Abstract
Various preparation methods have been widely explored to improve the combustion performance of nanothermites in recent years. In this work, two kinds of Fe2O3⁃containing nanothermites were successfully prepared by in⁃situ ball milling method and conventional ultrasonic blending method respectively. The morphologies and performance of as⁃prepared products have been fully characterized by thermogravimetric analysis (TGA), X⁃ray diffraction (XRD), contact angle tests, scanning electron microscopy (SEM), high⁃speed imaging experiments and infrared temperature measurement. The results show that the Fe2O3⁃doped nanothermites via in⁃situ ball milling method exhibit better performance than that made by ultrasonic blending method. The optimal nanothermites with 17% Fe2O3 doped amount possess the maximum mass gain percentage of 13.1% per 100 ℃. Compared with the products made by ultrasonic blending method, the heating voltage and initial combustion temperature of in⁃situ ball milled nanothermites decrease to 12 V and 600 ℃, respectively. In addition, the combustion flame of in⁃situ ball milled nanothermites is more stable and homogeneous than the corresponding one.
Graphic Abstract
Compared with Fe2O3⁃containing nanothermites prepared by the ultrasonic blending method, in⁃situ ball⁃milled samples possessing better uniformity, hydrophobicity and stable combustion flame.
Thermites, commonly composed of metal and metal oxid
Ball milling techniqu
Based on this experience,a series of Fe2O3⁃containing nanothermites with different oxide content were successfully prepared by in⁃situ ball milling method on a large scale. At the same time, the common ultrasonic blending method has also been utilized to prepare the nanothermites with the same doped amounts for comparison. The mixing uniformity, thermal behavior and hydrophobicity of as⁃prepared products were investigated by a series of measurements and calculations. Additionally, the combustion performances between the composites prepared via two methods were also deeply discussed.
The 4A molecular sieve was purchased from Sinopharm Chemical Reagent Co., Ltd. and treated by calcinating at 400 ℃ for 2 h in a muffle furnace. Dimethyl Sulfoxide (DMSO) was obtained from Aladdin and pre⁃purified via decompressing distillation technique by refluxing with calcium hydride (CaH2) under a dry nitrogen (N2) atmosphere for 24 h to keep anhydrous and oxygen free. Afterwards, the distilled DMSO solvent mixed with activated 4A molecular sieve under magnetic stirring for 8 h for deeply purified. Micron aluminum powders were received from Anshan Industry Fine Aluminum Powders Company Limited. Ferric oxide (Fe2O3) was purchased from Innochem. CaH2 and trimethoxy(vinyl)silane (A171) were obtained from Alfa Aesar. Cyclohexane and ammonium chloride (NH4Cl) were obtained from Aladdin.
The loading, sampling and covering processes were all operated in a glovebox under high purity N2. The ball milling method was carried out by QM⁃3SP4 planetary ball⁃mill in the steel vials and the milling mediums were steel balls with a diameter of 5 mm. The mixture of micron aluminum powders, Fe2O3, and NH4Cl, and steel balls with 1∶50 mass ratio, were all added into the steel vials and sealed tightly, then ball milling for 14 h. In addition,the added mass loading of Fe2O3 is 1%,5%,9%,13%,17% and 21% of micron aluminum powders,respectively. After ball milling process,all products were washed by purified DMSO for 3 times to remove NH4Cl.
Then, the mixture of 0.5 mL A171 and 100 mL cyclohexane were added into a two⁃neck flask as primed solution. Afterwards, 10 g ball⁃milled products were added into well⁃mixed primed solution and stirring for one hour at 80 ℃. The separation of solids and liquids were executed by vacuum filtration and washed 3 times by using cyclohexane. Finally, the products were dried in the vacuum drying oven for an hour at room temperature. The ultrasonic blending method was also performed as follows: the mixture of 10 g ball⁃milled aluminum nanoparticles and 100 mL purified cyclohexane were added to a two⁃neck flask. Then, various Fe2O3 (1%~21%) powders and 0.5 mL A171were added in afterwards. Subsequently, the suspension was continued ultrasonic dispersed for 1 h and the solution was removed via vacuum filtration by cyclohexane washing for 3 times. Finally, the samples were dried with the same conditions.
The crystalline of as⁃prepared composites were performed by X⁃ray powder diffraction (XRD) analysis (Cu Kα) using a PANalytical Empyrean instrument with a range of 10° to 90°. Thermogravimetric analysis (TGA) was detected by SDT Q600 from room temperature to 850
In order to verify the structures of as⁃prepared nanothermites by ball milling method, the XRD was conducted and the results shown in

a. in⁃situ ball milling method

b. ultrasonic blending method
Fig. 1 X⁃ray diffraction (XRD) patterns of nanothermites doped with different amounts of Fe2O3 via different methods (Note: the peaks marked by red dotted lines represent Al peaks and the unmarked peaks are all Fe2O3 peaks)
In order to compare the oxidation effect of Fe2O3 on aluminum nanoparticles prepared by in⁃situ ball milling method and ultrasonic blending method, the TGA measurements are conducted (

a. in⁃situ ball milling method

b. ultrasonic blending method
Fig. 2 TGA curves of nanothermites doped with different amounts of Fe2O3 via two methods
To compare and theoretically analyze the rising rate of the TGA curves of nanothermites obtained from two methods, fitting functions were employed by Origin softwar

Fig. 3 TGA fitting curves of nanothermites doped with different amounts of Fe2O3 prepared by in⁃situ ball milling method.

