Ji-xing Song ,To Guo ,Mio Yo ,Ji-lin Chen ,Wen Ding ,Feng-li Bei ,Xio-nn Zhng ,Qin Yin ,Jun-yi Hung ,Chng-ho Li

a College of Field Engineering,PLA Army Engineering University,Nanjing,210001,China

b School of Chemical Engineering,Nanjing University of Science and Technology,Nanjing,210094,China

c Tellhow Sci-Tech CO.,LTD.,Beijing,100032,China

Keywords: Nanothermite Fluoropolymers Combustion Thermal analysis

ABSTRACT Fluoropolymers get increasing attention in energetic materials application due to the high fluorine content.To explore the effect of poly(vinylidene fluoride-co-hexa fluoropropylene)(P(VDF-HFP))on Al/MnO2 nanothermite,the samples with different contents are prepared and characterized by SEM,TGDSC,XRD,and their ignition and combustion behavior are tested and recorded.The results show that P(VDF-HFP)as an energetic binder can combine the nanothermite components together,even exist in the gaps.The integrity of energetic materials has been improved.Thermal analysis shows that the addition of P(VDF-HFP)greatly changes the thermal reaction processes,and the exothermic peaks appear early,but the utilization of fuel and oxidizer is not ef ficient from the XRD results.Furthermore,the appropriate addition of P(VDF-HFP)can directly reduce the ignition energy threshold and increase the combustion time,which is necessary for the potential ignition charge application.The possible reasons for above phenomena are discussed and analyzed.This research provides a reference for improvement of thermitebased ignition charge formulation.

1.Introduction

Usually,the thermite mixtures fabricated by a fuel and an oxidizer at nanoscale are called nanothermites,as a class of nanometer scaled energetic materials,which can be used in propellants,explosives[1-4].Among the fuels,Al nanoparticles(Al-NPs)is one of the most widely used fuels due to its high reactivity,abundant resources[5,6].The different selections of oxidizers(Cr2O3,CuO,Fe2O3,MnO2,etc.)will lead to the different formulation which show the various thermal properties and combustion performance[4-8].Based on previous reports[9],the formula of Al and MnO2has both high burning rate,adiabatic temperature and relatively low ignition point.

Normally,alumina layer covered on the surface of the Al-NPs has directly negative in fluence on the whole reactivity,but it is hard to avoid the formation of alumina layer without some special process which might greatly increase costs[10].In order to deal with the alumina layer,recently,researchers try to put some fluoropolymers into the Al-NPs.Due to the strongest oxidizability of fluorine,the alumina layer can be consumed with the rise of temperature[11,12].So,the fluoropolymer is a potential energetic additive and binder for Al-based nanothermite formulation in the future.

Among those fluoropolymers,the most famous one is poly(-tetra fluoroethene)(PTFE)because of its highest fluoride content[13].PTFE/Al/CuO mixture are prepared as cylindrical energetic specimen,and with the addition of PTFE,combustion processes show intense and large fire[14].Besides,a comparative study on three different PTFE-containing thermites are reported,PTFE/Al/Fe2O3,PTFE/Al/MnO2and PTFE/Al/MoO3cylindrical energetic specimen[15].Among them,the PTFE/Al/MnO2exhibits the highest compressive strength and toughness.Zhang[16]has studied on thermal reaction process of PTFE/Al/MnO2formulation,but all of the raw materials are at micro-scale,and the combustion process are not recorded.In fact,it is dif ficult to obtain the uniform PTFEbased thermite at nanoscale since no solvent can effectively dissolve PTFE,which signi ficantly hinders its application in nanoscale[17,18].

