Studies on impact sensitivity of nanosized trinitrotoluene （TNT）conf i ned in silica processed by sol-gel method

S.V.INGALE*，P.B.WAGHP.U.SASTRY，C.B.BASAK，D.BANDYOPADHYAY，S.B.PHAPALE，Stish C.GUPTA

aApplied Physics Division，Bhabha Atomic Research Centre，Mumbai，India

bSolid State Physics Division，Bhabha Atomic Research Centre，Mumbai，India

cGlass and Advanced Materials Division，Bhabha Atomic Research Centre，Mumbai，India

dHeavy Water Division，Bhabha Atomic Research Centre，Mumbai，IndiaeChemistry Division，Bhabha Atomic Research Centre，Mumbai，India

Studies on impact sensitivity of nanosized trinitrotoluene （TNT）conf i ned in silica processed by sol-gel method

S.V.INGALEa，*，P.B.WAGHa，P.U.SASTRYb，C.B.BASAKc，D.BANDYOPADHYAYd，S.B.PHAPALEe，Satish C.GUPTAa

aApplied Physics Division，Bhabha Atomic Research Centre，Mumbai，India

bSolid State Physics Division，Bhabha Atomic Research Centre，Mumbai，India

cGlass and Advanced Materials Division，Bhabha Atomic Research Centre，Mumbai，India

dHeavy Water Division，Bhabha Atomic Research Centre，Mumbai，IndiaeChemistry Division，Bhabha Atomic Research Centre，Mumbai，India

Nano-sized trinitrotoluene （TNT）material restrained in silica gel has been prepared by using the sol-gel process to study the effect of varying porosity in gel on the sensitivity ofTNT.TheTNT content in the gel has been varied from 60 to 90 wt% （at f i xed acetone/tetramethoxysilane ratio of 50）.Also，for a f i xed TNT content of 75 wt%，the pore structure in the gel has been varied by changing the ratio of silica gel precursor to the solvent.The resultantTNT-silica gel composites have been characterized using scanning electron microscopy，thermal analysis，small angle X-ray scattering and surface area analysis techniques.Impact sensitivity studies were carried out using Fall Hammer ImpactTest.The results showed that the sensitivity of nanostructured explosives prepared by sol-gel process can be tailored precisely by controlling the process parameters.

Nanostructured TNT；Sol-gel method；Composites；Impact sensitivity

1.Introduction

Chemical explosives are the materials capable of storing energy due to its chemical composition and this energy can be harnessed with proper stimulant over an exceedingly short period of time.The detonation behavior of explosive materials is inf l uenced by their microstructural features like particle size distribution，surface area，density of explosive charge，impurities，defects or inhomogeneities present，as well as processing methods ［1］.Among these parameters，particle size and defects play a critical role in controlling the explosive properties like sensitivity and rate of energy release.As the particle size of explosive material is reduced，there is more surface area in contact between the particles of an explosive which may cause a faster reaction rate ［2］.It has been observed that explosive materials with reduced particle size show a decrease in impact sensitivity，but this trend has been found to be reversed for theexplosive materials with particle size less than 200 nm ［3］. Many attempts have been carried out to study the effect of particle size distribution in nanometer range on the sensitivity of explosive materials.However，the limited data are available on the effect of the microstructure of nano-sized particles on the sensitivity of explosives materials ［4，5］.Therefore，investigation of nano-sized explosives with varying microstructure is an important aspect in studying the sensitivity of these materials.

