- Email: [email protected]
Influence of fuels and combustion aids on solution combustion synthesis of bi-phasic calcium phosphates (BCP)
Materials Science and Engineering C 32 (2012) 2464–2468
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of fuels and combustion aids on solution combustion synthesis of bi-phasic calcium phosphates (BCP) M.A. Aghayan, M.A. Rodríguez ⁎ Instituto de Ceramica y Vidrio (ICV-CSIC), C/Kelsen, 5, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 14 November 2011 Received in revised form 6 June 2012 Accepted 17 July 2012 Available online 22 July 2012 Keywords: Biomaterials Ceramics Combustion Synthesis
a b s t r a c t The possibility to obtain biphasic HA/TCP composite by solution combustion synthesis method using urea and glycine as fuels was investigated. Calcium nitrate was taken as a source of calcium, and diammonium hydrogen phosphate served as a source of phosphate ions. Nitric acid and nitrate ions were used as oxidizers. The effect of the nature of fuel (urea and glycine) and fuel to oxidizer ratio on the combustion behavior, as well as, chemical composition and morphology of as-formed powders was investigated. It was found that only monoclinic-TCP was formed in the combustion end-product when glycine was used. In contrast to this, the use of certain amount of urea led to the formation of rhombohedral-TCP. A series of combustion reactions were carried out to study the influence of fuel to oxidizer ratio on HA to TCP ratio in synthesized product. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The most promising biomaterials for bone replacements are hydroxyapatite (HA), tricalcium phosphate (TCP), bioactive glasses, etc. Nowadays, HA and TCP are the most widely used in various bone repair applications due to their close chemical similarity to bone mineral, as well as, their bioactive and osteoconductive properties. However, HA is less resorbable than TCP. A mixture of HA and TCP produces biphasic calcium phosphate (BCP) which possesses the reactivity of TCP and the stability of HA, providing more bioactivity, involving more new bone growth, and ensuring better resistance of the implants to strain. BCP has an intermediate resorbability which can be controlled by variation of the HA/β-TCP ratio [1]. BCP was prepared either by mechanical mixing of HA and TCP powders or by obtaining calcium-deficient apatites (Ca10 − X(HPO4)X (PO4)6 − X(OH)2 − X) by co-precipitation [2–4], sol–gel [5], sol–gel combustion [6], and flame-spray pyrolysis [7] methods. One of the promising methods to obtain bi-phasic calcium phosphate is solution combustion synthesis [8–11]. Currently, solution combustion is being considered to be a promising method to obtain nano-sized powder due to its several advantages. This method involves exothermic chemical reaction between oxidizer and fuel, which are dissolved into a solution, providing high level molecular mixing of the components. The chemical energy released from the exothermic reaction provides self-sustained reaction. As a result, powder with high purity, better homogeneity and high surface area forms in a rapid, inexpensive single step operation. ⁎ Corresponding author. Tel.: +34 917355869; fax: +34 917355843. E-mail address: [email protected] (M.A. Rodríguez). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.07.027
Several researchers have tried to control the combustion flame temperature (Tf), and optimize the reaction parameters by adjusting the nature and amount of the fuel. Sasikumar et al. have synthesized HA using tartaric acid [12], succinic acid and citric acid [13] as fuels. It was suggested that the mentioned fuels acted as complexing agents reacting with metal cations, and prevented selective precipitation at the heating stage of solution. Similar to the mentioned amino acids, glycine is also an effective complexing agent [14,15]. Moreover, glycine provides high heat energy during combustion, hence, it has a wide application in combustion synthesis of different compounds. Ghosh et al. [9] studied the effect of urea, glycine and glucose on the combustion flame temperature and characteristics of synthesized powder. The flame temperature increased with decreasing the ratio of urea to glycine in the fuel mixture, keeping constant total quantity of fuels. This phenomenon was explained by the low value of standard enthalpy of combustion of urea, which was about three times less for glycine. In their further work Ghosh et al. [16] detected that, although, there was a noticeable difference of combustion standard enthalpies between urea and glycine, the maximum reaction temperature, generated during the process, was approximately that same when stoichiometric ratio of fuel to oxidizer (NO3 −) was used. The DTA and TGA analyses showed that the decomposition mechanism was quite different for glycine and urea. It was also detected that fuel to oxidizer ratio influenced on combustion temperature. In various works [8,10,12,13,16,17] concentrated nitric acid was added in the initial solution to obtain transparent solution (without precipitates). In all cases it was not given attention to the presence of HNO3 and its effect on the combustion process and yielded products. In this work, the possibility of the formation of BCP by combustion synthesis was studied using Ca(NO3)2/(NH4)2HPO4 pair of precursors
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
