Simplified quantitative analysis method and its application in the insitu synthesized copper-based azide chips

Jie Ren, Yunfeng Li, Mingyu Li, Xingyu Wu, Jiabao Wang, Qingxuan Zeng

State Key Laboratory of Explosion and Technology, Beijing Institute of Technology, Beijing 100081, China

Keywords:Copper-based azide chips Spectrophotometry Separation method Quantitative analysis Ignition ability

ABSTRACT Copper-based azide (Cu(N3)2 or CuN3, CA) chips synthesized by in-situ azide reaction and utilized in miniaturized explosive systems has become a hot research topic in recent years.However, the advantages of in-situ synthesis method, including small size and low dosage, bring about difficulties in quantitative analysis and differences in ignition capabilities of CA chips.The aim of present work is to develop a simplified quantitative analysis method for accurate and safe analysis of components in CA chips to evaluate and investigate the corresponding ignition ability.In this work, Cu(N3)2 and CuN3 components in CA chips were separated through dissolution and distillation by utilizing the difference in solubility and corresponding content was obtained by measuring N3- concentration through spectrophotometry.The spectrophotometry method was optimized by studying influencing factors and the recovery rate of different separation methods was studied, ensuring the accuracy and reproducibility of test results.The optimized method is linear in range from 1.0-25.0 mg/L, with a correlation coefficient R2 = 0.9998, which meets the requirements of CA chips with a milligram-level content test.Compared with the existing ICP method,component analysis results of CA chips obtained by spectrophotometry are closer to real component content in samples and have satisfactory accuracy.Moreover,as its application in miniaturized explosive systems,the ignition ability of CA chips with different component contents for direct ink writing CL-20 and the corresponding mechanism was studied.This study provided a basis and idea for the design and performance evaluation of CA chips in miniaturized explosive systems.

1.Introduction

Combining miniaturized explosive systems with micro-electromechanical systems (MEMS) technology possesses attractive applications in the military field, which leads to a large number of reports on emerging technologies such as direct ink writing explosives [1,2], in-situ synthesized primary explosives [3-6] and MEMS safety and arming(S&A)device[7,8].As the initial igniter in miniaturized explosive systems, in-situ synthesized primary explosive chips, typified by copper-based azide [6] (Cu(N3)2or CuN3, CA) chips owing to its dual advantage of outstanding initiating ability,has become a hot research topic in recent years[9-15].Copper azide(Cu(N3)2)and cuprous azide(CuN3),which are usually synthesized by the metathetical reaction of soluble copper salts and sodium azide in the past study,are hardly utilized in practice due to their high sensitivity [16].Compared with the traditionally direct synthesis method, in-situ synthesized CA chips are usually obtained through gas-solid phased and electrosynthesis azide reactions with nano-copper.In-situ synthesis methods significantly reduce the possibility of contact with the operator and the risk of accidental explosion and meet the requirements of miniaturized explosive systems including small size, low dosage and safety assembly.In the literature reported so far, CA chips only need about 1 mg dosage and the size of φ1.0 mm × 0.55 mm to initiate the secondary explosive used in miniaturized explosive systems[5,17-19].

However, the in-situ synthetic method of CA also brings uncertainty in the compositions and ignition ability of reaction products while improving security.Taking the most reported gassolid reactions as an example, the compositions of gas-solid reaction products are affected by particle size,specific surface area,carbon content of precursor and the concentration of HN3[18],which are composed of Cu(N3)2,CuN3,and unreacted precursor as copper [4,18,20,21] or excess carbon [10,12,22-24].Each component exists in a different energy density, thus varying component contents lead to quite different ignition capabilities in the past reports, limiting further studies and utilizations in miniaturized explosive systems.

