Thermal behavior and combustion performance of Al/MoO3 nanothermites with addition of poly (vinylidene fluorine) using electrospraying

To investigate the effect of the addition of poly (vinylidene fluoride) (PVDF) on nanothermites, Al/MoO3/PVDF energetic nanocomposites were prepared using electrospraying method. As a control group, Al/MoO3 was also designed. Then, both samples were tested by FE-SEM, XRD and TG-DSC. TG-DSC results showed that the Al/MoO3/PVDF energetic nanocomposites released more than 934.0 J g−1 with two obvious exothermic peaks. Compared with the control group of 800.7 J g−1 heat, it changed the thermal performance to some extent. There were Mo2C among the residues products after the reaction via XRD. The activation energy (Ea) was analyzed using the Kissinger method under different heating rates by DSC. The addition of PVDF reduced the Ea of the thermites. To explore the combustion performance, a preliminary experiment was designed. The Al/MoO3/PVDF energetic nanocomposites were easier to ignite and the burning was more durable, which was significant in solid propulsion and applications requiring extended combustion time.


Introduction
With the development of nanotechnology, nanothermite, as a high-energy material containing metal oxidant and fuel, has short diffusion distance, large contact area and good uniformity, which has aroused widespread concern [1][2][3][4]. It has a lower ignition temperature, better reactivity and faster propagation speed, which is significantly better than traditional thermite. Carrying out a powerful redox reaction in a short time, it can be applied in ammunition primer, [5] nano welding, [6] gas generator [7] and explosive propellant [8,9].
Preparing a more homogeneous structure and reducing the diffusion distance between fuel and oxidant help improve the thermal performance of thermite, which has attracted many scholars to study. For example, Kim [10] and Zachariah realized the directional assembly and close contact between fuel and oxidant using the electrostatically enhanced method. This approach intensified the interaction and improved the reactivity of energetic nanocomposites. Wang [11] and his co-authors chose sol-gel technology to prepare Al/Fe 2 O 3 nanocomposites. The results showed that the Fe 2 O 3 particles fabricated by the sol-gel method could successfully encapsulate nano-Al, avoiding the oxide film's generation. However, this method had many influencing factors. Some of them were difficult to control. Besides, Ke Xiang and co-authors [12] used the magnetron sputtering method to prepare CuO/Al core/shell structured nanothermites with stable combustion process and flame propagation speed. The good thermal performance and excellent energy retention performance had been proved. Further, Song [13] and his co-authors added potassium perchlorate (KClO 4 ) to the Al/MnO 2 nanothermites system by electrospray method, which effectively reduced the activation energy of the thermite system. Wang and his co-authors successfully prepared Al/CuO, Si/CuO nanothermites with core-shell Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. structure by self-assembly method and electrophoretic deposition method, which showed excellent thermal performance [14][15][16].
Zhou [17] filled the micron and nanometer passivated aluminum particles with PVDF to make composite materials, which significantly improved the thermal conductivity and relative dielectric constant. This phenomenon is the free electron transport of aluminum metal particles, which are embedded in the PVDF matrix, forming a uniform and dense microstructure. Fluorine, as the most abundant halogen, is the most electronegative and non-polarizable element known at present. The nature of its chemical bond reflects the physical properties of a fluoropolymer. C-F bond is the strongest single key inorganic substances (450 kJ mol −1 ) and more than 100 kJ stronger than a C-Cl bond, so its reactivity is reduced by more than 100 times. The addition of a fluoropolymer can also bring some other excellent properties, such as moisture resistance and super-hydrophobicity [18]. To explore new fluoroplastic materials, throughout the 1950 s, researchers studied and prepared many fluoropolymer mixtures, fluorinated ethylene propylene (FEP), which was TFE's first copolymer [19]. The following year, polyvinyl fluoride (PVF) and PVDF were created, the former with one type of fluorine and the latter with two types of that. Nevertheless, in terms of solubility, most fluoropolymers are insoluble, but PVDF has great solubility in polar organic solvent dimethylformamide (DMF), [20] so it is most commonly used. Song [21] prepared Al/MnO 2 /PVDF using an electrostatic spray. The activation energy of the thermite system has significantly been reduced. Li [22] synthesized Al/PVDF/CuO composites using solvent synthesis and explored its properties.
Molybdenum trioxide (MoO 3 ) has unique electrochemical, catalytic and environmental characteristics, excellent electrochemical colour development and electrocatalytic function, which is widely used in battery material research [23]. Compared with CuO, Fe 2 O 3 and MnO 2 , the enthalpy of Al/MoO 3 thermite reaction is the highest when MoO 3 is used as a metallic oxidizer in the thermite system, according to the stoichiometric ratio [9]. What is more, the Al/MoO 3 composites can provide more remarkable ignition characteristics. Wolenski, C and his co-authors [24] engineered the composition and morphology of particles in the Al-NP/MoO 3 thermite system. The results showed that this method could promote an enhanced response and adjust the combustion behavior.
In this work, as a binder, PVDF was added to the Al/MoO 3 nanothermites system to explore its impact on combustion and thermal performance. Firstly, the Al/MoO 3 nanothermites were fabricated by ultrasound method. Then, based on the role of PVDF binder, PVDF was uniformly dispersed into Al/MoO 3 by electrospray method to prepare Al/MoO 3 /PVDF energetic nanocomposites. At the same time, Al/MoO 3 also was prepared as the control group. Next, the Al/MoO 3 nanothermites and Al/MoO 3 /PVDF energetic nanocomposites were characterized and tested by Field Emission Scanning Electron Microscope (FE-SEM), x-ray diffraction (XRD) and Thermogravimetric Analysis and Differential Scanning Calorimetry (TG-DSC). To calculate and discuss their activation energy, DSC was implemented at different heating rates. In the end, the preliminary combustion test was designed to observe the actual combustion performance of the thermites.