with Ca/P = 1.5. Glycine and urea were used as fuels. The effect of the nature and quantity of fuels on the combustion process and characteristic of yielded powders were investigated. To study the influence of HNO3 as an aid on the phase composition and morphology of combustion product, different amount of HNO3 was added in the initial solution keeping constant the amount of fuels. Controlling the HA/β-TCP ratio, it will be possible to control reactivity and resorption of bi-phasic calcium phospate, improving its behavior as bone filler or bone augmentation material. 2. Experimental
2465
Table 1 Composition of the initial solution and the ratio of fuel to oxidizer using urea as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (urea), mol
n(HNO3), mol
n(HNO3) + n(NO3−) mol
n(urea)/ n(NO3−) mol
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.5 0.8 1.1 1.6 1.6 1.6 1.6
0.1 0.1 0.1 0.1 0.3 0.7 1.3
0.4 0.4 0.4 0.4 0.6 1 1.6
1.3 2.0 2.8 4.0 2.7 1.6 1.0
2.1. Preparation method Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) (99.5%, Merck) and diamonium hydrogen phosphate ((NH4)2HPO4) (99%, Merck) were used as green material precursors for calcium and phosphorus. 0.15 mol Ca(NO3)2·4H2O and 0.1 mol (NH4)2HPO4 were added to 200 ml water to form Ca/P molar ratio 1.5. After magnetic stirring for 15 min, 8 ml of concentrated (65%) nitric acid was added into the solution to dissolve the resulting white precipitate to make a clear homogeneous solution (Fig. 1). Different amounts of urea and glycine were added to the solution. In order to study the influence of fuel/NO3− ratio, the amount of concentrated HNO3 added was changed in the experiments containing 1.6 mol urea and 0.5 mol glycine (Tables 1, 2). Then the volume of solution was brought to 300 ml, adding required amount of water. 50 ml of the obtained transparent solution was transferred into ceramic beaker of 100 ml capacity, which, afterwards, was heated up to 450 °C in a muffle furnace (with 25°/min heating rate), where ignition took place. After 30 min the sample was taken out from the furnace (Fig. 2). The products were produced, which was lightly grinded into an Agatha mortar to obtain a fine powder. 2.2. Characterization The phase identification of the combustion synthesized powder was carried out by X-ray diffraction using a Bruker diffractometer
(D8) with CuKα radiation. Specific surface area (SSA) of the powder was determined by BET model with data provided by a dynamic adsorption equipment (Monosorb, Quantachrome, USA). Field emission scanning electron microscopy (FESEM S-4700, Hitachi, Japan) was used to characterize the microstructure of the combustion product, which beforehand was coated with silver. 3. Results and discussion 3.1. Effect of the quantity of urea Using urea as a fuel, a white mass was obtained as a combustion product, which turned into yellowish mass when amount of urea (nur) in the initial solution was increased. One can state that yellowish color of the product was caused by residues, formed as a result of incomplete oxidation of urea during combustion. The XRD pattern of the combustion products, obtained from the systems containing different amounts of urea, is plotted in Fig. 3. As it can be seen, HA formed as dominant phase at nur = 0.5 mol. In addition, small amount of β-TCP formed as minor (secondary) phase. The amount of β-TCP became appreciable when amount of urea increased up to 0.8 mol, as the quantity of energy, introduced in the system, also increased leading to decomposition of non stoichiometric HA to β-TCP, small amount of Ca2P2O7 begins to be observed. Gradual transformation of β- to α-TCP took place when the amount of urea was increased from 0.8 to 1.1 mol. Further increase of the quantity of urea resulted in increase of the amount of Ca2P2O7 at the expense of HA. Thus, when nur ≥ 1.1 mol, the combustion product consisted of Ca2P2O7 (Ca/P = 1) and TCP (Ca/P = 1.5), so the summing Ca/P b1.5 in the obtained crystal phases, while Ca/P = 1.5 in the initial solution. It is supposed that undetected part of Ca ions was in the residue of incomplete oxidized urea or in defects of detected phases. The incompleteness of the oxidation of urea was confirmed above by the yellowish color of the combustion product. Fig. 4 shows the XRD pattern of combustion products obtained from systems containing different amounts of HNO3 (nHNO3), with a content of urea of nur = 1.6 mol. When nHNO3 ≥ 0.3 mol, corresponding reflections of α-TCP (high temperature phase) disappeared (Fig. 4).
Table 2 Composition of the initial solution and the ratio of fuel to oxidizer using glycine as fuel. n((NH4)2HPO4), n (Ca(NO3)2), n (glycine), n(HNO3), n(HNO3)+ n(glycine)/ n(NO3−), mol mol mol mol n(NO3−), mol mol
Fig. 1. Schematic diagram for preparation of BCP by solution combustion synthesis method.
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.06 0.17 0.2 0.5 0.5 0.5 0.5
0.1 0.1 0.1 0.1 0.3 0.7 1.3
0.4 0.4 0.4 0.4 0.6 1 1.6
0.15 0.43 0.5 1.25 0.83 0.5 0.3
2466
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
Fig. 2. The heat-resistant container, and appearance of the products before light grinding.
This phenomenon can be explained by the complete oxidation of urea. Thus, increasing the amount of HNO3, which served as an oxidizer, urea was oxidized more completely releasing more energy during the process. Moreover, the amount of generated gasses also increased, causing energy loss and a lower combustion temperature. As a result, mainly β-TCP formed in the end-product. It should be noted that the amount of β-TCP and HA phases increased when the amount of HNO3 varied from 0.3 to 0.7 mol. However, further increase of amount of HNO3 led to diminution of β-TCP, while HA increases, accordingly, increase of Ca/P ratio. The change of the Ca/P ratio was caused by change of pH of the solution.