Therefore,some other fluoropolymers with good solubility and certain fluoride content are concerned as potential candidates,for example,poly(vinylidene fluoride)(PVDF),poly(vinylidene fluoride-co- hexa fluoropropylene) (P(VDF-HFP)) and poly(tetra fluoroethylene-ter-hexa fluoropropylene-ter-vinylidene fluoride)(THV)[19,20].PVDF contains 59.4%fluorine;and P(VDFHFP)has 66.5%fluorine;and THV has 73.0%fluorine.Above fluoropolymers can be dissolved by polar solvents,such as dimethylformamide(DMF)and acetone[21,22].Until now,some researchers have reported the studies on PVDF/Al,P(VDF-HFP)/Al,THV/Al composites and PVDF-based thermite[23-25].Wang[19]has reported a comparison study on PVDF/Al.P(VDF-HFP)/Al and THV/Al composites,and the mechanical properties as well as the ignition and combustion performance have been compared.Huang and his co-authors have focused on the thermal behavior of PVDF/nano-Al by using thermal analysis[23].The fiber reinforced films composed of Al/CuO/PVDF nanothermites were prepared,and their thermal reaction processes,reactive and mechanical properties were also studied[24,25].As for P(VDF-HFP),it has been already used into lithium-ion battery,piezoelectric materials[26,27].However,to the best of our knowledge,P(VDF-HFP)has relatively higher fluorine content compared with PVDF but few researches focus on the P(VDF-HFP)-based thermite.

In this work,P(VDF-HFP)is selected because of its higher fluorine contain and good solubility as energetic additive and binder.The aim of this work is to understand the thermal and combustion behavior of Al/MnO2/P(VDF-HFP)nanothermite.Different mass fractions of P(VDF-HFP)are added into the nanothermite system to find out the rule of effect.The field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and thermogravimetric-differential scanning calorimetry(TG-DSC)are employed to characterized the samples.In the end,the samples ignited by fast heating wire tests,and the combustion processes were recorded by high-speed camera.This work provides a strategy and perspective for potential application of P(VDF-HFP)in the field of energetic materials.

2.Experimental section

2.1.Materials

In our experiments,all chemicals can be used directly without any further puri fication.Al nanoparticles(Al NPs)(~100 nm diameter)are purchased from Shanghai Nai-ou Nano Technology Co.,LTD.KMnO4and HCl are supplied by Shanghai Lingfeng Chemical Reagent Co.,LTD for further MnO2synthesis.MnO2nanorods(MnO2NRs)are synthesized via hydrothermal method as before[8,28].Poly(vinylidene fluoride-co-hexa fluoropropylene)(P(VDF-HFP)copolymer)is selected from Shanghai 3F New Chemical Materials Co.,LTD,with about 66.5%fluorine content.As for solvents and dispersant,ethyl alcohol,deionized water and dimethylformamide(DMF)are purchased from Nanjing Chemical Reagent Co.,LTD.

2.2.Sample preparation by electrospray

After considering the about 64.5%active content of Al in nanoparticles[12],the ratio of each component is listed in Table 1.In a typical preparation experiment,the mixture of Al NPs and MnO2NRs is dispersed in ethyl alcohol by using ultrasonic method.In the meanwhile,the corresponding P(VDF-HFP)component is dissolved by DMF.Then,the P(VDF-HFP)-DMF solution is poured into the mixture of Al NPs and MnO2NRs as the precursor.Next,the precursor is put into the syringe with a 0.43 mm flat nozzle.The distance between the end of nozzle and the receiving plate is about 10 cm,and there is a 14 kV DC voltage loaded onto the distance.The precursor is ejected by a syringe pump with the velocity of 4.0 mL/h.During the process,the solvent volatilizes and solid-state sample attaches to the receiving plate.Finally,the sample is gently scraped off from the receiving plate for further tests.

Table 1 The detailed ratio of each component.

