Cheng-ai Wang ,Jian-bing Xu ,Yun Shen ,Yue-ting Wang ,Teng-long Yang ,Ze-hua Zhang ,Fu-wei Li ,Rui-qi Shen ,Ying-hua Ye ,*

a School of Chemical Engineering,Nanjing University of Science & Technology,Nanjing,210094,People’s Republic of China

b Micro-Nano Energetic Devices Key Laboratory of MIIT,Nanjing,210094,People’s Republic of China

Keywords:Al/CuO Nanothermites Stroage stability Thermodynamics

ABSTRACT The storage stability of energetic materials is important for its application.Here,the storage stability of Al/CuO nanothermite,which was prepared by electrospray method and stored with different storage time,was systematically researched.The activation energy of Al/CuO nanothermite was calculated by differential scanning calorimetry(DSC).The ignition temperature and the curve pressure history of Al/CuO nanothermite was measured using ignition temperature measuring device and constant-volume pressurization tests,respectively.Further,the thermites were characterized by X-ray Diffractometer(XRD),X-ray photoelectron spectroscopy(XPS),scanning electron microscope(SEM)and Transmission electron microscopy(TEM).The results show that the morphology of the thermites did not change significantly.The activation energy was decreased from 254.1 kJ/mol to 181.8 kJ/mol after storage for 13 months.When stored for 0,7 and 13 months,the peak pressures of Al/CuO nanothermite were 685.8 kPa,626.3 kPa and 625.5 kPa,respectively.In addition to the ignition temperature,it was 775 °C,739 °C and 754 °C,respectively.This result indicated that the ignition and combustion properties of Al/CuO nanothermite are obviously reduced when stored for a long time,at room temperature.

1.Introduction

Al/CuO nanothermite,as one of the important nanothermite,has been widely studied and applied by researchers in the field of energetic materials[1-4],because CuO and Al are cheap and easy to manufacture.The research direction of Al/CuO nanothermite mainly includes nanowire[5,6],powder[7-9],thin films[10-13]and the ink of 3D printing[14,15].And they have been employed as pyrotechnics[16],micro-energetic igniter[10,17],micro propulsion[18],and energetic additives[19]in explosives and propellants.

However,Al particles are nano scale and have great activity,it may lead to the performance degradation and failure of the Al/CuO nanothermite in long-term storage.In order to ensure the good storage performance of Al/CuO nanothermite,researchers have carried out many related researches.There are two main methods,one is to use the core-shell structure of Al/CuO[20,21],another is to use polymerizate to coat Al and CuO,forming core-shell structure[22].In our previous work,Al/CuO nanothermite was applied to a MEMS-based solid propellant micro thruster(SPM)array because of its short ignition delay and high reaction rate[23-25].The additive of the nanothermite is nitrocellulose(NC),which can be used as binder and coating material.

In the current work,the main research is the combustion performance and failure mechanism of Al/CuO nanothermite after long-term storage. Therefore, thermodynamic analysis,morphology and composition analysis as well as ignition and combustion performance tests were carried out for the nanothermite with different storage time.Finally,the storage stability and failure mechanism of the nanothermite was analyzed.

2.Experimental section

2.1.Materials

Aluminum nanoparticles(Al NPs,～100 nm,99.9%)and copper oxide nanoparticles(CuO NPs,～40 nm,99.9%)were purchased from HAOXI Research Nanomaterials,Inc.and used as received.The active aluminum content of Al NPs was 68.2 mass%,determined by thermogravimetric analysis(TGA).Nitrocellulose(NC)with nitrogen content of 12.5%was used as binder for electrospray formation of nanothermite.A diethyl ether(99.7%)and ethanol(99.9%)mixture(Volume ratio:1:3)was used to dissolve NC,forming NC solution.

2.2.Precursor preparation procedure

The mass content of the NC binder was 2.5%.15 mg NC was dissolved completely by the mixture solvent of diethyl ether(1 mL)and ethanol(3 mL),and ultrasound for 30 min,forming NC solution.168.5 mg Al NPs and 416.5 mg CuO NPs were introduced into the NC solution to form stable suspension.Then,the suspension was magnetically stirred for 24 h.

