Jing-ran Xu,Chen-guang Yan,Chen-guang Zhu

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

Keywords:Pyrotechnics Combustion Flame Boron carbide

ABSTRACT Presented herein is an experimental study on the combustion of B4C/KNO3 binary pyrotechnic system.Combustion products were tested using X-ray diffraction(XRD),scanning electron microscopy(SEM),and energy dispersive spectrometer(EDS).According to the results of tests and CEA calculation,the combustion reaction equation was established.The flames and burning rates were recorded by a high speed camera and a spectrophotometer.The effect of B4C particle size on the thermal sensitivity of B4C/KNO3 was investigated by differential scanning calorimetry(DSC)techniques.In addition,a reliable method for calculating the flame temperature was proposed.Based on the results of experiments,the combustion reaction mechanism was briefly analyzed.The burning rate,flame temperature and thermal sensitivity of B4C/KNO3 increase with the decrease of B4C particle size.The mass ratio of B4C/KNO3 has a great effect on combustion properties.Oxidizer-rich compositions have low flame temperatures,low burning rates,and provide green light emission.The combustion reactions of fuel-rich compositions are vigorous,and the B4C/KNO3 with mass ratio of 25:75 has the highest burning rate and the highest flame temperature.

1.Introduction

In recent years,boron carbide(B4C)has been found to be a pyrotechnic fuel with high calorific value,and its combustion products are low-toxicity and environment-friendly.Natan used highly loaded B4C/HTPB fuel grains in a solid-fuel ramjet motor and achieved high combustion efficiency[1].Sabatini developed a green luminescent agent with B4C/KNO3as the main component,his study showed that the use of B4C as a replacement for both amorphous boron and barium compounds in green-light-emitting pyrotechnic applications is a cost-effective and environmentfriendly approach with acceptable performance[2,3].An advanced and versatile boron carbide-based visual obscurant composition was introduced by Shaw and co-workers[4-7],the formulation composed of B4C as a fuel,KNO3as an oxidizer,and KCl as a combustion temperature moderator was proved to be a pyrotechnic smoke composition with good visible extinction properties,low toxicity and low damage to environment.Shaw also theoretically analyzed the combustion products and adiabatic reaction temperature of B4C/KNO3/KCl systems,but did not give an experimental verification[8].

Although B4C/KNO3has been used as a fuel/oxidizer pair in many pyrotechnic formulations,the combustion products,properties and reaction mechanism of B4C/KNO3have not been reported in an experimental study.The kinetic parameters(activation energy,pre-exponential factor and the reaction mechanism function)of B4C/KNO3ignition reaction were calculated by thermal analysis(TG/DSC)in my previous works[9].In this paper,a series of product detection tests and combustion experiments were carried out,the combustion products and properties such as burning rates,flame temperatures,thermal sensitivity and spectral irradiances of B4C/KNO3were obtained.Then,the combustion reaction mechanism was briefly analyzed,and the effects of B4C particle size and reactants mass ratio on combustion properties were discussed.

2.Experimental

2.1.Materials

Table 1 shows the materials used in the experiments,B4C powders and KNO3powder were respectively purchased from Meryer Chemical Technology Co.,Ltd.and Sinopharm Chemical Reagent Co.,Ltd.

Table 1The parameters of materials.

The object of this study is the combustion of the pyrotechnic fuel/oxidizer pair(B4C/KNO3),so no binder was used in this study.Considering the hygroscopicity of oxidizers,KNO3was ground into fine powders in a mortar and then was dried at 70°C for 8 h.For the materials with larger particle sizes(≥50μm),the uniform particle size distribution was obtained by sieving method.The particle size distribution of ultrafine B4C powders(≤10μm)was observed by SEM,as shown in Fig.1.The average particle size was about 5μm,the error was less than±2μm.

Fig.1.SEM images of ultrafine B4C powders(≤10μm).

2.2.Detection of combustion products

B4C/KNO3composition with minimum particle size and mass ratio of 25:75 was ignited in a combustion chamber and solid combustion products were collected by filtration membrane method.X-ray diffraction(XRD),scanning electron microscope(SEM)and energy dispersive spectroscopy(EDS)were carried out to analyze the combustion products.