2.3.Characteristics analysis

The structures and morphologies of samples prepared from electrospray method are observed by using field emission scanning electron microscopy(FE-SEM)(HITACHI High Technologies Corporation,S-4800II Japan).The secondary electron images are captured at 15 kV accelerating voltage.As for thermal analysis,thermogravimetric-differential scanning calorimetry (TG-DSC)(Mettle-Toledo 1600LF-PTM90611234 USA)is introduced to study the thermal properties of samples.The heating rate is 15°C/min from room temperature to 800°C at argon atmosphere.The mass of sample in an 80μL corundum crucible is about 3 mg.Finally,the Xray diffraction(XRD)(Bruker,D8 Advance,Germany)analysis is used to figure out the phase in residues after thermal analysis.

2.4.Ignition and combustion test

To verify and record the characteristics of samples combustion,the fast heating wire ignition combustion systemwas designed and made,as shown in Fig.1.The diameter of heating wire was about 0.1 mm.The DC was employed as the power supply,and the current on the heating wire increases gradually.In each combustion test,the mass weight of sample is about 20 mg.The whole ignition and combustion processes are recorded by the high-speed camera(FASTCAMSA-Z Japan)with shutter speed at 50μs per picture once the sample ignites.In order to protect the camera from the burning sparks,a transparent tempered glass is placed between the sample and camera.

Fig.1.The diagram of ignition and combustion test.

3.Results and discussion

3.1.Morphology and structure analysis

Fig.2 shows the SEM images of Al/MnO2and Al/MnO2/P(VDFHFP)nanothermites.The morphology of Al NPs is the spherical particles while that of MnO2NRs is the rods.The fuel contacts the oxidizer directly with good dispersion from Fig.2(a).Fig.2(b),(c)and(d)are the nanothermites samples with different mass fractions of P(VDF-HFP),from 10 wt%to 30 wt%.As a kind of energetic binder,P(VDF-HFP)seems to stick the fuel and oxidizer together.When the content of P(VDF-HFP)rises,from Fig.2(d),the bond range seems to enlarge,even filling in some gaps just like a spider web.The introduced energetic binder P(VDF-HFP)can change the con figuration of samples,fromloose nanothermite mixture powder to energetic fluoro-elastomer nanocomposite based on SEM images.

3.2.Thermal analysis

The thermal property is one of important characteristics for energetic materials.Fig.3 shows the results of thermal analysis for nanothermite samples.The black curves mean the mass change with the rise of temperature while the red curves represent the exothermic or endothermic DSC signal.Fig.3(a)shows the TG-DSC results of Al/MnO2nanothermite without any P(VDF-HFP)addition.Below 350°C,no clear exothermic or endothermic DSC signal can be found but the total mass keeps going down from the TG curve,which is caused by the evaporation of residual solvents,such as water,ethyl alcohol,especially MnO2NRs component obtained by hydrothermal method [29].Noticeably,there is merely one exothermic DSC signal in Fig.3(a),and the thermite reaction between Al NPs and MnO2NRs with great heat release accounts for it.The peak temperature point is at 554°C and the total heat release is about 1073.1 J/g.

Fig.2.SEM images of nanothermite samples with different mass fraction of energetic binder(a)Al/MnO2 nanothermite without energetic binder,(b)Al/MnO2/10 wt%-P(VDF-HFP)nanothermite,(c)Al/MnO2/20 wt%-P(VDF-HFP)nanothermite,(d)Al/MnO2/30 wt%-P(VDF-HFP)nanothermite.

From Fig.3(b)-(d),the P(VDF-HFP)energetic binder can great affect the thermal processes.First of all,at the range of 220-290°C,the small endothermic peaks appear in those three DSC curves indicating the melting process of P(VDF-HFP)energetic binder[30].

