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Ceramics International 37 (2011) 65–71
www.elsevier.com/locate/ceramint
Synthesis and characterization of hydroxyapatite/b-tricalcium
phosphate nanocomposites using microwave irradiation
A. Farzadi *, M. Solati-Hashjin, F. Bakhshi, A. Aminian
Biomaterial group, Faculty of Biomedical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, 15875-4413 Iran
Received 29 May 2010; received in revised form 7 June 2010; accepted 18 July 2010
Available online 22 August 2010
Abstract
Microwave assisted synthesis method is a relatively new approach employed to decrease synthesis time and form a more homogenous structure
in biphasic calcium phosphate bioceramics. In this study, nanocrystalline HA/b-TCP composites were prepared by microwave assisted synthesis
method and, for comparison reason, by conventional wet chemical methods. The chemical and phase composition, morphology and particle size of
powders were characterized by FTIR, XRD and SEM, respectively. The use of microwave irradiation resulted in improved crystallinity. The
amount of hydroxyapatite phase in BCP ranged from 5% to 17%. The assessment of bioactivity was done by soaking of powder compacts in
simulated body fluid (SBF). The decreasing pH of the solution in the presence of b-TCP indicated its biodegradable behavior. Rod-like
hydroxyapatite particles were newly formed during the treatment in SBF for microwave assisted substrate synthesis. In contrast, globular particles
precipitate under same conditions if BCP substrates were synthesized using conventional wet chemical methods.
# 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Microwave processing; Hydroxyapatite; b-tricalcium phosphate; Chemical synthesis
1. Introduction
Bioceramics such as alumina, zircon, calcium phosphates
and bioglass have great importance in biological environments.
Research and development on such bioceramics have made
significant contributions to the health and quality of human life.
Such biomaterials can be used in the human body to substitute
for damaged segments of human skeleton system [1,2].
Calcium phosphates are broadly used in medicine due to the
apatite-like structure of enamel, dentin and bones known as
‘‘hard tissue’’ [3]. Furthermore, hydroxyapatite crystals with
chemical formula of Ca10(PO4)6(OH)2 and Ca/P ratio of 1.67,
can generally make up to 69% of the weight of the natural bone.
Hydroxapatite has a hexagonal structure and is the most stable
phase among various calcium phosphates. Hydroxyapatite is
stable in body fluid as well as in dry or moist air up to 1200 8C.
It does not decompose and has shown to be bioactive. The btricalcium phosphate (b-TCP), represented by the chemical
* Corresponding author. Tel.: +98 912 1403448.
E-mail addresses: a.farzadi@aut.ac.ir, arghavanfarzadi@gmail.com
(A. Farzadi).
formula of Ca3(PO4)2 with Ca/P ratio of 1.5, has also a
hexagonal crystal structure [1]. The biocompatibility and
similarity of calcium phosphates like hydroxyapatite and
tricalcium phosphate to the mineral composition of human
bone and teeth have made them suitable for substitution of
damaged segments of human skeleton system [2–6]. Bioactivity of calcium phosphate materials depends on many factors
during the synthesis procedure including precursor reagents,
impurity contents, crystal size and morphology, concentration
and mixture order of reagents, pH and temperature. Such
conditions are application specific and should be controlled by
synthesis preparation parameters [7]. As discussed before, HA
is stable in the body fluid while TCP is rather soluble. The
dissolution rate of HA in body fluid is too low but that of b-TCP
is too fast for bone bonding. Therefore biphasic calcium
phosphate consisting of HA and TCP can be used to control the
bioresorbability and achieve optimal results. Biphasic calcium
phosphate composites, BCP, consisting of HA and b-TCP have
many applications in human body [8–14]. There are many
techniques for production of such biomaterials including wet
chemical methods [10,15–17], hydrothermal processes [18–
21], solid-state reaction [22–24], and sol–gel synthesis [15,25].
