Synthesis, characterization and thermal behavior of apatitic tricalcium phosphate

Materials Chemistry and Physics 80 (2003) 269–277

Synthesis, characterization and thermal behavior of apatitic tricalcium phosphate A. Destainville, E. Champion∗ , D. Bernache-Assollant, E. Laborde Science des Procédés Céramiques et de Traitements de Surface, Université de Limoges, UMR CNRS 6638, 123 Avenue Albert Thomas, 87060 Limoges Cedex, France Received 29 July 2002; received in revised form 10 September 2002; accepted 9 October 2002

Abstract Apatitic tricalcium phosphate Ca9 (HPO4 )(PO4 )5 (OH) is a calcium orthophosphate that transforms into ␤-tricalcium phosphate Ca3 (PO4 )2 by heating above 750 ◦ C. This work deals with powder synthesis using a wet precipitation method. An experimental design is applied to precise the influence of the synthesis parameters on the chemical composition (e.g. the Ca/P molar ratio). The Ca/P ratio of the precipitates varies greatly according to the pH value and the temperature of synthesis. A more or less important increase of the Ca/P ratio can occur with the ripening time in dependence on the value of the previous parameters. A reproducible synthesis of pure apatitic tricalcium phosphate (TCP) powders is attained by refinement of the parameters. The study is completed by physicochemical characterizations and the thermal behavior of the powders. X-ray diffractometry and differential thermal analysis are necessary to insure the purity of TCP powders. The decomposition of the apatitic TCP into ␤-TCP during heating influences the sintering behavior. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Biomaterials; Calcium phosphate; Precipitation; Heat treatment

1. Introduction Two ceramic materials based on calcium phosphates, hydroxyapatite (HA) Ca10 (PO4 )6 (OH)2 and ␤-tricalcium phosphate (␤-TCP) Ca3 (PO4 )2 have a wide range of potential applications for bone substitutes either in the form of dense or porous parts. These bioceramics favor bone reconstruction, thanks to high properties of resorbability for ␤-TCP and good osteoconductivity for HA [1]. The synthesis of pure TCP powders is not so much reported in the literature compared with that of HA. As reviewed by Elliott [2], it can be performed using either high-temperature solid-state reactions or wet precipitation methods at low temperature. The main problem encountered with both methods is the variability of the powder composition. The solid-state reactions at high temperatures are hardly usable for the synthesis of great quantities because of the difficulty in the control of intimate mixtures of reagent powders and of the complete reaction between them [3]. Therefore, the preparation of stoichiometric final products without residual

∗ Corresponding author. Tel.: +33-5-55-45-74-60; fax: +33-5-55-45-75-86. E-mail address: [email protected] (E. Champion).

second phases often requires successive grindings and/or stoichiometry corrections followed by firing. Considering the wet methods, ␤-TCP cannot be synthesized directly in aqueous solution. The compound that may precipitate is the apatitic tricalcium phosphate Ca9 (HPO4 )(PO4 )5 (OH), which appears to be hydroxyapatite Ca10 (PO4 )6 (OH)2 where a HPO4 2− ion substitutes a PO4 3− . The crystallization of anhydrous ␤-TCP requires further calcination of the apatitic compound at temperatures over 750 ◦ C. The synthesis of apatitic TCP is conceivable through the aqueous ways classically used for stoichiometric HA. Non-stoichiometric apatites Ca10−x (HPO4 )x (PO4 )6−x (OH)2−x with 0 ≤ x ≤ 1 can be precipitated, the value of x depending on the synthesis conditions. The most important parameters to control are the pH and the temperature. Though the values reported in the literature are scattered, the pH value is rather maintained at a value close to the neutrality, slightly acid, and low temperatures are generally used. The kinetics of formation of the precipitates is still misunderstood and the ripening times after total addition of the reagents range from no ripening up to 12 h [4–8]. A specific difficulty linked with non-stoichiometric apatite synthesis is the high variability of the powder composition after calcination even for very low variations of the

