Yue-ting Wng ,Xio-ting Zhng ,Jin-ing Xu ,Yun Shen ,Cheng-i Wng ,Fu-wei Li ,Ze-hu Zhng ,Jin Chen ,Ying-hu Ye ,,Rui-qi Shen

a Department of Applied Chemistry,School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing,210094,China

b Anhui Xinhe Defense Equipment Technology Corporation Limited,Anhui,230000,China

Keywords: Nanothermite Energetic materials Sol-gel Characterization

ABSTRACT In this study,Al-CuO nanocomposites were fabricated by sol-gel method.As a contrast,the thermite was prepared by physical mixing at the equivalence ratio of 0.5,1,2,respectively.The intermediates and samples as prepared were characterized by SEMand XRD.The exothermic properties of the two samples prepared at different equivalence ratios were tested and the reaction products were characterized by XRD.The SEMresults show that the sample prepared by the sol-gel method demonstrates a micron-sized agglomerated sphere formed by a mutual wrapping of Al NPs and CuO NPs,and the particles are evenly distributed in the agglomerate.In addition,when the content of Al powder is seriously insufficient,the heat release of the sample prepared by physical mixing is 1.6 times that of by sol-gel method.With the increase of Al powder content,the exothermic properties of Al/CuO NPs prepared by sol-gel method began to increase signi ficantly compared with physical mixing and the difference is 1.5 times when the equivalence ratio increases to 2.It can be concluded that the reason for this result may be attributed to the different mass transfer modes of components due to the different morphologies of samples.

1.Introduction

Nanothermites are a class energetic material containing both fuel and oxidizer.They undergo a very rapid redox reaction and release energy in a fast manner,which lead to signi ficantly improve the kinetics properties[1-4].In most formulations,aluminum(Al)was preferred fuel because of its abundance and high calori fic value[5,6].When compared with the traditional Al thermites composed of microparticles,Al nanothermites demonstrate a higher reaction rate and higher energy density due to reducing the diffusion distance between fuel and oxidizer[7-10].Many applications have been proposed,including environmentally clean primers and detonators[11],gas generator[12,13],propulsion[14,15],nanoenergetics on a chip[16,17].

With the advent of nanoscience and nanotechnology,it has been realized that the chemical and physical properties of achieved nano-materials majorly depend on the used preparation techniques[18].To date,several methods have been developed to prepare nanothermites,with each method imparting speci fic structural characteristics to the generated nanocomposite.The most immediate approach to prepare a mixture component is physically mixing[19,20].Nanoparticles(NPs)are firstly dispersed in volatile solvents,commonly hexane,then the full mixing of fuel and oxidizer is promoted by ultrasonication and magnetically stirring.At last,nanothermites can be obtained after the solvent being dried.Physical mixing by ultrasonication has proved to be a timesaving method to obtain nanocomposites in a facile way.Besides,the sonochemical synthesis method based on ultrasound pulse also has advantages including well controlling the crystal size distribution,decreasing the induction time and avoiding excessive amounts of organic solvents to contribute to green chemistry[21-23].However,in the process of preparing nanothermites by physical mixing,the homogeneity of the target components is not well guaranteed.Sol-gel method is an effective way to improve the contact conditions of components.In this method,it could combine fuel and oxidizer and allow one to uniformly disperse solid fuels within a nanoscale oxidizer framework matrix while eliminating concentration gradients in the final material due to the sedimentation of particles[24].Speci fically,materials containing active elements are chosen as the precursor.The fuel is evenly mixed in the liquid phase and then,hydrolysis and condensation reactions are carried out to form a stable transparent system that is known as “sol” .After that,the “sol” can be further linked through the condensation to form a “gel” by adjusting parameters such as pH value and temperature,and finally form a three-dimensional network structure[25-30].Self-assembly method is another ef ficient approach to improve the homogeneity of nanoparticles which can be divided into several speci fic approaches[31,33,34].This method is trying to modify the surface of particles to make the particles combine independently under the action of the electric field or other driving forces with advantages of de finite structure and stable performance.In addition,the electrospray method has shown its advantages in controlling the morphology and particle size of nanocomposites.The electrospray process is a one-step process,in which fuel and oxidizer nanoparticles within micrometer droplets aggregate during solvent evaporation,to generate a highly porous microparticle[35-39].

