MORPHOLOGICAL AND MICROSTRUCTURAL CHANGES DURING THE HEATING OF SPHERICAL CALCIUM ORTHOPHOSPHATE AGG

CHINA PARTICUOLOGY Vol. 2, No. 5, 200 -206, 2004MORPHOL OGICAL AND MICROSTRUCTURAL CHANGESDURING THE HEATING OF SPHERICALCALCIUM ORTHOPHOSPHATE AGGLOMERATESPREPARED BY SPRAY PYROLYSISKiyoshi latani:*, Mari Abe' , Tomohiro Umeda2, lan J. Davies3 and Seichiro Koda''Department of Chemisty, Sophia Universit, 7.1 Kioicho, Chiyoda ku, Tokyo 102-85545 Japan.2Biomaterial Business Center, Mitsubishi Materials Corporation, 2270 Yokoze, Chichibu, Saitama Prefecture 368 8501, JapanDepatment of Mechanical Engineering, Curtin University ot Technology, GPO Box U1987, Perth, WA 6845, Austalia*Author to whom correspondence should be addressed. E-mail: itatani@sophia.ac.jipAbstract The microstructural changes taking place during heating of calcium orthophosphate (Cas(PO]2) agglom-erates were examined in this study. The starting powder was prepared by the spray-pyrolysis of calcium phosphate (Ca/Prati=1.50) solution containing 1.8 molL Ca(NOslz, 1.2 molL' (NH)HPO4 and concentrated HNOs at 600 °C, using anair-liquid nozzle. The spray-pyrolyzed powder was found to be composed of dense spherical agglomerates with a meandiameter of 1.3 um. This powder was further heat-treated at a temperature between 800 and 1400 °C for 10 min. Whenthe spray-pyrolyzed powder was heated up to 900 °C, only B-Cag(PO4)2 was detected, and the mean pore size of thespherical agglomerates increased via the () elimination of residual water and nitrates, (间) rearrangement of primary par-ticles within the agglomerates, (i) coalescence of small pores (below 0.1 um), and (iv) coalescence of agglomerates withdiameters below 1 um into the larger agglomerates. Among the heatreated powders, pore sies within the sphericalagglomerates were observed to be the largest (mean diameter: 1.8 um) for the powder heat-reated at 900 °C for 10 min.With an increase in heat- treatment temperature up to 1000。C, the spherical agglomerates were composed of denseshells. Upon futher heating up to 1400 °C, the hollow spherical agglomerates collapsed as a result of sintering via thephase transfomation from B- to aFCas(PO4)2 (1150 °C), thus leading to the formation of a three- dimensional porous net-work.Keywords spray-pyolsis, calciurm orthophosphate, hllow spherical agglomerates, heatreatment, mophology,microstructure1. Introductionparticles due to flash thermal decomposition and solid-state reactions, (i) strict control of the chemical composi-Calcim phosphates, e.g. hydroxyapatt (Caro(PO)e(OH)2i tion provided that one can control the chemical composiHAp) and calcium orthophosphate (Ca3(PO4)2), are welltion of the starting solution, and (i) formation of hollowknown to be suitable bone and tooth implant materials spherical agglomrates, rlecting the outward form of(Jarcho et al., 1979; Hench, 1998). For example, the po-starting droplets (ltatani & Aizawa, 2003). Through use ofrous form of HAp has been used as a bone substitute forthis technique we have examined the properties of calciumbiological fixation, whereas the dense form has applica-phosphate powder with dfferent Ca/P ratios, such as HAptions including space fller for bioactive fixation (Hench,(Ca/P=1.67) (tatani et al, 1988), Ca3(PO4)2 (Ca/P=1.50)1998; Metger et al, 1999). In the case of Ca3(PO4)2, two(tatani et al, 1994), CazP2O7 (Ca/P=1.00) (Aizawa et al.phases exist, namely a and B, that are used in difrent1992), and Ca(PO3)2 (Ca/P=0.50) (Aizawa et al, 1992).applications. For example, porous B-Ca3(POa)2 ceramicWhile the presence of hollow spherical agglomerates werehas been used as a bio-resorbable material, i.e, permea-noted in these powders, no systematic information has sotion of cells/tissues