Materials Chemistry and Physics 94 (2005) 333–341
Characterization, thermal stability and sintering of hydroxyapatite
powders prepared by different routes
Nasser Y. Mostafa ∗
Chemistry Department, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt
Received 4 October 2004; received in revised form 18 April 2005; accepted 10 May 2005
Hydroxyapatite (HAP) powder precursors have been used as starting material for biomedical applications, such as synthetic bone graft
materials and scaffold for hard tissue engineering. Considering the numerous applications of hydroxyapatite, three different routes for HAP
powders preparations was investigated. Two powders were prepared by chemical precipitation reactions at 100 ◦ C and one by mechanochemical
reaction. The powders were characterized using chemical analysis, surface area measurements, laser diffraction, X-ray diffraction (XRD) and
SEM. The Ca/P ratios were varied from 1.67 to 1.58. The chemical composition, the crystallinity and the agglomeration characters depend on
the preparation route. The effect of powder characteristics on the sinterability was investigated. Although, the thermal stability and hence the
start of sintering dependents on the Ca/P ratio, the ﬁnal sintering density and hence the mechanical properties depends on the agglomeration
characteristics and the particle size distribution. Hydroxyapatite powder prepared by mechanochemical route have nano-sized crystallites with
a uniform smaller agglomerated particle size distribution and have a butter sinterability.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite; Mechanochemical synthesis; Agglomeration; Crystallinity; Sintering
1. Introduction of phosphate ions PO4 3− by hydrogen phosphate HPO4 2−
allows a continuous variation of the Ca/P atomic ratio
Hydroxyapatite [Ca10 (PO4 )6 (OH)2 ] is well known as between 9/6 and 10/6. This leads to calcium-deﬁcient hydrox-
the mineral component of bones and teeth. Thus, it has a yapatites, Ca10−x (PO4 )6−x (HPO4 )x (OH)2−x . Calcium-deﬁ-
considerable interest in dental and medical research [1–4]. cient hydroxyapatite powders can be precipitated from
Hydroxyapatite ceramics and hydroxyapatite–polymer conventional wet chemical methods and decomposed
composites are very promising for biomedical applications, into a mixture of HAP and tricalcium phosphate,
such as synthetic bone graft [5–7] and scaffold for hard tissue Ca3 (PO4 )2 (TCP) by thermal treatment above 700 ◦ C.
engineering [8,9]. Considering the numerous applications This allows a direct processing of biphasic calcium phos-
of hydroxyapatite in biomedical ﬁelds, numerous HAP phate ceramics HAP/TCP without the step of powder
synthesis techniques have been developed [2,10]. The most blending.
commonly used technique for the formation of HAP powder The main interest in the last years is not only control-
is the wet methods in aqueous solutions both by simple ling the stoichiometry of synthetic HAP but also controlling
preciptation method [11,12] or hydrolysis of acidic calcium the crystal size and shape and the agglomeration character-
phosphate salts [13–15]. Synthesis of HAP using wet method istics of the powder. Since these synthetic HAPs are further
is very complicated and needs a special attention to control processed to produce ceramics or incorporated with other
the Ca/P ratio as well as the crystallinity. The substitution materials (polymers) to produce biocomposites. Therefore,
a number of novel processing routes have been developed
∗ Tel.: +20 64 382216; fax: +20 64 322381. for preparing ﬁne hydroxyapatite powders, including sol–gel
E-mail address: firstname.lastname@example.org. syntheses [16,17], hydrothermal reactions [18,19], emulsion
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
334 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341
and microemulsion syntheses [20,21] and mechanochemical freshly boiled distilled water. H3 PO4 was dropped slowly to
syntheses [22–25]. the stirred suspension until a Ca/P ratio of 1.70 was reached.
