Calcination and sintering of hydroxyfluorapatite powders

s __ __ ll!!B SOLID STATE

ELSEVIER Solid State Ionics 101-103 (1997) 1357-1362

Calcination and sintering of hydroxyfluorapatite powders

N. Senamaud’, D. Bemache-Assollant”‘“, E. Champion”, M. Heughebaertb, C. Reyb

“Laboratoire de MatCriaux Ce’ramiques et Traitements de Surface, (IRA CNRS 320, 123. Av. A. Thomas, 87060 Limoges Cedex.

France

hLaboratoire de Materiaux, ENSCT, INPT-URA CNRS 445, 38, Rue des 36 Ponts. 31400 Toulouse, France

Abstract

A comparative study of calcination and sintering of hydroxyfluorapatite powders Ca,,,(PO,),(OH)J_, substituted at various fluorine ratios has been carried out. Calcination led to the decrease of the specific surface area by particles coalescence. Superficial diffusion and gaseous phase transport appear as the main mechanisms of surface reduction. For

densification a minimum was reached for x = 1 which corresponds to the minimum of FAP-HAP mixing enthalpy. Sintering is limited by the diffusion of hydroxide and fluorine ions.

Keywords: Fluorapatite; Hydroxyapatite; Sintering

Materials: Ca,,,(PO,),OH>F,_,

1. Introduction

Calcium phosphates, which are characterised by

their Ca/P atomic ratio, occupy an important place in bioceramics [l-3]. Among them, hydroxy-

apatite (HAP), with a chemical composition

Ca,,(PO,),(OH), close to that of the mineral bone,

represents a family of interesting compounds for orthopaedic applications, either in the form of coat-

ings, porous or dense parts [4]. The partial or complete substitution of hydroxide ions by fluorine ions yields to hydroxyfluorapatite (HFAP) or fluorapatite (FAP). In comparison with HAP, the

presence of fluorine is expected to induce better

biological properties [5]. Thus, it is thought to reduce the solubility of apatite in the physiological environ-

*Corresponding author. Fax: +33 5 55 45 75 86; e-mail:

bemache@unilim.fr

ment or to retard acidic corrosion when incorporated in dental enamel.

From a physico-chemical point of view, it has

been demonstrated in a previous study that hydroxide

ions are the rate limiting species in surface reduction

mechanisms of HAP [6,7]. The aim of the present work was to investigate the influence of fluorine

substitution in the HAP lattice on calcination and

densification processes of the materials. A first part is devoted to the decrease of specific surface area of

HAP, FAP and HFAP powders, according to the calcination temperature, a second one is dedicated to the sintering in air.

2. Materials and methods

The starting powders were prepared by a dilute

0167.2738/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.

PII SO 167-2738(97)00242-7

1358 N. Senamaud et al. I Solid State Ionics 101-103 (1997) 1357-1362

decomposition method. A solution containing 0.65 mol of (NH,),HPO, and a predetermined quantity of NH,F was added dropwise into a boiling solution of 1.0 mol Ca(NO,),. The pH was adjusted to 8-9 with a 20% ammonia solution. The resultant precipitates were filtered and washed with deionised water, and were dried at 80°C for 24 hours. The chemical compositions elaborated were Ca’,(P0,),(OH),F2_x with x = 0, 0.5, 1, 1.5 and 2. They were deter- mined by quantitative chemical analysis of F, Ca and P with EDTA titrating Ca, potentiometer test- ing F and calorimeter measuring P concentra- tions.

Specific surface areas of dried and calcined pow- ders or compacted samples were measured by the BET method (1 point method with argon, rapid surface analyser Micromeritics 2205). Two measure- ments were made for each sample and average value was retained. Replicate measurements agreed within 8%.

Phase analysis of the different products was carried out by powder X-ray diffraction (XRD) using Cu Ko radiation on a Siemens D5000 apparatus. The phases were determined from comparison of the registered patterns with the JCPDS reference data file.

Infra-red spectra (IRS) were recorded on a Bomen MB2 fourier-transform spectrometer with a resolu- tion of 2 cm-‘. 4 mg of powdered samples were mixed with 200 mg of KBr, then pressed in a 13 mm die.

Thermogravimetric analysis of the powders was performed at 1250°C for 10 min in atmospheric air (Setaram B85). The heating rate was 5°C min-‘.

For sintering analysis, powders were pressed in a cylindrical die (F = 13 mm) under a 150 MPa compressive stress. Linear shrinkage was determined by dilatometry (Se&ram TMA 92 dilatometer) using the same thermal cycle as the one used for thermog- ravimetry. To characterize the influence of fluorine content on the densification process, samples of the different compositions were treated at 1000°C for 10 min at heating and cooling rates of 20°C min - ’ . The relative density of the materials was measured by the Archimedean method in water. From crystallo- graphic data, theoretical densities of HAP and FAP were assumed to be 3.156 g cmm3 and 3.19 g cmm3, respectively.

