Cu-Co-O nano-catalysts as a burn rate modif i er for composite solidpropellants

D.Chaitanya Kumar RAO，Narendra YADAV*，Puran Chandra JOSHI

Space Engineering and Rocketry，Birla Institute of Technology，Mesra，Ranchi 835215，India

Cu-Co-O nano-catalysts as a burn rate modif i er for composite solid
propellants

D.Chaitanya Kumar RAO，Narendra YADAV*，Puran Chandra JOSHI

Space Engineering and Rocketry，Birla Institute of Technology，Mesra，Ranchi 835215，India

Nano-catalystscontainingcopper-cobaltoxides（Cu-Co-O）havebeensynthesizedbythecitricacid （CA）complexingmethod.Copper（II）nitrate and Cobalt （II）nitrate were employed in different molar ratios as the starting reactants to prepare three types of nano-catalysts.Well crystalline nano-catalystswereproducedafteraperiodof3hoursbythecalcinationofCA-Cu-Co-Oprecursorsat550 °C.Thephasemorphologiesandcrystal compositionofsynthesizednano-catalystswereexaminedusingScanningElectronMicroscope （SEM），EnergyDispersiveSpectroscopy （EDS）and Fourier Transform Infrared Spectroscopy （FTIR）methods.The particle size of nano-catalysts was observed in the range of 90 nm-200 nm.The prepared nano-catalysts were used to formulate propellant samples of various compositions which showed high reactivity toward the combustion of HTPB/AP-basedcompositesolidpropellants.Thecatalyticeffectsonthedecompositionofpropellantsampleswerefoundtobesignif i cantathigher temperatures.Thecombustioncharacteristicsofcompositesolidpropellantsweresignif i cantlyimprovedbytheincorporationofnano-catalysts.Out of the three catalysts studied in the present work，CuCo-I was found to be the better catalyst in regard to thermal decomposition and burning nature ofcompositesolidpropellants.Theimprovedperformanceofcompositesolidpropellantcanbeattributedtothehighcrystallinity，lowagglomeration and lowering the decomposition temperature of oxidizer by the addition of CuCo-I nano-catalyst.

Metal oxides；Nano-catalysts；Solid propellant；Burn rate；Surface morphology；Thermal analysis

1.Introduction

Transition metal oxides （TMOs）have widely been employed as solid-state catalysts for various energy applications ［1，2］.It has been demonstrated that mixed metal oxides （MMOs）are capable of showing better catalytic effects than a single-phase TMO，because of their synergic effects ［3，4］.The assessment of bimetallic （Cu-Cr）catalysts has been optimized usefully in the synthesis of some industrial hydrocarbons ［5］.The nanocomposite of Cu-Cr-O，and nano-alloys of Cu-Ni and Cu-Zn have also been investigated as combustion catalysts in many solid propellants ［6-9］.Such catalysts，which were found to alter the burning rate，are always considered as a great interest in modifying the ballistic properties of solid rocket motors.