into the pores and subsequentfar been available regarding morphological and micro-bio-resorption within the body (Metger et al，, 1999),structural changes of the agglomerates due to heat-whereas arCa(PO4)2 powder is a major component of treatment. The present paper ivstigates the efecet ofcalcium phosphate cement (CPC) for the repair of defectsheat-treatment temperature on the morphology and micro-in living bones (Ooms et al, 2003). A main advantage ofstructure of a- and β-Cas(PO4)2 agglomerates, with theCPC is that it rapidly selts to a hard mass, which is highly prospect that these hllow spherical agglomerates will bebiocompatible and gradually replaced by new bone in vivo.tilized as bioresorbable materials.The present authors have investigated the properties of中国煤化工various kinds of powders prepared by spray-pyrlysis, i.e,2. Nthe simultaneous spray-pyrolysis of solutions containingMHCNMHGdesired types and amounts of metal ions into the "hot 2.1 Methodszone" of an electric furmace. Powders prepared using thisPreparation of powder The staring calcium phos-technique have the following characteristics: () homoge-phate solution (1 L), whose chemical composition corre-neous chemical composition and submicron-sized primarylItatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Ca3(PO4)2 201sponds to that of Ca3(PO4)z (Ca/P ratio=1.50), was preparedmission electron microscope (TEM: Model JEM-2011,using 1.80 molL:1 Ca(NO312, 1.20 molL:1 (NHa)2HPO4, and JEOL, Tokyo; accelerating vltage, 200 kV), together with120 mL of concentrated nitric acid. A schematic diagram ofelectron difraction analysis. Finally, pore sizes for thethe spray-pyrolysis apparatus is shown in Fig. 1. The cal- powder were measured using the nitrogen adsorptioncium phosphate solution, (a), was sprayed into the thottechnique (Model BELSORP-mini, BEL Japan, Osaka),zone", (C), of an electric fumace heated at 600 °C, (d), usingtogether with mercury porosimetry (Model AutoPore 9420,an ar-liquid nozzle, (). The spray-pyrolysis temperature was Micromeritics Instrument, Norcross, GA, USA).monitored using a chromel-alumel thermocouple, (@). The2.2 Results and discussionspray-pyrolyzed powder was collected using a test-tube typefiter, (), whereas the water vapor containing various saltsPhase changes during heating of the spray-pyro-was condensed using a Liebig condenser, (g).lyzed powder First of all, the presence of phase changesduring heating of the spray-pyrolyzed powder was(eexamined using DTA-TG with results as shown in Fig. 2.The DTA curve contains two endothermic events, i.e, oneC)event that occurred shortly after the commencement ofheating, while the other, weaker, event started at 1150 °C.In contrast to this, the TG curve indicates step-wise masslosses in the ranges of room temperature to 200 °C and200 to 800。C, with no significant mass loss between 800and 1400 °C←(d)1150"C3-4b)由a0 200400600800100012001400Temperature/9CFig.1 Schematic dagam o1 the saryoyis apeaus, wheren Fg. 2 DTA.TG aunes of the spyoyreyo powder (eating rate(a) solution, (b) air-liquid nozzle, (C) fused silica tube 1.5 m,10 °C min^').(d) electric fumace 1 m, (e) thermocouple (chromel-alumel), (I)test-tube type fiter, (9) Liebig condenser.On the basis of the above information, phase changesHeat treatment The spray pyrolyzed powder was fur-that took place during heating of the spray- pyrolyzedther heat-treated in air at a temperature between 800 andpowder were examined using XRD. Typical XRD pattems1400 °C for 10 min; the heating rate from room tempera-of heat-treated powders are shown in Fig. 3, together withture up to the desired temperature was fixed at 10 °Cmin*.an XRD pattern of the spray-pyrolyzed powder. WhereasEvaluation Phase identification of