3. Results and discussion

3. I. Characterization of the dried powders

The specific surface area of the dried powders are summarised in Table 1. XRD patterns of powders are given in Fig. 1 (for x = 0, 1, and 2). They showed that powders were single-phased. Only a shifting angle of all the diffraction peaks, from the pattern of pure HAP to the one of pure FAP, were observed for HFAP products. These results mean that partially substituted products were solid-solutions of HAP and FAP.

Infra-red spectra of these powders are presented in Fig. 2. Most of the vibrational bands of these spectra were characteristic of Ca,,(PO,),(OH),F,_, struc- tures. The bands observed at 3540 cm-’ and 630 cm-’ are assigned to OH stretching and wagging, respectively. Their intensity decreased as fluorine content increased (from x = 2 to x = 0). The PO:- group is characterised by various bands: 476 cm-‘, 962 cm-‘, around 1000 cm-’ and 2000 cm-‘.

However, residual species remained in the dried powders. Thus, a band at 1380 cm-‘, attributed to

Table 1

Specific surface area of Ca,,(PO,),(OH),F,_, powders

x 0 0.5 1 1.5 2

SP (m’s_‘) 20 28.5 29.5 40.5 34

30 35 40 45 50 55

Angle (2 theta degrees)

Fig. 1. XRD patterns of Ca,,(PO,),(OH),F,_x powders.

N. Semmaud et al. I Solid State Ionics 101-103 (1997) 1357-1362 1359

L

45cKJ 3500 2500 1500 500

Wave number (cm-‘)

Fig. 2. IR spectra of Ca,,(PO,),(OH)lF,_x powders.

residual nitrate ions resulting from the synthesis

process, was detected. In the same way, the presence of a band at about 880 cm-’ was attributed to traces of HPOi- or CO:- (also observed at 1430 cm-‘) ions. These ions would result from either an incom-

plete crystallization of the products or the presence of impurities.

3.2. Thermal treatments

3.2.1. Thermogravimetry and dilatometry

Typical curves of weight change versus tempera-

2 ‘i; -2- FAP

E z CI) -4- E ul 0 3 -6-

0 200 400 600 800 1000 1200

Temperature (“C)

Fig. 3. TGA profile of FAP and HAP powders.

observed, which proceeded in several stages. The

first one, below 250°C corresponds to residual water

departure. The second one, between 250°C and 500°C was due to the release of volatile products

resulting from the residual species previously men-

tioned. Indeed, it was confirmed by IR-spectra of

powders heat treated at 500°C (Fig. 4) which showed

that the bands corresponding to these species had strongly decreased in intensity or even disappeared.

In the case of HAP powder (Fig. 3), a third weight

Initial powder

-_

Powder calcined at 500°C

I 40@3 3ooo 2olM 1000

Wave number (cm”)

ture are given in Fig. 3. They showed that in dependence from the composition a weight loss was

Fig. 4. IR spectra of Ca,,(PO,),(OH)F powders.

1360 N. Senamaud et al. I Solid State Ionics 101-103 (1997) 1357-1362

loss was registered at higher temperature. It can be attributed to the dehydration of hydroxyapatite ac- cording to the reaction:

Ca,,(PO,),(OH),

~Ca,,(PO,),(OH),-,,0, + Y&O.

X-ray diffraction patterns of powders heated at

1250°C showed that there was no difference with the

dried powder for FAP. In the case of HFAP and HAP powders, peaks corresponding to tricalcium phos-

phate and tetracalcium phosphate appeared, due to the beginning of a decomposition process according to the general reaction:

Ca,,(PO,),(OH),

+ 2Ca,(PO,), + Ca,P,O, + H,O.

Therefore, as already mentioned in the literature [S],

FAP structure is thermally more stable than that of HAP or HFAP.

As very similar behaviours were registered for the

five compositions, only a typical plot of linear shrinkage and shrinkage rate versus temperature is

given on Fig. 5 (case of HAP compound). It dem- onstrates that sintering began only from about 800°C.

After the 1250°C treatment, all the samples had a

relative density between 93% and 95% of their

theoretical value, except for x= 1 were it was only of

89%.

3.2.2. Calcination

The evolution of specific surface area of powders versus calcination temperature was investigated in the 300°C to 800°C range. The resulting curves are

given in Fig. 6. Independent from the composition of the precursor, no significant change of the specific

surface area was observed up to 500°C. Then, the

surface decreased as the temperature was increased. Because densification mechanisms do not occur in

this temperature range, the surface reduction of the

powders during calcination may be attributed to particles coalescence. This grain growth could result from either superficial diffusion or gaseous phase

transport. In order to clarify the mechanisms involved,

samples of pure HAP and FAP were compacted under a compressive stress of 150 MPa and calcined

at 500°C and 700°C using the same conditions as in

the case of powders calcination. The values of

measured specific surface area are given in Table 2. At 500°C the decrease of the surface was greater

for precompacted samples than for powders, either in the case of FAP or HAP. Nevertheless, the difference

in the values was much smaller between compacted and powdered HAP than it was for FAP products. The surface diminished from about 30 m2g-’ for powders to 24.9 m*g-’ and 13.8 m*g-’ for HAP and

FAP compacts, respectively. At 7OO”C, no difference

in surface was observed between compacted and

powdery HAP (about 17 m*g-‘). On the contrary,

-25

-30, ,l.,,II,I.l,lll,,.I,I ., , .-0.2 0 200 400 600 600 1000 1200

Temperature (“C)

Fig. 5. Shrinkage and shrinkage rate versus temperature of HAP

precalcined at 500°C.

