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Acta Materialia 52 (2004) 5655–5663 www.actamat-journals.com Inﬂuence of temperature and concentration on the sintering behavior and mechanical properties of hydroxyapatite a,** a,* Chandrasekhar Kothapalli , M. Wei , A. Vasiliev a, M.T. Shaw b,c a Department of Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, U-3136, Storrs, CT 06269, USA b Department of Chemical Engineering, University of Connecticut, 191 Auditorium Road, Storrs, CT 06269, USA c Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA Received 19 February 2004; received in revised form 16 July 2004; accepted 23 August 2004 Available online 28 September 2004 Abstract Human bone mineral contains calcium-deﬁcient crystalline hydroxyapatite (HA) embedded in collagen ﬁbers. Research over the past two decades has focused on preparing synthetic HA, which closely resembles bone apatite and exhibits excellent osteoconduc- tivity. This paper describes the synthesis of nano-HA particles via a wet precipitation method. The concentration of the reactants (0.5, 1.0 and 2.0 g/dL) and the temperature of the reaction (25, 70 and 100 °C) were varied. FESEM images were used to determine the size and shape of the resulting nano-particles. The length and breadth of the HA particles were found to increase with the tem- perature, while the aspect ratio increased with both the concentration and the temperature. The average length of the particles was in the range 53–165 nm and the average breadth in the range 29–52 nm. Agglomerates of HA precipitates were formed during the syn- thesis process. HA precipitated at 25 °C and concentration 0.5 g/dL resulted in large agglomerates with a speciﬁc surface area of 79.8 m2/g. HA agglomerates synthesized at each condition were pressed into discs and sintered at 1200 °C. It was found that there was a positive correlation (p = 0.015) between sintered density and biaxial ﬂexural strength. A maximum strength of 57.4 MPa was observed for the specimens 2.0–70 which also attained the highest density, 92%. XRD results indicated that most of the sintered discs had slightly decomposed. The decomposition of the specimens and their resulting microstructures also contributed to the mechanical strength drop of the specimens. Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Hydroxyapatite; Reactant concentration; Reaction temperature; Density and biaxial ﬂexural strength 1. Introduction tivity. Many methods of synthesizing HA have been reported, such as solid-state reaction, sol–gel, wet syn- Over the past two decades, numerous eﬀorts have thesis and hydrothermal methods [1–5]. The sol–gel been made to prepare hydroxyapatite (HA) for bone tis- method involves molecular mixing of the calcium and sue applications due to its excellent biocompatibility and phosphorous resulting in chemical homogeneity, but bioactivity. Of all the calcium phosphates, HA resem- has drawbacks such as the possible hydrolysis of the bles most closely bone apatite and exhibits osteoconduc- phosphates and the high cost of raw materials . Also, the HA prepared by this method resulted in relatively * inferior crystallinity and thermal stability. Although Corresponding authors. Tel.: +1 860 486 9253; fax: +1 860 486 HA ceramics with high aspect ratios were prepared by 4745 ** Tel.: +1 860 486 3543; fax: +1 860 486 4745. hydrothermal and solid-state reactions [1–5], the E-mail addresses: email@example.com (C. Kothapalli), procedures employed were relatively complicated and firstname.lastname@example.org (M. Wei). the temperatures involved were comparatively high. In 1359-6454/$30.00 Ó 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.027 5656 C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 addition, it has been proven diﬃcult to obtain controlled ing few drops of strong ammonia (Sigma, ACS rea- morphology with these two methods. gent). The nitrate solution was either kept at room A variety of methods have been explored in attempts temperature or heated to 70 or 100 °C while stirring to prepare nano-sized HA particles with relatively high vigorously. The temperature during the reaction was aspect ratios [7–9]. Such HA nano-particles have pri- controlled using a hot plate so that there is a maximum mary importance for the biomedical ﬁeld because they ﬂuctuation of ±2 °C. The phosphate