the powders wasthe spray-pyrolyzed powder contains pooly crystallineconducted using an X-ray dfractometer (ModelβCas(PO4)2 (JCPDS card, No. 9-169) and HAp (JCPDSRINT2100V/P, Rigaku, Tokyo; 40 kV, 40 mA) with mono-card, No. 9-432) (Fig. 3(), the powder heat-treated atchromatic Cu Ka radiation, together with a Fourier-transform800 °C for 10 min contains strongly crysalized βCa3(PO4)2infrared spectrometer (F T-IR; Model 8600PC, Shimadzu,(Fig. 3()). On the other hand, the powder heat-treated atKyoto) using KBr. The presence of phase changes during1150 °C for 10 min contains a~Cax(PO4)2 (JCPDS card, No.heating between room temperature and 1400 °C was ex 29-359) and BCa(PO4)2 (Fig. 3(), whereas heateatmentamined using dfrential thermal analysis and thermogra- at 1200 °C for 10 min resulted in the presence of onlyvimetry (DTA-TG; Model Thermo Plus TG8120, Rigaku,a-Ca(PO4)2 (Fig. 3().Tokyo), by using 25 mg of powder for each measurement.The presence of phases within the amorphous materialThe agglomerate morphologies were observed using a was further examined using FT-IR. Typical results arescanning electron microscope (SEM: Model S-4500, Hi-shown中国煤化工of the spray-pyro-tachi, Tokyo; accelerating voltage, 10 kV) from which thelyzed:sorption peaks atdistribution of agglomerate diameters were determined 1387 CrYHCNMHG40 cm*', 972 cm*",from at least 200 individual agglomerates. The structure 604 cm*, and 569 cm . In contrast to this, FT-IR spectra ofwithin the agglomerates was investigated using a trans- the powders heat-treated at 800。C (Fig.4(b)) and 1000 °C202CHINA PARTICUOLOGY Vol. 2, No. 5, 2004(Fig. 4() for 10 min both indicates absorption peaks at FT-IR, XRD and DTA-TG results, the phase changes oC~-1120 cm*', 1043 cm*', 972 cm*', 945 cm' , 605 cm' and curring during heating of the spray pyrolyzed powder may551 cm*'.be divided into three stages: () the elimination of residualwater and nitrates (room temperature to 800 °C), (i) solid-state reactions in order to form βCa3(PO4)2, and (i) trans-formation of β to a-Ca3(PO4)2 (1150 °C). The transforma-(dtion of β to a-Ca3(PO4)}z may also be verified by dimen-驾sional changes of a BCa(PO4)2 powder compact in pre-vious work (ltatani et al, 1994).Morphological and microstructural changes of ag-(Cglomerates due to heat treatment The spray-pyrolyzedpowder was heat-treated at temperatures in the range of800 to 1400。C with the result that residual water and nitrateswould be absent in these powders. Changes in the specificsurface area with increasing heat-treatment temperatureare shown in Fig. 5, together with typical morphologies.While the specitic surface area of the spray-pyrolyzedpowder was 21.0 m'.g"', this decreased to 1.7 m'.g' withincreasing heat-treatment temperature up to 1000 °C.，..o忠口o .dUpon further heating to 1400 °C, the specific surface area3040was reduced down to approximately 1 m*.g'The spray-pyrolyzed powder and the powders heat-2010 cu KaFig. 3 X-ray dfraction pattems of the spray-pyrolyzed and heat-treated at 800 °C and 1100 °C for 10 min were composedtreated powders, (a) spray-pyrolyzed; (b) heat-treated atof spherical agglomerates with diameters below 10 um. In900。C for 10 min; (c) heat-treated at 1150。C for 10 min; contrast to this, upon further heating to 1400 C, no spheri-(d) heat-treated at 1200 °C for 10 min.cal agglomerates were noted， and instead a three-●: BCax(PO4)2; o: a-Cax(PO4)z; o: HAp.dimensional network containing pores with sizes in therange of 5 to 10 μum was observed.The appreciable morphological changes at tempera-tures exceeding 1100。C may be atributed not only to thec)rapid sintering of primary particles but also to the trans-formation of B to a-Cas(PO4)2. On the basis of this informa-tion, changes in the morphology and microstructure of thespherical agglomerates heat-treated at temperatures be-tween 800 and 1000 。