Literature review on synthesis of HAps
HAp structure
HAp, Ca10 (PO4)6(OH)2, is one of the most important member of the apatite family, the name was given by Werner in 1788 to crystals of general formula Ca10 (PO4)6X2, where X is usually hydroxide (OH) or a halogen such as fluorine (F) and chlorine (Cl). The substitution of fluorine or chlorine gives fluorapatite, Ca5 (PO4)3F and chloroapatite, Ca5 (PO4)3Cl. [Klein and Hurlbut, 1993]
The crystallographic structure of biological apatite and Haps are closely similar and was first reported by DeJong in1926. However, biological apatite such bone and tooth are non stoichiometric [LeGeros, 1994], and differs from pure synthetic stoichiometric HAp in both crystal size and composition.
HAp structure has the hexagonal space group P63/m, with lattice parameters value a = 9.421, and c = 6.8814 Å [32], while synthetic stoichiometric HAp exhibits a structure with monoclinic P21/b. The unit cell of HAp crystal consists of calcium surrounded by PO4 and OH groups closely packet together. The theoretical density and composition of HAp are respectively 3.156 g/cm3 and (39.68 wt % Ca; 18.45 wt % P). The Ca/P molar ratio of synthetic stoichiometric HAp is 1.67 and the variation in Ca/P gives different type of calcium phosphates such as β-tricalcium phosphate TCP (Ca3 (PO4)2) and tetracalcium phosphate TTCP (Ca4(PO4)2O) with Ca/P molar ratio of 1.5 and 2 respectively. Don't use plagiarised sources.Get your custom essay just from $11/page
Various observation in the X-ray diffraction and IR spectrum confirm that if the Ca/P ratio is 1.67 only HAp phase will be present, while if Ca/P ratio is higher than 1.67, a mixture of HAp and CaO will be present [28]. The mixture of HAP, β-tricalcium phosphate TCP and other phase such as tetracalcium phosphate TTCP will be obtained if Ca/P ratio is lower than 1.67.
Synthesis Routes
Synthetic HAp has several practical advantages over other synthetic materials such as polymers and metals. The low cost, easy to synthesize in large quantity, and no negative effects makes HAp very attractive to be used in comparison with metals and polymers.
The purity, structure, and particle with different shape and size of the Haps are prepared using these techniques as early mentioned.
The size of the synthetic HAp nanoparticles can be modulated by varying the synthetic route and conditions of synthesis. There are several synthetic methods adopted to generate nHAP as reported in the literature.
For example, using sol-gel technique, Salimi et al.[139, review of lit.] reported that increasing in process temperature affects the particle size of HAp.
Hydroxyapatite nanofiber obtained by chemical precipitation technique was reported by Gao et al., [142]. In this experiment, surfactants sodium dodecyl sulphonate and cytel trimethyl ammonium bromide (CTAB) were used. They found that the length/diameter ratio of HAps varies between 10 to 100 nm by varying the concentration of these surfactants.
HAp with different morphologies was synthesized by wet precipitation method where both solvent and drying techniques affect strongly the particle size. Wang et al., [152]
Heat treatment and using the surfactant CTAB can also affect both surface area and morphology of HAp Khalid et al.,[153].
HAp particles of different shape and size are synthesized using some methods as summarised in Table 1.
Precipitation techniques
Wet methods such as chemical precipitation involve use of aqueous solutions to facilitate chemical reactions between calcium and phosphorous ions. The most widely used calcium sources are CaCl2, CaSO4, CaCO3, Ca(OH)2 and Ca(NO3)2・ 2H2O, while Phosphorus sources include H3PO4, NH4H2PO4, (NH)4HPO4, and K3PO4.
In the precipitation route, phosphorus source such as H3PO4 is added drop-wise to a stirred calcium solution such as Ca(OH)2 or CaCO3 in basic conditions and ammonia was used to regulate the pH of the solutions.The technique used here offers high purity and stoichiometric hydroxyapatite (HA) powder with common reaction temperature between 25°C and 80°.
The two widely reactions used are:
(1) calcium nitrate and diammonium hydrogen phosphate (Engin and Tas, 2000; Zang eta/., 2001) and (2) calcium hydroxide with phosphoric acid (Kweh eta/., 1999; Lazic eta/., 2001; Saeri eta/., 2003). The two possible reactions below refer to chemical precipitation route
The disadvantage of using the first (1) method is that the purity of the precipitated HA
depended on the purity of calcium nitrate used. The second method (2) using calcium hydroxide is preferred than the first method because in this reaction the only by-product is water (Kweh et a/.,1999).
Sol-gel method
The sol gel method has the advantage of using temperatures very close to ambient and allowing a mixture of calcium and phosphorus precursors at the molecular level generating an increase in the homogeneity of HAp [14]. In addition, it allows better control of the formation of certain phases and guarantees the purity of HAp[15].
On the other hand, the sol gel process presents a risk of hydrolysis of the phosphate precursors used [16] and requires a much greater quantity of precursors than in the other methods. The precursors used vary between conventional precursors such as Ca(NO3)2 and (NH4)2HPO4 or H3PO4 and less conventional precursors such as (CH3O)3P or KH2PO4.By carefully choosing the right combination of precursors and the right synthesis conditions (pH, solvent, temperature), the kinetics of the reaction can be controlled. This method differs from the others techniques because it is not part of the wet synthesis methods but by the dry method. The principle is based on calcinations, generally between 900°C and 1100 °C of solid reagents in stoichiometric quantities [2]. The precursors used can be a mixture of calcium carbonates and TCP or of TCP and Ca (OH)2. To obtain a pure HAp, this method is not necessarily ideal because it requires very high temperature.