Morphological and microstructural changes during the heating of spherical calcium orthophosphate agglomerates prepared by spray pyrolysis

CHINA PARTICUOLOGY Vol. 2, No. 5, 200-206, 2004

MORPHOLOGICAL AND MICROSTRUCTURAL CHANGES DURING THE HEATING OF SPHERICAL CALCIUM ORTHOPHOSPHATE AGGLOMERATES PREPARED BY SPRAY PYROLYSIS Kiyoshi Itatani1,*, Mari Abe1, Tomohiro Umeda2, Ian J. Davies3 and Seiichiro Koda1 1 Department of Chemistry, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan Biomaterial Business Center, Mitsubishi Materials Corporation, 2270 Yokoze, Chichibu, Saitama Prefecture 368-8501, Japan 3 Department of Mechanical Engineering, Curtin University of Technology, GPO Box U1987, Perth, WA 6845, Australia *Author to whom correspondence should be addressed. E-mail: [email protected]

2

The microstructural changes taking place during heating of calcium orthophosphate (Ca3(PO4)2) agglomerates were examined in this study. The starting powder was prepared by the spray-pyrolysis of calcium phosphate (Ca/P -1 -1 o ratio=1.50) solution containing 1.8 mol·L Ca(NO3)2, 1.2 mol·L (NH4)2HPO4 and concentrated HNO3 at 600 C, using an air-liquid nozzle. The spray-pyrolyzed powder was found to be composed of dense spherical agglomerates with a mean o diameter of 1.3 μm. This powder was further heat-treated at a temperature between 800 and 1400 C for 10 min. When o the spray-pyrolyzed powder was heated up to 900 C, only β-Ca3(PO4)2 was detected, and the mean pore size of the spherical agglomerates increased via the (i) elimination of residual water and nitrates, (ii) rearrangement of primary particles within the agglomerates, (iii) coalescence of small pores (below 0.1 μm), and (iv) coalescence of agglomerates with diameters below 1 μm into the larger agglomerates. Among the heat-treated powders, pore sizes within the spherical o agglomerates were observed to be the largest (mean diameter: 1.8 μm) for the powder heat-treated at 900 C for 10 min. o With an increase in heat-treatment temperature up to 1000 C, the spherical agglomerates were composed of dense o shells. Upon further heating up to 1400 C, the hollow spherical agglomerates collapsed as a result of sintering via the o phase transformation from β- to α-Ca3(PO4)2 (1150 C), thus leading to the formation of a three-dimensional porous network.

Abstract

Keywords

spray-pyrolysis, calcium orthophosphate, hollow spherical agglomerates, heat-treatment, morphology,

microstructure

1. Introduction Calcium phosphates, e.g., hydroxyapatite (Ca10(PO4)6(OH)2; HAp) and calcium orthophosphate (Ca3(PO4)2), are well known to be suitable bone and tooth implant materials (Jarcho et al., 1979; Hench, 1998). For example, the porous form of HAp has been used as a bone substitute for biological fixation, whereas the dense form has applications including space filler for bioactive fixation (Hench, 1998; Metger et al., 1999). In the case of Ca3(PO4)2, two phases exist, namely α and β, that are used in different applications. For example, porous β-Ca3(PO4)2 ceramic has been used as a bio-resorbable material, i.e., permeation of cells/tissues into the pores and subsequent bio-resorption within the body (Metger et al., 1999), whereas α-Ca3(PO4)2 powder is a major component of calcium phosphate cement (CPC) for the repair of defects in living bones (Ooms et al., 2003). A main advantage of CPC is that it rapidly sets to a hard mass, which is highly biocompatible and gradually replaced by new bone in vivo. The present authors have investigated the properties of various kinds of powders prepared by spray-pyrolysis, i.e., the simultaneous spray-pyrolysis of solutions containing desired types and amounts of metal ions into the “hot zone” of an electric furnace. Powders prepared using this technique have the following characteristics: (i) homogeneous chemical composition and submicron-sized primary

