Solution Combustion Synthesis of Nanosized Copper Chromite and Its

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Solution Combustion Synthesis of Nanosized Copper Chromite and Its Use as a Burn Rate Modifier in Solid Propellants P. S. Sathiskumar,†,‡ C. R. Thomas,† and Giridhar Madras*,‡ †

Vikram Sarabhai Space Centre, ISRO, Trivandrum-695022, India Department of Chemical Engineering, Indian Institute of Science, Bangalore 560012, India

‡

S Supporting Information *

ABSTRACT: Nano sized copper chromite, which is used as a burn rate accelerator for solid propellants, was synthesized by the solution combustion process using citric acid and glycine as fuel. Pure spinel phase copper chromite (CuCr2O4) was synthesized, and the effect of different ratios of Cu−Cr ions in the initial reactant and various calcination temperatures on the final properties of the material were examined. The reaction time for the synthesis with glycine was lower compared to that with citric acid. The synthesized samples from both fuel cycles were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), BET surface area analysis, and scanning electron microscope (SEM). Commercial copper chromite that is currently used in solid propellant formulation was also characterized by the same techniques. XRD analysis shows that the pure spinel phase compound is formed by calcination at 700 °C for glycine fuel cycle and between 750 and 800 °C for citric acid cycle. XPS results indicate the variation of the oxidation state of copper in the final compound with a change in the Cu−Cr mole ratio. SEM images confirm the formation of nano size spherical shape particles. The variation of BET surface area with calcination temperature was studied for the solution combusted catalyst. Burn rate evaluation of synthesized catalyst was carried out and compared with the commercial catalyst. The comparison between BET surface area and the burn rate depicts that surface area difference caused the variation in burn rate between samples. The reason behind the reduction in surface area and the required modifications in the process are also described. oxide and copper chromite.16 The characteristics of metal oxides such as phase composition, defects in the crystal structure, particle size, and surface area affect the burning rate, and thus control of these parameters is crucial in synthesizing these catalysts.4,11,17 Studies have shown that copper chromite has been considered the most effective catalyst, due to the tetragonally distorted normal spinel structure4,7 with c/a < 1 and the arrangement of copper in its structure.18 Previous work on the combustion of HTPB propellant has shown that copper chromite (CuCr2O4) has the advantage of both increasing the burning rate and the tendency to lower the pressure exponent.9,19 Copper chromite oxide is widely synthesized by two methods namely the ceramic method (oxide method) and the coprecipitation method. In the ceramic method,4,10 copper(II) oxide (CuO) and chromium(III) oxide (Cr2O3) are mixed in stoichiometric amounts and calcined at 700−800 °C. If the calcination temperature is increased to 900 °C, copper chromite reacts with copper oxide to form cuprous(I) chromite (Cu2Cr2O4).7 In the coprecipitation method,4,10 the catalyst is obtained by calcining basic copper chromate at 500 °C for 2 h. The increase in calcination temperature to 600 °C or higher causes the formation of cuprous chromite. Both the ceramic and coprecipitation synthesis process produce cupric chromite with associated impurities such as

1. INTRODUCTION Copper chromite composite oxides have been used as catalysts for chemical reactions such as hydrogenation, dehydrogenation, oxidation, alkylation, etc. and thus find wide commercial applications.1−3 Apart from its usage in chemical industries, copper chromite finds its major application as a burn rate modifier in solid propellant processing for space launch vehicles globally.4−9 The solid composite propellant is a composite energetic material consisting of binder (usually hydroxylterminated polybutadiene, HTPB), curator (toluene diisocyanate), oxidizer (ammonium perchlorate), metallic fuel (aluminum), and other additives in small amounts such as curatives, plasticizers, bonding agents, and burn rate modifiers. The role of a burn rate modifier is to alter the burning rate of the composite solid propellant and decrease the sensitivity of burning rate to pressure and temperature. Several materials have been studied and examined as potential burn-rate additives for composite solid propellants. The transition-metal oxides such as Fe2O3, Co2O3, Ni2O3, MnO2, and CuCr2O4 were found to increase the burning rate of composite solid propellants.4,10 The catalytic effect of these materials were examined on the thermal decomposition of ammonium perchlorate and also on the combustion of the ammonium perchlorate based propellants.5,11−15 Besides these solid materials, liquid derivatives from iron and boron are also used as burning rate catalysts.4 The disadvantage in using the liquid derivatives is their diffusion within the propellant mixture causes varying concentrations in the propellant. Among the metal oxide catalysts, the most widely used burning rate catalysts for composite propellants have been iron © 2012 American Chemical Society