The MMO nano-catalysts and their synthetic modes have often determined the physical and chemical properties，evenwith the same starting composition.The crystallinity，interdispersion of chemical constituents and specif i c surface area are the relevant parameters which affect the catalytic performance，which in turn depends primarily on the synthetic methodologies ［10］.A considerable effort has been made to f i nd some novel synthetic routes which can give precursor of two or more metals in a single crystalline phase.This has resulted in a well inter-dispersed matrix of MMOs of required specif i c surface area and particle size.The performance of application materials can be increased or decreased by the inclusion of such catalysts in a fewer percent ratios.The MMOs have great inf l uence on the thermal decomposition behaviors of the oxidizer masses in the solid propellants ［11-21］.Recent investigations have shown that MMOs in nano-scales can increase the burning rate effectively.Styborski et al.［14］carried out the burning rate measurements for AP/HTPB composite propellants with iron nanoparticles as additives.They found that the addition of 1% Fe nanoparticles increased the burning rate of propellant by factors of 1.2-1.6.Dave et al. ［15］prepared TiO2，ZnO2，and CrO2nanoparticles by quick precipitation method andobserved an increase in the thermal decomposition of propellants in the presence of transition metal oxide nanoparticles. The size distribution，morphology，and nano-structure of particles are primary characteristics which inf l uence the decomposition kinetics of solid propellants.Li and Cheng have synthesized Cu-Cr-O nano-composites via a citric acid complexing approach and demonstrated that these catalysts are useful in enhancing the catalytic-combustion of AP-HTPB based solid propellants ［6］.Dubey et al. ［22］prepared nanocomposites of Mn with Co，Ni，and Zn by polyol method，which showed high reactivity toward total combustion of solid propellants.Kawamoto et al. ［23］prepared copper chromite from ceramic and co-precipitation methods，which were found to increase the burning rate of HTPB-based solid propellants. Rajeev et al. ［24］synthesized Cu-Cr-O composites from thermal decomposition of copper ammonium chromate and observed that the higher burning rates of solid propellants are the consequence of crystal structures，particle sizes，and interdispersed metallic phases of the precursors.

In the present study，MMO nano-catalysts containing Cu-Co-O have been synthesized by citric acid complexing method.Three types of MMO nano-catalysts，namely CuCo-I，CuCo-II and CuCo-III，were prepared，by varying the molar ratios of the starting reactants.Characterization of precursors，as well as crystallized MMO nano-catalysts，was carried out using SEM，EDS，F(xiàn)TIR and thermal techniques.Efforts have been made to analyze the particle sizes，inter-dispersed metallic phases and structural features of all three nano-catalysts.The catalytic performance of all the nano-catalysts was studied in HTPB-AP based solid propellants.Burning rate measurements were carried out at atmospheric conditions.

2.Experimental sections

2.1.Materials and methods

Copper （II）nitrate trihydrate （Cu（NO3）2·3H2O）and cobalt（II）nitrate hexahydrate （Co （NO3）2·6H2O）of AR grade，procured from Central Drug House India Pvt.Ltd.，New Delhi，were used without further purif i cation.The citric acid monohydrate （C6H8O7·H2O）and benzoic acid of AR grade，obtained from Merck Millipore （India） Pvt.Ltd.，were used as complexing agent and standard sample for calibration of bomb calorimeter respectively.Ammonium perchlorate （AP）was used as an oxidizer in the present study，which was obtained from Tamilnadu Chlorates，Madurai.Hydroxyl-Terminated Poly Butadiene （HTPB）was used as a fuel-binder，which was obtained from VSSC，Trivandrum.Bis （2-ethylhexyl）adipate（BEHA）and Isophorone Di-Isocyanate （IPDI）were procured from Fluka Analytical，Germany，which were used as the plasticizer and curing agent respectively for solid propellant formulations.

The physical characteristics of nano-catalysts were analyzed using FTIR technique.The crystal phases and particle characterization of nano-catalysts and solid-state propellants were studied with the help of SEM.In a typical procedure，the beam of high energy electrons is focused on the solid specimen.The electrons react with the sample and emit signals of variousenergies.Of all the signals emitted，the secondary electron detector in conjunction with energy dispersive spectroscopy（EDS）was used to examine the elemental composition as well as molecular orders of metal oxides.The samples were coated with platinum to make them electrically conductive.The beams were operated at 25 keV to characterize the surfaces of nanocatalysts.The TGA/DTA analyzer，in the temperature range of ambient to 400 °C and at a heating rate of 10 °C/min，was used to examine the decomposition behavior of solid propellants with and without nano-catalysts.Bomb calorimeter was used to measure the heat of combustion of solid propellants of various compositions.The bomb calorimeter was used in isothermal conditions.

Table 1Molar concentration and product yield of prepared nano-catalysts.