particles due to flash thermal decomposition and solidstate reactions, (ii) strict control of the chemical composition provided that one can control the chemical composition of the starting solution, and (iii) formation of hollow spherical agglomerates, reflecting the outward form of starting droplets (Itatani & Aizawa, 2003). Through use of this technique we have examined the properties of calcium phosphate powder with different Ca/P ratios, such as HAp (Ca/P=1.67) (Itatani et al., 1988), Ca3(PO4)2 (Ca/P=1.50) (Itatani et al., 1994), Ca2P2O7 (Ca/P=1.00) (Aizawa et al., 1992), and Ca(PO3)2 (Ca/P=0.50) (Aizawa et al., 1992). While the presence of hollow spherical agglomerates were noted in these powders, no systematic information has so far been available regarding morphological and microstructural changes of the agglomerates due to heattreatment. The present paper investigates the effect of heat-treatment temperature on the morphology and microstructure of α- and β−Ca3(PO4)2 agglomerates, with the prospect that these hollow spherical agglomerates will be utilized as bioresorbable materials.

2. Methods and Results 2.1 Methods Preparation of powder The starting calcium phosphate solution (1 L), whose chemical composition corre-

Itatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Ca3(PO4)2 201 sponds to that of Ca3(PO4)2 (Ca/P ratio=1.50), was prepared -1 -1 using 1.80 mol·L Ca(NO3)2, 1.20 mol·L (NH4)2HPO4, and 120 mL of concentrated nitric acid. A schematic diagram of the spray-pyrolysis apparatus is shown in Fig. 1. The calcium phosphate solution, (a), was sprayed into the “hot o zone”, (c), of an electric furnace heated at 600 C, (d), using an air-liquid nozzle, (b). The spray-pyrolysis temperature was monitored using a chromel-alumel thermocouple, (e). The spray-pyrolyzed powder was collected using a test-tube type filter, (f), whereas the water vapor containing various salts was condensed using a Liebig condenser, (g). (e) (c)

(f)

(g)

mission electron microscope (TEM: Model JEM-2011, JEOL, Tokyo; accelerating voltage, 200 kV), together with electron diffraction analysis. Finally, pore sizes for the powder were measured using the nitrogen adsorption technique (Model BELSORP-mini, BEL Japan, Osaka), together with mercury porosimetry (Model AutoPore 9420, Micromeritics Instrument, Norcross, GA, USA).

2.2 Results and discussion Phase changes during heating of the spray-pyrolyzed powder First of all, the presence of phase changes during heating of the spray-pyrolyzed powder was examined using DTA-TG with results as shown in Fig. 2. The DTA curve contains two endothermic events, i.e., one event that occurred shortly after the commencement of o heating, while the other, weaker, event started at 1150 C. In contrast to this, the TG curve indicates step-wise mass o losses in the ranges of room temperature to 200 C and o 200 to 800 C, with no significant mass loss between 800 o and 1400 C.

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Schematic diagram of the spray-pyrolysis apparatus, wherein (a) solution, (b) air-liquid nozzle, (c) fused silica tube 1.5 m, (d) electric furnace 1 m, (e) thermocouple (chromel-alumel), (f) test-tube type filter, (g) Liebig condenser.

Heat treatment The spray-pyrolyzed powder was further heat-treated in air at a temperature between 800 and 1400 oC for 10 min; the heating rate from room temperao -1 ture up to the desired temperature was fixed at 10 C·min . Evaluation Phase identification of the powders was conducted using an X-ray diffractometer (Model RINT2100V/P, Rigaku, Tokyo; 40 kV, 40 mA) with monochromatic Cu Kα radiation, together with a Fourier-transform infrared spectrometer (FT-IR; Model 8600PC, Shimadzu, Kyoto) using KBr. The presence of phase changes during o heating between room temperature and 1400 C was examined using differential thermal analysis and thermogravimetry (DTA-TG; Model Thermo Plus TG8120, Rigaku, Tokyo), by using 25 mg of powder for each measurement. The agglomerate morphologies were observed using a scanning electron microscope (SEM: Model S-4500, Hitachi, Tokyo; accelerating voltage, 10 kV) from which the distribution of agglomerate diameters were determined from at least 200 individual agglomerates. The structure within the agglomerates was investigated using a trans-

Fig. 2

DTA-TG curves of the spray-pyrolyzed powder (heating rate: 10 oC min-1).

