Burning surface formation mechanism of laser-controlled 5-aminotetrazole propellant

Nian-bai He ,Rui-qi Shen ,Luigi T.DeLuca ,Li-zhi Wu * ,Wei Zhang ,Ying-hua Ye ,Yue-ting Wang

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

b Institute of Space Propulsion, Nanjing University of Science and Technology, 210094, Jiangsu, China

c Sichuan Huachuan Industries Corporation, 610106, Sichuan, China

d Space Propulsion Laboratory (Ret), Department of Aerospace Science and Technology, Politecnico di Milano, 20156, Milan, Italy

Keywords:5-Aminotetrazole (5-ATZ)Laser-augmented chemical propulsion(LACP)Combustion mechanism Burning surface Micro computed tomography (MicroCT)

ABSTRACT As an innovative propulsion technique,combustion mechanism of laser-augmented chemical propulsion has still to be ascertained.Benefiting from high nitrogen content and thermal stability,5-aminotetrazole is a suitable ingredient for LACP.Under a flowing nitrogen environment,two kinds of unique burning surfaces were observed to occur for 5-ATZ,used as a single reacting propellant ingredient with the addition of carbon,under laser ablation.Both surfaces are hollow structures and differ by the possible presence of edges.Using micro computed tomography,the 3D perspective structures of both surfaces were revealed.Resorting to various characterization methods,a unified formation mechanism for both surfaces is proposed.This mechanism specifically applies to laser ablation,but could be crucial to common burning mechanisms in LACP.

1.Introduction

Benefiting by high safety and precision,laser-induced ignition has been commonly applied to propellants and explosive [1-4].Owing to the continuous assistance of an external radiation source,this innovative propulsion technique reveals superior to the traditional ones,in that laser power allows combustion of non-selfsustaining propagation waves.However,new combustion characteristics are specifically required for applicable propellants.For example,the combustion products must be clean enough to avoid energy loss over the laser path,and the energetic formulation must permit sustained combustion only under laser impingement while it should rapidly extinguish without radiation.On the 6th European conference for aeronautics and space sciences [5] in 2015,Shen suggested “l(fā)aser-augmented chemical propulsion (LACP)” as the name of this propulsion technique and proposed 5-aminotetrazole(5-ATZ)as an applicable ingredient of LACP propellant.Fig.1 shows the chemical structure formula of 5-ATZ[6].The percent of nitrogen in tetrazole is the highest among all organic compounds,and in particular 5-ATZ features a content of 82.3 wt%.For traditional composite or double-base propellant products,massive black smoke exists among the incomplete combustion products,due to the high content of carbon in the propellant formulations.Using 5-ATZ,the amount of black smoke was significantly restrained in the gas products of the propellant combustion.Besides,5-ATZ is commonly used as gas generating composition due to its high thermal stability and non-self-sustaining combustion feature.

Fig.1.Chemical structure formula of 5-ATZ.

In traditional chemical propulsion,systematic burning research of single ingredients is necessary before applying them as components in any propellant formulation [7].In the early research of LACP,Shen’s team studied a propellant manufactured by pressing mixed powders of 5-ATZ and carbon [8].Surprisingly,unique bulges invariably occurred on all propellant surfaces for laser ablation under nitrogen flow,and the Bulge Surface structure had not been reported in former research.Despite the preliminary nature of investigations,experimental results showed that such a kind of burning surface causes unsteady combustion.

Because of the lack of applicable measurements and open literature information,subsequent research activities mostly focused on combustion performance of 5-ATZ based composite propellants.After testing various oxidants,binders,and other additives in the formulation,it was possible to eliminate the Bulge Surface structure from 5-ATZ based propellants during combustion.Nevertheless,combustion mechanism was rarely mentioned in subsequent researches,or indiscriminately applied to the developed mechanism of traditional propulsion,with the simplification of laser radiation as external heating.Changing formulation had significantly improved the burning performance of 5-ATZ based propellants,but the side effect was a weaker or missing combustion controllability by laser ablation,and required remedies such as adding combustion inhibitor or applying excessively negative oxygen balance.