The D8 Advance X-ray diffractometer made by German BrukerAXS GmbH was used to analyze the combustion products from 5°to 90°.The Quanta 400 FEG SEM made by American FEI was used to observe the sample with 200-20000 magnification.Quantitative elements detection and mapping using the Edax appllo xl EDS made by American EDAX were applied to analyze the sample.

2.3.Combustion experiments

Based on the chemical reaction equation deduced from the detection of combustion products,the B4C/KNO3compositions with mass ratios of 10:90,15:85,18.55:81.45,20:80,25:75 and 30:70 were chosen for the experimental study.The compositions with different B4C particle sizes and mass ratios were mixed in a mechanical mixer for 30 min before testing.The B4C/KNO3loose powders were placed in a cylindrical cast iron container(d=2 cm,h=2.5 cm)and ignited by a nitrocellulose sliver in the air environment.The mass of the sample in each test was 3 g.The flame was observed by a high speed camera(FASTCAM Mini UX50,Japanese Photron)at a distance of 2 m and a spectrophotometer(OPT-2000, Beijing Normal University, wavelength range 380 nm-780 nm)at a distance of 1 m.According to the flame characteristics,the exposure time of the high speed camera was set to 1/256000 s,and the frame number was set to 500 fps.The test interval of spectrophotometer is set to 100 ms.

Fig.2.XRD result of combustion products.

2.4.DSC measurements

The DSC measurements of B4C/KNO3were carried out with METTLER TOLEDO 3+simultaneous thermal analyzer made in Switzerland.B4C/KNO3samples with a mass of about 2 mg were employed for the measurements.Platinum pans with 50 ml volume were used as sample containers.The samples were measured in the high-purity argon(Ar)atmosphere with flow rate of 50 mL/min.The samples were heated from ambient temperature(30°C)to 1000°C at a heating rate 15 K/min.

Fig.3.SEM images of combustion products.

Fig.4.The element detection of a hexagonal substance by EDS.

3.Results and discussion

3.1.Combustion products of B4C/KNO3

The result of XRD test is shown in Fig.2.There is no sharp diffraction peak in XRD test from 5°to 90°,indicating that there is almost no crystal material in the combustion products of B4C/KNO3.However,some broad diffraction peaks appear in Fig.2,because the arrangement of atoms in the sample material is irregular,X-ray diffuse scattering occurs.This arrangement of atoms accords with the characteristics of amorphous materials,so the combustion products are amorphous,and the information of elements and compounds in the sample cannot be obtained by XRD test.

The SEM images of combustion products are shown in Fig.3.During the process of collection,storage and testing,the combustion products agglomerated and formed larger particles,as shown in the first SEM image.When the magnification reached 20000 times,some white hexagonal substances were found in some areas.After consulting references,it was found that the white hexagonal substances conform to the physical appearance of potassium metaborate(KBO2)[10].KBO2is the chemical equilibrium compound of K2O-B2O3system,K2O+B2O3→2KBO2[11].Therefore,

Fig.5.The element detection of a 100×100μm2 area by EDS.

The surface element detection of a hexagonal substance by EDS are shown in Fig.4.In the surface of hexagonal substance,50.5%(wt.%)K,6.6% B,41.7% O and 1.2% C were detected.EDS also detected Au element which was sprayed on the sample surface for obtaining clear images,so Au was not considered as an element in the sample.The hexagonal substance is supposed to be KBO2because of its appearance.The element mass ratio between K and O is consistent with that of KBO2(39:32)but the content of B is low.Because the inaccurate measurement of elements with small relative atomic mass by EDS instrument,and the measured proportion of B is lower than the real value.Therefore,its surface elements and their mass ratio are basically consistent with KBO2.Fig.5 shows the element detection of a 100×100μm2area.The proportion of C changes obviously from 1.2% to 4.6%.1.2% is proportion of the C attached to the KBO2surface,4.6% is closer to the real mass proportion of C in combustion products.According to the conservation of elements,if all C in reactant B4C is converted into amorphous C,the proportion of amorphous C in combustion products is 4.8%,which is almost equal to the experimental proportion 4.6%.Besides,the proportion of B and O in 100×100μm2area slightly increased.It illustrates that the combustion products contain oxides of B,and the stable oxide of B in condensed phase is B2O3.There no trace of amorphous B2O3in SEM images because it is generally colorless,vitreous and difficult to be identified[12].Thus,it was inferred that the solid combustion products contain amorphous B2O3.the main combustion product of B4C/KNO3is inferred to be KBO2,this inference will be verified in EDS element detection.Considering that no crystal material was detected by XRD,it is indicated that most of KBO2produced by combustion reaction exists in amorphous state.

