High density thermite mixture for shaped charge ordnance disposal

Tamer Elshenawy,Salah Soliman,Ahmed Hawass

Technical Research Center,Cairo,Egypt

High density thermite mixture for shaped charge ordnance disposal

Tamer Elshenawy*,Salah Soliman,Ahmed Hawass

Technical Research Center,Cairo,Egypt

1.Introduction

Thermite is a mixtureof fuel and oxidizer that can react together to form more stable oxide and free metal.Exothermic and spontaneous reaction is the main feature for thermite mixture.It is commonly used in incendiary devices,welding,gas generator materials and reactive fragments due to the high amounts of heat released.Fischer et al.compared the chemical energy and heat of reactions for thermite compositions and conventional high explosives[1].The ignition ability and combustion behavior of various aluminum-based thermites have been studied by Dreizin[2].There are many factors that affect the performance of thermite compositions as fuel/oxidizer ratio,which has been demonstrated in Ref.[3].It has been shown that the high efficient thermite reaction can be produced when fuel and oxidizer used in a composition are stoichiometrically balanced,where the optimum percentage was 1:1.Other works[4,5]showed that the optimum thermite performance(highest combustion velocity)can be obtained at fuel rich compositions.The ambient air is the main factor,which is the reason of fuel rich compositions have the highest performance.The reactions are performed in an environment,where oxygen from the surrounding may contribute to the aluminum oxidation,and hence optimum thermite compositions are determined to be fuel rich.Metals as aluminum have higher thermal conductivity than solid oxidizer,which enhances the thermal transport properties and combustion velocities of the mixtures[6].Other factors such as mass ratio and type of impurities were also considered in the performance of thermite compositions.Aumann et al.[7]found that the optimum percentage for fuel and oxidizer was mainly affected by particle size distribution.He found that a flame temperature of 600°C or primary explosive are necessary for thermite mixture ignition.This is because ignition is usually carried out by diffusion-controlledreaction[8].The particle size is one of the main factors that affects the combustion characteristics as it directly affects the bulk density of the thermite.The usages of thermite mixture in explosive compositions were illustrated in both reference[9]and[10],which investigate the creation of free electrons(oxygen),which can react with aluminum to form aluminum oxide and significant additional heat.High value of positive after burning and extended fire balling is the distinguishing feature of thermite explosive compositions.Replacing the conventional lead-based ammunition primers with"Green"compositions is one of the most recommended applications as cited in Ref.[11].A novel thermite device;mine Incinerator(MI)was manufactured and tested against different mines,interested reader refer to reference[12].The approach reaction product was in liquid state and its temperature was up to 4000 K.Thermal properties such as thermal diffusivity and thermal conductivity increases as density of the mixture increases[13].By increasing the bulk density within the micron size,the velocity of combustion increases in contrast withnano particles[13].Wang et al.studied the formability and fragment production using thermite compositions[14].

Table 1 Chemical compositions of thermite compositions.

Zhang et al.designed new destructing device based on combustion mechanism.Stand-off distance was 10 mm and the Q235 steel plate used as casing for ammunition can be destructed by a thermite charge weight of 60 g[15].

In this research,Theoretical maximum density,reaction temperatures,product and the heat of reaction for different thermite compositions have been discussed theoretically,while some optimum thermite formulations have been prepared experimentally using the cold-isostatic pressing technology to produce high density mixtures.The prepared thermite mixtures have been used to neutralize small caliber shaped charge containing HMX pressed powder and thick steel confinement.

2.Theoretical calculations

Ignoring the in fluence of the air on the combustion of the thermite compositions and the theoretical thermochemical calculations have been carried out using Chemical Equilibrium Speci fic Impulse Code,which calculates enthalpy of formation based on the chemical formula of the elements,density,and heat of formation[16].Thermochemical calculations have been carried out for different percentage of Al to the candidate ferric oxide based compositions,which are preferred to produce the highest flame temperature.Different thermite compositions have been calculated using Chemical Equilibrium code to investigate the higher performance of different compositions shown in Table 1.The aluminum has been selected for the current study with ferric oxide because its flame temperature is 4000 K,which is much greater than its boiling point of 2791 K.This means the aluminum fuel particles are therefore able to escape the oxidation state to form oxide shell but react quickly with ferric oxide forming aluminum oxide and iron with an exothermic reaction.