2.3.Preparation and storage of nanothermite

In this study,electrospray was employed to prepare Al/CuO nanothermite,which can get dry powder with small particle size and uniform mixing.The precursor was loaded into a syringe,and drove by a syringe pump at the feed rate of 3.0 mL/h.An aluminum foil(30 cm×30 cm)was served as the receiving substrate.The voltage applied between the needle and substrate was 18 kV-20 kV at a distance of 8 cm-10 cm.The detailed preparation method and process of electrospray have been described in our previous study[26].

After the nanothermite was prepared,it was sealed in a small glass bottle(5 mL).In addition,the bottle was stored in a glass desiccator,which placed at room temperature.

2.4.DSC/TG,SEM,TEM,XRD and XPS

The exothermic reaction properties of the samples were investigated by differential scanning calorimetry/thermogravimetric(DSC/TG,NETZSCH STA 449F3).The samples were placed in alumina crucible and the analysis was conducted under nitrogen atmosphere at the heating rate of 5°C/min,10°C/min,15°C/min and 20°C/min,respectively.The surface morphology of the composites was obtained by scanning electron microscope(SEM,HITACHI,S-4800).Transmission electron microscopy(TEM)was performed on a FEI Talos-S.X-ray Diffractometer(XRD,D8ADVANCE)was used to determine the crystallization state of Al/CuO nanothermite prepared by different methods.And the XRD patterns were recorded over a diffraction angle(2θ)range from 10°to 80°in 0.02°steps.Surface chemistry analysis of the nanothermite were also analyzed using X-ray photoelectron spectroscopy(XPS,Thermo ESCALAB 250XI).

2.5.Test of ignition and combustion performance

The ignition temperature of the nanothermite was tested by ignition temperature measuring device,as shown in Fig.1.In terms of tests of ignition temperature,a platinum filament with a diameter of 0.08 mm and length of 10 mm,which was ramped to～1600°C in 10 ms,was connected between the two poles and fixed on the sample holder.An appropriate amount of powder sample was placed in the sample pool and paved.The high-speed pulse current source,which connected with the two poles,operated at 6.0 A and 10 ms.A photoelectric sensor(THORLABS,DET08CFC/M)real-time acquisition of optical signal during sample combustion.An oscilloscope was recorded the current signal on the circuit,the voltage signal between the two poles and the voltage signal of the photoelectric sensor at the same time.

Fig.1.Schematic diagram of ignition temperature test device.

Combustion performance was evaluated with a confined combustion cell of constant volume(12 mL),which can real-time measure the pressure rises.The detailed testing process has been explained in our previous works[27].Typically,25 mg of sample was weighted out and placed in the combustion cell.A nichrome coil with a diameter of 150μm was applied and coupled to a custom-made external high-speed pulse current source operated at 6.0 A and 30 ms to ignite the sample by joule heating.During the combustion,a pressure senor(JUFENGKEJI,JF-YD-205)was used to measure the induced pressure.Then,the pressure signal was amplified and transformed into a voltage signal through a signal processor(SINOCERA,YE5854A).Finally,the signal was monitored and recorded by an oscilloscope(LeCroy,44Xi-A).

3.Results and discussion

3.1.Thermodynamics analysis

The activation energy(Ea)of Al/CuO nanothermite after storage for different time was determined using a thermodynamics analysis.The kinetics study is based on the method of Kissinger,which uses the following equation:

whereTpis the peak temperature,βis the heating rate,Ais the preexponential factor,Eais the activation energy andRis the universal gas constant.

The value of the activation energy is obtained from the slope of the straight,

as shown in Fig.2.

The values of activation energy of the Al/CuO nanothermite with different storage time are represented in Table 1.The value of activation energy increases at first and then decreases as the storage time of the Al/CuO nanothermite increases.The activation energy of the nanothermite stored for 0,7 and 13 months are 254.1 kJ/mol,317.8 kJ/mol and 181.8 kJ/mol,respectively.The average increase rate of activation energy of the nanothermite is 9.1 kJ/mol/month when the storage time is between 0 and 7 months.And the average decrease rate of the nanothermite is 22.7 kJ/mol/month when the storage time is between 7 and 13 months.It can be inferred that activation energy of the nanothermite increases slowly and then decreases rapidly with the increase of storage time of the nanothermite.

Fig.2.DSC curves and Plots of Kissinger to evaluate the activation energy for the Al/CuO nanothermite.

Table 1Activation energy(Ea)and coefficient of determination(R2)of the Al/CuO nanothermite after storage for different time.