Mechanochemical powder synthesis is a solid-state Boiling and stirring was continued for 2 h. The supernatant
synthesis method that takes advantage of the perturbation of liquid was removed. Two liters of water was added to the ves-
surface-bonded species by pressure to enhance thermody- sel and the suspension was boiled and stirred for 2 h to remove
namic and kinetic reactions between solids . The main the Ca(OH)2 . The resulting hydroxyapatite was washed
advantages of mechanochemical synthesis of ceramic pow- three times with water and ﬁnally with 0.001 M H3 PO4 .
ders are simplicity and low cost, which make it a valuable Calcium-deﬁcient hydroxyapatite (d-HAP) was prepared
method for industrial production of HAP powder. Equip- by dropping slowly and simultaneously with stirring aqueous
ments used for mechanochemcal synthesis are conventional solutions of (CH3 COO)2 Ca (0.1 M), and K2 HPO4 (0.06 M),
milling equipment, such as ball mills and vibratory mills. with a stoichiometric Ca/P molar ratio of 1.67, into 1 l of
Since the mechanochemical synthesis involves only boiling water. The precipitate was aged for 2 h at 100 ◦ C after
solid-state reaction, Suchanek et al.  distinguished dropping solutions, then left to cool down for 24 h. The ﬁnal
mechanochemical from the mechanochemical–hydrothermal pH was ﬁnd to be weak acidic (pH ∼5.0).
synthesis (wet mechanochemical). In the latter case, an The third hydroxyapatite was prepared through wet
aqueous phase incorporates in the system. The aqueous mechanochemical route (m-HAP) with Ca(OH)2 and
phase accelerates kinetic processes that commonly rate limits (NH4 )2 HPO4 as starting materials. Appropriate amounts of
a process, such as dissolution, diffusion, adsorption, reaction the two starting materials at a molar ratio of 1.67 were mixed
rate, and crystallization (nucleation and growth) . The together in a conventional ball mill with zirconia balls as
mechanochemical activation of slurries can generate local the milling medium and 100% distilled water. Millings were
zones of high temperatures (up to 450–700 ◦ C) and high carried out for 8 h with the rotating speed of 170 rpm in a
pressures due to friction effects and adiabatic heating of high-density polyethylene bottle. After milling, the slurry
gas bubbles (if present in the slurry), while the overall was ﬁltered under suction and washed with freshly boiled
temperature is close to the room temperature . The distilled water.
mechanochemical method has many advantages over the All the preparations of hydroxyapatite were dried under
hydrothermal methods, especially, on the industrial scale. N2 , at 105 ◦ C for 24 h then calcined at 600 ◦ C for 2 h. Pellets
Mechanochemical method can produce large amounts of specimens were molded into a stainless steel mold by hand
HAP powder at lower temperature, i.e. room temperature, as to produce specimens 13 mm in diameter and about 2 mm
compared to 90–200 ◦ C for the hydrothermal process. Thus, high, at a pressure of 200 MPa. The green pellets of about
for the mechanochemical–hydrothermal process, there is no 50–55% of the theoretical density were sintered in air for 1 h
need for a pressure vessel and external heating. at temperatures from 850 to 1200 ◦ C, with 50 ◦ C intervals.
The aim of this research is to characterize several HAP Specimens 13 mm in diameter and about 18 mm high were
powders prepared by different routes and comparing their molded, for compressive strength measurements. The heating
sintering behaviors. This is the primary step in a project to and cooling rates were limited to 300 ◦ C h−1 to prevent crack
investigate the utilization of these hydroxyapatites with poly- formation.
mers to produce scaffold for tissue engineering with different The green and sintered densities of compacts were
mechanical properties and different resorbation rates. Com- obtained from the measurement of dimensions and sample
positional, microstructural, morphological and mechanical mass. Relative densities were given as a percentage of the
characterizations were carried out on the powder as well as theoretical density of HAP, 3.16 g cm−3 .