Such a kind of setback was caused by imprudent attempts to modify the combustion characteristics without seeking the true reaction mechanism.Without oxidant,the chemical reaction of 5-ATZ in nitrogen should be regarded as decomposition by laser ablation instead of proper combustion,and hence the systematic ingredients research conducted for traditional propellants might not be applicable.

Additionally,several burning characteristics of other types of solid propellant were hard to explain with application of an external laser source.In a former study[9]of laser assisted burning of double-base solid propellants with back pressure of 0.6 MPa,burning rate decreased with the increasing of laser power in the range 0.3-0.7 W/mm2.Such dependence was against common expectation that burning rate should increase with higher power laser ablation,because of reaction rate on burning surface augmented by external heat from radiation.In another former study [10] of composite propellants made of hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP),all burning rates were found to decrease with the environment pressure increasing above 100 kPa,under laser power densities of 0.35,0.45 W/mm2,0.65 W/mm2,and 0.80 W/mm2.Normally,burning rates of such composite propellants increase with higher environment pressure without external radiation[11-15],according to the Vieille’s (or the Saint Robert’s law) [16,17] commonly applied for chemical propulsion.Presumably,the combustion mechanism of LACP might be quite different from traditional chemical propulsion.

Even though the laser ablating reaction mechanism of 5-ATZ propellant might be distinct from that of 5-ATZ based composite propellants,the unique Bulge Surface structure was an associated outcome of laser ablation and clean fuel component,which are both crucial to LACP.Studying 5-ATZ as a single ingredient could provide fundamental knowledge for combustion mechanism of 5-ATZ based composite propellants,and even for LACP.For simplicity,in this paper the reaction of 5-ATZ propellant ablated in nitrogen by laser is also called combustion or burning.

2.Experimental

2.1.Propellant preparation

Powders of 5-ATZ and carbon were mixed evenly,with mass ratio of 19:1.The component 5-ATZ was grinded by ball-milling and sieved to particles size between 75 μm and 150 μm.Grinded 5-ATZ looks like a white powder and scarcely absorbs laser energy,hence carbon powder is added as a laser absorption agent.Carbon was purchased as micron-sized black particles in the range 10-40 μm.Without any oxidant,oxygen balance does not apply for this formulation.After completely drying the mixed powders at 80°C,crystal water was removed from 5-ATZ.The mixed powders were pressed into cylindrical propellant samples of 6 mm diameter under 500 kg pressure.The pressed samples so obtained feature a length of 12 mm±50 μm and an average density of 1.45 g/cm3.In this paper,if not otherwise stated,“propellant”refers to the 5-ATZ single reacting ingredient modified by the addition of carbon.

2.2.Combustion system

Combustion of pressed propellant samples was tested in a pressurized combustor under a steady nitrogen flow and no oxygen.Samples were ablated by operating a semiconductor laser in CW mode,with wavelength of 808 nm and collimating beam of 6 mm diameter.The combustor was made of stainless steel with a theoretical maximum operating pressure of 2 MPa.The internal volume of combustor was approximately 500 cm3,which was large enough for keeping a steady nitrogen flow during combustion.Fig.2 shows a brief schematic of the experimental system.Burning tests were carried out in the range of pressure 50-400 kPa and laser power 3-10 W.

Fig.2.Schematic of the experimental combustion system.

2.3.MicroCT measurement and 3D reconstruction

As a non-destructive measurement,micro computed tomography (MicroCT) has commonly been applied for medicine and biology research [18,19].Benefiting by the high penetrability of Xray,the samples under test were scanned with various rotation angle by MicroCT,and then the 3D perspective reconstructed by image software.In a companion study [20],the same 5-ATZ propellant burns as single ingredient under different conditions,and Fig.3 shows the intact structures of propellant remains revealed by MicroCT without destroying burning surfaces.