solution was can be blended into biodegradable polymers to make added into each of the calcium solution in a dropwise composites with superior mechanical properties. In manner. HA precipitates were formed after mixing addition, it was reported that nano-sized HA exhibited the solutions, which was continuously stirred and main- better bioactivity than coarser crystals . tained at the reaction temperature for 3 h. Finally, the Many investigations [5,11,12] studied a wide range of precipitates were aged for 7 days without heating and experimental conditions to prepare rod-like HA parti- stirring at room temperature. They were then washed cles. However, ﬁbrous HA with relatively low crystallin- with distilled water until the traces of ammonia were re- ity and poor purity were formed. The key factors for moved, dried at 80 °C, and ground to powders using a synthesizing HA particles with desired properties such mortar and pestle. Table 1 describes the notation used as stability, controllable aspect ratios and bioactivity for the HA samples prepared at various concentrations are: reaction temperature, pH and concentration of the and temperatures. reactants. The properties of HA, particularly those men- tioned above, aﬀects the eﬃciency of the powder in its 2.2. Characterization of HA particles ultimate applications [13–16]. In this study, a wet precip- itation method was employed to prepare nano-sized HA Hydroxyapatite particles thus synthesized were exam- particles. This method was advantageous because it re- ined using XRD (BRUKER AXS D5005) with a copper sulted in homogenous pure HA precipitates with good target. The voltage and current used were 40 kV and crystallinity. 40 mA, respectively. A step size of 0.02° and a scan The eﬀects of temperature and concentration of the speed of 1.0° minÀ1 were used. The analysis was con- reacting solutions were examined, along with their ducted for both the as-prepared and calcined specimens inﬂuence on HA properties such as morphology, sinter- at 1200 °C. The morphological features of the HA pre- ing behavior and mechanical strength. HA was synthe- cipitates prepared at diﬀerent conditions were studied sized by setting the initial concentration of the reactant using JEOL JSM-6340 (15 kV) type high-resolution solutions at 0.5, 1.0 and 2.0 g/dL, while the tempera- FESEM. The speciﬁc surface area of the HA powder tures of the reaction were maintained at 25, 70 and was calculated using the BET method from a N2 adsorp- 100 °C at each concentration. Hence nine independent tion isotherm obtained from a Micromeritics ASAP batches of HA particles were synthesized. The samples 2010 (Micrometrics) surface area analyzer operating be- were characterized using X-ray diﬀraction (XRD), tween 10 and 127 kPa. Brunauer–Emmett–Teller (BET) speciﬁc surface area, scanning electron microscope (SEM) and ﬁeld emission 2.3. Mechanical testing and characterization of sintered scanning electron microscope (FESEM). In the present HA paper, we choose to sinter the pellets at 1200 °C as our rationale was to have consistent sintering temperature Hydroxyapatite powders prepared at diﬀerent condi- with other researchers in the ﬁeld to compare the tions were pressed into half-inch diameter discs at a mechanical strength results we obtained. This paper is pressure of 150 MPa for 1 min and sintered in air at the ﬁrst known report which systematically studied 1200 °C for 1 h with a heating rate of 5 °C/min. Nine the eﬀect of reactant concentration on HA particle morphology, sintering behavior and mechanical properties. Table 1 Preparation conditions of the samples studied in this work Sample ID Concentration (g/dL) Temperature (°C) 2. Materials and methods 0.5–25 0.5 25 0.5–70 70 2.1. Preparation of HA 0.5–100 100 1.0–25 1.0 25 Fifty milliliters of calcium nitrate (99% pure, Sigma) 1.0–70 70 and 82 mL of ammonium hydrogen orthophosphate 1.0–100 100 (99% pure, Sigma) solutions in distilled water were pre- 2.0–25 2.0 25 pared at concentrations of 0.5, 1.0 and 2.0 g/dL, and 2.0–70 70 2.0–100 100 the pH of each solution was brought up to 12 by add- C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 5657 discs in each group were prepared, and the average bulk on the losses incurred during washing of precipitates. density of each specimen before and after sintering was HA particles