Materials
All chemicals were analytical grade reagents and were used directly without any treatment or purification. Nano-Al (∼100 nm) was obtained from Aladdin Industrial Corporation (Shanghai, China). Nano-MoO 3 particles (∼50 nm) were purchased from Nano Chemical Technology Co., Ltd (Guangzhou, China). The energetic additive, PVDF was supplied by Sinopharm Chemical Reagent Co., Ltd And the molecular mass of PVDF was 476×10 3 g·mol −1 . Besides, the absolute ethanol and DMF were obtained by Nanjing Chemical Reagent Co., Ltd Deionized water, absolute ethanol as well as DMF were chosen as solvent and dispersant.

Precursor preparation
At first, ultrasonic mixing method was applied to synthesis the Al/MoO 3 nanothermites. According to the stoichiometric ratio, 62 mg of MoO 3 and 38 mg of nano-Al (considering the effect of aluminum oxide layer) were magnetically stirred in cyclohexane for about 30 min At the same time, 43 mg of PVDF was dissolved in 1 ml of DMF. Then, the PVDF/DMF solution was poured into the Al/MoO 3 solution under ultrasonic conditions. The mixed solution appeared black without precipitation. 43 mg of PVDF accounts for 30 wt% of the total solution mass of 143 mg. Also, Al/MoO 3 thermite without PVDF additive was prepared as an experimental control group.

Electrospray experiment
As shown in figure 1, the precursor solution was contained in a syringe with a flat needle whose diameter was 0.42 mm at the end. A syringe pump was used to apply a flow rate of 4.0 ml·h −1 , and a voltage of 13.5 V was applied between the nozzle and the receiving plate (a square aluminum foil with a side length of 30 cm) to form the Taylor cone. The distance between the nozzle and the receiving plate was about 10 cm. The relative humidity of the experimental environment was 75%.
Under the action of a strong electrostatic field, the precursor liquid advanced through the nozzle, and the nanoparticles adhering to the charged droplets were accelerated by the electric field and formed a Taylor cone. Because the electrostatic force was bigger than the liquid's molecular cohesion, the sprayed liquid broke into a large number of small droplets [25,26]. Simultaneously, the solvent evaporated quickly, leaving concentrated nano-solid particles diffused to the receiving plate to form uniform, highly polymerized thermite composites. Finally, a scraper was used to collect the deposits on the receiving plate and installed it in an anti-static bottle.