Fig.3.Profiles of burning surfaces on propellant remains by MicroCT reconstruction.Abscissa indicates the nitrogen pressure and ordinate indicates the impinging laser power.All propellant samples were ablated by laser for 120 s under different combustion conditions,and analyzed by MicroCT after natural cooling down [20].

In the combustion experiments of Fig.3,unique burning surfaces formed on every propellant sample,and under higher environment pressure most burning surfaces were different because of an extra structure appearing on the edge.This kind of burning surface was not observed in former research activities.The reason might be the fivefold average mass density of pressed propellant achieved by improving propellant manufacture in this research.Besides,the structural difference was not always controlled by environmental pressure.For example,no extra edge structures occurred under test conditions of 350 kPa at 9 W and 300 kPa at 10 W in Fig.3.

2.4.Two kinds of unique burning surface

To avoid misunderstanding,two kinds of burning surface structure-Bulge Surface and Edge Extension Surface-were named based on the observed morphology.The corresponding structures are shown in Fig.4.

Fig.4.The two kinds of burning surface configurations under investigation(a)and(c)and their corresponding structures(b)and(d)identified by MicroCT reconstruction.Pictures(a) and (b) respectively present 3D and axial sectional views of Bulge Surface under test condition of 100 kPa and 10 W.Pictures (c) and (d) respectively present 3D and axial sectional views of Edge Extension Surface under test condition of 400 kPa and 4 W.The regions and corresponding customized definitions of both structures are labeled with different colors in (b) and (d).

Regions of both burning surface structures were identified according to MicroCT reconstruction.The irregular bottom of propellant remains was caused by cracking during sample transportation.A peripheral sample holder was used to clamp propellant remains during MicroCT scanning.Except for “extensional edge”,all other regions exist in both burning surfaces.With reference to Fig.4,“burning surface” is the major reaction zone of 5-ATZ decomposition.“Shell” is the extra structure covering the“burning surface”.“Boundary vesicles” are numerous miniature cavities gathering at the boundary of the “shell” and “burning surface”.“Central cavity” is embraced by the “shell”,“burning surface” and “boundary vesicles” regions and is filled with the gaseous products produced during laser ablation.“No reaction zone”is the propellant portion underneath burning surface with no active chemical reaction(unreacted propellant).“Extensional edge”is the main distinction between the two surface structures,which is located on the periphery of the edge extension structure and oriented in the opposite direction with respect to the impinging laser beam.

2.5.Component tests

To find out the relationship between the two burning surfaces,the first step is to investigate the nature of the components of each region.

2.5.1.Testing samples preparation

Samples for most component tests were required as dried grinded powders,but the quality of single burning surface structure was inadequate.To prepare the testing samples,two groups of propellants were ablated under appropriate combustion conditions to obtain the two kinds of burning surface.The first group contained 30 samples ablated 120 s by 10 W laser under nitrogen at 50 kPa,and provided 30 Bulge Surface structures.The second group contained 30 samples ablated 120 s by 3 W laser under nitrogen at 400 kPa,and provided 27 Edge Extension Surfaces,with three Bulge Surfaces disregarded.

The diameter of each surface structure was approximately 6 mm,which was small and made hard to separate each region precisely according to the definition illustrated in Fig.4.The samples practically used for component tests approximately complied with the indications in Fig.5.

Fig.5.Indications of practical testing samples of both burning surface structures.In each image,3D structure on the left corresponds to region in green frame of axial sectional view on the right.(a) and (b) indicate different regions of Bulge Surface,while (c) and (d) indicate different regions of Edge Extension Surface.

For the 30 Bulge Surfaces,shells were separated and grinded to prepare testing Sample (A),as illustrated in Fig.5(a).Due to multitudinous boundary vesicles between shell and burning surface,the shell could be peeled easily.Testing Sample (B) was supposed to be the burning surface of Bulge Surface.However,the burning surface was very thin,and difficult to fully separate from adjacent areas.The actual testing Sample (B) was grinded by 30 Bulge Surfaces after removing shells and most no reaction zones,which contained burning surfaces,boundary vesicles and no reaction zone,as illustrated in Fig.5(b).