Fig. 4 TGA fitting curves of nanothermites doped with different amounts of Fe2O3 prepared by ultrasonic blending method.
For the purpose of obtaining the rising rates of the fitting curves, the derivative functions of above⁃mentioned functions are calculated as follows:
Therefore,the maximum rising rate of TGA fitting curves were calculated at 850 ℃ and the oxidation rates of all samples were reflected by mass gain percentage per 100 ℃. Obviously, the maximum mass gain percentage per 100 ℃ of optimal in⁃situ ball⁃milled nanothermites was 13.1%. The maximum mass gain percentage per 100 ℃ of ultrasonic⁃blended nanothermites was only 12.2%,indicating the two fitting curves with 5% and 9% doped composites shown similar oxidation rates. However, the mass gain of the 5% doped composite is higher than that of 9%, illustrating the oxidation degree of nanothermites doped with 5% Fe2O3 is more complete than that of nanothermites doped with 9% Fe2O3.
As a consequence, the optimal doped amounts of nanothermites prepared by the two methods were 17% and 5% for ball⁃milled and ultrasonic⁃blended composites, respectively, and the in⁃situ ball⁃milled nanothermites possess higher oxidation rate in comparison with the ultrasonic⁃blended nanothermites, which is consistent with the results presented by TGA curves.
To exhibit the particles size, element distribution and microscopic morphology of nanothermites prepared by the two methods, scanning electron microscope (SEM) and mapping scanning of energy dispersive spectrometer (EDS) analysis were conducted and the results are presented in

Fig. 5 Microscopic morphology and element distribution of in⁃situ ball⁃milled and ultrasonic⁃blended nanothermites: (a) Element distribution of in⁃situ ball⁃milled nanothermites doped with 17 % Fe2O3 from EDS mapping; (b) Element distribution of ultrasonic⁃blended nanothermites doped with 17 % Fe2O3 from EDS mapping; (c) corresponding SEM images of in⁃situ ball⁃milled nanothermites doped with 17 % Fe2O3; (d) corresponding SEM images of ultrasonic⁃blended nanothermites doped with 5 Fe2O3; (e) Schematic diagram of in⁃situ ball⁃milled nanothermites doped with 17 % Fe2O3 and (f) Schematic diagram of ultrasonic⁃blended nanothermites doped with 5 % Fe2O3.
Obviously, aluminum and iron elements in the prepared nanothermites are both displayed clearly,(
The hydrophobicity of nanothermites prepared by two methods were investigated and the contact angles were detected and the results presented in

a. in⁃situ ball milling method (17% Fe2O3)

b. ultrasonic blending method (5% Fe2O3)

c. Fe2O3 particles
Fig. 6 Contact angles of nanothermites doped different amounts of Fe2O3 via different methods.
Clearly, the contact angles of nanothermites obtained from ball milling method, ultrasonic blending method and pristine Fe2O3 are 113.01°,96.54° and 17.93°,respectively. The result could be ascribed to the exposure of Fe2O3 nanoparticles to the surrounding atmosphere and water,stemming from the uneven mixing of Al and Fe2O3 particles. In contrast, the contact angle of in⁃situ ball⁃milled nanothermites doped with 17% Fe2O3 (113.01°) is much larger than that of the ultrasonic⁃blended composite, indicating that in⁃situ ball⁃milled composites possess better hydrophobicity than that os ultrasonic⁃blended composites. Therefore, in⁃situ ball⁃milled nanothermites are more conducive to preservation in comparison to ultrasonic blended nanothermites
To obtain the real⁃time flame temperature and combustion phenomena of nanothermites prepared by the two methods,infrared temperature measurement and high⁃speed imaging experiments were conducted on two compisites.
As shown in

a. in⁃situ ball milling method (17% Fe2O3, 12 V)

b. ultrasonic blending method (5% Fe2O3,12V and 15 V)
Fig. 7 Images of real⁃time infrared temperature measurement of nanothermites prepared via different methods. (Note: The cross symbols (+) represent the tracked automatically highest temperature points.)
(1) The Fe2O3 powders and aluminum particles of nanothermites prepared via ball milling method are mixed more evenly, nevertheless, the nanothermites prepared by ultrasonic blending method exist clearly distinguished respective agglomerated aluminum particles and Fe2O3 particles.
(2) The contact angle of the nanothermites prepared by in⁃situ ball milling method is 113.01°, which is significantly larger than that of corresponding ultrasonic blending method (96.54°).With the optimal doped Fe2O3 amount (17%) by in⁃situ ball milling, the maximum mass gain percentage per 100 ℃ is 13.1%, which is larger than that for ultrasonic⁃blended doped 5% Fe2O3 product (12.2%).
(3) The in⁃situ ball milled nanothermites could be ignited at lower heating voltage (12 V) and lower initial combustion temperature (~ 600 ℃) than those of ultrasonic⁃blended nanothermites (15 V and ~ 700 ℃), and the combustion flame is more stable and homogeneous. This investigation is expected to promote the development of nanothermites.
(责编: 姜 梅)
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