On the basis of the above information, phase changes that took place during heating of the spray-pyrolyzed powder were examined using XRD. Typical XRD patterns of heat-treated powders are shown in Fig. 3, together with an XRD pattern of the spray-pyrolyzed powder. Whereas the spray-pyrolyzed powder contains poorly crystalline β-Ca3(PO4)2 (JCPDS card, No. 9-169) and HAp (JCPDS card, No. 9-432) (Fig. 3(a)), the powder heat-treated at o 800 C for 10 min contains strongly crystallized β-Ca3(PO4)2 (Fig. 3(b)). On the other hand, the powder heat-treated at o 1150 C for 10 min contains α-Ca3(PO4)2 (JCPDS card, No. 29-359) and β-Ca3(PO4)2 (Fig. 3(c)), whereas heat-treatment o at 1200 C for 10 min resulted in the presence of only α-Ca3(PO4)2 (Fig. 3(d)). The presence of phases within the amorphous material was further examined using FT-IR. Typical results are shown in Fig. 4. The FT-IR spectrum of the spray-pyrolyzed powder (Fig. 4(a)) contains absorption peaks at -1 -1 -1 -1 1387 cm , 1094 cm (shoulder), 1040 cm , 972 cm , -1 -1 604 cm , and 569 cm . In contrast to this, FT-IR spectra of o o the powders heat-treated at 800 C (Fig. 4(b)) and 1000 C

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(Fig. 4(c)) for 10 min both indicates absorption peaks at -1 -1 -1 -1 -1 1120 cm , 1043 cm , 972 cm , 945 cm , 605 cm and -1 551 cm .

curring during heating of the spray-pyrolyzed powder may be divided into three stages: (i) the elimination of residual o water and nitrates (room temperature to 800 C), (ii) solidstate reactions in order to form β-Ca3(PO4)2, and (iii) transo formation of β- to α-Ca3(PO4)2 (1150 C). The transformation of β- to α-Ca3(PO4)2 may also be verified by dimensional changes of a β-Ca3(PO4)2 powder compact in previous work (Itatani et al., 1994). Morphological and microstructural changes of agglomerates due to heat treatment The spray-pyrolyzed powder was heat-treated at temperatures in the range of o 800 to 1400 C with the result that residual water and nitrates would be absent in these powders. Changes in the specific surface area with increasing heat-treatment temperature are shown in Fig. 5, together with typical morphologies. While the specific surface area of the spray-pyrolyzed 2 -1 2 -1 powder was 21.0 m .g , this decreased to 1.7 m .g with

o increasing heat-treatment temperature up to 1000 C. o Upon further heating to 1400 C, the specific surface area 2 -1 was reduced down to approximately 1 m .g .

Cu Kα

Fig. 3

X-ray diffraction patterns of the spray-pyrolyzed and heattreated powders, (a) spray-pyrolyzed; (b) heat-treated at 900 oC for 10 min; (c) heat-treated at 1150 oC for 10 min; (d) heat-treated at 1200 oC for 10 min. ●: β-Ca3(PO4)2; ○: α-Ca3(PO4)2; □: HAp.

Fig. 4

FT-IR spectra of the spray-pyrolyzed and heat-treated powders, (a) spray-pyrolyzed; (b) heat-treated at 800 oC for 10 min; (c) heat-treated at 1000 oC for 10 min.