For the 27 Edge Extension Surfaces,extensional edges were easily separated due to the independent spatial structures,and grinded to prepare testing Sample(C),as illustrated in Fig.5(c).The shell in Edge Extension Surface was flat and hard to peel.Hence,the actual testing Sample (D) was grinded from 27 Edge Extension Surfaces after removing extensional edges and most no reaction zones,which contained shells,burning surfaces,boundary vesicles and no reaction zones,as illustrated in Fig.5(d).

Testing sample (E) was grinded from the no reaction zone of propellant remains,instead of initial mixed powders before propellant pressing,to maintain comparability as much as possible with other testing samples.

Five grinded samples were fully dried at 50°C to meet test standards.Considering that the propellant powders had been dried at 80°C before pressing,and the combustion of propellant did not produce vapor with no oxidant at the ablation temperature much higher than 50°C,components of combustion products would not be changed by drying grinded samples before testing.

All component tests in Section 2.5.Component tests were fetched from the same grinded samples following the aforementioned preparation.

2.5.2.FTIR and Raman spectrum

FTIR and Raman spectrum are both used for identifications of functional groups in molecular structure.Functional groups usually peak at same wave numbers in both spectrums.Fig.6 shows FTIR and Raman spectrum of five testing samples.

Fig.6.FTIR and Raman spectrum of five samples: (1) Sample (A);(2) Sample (B): (3) Sample (C);(4) Sample (D);(5) Sample (E).

Since carbon powders featured no peaks in both spectrums,spectral features of Sample(E)were identical to 5-ATZ in FTIR[21]and Raman spectrum [22].In Fig.6(a),Sample (A) featured no obvious peak in FTIR,while featured similar peaks as Sample (E)between wave number of 500 cm-1and 1700 cm-1in Ramen spectrum.Spectral features of Sample (B),(C) and (D) accorded to Sample (E) in both spectrums according to Fig.6(b).

2.5.3.Elemental analysis(EA)

By collecting the gaseous products from the testing samples fully burned in pure oxygen,EA can calculate the mass percent of organic elements.Table 1 shows the EA results of five testing samples.

Table 1 EA results of five testing samples.

In Table 1,the sum of elements N,H and C were all insufficient to reach 100 wt%.Considering the elemental composition of the initial propellant and combustion environment,no other elements should be contained in testing samples.Testing sample might have absorbed a bit of water vapor during transportation.If so,moisture absorption of Sample (A) was much weaker than other samples.The theoretical percent of elements C,H and N was then recalculated by eliminating the possible presence of water vapor.The results obtained after this correction are shown as bar scale diagram in Fig.7.

Fig.7.Theoretical element percent of testing samples by EA,after elimination of possible vapor.

Compared to Sample (E),the theoretical percent of elemental carbon increased by different levels in the remaining four testing samples,which indicates that the reaction degree of carbon powder was much lower than 5-ATZ in the propellant combustion.

2.5.4.Thermal analysis

Thermogravimetry (TG) and differential scanning calorimetry(DSC) were used to analyze thermal performance with increasing temperature.Fig.8 shows the DSC-TG curves of five testing samples.

Fig.8.DSC-TG curves of five testing samples:(a)Sample(A);(b)Sample(B);(c)Sample(C);(d)Sample(D);(e)Sample(E).Curve(1)and(2)in each graph respectively represent TG and DSC results.Heating rate was 10 K/min,and heating environment was high purity nitrogen.

The DSC curves of five testing samples all featured one endothermic peak in the lower temperature range and one exothermic peak in the higher temperature range.Table 2 and Table 3 respectively show DSC and TG features of Fig.8.

Table 2 DSC features of five testing samples.

Table 3 TG features of five testing samples.