prepared at diﬀerent temperatures and calculated by measuring its weight and size. XRD was concentrations were observed using FESEM as shown used to examine the decomposition of the sintered in Fig. 2, which provided the basis for subsequent par- specimens. The biaxial ﬂexural strength of sintered spec- ticle size distribution analysis. The particle size distri- imens was measured using Instron testing machine bution studies were not done before grinding the HA according to ASTM C 1499-03 . The surface mor- powders using mortar and pestle. Five diﬀerent FES- phology of the sintered specimens was studied by scan- EM images of each sample were considered for meas- ning electron microscopy (SEM). The sintered pellets uring the length, breadth and aspect ratio of the HA were polished using various grades of silicon carbide pa- particles; though we had included only one representa- pers (grade 400–1200) and 1.0 lm diamond paste was tive image of each sample in this paper. Results indi- used for the ﬁnal polishing. The pellets were then coated cated that the length and breadth of the particles of with a gold ﬁlm and their surface morphologies were ob- all samples follow normal distribution. It was observed served using an environmental scanning electron micro- that the morphology of HA crystals was strongly scope (ESEM). dependant on the concentration of the reactants and the temperature of the reaction. Table 2 summarizes the particle size distribution of HA particles synthe- 3. Results sized at various conditions. At a constant reactant concentration, as the reaction temperature increased, 3.1. Before calcination the particle size and aspect ratio also increased (p = 0.006); at a constant reaction temperature, with 3.1.1. XRD patterns of as-prepared HA precipitates the increase of the reactant concentration, the aspect Fig. 1 shows the typical XRD pattern of the HA pow- ratio of the precipitates increased (p = 0.033) with a der synthesized at diﬀerent concentrations and tempera- predicted maximum of 3.6 at reaction conditions of tures (Table 1). All peaks perfectly matched with the 2.0 g/dL and 100 °C. JCPDS pattern 9-432 for HA, which suggested that pure HA was obtained for all the samples prepared at varied 3.2. Formation of HA agglomerates conditions. The lattice parameters of the HA were found ˚ ˚ ˚ to be a = b = 9.426 A, c = 6.887 A and c = 119.97 A. It 3.2.1. Agglomerate size and distribution was observed that as the reaction temperature increased Most of the methods used for synthesizing HA at a constant concentration, the diﬀraction peaks be- produced heterogeneous agglomerates either during came more intense, indicating an increase in the crystal- the chemical reaction or subsequent drying process. linity of the HA powders (p = 0.0055). The average agglomerate size (by weight) obtained from the light scattering technique is shown in 3.1.2. Particle size distribution based on FESEM images Fig. 3. There was not enough evidence to draw con- At each concentration and temperature, the typical clusions regarding the eﬀect of concentration on yield of HA powder was around 5.5–5.8 g depending agglomerate size. However, with the increase of the reactant temperature, the HA agglomerate size de- creased (p = 0.046). 3.2.2. Speciﬁc surface area (211) Fig. 4 shows the speciﬁc surface area of as-precipi- tated HA synthesized at various conditions. Increasing (112) the reaction temperature resulted in large HA crystals Arbitrary units with a reduction in their speciﬁc surface areas. It was (300) (002) found from Fig. 4 that the smallest HA particles syn- thesized at the concentration of 0.5 g/dL and the tem- perature of 25 °C had the surface area of 79.8 m2/g, (213) (402) (410) (202) (222) while the particles obtained at higher reaction temper- (310) (210) (004) (102) (321) atures such as 2.0–100, had lower surface areas. The (312) (111) (212) (201) (301) (203) (322) (311) (200) (501) speciﬁc surface area values obtained were in the range (331) reported by few other research groups [25–28] who also 10 20 30 40 50 60 70 employed the wet method to synthesize HA. The slight diﬀerence between the observed and reported values 2θ might be due to the diﬀerent synthesizing conditions Fig. 1. XRD powder patterns for the samples of HA as-prepared. used. 5658 C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 Fig. 2. FESEM images of HA particles synthesized at various conditions. 