Fig. 4. The XRD patterns of the synthesized powders obtained from systems containing different amounts of HNO3, nur = 1.6 mol.
Thus, by increasing the amount of HNO3 the acidity of the solution increased strongly and the reactivity of phosphate ions decreased. The results of FESEM show that nanoparticles with different morphologies (nodular and rod like) and sizes formed, at nur = 0.5 mol (Fig. 5a). The specific surface area (SSA) determined by BET was 14 m 2/g (Table 3). Increasing the amount of urea, the presence of big particles increased significantly (Fig. 5b). The rod like shape of the particles became more noticeable and the SSA of product dropped to 9 m 2/g. When the quantity of nitric acid was increased (nHNO3 = 1.3 mol, nur = 1.6 mol), agglomerates with 500 nm in diameter obtained. Those agglomerates were formed by small particles (≤ 50 nm) (Fig. 5c). By increasing the amount of HNO3, the volume of evolved gasses rose leading to the increase of SSA up to 20 m 2/g. Further increase of the quantity of HNO3 did not strongly influence on the SSA of the product.
3.2. Effect of the quantity of glycine
Fig. 3. The XRD patterns of the synthesized powders obtained from systems containing different amounts of urea, nHNO3 = 0.1 mol.
Using glycine as a fuel, a grayish foamy mass formed as a combustion product. Such color of the product could be caused by the presence of carbonaceous residue remained from incomplete oxidized glycine. The product became darker when the ratio of glycine to the oxidant (NO3−) increased in the initial solution. Fig. 6 shows XRD patterns of products formed from systems containing different amounts of glycine. One can observe that the dominant phase was HA when the amount of glycine was small. Small amount of α-TCP and Ca2P2O7 phases also formed as secondary phases. Further increase of quantity of glycine led to the gradual decrease of amount of HA and increase of amount of Ca2P2O7. At 0.5 mol glycine, Ca2P2O7 became the dominant phase, while HA disappeared totally. It can be seen in Fig. 6 that the increase of the amount of glycine resulted in an apparent decrease of Ca/P ratio according to the crystallize phases detected by XRD. Being amino acid glycine reacted with Ca ions forming calcium glycinate complexes which decomposed during the combustion process releasing gasses and Ca ions [14]. When the ratio of glycine/oxidant was high, glycine was not oxidized completely, resulting to the formation of carbonaceous residue which might contain Ca ions. On the other hand, the higher amount of glycine (consider globally) led to the increase of the quantity of released energy, which could be a reason of preponderance of high temperature phases, such as Ca2P2O7 as a
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
2467
Fig. 6. The XRD patterns of the combustion products obtained from systems containing different amounts of glycine, nHNO3 = 0.1 mol.
0.8 mol urea (Eq. (1)) is higher than for 0.4 mol glycine (Eq. (2)), which can be a reason of withdrawing of energy from the system. COðNH2 Þ2 ðgÞ þ 3=2 O2 ðgÞ ¼ CO2 ðgÞ þ N2 ðgÞ þ 2H2 OðgÞ ΔH200 ¼ −154 kcal
ð1Þ NH2 CH2 COOHðgÞ þ 9=4O2 ðgÞ ¼ 2CO2 ðgÞ þ 1=2 N2 ðgÞ þ 5=2H2OðgÞΔH200 ¼ −307 kcal
Fig. 5. FESEM micrograph of the synthesized powders: (a) nur = 0.5 mol, nHNO3 = 0.1 mol, (b) nur = 1.6 mol, nHNO3 = 0.1 mol and (c) nur = 1.6 mol, nHNO3 = 1.3 mol.
ð2Þ
where ΔH is the enthalpy of combustion reaction. Thus, the formation of α-TCP phase in a low energetic system (when glycine is used) might be caused by its formation mechanism. It should
dominant phase, and the disappearance of HA (low temperature phase). It is notable that α-TCP was obtained when different amount of glycine was used. While using low amounts of urea (nur ≤ 0.8), β-TCP phase formed. It should be considered that 0.8 mol urea provides similar amount of energy to that of 0.4 mol glycine (Eqs. (1), (2)). Moreover the quantity of gasses released from the oxidation of
Table 3 The SSA of the combustion product using urea as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (urea), mol
n(HNO3), mol
SSA, m2/g
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.5 0.8 1.1 1.6 1.6 1.6 1.6
0.1 0.1 0.1 0.1 0.3 0.7 1.3
14 11 6 9 20 19 20
Fig. 7. The XRD patterns of the combustion products obtained from systems containing different amounts of HNO3 ngly = 0.5 mol.