The degree of success in preparing HA is significantly different
0272-8842/$36.00 # 2010 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
doi:10.1016/j.ceramint.2010.08.021
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A. Farzadi et al. / Ceramics International 37 (2011) 65–71
for each method [1]. These methods have some disadvantages
such as being time-consuming, having low quality control and
chemical contamination [2,26–28]. Microwave synthesis
methods can be used to limit and reduce such disadvantages
and be able to easily control the condition of production
process. By using microwave as an assisted method, the
powders can be produced faster by employing an improved and
efficient heat transfer throughout the volume [29–33]. It is the
aim of this research to investigate microwave assisted synthesis
of nanopowders for biphasic calcium phosphate composites and
characterize the resulting powders by XRD, FTIR and SEM
techniques. For the purpose of comparison, another set of
nanopowders was synthesized by conventional wet chemical
precipitation method.
2. Materials and methods
Calcium hydroxide (Merck) and orthophosphoric acid
(Merck) were used as the starting materials. HNO3 (Merck)
and NaOH (Merck) were used to control the pH of the solution
mixture during the process. Two different synthesis methods
were used for preparation of the BCP powders: (1) Nanopowders
wet chemical, acid–base reaction [27] and (2) Microwave
radiation based chemical reaction [32]. To prepare BCP by acid
base reaction method, orthophosphoric acid solution was added
dropwise to the calcium hydroxide suspension under stirring
conditions. The suspension was centrifuged, dried out in an oven
at 90 8C overnight and calcined at 900 8C for 1 h. On the other
hand, to prepare the biphasic calcium phosphate by microwave
assisted synthesis, the solution mixture was immediately
transferred to a domestic microwave oven (2.45 GHz, 800 W)
and irradiated for 45 min. At the end of the irradiation, the white
precipitation was centrifuged and dried at 90 8C. The powders
were calcined at 900 8C for 1 h. The initial Ca/P ratios and the
conditions for preparation of BCP powders are given in Table 1.
We studied the effects of microwave irradiation on the growth of
hydroxyapatite particles on BCP samples by immersion in a
simulated body fluid (SBF). The SBF analysis was adopted from
Kokubo et al. who conducted similar experiments with a
simulated chemically prepared body fluid (SBF) solution. He
designed the solution to imitate human body fluid with ion
concentrations similar to those of the inorganic constituents of
human blood plasma to demonstrate the similarity between in
vitro and in vivo behavior of bioceramic compositions. Each liter
of SBF was prepared by dissolving of NaCl (7.996 g), NaHCO3
(0.350 g), KCl (0.224 g), K2HPO43H2O (0.228 g),
MgCl2.6H2O (0.305 g), CaCl2 (0.278 g), Na2SO4 (0.071 g)
and TRIS-C4H11NO3 (6.057 g) into distilled water. The pH value
of 7.25 was maintained by adjusting amount of HCl. Its
composition is comparable with the ionic composition of human
blood plasma. Also, the samples’ containers were placed in an
incubator to keep the temperature of the solution at 37 8C [34].
2.1. Sample characterization
Phase analyses of composite powders were determined by Xray diffraction (Siemens D 500 diffractometer, Cu-Ka radiation,
40 kV, 30 mA and 0.028 s1 step scan). Fourier transform
infrared spectroscopy (Vector 33) using pellets of powdered
samples mixed with KBr was performed to evaluate the
functional groups of specimens. The FTIR spectra were obtained
over the region 400–4000 cm1. The stoichiometry of the HA
was checked using differential thermal analysis (PL-STA 1640)
method at a heating rate of 10 8C/min between 25 and 1100 8C in
the air atmosphere to analyse the endothermic and exothermic
reactions. a-Alumina was used as the reference material.
Simultaneous thermogravimetric analysis was used to find the
weight loss during the heating using procedures similar to those
described above. Scanning electron microscopic analysis
(Philips XL30) was used for morphological observations. The
pH of the SBF solution was measured at predefined intervals (7,
14 and 21 days) of time after soaking powder compacts in SBF
under a controlled environment of 37 8C and pH 7.25. Finally, the
morphologies of hydroxyapatite crystals after immersion in SBF
solution were characterized using SEM.
3. Results and discussions
3.1. Phase characterization
The XRD patterns of samples are shown in Figs. 1 and 2.
[(Fig._1)TD$IG]Powders exhibited sharp diffraction peaks indicating a high
Table 1
The condition for preparation of BCP powders.