Received: Revised: Accepted: Published: 10108

March July 4, July 5, July 5,

24, 2012 2012 2012 2012

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batches of the commercially available copper chromite (ACR-1, ACR-2), prepared by coprecipitation method, were also characterized and compared with the product synthesized by solution combustion. X-ray diffraction (XRD) was carried out in Philips X’pert diffractometer using Cu Kα radiation with a wavelength of 1.5418 Ǻ . The diffraction patterns were obtained in a 2θ range of 16° to 67° at a scan rate of 3.26°/min. The resulting XRD profiles were analyzed to identify the crystal phase of the compound using reference standards. The line width of the most intense XRD peak was taken for estimation of crystallite size by the Scherrer equation. X-ray photoelectron spectroscopy (XPS) was carried out for the compounds prepared using citric acid as fuel (CA-1 and CA-4) and the commercial sample. Thin pellets of samples were made using a hydraulic press and were degasified in the preparation chamber for 24 h. The spectra were recorded on a Thermo Fisher Scientific Multilab 2000 spectrometer with AlKα radiation of energy 1486 eV. The resulting spectra were analyzed to identify the different oxidation states of the copper and chromium ions present in the sample. Prior to the analysis, the spectra were calibrated with reference to C1s observed at a binding energy of 284.5 eV. The BET surface area of the samples was recorded in analyzer (Belsorp, Japan) using nitrogen as adsorbent. Initially the samples were pretreated under vacuum for 6 h at 77 K, and the adsorption−desorption tests were conducted at multipoint pressure levels to obtain the final surface area. Samples CA-1, CA-4, GLY-1, and GLY-4 were scanned by Scanning Electron Microscope (SEM) (FEI, QUANTA, USA) at 10 kV to determine the particle size, shape, and morphology of the synthesized and commercial compounds. 2.3. Catalytic Effect on Solid Propellant Burn Rate. The efficiency of synthesized copper chromite as burn rate modifiers in solid propellant was analyzed by testing its catalytic activity in solid propellant combustion and compared with the activity of the commercial catalyst. A typical solid propellant formulation consisting of hydroxyl terminated polybutadiene as binder, ammonium perchlorate as oxidizer, and aluminum as metallic fuel along with processability additives was chosen. The synthesized copper chromite was used as burn rate modifiers with 0.27 wt % in the formulation. Initially all the ingredients were mixed in lab-scale horizontal sigma blade mixer. The viscosity of the mixed slurry was measured in Brookfield Viscometer. The cured samples were tested for its burn rate at 3.92 MPa in Crawford bomb. Similar tests were conducted with the samples prepared using commercial catalysts that are presently used in solid propellant formulation.

mixed oxides of chromium and copper due to the solid-state reaction. The particle size of the product as synthesized is also high with a lower surface area due to the high temperature involved in the process.20,21 Therefore, a simple process for synthesizing nanosized CuCr2O4 with high surface area needs to be developed. Solution combustion, a technique frequently used in catalyst synthesis in recent years,19−21 was used to synthesize nanosized pure copper chromite. Solution combustion provides a uniform, pure product and favors synthesis of nanosize powders with high specific surface area.20,21 Although copper chromite (CuCr2O4)19 and CuCrO222 have been synthesized from metal nitrates using citric acid and glycine as fuel, respectively, the effect of different fuels on the synthesis of CuCr2O4 has not been studied. In this study, nanosized copper chromite CuCr2O4 catalyst was synthesized using citric acid and glycine as two different fuels. The effect of molar ratio of Cu and Cr ions in the reactants and the effect of calcination temperature on the final product characteristics were also studied and compared with the commercial copper chromite.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Copper Chromite. Copper nitrate (Cu(NO3)2·3H2O, 99.5% pure), citric acid anhydrous (MW: 192.13, 99.7% pure), and glycine (99.7% pure) were purchased from S.D. Fine chemicals Ltd., India, and chromium nitrate (Cr(NO3)3·9H2O, 99.0%) was purchased from Nice Chemicals, India. Nanosize copper chromite was prepared by dissolving copper nitrate trihydrate and chromium nitrate with various Cu:Cr mole ratios, as shown in Table 1, in approximately 100 Table 1. Samples Details Prepared with Different Cu:Cr Mole Ratios and Fuels sample identification

Cu:Cr mole ratio

CA-1 CA-2 CA-3 CA-4 GLY-1 GLY-2 GLY-3 GLY-4

0.5 0.7 0.9 1.0 0.5 0.6 0.9 1.0

fuel used

fuel to metal ions ratio

citric acid

2:1

glycine

2:3

mL of deionized water. After the dissolution of metal ions, the fuel was dissolved in the mixture, and the molar ratio of fuel to the total metal ions was fixed at 2:1 and 2:3 for citric acid and glycine, respectively. The solution was heated at 85−95 °C for the slow removal of water until the formation of viscous slurry. The viscous slurry was then combusted in a muffle furnace at 160−180 °C leading to the formation of foamy Cu−Cr−O nanocomposites precursor powder. These precursors were calcined in silica crucible at 700 °C for 3 h to obtain the final product. In order to study the effect of morphology, crystallinity and composition of the final product, several experiments were conducted by varying the calcination temperature and the molar ratio of Cu−Cr with both fuels separately. 2.2. Characterization of Cu−Cr−O Nanocomposites. The synthesized copper chromite samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, BET surface area analysis, and scanning electron microscope. Two