C were examined, because the10spherical morphology remains unchanged in this tempera-b)ture range. SEM micrographs of powders heat-treated at 800to 1000。C for 10 min are shown in Fig. 6, together with aSEM micrograph of the spray-pyrolyzed powder. Thea)spherical agglomerates in the spray-pyrolyzed powder(Fig. 6(a)) are composed of closely-packed primary parti-cles with sizes below approximately 0.1 μm, whereas thespherical agglomerates in the powder heat-treated at 800 °Cfor 10 min (Fig. 6(b)) consist of polyhedral primary particleswith sizes below 1 μm and pores (also below 1 μm). Incontrast to this, the spherical agglomerates in powder200015001000000heat-treated at 900 °C for 10 min (Fig. 6(C)) are comprisedWavenumber 1 cm1of primary particles with a size of approximately 1 μm,Fig. 4 FT-IR spectra of the spray-pyrolyzed and heatreated powders,together with iregularly shaped pores with sizes in the(a) spray-pyrolyzed; (b) heat-treated at 800 °C for 10 min;range中国煤化工agglomerates in the(c) heat-treated at 1000 °C for 10 min.powdeYH0 min (Fig. 6(d)) arecompaCNMHGtypicalsizeof1um;Although the absorption peak at 1387 cm' is assignedto the number of pores for this powder is much smaller, asNO3~ (ltatani et al, 1988), all the other peaks are assigned compared to that of the powder heat-treated at 900°C forto βCa(POx)z (Fowler et al, 1966). On the basis of the 10 min.Itatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Cas(PO4)2e 20330 厂25 t20 um。20↑15火囂120 μmQ1.T.900 100011001200 13001400Heat-treatment temperature /0CFig. 5 Changes in specific surface area of the spray-pyrolyzed powder with increasing heat-treatment temperature, together with typical SEMmicrographs.中国煤化工MHCNMHGFig. 6 Typical SEM micrographs of the spray-pyrolyzed and heat-treated powders, (a) spray-pyrolyzed; (b) heat-treated at 800 °C for 10 min;(C) heat-treated at 900 °C for 10 min; (d) heat-treated at 1000 °C for 10 min.204CHINA PARTICUOLOGY Vol. 2, No. 5, 2004Fig. 7 TEM micrographs of the spray-pyrolyzed and heat-treated powders, together with typical electron dftraction patterns, (日) spray-pyrolyzed;(b) heat-treated at 800 °C for 10 min; (C) heat-treated at 900。C for 10 min; () heat-treated at 1000 °C for 10 min (Arrows indicate sticking ofsmall agglomerates to form large agglomerates.Typical TEM micrographs and difraction patterns areglomerate diameter with increasing heat-treatment tem-shown in Fig. 7. No distinct porosity on the surfaces andperature may be ascribed to the coalescence of agglom-inside of the spherical pores was observed in the spray-erates. The coalescence of agglomerates is believed topyrolyzed powder (Fig. 7(a)) with the electron difractionhave occurred as a result of active mass transfer, aspattern showing the presence of broad rings that indicatedemonstrated by the electron difraction pattern whichan amorphous structure. In contrast to this, the spherical indicate a change from amorphous to crystaline structureagglomerates in powders heat-treated at 800 °C for 10 min (see Fig. 7(a) and 7(C)) and confirmed by the XRD results(Fig. 7(b)) contain iregularly shaped pores; small agglom-(see Fig. 3). Mass transfer appears to be promoted witherates also stuck to the larger ones (see arrow marks). The increasing heat treatment temperature, as indicated by thespherical agglomerates in the powder heat-reated atsticking together of the larger agglomerates (see Fig. 7().900 °C for 10 min (Fig. 7(C)) also contain iregularly shapedpores, while the electron difraction patterm indicates acrytalline structure. Finally, the spherical agglomerates in↑，,(间)b)、powder heat-treated at 1000。