the enhancement of reaction and the adjustment of combustion behavior. The chemical and physical properties of MoO 3 make it versatile so it can be used in optical, electronic, catalytic, biological and energy. 13 Besides, previous studies showed the Al/MoO 3 thermite owned better ignition performance. 14 The thermite reaction, a solid-state diffusion process, is enhanced by decreasing the distance between metals and metal oxides. In other words, by improving the preparation method of the thermite to make the structure of the thermite more uniform, the thermal performance of the thermite can be effectively improved. Gash 15 has proposed Al/Fe 2 O 3 thermite through the sol-gel reaction of metal salt and propylene oxide precursor. The results showed the thermite shortens the distance between Fe 2 O 3 and Al, and inhibits the generation of Al oxide film, making the reaction more sufficient. However, the success of the sol-gel method is affected by a variety of factors, some of which are difficult to control. Zheng 16 has used the magnetron sputtering method to study the Al/Mn 2 O 3 thermal film for the first time. Although the thermite prepared by this method has good thermal properties and excellent energy retention properties, the preparation equipment is expensive, the process is complicated, and the preparation amount is small. Electrostatic spraying, as a new preparation process, has been widely used to make the sample more uniform through the action of a strong electric field. 17,18 In this paper, nano-MoO 3 was fabricated by hydrothermal method. Then, Al/MoO 3 nano-thermite was prepared by electrostatic spraying method. Also, Al/MoO 3 nano-thermite was prepared by ultrasonic dispersion method as the control group. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to detect and characterize the samples, and differential scanning calorimetry (DSC) was used to conduct thermal decomposition experiments. Multi-scan rate thermal analysis experiments were carried out for Al/MoO 3 nano-thermite, and the activation energy (E a ) of Al/MoO 3 nano-thermite was calculated. Finally, the mechanism of improving the thermal properties of the Al/MoO 3 nano-thermite prepared by electrostatic spraying was preliminarily analyzed.

Materials
All the chemicals (analytical grade) were purchased from Shanghai Chemical Reagents Co. without further purification. The mean size of 80 nm Al nanoparticles and 50 nm MoO 3 nanoparticles, were designated by the manufacturer. Absolute ethanol with a purity of 99.7% was chosen as a solvent to disperse the nanoparticles.

Preparation of nano-MoO 3
One of the most common procedures for synthesizing MoO 3 was used. 19 The schematic diagram of the preparation process is shown in Figure 1. 1.5 g of (NH 4 ) 6 Mo 7 O 24 ⋅4H 2 O and 30 mL of deionized water were poured into a beaker to prepare a certain concentration of ammonium molybdate solution, which was stirred strongly through a magnetic stirring device for 30 min. Then, 5 mL HNO 3 was extracted by pipette to dilute using 10 mL deionized water. Next, the diluted nitric acid solution was added dropwise to the stirring ammonium molybdate solution. At this time, white flocs were observed in the solution. The mixture solution was placed in the ultrasonic device at 25 • C for 10 min. Following this treatment, the solution was transferred into a 100 mL Teflon-lined steel stainless autoclave, sealed and kept at 200 • C for 25 h. After the temperature of the autoclave was naturally returned to room temperature, the sample was then washed 3 times with deionized water and centrifuged. Later, the precipitate was dried at 80 • C for 12 h. In the end, the sample was ground to place into a tube-type calciner, and baken at 400 • C for 12 h to obtain the MoO 3 sample. The chemical reaction equation is as follows.

Nano-Al activity assay
Due to oxidation, the activity of nano-Al powder will not be 100%. 20 To analyze the content of active Al, the test was carried out by thermogravimetric analysis (TG) at 30 ∼ 1200 • C with a heating rate of 10 K min −1 and an airflow rate of 50 mL min −1 to ensure complete oxidation of the nano-Al particles. The reaction of Al in air is as in Equation (2).
The TG analysis curve is shown in Figure 2. Mass increase in the TG analysis is attributed to oxidation of active Al. The active Al content (c) can be calculated by Equation (3).
where, Δm(%) is the percentage of mass gain in TG analysis. It was concluded that the content of active Al is 65.5%.