Limited by low-dose of individual reaction products, the main characterization method of compositions in current literature is powder X-ray diffraction (PXRD) [25,26], which possesses difficulties and risks in promoting application and sample pretreatment.Recently the conversion rate of in-situ reaction products based on copper ion concentration, which is determined by inductively coupled plasma optical emission method (ICP), has been reported and applied[13,17,18,27].The ICP method provides a new thought for composition analysis with safety.However,limited by the properties of copper ions in the test object, the conversion rate of in-situ azide reaction products with CuO[28]or Cu2O[11]as precursors cannot be tested by the ICP method.Meanwhile, the nano-copper precursor is gradually consumed to produce Cu(N3)2and CuN3in the gas-solid phased reaction process, resulting in gradual reduction to the size of copper core and affecting the test results.The problems in popularizing the application of the characterization method have not been solved and the correlations between copper ion concentration and detonation capacity are not obvious.As the only additive and energetic group of gas-solid phased azide reaction, azide ion (N3-) concentration reflects in the depth of gas-solid reaction and output performance of reaction products.However,there is no report on quantitative analysis of CA chips by measuring azide content.The difference between existing test methods is shown in Fig.1.

The aim of present work is to develop a simplified quantitative analysis method for accurate and safe analysis of components in CA chips to evaluate and investigate the corresponding ignition ability.In this work, Cu(N3)2and CuN3components in CA chips were separated through dissolution and distillation by utilizing the difference in solubility and corresponding content was obtained by measuring N3-concentration through spectrophotometry.The spectrophotometry method was optimized by studying influencing factors and the recovery rate of different separation methods was studied.The test results obtained from spectrophotometry and the ICP method were compared and discussed.Moreover, as its application in miniaturized explosive systems,the ignition ability of CA chips with different component contents for direct ink writing CL-20 and the corresponding mechanism was studied.

2.Experimental

2.1.Materials and equipment

Analytical-grade chemicals(Cu(NO3)2∙3H2O,H2SO4,HCl,NaOH,CH3COOH, CH3COONa, CaCl2, NaN3) were purchased and used as received.Copper standard solution (1000 mg/L) was purchased from China National Standards Institute.

X-ray diffraction (XRD) analysis was performed on a diffractometer(D8 ADVANCE,Germany)with Cu Kα radiation in the range from 5°to 80°.The copper ion concentration in the solutions was measured by inductively coupled plasma-emission spectrometer(ICP, PE Optima 7000DV,USA).The microstructure of samples was observed by scanning electron microscopy-energy dispersive spectroscopy(SEM-EDS,S4800,Japan).The thermal decomposition process of CA chips was analyzed by thermogravimetric analyzer(TG/DTA6300, Japan) at a heating rate of 10°C/min under the nitrogen flow rate of 100 mL/min.The concentration of N3-was determined by UV-visible spectroscopy using copper nitrate as the complexing agent at the wavelength of 375 nm.The absorbance of[CuN3]+was recorded with spectrophotometer(Agilent Cary 5000,USA)and N3-concentration in the solutions was obtained by Beer-Lambert Law.

Fig.1.Schematic illustration of quantitative analysis method.

2.2.Sample preparation

The nanoporous copper (NPC) precursor was prepared by sintering decomposition of copper oxalate and NPC with a certain density pressed into same size(φ1.0 mm×0.5 mm)polycarbonate chips based on the published Refs.[18,29].The gas-solid phased azide reaction device consists of HN3gas generator, calcium chloride driers, reaction reactor and sodium hydroxide tail gas treatment device.Sodium azide(1.0 g)and excess H3PO4(5.0 mL)were mixed and heated at 55°C to generate HN3gas.Calcium chloride(driers were employed to remove the moisture in the gas mixture.NPC chips were separately deposited in the reactor and gradually converted into copper-based azide chips through gas-solid phased reaction.A slow flow of N2should be introduced for 15 min before and after the reaction to eliminate air and residual HN3.

