Journal of Mi nerals & Materials Characteriza tion & Engineering, Vol. 10, No.8, pp.727-734, 2011 Printed in the USA. All rights reserved
Synthesis and Characterization of Hydroxyapatite Powder by Sol-Gel
Method for Biomedical Application
Khelendra Agrawal*, Gurbhinder Singh, Devendra Puri, Satya Prakash
Metallurgical and Materials Engineering Department, Indian Institute of Technology
Roorkee- 247667, India
*Corresponding Author:
Hydroxyapatite (HA) is effectively used as a bioimplant material because it closely resembles
bone apatite and exhibits good biocompatibility. This paper describe synthesis technique of
HA powder by sol-gel method. The product was sintered twice at two different temperatures
400°C to 750°C to improve its crystallinity. The final powder sintered at two temperatures
was characterized by X-ray analysis, Scanning electron microscopy (SEM) and Fourier
Transform Infrared Spectroscopy (FT-IR) to reveal its phase content, morphology and types
of bond present within it. Thermal analysis (TG–DTA) was carried out to investigate the
thermal stability of the powder.
Keywords: Hydroxyapatite, bioimplant, sol-gel, X-ray diffraction, FT-IR, TG-DTA
Hydroxyapatite (Ca10(PO4)6(OH)2, HA) is an important inorganic biomaterial which has
attracted the attention of researchers related to biomaterials field in recent years. Due to its
chemical and structural similarity with the mineral phase of bone and teeth, HA is widely
used for hard tissues repair. As a result, this inorganic phosphate has been studied extensively
for medical applications in the form of powders, composites or even coatings [1–11]. It is
also observed that dense sintered HA has many bone replacement applications and is used for
repairing bone defects in dental and orthopedic sites, immediate tooth replacement,
augmentation of alveolar ridges, pulp capping material and maxillo facial reconstruction, etc
[12]. For substituting or repairing the bone, the designed material must has the ability to
create a bond with the host living bone [13]. Hence, it is always desirable to include a high
degree of crystallinity and chemical stability among the desirable properties of an ideal
hydroxyapatite [14, 15]. Furthermore, HA has also been studied for other non-medical
728 Khelendra Agrawal, Gurbhinder Singh, Devendra Puri, Satya Prakash Vol.10, No.8
applications, for example, as packing media for column chromatography, gas sensors,
catalysts, etc. [2, 16]. However, poor mechanical properties, e.g. low strength and toughness,
restrict monolithic HA applications to those that require little or no load-bearing parts [17].
Due to its diverse applications, the materials properties accordingly need to be tailored for
real world application. Hence researchers have tried to customize its properties such as
bioactivity, mechanical strength, solubility and sinterability by controlling its composition,
morphology and particle size [9, 10].
The chemical, structural and morphological properties of synthetic HA can be modulated by
varying the method and the conditions of synthesis. Classical methods for HA powder
synthesis include direct precipitation, hydrothermal techniques, hydrolysis of other calcium
phosphates, as well as solid-state reactions [18, 19] and mechano-chemical methods [20, 21].
One of the most widely used methods is wet precipitation, where chemical reactions take
place between calcium and phosphorus ions under a controlled pH and temperature of the
solution. The precipitated powder is typically calcined at 400-600°C or even at higher
temperature in order to obtain a stoichiometric, apatitic structure. In some cases, a well-
crystallized HA phase was only developed while approaching a sintering temperature of
1200°C. However, fast precipitation during phosphate solution titration (to calcium solution)
leads to chemical in homogeneity in the final product. Slow titration and diluted solutions
must be used to improve chemical homogeneity and stoichiometry of the resulting HA.
Careful control of the solution condition is critical in the wet precipitation. Otherwise, a
decrease of solution pH below about 9 could lead to the formation of Ca-deficient HA
structure [17].
Most of the wet methods are time-consuming because the formation of HA phase and the
rinsing of unnecessary anions all take time. A method involving non-aqueous systems to
synthesize HA has also been reported [22], in which a viscous solution was first obtained by
hydrolysis and oxidation of a mixed acetone solution of calcium nitrate (Ca(NO3)2.4H2O) and
phenydichlorophosphine (C6H5PCl2). After the viscous solution was dried and calcined at
above 700°C, HA Powder was obtained. However, the method is usually not so simple due to
the need of hydrolysis and oxidation steps.