Characterization and thermal analysis
The MoO 3 sample phase structures and chemical reaction composition were characterized by using XRD analysis (Bruker, D8 Advance, Germany) with CuK α radiation (λ = 0.1542 nm). The morphology, particle size and mixing quality of the materials and mixture were observed by FE-SEM analysis (HITACHI High-Technologies corporation, S-4800 II, Japan).
TG-DSC (NETZSCH STA 449F3, Germany) analysis was applied to investigate both components and thermite samples' thermal behaviours. All the experiments were conducted under the argon atmosphere with the heating rates of 15 K min −1 with the temperature range from 40°C to 1000°C. To further calculate the activation energy, in a corundum crucible, a sample with a mass of about 5 mg was heated through the DSC at a heating rate of 10, 15, 20, 25 K min −1 .

Theoretical model
As one of the most famous isoconversional methods, the reliable Kissinger method was accessed to obtain the Ea of Al/MoO 3 . The Kissinger method of the model equation can be provided as follows [27,28].
Where β is the linear heating rate (K min −1 ), T P the absolute temperature (K), R the gas constant (J mol −1 K −1 ), A the pre-exponential factor (s −1 ) and Ea is the activation energy (kJ mol −1 ). The plot of ln (β/T P 2 ) Versus 1/T should be a straight line when hypothesizing that the rate of reaction reaches the maximum at the peak temperature, and the slope can be considered as the value of the activation energy Ea.

Preliminary combustion tests
The potential energy capacity of thermite can be characterized by thermal analysis, while the combustion performance is an indicator that reflects the actual properties, which is essential for practical applications.
Heating wire ignition experiments tested the burning characteristics of samples. The heating wire diameter was 0.1 mm, which was heated by a DC power supply to ignite 8 mg samples. A high-speed camera (FAST CAM-AZ) was used to record the combustion processes with a sampling rate of 20,000 frames per second and a frame size of approximately 1024 * 512 pixels. The aperture value was adjusted to 6.4. The schematic diagram of the experimental device is shown in figure 2.

Results of nano-MoO 3
To explore the properties of the samples, XRD was used to test the phase structure, and SEM was introduced to observe the morphology.

XRD analysis
The XRD pattern of the sample is shown in figure 3. The sample shows the diffraction peaks from 5°to 90°can be attributed to MoO 3 (ICSD No. 76-1003 MDI Jade 6.0). The diffraction pattern for the samples have six broad     Figure 4 shows the FE-SEM image of the MoO 3 . Figure4(a) is the overall FE-SEM image of the nano-MoO 3 . For more accurate observation, the red box in figure 4(a) is enlarged, as shown in figure 4(b). The pictures show that the nano-MoO 3 particles have a round shape with a diameter of 50-80 nm. The surface is smooth and flat with less reunion, but some big blocks have some minor effects on the subsequent thermal effects.

Results of nanocomposites and nanothermites XRD analysis
The XRD pattern of the synthesized nanocomposites and PVDF is shown in figure 5. The red line indicates the XRD of Al/MoO 3 /PVDF, and the blue line represents the control group Al/MoO 3 . The black line is the XRD of PVDF. Obviously, peaks appearing in the red line can be well indexed to MoO 3 (ICSD No. 76-1003 MDI Jade 6.0) and Al (ICSD No. 04-0787 MDI Jade 6.0). Also, the diffraction peaks are sharp and intense, indicating their highly crystalline impurity peaks are observed, confirming the high purity of the products.
But no characteristic diffraction peaks of PVDF are observed, which could be caused by its lower loading content and weak crystallization, on the other hand, implying the good dispersion of the tiny PVDF clusters on the Al/MoO 3 surface. Generally, PVDF undergoes a process of rapid evaporation and recrystallization. However, according to the preparation method of the precursor, under the action of a high-voltage electric field, PVDF is difficult to recrystallize after evaporation and dispersion, especially polymers with large molecular masses. Therefore, it is hard to find the obvious characteristic peaks in the XRD pattern.
The blue line is not much different from the red line, showing similar characteristics.

