Thermal decomposition of ammonium perchlorate catalyzed with CuO nanoparticles

Sherif Elasuney ,M.Yehia

a Head of Nanotechnology Research Center,School of Chemical Engineering,Military Technical College,Cairo,Egypt

b School of Chemical Engineering,Military Technical College,Cairo,Egypt

ABSTRACT Ammonium perchlorate(APC)is the most common oxidizer in use for solid rocket propulsion systems.However its initial thermal decomposition is an endothermic process that requires 102.5 J?g-1.This manner involves high activation energy and could render high burning rate regime.This study reports on the sustainable fabrication of CuO nanoparticles as a novel catalyzing agent for APC oxidizer.Colloidal CuO nanoparticles with consistent product quality were fabricated by using hydrothermal processing.TEM micrographs demonstrated mono-dispersed particles of 15 nm particle size.XRD diffractogram demonstrated highly crystalline material.The synthesized colloidal CuO particles were effectively coated with APC particles via co-precipitation by using fast-crash solvent-antisolvent technique.The impact of copper oxide particles on APC thermal behavior has been investigated using DSC and TGA techniques.APC demonstrated an initial endothermic decomposition stage at 242°C with subsequent two exothermic decomposition stages at 297.8°C and 452.8°C respectively.At 1 wt%,copper oxide offered decrease in initial endothermic decomposition stage by 30%.The main outcome of this study is that the two main exothermic decomposition peaks were merged into one single peak with an increase in total heat release by 53%.These novel features can inherit copper oxide particles unique catalyzing ability for advanced highly energetic systems.

Keywords:Ammonium perchlorate Catalyst Thermal behavior Energetic systems Catalyzed propellants

1. Introduction

Ammonium perchlorate(APC)is the main oxidizer in use for solid rocket propellants,as it can offer high oxidizing power as well as gas-generating capabilities[1,2].During combusting,oxidizer would decompose with the release of free oxygen.However the initial oxidizer decomposition to generate free oxygen is an endothermic process[3,4].Consequently the rate-determining step in many high-energy reactions appears to be the endothermic process associated with oxidizer decomposition[5].The higher oxidizer decomposition temperature,and its endothermic decomposition,the slower the burning rate will be[6].This endothermic process means high activation energy is required.Even though high energy reactions are extremely exothermic and spontaneously propagated; these reactions require activation energy to start the chemical reaction(Fig.1).

APC has been shown to be capable of catalytic decomposition,with different metal oxides.It is widely accepted that transition metal and metal oxides can be employed for catalyzing the thermal decomposition of APC[7].Therefore they can be employed as burning rate catalyst in APC-based rocket propellant formulations[8,9].A low percentage(0.1 wt%-2 wt%),catalyst can offer a significant increase in propellant combustion performances[10].Nanostructured energetic materials(NSEMs)include nanoscale objects at least in one dimension[11].The performances of NSEMs differ from that of micro-structured energetic materials[12].Small critical diameter,high reaction rate,and high released heat allow NSEMs to fit very well to energetic chips[13].

1.1. Nanocoating for activating energetic materials

Core-shell nanoenergetic structure can offer an intimate mixing between catalyzing particles and potential oxidizer(i.e.APC).This emergent approach can offer effective catalyzing with high reactivity[14].Fig.2 demonstrates the impact of fine APC encapsulated with Fe2O3particles on burning rate and flame structure of APC-based composite propellants.

Fig.2.The burning rates for the baseline propellant,propellant with micron-sized catalyst,propellant with nano-size catalyst,and propellant with encapsulated catalyst.

It is apparently clear that APC encapsulated with catalyst yieldes the highest burning rate,followed by nanoscale catalyst mixed in directly.

Burning and combustion characteristics of APC-based propellants depend on the decomposition characteristics of APC[15,16].The decomposition characteristics of APC are known to be sensitive to the presence of specific catalysts in small amounts[17,18].These additives are used as ballistic modifiers[19,20].The ballistic performance of composite propellant can be improved by adding certain catalyst such as ferric oxide(Fe2O3),copper oxide(CuO),copper chromite(CuO,Cr2O3),nickel oxide(NiO)etc.CuO particles demonstrated the lowest ignition temperature as well as high oxygen release rate at low temperature(Fig.3).