C for 10 min (Fig. 7(d)) ap-pear to be hollow and composed of dense shells; these/(C)agglomerates stuck to one another.y(d5FAgglomerate diameters for the spray-pyrolyzed andheat-treated powders were examined quantitatively, asoshown in Fig. 8. The median diameter of the spray-pyro-lyzed powder was 1.3 um, almost identical to that of theqpowder heat-treated at 800 °C for 10 min. However, thenean diameter increased with further increases in the中国煤化工1010heat-treatment temperature.According to the TEM observation, agglomerates withFig.JYHC NMH G of spyoyred andheat-treated powders, (间) spray-pyrolyzec) heat-tted atdiameters of 1 μm or less become coalesced to larger800 °C for 10 min; (C) heatrated at 900。C for 10 min;agglomerates (see Fig. 7()). Thus, the increase in ag-(d) heat-treated at 1000 °C for 10 min.ltatani, Abe, Umeda, Davies & Koda: Morphological and Microstructural Changes during Heating of Cas(PO4)2 205Moreover, pore radi of the agglomerates were meas-2.0 Fured quantitatively using the nitrogen adsorption technique,-0.3 .as shown in Fig. 9. The sVpORp value (V and Rp repre-←°sent pore volume and radius, respectively) along the.5 F→ordinate indicates the specific pore volume per unit pore-0.2radius. Whereas most of the pore radi for the.0 tspray-pyrolyzed powder are distributed in the range of0 to0.075 um (75 nm), those in the powders heat-treated at+0.1800 to 1000。C for 10 min are distributed in the range of 0to 0.01 um (10 nm).140.001 0.01 0.1 1 10 100 1000Pore radius 1 um12Fig. 10 Pore diameter distibution of powder heat-treated at 900 °C for110 min (mercury porosimetny).EAs indicated in the TEM micrographs, no significant po-6rosity is observed on the surftace of the spherical agglom-erates in the spray-pyrolyzed powder (see Fig. 7(). In fact,the porosity was distributed over a range of small poresizes, i.e., 0 to 0.075 μum (75 nm). On the other hand, the2pores with diameters in the range of 0.1 to 4 μm are pre-sent in the powder heat-treated at 900 °C for 10 min. These0.0010.010.pores may be formed by the coalescence of smaller pores.Pore radius I umThe pores with the diameters in the range of 10 to 400 μum,Fig. 9 Pore radius (R) distribution of spray-pyrolyzed and heat-which are detected by mercury porosimetry, indicate thetreated powders (nitrogen adsorption technique; Vp, porepresence of pores not only in the spherical agglomeratesvolume), (a) spray-pyrolyzed; (b) heat-treated at 800。C for 10and those among the agglomerates that coalesce together.min; (c) heat-treated at 900 C tor 10 min; (d) heat-treated at1000 °C for 10 min.As the present data indicate, the spray-pyrolyzed pow-der contains spherical agglomerates with closely-packedAs the SEM observation indicates, pore sizes in theprimary particles. Thmorphological and microstructuralspherical agglomerates of the powder heat-treated at 900 °Cchanges of spray-pyrolyzed powder during heating arefor 10 min are the largest (1 ~3 μm) among the heat-reatedschematically ilustrated in Fig. 11. The rearrangement ofpowders (see Fig. 6(). The pore diameters of the ag- primary particles and the calescence of pores occurglomerates in this powder were also measured quantita-(Fig. 11(a)→()), together with coalescence of smallertively using mercury porosimetry, as shown in Fig. 10 foragglomerates to larger agglomerates. Following this, thepowder heat-treated at 900。