As for Al/MnO2/10 wt%-P(VDF-HFP)nanothermite,as the temperature increases gradually,there are three exothermic peaks appear in sequence.Based on the TG curve in Fig.3(a),below the 300°C,the main reason for mass loss is the evaporation of solvent.Then,from 300°C to 500°C,the total mass of system decreases by 8.9%continuously with two clear exothermic DSC peaks(Peak 1 and 2).To be more precisely,the decrease trend of TG curve can be divided into two periods,a rapid mass loss periods from 300°C to 420°C and a slow mass loss periods from 420°C to 500°C,which corresponds to the exothermic DSC peak 1 and peak 2,respectively.Namely,the thermal decomposition of P(VDF-HFP)accounts for above two peaks.Besides,if all of P(VDF-HFP)decomposes,the actual mass loss should be near 10%rather than only 8.9%,which means a small part of P(VDF-HFP)reacts with nanothermite components,Al NPs,MnO2NRs as well as the alumina shell on the surface of Al NPs[23].Thus,when 10 wt%-P(VDF-HFP)is added into nanothermite system,most of energetic binder transfers into gaseous products because of thermal decomposition while a small section can react with Al NPs and MnO2NRs,and the phenomenon is corresponding to the previous report[23,25].Next,the exothermic DSC peak 3 presents the thermite reaction between Al NPs and MnO2NRs,similar to the exothermic peak in Fig.3(a).By contrast,the peak temperature point(Peak 3)is delayed by 27°C since some sections of thermite components react with P(VDF-HFP)before.Besides,at about 660°C,a weak endothermic DSC peak appears which is caused by the melting of remaining Al NPs.But the results of Al/MnO2nanothermite without P(VDF-HFP)addition,there is no any remaining Al NPs,so the introduced energetic binder might interrupt the original balance between Al NPs and MnO2NRs.

Furthermore,the mass fraction of P(VDF-HFP)increases to 20 wt%,as shown in Fig.3(c).Below the 484°C,the thermal processes are almost same to that of Al/MnO2/10 wt%-P(VDF-HFP)nanothermite,including evaporation of solvent,P(VDF-HFP)melt,two periods mass loss with two corresponding exothermic DSC peaks(Peak 1 and 2).But from the TG curve,the total mass loss from room temperature to 500°C is about 10.3%,including the evaporation of solvent.Namely,more than half of P(VDF-HFP)takes part into the react with Al NPs,MnO2NRs and alumina shell[23],which can account for the exothermic DSC peak 3.Combined with the previous report[11,25],DSC peak 3 can be explained by the Al NPs and/or MnO2react with P(VDF-HFP)and its products at high temperature.In sequence,the small exothermic DSC peak 4 should be the thermite reaction with residual Al NPs and MnO2NRs,or the products of MnO2thermal decomposition.From the area of exothermic DSC peak 3 and 4,it can be estimated that most of fuel and oxidizer involve into the reaction with P(VDF-HFP)in DSC peak 1,2 and 3.

Finally,the mass fraction of P(VDF-HFP)increases to 30 wt%,and the TG-DSC results are shown in Fig.3(d).Familiar with above results below 485°C,all of the phenomena appear in sequence,but the area of DSC peak 1 and 2 become much larger due to the more P(VDF-HFP)addition,especially DSC peak 1.Within the solvent evaporation,the total mass is reduced by 13.7%before the 500°C.Both the amount of mass loss and the area of DSC peak 1 and 2 re flect that more P(VDF-HFP)join the reaction with nanothermite components,alumina shell,Al NPs and MnO2NRs,similarly[23].As for DSC peak 3,it is caused by the thermal reaction among Al NPs,MnO2NRs,P(VDF-HFP)and its products at high temperature stage.Interestingly,no clear exothermic peak appears after DSC peak 3,which means all of Al NPs and MnO2NRs are consumed up in the DSC peak 1,2 and 3 stages.

Besides,over 750°C,the TGcurves in Fig.3(b),(c)and(d)show a reduction trend.Based on previous report[16],it is estimated by sublimation of produced AlF3.Therefore,in summary,the introduction of P(VDF-HFP)with different contents can greatly affect the thermal reaction processes.P(VDF-HFP)can directly react with nanothermite components,and when the P(VDF-HFP)is added,at least three exothermic reactions appear with the rise of temperature.

