Materials Chemistry and Physics 94 (2005) 333–341 Characterization, thermal stability and sintering of hydroxyapatite powders prepared by different routes Nasser Y. Mostafa ∗ Chemistry Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt Received 4 October 2004; received in revised form 18 April 2005; accepted 10 May 2005 Abstract Hydroxyapatite (HAP) powder precursors have been used as starting material for biomedical applications, such as synthetic bone graft materials and scaffold for hard tissue engineering. Considering the numerous applications of hydroxyapatite, three different routes for HAP powders preparations was investigated. Two powders were prepared by chemical precipitation reactions at 100 ◦ C and one by mechanochemical reaction. The powders were characterized using chemical analysis, surface area measurements, laser diffraction, X-ray diffraction (XRD) and SEM. The Ca/P ratios were varied from 1.67 to 1.58. The chemical composition, the crystallinity and the agglomeration characters depend on the preparation route. The effect of powder characteristics on the sinterability was investigated. Although, the thermal stability and hence the start of sintering dependents on the Ca/P ratio, the ﬁnal sintering density and hence the mechanical properties depends on the agglomeration characteristics and the particle size distribution. Hydroxyapatite powder prepared by mechanochemical route have nano-sized crystallites with a uniform smaller agglomerated particle size distribution and have a butter sinterability. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydroxyapatite; Mechanochemical synthesis; Agglomeration; Crystallinity; Sintering 1. Introduction of phosphate ions PO4 3− by hydrogen phosphate HPO4 2− allows a continuous variation of the Ca/P atomic ratio Hydroxyapatite [Ca10 (PO4 )6 (OH)2 ] is well known as between 9/6 and 10/6. This leads to calcium-deﬁcient hydrox- the mineral component of bones and teeth. Thus, it has a yapatites, Ca10−x (PO4 )6−x (HPO4 )x (OH)2−x . Calcium-deﬁ- considerable interest in dental and medical research [1–4]. cient hydroxyapatite powders can be precipitated from Hydroxyapatite ceramics and hydroxyapatite–polymer conventional wet chemical methods and decomposed composites are very promising for biomedical applications, into a mixture of HAP and tricalcium phosphate, such as synthetic bone graft [5–7] and scaffold for hard tissue Ca3 (PO4 )2 (TCP) by thermal treatment above 700 ◦ C. engineering [8,9]. Considering the numerous applications This allows a direct processing of biphasic calcium phos- of hydroxyapatite in biomedical ﬁelds, numerous HAP phate ceramics HAP/TCP without the step of powder synthesis techniques have been developed [2,10]. The most blending. commonly used technique for the formation of HAP powder The main interest in the last years is not only control- is the wet methods in aqueous solutions both by simple ling the stoichiometry of synthetic HAP but also controlling preciptation method [11,12] or hydrolysis of acidic calcium the crystal size and shape and the agglomeration character- phosphate salts [13–15]. Synthesis of HAP using wet method istics of the powder. Since these synthetic HAPs are further is very complicated and needs a special attention to control processed to produce ceramics or incorporated with other the Ca/P ratio as well as the crystallinity. The substitution materials (polymers) to produce biocomposites. Therefore, a number of novel processing routes have been developed ∗ Tel.: +20 64 382216; fax: +20 64 322381. for preparing ﬁne hydroxyapatite powders, including sol–gel E-mail address: email@example.com. syntheses [16,17], hydrothermal reactions [18,19], emulsion 0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.05.011 334 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 and microemulsion syntheses [20,21] and mechanochemical freshly boiled distilled water. H3 PO4 was dropped slowly to syntheses [22–25]. the stirred suspension until a Ca/P ratio of 1.70 was reached. Mechanochemical powder synthesis is a solid-state Boiling and stirring was continued for 2 h. The supernatant synthesis method that takes advantage of the perturbation of liquid was removed. Two