Accepted Manuscript

Title: Manganese oxalate nanorods as ballistic modifier forcomposite solid propellants

Author: Supriya Singh Mohit Chawla Prem Felix Siril GurdipSingh

PII: S0040-6031(14)00477-8DOI: http://dx.doi.org/doi:10.1016/j.tca.2014.10.016Reference: TCA 77043

To appear in: Thermochimica Acta

Received date: 5-8-2014Revised date: 13-10-2014Accepted date: 15-10-2014

Please cite this article as: Supriya Singh, Mohit Chawla, Prem Felix Siril, Gurdip Singh,Manganese oxalate nanorods as ballistic modifier for composite solid propellants,Thermochimica Acta http://dx.doi.org/10.1016/j.tca.2014.10.016

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Manganese oxalate nanorods as ballistic modifier for composite solid propellants

Supriya Singha, Mohit Chawlab, Prem Felix Sirilb*prem@iitmandi.ac.in, Gurdip Singha

aDepartment of Chemistry, DDU Gorakhpur University, Gorakhpur 273009, U.P., India

bSchool of Basic Sciences, Indian Institute of Technology Mandi, Mandi-175005, H.P., India

*Corresponding author. Phone: 91-1905-300040

Graphical abstract

Highlights

Manganese oxalate nanorods were prepared using mild thermal precipitation and aging

The nanorods were found to be efficient ballistic modifier for solid propellants

The nanorods sensitized the thermolysis of ammonium perchlorate

Controlled thermal decomposition of nanorods yielded manganese oxide nanoparticles

MnO nanoparticles formed insitu in the condensed phase enhance the burning rates

Abstract

Rod-shaped nanostructures of manganese oxalate (MnC2O4) were synthesized via mild thermal

precipitation and aging process. Chemical composition of the MnC2O4 nanorods were confirmed

using Fourier transform infra-red (FTIR) spectroscopy and energy dispersive X-ray spectroscopy

(EDS). X-ray diffraction (XRD) and selected area electron diffraction (SAED) studies revealed

the crystal structure. Field emission scanning electron microscopy (FE-SEM) imaging and high

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resolution transmission electron microscopy (HR-TEM) were employed to study the structural

features of the nanorods. The MnC2O4 nanorods were found to be efficient ballistic modifier for

the burning rate enhancement of composite solid propellants (CSPs). Thermal analysis using

TGA-DSC showed that MnC2O4 nanorods sensitized the thermal decomposition of ammonium

perchlorate (AP) and the CSPs. Controlled thermal decomposition of the MnC2O4 nanorods

resulted in the formation of managanese oxide nanoparticles with mesoporosity. A plausible

mechanism for the burning rate enhancement using MnC2O4 nanorods was proposed.

Key words: manganese oxalate nanorods; nanocatalysts; ammonium perchlorate; manganese

oxide nanoparticles; composite solid propellants.

1. Introduction

CSPs are the major source of chemical energy in space vehicles and missiles. Composite

propellants are composed of crystalline oxidizer particles dispersed in a polymeric fuel binder.

AP is one of the main oxidising agents that are commonly used in CSPs while

hydroxyterminated polybutadiene (HTPB) is used as the fuel binder. Ballistic modifiers (burning

rate modifiers) are incorporated in the formulation to achieve the required burning rates. Burning

rate is one of the most important combustion characteristics for CSPs, as enhancement in it

results in higher specific impulse. Specific impulse of CSPs determines the payload and range of

rockets and missiles. Nanoparticles of transition metal salts, oxides and mixed metal oxides are

effective ballistic modifiers for CSPs [1-7]. They enhance the burning rate of CSPs by enhancing

the thermal decomposition of AP [1-7]. Iron oxide and copper chromite are the present catalysts

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in vogue for burning rate enhancement of CSPs [8]. Recently, there has been a steady increase in

interest in manganese oxide nanoparticles as catalyst for thermal decomposition of AP ever since

Singh et. al. reported that nano-MnO have better catalytic activity compared to nano particles of

CuO, Fe2O3 and Cr2O3 [9]. Interestingly MnO2 is one of the very first catalysts that were

reported for accelerating the thermal decomposition of AP way back in the year 1959 [10].