2.2.Preparation of nano-catalysts

By varying the molar concentrations of Cu/Co nitrates as presented in Table 1，three types of nano-catalysts were synthesized.During the synthesis process，the Cu（NO3）2·3H2O and Co（NO3）2·6H2O were dissolved separately in 25 mL of doubly distilled water （DDW）.The solution mixtures were added together in a porcelain evaporator dish （100 mL）and stirred adequately to make a homogenous nitrate solution.Citric acid（C6H8O7·H2O）was then added to the nitrate solution mixture and stirred vigorously for a period of 10 min.The f i nal solution mixture was heated at 105 °C for 4 hours so that the water content is evaporated，producing a viscous gel.Fig.1（a）shows a representative gel of CA-（Cu-Co-O）precursor.The gel was then dried at 165 °C for 2 hours to obtain foamy textures as shown in Fig.1（b）.It was then calcined at 550 °C for 3 hours，which produced black powder of MMOs.A representative scheme for synthesis of nano-catalysts is shown in Eq.（1）.The stoichiometry of nano-catalysts may varied as:x=2，3，4，y=2，4 and z=3，4，6.

This was again grinded properly and stored for characterization and its application as a catalyst for the combustion of solid rocket propellants.

2.3.Preparation of solid composite propellants

Fig.1. （a）Viscous gel containing CA-（Cu-Co-O）precursor，（b）foamy texture obtained after drying the gelled mass，and （c）powdered Cu-Co-O nano-catalyst.

HTPB and AP were used as a fuel-binder and solid oxidizer for the formulation of solid propellants.The loading percentage of AP was kept f i xed at 70%by weight in the entire set of experiments.Two particle sizes of AP were used in the present study，i.e.250 μm and 44 μm，which were employed in the ratio of 3:1 in all such compositions.The requisite quantities of AP，HTPB，BEHA，IPDI and Cu-Co-O nano-catalysts were duly weighed and stored in controlled conditions.BEHA was f i rstly dehumidif i ed to remove all the moisture content and then it was allowed to cool down.HTPB and BEHA were thoroughly mixed for about 10 min，and then IPDI was added and mixed for another 10 minutes，which was followed by the addition of AP in three slots.Finally，the synthesized nano-catalysts were added and mixed for about 10-15 minutes to get a homogenous propellant mixture.The propellant mixture was found to be pourable，which was then cast on the plate molds and was placed on the mechanical vibrator for approximately 30 minutes to remove possible air bubbles or voids in the matrix of solid propellants.The molds were kept in a hot air oven at 60 ± 1 °C for 6 days to allow the propellant to cure ［25］.The cured propellant samples were stored in a controlled atmospheric condition，and then they were analyzed by different techniques as described.A schematic of composite solid propellant processing starting from synthesis procedure of nanocatalysts is presented in Fig.2.

3.Results and discussions

3.1.Characterization of nano-catalysts

Fig.2.A schematic of:（a）synthesis of nano-catalyst and （b）processing of composite solid propellant.

Fig.3.SEM micrograph of nanocatalyst CuCo-I.