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 6 6 - 2

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calcium/phosphorus molar ratio of the initial precipitate [9]. Pure ␤-TCP is formed after calcination of a powder with Ca/P = 1.500. For Ca/P values higher than 1.500, HA is formed as a second phase. A relative variation of 1% of the Ca/P molar ratio induces the formation of 10 wt.% of HA. For Ca/P values lower than 1.500 another second phase appears: calcium pyrophosphate Ca2 P2 O7 . The biological and mechanical properties of calcium phosphate compounds depend strongly on their chemical composition [10,11]. Consequently, their preparation needs a high level of control if one wants to obtain reproducible properties and precise techniques of characterization must be used. On this basis, our work is concerned with the use of an experimental design to investigate the synthesis process of pure apatitic tricalcium phosphate. It consisted in determining the effect of pH, temperature, and ripening time on the final chemical composition of powders, e.g. on their Ca/P molar ratio. We have recently reported that quantitative X-ray diffractometry was the most accurate method for the determination of the Ca/P ratio of compounds with Ca/P ≥ 1.500 [12]. Complementary results for Ca/P < 1.500 are discussed in this paper and the thermal behavior of the powders is also reported.

2. Materials and methods 2.1. Powder synthesis Powders were prepared by an aqueous precipitation method from the addition of a diammonium phosphate solution (NH4 )2 HPO4 (Aldrich, France) into a reactor containing a calcium nitrate solution Ca(NO3 )2 (Aldrich, France). The synthesis device was a fully automated apparatus. The reactor was placed in an argon atmosphere under dynamic flow in order to prevent any presence of CO2 which could result in carbonate apatite formation. The calcium/phosphorus molar ratio of initial reagents was 1.500. The (NH4 )2 HPO4 addition rate was controlled using a peristaltic pump. The pH value of the solution was maintained at a constant value by the addition of an ammonium hydroxide solution using a pH controller and dosing pump system (Hanna Instruments). The temperature was controlled and regulated. The suspension was continuously stirred and refluxed. After total addition of the (NH4 )2 HPO4 solution, the suspension was ripened from 15 min up to 48 h. Then, the resulted precipitate was filtered (without washing) and dried at 100 ◦ C during 24 h.

from a comparison of the registered patterns with the international center for diffraction data (ICDD) powder diffraction files (PDF). The Ca/P molar ratio of powders was determined after calcination at 1000 ◦ C for 15 h from quantitative X-ray diffractometry according to a standardized procedure established for HA [13]. After calcination, the apatitic powders are single-phased HA for Ca/P = 1.667 or biphasic mixtures of HA and ␤-TCP if 1.500 < Ca/P < 1.667 whose proportions relate to the Ca/P ratio. On this basis, the principle consists in the determination of the phase proportions, using a ratio of two integrated diffraction intensities. This method was extended to a larger domain of composition, i.e. for Ca/P molar ratios between 1.5 and 2, and completed as detailed in a previous paper [12]. For compounds with Ca/P ≤ 1.500 a similar approach has been investigated. For these compositions, the powders heated at 1000 ◦ C during 15 h consist of pure ␤-TCP if the Ca/P molar ratio of the initial powder is 1.500 or of a mixture of ␤-TCP and calcium pyrophosphate (CPP) ␤-Ca2 P2 O7 for Ca/P < 1.500 as shown in the XRD patterns (Fig. 1). Calibrated mixtures of pure ␤-TCP and ␤-Ca2 P2 O7 powders containing up to 10 wt.% of ␤-CPP were prepared. The ratio of the integrated intensity of the (2 0 2) diffraction peak of ␤-CPP to the integrated intensity of the (0 2 10) diffraction peak of ␤-TCP was calculated. The (2 0 2) peak at 2θ = 28.9◦ of CPP was the most intense usable peak for the quantification (relative intensity 45%). A linear relation with a correlation coefficient of 0.9997 was found between the ␤-CPP phase content and the intensity ratio: R =

ICPP(2 0 2) ≈ C × CPP wt.% ITCP(0 2 10)

= 3.32 × 10−3 × CPP wt.% This equation agrees with the theoretical calculation of an intensity ratio at low content of one the two phases on the assumption that the bulk densities of both phases

2.2. Powder characterizations Powder X-ray diffraction (XRD) patterns were recorded with Cu K␣ radiation in the 2θ range 20–60◦ on a θ/2θ diffractometer (Siemens, Model D5000, Germany) (θ = diffraction angle). The crystalline phases were identified

Fig. 1. XRD patterns of powders calcined at 1000 ◦ C for 15 h. (a) Ca/P < 1.500; (b) Ca/P = 1.500; (c) Ca/P > 1.500.