Sol–gel technique have attracted much attention recently [5, 6, 9-11] due to its well-known
inherent advantages to generate glass, glass–ceramic and ceramics powders. These include
homogeneous molecular mixing, low processing temperature, the ability to generate sized
particles, the tremendous flexibility to generate nanocrystalline powders, bulk amorphous
monolithic solids and thin films [23]. The sol–gel process is easily applicable to surface
coating and it allows the preparation of high-quality HA thin films on metal substrates [7-10].
Thus, the sol–gel process can be usefully utilized to synthesize both HA powders and HA
films under significantly mild conditions. The versatility of the sol-gel method opens a great
opportunity to form thin film coatings in a rather simple process, an alternative to thermal
spraying which is currently widely used for biomedical applications [24, 25].
Accordingly, the objective of the present work is to synthesis of HA powder from sol-gel
method and carried out its characterization.
Vol.10, No.8 Synthesis and Characterization of Hydroxyapatite Powder 729
2.1. Powder Preparation
In this method two different chemical reagents (precursors) were used. At first phosphoric
pentoxide P2O5 (Merck) was dissolved in absolute ethanol (Fisher Scientific) to form a 0.5
mol/l solution and secondly calcium nitrate tetrahydrate Ca(NO3)2.4H2O(Merck) was also
dissolved in ethanol to form 1.67 mol/l solution. After this both the solutions were mixed to
obtain the desired Ca/P molar ratio of 1.67. It was reported in the previous literature [26] that
sequence of the dissolution of the reagents did not affect the process, but rapid addition of
any one reagent to another reagent can cause precipitation. Hence the solution was stirred
slowly for 10 to 15 h until the formation of a gel. Further the gel was dried in an electric oven
at 80°C in air for 20 h, followed by two stage heat treatment in stagnant air starting from
400°C to 750°C for 8 h. The whole procedure was as shown in the flow diagram given below.
Fig. 1. Schematic flow process chart for the synthesis of HA by the use of Ca(NO3)2.4H2O
and P2O5
2.2. Powder Characterization
The crystallographic phases of HA powder was determined by X-ray diffractometer (XRD)
using a (Bruker D8 Advance, Germany) diffractometer in reflection mode with Cu Kα
(λ=1.5405 Å) radiation. The data were collected in the 2Ө range from 15º to 80º with a
scanning speed of 1.5º/ min. The presence of functional groups was confirmed by using
Fourier transform infrared spectroscopy (Thermo NICOLET 5700, FTIR). The FT-IR spectra
730 Khelendra Agrawal, Gurbhinder Singh, Devendra Puri, Satya Prakash Vol.10, No.8
were obtained over the region 400–4,000 cm-1 using KBr pellet technique. The resolution of
spectrometer was 4 cm-1. The surface morphology and microstructural features of the
synthesized HA powder with elemental composition was studied and evaluated by Field
Emission Scanning Electron Microscope (FE-SEM) (FEI Quanta 200F, Czech Republic)
fitted with energy dispersive X-ray (EDAX). Thermo gravimetric analysis (TGA) equipped
with differential thermal analysis (DTA) of the powder was done with (Perkin Elmer Pyris
Diamond) thermal analyzer in air atmosphere at a heating rate of 100C/min up to 12000C.
15 20 25 30 35 40 45 50 55 60 65 70 75 80
Relative Intensity
2 Theta (D egree)
* Hydroxyapatite
(2 0 0)
(1 1 1)
(0 0 2)
(1 0 2)
(2 1 0)
(2 1 1)
(1 1 2)
(3 0 0)
(2 0 2)
(2 1 2)
(3 1 0)
(3 1 1)
(1 1 3)
(2 0 3)
(2 2 2)
(3 1 2)
(3 2 0)
(2 1 3)
(3 2 1)
(4 1 0)
(4 0 2)
(0 0 4)
(3 2 2)
(3 1 3)
Fig. 2. XRD patterns of the HA powder sintered at (a) 400°C and (b) 750°C.