The SEM micrograph of CuCo-I catalyst showed welldistributed particles in the range of 96 nm-176 nm as shown in the inset of Fig.3.The sizes of particles of CuCo-II and CuCo-III nano-catalysts as shown in Figs.4 and 5，respectively，were found to be slightly larger in comparison to the CuCo-I，probably due to the crystalline agglomeration.It further emphasizes that the particle size and shape of the nano-catalysts are dependent on the molar ratio of Cu/Co.The agglomerated particles of nano-catalysts can be seen in SEM micro-images，and it can be clearly observed in the micro-image of CuCo-III catalyst. Based on the measurement of elemental data by EDS spectra（Fig.6），the plausible molecular formulae of nano-catalysts were evaluated as shown in Table 2.As observed，the atomic ratios of Cu，Co and O atoms in the products were found to be 2/4/3 for CuCo-I，3/2/4 for CuCo-II and 4/2/6 for CuCo-III respectively.There was no trace of any other atoms in the nano-catalysts.Above results can usefully be incorporated to deduce the molecular formulae of primary products which are Cu2Co4O3，Cu3Co2O4and Cu4Co2O6for nano-catalysts CuCo-I，CuCo-II and CuCo-III respectively.It has also been emphasized that the molecular constituent of Cu increased with the increase in the molar concentration of CuNO3·3H2O.Besides this，the oxidation peak of Cobalt with oxidation state （0）appeared at 7.78 keV and 7.93 keV，whereas 7.80 keV，7.96 keV and 7.79 keV，7.94 keV oxidation peaks were observed for Cobalt of oxidation states （+2）and （+3）respectively ［26］.A similar trend of surface energy peaks in different scales has been observed with the prepared nano-catalysts.It also conf i rmed the stoichiometry of formulae above as deduced.Consistency in spectral patterns indicates a good homogeneity in the composition of solid-state surfaces.The sharp peaks of EDS spectra revealed that the nature of nano-catalysts is highly crystalline.Uniformity in crystal sizes and inter-distribution of nano-particles were found to be dependent on the molar concentration of the respective metal nitrates.

Fig.4.SEM micrograph of nanocatalyst CuCo-II.

Fig.5.SEM micrograph of nanocatalyst CuCo-III.

The results of FTIR spectra of CuCo-I，CuCo-II and CuCo-III nano-catalysts as seen in Fig.7 showed a pair of strong transmittance peaks.The peaks are in range of 570 cm-1，663 cm-1，574；667 cm-1and 572；667 cm-1respectively，which are characteristic peaks of bi-metallic （Mx-My-O）metaloxygen （M-O）vibrations.The weak transmittance peak at 1384 cm-1in spectrum Fig.6（c）was observed due to the stretching peak of M-O bond.The peaks 1122 and 1130 cm-1in spectra Fig.6（a）and 6（b）ensured that the nano-catalysts are well crystalline along with the presence of inter-metallic oxide lattices.

3.2.Phase morphologies of solid propellants

Fig.6.EDS spectra of MMO nano-catalysts: （a）CuCo-I，（b）CuCo-II，and （c）CuCo-III.

Table 2Elemental composition of MMO nano-catalysts obtained from EDS analysis.

Fig.7.FTIR spectra of MMO nano-catalysts:（a）CuCo-I，（b）CuCo-II，and （c）CuCo-III.

The surface morphologies of the propellant samples with and without catalysts were analyzed by SEM of an energy beam of 3 keV in LV mode.The SEM micro-images of virgin AP-HTPB and propellantscontaining nano-catalystsare depicted.The magnif i cation of 2000× has been undertaken to examine the particle distribution as well as the surface textures of all prepared propellant samples.The AP-HTPB propellant（Fig.8（a））shows well-dispersed oxidizer particles over the fuelbinder matrix.The consistency in particles （coarser to f i ner）distribution has also been observed.A comparison of the three nano-catalysts as shown in Fig.8（b）-（d）shows the stacked textures of various morphologies，which may be due to the agglomeration of catalysts with solid oxidizer particles. However，the gross morphology of individual propellant sample exhibited its own type of surface structures，which can be clearly seen in their SEM micro-images.