3. RESULTS AND DISCUSSION 3.1. Synthesis. Cu−Cr−O nanocomposite oxide catalysts were synthesized by varying the Cu−Cr mole ratio to understand the mole ratio effect on the final product with different fuels namely glycine (GLY-1 to 4) and citric acid (CA1 to 4). The variation of the Cu:Cr mole ratio in the citric acid cycle was earlier studied by Li et al.,19 who observed that a mole ratio of 0.7 gives a higher burn rate and phase to phase interaction occurs in the catalysts containing CuCr2O4 and CuCrO2 leading to higher burn rates. Hence a different mole ratio of Cu:Cr was chosen to find its effect in burn rate. Initially, the nitrate reactants and fuel in solution was made into viscous solution (gel) with slow evaporation of water and finally combusted in an oven at 160−180 °C. The combustion temperature is determined by the type of fuel, and, therefore, 10109

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the choice of fuel is critical in deciding the exothermicity and duration of the redox reaction. Glycine, being a strong reducing agent, causes a fast reaction inducing splashing of fine product and evolution of a large amount of gases during the combustion process. Tiny sparks of flame were observed during the combustion, and the process was completed within 30 min of initiation of combustion and the final product was porous flakes. The reaction with citric acid fuel was relatively slow due to its weak reducing power in the redox reaction. The reaction progresses slowly with slow expansion of the final porous mass inside the reaction vessel with no splashing. The combustion time was 2−3 h, and the completion of the reaction was found from the disappearance of greenish color to form black color forming the product mass. The final product was found to be porous fine crispy flakes. Due to the smoldering type of reaction, the time required for combustion redox reaction varies depending on the quantity of the reaction mass. In the glycine fuel cycle, the splashing of product outside the reaction vessel during combustion was addressed by loosely covering the vessel with a fine wire mesh. On increase in quantity and due to the vigorous reaction during combustion process, the final compound spills out over and above the wire mesh. In citric acid fuel cycle, it was observed that the expansion of mass spills over the vessel if the quantity of initial reactants were increased. To increase the batch size, a custom built glass reactor with an increased length and typical openings with mesh closure in the top lid was used for safe disposal of exhaust gases. This contained the spillage inside the reactor and hence maintains the final product composition. The calcination process follows the combustion process and was carried out at different temperatures for CA-1 and GLY-1 samples to study the effect on the final product composition. The thermal decomposition process of the Cu−Cr−O reactant mixture precursors for citric acid cycle was studied earlier using TGA-DTA analysis by Li et al.19 Here, we have studied the thermal decomposition process of the Cu−Cr−O reactant mixture precursors of both citric acid and glycine fuel cells using TGA-DTA analysis for the samples prepared with both the fuels namely citric acid and glycine. The precursors were prepared similar to the main sample preparation, and water was allowed to evaporate slowly at 60 to 70 °C to form a thick mass. This precursor sample prepared from both the fuels was analyzed by TGA-DTA to understand the various decomposition regimes as shown in Figure 1. Figure 1a shows that the weight loss has three regimes, first being the loss of water followed by the decomposition of reactants to form NO3− and organic phases at 150 to 250 °C and finally the combustion process between 250 and 350 °C. A further small mass loss is noticed between 400 and 550 °C due to the elimination of remaining carbon and organic compounds. The weight loss and DTA curve for glycine sample is shown in Figure 1b. The mass loss has two major regimes: the initial mass loss due to water removal and the second mass loss with a sharp fall due to the combustion process. The DTA curve of citric acid cycle shows that the combustion reaction is maximum at 315 °C and has a spread from 250 to 350 °C. However, for the glycine cycle, a sharp exothermic peak, corresponding to combustion reaction, is observed at 190 °C. This confirms that the glycine fuel cycle is more reactive than the citric acid fuel cycle, and this leads to a difference between the reaction time for both the fuels. 3.2. XRD Crystal Structure Analysis. The formation of crystalline phase and the solid-state reaction to form the final

Figure 1. TGA-DTA analysis of Cu−Cr−O reactant mixture precursor: (a) CA-1 and (b) GLY-1.

