INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY

ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

Characterization and Thermal Analysis of Hydroxyapatite

Bioceramic Powder Synthesized by Sol-Gel Technique
Sandeep Sahu*, Deepak Mehra$, R. D. Agarwal#

Metallurgical and Materials Engineering Department, Indian Institute of Technology

Roorkee-247667, India
*Phone no. +91-8791158826,
$
Phone no. +91-8058111512,
#
Phone no. +91-9411174284,

ABSTRACT

Hydroxyapatite (HA) is an inorganic bioceramic compound which is widely used in various

biomedical applications, mostly in orthopaedics and dentistry due to its close similarity with
inorganic mineral component of bone and teeth. This paper describes a simple sol-gel
technique for synthesizing nanosized crystalline hydroxyapatite powder. For synthesis of HA
powder, calcium nitrate tetrahydrate and di-ammonium hydrogen phosphate were used as
calcium and phosphorous precursors respectively. The sintering of the product was done twice
at two different temperatures 400°C and 750oC to improve its crystallinity. The final powder
was characterized by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy
(FT-IR) and Field Emission Scanning electron microscopy (FESEM) equipped with Energy
dispersive X-ray spectroscopy (EDS) techniques to reveal its phase content, various kinds of
bonds present in it and its morphology. Thermal analysis (TG–DTA) was carried out to
examine the thermal stability of the powder.

Keywords: Hydroxyapatite, Sol-gel, XRD, SEM, EDS, FT-IR, TG-DTA

*Corresponding Author: Sandeep Sahu

1. INTRODUCTION

Biomaterials are a class of engineering materials which can be used in human body tissue
replacements, reconstructions, and regenerations for long term serviceability without any
hostile effect. Among the different classes of biomaterials, bioceramic is one of the promising
classes of existing biomaterials used as human body implants. Few of the bioceramics have
similarity with the mineral part of our bone. However, they do not match with the intricate
structure of the bone. There are several calcium phosphate ceramics that are considered
biocompatible. Of these, most are resorbable and will dissolve when exposed to physiological
conditions [1]. Hydroxyapatite [Ca10(PO4)6(OH)2; abbreviated as HA] is an inorganic
bioceramic compound which is widely used for various biomedical applications such as a
bone substitute material in orthopedics and dentistry due to its excellent biocompatibility,
bioactivity and osteoconduction properties [2]. Unlike the other calcium phosphate
compounds, hydroxyapatite does not breakdown under physiological environments. In fact, it

Page 281
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

is thermodynamically stable at physiological pH and actively participates in bone bonding,

forming strong chemical bonds with surrounding bone [3, 4]. This property has been exploited
for rapid bone repair after major trauma or surgery. However, HA ceramics cannot be used for
heavy load bearing applications due to its poor mechanical properties [5], but general uses
include bone graft substitution and coatings on load bearing metallic implants [5, 6] such as
titanium, stainless steel and their alloys or composites with other materials [7]. Moreover, HA
is also used for various non-medical applications such as gas sensors, packing media for
column chromatography, catalysts etc. [8].

Hydroxyapatite can be synthesized using various techniques such as hydrothermal technique

[9, 10], solid-state reaction [11], precipitation [12, 13], sol–gel [14, 15], sputtering [16],
mechanochemical [17], mechanochemical–hydrothermal [18], microemulsion [19] and others.
Differences in preparative routes lead to deviations in morphology, stoichiometry and level of
crystallinity etc.

Sol-gel technique for synthesizing HA bioceramics has attracted much attention in recent
years [20 – 25] due to its numerous advantages over other techniques such as it offers
homogeneous molecular-level mixing of calcium and phosphorus precursors and usually
requires lower processing temperature to produce nanocrystalline powders [26]. The sol–gel
technique can be used to synthesize HA powders or coatings by a comparatively easier route
with a much better structural integrity [27]. It also has an added advantage that it needs fewer
materials to synthesize HA as compared to other techniques. It has been reported that HA
ceramics synthesized by sol-gel process are able to improve the contact and stability at the
artificial/natural bone interfaces in both, in vitro and in vivo environment [28].

Accordingly, in the present work, a novel sol-gel technique for synthesizing nanosized
crystalline HA powder using easily obtainable and comparatively cheaper raw material has
been reported. Simplicity of experimental execution in this sol-gel technique is one of the
most important advantages achieved by this technique.