2468
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
Fig. 8. FESEM micrograph of the synthesized powders: (a) ngly = 0.17 mol, nHNO3 = 0.1 mol, (b) ngly = 0.5 mol, nHNO3 = 0.1 mol and (c,d) ngly = 0.5 mol, nHNO3 = 1.3 mol.
be noted that according to phase diagram, α-TCP forms at higher temperature, while β-TCP exists when the temperature of the system is less than 1120 °C. The intensity of the peaks corresponding to α-TCP rose when the amount of glycine increased up to 0.2 mol (Fig. 6). Different amount of HNO3 was added in the initial solution containing 0.5 mol glycine (Table 2). The XRD patterns of combustion products are presented in Fig. 7. α-TCP, Ca2P2O7 and HA phases formed adding small amount of HNO3 in the initial solution containing 0.5 mol glycine, (Fig. 7). Increasing the amount of HNO3, the amount of high temperature phases decreased, the peaks of Ca2P2O7 phase disappeared and α-TCP phase decreased continuously, while the peaks of HA increased, caused by decrease of pH of the solution. FESEM observation of the product shows that the rod-like particles, with ~ 50 nm in diameter, were lightly agglomerated (making agglomerates smaller than 3 μm) (Fig. 8a). The SSA of the formed powder was 90 m 2/g (Table 4). Increasing the amount of glycine up to 0.5 mol, the SSA dropped to 21 m 2/g and agglomerates-with smooth surface-formed. It can be seen that particles with different morphologies were obtained between the smooth bubbles (Fig. 8b). Adding 1.3 mol HNO3 in the system which contained 0.5 mol glycine, led to the formation of powder with grid-like (netlike) macrostructure. (Fig. 8c). The nanosize particles were almost spherical and were sintered making necks among them (Fig. 8d). Although the microstructure of the powder was altered, changing the amount of HNO3, the SSA remained almost unchanged (Table 4). 4. Conclusions The influence of the type and amount of fuel on the possibility of formation of biphasic HA/TCP composite by solution combustion Table 4 The SSA of the combustion product using glycine as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (glycine), mol
n(HNO3), mol
SSA, m2/g
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.06 0.17 0.2 0.5 0.5 0.5 0.5
0.1 0.1 0.1 0.1 0.3 0.7 1.3
90 76 9 21 15 20 20
synthesis method using urea and glycine as fuels was studied. It was found that the type of fuel and its quantity influenced significantly on the phase composition and microstructure of the synthesized powders. Ca-poor compounds formed as a result of combustion of fuel-rich systems. Mainly α-TCP was produced using glycine as a fuel, while β-TCP phase prevails using urea, and β-TCP was not formed significantly when glycine was used as a fuel. The investigation also showed that the amount of induced HNO3 had a big influence on the combustion synthesized product. Changing the amount of HNO3, it was possible to control Ha, TCP contents in the yielded BCP, keeping constant Ca/P ratio in the initial mixture. By means of the solution combustion method using urea and glycine, the HA/β-TCP ratio can be easily controlled in single step operation, and the in-vivo behavior of BCP will be improved as well. Acknowledgments Supported by the Spanish Science and Technology Agency (CICYT) through the projects MAT 2010‐17753 and MAT 2010-C21088-C03. References [1] R.Z. LeGeros, G. Daculsi, J.P. LeGeros, Orthop. Biol. Med. 2 (2008) 153–181. [2] N. Kivrak, A.C. Taş, J. Am. Ceram. Soc. 81 (1998) 2245–2252. [3] M. Campos, F.A. Müller, A. Bressiani, J.C. Bressiani, P. Greil, J. Mater. Sci. Mater. Med. 18 (2007) 669–675. [4] S.H. Kwon, Y.K. Jun, S.H. Hong, H.E. Kim, J. Eur. Ceram. Soc. 23 (2003) 1039–1045. [5] S. Gomes, G. Renaudin, E. Jallot, J.M. Nedelec, Appl. Mater. Interfaces 1 (2009) 505–513. [6] J. Zhang, J. Guo, S. Li, B. Song, K. Yao, Front. Chem. Chin. 3 (2008) 451–453. [7] J.S. Cho, Y.N. Ko, H.Y. Koo, Y. Kang, J. Mater. Sci. Mater. Med. 21 (2010) 1143–1149. [8] S.K. Ghosh, S.K. Nandi, B. Kundu, S. Datta, D.K. De, S.K. Roy, D. Basu, J. Biomed. Mater. Res. B Appl. Biomater. 86B (2008) 217–227. [9] S.K. Ghosh, A. Prakash, S. Datta, S.K. Roy, D. Basu, Bull. Mater. Sci. 33 (2010) 7–16. [10] S.K. Nandi, B. Kundu, S.K. Ghosh, T.K. Mandal, S. Datta, D.K. De, D. Basu, Ceram. Int. 35 (2009) 1367–1376. [11] S. Sasikumar, R. Vijayaraghavan, Mater. Sci. Technol. 26 (2010) 1114–1118. [12] S. Sasikumar, R. Vijayaraghavan, Sci. Technol. Adv. Mater. 9 (2008) 035003. [13] S. Sasikumar, R. Vijayaraghavan, Ceram. Int. 34 (2008) 1373–1379. [14] D. Gopi, J. Indira, V. Prakash, L. Kavitha, Spectrochim. Acta A Mol. Biomol. Spectrosc. 74 (2009) 282–284. [15] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, J. Mater. Lett. 10 (1990) 6–12. [16] S.K. Ghosh, S.K. Roy, B. Kundu, S. Datta, D. Basu, Mater. Sci. Eng., B 176 (2011) 14–21. [17] Y. Liu, X. Dan, Y. Du, F. Liu, J. Mater. Sci. 39 (2004) 4031–4034.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Influence of fuels and combustion aids on solution combustion synthesis of bi-phasic calcium phosphates (BCP) M.A. Aghayan, M.A. Rodríguez ⁎ Instituto de Ceramica y Vidrio (ICV-CSIC), C/Kelsen, 5, 28049 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 14 November 2011 Received in revised form 6 June 2012 Accepted 17 July 2012 Available online 22 July 2012 Keywords: Biomaterials Ceramics Combustion Synthesis
