Synthesis and optimization of the production of millimeter-size
hydroxyapatite single crystals by Cl--OH- ion exchange
Esther García-Tuñon1,3, Jaime Franco2, Salvador Eslava3, Vineet Bhakhri3, Eduardo
Saiz3, Finn Giuliani3, Francisco Guitián1
1
Instituto de Cerámica de Galicia, Universidad Santiago de Compostela, Santiago de
Compostela, Spain
2
Keramat S. L., Spain
3
Centre for Advanced Structural Ceramics, Materials Department, Imperial College London,
London, United Kingdom
Corresponding Author: *Esther García-Tuñón
Instituto de Cerámica de Galicia, Universidad de Santiago de Compostela
Avda Mestre Mateo S/N, 15706 Santiago de Compostela (Spain)
*Current address: Imperial College London, CASC, London, United Kingdom,
+44(0)2075895111, esther.garcia-tunon@imperial.ac.uk
RECEIVED DATE
ABSTRACT
Millimeter-size hydroxyapatite single crystals were synthesized from chlorapatite
crystals via the ionic exchange of Cl- for OH- at high temperature. X-ray diffraction,
Fourier-transform infrared spectroscopy, and chloride content measurements were used
to follow the progress of this conversion, and to assess the effect of the experimental
conditions (temperature, time and atmosphere). Cl-OH- exchange took place
homogeneously and was enhanced by firing in wet air. After firing at 1425ºC for 2
hours 92% of the Cl- ions were exchanged by OH- while maintaining crystal integrity.
Temperatures above 1450ºC damaged the surface of the crystals, destroying the
hexagonal habit at 1500ºC. The composition of these apatite crystals was close to bone
mineral content. Their hardness (8.71.0GPa) and elastic modulus (12010GPa) were
similar to those of the starting chlorapatite (6.61.5GPa, and 11015GPa respectively).
However, their average flexural strength was 25% lower due to the formation of
defects during the thermal treatments.
1. Introduction.
Calcium orthophosphates are chemical compounds of wide interest in different fields of
research such as geology, chemistry, biology and medicine. Calcium apatites are the
main ore of inorganic phosphorous in nature. Hydroxyapatite (HA) is a calcium
orthophosphate with a chemical composition similar to the inorganic component of
bone and teeth. Bone serves as a structural support for the body and is the major
reservoir of calcium and phosphate ions necessary for several metabolic functions [1].
Bone is a composite material with a complex and hierarchical structure composed by
inorganic apatite mineral, type I collagen fibrils (organic part) and cells (osteoblasts,
osteoclasts and osteocytes) [2]. Synthetic calcium phosphate ceramics have been
successfully employed as bone replacement materials for more than 30 years [3, 4].
These materials have good biocompatibility and bioactivity and are being used in dental
and medical applications. It is also well known that calcium phosphate ceramics exhibit
relatively weak mechanical properties. They show low reliability and brittle behavior,
i.e. low Weibull modulus [5, 6], especially in wet conditions [6, 7]. The fracture
toughness of sintered hydroxyapatite and tricalcium phosphate materials are of the order
of 1 MPa·m1/2, while, for example, reported values for bone range between 2 to 12
MPa·m1/2 [8, 9]. Due to their poor mechanical performance, their application is limited
to non load-bearing applications. One possibility to overcome this could be the use of
high aspect ratio single crystal fibers as biocompatible reinforcement to obtain biocomposites [10, 9].
Also, large apatite crystals, unlike polycrystalline bulk materials, can be used to
determine intrinsic properties and analyze basic surface chemical and biological
processes. Millimeter-size crystals with a similar composition to the mineral component
of bone are the best substrate to perform basic dissolution studies [11-14], and to study
how extracellular matrix proteins adsorb and interact on their surfaces [15, 16]. They
can also be used to study the dependence of the mechanical properties depending with
crystal orientation [17].
To the best of our knowledge, large amounts of millimeter-size HA single crystals have
not yet successfully been obtained by using either the molten salts method or
hydrothermal synthesis. Following the molten salt approach several authors have
reported the synthesis of micro-sized whiskers of HA doped with K+ [18, 19] and
millimeter-size carbonate apatite (with formula Ca9.8[(PO4)5.6(CO3)0.4](CO3)) using
CaCO3 as flux [20], where the carbonates are allocated in both anionic channel and
phosphate positions [21]. Using the hydrothermal route several authors have obtained
micrometer-sized HA single crystals with needle, platelet, flake or whisker
morphologies [22-25]. Some hydrothermal and combined procedures to obtain
millimeter-sized apatite single crystals have been reported. Ito et al. obtained carbonatecontaining hydroxyapatite single-crystals with 4mm in length by CaHPO4 hydrolysis
[26]. The disadvantages of this approach are that only 4-8 large apatite single crystals
per experiment are obtained, the set-up is complicated and long times are needed (it
requires heating rates of 0.005ºC·min-1) [26]. Following a similar approach under
natural convection Teraoka et al. obtained Ca-deficient hydroxyapatite whiskers with
length up to 8.3mm [27]. Again, the main drawback is the low number of crystals
obtained per experiment and the time needed [27]. Given the difficulties in synthesizing
large HA single crystals, other authors have investigated the conversion of other apatites,
such as chlorapatite or fluorapatite into HA. Elliot and Young provided the first
evidence of the solid-state ion exchange in the apatite structure [28]. An alternative
procedure was proposed by Yanagisawa et al. and Rendón-Ángeles et al., who
investigated the ion exchange of Cl- and OH- with F- in synthetic calcium ClAp and
calcium hydroxyapatite single crystals under hydrothermal conditions [29, 30].