on dense sintered bodies. X-ray diffraction (XRD) analyses were performed using
an automated diffractometer (Scintag Inc., Sunnyvale, CA),
at a step size of 0.02◦ , scan rate of 2◦ min−1 , and a scan range
2. Materials and methods from 4◦ to 60◦ 2θ. The average crystallite size of the precip-
itates was estimated by using the simple Scherrer equation:
Pure CaCO3 powder was heated at 1100 ◦ C for 4 h, and
the resultant CaO powder was used as it is or hydrated under kλ
nitrogen with a third equivalent quantity of distilled water to [β1/2 cos θ]
produce Ca(OH)2 . All the other chemicals were of analytical
grades. Two hydroxyapatite powders were prepared by ˚
where D is the size in A measured using reﬂection (0 0 2),
aqueous precipitation method. The precipitation was carried k the shape factor equal to 0.9, λ the wavelength of X-rays
out in a three-necked bottle equipped with a reﬂux condenser equal to 1.5418 A, θ the diffraction angle equal to 12.92 for
protected from the atmosphere by a CO2 -absorbing trap the reﬂection (0 0 2), and β1/2 is deﬁned as the diffraction peak
and ports for introducing N2 and the titrant. Stoichiometric width at half height, expressed in radians. The particle size of
hydroxyapatite (s-HAP) was prepared by titrating a boiling the powders was analyzed by using a laser diffraction particle
suspension of Ca(OH)2 with 0.5 M H3 PO4 . The Ca(OH)2 size analyzer (SHIMADZU), by suspending the powder in
suspension was prepared by adding 56 g of CaO to 3 l of water using sonication.
N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 335
The speciﬁc surface area measurements were performed
with BET method employing a Quantachrome (Monosorb)
Surface Area Analyzer. The Ca/P ratio was measured
by Inductively Coupled Plasma Spectrometry (ICPMS)
(JOBINYBON, mod. JY-38). A universal test machine
(Instron) was used for compression tests.
The powders and the fracture surfaces of the sintered
samples were investigated using SEM (HITACHI S-3500N)
equipped with secondary electron detector and EDX. All
samples were coated with gold.
3. Results and discussions
Fig. 1. XRD patterns of hydroxyapatite powders dried at 100 ◦ C for 24 h.
XRD patterns of HAP powders as shown in Fig. 1, revealed
no secondary phases besides hydroxyapatite. The Ca/P ratio
of the HAP powder is one of the most crucial parame- (57 nm) and d-HAP has the largest one. These results agree
ters in determining properties and thermal stabilities . well with the after-mentioned surface area measurements.
Chemical analysis results are presented in Table 1, it is d-HAP have the largest crystallites, since it was prepared
clear that the Ca/P ratio of s-HAP powder coincide well under acidic conditions. Many authors [31,32] found that the
with the theoretical values for stoichiometric hydroxyap- crystallite size increases with increasing the acidity of the
atite (1.67). Hydroxyapatite prepared at low pH (d-HAP) is precipitation medium.
highly Ca-deﬁcient (Ca/P = 1.58), and wet mechanochemical Fig. 2 shows plots of volume percent of particles sus-
route produces hydroxyapatite (m-HAP) with Ca/P ratio of pended in water over a range of particle diameter as deter-
1.65. Table 1 also shows the chemical formula corresponds to mined by laser diffraction particle size analyzer. Samples
the after-mentioned chemical analysis, with considering the s-HAP and m-HAP have a clear bimodal particle size distribu-
charge compensation of the deﬁcient calcium by substituting tion, while d-HAP shows a unimodal particle size distribution
PO4 3− with HPO4 2− and creating a vacancy on OH− site, (single maximum at about 15 m). The absence of particles
Ca10−x (PO4 )6−x (HPO4 )x (OH)2−x [2,13,14]. with diameters less than 0.2 m (Fig. 3) suggests there is no
The BET surface of HAP powders measured using N2 suspended single crystallites of HAP in all samples.
adsorption are given in Table 1. Mechanochemical route To test the effect of ultrasonic energy on the agglomer-
produces powder (m-HAP) with the highest surface area ation, the particle sizes were measured for each sample at
(86 m2 g−1 ). Hydroxyapatite prepared in acidic media d-HAP different sonication time up to 30 min. This gives no changes
have the lowest surface area (19 m2 g−1 ). in the particle size distribution of Fig. 2 for samples sonicated
Particle size in agglomerated or aggregated systems like for only 5 min. There also no particle sizes were determined
hydroxyapatite powders is one of the most important param- corresponding to the results of the XRD, i.e. agglomerates
eters controlling ceramic processing. In this study, particle are not broken by ultrasonic energy to the primary particles
size of the powders were investigated using XRD (Scherrer (crystallites).