Recently, it was reported that mesoporous β-MnO2 showed exceptional catalytic activities, for

the thermolysis of AP. [11]. Well dispersed Mn3O4 nanoparticles supported on Graphene was

also found to be showing exceptional catalytic activity for the thermal decomposition of AP [12].

Catalysed thermal decomposition of AP in presence of nanoparticles of manganese

oxyhydroxide (MnOOH) supported on Graphene was also reported. [13]. Thus it was found

interesting to develop manganese based catalyst for the thermal decomposition of AP. Here we

report the exceptional catalytic activity of MnC2O4 nanorods for the thermal decomposition of

AP and their potential as ballistic modifier for CSPs.

It has been widely reported in the literature that catalytic activity of TMOs is

concentration dependent [14]. However, increasing the concentration of TMOs in CSPs leads to

decrease in total energy of CSPs as the TMOs themselves are non-energetic in nature [15]. We

have proposed the use of energetic catalytic precursors that can generate TMOs insitu during the

combustion of CSPs to alleviate this problem. In the present paper MnC2O4 nanorods were used

as precursor for insitu generation of managanese oxide nanoparticles during the combustion of

CSPs. Oxalate nanostructures have been synthesized as a precursor for obtaining the

corresponding nanostructured metal oxides [16]. Nanorods of manganese oxalate were

previously reported as a single source for preparing various manganese oxides through different ACCEPTED MANUSCRIP

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reaction routes [17, 18]. Manganese oxide has extensive application in catalysis, ion exchange

and batteries [19].

A facile green route for the synthesis of MnC2O4 nanorods, having potential to act as

efficient catalyst precursors, by mild thermal precipitation and aging process is reported here.

The prepared nanorods were found to be excellent ballistic modifiers for CSPs as they enhanced

the thermal decomposition of AP. Moreover, it was found that mesoporous manganese oxide

nanostructures can be prepared by controlled thermal decomposition of manganese oxalate

nanorods. The manganese oxide nanostructures that are formed insitu during combustion

catalyze the thermal decomposition of AP and enhance the burning rates.

2. Experimental

2.1. Materials

AP was obtained from CECRI, Karaikudi and HTPB from VSSC, Thiruvanthapuram.

Isophoron diisocynate (IPDI), dioctyl adipate (DOA) MnCl2.4H2O, polyvinylpyrrolidone (PVP),

NH3.H2O and (NH4)2C2O4 were purchased from Merck and were used without further

purification.

2.2 Synthesis of Manganese oxalate nanorods.

The manganese oxalate nanorods were prepared by slightly modifying a reported procedure for

the synthesis of nickel oxalate nanofibres [20]. In a typical synthesis, MnCl2.4H2O (2.97 g),

(NH4)2C2O4 (1.95 g) and PVP (0.15 g) was dissolved in water (70 mL) and then heated to 60 oC.

After a few minutes, ammonia solution was dropped slowly into the mixture to adjust the pH

value to 8.0. The obtained solution was then incubated for 18 hours at 60 °C. The precipitate

obtained after the incubation process was filtered and washed multiple times with water and

butanol. Finally the product was dried at 90 °C for 12 hours.

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2.3 Characterizations

The FTIR spectrum of the manganese oxalate was recorded (Perkin Elmer Spectrum II) from

4000 cm-1 to 600 cm-1 frequency range with a resolution of 4 cm-1 and 8 scans. The samples were

properly grounded with KBr powder and then pressed to obtain a suitably sized pellet for

recording FTIR spectrum. Pure KBr pellet was used for background correction. XRD pattern of

the sample was recorded on a Smart Lab X-Ray Diffractometer (Rigaku, Japan) using Cu Kα

radiation as X-ray source (λ = 0.15418 nm) at room temperature. The voltage and current applied

were 45 kV and 100 mA respectively. The morphology and structure of the sample was

characterized by FE-SEM. The particle size, shape and microstructure were observed using HR-

TEM and SAED patterns (FEI Tecnai G20 S-twin, 200kV). Thermal analysis was carried out by

using Netzsch TGA-DSC, STA F1 Jupiter model by heating small amount of samples from

ambient to 1000 0C at a heating rate of 10 0C min-1, under nitrogen atmosphere in alumina

crucibles covered with lid having a pin hole.