3.3. Characterization of sintered HA discs for the samples 0.5–100 and 1.0–100, all the remaining samples were partially decomposed: a low peak of 3.3.1. XRD analyses monetite (CaHPO4) was observed at 31° 2h in the Fig. 5(a)–(c) shows the XRD patterns of HA discs specimens (0.5–25, 1.0–25 and 2.0–25) as shown in sintered at 1200 °C for 2 h. It was found that except Fig. 5(a), while CaO was detected in the specimens Table 2 Statistics of particle size and distribution of HA synthesized at diﬀerent conditions Sample ID Number Average Average Average 2.1 Tm 0567 672.985 l 3.843 of particles length (nm) breadth (nm) aspect ratio Relative intensity 0.5–25 57 53 29 1.8 0.5–70 110 76 32 2.3 0.5–100 49 157 58 2.7 1.0–25 40 64 29 2.2 1.0–70 76 104 37 2.8 1.0–100 18 141 52 2.7 2.0–25 10 56 29 1.9 2.0–70 23 132 40 3.3 2.0–100 12 165 42 3.9 20 30 40 50 60 2.0 1.8 0.5 g/dL Log (agglomerate size, µm) 1.6 1.4 1.2 1.0 2.0 g/dL 1.0 g/dL 0.8 0.6 0.4 0.2 20 40 60 80 100 120 Reaction temperature,˚C Fig. 3. Average volume-weighted agglomerate size of particles syn- thesized at various conditions. 100 Specific surface area, m2 /g 80 60 40 0.5 g/dL 20 2.0 g/dL 1.0 g/dL 0 20 40 60 80 100 120 Reaction temperature,˚C Fig. 4. Speciﬁc surface area results of as-precipitated HA powder synthesized at various conditions. Ca10 ðPO4 Þ6 ðOHÞ2 !2Ca3 ðPO4 Þ2 þCa3 P2 O8 þCaOþH2 O ð1Þ prepared at 70 °C (0.5–70, 1.0–70 and 2.0–70) as shown in Fig. 5(b). In comparison, a large proportion of sample 2.0–100 was decomposed into a-tricalcium phosphate 3.3.2. Bulk density (Ca3(PO4)2), b-tricalcium phosphate (Ca3P2O8) and The bulk density of HA discs before and after sinter- calcium oxide according to the following equation : ing are plotted in Fig. 6. It was found that the density of 5660 C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 as the temperature increased, the ﬂexural strength in- 4.0 creased proportionally (p = 0.004). The observed maxi- 3.5 mum ﬂexural strength of 57.4 MPa was close to the values reported by Jarcho et al.  and Rodriguez Conc, g/dL 3.0 sintered 0.5 et al. . ρ, g/cm3 1.0 2.5 3.4. Surface morphology 2.0 2.0 Fig. 9 shows the representative SEM images of HA 1.0 ceramics sintered at 1200 °C for 1 h. Nearly pore-free pre-sintered 2.0 1.5 surfaces were observed for the sample 2.0–70, which 0.5 incidentally demonstrated maximum ﬂexural strength. 1.0 Especially, the sample 2.0–100, which decomposed into 20 40 60 80 100 120 tricalcium phosphates, after sintering, showed a highly Reaction temperature, ˚C porous surface with pores distributed all over the sur- Fig. 6. Bulk densities of the samples before and after calcination as a face. Open round pores were also observed for the sam- function of synthesis temperature. ples synthesized at 25 °C and concentrations of 0.5, 1.0 and 2.0 g/dL (the images of which are not shown here). Fig. 10 shows the variation of agglomerate size of HA the pre-sintered HA discs increased with synthesis tem- precipitates and their speciﬁc surface area with the par- perature (p = 0.01) in spite of the concentration of the ticle surface area. The surface area of the particles was reactants. The highest density observed was for the discs calculated using the relation: pressed from the HA powders precipitated at 100 °C, and concentrations of 0.5, 1.0 and 2.0 g/dL. After sinter- 4Ã ½1 þ 0:5=ðL=DÞ=qÃ D ðm2 =gÞ; ing at 1200 °C, the density of the discs pressed from where D is the average breadth of the particles, L is the samples 0.5–25, 1.0–25 and 2.0–25 reached 80–85% of average length of the particles and q is the theoretical the theoretical density, i.e., 2.91 g/cm3. The samples density of HA (3.16 g/cm3). It was observed that the 2.0–70 exhibited the highest sintered density of 92% agglomerate size of the HA precipitates increased with while the samples 2.0–100 showed the lowest density the surface area of the particles (p = 0.001). The HA pre- among all the samples tested. cipitates having small particle sizes and thereby high agglomerate sizes possessed high speciﬁc surface areas. 