equation) and laser diffraction. Each method of measure- Critical parameters describing all curves are shown in
ment senses a particular aspect of particle size. The proﬁle Table 2 and include particle-size range, particle size at
broadening by powder X-ray diffraction senses monocrys- maximum volume percent, and total volume percent under
talline domains. Laser diffraction method determines only a given peak. The two peaks of s-HAP powder bimodal
the agglomerated or aggregated particles in the suspended distribution were in the range of 0.6–0.9 and 0.9–45 m. It
solvent. was also calculated from the distribution that around 57% of
The mean crystallite sizes estimated by using the Scherrer the powder full in the micrometer size range (0.9–45 m).
equation for the (0 0 2) planes are shown in Table 1. The For m-HAP, around 86% of the powder consisted of particles
m-HAP has the smallest crystallites in the nanoscale size within 0.2–1.0 m and the other 14% of the mass was
Characteristics of hydroxyapatite powders
Sample Ca/P ratio Composition Crystallite size (nm) Surface area (m2 g−1 )
s-HAP 1.67 Ca10 (PO4 )6 (OH)2 141.3 48
m-HAP 1.65 Ca9.9 (HPO4 )0.1 (PO4 )5.9 (OH)1.9 57.4 86
d-HAP 1.58 Ca9.48 (HPO4 )0.48 (PO4 )5.48 (OH)1.48 211.9 19.5
336 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341
was 0.20 m (Fig. 3(a)). m-HAP powder agglomerates are
small in size than s-HAP agglomerates. It is obvious that the
agglomerate particles in case of m-HAP have lower particle
size than those of s-HAP. The typical volume of primary
m-HAP agglomerates, at 0.1–1.0 m, which agrees with the
laser diffraction results (Fig. 2).
Fig. 3(c) shows that d-HAP, which formed in acidic
medium had a rod-like shape with mean particle lengths of
2 m and diameters of 0.25 m. This ﬁgure shows clearly
the agglomeration of the primary crystallites with interparti-
cle bonds glow between them.
In general, the formation of agglomerated HAP particles
can be explained by nucleation–aggregation–agglomeration–
growth mechanism proposed by Randolph and Larson 
and Rodriguez et al. . According to this mechanism,
particle formation goes in the following steps: (a) nucleation
and growth to form crystallites in nanometric size range; (b)
aggregation of elemental nanocrystals by physical attrac-
tions; (c) further crystal growth, at a constant residual super-
saturation, acting as cementing agent inside the aggregates
to form stable agglomerate. These agglomerates continue
to grow as distinct particles and appear as submicrometer
particles in case of m-HAP and s-HAP. Finally, the increase
in particle size takes place by aggregation of these sub-
micrometer size agglomerated particles to form secondary
agglomerate in the micrometric size range (second bimodal
Gomez-Morales et al.  found that the aggregation
processes directed by different forces, in the nanomet-
ric/colloidal scale, surface free energy minimization would
be responsible for the aggregation, while electrical surface
charges would direct the micrometric scale particles aggre-
Fig. 2. Particle size distribution by laser diffraction. gation.
3.2. Sintering and thermal stability
particles of 1.0–25.0 m (Table 2). The agglomerate of m-
HAP is homogenous and most of it full in the submicrometer Understanding the sintering behavior of hydroxyapatite
range. This was conformed by SEM. powders is important, because this allows to design ceramics
The presence of two particle size ranges in case of m-HAP with controlled grain growth, microstructure and mechanical
and s-HAP powders possibly reﬂects the aggregation in the properties. In the present study, the effect of powder charac-
submicrometer sizes range, yielding the so-called primary teristics on densiﬁcation, microstructural development and
agglomerates (the ﬁrst peak). These agglomerates continue mechanical properties of HAP compacts were studied.