EDS mapping test was used to verify the content and distribution of K,B,O and C,as shown in Fig.6.The mapping results are consistent with the results of element detection.Obviously,the contents of K and O are higher,followed by B and C.The four elements are evenly distributed in the combustion products.

Based on the minimization of Gibbs free energy theory,NASA CEA Code[13]was used to calculate the chemical equilibrium compositions of B4C/KNO3under the condition of constant pressure and constant enthalpy[14].The combustion of B4C/KNO3with different mass ratio will produce different chemical equilibrium products,as shown in Table 2.The unit of combustion product contents is mass percentage(%).

The combustion products of B4C/KNO3calculated by CEA are generally consistent with the experimental results.The main solid product is KBO2and the main gas product is nitrogen(N2),but there are still some differences between the calculated and experimental results.Because the CEA calculation does not consider the heat loss in actual combustion and the influence of air environment on the reaction.According to the EDS results,the existence of BN can be excluded and most of C element exists in amorphous C rather than CO,CO2.CEA results indicate that,except condensed-phase B2O3,combustion products also contain many metastable oxides of B such as BO,B2O2(in fuel-rich compositions)and BO2(in oxidizerrich compositions).If flammable gases BO and B2O2appear in combustion process,they will be oxidized by the O2in the air with a flame to form B2O3.And BO2will generate green-light emission in the combustion.The existence of BO,B2O2and BO2can be proved by observing the flames using high speed camera in experiments.In addition,CEA results also show that there is K atom in combustion products,but K atom cannot exist stably in the air,the oxide of K(K2O)will react with B2O3to form KBO2.

In summary,according to the results of XRD,SEM,EDS tests and CEA calculation,it can be inferred that the main final combustion products of B4C/KNO3contain KBO2,B2O3,amorphous C and N2.Without considering the trace products,the combustion reaction can be written as Eq.(1)based on the experimental results,although the results may be affected by the air environment.

3.2.Flame temperature measurement method

Common measurement methods for pyrotechnic flame temperature include contact measurement(thermocouple)and optical measurement.Because of the high burning rate of the pyrotechnic compositions in this study,it is difficult for thermocouple to accurately record the flame temperature.Therefore,through the analysis of pyrotechnic flame,a suitable optical temperature measurement method was developed.

In B4C/KNO3composition,B4C has a high melting point of 2763°C and a high boiling point of 3500°C[15].B4C particles cannot evaporate in combustion reaction,many solid particles will be carried into the flame by the produced gases.As a result,the radiation of pyrotechnic flame is very complex,including not only the thermal radiation of condensed-phase particles with high temperature,but also the atomic and molecular radiation of gas combustion.The thermal radiation is only related to temperature,it can be regarded as gray body,obeying Planck’s black body law,and the radiation spectrum is continuous spectrum.While the atomic and molecular radiation generated by gas combustion is linear spectrum and band spectrum.Thus,the radiation spectrum of pyrotechnic flame is a mixture of continuous spectrum and linear(band)spectrum.In this paper,flame temperatures were calculated using the continuous spectrum part of the flame radiation spectrum.

Fig.6.EDS mapping results of K,B,O and C in combustion products.

Table 2Combustion products of B4C/KNO3 with different mass ratio calculated by NASA CEA Code.

Fig.7.The high speed camera image and spectral irradiance curve of B4C/KNO3 flame at a same time.

Fig.8.The flame area in high speed camera.

Fig.9.The spectral radiant exitance curves of the flame and the gray body at 1522.6 °C.

Fig.10.The temperature-time(T-t)curve of the flame.

According to Planck’s law and Kirchhoff’s law,the spectral radiant exitance of blackbodyand gray body(MT，λ)can be given as

Whereλis the wavelength,m,T is the temperature,K,ελis the emissivity atλwavelength,c1is the first radiation constant,c1=3.7418×10-16W·m2,c2is the first radiation constant,c2=1.4388×10-2m·K.

In the visible spectrum range,c2/λT?1,the spectral radiant exitance of gray body can be expressed as follows:

The output signal of the visible spectrophotometer in the experiment is spectral irradiance(ET，λ).