2.3.Optimization of reaction variables for spectrophotometry

As a spectrophotometry based on Lambert-Beer’s law,the key to ensuring the reliability of copper nitrate spectrophotometry is forming a stable and only [CuN3]+complex with azide ion in solution.To ensure the accuracy of test results, the influence of different reaction variables on test results, including wavelength selection, buffer solution volume, chromogenic condition, copper ion concentration and solution acidity,were studied and discussed in detail.The test wavelength and buffer volume are selected according to the experimental results, which were discussed in and shown in Sections S1 and S2(Supplementary Information).The test results of chromogenic condition, as discussed and shown in Section S3, illustrate that the tested samples keep good stability without HN3escaping for 5 h at room temperature.The effects of copper ion concentration and acidity on absorbance are related to the development of separation methods, which were shown and discussed in subsection 3.1.

2.4.Standard solutions and test methods

Spectrophotometry:50 mg/L N3-calibration solution was prepared from NaN3.1.0 mL acetate buffer solution (1 mol/L CH3COONa -0.1 mol/L CH3COOH) was pipetted into a 10 mL volumetric flask, add a certain volume of N3-calibration solution and mixed,then add 2.5 mL Cu(NO3)2solution(1 mol/L,containing 10-3mol/L HNO3) and dilute to the mark with deionized water.Measure at 375 nm against a copper-acetate buffer blank[30].The absorbance of sample cell was performed three times and averaged.

ICP-OES method: After preparation of the analyte solutions, Cu calibration solutions were prepared using a Cu calibration standard containing a concentration of 1000 mg/L.A 1000 μL pipette was used to deliver aliquots of the Cu calibration standard in volumes of 250 μL and 2500 μL,then diluted with 1%HNO3to a final volume of 50.00 mL using a 50 mL volumetric flask.This yielded calibration solutions with concentrations of 5.0 and 50.0 mg/L, respectively.Emission spectra for Cu were collected at the 324.752 nm wavelength[31].The integrated areas of the spectral lines recorded as a function of concentration were then fit into a straight line to obtain the calibration lines for analyte.

2.5.Extraction procedure and component analysis

Quantitative analysis of Cu(N3)2: Copper-based azide chips and 4.0 mL CH3COOH solution(0.5 mol/L)were placed in a 10 mL glass tube.Tubes were placed in the oscillator and agitated at speed of 80 r/min under room temperature for 3 h.After oscillation, transfer a certain volume of upper clear solution into 10 mL volumetric flask and use NaOH/CH3COONa solution (1.0 mol/L) to form acetate buffer (1 mol/L CH3COONa-0.1 mol/L CH3COOH) containing Cu2+and N3-through neutralization reaction.Then add 2.5 mL 1 mol/L Cu(NO3)2solution and dilute to the mark with deionized water.The absorbance at 375 nm was measured and the corresponding N3-content of Cu(N3)2was calculated from a calibration plot.The method was linear in range from 1.0-25.0 mg/L, with correlation coefficient R2= 0.9998.

Quantitative analysis of CuN3:After the quantitative analysis of Cu(N3)2process, acidify the remaining mixtures with 1 mol/L H2SO4and distill.HN3was transferred through the distillation apparatus with a flow of nitrogen and collected by NaOH solution(1.0 mol/L).Add CH3COOH solution (1.0 mol/L) to neutralize and form a standard acetate buffer containing N3-.Solution preparation and corresponding N3-content of CuN3were obtained in the same way.The schematic diagram of the operation process is shown in Fig.2.

The content of Cu(N3)2and CuN3was calculated using the following Eq.(1).

where CN3-is the concentration of N3-measured by the spectrophotometer, μg/mL; VN3-is the total volume of collected solution,mL; wN3-is the mass fraction of N3-in Cu(N3)2and CuN3, with values of 56.94% and 39.80%, respectively.N is dilution multiple.

Fig.2.Schematic illustration of the separation process.