a b s t r a c t The possibility to obtain biphasic HA/TCP composite by solution combustion synthesis method using urea and glycine as fuels was investigated. Calcium nitrate was taken as a source of calcium, and diammonium hydrogen phosphate served as a source of phosphate ions. Nitric acid and nitrate ions were used as oxidizers. The effect of the nature of fuel (urea and glycine) and fuel to oxidizer ratio on the combustion behavior, as well as, chemical composition and morphology of as-formed powders was investigated. It was found that only monoclinic-TCP was formed in the combustion end-product when glycine was used. In contrast to this, the use of certain amount of urea led to the formation of rhombohedral-TCP. A series of combustion reactions were carried out to study the influence of fuel to oxidizer ratio on HA to TCP ratio in synthesized product. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The most promising biomaterials for bone replacements are hydroxyapatite (HA), tricalcium phosphate (TCP), bioactive glasses, etc. Nowadays, HA and TCP are the most widely used in various bone repair applications due to their close chemical similarity to bone mineral, as well as, their bioactive and osteoconductive properties. However, HA is less resorbable than TCP. A mixture of HA and TCP produces biphasic calcium phosphate (BCP) which possesses the reactivity of TCP and the stability of HA, providing more bioactivity, involving more new bone growth, and ensuring better resistance of the implants to strain. BCP has an intermediate resorbability which can be controlled by variation of the HA/β-TCP ratio [1]. BCP was prepared either by mechanical mixing of HA and TCP powders or by obtaining calcium-deficient apatites (Ca10 − X(HPO4)X (PO4)6 − X(OH)2 − X) by co-precipitation [2–4], sol–gel [5], sol–gel combustion [6], and flame-spray pyrolysis [7] methods. One of the promising methods to obtain bi-phasic calcium phosphate is solution combustion synthesis [8–11]. Currently, solution combustion is being considered to be a promising method to obtain nano-sized powder due to its several advantages. This method involves exothermic chemical reaction between oxidizer and fuel, which are dissolved into a solution, providing high level molecular mixing of the components. The chemical energy released from the exothermic reaction provides self-sustained reaction. As a result, powder with high purity, better homogeneity and high surface area forms in a rapid, inexpensive single step operation. ⁎ Corresponding author. Tel.: +34 917355869; fax: +34 917355843. E-mail address: [email protected] (M.A. Rodríguez). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2012.07.027
Several researchers have tried to control the combustion flame temperature (Tf), and optimize the reaction parameters by adjusting the nature and amount of the fuel. Sasikumar et al. have synthesized HA using tartaric acid [12], succinic acid and citric acid [13] as fuels. It was suggested that the mentioned fuels acted as complexing agents reacting with metal cations, and prevented selective precipitation at the heating stage of solution. Similar to the mentioned amino acids, glycine is also an effective complexing agent [14,15]. Moreover, glycine provides high heat energy during combustion, hence, it has a wide application in combustion synthesis of different compounds. Ghosh et al. [9] studied the effect of urea, glycine and glucose on the combustion flame temperature and characteristics of synthesized powder. The flame temperature increased with decreasing the ratio of urea to glycine in the fuel mixture, keeping constant total quantity of fuels. This phenomenon was explained by the low value of standard enthalpy of combustion of urea, which was about three times less for glycine. In their further work Ghosh et al. [16] detected that, although, there was a noticeable difference of combustion standard enthalpies between urea and glycine, the maximum reaction temperature, generated during the process, was approximately that same when stoichiometric ratio of fuel to oxidizer (NO3 −) was used. The DTA and TGA analyses showed that the decomposition mechanism was quite different for glycine and urea. It was also detected that fuel to oxidizer ratio influenced on combustion temperature. In various works [8,10,12,13,16,17] concentrated nitric acid was added in the initial solution to obtain transparent solution (without precipitates). In all cases it was not given attention to the presence of HNO3 and its effect on the combustion process and yielded products. In this work, the possibility of the formation of BCP by combustion synthesis was studied using Ca(NO3)2/(NH4)2HPO4 pair of precursors
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
with Ca/P = 1.5. Glycine and urea were used as fuels. The effect of the nature and quantity of fuels on the combustion process and characteristic of yielded powders were investigated. To study the influence of HNO3 as an aid on the phase composition and morphology of combustion product, different amount of HNO3 was added in the initial solution keeping constant the amount of fuels. Controlling the HA/β-TCP ratio, it will be possible to control reactivity and resorption of bi-phasic calcium phospate, improving its behavior as bone filler or bone augmentation material. 2. Experimental
2465
