Synthesis of Cu-Chromite Catalyzer by Citrate Sol-Gel

It has been introduced several ways for rising fuel burning rate. Using catalyzers is a common way to rising fuel burning rate. Cu-Chromite catalyzer used in solid fuels, as burning rate catalyzer in thermal decomposition of Ammonium Perchlorate and results were satisfying. This catalyzer is produced by several methods such as: ceramic, coprecipitating, sol-gel, vacuum depositioning, but this paper explains producing catalyzer by Citrate sol-gel. Thermal analysis is used for studying process also SEM, XRD, TEM, FTIR tests used for determination of particle sizes.


Introduction
In recent years the Cu-Cr-O composites are found great promising in application as burning rate catalysts (ballistic modifier) for solid propellants used in defense (high explosives, ballistic missiles) and space vehicles (rocket propellants).
Solid composite propellants are mixtures of prepolymer (binder), aluminum fuel, oxidizer salts (e.g. ammonium perchlorate), and other components, including curatives, plasticizers, bonding agents, stabilizers and catalysts. Even though added at few percent of the propellant binder, the catalysts used to control the burn rate are of high importance, since they allow improving the ballistics of rockets [1]. Combustion of this system involves the decomposition of AP

Ceramic Method
The vast majority of the powder synthesis of high temperature superconducting oxides to date has been carried out using the traditional "solid-state reaction route". Copper chromite spinels are usually synthesized by the conventional high temperature method of solid state reaction (Equation (1)): [2] ( ) Cu/Cr = 1.5. The mixtures of oxides were homogenized with acetone followed by subsequent calcination at 900˚C for 6 hours. This method results in spinel particles with low surface areas. In order to synthesize spinels with high surface area, they attempted different wet chemistry techniques.

Materials and Methods
By this method, Cu and Potassium dichromate with Ammoniac and deionized water mixed with specific ratio (determines the particle size of final Chromite) then dried the outcome precipitate in 110˚C and 500˚C for twice finally, Cu-Chromite produces Vacuum precipitating is a laboratory method, that isn't economical because of problems of performing such as: low temperature, high vacuum for producing catalyzer. By studying different factors: ease of perform, low costs, uniform particle size, Citrate sol-gel was choice for producing catalyzer.

Characterization Methods
Thermal decomposition process acts with thermal analysis of DTA/TG model

Citrate Self-Burning Behavior
Results shows that produced Citrate gel from metal Nitrates and Citric acid goes through self-burning. By heating, bright points appear. This process studied by thermal analysis. Figure 1 shows that in DTA diagram, exothermic reaction acts in 237˚C. This reaction is because of reduction-oxidation reactions between Citrate system and Nitrate [4]. Diagram TG also shows that decomposition of gel occurs suddenly and randomly. In this temperature (237˚C) gel begins to burn.
Weight loss of gel is about 80% that occurs with self-burning. This weight loss is because of removing water vapor, carbon dioxide and nitrogen oxide.

Measuring Produced Particles Size
In polycrystalline materials, width of peak of XRD increases with decreasing crystalline plane spacing. The most useful pattern for measuring grain size, is use the FWHM 4 formula that shown in Figure 5. FWHM, depends on the number of reflecting crystalline plane. Scherer formula relates the crystalline grain size to width of maximum peak in half height and other conditions [6]. Table 1 shows the FWHM details. By substituting values in formula of FWHM, the mean particle size is 47 nm.

Measuring and Determining Synthesized Productions
Crystalline Type by TEM Images Figure 6 shows that synthesized particles size is 50 nm that confirms diffraction Open Journal of Synthesis Theory and Applications    that synthesized production by Citrate sol-gel method in PH of 9, has spinel crystalline that is In accordance with the detected structure in XRD. Synthesized Cu-Chromite in PH of 9 is solid phase of CuO, CuCr 2 O 4 that has spinel crystalline structure.

Synthesis of Cu-Chromite Nano Catalyzer in Different PHs
For studying PH effect in synthesis process of Cu-Chromite nano catalyzer, all of last studied method include metal percentage, amount of used Citric acid and drying and calcinating method (environmental conditions) are used and Ammonium solution of 0.1 molar used for controlling PH values. In this step, values of PH set to 3.32 (that is equal to PH of sample without adding Ammonium solution), 5, 7 and 9.  (Figure 7).
Cu-Chromite that used in increasing fuels burning rate has solid phase of CuO.CuCr 2 O 4 that synthesis in PH of 9 according to XRD results.    [8]. We informed in this project that changing PH is effective in type of solid phase. It has done for different calcination temperature and metal ratio, leads to finding the optimum calcination temperature and metal ratio.

Innovation in Catalyzer Synthesis
By this method, the best values for calcination temperature is 700˚C for 3 hrs and Cu to Cr ratio of 0.5. Figure 8 shows the XRD results analysis of synthesized catalyzer by last optimum method and introduced method of this paper. Also shows that introduced optimum method (calcination temperature of 700˚C for 3 hrs and Cu to Cr ratio of 0.5 in PH of 9) synthesis the single phase catalyzer and there isn't other phases like Cr 2 O 3 or CuCrO 2 .

Final Conclusion
Producing Cu-Chromite catalyzer according to applications in solid fuels is important for burning rate catalyzer. Thermal analysis shows that Citrate gel have self-burning behavior and sol-gel method can be used because of low costs, ease of action and uniform particle size for producing nano Cu-Chromite.