liters of water was added to the ves- surface-bonded species by pressure to enhance thermody- sel and the suspension was boiled and stirred for 2 h to remove namic and kinetic reactions between solids . The main the Ca(OH)2 . The resulting hydroxyapatite was washed advantages of mechanochemical synthesis of ceramic pow- three times with water and ﬁnally with 0.001 M H3 PO4 . ders are simplicity and low cost, which make it a valuable Calcium-deﬁcient hydroxyapatite (d-HAP) was prepared method for industrial production of HAP powder. Equip- by dropping slowly and simultaneously with stirring aqueous ments used for mechanochemcal synthesis are conventional solutions of (CH3 COO)2 Ca (0.1 M), and K2 HPO4 (0.06 M), milling equipment, such as ball mills and vibratory mills. with a stoichiometric Ca/P molar ratio of 1.67, into 1 l of Since the mechanochemical synthesis involves only boiling water. The precipitate was aged for 2 h at 100 ◦ C after solid-state reaction, Suchanek et al.  distinguished dropping solutions, then left to cool down for 24 h. The ﬁnal mechanochemical from the mechanochemical–hydrothermal pH was ﬁnd to be weak acidic (pH ∼5.0). synthesis (wet mechanochemical). In the latter case, an The third hydroxyapatite was prepared through wet aqueous phase incorporates in the system. The aqueous mechanochemical route (m-HAP) with Ca(OH)2 and phase accelerates kinetic processes that commonly rate limits (NH4 )2 HPO4 as starting materials. Appropriate amounts of a process, such as dissolution, diffusion, adsorption, reaction the two starting materials at a molar ratio of 1.67 were mixed rate, and crystallization (nucleation and growth) . The together in a conventional ball mill with zirconia balls as mechanochemical activation of slurries can generate local the milling medium and 100% distilled water. Millings were zones of high temperatures (up to 450–700 ◦ C) and high carried out for 8 h with the rotating speed of 170 rpm in a pressures due to friction effects and adiabatic heating of high-density polyethylene bottle. After milling, the slurry gas bubbles (if present in the slurry), while the overall was ﬁltered under suction and washed with freshly boiled temperature is close to the room temperature . The distilled water. mechanochemical method has many advantages over the All the preparations of hydroxyapatite were dried under hydrothermal methods, especially, on the industrial scale. N2 , at 105 ◦ C for 24 h then calcined at 600 ◦ C for 2 h. Pellets Mechanochemical method can produce large amounts of specimens were molded into a stainless steel mold by hand HAP powder at lower temperature, i.e. room temperature, as to produce specimens 13 mm in diameter and about 2 mm compared to 90–200 ◦ C for the hydrothermal process. Thus, high, at a pressure of 200 MPa. The green pellets of about for the mechanochemical–hydrothermal process, there is no 50–55% of the theoretical density were sintered in air for 1 h need for a pressure vessel and external heating. at temperatures from 850 to 1200 ◦ C, with 50 ◦ C intervals. The aim of this research is to characterize several HAP Specimens 13 mm in diameter and about 18 mm high were powders prepared by different routes and comparing their molded, for compressive strength measurements. The heating sintering behaviors. This is the primary step in a project to and cooling rates were limited to 300 ◦ C h−1 to prevent crack investigate the utilization of these hydroxyapatites with poly- formation. mers to produce scaffold for tissue engineering with different The green and sintered densities of compacts were mechanical properties and different resorbation rates. Com- obtained from the measurement of dimensions and sample positional, microstructural, morphological and mechanical mass. Relative densities were given as a percentage of the characterizations were carried out on the powder as well as theoretical density of HAP, 3.16 g cm−3 . on dense sintered bodies. X-ray diffraction (XRD) analyses were performed using an automated diffractometer (Scintag Inc., Sunnyvale, CA), at a step size of 0.02◦ , scan rate of 2◦ min−1 , and a scan range 2. Materials and methods from 4◦ to 60◦ 2θ. The average crystallite size of the