3.3.3. Biaxial ﬂexural strength It was observed that a strong correlation exists be- Fig. 7 shows the biaxial ﬂexural strength of the HA tween the observed ﬂexural strength of the pellets and discs sintered at 1200 °C. Except for the sample 2.0– the crack dimension observed from the SEM images of 100 which was severely decomposed, it was found that the surface-polished pellets. Random cracks were se- temperature had more impact on the strength of discs lected on the surface polished pellets and the crack size than the concentration. At a constant concentration, was calculated by measuring the length (l) and width (w) of the crack. The ﬂexural strength was correlated 7.8 HA discs, sintered p = 0.015 Log (flex strength, Pa) 7.6 7.4 7.2 7.0 0.38 0.40 0.42 0.44 0.46 0.48 Log (ρ, g/cm3) Fig. 8. Correlation between density and ﬂexural strength of sintered Fig. 7. Flexural strength of samples after calcination. pellets. C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 5661 Fig. 9. SEM micrographs of the top surface morphology of samples sintered at 1200 °C for 1 h. 100 80 4. Discussion Specific surface area, m /g 70 Apparently, the preparation conditions had signiﬁ- 2 80 60 Agglomerate size, µm cant impact on the resulting HA precipitates. The exper- 60 50 imental design was a three-level full factorial, with no 40 repetitions of the HA batch preparations. The observa- 40 tions were handled in terms of linear models that used 30 only ﬁrst-order (trend) terms signiﬁcant at the 95% con- 20 ﬁdence level and a curvature term with temperature thus 20 10 leaving ﬁve degrees of freedom for error. In this fashion, 0 0 we reported with good certainty on trends with concen- 20 25 30 35 40 45 50 55 60 tration and temperature, and the curvature with temper- Particle surface area, m2/g ature without repetitions. The HA particles were Fig. 10. Variation of agglomerate size and speciﬁc surface area with precipitated by a wet process and aged for 7 days. It particle surface area of HA precipitates. was found that aging was a crucial process during HA synthesis which allowed the precipitates ripening . With the increase of the reaction temperature, the size to the average crack size for each sample according to of the precipitates increased. At a high reaction temper- the relation r = r0/(1 + 2(l/w)), where r0 is a constant. ature, the rate of particle ripening was high and the pre- Fig. 11 shows the positive correlation (p = 0.04) between cipitates grew large during the allotted time. As a result, the biaxial ﬂexural strength and the crack size. It was large precipitates were observed for the HA particles observed that the biaxial ﬂexural strength rose exponen- precipitated at high temperatures, i.e., 70 and 100 °C tially with the decrease in the crack size. (as shown in Table 2). It was observed that when the reactant concentration was high, the precipitates 70 selectively grew quicker along the c-axis compared to p = 0.04 the a-axis resulting in high-aspect-ratio HA crystals Biaxial flexural strength, MPa 60 (p = 0.033). Reactant concentration also had an eﬀect 50 on the resulting morphology of the precipitates. Increas- ing the reactant concentration while maintaining the 40 reacting temperature constant, resulted in HA precipi- tates with increased agglomerate size and aspect ratio. 30 Fig. 3 shows the average agglomerate size (by weight) 20 obtained at various concentrations and temperatures studied. It was observed that larger agglomerates were 10 formed for relatively smaller particles, such as those syn- 0 thesized at 25 °C (refer to Fig. 10). Rodriguez et al.  0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 showed that the precipitation of HA was a nucleation– 1/[1+2(l/w)] aggregation–agglomeration growth process where the Fig. 11. Correlation between biaxial ﬂexural strength of the pellets and aggregation process was ruled by surface energy minimi- the crack size observed on the SEM images of the pellets. zation. Due to their large surface areas, small precipitates 5662 C. Kothapalli et al. / Acta Materialia 52 (2004) 5655–5663 were more likely to form large agglomerates so as to min- sition was accompanied by a dramatic microstructure imize their surface energy . This was in agreement change of the specimens. In contrast, a smooth, dense with Gibson et al.  who also reported that smaller surface morphology almost free of pores was observed particles appeared to form into large agglomerates with for the specimens 2.0–70 (Fig. 9), which were prepared rough surfaces, which is indicative of high surface