Lei Xio,Jie Liu,Gzi Ho,Xing Ke,Teng Chen,Hn Go,Yun-o Rong, Cheng-su Jin,Jing-lin Li,Wei Jing,*

aNational Special Superfine Particle Engineering Research Center,Nanjing University of Science and Technology,Nanjing 210094,Jiangsu,China

bResearch Institute,Gansu Yinguang Chemical Industry Group Co.,Ltd.,Baiyin 730900,Gansu,China

Preparation and study of ultra fine flake-aluminum with high reactivity

Lei Xiaoa,Jie Liua,Gazi Haoa,Xiang Kea,Teng Chena,Han Gaoa,Yuan-bo Ronga, Cheng-su Jina,Jing-lin Lib,Wei Jianga,*

aNational Special Superfine Particle Engineering Research Center,Nanjing University of Science and Technology,Nanjing 210094,Jiangsu,China

bResearch Institute,Gansu Yinguang Chemical Industry Group Co.,Ltd.,Baiyin 730900,Gansu,China

A R T I C L E I N F O

Article history:

29 March 2017

Accepted 19 May 2017

Available online 24 May 2017

Aluminum

Flake

Surface coating

Reactivity

Thermal property

To achieve aluminum particles with ultra fine granularity and high reactivity,the mechanical ball-milling method was adopted and three kinds of coatings,including stearic acid(SA),viton and dinitrotoluene (DNT),were added.The effects of milling time and different coatings on granularity and reactivity of ultra fine aluminum particles were studied.The structures of prepared ultra fine aluminum were characterized by scanning electron microscopy,X-ray particle diffraction and the thermal properties were analyzed by TG/DSC.Besides,the reactivity of prepared ultra fine aluminum particles was comprehensively analyzed and judged according to several thermodynamic parameters,the maximal oxidation rate, the oxidation degree of aluminum and the enthalpy change.The results revealed that aluminum particles prepared by the mechanical ball milling method were all flake-like and the particle sizes were below 5μm with nanometer-scale thickness.And the crystal form of aluminum was found to be unchanged. Besides,the ultra fine flake aluminum coated with stearic acid after milling for 5 h showed the highest reactivity with 56.1%of oxidation degree before 660°C,0.945 mg/°C of maximal oxidation rate and 20491 J/g of enthalpy change.

?2017 The Authors.Production and hosting by Elsevier B.V.on behalf of China Ordnance Society.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

1.Introduction

As a kind of promising metal fuel,ultra fine aluminum particles are widely applied in pyrotechnics,propellants and explosives and other energetic material fields for high calori fic value and excellent energy ef ficacy [1-3].Solid propellantscontaining ultra fine aluminum particles show higher burning rate and lower pressure exponent[4,5].Mench and Ivanov replaced normal aluminum with Alex in propellants and found that buring rates had a large increase [6-8].Gao Donglei studied several the effect of nano aluminum with different materials coated on solid propellant and the results showed that nano-aluminum decreased the pressure exponent[9]. And the detonation properties and power strength of aluminized explosives with ultra fine aluminum particles can be clearly improved[10-12].Brousseau compared the performance of nanometric and micrometric aluminum in various explosives and found nano aluminum could increase detonation velocities and heats of detonation in TNT/Al mixture explosive[11].Huang Hui added 20%50 nm aluminum in RDX based polymer bonded explosive and the detonation property and power strength were much higher than those containing 20%5μm and 50μm aluminum [12].

However,the active aluminum atoms on the surface can be easily and rapidly oxidized by O2or H2O in the air because of large speci fic surface area of ultra fine aluminum particles,which causes lower metallic aluminum content[13,14].So the study on preparation and protection of ultra fine aluminum particles with high reactivity is always an important research subject.Alexander Gromov et al.analyzed DTA-TG results of different electro-exploded aluminum nanoparticles coated with nitrocellulose(NC),oleic acid and other reagents and found that Al(Ar+H2)and Al(B)were good for solid propellants and Al(SA,stearic acid)was good for“Alwater”propellants[15].Liangui Guo et al.prepared HTPB-coated aluminum nanoparticles by laser-induction complex heating and studied the reactivity,stability and storage property of HTPB-coated nano-Al which outperformed non-coated nano-Al[16].Clayton et al.succeeded in fabricating nano-Al coated with PFPE (Per fluoropolyethers)via electrospinning and observed the reduction of decomposition temperatures[17].