3.3.Thermal analysis

Thermal degradation behavior of propellant samples with and without nano-catalysts was conducted in the temperature range of ambient to 400 °C.Fig.9（a）shows the thermogram of weight loss of propellants as a function of temperature.It was observed that the weight loss of propellants takes place in three distinct scales，whereas the rate of weight loss differs markedly in each case.From temperature range of 28 °C-200 °C a weight loss of 4.16%was observed.A weight loss of 9.23%was observed in the temperature range of 200 °C-280 °C，and f i nally，weight loss of 67.73%was recorded in the temperature range of 280 °C-400 °C.The weight loss in the f i rst two regimes may be attributed to the removal of partly moisture content and some low-temperature volatile ingredients.The signif i cant weight loss in the temperature range 280 °C-400 °C might be a result of decomposition and removal of decomposition products of oxidizer particles or degradation of fuel-binder. A blackish residual mass was found to be left in the crucible at the end of 400 °C.This may be because of char of polymeric fuels which is usually a non-volatile product at the existing temperature range.Fig.9（b）-（d）shows the weight loss curvesof propellant samples containing 2%/wt.each of CuCo-I，CuCo-II，and CuCo-III nano-catalysts.A three-stage decomposition pattern of propellants containing respective nanocatalysts has also been observed.The weight loss was found to be highest in the third stage in all such compositions.The residual mass left after the decomposition of propellants with CuCo-II catalyst was seen almost negligible，whereas the residual mass was found to be 20%，13%and 15%left during decomposition of AP-HTPB propellant，and propellants containing CuCo-I and CuCo-III respectively.It can be concluded that a stable combustion can be achieved by incorporating CuCo-II type of nano-catalyst in the solid-state propellants. This also shows the merits of the current synthetic route where controllable phase formulation is possible in several orders［27］.

Fig.8.Surface morphologies of （a）AP-HTPB propellant，（b）propellant sample with CuCo-I，（c）propellant sample with CuCo-II，and （d）propellant sample with CuCo-III.

Fig.9.TGAcurvesof（a）HTPB-APpropellant，（b）propellantwithCuCo-I，（c）propellant with CuCo-II，and （d）propellant with CuCo-III.

Fig.10.DTA curves of （a）AP-HTPB propellant，（b）propellant with CuCo-I，（c）propellant with CuCo-II，（d）propellant with CuCo-III.

Table 3DTA data of solid propellant samples.

DTA thermograms of propellant samples with and without nano-catalysts are presented in Fig.10（a）-（d）.The f i rst endothermic peak of all propellant compositions was found at almost same temperature.It appeared at approximately 245 °C，which can be recognized as the transition of AP from orthorhombic to cubic crystals ［1，22］.This indicates that the CuCo nano-catalysts have little effect on the crystalloid transition temperature.A clear difference is seen in the exothermic peaks of propellants with and without nano-catalysts.A single exothermic peak is observed in HTPB-AP and propellants containing CuCo-II nano-catalyst，whereas two and three exothermic peaks are found in the propellant sample containing CuCo-I and CuCo-III catalysts respectively.Details of heat f l ow characteristics and peak position are shown in Table 3.The f i rst exothermic peak in the range of ＞300 °C corresponds to the low temperature decomposition of oxidizer mass and formation of intermediates in condensed phase.The second and third exothermic peaks can be attributed to the low and high temperature decompositions of AP into its intermediate （HClO4）forms.At that moment，a heterogeneous decomposition of deprotonized HClO4on the solid surface can also occur in gas-phase.Based on the f i ndings，it can be suggested that CuCo nano-catalysts accelerate both low and high temperature decompositions.An average difference in decomposition temperature from AP-HTPB propellant compared to the propellants containing Cu/Co nano-catalysts is approximately 30 °C.

3.4.Burn rate studies

The burn rate studies were carried out in ambient conditions. The propellant strands were electrically ignited and the time taken to burn the propellant strands of 50 mm length was recorded.Theburnratewas then calculatedby dividingthetotal effective length to burning duration.Table 4 presents the burning rate comparison of propellant samples with and without catalysts at atmospheric pressure.It has been observed that the addition of 2%nano-catalyst to the AP-HTPB propellants increases the burn rate by approximately two times at ambient condition.The burning rate was found to be highest for nano-catalyst CuCo-I，which is followed by CuCo-II and CuCo-III.The percentage deviation in the burn rate values was observed to be within ±2%.