Table 1 Composition of the initial solution and the ratio of fuel to oxidizer using urea as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (urea), mol
n(HNO3), mol
n(HNO3) + n(NO3−) mol
n(urea)/ n(NO3−) mol
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.5 0.8 1.1 1.6 1.6 1.6 1.6
0.1 0.1 0.1 0.1 0.3 0.7 1.3
0.4 0.4 0.4 0.4 0.6 1 1.6
1.3 2.0 2.8 4.0 2.7 1.6 1.0
2.1. Preparation method Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) (99.5%, Merck) and diamonium hydrogen phosphate ((NH4)2HPO4) (99%, Merck) were used as green material precursors for calcium and phosphorus. 0.15 mol Ca(NO3)2·4H2O and 0.1 mol (NH4)2HPO4 were added to 200 ml water to form Ca/P molar ratio 1.5. After magnetic stirring for 15 min, 8 ml of concentrated (65%) nitric acid was added into the solution to dissolve the resulting white precipitate to make a clear homogeneous solution (Fig. 1). Different amounts of urea and glycine were added to the solution. In order to study the influence of fuel/NO3− ratio, the amount of concentrated HNO3 added was changed in the experiments containing 1.6 mol urea and 0.5 mol glycine (Tables 1, 2). Then the volume of solution was brought to 300 ml, adding required amount of water. 50 ml of the obtained transparent solution was transferred into ceramic beaker of 100 ml capacity, which, afterwards, was heated up to 450 °C in a muffle furnace (with 25°/min heating rate), where ignition took place. After 30 min the sample was taken out from the furnace (Fig. 2). The products were produced, which was lightly grinded into an Agatha mortar to obtain a fine powder. 2.2. Characterization The phase identification of the combustion synthesized powder was carried out by X-ray diffraction using a Bruker diffractometer
(D8) with CuKα radiation. Specific surface area (SSA) of the powder was determined by BET model with data provided by a dynamic adsorption equipment (Monosorb, Quantachrome, USA). Field emission scanning electron microscopy (FESEM S-4700, Hitachi, Japan) was used to characterize the microstructure of the combustion product, which beforehand was coated with silver. 3. Results and discussion 3.1. Effect of the quantity of urea Using urea as a fuel, a white mass was obtained as a combustion product, which turned into yellowish mass when amount of urea (nur) in the initial solution was increased. One can state that yellowish color of the product was caused by residues, formed as a result of incomplete oxidation of urea during combustion. The XRD pattern of the combustion products, obtained from the systems containing different amounts of urea, is plotted in Fig. 3. As it can be seen, HA formed as dominant phase at nur = 0.5 mol. In addition, small amount of β-TCP formed as minor (secondary) phase. The amount of β-TCP became appreciable when amount of urea increased up to 0.8 mol, as the quantity of energy, introduced in the system, also increased leading to decomposition of non stoichiometric HA to β-TCP, small amount of Ca2P2O7 begins to be observed. Gradual transformation of β- to α-TCP took place when the amount of urea was increased from 0.8 to 1.1 mol. Further increase of the quantity of urea resulted in increase of the amount of Ca2P2O7 at the expense of HA. Thus, when nur ≥ 1.1 mol, the combustion product consisted of Ca2P2O7 (Ca/P = 1) and TCP (Ca/P = 1.5), so the summing Ca/P b1.5 in the obtained crystal phases, while Ca/P = 1.5 in the initial solution. It is supposed that undetected part of Ca ions was in the residue of incomplete oxidized urea or in defects of detected phases. The incompleteness of the oxidation of urea was confirmed above by the yellowish color of the combustion product. Fig. 4 shows the XRD pattern of combustion products obtained from systems containing different amounts of HNO3 (nHNO3), with a content of urea of nur = 1.6 mol. When nHNO3 ≥ 0.3 mol, corresponding reflections of α-TCP (high temperature phase) disappeared (Fig. 4).
Table 2 Composition of the initial solution and the ratio of fuel to oxidizer using glycine as fuel. n((NH4)2HPO4), n (Ca(NO3)2), n (glycine), n(HNO3), n(HNO3)+ n(glycine)/ n(NO3−), mol mol mol mol n(NO3−), mol mol
Fig. 1. Schematic diagram for preparation of BCP by solution combustion synthesis method.
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.06 0.17 0.2 0.5 0.5 0.5 0.5
0.1 0.1 0.1 0.1 0.3 0.7 1.3
0.4 0.4 0.4 0.4 0.6 1 1.6
0.15 0.43 0.5 1.25 0.83 0.5 0.3
2466
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
Fig. 2. The heat-resistant container, and appearance of the products before light grinding.
This phenomenon can be explained by the complete oxidation of urea. Thus, increasing the amount of HNO3, which served as an oxidizer, urea was oxidized more completely releasing more energy during the process. Moreover, the amount of generated gasses also increased, causing energy loss and a lower combustion temperature. As a result, mainly β-TCP formed in the end-product. It should be noted that the amount of β-TCP and HA phases increased when the amount of HNO3 varied from 0.3 to 0.7 mol. However, further increase of amount of HNO3 led to diminution of β-TCP, while HA increases, accordingly, increase of Ca/P ratio. The change of the Ca/P ratio was caused by change of pH of the solution.
Fig. 4. The XRD patterns of the synthesized powders obtained from systems containing different amounts of HNO3, nur = 1.6 mol.