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are similar [12], which is the case for ␤-TCP and calcium pyrophosphate ␤-CPP (with theoretical values of 3.07 and 3.12 g cm−3 , respectively). The presence of HA as a second phase in ␤-TCP, when the Ca/P molar ratio of the as-synthesized powder is above 1.500, can be detected down to 0.5 wt.% from XRD [12], whereas the presence of CPP in powders whose Ca/P initial ratio is lower than 1.500 could not be detected from XRD below 4 wt.% because its main diffraction peak (relative intensity 100%) at 2θ = 29.6◦ is superposed with a peak of ␤-TCP, the second peak of relative intensity 45% at 2θ = 28.9◦ being no more observable below 4 wt.%. Infrared spectra of powders were recorded on a Fourier transform spectrometer (Perkin-Elmer Spectrum One, USA) with a resolution of 4 cm−1 . The powder samples were mixed with KBr, then pressed in a 13 mm die. The specific surface area of powders was measured by the Brunauer, Emmett and Teller (BET) method (8 points, analyzer Micromeritics ASAP2010, USA) after degassing under vacuum at 150 ◦ C. Scanning electron microscopy (SEM) was used for morphological investigations (Hitachi S2500, Japan). Thermogravimetry coupled with differential thermal analysis (TG/DTA) was performed in air up to 1500 ◦ C at a heating rate of 5 K min−1 (SDT 2960 TA Instrument, USA). The linear shrinkage was measured by dilatometry in air up to 1250 ◦ C at a heating rate of 2.5 K min−1 (Setaram TMA 92, France). The powders were heat-treated at 650 or 750 ◦ C and then, the pellets were pressed in a cylindrical die prior to the dilatometric experiment.

271

Fig. 2. Schematic representation of the experimental design.

Table 1 Reduced and real coordinates of experimental design points Number

Tred

pHred

T (◦ C)

pH

1 2 3 4 5 6 7

+1 −1 +0.5 −0.5 +0.5 −0.5 0

0 0 +0.866 −0.866 −0.866 +0.866 0

50 30 45 35 45 35 40

7.0 7.0 8.0 6.0 6.0 8.0 7.0

of experiments, the computed equation was given in a polynomial form defined as follows: (Ca/P)cal = A0 + A1 × pH + A2 × T + A3 × pH2

2.3. Experimental design

+A4 × T 2 + A5 × pH × T Experimental designs are often used in industry to establish empirical relationships between a measured response and variables [14]. A two-variable design was established to characterize the influence of synthesis experimental parameters on the chemical composition of precipitated calcium phosphate compounds (i.e. their Ca/P ratio). The two independent variables chosen were the pH value and the temperature (T) of synthesis. The range of investigations was fixed, according to previous experiments, from 6 to 8 for pH (close to the neutrality), and from 30 to 50 ◦ C for the temperature. For the experimental analysis, each variable was expressed in reduced coordinates in the range −1 (low level) and +1 (high level) from the real coordinate values according to the equations: pHred =

pH − 7 ; 1

Tred =

T − 40 ◦ C 10 ◦ C

(1)

The experiments were uniformly spaced on a two-dimensional surface area (Fig. 2). The notations and the corresponding values of variables are given in Table 1. A commercial software (Nemrod 3.0 [15]) was used for the mathematical and statistical analyses. An empirical relationship (Ca/P)cal = f (pH, T ) was calculated to estimate a measured characteristic (Ca/P)exp . According to the number

(2)

The statistical analyses of the computed equations were performed, using F-test and Student’s test to evaluate the validity of the relationship and the confidence level of its coefficients Ai . For the presentation of the results, reduced coordinates (pH ∈ [−1, +1]; T ∈ [−1, +1]) were used. 3. Results and discussion 3.1. Powder synthesis The precipitates were characterized by the presence of one or two distinguishable crystalline phases depending on the synthesis conditions as shown on the diffraction patterns of powders (Fig. 3). For Ca/P ≥ 1.500, the powder exhibited a single apatitic phase whereas a second phase identified as dicalcium phosphate anhydrous CaHPO4 (DCPA, PDF 9–80) was detected in compounds with Ca/P < 1.500. The influence of synthesis parameters (i.e. pH, temperature, and ripening time) on the precipitate composition was evaluated from the results of four experimental designs established for increasing ripening times: 15, 30 min, 1 and 24 h. The Ca/P molar ratio of each synthesized powder is given in Table 2.