In this work we aim to synthesize and optimize the production of large HA single
crystals by delving into the time and temperature dependent evolution of the ionicexchange process (between Cl- and OH- species along the whole structured apatite
system) during the conversion of chlorapatite (ClAp) into HA following a similar
approach to that proposed by Elliot and Young [28, 31]. Starting from millimetre-size
ClAp single crystals obtained by the molten salt synthesis method [32] we follow the
conversion as a function of time, temperature and atmosphere and its effects on the
mechanical properties of the material. The analysis allows us to identify optimum
processing conditions to maximize the exchange and synthesize large HA crystals.
2. Experimental Procedure.
2.1. ClAp Growth.
Calcium chloride (Merck, PA, CaCl2·2H2O CAS 10043-52-4) and β-tricalcium
phosphate, (β-Ca3(PO4)2, β-TCP, D50=0.5m, >99wt%, Keramat®, Spain) powders
were mechanically blended in different proportions in an agate mortar (Restch, RM-
100, Germany) for 12 min. Prior to mixing the CaCl2 was dehydrated at 110oC for 2
weeks. Thermo-gravimetric analysis (PL-STA, Polymer Laboratories) of the dehydrated
flux showed that the remaining H2O percentage was 8wt%. Water was fully eliminated
at 300oC. After milling, the mixtures were uniaxially pressed at 60 MPa into cylindrical
pellets (2.54cm diameter) using a stainless steel dye. For the growth process, pellets
(12-15g per batch) were placed inside a platinum crucible and heated up to 1100oC for
30 min in a rapid melting furnace. The heating rate was 10oC·min-1 and the quenching
was done in air. After cooling, the sample was rinsed with deionized water to dissolve
the unreacted calcium chloride. The crystals were vacuum-filtered and washing was
repeated three times. Subsequently they were sieved trough a 63 µm mesh, and dried at
110ºC for 24 hours. The final samples consist mostly of millimeter size ClAp single
crystals with needle-shape and characteristic microscopic roughness [32].
2.2. Chlorapatite conversion to hydroxyapatite.
ClAp millimetre-size crystals were converted into HA by thermal annealing under
different atmospheres. For the first set of experiments, we used a lift furnace to anneal
the samples in air for 2 hours at temperatures ranging from 1300ºC to 1500ºC. For the
second set, an alumina tube furnace with moisture atmosphere control was used. A flow
of wet air was fed into the furnace during the annealing process to increase the humidity
and enhance the Cl-  OH- exchange. Synthetic air was pumped through a water
bubbler into the furnace (gas pressure 1.5MPa) and a second bubbler was placed in the
exit to monitor the flow during the thermal treatment. To study the effect of
temperature, the samples were fired for two hours at temperatures ranging from 1375ºC
to 1490ºC. A set of samples was also fired at 1425ºC for times ranging from 2 to 8
hours. All the samples were placed on alumina crucibles and heating and cooling rates
were set at 10ºC·min-1 for all the experiments.
2.3. Characterization.
The samples were analyzed by powder X-ray diffraction (XRD). In order to get the best
peak profile and minimize the preferred orientation effect, the samples were milled in a
zirconia ball mill and the powder was sieved through a 63μm mesh. The XRD analysis
(D5005, Siemens, Germany) was performed using Cu(Kα1) radiation (λ=1.5406Å). The
powder X-ray diffraction pattern was collected using a step size of 0.02° and counting
time of 4s per step. Single-crystal XRD measurements were also carried out as
described in Ref. [33].
The crystal morphology and chemical composition were analyzed by scanning electron
microscopy (SEM) on a JEOL 1640 equipped with an energy dispersive spectroscopy
(EDS) microprobe (INCA Sight Oxford-instruments, UK). The samples were also
characterized by Fourier transform infrared spectroscopy (FTIR Bomem, MB-100
series, USA), in transmission mode, in the range of 400-4000cm-1, and with a resolution
of 2cm-1. For this characterization, pellets were prepared by vacuum dye pressing
mixtures of 1mg of each sample with 100mg of dry KBr.
Chloride contents were measured by using an ion selective electrode (9617BNWP)
attached to Thermo Benchtop pH/ISE Model 720A. Standard solutions (0.1M NaCl,
100ppm, and 1000ppm Thermo) and an ionic strength adjustor solution (Thermo ISA)
were employed during the analysis. Measurements were performed on 0.5mg of each
sample dissolved in nitric acid solution (5wt%).