2. EXPERIMENTAL PROCEDURE

2.1 Synthesis of Hydroxyapatite Powder

For synthesis of HA powder, Calcium Nitrate Tetrahydrate Ca(NO3)2.4H2O (Rankem) and

di-ammonium hydrogen phosphate (NH4)2HPO4 (Rankem) were used as calcium and
phosphorous precursors. 0.5 M Calcium Nitrate Tetrahydrate and 0.5 M di-ammonium
hydrogen phosphate solutions in ethanol (Changshu Yangyuan Chemical) with a pH 10.5
were prepared. Then the calcium containing precursor was added dropwise to phosphorous
containing precursor while it was stirred vigorously at a constant temperature, 85oC. The
resultant solution was continuously stirred at a constant pH 10 (pH was kept constant by
adding Ca(OH)2 solution) and at a constant temperature of 85oC for 3 h so that a gel was
obtained. The gel was cooled to room temperature and then, it was kept inside an electric
oven at 85oC for 15 h. The product obtained was ground into fine powder using an agate
mortar and pestle and then heat treated for 6 h at 400oC and again at 750oC. The whole
procedure was as shown in the flow diagram (Fig.1).

Page 282
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

Fig.1. Schematic flow process chart for the synthesis of HA

2.2 Characterization of Synthesized Hydroxyapatite Powder

The crystallographic phase composition 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 samples were examined 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 Scientific Nicolet 6700 FTIR). The FT-IR spectra were obtained over

Page 283
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

the region 400–4,000 cm-1. The sample was prepared as standard KBr pellet and the
transmission technique was applied. The resolution of spectrometer was 4 cm-1.

The surface morphology and microstructural features of as 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
spectroscopy X-ray (EDS).

Thermogravimetric analysis (TG) equipped with differential thermal analysis (DTA) of as

synthesized HA powder was done with (Perkin Elmer Pyris Diamond) thermal analyzer in air
atmosphere at a heating rate of 10oC/min up to 1200oC.

3. RESULTS AND DISCUSSION

3.1 X-Ray Diffraction Analysis

An XRD spectra of the sol–gel synthesized HA powders has been shown in Fig. 2 where
Fig. 2(a) is for sample sintered at 400°C and 2(b) is for sample again sintered at 750°C. It can
be observed from the figure that the XRD analysis of the powders synthesized at 750°C
resembles with standard HA powder spectra [24]. The effect of sintering temperature on the
development of HA is quite visible from the Fig. 2 which shows that when sintering
temperature was increased from 400°C to 750°C, it resulted in an increase in crystallinity of
HA powder i.e. by this process, the width of the several peaks of XRD spectra belonging to
HA becomes more narrow and intensity of these peaks increases, which proposes that there is
an increase in the crystallinity of HA [25].

Page 284
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

Fig.2. XRD spectra of the HA powder sintered at (a) 400°C and (b) 750°C

3.2 FT-IR Analysis

Fig. 3. FT-IR spectra of the HA powder sintered at 750°C

The FT-IR spectra of as synthesized HA (i.e. sample sintered at 750°C) is shown in Fig. 3.
The O-H stretching bonds are shown at 3644.49, 3570.94 and 2921.23 cm-1, which confirms
the presence of HA powder [26]. A weak band (v3) of CO32- was detected at around 1650.40
and 1415.80 cm-1. This band indicates the minor amount of carbonate substitution which
confirms that the sol-gel derived HA powder is partially carbonated hydroxyapatite, as
commonly observed in synthesis involving organic reagents. The peak at 725.67 cm-1
corresponds to HPO42-. The asymmetric bending mode (v4) of PO43- ion was detected at
around 561.14 cm-1. The symmetrical stretching mode (v2) of PO43- ion was also found at
around 492.83 cm-1.

3.3 TG-DTA Analysis

TG (Fig. 4) analysis shows that there is a weight loss of around 13% upto temperature 200oC
and approximately 32% in the range 200 oC to 560oC due to liberation of chemically bonded
water & decomposition of carbonate from the sample. This major weight loss indicates the
crystallization of HA powder from amorphous phase. Beyond 560oC to 1200oC, no significant
weight loss was observed and almost stable curve was noticed within this temperature range,
which confirms the thermal stability of HA powder.

Page 285
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

In DTA curve, the initial rise and continuous fall thereafter (after 300 oC) shows chemical
changes occurring throughout the process of heat treatment. In DTA curve, there is an
indication of sharp endothermic peak at 420oC with two other small peaks.

Fig. 4. TG-DTA graph of the synthesized HA powder.

