Tuning energy output of PTFE/Al composite materials through gradient structure

Yao-feng Mao,Qian-qian He,Jian Wang,Chuan-hao Xu,Jun Wang ,Fu-de Nie

China Academy of Engineering Physics Institute of Chemical Materials,China

Keywords:PTFE/Al composite Gradient structure Radial gradient Pressure output

ABSTRACT As a typical energetic composite,polytetrafluoroethylene(PTFE)/aluminum(Al)has been widely applied in explosives,pyrotechnics,and propellants due to its ultra-high energy density and intense exothermic reaction.In this work,the radial gradient (RG) structure of PTFE/Al cylinders with three different PTFE morphologies (200 nm and 5 μm particles and 5 μm fiber) and content changes are prepared by 3D printing technology.The effect of radial gradient structure on the pressure output of PTFE/Al has been studied.Compared with the morphology change of PTFE,the change of component content in the gradient structure has an obvious effect on the pressure output of the PTFE/Al cylinder.Furthermore,the relationships of the morphology,content of PTFE and the combustion reaction of the PTFE/Al cylinder reveal that the cylinder shows a more complex flame propagation process than others.These results could provide a strategy to improve the combustion and pressure output of PTFE/Al.

1.Introduction

As a typical energetic material,PTFE/Al composite has high energy density(21 kJ/cm3)and can release a large amount of heat in the reaction process,which has great potential in the applications of composite solid propellants,explosives and reactive fragments[1-3].The energy and pressure output of PTFE/Al is determined by the combustion reaction.Therefore,it is important to tune the combustion reaction of PTFE/Al composites accurately for the practical applications [4,5].

At present,regulating the mass ratio of reactants is deemed as a vital way to tune the combustion reaction of PTFE/Al composites.Various studies have shown that the optimum mass ratio of PTFE/Al composites is about 73.5/26.5,which is determined by the stoichiometric ratio of the chemical reaction between PTFE and Al[6-8].However,the optimal reaction ratio is usually observed to be a smaller proportion than that one above due to the loss of active Al,reaching 60/40 or even 50/50 in actual formulations [9,10].In addition,the particle size and morphology of Al are important factors for controlling the combustion reaction and pressure output of PTFE/Al composites.Small-sized Al particles can increase the interface area and mass diffusion,thereby improving combustion kinetics.The flake aluminum powder also affects the combustion performance of the PTFE/Al composites [11].Although the content and particle size of Al in the PTFE/Al composites have been fully studied,the ability to regulate combustion reaction is limited.It is well known that the energy output of Al/PTFE composites lies in the redox reaction between oxidant PTFE and fuel Al.Therefore,the combustion performance in Al/PTFE composite is not only decided by the Al powder,but also depended on the nature of PTFE.The same effect as Al powder on composites,the energy output of PTFE/Al composites can also be regulated by controlling the oxidant(morphology and particle size,etc) [12,13].However,few studies have been concentrated on the regulating studies of combustion reaction in PTFE/Al composite from the PTFE point of view.

Furthermore,tailoring the structure of components is also an effective way to adjust the energy output of composites,which has been proved by a large number of experimental studies[14-18].For example,the regulation of energy output for the core-shell structure Al/PTFE could be realized through changing the thickness of PTFE shell,which performs greater pressure output than the physical mixed one[19,20].Similarly,the reaction completed level and rate between Al and PTFE could be adjusted by changing the thickness of PDA coating in Al@PDA/PTFE composites [21].However,it should be noted that the above researches are all focused on the micro scale in which the regulations on energy output are limited and the preparation methods are full of challenges.In terms of macro structure,solid rocket propellants with different filling densities prepared by 3D printing achieve different combustion rates,and the combustion rate decreases with the increase of filling density[22].The combustion properties of Al/PVDF and Al/AP/PVDF films prepared by 3D printing,electrostatic spraying,and electrospinning also prove the important relationship between the macroscopic structure and the energy release of the composites [23].Therefore,in order to realize the regulation of macro structure on the energy output of composite materials,a gradient structure was proposed [24-26].Gradient structure is a heterogeneous structure that slowly changes within the sample,and the change objects include sample density,particle size,porosity,etc.It can customize specific functions and performance by controlling the gradient change of structure or component[27,28].The PTFE/Al composites with radial gradient structure can change the pressure output and flame morphology during combustion,which has been proved in previous articles [29,30].It is gratifying that the rapid development of 3D printing technology makes the preparation of gradient structure possible [31-35].