Where l is the distance between spectrophotometer and flame,m,A0is the flame area.The irradiances of the flame with temperature T are ET，λ1and ET，λ2at wavelengthsλ1andλ2.

Logarithm on both sides of the equation:

Two wavelengths are selected in the continuous spectrum part of the mixed spectrum,the emissivities can be considered as the same(ελ1=ελ2),so the experimental flame temperatures can be given as:

The high speed camera image and spectral irradiance curve of B4C/KNO3(25:75)flame at a same time are shown in Fig.7.The spectral irradiance curve illustrates that the flame radiation is continuous spectrum in the wavelength range of 380 nm-750 nm,and there are two linear spectrums at 750 nm-780 nm generated by the radiation of K atom[16].The continuous spectrum was regarded as gray body radiation,two wavelengths(λ1=620 nm,λ2=680 nm)and their irradiances(Eλ1=0.238μW cm-2nm-1,Eλ2=0.469μW cm-2nm-1)were selected in the continuous spectrum,and the flame temperature was calculated by Eq.(9).The calculated flame temperature is 1522.6°C.

The accuracy of the calculated flame temperature was verified by comparing the spectral radiant exitance curves of the flame and gray body.The spectral radiant exitance of flame was obtained by the spectral irradiance curve using the following equation:

l is the distance between spectrophotometer and flame,l=1 m.The flame area A0was obtained by analyzing the high speed camera image.Fig.8(a)shows the average value distribution of RGB channels in the flame image recorded by high speed camera(value range:0-255).The peripheral low radiation area cannot be detected by the OPT-2000 spectrophotometer and was not counted as the flame area.Therefore,the area with RGB values greater than 30 was set as the flame area,as shown in Fig.8(b),A0=0.0071 m2.

The spectral radiant exitance curve of the gray body at the calculated temperature is given by Eqs.(11)and(12)

Fig.11.The high speed camera images of the flames of B4C(5μm,53μm,106μm)/KNO3,(a)5μm,(b)53μm,(c)106μm.

Fig.12.DSC curves of B4C(5μm,53μm,106μm)/KNO3.

Table 3The mass burning rates and average flame temperatures.

Table 4The initial temperature(To),peak temperature(Tp),final temperature(Tf)and the enthalpy values(ΔH)of the samples.

Table 5The mass burning rates and average flame temperatures of B4C/KNO3 with different mass ratios.

As shown in Fig.9,There is a high coincidence between the Mλcurve of the gray body at 1522.6°C and the Mλcurve of the flame(continuous spectrum part).It proves that the calculated value of the flame temperature is quite reliable.The temperature-time(Tt)curve of the B4C/KNO3flame is shown in Fig.10.

3.3.Effect of B4C particle size on combustion properties

The high speed camera images of the flames of B4C(5μm,53μm,106μm)/KNO3compositions with a mass ratio of 25:75 are shown in Fig.11.It is obvious that B4C(5μm)/KNO3has the most vigorous combustion and the largest flame area.The flame of B4C(53μm)/KNO3is smaller and a large number of burning particles appear in the flame.In terms of B4C(106μm)/KNO3,there is no flame in high speed camera images,only some large-size burning particles.The mass burning rates recorded by high speed camera and average flame temperatures calculated by the above method are shown in Table 3.The thermal sensitivity and relative heat release of B4C/KNO3were characterized by DSC tests which is shown in Fig.12 and Table 4.The initial temperature(To),peak temperature(Tp)and final temperature(Tf)all increase with the increase of the particle size of B4C.When the particle size of B4C increases,the heat release decreases obviously.

Fig.13.The experimental flame temperatures and the calculated adiabatic temperatures.

Fig.14.The flames of B4C/KNO3 with a mass ratio of 10:90.

According to the results of combustion experiments and CEA calculation,the combustion reaction mechanism B4C/KNO3can be inferred.At the beginning,KNO3 decomposes into O2,N2and K2O at the ignition temperature.With the increase of temperature,K2O further decomposes into O2and K atoms at high temperature,so K atom radiation was shown in the spectral irradiance curve.Meanwhile,heterogeneous combustion reactions occur on the surface of B4C particles.The outer layer of B4C is oxidized to BO,B2O2and B2O3by O2decomposed by KNO3.B4C with high boiling point cannot evaporate during combustion,solid particles can only be oxidized layer by layer from the outer to the inner,so the smaller particles are easier to burn completely and release more heat.The lack of heat release will lead to a low flame temperature,B2O3with the sublimation point of 1500°C[17]cannot sublime and hinder the combustion reaction of B4C particles.As the gas products leave the surface of B4C,combustion reactions of flammable BO,B2O2occur in gaseous environment and flames appear.At the periphery of the flame,K+O2→K2O,B2O3+K2O→2KBO2,KBO2condenses into smoke particles after leaving the flame area.