It's always a complex question to study how to systematically estimate the reactivity of prepared or coated ultra fine aluminum particles and a lotof researchworkhavebeen devotedtoit[18-20], such as chemical oxidation reduction titration[21],TG analysis [22,23],and TEM analysis[24,25],etc.To fast determine the reactivity of aluminum particles,some researchers suggested the following four thermodynamic parameters,including temperature of the beginning of oxidation(Ton),maximal oxidation rate(νox), degree of transformation of aluminum in certain temperature interval(α)and relative thermal effect of aluminum per unit mass(H/ m),which were con firmed to be appropriate and convenient [10,26].

In this work,ultra fine aluminum particles with high reactivity are prepared by mechanical ball milling method and adding three different coating materials which are stearic acid(SA),viton(F)and dinitrotoluene(DNT).The reactivity of as-prepared ultra fine aluminum samples was analyzed and compared based on maximal oxidation rate(νox),the oxidation degree of aluminum(α)and the enthalpy change(ΔH)to finally determine the most bene ficial preparation process.

2.Experimental

Ultra fine aluminum particles with high reactivitywere prepared using a mechanical ball mill.In this work,ethyl acetate was used as a dispersant with 0.3 mm zirconiaas the milling medium.First,20 g of 15μm aluminum particles were weighed and transferred to the ceramic chamber,with a certain amount of ethyl acetate poured inside.The milling rate was set at 400 rpm for 2 min to guarantee the uniform dispersion of aluminum particles.Then about 4 wt%of coating material(SA,viton or DNT)was added to the chamber, keeping milling at 400 rpm for a while.Usually,the milling chamber was filled with nitrogen gas to prevent aluminum particles from the air at a high milling rate.At last,the milling rate was increased to 1100 rpm and continued for 5 h.Prepared aluminum particles samples of 1 h,3 h,5 h were collected and dried at a vacuum oven for characterization and further study.The prepared ultra fine aluminum samples coated with stearic acid,viton and DNT were named as Al-SA,Al-F and Al-DNT for convenience, respectively.

The structure and morphology of prepared aluminum samples were investigated with a field emission scanning electron microscope(FE-SEM,Hitachi S-4800Ⅱ).

The samples'x-ray particle diffraction patterns were analyzed on a Bruker D8-Advanced diffractometer in the 2θrange of 20-80°at scan rate of 0.02°/s with Cu Kα1 radiation(λ=0.15406 nm).

The thermogravimetric(TG)analysis and differential scanning calorimetric(DSC)were carried out with a SDT Q600 thermal analysis system.All the samples of about 2.0 mg were heated at 20°C/min from room temperature to 1000°C with dry air flow rate of 100 mL/min.

3.Results and discussion

The SEM images of raw aluminum particles and three kinds of prepared ultra fine aluminum particles coated with SA,viton and DNT,respectively,are shown in Fig.1.It is clearly seen from Fig.1a that the raw aluminum particles are spherical,having large granularity distribution with quite rough surface which means that the surface has been oxidized.While all the prepared aluminum particles after mechanical ball-milling have flake-like structure and very smooth surface.And the three kinds of ultra fine aluminum particles are all less than 5μm with nanometer-scale thickness.

Fig.2 shows the XRD patterns of raw aluminum particles,Al-SA, Al-Fand Al-DNT.It can be observed that XRD characteristic peaks of the three ultra fine aluminum are consistent with raw aluminum, which are almost at 38.5°,44.8°,65.2°and 78.3°,indexed to the(11 1),(2 0 0),(2 2 0)and(3 11)planes of standardized FCC aluminum, respectively.The results showed that the crystal form of aluminum particles did not change in the milling process.

Fig.1.SEM images of raw aluminum particle(a)and three kinds of ultra fine aluminum particles coated with(b)SA,(c)viton and(d)DNT.

Fig.2.XRD patterns of raw aluminum particle and three kinds of ultra fine aluminum particles coated with SA,viton and DNT.

