The effect of al particle size on thermal decomposition,mechanical strength and sensitivity of Al/ZrH2/PTFE composite

Jun Zhang,Yu-chun Li,Jun-yi Huang,Jia-xiang Wu,Qiang Liu,Shuang-zhang Wu,Zhen-ru Gao,Sheng Zhang,Li Yang

College of Field Engineering,PLA Army Engineering University,Nanjing,210007,China

Keywords:Al/ZrH2/PTFE Thermal decomposition Mechanical strength Reaction characteristics Impact sensitivity

ABSTRACT To study the thermal decomposition of Al/ZrH2/PTFE with different Al particle size as well as mechanical strength and impact sensitivity under medium and low strain rates,molding-vacuum sintering was adopted to prepare four groups of power materials and cylindrical specimens with different Al particle size.The active decomposition temperature of ZrH2 was obtained by TG-DSC,and the quasi-static mechanics/reaction characteristics as well as the impact sensitivity of the specimen were studied respectively by quasi-static compression and drop-hammer test.The results show that the yield strength of the material decreased with the increase of the Al particle size,while the compressive strength,failure strain and toughness increased first and then decreased,which reached the maximum values of 116.61 MPa,191%,and 119.9 MJ/m respectively when the Al particle size is 12-14μm because of particle size grading.The specimens with the highest strength and toughness formed circumferential open cracks and reacted partly when pressed.Those with developmental cracks formed inside did not react.It is considered that fracture of specimens first triggered initial reaction between Al and PTFE to release an amount of heat.Then ZrH2 was activated and decomposed,and participated in subsequent reaction to generate ZrC.The impact sensitivity of the specimens decreased with the increase of Al particle size.

1.Introduction

Aluminum/Polytetrafluoroethylene(Al/PTFE)is one of the most representative reactive materials in metal/fluoropolymers.It has higher energy releasing level,sufficient insensitiveness as well as better physical and chemical properties compared with traditional explosive materials and propellants.Al/PTFE could undergo impact energy release reaction under the function of high-speed impact load[1],the essence of which is that Al and PTFE in the material have severe redox reaction and release plenty of energy[2].When the mass ratio of Al to PTFE is 26.5:73.5,the unit mass heat value could reach 8.68 MJ/kg in reaction,about 2.0 times of the reaction heat value of TNT(4.18 MJ/kg).Therefore,the material has great potential in military field to fabricate energetic warheads,such as active fragments,shell body and high-speed projectiles.The impact-induced reaction not only eliminate the complex fuze structure of the warhead,but also cause two-stage damage of both mechanical and chemical energy to the target.Hunt et al.revealed that,When the energetic projectile made by Al/PTFE material impacts and destroy the target at the speed of 200-5000 m/s,severe plastic deformation would occur within the pellet.Plastic-flow and crack propagation of the material would make the components in the reactive material mix fully and burn intensely or even develop into detonation.The heating rate could reach 104-108K/s[3].

Though Al/PTFE reactive material has preferable physiochemical properties compared with traditional energetic materials,its mechanical properties,such as density and strength,are not so good as metal.It cannot be used in military field as independent structural destroying element.The development direction of reactive materials is inevitably to increase its mechanical strength and overall density to ensure the penetration ability of the destroying element.Meanwhile,it also requires to have sufficiently high energy releasing level to guarantee the damaging effect after penetration.In addition,the material should also be insensitive enough so that it would not react in service operations,such as production,processing,transportation,storage and military operation.To explore the application of Al/PTFE,scholars have conducted systematic and thorough studies on proportion of ingredients,particle size and preparation technology[4-7].However,few achievements have been made in improving the mechanical properties and overall density of Al/PTFE material.Based on the controllability of the composition in preparation of the reactive material,many scholars have introduced other materials with excellent physicochemical properties into Al/PTFE material by means of filling modification,among which high-density W is the additive that was reported the most.For example,Cai et al.[8,9]studied and analyzed the compressive strength and failure mode of Al/W/PTFE with modified drop-hammer apparatus.Herbold et al.[10]studied the strength,failure and impact behaviors of Al/W/PTFE material with different W particle size through quasi-static and dynamic experiment combining simulation method.Wang Haifu et al.[11-14]conducted experimental investigation on the mechanical properties and the damaging effect on aluminum target of Al/W/PTFE.The research results show that W does not participate in the exoenergic reaction of Al and PTFE.In case the additive amount of W is small,it could not improve the physical properties of the material.While if the additive amount of W is overdosed,it could cause decrease of the relative content of Al and PTFE,which not only could affect the mechanical properties of the material by lowering the continuity of PTFE matrix,but also could greatly reduce the energy density of the material.