Some of the popular techniques used to produce nanosized explosive materials are crystallization out of solutions，crystallization using supercritical f l uids or rapid expansion of supercritical solvent （RESS）and sol-gel technique ［6］.Out of these techniques，crystallization out of solutions is a commonly used method.Using this method，particle size can be reduced to submicron level.Although this method is very simple and safe，it is diff i cult to control the particle size and size distribution. Using the RESS method，nano-crystals of explosive with mean particle size in the range of 110-220 nm have been produced［7］.However，it requires sophisticated instrumentation including low temperature and high pressure set-up.Moreover，various parameters like solvent temperature，pressure，nozzlesize，etc.，need to be controlled accurately，which makes this method complicated.In the sol-gel method，the explosive material is dissolved in a specif i c solvent and to this solution silica gel precursors like tetramethoxysilane and water are added.The hydrolysis and condensation of tetramethoxysilane results in the formation of nanosized primary particles of silica suspended in the solution，called sol.The primary particles of silica cross-link to form a three dimensional porous network which is referred to as gel.The pores of the gel contain the solvent in which explosive material is dissolved.Evaporation of liquid from the gel at ambient conditions results in xerogel material with recrystallized explosive material in the gel pores. Unlike other methods，the sol-gel method is simpler and safer as it does not require complicated instruments and it does not involve high temperature or high pressure.The advantage of sol-gel method is that the microstructure can be tailor-made so as to achieve the particle size in nanometer range with narrow size distribution as well as the porosity in the gel matrix can be varied.The usual average particle size obtained by this method is about 20-30 nm ［8］.However，the sensitivity of explosive materials may get affected due to enhanced defect density in terms of pores ［9］.

We synthesized nano-sized trinitrotoluene （TNT）material in silica gel by using sol-gel method to study the effect of varying porosity in the gel on the sensitivity of TNT.TNT has been chosen for these studies because，due to high solubility of TNT in acetone，by controlling acetone to silica gel precursor molar ratio，the microstructure and porosity in the TNT-SiO2composite can be varied signif i cantly so as to study these materials in wide range.The TNT content in the gel was varied from 60 to 90 wt%.The porosity was also varied by varying the process parameters.The resultant explosive materials were characterized by various techniques and impact sensitivity of these materials has been studied.

2.Materials and methods

2.1.Preparation of TNT-SiO2composites

The TNT-SiO2nanocomposites were prepared by using the sol-gel method ［10］.To prepare the TNT-SiO2composites，the predetermined amount of TNT was dissolved in acetone. The molar ratio ofTNT/tetramethoxysilane （TMOS）was varied from 0.4 to 2.4 to obtain the TNT content in the gel ranging from 60 wt%to 90 wt%.TMOS and water in the form of diluted hydrof l uoric （HF）acid （1 M）were added to this solution as silica gel precursors.H2O/TMOS ratio was kept at 16，whereas acetone/TMOS ratio was kept at 50.To vary the pore volume in the gel matrix in the samples containing 75 wt% TNT，the ratio ofTMOS to acetone was varied from 50 to 20 by maintaining the molar ratio of TNT/TMOS as 0.8 and H2O/ TMOS as 16.The hydrolysis and condensation of TMOS resulted in the formation of a clear gel within three hours.After the formation of the gel，the solvent from the gel pores was allowed to evaporate at ambient conditions to obtain nano-sized TNT retained in the pores of the gel.The samples containing 60 wt%，75 wt%and 90 wt%of TNT and acetone/TMOS ratio as 50 have been designated as T60，T75-I and T90 respectively. The samples containing 75 wt%of TNT has been further designated as T75-I，T 75-II and T75-III with acetone/TMOS ratio as 50，35 and 20，respectively.