to aggregate by forming interparticle bonds until they reach Fig. 4 shows the effect of sintering temperature on the
the micrometer size range, the second peak (>1 m). densiﬁcation (percent of sintered densities with respect to
Morphological results from SEM investigation are shown theoretical density of HAP; 3.16 g cm−3 ) after cooling to
in Fig. 3(a–c). s-HAP and m-HAP powder consists of hard room temperature. The green density for s-HAP and m-
agglomerates which are composed of ﬁne crystallites. In case HAP are comparable and smaller than that of d-HAP. This
of s-HAP, the dominant observed agglomerated particle size is due to the nature of agglomerated particles of m-HAP
Mean parameters of particle size for hydroxyapatite powders
Sample First peak Second peak
Range ( m) Peak ( m) Volume (%) Range ( m) Peak ( m) Volume (%)
s-HAP 0.6–0.9 0.6 43 0.9–45 2.0 57
m-HAP 0.2–1.0 0.3 86 1.0–25 2.0 14
d-HAP 7–100 15 100 – – –
N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 337
Fig. 4. Densiﬁcation of hydroxyapatite compacts sintered at different tem-
900 and 1000 ◦ C, compared to between 950 and 1000 ◦ C for
m-HAP and between 1000 and 1050 ◦ C for stoichiometric
This is clearly shown in SEM micrographs of the fracture
surfaces of powder compacts sintered at 1000 ◦ C (see Fig. 5).
Compared with the as synthesized powder (Fig. 3) evidence
exists for a particle coalescence which associated with the
surface reduction . The particle coarsening occurred more
signiﬁcantly with samples with low Ca/P ratio. The necking
among the particles was apparent in Ca-deﬁcient samples (d-
HAP and m-HAP) due to localized sintering at 1000 ◦ C. At
this sintering temperature the particle size decreases as the
Ca/P ratio increases.
It is remarkable to notice that at 1000 ◦ C, sample s-
HAP shows a collapse of the large agglomerate (compare
Figs. 3(a) and 5(a)). In another investigation , the authors
ﬁnd, the calcination reduces the particle size and narrows the
The second stage of sintering corresponds to densiﬁca-
tion and the removal of most of the specimen porosity. The
onset of densiﬁcation as indicated by a sharp increase in the
sintered density, for d-HAP and m-HAP were started nearly
at 1000 ◦ C, whereas stoichiometric hydroxyapatite (s-HAP)
required higher temperature 1050 ◦ C.
For d-HAP, the maximum rate of densiﬁcation was
between 1050 and 1150 ◦ C. Comparing this behavior with
that of stoichiometric hydroxyapatite, calcium-deﬁcient HAP
sinters at much lower temperature, and the temperature of
maximum densiﬁcation decreases as the calcium deﬁciency
m-HAP achieves the highest ﬁnal sintered density of
Fig. 3. SEM micrographs of hydroxyapatite powders: (a) s-HAP; (b) m- 93.5% of the theoretical density at 1200 ◦ C, whereas d-HAP
HAP; (c) d-HAP.
and s-HAP reach only 89.4 and 76.8% respectively, at the
same sintering temperature. In conclusion, the start of densiﬁ-
and s-HAP, which are composed of nano-sized crystallites. cation depends on the chemical composition of the precursor
A small increase in density is observed before the onset of powder, i.e. Ca/P ratio. The ﬁnal density of the sintered bod-
densiﬁcation and this corresponds to the ﬁrst stage of sinter- ies mainly depends on the homogeneity and the particle size
ing, where necks are forming between powder particles. For distribution of the precursor powder. This conclusion is sup-
highly calcium-deﬁcient d-HAP, this process occurs between u
ported by G¨ ltekin and Oktar ; they found that the ﬁnal
338 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341
Fig. 5. SEM of hydroxyapatite sintered at 1000 ◦ C: (a) s-HAP; (b) m-HAP; Fig. 6. SEM of hydroxyapatite sintered at 1200 ◦ C: (a) s-HAP; (b) m-HAP;
(c) d-HAP. (c) d-HAP.
increase of sintering temperature improves the homogene-
limiting densiﬁcation level depend predominantly on the sur- ity and favors grain growth, resulting in an increase in mean
face area of the HAP powder and to a lesser extent on Ca/P grain size. The increase in grain size is also depending on the
ratio . homogeneity of the precursor powder.