The ideal additive should be able to improve the strength and density of the material while guaranteeing the energy release level of Al/PTFE material applied to military.For the reason,new energetic material metal hydride could be introduced into Al/PTFE as an energetic additive.Previously,many scholars[15-17]introduced metal hydride into traditional explosives or rocket propellants as a high energy additive and carried out plenty of researches.However,Yu et al.[18-20]introduced TiH2into Al/PTFE reactive material for the first time and conducted related studies on quasi-static and dynamic mechanical properties,reaction mechanism,energy release characteristics and so on.The results show that moderate TiH2not only could improve the compressive strength of the material,but also could help increase the level of energy release,so it is a potential energetic additive.As a metal hydride,ZrH2has high hydrogen storage density(the hydrogen content is about 3.8%[21]),safety and stability.At present,only a few scholars[22,23]have carried out some researches on the combustion mechanism and energy characteristics of traditional propellants and RDX-based explosives with ZrH2.

Recently,our research group introduced ZrH2into Al/PTFE reactive material for the first time.We have studied the effect of ZrH2content on the mechanical properties and reaction characteristics of Al/PTFE[24].The results show that appropriate ZrH2additives can improve strength of Al/PTFE,and decompose to participate in chemical reaction between Al and PTFE,verifying the potential of ZrH2for the enhancement of strength and energy release of the composites.

In this work,four groups of Al/ZrH2/PTFE reactive materials of different Al particle size were prepared.TG-DSC was adopted to analyze the thermal decomposition of Al/ZrH2/PTFE to explore the reaction process of ZrH2in A/PTFE reaction.And the effect of different Al particle size on the mechanical strength and sensitivity of Al/PTFE material were also studied through quasi-static compression and drop-hammer experiment,respectively.

2.Experiment

2.1.Material selection and sample preparation

Raw materials:PTFE(average particle size:25μm,density:2.2 g/cm3,from 3F,Shanghai,China),Al powder(average particle size:1-2μm,12-14μm,22-24μm and 33-35μm,density:2.7 g/cm3,from JT-4,Hunan,China),ZrH2powder(average particle size:10μm,density:5.6 g/cm3,from Haixin,Shenyang,China).Four groups of Al/ZrH2/PTFE powder material and cylinder specimens(two specifications:φ10 mm×10 mm andφ10 mm×3 mm)of different Al particle size were prepared.The mass fraction of Al and PTFE of each group specimens were prepared according to chemical equilibrium ratio(26.5%/73.5%).The content of ZrH2accounts for 20% of the total mass.The mass fractions of Al,ZrH2and PTFE of each groups of specimens are 21.2%,20% and 58.8%.According to the size sequence of Al particle size,four groups of materials were marked as 1#-4#.

Preparation process of Al/ZrH2/PTFE samples:(1)Mixing:Weigh four groups of reactive materials according to the formula and mix in flask.Add moderate anhydrous ethanol to prepare it into suspension.Agitate mechanically for 20 min to full mixing,and then place it into a vacuum oven at 60°C until dry completely.(2)Molding:Sieve the dry mixture(60 meshes)to obtain the homogeneous dry powder for synchronous thermal analysis,and then press the powder into cylinder specimens that could be used for quasi-static and drop-hammer test using a molding die and a hydraulic press.The compressive pressure is 240 MPa,and the pressure maintaining time is 20 s(3)Sintering:To significantly improve the mechanical strength of the molded specimens after cold pressing,the preform-sample was placed into a vacuum sintering furnace to sinter at 360°C for 4 h with the heating rate of 90°C/h.The polymer molecules gradually change from crystalline to amorphous.The dispersed single resin particles pass through interdiffusion and melting,encapsulating additives(Al,ZrH2)and bonding into a entirety.Then,it’s cooled at a cooling rate of 50°C/h.The polymer molecules change from amorphous to crystalline.At this time,the mechanical strength of the specimen was greatly improved.The sintering process curve is shown in Fig.1.