Precursor preparation
Thermite samples were prepared using ultrasonic dispersion method. The mass of MoO 3 samples is controlled at 120 mg.
According to the calculation of the chemical Equation (4), the mass of fuel Al required for 120 mg of MoO 3 to reach zero oxygen balance is about 45 mg. However, the fuel used is only 65.5% active aluminum, so the mass of sample Al is 70 mg.
At first, 120 mg of nano-MoO 3 and 70 mg of nano-Al were respectively dispersed in absolute ethanol and magnetically stirred for 30 min. Then, two cups of solution were mixed into the same beaker and continued to be magnetically stirred for 20 min. Subsequently, the mixture was put into an ultrasonic cleaning machine and ultrasonically dispersed for 30 min to obtain a uniformly dispersed precursor suspension.
Two sets of solutions were prepared, one for subsequent electrostatic spray preparation of samples, labeled I. The other group was the control group, and the thermite sample was obtained by directly drying at 80 • C for 12 h, which was marked as II.

Preparation of thermite by electrostatic spraying
The precursor solution was loaded into a syringe with a flat needle with a tip diameter of 0.42 mm. The syringe was pressurized through a syringe pump and programmed to produce a spray rate of 4.0 mL h −1 . On the opposite side of the injector 10 cm, there was a square aluminum foil receiving plate with a length of 30 cm on one side, and a voltage of 13.5 kV was applied between the nozzle and the receiving plate. The ejected droplets formed a Taylor cone under the action of a strong voltage. The whole experiment was carried out in an environment with a relative humidity of 45%. The setup for the electrostatic spray experiment is shown in schematic Figure 3.
The precursor liquid was ejected through the nozzle under the action of a strong electric field, and the nanoparticles attached to the charged droplet were accelerated by the electric field and formed a Taylor cone. Since the electrostatic force was greater than the molecular cohesion of the liquid, the ejected liquid was broken into a large number of small F I G U R E 3 Schematic diagram of electrostatic spray experiment F I G U R E 4 Electrostatic spray experiment effect droplets. 18 At the same time, the solvent evaporated rapidly, leaving behind concentrated nano-solid particles that diffused onto the receiver plate to form a uniform, highly polymerized thermite composite. Figure 4 is the thermite powder formed by spraying on the aluminum foil. A thin layer of film was formed on the surface of the aluminum foil, and the powder grew upright on the aluminum foil. The sediment was collected on the receiver plate with a spatula and placed in an antistatic bottle for later use.

2.6
Characterization and thermal analysis X-ray diffraction analysis (XRD, Bruker D8 Discover, Germany) was performed on the thermite samples to determine phase structures. The morphology of the samples was observed by scanning electron microscopy (SEM, JSM-7800F, Japan). Thermogravimetric-differential scanning calorimetry (TG-DSC) method (NETZSCH STA 449F3, Germany) was introduced to measure the thermal properties of samples. The thermal analysis of thermites was conducted under the argon atmosphere with the temperature range from 40 to 1000 • C. Each sample was 3.0 mg for thermal safety, and the heating rates was 15 K min −1 . In the end, thermal analysis experiments are performed under different heating rates to determine the actuation energy (E a ). The heating rate were 10, 15, 20, 25 K min −1 .

Theoretical analysis
The Kissinger method is one of the most well-known isoconversional methods on behalf of the differential method to determine E a . The E a of Al/MoO 3 thermite was obtained by using the Kissinger method. The Kissinger calculation method is shown in Equation (5). 21 ln ( Where is the linear heating rate (K min −1 ), T P is the absolute temperature (K), R is the gas constant (J mol −1 K −1 ), A is the pre-exponential factor (s −1 ) and E a is the activation energy (kJ mol −1 ).
Assuming that the reaction rate is maximized at the peak temperature, the plot of ln ( /T P 2 ) versus 1/T is a straight line, the slope of which is calculated as the value of the activation energy E a .

Microscopic morphology of synthetic MoO 3
Microscopic morphology and surface analysis of synthetic MoO 3 were observed by FE-SEM. In Figure 5, SEM images show particles of synthetic samples adopt a rod morphology with diameters of 80 ∼ 120 nm and lengths of 15 ∼ 20 μm.

Crystal structures of synthetic MoO 3
To determine the crystal structures of obtained samples by hydrothermal method, XRD analysis was performed to test the phase structure. The results are shown in Figure 6. Figure 6 shows the diffraction peaks from 10 to 80 • . The diffraction pattern for the samples has seven broad peaks at 12.