C for 10 min, showing a bi-median agglomerate diameter increases, together with themodal dstrbution in the ranges of 0.1 to 4 um and 10 to formation of shlls in Fig. 11(), and fnally the formationof400 μm. The mean pore diameter of this powder is 1.8 μum.three-dimensional networks in Fig. 11(d).ig. 11 Schematic diagram of the changes in microstructure of the agglomerates中国煤化工，-)owder: presence ofclosely-packed primary particles; (b) hear-treated powder. rearrangemente of pores, and thecoalescence of smaller agglomerates to larger agglomerates; (C) heat-treateMYHC N M H Gglomerate diametersand formation of shell structures; (d) heat-treated powder: formation of a three-dimensional network.206CHINA PARTICUOLOGY Vol. 2, No. 5, 20043. ConclusionsAcknowledgementThe morphological and microstructural changes takingThe present authors wish to express their thanks to Dr. S. Sudaplace upon heating of spherical agglomerates of calciumJapan Fine Ceramics Center for the measurement of pore di-orthophosphate (Ca3(PO4)2) prepared by spray pyrolysisameter distribution using mercury porosimetry and Dr. Rob Hart(Curtin University) for help with TEM.were examined in this study. The results obtained weresummarized as fllows:References(1) The spray-pyrolyzed powder contained poorly crys-Aizawa, M., Itatani, K, Miyamoto, Y. Kishioka, A. & Kinoshita, M.tllized BCa<(P04)2, hydroxyapatite (Ca1o(PO4)e(OH)2),(1992). Properties of calcium metaphosphate and calciumand other amorphous materials. Due to heat treat-diphosphate powders preparedby spray-pyrolysis technique.ment of the spray-pyrolyzed powder, only βCa3(PO4)2Gypsum& Lime, 237, 22-30.was present, following the elimination of residualry, E. E. (1966). Spectra structure crreltionsin hydroxy and fluorapatite. Spectrochim. Acta, 22, 1407-1416.water/nitrates and solid-state reactions.Fowler, B. 0., Moreno, E. C. & Brown W. E. (1966). Infra-red spectra(2) The spray-pyrolyzed powder was composed ofof hydroxyapatite, octacalcium phosphate and pyrolyzedspherical agglomerates with a mean diameter ofoctacalcium phosphate. Arch. Oral Biol, 11, 477-492.1.3 μm. When the spray-pyrolyzed powder wasHench, L L (1998). Bioceramics. J. Am. Ceram. Soc, 81, 1705-1728.heated up to 900。C, the mean pore size of theItatani, K. & Aizawa, M. (2003). Fabrication of multi-functionalspherical agglomerates increased, due to the rear-ceramics by the utilization of spray-pyrolysis technique. J. Soc.rangement of primary particles within the agglomer-Inorg. Mater. Japan, 10, 285- -292.ates, coalescence of small pores below 0.1 um, andItatani, K., Nishioka, T. Seike, S, Howell, F. S, Kishioka, A. &coalescence of agglomerates with diameters belowKinoshita, M. (1994). Sinterability of Bcalcium orthophosphatepowder prepared by spray-pyrolysis. J. Am. Ceram. Soc, 77,1 μm into larger agglomerates. Pore sizes within the801-805.spherical agglomerates of the powders heat-reatedItatani, K., Takahashi, 0., Kishioka, A. & Kinoshita, M. (1988).at 900。C for 10 min were observed to be the largestProperties of hydroxyapatite prepared by spray-pyrolysis(average diameter: 1.8 μm) among the heat-treatedtechnique. Gypsum & Lime, No.213, 19-27.Jarcho, M, Salsbury, R. L, Thomas, M. B. & Doremus, R. H. (1979).powders. With increase in heat-treatment tempera-Synthesis and fabrication of Btricalcium phosphate (witlockite)ture up to 1000 C, the spherical agglomerates be-ceramics for potential prothetic applications. J. Mater. Sci, 14,142-150.came covered by dense shells.(3) Upon heating from 1100。C up to 1400 °C, the hol-Metger, D. s, Rieger, M. A. & Foreman, D. W. (1999). Mechanicalproperties of sintered hydroxyapatite and tricalcium phosphatelow spherical agglomerates collapsed due to sinter-ceramic. J. Mater. Sci: Mater. Med, 10,9-17.ing and also the transformation from β to Ooms, E. M., Egglezos, E. A., Wolke,J. G. C. & Jansen, J A.a~Ca3(PO4)z (1150 °C), thus resuting in the forma-(2003). Soft- tissue response to injectable calcium phosphatecements. Biomaterials, 24, 749-757.tion of three-dimensional porous networks.Manuscript received June 8, 2004 and accepted July 20, 2004.中国煤化工MYHCNMHG