Fig.1 shows the relation between the enthalpy of formation of different thermite mixtures against different aluminum(fuel)percentages.The obtained results demonstrated that as the aluminum percentage increases,the enthalpy of formation decreases.This is attributed to the produced fast reaction,which consumes Fe2O3faster and some aluminum powders may be kept without burning after the thermite burn.

A mathematical relation was obtained by plotting the percentage of aluminum against the enthalpy of formation as shown in Fig.1.As the percentage of aluminum increases(i.e.fuel rich compositions),the performance of thermite reaction decreases with neglecting the effect of oxygen on the burning of the thermite compositions.From Fig.1,it can be deduced that composition 1 has the highest heat of enthalpy,which is assumed to have the greatest flame temperature and the best thermite performance,but it was extremely dif ficult to ignite using the safety fuse and magnesium powder.It was ignited only using propane torch,which was not approved for our military field safety precautions.Similarly,there was difficulty in igniting thermite compositions 2 and 3 due to their low percent of fuel.These fuel rich mixtures require special igniter and intermediate charges containing more sensitive ingredients such as barium nitrate,barium peroxide and magnesium,which make the thermite canister less reliable during the ignition and usage in military ammunition disposal.These two compositions have been also waived.Therefore,composition 4 containing 33%aluminum powder has been selected for the entire testing for its easy ignition ability and meeting the safety requirements when ignited using the traditional safety fuse with little magnesium powder.

Density of the thermite mixture is a very important property,which has a great effect on the resultant burning rate.Fig.2 investigate the decrease of the composition density as the aluminum percentage increases.The lower densities of aluminum demonstrate the reason behind this graduation.

3.Material specification and experimental testing

Thermite mixture was fabricated directly from the row materials as received afterheating at 60°C in a controlled atmosphere to remove humidity from the initial powders.After good mixingof the powders and pressing using cold iso-static pressing apparatus,the compacted thermite is located inside a PVC nozzle tube with a wall thickness of 2 mm and an outer diameter of 5 cm.The nozzle tube has a built in hole through its axis ending with a conical surface facing the disposal object.This conical shape guarantees the flow of burnt mixture over the same point on the bomblet body,and therefore avoids the random distribution of burnt products on the bomblet body.Thus,a single small hole on the bomblet steel body has been created,which allowed the entrance of the burnt products to the explosive charge to neutralize.A layout for the thermite charge assembly and the test setup are shown in Fig.3.

Fig.1.Effect of aluminum percentage on the enthalpy of thermite compositions.

Fig.2.Effect of aluminum percentage on the density of thermite compositions.

The aluminum powder is supplied from Western Germany with an average particle size of 50μm.Ferric oxide was supplied from Morgan chemical industry,Egypt with an average grain size 30μm.Magnesium powder was supplied from BDH laboratory supplies,England.

Three un-sintered thermite mixtures have been prepared by powder metallurgy(PM)technique according to stoichiometric ratio.The mixture contains aluminium powder and ferric oxide(Fe2O3)of 54 and 160 gm each respectively.Each thermite mixture weighing 40 gm was produced from the PM technology using cold iso-static pressing apparatus to form a cylinders of compacted powders of initial inner diameters 48 mm before powder compaction,after which the powder shrinks radially around 5%.This means that the final diameter of green thermite mixture is 45.6 mm.Initially,the mixture powders are dried in a furnace at 60°C to remove humidity and then blended inside a horizontal mixer to guarantee uniform homogeneity of the thermite mixture.