Thus, by increasing the amount of HNO3 the acidity of the solution increased strongly and the reactivity of phosphate ions decreased. The results of FESEM show that nanoparticles with different morphologies (nodular and rod like) and sizes formed, at nur = 0.5 mol (Fig. 5a). The specific surface area (SSA) determined by BET was 14 m 2/g (Table 3). Increasing the amount of urea, the presence of big particles increased significantly (Fig. 5b). The rod like shape of the particles became more noticeable and the SSA of product dropped to 9 m 2/g. When the quantity of nitric acid was increased (nHNO3 = 1.3 mol, nur = 1.6 mol), agglomerates with 500 nm in diameter obtained. Those agglomerates were formed by small particles (≤ 50 nm) (Fig. 5c). By increasing the amount of HNO3, the volume of evolved gasses rose leading to the increase of SSA up to 20 m 2/g. Further increase of the quantity of HNO3 did not strongly influence on the SSA of the product.
3.2. Effect of the quantity of glycine
Fig. 3. The XRD patterns of the synthesized powders obtained from systems containing different amounts of urea, nHNO3 = 0.1 mol.
Using glycine as a fuel, a grayish foamy mass formed as a combustion product. Such color of the product could be caused by the presence of carbonaceous residue remained from incomplete oxidized glycine. The product became darker when the ratio of glycine to the oxidant (NO3−) increased in the initial solution. Fig. 6 shows XRD patterns of products formed from systems containing different amounts of glycine. One can observe that the dominant phase was HA when the amount of glycine was small. Small amount of α-TCP and Ca2P2O7 phases also formed as secondary phases. Further increase of quantity of glycine led to the gradual decrease of amount of HA and increase of amount of Ca2P2O7. At 0.5 mol glycine, Ca2P2O7 became the dominant phase, while HA disappeared totally. It can be seen in Fig. 6 that the increase of the amount of glycine resulted in an apparent decrease of Ca/P ratio according to the crystallize phases detected by XRD. Being amino acid glycine reacted with Ca ions forming calcium glycinate complexes which decomposed during the combustion process releasing gasses and Ca ions [14]. When the ratio of glycine/oxidant was high, glycine was not oxidized completely, resulting to the formation of carbonaceous residue which might contain Ca ions. On the other hand, the higher amount of glycine (consider globally) led to the increase of the quantity of released energy, which could be a reason of preponderance of high temperature phases, such as Ca2P2O7 as a
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
2467
Fig. 6. The XRD patterns of the combustion products obtained from systems containing different amounts of glycine, nHNO3 = 0.1 mol.
0.8 mol urea (Eq. (1)) is higher than for 0.4 mol glycine (Eq. (2)), which can be a reason of withdrawing of energy from the system. COðNH2 Þ2 ðgÞ þ 3=2 O2 ðgÞ ¼ CO2 ðgÞ þ N2 ðgÞ þ 2H2 OðgÞ ΔH200 ¼ −154 kcal
ð1Þ NH2 CH2 COOHðgÞ þ 9=4O2 ðgÞ ¼ 2CO2 ðgÞ þ 1=2 N2 ðgÞ þ 5=2H2OðgÞΔH200 ¼ −307 kcal
Fig. 5. FESEM micrograph of the synthesized powders: (a) nur = 0.5 mol, nHNO3 = 0.1 mol, (b) nur = 1.6 mol, nHNO3 = 0.1 mol and (c) nur = 1.6 mol, nHNO3 = 1.3 mol.
ð2Þ
where ΔH is the enthalpy of combustion reaction. Thus, the formation of α-TCP phase in a low energetic system (when glycine is used) might be caused by its formation mechanism. It should
dominant phase, and the disappearance of HA (low temperature phase). It is notable that α-TCP was obtained when different amount of glycine was used. While using low amounts of urea (nur ≤ 0.8), β-TCP phase formed. It should be considered that 0.8 mol urea provides similar amount of energy to that of 0.4 mol glycine (Eqs. (1), (2)). Moreover the quantity of gasses released from the oxidation of
Table 3 The SSA of the combustion product using urea as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (urea), mol
n(HNO3), mol
SSA, m2/g
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.5 0.8 1.1 1.6 1.6 1.6 1.6
0.1 0.1 0.1 0.1 0.3 0.7 1.3
14 11 6 9 20 19 20
Fig. 7. The XRD patterns of the combustion products obtained from systems containing different amounts of HNO3 ngly = 0.5 mol.
2468
M.A. Aghayan, M.A. Rodríguez / Materials Science and Engineering C 32 (2012) 2464–2468
Fig. 8. FESEM micrograph of the synthesized powders: (a) ngly = 0.17 mol, nHNO3 = 0.1 mol, (b) ngly = 0.5 mol, nHNO3 = 0.1 mol and (c,d) ngly = 0.5 mol, nHNO3 = 1.3 mol.