Sample
BCP1
BCP2
BCP3
BCP4
MBCP1
MBCP2
MBCP3
MBCP4
HA
HA-Riched-BCP
b-TCP-Riched-BCP
b-TCP
HA + Microwave
HA-Riched-BCP + Microwave
b-TCP-Riched-BCP + Microwave
b-TCP + Microwave
Initial
Ca/P
ratio
pH
Aging
time
1.67
1.54
1.53
1.51
1.67
1.54
1.53
1.51
11
9
8
6
11
9
8
6
24 h
10 h
6h
3h
45 min
45 min
45 min
45 min
Fig. 1. XRD patterns of BCP samples.
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[(Fig._2)TD$IG]
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A. Farzadi et al. / Ceramics International 37 (2011) 65–71
Table 3
The amount of crystallinity of HA and TCP powders.
Sample
2u8
Variable
Intensity
%Xc
BCP1
32.93
32.75
32.85
32.5
I300
V(112/300)
I300
V(112/300)
2950
600
503
95
79.66
MBCP1
Fig. 2. XRD patterns of MBCP samples.
crystallinity. The peaks of TCP and HA are indexed according to
standard patterns. The pattern for BCP1 in Fig. 1 shows wellcharacterized peaks of pure HA (hereafter BCP1 is called as pure
HA) and the peaks were indexed according to the standard pattern
(JCPDS 09-0432). The diffractograms of the samples BCP2 and
BCP3 show additional peaks rather than the HA peaks. The peaks
were identified to be corresponding to b-TCP and indexed
according to the standard value (JCPDS 09-0169). The intensity
of the additional peaks increases from BCP1 to BCP3 indicating
that the amount of b-TCP increases as the excess amount of the
phosphate solution is increased during the preparation procedure.
The sample BCP4 shows peaks of b-TCP and the Ca/P ratio of
this sample approaches the value of stoichiometric TCP. This
indicates that single phase b-TCP was successfully synthesized at
temperatures 900 8C [31]. The pattern in Fig. 2 indicates the two
phases of HA and TCP according to standard patterns (JCPDS
cards) and shows how easy it is to produce BCP powders by
microwave assisted method.
The percentages of volume fraction of b-TCP and HA present
in all the samples were calculated using the relative intensity ratio
(RIR), Eq. (1), for the HA/b-TCP in biphasic calcium phosphate,
where ITCP and IHA represent the normalized intensity of (2 1 1)
and (2 0 1 0) peaks of HA and b-TCP respectively [27,35] and
the results are given in Table 2. The behavior of dipole moment of
the hydroxyl ions in HA structure could be responsible for the
dielectric nature of hydroxyapatite. OH groups absorbed more
Table 2
Percentage of HA and b-TCP phases in BCP powders.
Sample
HA%
b-TCP%
BCP1
BCP2
BCP3
BCP4
MBCP1
MBCP2
MBCP3
MBCP4
100
64
42
0
100
75
44
13
0
36
58
100
0
25
56
87
81.11
Sample
2u8
Variable
FWHM
%Xc
BCP4
MBCP4
25.802
25.802
b(1010)
b(1010)
0.375
0.3125
26.21
45.3
microwave radiation that indicates the development of HA phase
[36–40]. The XRD and FTIR patterns also illustrate this result.
The crystallinity degree, corresponding to the fraction of
crystalline HA phase present in the examined volume, was
evaluated by the Eq. (2), where I300 is the intensity of (3 0 0)
reflection and V112/300 is the intensity of the hollow between
(1 1 2) and (3 0 0) reflections, that completely disappears in noncrystalline samples. For this method being sensitive to the
crystallite dimensions, verification can be done with the Eq. (3)
where K is a constant found to be equal to 0.24 for TCP powders,
and b002 is FWHM of reflection (0 0 2). Table 3 shows the
amount of crystallinity in HA and TCP powders by using both
Eqs. (2) and (3), respectively [41]. The crystallization process is
controlled by diffusion. It seems that by using microwave as an
external source, increasing temperature influences the rate of
atomic motion and decreases the diffusion barrier. So the atoms
are easily transported to the lattice site and the crystallinity is
increased.