2.4 Preparation of CSPs

CSP samples were prepared by dry mixing of AP (100-200 mesh) with and without MnC2O4

nanorods (1% by wt.) followed by addition of HTPB [21]. The solid materials were mixed with

HTPB in the ratio of 3:1 at 70 oC for 1 h. Then curing agent (IPDI) in equivalent ratio to HTPB

and plasticizer (DOA, 30% by wt.) to HTPB was added. The slurry was casted into aluminum

plates having dimensions 1x3x10 cm. The samples were cured in an incubator at 70 oC for 9 days

[15].

2.5. Ballistic modification

2.5.1 Burning rate measurements ACCEPTED MANUSCRIP

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The burning rates of CSP samples at an ambient pressure were measured by the same method as

reported earlier [15]. The propellant strands were inhibited by applying PVC tape to check the

side burning. The vertically held strands were ignited electrically with the help of a Nichrome

wire at the top. The time required to burn a certain length of the strand was recorded by a stop

watch. The burning rate has been calculated by using l/t where l is length of propellant strand

(mm) and t the time (s) of burning. Average of 3 measurements was taken and reported.

2.5.2 Simultaneous TGA-DSC

TGA-DSC thermal curves for AP with and without MnC2O4 nanorods (1, 2 & 5 % by wt.) were

recorded on the samples using Netzsch TGA-DSC, STA F1 Jupiter model by heating small

amount of sample from ambient to 700 °C at a heating rate of 10 °C min-1, under nitrogen

atmosphere in alumina crucibles with a lid having a pin hole.

2.5.3 Non-isothermal TGA in static air

Non-isothermal TGA studies on AP and CSPs with and without MnC2O4 nanorods (wt. ~ 20

mg) were undertaken in static air at the heating rate of 10 °C min-1 using an indigenously

fabricated TG apparatus [22]. A round bottom gold crucible was used as the sample holder.

2.5.4 Ignition delay (Di) measurements

The ignition delay (Di) was measured using a tube furnace (TF) technique [23]. 20 mg (100-200

mesh) of samples for AP (with and without catalyst) and CSPs (temperature range 345-420°C)

was used for the measurements. The accuracy of the temperature measurement of tube furnace

was ±1°C. The samples were taken in an ignition tube (5 cm length x 0.4 cm diameter) and the

time interval between the insertion of the ignition tube into the TF and the moment of appearance

of a flame was noted with the help of a stop watch with accuracy of 0.1 s to get the value of

ignition delay (Di). The ignition tube clamped in a bent wire was inserted manually into the

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furnace up to a fix depth (14 cm) just above the probe of the temperature indicator cum

controller (Century, Chandigarh). The time for insertion of the ignition tube was also kept

constant. Each run was repeated three times, and mean Di values are reported. The Di data were

found to fit in equation, [24-26]

Di = A exp (Ea* / RT) ...............................(1)

where Di is the time of ignition (ignition delay), A is a constant, Ea* is the energy of activation

(ignition) and T is the absolute temperature.

3. Results and discussion

The manganese oxalate sample prepared was white in color. FTIR spectrum of the

manganese oxalate nanorods is shown in Figure 1. The frequency values and assignment of the

peaks observed are enlisted in Table T1 (supporting information). The characteristic bands at

3382 cm-1, 1310 cm-1 and 615 cm-1 reveal the formation of manganese oxalate dihydrate [20].

XRD pattern of the manganese oxalate sample is shown in Figure 2. The diffraction peaks of

sample can be indexed to the pure MnC2O4.2H2O (JCPDS-32-0646). An EDS spectrum (Figure

3) was recorded over 50 particles with different sizes and in different areas for a statistical view

of the composition of individual particles. It was found that the oxalate product is composed of

elements of Mn, C and O. The atomic ratio of Mn:O was approximately 1:5 with the uniform

distribution of the elements. This elemental ratio also suggested that the prepared product was in

the form of MnC2O4.2H2O. The particle size and morphology of MnC2O4 nanorods were

investigated using FE-SEM and TEM. From FE-SEM image given in Figure 4(a), it can be seen

that the MnC2O4 is generally rod shaped. The rods have a cubical cross section. A typical TEM

image of a MnC2O4 nanorod is shown in Figure 4(b). The sample contained nanorods of

diameter in the range of 100 - 500 nm with length extending upto many micrometers. HR-TEM

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image of an individual MnC2O4 nanorod is shown in Fig. 4(c). Lattice spacing of 0.19 nm

indicates the dominant presence of well-defined (123) plane. The SAED pattern shown in Fig.