SEM analysis
To better understand the morphological characteristics of the composites, FE-SEM is used to observe its features. It is worth noting that there are no obvious characteristic peaks of PVDF seen in the XRD diffraction pattern, but PVDF can be clearly captured in FE-SEM.    Figure 7 shows the distribution overlay of element Al, element Mo and element F, indicating that these elements have been evenly distributed and confirmed the uniformity of the distribution of several components.
Combining the results of Figures 6 and 7 can be found that Al is directly attached to MoO 3 , and its dispersion is relatively uniform with few agglomerations. As we all know, agglomeration is inevitable [29], but the method of electrostatic spray can effectively reduce the reunion phenomenon. In figures 6(c) and (d), it can be observed that PVDF used as an adhesive can tightly glue Al and MoO 3 together. The bonding effect of PVDF can be clearly seen in figure 6(d). Besides, when the poly fluoride is heated to a specific temperature, a decomposition reaction will release heat. At this time, PVDF becomes a reactant in the redox reaction and participates in the reaction [30].

Thermal analysis and kinetics calculation
TG-DSC analysis TG-DSC tested the pure PVDF, Al/MoO3 and Al/MoO3/PVDF for investigating the effect of PVDF addition in nanothermites system. The results are shown in figure 8 and the main details are listed in table 1. Figure 8(a) shows the TG-DSC results of pure PVDF. There are three peaks in the picture. A small endothermic peak (peak A) corresponds to the melting of PVDF at 170°C [31]. Peak B and peck C, two large exothermic peaks at 480°C and 690°C, accompanied by rapid mass loss, representing the decomposition reaction of PVDF, releasing a large amount of heat 1161.12 J g −1 . After the decomposition reaction, the quality of PVDF no longer decreases, leaving 30.46%.
The TG-DSC of Al/MoO 3 nanothermite is shown in figure 8(b). Before 400°C, the mass of the sample decreased slightly by 3% due to both physisorbed and structural H 2 O and ethanol, accompanied by a small endothermic peak D [32]. As the temperature increases, the main exothermic peak of the Al/MoO 3 thermite starts at 553°C. The two exothermic peaks E and G undergo the same thermite reaction process, but an endothermic peak F that respects the melt of Al at 660°C produces. The exothermic peak E shows a solid-solid phase reaction, while the exothermic peak G indicates a liquid-solid phase reaction between molten Al and solid MoO 3 , which might be caused by the big-block part of MoO 3 with melted Al. The process's onset temperature is 519°C and the endpoint is 824°C, with heat release 800.72 J g −1 .
It can be seen that the space I correspond to the decomposition of PVDF. In the exothermic zone J, the primary thermite reaction occurs between Al and MoO 3 with the exothermic heat is approximately 771.3 J g −1 . At the same time, there is no noticeable quality change in the TG curve, implying that Al and MoO 3 are completely reacted, and there is no residual MoO 3 . Besides, the contact between Al and MoO 3 will be closer and the reaction will be more complete when Al is melted. Due to the addition of PVDF, the peak exotherm at this stage is 680°C, which is much earlier than that in figure 8(b). What is more, the total heat release of Al/MoO 3 /PVDF energetic nanocomposite is 934.0 J g −1 , which changes the heat release of Al/MoO 3 (800.7 J g −1 ) to a certain degree.

Analysis of reaction products
The residues products after the TG-DSC tests are collected and characterized by XRD to analyze the reactant reaction. The results are shown in figure 9.
In the control group, the reactants are Al and MoO 3 , and the residual products after the reaction are detected as Mo and Al 2 O 3 . No reactants are caught in the residue, indicating that the reaction was complete and only one thermite reaction occurred. PVDF is added to the experimental group whose residual products after the reaction are Mo, Al 2 O 3 and Mo 2 C. It is speculated that some part of the Mo produced after the thermite reaction reacted with PVDF to produce Mo 2 C.