Microscopic morphologies of thermite
To better observe the microscopic morphology, the thermite samples I and II were observed using SEM. Figure 8A,B are the SEM images of the prepared thermite, in which the part selected by the red frame in the middle of the picture is individually enlarged for observing the distribution. There is an agglomeration of nano Al particles in Figure 8A. Although part of Al is also agglomerated in Figure 8B, more Al particles are evenly dispersed. The SEM images better illustrate the uniformity of the dispersion of the samples. To further analyze the dispersion of MoO 3 and Al in the thermite, SEM-Mapping characterization was carried out. Figure 9A shows Al has an obvious agglomeration phenomenon. In the red circle, Al accumulation occurs, and in the corresponding area, there is less MoO 3 distribution. By contrast, in Figure 9B, The distribution of MoO 3 is in the shape of thin stripes MoO 3 , Al is uniformly dispersed along its grain, which illustrates Al and MoO 3 are well dispersibility.

Thermal analysis
The thermal properties of the thermite samples were tested by DSC, as shown in Figure 10. The details of the main exothermic peaks of the thermite samples are listed in Table 1. Figure 10B shows the DSC image curve of the Al/MoO 3 thermite prepared by the electrostatic spray method is consistent with the overall trend of the DSC curve of that prepared by the ultrasonic dispersion method. There is only The exothermic heat of Al/MoO 3 thermite prepared by electrostatic spraying increased obviously, which is 1044 J g −1 . At the same time, the initial exothermic temperature of the Al/MoO 3 thermite prepared by the electrostatic spray method is relatively stable. Compared with the Al/MoO 3 thermite prepared by the ultrasonic dispersion method, there is no major change, which ensures safety and does not release heat prematurely.
The thermal properties of the Al/MoO 3 thermite prepared by electrostatic spraying are significantly improved, benefiting from its more uniformly dispersed structure. As shown in Figure 9B, Al is distributed along the shape of MoO 3 thin rods and is relatively uniform. On the contrary, in the Al/MoO 3 thermite prepared by the ultrasonic dispersion method, the distribution of Al is not uniform, and there is a relatively obvious agglomeration phenomenon. Its reaction mechanism is shown in Figure 11. The contact area of the agglomerated Al and MoO 3 is small, so the effective area of the aluminothermic reaction is small. In sample I, the contact area between Al and MoO 3 is larger, which is conducive to the more complete reaction, so the heat released by the reaction is more. 22

Non-isothermal thermodynamic analysis
To explore the thermal kinetics of the Al/MoO 3 thermite, DSC analysis experiment of multiple heating rates was carried out, which could obtain the E a of the sample, as shown in Figure 12. E a is an indicator of the minimum energy required for a chemical reaction, and it can reflect the difficulty of the chemical reaction. 23 In Figure 12, the thermal kinetics of the Al/MoO 3 thermite were obtained through Equation (5). The detail of calculation parameters of E a for sample thermite reaction is shown in Table 2. The linear fitting diagram of the peak temperature point of the sample DSC test is shown in Figure 13.   Figure 13A shows the straight line fitted by sample I is y = −21,876.8x + 15.7, the E a is 181.88 kJ mol −1 by calculation. The R is 0.99712, indicating a high degree of fitting. The straight line fitted by sample II is y = −20809.2x + 14.4, R is 0.99454, and the obtained E a is 173.01 kJ mol −1 , which is a bit smaller than sample I. This further illustrates the stability and safety of the Al/MoO 3 thermite prepared by the electrostatic spray method and will not be excited to release heat prematurely. Under the premise of ensuring safety and stability, the Al/MoO 3 thermite prepared by the electrostatic spray method has greatly improved the heat release and its thermal performance has been significantly improved.

CONCLUSION
The MoO 3 was prepared via the hydrothermal process, and then Al/MoO 3 thermite was prepared by electrospray. At the same time, Al/MoO 3 thermite was prepared by ultrasonic dispersion method as a control group. SEM, XRD, and DSC were carried out to explore their thermal properties. DSC analysis showed the thermal decomposition performance of Al/MoO 3 thermite prepared by electrospray was better than Al/MoO 3 thermite prepared by ultrasonic dispersion method, which was manifested in the fact that the released heat of the former thermite, 1044 J g −1 was more than 692 J g −1 of the latter thermite. The obvious improvement in thermal properties of Al/MoO 3 thermite prepared by electrostatic spraying is due to its more uniformly dispersed structure.
Neither the initial reaction temperature of thermite nor the E a obtained by non-isothermal thermodynamic analysis has a significant decrease. It shows Al/MoO 3 thermite prepared by electrospray presents good thermal stability, will not be easily excited, premature thermal decomposition, which ensures safety.
In a word, the thermal properties of the thermite prepared by the electrostatic spray method have been greatly improved, and safety is ensured, laying a foundation for the subsequent scientific research technology and product manufacturing application.