2.2.Characterization of TNT-SiO2composites

The presence of TNT in the gel was conf i rmed by X-ray diffraction （XRD）measurements.The XRD data for the resultant TNT-SiO2xerogel were obtained on a Philips X-ray diffractometer using a PW 1710 goniometer （CuKα，30 kV，20 mA）.The data were recorded by step-scan mode from 2θ of 10.01°to 79.99°，with step size of 0.02°.The amount ofTNT in the gel was conf i rmed by thermogravimetric and differential scanning calorimetry （TG-DSC）analysis using SETSYS Evolution，SETARAM system.The samples were heated in Argon atmosphere from room temperature to 500 °C at heating rate of 10 °C/min.The microstructure of the gel containing nano-sized TNT was studied by using Carl Zeiss Auriga f i eld emission scanning electron microscope （FESEM）.For FESEM analysis，the TNT-SiO2gel powder was suspended in methanol and the suspension was dispersed on a copper plate.The samples were then gold coated.Small angle X-ray scattering （SAXS）measurements were carried out on pure silica xerogels and TNT incorporated silica xerogels using a Rigaku small angle goniometer mounted on rotating anode X-ray generator.Scattered intensity I（Q）was recorded using a scintillation counter by varying the scattering angle 2θ.Here Q is the scattering vector equal to 4π sin （θ）/λ，λ is the wavelength of incident （CuKα）X-rays.The intensities were corrected for sample absorption and smearing effects of collimating slits ［11］.Specif i c surface area was measured by nitrogen physisorption method using a Sorptomatic 1990 analyzer from CE Instruments.Prior to surface area measurements，the samples were degassed at 40 °C under vacuum for 6 hours.The specif i c surface area was calculated using the Brunauer-Emmett-Teller （BET）method from the amount of N2gas adsorbed at 77 K at various partial pressures （eleven points；0.05 ＜ p/p0＜ 0.3）.Impact sensitivity studies were carried out by Fall Hammer Impact Test using a 2 kg weight.For impact sensitivity test，powder sample of about 30-40 mg was placed on anvil and the height of impact （2 kg hammer）was varied to arrive at a height where 50%probability of initiation is found.Tetryl with f i gure of insensitivity （FoI）of 70 was considered as the standard.

3.Results and discussion

3.1.X-ray diffraction studies

Fig.1 shows the XRD patterns for raw TNT and the sol-gel processed TNT-SiO2composites containing 90，75 and 60 wt %TNT.The diffraction peaks of crystalline phase in the XRD pattern corresponds to the monoclinic phase of TNT ［12］.It indicates the presence ofTNT in the sol-gel processed composites.As the silica content in the composite samples increases，the visibility of crystalline nature ofTNT is less prominent due to the amorphous nature of silica gel.As TNT recrystallizes in the pores of silica gel，there could be some variation in the peak intensity of scattering planes.

Fig.1.XRD pattern forTNT andTNT-SiO2composites:（a）rawTNT；（b）T90；（c）T75；（d）T60.

3.2.TG-DSC analysis

Fig.2 shows typical TG-DSC curves for the TNT-SiO2nano-composite with 90 wt%TNT （T90）.The endothermic peak at about 70 °C indicates melting of TNT.The melting temperature of TNT in the composite has shifted to lower temperature as compared to melting temperature of 80 °C for neat TNT.It reveals that recrystallized TNT in the composites is with nanometric size.The exothermic peak at about 280 °C is attributed to TNT decomposition ［13］.This conf i rms that TNT is retained in the gel matrix.

Fig.2.TG-DSC curves for TNT-SiO2composite with 90%TNT content.

There is a weight loss of about 86%within the range of 210-280 °C as shown in the TG curve of the T90 sample.As TNT decomposes completely into gaseous product，this weight loss is consistent with the desired TNT wt%in the composite. The remaining part is silica which is inert in this temperature range.The exothermic peak at 280 °C corresponds to the decomposition of TNT and the formation of gaseous products like CO，CO2，H2O，etc.，that accounts for sudden energy release.

3.3.FESEM

Fig.3（a）and （b）shows FESEM pictures of TNT-SiO2xerogels containing 75 wt%TNT with acetone/TMOS ratio as 50 and 20，respectively.Whereas Fig.3（c）shows FESEM of the sample containing 90 wt%TNT prepared with acetone/TMOS ratio of 50.