In Fig.8(b)-Fig.8(e),TG curves of testing Samples (B),(C),(D)and(E)featured two weight dips,a rapid dip at lower temperature and a slow dip at higher temperature,while Sample (A) featured only one weight dip at higher temperature in Fig.8(a).In literature,the melting point of anhydrous 5-ATZ approximates 205°C,which is slightly lower than the thermal decomposition temperature[23].The thermal performance of Sample (E) was similar,where the initial melting temperature (Te1) was also 205°C in Table 2,and initial decomposition temperature (Te3) was 213.9°C in Table 3,shortly after the melting temperature.Between 200°C and 260°C,the thermal reactions of the five testing samples could be considered as the phases of melting and primary decomposition of 5-ATZ,with little or no participation of carbon powders.After respectiveTc3,5-ATZ kept on decomposing in each testing samples over a wide range of temperature.However,the exothermic peak ofTc2at higher temperature in Table 2 should not occur in the thermal performance of pure 5-ATZ and might point out reactions with carbon powder.

Based on thermocouple measurements,the highest temperature over the entire burning surface was 420°C during propellant combustion under 10 W laser power[24].Considering thatTe4of all testing samples were much higher than 420°C,the undetermined exothermic reaction did not affect the combustion process during ablation.

2.5.5.Comprehensive analysis

For Samples(B)and(D),testing results of through FTIR,Raman,and thermoanalysis were all very close to those of Sample (E).According to Fig.4 and Fig.5,The main reason might be the high proportion of no reaction zone in the testing samples,which resulted in Samples (B) and (D) being not precise enough for quantitative analyses.

Surprisingly,all testing results of Sample(C)were also very close to those of Sample (E),but Sample (C) was prepared by the independent structure of the extensional edge,which did not contain no reaction zone as Samples (B) and (D).

In experimental measurements,the gaseous product of 5-ATZ were usually tested with an oxidant or oxidizing environment.The principle of gaseous product testing is similar to EA,by collecting different complete burning gaseous products of testing sample.The decomposition of 5-ATZ in absence of oxygen conditions is very complex [23,25-27],and various theoretical mechanisms were proposed based on thermal kinetics,where N2,HN3and NH3were determined as main gas products [27].Therefore,in Fig.7,the carbon percent in Sample (A) being significantly larger than Sample (E),was not only caused by the presence of carbon powders in the formulation,but also by the larger amount of carbon left in the solidification remained after 5-ATZ decomposition.

According to Fig.8 and Table 2,Sample(A)still featured a small endothermic peak in DSC around 323.7°C,while △H1approximates a quarter of that of Sample(E).Small quantities of 5-ATZ with no or slight decomposition might still exist in the shell.Correspondingly,the features of sample A in FTIR and Raman spectrum could be explained.Due to the difference of testing principle,asymmetric vibrations of polar groups are more obvious in FTIR,while symmetric vibration of non-polar group and molecular framework are significant in Raman spectrum.A common theoretical mechanism [25] of 5-ATZ decomposition under absence of oxygen conditions claims that the tetrazole ring is the first molecular structure to collapse with temperature rising.After losing the tetrazole ring of 5-ATZ,the solidification remains are a complex mixture of multiple compounds with new polar groups,seriously interfering feature peaks in FTIR.But Raman spectrum was interfered less with rarely new polar groups generating.In Raman spectrum of Sample (A),peaks between wave number 1700 cm-1and 500 cm-1were identified as the symmetric vibration of tetrazole ring in 5-ATZ,at 1674 cm-1,1064 cm-1,741 cm-1and 537 cm-1.

According to Table 3,dw4of Sample (E) was 34.92 wt% of total weight,but reached 76.13 wt% of residual weight withdw3eliminated from total weight,thus approximatingdw4of Sample (A).

In summary,all results support the conclusion,that the shell of the Bulge Surface structure consists of solidification remain of 5-ATZ after thermal decomposition.

2.6.SEM and EDS

Although the shell of Bulge Surface structure was verified as a solidification remain,other regions of both burning surface structures still required further testing with higher pertinence.Using scanning electron microscope (SEM),morphology of scanning sample could be characterized.The elemental distribution on the scanning surface could be measured by energy dispersive spectrometer(EDS) in coordination with SEM.