precip- itates was estimated by using the simple Scherrer equation: Pure CaCO3 powder was heated at 1100 ◦ C for 4 h, and the resultant CaO powder was used as it is or hydrated under kλ D= nitrogen with a third equivalent quantity of distilled water to [β1/2 cos θ] produce Ca(OH)2 . All the other chemicals were of analytical grades. Two hydroxyapatite powders were prepared by ˚ where D is the size in A measured using reﬂection (0 0 2), aqueous precipitation method. The precipitation was carried k the shape factor equal to 0.9, λ the wavelength of X-rays out in a three-necked bottle equipped with a reﬂux condenser equal to 1.5418 A, θ the diffraction angle equal to 12.92 for ˚ protected from the atmosphere by a CO2 -absorbing trap the reﬂection (0 0 2), and β1/2 is deﬁned as the diffraction peak and ports for introducing N2 and the titrant. Stoichiometric width at half height, expressed in radians. The particle size of hydroxyapatite (s-HAP) was prepared by titrating a boiling the powders was analyzed by using a laser diffraction particle suspension of Ca(OH)2 with 0.5 M H3 PO4 . The Ca(OH)2 size analyzer (SHIMADZU), by suspending the powder in suspension was prepared by adding 56 g of CaO to 3 l of water using sonication. N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 335 The speciﬁc surface area measurements were performed with BET method employing a Quantachrome (Monosorb) Surface Area Analyzer. The Ca/P ratio was measured by Inductively Coupled Plasma Spectrometry (ICPMS) (JOBINYBON, mod. JY-38). A universal test machine (Instron) was used for compression tests. The powders and the fracture surfaces of the sintered samples were investigated using SEM (HITACHI S-3500N) equipped with secondary electron detector and EDX. All samples were coated with gold. 3. Results and discussions 3.1. Characterization Fig. 1. XRD patterns of hydroxyapatite powders dried at 100 ◦ C for 24 h. XRD patterns of HAP powders as shown in Fig. 1, revealed no secondary phases besides hydroxyapatite. The Ca/P ratio of the HAP powder is one of the most crucial parame- (57 nm) and d-HAP has the largest one. These results agree ters in determining properties and thermal stabilities . well with the after-mentioned surface area measurements. Chemical analysis results are presented in Table 1, it is d-HAP have the largest crystallites, since it was prepared clear that the Ca/P ratio of s-HAP powder coincide well under acidic conditions. Many authors [31,32] found that the with the theoretical values for stoichiometric hydroxyap- crystallite size increases with increasing the acidity of the atite (1.67). Hydroxyapatite prepared at low pH (d-HAP) is precipitation medium. highly Ca-deﬁcient (Ca/P = 1.58), and wet mechanochemical Fig. 2 shows plots of volume percent of particles sus- route produces hydroxyapatite (m-HAP) with Ca/P ratio of pended in water over a range of particle diameter as deter- 1.65. Table 1 also shows the chemical formula corresponds to mined by laser diffraction particle size analyzer. Samples the after-mentioned chemical analysis, with considering the s-HAP and m-HAP have a clear bimodal particle size distribu- charge compensation of the deﬁcient calcium by substituting tion, while d-HAP shows a unimodal particle size distribution PO4 3− with HPO4 2− and creating a vacancy on OH− site, (single maximum at about 15 m). The absence of particles Ca10−x (PO4 )6−x (HPO4 )x (OH)2−x [2,13,14]. with diameters less than 0.2 m (Fig. 3) suggests there is no The BET surface of HAP powders measured using N2 suspended single crystallites of HAP in all samples. adsorption are given in Table 1. Mechanochemical route To test the effect of ultrasonic energy on the agglomer- produces powder (m-HAP) with the highest surface area ation, the particle sizes were measured for each sample at (86 m2 g−1 ). Hydroxyapatite prepared in acidic media d-HAP different sonication time up to 30 min. This gives no changes have the lowest surface area (19 m2 g−1 ). in the particle size distribution of Fig. 2 for samples sonicated Particle size in agglomerated or aggregated systems like for only 5 min. There also no particle sizes were determined hydroxyapatite powders is one of the most important