All the composite samples show nano-structures.It is observed from Fig.3（a）and （b）that the sample T75-I is more porous as compared to T75-III.As the molar ratios of Acetone/ TMOS for theT75-I andT75-III samples are 50 and 20，respectively，the pores in T75-I are more widespread with larger pore volume which is conf i rmed by SEM pictures.The pores and particles are in mesoporous range.In the T75-III sample，the particles are closely spaced due to low acetone/TMOS ratio and the microstructure is more compact and indicates a signif i cant decrease in porosity.Fig.3（c）shows the SEM picture of T90 sample.The acetone/TMOS ratio for bothT 75-I andT 90 is 50. Compared to T75-I sample，the particles in T90 sample are more grown and the particle size is bigger.Due to higher loading of TNT，more pores get occupied with TNT and the particles grow more and particle size is bigger that result in less porous microstructure as compared to T75-I.As compared to T75-III sample，the microstructure in T90 sample is less compact which is due to higher acetone/TMOS ratio.These observations suggest that by varying the process parameters like solvent to TMOS ratio and TNT content in the composite，the microstructure can be suitably controlled.These results are found to be consistent with SAXS studies and surface area measurements.

3.4.SAXS measurements

The SAXS prof i les displayed on log-log scale are shown in Fig.4.

The structure of silica based xerogels has been investigated extensively in earlier studies ［14］.In the silica xerogel，the scattering at low-Q （Q ＜ Q1）region occurs from larger，submicron size particles and the inter-particle voids.Whereas in the region Q2＜ Q ＜ Q1，the scattered intensity arises due to surfaces of the smaller particles or pores within the aggregates. For particles with smooth surface，I（Q）in this （Porod）region varies as Q-β，with β being equal to 4.In the region beyond Q2，the intensity is contributed by micropores within the silica network.

As shown in Fig.4，the SAXS prof i le of pure sample follows a linear behavior with a change of slope at a Q.Below this crossover point，I（Q）varies as Q-α，with a value of 1.1 for α. This suggests that the silica particles are in the form of mass fractal aggregates with fractal dimension of 1.1.From the crossover point，the average size （D）of the basic particles within the aggregates is found to be about 18.5 nm.In thehigh-Q region，the slope of the linear prof i le is steeper than 4.0，suggesting a fuzzy or diffuse boundary ［15］between particles and pores.The micropore region （Q ＞ Q2）is beyond the Q-range of measurements of this study.

Fig.3.FESEM of TNT-SiO2composites.

Fig.4.Small angle X-ray scattering of silica xerogel with TNT （T）.Lines are a guide to the eyes.

For silica xerogels with TNT，the SAXS prof i les are in the same shape as for pure sample but the mass fractal dimension has increased with the highest value of 2.26 for 90%TNT.Thus，the matrix became compact with the presence ofTNT.The size of the basic particles （pores）increased marginally to about 20 nm forT60 andT75-I samples.It is increased to 22.5 nm for T75-III sample and a steep rise to 30.4 nm for 90 wt%TNT sample.The typical size of the particles is concurrent in order of magnitude with SEM pictures.

3.5.Surface area measurements

The results of surface area and pore volume measurements for silica xerogel and typical TNT-SiO2composites are shown in Table 1.

The pore volume and surface area measured for TNT-SiO2composite are less as compared to SiO2xerogel.The decrease in pore volume of TNT-SiO2composites as compared to SiO2xerogels indicates that pores of the gel have been occupied by TNT.For samples T75-I and T90 which were prepared with acetone/TMOS ratio of 50，the surface area has been found to be decreased from 189 to 74 m2/g with an increase in the content of TNT.The pore volume has been found to be decreased from 0.136 to 0.053 cm3/g，respectively.This is due to growth of larger size particles of TNT in the pores.In the T75-I sample，some of the pores may also be non-occupied which might have resulted in high pore volume and surface area.For T75-I and T75-III samples，the surface area has been found to be drastically decreased from 189 to 18 m2/g.The acetone/TMOS ratios for these samples were 50 and 20，respectively.The decrease in solvent/precursor ratio has resulted in a much compact network，leading to a decrease in porosity as revealed from the SEM pictures.This has led to a decrease in surface area and pore volume.The pore volume has been found to be decreased from 0.136 to 0.011 cm3/g forT75-I andT75-III samples，respectively.The results show that porosity in TNTSiO2composites can be suitably controlled by controlling the process parameters likeTNT content or solvent/precursor ratio. The observed trend in surface area and pore volume measurements for TNT-SiO2composites is consistent with the FESEM results.