Fig.3.Layout for the thermite mixture components(Left)and the test setup(right).1:thermite mixture;2:Magnesium and black powder mixture,3 Magnesium powder,4:PVC tube,5:the tested shaped charge.

Fig.4.Thermite composition burning and explosive charge deflagration.

Then the weighed amount of 40 gm were filled into cylinder sealed rubber mould of outer diameter 5 cm(and inner diameter of 48 mm)and lowered into the water iso-static pressing pool.The maximum applied pressure was set to 100 MPa at a low rate(i.e.1 MPa per second)to avoid trapping air voids inside the thermite mixture material.The product is a brittle material in the presintering state and is called'the green product'.The product is then taken and put inside a PVC nozzle tube of inner diameter 46 mm and a length of 15 cm within a cylindrical cavity(built in perforation of 5 mm diameter)needed for pyrotechnic igniter.

The thermite mixture is very difficult to ignite using normal flame,thus in this research paper,it was ignited using few grams of magnesium powder with black powder(KNO3:C:S)with mass ratio 6:1:1.

4.Results and discussion

Thermite compositions have been initiated using safety fuse and 3 g of magnesium powder.The burnt product is allowed to flow through the conical nozzle;Fig.3 over the metallic steel body of the tested small calibre shaped charge bomblet for neutralisation.The density of the prepared thermite mixtures was calculated and found 4.9 g/cm3(based on its mass and volume),at which steady burning was observed with total burning time 25 s and the average burning rate equals 4 mm/s was obtained.A continuous jet of molten iron is allowed to flow towards the steel body until complete perforation of this body with a thickness of 2 mm.After which,the molten iron was capable of burning the HMX explosive charge,where a deflagration occurred to the high explosives inside shaped charge bomblet without any detonation observed or DDT(deflagration detonation transition)as shown in Fig.4.Complete neutralization has been obtained by thermite compositions,which encourage the usage of thermite compositions with the ammunition ordnance disposal especially when the explosive charge cannot be neutralized byexplosion in the nearvicinityof petroleum stations or inside the dwelling area or even can be used in the mine clearance systems when its explosion in undesirable as shown in Fig.5 four,our shaped charge ordnance disposal.

Fig.5.Neutralization of the shaped charge using thermite.

5.Conclusions

Theoretical calculations have been carried out for different thermite compositions including various percentages of aluminum powder as a fuel in the thermite mixture matrix.Different percentages of aluminum exhibited different performances of these compositions including densities,the output heat and loading density.Steady burning and high flame temperature produced by high density thermite mixture containing 33%Al powder has been manufactured using cold iso-static pressing technology and found to have a relatively high density of 4.9 g/cm3and regular burning rate of 4 mm/s.The suggested thermite design has been tested for shaped charge disposal as it exhibited more safety,cheap tool and reliable explosive deflagration device rather than its explosion.Besides,the proposed device has been implemented for shaped charge ordnance disposal with the approved ignition method.

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A R T I C L E I N F O

Article history:

10 November 2016

in revised form

17 March 2017

Accepted 27 March 2017

Available online 28 March 2017

The effect of thermite mixture based on aluminum and ferric oxides for ammunition neutralization has been studied and tested.Thermochemical calculations have been carried out for different percentage of Al using Chemical Equilibrium Code to expect the highest performance thermite mixture used for shaped charge ordnance disposal.Densities and enthalpy of different formulations have been calculated and demonstrated.The optimized thermite formulation has been prepared experimentally using cold isostatic pressing technique,which exhibited relatively high density and high burning rate thermite mixture.The produced green product compacted powder mixture was tested against small caliber shaped charge bomblet for neutralization.Theoretical and experimental results showed that the prepared thermite mixture containing 33%of aluminum as a fuel with ferric oxide can be successfully used for shaped charge ordnance disposal.

?2017 Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

*Corresponding author.

E-mail address:tamrshenawy@yahoo.com(T.Elshenawy).

Peer review under responsibility of China Ordnance Society.