During the past few years,several methods have been utilized to synthesize the CuO NPs,such as thermal oxidation of CuO in an air[40],microwave irradiation[41],solvothermal[42],sol-gel[43]and sonochemical method[44].In the process of synthesizing CuO,the biggest challenge is how to prepare CuO crystals with high purity and good crystal form without adding a large amount of stabilizers.In our present work,nano CuO with high purity and good crystal shape was prepared by sol-gel method.In addition,Al/CuO nanocomposites with different equivalent ratios were prepared by two methods:one composite was obtained by adding Al NPs into bulk xerogel CuO and calcining at high temperature,another was mixing Al NPs and CuO NPs(prepared by sol-gel method)by physical mixing.In this context,the primary purposes of this work were to investigate:(i)effects of synthesis methods on properties of Al/CuO nanocomposites,(ii)effect of the equivalence ratio on the properties of nanocomposites.

2.Experimental

2.1.Materials

Aluminum nanoparticles(Al NPs,~50 nm;Shanghai Aladdin Bio-Chem Technology Co.Ltd)were used as received.The active content of the Al was measured to be 67.4%by thermogravimetric analysis.In the synthesis of xerogel CuO,copper(II)nitrate(Cu(NO3)2·3H2O),Shanghai Xinbao Fine Chemical Plant),Ethanol,1,2-propyleneoxide(C3H6O;Shanghai Lingfeng Chemical Reagent Co.Ltd)were used as a starting material.In the preparation of Al/CuO thermite by ultrasonic mixing,n-hexane(C6H14;China Pharmaceutical Group Chemical Reagents Co.Ltd)was used as a dispersing solvent.All the reagents used in the experiments were of analytical grade.

2.2.Preparation of xerogel CuO

In general,there are two main approaches to prepare nanothermites by sol-gel method.One is sol-gel based on metal alkoxides,the other is sol-gel based on metal inorganic salts.As the high cost of the metal alkoxides and alkoxides with large metal atomic radius is extremely reactive,they are easy to hydrolyze in air.In contrast,metal inorganic salts are cheaper and easier to industrialize,so metal inorganic salts Cu(NO3)2·3H2O was chosen as a raw material in this paper.

In the preparation process,Cu(NO3)2·3H2O was dissolved in ethanol,the positive charge of Cu2+repels the H+of the water molecule and causes the H+to ionize making the solution to be acidic.Cu2+combines with the crystal water to form[Cu(H2O)n]2+.Due to the strong acidity of[Cu(H2O)n]2+,the hydrolysis of[Cu(H2O)n]2+occurs step by step in the solution,resulting in the formation of “hydrated metal oxides” and H+.Meanwhile,1,2-propylene oxide contains oxygen with high electronegativity,which absorbs H+to increase the pH value of the system and promote the hydrolysis of[Cu(H2O)n]2+.

In particular,9.69 g of Cu(NO3)2·3H2O(7.8 mmol)was firstly dissolved in 126 ml of ethanol at the ambient condition with constant stirring and a blue solution was obtained.For properly hydrolysis of the Cu(II)solution,11 ml of 1,2-propyleneoxide was slowly dropped into the solution when the solution was stirred by ultrasonic.Cu(II)sol was further formed by aging the solution at room temperature for 5 days.Gels were obtained with further aging and drying at 60°C under vacuum for 24-48 h.The gels were then placed in a box-type resistance furnace,calcined at 300°C for 2 h,and CuO nanoparticles can be finally obtained by grinding.

2.3.Preparation of Al-CuO nanocomposites

Stoichiometric Al-CuO(2:3 per mole basis)was prepared in two different ways.To further illustrate the difference of sample performance caused by different preparation methods,the speci fic synthesis process is shown in Fig.1.

As shown in Fig.1,Al-CuO nanoparticles were synthesized by two methods:physical mixing and sol-gel route.The similarity between the two methods is that the copper oxide in the component is also prepared by sol-gel route while the difference lies in the different ways in which Al NPs are added into the mixtures.Speci fically,the former is based on the preparation of CuO particles by sol-gel route,Al NPs were gently added while the mixture was mixed in n-hexane by ultrasonication.Afterwards,the mixture was placed at 60°C for 2.5 h to remove the hexane.The latter route is adding Al NPs before the calcination of the xerogel,then the components were calcined in a muf fle furnace,and the Al-CuO composite was obtained after cooling down.