In this work,the combustion reaction behaviors and energy output of PTFE/Al composites are studied from the PTFE content,the PTFE morphology(particle size)and the macroscopic structure.The radial gradient (RG) cylinders of PTFE/Al composites are designed and prepared through a 3D printing platform.The thermal decomposition and combustion properties of PTFE/Al composites are characterized by TG-DSC and high-speed camera technology,respectively.On this basis,the pressure test of the composite cylinders is carried out to obtain the regulation of energy output(pressure release)of PTFE/Al composites by modulating the macrostructure.

2.Experimental

Materials:The Al nanoparticle was purchased from Aladdin(Shanghai,China)and the average size was about 50-100 nm.The PTFE nanoparticles were produced by XIANFENG Nano Technology Co.,Ltd(Nanjing,China).The diameter of PTFE nanoparticles,PTFE microparticles,and PTFE fiber was about 100 nm,5 μm,and 1-5 μm,respectively.Binder (polyvinylidene fluoride,PVDF) was produced at the Chenguang Chemical Research Institute (Zigong,China).

Designing and printing the gradient structure:Nowadays,RG structures are applied to tune the combustion performance and pressure output as the main strategies.Combine the morphology of PTFE and the formula of composite materials to control the pressure output of PTFE/Al cylinder in this work.According to the principle of fuel-rich,the formula which PTFE/Al composite materials are used to prepare the gradient cylinder is shown in Table 1.From Fig.1(a),the composite materials that are added to the binder system of appropriate amount have been mixed uniformly by acoustic resonance technology.Then,the excess solvent (butyl acetate) was volatilized to control rheological properties (viscosity and modulus)of PTFE/Al composite for 3D printing(direct writing).

Fig.1.(a) Schematic illustration of fabrication of PTFE/Al composite gradient structure,(b) Schematic diagram of RG structure.

Table 1The component content of PTFE/Al composite materials.

As an advanced technology way,the process of direct writing needs to be designed and programmed before printing.The RG structures of the component ratio and component morphology were designed and printed.The radial gradient cylinder has a cylindrical structure,and the whole structure is divided into three layers from inside to outside.The 3D printing machine with three nozzles were used to print layer by layer from outside to inside.The diameter of the radial gradient cylinder was 15 mm,the height was 12 mm,and the weight was 3.0 g.The combination details of the radial gradient cylinder are shown in Table S1 and Table S2,and the schematic diagram of RG structure is shown in Fig.1(b).Such as the RG cylinder of RG-567,the components from inside to outside are nPA-5050,nPA-6040 and nPA-7030,respectively.In the RG cylinder of RG-UNX,the components from inside to outside are μPA,nPA and PxA,respectively.

Characterization:The DHR-1 (TA,United States) stresscontrolled rheometer was used to test the rheological performance of PTFE/Al composite materials.The field emission scanning electron microscopy (FE-SEM,Ultra-55,Carl Zeiss,Germany) was used to characterize the structure and morphology of PTFE/Al composite.The thermal decomposition behavior of as-prepared samples was studied by TG-DSC (Mettler Toledo) measurement with a heating rate of 10°C/min under the flow of N2,and the temperature range from 200 to 800°C.Oxygen in the furnace was removed by initiating N2flow 20 min prior to elevated temperature during DSC.