SEM micrographs of the fracture surfaces of powder com- Sample m-HAP shows the isolated closed pores in the
pacts sintered at 1200 ◦ C are given in Fig. 6. In general, the matrix of uniform closed packed HAP grains. Samples
N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 339
Chemical composition and physicomechanical properties of sintered ceramics
Sample Ca/P molar ratio TCP (wt.%) HAP (wt.%) Porosity (%) Compressive strength (MPa)
s-HAP 1.67 0.0 100 23.2 58
m-HAP 1.65 9.3 90.7 6.5 108
d-HAP 1.58 50.1 49.9 10.6 87
s-HAP and d-HAP show larger pores in the range from 0.3
to 1 m size. These pores seem to be connected to form a
continuous network separated by thick walls. This may be
an advantage for the circulation of the physiological ﬂuid
when it is used for biomedical purposes but it is not useful
for the intergrowth of cells .
Comparing the ﬁnal density of sample m-HAP and s-HAP,
which have the same green density, leads to the conclusion
improved packing homogeneity leads to improved sinterabil-
ity in the form of higher sintered density at a given sintering
temperature and the ability to achieve high density with ﬁne
3.3. Compressive strength
Table 3 shows porosity and compressive strength of HAP
sintered compacts at 1200 ◦ C. It can be seen that the decrease
in porosity is accompanied by an increase in the mechanical
strength. Sample m-HAP, with improved packing homogene-
ity attains the highest compressive strength (1080 MPa) and Fig. 7. XRD patterns of hydroxyapatite powders compacts sintered at
the lowest porosity (6.5%). Stoichiometric HAP, s-HAP have 1100 ◦ C.
the lowest compressive strength (578 MPa) and the highest
where x is the calcium deﬁciency, and thus, Ca/P = (10 − x)/6.
The mole fraction of -TCP to the mole fraction of HAP
3.4. Thermal stability in the sintered bodies is given by: XTCP /XHAP = 3x/(1 − x).
Since, XHAP = 1 − XTCP, and XTCP /(1 − XTCP ) = 3x/(1 − x),
XRD patterns of sintered materials at 1100 ◦ C, as shown the percent of -TCP can be calculated by knowing
in Fig. 7, highlighted the difference in the thermal stability calcium deﬁciency (x) from the chemical analysis.
of HAP powders. HAP synthesized by mechanochemical Table 2 shows the Ca/P ratio and weight percent of -
method and from Ca(CH3 COO)2 precursor (in acidic TCP and HAP in the sintered compacts. Results agree
media) transformed partially into -tricalcium phosphate, - well with the semiquantitative analysis using XRD in
Ca3 (PO4 )2 , which related to the Ca-deﬁciency in the lattice Fig. 7.
of HAP powders. This agreed with the research of Yoshimura
et al. , in which he indicated that Ca-deﬁcient HAP tends
to transform to -Ca3 (PO4 )2 on heating at 900 ◦ C depending 4. Conclusion
on the deﬁciency of calcium. Stoichiometric hydroxyapatite
(s-HAP) with Ca/P ratio of 1.67 is stable up to 1200 ◦ C. The present study compares the powder properties of
Raynaud et al.  proved the reliability of the Ca/P hydroxyapatite prepared by wet mechanochemical method
atomic ratio of the initial HAP powder calculated by with those prepared at 100 ◦ C using wet solution precipita-
determining the phase proportions in calcined biphasic tion methods. The wet mechanochemical method produces
calcium phosphates. Thus, the percent of -TCP and HAP in nanocrystallites of hydroxyapatite with homogenous ﬁne
the sintered bodies can be determined from the Ca/P ratio of particle size distribution with most of the agglomerated
the precursor HAP powder. Considering calcium-deﬁcient particles full on the submicrometer size range. This alters the
HAP change with thermal treatment to stoichiometric HAP sintering characteristics of hydroxyapatite powder compacts
and -TCP according to the following equation: by attaining high sintering density at lower temperature in
comparison with those prepared by wet solution precipita-
Ca10−x (HPO4 )x (PO4 )6−x (OH)2−x
tion method at 100 ◦ C. Hence, this preparation route could
→ (1 − x)Ca10 (PO4 )6 (OH)2 + 3x -Ca3 (PO4 )2 + xH2 O emerge as a simple synthetic route to produce HAP ceramics
340 N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341
with an enhanced characteristics, especially, for industrial References
 L.L. Hench, J. Am. Ceram. Soc. 81 (7) (1998) 1705.