Fig.1.The Schematic illustration of the synthesis of Al/CuO.

2.4.Measurements and characterizations

The crystalline structure was determined by X-ray diffraction(XRD,D8 Advance,Cu K radiation,Bruker Company,Germany).Particle size was estimated using Scherrer’s equation,size=0.89λ/(Bcosθ),whereλis the wavelength,Bis the full width at half maximum(FWHM)of the peak,θis the Bragg angle.The morphology of the intermediate and products was examined by field emission scanning electron microscope(FE-SEM,Ultra plus,Zeiss Company of Germany)and transmission electron microscope(TEM,Philips Tecnai 12).The chemical composition of samples was measured by X-ray energy-dispersive spectroscopy (EDS)elemental mapping (Oxford Instruments X-max 50).The exothermic reaction properties of the samples were placed in an alumina crucible and the analysis was conducted under flowing argon(30 ml/min)at the heating rate of 10°C/min.

3.Results and discussion

3.1.Characterization of the synthesized copper(II)oxide

Characterization of as-prepared CuO NPs was made by SEM-EDS and XRDanalysis,while Figs.2,3 show the XRDresult and the SEMEDS,TEM results,respectively.As is shown in Fig.2,two main observed reflection peaks(2θ=35.6°,2θ=38.8°)are ascribed to the(002)and(111)reflections in the CuO monoclinic crystal phases,according to the JCPDS card N°:048-1548.The results illustrate that the product is pure CuO with good crystal phases and calculation based on Scherrer’s equation leads to a mean dimension value of 28.17 nm.

Fig.2.XRD result of CuO NPs.

The morphology and composition of CuO NPs were investigated byways of SEM-EDS and TEM.The typical SEMimages of CuONPs at different resolutions are shown in Fig.3(a),(b),(c),(d),respectively.As can be seen from the SEM images,CuO NPs agglomerate into microspheres with a smooth surface and uniform distribution of particles among aggregates.From the EDS result as shown in Fig.3(e),the component atoms are ascertained to be Cu and O elements from the EDS spectrum.The relative atomic ratio of Cu:O is determined to be 1.04:1.0(calculated from 51.1:48.9),which is comparable to the stoichiometric value of 1:1 for cupric oxide.To further analyze the morphology of CuO NPs,the TEM results are shown in Fig.3(f)and(g).The results illustrate that the agglomeration would happen which may lead to a decrease in the performance of CuO NPs.

Fig.3.The morphology of CuO NPs.

3.2.Characterization of Al/CuO nanocomposites

Al/CuO nanocomposites were prepared by two methods of physical mixing and sol-gel.The method of physical mixing is mixing the CuO NPs(obtained by calcining Cu(II)xerogels)with Al NPs in n-hexane.Sol-gel method is to add Al NPs before calcination,and then Al/CuO nanocomposites were prepared by calcining at 300°C for 2 h.The morphology of CuO precursor is shown in Fig.4(a)and 4(b).It can be seen that the surface of CuO precursor is smooth and the size of the spheres is about 6μm.Besides that,the surface of the microspheres has visible voids,which probably can provide more attachment sites for Al NPs.