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A. Destainville et al. / Materials Chemistry and Physics 80 (2003) 269–277 Table 4 Eq. (2) and associated statistical analyses for 30 min of ripening Coefficient

Standard deviation

1.5070 0.0283 0.0133 −0.0100 −0.0193 −0.0023

0.0092 0.0075 0.0075 0.0130 0.0130 0.0150

Student’s test (unit normal deviate) 163.78 3.77 1.77 −0.77 −1.49 −0.15

Validity (%) >99 94 78 48 72 11

Polynomial : Ca/P = 1.5070 + 0.0283 × T + 0.0133 × pH − 0.0100 × T 2 − 0.0193 × pH2 − 0.0023 × T × pH + ε(T , pH); validity of equation (F-test): 79%; sample standard deviation: 0.013. Table 5 Eq. (2) and associated statistical analyses for 1 h of ripening Fig. 3. XRD patterns of as-synthesized powders after drying. (a) Ca/P < 1.500; (b) Ca/P = 1.500; (c) Ca/P > 1.500.

Table 2 Ca/P molar ratio of powders prepared at different ripening time from the experimental designs Number

T (◦ C)

pH

t = 15 min

t = 30 min

t = 1h

t = 24 h

1 2 3 4 5 6 7 7

50 40 45 35 45 35 0 0

7 7 8 6 6 8 0 0

1.514 1.454 1.522 1.454 1.500 1.460 1.500 1.500

1.519 1.475 1.521 1.457 1.500 1.482 1.514 1.500

1.520 1.500 1.527 1.463 1.500 1.508 1.520 1.500

1.552 1.500 1.548 1.506 1.525 1.511 1.545 –

The treatment of the experimental data allowed the computation of the empirical Eq. (2) to transcribe the development of Ca/P ratio versus pH and temperature of the synthesis. The polynomial expressions (expressed in reduced coordinates) and the related statistical analyses are summarized in Tables 3–6 for the different ripening times. The plots of the empirical equations are shown in Fig. 4. They show an increase of the Ca/P ratio of powders with increasing ripening time and pH. The curvature of the response surfaces would indicate a maximum for intermediate temperatures. Such an effect is not expected to occur and should rather be ascribed Table 3 Eq. (2) and associated statistical analyses for 15 min of ripening Coefficient

Standard deviation

1.5000 0.0380 0.0081 −0.0160 −0.0160 0.0092

0.0098 0.0080 0.0080 0.0139 0.0139 0.0160

Student’s test (unit normal deviate) 153.09 4.75 1.01 −1.15 −1.15 0.58

Coefficient

Standard deviation

1.5100 0.0160 0.0208 0.0000 −0.0140 −0.0104

0.0102 0.0083 0.0083 0.0144 0.0144 0.0166

Polynomial : Ca/P = 1.5000 + 0.0380 × T + 0.0081 × pH − 0.0160 × T 2 − 0.0160 × pH2 + 0.0092 × T × pH + ε(T , pH); validity of equation (F-test): 83%; sample standard deviation: 0.014.

148.07 1.92 2.50 0.00 −0.97 −0.62

Validity (%) >99 81 87 – 56 40

Polynomial : Ca/P = 1.5100 + 0.0160 × T + 0.0208 × pH + 0.0000 × T 2 − 0.0140 × pH2 − 0.0104 × T × pH + ε(T , pH); validity of equation (F-test): 67%; sample standard deviation: 0.015.

to the definition of the second degree polynomial models that imposes this curvature, which can be considered as a kind of an artifact. The confidence levels calculated from the F-test indicate that the mathematical models have a relatively low validity for ripening time up to 1 h, only the model defined for 24 h describes the Ca/P ratio of the precipitate with a satisfactory accuracy. Similar results are obtained for the confidence levels of the polynomial coefficients determined from the Student’s test. For the short ripening durations, the coefficients associated with the temperature are preponderant. The variations of Ca/P ratio may be attributing mainly to the synthesis temperature. This also explains the low reliability of polynomial models to represent the evolutions of the composition of precipitates. For long maturation times, Table 6 Eq. (2) and associated statistical analyses for 24 h of ripening Coefficient