2.2.Experimental process

1.Synchronous thermal analysis

Fig.1.The typical sinter temperature curve of reactive materials.

Germany Bavaria Model NETZSCH-STA449C thermogravimetrydifferential scanning calorimetry(TG-DSC,NETZSCH-STA449C,NETZSCH,Bavaria,Germany)was adopted to analysis the thermal decomposition reaction process of pure ZrH2and a group of Al/ZrH2/PTFE mixed powder with 1-2μm Al particle size.The test temperature was 25°C-1000°C,and the heating rate was 10°C/min.To prevent the air participating in the reaction,the test was conducted in a highly pure argon environment with the argon purge rate of 30 ml/min.

2.Quasi-static compression

The CMT5105 microcomputer-controlled electron universal testing machine was used to carry out quasi-static compression experiments on four groups of Al/ZrH2/PTFE specimens,with the head loading rate of 6 mm/min.An appropriate amount of petroleum jelly was smeared on both ends of the specimens before loading so as to reduce the influence of the friction.During the test,a high-speed camera(FASTCAM SA-Z,Photron,Tokyo,Japan)with a frame rate up to 10,000 frames/s was used to record the deformation and reaction process of each group of specimens under quasi-static compression.To guarantee the consistency and reliability of the test results,four groups of specimens were tested repeatedly 3 times respectively,and the mean value was taken.The experimental ambient environment was 25°C.

After quasi-static compression,the X-ray diffraction(XRD,Bruker D8 ADVANCE,Bruker,Berlin,Germany)was used to characterize the residues of reaction products.The instrument parameters were set as follows:The tube voltage was 40 kV,the current was 40 mA,CuK radiation(λ=0.15406 nm),the scanning scope 2θ was 10°-90°,and the scanning speed was 5°·min-1.

3.Drop-hammer tests

A drop-hammer machine(HGZ-1,TD,Xiangfan,China)was adopted to measure the impact sensitivity of four groups of Al/ZrH2/PTFE material within moderate strain rate.The mass and dropping height of drop-hammer are 10 kg and 0-160 cm,respectively.The impact energy of drop-hammer is 156.8 J when dropping freely at the maximum height.According to the test standard of sensitivity in GJB772A-1997 Method 601.2,the impact experiment was performed 15 times for each group of the same specimens.The impact process was recorded by a high-speed camera.Whether the specimens generate fire was taken as the criterion of ignition to judge whether the specimen reacted.It is obtained that the characteristic drop height(H50)for the specimens having 50% probability to ignite.The experimental ambient temperature was 24°C.The calculation formula of characteristic drop heightH50is as follows:

whereAis the lowest height in the test,Bis the increment of the height,iis the order of the drop height beginning from 0,Nis the total number of reaction events in the test,Niis the number of reaction events at a certain height,Eiis the ignition energy of specimens,Epis the potential energy of the drop mass,andmis the mass of the drop hammer.

3.Results and discussion

3.1.Thermal decomposition reaction of Al/ZrH2/PTFE

Fig.2.TG-DSC(Thermogravimetry-differential scanning calorimetry)curve of three materials(a)ZrH2 powder(b)Al/PTFE(c)Al/ZrH2/PTFE.

TG-DSC curve of ZrH2powder is shown in Fig.2a.It could be known from the experimental curve that TG curve showed a slow downward trend all the time before 600°C,although there is no obvious endothermic peak on DSC curve.Rising to 250°C,the mass of ZrH2has lost 1.09 wt.%.At around 700°C,an endothermic peak appeared and the mass of the sample decreased significantly in a short time,with a reduction of 1.33 wt.%.According to the analysis,during the linear heating process in a vacuum environment,ZrH2decomposed slowly from about 100°C,and the decomposition process intensified from 600°C to 800°C which is the temperature range of obvious decomposition of ZrH2.Therefore,the peak A is considered to be a decomposition endothermic peak of ZrH2.