Room temperature (20 °C) nanoindentation tests were performed by using a Nano Test
instrument (Micro Materials Ltd. Wrexham, UK) on the millimeter-sized crystals
embedded in Bakelite resin. Samples were polished to 6 µm finish with SiC paper, to
obtain flat surfaces and avoid the effect of angled indentation [34]. During each
experiment, load was increased at a fixed rate of 1.25mN.s-1 to a predetermined
maximum value of 50 mN. The maximum load was then maintained for 60 s before
unloading at the original loading rate. Thermal drift contribution to the depth signal was
estimated by monitoring the depth signal over 30 seconds at 10% of the maximum load
during unloading and final indentation data was corrected for this estimated artefact. An
average of 80 indentations placed on 30 crystals per sample, were used to obtain the
elastic modulus and hardness. The inter-indent distance was chosen as ten times the size
of the residual impression (Berkovich shape). The hardness (H) and the reduced
modulus (Er) were calculated by using the procedure outlined by Oliver and Pharr
method [35, 36]. The values of elastic modulus (Es) were calculated using Er and
Poisson’s ratios of diamond (Ei=1141GPa, νi=0.07), chlorapatite and hydroxyapatite
(νi=0.27) [37-39].
All the load-displacement curves were checked and also the
morphology of the indents was observed by SEM, in order to identify possible sources
of error such us pile up or pop in among others.
Flexural strength of 20 single-crystals of selected samples was measured by three-point
bend test in a horizontal nanoindenter (Micro Materials Ltd. Wrexham, UK). The
single-crystals were mounted on the top of a custom-built aluminium holder with
0.5mm wide deep trenches. The crystals were subjected to bending with a constant load
of 200mN·s-1, using a diamond cone-spherical probe with a 25m tip radius. The
average diameter of the crystals analyzed was 50m, being the ratio of trench width to
crystal diameter 10.
Results and Discussion
Searching for an optimum environment to facilitate the anionic exchange in the ClAp
conversion to HA, we assessed the annealing under a wet atmosphere and compared it
to the conversion in air. Figures 1 and 2 show the evolution of the powder XRD patterns
and FTIR spectra of the single crystals under these two conditions for temperatures
between 1300 and 1500 ºC. Figure 1 includes the plots of the Le Bail refinements that
we perform in the samples annealed in air to determine how the unit cell parameters (a,
c and therefore dhkl) evolve with temperature. We highlight in grey the 2θ range (29º36º) that we use to follow the conversion by powder XRD in Figures 2a and 3a. In this
2θ interval, we can identify the main diffractions of the phases involved in the process
and therefore monitor the conversion. In both cases, in air and wet atmosphere (Figure
1a and 2a), powder XRD shows systematic peak displacements with temperature
towards the HA structure above 1450C [40]. There is also a secondary transformation
to a monoclinic calcium phosphate without Cl- or OH- in the structure, identified by
diffractions at 2θ=30.7 and 34.5º [33]. Comparison of the XRD pattern evolution in the
two different conditions (room and wet air) shows that the HA structure is achieved at
lower temperatures in wet air (Figures 1a and 2a). Thus, vapor accelerates the Cl- 
OH- exchange. FTIR spectroscopy confirms it, since OH- vibration bands at 630, 3570
and 3640cm-1 appear at lower temperature and with higher intensity in samples prepared
in wet air conditions (Figures 1B and 2B). A temperature of 1400ºC in wet air is enough
for a clear OH- incorporation in the apatite structure (Figure 2B). In view of these
results, firing at 1425 ºC in wet air was selected as the optimum condition to follow the
degree of conversion with time (Figure 3). FTIR spectra also show a small absorption
band in the vicinity of 1600cm-1, which may indicate the presence of CO32- in the
samples [41, 42]. Carbonate incorporations in the structure either in the anionic channel
or PO43- substitutions were ruled out by SXRD. This band may be justified by the
carbonation of spodiosite remaining on the surfaces of the ClAp from the fabrication
process [33, 40].
Chloride content measurements allowed us to quantify the conversion to HA. From this
measurements and considering the general formula Ca5(PO4)3Cl1-x(OH)x, we have
calculated the average hydroxyl incorporation in the structure, x. The analysis shows
that the Cl- content is close to saturation after firing for 2 hours (Fig 4A). The overall
reaction could be written:
Ca5(PO4)3Cl + xH2O(g)  Ca5(PO4)3Cl1-x(OH) x + xClH(g)
(1)
The saturation values of x increase with temperature and are higher for wet atmospheres
(Fig. 4B). When the experiments are carried out in room air, a limit on the conversion is
reached at 1450ºC, above which the chloride content of the samples remains unaltered
(Figure 4B). In wet air, however, the hydroxyl incorporation continues increasing above
1450ºC, reaching values close to the OH- content in HA above 1475 ºC (x=1, dashed
line in Figure 4B), which indicate a high conversion. In wet air values of x>0.9 are
reached after 2 hours at 1425ºC (Figure 4A). Therefore, a high conversion can be
achieved in wet air by different paths, either increasing the temperature or the time.
In addition to high conversion, we also aimed to obtain best mechanical properties,
while maintaining the quality of the crystals (in terms of quality of the data during
SXRD measurements), their apatite structure and avoiding damage to the surfaces.