Standard deviation

Student’s test (unit normal deviate)

Validity (%)

1.5450 0.0263 0.0075 −0.0019 −0.0243 0.0092

0.0008 0.0005 0.0005 0.0010 0.0010 0.0009

1.90 5.60 1.60 −1.90 −2.40 9.80

>99 >95 >95 >95 >95 93

Validity (%) >99 >95 58 63 63 38

Student’s test (unit normal deviate)

Polynomial : Ca/P = 1.545 + 0.0263 × T + 0.0075 × pH − 0.0019 × T 2 − 0.0243 × pH2 + 0.0092 × T × pH + ε(T , pH); validity of equation (F-test): >95%; sample standard deviation: 0.0008.

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During the first time of ripening, an important increase of the Ca/P ratio of the precipitate was observed. Then, for longer times, a slow-down occurred. In this second domain, a quasi-equilibrium could be reached for low temperature and pH. Few works deal with the precipitation mechanisms of non-stoichiometric calcium phosphate apatites. Heughebaert [16] proposed a precipitation process of apatitic tricalcium phosphate. The first step is the precipitation of an amorphous compound which stoichiometry is close to the one of TCP: 9Ca2+ + 6PO4 3− → Ca9 (PO4 )6 , nH2 O This step is followed by the hydrolysis of phosphate ions into hydrogenophosphate and hydroxide ions: PO4 3− + H2 O → HPO4 2− + OH−

Fig. 4. Three-dimensional representation of Eq. (2) for the different ripening times. (a) 15 min; (b) 30 min; (c) 1 h; (d) 24 h.

the coefficients of variables T and pH become well-balanced while the weight attributed to the product T × pH increases lightly. This relates with a greater combined influence of both factors and indicates that the system is close to an equilibrium state. In this case, the polynomial mathematical model is more adapted and accurate. Considering the synthesis process, the results of the experimental designs could be used for the elaboration of apatitic powders whose composition would remain inside an acceptable interval around the value of Ca/P = 1.500. But, if the reproducible synthesis of highly pure TCP is expected, an additional refinement of the synthesis parameters is required. According to the previous results complementary syntheses were performed. Fig. 5 shows the Ca/P molar ratio versus the ripening time of powders synthesized at different temperature and pH values. All the curves got similar variations that could be divided in two distinguished domains.

Finally, these reactions lead to a non-stoichiometric compound of apatite structure: Ca9 (HPO4 )x (PO4 )6−x (OH)x . Apatitic tricalcium phosphate is the terminal orthophosphate precipitated for x = 1 and the process is thermochemically dependent. According to other authors [2,17], apatitic tricalcium phosphate can also be precipitated through the hydrolysis of dicalcium phosphate dihydrate CaHPO4 , 2H2 O (DCPD) in solution. Taking these analyses in account, the evolution of the chemical composition of precipitates with Ca/P < 1.500 can support two hypotheses. It can be supposed that the precipitation begins by DCPD formation, followed by its hydrolysis into apatitic TCP. The other hypothesis is concerned with the precipitation of a single-phased non-stoichiometric apatite. Some works deal with the formula of calcium-deficient apatites whereas none has been established as sure. Mixtures are always considered as solid solutions but their composition range may be different according to the authors [2]: Ca9−x (HPO4 )1+2x (PO4 )5−2x (OH)

,

Ca10−x−y (HPO4 )x (PO4 )6−x (OH)2−x−2y

0≤x≤2 y ≤ 1 − 21 x

For Ca/P > 1.500, Heughebaert [16] also proposed a precipitation mechanism thermochemically dependent in two steps very close to the one exposed for apatitic tricalcium phosphate precipitation. The first compound appearing is the same as previous one. The next step corresponds with the appearance of the apatite structure with a Ca/P molar ratio greater than 1.500: Ca9 (PO4 )6 + yCa2+ + 2yOH− → Ca9+y (PO4 )6 (OH)2y This reaction induces an increase of the Ca/P ratio of synthesized product and a decrease of the pH value in association with hydrogenophosphate and hydroxide ions incorporation within the precipitate according to: PO4 3− + H2 O → HPO4 2− + OH−

Fig. 5. Ca/P molar ratio of precipitates vs. the ripening time for different synthesis conditions.