3.4 SEM Analysis

Fig. 5. SEM micrographs of synthesized HA powder sintered at 750 oC temperature at

(a) 100X (b) 20,000X
Page 286
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

The SEM images of as synthesized powder at 100X and 20,000 X magnifications has been
shown in Fig. 5. The SEM image at 100X (Fig. 5(a)) shows that individual HA particle
formed in angular shape. The SEM image at 20,000X (Fig. 5(b)) reveal that single particle of
HA is made of agglomeration of nanosized grains. These grains may have been agglomerated
due to the formation of gel during the synthesis process. It is very difficult to disperse all
agglomerates even after rigorous stirring for hours. Also, it can be seen that the powder
obtained exhibits the morphology of dense sintered platelets with distributed pores in them.
The formations of pores are advantageous, as they would permit the circulation of the
physiological fluid throughout the coatings when it is used as a biomaterial.

3.5 EDS Analysis

Fig. 6. EDS analysis of HA powder sintered at 750oC

From the EDS analysis (Fig. 6) of synthesized HA powder, it was revealed that in the
powder, the molar ratio of calcium and phosphate is 17.28/10.21 = 1.69 which is very close to
the theoritical value of 1.67, which confirms that the synthesized powder must have HA
properties and characteristics.

4. CONCLUSION

This technique provides a novel, economical and simple route to form pure, stable and nanosized
crystalline HA powder which took place at 85oC at alkaline pH via an alcohol based sol–gel
process. The presence of alcohol as a solvent provides thermal stability to HA powder. Also
the process took place at lower temperature (750oC) as compared to other existing techniques
where sintering temperature is usually more than 900OC to achieve all the above characteristics.
The pores in the crystal planes of HA itself will help the material to achieve more
biocompatibility and enables the circulation of physiological fluids through these pores. It was
also observed that the crystallinity of the synthesized powder can further be improved by
increasing the sintering temperature. Also, the nanosize of the HA powder makes them ideal
bone replacement material.

Page 287
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

REFERENCES

[1] H. Aoki, 1991, “Science and Medical Applications of Hydroxyapatite”, Japan Association
of Apatite Sciences (JAAS), Takayama Press Centre Co., Tokyo, Japan, Vol. 50,
pp. 27-30.
[2] J.C. Le Huec, T. Schaeverbeke, D. Clement, J. Faber, A. Le Rebeller, 1995, “Influence of
porosity on the mechanical resistance of hydroxyapatite ceramics under compressive
stress”, Biomaterials, Vol. 16, pp. 113–118.
[3] L.J. Jha, S.M. Best, J.C. Knowles, I. Rehman, J.D. Santos, W. Bonfield, 1997,
“Preparation and characterization of fluoride-substituted apatites”, Journal of Materials
Science: Materials in Medicine, Vol. 8, No. 4, pp. 185-191.
[4] Y.C. Tsui, C. Doyle, T.W. Clyne, 1998, “Plasma sprayed hydroxyapatite coatings on
titanium substrates Part 1: Mechanical properties and residual stress levels”, Biomaterials,
Vol. 19, pp. 2015-2029.
[5] G. Muralitharn, S. Ramesh, 2002, “The effects of sintering temperature on the properties
of hydroxyapatite”, Ceramics International, Vol. 26, pp. 221–230.
[6] Shiny Velayudhan, P. Ramesh, M.C. Sunny, H.K. Varma, 2000, “Extrusion of
hydroxyapatite to clinically significant shapes”, Materials Letters, Vol. 46, pp. 142-146.
[7] Shekhar Nath, Bikramjit Basu, Arvind Sinha, 2006, “A Comparative Study of
conventional sintering with Microwave Sintering of Hydroxyapatite Synthesized by
Chemical Route”, Trends Biomater. Artif. Organs, Vol. 19(2), pp. 93-98.
[8] W. Weng, J.L. Baptista, 1997, “A new synthesis of hydroxyapatite”, Journal of the
European Ceramic Society, Vol. 17, pp. 1151-1156.
[9] K. Ioku, S. Yamauchi, H. Fujimori, S. Goto,M. Yoshimura, 2002 “Hydrothermal
preparation of fibrous apatite and apatite sheet”, Solid State Ionics Vol. 151, pp. 147–150.
[10] Jingbing Liu, Xiaoyue Ye, Hao Wang, Mankang Zhu, Bo Wang, Hui Yan, 2003, “The
influence of pH and temperature on the morphology of hydroxyapatite synthesized by
hydrothermal method”, Ceramics International, Vol. 29, pp. 629–633.
[11] R. Ramachandra Rao, H.N. Roopa, T.S. Kannan, 1997, “Solid State synthesis and thermal
stability of HAP and HAP-β-TCP composite ceramic powders”, Journal of Materials
Science: Materials in Medicine, Vol. 8, pp. 511–518.
[12] A. Afshar, M. Ghorbani, N. Ehsani, M.R. Saeri, C.C. Sorrell, 2003, “Some important
factors in the wet precipitation process of hydroxyapatite”, Materials and Design, Vol. 24,
pp. 197–202.
[13] Elena I. Dorozhkina, Sergey V. Dorozhkin, 2002,”Application of the turbidity
measurements to study in situ crystallization of calcium phosphates”, Colloids and
Surfaces A: Physicochemical and Engineering Aspects, Vol. 203, pp. 237–244.
[14] G. Bezzi, G. Celotti, E. Landi, T.M.G. La Torretta, I. Sopyan, A. Tampieri, 2003, “A
novel sol–gel technique for hydroxyapatite preparation”, Materials Chemistry and
Physics, Vol. 78, pp. 816–824.
[15] Wenjian Weng∗, Gaorong Han, Piyi Du, Ge Shen, 2002, “The effect of citric acid
addition on the formation of sol–gel derived hydroxyapatite”, Materials Chemistry and
Physics, Vol. 74, pp. 92–97.
[16] K. Yamashita, T. Arashi, K. Kitagaki, S. Yamada, T.Umegaki, K. Ogawa, 1994,
“Preparation of apatite thin films through rf-Sputtering from calcium phosphate glasses”,
Journal of the American Ceramic Society, Vol. 77 (9), pp. 2401–2407.