The line of PTFE/Al composite materials with the diameter of 0.84 mm,1.21 mm,and 1.54 mm and the length of 30 mm were ignited by nickel-chromium wire on the combustion platform at room temperature in an open environment (Fig.S1(a)).The combustion process that includes the way of flame propagation the flame morphology of PTFE/Al composite material lines was recorded by a high-speed camera(Unix-50,Japan).The burning velocity of composite lines was calculated based on the length of the lines and the burning time.

The pressure evolution with time of PTFE/Al composite gradient structure(RG cylinder) was tested in a constant-volume explosion vessel with 330 mL(Fig.S1(b)).The quality and specification of the gradient cylinder were controlled to a certain value during preparation.The gradient cylinder was ignited by nichrome wire in closed space with air atmosphere.The signal of pressure change was collected by a pressure sensor with 20 MPa,and the pressure sensor was installed above the constant-volume explosion vessel.The pressure-time curves can be obtained based on this experiment.

3.Results and discussion

3.1.Morphology and structure

The change of PTFE morphology will lead to the difference in microstructure of PTFE/Al composites.It can be seen from Fig.2(a)that the nano-aluminum powder particles are relatively uniform without obvious agglomeration.For raw materialn-PTFE(Fig.2(b)),nano-scale PTFE particles have obvious agglomeration,forming irregular particles with a diameter of about 10 μm,which is contrary to nano-aluminum powder.In the high magnification(10,000 times) SEM images,we see that the n-PTFE particles achieve uniform distribution in the large agglomerated particles,and there are a certain number of voids.The u-PTFE (Fig.2(c)) and PTFE-x(Fig.2(d)) are irregular in shape with an average size of 3-5 μm.

Fig.2.(a) SEM images of nano-Al particles;(b) n-PTFE;(c) u-PTFE and PTFE-x (d);The surface structure of line that is printed with composite materials of (e) nPA-6040,(f) PxA-6040,and (g) uPA-6040;(h) The model and actual picture of RG cylinder with RG-657 and (i) single-component cylinder with uPA-6040 that the height is 15 mm.

For the composites with three different PTFE morphologies,the same frequency technology is used in the mixing preparation process.However,the binders in Fig.2(e)and Fig.2(f),and Fig.2(g)show different distribution states.The n-PTFE particles have the smallest particle size,and the binder is uniformly coated in the middle of the gap between nano-PTFE particles.Therein,the binder showed adhesion and fixed the nano-PTFE particles.Contrary ton-PTFE particles,the u-PTFE particles have the largest particle size.It can be seen from Fig.2(g) that the binders are agglomerated and there are large gaps among them.In the molding surface of composites,the above phenomenon is mainly caused by the difference in PTFE particle morphology.It is known that small particles have large specific surface area.Compared with PTFE-x and u-PTFE,n-PTFE has a larger specific surface area,and has a larger contact area with binder,which is easy to form a strong interface effect.This leads to the binder being more evenly dispersed between particles.The specification and writing mode of the cylinder are set in the 3D printing program.Fig.2(h) and Fig.2(i) show the radial gradient structure cylinder and single component cylinder prepared by 3D printing technology.The cylinder is processed in detail after printing,and the composite gradient cylinders for pressure and combustion tests are finally obtained.

3.2.Thermal analysis and combustion performance

The exothermic reaction of PTFE/Al composite materials is analyzed with Differential Scanning Calorimetry (DSC).Fig.3(a)shows the DSC plots of PTFE/Al composite materials with a heating rate of 10°C/min.

Fig.3.(a) DSC and (b) TG curves of PTFE/Al composite materials with different PTFE morphology.