Starting ClAp single crystals have a clear hexagonal habit and a characteristic
roughness on the surface (Fig 5). This roughness is formed during fabrication due to a
peritectic reaction between ClAp and the flux [32].
Thermal treatments have an effect not only on the ionic exchange but also on the
morphology of the crystals. SEM was used to inspect the surfaces and morphology of
transformed single crystals (Figure 6 and 7). Above 1050ºC the incongruent melting of
small amounts of spodiosite remaining on the crystal surfaces takes place according to:
Ca2(PO4) Cl  Ca5(PO4)3Cl + CaCl2
(2)
This leads to the formation of small CaCl2 deposits on the surfaces (Figure 6A). Thus
the decomposition of small amounts of spodiosite on the surfaces (equation 2), the recrystallization of chlorapatite (equation 2), and also the formation of small amounts of
HCl(g) in the furnace atmosphere (equation 1) promote changes in the morphological
features. The surface of the crystals presents more defects after the heat treatments
(Figure 6A-B) and the hexagonal habit is damaged at 1480ºC and above (Figure 6C).
The presence of defects increases with temperature and time in any conditions.
Hexagonal defects were spotted on the prism surfaces after treatment in room air at
1400ºC (Fig. 6B), while the hexagonal habit and surface of the crystals were damaged
at higher temperatures (1480ºC, Figure 6B, D). At even higher temperature, 1500ºC,
displacements and bending of the crystals along the hexagonal axis were observed
(Figure 6E), the crystals are also considerably rounded due to the closeness to the
melting point of chlorapatite (Tf = 1530ºC). After firing in wet air the crystals preserved
their hexagonal habit, the formation of surface defects was minimized and their
roughness was softened (Figures 7A-7B). Although, firing at 1475ºC and above in wet
air also led to a certain degree of mass transport and chlorapatite recrystallization on the
surfaces due to the decomposition of spodiosite. Thus, control of the furnace
atmosphere allows not only a higher percentage of conversion but also minimization of
morphological changes.
EDX analyses also confirmed the chloride loss in the surface of different single crystals
(Figures 6F and 7C-D). To assess if the loss of chloride content was homogeneous from
the surface to the inner core, a single crystal prepared in wet air for 4 hours at 1425 ºC
was embedded into resin and polished perpendicularly (Figure 7B). EDX analysis on
the transversal section showed that the ion exchange is homogeneous across the
structure and the chloride content is below the detection limit (Figure 7B). EDX also
served to assess the composition homogeneity among different single crystals for each
treatment. The homogeneity was confirmed in all the samples treated in room air.
However, for wet air treatment, the concentration is slightly different from one crystal
to another. This may be explained by the lack of laminar flow in the furnace fluxed with
wet air, resulting in positions with slightly different humidity.
SXRD analyses indicated an increase of the mosaicity (higher tension in the network of
unit cells due to the enhanced chemical changes in the structure) of the single crystals
when using times ≥4 hours at 1425ºC (Figure 7D), being the SXRD data not as good as
for any sample annealed for 2 hours (Figure 7C). Therefore, it is necessary to find a
compromise between the percentage of conversion and the quality of the crystals.
Considering SXRD, XRD, FTIR analyses and chloride content measurements, the best
samples (those that showed a degree of conversion above 85% and up to 96%, while
maintaining the surface roughness and the single crystal structure) were selected for
mechanical characterization (Table 1). Table 1 also shows the chloride content of the
selected samples relative to the measured Cl- content of stoichiometric ClAp [43].
The results of the mechanical characterization are summarized in Table 1, including the
values of hardness (H) and elastic modulus (Es). Figure 8A shows typical loaddisplacement curves obtained in the nanoindentation tests to measure the elastic
modulus and hardness. It must be pointed out that hollow core defects and the formation
of micro-tubes is a common phenomenon that takes place during the synthesis of these
crystals [25]. The presence of these defects affects the mechanical behavior. Most of the
curves are continuous, i.e. without any visible pop in or pop out, both in loading and
unloading. Nevertheless, some of them present discontinuities likely due to the presence
of defects under the site of the indentation. Data obtained from these curves were
rejected. The morphology of the indents was analyzed by SEM in order to identify
sources of error (Figure 8B). No signs of pile-up, sink in or cracking were detected.
Table 1 shows the values of H and Es obtained for the selected samples ordered with
decreasing chloride content. All the samples have higher H after the conversion
treatment compared to starting ClAp crystals however there is not a clear trend with
remaining Cl content, while the elastic modulus appears to remain constant. These
results suggest no significant differences associated with the ionic exchange. All these
values are of the order of those typically reported for hydroxyapatite single crystals and
thin films as well as those calculated using ‘ab-initio’ models [17, 44-47]. Viswanath et
al. [47] and Saber-Samandari & Gross [17] reported certain degree of anisotropy
(smaller than 10%) from measurements on synthetic and natural single HA crystals.
However, it must be pointed out that due to the complex multi-axial stress field
generated during nanoindentation the measured values are weighted averages over
several directions. In addition the surface roughness and the introduction of defects
during the exchange process can add to the variability of the results.