Fig.3.TG-DSC curves of each nanothermite sample,(a)Al/MnO2 nanothermite without energetic binder,(b)Al/MnO2/10 wt%-P(VDF-HFP)nanothermite,(c)Al/MnO2/20 wt%-P(VDF-HFP)nanothermite,(d)Al/MnO2/30 wt%-P(VDF-HFP)nanothermite.

3.3.XRD and residues analysis

In order to figure out the reaction products phases with different mass fraction of P(VDF-HFP),after thermal analysis tests,the residues in corundum crucible are collected and further tested by XRD,respectively,as shown in Fig.4.Without P(VDF-HFP)addition,the reaction products from thermite reaction between Al NPs and MnO2NRs are mainly MnO,Mn3O4,Al2O3and Aluminum manganese(AlxMny).When the 10 wt%-P(VDF-HFP)is introduced into original nanothermite system,the phases of reaction products change a lot.Noticeably,there are a few Al NPs and MnO2NRs remaining from the XRD results of AMP-1,which is corresponding to the TG-DSC results.Besides,certain content of MnF2and AlF3can be found which are consistent to the previous report[15,16].With the increase mass fraction of P(VDF-HFP),the characteristic peaks of remaining Al NPs and MnO2NRs still can be found,and the phases of Manganese aluminum oxide(MnAl2O4)shows up.However,there is no any clear characteristic peak for MnO or Mn3O4.Therefore,it can be estimated that the introduction of P(VDF-HFP)have certain negative effect on the complete reaction between fuel and oxidizer,especially high content of P(VDF-HFP).In the end,there are certain content nanothermite components unreacted.Because P(VDF-HFP)exists in the gap among the components,which hinder the direct contact and reaction between Al NPs and MnO2NRs.Besides,after thermal decomposition of P(VDFHFP),some solid residues might completely cover the components,such as carbon materials.Namely,the P(VDF-HFP)polymers matrix and/or its reaction products can hinder the thermite reaction progress.

Fig.4.XRD results of residues in corundum crucible.

3.4.Ignition and combustion

Ignition and combustion are the direct and important characteristics for energetic materials,especially further practical application.In this paper,the fast heating wire test is employed to carry out the ignition and combustion tests.The current value can be read from DC power supplier,and recorded as a comparison of the ignition energy threshold.At the same time,the combustion process and flame growth are caught by high-speed camera,as shown in Fig.5.

In Fig.5(a),when the current reaches 1.207 A,the Al/MnO2nanothermite sample ignites successfully with a clear noise,and then the bright flame grows rapidly and gets to the maximum at about 400μs.Bright firelight filled the whole pictures.After that,the flame accordingly decreases.At about 2 ms,some splashes of sparks can be seen.At 5 ms,the flame fades away gradually.The speed of flame growth of Al/MnO2nanothermite sample is so fast and intense with a short combustion whole time.

As for the samples with different mass fraction of P(VDF-HFP),from 10 wt%to 30 wt%,in Fig.5(b),(c)and(d),the currents at the ignition moment are 1.034 A,0.996 A and 1.081 A,respectively.The introduction of P(VDF-HFP)can reduce the ignition energy threshold at first,but if the P(VDF-HFP)content is high,at 30 wt%,the binder will have a negative effect in reverse.When P(VDF-HFP)is added into nanothermite system,the whole combustion time increases a lot.Among three,the flames grow to the maximum at about 3 ms,which are much slower than that of Al/MnO2nanothermite.So,slow flame growth directly leads to long combustion time.At 10 ms,the flames of three samples still clear and obvious.Increasing the combustion time contributes to the reliable transfer of energy.Namely,appropriately reduced burning rate is necessary for potential ignition charge application.Besides,the increasing number of F atoms further improves the hydrophobicity and moisture resistance[22].