In order to have an in-depth understanding of the differences of the combination behavior between components due to different sample preparation methods,the morphology of the mixture precursor and the calcined product was compared.Speci fically,we can learn from Fig.4(c)that,owing to the unique structure of Cu(II)xerogels,it can indeed provide more attachment sites for Al NPs.The analysis considers that it is because of this kind of combination behavior,the sample prepared by sol-gel method can have a closer contact between fuel and oxidizer than physical mixing and can reduce large-scale agglomeration.Meanwhile,the morphology of Al/CuO component is shown in Fig.4(d),the structure shows that Al/CuO nanocomposites exist as a micron-scale aggregate with a size of about 25μm.Fig.4(e)shows the detailed image of the aggregates,it can be seen that the Al NPs and CuO NPs are uniformly dispersed in the aggregate.The SEM results show that the sample prepared by sol-gel method is a micron-sized agglomerated sphere formed by a mutual wrapping of Al NPs and CuO NPs,and the particles are evenly distributed in the agglomerate.In addition,the XRD result of the sample prepared by sol-gel method shows in Fig.4(f),in which characteristic diffraction peaks of Al(2θ=38.472°,2θ=44.783°,2θ=65.133°,2θ=78.277°,JCPDS card N°:04-0787)and CuO(2θ=32.508°,2θ=35.543°,2θ=53.366°,JCPDS card N°:048-1548)can be obtained.From the XRD spectrum,the component can be identi fied as pure Al NPs and CuO NPs,while there is no unknown crystalline phases or impurities peak present in the spectrum.Meanwhile,the calculation result based on Scherrer’s equation leads to a mean dimension value of 46.84 nm for Al NPs and 30.47 nm for CuO NPs,respectively.The result also shows that at this calcination temperature,Al NPs did not participate in the reaction,which is consistent with the result as shown in Fig.S1.

Fig.4.The morphology and XRD results of the intermediate and Al/CuO NPs.

3.3.Thermal analysis of Al/CuO nanocomposites

Generally speaking,thermite reaction is a complex process involving multiple chemical reactions at solid/solid,solid/liquid,liquid/gas and inside the gas phase[45].The reaction rate of nanothermite is affected by many factors,such as the properties of oxidizer,particle size,the speci fic surface area of particles,crystal structure and contact state between particles[46].Researches show that the ratio of fuel to oxidizer has a great in fluence on the chemical reaction and combustion performance of thermite system.For the convenience of analysis,Pantoya et al.proposed that the ratio of fuel to oxidizer be expressed by equivalent ratioФ[2,47]:

The ACT and ST signify the actual ratio and stoichiometric ratio,respectively.In this work,in order to study the effect of fuels and oxidizer ratios on the performance of samples prepared by the two different methods,the Al NPs and CuONPs were mixed when theФ was fixed at 0.5,1,2,respectively.Through the preparation methods described in the 2.4 section,Al/CuO nanocomposites with different equivalence ratios were prepared under two different preparation methods.To characterize the exothermic reaction properties and the reaction progress of the nanocomposites,DSC measurements were performed.In addition,the DSC products were characterized by XRD.

DSC analysis was performed at a heating rate of 10°C/min and the experiments were conducted under argon flow with the temperature ranging from 30°C to 1000°C.The DSC curve and the XRD results are shown in Fig.5.The total heat release value of the Al/CuO NPs was quanti fied based on the DSC measurements,which was concluded in Fig.6.

It can be found in Fig.5(a),when theФis fixed at 0.5,there is only one exothermal peak of the two preparation methods(physical mixing and sol-gel)with the peak temperatures of 559.1°C and 558.9°C(below the melting point of Al NPs),respectively.This is corresponding to the solid-solid reaction between Al and CuO NPs.At the same time,there is no exothermal peak of solid-liquid reaction between Al and CuO NPs which is probably attributable to the lack of Al NPs.In addition,it can also be found in Fig.5(a)that there is an endothermic peak between 700°C and 900°C,which is considered to be an endothermic decomposition peak of CuO.From the heat release point of view,it can be quanti fied from the DSC curve as shown in Fig.6,that the total heat release of the Al/CuO NPs prepared by physical mixing and sol-gel methods are 381.4 J/g and 236.6 J/g,whenФfixed at 0.5 respectively.It is analyzed that the Al/CuO nanocomposites prepared by sol-gel method are composed of micron-sized aggregates,and the aggregates have unique structures different fromphysical mixing.As the insufficient of Al NPs,the structure of the aggregates can restrict the energy transfer,resulting in a smaller heat release between Al NPs and CuO NPs prepared by sol-gel method.From the XRD results of the products as shown in Fig.5(b),we can see that due to the lack of Al content,Al2O3,Cu and partially Cu2O are produced in the products.