To prepare testing samples of SEM and EDS,two extra propellant remains were experimented by 120 s of laser ablation in nitrogen flow,where one Bulge Surface was obtained with 10 W laser power and 50 kPa environmental pressure,while one Edge Extension Surface was obtained with 3 W laser power of 3 W and 400 kPa environmental pressure.To reduce the impact of water vapor in EDS,the two propellant remains had been fully dried at 50°C in advance.

Fig.9 indicates all scanning positions from both burning surfaces with corresponding SEM morphologies 300 times magnification.Table 4 shows the meanings of each scanning position in Fig.9.Fig.10 shows SEM images of all scanning positions with three different magnification times.Table 5 shows the elements proportions by EDS.

Table 4 Meanings of scanning positions.

Table 5 Elements proportions of all scanning positions by EDS mapping.

Fig.9.SEM scanning positions from both burning surfaces with corresponding SEM morphologies of 300 times magnification: (a) external side of shell in Bulge Surface,where scanning directions of Positions ①and ②were the same;(b) Internal side of shell in Bulge Surface,where scanning directions of Positions ③and ④were the same;(c) Burning surface of Bulge Surface,where Position ⑤was scanned after the shell removal;(d)Edge Extension Surface,where scanning directions of Positions ⑥,⑦and ⑨were all different and labeled.

Fig.10.SEM images of nine scanning positions with three different magnification factors.Cracks in SEM images were caused by transportation of testing samples,not by laser ablation.Scanning positions of the three different magnifications remained the same,except for slightly adjustment to Position ④for better EDS position.

Scanning positions ①to ⑧were marked in Fig.9.Scanning position ⑨was selected from an initial pressed propellant for comparison.

EDS mapping of each position was scanned with the same area as SEM image of 600X magnification in Fig.10.Value of N:C is the ratio between elements N and C.

Limited by testing principle,results of elemental composition by EDS are not accurate as EA,and elements like hydrogen and helium cannot be detected in EDS.EDS mapping only showed the elements N,C,O and Au.Element Au was added in testing sample to endow electrical conductivity for electron microscope scanning.Trace element O was detected in positions ①and ⑧,but too little to quantify.According to section titled “component tests”,decomposition of 5-ATZ causes an increasing percent of element C in solidification remain,therefore the value of N:C could indicate the reaction extent at the scanning position.With the value of N:C closer to Position ⑨of 4.087,the reaction extent of the corresponding position was lower and closer to that of the initial surface of pressed propellant with no burning reaction nor laser ablation.

Morphologies were very similar for Positions ①and ⑧.The external sides of shells in both burning surfaces were uniform and compact.The N:C values of Positions ①and ⑧were close,and both much lower than Position ⑨.

Observing the shell of bulge surface with eyes,the color of central area was darker than fringe area.Sometimes,a tiny hole was left in the middle of shell ablated through by laser,as shown in Fig.11.Laser energy received in the middle area of the shell is much higher than that at the border,due to the Gaussian energy distribution of the laser beam.Comparing morphologies between Position ①and ②,distinct wavy shapes appeared from center to fringe on external side of shell.The N:C value of Position ①and ②were respectively 1.700 and 2.073,which indicates that the reaction extent of central area was the highest on external side of shell,and the entire shell had reacted with relatively higher extent.

Fig.11.Picture by camera of Bulge Surface with a tiny hole left on the middle shell.

Comparing Position ①,morphology of Position ③was coarse with crumples and pits.The N:C values of Position ①and ③were the two highest among all scanning positions.The reaction extents on both sides of shell in central area were close,while the obvious discrepancy of morphologies related to different gaseous situations separated by shell.Gaseous situation on the shell external side was the steady nitrogen flow in combustor.Ablated directly by laser,scorched external shell with moderate liquidity formed into the shape of spherical surface with minimal surface tension.On the shell internal side,gas products and incidental solidification products of 5-ATZ decomposition filled the central cavity.Profiles in Fig.3 shows the complexity of central cavity,and sometimes only numerous diminutive cavities filled between shell and burning surface instead of one major central cavity,such as the sectional images with conditions of 150 kPa at 5 W and 250 kPa at 3 W.Therefore,the gaseous situation on the shell internal side must be complicated and unsteady during laser ablation.