This hydrolysis of phosphate ions leads to the crystallization of a calcium-deficient apatite Ca9+y (HPO4 )z (PO4 )6−z

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(OH)2y+z . The Ca/P molar ratio is proportional to the ratio (9 + y)/6 and consequently to the pH value: a pH increment leads to the increase of the Ca/P molar ratio. The evolution of Ca/P molar ratio when above 1.500 agrees with this analysis particularly with the influence of pH, which increases the Ca/P molar ratio of precipitates. The evolution of the composition of the compounds with Ca/P < 1.500 does not seem consistent with the hypothesis of an apatite precipitation with the formula Ca9 (HPO4 )x (PO4 )6−x (OH)x which Ca/P ratio is always 1.500. Indeed, not only the crystallization evolves during the process, but also the Ca/P molar ratio, as shown in Fig. 5. These results should be in better agreement with an initial precipitation of DCPD followed by its slow and partial hydrolysis into apatitic tricalcium phosphate and would explain that the final mixture contains dicalcium phosphate anhydrous (DCPA) as a second phase after drying. Considering the synthesis of pure apatitic TCP, for low ripening times the Ca/P ratio varied too quickly to ensure reproducible syntheses. The Ca/P ratio must tend to stability. A pH of 7 and a temperature of 30 ◦ C for at least 10 h of ripening must, therefore, be imposed to obtain the best reproducibility. A deviation of one unit in the pH value, i.e. pH 6 or 8 at 30 ◦ C led to compositions having a Ca/P molar ratio of 1.51 or 1.48, respectively. Though this corresponds with apparently low variations of the Ca/P ratio, the composition after calcination treatment differs slightly. A Ca/P ratio of 1.48 leads to a biphasic mixture containing ␤-TCP and 5 wt.% of ␤-CPP as a second phase and a Ca/P of 1.51 leads to the presence of 5 wt.% of HA in the ␤-TCP powder. Small temperature deviation from 30 ◦ C led to similar results. For example, at 35 ◦ C and pH 7 the Ca/P ratio reached 1.506 after 24 h. Finally, only very restrictive conditions allowed the reproducible precipitation of apatitic TCP. Consequently, the control of synthesis parameter settings has to be very rigorous.

Fig. 6. IR spectra of as-synthesized powders. (a) Ca/P < 1.500; (b) Ca/P = 1.500; (c) Ca/P > 1.500.

The presence of hydrogenophosphate and hydroxide ions confirms that the apatite phases are calcium-deficient Ca10−x (HPO4 )x (PO4 )6−x (OH)2−x . Fig. 7 shows a typical morphology of pure apatitic tricalcium phosphate powders (Ca/P = 1.500). They were made of nanometric (and agglomerated) grains, whose size was about 20 nm. SEM observation of the raw powders for different ripening times showed no evidence for significant

3.2. Physicochemical characterizations of powders Before thermal treatment, it is impossible to distinguish all kinds of a composition using X-ray diffractometry (Fig. 3). Only powders with Ca/P < 1.500 can be identified because of the presence of DCPA. For Ca/P = 1.5000, only an apatitic structure is detected. Complementary analyses are necessary to characterize the raw powders. Fig. 6 gives infrared (IR) spectra of three as-synthesized powders (Ca/P = 1.430; Ca/P = 1.500; Ca/P = 1.554). The characteristic bands of phosphate groups appeared for all the compositions: v2 PO4 (460 cm−1 ), v4 PO4 (560–600 cm−1 ), v1 PO4 (960 cm−1 ), v3 PO4 (1020–1120 cm−1 ). The band at 875 cm−1 was ascribed to hydrogenophosphate groups. The bands at 630 and 3540 cm−1 were assigned to OH− and those at 820 and 1380 cm−1 were attributed to nitrate groups resulting from synthesis residuals. The spectra were similar whatever the composition might be.

Fig. 7. SEM micrograph of as-synthesized apatitic TCP (synthesis pH 7, 30 ◦ C, 48 h).