There is an obvious exothermic peak in the range of 535-584°C,which belonged to the most important reaction of composites(redox reaction between PTFE and Al).In addition,an exothermic step appeared in the temperature range of 403-435°C.Combined with relevant literature reports,there is a pre-ignition reaction between PTFE and Al.Some PTFE transformed into viscoelastic state reacted with the alumina layer on the surface of Al particles at high temperature,which destroyed the protective shell of Al core.This makes the reaction between PTFE and Al easier,which can be known from the pre-ignition reaction temperature.It is worth noting that nPA-6040 has the smallest pre-ignition reaction temperature,due to the maximum specific surface area of nano-PTFE.There are two relatively small endothermic peaks at～327°C and～660°C,which belong to the melting process of binder endothermic melting and residual Al metal particles.The products after the melting of the binder provide some help for the preignition reaction and participate in the destruction of the alumina layer.The Al metal particles exist in a liquid form at high temperature after melting,which absorbs a small amount of oxygen in the reaction gas to form amorphous alumina(Al2O3)and adheres to the bottom of the test crucible.The mass change in the TG curve(Fig.3(b)) also proves the reaction state of the composites.At～330°C,the mass loss is about 5 wt%,which indicates that the binder melts and decomposes to form gaseous products.There is an obvious mass loss step (30%-40 wt%) due to the thermal decomposition of PTFE at a temperature range of 500°C-580°C,and a large amount of decomposition products react with Al.A slight increase in quality after 660°C,which confirms the formation of Al2O3.

The combustion performance of PTFE/Al composite printed lines with different components and diameters is tested.Fig.4(a)shows the combustion process of the composite nPA-6040.The combustion flame changes significantly with the increase of the diameter of the line,and a larger and brighter flame is obtained.The combustion process of other composite lines is shown in Fig.S2,and the change law of flame is consistent with that of composite nPA-6040.By comparing the combustion flame of the composite material with a diameter of 1.56 mm,it can be found that the flame self-propagating phenomenon occurs during the propagation process,except for the composite material nPA-7030.The selfpropagating phenomenon is a factor leading to the rapid increase in combustion rate.According to the combustion time (flame propagation time) and the length of the lines to calculate the corresponding combustion rate,the specific values are shown in Fig.4(d).

Fig.4.(a)Combustion flame diagram of 3D printed composite lines of nPA-6040,(b)PxA-6040 and(c)uPA-6040 with different diameter,and(d)the combustion velocity of PTFE/Al composite materials with different diameter (0.84 mm,1.21 mm and 1.56 mm).

For the five composites,the combustion rate can be divided into three levels.The composite nPA-5050 has the fastest combustion rate,reaching 55.242 mm/s (0.84 mm),102.302 mm/s (1.21 mm),and 264.451 mm/s (1.56 mm).For the composite nPA-7030,the minimum combustion rate is 16.62 mm/s(0.84 mm),20.977 mm/s(1.21 mm),and 22.341 mm/s (1.56 mm),due to excessive fuel and less oxidant.The composite materials nPA-6040,PxA-6040,and uPA-6040 have relatively moderate burning velocities in the ranges of 30.0-45.0 mm/s (0.84 mm),40.0-60 mm/s (1.21 mm),and 100.0-200.0 mm/s (1.56 mm),respectively.Importantly,the particle shape of PTFE also affects the combustion rate of the PTFE/Al composites.As the specific surface area of PTFE particles increases,the burning rate of Al/PTFE composites decreases.The change of specific surface area causes the variations of heat and mass transfer distance,as well as the contact area between particles.

When the diameter of the printed line increases from 0.84 to 1.56 mm,the PTFE particle shape has a more obvious effect on the combustion rate of the composite.Similar to the specific surface area of particles,the printed lines with larger diameters have smaller relative exposure areas.Therefore,in the combustion process of the line,the lost heat will decrease with the increase of the line diameter.Another interesting phenomenon is that the combustion rate of composite PxA-6040 is higher than that of nPA-6040 in the combustion of large diameter(1.21 mm and 1.56 mm)lines,which is caused by the shape of PTFE fiber particles.Irregular PTFE fiber particles form a variety of fine spaces within the composite lines and have more heat flow diffusion space than the nano-PTFE/micro-PTFE composite lines.