spinel type compound happens at 600−700 °C. To understand the effect of the calcination temperature in the formation of spinel copper chromite product with a Cu−Cr mole ratio of 0.5, CA-1, GLY-1 were prepared by calcining the samples at different temperatures until 900 °C. Similarly, samples CA-1 to 4 and GLY-1 to 4 with different Cu−Cr mole ratios in the reactant were prepared to study the effect of the initial mole ratio in the final product composition phase. The presence of different oxide forms in the product was analyzed by subjecting the samples to X-ray diffraction, and the diffractogram was characterized for the presence of spinel type form and other oxides. Effect of Calcination Temperature. The XRD profile of samples prepared from CA-1 with different calcination temperatures are shown in Figure 2. The patterns of the diffractogram shows the main reflections of spinel structure [JCPDF: 00-034-0424] indicating the formation of spinel type copper chromite. The position of different peaks in Figure 2a reveals that the spinel type copper chromite (CuCr2O4) has formed as a major compound with the maximum intensity peak at 2θ value of 35.16° along with the presence of chromium oxide at 2θ value of 24.35°. This indicates that the reaction is not complete to form a pure phase spinel type at 700 °C. The XRD profile (Figure 2b) of the sample calcined at 800 °C can be readily indexed to pure phase spinel along with an appearance of the slow formation of the CuCrO2 phase by the peak at 2θ value of 36.39°. The sharp peaks in Figure 2b compared to Figure 2a shows the improvement in the crystallinity of the compound with the calcination temperature.19 10110

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The comparison between citric acid and glycine as fuel indicates that the formation of pure spinel type CuCr2O4 compound occurs at 700 °C with glycine, whereas the pure phase is obtained between 700 and 800 °C with citric acid. The presence of other oxides occurs at 800 °C for the glycine cycle, whereas a pure phase spinel compound exists at 800 °C in the citric acid cycle. This may be due to the difference in the energy utilization where the solid-state reaction occurs at a comparatively lower rate at 700 °C and gets completed between 700 and 800 °C. The formation of delafossite is favored with GLY-1 compared to CA-1 as seen from the relative intensity of the peaks corresponding to CuCrO2 (Figure 2c and Figure 3c). Cu−Cr Molar Ratio Effect. The XRD profiles of samples CA-1 to 4 calcined at 700 °C are shown in Figure 4. The XRD

Figure 2. XRD patterns of the synthesized Cu−Cr−O composites, CA-1, after calcined at different temperatures for 3 h: (a) 700 °C; (b) 800 °C; and (c) 900 °C [open circle, ○ - CuCr2O4, filled circle, • CuCrO2; filled square, ■ - Cr2O3].

The effect of increase in temperature to 900 °C is clearly visible from Figure 2c. Besides spinel type CuCr2O4, other diffraction peaks corresponding to delafossite CuCrO2 is observed, as indicated by the presence of peaks of (101) and (012) plane at 2θ value of 31.37° and 36.39°, respectively (JCPDF: 01-074-0983). The temperature increase to 900 °C during calcination causes the spinel type copper chromite to transform into a delafossite phase compound,7 and hence the calcination temperature plays a major role in obtaining the spinel phase. Figure 2c shows the presence of the peak at 2θ of 33.53° (JCPDF: 00-006-0504) indicating small amounts of Cr2O3 in the final compound though other peaks correspond to Cr2O3 are absent. The XRD profile of samples prepared from GLY-1 at different calcination temperatures is depicted in Figure 3. The

Figure 4. XRD patterns of the synthesized Cu−Cr−O composite oxides (a) CA-1; (b) CA-2; (c) CA-3; and (d) CA-4. Calcination temperature: 700 °C for 3 h [open circle, ○ - CuCr2O4; filled circle, • - CuCrO2; open square, □ - CuO; filled square, ■ - Cr2O3].

diffractogram seen in Figure 4a is for sample CA-1, and the same is shown in Figure 2a also. However, for CA-2, the final compound has copper oxide along with spinel copper chromite, as indicated in Figure 4b. The presence of copper oxide in the product is prominently reflected in Figure 4c by the presence of peaks at 2θ of 38.73° (JCPDF 00-045-0937), which also shows the slow appearance of delafossite copper chromite, CuCrO2, when the ratio is increased to 0.9 for sample CA-3. The relative diffraction intensities related to delafossite CuCrO2 becomes more prevalent in Figure 4d for sample CA-4, which clearly shows the formation of delafossite phase compound. As discussed earlier, a pure delafossite phase can be obtained with the Cu−Cr ratio of 1.0 when the calcination temperature is maintained between 700 and 800 °C due to the slow reducing power of citric acid. The aforementioned phenomena clearly show the effect of the Cu−Cr mole ratio on the final product, and hence the final product can be tuned by varying this ratio in the reactant. The XRD profiles of the as synthesized compounds GLY-1 to 4 are shown in Figure 5. Figure 5a, for sample GLY-1, shows that the formation of pure spinel phase copper chromite is possible at the Cu−Cr molar ratio of 0.5 with the calcination temperature of 700 °C. A further increase in the mole ratio produces a mixture of copper oxide and spinel type copper chromite as seen in Figure 5b and c. The relative increase in the intensities related to copper oxide is seen from Figure 5b, c, and d.