be noted that according to phase diagram, α-TCP forms at higher temperature, while β-TCP exists when the temperature of the system is less than 1120 °C. The intensity of the peaks corresponding to α-TCP rose when the amount of glycine increased up to 0.2 mol (Fig. 6). Different amount of HNO3 was added in the initial solution containing 0.5 mol glycine (Table 2). The XRD patterns of combustion products are presented in Fig. 7. α-TCP, Ca2P2O7 and HA phases formed adding small amount of HNO3 in the initial solution containing 0.5 mol glycine, (Fig. 7). Increasing the amount of HNO3, the amount of high temperature phases decreased, the peaks of Ca2P2O7 phase disappeared and α-TCP phase decreased continuously, while the peaks of HA increased, caused by decrease of pH of the solution. FESEM observation of the product shows that the rod-like particles, with ~ 50 nm in diameter, were lightly agglomerated (making agglomerates smaller than 3 μm) (Fig. 8a). The SSA of the formed powder was 90 m 2/g (Table 4). Increasing the amount of glycine up to 0.5 mol, the SSA dropped to 21 m 2/g and agglomerates-with smooth surface-formed. It can be seen that particles with different morphologies were obtained between the smooth bubbles (Fig. 8b). Adding 1.3 mol HNO3 in the system which contained 0.5 mol glycine, led to the formation of powder with grid-like (netlike) macrostructure. (Fig. 8c). The nanosize particles were almost spherical and were sintered making necks among them (Fig. 8d). Although the microstructure of the powder was altered, changing the amount of HNO3, the SSA remained almost unchanged (Table 4). 4. Conclusions The influence of the type and amount of fuel on the possibility of formation of biphasic HA/TCP composite by solution combustion Table 4 The SSA of the combustion product using glycine as fuel. n((NH4)2HPO4), mol
n (Ca(NO3)2), mol
n (glycine), mol
n(HNO3), mol
SSA, m2/g
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.15 0.15 0.15 0.15 0.15 0.15 0.15
0.06 0.17 0.2 0.5 0.5 0.5 0.5
0.1 0.1 0.1 0.1 0.3 0.7 1.3
90 76 9 21 15 20 20
synthesis method using urea and glycine as fuels was studied. It was found that the type of fuel and its quantity influenced significantly on the phase composition and microstructure of the synthesized powders. Ca-poor compounds formed as a result of combustion of fuel-rich systems. Mainly α-TCP was produced using glycine as a fuel, while β-TCP phase prevails using urea, and β-TCP was not formed significantly when glycine was used as a fuel. The investigation also showed that the amount of induced HNO3 had a big influence on the combustion synthesized product. Changing the amount of HNO3, it was possible to control Ha, TCP contents in the yielded BCP, keeping constant Ca/P ratio in the initial mixture. By means of the solution combustion method using urea and glycine, the HA/β-TCP ratio can be easily controlled in single step operation, and the in-vivo behavior of BCP will be improved as well. Acknowledgments Supported by the Spanish Science and Technology Agency (CICYT) through the projects MAT 2010‐17753 and MAT 2010-C21088-C03. References [1] R.Z. LeGeros, G. Daculsi, J.P. LeGeros, Orthop. Biol. Med. 2 (2008) 153–181. [2] N. Kivrak, A.C. Taş, J. Am. Ceram. Soc. 81 (1998) 2245–2252. [3] M. Campos, F.A. Müller, A. Bressiani, J.C. Bressiani, P. Greil, J. Mater. Sci. Mater. Med. 18 (2007) 669–675. [4] S.H. Kwon, Y.K. Jun, S.H. Hong, H.E. Kim, J. Eur. Ceram. Soc. 23 (2003) 1039–1045. [5] S. Gomes, G. Renaudin, E. Jallot, J.M. Nedelec, Appl. Mater. Interfaces 1 (2009) 505–513. [6] J. Zhang, J. Guo, S. Li, B. Song, K. Yao, Front. Chem. Chin. 3 (2008) 451–453. [7] J.S. Cho, Y.N. Ko, H.Y. Koo, Y. Kang, J. Mater. Sci. Mater. Med. 21 (2010) 1143–1149. [8] S.K. Ghosh, S.K. Nandi, B. Kundu, S. Datta, D.K. De, S.K. Roy, D. Basu, J. Biomed. Mater. Res. B Appl. Biomater. 86B (2008) 217–227. [9] S.K. Ghosh, A. Prakash, S. Datta, S.K. Roy, D. Basu, Bull. Mater. Sci. 33 (2010) 7–16. [10] S.K. Nandi, B. Kundu, S.K. Ghosh, T.K. Mandal, S. Datta, D.K. De, D. Basu, Ceram. Int. 35 (2009) 1367–1376. [11] S. Sasikumar, R. Vijayaraghavan, Mater. Sci. Technol. 26 (2010) 1114–1118. [12] S. Sasikumar, R. Vijayaraghavan, Sci. Technol. Adv. Mater. 9 (2008) 035003. [13] S. Sasikumar, R. Vijayaraghavan, Ceram. Int. 34 (2008) 1373–1379. [14] D. Gopi, J. Indira, V. Prakash, L. Kavitha, Spectrochim. Acta A Mol. Biomol. Spectrosc. 74 (2009) 282–284. [15] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, J. Mater. Lett. 10 (1990) 6–12. [16] S.K. Ghosh, S.K. Roy, B. Kundu, S. Datta, D. Basu, Mater. Sci. Eng., B 176 (2011) 14–21. [17] Y. Liu, X. Dan, Y. Du, F. Liu, J. Mater. Sci. 39 (2004) 4031–4034.