Morphology of Position ⑤showed obvious change of actual burning surface from the initial surface of the pressed propellant,as Position ⑨.After laser ablation,granules on initial propellant surface were melted and became flat with embossment as bird nest.The N:C value of Position ⑤was slight lower than Position ⑨.Since the decomposition of 5-ATZ starts on the burning surface,the reaction extent is relatively low.

Fig.12 shows two different morphologies on Position ⑥,where the area far from burning surface was closer to Position ⑦,while the area adjacent to burning surface was similar to the morphology of Position ⑤.Burning surface of Edge Extension Surface has not been scanned in SEM,due to the difficulty of removing the smaller shell.Considering the same reactants of both burning surfaces,and SEM morphologies comparability of both surface shells,morphologies of both burning surfaces were probably similar.As to the Edge Extension Surface,the internal side of extensional edge processed the combination morphology of external side and burning surface.Among all scanning positions,the N:C values of position ⑥and ⑦were the closest to position ⑨.

Fig.12.Relevance among morphologies of positions ⑤,⑥and ⑦.The three scanning images were the same as the corresponding positions of 600X magnification in Fig.10.

3.Mechanism

3.1.Formation mechanism for bulge surface

Based on all the previous test results and subsequent conclusions,the formation mechanism of Bulge Surface could be established as shown in Fig.13.

Fig.13.Schematic of formation mechanism for Bulge Surface.

When the laser power is turned on,5-ATZ propellant begins to decompose on the ablated surface.Under absence of oxygen conditions,carbon powder can barely react and remains above the burning surface,along with the solidification products of 5-ATZ decomposition.The gas products from subsequent decomposition gather beneath solidification remain.Gradually,gas products form the central cavity,and solidification remain forms the shell.

Even though a tiny hole on the shell,as the one evidenced in Fig.11,was not definitely observed for all propellant remains,this configuration of laser ablating through the shell was common.Since all propellant remains were collected after cooling down,the hole might recover due to moderate liquidity of the scorched shell after laser ablation.SEM results show the compactness on the shell.As reaction extent increases with ablation time,the only gas passage left on the shell is the middle hole ablated through by the impinging laser beam.As gaseous products compress inside the central cavity and cause a tremendous pressure difference between the two shell sides,the loosest part could form a new gas passage.According to Table 5,the reaction extent of Position ④is the lowest,therefore boundary vesicles are formed on marginal shell area with a slight gas generation.

3.2.Formation mechanism for Edge Extension Surface

According to Fig.3,the Edge Extension Surface only,but not inevitably,occur on 5-ATZ propellant with environment pressure higher than 300 kPa.Based on the test results previously discussed,the formation mechanism of Edge Extension Surface is similar to that of Bulge Surface,with the extra extensional edge related to higher environment pressure.The formation mechanism is illustrated in Fig.14.

Fig.14.Schematic of formation mechanism for Edge Extension Surface.

During the initial stage of laser ablation,decomposition reaction and surface structure are the same for both kinds of burning surface.With the central cavity enlarging,the volume of gas products is limited by the high environment pressure.Due to the Gaussian distribution of laser energy,the gas production rate on the edge of burning surface is not enough to prop up the shell.Gradually,burning surface on the edge area moves beside the shell.Therefore,the shell is smaller in Edge Extension Surface and only exists in central area where laser ablation is the highest.

As mentioned before,the gaseous situation in the central cavity during laser ablation is complicated and might attach to the decomposed solidification products.Occasionally,higher gas pressure beneath the shell might counteract the volume limitation of central cavity by higher environment pressure.Therefore,the Edge Extension Surface only occur with environment pressure higher than 300 kPa,but not necessarily.

