Powder Metallurgy Progress, Vol.11 (2011), No 3-4 244

INFLUENCE OF THE SYNTHESIS CONDITIONS ON SINTERING AND PROPERTIES OF THE HYDROXYAPATITE – CALCIUM CARBONATE SYSTEM

M.A Goldberg, V.V. Smirnov, S.M Barinov, L.I. Shvorneva, N.V. Bakunova

Abstract Composite materials in the hydroxyapatite (HA) – calcium carbonate (CC) system were synthesized by the precipitation from aqueous solution followed by ripening in the mother solution for different times. The specific area of the powders hardly increased after ripening for 21 days and reached 238 m2/g for the composite powder of the lowest CC content. The use of highly active to sintering nanosized powders resulted in the lowering of sintering temperature down to 7000C, thus preventing thermal decomposition CO3

2--containing phases and crystallization of CaO. According to in vitro testing, the materials were non cytotoxic for human fibroblast cells Keywords: bio-materials, hydroxyapatite, calcium carbonate, ripening

.INTRODUCTION Materials based on hydroxyapatite (HA) – calcium carbonate (CC) system are

promising for bone tissue repair due to their controlled resorption rate [1]. HA is the calcium phosphate which is most resistant to resorption, whereas CC is the much more soluble biocompatible phase [2]. Varying their ratio, it becomes possible to control the resorption kinetics of the composite. The main technological disadvantage of the HA-CC system is related to the low-temperature decomposition of CC, accompanied by toxic calcium oxide formation. Furthermore, the calcium oxide presence may cause ceramic strength to decrease due to its interaction with CO2 and water vapor and an appearance of numerous cracks resulting in destruction of the sintering body. Due to thermal instability of carbonate containing phases, the sintering temperature should be decreased. One way to reduce the sintering temperature is the use of nanosized powders which are highly active in sintering [3]. This study was aimed at the synthesis of nanosize powders and at the thermal analysis of the CC– HA system to find an optimal sintering route.

EXPERIMENTAL According to reaction (1), materials containing 12, 24, 30, 42 and 48 wt. % of

carbonate groups or 20, 40, 50, 70 and 80 wt. % of a calcium carbonate content respectively (materials 1, 2, 3, 4 and 5 respectively) were synthesized by precipitation from aqueous solution followed by ripening for 1 and 21 days.

(10y+x)Ca(OН)2+6y(NH4)2НPO4+x(NH4)НCO3→ yCa10(PO4)6(OH)2+xCaCO3+(12y+x)NH4OH+(6y+x)H2O (1) The powders were investigated by X-ray diffraction (XRD) (Shimadzu XRD-6000

diffractometer, CuKα radiation) and Fourier transform IR spectroscopy (Avatar spectrometer, sample was the mixture of powders with KBr, mass ratio of KBr to powder

Margarita A Goldberg, Valeiy V. Smirnov, Sergey M Barinov, Ludmila I. Shvorneva, Natalia V. Bakunova, A.A. Baikov Institute of Metallurgy and Material Science Russian Academy of Sciences, Moscow, Russia

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 245 was 90/10). The powders were analyzed for carbon by oxidative melting (LECO CS400) and the specific surface area of the powders was measured by low temperature nitrogen adsorption (BET method, Micromeritics TriStar analyzer). Thermal stability was studied by differential scanning calorimetry (DSC), thermogravimetry (TG) (STA 409 Luxx system), and mass spectrometry (Aeolos quadrupole mass spectrometer). To produce ceramic samples, powders with a low melting point addition [1] were sintered at temperature 700-720°С in CO2 atmosphere. The microstructures were investigated with scanning electron microscopy (CrossBeam 1540EsB). The standard MTT test for cytotoxicity was performed using a human fibroblast model.

RESULTS AND DISCUSSION According to XRD data, the powders ripened for 1 day were of a low degree of

crystallization of apatite phase and mainly contained CC and amorphous HA (characteristic halo in the range 2θ = 31-33). As the content of carbonate groups increased, the intensity of the peak of CC increased (Fig.1a). The investigation of powders aged for 21 day revealed an increased crystallization degree of apatite phase with forming of carbonate-substituted HA (CHA) (Fig.1, b). According to the XRD analysis, there was recrystallization of CC with a decreasing of peak intensity and CHA forming, in fact, as materials were ripened in mother solution.

Fig.1 XRD pattern of powders aged for 1 (a) and 21 (b) days, + – apatite phase, * – CC.

The chemical analysis of CO32- content in powders showed that all materials aged

for 21 days had CO32- values close to calculated (Tab.1).

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 246 Tab.1. Composition of powders

Powder Parameter 1 2 3 4 5 Specified content of carbonate groups [wt %] 12 24 30 42 48 Content of carbonate groups of powders aged for 21 days, [wt%] 12.5 18.5 29.5 36 50

Fig.2. Fourier transform infrared spectra of as synthesized powders: * – ОН-, ^ – НОН, # –

РО43-,О –– СО3

2- of СaСО3, + – СО32- carbonated hydroxyapatite.