Although the absorption peak at 1387 cm-1 is assigned to NO3− (Itatani et al., 1988), all the other peaks are assigned to β-Ca3(PO4)2 (Fowler et al., 1966). On the basis of the FT-IR, XRD and DTA-TG results, the phase changes oc-

The spray-pyrolyzed powder and the powders heattreated at 800 oC and 1100 oC for 10 min were composed of spherical agglomerates with diameters below 10 μm. In contrast to this, upon further heating to 1400 oC, no spherical agglomerates were noted, and instead a three- dimensional network containing pores with sizes in the range of 5 to 10 μm was observed. The appreciable morphological changes at temperao tures exceeding 1100 C may be attributed not only to the rapid sintering of primary particles but also to the transformation of β- to α-Ca3(PO4)2. On the basis of this information, changes in the morphology and microstructure of the spherical agglomerates heat-treated at temperatures between 800 and 1000 oC were examined, because the spherical morphology remains unchanged in this temperature range. SEM micrographs of powders heat-treated at 800 o to 1000 C for 10 min are shown in Fig. 6, together with a SEM micrograph of the spray-pyrolyzed powder. The spherical agglomerates in the spray-pyrolyzed powder (Fig. 6(a)) are composed of closely-packed primary particles with sizes below approximately 0.1 μm, whereas the o spherical agglomerates in the powder heat-treated at 800 C for 10 min (Fig. 6(b)) consist of polyhedral primary particles with sizes below 1 μm and pores (also below 1 μm). In contrast to this, the spherical agglomerates in powder o heat-treated at 900 C for 10 min (Fig. 6(c)) are comprised of primary particles with a size of approximately 1 μm, together with irregularly shaped pores with sizes in the range of 1 to 3 μm. Finally, spherical agglomerates in the o powder heat-treated at 1000 C for 10 min (Fig. 6(d)) are composed of primary particles with a typical size of 1 μm; the number of pores for this powder is much smaller, as compared to that of the powder heat-treated at 900°C for 10 min.

Itatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Ca3(PO4)2 203 30

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Changes in specific surface area of the spray-pyrolyzed powder with increasing heat-treatment temperature, together with typical SEM micrographs. (a)

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Typical SEM micrographs of the spray-pyrolyzed and heat-treated powders, (a) spray-pyrolyzed; (b) heat-treated at 800 oC for 10 min; (c) heat-treated at 900 oC for 10 min; (d) heat-treated at 1000 oC for 10 min.

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(b)

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TEM micrographs of the spray-pyrolyzed and heat-treated powders, together with typical electron diffraction patterns, (a) spray-pyrolyzed; (b) heat-treated at 800 oC for 10 min; (c) heat-treated at 900 oC for 10 min; (d) heat-treated at 1000 oC for 10 min (Arrows indicate sticking of small agglomerates to form large agglomerates.).

Typical TEM micrographs and diffraction patterns are shown in Fig. 7. No distinct porosity on the surfaces and inside of the spherical pores was observed in the spraypyrolyzed powder (Fig. 7(a)) with the electron diffraction pattern showing the presence of broad rings that indicate an amorphous structure. In contrast to this, the spherical agglomerates in powders heat-treated at 800 oC for 10 min (Fig. 7(b)) contain irregularly shaped pores; small agglomerates also stuck to the larger ones (see arrow marks). The spherical agglomerates in the powder heat-treated at 900 oC for 10 min (Fig. 7(c)) also contain irregularly shaped pores, while the electron diffraction pattern indicates a crystalline structure. Finally, the spherical agglomerates in powder heat-treated at 1000 oC for 10 min (Fig. 7(d)) appear to be hollow and composed of dense shells; these agglomerates stuck to one another. Agglomerate diameters for the spray-pyrolyzed and heat-treated powders were examined quantitatively, as shown in Fig. 8. The median diameter of the spray-pyrolyzed powder was 1.3 μm, almost identical to that of the powder heat-treated at 800 oC for 10 min. However, the mean diameter increased with further increases in the heat-treatment temperature. According to the TEM observation, agglomerates with diameters of 1 μm or less become coalesced to larger agglomerates (see Fig. 7(b)). Thus, the increase in ag-

glomerate diameter with increasing heat-treatment temperature may be ascribed to the coalescence of agglomerates. The coalescence of agglomerates is believed to have occurred as a result of active mass transfer, as demonstrated by the electron diffraction pattern which indicate a change from amorphous to crystalline structure (see Fig. 7(a) and 7(c)) and confirmed by the XRD results (see Fig. 3). Mass transfer appears to be promoted with increasing heat treatment temperature, as indicated by the sticking together of the larger agglomerates (see Fig. 7(d)). 30 30

