SYNTHESIS AND CHARACTERIZATION OF BETA TRICALCIUM PHOSPHATE

A. DESTAINVILLE1, A. ROLO2, E. CHAMPION3 and D. BERNACHE-ASSOLLANT4

1 Science des Procédés Céramiques et de Traitements de Surface UMR CNRS 6638, Faculté des Sciences et Techniques

123 avenue Albert Thomas, F – 87060 Limoges cedex, FRANCE arnaud.destainville@unilim.fr

2 Departamento de Engenharia Cerâmica e do Vidro Universidade de Aveiro, P – 3810-193 Aveiro – PORTUGAL

alexandra.rolo@hotmail.com 3 champion@unilim.fr 4 bernache@unilim.fr

Keywords: Apatite, Tricalcium phosphate, Synthesis, Mechanical properties Abstract. This work aimed to develop apatitic tricalcium phosphate via an aqueous precipitation process. The results showed high variability of Ca/P ratio of powders with the ripening time, and more particularly an increase of the Ca/P value with this duration. Temperature and pH of synthesis also play an important role in the composition of the precipitate. Hot pressing was used to fully densify the material at a temperature below the β � α transition, that occurs at 1150°C, and mechanical characterizations of dense

� TCP were performed.

Introduction

Thanks to their high bioactive properties and capacity to allow intimate bone growth within their structure so close to that of mineral bone, calcium phosphate ceramics play an active role in the development of biomaterials. Hydroxyapatite Ca10(PO4)6(OH)2 (HAp) and

� tricalcium phosphate

Ca3(PO4)2 (�-TCP) are the two main representative compounds of this family with a wide field of

potential applications (spinal surgery, dental implants, orthopaedics). These bioceramics are thus able to promote bone reconstruction and its rapid fixation through high properties of resorbability for β TCP and good osteoconductivity for HA. The use of pure β TCP as bone substitute is crucial because any presence of a second phase should induce a deterioration of its property of total resorbability. Apatitic tricalcium phosphate Ca9(HPO4)(PO4)5(OH) is the calcium orthophosphate leading to β tricalcium phosphate Ca3(PO4)2 (β-TCP) for a temperature above 700°C [1]. This compound is very interesting but no experimental technique exists for its reproducible synthesis as pure compound by aqueous media. Obtaining apatitic TCP, which is in fact a non – stoichiometric HA where a PO4

3- ion has been substituted by a HPO42- ion, is conceivable through an aqueous way

classically used for stoichiometric HA synthesis [2-5]. Chemical, physico – chemical, biological and mechanical properties of calcium phosphate compounds are radically different from one to the other and depends strongly on their Calcium/Phosphorus (Ca/P) molar ratio, particularly in apatites [6]. So, their preparation needs a high level of control if one wants to obtain reproducible results.

Materials and Methods

Powders were synthesized by an aqueous precipitation method from the addition of a diammonium phosphate solution (NH4)2HPO4 into a reactor containing a calcium nitrate solution Ca(NO3)2. The Calcium/Phosphorus molar ratio of initial reagents was 1.500. The reactor was placed in an argon atmosphere under dynamic flow. The (NH4)2HPO4 addition rate was controlled. The regulation of pH and temperature was insured by an automated apparatus. The suspension was continuously stirred and refluxed. After total addition of the phosphate solution, the suspension was ripened from

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Key Engineering Materials Vols. 240-242 (2003) pp 489-492Online available since 2003/May/15 at www.scientific.net© (2003) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.240-242.489

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15 min. up to 48 hours. The resulted precipitate was filtered and dried at 100°C during 24 hours. Powders characterizations were performed by XRD, BET and SEM. The synthesized powders were analysed using quantitative X-ray diffractometry to determine their Ca/P ratio [7]. The sintering step was performed using hot pressing of β TCP pellets (30 mm in diameter; compressive stress of 20 MPa). In order to transform apatitic TCP into β TCP, powders were previously heated at 740°C for 30 minutes. Flexural strength and fracture toughness of dense ceramics were investigated by three point bending and Vickers surface indentation technique, respectively.

Results and Discussion

Powders synthesis On the basis of HA synthesis, the main problem encountered to synthesize apatitic TCP is the control of the HPO4

2- substitution ratio as function of experimental parameters, i.e. pH, temperature and ripening time. Figure 1 exposes the evolution of the Ca/P molar ratio of synthesized powders in different experimental conditions. This ratio increases with the maturation time. For low temperatures and neutral pH, this increase slows down after some hours and the Ca/P

tends to stability. The curves got similar variation that could be divided in two domains. The first one deals with the low maturation time values (<1h) for which the Ca/P ratio quickly increases. The second domain concerns higher maturation times (>3h) for which the Ca/P evolution slows down, except for high temperature (>40°C) for which it increases continuously. These results are in agreement with previous works indicating the slow kinetic and the thermo chemical dependence of HA precipitation [8]. Only very restrictive conditions allow the reproducible synthesis of pure apatitic TCP: a temperature of 30°C, a pH of 7.0 and a maturation time of at least 10 hours.

The specific surface area of the as-synthesized powders was around 90 m².g-1, with an elementary particles size of about 20 nm. The main difficulty concerns low deviations around the value 1.500 of the Ca/P molar ratio, which induce the presence of secondary phases in significant amounts. These variations appear for very low changes of the synthesis temperature and pH of the solution. For example, if the pH value is decreased or increased of one unit with respect to the optimal conditions, i.e. T=30°C, pH=8 or T=30°C, pH=6, both for 48 hours of ripening time, the Ca/P changes from 1.51 down to 1.48 respectively. Though this makes relative variations of the Ca/P molar ratio lower than 2% around 1.500, the compositions of synthesized powders after firing are radically different. Thus, a Ca/P ratio of 1.48 gives a mixture of 95 wt% β TCP and 5 wt% of β CPP (β CPP=calcium pyrophosphate Ca2P2O7), whereas a Ca/P ratio of 1.51 leads to a composition of 95 wt% β TCP and 5 wt% of HA. Therefore a strict control and a precise regulation of the synthesis parameters are required for reproducible synthesis of pure β TCP.