Figure 3. XRD patterns of the Cu−Cr−O composites, GLY-1, synthesized after calcined at different temperatures for 3 h: (a) 700 °C; (b) 800 °C; and (c) 900 °C [open circle, ○ - CuCr2O4, filled circle, • CuCrO2; open square, □ - CuO].

formation of spinel type copper chromite is similar to samples obtained from CA-1. Figure 3a shows a pure spinel type phase CuCr2O4 compound formed at 700 °C, whereas the presence of copper oxide is also seen in Figure 3b. The narrow peaks in Figure 3b compared to Figure 3a confirms the improvement in crystallinity and the growth of particle size of the compound.19 The increase in the temperature during calcination also results in the delafossite CuCrO2 compound along with spinel type, as observed in Figure 3c. 10111

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modifiers. The comparison with commercial catalyst validates that the solution combustion technique is better than conventional methods in synthesizing pure phase copper chromite catalyst. 3.3. XPS Analysis. Samples prepared from solution combustion and commercial samples were analyzed by XPS to understand the oxidation state of the copper and chromium in the final product. The spectrum was initially calibrated with the standard value of binding energy for carbon 1S peak as 284.5 eV. The XRD analysis of CA-1 and CA-4 compounds shows the presence of cupric chromite for the ratio of 0.5 and a combination of cupric chromite and cuprous chromite for the ratio of 1.0. This was further confirmed by XPS analysis by evaluating the oxidation state of copper and chromium in the final compound. Figure 7a and b shows the original XPS binding energy spectrum of chromium and copper, respectively, along with the fitted curve for both 2p1/2 and 2p3/2 energy levels for sample CA-1. The peak binding energy values in Figure 7a for 2p3/2 and 2p1/2 energy levels match with the standard values [NIST database] of chromium in the 3+ oxidation state. The peak binding energy values from Figure 7b for 2p3/2 and 2p1/2 energy levels match with the standard values [NIST database] of copper in the 2+ oxidation state. This confirms the presence of copper in the 2+ oxidation state and chromium in the 3+ oxidation state which match with the cupric chromite individual element oxidant states whose formation is confirmed by XRD peaks in Figure 4a. The resultant XPS spectrum was analyzed for the surface Cu−Cr mole ratio and found to be 0.48 which is close to the initial reactant Cu−Cr ratio of 0.5. Figure 8a and b shows the XPS spectrum of both chromium and copper ions and their oxidation state in the final product for sample CA-4. The peak binding energy values in Figure 8a correspond to chromium of 2p3/2 and 2p1/2 energy levels with an oxidation state of 3+. The spectrum for copper ion, as shown by Figure 8b, shows two peaks for both 2p3/2 and 2p1/2 energy levels. The peak value of 932.1 eV at 2p3/2 energy and 952.15 eV at 2p1/2 energy corresponds to the 1+ oxidation state of copper. Similarly, the peak value of 934.4 eV at the 2p3/2 energy level and 954.7 eV at the 2p1/2 energy corresponds to the 2+ oxidation state of copper. The intermediate peaks between 2p3/2 and 2p1/2 energy levels are the satellite peaks generally observed for the copper ion with an oxidation state of 2+.23 Figure 8b indicates that the sample CA-4 consists of copper in both the oxidation states. The relative amount of different oxidative state copper ion was analyzed from the resultant XPS spectrum by their total areas and found that the CuI/ (CuI+CuII) mole ratio is 0.63. This confirms the presence of copper is higher in the 1+ oxidation state which matches with the oxidative state of copper in CuCrO2. The presence of the delafossite CuCrO2 compound along with spinel type CuCr2O4 is also confirmed by the XRD spectrum as seen from Figure 4d. The XPS spectrum was also analyzed for the surface Cu−Cr mole ratio and found to be 1.1, which is close to the initial reactant Cu−Cr ratio of 1.0. The XPS and XRD data show that the delafossite CuCrO2 is predominantly formed when the mole ratio of Cu to Cr initially taken is 1.0. The XPS of the compounds formed using glycine GLY-1 and GLY-4 are similar to CA-1 and CA-4, respectively, and, therefore, not shown. XPS analysis was carried out for the commercial (ACR-1) sample and compared with the compounds synthesized by solution combustion. Figure 9a and b shows the XPS binding energy spectrum of chromium and copper, respectively, for the

Figure 5. XRD patterns of the synthesized Cu−Cr−O composite oxides (a) GLY-1; (b) GLY-2; (c) GLY-3; and (d) GLY-4. Calcination temperature: 700 °C for 3 h [open circle, ○ - CuCr2O4; filled circle, • - CuCrO2; open square, □ - CuO].