Combined with the previous SEM,TG-DSC and XRD analysis,P(VDF-HFP)energetic binder plays important roles in the changes of ignition and combustion process.As for the reduction of ignition energy threshold,from TG-DSC results,there are some exothermic peaks appeared at relatively low temperature points,since the P(VDF-HFP)thermally decomposes and reacts with alumina shell,Al NPs and MnO2NRs,leading to the temperature rise at many parts[31].At the same time,P(VDF-HFP)energetic binder with high fluorine content can release fluoride gas products via decomposition which can help the reactions happen at relatively low energy[11].As for the phenomena of low speed of flame growth,the addition of P(VDF-HFP)energetic binder can directly hinder the contact between Al NPs and MnO2NRs from SEMimages.Namely,if there is no binder,the original reaction of Al/MnO2nanothermite can happen directly between the fuel and oxidizer,and the loose structure naturally helps the spread of flame[9].That is why the speed of flame growth of Al/MnO2nanothermite is so fast.By the way,in our case,the mass fraction of polymer content is from 10 wt%to 30 wt%,which is high enough for nanothermite.For future work,it is necessary to figure out the thermal behavior and combustion performance of samples at relatively low polymer content,no more than 10 wt%.But with the addition of P(VDF-HFP)energetic binder,the structure changes a lot,from loose structure to a kind of nanocomposite,and the additive also hinder the contact between fuel and oxidizer.The spread of flame has to be gradual from side to side,as shown in Fig.5(b).Meanwhile,some solid products from exothermic reactions at low temperature might producing the agglomeration,which will have signi ficantly negative effect for further reaction.Some large agglomerations ignite after pretty long delays and/or never ignite at all[32],which can also explain some remaining Al NPs and MnO2NRs phase from XRD results.On the other hand,if the P(VDF-HFP)is selected for application in nanothermite materials,it is necessary to redesign the formulation for the fuel and oxidizer to improve the utilization rate.Those remaining agglomerations also reduce the total speed of flame growth.From Fig.5(b)-(d),certain content of P(VDF-HFP)energetic binder help the reduction of ignition energy threshold,but too many mass fractions will show the negative effect for the reactivity,especially for the size of flame and speed of flame growth.

4.Conclusion

In this paper,the investigation of P(VDF-HFP)additive on Al/MnO2nanothermite system has presented.The samples are prepared via electrospray method,and characterized by SEM,TG-DSC and XRD.Then,the ignition performance and combustion behavior are also studied by fast heating wire tests and recorded by highspeed camera.The following conclusions can be drawn from the study:

Fig.5.Ignition and combustion processes recorded by high-speed camera,(a)Al/MnO2 nanothermite without energetic binder,(b)Al/MnO2/10 wt%-P(VDF-HFP)nanothermite,(c)Al/MnO2/20 wt%-P(VDF-HFP)nanothermite,(d)Al/MnO2/30 wt%-P(VDF-HFP)nanothermite.

(1)The different contents of P(VDF-HFP)will lead to different thermal reaction processes,especially from 0 wt%to 20 wt%.The single exothermic reaction of Al/MnO2nanothermite happens at the range of 500°C-590°C without P(VDF-HFP)addition,while there two obvious exothermic peaks appear before the main thermite reaction.But there are more exothermic reactions show up due to the more addition of P(VDF-HFP),which are caused by thermal decomposition of P(VDF-HFP),reaction with P(VDF-HFP)and nanothermite components,which result in the great decrease of the thermite reaction from the area of its exothermic peak.

(2)The P(VDF-HFP)binder will lead to the inef ficient utilization of fuel and oxidizer because it prevents the direct contact of nanothermite components.It is the negative in fluence for introduction of P(VDF-HFP).

(3)Appropriate content of P(VDF-HFP)has a positive effect on reduction of ignition energy threshold and speed of flame growth.Appropriately reduced speed of flame growth and corresponding increase of combustion total time is necessary for potential ignition charge application.

Declaration of competing interest

The authors declare that there is no con flict of interest regarding the publication of this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation,project no.51704302,and was also supported by China Scholarship Council,no.201903170086.