3.4.Effect of the mass ratio of B4C/KNO3 on combustion properties

Table 5 shows the mass burning rates and average experimental flame temperatures of B4C/KNO3with mass ratios of 10:90,15:85,18.55:81.45,20:80,25:75 and 30:70.The flame temperatures determined experimentally and the adiabatic temperatures calculated by NASA CEA Code are shown in Fig.13.The experimental flame temperatures are lower than the calculated adiabatic temperatures because much heat lost to the container and surroundings.When the mass percentage of B4C is 15%,the calculated adiabatic temperature reaches the highest value.But the combustion efficiency and adiabatic temperature of the B4C/KNO3with the B4C mass ratio of 18.55% are predicted to be the highest based on Eq.(1).The reason for this difference is that there are some differences between the theoretically calculated products and the detected products,Eq.(1)was established based on the experimental results.In the combustion experiments,the flame reached the highest temperature when the mass percentage of B4C was 25%,because KNO3was not the only oxygen source for the combustion reaction,O2in the air also participated in the reaction to make more B4C release heat.

In the combustions of the oxidizer-rich B4C/KNO3formulations(10:90 and 15:85),the flames were small and green lights were generated,as shown in Figs.14 and 15.With the increase of oxidizer content,the green-light radiation became stronger.The spectral irradiance curve of a green flame is shown in Fig.16.Compared with the flame of B4C/KNO3(25:75)in Fig.7,the spectral irradiance was greatly reduced.In addition,there is molecular radiation in 450 nm-600 nm(green light band).It is shown in Sivan’s study that the molecular radiation in this band range is provided by BO2molecule[18].Because excessive oxidizers react with B4C to form BO2molecule in the oxidizer-rich formulation.However,insufficient fuel will result in a low heat release,a low flame temperature and a low burning rate.

Fig.15.The flames of B4C/KNO3 with a mass ratio of 15:85.

Fig.16.The spectral irradiance curve of a green flame.

The flame images of the zero-oxygen-balance(18.55:81.45)and fuel-rich(20:80,25:75 and 30:70)B4C/KNO3compositions are shown in Fig.17.There is no green flame in the combustion processes of these compositions.Compared with the oxidizer-rich and zero-oxygen-balance formulations,the fuel-rich B4C/KNO3have more vigorous combustion reactions.The B4C/KNO3with mass ratio of 25:75 has the highest burning rate and the highest flame temperature.Because O2in the air can participate in the combustion reaction,more B4C was oxidized and more heat was released.However,the amount of O2in the air which can participate in the reaction is limited,if the amount of B4C is too large,the combustion efficiency will decrease.

4.Conclusion

The results of XRD,SEM,EDS tests and CEA calculation were analyzed,the main combustion products of B4C/KNO3were inferred to be KBO2,B2O3,amorphous C and N2,and the combustion reaction equation was established.Based on the analysis of the radiation properties of the flames,a method for calculating the flame temperature using two wavelengths and their irradiance data in the continuous spectral part of the spectral irradiance was proposed.The calculated flame temperature was reliable.With the decrease of B4C particle size,the heat release,burning rate,flame temperature and thermal sensitivity of B4C/KNO3increase significantly.The mass ratio of B4C/KNO3has a great effect on combustion properties.Oxidizer-rich compositions have low flame temperatures,low burning rates,and provide green light emission.The combustion reactions of fuel-rich compositions are quite vigorous,and the B4C/KNO3with mass ratio of 25:75 has the highest burning rate and the highest flame temperature.Binders are also an important part of pyrotechnic composition in practical application,future work will study the effect of different binders on the combustion properties,so that B4C/KNO3can be used in industrial production.

Fig.17.The flames of zero-oxygen-balance and fuel-rich B4C/KNO3.(a)18.55:81.45;(b)20:80;(c)25:75;(d)30:70.

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.

Acknowledgement

The support for this work was provided by the National Natural Science Foundation of China(Project No.51676100).