Consequently CuO particles could the ideal catalyst to accelerate the decomposition rate of APC oxidizer[21].Recent investigations have demonstrated that nanoparticles of transition metal oxides,without any agglomeration,can offer enhanced catalyzing ability[22].There are great advantages for any synthesis technique that could offer fabrication of mono-dispersed particles in dispersion.Such technique is hydrothermal processing.

1.2. Hydrothermal processing

Fig.3.Temperature at which O2 was observed as oxidizer was heated alone(Y-axis)versus the ignition temperature when oxidizer was mixed stoichiometrically with Al nanoparticles(X-axis).

Fig.4.Phase diagram of water PVT curve.

Whatever the synthesis technology,as nanoparticles are dried they tend to agglomerate and aggregate with dramatic decrease in their surface area and reactivity[23].Therefore,these particles would act as micron particles rather than nanoparticles.Hydrothermal processing technique can offer sustainable fabrication of mono-dispersed particles with consistent product quality[24].

Hydrothermal processing includes instant mixing of metal salt fed with supercritical fluid.The synthesis conditions including precursor feed rate,reaction temperature,reaction pressure,and residence time can be precisely controlled[25].Mono-dispersed particles were likely to form as nucleation and subsequent crystal growth were similar for all particles[26].Therefore,hydrothermal processing could be a perfect technique for very fine powder processing with high purity,narrow particle size distribution,and controlled stoichiometry[27,28].

The employed fluids for hydrothermal processing are supercritical fluids[29].A supercritical fluid(ScF)is defined as any fluid above its critical values[27].Beyond the critical point,the phase boundary disappears and a homogenous supercritical phase exists[30,31]. Even though, supercritical water (ScW) necessitates extreme conditions(Tc374.2°C,Pc220.5 bar)(Fig.4);it is one of the most common fluid in use for hydrothermal processing[32].

ScW imposes exceptional properties related to improved number of hydrogen ions(H+)and hydroxide ions(OH-)[33-38].The superior levels of OH-at the critical point can be employed for nanoparticle synthesis as established by Adschiri[39,40].Under these extreme conditions hydrolysis(Eq.(1))of metal salts is instantly followed by a dehydration step(Eq.(2))[41-43].

Hydrothermal processing includes instant mixing of ScW with metal salts;nanoparticles are formed at the interface of the two fluids[44].Hydrothermal processing offers controlled synthesis conditions via controlled reactant feed rate,reaction temperature,pressure, and residence time [45,46]. Therefore, hydrothermal processing can offer a relatively simple route for fabrication of different nanoparticles with high purity,controlled stoichiometry,and high crystallinity[22,23].

This study reports on the sustainable fabrication of monodispersed colloidal CuO particles of 15 nm particle size by using straightforward continuous hydrothermal processing.CuO were developed in colloidal form and were effectively coated with APC particles by using fast-crash solvent-antisolvent technique.Thermal behavior of APC encapsulated with 1 wt%CuO has also been investigated to APC by using TGA and DSC.The main outcome of this study is that CuO nanoparticles offered a decrease in the initial endothermic decomposition of APC by 30%.Furthermore the subsequent two main exothermic peaks were merged into one single peak at 350°C.Additionally CuO offered an increase in total heat release of APC by 53%.It can be concluded that hydrothermal processing not only offered effective fabrication of novel colloidal catalyzing agent but also enabled the effective integration into APC particles.This novel catalyzing agent can find wide applications in advanced energetic systems.

2. Experimental work

2.1. Materials

Copper acetate(Aldrich)was employed as metal salt precursor for the hydrothermal processing of CuO.Ammonium perchlorate with an average particle size of(200 μm)was purchased from(Aldrich).Acetone(Aldrich)was employed as a solvent for APC.All the reagents were analytical-grade chemicals.