When theФis fixed at 1,as shown in Fig.5(c),there is not only a solid-solid reaction peak appears between Al NPs and CuO NPs before the melting point of aluminum,but also a solid-liquid reaction peak appears after the melting point.Speci fically,the solidsolid reaction peak occurs between 550°C-570°C,while the solidliquid reaction peak occurs between 700°C-900°C.From the heat release point of view,we can quantify from the curve that the total heat release of the samples prepared by physical mixing and sol-gel method are 676 J/g and 863 J/g,respectively.From the results,the heat release value of sol-gel method is higher than that of physical mixing,which indicates that the different content of Al NPs in the reaction system has different in fluence mechanisms on the properties of samples prepared by two kinds of preparation methods.In addition,from the XRDresults as shown is Fig.5(d),we can also see that Al2O3,Cu and Cu2O exist in the products.The existance of Cu2O is due to the existence of the oxide layer on the surface of aluminum particles,which leads to the insuf ficiency of Al NPs content under the stoichiometric ratio.

Fig.5.DSC curves and XRD patterns of the DSC products prepared by two different methods:(a),(b):Ф=0.5;(c),(d):Ф=1;(e),(f):Ф=2.

We can learn from the DSC curve as shown in Fig.5(e)that,when the value ofФincrease to 2,there is only one exothermal peak of solid-solid reaction between Al NPs and CuO NPs prepared by physical mixing.As to the Al/CuO NPs prepared by sol-gel method,besides the exothermic peak of solid-solid reaction before the melting point of aluminum,a solid-liquid reaction exothermic peak appears after the melting point.In addition,the total heat release value of the sample prepared by these two methods are 820.1 J/g and 1269 J/g,respectively.Beyond that,Fig.5(f)also shows the XRD characterization of the DSC products.It can be seen that the DSC products of Al/CuO NPs prepared by the two methods are both composed of Cu and Al2O3which is suggested that all the CuO NPs have reacted when the value ofФis set at 2.Characteristic diffraction peaks appeared at 2θ=43.297°,2θ=50.433°,2θ=74.130°,which corresponds to(113),(200)and(220)crystal planes of the face-centered structure of Cu respectively,and are consistent with JCPDS card N°:04-0836.

From the heat release point of view,we can get the changing trend of the total heat release of two sample preparation methods under different equivalent ratios as shown in Fig.6.It is obvious that the exothermic properties can be seriously affected when Al NPs is insufficient(Ф<1).In addition,when the content of Al powder is seriously insufficient(Ф=0.5),the heat release performance of the sample prepared by physical mixing is about 1.6 times of that by sol-gel method.With the increase of Al powder content,the exothermic properties of Al/CuO NPs prepared by sol-gel method began to increase signi ficantly compared with physical mixing.When the equivalence ratio increases to 2,the heat of Al/CuO prepared by sol-gel is 1.5 times of that by the physical mixing method.As an analysis,the Al/CuO NPs prepared by sol-gel method is an agglomerated sphere formed by the mutual wrapping of Al and CuONPs while the Al/CuO NPs prepared by the physical mixing method are simply mixed.It is probably because of the difference of the components morphology that leads to the difference in the heat release with the change of equivalent ratios as described above.The unique structure of the sample prepared by sol-gel method can enhance the energy transfer ef ficiency between components when the content of Al NPs is sufficient.Therefore,it can be concluded that when the fuel is excessive,the reaction performance of Al/CuO NPs prepared by sol-gel method is superior to that prepared by physical mixing.

Fig.6.The total energy release of Al/CuO nanocomposites quanti fied from DSC experiments.

4.Conclusions

Al/CuO nanocomposites were successfully prepared by physical mixing and sol-gel synthetic approach.Characterization shows that the Al/CuOnanocomposites prepared by physical mixing are a simple combination of the components,while the sample prepared by solgel method is a micron-sized agglomerated sphere formed by the mutual wrapping of Al and CuONPs.In addition,when the content of Al powder is seriously insufficient(Ф=0.5),the heat release performance of the sample prepared by physical mixing is about 1.6 times higher than that by sol-gel method.With the increase of Al powder content,the exothermic properties of Al/CuO NPs prepared by sol-gel method begin to increase signi ficantly compared with physical mixing.When the equivalence ratio increases to 2,the heat of Al/CuOprepared by sol-gel is 1.5 times higher than that by physical mixing method.The analysis shows that the reason for this result is attributed to the different mass transfer modes of components due to the different morphology of samples.

Declaration of competing interest

The authors declared that they have no con flicts of interest to this work.

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2020.06.029.