We also carried out three point bending test on ClAp single crystals and on the
converted samples with highest H and Es (those fired in wet air at 1425ºC for 2 and 4
hours, Figure 9A). The addition of defects during the thermal treatments decreases the
average flexural strength (D50, Figure 9A). As it could be expected, the measured
flexural strength (f) also decreases in crystals with larger diameter (Figure 9B). In any
case, the average values are in the same order of magnitude reported in literature for
hydroxyl apatite and carbonate-hydroxyapatite synthesized by hydrothermal methods
and natural convection (300-500MPa) [36, 27]. However, our results indicate that the
smaller crystals can exhibit high strengths, reaching values up to 2GPa. These values
are comparable to the tensile strengths reported for single crystalline fibers of technical
ceramics such as Al2O3 or SiC (2-6 GPa) [48, 49] and will likely correspond to crystals
with very low defect concentrations. The results illustrate how one of the main
limitations in the synthesis of apatite fibers and crystals is the need to develop processes
for the reliable fabrication of “pristine” materials.
Conclusions
We have successfully obtained apatite single crystals with workable sizes through the
thermal treatment of Ca5(PO4)3Cl to promote Cl-OH- exchange. The crystals have the
general formula Ca5(PO4)3(OH)xCl1-x and hydroxyapatite structure. There is a range of
firing conditions that results in samples with low chloride content, close to the one of
biological apatites (~0.13wt%) [2]. By adjusting the heat treatment it is possible to
reach values of 1-x as low as 0.04. However, the process can introduce defects that are
detrimental to the mechanical response. In general “softer” firing conditions (lower
temperature under a flow of wet air) preserve the crystal structure and morphological
features while reaching higher exchange. The best combination of larger OH- content
and higher mechanical properties was achieved after firing the chlorapatite crystals for 2
hours at 1425 ºC in wet air. In this way it is possible to get crystals with average
formula Ca5(PO4)3(OH)0.920.03Cl0.080.03, H=8.71.0, Es=12010 GPa, and f=644±389.
Acknowledgements
This work has been partially supported by the Galician Government project
number PGIDT09TMT003239PR. ES acknowledges support from the National
Institutes of Health/National Institute of Dental and Craniofacial Research
(NIH/NIDCR) Grant No. 1 R01 DE015633 and 2 R01 DE015821.
TABLES
Table 1. Mechanical properties on ClAp and HA single crystals. Large standard
deviations are expected due to the different size and distribution of defects in ClAp
crystals.
Firing
Average Formula
conditions
Chlorapatite
145ºC 2h
H
Es
(Relative %
(GPa)
(GPa)
100
6.61.5
11015
142
7.42
11530
83
8.71
12010
73
7.01.5
10525
43
8.21.6
10615
to ClAp)
Ca5(PO4)3Cl
Ca5(PO4)3Cl0.14±0.02(OH)0.86±0.02
(room air)
1425ºC 2h
Cl- content
Ca5(PO4)3Cl0.08±0.03(OH)0.92±0.03
(wet air)
1450ºC 2h
Ca5(PO4)3Cl0.07±0.03(OH)0.93±0.03
(wet air)
1425ºC 4h
Ca5(PO4)3Cl0.04±0.03(OH)0.96±0.03
(wet air)
References
1.
2.
3.
4.
5.
6.
7.
8.
D.H. Copp and S. Shim, "The Homeostatic Function of Bone as a Mineral
Reservoir". Oral Surgery, Oral Medicine, Oral Pathology, 16(6): p. 738-44
(1963)
S. Weiner and W. Traub, "Bone Structure: From Angstroms to Microns". The
FASEB journal, 6(3): p. 879-85 (1992)
L.L. Hench, "Bioceramics: From Concept to Clinic". Journal of the American
Ceramic Society, 74(7): p. 1487-510 (1991)
L.L. Hench and J.M. Polak, "Third-Generation Biomedical Materials". Science,
295(5557): p. 1014-17 (2002)
J. Franco, P. Hunger, M. Launey, A. Tomsia, and E. Saiz, "Direct Write
Assembly of Calcium Phosphate Scaffolds Using a Water-Based Hydrogel".
Acta Biomaterialia, 6(1): p. 218-28 (2010)
A.J. Wagoner Johnson and B.A. Herschler, "A Review of the Mechanical
Behavior of Cap and Cap/Polymer Composites for Applications in Bone
Replacement and Repair". Acta Biomaterialia, 7(1): p. 16-30 (2011)
G. With, H. Dijk, N. Hattu, and K. Prijs, "Preparation, Microstructure and
Mechanical Properties of Dense Polycrystalline Hydroxy Apatite". Journal of
Materials Science, 16(6): p. 1592-98 (1981)
L. Rodríguez-Lorenzo, M. Vallet-Regí, J. Ferreira, M. Ginebra, C. Aparicio, and
J. Planell, "Hydroxyapatite Ceramic Bodies with Tailored Mechanical
Properties for Different Applications". Journal of biomedical materials
research, 60(1): p. 159-66 (2002)
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
W. Suchanek, M. Yashima, M. Kakihana, and M. Yoshimura, "Processing and
Mechanical Properties of Hydroxyapatite Reinforced with Hydroxyapatite
Whiskers". Biomaterials, 17(17): p. 1715-23 (1996)
P.F. Becher, C.H. HSUEH, P. Angelini, and T.N. Tiegs, "Toughening Behavior
in Whisker‐Reinforced Ceramic Matrix Composites". Journal of the
American Ceramic Society, 71(12): p. 1050-61 (2005)
S.V. Dorozhkin, "Surface Reactions of Apatite Dissolution". Journal of colloid
and interface science, 191(2): p. 489-97 (1997)
W. Jongebloed, P. Van Den Berg, and J. Arends, "The Dissolution of Single
Crystals of Hydroxyapatite in Citric and Lactic Acids". Calcified Tissue
International, 15(1): p. 1-9 (1974)
K.Y. Kwon, E. Wang, A. Chung, N. Chang, E. Saiz, U.J. Choe, M. Koobatian, and
S.W. Lee, "Defect Induced Asymmetric Pit Formation on Hydroxyapatite".