2.2. Hydrothermal synthesis of CuO

Supercritical water was employed at 400°C (20 ml?min-1)(Flow A). An aqueous solution of 0.05 M copper acetate was employed as metal salt precursors at 25°C(10 ml?min-1)(Flow B).The whole system was pressurized to 240 bars using back pressure regulator(Fig.5).

Colloidal CuO nanoparticles were synthesized at the interface of the two streams inside the reactor(R).The developed particles were cooled down to 60°C prior to collection at point D.

2.3. Characterization of CuO nanoparticles

The size and shape of colloidal CuO were visualized with TEM(JEM-2100F by Joel Corporation).The crystalline phase was investigated with XRD D8 advance by Burker Corporation over the angle range 2θ from 5°to 65°.The dry powder size and shape were investigated with SEM,ZEISS SEM EVO 10 MA.

Fig.5.Flow diagram of the continuous hydrothermal synthesis system used for the instant production of CuO.

2.4. Coating of CuO with ammonium perchlorate

Synthesized CuO nanoparticles were effectively re-dispersed in acetone by using ultrasonic bath.APC particles were dissolved in acetone colloid.The weight ratio of APC/CuO was 99:1.CuO particles were coated with APC by using the fast-crash solvent-antisolvent technique using dichloromethane antisolvent.The precipitated composite particles were filtered and dried in a vacuum oven.The size and shape of composite particle diameter were investigated using SEM.

2.5. Thermal behavior of ammonium perchlorate encapsulated with CuO

The impact of CuO nanoparticles on APC thermal behavior was investigated by using DSC in an attempt to evaluate the change in endothermic decomposition peak as well as total heat released upon oxidizer complete decomposition.DSC Q20 by TA,USA was employed;the tested sample was heated from 50°C to 500°C with heating rate 5°C?min-1under constant flow of N2gas at 50 ml?min-1.The impact of CuO on weight loss with temperature was investigated by using TGA 55 by TA,USA.The tested sample was heated from 50°C to 500°C at heating rate 5°C?min-1under N2flow rate at 25 ml?min-1.

3. Result and discussions

3.1. Characterization of synthesized CuO nanoparticles

The size and shape of synthesized CuO particles were visualized using TEM.TEM micrographs demonstrated mono-dispersed CuO particles of 15 nm average particle size(Fig.6(a)).The diffraction pattern of the incident beam demonstrated mono-crystalline structure(Fig.6(b)).

The synthesized colloidal CuO nanoparticles were dried.The crystalline structure of dried particles was investigated with X-ray diffraction(XRD).XRD diffractogram demonstrated high quality crystalline structure.The main diffraction peaks were found to be in good agreement with Joint Committee on Powder Diffraction Standards(JCPDS)from the International Centre for Diffraction Data(ICDD)according to(PDF-04-007-1375)(Fig.7).

The average particle size of synthesized CuO nanoparticles was further evaluated using Debye-Scherer(Eq.(3))by employing the major diffraction peak[47].

Fig.6.TEM micrographs of synthesized CuO nanoparticles.

Fig.7.XRD diffractogram of synthesized CuO nanoparticles.

Wherekis a constant equal to 0.94,λ is the wavelength of Cu Kα radiation,β is the full width at half maximum height(FWHM)of the diffraction peak in radians,and θ is the Bragg angles of the main planes.Evaluation of particle size of CuO nanoparticles by Scherer equation was found to be 15 nm.This result was found to be in good accordance with TEM micrographs.

CuO particles were dried. SEM micrographs of dried CuO demonstrated a great tendency of CuO particles to aggregate and agglomerate with dramatic decrease in their interfacial surface area and reactivity(Fig.8).Agglomerates include strongly bonded or fused particles where the resulted external surface area could be reduced[33,48].

The size and shape of starting APC particles was investigated with SEM.SEM micrographs demonstrated cubic particles with wide range particle size 150-200 μm(Fig.9(a));elemental mapping demonstrated uniform element distribution(Fig.9(b)).