By using the Scherrer equation, the average crystallite size (based on 35.16°) for the catalysts synthesized by the citric acid cycle was found to be around 90 and 150 nm. By increasing the calcination temperature from 700 to 900 °C, the average crystallite size increases to 200 and 225 nm. The average crystallite size calculated from the XRD spectrum for the catalysts synthesized by the glycine cycle is between 100 and 130 nm. An increase in crystallite size to 200 to 225 nm was observed in samples after calcination. The as-obtained commercial samples (ACR-1 and ACR-2) were also characterized by XRD and compared with the diffractogram of solution combustion synthesized compounds from both citric acid and glycine fuel cycles. Figure 6 shows the

Figure 6. XRD patterns comparison between solution combustion synthesized (CA-1, GLY-1) and commercial Cu−Cr−O oxides (ACR1 and ACR-2).

comparative plot of XRD profile between commercial compounds (ACR-1, ACR-2) and solution combusted samples (CA-1, GLY-1). The XRD diffractogram of commercial compounds (ACR 1 and ACR 2) shows that the copper chromite used is a mixture of copper oxide, chromium oxide, cuprous chromite, and cupric chromite along with barium oxide (indicated by the peak at 2θ value of 27.9°). The elemental composition of commercial copper chromite ACR-1 was Cu 32%, Cr - 30%, and Ba - 5.8%. The barium was included in the commercial catalyst as a partial replacement for copper to reduce the poisoning of the catalyst in hydrocarbon reactions,2 from which these catalysts are derived for propellant burn rate 10112

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Figure 7. XPS spectrum of the solution combustion synthesized Cu−Cr−O composite oxide, CA-1: (a) chromium ion and (b) copper ion.

Figure 8. XPS spectrum of the synthesized Cu−Cr−O composite oxide, CA-4: (a) chromium ion and (b) copper ion.

Figure 9. XPS spectrum of the commercial Cu−Cr−O composite oxide, ACR −1: (a) chromium ion and (b) copper ion.

Supporting Information). Figure S1a show the micrograph obtained for the sample GLY-1. The particles are close to spherical in shape, and the particle size of the sample consists of two fractions: the average small size fraction lies between 40 and 100 nm, whereas the second fraction has an average size of 200 nm. The micrograph (Figure S1b) of sample GLY-4 exhibits a pattern and size fraction similar to GLY-1. The micrograph (Figure S2a) obtained for the sample CA-1 exhibits particles of definite trapezoidal shape and average size of 0.1 to 0.5 μm. The sample CA-4 shows aggregates of close to spherical shape with a particle size ∼100 nm, as seen from Figure S2b. SEM micrographs of commercial copper chromite compound are shown in Figure S3 (see Supporting Information). The micrograph of ACR-1 compound in Figure S3a does not show a clear structure or shape but only

commercial compound. Figure 9a confirms the oxidation state of chromium in 3+, similar to the compound synthesized by solution combustion. The copper spectrum (Figure 9b) shows that the material is present both in the oxidation state of 1+ and 2+ with more prominence toward 2+. The relative amount of different oxidative state copper ion was analyzed and found that the CuI/(CuI+CuII) mole ratio is 0.24, which indicates that a higher amount of copper exists in the +2 oxidation state. 3.4. Scanning Electron Microscopy Analysis. Samples prepared by solution combustion from both citric acid and glycine fuels and the commercial samples were subjected to scanning electron microscope analysis to understand the shape, size, and morphology properties and compare between them. SEM micrographs of copper chromite prepared by solution combustion are shown in Figures S1 and S2 (see the 10113

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Table 2. Specific Surface Area of the Solution Combusted and Commercial Samplesa CA-2 sample 2

surface area (m /g)

CA-1

A

B

C

D

E

CA-4

GLY-1

GLY-4

ACR-1

ACR-2

0.57

167

48

31.6

22.7

1.33

2.81

14.83

18.68

46.4

42.57

a

ACR-1: commercial sample I. ACR-2: commercial sample II (same source but from a different batch). CA-2/A: product after solution combustion with no calcination. CA-2/B: product after solution combustion followed by smashing of flakes in mortar and pestle and no calcination. CA-2/C: product after calcination at 300 °C. CA-2/D: product after calcination at 500 °C. CA-2/E: product after calcination at 700 °C. CA-1,CA-4,GLY1,GLY-4: product after calcination at 700 °C.