Hot pressing Hot pressing was chosen to produce dense materials of � tricalcium phosphate below 1150°C, which corresponds to the temperature of the allotropic transformation � �� � of the TCP. This transformation is undesirable because the � TCP lattice volume is 7 % larger than the

1,4

1,44

1,48

1,52

1,56

0 10 20 30 40 50

30°C - pH = 730°C - pH = 630°C - pH = 835°C - pH = 830°C - pH = 6,545°C - pH = 850°C - pH = 7

Ca/

P m

olar

rat

io

Ripening time

Fig.1 : Ca/P molar ratio of as synthesized powders vs ripening time

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490 Bioceramics 15

� TCP one (the theoretical densities of the � - and

�-form are 2.86 and 3.07, respectively). The direct

consequence is the development of residual stresses within the material during the transformation that are detrimental to the mechanical reliability [9]. Hot pressing temperatures were chosen between 900°C and 1100°C for a dwell duration of 30 minutes. Table 1 gives the experimental conditions and the corresponding final densification ratio (determined from the Archimedean method) of the hot pressed materials.

Dwell time [min.]

Pressure [MPa]

Temperature [°C]

Average grain size [µm]

Densification ratio [%th d]

900 0.25 65.1 950 0.40 82.3 1000 0.50 97.0 1050 0.65 99.7

30 20

1100 1.25 99.9

Tab.1: Hot pressing conditions and resulted characteristics of sintered materials

These results show a constant increase of the density and the average grain size with temperature. Nearly fully dense materials can be prepared from 1050°C (τ = 99.7 %). X-ray diffractometry allowed to verify the only presence of

� TCP. For low temperatures (900°C, 950°C), SEM analyses

revealed a coalescence of grains and the presence of porosity within a quite homogeneous microstructure. This porosity disappears at higher temperatures but a noticeable grain growth occurs resulting in a larger grain size distribution. Compared with hydroxyapatite hot pressed in the same conditions [9], grain growth is particularly important in TCP materials: HA presented a densification ratio of 97.8% and an average diameter of 0.20 µm. As numerous properties of calcium phosphate, the hot pressing behaviour of powders is directly in relation with the Ca/P molar ratio: the higher the ratio, the lower the grain growth. Moreover, non stoichiometric apatites powders, as it is the case of Ca9(HPO4)(PO4)5(OH), are known to coalesce at low temperature, microstructures with important grains coarssening appears that limits the densification [9]. To overcome this undesirable evolution, powders preparation, and particularly the initial calcination that quickly transforms the apatite into

� TCP, appears helpful to densify the material. But, optimal

conditions to decrease grains coalescence during the initial calcination of powders should be established for further improvement of the microstructural design of dense TCP.

Strength and toughness of hot pressed materials The mechanical properties were investigated on dense hot pressed pellets of β TCP. Bending strength and fracture toughness are given in Fig.2 and Fig.3, respectively, as functions of the average grain size. The bending strength increases from 30 MPa (900°C - φ=0.25 µm) to 90 MPa (1100°C - φ=1.25 µm). This value is included in the range

0

10

20

30

40

50

60

70

80

90

100

0,25 0,4 0,5 0,65 1,25Average grains diameter (µm)

Be

ndin

g s

tre

ngth

(M

Pa

)

0

10

20

30

40

50

60

70

80

90

100

De

nsification ratio (%

th d)

Bending strength τ

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0,25 0,4 0,5 0,65 1,25

Average grains diameter (µm)

To

ughn

ess

(M

Pa

.m1

/2)

0

10

20

30

40

50

60

70

80

90

100

De

nsificatio

n ratio

(%th d)

Toughness τ

Too porous materials

Fig.2: Fracture strength of hot pressed materials and densification ratio versus grains diameter

Fig.3: Toughness of dense materials and densification ratio versus grains diameter

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Key Engineering Materials Vols. 240-242 491

given by the literature (80 MPa to 150 MPa [6]). Toughness evolution is less pronounced than the bending strength. The determined values of KIC are low, starting from 0.80 MPa.m1/2 up to 1.00 MPa.m1/2 (some materials were too porous to obtain correct mesures). This last value is characteristic of calcium phosphate apatites [6].

Conclusion

Syntheses at low temperature and neutral pH performed for a long ripening time allowed the reproducible synthesis of pure apatitic tricalcium phosphate Ca9(HPO4)(PO4)5(OH)2, providing a good control of the synthesis parameters is achieved. Hot pressing experiments showed, in the determined conditions (dwell of 30 min. under a pressure 20 MPa), that pure and nearly fully dense β TCP can be obtained at temperatures from 1050°C to 1100°C. The mechanical properties are in good accordance with those of calcium phosphate compounds: bending strength of 90 MPa and fracture toughness of 1.00 MPa.m1/2. Nevertheless they should be improved by an optimisation of the elaboration process with a particular attention to powders preparation to limit grain growth.

References

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Bioceramics 15 10.4028/www.scientific.net/KEM.240-242 Synthesis and Characterization of Beta Tricalcium Phosphate 10.4028/www.scientific.net/KEM.240-242.489

DOI References

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