Fig.2b presents the TG-DSC curve for Al/PTFE.The endothermic peak A covers a temperature range from 329.8°C to 350.6°C,where no change shows up on the TG curve.It caused by the melting endotherm of PTFE.Starting from 500°C,the TG curve suggests that the sample weight dropped sharply,and peak B starts at 478.5°C,indicating that peak B is the endothermic decomposition peak of PTFE.The fluorine-containing ions generated from PTFEdecomposition and Al powder had oxidation-reduction reaction at high temperature,so the exothermic peak C appears on DSC curve.The peak temperature of peak D is 659°C,and there is no obvious change in the sample mass during this period,so it is the melting endothermic peak of Al powder that is not reacted completely.

Table 1The endothermic and exothermic peak parameters of Al/ZrH2/PTFE reactive material and the corresponding physicochemical changes.

Fig.2c shows the results of synchronous thermal analysis of Al/ZrH2/PTFE reactive material linearly heated to 800°C.The heat absorption and desorption parameters of the five corresponding peaks and the physiochemical changes of each peak are shown in Table 1.Compared with the results of synchronous thermal analysis of Al/PTFE,it could be found that the endothermic peaks,A and B,were caused by the melting and decomposition of PTFE,the peak C is the reaction exothermic peak of Al and PTFE,and peak D is the melting endothermic peak of Al at about 660°C,which correspond to the TG-DSC test results of Al/PTFE.However,the difference is that an endothermic peak E appears at about 700°C,with the peak temperature of 729.9°C,and ends at 746.2°C.Peak E is rather flat and spans a wider temperature range,and TG curve shows weak descending tendency.It could be concluded that E is the decomposition endothermic peak of ZrH2powder compared with TG-DSC results of pure ZrH2,but the peak temperature of Al/ZrH2/PTFE delays about 30°C than the decomposition absorption peak of pure ZrH2.

3.2.Mechanical properties of the material under quasi-static compression

Fig.3 shows the true stress-strain curve of four groups of Al/ZrH2/PTFE specimens with different Al particle size,and the corresponding mechanical properties parameters are shown in Table 2.It could be seen from the figure and table that Al/ZrH2/PTFE is elastoplastic material under quasi-static compression.First,it reached the yield point after experiencing a short period of elastic deformation,and then it entered plastic stage and showed strain hardening.The influences of Al particle size on mechanical strength of Al/ZrH2/PTFE specimen are obvious with the same ZrH2content,as shown in Fig.4.It can be seen that the yield strength of the specimen decreases with the increase of Al particle size,dropped from 24.05 MPa to 16.32 MPa.While the compressive strength,failure strain and toughness increase first and then decreased with the increase of Al particle size,which reached respectively the maximum value of 111.61 MPa,191% and 119.90 MJ/m3when Al particle size is 12-14μm.

Fig.3.True stress-strain curve of Al/ZrH2/PTFE specimens under quasi-static compression.

Table 2Mechanical parameters of Al/ZrH2/PTFE specimens.

When studying the compressive mechanical and reaction characteristics of Al/PTFE reactive materials,Wu et al.[25]found that when Al particle size was 1-2μm,the failure stress reached the maximum value of 92.8 MPa,which was significantly higher than that when the Al particle size was 12-14μm.It is analyzed that the destroyed fine dimension mechanism is that the metal particles stripped from the polymer and the matrix breaks if the material is formed by homojunction metal and polymer.In case the volume fraction of the material is definite,the smaller the particle size is,the more compact the metal is bonded to the matrix,and the material would show more better macro-mechanical properties.However,from the experiment on the effect of Al/W/PTFE particle size grading on strength of materials,Qiao et al.[26]found that the size gradation of metal particle could affect a lot on mechanical properties when the material is polymer filled by two or more metal materials.When the matrix is coated with fillers with large particle size difference,it will form extremely uneven bonding force in different parts.In the case of external loads,the part with weak bond force first breaks and fails,represented as overall decrease of the material strength.When the size of the metal particles is close,the material could acquire homogeneous bonding force and exhibit more better macro-mechanical properties.For Al/ZrH2/PTFE material,the particle size grading of Al and ZrH2has a more significant influence on the material mechanical strength than the particle size.The average particle size of ZrH2is 10μm,so when the particle size of Al is 12-14μm,Al/ZrH2/PTFE specimen shows higher strength.Particle size grading not only can be used as metal filling material with regular particle shape,but also apply to metal and metal hydride.

