Synthesis and Characterization of Bioceramic Calcium Phosphates by Rapid Combustion Synthesis

J. Mater. Sci. Technol., 2010, 26(12), 1114-1118.

Synthesis and Characterization of Bioceramic Calcium Phosphates by Rapid Combustion Synthesis S. Sasikumar and R. Vijayaraghavan† Materials Division, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India [Manuscript received February 24, 2010, in revised form June 16, 2010]

Calcium hydroxyapatite (Ca10 (PO4 )6 (OH)2 ) has been synthesized in short duration by rapid solution combustion by employing different fuels. Calcium nitrate was taken as source of calcium and diammonium hydrogen phosphate served as the source of phosphate ions. Citric acid, tartaric acid, sucrose, glycine and urea were used as the fuels and nitrate ions and nitric acid were used as oxidizers. The influence of fuels on the morphology of the phase formed was studied. Results of the studies by powder X-ray diffraction and Fourier-transform infrared spectroscopy showed the formation of hydroxyapatite as a major phase for all the fuels. The thermal analysis of the decomposed precursor showed variation in heat content for different fuels. Scanning electron microscopy showed different morphologies of the products obtained by different fuels. Chemical analyses to determine the Ca:P atomic ratio in synthesized ceramics showed that the ratio was 1:1.66. KEY WORDS: Bioceramics; Combustion synthesis; Thermochemistry; Morphology

1. Introduction Human bone is a composite of inorganic and organic material consisting of 70% inorganic material with hydroxyapatite as a major constituent and 30% organic constituents, in which collagen constitutes more. Hydroxyapatite (HAp) is a member of the “apatite” family having almost similar chemical composition to that of human bone. Due to its biocompatibility and osteoconductive properties, it is highly used in orthopaedics and dentistry[1] . However, biological apatites differ from synthetically produced hydroxyapatite in stoichiometry, composition, crystallinity, and also in other physical and mechanical properties. Manipulating the morphology and architecture of the product is a challenge as its influence on material properties is significant. In this context, there is always a need to explore new cost effective methods to produce hydroxyapatite. In order to strike a balance between cost, synthesis duration and properties, we attempted to synthesize HAp by rapid solution † Corresponding author. Prof., Ph.D.; Tel.: +91 416 2202460; Fax: +91 416 224 3092; E-mail address: [email protected] (R. Vijayaraghavan).

combustion employing different fuels. Rapid combustion synthesis (RCS) is a rapid and energy-saving technique used by many researchers[2–4] for the preparation of various metal oxides. During combustion reaction the nitrate ions behave like conventional oxidants and the organic compounds function as fuels. It is an exothermic redox reaction associated with nitrate decomposition and fuel oxidation, which releases enormous amount of heat energy. Nature of the fuel and the ratio of oxidizer to fuel control the exothermicity of the combustion. The conventional solution combustion synthesis (SCS) involves uniform reaction solution preheating prior to self-ignition, whereas the RCS needs little heating to start a rapid reaction and requires little or no further calcination and milling of the products. Hydroxyapatite has been prepared by microemulsion route[5] , hydrothermal synthesis[6] , selfpropagating combustion synthesis[7] , mechanochemical synthesis[8] , microwave synthesis[9] , precipitation method[10] , sol-gel method[11] , sonochemical synthesis[12] , electrochemical method[13] , template assisted synthesis[14] and solution combustion synthesis[15] . Synthesis of hydroxyapatite by using

S. Sasikumar et al.: J. Mater. Sci. Technol., 2010, 26(12), 1114–1118 [16]

[7]

citric acid and tartaric acid as a fuel in combustion synthesis was already reported, where an intermediate step of gel formation took place. Recent reports show that single phasic hydroxyapatite can be synthesized via a mixed fuel approach by mixing two different fuels together in a different ratio instead of single fuel[17] . The present investigation was intended to synthesize hydroxyapatite by using rapid combustion synthesis exploring citric acid, tartaric acid, urea, glycine and sucrose as a fuel and to study the effect of fuel on the morphology of the products. In addition, the conventional solution combustion synthesis procedure was modified by adding a combustion aid to change the energetics of the reaction. The attempt was to synthesize hydroxyapatite of different morphology in a shorter duration of time by cost effective routes. 2. Experimental 2.1 Materials and methods Stoichiometric amounts of powders of Ca(NO3 )2 4H2 O (99+%, AR, Rankem) and (NH4 )2 HPO4 (99%, AR, Merck) were dissolved in demineralized water at room temperature to yield 1 mol/L stock solutions. The 1 mol/L stock solution of fuel was prepared by dissolving appropriate amount of citric acid, tartaric acid, urea, glycine and sucrose in demineralized water. From the stock solution 20 mL of 1 mol/L calcium nitrate was mixed with 20 mL of 1 mol/L fuel solution. The pH of the solution was adjusted to around 10 by NH4 OH solution. 12 mL of 1 mol/L of (NH4 )2 HPO4 solution was then added dropwise at a rate of 2 mL/min with constant magnetic stirring at room temperature to the mixture under stirring condition, which formed a gelatinous white precipitate. The formed precipitate was dissolved using concentrated nitric acid and excess of nitric acid was added until pH was around 1. The clear solution was kept in the preheated muffle furnace at 500◦ C. Within few minutes the solution underwent self-propagating auto ignition and formed a black coloured precursor. The precursors formed were calcined in air at 900◦ C for 2 h. 2.2 Chemical analysis The calcium/phosphorous ratio of the samples was measured by wet chemical method. The phosphate content was analyzed colorimetrically by using vanadate molybdate reagent. Calcium was estimated through standard complexometric titration by EDTA method. 2.3 Characterization Phase purity of the synthesized hydroxyapatite