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Diameter / μm Diameter / μm Agglomerate diameter distribution of spray-pyrolyzed and heat-treated powders, (a) spray-pyrolyzed; (b) heat-treated at 800 oC for 10 min; (c) heat-treated at 900 oC for 10 min; (d) heat-treated at 1000 oC for 10 min.

Itatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Ca3(PO4)2 205

3 -1 ?g -1 ΔV p∆V / ΔR mm3.?nm nm-1.g-1 p p/ /mm p/∆R

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Pore radius (Rp) distribution of spray-pyrolyzed and heattreated powders (nitrogen adsorption technique; Vp, pore volume), (a) spray-pyrolyzed; (b) heat-treated at 800 oC for 10 min; (c) heat-treated at 900 oC for 10 min; (d) heat-treated at 1000 oC for 10 min.

As the SEM observation indicates, pore sizes in the o spherical agglomerates of the powder heat-treated at 900 C for 10 min are the largest (1~3 μm) among the heat-treated powders (see Fig. 6(c)). The pore diameters of the agglomerates in this powder were also measured quantitatively using mercury porosimetry, as shown in Fig. 10 for o powder heat-treated at 900 C for 10 min, showing a bimodal distribution in the ranges of 0.1 to 4 μm and 10 to 400 μm. The mean pore diameter of this powder is 1.8 μm. (a)

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Moreover, pore radii of the agglomerates were measured quantitatively using the nitrogen adsorption technique, as shown in Fig. 9. The ∆Vp/∆Rp value (Vp and Rp represent pore volume and radius, respectively) along the ordinate indicates the specific pore volume per unit pore radius. Whereas most of the pore radii for the spray-pyrolyzed powder are distributed in the range of 0 to 0.075 μm (75 nm), o those in the powders heat-treated at 800 to 1000 C for 10 min are distributed in the range of 0 to 0.01 μm (10 nm).

As indicated in the TEM micrographs, no significant porosity is observed on the surface of the spherical agglomerates in the spray-pyrolyzed powder (see Fig. 7(a)). In fact, the porosity was distributed over a range of small pore sizes, i.e., 0 to 0.075 μm (75 nm). On the other hand, the pores with diameters in the range of 0.1 to 4 μm are preo sent in the powder heat-treated at 900 C for 10 min. These pores may be formed by the coalescence of smaller pores. The pores with the diameters in the range of 10 to 400 μm, which are detected by mercury porosimetry, indicate the presence of pores not only in the spherical agglomerates and those among the agglomerates that coalesce together. As the present data indicate, the spray-pyrolyzed powder contains spherical agglomerates with closely-packed primary particles. The morphological and microstructural changes of spray-pyrolyzed powder during heating are schematically illustrated in Fig. 11. The rearrangement of primary particles and the coalescence of pores occur (Fig. 11(a) → (b)), together with coalescence of smaller agglomerates to larger agglomerates. Following this, the median agglomerate diameter increases, together with the formation of shells in Fig. 11(c), and finally the formation of three-dimensional networks in Fig. 11(d). (c)

Pore

(d)

Shell

Schematic diagram of the changes in microstructure of the agglomerates upon heating, (a) spray-pyrolyzed powder: presence of closely-packed primary particles; (b) heat-treated powder: rearrangement of primary particles and coalescence of pores, and the coalescence of smaller agglomerates to larger agglomerates; (c) heat-treated powder: slight increase in mean agglomerate diameters and formation of shell structures; (d) heat-treated powder: formation of a three-dimensional network.