3.3.Reaction phenomenon of the reactive material under quasistatic compression

Fig.4.Influences of Al particle size on mechanical properties of Al/ZrH2/PTFE specimens(a)Influences of Al particle size on compressive strength and failure strain(b)Influences of Al particle size on yield strength and toughness.

Fig.5.Ignition phenomenon in reaction and state after reaction of the specimen(a)Ignition phenomenon(b)State after reaction.

Some part of 2#specimens reacted in the quasi-static compression experiment.The ignition phenomenon at certain moment and the state of the specimen after reaction recorded by high-speed camera are shown in Fig.5.When the specimen was compressed to certain moment,initial reactions occurred at multiple circumferential cracks on the edge of the specimen,accompanied by fire and explosion,and then the reaction extinguished immediately,and a small amount of black solid residue was generated at the reaction cracks.

Fig.6.XRD results of the reaction products of specimen 2#.

The reaction products were collected for XRD detection to explore the reaction mechanism of Al/ZrH2/PTFE reactive material.The XRD results are shown in Fig.6,from which it could be seen that AlF3and ZrC were generated in the reaction residue,indicating that additive ZrH2participated in the activating reaction.Feng et al.discovered that Al/PTFE reactive material can undergo deformation,cracking and violent chemical reaction under quasi-static compression for the first time,releasing abundant heat and black smoke.In addition,AlF3and C(carbon black)were also detected in the reaction products.The results of synchronous thermal analysis of Al/ZrH2/PTFE material show that the peak reaction temperature of Al/PTFE is 602.5°C and the activation decomposition temperature of ZrH2is about 700°C.It can be speculated that the specimen fractured instantaneously under quasi-static compression,and the temperature at the tip of the circumferential opening cracks rose sharply to form local heat point,which could induce Al and PTFE react initially,generating AlF3and releasing heat.Then ZrH2was activated and decomposed to release hydrogen.And next,the decomposition products Zr and C(carbon black)reacted continuously and generated ZrC.It could be known that the possible reactions occurred during the reaction process are:

The other specimens did not react and the state after quasistatic compression are shown in Fig.7.The specimens were deformed under compression and failed immediately after experiencing the strain hardening stage when the pressure reached the fracture strength of the specimen.Developmental cracks appeared in all the unreacted specimens.The morphology of fracture surface(Fig.8)shows that the specimens produced shear cracks along 45°angle of the compression direction(the direction of the maximum shear stress),just as typical hard brittle materials.

Fig.7.Morphology of specimens that did not react under quasi-static compression(a)Specimen 1#(b)Specimen 3#(c)Specimen 4#.

Fig.8.Shear cracks on the fracture surface of the unreacted specimens.

By comparing the mechanical parameters of four groups of specimens,it can be seen that specimen 2#had the highest strength and toughness compared with the other specimens because of particle size gradation.It can absorb more energy in the process of quasi-static compression and easy to form circumferential opening cracks in failure.The temperature at the tip of cracks rose quickly and formed local heat point,which expanded along the extension direction of the cracks,resulting in reaction between Al and PTFE.The compressive strength and toughness of other specimens decreased,and the energy absorbed during compression was insufficient to initiate a reaction.Developmental cracks were formed inside the specimens,which cannot activate local heat point of the activation reaction.Therefore,even specimen 1#with an Al particle size of 1-2μm cannot have a chemical reaction.It can be seen that the introduction of ZrH2and the particle size gradation could cause significant influences on the mechanical strength of the reactive material,and then affect the reaction characteristics of Al/ZrH2/PTFE material.