aggregates of particles. Sample ACR-2 shows the aggregates along with a few particles of needle shape, as seen in Figure S3b. The results indicate that commercial samples prepared by conventional methods do not have a definite shape and have variations as it is subjected to grinding during its manufacturing process. The effect of a needle-like structure of copper chromite is to reduce the packing fraction of the solid propellant slurry compared to the spherical shape formed in the solution combustion process. Therefore, samples prepared by solution combustion yield nanosized particles that are more uniform in shape/size. 3.5. BET Surface Area. The surface area of the copper chromite catalyst is a major parameter for solid propellant processing because it plays a significant role in the augmentation of burn rate. The specific surface area of the samples CA-1, CA-2, CA-4 and GLY-1, GLY-4 after calcination in air at 700 °C are analyzed and compared with the commercial samples (Table 2). The results of the BET surface area for analyzed samples show that samples prepared using citric acid as the fuel have a very low surface area compared to the material prepared using glycine as the fuel. This is due to the dependence of surface area on the type of fuel used in the solution combustion process. Glycine, being more reactive, produces a large amount of gases during combustion which restricts the particle size and induces more micropores in the final product. However, when citric acid is used as the fuel, the amount of gases evolved is not significant and the surface area of the material produced is not high. The commercial samples, as seen from Table 2, have a large surface area compared to samples prepared by solution combustion. This is due to the fact that the material undergoes many phases of grinding and regrinding in a ball mill until a specific level of surface area is achieved. Hence to understand the surface area variation of the samples prepared by solution combustion, the sample CA-2 was calcined at various temperatures for 1 h. These samples (CA-2 A to E) were tested for their surface area, and the results are shown in Table 2. The results from Table 2 indicate that the surface area created by the solution combustion process without calcination is four times higher than the commercial copper chromite samples. The effect of calcination temperature on the surface area is clearly seen from the fact that the value decreases as the calcination temperature increases. The decrease in the surface area with increasing temperature of calcination can be attributed to the closing of micropores. 3.6. Processability and Catalytic Effect on Solid Propellant Burn Rate. Copper chromite synthesized using citric acid as fuel were analyzed for its burn rate augmentation in solid propellant combustion. Solid propellant prepared by incorporating 0.27 wt % of copper chromite was tested for its burn rate and viscosity levels. Three propellant mixtures were prepared with copper chromite samples CA-1, CA-2, and CA-4. The cured samples were tested in a Crawford bomb to

determine its burn rate at 3.92 MPa pressure. Similar burn rate tests were carried out for propellant samples prepared from the commercial catalyst (ACR-1). These burn rate results of both the catalysts were compared with their BET surface areas, as shown in Table 3. The propellant burn rate results indicate that Table 3. Burn Rate and Viscosity Comparison for Propellant Samples Prepared Using Solution Combusted and Commercial Catalysta sample identification CA-1 CA- 2 CA-4 ACR-I a

burn rate (mm/s) @ 3.92 MPa

viscosity at end of mix (Poise) @40 °C

BET surface area of catalyst (m2/g)

± ± ± ±

9920 8640 8320 8000

0.57 1.33 2.81 46.4

6.40 6.40 6.55 7.91

0.04 0.04 0.06 0.05

The burn rate was based on 0.27% of catalyst in the propellant.

propellants prepared from solution combusted catalyst, CA-1, CA-2, and CA-4, are lower than that obtained with the commercial catalyst. The difference in burn rate between solution combusted catalyst and commercial catalyst may be due to the difference in their surface areas of 1.33 and 46.4 m2/ g, respectively. The small difference in burn rate between the solution combusted samples may also be due to the small change in the surface area. The reduction in surface area of solution combusted catalyst was due to the calcination process at 700 °C for 3 h after the solution combustion process. The requirement of calcination in the citric acid cycle becomes necessary to remove the excess carbon complex formed during the combustion process. This exothermic process can be verified by the thermal analysis of the solution combusted catalyst, CA-1, as seen from Figure S4 (see the Supporting Information). The two exothermic peaks, as observed in Figure S4, at 315 °C and 450−550 °C with a total mass reduction of nearly 10% confirms the presence and the removal of carbon and organic complex. This carbon complex, if not removed, eventually reduces the actual percentage of copper chromite in the propellant mixture and may interfere in the catalytic process. The stoichiometric requirement of fuel for the Cu/Cr mole ratio of 0.5 for combustion reaction is 2.22 mol of citric acid for 3 mol of metal ions (fuel: metal ions = 0.74:1 (or) copper:chromium:citric acid = 1:2:2.22) as described by Jain et al.24 Hence the formation of carbon complex is due to the excess and nonstoichiometric fuel (fuel:metal ions = 2:1 taken against 0.74:1). The auto ignition of citric acid and metal ions oxidizers occurs at 315 °C which was confirmed by thermal analysis (Figure 1a) of the fuel-oxidizer precursors. Hence the lower temperature maintained during the combustion experiments, 160−180 °C, reduced the reaction rate and heat evolution. 10114