The sintering behavior of HAP powders with different
 J.C. Elliott, Structure chemistry of the apatites and other calcium
characteristics, allows the identiﬁcation of deferent factors orthophosphates, in: Studies in Inorganic Chemistry, Elsevier, Ams-
effecting sintering behavior and density of the ﬁnal ceramics. terdam, 1994.
At the beginning of the sintering, below 1000 ◦ C, particle  F.H. Jones, Surf. Sci. Rep. 42 (2001) 75.
coalescence occurs without densiﬁcation or with little densi-  C. Rey, Calcium phosphate for medical applications, in: Z. Amjad
(Ed.), Calcium Phosphate in Biological and Industrial Systems,
ﬁcation. The particle coalescence usually associated with a
Kluwer Academic Publishers, Boston, 1998.
reduction of the speciﬁc surface area . The temperature, at  Y. Shikinami, M. Okuno, Biomaterials 20 (1998) 859.
which start of this coalescence occurs, decreases as the Ca/P  K.S. TenHuisen, R.I. Martin, M. Klimkiewicz, P.W. Brown, J.
ratio of the powder decreases. Nearly at the same temperature, Biomed. Mater. Res. 29 (1995) 803.
the calcium-deﬁcient hydroxyapatites dissociate into a mix-  K.S. TenHuisen, P.W. Brown, J. Biomed. Mater. Res. 28 (1994)
ture of stoichiometric hydroxyapatite and -tricalcium phos-
 D.W. Hutmacher, Biomaterials 21 (2000) 2529.
phate. Stoichiometric; s-HAP (Ca/P = 1.67) is stable upon all  S.H. Li, K. deGroot, P. Layrolle, Key Eng. Mater. 218–220 (2002)
the temperature range studied. This agrees with fact that the 35.
transfer of matter by superﬁcial diffusion is enhanced by the  A.D. Papargyris, A.I. Botis, S.A. Papargyri, Key Eng. Mater.
increase of vacancies and defects in the structure (i.e. by a 206–213 (2002) 83.
 Y. Liua, W. Wanga, Y. Zhana, C. Zhenga, G. Wanga, Mater. Lett.
decrease of the Ca/P ratio), i.e. the dissociation reaction helps
56 (2002) 496.
in activation the transfer of matter by superﬁcial diffusion  R.E. Rimana, W.L. Suchaneka, K. Byrappaa, C.W. Chena, P. Shuka,
. C.S. Oakesa, Solid State Ionics 151 (2002) 393.
The densiﬁcation stage is seemed to be also assisted by  K.S. TenHuisen, P.W. Brown, Biomaterials 19 (1998) 2209.
the dissociation of calcium-deﬁcient HAP, since the start  R. Martin, P.W. Brown, J. Biomed. Mater. Res. 35 (1997)
of densiﬁcation for Ca-deﬁcient hydroxyapatite (d-HAP
 P.W. Brown, R.I. Martin, K.S. TenHuisen, Factors inﬂuencing the
and m-HAP) start at lower temperatures than stoichiometric formation of monolithic hydroxyapatite at physiological temperature,
hydroxyapatite (s-HAP). In this stage, the densiﬁcation is in: Biomedical and Biological Applications of Glass and Ceramics,
known to result from volume diffusion and grain boundary Am. Ceram. Soc. (1996) 37–48.
diffusion .  G. Bezzi, G. Celotti, E. Landi, T.M.G. La Torretta, I. Sopyan, A.
Tampieri, Mater. Chem. Phys. 78 (2003) 816.
Although, the start of densiﬁcation depends mainly on the
 T. Anee Kuriakose, S. Narayana Kalkura, M. Palanichamy, D.
Ca-deﬁciency (Ca/P ratio), the ﬁnal sintering density depends Arivuoli, G. Karsten Dierks, C. Bocelli, Betzel, J. Cryst. Growth
on the particle size, the homogeneity and the agglomeration 263 (2004) 517.
character of the powder precursor. Thus, the ﬁnal densiﬁ-  H. Hattori, Y. Iwadate, J. Am. Ceram. Soc. 73 (1990) 1803.
cation ratio of sintered materials was maximum for m-HAP  M. Yoshimura, H. Suda, K. Okamoto, K. Ioku, J. Mater. Sci. 29
at 93.5% of the theoretical density. It was slightly lower, at
 M.G. Murray, J. Wang, C.B. Ponton, P.M. Marquis, J. Mater. Sci.