0 300 400 500 600 700 800

Temperature (“C)

Fig. 6. Evolution of the specific surface area of

Ca,,(PO,),(OH),F,_, powders as a function of calcination tem-

perature.

Table 2

N. Semmaud et al. I Solid State tonics 101-103 (1997) 1357-1362 1361

Specific surface area of calcined HAP and FAP

Compound HAP powder HAP compacted FAP powder FAP compacted

Calcination (“C) 500 700 500 700 500 700 500 700

SP (m*f ‘) 30.1 16.8 24.9 16.7 29.7 11.8 13.8 8.2

surface decrease remained more important in the

compacted FAP than in the corresponding powder (8.2 against 11.8 rn’g-‘).

These results indicate that contacts between grains

were involved in the mechanism of grain growth

during calcination of FAI? Thus, it could be assumed that grain growth occurs through a mechanism of

superficial diffusion. In contrast, for pure HAP,

contacts between grains had few influence on surface

reduction at 700°C which would rather be explained by a mechanism of gaseous transport above that temperature.

3.2.3. Sintering

As the sinterability of a powder depends on its

specific surface area, initial powders were first

calcined to reach a same specific surface area of

about 21 m*g-‘. Then, compacted samples were

sintered at 1000°C for 10 minutes. Calcination conditions, and relative densities obtained after sin-

tering are presented in Table 3.

Relative densities versus fluorine content are plotted in Fig. 7. Densification was at maximum for

lOOr

1 -119

0

-120

-123

These results are in agreement with the analysis of

densification mechanisms of HAP in which it was demonstrated that densification was governed by

hydroxide ions diffusion [lo]. The partial substitu-

tion of hydroxide ions induces a stabilizing effect which reduces the mobility of the remaining hy-

droxide ions and consequently affects the sinterabili-

ty of HFAP products. -124

40 , / r , , -125 0 0.5 1 1.5 2

Hydroxide content (mol.) 4. Conclusion

Fig. 7. Relative density of Ca,,(PO,),(OH),F,_x samples sintered This study allowed us to point out the influence of at 1000°C. (0: solubility product, from [9]). fluorine substitution on the thermal behaviour of

Table 3

Calcination conditions, specific surface area of the calcined

powder and relative density of Ca,,(PO,),(OH)xFz_, sintered

samples

x 0 0.5 I 1.5 2

Calcination (“C) no 650 650 700 750

SP (m’g _ ’ ) 21 21 22 22 23

Relative density (% d,,) 92.5 55.5 41.5 58.5 61

pure FAP (92.5% of the theoretical value) and a

minimum was observed for the compound Ca,,(PO,),(OH)F. Compared to HAP, the complete substitution of hydroxide ions by fluorine ions

conducted to a better sinterability, whereas partial

substitutions decreased it. The poorer sinterability of

HFAP materials indicates that the partial incorpora- tion of fluorine in the lattice induces interactions

which reduce species mobility. This hypothesis can

be confirmed by comparison with investigations

concerning the solubility of fluoride-apatite systems [9]. The curve representing solubility product (KS)

versus fluorine ions content (Fig. 7) is very similar to that of the relative density with a maximum value for

FAP and a minimum one for Ca,,(PO,),(OH)F, indicating that the non-ideality of HAP-FAP solid

solutions scales a maximum for x = 1.

1362 N. Senamaud et al. I Solid State Ionics 101-103 (1997) 1357-1362

Ca,,(PO,),(OH),F,_, compounds. We have shown that the calcination of the powders in the 500°C to 800°C lead to a decrease of their specific surface area without densification. The mechanisms responsible for grains coalescence in HAP powder are consistent with superficial diffusion below 700°C and gaseous phase transport between 700°C and 800°C. For FAP powder, superficial diffusion remains preponderant in the whole of the temperature range.

Above 800°C densification of the materials occurs but the sintering rate is not proportional to the substitution ratio of hydroxide ions. Partial substitu- tions decrease the sinterability and a minimum is reached for Ca,,(PO,),(OH)F. These results, in agreement with densification mechanisms controlled by OH- or F- diffusion, can be explained by interactions between these ions inducing a stabilizing effect which becomes maximum for x = 1. Finally, hydroxyfluorapatite is thermally more stable than pure HAP or FAP.

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