param- corresponding to the results of the XRD, i.e. agglomerates eters controlling ceramic processing. In this study, particle are not broken by ultrasonic energy to the primary particles size of the powders were investigated using XRD (Scherrer (crystallites). equation) and laser diffraction. Each method of measure- Critical parameters describing all curves are shown in ment senses a particular aspect of particle size. The proﬁle Table 2 and include particle-size range, particle size at broadening by powder X-ray diffraction senses monocrys- maximum volume percent, and total volume percent under talline domains. Laser diffraction method determines only a given peak. The two peaks of s-HAP powder bimodal the agglomerated or aggregated particles in the suspended distribution were in the range of 0.6–0.9 and 0.9–45 m. It solvent. was also calculated from the distribution that around 57% of The mean crystallite sizes estimated by using the Scherrer the powder full in the micrometer size range (0.9–45 m). equation for the (0 0 2) planes are shown in Table 1. The For m-HAP, around 86% of the powder consisted of particles m-HAP has the smallest crystallites in the nanoscale size within 0.2–1.0 m and the other 14% of the mass was Table 1 Characteristics of hydroxyapatite powders Sample Ca/P ratio Composition Crystallite size (nm) Surface area (m2 g−1 ) s-HAP 1.67 Ca10 (PO4 )6 (OH)2 141.3 48 m-HAP 1.65 Ca9.9 (HPO4 )0.1 (PO4 )5.9 (OH)1.9 57.4 86 d-HAP 1.58 Ca9.48 (HPO4 )0.48 (PO4 )5.48 (OH)1.48 211.9 19.5 336 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 was 0.20 m (Fig. 3(a)). m-HAP powder agglomerates are small in size than s-HAP agglomerates. It is obvious that the agglomerate particles in case of m-HAP have lower particle size than those of s-HAP. The typical volume of primary m-HAP agglomerates, at 0.1–1.0 m, which agrees with the laser diffraction results (Fig. 2). Fig. 3(c) shows that d-HAP, which formed in acidic medium had a rod-like shape with mean particle lengths of 2 m and diameters of 0.25 m. This ﬁgure shows clearly the agglomeration of the primary crystallites with interparti- cle bonds glow between them. In general, the formation of agglomerated HAP particles can be explained by nucleation–aggregation–agglomeration– growth mechanism proposed by Randolph and Larson  and Rodriguez et al. . According to this mechanism, particle formation goes in the following steps: (a) nucleation and growth to form crystallites in nanometric size range; (b) aggregation of elemental nanocrystals by physical attrac- tions; (c) further crystal growth, at a constant residual super- saturation, acting as cementing agent inside the aggregates to form stable agglomerate. These agglomerates continue to grow as distinct particles and appear as submicrometer particles in case of m-HAP and s-HAP. Finally, the increase in particle size takes place by aggregation of these sub- micrometer size agglomerated particles to form secondary agglomerate in the micrometric size range (second bimodal peak). Gomez-Morales et al.  found that the aggregation processes directed by different forces, in the nanomet- ric/colloidal scale, surface free energy minimization would be responsible for the aggregation, while electrical surface charges would direct the micrometric scale particles aggre- Fig. 2. Particle size distribution by laser diffraction. gation. 3.2. Sintering and thermal stability particles of 1.0–25.0 m (Table 2). The agglomerate of m- HAP is homogenous and most of it full in the submicrometer Understanding the sintering behavior of hydroxyapatite range. This was conformed by SEM. powders is important, because this allows to design ceramics The presence of two particle size ranges in case of m-HAP with controlled grain growth, microstructure and mechanical and s-HAP powders possibly reﬂects the aggregation in the properties. In the present study, the effect of powder charac- submicrometer sizes range, yielding the so-called primary teristics on densiﬁcation, microstructural development and agglomerates (the ﬁrst peak). These agglomerates continue mechanical properties of HAP compacts were studied. to aggregate by forming interparticle bonds until they reach Fig. 4 shows