4(d) demonstrated that particles are single crystalline. The SAED pattern also showed the

presence of the some of the distinct planes corresponding to MnC2O4 in the selected area.

Results reported in Table 1, clearly showed that burning rate was enhanced when

MnC2O4 nanorods were used as additives. The TGA thermal curves shown in Fig.S1 (supporting

information) also indicate that the decomposition temperature of CSPs was also lowered in

presence of MnC2O4 nanorods. Increase in the burning rate of AP based CSPs in presence of

burning rate modifiers is usually attributed to the enhanced decomposition of AP in the CSPs [2-

8]. In order to find out the mechanism of burning rate enhancement in presence of MnC2O4

nanorods, TGA-DSC analysis of AP+ (MnC2O4 nanorods) was undertaken. The catalytic activity

of transition metal salts and oxides is often concentration dependent in the combustion of CSPs

[2-8]. Hence, the effect of MnC2O4 on the thermal decomposition of AP was studied for different

amounts of MnC2O4 nanorods viz. 1%, 3% and 5% respectively. TGA, DTG thermal curves for

AP in presence of different concentration of MnC2O4 nanorods is presented in Figure 5 and the

phenomenological data is summarized in Table T2 (supporting information). It is evident from

the figures that MnC2O4 nanorods affect the thermal decomposition of AP. It can be seen from

the TGA and corresponding DTG curves that the thermal decomposition of AP is a two-step

process. The presence of MnC2O4 resulted in considerable lowering of the decomposition

temperature of AP. The two step decomposition process of AP was converted into a single step

rapid decomposition process in presence of MnC2O4. However, increasing the MnC2O4 loading

resulted in only minor decrease in the decomposition temperature. The corresponding

phenolenological data is reported in Table T2 (supporting information).

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The DSC thermal curves for thermal decomposition of pure AP and AP in the presence of

1, 3 and 5 (% by wt.) of MnC2O4 are shown in Figure 6 and the corresponding phenomenological

data is reported in Table T3 (supporting information). The DSC thermal curve for thermal

decomposition of pure AP showed three stages. The endothermic peak at 244 oC is ascribed to

the solid state phase transition from orthorhombic to cubic [8]. The exothermic peak at 291 oC is

attributed to the partial decomposition of AP and is also known as low temperature

decomposition (LTD). The third peak appears at relatively higher temperature of 406 °C

corresponds to the high temperature decomposition (HTD). However, the addition of MnC2O4

has apparently influenced the thermal decomposition of AP. From Fig.6 and Table T3

(supporting information) it is clear that MnC2O4 nanocrystals has no influence on

crystallographic transition temperature of AP, while dramatic changes were observed in

exothermic peaks in relatively high temperature region after adding MnC2O4 nanocrystals. The

LTD and HTD peaks merged and a single exothermic peak was observed at 308 0C in presence

of 1 % (w/w) catalyst. Moreover, with increase in MnC2O4 loading, no significant lowering of

peak temperature was observed. However, the increase in MnC2O4 loading resulted in a

considerable increase in heat release as can be seen from Table T3 (supporting information). The

increase in heat release can be attributed to the relatively increased condensed phase thermolysis

of AP relative to dissociative sublimation and gas phase decomposition.

The catalytic activity of the MnC2O4 nanocrystals in the thermal decomposition of AP

and CSPs was also measured by using ignition delay of AP and CSPs in presence of MnC2O4

nanocrystals (1% by wt.). Ignition delay data in the temperature range 345-420 oC is presented in

Table 2. The plot of lnDi vs 1/T is given in supporting information (Figure S2). Ea* evaluated

using equation (1) along with the correlation coefficient (r) are also reported in Table 2. Both the

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Di and the activation energy for ignition of AP and CSPs were lowered by adding MnC2O4

nanocrystals. These results clearly indicate that these MnC2O4 nanocrystals act as good catalyst

during deflagration and decreases activation energy for ignition.