The TG,DTG and DSC curves of Al-SA,Al-F and Al-DNT samples after milling for 1 h,3 h and 5 h are shown in Fig.3.According to Fig.3a,it can be observed that each sample has a mass loss in varying degrees before 400°C,especially prolonged milling duration,which is mainly due to the desorption of water vapor and CO2in air adsorbed on the surface of ultra fine aluminum particles. Between 400°C and 660°C,Al-SA,Al-F and Al-DNT samples of 1 h have a small weight increase while the sudden weight gains were found after milling for 3 h,which suggests that aluminum particles are activated due to the continuous grinding.DTG curves based on TG are shown in Fig.3b.The peak values of DTG curves mean the maximal oxidation rate(νox)of aluminum samples,representing the maximal reaction rate and reactivity of ultra fine aluminum.It can be seen thatνoxvalues of different samples increase with the extension of milling time andνoxof Al-DNT has the smallest value comparing to Al-SA and Al-F samples.Comparing different DSC curves of Al-SA,Al-F and Al-DNT samples of 1 h,3 h and 5 h in Fig.3c,it is found that Al-SA sample after milling for 5 h has the lowest peak temperature(611.6°C),proving the fast reaction of aluminum.Meanwhile,DSC curves of Al-F and Al-DNT samples of 1 h exist two exothermic peaks near the melt point(660°C),while this phenomenon doesn't happen to samples of 3 h and 5 h. Spherical aluminum particles can become deformed,broken and re fined to flake aluminum step by step under powerful mechanical force produced by high-speed rotating zirconia.1 h of milling time is so short that a part of raw aluminum particles just undergo deformed or brokenprocess with large particle size and surface still covered with alumina shell.So the DSC curves of Al-Fand Al-DNTof 1 h are different.And it can be also concluded that SA can help there finement of aluminum particles.

Fig.3.TG,DTG and DSC curves of three kinds of ultra fine aluminum particles coated with SA,viton and DNT.

Table 1 Thermodynamic parameters of Al-SA,Al-F and Al-DNT.

To further investigate and judge the reactivity of prepared ultra fine aluminum samples,different thermodynamic parameters, including the oxidation degree of aluminum before 660°C,the maximal oxidation rate and enthalpy change of aluminum before 660°C,are calculated and listed in Table 1.

It can be seen that for prepared ultra fine aluminum sample coated with SA,αandνoxvalues increase with milling time extended,up to the maximum at 5 h.And the exothermic peak shifts to the lower temperature,602.6°C,with the highest enthalpy change,20491 J/g.The Al-F sample of 3 h shows the bestαandΔH values compared with samples of 1 h and 5 h,while the peak temperature still decreases,but the coating effect will become worse when increasing the milling time.The thermodynamic parameters of Al-DNT in Table 1 show the similar regularity with Al-SA and the effect of milling time for 5 h is the best.Then,comparing the best samples of three kinds of prepared ultra fine aluminum,it can be found that the oxidation degree upto660°C of Al-SA sample is the largest,up to 56.1%.And the maximal oxidation rate of Al-SA sample is close to Al-F,νoxvalues of which are 0.945 mg/°C and 1.020 mg/°C,respectively,much higher than 0.422 mg/°Cof Al-DNT sample.ΔH data show that Al-SA sample of 5 h releases the most energy during combustion,far higher than 13609 J/g of Al-F sample and 8290 J/g of Al-DNT sample.In fact,when SA is added into ethyl acetate containing raw aluminum particles,it can dissolve in ethyl acetate and coat rapidly on the surface of aluminum particles, preventing re fined flake aluminum particles from clustering and growing during milling process,and protecting new-prepared dried ultra fine flake aluminum particles from being oxidized by water or oxygen in air.So it can be judged that the reactivity of prepared ultra fine flake aluminum particles coated with SA after milling for 5 h is the best.

4.Conclusions

Ultra fine flake aluminum particles were successfully produced by mechanical ball milling and adding three different coating materials,stearic acid,viton and dinitrotoluene.Characterization results revealed that the particle sizes of three aluminum particles were all below 5μm with nanoscale thickness and the crystal form had no change in the milling process.Thermal analysis showed that ultra fine flake aluminum particles coated with stearic acid had the highest oxidation degree,the biggest maximal oxidation rate and the largest enthalpy change,indicating the best reactivity.Therefore the obtained ultra fine flake aluminum particles can be considered as promising energy fuel in energetic material fields. Some of the potential applications include:(a)composite solid propellants (CSPs) by partialreplacementofcommercial aluminum,(b)compositemodi fieddoublebasepropellants (CMDB)by substantial replacement commercial aluminum,(c) Aluminum-based explosives by partial replacement of commercial aluminum,and other energetic material fields.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China(Project No51606102),the Fundamental Research Funds for the Central Universities(No.30916011315),the Qing Lan Project,the Weapon Research Support Fund(No. 62201070827),a Project funded by the Priority Academic Program DevelopmentofJiangsu HigherEducation Institutions,the Shanghai Aerospace Science and Technology Innovation Fund (SAST2015020)and Basic Product Innovation Technology Research Project of Explosives.

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31 December 2016

*Corresponding author.

E-mail address:super fine_jw@126.com(W.Jiang).

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2017.05.008

2214-9147/?2017 The Authors.Production and hosting by Elsevier B.V.on behalf of China Ordnance Society.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

in revised form