Based on the burning behaviors of the PTFE/Al composite printed lines,the combustion performance of the Al/PTFE cylinders is tested.The diameter of the sample is 10 mm,and the height is 15 mm.Fig.5 shows the combustion process of the PTFE/Al composite cylinder.It can be seen from the figure that the cylinder is ignited by the laser pulse,the flame spray direction is opposite to the propagation direction of the flame front.The flame area in the camera image is relatively compact as if constrained together.When the flame propagates along with the cylinder to a certain distance (a certain time),the flame is no longer constrained and bursts out instantly to obtain a larger combustion area.After the combustion image is processed,the combustion time and flame burst time of each cylinder can be obtained (Fig.6(b)).The results show that the burning time of uPA-6040 was the shortest,which is 253 ms.Moreover,it also has the relatively widest flame area after the flame burst,showing more vitality.The combustion time of nPA-5050,nPA-6040,and PxA-6040 are 279 ms,292 ms,and 317 ms,respectively,and nPA-5050 has the smallest flame burst time(78 ms,time of the flame spraying around).Interestingly,the change rule of the combustion rate of the composite cylinder(nPA-6040,PxA-6040,and uPA-6040) is opposite to the line (1.21 mm and 1.56 mm) if the combustion rate is calculated by dividing the cylinder height by time.The rapid combustion rate of lines (nPA-6040 and PxA-6040) leads to the rapid diffusion of flame in the ignition plane of the cylinder,so that the flame plane propagating downward is close to the horizontal plane.In the uPA-6040 cylinder,the downward propagating flame surface is conical,making it easier to penetrate the entire cylinder.The combustion model of the cylinder is shown in Fig.6(a).In addition,nPA-7030 has the maximum combustion time and flame burst time,and the flame appears inactive with only a small combustion area and low brightness.A large number of PTFE did not have the opportunity to participate in the reaction because of the Al content and activity,and finally become residue.

Fig.5.Flame morphology of PTFE/Al composite materials cylinder by laser ignited.

Fig.6.(a)The combustion model of 3D printed single cylinders with different PTFE morphology;(b)The diagram of the combustion time with PTFE/Al composite materials cylinder.

3.3.Performance of pressure output

Pressure output is a way of energy release for energetic materials,which will be affected by the component content and morphology of PTFE in the PTFE/Al composite.The pressure output of a small cylinder(Φ10 mm×15 mm)with a single component is obtained simultaneously during the combustion of a cylinder,and the results are shown in Fig.S4.In order to further explore the influence of PTFE content and morphology on the properties of PTFE/Al composites,gradient structures are designed and prepared based on different PTFE content and morphology.The gradient structure realizes the orderly distribution of the composition content and morphology inside the cylinder,so as to realize the effective control of the energy output process of the composite material.

Fig.7(a) shows the pressure-time curves of the RG structure with different PTFE content in PTFE/Al composites.From the overall pressure change,the control effect of the RG structure of PTFE content on the pressure output of the cylinder can be divided into two steps.Firstly,the composition of the center position of the RG structure can determine the approximate level of the cylinder pressure output.Secondly,the external components determine the trend (increase or decrease) of cylinder pressure output.It can be seen from the pressure curve that when the component at the center changes from nPA-5050 to nPA-7030,the maximum pressure value increases from 2903.5 to 3351.4 kPa (or from 2640.3 kPa to 2817.1 kPa).In addition,when the inner component is nPA-5050,the maximum pressure value of RG-567 is greater than that of RG-576.Similarly,the maximum pressure value of RG-657 is greater than that of RG-675,and the maximum pressure value of RG-756 is greater than that of RG-765.In other words,under the premise of determining the central component,when the components with less PTFE content are distributed in the outer layer,the pressure output obtained by the RG structure is relatively small.

Fig.7.(a) The Pressure-Time curves of RG composite cylinder with component content and (b) PTFE morphology.