3.1. X- ray Diffraction Analysis
An XRD pattern of a sol–gel prepared powder that was sintered at 400°C and 750°C is shown
in Fig. 2 (a) and (b). It can be observed from both figures that the XRD analysis of the
powders synthesized at 400°C and 750°C resembles with the standard HA powder pattern.
The effect of sintering temperature on the formation of HA can be seen in Fig. 2 (a) and (b).
The sintering temperature plays an important role on the formation of HA. The samples
heated at 400°C show broad peaks indicating the formation crystalline phase, which was
increased with the increase of sintering temperature. As the sintering temperature is increased
from 400°C to 750°C, several peaks of XRD pattern which belongs to the HA powder
become more distinct and, also the widths of the peaks become more narrow, which suggests
that there is an increase in the crystallinity of powder. It can also be seen that no additional
crystalline phases is present besides HA at 750°C.
3.2. FT-IR Analysis
The FT-IR spectra of HA samples at 400°C and 750°C are shown in Fig. 3 (a) and (b). From
the graph it is indicated that there is a broad envelop between 3825 cm-1 and 2550.16 cm-1.
The O-H stretching bond is shown at 3580.74 cm-1 in sample (a) and 3575.02 cm-1 in sample
Vol.10, No.8 Synthesis and Characterization of Hydroxyapatite Powder 731
(b), which confirms the presence of hydroxyapatite powder [7, 27]. A weak band of CO32-
was detected in the region around 1567.78 cm-1 in sample (a) and 1554.23 cm-1, 834.65 cm-1
in sample (b). This band indicates or confirms the minor amount of carbonate substitution.
Initially at lower sintering temperature these peaks are broad but with increase of sintering
temperature the peaks get ill-defined due to elimination of CO32-. The peak at 976.89 cm-1
corresponds to symmetric stretching mode of PO43-. The peaks at 623.02 cm-1 and 560 cm-1
for sample at 400°C and 589.7 cm-1 for sample at 700°C indicate the bending mode of PO43-.
The large separation of these bands indicates the presence of crystalline phase [28, 29].
Fig. 3. FT-IR patterns of the HA powder sintered at (a) 400°C and (b) 750°C.
3.3. DTA-TG Analysis
TG (Fig.4.) analysis shows that there is weight loss of around 12% up to temperature 2200C
and approximately 40% in the range 2200C to 3500C. This major loss confirmed the
formation of HA, similarly about 5% wt. loss was observed up to 6000C. Beyond 6000C to
12000C no significant wt. loss was observed. Almost stable curve was noticed within this
temperature range, which indicates thermal stability of HA powder.
Fig. 4. TG-DTA patterns of the sintered powder
732 Khelendra Agrawal, Gurbhinder Singh, Devendra Puri, Satya Prakash Vol.10, No.8
In DTA curve initially there are series of small curves occur which is followed by a broad
curve between approx. (2150C to 3300C). This is occurring because evaporation of water in
calcium nitrate tetrahydrate Ca(NO3)2.4H2O happens. Similarly the other endothermic peaks
in the curve related to the removal or addition of other groups during the synthesis of HA
powder. However in the starting at 2000C a sharp exotherm indicates the crystallization of
3.4. SEM- Analysis
Fig. 5. SEM micrographs of synthesized HA powder sintered at 7500C temperature for 8 hr at
(a) 100X (b) 500X
The above Figure 5 shows the SEM images of the synthesized powder obtained after heat-
treatment at 7500C for 8 h in stagnant air. The powder appears to be of crushed angular shape
when observed at 100X in Figure 5(a). Figure 5(b) which was taken at higher magnification
reveals single particle of HA is made of agglomeration of nano sized grains. These grains
may be agglomerated due to the formation of the gel during the synthesis process.
This study presents an alternative method to form pure, stable, good crystalline nanosized HA
powder at low temperature maximum (7500C) as compared to other existing methods where
temperature of treatment is more than 8000C to achieve all the above characteristics. Also it
was observed that the crystallinity of the synthesis powder can be improved further by
increasing the sintering temperature. The nanosized HA powder produced can be highly
useful as a bone replacement material.
(a) (b)
Vol.10, No.8 Synthesis and Characterization of Hydroxyapatite Powder 733
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