Langmuir, 24(19): p. 11063-66 (2008)
W.J. Tseng, C.C. Lin, P.W. Shen, and P. Shen, "Directional/Acidic Dissolution
Kinetics of (Oh, F, Cl)‐Bearing Apatite". Journal of Biomedical Materials
Research Part A, 76(4): p. 753-64 (2006)
N. Bouropoulos and J. Moradian–Oldak, "Analysis of Hydroxyapatite Surface
Coverage by Amelogenin Nanospheres Following the Langmuir Model for
Protein Adsorption". Calcified Tissue International, 72(5): p. 599-603
(2003)
V. Uskoković, W. Li, and S. Habelitz, "Amelogenin as a Promoter of
Nucleation and Crystal Growth of Apatite". Journal of Crystal Growth,
316(1): p. 106-17 (2011)
S. Saber-Samandari and K.A. Gross, "Micromechanical Properties of Single
Crystal Hydroxyapatite by Nanoindentation". Acta Biomaterialia, 5(6): p.
2206-12 (2009)
A.C. Taş, "Molten Salt Synthesis of Calcium Hydroxyapatite Whiskers".
Journal of the American Ceramic Society, 84(2): p. 295-300 (2001)
K. Teshima, S.H. Lee, M. Sakurai, Y. Kameno, K. Yubuta, T. Suzuki, T.
Shishido, M. Endo, and S. Oishi, "Well-Formed One-Dimensional
Hydroxyapatite Crystals Grown by an Environmentally Friendly Flux
Method". Crystal Growth and Design, 9(6): p. 2937-40 (2009)
Y. Suetsugu and J. Tanaka, "Crystal Growth of Carbonate Apatite Using a
Caco 3 Flux". Journal of Materials Science: Materials in Medicine, 10(9): p.
561-66 (1999)
Y. Suetsugu, T. Ikoma, M. Kikuchi, and J. Tanaka, "Single Crystal Growth and
Structure Analysis of Ab-Type Carbonate Apatite". Key Engineering
Materials, 288: p. 525-28 (2005)
Z. Hongquan, Y. Yuhua, W. Youfa, and L. Shipu, "Morphology and Formation
Mechanism of Hydroxyapatite Whiskers from Moderately Acid Solution".
Materials Research, 6(1): p. 111-15 (2003)
I.S. Neira, F. Guitián, T. Taniguchi, T. Watanabe, and M. Yoshimura,
"Hydrothermal Synthesis of Hydroxyapatite Whiskers with Sharp Faceted
Hexagonal Morphology". Journal of Materials Science, 43(7): p. 2171-78
(2008)
I.S. Neira, Y.V. Kolen’ko, O.I. Lebedev, G. Van Tendeloo, H.S. Gupta, F. Guitián,
and M. Yoshimura, "An Effective Morphology Control of Hydroxyapatite
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
Crystals Via Hydrothermal Synthesis". Crystal Growth and Design, 9(1): p.
466-74 (2008)
H. Zhang, Y. Wang, Y. Yan, and S. Li, "Precipitation of Biocompatible
Hydroxyapatite Whiskers from Moderately Acid Solution". Ceramics
international, 29(4): p. 413-18 (2003)
A. Ito, S. Nakamura, H. Aoki, M. Akao, K. Teraoka, S. Tsutsumi, K. Onuma, and
T. Tateishi, "Hydrothermal Growth of Carbonate-Containing Hydroxyapatite
Single Crystals". Journal of Crystal Growth, 163(3): p. 311-17 (1996)
K. Teraoka, A. Ito, K. Onuma, T. Tateishi, and S. Tsutsumi, "Hydrothermal
Growth of Hydroxyapatite Single Crystals under Natural Convection".