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As seen from Table 2, the initial surface area after solution combustion is high and comparable with the commercial catalyst. Therefore, the real challenge in solution combustion process is to synthesize pure copper chromite without calcination to maintain a high surface area equivalent to commercial regrinded catalyst. This requires a proper selection of the synthesis temperature, choosing the right fuel and choosing appropriate ratios of fuel and oxidizer. Similar work on synthesizing pure nanosize different structure type compounds (including spinel25) with large surface area using the solution combustion process without calcination has been demonstrated using a mixture of fuels.26 Future studies will examine the above parameters in obtaining a material with a high surface area that need not be calcined before use. The other critical property in using nano sized solid particles for solid propellant slurry is its processability due to the increase in viscosity. The ideal requirement in using nano sized particles is not to have an appreciable increase in viscosity compared to samples prepared from commercial catalyst. All the propellant samples prepared were tested for its viscosity in Brookfield viscometer and compared in Table 3. The results indicate that there is no appreciable change in the end of mix viscosity compared to commercial catalyst except for sample CA-1. Though the catalyst particles are nano sized, it improves the packing density and results in better flow ability due to its spherical shape. The experiments have shown that the catalyst synthesized by solution combustion does not hinder processability, and all the samples could be easily cast as propellant slabs.

AUTHOR INFORMATION

Corresponding Author

*Phone: 091-80-22932321. Fax: 091-80-23600683. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Solid State and Chemistry Unit, Material Research Centre and Centre for Nano science and engineering, IISc for providing facility and experimental support. We sincerely acknowledge Ms. Shikha Singh, an undergraduate student from Motilal Nehru NIT, Allahabad, who worked as a summer intern for her contribution with the experimental work. We also acknowledge the RMPF team from RPP, VSSC and TAPS and PMTS teams from PSCG, VSSC, Trivandrum for rendering their technical support with the experimental work.

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REFERENCES

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4. CONCLUSIONS The copper chromite, used for solid propellant as a burn rate modifier, synthesized by the solution combustion technique produces a nanosize, pure phase of the spinel compound. The process is simple compared to conventional methods of preparation. The role of the fuel, change in Cu/Cr ratio, the calcination process and its temperature on the preparation process, and its influence on the properties such as BET surface area, phase purity, and size/shape was determined. The solution combustion technique can be used to synthesize the required phase of the copper chromite compound with reduced particle size and high surface area before calcination compared to the conventional synthesis process. However, calcination of the catalyst is required before its use as a burn rate modifier. This results in a reduction of surface area and lower burn rate than the commercial catalyst. Future studies will examine the role of synthesis temperature, choice of fuel, and the ratio of fuel to metal ions to obtain a pure high surface area copper chromite catalyst, which does not require calcination, using a solution combustion process.

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Article

ASSOCIATED CONTENT

* Supporting Information S

Figure S1 shows the SEM images of synthesized Cu−Cr−O composite oxides: (a) GLY-1 and (b) GLY-4. Figure S2 shows the SEM images of synthesized Cu−Cr−O composite oxides: (a) CA-1 and (b) CA-4. Figure S3 shows the SEM images of commercial Cu−Cr−O composite oxides: (a) ACR-1 and (b) ACR-2. Figure S4 shows the TGA-DSC analysis of assynthesized solution combusted copper chromite, CA-1 without calcination. This material is available free of charge via the Internet at http://pubs.acs.org. 10115

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the 21st International Annual Conference of Technology of Polymer Compounds and Energetic Materials, Karlsruhe, Germany, 1990; 81.1− 81.12. (18) Boldyreva, A. V. Effects of spinels on the pyrolysis and combustion rates for ammonium perchlorate mixtures. Combust. Explosives Shock Waves 1975, 11, 611−613. (19) Li, W.; Cheng, H. Cu-Cr-O nano-composites: Synthesis and characterization as catalysts for solid state propellants. Solid State Sci. 2007, 9, 750−755. (20) Hegde, M. S. Madras, G.; Patil, K.C. Noble Metal Ionic Catalysts. Acc. Chem. Res. 2009, 42, 704−712. (21) Patil, K. C.; Aruna, S. T.; Mimani, T. Combustion synthesis: an update. Curr. Opin. Solid State Mater. Sci. 2002, 6, 507−512. (22) Chiu, T. W.; Yu, B. S.; Wang, Y. R.; Chen, K. T.; Lin, Y. T. Synthesis of nano-sized CuCrO2 porous powders via a self-combustion glycine nitrate process. J. Alloys Compd. 2011, 509, 2933−2935. (23) Wandelt, K. Photoemission studies of adsorbed oxygen and oxide layers. Surf. Sci. Rep. 1982, 2, 1−121. (24) Jain, S. R.; Adiga, K. C. A new approach to thermochemical calculations of condensed fuel-oxidiser mixtures. Combust. Flame 1981, 40, 71−79. (25) Ianos, R.; Lazau, I.; Pacurariu, C.; Barvinschi, P. Solution combustion synthesis of MgAl2O4 using fuel mixtures. Mater. Res. Bull. 2008, 43, 3408−3415. (26) Aruna, S. T.; Kini, N. S.; Shetty, S.; Rajam, K. S. Synthesis of nanocrystalline CeAlO3 by solution combustion route. Mater. Chem. Phys. 2010, 119, 485−489.

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