RIR ¼
I bTCP
I bTCP þ I HA
X C ¼ 1 ðV 112=300 =I 300 Þ
pffiffiffiffiffi
bð1 0 1 0Þ 3 X c ¼ K
(1)
(2)
(3)
The broadening of a diffraction peak can be related to the
crystallite size via the Scherer equation relying on Eq. (4). The
(0 0 2) and (1 0 1 0) peaks of HA and b-TCP were chosen
respectively for the analysis of the broadening of the Bragg line
[42–44]. The crystallite size of hydroxyapatite and b-tricalcium
phosphate in BCP1, MBCP1, BCP4 and MBCP4 are given in
Table 4. These results clearly indicate that using microwave and
increasing temperature result in increasing crystallite size of the
powders. The lattice parameters of HA and b-TCP were
calculated using the XRD data. Lattice parameters of all the
samples were listed in Tables 5 and 6. Using microwave
irradiation leads to a slight increase in the a lattice parameter,
whereas the c lattice parameter does not show any significant
difference [32].
b¼
0:9l
t cosðuÞ
(4)
The FTIR spectra of BCP powders are shown in Figs. 3 and
4. The spectra illustrate the hydroxyl bond stretch at 3550 cm1
and HOP42 at 970 cm1 corresponding to HA and b-TCP
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A. Farzadi et al. / Ceramics International 37 (2011) 65–71
Table 4
The effect of synthesis condition on crystal size.
Sample
D (2u8)
FWHM
2u8
Cos(u)
Crystal size (Å)
BCP1
MBCP1
BCP4
MBCP4
0.5
0.375
0.375
0.3125
0.008726646
0.006544985
0.006544985
0.005454154
26.039
25.9
25.802
25.89
0.974293
0.974566
0.974757
0.974586
163.0673666
217.3623499
217.3196923
260.8295868
an external source, the endo peak at 700 8C shows the formation
of HA. The behavior of dipole moment of the hydroxyl ions in
HA structure is the root cause of dielectric nature of
hydroxyapatite that indicates the development of HA phase
by using microwave method [36–40].
structure respectively. The vibrational bands of the phosphate
ions are observed in all of the samples. As there is insignificant
amount of hydroxyapatite in BCP4 and MBCP4 samples, the
hydroxyl band disappears and forms a relatively broad band
stretch over the range of 3300–3800 cm1 due to the adsorbed
molecules of water. By increasing the amount of tricalcium
phosphate phase in BCP2, BCP3 and BCP4 samples, the
hydroxyl band at 633 cm1 reduces in peak area. The hydroxyl
vibrational band disappears in BCP4 sample [32]. The powders
prepared by microwave assisted method have the FTIR patterns
similar to the previous results [32]. Also the vibrational bonds
of OH decrease in spectra because of TCP forming. In
addition, the bond of HPO42 at 980 cm1 increases by
showing an increase in phosphate group forming TCP. The in
situ formation of BCP is confirmed by these FTIR spectra.
3.3. Morphological observation
[(Fig._3)TD$IG]
The scanning electron micrograph of BCP2, BCP3, MBCP2
and MBCP3 powders are illustrated in Fig. 7. They
demonstrate many agglomerations of small spherical particles
in nanometric scale, but samples produced with microwave
3.2. Thermal analysis
The TG/DTA analysis of BCP3 and MBCP3 powders are
shown in Figs. 5 and 6, respectively. The thermogram shows a
decreasing path that is related to the removal of the water from
the precipitated powders. The results show no exothermal peak
that indicates there is no decomposition reaction taking place in
the BCP3 sample. The XRD and FTIR patterns show that HA
and TCP phases are present in the powders and the DTA
indicates the in situ formation of BCP. By using microwave as
Table 5
List of lattice parameters (a and c) of HA.
Sample
a (Å)
c (Å)
BCP1
BCP2
BCP3
MBCP1
MBCP2
MBCP3
9.42
9.389
9.39
9.425
9.403
9.402
6.85
6.848
6.87
6.869
6.861
6.865
[(Fig._4)TD$IG]
Fig. 3. FTIR spectra of BCP samples.
JCPDS (09-0432): a (Å) = 9.418 and c (Å) = 6.884.
Table 6
List of lattice parameters (a and c) of b-TCP.