Journal of materials research, 14(06): p. 2655-61 (1999)
J. Elliott and R. Young, "Conversion of Single Crystals of Chlorapatite into
Single Crystals of Hydroxyapatite". Nature, 214: p. 904-06 (1967)
J. Rendon-Angeles, K. Yanagisawa, N. Ishizawa, and S. Oishi, "Topotaxial
Conversion of Chlorapatite and Hydroxyapatite to Fluorapatite by
Hydrothermal Ion Exchange". Chemistry of materials, 12(8): p. 2143-50
(2000)
K. Yanagisawa, J. Rendon-Angeles, N. Ishizawa, and S. Oishi, "Topotaxial
Replacement of Chlorapatite by Hydroxyapatite During Hydrothermal Ion
Exchange". American Mineralogist, 84(11-12): p. 1861-69 (1999)
R. Young, "Implications of Atomic Substitutions and Other Structural Details
in Apatites". Journal of dental research, 53(2): p. 193-203 (1974)
E. García-Tuñón, R. Couceiro, J. Franco, E. Saiz, and F. Guitián, "Synthesis and
Characterisation of Large Chlorapatite Single-Crystals with Controlled
Morphology and Surface Roughness". Journal of Materials Science: Materials
in Medicine: p. 1-12 (2012)
E. García-Tuñón, Dacuna, B., Zaragoza, G., Franco, J., Guitian, F., "Cl-Oh IonExchange Process in Chlorapatite. A Deep Insight. ". Acta Crystallographica
Section B, 68(5): p. 467-79 ( 2012)
S. Saber-Samandari and K.A. Gross, "Effect of Angled Indentation on
Mechanical Properties". Journal of the European Ceramic Society, 29(12): p.
2461-67 (2009)
W. Oliver and G. Pharr, "Measurement of Hardness and Elastic Modulus by
Instrumented Indentation: Advances in Understanding and Refinements to
Methodology". Journal of materials research, 19(01): p. 3-20 (2004)
W.C. Oliver and G.M. Pharr, "Improved Technique for Determining Hardness
and Elastic Modulus Using Load and Displacement Sensing Indentation
Experiments". Journal of materials research, 7(6): p. 1564-83 (1992)
A.C. Fischer-Cripps, "Critical Review of Analysis and Interpretation of
Nanoindentation Test Data". Surface and coatings technology, 200(14): p.
4153-65 (2006)
B.S. Mitchell, "An Introduction to Materials Engineering and Science: For
Chemical and Materials Engineers"2004: John Wiley & Sons, Inc.,
Publication.
G. Simmons and H. Wang, "Single Crystal Elastic Constants and Calculated
Aggregate Properties "1971: The MIT Press; 2nd edition (June, 1971). 320.
E. García-Tuñón, J. Franco, B. Dacuña, G. Zaragoza, and F. Guitián.
Chlorapatite Conversion to Hydroxyapatite under High Temperature
Hydrothermal Conditions. 2010. Trans Tech Publ.
41.
42.
43.
44.
45.
46.
47.
48.
49.
I.R. Gibson and W. Bonfield, "Novel Synthesis and Characterization of an AbType Carbonate-Substituted Hydroxyapatite". Journal of biomedical
materials research, 59(4): p. 697-708 (2001)
I. Rehman and W. Bonfield, "Characterization of Hydroxyapatite and
Carbonated Apatite by Photo Acoustic Ftir Spectroscopy". Journal of
Materials Science: Materials in Medicine, 8(1): p. 1-4 (1997)
J.W. Agna, H.C. Knowles Jr, and G. Alverson, "The Mineral Content of Normal
Human Bone". Journal of Clinical Investigation, 37(10): p. 1357-61 (1958)
S. Saber-Samandari and K.A. Gross, "Nanoindentation on the Surface of
Thermally Sprayed Coatings". Surface and coatings technology, 203(23): p.
3516-20 (2009)
R. Snyders, D. Music, D. Sigumonrong, B. Schelnberger, J. Jensen, and J.
Schneider, "Experimental and Ab Initio Study of the Mechanical Properties
of Hydroxyapatite". Applied physics letters, 90: p. 193902(1-3) (2007)
B. Viswanath, R. Raghavan, N. Gurao, U. Ramamurty, and N. Ravishankar,
"Mechanical Properties of Tricalcium Phosphate Single Crystals Grown by
Molten Salt Synthesis". Acta Biomaterialia, 4(5): p. 1448-54 (2008)
B. Viswanath, R. Raghavan, U. Ramamurty, and N. Ravishankar, "Mechanical
Properties and Anisotropy in Hydroxyapatite Single Crystals". Scripta
materialia, 57(4): p. 361-64 (2007)
C. Papakonstantinou, P. Balaguru, and R. Lyon, "Comparative Study of High
Temperature Composites". Composites Part B: Engineering, 32(8): p. 637-49
(2001)
V. Valcárcel, C. Cerecedo, and F. Guitián, "Method for Production of Α‐
Alumina Whiskers Via Vapor‐Liquid‐Solid Deposition". Journal of the
American Ceramic Society, 86(10): p. 1683-90 (2004)
FIGURE CAPTIONS
Fig. 1. Effect of the temperature on the structure of the crystals after firing in room air:
Le Bail refinements of powder XRD (A) and FTIR (B). A) The interval highlighted in
grey shows the range that we use to study the conversion in wet atmosphere. There is a
good agreement between the experimental intensities (black dots) and the calculated
after refinement (red line). The position of the peaks shifts to hydroxyapatite (HA) dhkl
values as temperature increases, at temperatures ≥1450C a secondary monoclinic phase
is also identified and highlighted with filled squares (2=30.7 and 34.5). Filled and
open circles show the main diffractions of HA and ClAp patterns respectively. B) FTIR
scans showing weak vibration bands at 3570 and 630cm-1 with increasing intensity
depending on the annealing temperature. Bands due to carbon dioxide, vibrational
overtones and carbonates (CO32-) are highlighted in the graph.