Fig.8(a)shows the pressure change rates of RG structures with different contents.The maximum pressurization rates of RG-675 and RG-756 are about 54,000 kPa/s.Interestingly,the maximum pressure value obtained by RG-756 is far greater than that obtained by RG-675,indicating that RG-675 has a faster reaction rate.Not only that,RG-675 also has the maximum decompression rate and is less efficient in maintaining the output pressure.This extreme phenomenon is caused by the difference of the reaction rate in a single component,which can be explained by the different combustion time.It can be seen from the pressure test curve of the RG structure with different PTFE morphologies in PTFE/Al composites(Fig.7(b)) that the maximum pressure is related to the position of the PTFE/Al composite materials in the RG structure.When the composite material PxA-6040 is in the inner layer of RG structure,the pressure value obtained is the smallest,which is 4479.0 kPa of RG-XUN and 4656.9 kPa of RG-XNU,respectively.When the composite PxA-6040 is in the middle layer of RG structure,the relatively moderate pressure values are 4794.3 kPa for RG-NXU and 4750.1 kPa for RG-UXN,respectively.When the composite PxA-6040 is in the outer layer of the RG structure,the maximum output pressure is obtained,which is 4903.5 kPa of RG-NUX and 5090.6 kPa of RG-UNX,respectively.Fig.8(b) shows the pressure variation of the RG structure with different morphologies in the reaction process.The pressurization rates are all about 126,000 kPa/s,and the change is not particularly obvious.The position of PxA-6040 has little or no effect on the pressurization rate.

When PxA-6040 firstly started ignition reaction,the pressure value obtained by the whole gradient structure mainly depends on the inner reaction of PxA-6040.The composite materials of middle and outer only play a supplementary role,and the pressure values obtained are relatively small.When PxA-6040 is used as the shell,it can obtain greater pressure value.On the contrary,higher pressure can be obtained when uPA-6040 is located in the inner core of the gradient structure.Therefore,in the radial gradient structure of PTFE/Al composites with three morphologies,the combustion reaction order of PxA-6040 is the key to regulate its pressure output.

For comparison,the composite materials with three contents(nPA-5050,nPA-6040,nPA-7030) and three morphologies (nPA-6040,PxA-6040,uPA-6040)are mixed in equal mass to prepare the cylinder(RG-mix and RG-MIX)with the same specifications as the radial gradient structure,and the pressure tests were carried out.The maximum pressure values obtained by RG-mix and RG-MIX are 2950 kPa (Fig.9(a)) and 4867.9 kPa (Fig.9(b)),respectively,which are close to the average of the maximum pressure values obtained by the same series of gradient structures.In addition,the pressure change rate is shown in Fig.S5.This result further illustrates that the distribution position of PTFE/Al composites with three morphologies contained in the gradient structure can regulate the maximum output pressure of the PTFE/Al cylinder.

Fig.9.The Pressure-Time curves of RG mixture cylinder with (a) component content and (b) PTFE morphology.

4.Conclusions

In summary,the PTFE/Al lines and cylinders with radial gradient structure are fabricated by 3D printing technology.The combustion reaction and pressure output of PTFE/Al cylinders are tuned through the content and morphology in composites.The burning rate of can be controlled in the range of 22.341-263.451 mm/s by changing the PTFE content from 70 wt% to 50 wt%.Changing the morphology of PTFE in the PTFE/Al composites can tune the linear burning rate in the range of 36.854-192.712 mm/s.The combustion rate of 3D printing cylinder with single component is opposite to the lines of 3D printing due to the difference in flame front diffusion.Most importantly,the overall pressure output can be tuned by changing the distribution of component in the radial gradient structure.Like the distribution of nPA-7030 in radial gradient structures with different PTFE content and PxA-6040 in radial gradient structures with different PTFE morphology.It is worth noting that the regulatory effect of PTFE content change in PTFE/Al composites on gradient structure is better than that on PTFE morphology.

Declaration of competing interest

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.

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Grant Nos.11872341 and 22075261).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2022.05.015.