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275

Fig. 9. TG/DTA in air of apatitic TCP raw powder (Ca/P = 1.500). Fig. 8. IR spectra of powders calcined at 1000 ◦ C. (a) Ca/P < 1.500; (b) Ca/P = 1.500; (c) Ca/P > 1.500.

morphological changes. The specific surface area of apatitic TCP was 95 ± 2 and 89 ± 2 m2 g−1 after 24 and 48 h ripening, respectively. 3.3. Thermal stability of powders Fig. 8 gives the IR spectra of powders heat-treated at 1000 ◦ C. They agree with the phases determined from XRD patterns of calcined powders (Fig. 1). For Ca/P > 1.500, the results are consistent with those of previous studies dealing with the thermal behavior of calcium-deficient hydroxyapatite Ca10−z (HPO4 )z (PO4 )6−z (OH)2−z [9]. The band attributed to HPO4 groups in the as-synthesized powder (v = 875 cm−1 ) had disappeared. This corresponds with the decomposition of calcium-deficient HA that occurs around 750 ◦ C according to the global reaction: Ca10−x (HPO4 )x (PO4 )6−x (OH)2−x → (1 − x)Ca10 (PO4 )6 (OH)2 + 3xCa3 (PO4 )2 + xH2 O The presence of HA Ca10 (PO4 )6 (OH)2 is confirmed by the bands at 630 and 3540 cm−1 ascribed to OH vibrations. For Ca/P < 1.500, the band assigned to OH group (630 cm−1 ) had disappeared at 1000 ◦ C, and the PO4 bands are present on account of the decomposition of the apatitic phase into ␤-TCP. The bands at 725 and 1200 cm−1 indicated the presence of P2 O7 groups associated with the transformation of DCPA into CPP according to [18]: 2CaHPO4 → (␦)Ca2 P2 O7 + H2 O (␦)Ca2 P2 O7 → (␤)Ca2 P2 O7

The TG/DTA curves of the raw apatitic TCP are shown in Fig. 9. The weight losses and associated thermal effects at temperature below 500 ◦ C are due to the departure of synthesis residuals. Between 300 and 700 ◦ C, a part of the weight loss is due to the condensation of hydrogenophosphate ions before the decomposition of the apatite according to the mechanism proposed by Mortier et al. [19] for calcium-deficient hydroxyapatite: Ca10−z (HPO4 )2z (PO4 )6−2z (OH)2 → Ca10−z (P2 O7 )z−s (PO4 )6−2z+2s (OH)2(1−s) +(z + s)H2 O At 750 ◦ C, the weight loss and the endothermic peak correspond with the decomposition of the apatitic phase into ␤-TCP. The endothermic peak that appears at 1150 ◦ C is associated with the transformation ␤ → ␣ of the TCP, followed by a second endothermic peak around 1450 ◦ C assigned to its second allotropic transformation ␣ → ␣, ¯ which agrees with the phase diagram of TCP [2]. Fig. 10 shows the DTA of powders previously heated at 1000 ◦ C. For Ca/P = 1.500 (or Ca/P > 1.500, which is not presented here) only the thermal effects relating with the

(450◦ C)

(850◦ C)

For powders with Ca/P = 1.500, as previously the band attributed to hydroxide group OH− (630 cm−1 ) had disappeared. The absence of pyrophosphate bands at 727 and 1200 cm−1 indicates that there is no detectable CPP. The spectrum is characteristic of pure ␤-TCP.

Fig. 10. DTA in air of powders initially calcined at 1000 ◦ C for 15 h. (a) Ca/P = 1.500; (b) Ca/P < 1.500.

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allotropic transformations of the TCP phase are observable. For powders with Ca/P below 1.500, additional endothermic peaks appear. A first one that begins at about 1220 ◦ C (see inset in Fig. 10), is associated with the transformation ␤ → ␣ of CPP. A second peak at about 1280 ◦ C, associated with a much stronger endothermic effect than the allotropic transformations, corresponds with the formation of a liquid phase between CPP and TCP, according to the phase diagram CaO–P2 O5 proposed by Kreidler and Hummel [20], as (for an excess TCP): ␣–Ca3 (PO4 )2 + ␣–Ca2 P2 O7 → ␣–Ca3 (PO4 )2 + liquid