2.6.Recovery rate of separation steps

Cu(N3)2and CuN3components in CA chips were separated through dissolution and distillation by utilizing the difference in solubility and the recovery rate of different separation steps was studied.For the recovery of directly dissolved Cu(N3)2, 4 μL NaN3(3 mol/L) and 12 μL Cu(NO3)2(1 mol/L) were transferred using microliter syringe into a set of glass tubes to obtain Cu(N3)2, then the acetic acid solution was added to dissolve Cu(N3)2.For the recovery of distillation CuN3, 4 μL NaN3(3 mol/L) was used to distill instead of CuN3.As a control,the same amount of NaN3was diluted with 1 mL of deionized water.N3-concentration of all solutions was measured by copper nitrate spectrophotometry and recovery (as a percentage) was determined by comparing the ratio of azide concentration between the experimental group and the control group.All experiments were performed five times.

3.Results and discussion

3.1.Method development

3.1.1.Determination of separation steps

The separation steps of different components in the sample are the key to determining this method.Owing to the almost equal acidity of CH3COOH and HN3, CH3COOH solution was selected to separate Cu(N3)2and CuN3.To understand the solubility difference between Cu(N3)2and CuN3, pure Cu(N3)2and CuN3were synthesized by metathetical reaction in an aqueous solution.Then Cu(N3)2and CuN3were dissolved in CH3COOH solution and the corresponding copper ion concentrations at different times were measured by the ICP method.As shown in Fig.3(a), with the increase of CH3COOH solution concentration, the dissolution rate of Cu(N3)2increased significantly while CuN3hardly dissolved.Copper-based azides in a weakly acidic solution dissociate into copper ions and azide anions according to Eqs.(2)-(4).

Fig.4.Effect of copper ion concentration on absorbance.

Ionization of HN3in solution inhibits the dissolution of CuN3,realizing the separation of Cu(N3)2and CuN3.In this case, CuN3is easier to reach the dissolution equilibrium than Cu(N3)2.To dissolve CuN3,H2SO4or HCl solution is required to inhibit the ionization of HN3.For the in-situ synthesized CA chips containing Cu(N3)2and CuN3, Cu(N3)2in the sample can be dissolved by using CH3COOH solution, but little CuN3can be dissolved.This view is also confirmed by powder XRD test results, which was shown in Fig.3(b).The diffraction peak corresponding to Cu(N3)2disappears after dissolution with acetic acid,while no change happened in the diffraction peak of CuN3.

3.1.2.Effect of copper ion concentration on absorbance

The effect of Cu2+and solution acidity on absorbance is shown in Fig.4.When N3-concentration is constant, the absorbance first increases with the increase of Cu2+concentration and then tends to a stable value.The reason for this phenomenon is that with the increase of Cu2+concentration,the existing form of N3-in solution changes from various complexes like [CuN3]+, Cu(N3)2, [Cu(N3)3]-and[Cu(N3)4]2-[32-34]to only[CuN3]+complex,thus the increase of [CuN3]+complex concentration leads to the increase of absorbance.When all N3-in solution form [CuN3]+complex,increasing the Cu2+concentration would not affect the absorbance.Therefore, the excess Cu2+introduced by dissolved Cu(N3)2in samples has little effect on absorbance, which is the reason for choosing copper nitrate spectrophotometry to determine N3-concentration.

Fig.3.(a) Dissolution process of Cu(N3)2 and CuN3 in acetic acid; (b) Powder XRD pattern before and after acetic acid dissolution.

3.1.3.Effect of acidity on absorbance

Different from the influence of copper ion concentration, the accuracy of copper nitrate spectrophotometry is greatly affected by pH value.N3-tends to form HN3rather than [CuN3]+complex in more acidic solution, while copper hydroxide colloid significantly increases the absorbance in a slightly alkaline solution.Since the amount of N3-is far less than that of acetate in the buffer solution,we neglected the effect of azide acid hydrolysis on pH value and predicted the theoretical ratio of N3-/HN3and theoretical absorbance at a certain pH value by calculating the pKavalue of HN3.The effect of acidity on the absorbance was studied by adding different acids, the corresponding experimental results are shown in Fig.5.The addition of H2SO4or HCl leads a rapid decline in absorbance,which can be attributed to the significant effect of acidity on N3-coordination ability.By contrast,the addition of acetic acid met the theoretical prediction value at first and gradually deviated as the amount of addition increased, probably because of anion competition and acidity variation.