3.4.Impact sensitivity of the reactive material

The values of the characteristic drop height and reaction excitation energy of each group were calculated by ignition data of four groups of specimens,as shown in Table 3.It could be seen that the characteristic drop height and ignition excitation energy of Al/ZrH2/PTFE specimens increased with the increase of Al particle size.When Al particle size is 1-2μm and 33-35μm,the material sensitivity was the highest and the lowest,respectively.The changing tendency of impact sensitivity with Al particle size was obvious.When Al particle size increased from 1-2μm to 12-14μm,the increase margin of characteristic drop height was the largest,which is 27.74 cm.However,the characteristic drop height tended to grow in linearity gradually when Al particle size increased further.

The characteristic drop height and ignition excitation energy of the material increased with the increase of Al particle size.That is to say,the smaller the impact sensitivity of the specimen is,the less likely it is to react with the increase of the particle size of Al.It is analyzed that the larger Al particle size is,the smaller the specific surface area is(The specific surface area of Al particles with different size is shown in Table 3),making the thermal diffusion distance between the particles increased,which went against transmission of heat and reduced the reaction activity of the material.

4.Conclusion

In this study,four groups of Al/ZrH2/PTFE powder materials and cylinder specimens of different Al particle size were prepared.TGDSC,quasi-static compression and drop-hammer experiments were adopted to study thermal decomposition of Al/ZrH2/PTFE material of different Al particle size as well as their mechanical strength and impact sensitivity at medium and low strain rates.The results are obtained as follows:

(1)ZrH2powder decomposes slowly from about 100°C and obviously from 600°C to 800°C.There is an obvious decomposition endothermic peak in DSC curve at about 700°C.Compared with the results of synchronous thermal analysis of Al/PTFE,a flat decomposition endothermic peak of ZrH2 appears at about 700°C,with the peak temperature of 729.9°C,and ends at 746.2°C in TG-DSC curve of Al/ZrH2/PTFE.

Table 3Specific surface area of Al particle,Characteristic drop heights and ignition excitation energy of four groups of Al/ZrH2/PTFE specimens.

(2)With the increase of Al particle size,the yield strength of the material was on a declining curve,while the compressive strength,failure strain and toughness increased first and then decreased.When Al particle size is 12-14μm,the compressive strength,failure strain and toughness of the material reached the maximum values of 116.61 MPa,191%,119.9 MJ/m3,respectively.Compared with particle size,the gradation of particle size between Al and ZrH2 has more significant influences.Therefore,Al/ZrH2/PTFE specimen with 12-14μm Al particle size showed higher strength and toughness than other specimens,and formed circumferential open cracks under quasi-static compression,which leaded to partial reaction of specimens.

(3)By analyzing the active decomposition temperature of ZrH2and components of reaction products,it could be obtained that the specimens failed and cracked instantly under quasistatic compression and formed circumferential opening cracks.The high temperature could be generated at the tip of the crack,causing components Al and PTFE to undergo initial reaction.The heat released made ZrH2activate and decompose,generating products Zr and C(charcoal black),which could react continuously at high temperature and generate ZrC.

(4)The impact sensitivity of Al/ZrH2/PTFE specimens decreased with the increase of Al particle size in the drop-hammer test.The larger Al particle size is,the specific surface area is.The diffusion distance between particles then increased and it would be harder for heat transfer,which could reduce the activity of the reactive material.It’s more difficult for materials ingredient to participate in and maintain reaction,resulting in increase of the characteristic drop height and ignition excitation energy.

Author contributions

Y.L.conceived and designed the experiments;J.Z.,J.H.,J.W.and S.W.performed the experiments;Q.L.,Z.G.,S.Z.,and L.Y.analyzed the data;and J.Z.wrote the paper.

Funding

This research was funded by the National Natural Science Foundation of China(No.51673213 and No.51803235).

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgments

The financial support from the National Natural Science Foundation of China(General Program.Grant No.51673213)and the National Natural Science Foundation of China(Youth Science Foundation.Grant No.51803235)are gratefully acknowledged.