the effect of sintering temperature on the the micrometer size range, the second peak (>1 m). densiﬁcation (percent of sintered densities with respect to Morphological results from SEM investigation are shown theoretical density of HAP; 3.16 g cm−3 ) after cooling to in Fig. 3(a–c). s-HAP and m-HAP powder consists of hard room temperature. The green density for s-HAP and m- agglomerates which are composed of ﬁne crystallites. In case HAP are comparable and smaller than that of d-HAP. This of s-HAP, the dominant observed agglomerated particle size is due to the nature of agglomerated particles of m-HAP Table 2 Mean parameters of particle size for hydroxyapatite powders Sample First peak Second peak Range ( m) Peak ( m) Volume (%) Range ( m) Peak ( m) Volume (%) s-HAP 0.6–0.9 0.6 43 0.9–45 2.0 57 m-HAP 0.2–1.0 0.3 86 1.0–25 2.0 14 d-HAP 7–100 15 100 – – – N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 337 Fig. 4. Densiﬁcation of hydroxyapatite compacts sintered at different tem- peratures. 900 and 1000 ◦ C, compared to between 950 and 1000 ◦ C for m-HAP and between 1000 and 1050 ◦ C for stoichiometric s-HAP. This is clearly shown in SEM micrographs of the fracture surfaces of powder compacts sintered at 1000 ◦ C (see Fig. 5). Compared with the as synthesized powder (Fig. 3) evidence exists for a particle coalescence which associated with the surface reduction . The particle coarsening occurred more signiﬁcantly with samples with low Ca/P ratio. The necking among the particles was apparent in Ca-deﬁcient samples (d- HAP and m-HAP) due to localized sintering at 1000 ◦ C. At this sintering temperature the particle size decreases as the Ca/P ratio increases. It is remarkable to notice that at 1000 ◦ C, sample s- HAP shows a collapse of the large agglomerate (compare Figs. 3(a) and 5(a)). In another investigation , the authors ﬁnd, the calcination reduces the particle size and narrows the size distribution. The second stage of sintering corresponds to densiﬁca- tion and the removal of most of the specimen porosity. The onset of densiﬁcation as indicated by a sharp increase in the sintered density, for d-HAP and m-HAP were started nearly at 1000 ◦ C, whereas stoichiometric hydroxyapatite (s-HAP) required higher temperature 1050 ◦ C. For d-HAP, the maximum rate of densiﬁcation was between 1050 and 1150 ◦ C. Comparing this behavior with that of stoichiometric hydroxyapatite, calcium-deﬁcient HAP sinters at much lower temperature, and the temperature of maximum densiﬁcation decreases as the calcium deﬁciency increases. m-HAP achieves the highest ﬁnal sintered density of Fig. 3. SEM micrographs of hydroxyapatite powders: (a) s-HAP; (b) m- 93.5% of the theoretical density at 1200 ◦ C, whereas d-HAP HAP; (c) d-HAP. and s-HAP reach only 89.4 and 76.8% respectively, at the same sintering temperature. In conclusion, the start of densiﬁ- and s-HAP, which are composed of nano-sized crystallites. cation depends on the chemical composition of the precursor A small increase in density is observed before the onset of powder, i.e. Ca/P ratio. The ﬁnal density of the sintered bod- densiﬁcation and this corresponds to the ﬁrst stage of sinter- ies mainly depends on the homogeneity and the particle size ing, where necks are forming between powder particles. For distribution of the precursor powder. This conclusion is sup- highly calcium-deﬁcient d-HAP, this process occurs between u ported by G¨ ltekin and Oktar ; they found that the ﬁnal 338 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 Fig. 5. SEM of hydroxyapatite sintered at 1000 ◦ C: (a) s-HAP; (b) m-HAP; Fig. 6. SEM of hydroxyapatite sintered at 1200 ◦ C: (a) s-HAP; (b) m-HAP; (c) d-HAP. (c) d-HAP. increase of sintering temperature improves the homogene- limiting densiﬁcation level depend predominantly on the sur- ity and favors grain growth, resulting in an increase in mean face area of the HAP powder and to a lesser extent on Ca/P grain size. The increase in grain size is also depending on the ratio . homogeneity of the precursor powder. SEM micrographs of the fracture surfaces of powder com- Sample m-HAP shows the isolated closed pores in the pacts sintered at 1200 ◦ C are given in Fig. 6. In general, the matrix of uniform closed packed HAP grains. Samples N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 339 Table 3 Chemical composition and physicomechanical properties of sintered ceramics Sample Ca/P molar ratio TCP (wt.