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sample was analyzed in a PANalytical X’pert PRO diffractometer (Netherland), using CuKα, Ni filtered radiation. The functional groups were identified by Fourier-transform infrared spectroscopy (FTIR, Thermo Nicolet, Avatar 330 FTIR Spectrometer, USA) studies. The surface morphology was imaged by scanning electron microscopy (SEM, JEOL JSM5600 LV, Japan). The thermal analysis of the precursor was carried out at a heating rate of 10◦ C/min in air from room temperature to 1000◦ C by a thermal analyzer (V8.2 SDT Q600, TA Instruments, USA). Phosphate analysis was carried out with Filter 1 (λ=420 nm) by using Colorimeter 115, Systronics, India. 3. Results and Discussion Organic substances used as a fuel in RCS plays a duel role in the synthesis since it functions as a chelating agent and also avoids the precipitation of ions by complexing with metal ions. The vigorous redox reaction between fuel and oxidant mixture gives rise to a very high local temperature with the evolution of large volume of gases. Though the furnace temperature was maintained at 500◦ C, the particles could have attained higher local temperatures during ignition. Large amount of heat is released during the combustion, which produces an intense local heating that enhances the phase formation. The volume of gas generated and the increase in the local temperature due to the combustion depend on various factors such as nature of the fuel, temperature, water content and the ratio of fuel to oxidizer, as the thermochemistry of combustion are different for different fuels. This is evident from the sudden rise in temperature inside the furnace from the set temperature of 500 to 700◦ C during combustion. The heat energy evolved will be different for different fuels based on its heat of combustion, and hence every system is expected to form different phase based on the total heat content of the system. It is possible to attain different morphology of the products by varying the fuel, as different organic molecule with different functional groups will form a different polymeric network based on the different morphology. The heat content of the system depends on the nature of polymeric network formed during gelation, in which urea is exceptional as it undergoes controlled hydrolysis to form ammonium hydroxide, which on further reaction with metal ion gives metal hydroxide[18] . Whereas the carboxylic acid like citric acid and tartaric acid forms a different polymeric network with respect to the number of different functional groups present in their structure. Citric acid with three carboxylic acid groups and one alcoholic hydroxyl group will form a different three dimensional network from the one formed by tartaric acid, which possess two carboxylic acid groups and two hydroxyl groups. In the case of sucrose, it undergoes oxidation in presence

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S. Sasikumar et al.: J. Mater. Sci. Technol., 2010, 26(12), 1114–1118

Fig. 1 DTA curve of precursor synthesized using Sucrose as a fuel

Fig. 2 DTA curve of precursor synthesized using Urea as a fuel

of nitric acid and forms gluconic acid, which chelates with metal ion and then undergoes further polymerization[19] . Based on the polymeric network formed by the organic molecules, the morphology of the particles will differ and the energetics of different polymeric network will decide the formed phases. Addition of ammonium hydroxide and nitric acid helps in the formation of ammonium nitrate, which helps in increasing heat of combustion by acting as a combustion aid. Also it helps in the formation of the porous network structure as the decomposition of NH4 NO3 releases gasses, such as NH3 , NOx and H2 O[20] . The product obtained by the combustion results in very fine powder due to rapid evolution of large volume of gases during the reaction. The colour of the precursor indicates the completion of combustion reaction. Black colour indicates the complete combustion of higher carbon- containing fuels, whereas light brown to dark brown indicates the incomplete combustion of carbonaceous fuels, which may be due to insufficient oxidizer or poor oxygen supply. In this study, all the precursors except the precursor of urea system were found to be black in colour, which indicates the complete combustion of the fuel. Immaterially complete or incomplete combustion always results in a pale yellow precursor as carbon content of urea is very less. Thermal analysis (Figs. 1 and 2) for different precursors shows different differential thermal analysis

Fig. 3 XRD patterns of precursors synthesized by: (a) tartaric, (b) sucrose, (c) citric acid, (d) urea, (e) glycine