Table 1Preparative condition and textural properties of SiO2xerogel and TNT-SiO2composites.

3.6.Impact sensitivity

Fig.5.Impact sensitivity data for raw TNT and TNT-SiO2composites.

The data for sensitivity to impact of nano-sized TNT processed by sol-gel method are shown in Fig.5.It has been observed that f i gure of insensitivity （FoI）decreases，that is，the sensitivity to impact of the TNT-SiO2composite material increases as compared to raw TNT.The particle size of TNT in the TNT-SiO2composites has been found to be less than 100 nm.The impact sensitivity results are not in agreement with the general belief that the sensitivity of explosives is reduced with a decrease in particle size.However，it has to be mentioned that the reported data in the literature are accounted for the explosive materials in pure form and for the particle size in the micrometer range.In the present work，the decrease in particle size up to nanometer scale might have enhanced the reactivity due to high specif i c surface area which dominates the initiation mechanism and results in higher sensitivity.The sensitivity to impact for the TNT-SiO2composites has also been found to be increased with a decrease in TNT content in the composite from 90 wt%to 60 wt%.Generally the impurities like grit or silica in the explosives lead to friction in localized area and therefore increase of temperature in surrounding area that causes initiation.However，in sol-gel processing，the silica is homogeneously distributed at nanometer scale，and therefore although silica as an impurity may contribute to an increase in sensitivity of the composite，its contribution is limited.In the composites with 75%TNT and 25%silica，although the silica content is the same，the impact sensitivity was found to be increased with an increase in the acetone/TMOS ratio from 20 to 50.It indicates that the impact sensitivity could be altered with change in process parameters and hence microstructure of the composites.Therefore，the increase in sensitivity of the composites might be due to an increase in the density of defects like pores/voids which may lead to adiabatic compression of interstitial gases and act at centers for initiation of chemical reaction.In samples with lesser TNT content，the number of non-occupied pores will be more，which leads to higher defect density，which has also been observed from fractal dimensions in SAXS studies.

4.Conclusions

The sol-gel method has been successfully used to prepare nano-crystalline TNT materials.The advantage of sol-gel method of high solid loading in the porous matrix is utilized to prepare theTNT-SiO2xerogels containing up to 90 wt%explosive material.The results on impact sensitivity measurement showed that the sensitivity of nano-sized explosives can be tailored precisely by controlling either the amount of explosive loading in the gel or the microstructure of the material by varying process parameters like precursor ratio.These studies could be useful to understand the role of pore density defects in the initiation and detonation phenomenon of nano-sized secondary explosives.

Acknowledgments

The authors acknowledge the help from Ratanesh Kumar，I. K.Singh，Rakesh Patel，Sonu Gavit and SandipVirnak ofAPD，BARC in the experimental work.

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Received 16 April 2015；revised 17 August 2015；accepted 18 August 2015 Available online 15 September 2015

Peer review under responsibility of China Ordnance Society.

*Corresponding author.Tel.:+912225591808.

E-mail address:svingale@barc.gov.in （S.V.INGALE）.

http://dx.doi.org/10.1016/j.dt.2015.08.004

2214-9147/? 2015 China Ordnance Society.Production and hosting by Elsevier B.V.All rights reserved.

? 2015 China Ordnance Society.Production and hosting by Elsevier B.V.All rights reserved.