It is also reported that overall decomposition process of AP takes place by the transfer of

proton (N-H bond cleavage) from the ammonium ion to ClO4- to form the NH3 and HClO4

molecule in the condensed phase before ignition [8, 27]. NH3 and HClO4 may escape the

condensed phase without further reaction leading to dissociative sublimation [28]. HClO4 may

involve in oxidation-reduction with ammonia to form gaseous products, if the conditions are

favourable. Following is the reaction scheme for AP thermolysis, suggested by many workers [8,

27].

)

) )

Products

It is proposed that the rate controlling step in the thermal decomposition of AP is

associated with proton transfer. In general, high-surface area nanocatalytic materials exhibiting

numerous crystal faces, edges and corners which are conventionally considered active sites for

the adsorption of reactants should generate better catalytic performance [29]. In the present

work, the catalytic activity of MnC2O4 nanorods is mainly in the HTD of AP and is not very

much concentration dependent. This is contrary to the catalytic behavior of most transition metal

oxide catalysts and salts [3-10]. This may be because MnC2O4 itself may be decomposing before

the decomposition of AP is initiated. The decomposition product of MnC2O4 may be not efficient

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in promoting proton transfer reaction, but must be providing active sites for the adsorption of

dissociation products of AP and thus promote condensed phase reaction of NH3 and HClO4.

In order to get a better insight into the mechanism of catalysis we also investigated the

thermal decomposition of MnC2O4 nanorods in nitrogen and in air (Fig. 7). The thermal

decomposition data of the MnC2O4 nanorods matched well with the data reported in the literature

[30]. The sample was heated from ambient temperature to 1000 °C at the rate of 10 °C min-1. In

nitrogen atmosphere the decomposition of MnC2O4 nanorods involved a two step dehydration

process initially between 100 °C and 170 °C (Fig. 7). A rapid decomposition in the range of 230

°C and 330 °C is attributed to the formation of a poorly defined structure [30]. Further heating

resulted in the formation of manganese (III) oxide at 450 °C which on further heating at 900 °C

changes into manganese (II) oxide. The decomposition in air is similar to that in the presence of

nitrogen except that there is an increase in the mass at higher temperatures. This increase in the

mass can be attributed to the formation of Mn3O4 [30]. The TEM images of the products obtained

after the thermal decomposition of MnC2O4 nanorods in nitrogen and air are shown in Figure 8

and Figure 9 respectively. The XRD pattern of the residue obtained after heating MnC2O4

nanorods in air is shown in Figure 10. The electron diffraction patterns and X-ray diffraction

patterns of MnO (JCPDS Card no. 7-230) and Mn3O4 (JCPDS Card no. 24-734) were indexed in

accordance with the ICDD database. From the figures, it is clear that the manganese oxide

formed has a very fine particle size in the range 5 nm to 30 nm. Moreover, the particles do not

fall apart and are tightly adhered to each other to form a highly porous mesostructure. Formation

of porous manganese oxide nanoparticles by controlled thermal decomposition of manganese

oxalate is reported in the literature also [17, 18]. ACCEPTED MANUSCRIP

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In case of propellants, the temperature in the condensed phase is very high of the order of

500-600 °C when the burning or deflagration starts. This can lead to the formation of porous

manganese oxide nanoparticles in the condensed phase and result in the enhanced burning rates.

Thus, the enhanced burning rates in the present study can be attributed to the formation of porous

manganese oxides in the condensed phase. The higher heat release during catalyzed thermolysis

of AP that was observed with the increase in concentration of the catalysts may be due to the

completion of thermolysis in the condensed phase. Insitu formation of manganese oxide during

combustion yield higher specific surface area and higher surface energy and provide active sites

that enhances the adsorption of gaseous products, thus leading to higher reaction rates. Thus

MnC2O4 nanorods acted as effective catalyst precursors in the thermal decomposition of AP and

CSPs.