Sample
a (Å)
c (Å)
BCP2
BCP3
BCP4
MBCP2
MBCP3
MBCP4
10.401
10.38
10.41
10.428
10.3839
10.436
37.23
37.148
37.48
37.003
37.186
37.26
JCPDS (09-0169): a (Å) = 10.429 and c (Å) = 37.38.
Fig. 4. FTIR spectra of MBCP samples.
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[(Fig._5)TD$IG]
A. Farzadi et al. / Ceramics International 37 (2011) 65–71
69
shape of grains precipitating on the original grain surfaces
differed. In BCP3 sample that is prepared by acid–base
reaction, the HA crystals has spherical shape in SBF solution.
On the other hand, a precipitated HA rod and cubic like layer
formed on the surface of MBCP3 prepared by the microwave
assisted method. These results could be due to the effect of
microwave on the surface energy of powders that in turn
increases the suitable sites for nucleation.
[(Fig._6)TD$IG]
Fig. 5. TG/DTA graph of BCP3 powders.
[(Fig._7)TD$IG]
Fig. 6. TG/DTA graph of MBCP3 powders.
become fine and more homogeneous. The grain size measurement reveals that the grain size of powders produced with
microwave assisted method is around 43 nm to 55 nm and the
grain size of powders produced by acid–base reaction method is
around 57–67 nm.
The morphology of hydroxyapatite crystals after soaking in
the SBF is shown in Fig. 8. After immersion (21 days), the
3.4. In vitro assessment
The SBF solution was used to characterize the biodegradable
behavior of samples under a controlled environment of 37 8C and
pH 7.25 [45]. For this experiment sintered powder compacts were
employed. The pH values were recorded at regular time intervals
shown in Fig. 9. The extent of dissolution in SBF in vitro is much
higher for the b-tricalcium phosphate ceramic compared to that
of the hydroxyapatite ceramic. Thus, the extent of dissolution of
BCP composite depends on the HA/b-TCP ratio. However,
biphasic calcium phosphate composite with similar HA/b-TCP
ratios could present different dissolution rates depending on the
grain size, crystallinity and lattice defects. This phenomenon
may be caused by processing techniques such as using
microwave which could affect the crystallinity and microstructure [46]. The BCP2 sample shows a decrease in the pH
value but less than that of BCP3. This is due to the presence of
more tricalcium phosphate phase in the BCP3 sample converted
to a stable HA phase causing the release of acidic ion of HPO42.
Also, the sample prepared by microwave assisted method shows
less decrease in the pH value indicating its higher stability by an
increase in crystallinity [32,47].
Fig. 7. SEM images of BCP powders (a) BCP2, (b) MBCP2, (c) BCP3 and (d) MBCP3.
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[(Fig._8)TD$IG]
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A. Farzadi et al. / Ceramics International 37 (2011) 65–71
27 nm. Also, using microwave irradiation as an assisted
method increases the amount of HA phases in biphasic calcium
phosphate. In SBF, the pH of the solution was decreased with
the presence of b-tricalcium phosphate due to its biodegradable
behavior. So, the dissolution rate of the biphasic calcium
phosphate powders was strongly dependent on the b-TCP
content. The precipitated hydroxyapatite particles formed after
soaking in SBF solution have rod-like shapes once the powder
compacts produced by microwave assisted synthesis have been
employed. By adjusting the b-tricalcium phosphate percentage,
the bioresorbability of the calcium phosphate powders can be
controlled.
Acknowledgement
The authors gratefully acknowledge Dr. M. Schmuecker, Dr.
M. Mahyari, Dr. A. Mahyari and Sh. Shafiei-Nejad for their
valuable help in the preparation of the manuscript.
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Fig. 8. SEM images of prepared samples after soaking in SBF for 21 days (a)
BCP3 and (b) MBCP3.
[(Fig._9)TD$IG]
Fig. 9. The pH value variations of SBF due to the immersion of the powders.
4. Conclusion
By using microwave heating, biphasic calcium phosphate
powders can be prepared in situ in a shorter time compared to
traditional methods. The variation in the Ca/P ratio during the
preparation has resulted to the variation in the HA/b-TCP ratio.
The results showed that the use of microwave improves
crystallinity where the crystallite size ranged from 16 nm to
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