89.4% in case of stoichiometric hydroxyapatite and very low; 30 (1995) 3061.
76.8% in case of highly deﬁcient hydroxyapatite (d-HAP),  G.K. Lim, J. Wang, L.M. Gan, Mater. Lett. 30 (1996)
i.e. m-HAP, which was produced by wet mechanochemical 431.
route and has a nanocrystallites with homogenous agglom-  W. Kim, Q. Zhang, F. Saito, J. Mater. Sci. 35 (2000)
erated particles, attains the highest densiﬁcation. These
 K.C.B. Yeong, J. Wang, S.C. Ng, Biomaterials 22 (2001)
results are in partial disagreement with the result of Raynaud 2705.
et al. [40,41], who found the densiﬁcation decrease with  S. Rhee, Biomaterials 23 (2002) 1147.
increasing the -TCP contents in HAP/TCP sintered bodies.  B. Yeong, X. Junmin, J. Wang, J. Am. Ceram. Soc. 84 (2001)
Thus, they concluded, the lower sinterability of calcium- 465.
 E. Gutman, Mechanochemistry of Materials, Cambridge International
deﬁcient HAP in comparison with the stoichiometric
Science Publishing, Cambridge, UK, 1997.
HAP.  W.L. Suchanek, P. Shuk, K. Byrappa, R.E. Riman, K.S. TenHuisen,
In conclusion, HAP ceramics prepared from mechano- V.F. Janas, Biomaterials 23 (2002) 699.
chemical synthesis powders were found to exhibit better den-  M. Yoshimura, W. Suchanek, Solid State Ionics 98 (1997) 197.
siﬁcation, uniform microstructure and improved mechanical  N.V. Kosova, A.K. Khabibullin, V.V. Boldyrev, Solid State Ionics
101–103 (1997) 53.
 M.A. Fanovich, J.M. Porto LoPez, J. Mater. Sci. Mater. Med. 9
 S. Suzuki, M. Ohgaki, M. Ichiyanagi, M. Ozawa, J. Mater. Sci. Lett.
17 (1998) 381.
Acknowledgements  K. Kandori, N. Horigami, A. Yasukawa, T. Ishikawa, J. Am. Ceram.
Soc. 80 (1997) 1157.
 A.D. Randolph, M.A. Larson, Theory of Particulate Processes, sec-
The author acknowledges the ﬁnancial support from HEC ond ed., Academic Press, NY, 1986.
(Egypt) and Dr. P. W. Brown (Pennsylvania State University) ı o o
 R. Rodr´guez-Clemente, A. L´ pez-Macipe, J. G´ mez-Morales, J.
for useful discussions. e n
Torrent-Burgu´ s, V.M. Casta˜ o, J. Eur. Ceram. Soc. 18 (1998) 1351.
N.Y. Mostafa / Materials Chemistry and Physics 94 (2005) 333–341 341
 J. Gomez-Morales, J. Torrent-Burgues, R. Rodriguez-Clemente,  B. Chang, C. Lee, K. Hong, H. Youn, H. Ryu, S. Chung, K. Park,
Cryst. Res. Technol. 36 (2001) 1065. Biomaterials 21 (2000) 1291.
 D. Bernache-Assollanta, A. Ababoua, E. Championa, M. Heughe- u
 G. G¨ ltekin, F.N. Oktar, Mater. Lett. 56 (2002) 142.
baertb, J. Eur. Ceram. Soc. 23 (2003) 229.  S. Raynaud, E. Champion, D. Bernache-Assollant, J. Laval, J. Am.
 N. Thangamania, K. Chinnakalib, F.D. Gnanama, Ceram. Int. 28 Ceram. Soc. 84 (2001) 359.
(2002) 355.  S. Raynaud, E. Champion, D. Bernache-Assollant, Biomaterials 23