Page 288
 INTERNATIONAL JOURNAL OF ADVANCED SCIENTIFIC RESEARCH AND TECHNOLOGY
ISSUE 2, VOLUME 3 (JUNE- 2012) ISSN: 2249-9954

[17] C.C. Silva, A.G. Pinheiro, M.A.R. Miranda, J.C. Go´es, A.S.B. Sombra, 2003, “Structural
properties of hydroxyapatite obtained by mechanosynthesis”, Solid State Sciences, Vol. 5,
pp. 553–558.
[18] S. Nakamura, T. Isobe, M. Senna, 2001, “Hydroxyapatite nano sol prepared via a
mechanochemical route”, Journal of Nanoparticle Research, Vol. 3, pp. 57–61.
[19] G.K. Lim, J.Wang, S.C. Ng, C.H. Chew, L.M. Gan, 1997, “Processing of hydroxyapatite
via microemulsion and emulsion routes”, Biomaterials, Vol. 18, pp. 1433–1439.
[20] P. Layrolle, P. Ito, T. Tateishi, 1998, “Sol-Gel Synthesis of Amorphous Calcium
Phosphate and Sintering into Microporous Hydroxyapatite Bioceramics”, Journal of the
American Ceramic Society, Vol. 81, pp. 1421 – 1428.
[21] Wenjian Weng, J. L. Babtista, 1998, “Alkoxide Route for Preparing Hydroxyapatite and
its Coatings”, Biomaterials Vol.19, pp. 125 – 131.
[22] Dean-Mo Liua, T. Troczynskia, Wenjea J. Tsengb, 2002, “Aging Effect on the Phase
Evolution of water-based Sol-Gel Hydroxyapatite” Biomaterials Vol. 23: pp. 1227–1236.
[23] Ming-Fa Hsieh, Li-Hsiang Perng, Tsung-Shune Chin, Huann-Guang Perng, 2001, “Phase
Purity of Sol-Gel Derived Hydroxyapatite Ceramic” Biomaterials Vol. 22,
pp. 2601 – 2607.
[24] A. Lillavenkatesa, R. A. Condrate, 1998, “Sol-Gel Processing of Hydroxyapatite”,
Journal of Materials science Vol. 33(16), pp. 4111 – 4119.
[25] A. Milev, G.S.K. Kannangara, B. Ben-Nissan, 2003, “Morphological Stability of
Hydroxyapatite Precursor”, Materials Letters, Vol. 57, pp. 1960 – 1965.
[26] C.J. Brinker, G.W. Scherer, 1990, Sol–Gel Science: The Physics and Chemistry of
Sol-Gel Processing, Academic Press, Boston, pp. 797-818.
[27] M. P. Mahabole, R. C. Aiyer, C .V. Ramakrishna, B. Sreedhar, R. S. Khairnar, 2005,
“Synthesis, characterization and gas sensing property of hydroxyapatite ceramic”, Bull.
Mater. Sci., Vol. 28(6), pp. 535–545.
[28] K. Cheng, G. Han, W. Weng, H. Qu, P. Du, G. Shen, J. Yang, J.M.F. Ferreira, 2003,
“Sol–gel derived fluoridated hydroxyapatite films”, Materials Research Bulletin, Vol. 38,
pp. 89-97.

Page 289