It was thought useful to compare the catalytic activity of the MnC2O4 nanorods with other

manganese nano-catalysts that are reported in the recent literature for thermal decomposition of

AP. Catalytic activities of some of the manganese based catalysts are summarized in Table T4

(supporting information). The catalytic activities are compared on the basis of peak temperature

corresponding to the HTD of AP. Lower the value of the peak temperature, higher is the catalytic

activity. Mesoporous MnO2 seems to be the best catalyst. However, a direct comparison of this

catalyst with the others is difficult as the heating rate used for DSC measurements was 5 0C min-1

whereas the heating rate was 10 0C min-1 in all other reports. Manganese oxalate fared as one of

the best catalysts when the concentration was 1% (wt./wt.) in AP. This is interesting as the

concentration of the effective catalyst (i.e. manganese oxide) is lower than the other manganese

oxide catalysts. However, the drawback of MnC2O4 nanorods seems to be its lower concentration ACCEPTED MANUSCRIP

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dependency. The unusual results for these nanorods inspire us to hope that using transition metal

salt nanostructure could lead to better catalysts for the combustion of CSPs.

4. Conclusions

Uniform nanorods of manganese oxalate were synthesized through a mild thermal

precipitation and ageing process. These nanorods acted as efficient ballistic modifier for CSPs.

Manganese oxides with ultrafine particle size and with mesoporous structure could be obtained

by controlled thermal decomposition of manganese oxalate nanords. Manganese oxide

nanoparticles that are formed in the condensed phase of CSPs catalyse the ammonium

perchlorate thermolysis and thus enhance the burning rate.

5. Acknowledgements

The authors are grateful to Advanced Materials Research Centre (AMRC), IIT Mandi and

Head, Chemistry Department of DDU Gorakhpur University for laboratory facilities. Thanks are

also due to financial assistance by DST for providing Emeritus Fellowship to G. Singh, INSPIRE

fellowship to Supriya Singh and Senior Research Fellowship to Mohit Chawla.

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3.

Caption of figures

Fig.1 FTIR spectra of MnC2O4nanorods. Fig.2 XRD pattern of MnC2O4 nanorods (Inset: XRD pattern indexed from 2θ=300 to 2θ=500).

Fig.3 Energy Dispersive X-ray spectrum of MnC2O4 nanorods.

Fig.4 Electron microscopy images of MnC2O4 nanorods (a) FE-SEM (b) TEM (c) HR-TEM

(Inset: magnified image of the rectangle marked in (c) showing the fringe width) (d) SAED

pattern.

Fig.5 TGA thermal curves (left) and DTG curves (right) of AP and AP with MnC2O4 nanorods

(1%, 2% and 3% respectively by wt.).

Fig.6 DSC thermal curves of AP and AP with MnC2O4 nanorods (1%, 2% and 3% respectively

by wt.).

Fig. 7 TGA thermal curves for the thermal decomposition of MnC2O4 nanorods.

Fig.8 (a-c) Representative TEM images of MnO obtained after heating MnC2O4 nanorods in

nitrogen till 1000°C, (d) selected area diffraction of the same.

Fig.9 (a,b) Representative TEM images of Mn3O4 obtained after heating MnC2O4nanorods till

1200°C, (c) HRTEM image, (d) selected area diffraction of the same.

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Fig.10 XRD pattern of Mn3O4 nanoparticles obtained after heating MnC2O4 nanorods in air till

1200 °C.

Table 1 Burning rate of CSPs with and without MnC2O4 nanorods.

Additive Burning rate (mm/s) rc/r*

Nil 1.2±0.2 1.0 1% by wt. of

MnC2O4 2.5±0.3 2.1

Table 2 Ignition delay (Di), activation energy for ignition (Ea) and correlation coefficient

of AP and CSPs with and without MnC2O4 nanorods

Sample

Di (s)

Ea(kJ/mol) r 345 ±10C

360 ±10

C

375 ±10C

390 ±10C

405 ±10C

420 ±10C

AP DNI1 125 112 95 84 63 40±3 0.9785 AP + MnC2O4 (1%) DNI1 57 34 32 31 29 36±3 0.8511

CSP DNI1 92 86 67 64 49 38±2 0.9724 CSP + MnC2O4 (1%) DNI1 34 32 31 25 20 31±2 0.9357

1DNI= Did Not Ignite

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Fi

Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Fig. 6

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Fig. 7

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Fig. 8

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Fig. 9

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Fig. 10

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