In the analysis process of real samples, direct dissolve real samples by using H2SO4solution and then neutralizing to test corresponding N3-content was also attempted.Unfortunately, the neutralization test results were less than 40% of the distillation results.This phenomenon may be attributed to the adsorption of N3-by cuprous complex formed during neutralization.Hence, for copper-based azide chips, Cu(N3)2can be separated by dissolving with acetic acid, while residual CuN3should be separated by distillation to ensure the accuracy of test results.

3.2.Validation process

3.2.1.Recovery rate

Fig.5.Effect of solution acidity on absorbance.

Table 1 Effect of time on the recovery rates of two separation methods.

The recovery rates of both methods were studied and listed in Table 1.Although Cu(N3)2synthesized by metathetical reaction dissolved quickly and the recovery rate did not change in several hours,Cu(N3)2synthesized by gas-solid reaction in real samples has a dense structure and the dissolution time needs to be extended to ensure complete dissolution.The maximum recovery rate of distillation is probably affected by distillation temperature and vessel volume,so it is necessary to test the recovery before testing the real sample.

3.2.2.Comparison of real sample analysis by different methods

After optimization of test condition of N3-concentration in CA chips,the method was validated.Different components in CA chips were separated by direct dissolution and distillation in two steps,then corresponding concentration of N3-and copper ions were determined to obtain the component proportion of Cu(N3)2and CuN3.As shown in Fig.6,the test results in Cu(N3)2proportion from two methods are almost same and confirms the validity of spectrophotometry, whereas significant differences are observed in CuN3.Higher results from the ICP method can be attributed to the tiny size of unreacted copper cores after gas-solid phased azide reaction.Therefore, component analysis results of CA chips by spectrophotometry based on N3-concentration is closer to real component content in samples and has satisfactory accuracy.

3.3.Initiation test of CA chips with different components content

Fig.6.Comparison of quantitative analysis results from spectrophotometry and ICP methods.

Fig.7.Initiation result of CA chips.

The above test results show that sample 1-5 has similar total conversion rate based on copper, while the difference lies in the mole proportion of different components.To investigate the initiation ability of CA chips with different components content, the micro-initiating devices composed of base plate with Ni-Cr bridge,CA chips, direct ink writing CL-20 explosive and aluminum plate were assembled and tested.The direct ink writing CL-20 explosive[35]was fabricated in a cavity of φ1.5 mm×1.2 mm with a density of 1.55 g/cm3and initiation result was observed from the dent of aluminum plate.The experimental results show that different components content of CA chips have obvious differences in the initiation ability.As shown in Fig.7, sample 3 with the lowest content of Cu(N3)2/CuN3/Cu about 0.11/0.63/0.08 mg could reliably detonate CL-20 explosive and blast dent in aluminum plate, while sample 4 with the content of Cu(N3)2/CuN3/Cu about 0.07/0.86/0.11 mg can only make CL-20 explosive deflagration but not detonation.This phenomenon indicates that the difference in Cu(N3)2component mass with 0.1 mg will lead to the significant difference in initiation ability.The results also prove that the content of Cu(N3)2significantly affects the initiation ability of CA chips,while the content of CuN3has less effect on the initiation ability.This experimental phenomenon illustrates the feasibility and necessity of component analysis method.

3.4.Mechanism analysis of difference in initiating ability

It is widely recognized that there exists a significant relationship between the properties of energetic materials and their microstructure [36,37].To further understand the reasons for the difference in initiating ability, the morphology and thermal decomposition behavior of CA chips with different components content, represented by sample 3 and sample 4, were determined and studied in detail.