%) HAP (wt.%) Porosity (%) Compressive strength (MPa) s-HAP 1.67 0.0 100 23.2 58 m-HAP 1.65 9.3 90.7 6.5 108 d-HAP 1.58 50.1 49.9 10.6 87 s-HAP and d-HAP show larger pores in the range from 0.3 to 1 m size. These pores seem to be connected to form a continuous network separated by thick walls. This may be an advantage for the circulation of the physiological ﬂuid when it is used for biomedical purposes but it is not useful for the intergrowth of cells . Comparing the ﬁnal density of sample m-HAP and s-HAP, which have the same green density, leads to the conclusion improved packing homogeneity leads to improved sinterabil- ity in the form of higher sintered density at a given sintering temperature and the ability to achieve high density with ﬁne particle size. 3.3. Compressive strength Table 3 shows porosity and compressive strength of HAP sintered compacts at 1200 ◦ C. It can be seen that the decrease in porosity is accompanied by an increase in the mechanical strength. Sample m-HAP, with improved packing homogene- ity attains the highest compressive strength (1080 MPa) and Fig. 7. XRD patterns of hydroxyapatite powders compacts sintered at the lowest porosity (6.5%). Stoichiometric HAP, s-HAP have 1100 ◦ C. the lowest compressive strength (578 MPa) and the highest porosity (23.2%). where x is the calcium deﬁciency, and thus, Ca/P = (10 − x)/6. The mole fraction of -TCP to the mole fraction of HAP 3.4. Thermal stability in the sintered bodies is given by: XTCP /XHAP = 3x/(1 − x). Since, XHAP = 1 − XTCP, and XTCP /(1 − XTCP ) = 3x/(1 − x), XRD patterns of sintered materials at 1100 ◦ C, as shown the percent of -TCP can be calculated by knowing in Fig. 7, highlighted the difference in the thermal stability calcium deﬁciency (x) from the chemical analysis. of HAP powders. HAP synthesized by mechanochemical Table 2 shows the Ca/P ratio and weight percent of - method and from Ca(CH3 COO)2 precursor (in acidic TCP and HAP in the sintered compacts. Results agree media) transformed partially into -tricalcium phosphate, - well with the semiquantitative analysis using XRD in Ca3 (PO4 )2 , which related to the Ca-deﬁciency in the lattice Fig. 7. of HAP powders. This agreed with the research of Yoshimura et al. , in which he indicated that Ca-deﬁcient HAP tends to transform to -Ca3 (PO4 )2 on heating at 900 ◦ C depending 4. Conclusion on the deﬁciency of calcium. Stoichiometric hydroxyapatite (s-HAP) with Ca/P ratio of 1.67 is stable up to 1200 ◦ C. The present study compares the powder properties of Raynaud et al.  proved the reliability of the Ca/P hydroxyapatite prepared by wet mechanochemical method atomic ratio of the initial HAP powder calculated by with those prepared at 100 ◦ C using wet solution precipita- determining the phase proportions in calcined biphasic tion methods. The wet mechanochemical method produces calcium phosphates. Thus, the percent of -TCP and HAP in nanocrystallites of hydroxyapatite with homogenous ﬁne the sintered bodies can be determined from the Ca/P ratio of particle size distribution with most of the agglomerated the precursor HAP powder. Considering calcium-deﬁcient particles full on the submicrometer size range. This alters the HAP change with thermal treatment to stoichiometric HAP sintering characteristics of hydroxyapatite powder compacts and -TCP according to the following equation: by attaining high sintering density at lower temperature in comparison with those prepared by wet solution precipita- Ca10−x (HPO4 )x (PO4 )6−x (OH)2−x tion method at 100 ◦ C. Hence, this preparation route could → (1 − x)Ca10 (PO4 )6 (OH)2 + 3x -Ca3 (PO4 )2 + xH2 O emerge as a simple synthetic route to produce HAP ceramics 340 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 with an enhanced characteristics, especially, for industrial References production.  L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705. The sintering behavior of HAP powders with different  J.C. 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