(DTA) curves, indicating that the heat evolution and absorption by the system vary with the fuel. DTA curve (Fig. 1) of the precursor synthesized by using sucrose as a fuel shows a broad exothermic peak in the range of 200 to 500◦ C where the decomposition of carbonaceous substance takes place. The endothermic peak in the range of 800 to 1000◦ C may be due to the crystallization of hydroxyapatite phase from amorphous calcium phosphate, which is evident from the X-ray diffraction (XRD) patterns (Fig. 3(b) and Fig. 4(b)), revealing the absence of hydroxyapatite phase in precursor and the presence of hydroxyapatite phase when the precursor is calcined at 900◦ C for 2 h. The DTA curve (Fig. 2) of the precursor synthesized using urea as a fuel shows a broad exothermic peak in the range of 100 to 600◦ C, which may be due to the decomposition of carbonaceous substances. The XRD pattern (Fig. 3(d)) of the precursor confirms that the hydroxyapatite phase formation and crystallization take place at the time of combustion. Presence of endothermic peak in the range of 700 to 720◦ C indicates the phase transformation of amorphous calcium phosphates, which may be attributed to the change in the concentration of βtricalciumphosphate (β-TCP) phase in the XRD pattern observed for the precursor (Fig. 3(d)) and cal-

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Fig. 5 FTIR spectra of hydroxyapatite synthesized by: (a) tartaric, (b) sucrose, (c) citric acid, (d) urea, (e) glycine

Fig. 4 XRD patterns of hydroxyapatite obtained from precursors calcined at 900◦ C synthesized by: (a) tartaric, (b) sucrose, (c) citric acid, (d) urea, (e) glycine

cined sample (Fig. 4(d)). As expected, both the DTA curves show the absence of sharp exothermic peaks, which indicates that the phase formation occurs already at the time of combustion and not during calcination. Broad exotherm in the range of 200 to 500◦ C may be due to the removal of organic constituents by decomposition. The XRD pattern (Fig. 3) of the as-synthesized precursors shows well crystalline hydroxyapatite phase formation along with β-TCP for glycine and urea systems, whereas tartaric acid system shows the mixture of amorphous and crystalline phase of hydroxyapatite. Citric acid and sucrose system shows amorphous pattern, maybe due to the large amount of carbon particles, which is evident from the dark black precursors. Urea and glycine with higher heat of combustion result in the formation of crystalline phase. The XRD patterns (Fig. 4) of the calcined samples confirm the formation of hydroxyapatite as a major phase for all the samples. The lattice parameter values refined using check cell is in close agreement with JCPDS card No. 09-432. Different fuels form

different ratios of hydroxyapatite to β-tricalcium phosphate, which may be due to the change in heat content of the different fuel systems. The FTIR spectra (Fig. 5) of the final powder of all samples show the characteristic peaks corresponding to OH bending vibrations at 633 cm−1 and its stretching vibrations at 3571 cm−1 . Bending vibrations of phosphate group are observed at 473, 571 and 601 cm−1 , whereas the stretching vibrations of the phosphate group are observed at 962, 1044 and 1089 cm−1 . The FTIR spectra show weak bands of the carbonate group at 1465 cm−1 for the hydroxyapatite prepared using glycine and tartaric acid, which shows the carbonate substitution in the phosphate site of hydroxyapatite. Chemical analysis of all samples shows Ca:P ratio as 1:1.67, which indicates that the starting composition falls in the stoichiometric range of hydroxyapatite. SEM images (Fig. 6) show the influence of fuel on the morphology of the products. When tartaric acid is used as a fuel, the particles formed are spherical in shape, whereas in sucrose method the particles of the products have flake-like morphology. In contrast, when citric acid is used as fuel, the particles of hydroxyapatite are agglomerated and irregular shape with porous structure. The particle size is not uniform as the polymeric network formed is not uniform due to rapid rise in the temperature of the system. The change in morphology may be attributed to the different polymerization network formed by different fuels. The particle size also varies with fuel, which is

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Fig. 6 SEM micrographs of hydroxyapatite synthesized by: (a) tartaric, (b) sucrose, (c) citric acid

evident from the SEM images (Fig. 6). Tartaric acid as a fuel forms submicron particles and citric acid forms hard agglomerates, which are in the range of submicron to 1 μm. But sucrose forms flakes which are in the range of one to few microns. The porosity of the product depends on the volume of gas produces, which will differ for different fuels. 4. Conclusion This investigation demonstrates a simple, energy and time saving route to synthesize hydroxyapatite. It is shown that sucrose, urea, citric acid, tartaric acid and glycine can be used as a fuel to synthesize calcium phosphate bioceramics by rapid combustion synthesis with different morphology. For the fuels like citric acid, tartaric acid and sucrose, the calcination of the precursor is required to obtain a crystalline hydroxyapatite; whereas for the fuels like urea and glycine, the phase formation is completed during the combustion itself and further calcination is required for the removal of organic residues. From the results, it is concluded that the different fuels used in the combustion synthesis lead to different morphology and different ratio of calcium phosphate bioceramic phases.

Acknowledgements The authors thank VIT University management and DRDO, Grant in aid scheme, Government of India, for financial assistance and Technology Business Incubator, VIT for FTIR measurements. REFERENCES [1 ] J. Chevalier and L. Gremillard: J. Eur. Ceram. Soc., 2009, 29(7), 1245. [2 ] V. Singh, T.K. Gundu Rao and J.J. Zhu: J. Lumin.,

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