areas. under reaction conditions that produced the highest ob- The HA agglomerates were pressed into discs, and served density and strength among the entire specimens sintered at 1200 °C for 1 h. The variation of density with tested. While it would be more intuitive to think that the reaction conditions for all the HA discs before and after micro structural defects inside the sintered pellets would sintering is shown in Fig. 6. The HA discs attained a have a profound eﬀect on the dynamics of the fracture maximum sintered density of 92% for the samples syn- propagation and ultimately dictate the mechanical thesized at 70 °C; this density was slightly lower than strength of the pellet, the images of the polished samples the values reported by both Ruys et al.  and Jarcho gave us a better view of the porosity (or density) of the et al.  who both used smaller HA particles. The phys- samples than those of the fractural sections. It would of ical basis of the Herring law of sintering , suggested interest to note that the fractured surface of the sintered that the sintering rate at a given temperature was inver- pellets did not reveal micro-structural features more evi- sely proportional to the square of the powder particle dently than the polished surface of the pellet, the images size. Thus, the smaller the agglomerate size, the easier of which are not included in this paper. for the powder to achieve high density at the same sin- tering temperature. As shown in Figs. 3 and 6, with the increase of the reaction temperature, the powder 5. Conclusions agglomerate size decreased, and their sintering density also improved in spite of the decrease of the surface area Diﬀerent HA powders were synthesized using a wet with agglomerate size. However, it was reported that the precipitation method wherein the concentration of the high sinterability of the synthetic HA was due to the reactants and the temperature of the chemical reaction high surface area and chemical purity of the HA were varied. The results indicate that these two factors [18,30,32]. In both reports, single-phase HA powders were critical in controlling the size and shape of the were studied and they did not decompose to secondary resulting HA particles. The length and breadth of the phases on sintering. In contrast, most of the HA used HA particles were found to increase with the reaction in this study were decomposed when sintered at temperature. HA agglomerates were formed during the 1200 °C, which was one of the reasons resulting in rela- synthesis process. Relatively large agglomerates were tively low sintering density in this study. found for small HA precipitates (such as those precipi- A low sintering density was observed for the speci- tated at 25 °C), due to their large surface areas. These mens 2.0–100, which had severely decomposed during HA agglomerates were pressed into discs and sintered the sintering process and caused their structure failure. at 1200 °C for 1 h. It was discovered that there was a It was reported that HA decomposed within the temper- positive correlation between the sintered density and ature range 1200–1450 °C, and the decomposition tem- biaxial ﬂexural strength. With the increase of the sinter- perature was strongly dependent on the characteristics ing density, the biaxial ﬂexural strength increased. How- of the HA powders . During the decomposition, a ever, due to severe decomposition, a signiﬁcant drop in large amount of water was formed, as indicated in strength as well as the density was found in specimens Eq. (1). It was hypothesized that as the water forced 2.0–100. This was also accompanied by a porous micro- its way out, it created many pores in the sintered discs, structure, which was witnessed by the SEM observation. as observed in the SEM images of the polished speci- mens (Fig. 9). As a result, the sintering density of the specimens 2.0–100 dramatically decreased. Acknowledgement It was found that there was also a positive correlation (p = 0.015) between the sintered density and biaxial ﬂex- The authors acknowledge the support of University ural strength of the specimens tested, as shown in Fig. 8. of Connecticut Research Foundation for sponsoring With the increase of the sintering density, the biaxial the project. ﬂexural strength increased. 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