3.5.Heat of combustion

The heat of combustion （Qc）refers the caloric equivalence of total combustion energy of propellant materials.Its valuedepends only on the composition of the material and not on any other properties of propellant.To calibrate the bomb calorimeter，the pellets of benzoic acid （standard）in 400 psi oxygen atmosphere were combusted.The water equivalent（W）of calorimeter was obtained as 2501 cal/°C by repeating the experiment for three times.It was calculated using the following equation:

where

Q=Heat of combustion of the standard benzoic acid sample，cal/g （6318 cal/g）

m=Mass of standard benzoic acid sample （g）

T=Temperature rise （°C）

e=Correction for nichrome wire （24 cal/g for 10 cm）

The heat of combustion of solid propellants with and without nano-catalysts was determined by the same procedure as adopted for above measurement.However，the equation for determining the heat of combustion of propellants was used as given below:

Table 5 presents the heat of combustion of all the propellant samples at ambient conditions.Some researchers have also published their results of heat of combustion in various atmospheric conditions as also the different propellant compositions［28，29］.It is observed that the heat of combustion of propellants containing Cu-Co-O nano-catalysts increases substantially compared to virgin HTPB-AP propellant in ambient condition.A quantitative relationship can be made between the burning rate and heat of combustion.Similarly，the data of（Qc）indicated why the burning rate of propellants containing CuCo-II and CuCo-III has lower values compared to CuCo-I nano-catalyst.

4.Conclusions

Crystalline MMO nano-catalysts based on Cu-Co-O have been synthesized.The morphological characterization of nanocatalysts indicates that all the prepared catalysts have almost uniform particle sizes.This proves the advantage and eff i ciency of citric acid complexing method over other methods of preparationofmixedmetaloxides.Thesurfacemorphologiesof propellants containing CuCo nano-catalysts ref l ect that the processing is well，and the mix has a uniform consistency.This observation also reveals that the AP crystals were unevenly encrusted with CuCo nano-catalysts.TheTGA thermograms of HTPB-AP and propellants containing CuCo-I and CuCo-III nano-catalysts are comparable whereas a rapid decomposition has been observed from 300 °C to 320 °C in the propellant containing CuCo-II nano-catalyst.From the DTA results，it was noticed that the endothermic peaks are almost at the same temperature for all the propellants but there is an apparent difference in the exothermic peak temperatures between the AP-HTPB and the propellants containing prepared nanocatalysts.Itindicatesthatthecatalyticeffectson the decomposition of propellant samples are signif i cant at higher temperatures.Trace of interstitial bimetallic-oxygen bonds has been examined in FTIR spectra.The burning characteristics of AP-HTPB based solid propellants have largely been improved by the incorporation of synthesized nano-catalysts.The CuCo-I nano-catalyst was found to be the best catalyst in regard to thermal decomposition of solid propellants.The CuCo-I catalyst，when incorporated in the formulation of solid propellant，wasfoundtobeamucheffectivenano-catalystwhich may be attributed to its high crystallinity，low agglomeration，anditspropertytolowerthedecompositiontemperatureofsolid propellants.Thus，it could be used as a combustion catalyst for rocket propulsion applications.

Acknowledgement

The authors are thankful to Central Instrumental Facility（CIF），BIT-Mesra for their analytical supports.

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Received 21 August 2015；revised 14 December 2015；accepted 4 January 2016 Available online 19 January 2016

Peer review under responsibility of China Ordnance Society.

*Corresponding author.Tel.:+91 8986764332.

E-mail address:yadavnarendra@bitmesra.ac.in （N.YADAV）.

http://dx.doi.org/10.1016/j.dt.2016.01.001

2214-9147/? 2016 China Ordnance Society.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license （http:// creativecommons.org/licenses/by-nc-nd/4.0/）.

? 2016 China Ordnance Society.Production and hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license（http://creativecommons.org/licenses/by-nc-nd/4.0/）.