3.2.Morphology and composition

The morphology of Al/CuO nanothermite with different storage time is shown in Fig.3.Fig.3a,b and c are SEM images of nanothermite stored for 13,7 and 0 months,respectively.Fig.3d,e and f are TEM images of nanothermite stored for 13,7 and 0 months,respectively.From the results of SEM,the boundary between Al NPs and CuO NPs is still obvious with the increase of storage time.It can be seen from the mapping results that with the increase of storage time,there is no obvious agglomeration of thermite components,and the distribution is uniform.This means that the dispersion and composition of nanothermite have no obvious change after longterm storage.Furthermore,the TEM images show that the morphology of Al NPs has not changed significantly.The results show that after a long-time storage,the morphology and mixture uniformity of the nanothermite do not change significantly.

Fig.3.SEM images(a,b,c)and TEM images(d,e,f)of Al/CuO nanothermite with different storage time(a and d were stored for 13 months;b and e were stored for 7 months;c and f were stored for 0 months).

XRD patterns of Al/CuO nanothermite with different storage time is shown in Fig.4.The curve a,b and c are nanothermite stored for 13,7 and 0 months,respectively.From Fig.4,the main components of the nanothermite are still Al and CuO.Typically,Al2O3is amorphous when the temperature is below 550°C[28,29].The main reaction equation of thermite reaction is:

The main characteristic peaks of Cu and cuprous oxide are 43.297°and 36.418°,respectively.However,the characteristic peaks of Cu and cuprous oxide were not detected by XRD(Fig.4).The result implies that there is no observable thermite reaction in the long-term storage of the Al/CuO nanothermite.

Fig.4.XRD patterns of Al/CuO nanothermite with different storage time(a was stored for 13 months;b was stored for 7 months;c was stored for 0 months).

Fig.5.Survey spectra(c),Cu 2p spectra(e)and Al 2p spectra(d)for the Al/CuO nanothermite with different storage time.The Al/Al2O3 ratio(b)and surface nitrogen composition(a)of the Al/CuO nanothermite.(The curve a,b and c in each subgraph indicate that the nanothermite has been stored for 13 months,7 months and 0 months,respectively.)

The XPS spectra of Al/CuO nanothermite with different storage time are shown in Fig.5.Among them,Fig.5a is the surface nitrogen composition,Fig.5b is the Al/Al2O3ratio,Fig.5c is the survey spectra,Fig.5d is the Al 2p spectra,and Fig.5e is the Cu 2p spectra.The main components of Al/CuO nanothermite with different storage time have no difference in the valence state of elements,as shown in Fig.5c.This result is the same as that of the XRD patterns of Al/CuO nanothermite(Fig.4).According to the Cu 2p XPS spectra of the nanothermite(Fig.5e),the observed peaks at 933.6 eV and 953.7 eV correspond to Cu 2p3/2 and Cu 2p1/2 peaks of Cu2+,respectively,indicating the formation of CuO[30].High-resolution Al 2p spectra of the nanothermite(Fig.5d)shows that an oxide layer is on the surface of Al NPs.Furthermore,the curves of Al and Al2O3are analyzed by peak splitting,and the ratio of Al to Al2O3(Fig.5b)on the surface of Al NPs is obtained.The ratio of Al to Al2O3of Al/CuO nanothermite,stored for 13,7 and 0 months,is 0.157,0.167 and 0.191,respectively.Because the detection depth of XPS is less than 10 nm,and the detection depth is certain,the decrease of the ratio of Al to Al2O3indicates that the oxide layer of Al NPs is thicker and thicker.The average increase of the ratio of Al to Al2O3is 0.0034 when the nanothermite is stored for 0-7 months;however,the average increase of the ratio is 0.0017 when the nanothermite is stored for 7-13 months.It can be concluded that with the increase of the storage time of the nanothermite,the growth of the oxide layer thickness of the Al NPs decreases.It can be deduced that the thickness of the oxide layer will no longer be thickened after a certain storage period.When Al/CuO nanothermite is stored for 0,7 and 13 months,the content of nitrogen analyzed by XPS,is 13.15%,9.65%and 8.23%respectively(Fig.5e).With the increase of storage time,the content of nitrogen decreases.When the storage time is between 0 and 7 months,the content of nitrogen decreased by 3.50%;when the storage time is between 7 and 13 months,the content of nitrogen only decreased by 1.42%.The results show that the decomposition of NC is slow during the storage of the nanothermite,and the decomposition rate of NC decreases with the increase of storage time.