3.2. Characterization of APC encapsulated with CuO

APC encapsulated with CuO was investigated with SEM in an attempt to investigate the particle size and morphology(size and shape).SEM micrographs demonstrated ultra fine particles with uniform particle size of 3 μm(Fig.10).

Fig.8.SEM micrographs of dry CuO nanoparticles.

Elemental analysis of APC encapsulated with CuO was investigated by using energy dispersive X-ray spectrometer(EDX)Bruker Quantax 200 equipped with SEM(Fig.11).

Elemental analysis confirmed the chemical structure of APC encapsulated with CuO no interfering elements were reported.It can be concluded that the employed fast-crash solvent-antisolvent technique offered the development of ultra fine APC particles with uniform particle size as well as effective coating of CuO particles into APC oxidizer.

3.3. Catalytic activity measurements

The effectiveness of CuO catalyst for APC oxidizer was investigated by using DSC.APC thermal decomposition took place in three main stages as follow[49]:

a)The first stage is the initial endothermic decomposition at 242.1°C;this decomposition is accompanied with heat absorption of 102.5 J?g-1.

Fig.9.SEM micrographs.

Fig.10.SEM micrographs of APC encapsulated with CuO.

b)The second stage is the partial exothermic decomposition at 297.8°C with the formation intermediate gaseous such as NH3and HClO4via incomplete dissociation and sublimation(Eq.(4))[50,51].This partial decomposition of APC releases amount of heat 345.5 J?g-1.

Fig.12.DSC thermogram of APC encapsulated with CuO to pure APC.

c)The third stage is the second exothermic decomposition appears at 452.8°C.Complete decomposition of APC took place with the production of several final volatile molecules,such as HCl,H2O,N2O,Cl2,NO,O2,and NO2(Fig.12).

CuO demonstrated high catalytic activity with dramatic change in thermal behavior of APC particles.CuO demonstrated an effective role as catalyst for APC oxidizer as follow:

1)The initial endothermic decomposition heat was decreased from 102.5 to 71.2 J?g-1.This step behavior means high reactivity and less activation energy is required for decomposition of APC particles.This endothermic process is the rate determining step in combustion of different energetic systems.

2)The subsequent two exothermic decomposition peaks were merged into one single exothermic broad peak with total heat release of 1268.4 J?g-1at 350.1°C.

3)CuO offered an increase in total heat release of APC by 53%.

Thermal behavior of pure APC was further investigated with TGA;the wt%was recorded as function of temperature(Fig.13).APC demonstrated two main decomposition stages as follow:

Fig.13.TGA thermogram of pure APC.

1)The first partial decomposition stage at 298°C with weight loss of 30 wt%.This decomposition stage could be correlated to the initial exothermic decomposition peak in DSC at 297.8°C(345.5 J?g-1).

2)The second main decomposition stage at 452°C with wt%loss of 69.9%;this could be ascribed to complete dissociation of APC.This decomposition peak could be corresponding to second main exothermic decomposition peak in DSC at 452.8°C(489.8 J?g-1).

The main outcome of this study is that CuO particles demonstrated a complete shift of the main two exothermic decomposition peaks into one single peak(Fig.14).

CuO offered not only decrease in required heat for APC initial decomposition,but also the two exothermic peaks were merged into one single peak.

4. Conclusion

Colloidal CuO with consistent product quality and average particle size of 15 nm were manufactured in continuous manner by using hydrothermal processing.Effective coating of CuO with APC oxidizer was accomplished via fast-crash solvent-antisolvent technique.CuO particles demonstrated superior catalytic activity by decreasing the endothermic decomposition stage by 30%.Furthermore CuO demonstrated unique thermal behavior;the two main exothermic decomposition peaks were merged into one peak with an increase in total heat release by 53%.This novel thermal behavior of catalyzed APC particles could open the route for advanced energetic systems where high burning rate regime is required.

Fig.14.TGA thermogram of APC encapsulated with 1 wt%CuO.