Fig. 2. Effect of the temperature on the structure of the crystals after firing in wet air:
powder XRD (A) and FTIR (B) of ground and sieved samples. The position of the
peaks shifts from chlorapatite (ClAp) to hydroxyapatite (HA) dhkl values as temperature
increases, at temperatures ≥1450C a secondary phase is also identified (2=30.7 and
34.5). Vertical lines show the HA pattern (ICDD# 09-432). B) FTIR scans showing
clear vibration bands at 3570 and 630cm-1 at 1425C, with increasing intensity
depending on the annealing temperature, thus confirming a higher OH- incorporation in
wet air. Bands due to carbon dioxide, vibrational overtones and carbonates (CO32-) are
highlighted in the graph.
Fig. 3. Effect of firing time on the structure of the crystals annealed in wet air at
1425ºC: powder XRD (A) and FTIR (B) of ground and sieved samples. A) Firing times
≥2 hours lead to the formation of the secondary monoclinic phase (2=30.7 and 34.5).
B) FTIR analyses confirm the OH- incorporation in the structure (vibration bands at
3570cm-1 and 630cm-1); bands due to carbon dioxide, vibrational overtones and
carbonates (CO32-) are highlighted in the graph.
Fig. 4. Hydroxyl content in the crystals, x in Ca5(PO4)3Cl1-x(OH)x, as a function of
annealing temperature and time. The dashed line corresponds to HA, x=1, in both
graphs. The shaded region highlights the conversion to hydroxyapatite. A) Hydroxyl
incorporation with time at 1425C in wet air. This incorporation reaches values close to
saturation after 2 hours, reaching x ≥0.9. B) Hydroxyl content as a function of
temperature. Higher x values are reached with increasing temperatures and in wet
atmosphere. This ionic exchange reaches a plateau in air at 1450C while it keeps
progressing to a maximum in wet air. It must be pointed out that the chlorine
measurements are very close to the detection limit of the electrode, being difficult to
quantify minute chloride concentrations.
Figure 5. SEM images showing features of the starting ClAp single crystals before
annealing (A, B). Image in A shows a crystal with a clear hexagonal habit and no
defects on the basal plane; the characteristic microscopic roughness is also appreciated
on prism planes. Image in B shows a ClAp twin, where the hexagonal habit is also well
defined, and prism surfaces exhibit microscopic roughness.
Fig. 6. SEM images (A-E) showing the effect of temperature on the morphology of the
crystals after heating in room air. (F) EDX analysis on the crystal in (D) showing low
chloride content. Image in A shows how the basal surface of the crystal presents defects
and small depositions that may be associated with CaCl2 formation due to the
decomposition of the spodiosite remaining on the surface. Image in B shows some of
the hexagonal defects spotted on prism planes after annealing at 1400C. Higher
annealing temperatures damage the surfaces as images in C and D show, at 1480C
crystals lose the hexagonal habit and the surfaces are significantly damaged. At 1500C
the crystals are distorted along the hexagonal axis; surfaces show signs of mass
transport due to the proximity to the melting point of chlorapatite, 1530ºC. Graph in F
shows a representative EDX analysis performed on the crystal in D, where the chloride
content is very low.
Fig. 7. (A, B) SEM images and EDX analysis for two single crystals transformed into
low chloride content HA after firing in wet air. Image in A shows how after 2 hours at
1450C the hexagonal habit is still appreciated and the surfaces are not as damaged as
after firing at similar temperatures in air. The EDX analysis for this crystal shows how a
residual chlorine peak is still visible. Some of the crystals were embedded in resin and
polished to carry out EDX in the transverse section. Image in B shows one of them
obtained at 1425C for 4 hours. Several EDS-analyses on the transversal surfaces of
fractured and unpolished crystals were carried out to confirm these results. The
composition measured by EDX is homogenous across the section, where the chloride
value is below the detection limit of the equipment. Synthetic precession images in C
and D were generated from SXRD data collection. After 2 hours at 1475ºC in air (C),
the image shows well-defined spots, while the same image for crystals obtained after 8h
in wet air at 1425ºC (D) exhibit splitting and lack of definition due to tension between
unit cells (mosaicity).
Fig. 8. A) Representative load-displacement curves. B) SEM image of one indent on a
single-crystal. Sources of error, such as pile up, sink in, or crack in the edges of the
indent, were not detected.
Fig. 9. A) Cumulative frequency distribution and D50 values of the flexural strength for
the samples tested by three point bending. Where x represents the Hydroxyl content in
the crystals, x in Ca5(PO4)3Cl1-x(OH)x. B) Representative flexural strength of single-
crystals vs diameter for single crystals prepared at 1425ºC for two hours in wet air. The
flexural strength decreases with increasing diameter as expected.