Comparing extensional edge to no reaction zone,testing results of EA and EDS both showed similar element proportions,and even identical functional groups were featured by FTIR and Raman spectrum.Convincingly,the extensional edge was an extending structure of the initial propellant with no or slight chemical reaction.

However,the extensional edge could not be treated simply as a marginal remain after propellant decomposition in the central area,because the propellant remain including edge extension was normally higher than the initial pressed propellant.Besides,shapes and angles of extensional edges vary among Edge Extension Surfaces according to Fig.3.

Based on SEM images in Fig.10,morphology of Position ⑦is different from Position ⑨,indicating that the external side of extensional edge has experienced moderate deformation from the initial pressed propellant.But the temperature of extensional edge is not enough for 5-ATZ decomposition,because morphology of Position ⑦is also different from the actual burning surface as Position ⑤.Passing by gaseous products with high temperature ejecting through boundary vesicles,extensional edge was reformed into an irregular shape with thinner and higher features.

The N:C values of Position ⑥indicates that the average reaction extent of the internal side of extensional edge is between external side and burning surface.In addition to the analysis of two different morphologies on Position ⑥in Fig.12,the internal edge area adjacent to burning surface has been part of burning surface,but moved outside of shell because of the central cavity volume limited by high environment pressure.The internal edge area far from burning surface is the outcome of moderate deformation from the external edge.

3.3.Unified formation mechanism for both surfaces

In summary,the formation mechanisms of both burning surfaces can be unified.Both unique surfaces are formed by solidification and gaseous products of 5-ATZ decomposition.Under external laser radiation,solidification products with high reaction extent remained above the actual burning surface,and retained adequate gaseous products beneath.Promoted by the higher environment pressure,a moderate deformation of the unreacted propellant on the burning surface edge causes the difference of extensional edge.

3.4.Correlation to combustion mechanism for LACP

In this research,5-ATZ was studied as a single decomposed ingredient with carbon powder under laser absorption,and the burning mechanism is presumably different from the composite propellant formulations including oxidizers also used for LACP.

As an innovative propulsion technique,research activities of LACP were not nearly adequate nor universal.For instance,a companion study [20] with the same propellant with 5-ATZ burns as single ingredient suggested that,under certain combustion condition,its burning rates significantly reduce with burning time,which is predictable since the laser energy received on the actual burning surface decreases along with the higher reaction extent of the shell.On the contrary,burning rate keeps constant under a relatively low nitrogen pressure with a relatively strong laser ablation,which indicates that the shell of burning surface may not invariably be an obstacle.

Based on the unified formation mechanism of burning surfaces,analogous structure will never occur for solid propellants commonly used in traditional propulsion,because combustion would quickly be terminated with solidification products massively gathered above the actual burning surface.Both unique burning surfaces are the specific outcome combining external laser radiation and applicable propellant ingredient,pertinent to LACP.The formation mechanism of burning surface in this research might be an inspiration source for combustion mechanisms of LACP.For instance,even with no obvious special surface of the propellant sample,the reaction areas of combustion might become multiple with extra laser radiation,instead of the single burning surface observed in traditional propulsion.

4.Conclusions

By studying two kinds of unique burning surface structure,respectively called Bulge Surface and Edge Extension Surface,a unified formation mechanism is proposed.Both structures occur in 5-ATZ burning as a single reacting ingredient with the addition of carbon under laser ablation in absence of oxygen environment.As verified by component tests and morphology characterization,both burning surface structures are formed by gaseous and solidification products of 5-ATZ decomposition,while the difference between the two structures is caused by a moderate deformation from the unreacted portion of the propellant sample.The unified formation mechanism of burning surface structures is based on research activities conducted for 5-ATZ,but it could be a source of inspiration to further and wider investigations on LACP.

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.

Acknowledgements

Thiswork was supported by the Shanghai Aerospace Science &Technology Innovation Fund (Grant No.SAST201363),and also the Fundamental Research Funds for the Central Universities(Grant No.30919012102 in part).