(1280 ◦ C)

This peak could be detected down to 0.5 wt.% of CPP in the biphasic mixture TCP–CPP. Therefore, this thermal analysis is more accurate for the detection of very low content of CPP than the XRD analysis and should be used to make sure of the high purity of TCP powders. The thermal behavior of TCP was completed by dilatometric experiments. Before these experiments, the powder having a Ca/P = 1.500 was either heated at 650 ◦ C to remove synthesis residuals without transforming the initial apatitic TCP, or heated at 740 ◦ C to transform the apatite TCP into ␤-TCP. The linear shrinkage plots versus the temperature are shown in Fig. 11. The ␤-TCP pellet expanded up to 750 ◦ C, the temperature at which the sintering began. The maximum rate of densification was between 950 and 1000 ◦ C. Comparing this behavior with that of hydroxyapatite, ␤-TCP sinters at much lower temperature. The maximum of shrinkage rate is about 1100 ◦ C for HA [21]. At 1200 ◦ C, an important expansion occurred on account of the ␤ → ␣ transformation that provokes a linear expansion due to the difference of density between the two forms (3.07 for the ␤-form and 2.86 for the ␣-form), thus a relative increase of 7% of the lattice volume. The linear expansion of the ceramic during this step was 2.5% (i.e. about a third of the theoretical volume expansion). The apatitic TCP pel-

Fig. 11. Dilatometric analysis of pellets initially calcined at 650 and 740 ◦ C and derivative plots (dashed lines).

let had a slightly different behavior. As it can be seen on the derivative plot of the linear shrinkage for the powder initially calcined at 650 ◦ C in comparison with that of the powder calcined at 740 ◦ C (Fig. 11), a small shrinkage was registered between 500 and 700 ◦ C, which would be associated with hydrogenophosphates condensation above mentioned. An accident occurred between 800 and 900 ◦ C, particularly visible on the derivative plot. It has been attributed to the decomposition of the apatitic TCP into ␤-TCP. Then, at higher temperature the behavior is identical to that of ␤-TCP. The important expansion at 1200 ◦ C came from the formation of cracks due to the stresses developed during the volume expansion associated with the allotropic transformation ␤ → ␣ of TCP. A lower densification was obtained with the use of apatitic TCP in comparison with ␤-TCP. The final densification ratio was 92 and 88% of the theoretical density (assumed to be 3.07) for ␤- and apatitic TCP, respectively. Finally, transforming apatitic TCP powders before shaping appeared useful to enhance the sintering behavior of TCP.

4. Conclusions This work points out the high variability of the Ca/P molar ratio of apatitic calcium phosphates precipitated from aqueous media with the experimental synthesis conditions. An increase of this ratio with the maturation time has been observed. The kinetics of hydrolysis reactions can explain this phenomenon. The hypothesis retained for powders with Ca/P ratios below 1.500 would be the formation of dicalcium phosphate dihydrate CaHPO4 , 2H2 O, followed by its hydrolysis into apatitic tricalcium phosphate. Concerning powders with Ca/P ratio above 1.500, the initial precipitate would be close to the apatitic tricalcium phosphate composition and subjected to hydrolysis that transforms it into a calcium-deficient hydroxyapatite. In both cases, the process appears thermochemically dependent. For low temperatures, the Ca/P of precipitates tends to stability after several hours, allowing the reproducible synthesis of apatitic TCP with Ca/P = 1.500 providing a strict control and regulation of the pH and temperature are achieved. Its synthesis conditions are pH 7, T = 30 ◦ C for 48 h ripening. The purity of TCP is difficult to characterize with a high degree of precision. We have demonstrated that several complementary methods should be used. They include the XRD quantitative analysis for the superior limit, i.e. detection of HA in calcined powders, and DTA at high temperature for the inferior limit, i.e. detection of the liquid phase formed between ␣-TCP and ␣-CPP at 1280 ◦ C. The sintering behavior of pure TCP differs slightly from that of HA. Though it remains still misunderstood the preparation of the initial powder, in particular, its calcination can play an important role. In this domain, further investigations are under progress for an optimization of the microstructural design of pure ␤-TCP ceramics.

A. Destainville et al. / Materials Chemistry and Physics 80 (2003) 269–277

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