Reactions between HN3gases and nano-copper precursor occur at the solid surface or interface, and are accompanied by microstructural changes.Based on the shrinking core model,the volume expanding multiple K could be calculated by the following Eqs.(5) and (6).

Fig.8.(a)-(c) SEM images of nano-copper precursor; (d)-(f) SEM images of sample 3; (g)-(i) SEM images of sample 4.

Fig.9.(a) TG and DTG curves of sample 3; (b) TG and DTG curves of sample 4.

where the molar mass of Cu(N3)2, CuN3, Cu are 147.59 g/mol,105.57 g/mol and 63.55 g/mol,respectively;the density of Cu(N3)2,CuN3, Cu are 2.60 g/cm3, 3.52 g/cm3and 8.96 g/cm3, respectively[16,20].It can be seen from the above formula that a higher proportion of Cu(N3)2compared with CuN3will lead to larger volume expansion and denser stacking structure.This view was also confirmed by the scanning electron microscopy (SEM) image(Fig.8).Compared with unreacted precursors, the volume of solid products expands significantly.Compared with sample 4,sample 3 shows more obvious volume expansion and more dense packing,which is related to the higher proportion of Cu(N3)2in the quantitative test results.When the total conversion rate of CA chips is constant, higher proportion of Cu(N3)2possesses a higher energy and denser structure to provide more initial energy and less energy consumption for the detonation growth.Meanwhile higher content of CuN3provides subsequent energy for the detonation growth and propagation of CA chips.It is helpful to further understand the detonation growth phenomenon in the micro-charge by component analysis of CA chips.

The thermal decomposition process of sample 3 and 4 were determined by thermogravimetric analysis (TGA) methods at a heating rate of 10°C/min and the corresponding TG and DTG curves were shown in Fig.9.A similar onset decomposition temperature was found at 148.7°C and different decomposition behaviors were observed in two samples.The actual weight loss on TG curves is close to the theoretical weight loss obtained from spectroscopy,which verified the validity of test method.Compared with sample 4, the decomposition process of sample 3 is longer and the peak temperature of DTG curve is higher.The peak temperatures of corresponding DTG curve sample 3 and 4 are 186.8°C and 178.7°C respectively, which is closer to the corresponding peak temperatures of CuN3(about 178°C)[16]and lower than of Cu(N3)2/C(about 200°C) [17].According to the results of quantitative analysis,sample 4 has a higher proportion of CuN3than sample 3,leading to a faster decomposition at 178.7°C.This phenomenon also illustrates that the increase of Cu(N3)2content leads to a denser structure and delays the peak temperatures by analyzing the results of thermal analysis and morphology.

4.Conclusions

In summary, we have developed a simplified quantitative analysis method for CA chips via spectrophotometry and studied its practical application.Cu(N3)2and CuN3components in CA chips were separated through dissolution and distillation by utilizing the difference in solubility and the corresponding content was obtained by measuring N3-concentration through spectrophotometry.The quantitative spectrophotometry method was optimized and determined by studying all influencing factors and the recovery rate of different separation methods.The optimized method was linear in range from 1.0-25.0 mg/L, with a correlation coefficient R2= 0.9998, which meets the requirements of CA chips with a milligram-level content test.Compared with the existing ICP method, component analysis results obtained by spectrophotometry have satisfactory accuracy to test a milligram-level content of CA chips and are closer to component content in real samples.The test results of ignition ability also show that the initiation ability of CA chips was significantly affected by Cu(N3)2instead of CuN3content.The corresponding mechanism was investigated through the study of morphology and thermal decomposition process,illustrating that the increase of Cu(N3)2content leads to a denser structure and delays the peak temperatures.This study provided a basis and idea for the design and performance evaluation of CA chips in miniaturized explosive systems.

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 authors appreciate the financial support provided by the National Natural Science Foundation of China(Grant No.11872013).

Appendix A.Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.dt.2023.02.015.