3. Conclusions

The morphological and microstructural changes taking

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place upon heating of spherical agglomerates of calcium orthophosphate (Ca3(PO4)2) prepared by spray pyrolysis were examined in this study. The results obtained were summarized as follows: (1) The spray-pyrolyzed powder contained poorly crystallized β-Ca3(PO4)2, hydroxyapatite (Ca10(PO4)6(OH)2), and other amorphous materials. Due to heat treatment of the spray-pyrolyzed powder, only β-Ca3(PO4)2 was present, following the elimination of residual water/nitrates and solid-state reactions. (2) The spray-pyrolyzed powder was composed of spherical agglomerates with a mean diameter of 1.3 μm. When the spray-pyrolyzed powder was o heated up to 900 C, the mean pore size of the spherical agglomerates increased, due to the rearrangement of primary particles within the agglomerates, coalescence of small pores below 0.1 μm, and coalescence of agglomerates with diameters below 1 μm into larger agglomerates. Pore sizes within the spherical agglomerates of the powders heat-treated o at 900 C for 10 min were observed to be the largest (average diameter: 1.8 μm) among the heat-treated powders. With increase in heat-treatment temperao ture up to 1000 C, the spherical agglomerates became covered by dense shells. (3) Upon heating from 1100 oC up to 1400 oC, the hollow spherical agglomerates collapsed due to sintering and also the transformation from β- to α-Ca3(PO4)2 (1150 oC), thus resulting in the formation of three-dimensional porous networks.

Acknowledgement The present authors wish to express their thanks to Dr. S. Suda of Japan Fine Ceramics Center for the measurement of pore diameter distribution using mercury porosimetry and Dr. Rob Hart (Curtin University) for help with TEM.

References Aizawa, M., Itatani, K., Miyamoto, Y., Kishioka, A. & Kinoshita, M. (1992). Properties of calcium metaphosphate and calcium diphosphate powders prepared by spray-pyrolysis technique. Gypsum & Lime, 237, 22-30. Baddiel, C. B. & Berry, E. E. (1966). Spectra structure correlations in hydroxy and fluorapatite. Spectrochim. Acta, 22, 1407-1416. Fowler, B. O., Moreno, E. C. & Brown W. E. (1966). Infra-red spectra of hydroxyapatite, octacalcium phosphate and pyrolyzed octacalcium phosphate. Arch. Oral Biol., 11, 477-492. Hench, L. L. (1998). Bioceramics. J. Am. Ceram. Soc., 81, 17051728. Itatani, K. & Aizawa, M. (2003). Fabrication of multi-functional ceramics by the utilization of spray-pyrolysis technique. J. Soc. Inorg. Mater. Japan, 10, 285-292. Itatani, K., Nishioka, T., Seike, S., Howell, F. S., Kishioka, A. & Kinoshita, M. (1994). Sinterability of β-calcium orthophosphate powder prepared by spray-pyrolysis. J. Am. Ceram. Soc., 77, 801-805. Itatani, K., Takahashi, O., Kishioka, A. & Kinoshita, M. (1988). Properties of hydroxyapatite prepared by spray-pyrolysis technique. Gypsum & Lime, No. 213, 19-27. Jarcho, M., Salsbury, R. L., Thomas, M. B. & Doremus, R. H. (1979). Synthesis and fabrication of β-tricalcium phosphate (whitlockite) ceramics for potential prothetic applications. J. Mater. Sci., 14, 142-150. Metger, D. S., Rieger, M. R. & Foreman, D. W. (1999). Mechanical properties of sintered hydroxyapatite and tricalcium phosphate ceramic. J. Mater. Sci.: Mater. Med., 10, 9-17. Ooms, E. M., Egglezos, E. A., Wolke, J. G. C. & Jansen, J. A. (2003). Soft-tissue response to injectable calcium phosphate cements. Biomaterials, 24, 749-757. Manuscript received June 8, 2004 and accepted July 20, 2004.