The IR spectra of all powders ripened for 1 day exhibit absorption bands for PO43-,

OH- and CO32- groups (Fig.2) [4,5]. As the powders were aged for 21 days, the intensity

of the PO43- group increased and the band close to 568 cm-1 formed a doublet at 563 and

587 сm-1 which is characteristic for crystallization of apatite phase. Furthermore, the formation of CHA was confirmed by the presence in the spectra of the characteristic bands for a CO3

2- at 1300-1650 (mode ν3) and 873-877 (mode ν2) cm-1. The specific surface area of powders aged for 1 day decreased from 87,23 to 23,1

m2/g (Table 2). After aging for 21 days, the specific surface area hardly increased and reached 238,7 m2/g for material containing 12 wt.% of СО3

2- and decreased to 24,6 m2/g for material containing 48 wt.% of СО3

2-. The growing of the specific surface area is the result of a decreasing of CC content by its recrystallization and forming high crystalline CHA. So the increase of specific surface area was as strong, as more CHA was formed.

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 247 Tab.2 Specific surface area of powders.

Powder Parameter 1 2 3 4 5

Specific surface area of powders aged for 1 day [m2/g]

87.2 82.1 39.6 30.3 23.0

Specific surface area of powders aged for 21 days [m2/g]

238.7 220.6 125.5 105.7 24.6

Fig.3. Differential scanning calorimetry (a) and thermogravimetry (b) of powders aged for

21 days.

According to mass-spectroscopy data of powders ripened for 21 days, two extractions by thermal decomposition in air gases were H2O and СО2, whereas for the materials ripened for 1 day there was only СО2. For powders ripened for 21 days, according

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 248 to DSC and mass-spectroscopy data, the first peak of mass loss was close to 100 ºC and corresponded to the removal of adsorbed water (Fig.3). The materials with low CC content (materials 1-2) also had a low temperature peak of thermal decomposition of CO3

2-, at average 680-700 ºC. The materials with higher CO3

2- content showed a shift to higher temperatures and decomposition started at about 720-730 ºC due to the increase in particle size. The mass loss of powders rises with an increase of CO3

2- content in the materials. The thermal decomposition of powders, ripened for 1 day, started at the same temperature for materials with low CC content and at 720-750 ºC for materials 3-5.

Powders were mixed with modify additive, mass ratio was 5 wt. % of additive to 100 wt. % of powders and samples were made at 100 MPa using unaxial pressing [1]. The sintering of composite materials HA-CC was carried out with a liquid phase, which was formed by melting the modify additive containing carbonate ions. These additives stabilized phase composition in the sintering process by an increase of the carbonate-ions concentration in liquid phase, preventing decomposition of CC at temperatures up to 600°С. It was found that optimal sintering technology allowed obtaining non-porous ceramics using powders ripened for 1 day and includes CO2 furnace atmosphere and a temperature of 720°С. However, the materials ripened for 21 days started to sinter at lower temperatures, and dense ceramics were produced at 700°С. According to SEM data, materials ripened for 21 days and sintered at 700°С had the same microstructure as materials ripened for 1 day and sintered at 720°С. It was shown that the grain size of materials with low CC content was 100-200 nm and increased to 500-1000 nm with an increase of CC content (fig.1). The porosity of ceramics is close to zero. These bioceramics were characterized as two phase composites containing CHA and CC.

Fig.4. Microphotographs of bioceramics of materials 1(a) and (5) aged for 21 days.

The tests in vitro of sintered granules have shown biocompatibility of all materials. The samples were non toxic for cells and support adhesion, spreading, and proliferation of human fibroblasts.

CONCLUSIONS Ripening in mother solution for 21 days after the powders` synthesis in the system

hydroxyapatite – calcium carbonate allowed one to get nanosize powders of controlled phase ratio. The dissolution and recrystallization of CC, with forming precipitated carbonate substituted hydroxyapatite, resulted in an increase of specific surface area to 238 m2/g and the appearance of small highly crystallized particles. The sintering at 700°С of

Powder Metallurgy Progress, Vol.11 (2011), No 3-4 249 powders ripened for 21 days allowed one to fabricate non-porous ceramics without thermal decomposition.

This work was supported by the Russian Foundation for Basic Research (project

no11–08–00596a) and program Presidium of Russian academy of science P21.

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125, 2010, p. 339 [2] Chissov, VI., Sviridova, IK., Sergeeva, NS., et al.: Klet. Tekhnol. Biol. Med., 2008, no.

3, p. 151 [3] Pang, X., Bao, X.: J. European Ceram. Soc., vol. 23, 2003, p. 1697 [4] Rehman, I., Bonfield, W.: J. Mater. Sci. Mater. Med., vol. 8, 1997, p. 1 [5] Rey, C., Renugopalakrishnan, V., et al.: Calcif Tissue Int., vol. 49, 1991, p. 251