Kinetics analysis
Activation energy, reflecting the degree of difficulty of a chemical reaction, represents the minimum energy required for a chemical reaction to occur. Reducing the activation energy of the Al/MoO 3 reaction can promote    According to the Kissinger method theory, the plots of ( ) is constructed, and the result is shown in figure 11. The analytical expression of the data correlation line of the A exothermic peak of Al/MoO 3 is = -+ y x 35444 26 and the correlation coefficient R is −0.94019. The value of the activation energy Ea can be calculated from the slope of the linear fitting line is 294.6 kJ mol −1 . Using the same approaches, the B exothermic peak of Al/MoO 3 and the Al/MoO 3 /PVDF are = -+ y x 70928 56 and = -+ y x 33981 24, corresponding the correlation coefficient R are 0.99058 and 0.94698. Prominently, the addition of PVDF allows the thermite to react fully. Namely, the reaction of MoO 3 with Al after melting is much earlier, and the activation energy is significantly reduced.

Result of preliminary combustion tests
The results of preliminary combustion tests are shown in figure 12. Figure 12  red light is set as the starting time, specified 0. It can be seen that during ignition and combustion, the flame energy release is concentrated and only a small amount of spark splashes.
Obviously, the figure shows that compared to Al/MoO 3 /PVDF, the reaction of Al/MoO 3 is severe brighter fast and shining brightly, reaching the maximum intensity at about 400 μm. However, the excitation current required for Al/MoO 3 /PVDF ignition is 0.678 A, which is much smaller than the excitation current required for Al/MoO 3 ignition, 0.838 A, which implies that the addition of PVDF can effectively make the Al/MoO 3 nanothermite easier to ignition. In terms of burning time, almost completely releasing after 10 ms, the release rate of Al/MoO 3 is too fast. In contrast, the release of Al/MoO 3 /PVDF is more durable, which can still maintain its most violent state of burning at 20 ms, gradually beginning to extinguish after 35 ms.
PVDF with a mass fraction of 30% may reduce the Al/MoO3 thermite content, so there will be attenuation to a certain extent. Still, it is of great significance in improving the activity of the thermite, reducing the reaction conditions and increasing the reaction time.

Conclusions
In this work, the impact of PVDF on Al/MoO 3 nanothermites system was investigated. Firstly, the MoO 3 was characterized by SEM and XRD. Then the Al/MoO 3 /PVDF nanocomposites were assembled via electrospraying method, and the Al/MoO 3 nanothermite was also prepared in the same way as a control group. Agglomerations of components could be effectively reduced through electrospraying method according to the SEM and Mapping results. Besides, all the nano-particles were evenly distributed.
TG-DSC results show that the Al/MoO 3 /PVDF energetic nanocomposites have two obvious exothermic areas in the range of room temperature to 1000°C, which release 934.0 J g −1 heat in total, while the control experiment group, the Al/MoO 3 , has a heat release area in the same range with the about 800.7 J g −1 heat releases. The addition of PVDF slightly improves the heat release. Besides, it can be found in the calculation of activation energy that the activation energy of Al/MoO 3 /PVDF nanocomposites is significantly reduced, which is carried out by thermal analysis experiments under different heating rates using Kissinger method. As a highenergy additive, the results reveal PVDF can dramatically improve the reaction's activity and help ignite the thermite at comparatively low energy.
A preliminary combustion test was conducted and recorded by high-speed photography. The excitation current of Al/MoO 3 /PVDF being ignited is 0.678 A, compared with the 0.838 A of the Al/MoO 3 , which is lower. Meanwhile, Al/MoO 3 /PVDF nanocomposites can burning duration time longer. Obviously, the addition of PVDF can ignite easily and increase reaction time, which corresponds with the results of TG-DSC.