3.3.Ignition and combustion performance

Constant-volume combustion tests have been commonly employed to investigate the pressure output which is related to the energy release,And the tests results are shown in Fig.6.The pressure duration of the nanothermite at different storage time is basically the same,about 60 ms,as shown in Fig.6a.The pressurization rate is related to the rate of energy release and determined by taking the slope of pressure-time rise curve ranging from～10%of peak pressure to～90% peak pressure,as shown in the inset of Fig.6a.With the increase of storage time,the slope of the nanothermite decreases obviously,which refers to the pressure drops,as shown in Fig.6b.This result shows that the pressurization rate decreases with the increase of storage time.

As shown in Fig.6b,when the thermite is stored for 0,7 and 13 months,the peak pressure is 685.8 kPa,626.3 kPa and 625.5 kPa respectively.The difference of peak pressure between 0 months and 7 months is 59.6 kPa,while that between 7 months and 13 months is only 0.7 kPa.It can be concluded that the peak pressure of thermite decreased by 8.7% after stored for 7 months and 8.8%after stored for 13 months.It can be concluded that the peak pressure drop of thermite after storage is not more than 10%.The pressurization rate of Al/CuO nanothermite decreases with the increase of storage time.The result is that with the increase of storage time,the peak pressure decreases at first and then remains unchanged.

As shown in Fig.7,the ignition temperature of Al/CuO nanothermite with different storage time is measured by ignition temperature measuring device.Observed from the curve,the ignition temperature of the nanothermite,decreases at first and then increases with the increase of storage time.The results of XPS shows that the thickness of oxide layer increased and the content of NC decreased with the increase of storage time.The thickness of the oxide layer of Al NPs increases,which makes the thermite insensitive,and NC has the function of passivating the nanothermite[27,31].Therefore,the ignition temperature of Al/CuO nanothermite is determined by the thickness of oxide layer and NC content.

Fig.7.The ignition temperature of Al/CuO nanothermite with different storage time.

3.4.Storage stability and failure mechanism

Through the analysis of storage stability and failure mechanism of Al/CuO nanothermite,it can be concluded that the slow decomposition of NC and the thickening of oxide layer of Al NPs exist simultaneously in the storage.The reduction rate of activation energy,decomposition rate of NC,the thickening rate of the oxide layer and peak pressure decrease rate of the nanothermite decrease with the increase of storage time.The ignition temperature of the nanothermite first decreases and then increases with the increase of storage time.As NC is coated with Al NPs and CuO NPs,which hinders the reaction of Al/CuO nanothermite.At the same time,the increase of oxide thickness of Al NPs will also impede the thermite reaction.Due to the decomposition of NC,the coating layer of the nanothermite becomes thinner,but the thickness of oxide layer of Al NPs increases with the increase of storage time.Therefore,the ignition temperature of thermite decreases at first and then increases with the increase of storage time,and the pressurization rate decreases with the increase of storage time.Decomposition rate of NC and the thickening rate of the oxide layer decrease with the increase of storage time,this means that at some point,the decomposition rate of NC decreases to a very low level and the oxide layer thickness does not change.This result infers that the combustion performance of Al/CuO nanothermite is no longer reduced.

Fig.6.The representative time resolved pressure signal(a,the inner figure shows the pressurization rate.),and the peak pressure and pressurization rate(b)of Al/CuO nanothermite with different storage time.

4.Conclusion

In summary,the storage stability and combustion performance of Al/CuO nanothermite was investigated and analyzed by thermodynamic analysis,morphology,composition analysis and combustion performance test.The apparent morphology of Al/CuO nanothermite have no obvious difference,and its main components have not changed.The thermokinetic analysis shows that the activation energy of the nanothermite decreases with the increase of storage time,which means that the nanothermite are more likely to react.Further,with the increase of storage time,the content of NC decreases,and the thickness of oxide layer of Al NPs increases and finally tends to be constant.The ignition temperature of Al/CuO nanothermite first decreases and then increases with the increase of storage time,and finally tends to be constant.The peak pressure of Al/CuO nanothermite decreases with the increase of storage time and remains unchanged.The peak pressure of Al/CuO nanothermite drop after storage shall not exceed 10%.In short,the performance of Al/CuO nanothermite tends to be stable,after storage for a period of time.Therefore,Al/CuO nanothermite has a wide application prospect because of its stable performance.

Notes

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.