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Influenceofmicrowavepoweronnanosizedhydroxyapatiteparticles

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Scripta Materialia 55 (2006) 175–178

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Influence of microwave power on nanosized hydroxyapatite particles

A. Siddharthan, S.K. Seshadri and T.S. Sampath Kumar*

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Sardar Patel Road,

Chennai, TN 600 036, India

Received 6 January 2006; revised 16 March 2006; accepted 25 March 2006Available online 12 May 2006

Hydroxyapatite synthesized by a co-precipitation process of calcium nitrate and orthophosphoric acid was subjected to micro-wave irradiation at various powers until the precipitate dried. The particle size, as characterized by X-ray powder diffraction andtransmission electron microscopy, was of nanodimensions. The results show the variation of particle size with the power of micro-wave irradiation. The shape of the particles also changed from needle-like to acicular to platelet form with the increase in microwavepower.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Nanocrystalline materials; Hydroxyapatite; Microwave processing; X-ray diffraction (XRD); Transmission electron microscopy (TEM)

Hydroxyapatite (HA) is an attractive biomaterialfor bone and tooth implants, which has received consid-erable attention because of its chemical similarity to nat-ural bone and its excellent biocompatibility [1]. Theimplant success is related to the interaction of calciumphosphate with biological media. Bioactivity is influ-enced by several factors such as the Ca/P ratio, contentof carbonate and other ions, crystal size, morphologyand sample texture [2].

Natural bone minerals are nanostructured non-stoi-chiometric HA of dimensions 20 nm in diameter and50 nm long, with substitution of ions like magnesium,fluoride and carbonate in minor concentrations. Syn-thetic apatites that are to be used for repairing damagedhard tissues are expected to have characteristics close tothose of biological apatite in both composition andstructure [3]. Nanocrystalline HA has proved to be ofgreater biological efficacy in terms of osteoblast adhe-sion, proliferation, osseointegration and formation ofnew bone on its surface [4].

Different techniques have been used for nanocrystal-line HA synthesis. Depending upon the technique, mate-rials with various morphology, stoichiometry, and levelof crystallinity have been obtained. In the precipitationmethod, the parameters that influence the above proper-ties are temperature, the concentrations of the reagents,addition rate, stirring, maturation, and presence of

1359-6462/$ - see front matter � 2006 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2006.03.044

* Corresponding author. Tel.: +91 44 22574772; fax: +91 4422570545; e-mail: tssk@iitm.ac.in

impurities. The HA precipitated at 22 �C has an averagewidth of 11 nm and length of 110 nm; at 70 �C the widthis in the range of 11–33 nm and length 55– 200 nm, whileat 95 �C the width is 33–110 nm and length is 55–220 nm[5]. Crystal ordering also increases with an increase inpreparation temperature. The reaction temperature af-fects the reaction rate and morphology of HA [5]. Rais-ing the temperature accelerates the formation of HA. Itwould take 24 h to form pure-phase HA at 25 �C, whileonly 5 min at 60 �C. Therefore, raising the reaction tem-perature could greatly shorten the reaction time for theformation of pure HA [6].

Microwave synthesis is a fast, simple and efficientmethod to prepare nanosized inorganic materials. Com-pared with conventional methods, microwave synthesishas the advantages of rapid growth, small particle sizeand narrow particle size distribution due to fast homog-enous nucleation [7]. Microwaves play an important rolein reactions in aqueous media [8] and have been used forpreparing HA in less than 45 min [9]. Precipitation ofnanosized HA using microwave irradiation has alsobeen reported [10,11]. The thermal stability of micro-wave synthesized HA increases with increases in theaging time, microwave irradiation time and power [12].It is found that the pH value and a complexing reagentsuch as ethylenediaminetetraacetic acid in microwavesynthesis of HA play an important role in the finalHA nanostructures with different shapes [13]. Hence,the objective of the present study was to investigatethe role of the microwave power on the size andmorphology of nanosized HA particles.

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176 A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178

In this work, a simple co-precipitation method wasused to prepare nanosized HA powder using calcium ni-trate tetra hydrate (Ca3(NO4)2 Æ 4H2O) and phosphoricacid (H3PO4) as starting materials [14]. Calcium nitratewas dissolved in distilled water to form a 0.5 M solution,into which phosphoric acid was added in order to obtaina Ca/P ratio of 1.67. After 30 min of mixing, ammoniumhydroxide was added to the mixed solution. Stirring fur-ther for about 30 min resulted in a precipitate, whichwas washed repeatedly to remove unwanted ions

(NH4+ and NO2�3 ). The precipitate in paste form was

subjected to microwave irradiation in a domestic micro-wave oven (BPL India, 2.45 GHz, 800 W power) at var-ious powers until the precipitate was dry. HA powderwas obtained by grinding with an agate mortar and pes-tle. The crystalline size and morphology were analyzedin a transmission electron microscope (TEM) operatedat 120 keV (Philips CM12wSTEM, Netherlands). TEMspecimens were prepared by depositing a few drops ofHA dispersed in ethanol on a carbon coated coppergrid. The X-ray powder diffraction (XRD) analysis

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Figure 1. Typical XRD pattern of microwave synthesized nano HA at175 W microwave power.

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Figure 2. Variation of nano HA crystallite size with microwave power.

was done (Shimadzu, XD-D1, Japan) in reflection modewith CuKa radiation.

The XRD peak broadening can measure the crystal-lite size in a direction perpendicular to the crystallo-graphic plane. The crystallite size t(hkl) perpendicular toa crystallographic plane (h kl) can be evaluated measur-ing the full width at half maximum (FWHM) accordingto the Scherrer formula, t(hkl) = Kk/BCosh(hkl), where t isthe crystallite size (nm); K is the shape factor (K = 0.9); kis the wavelength of the X-rays (k = 1.54056 A for CuKaradiation); B is the FWHM (rad) and h(hkl) is the Bragg’sdiffraction angle (�). The diffraction peak at 25.9� (2h)corresponding to the (002) Miller plane family was cho-sen for calculation of the crystalline size, as it is isolatedfrom other peaks. Also, this peak is relatively sharperthan the other peaks, as shown in Figure 1. This corre-sponds to the crystal growth following the c-axis of theHA structure as reported [15].

Figure 2 shows that the variation of crystallite sizewith microwave power is irregular and repeated calcula-tions were done for the validity of data. The same trendwas also observed in the TEM morphology, as shown inFigure 3. The TEM micrograph of sample at 175 Wpower shows a length of 39–56 nm and width of 12–14 nm, and for the 525 W power sample the respectivedimensions were 10–16 nm and 10–12 nm. The sampleat 660 W power shows platelet shapes of lengths 32–42 nm and widths of 12–25 nm. The shapes were needlefor the 175 W power sample, acicular for the 525 Wpower sample and platelet for the 660 W power sample.A shape factor (Fs) can be defined by the ratio of lengthto width of HA nanocrystals [16]. The Fs values werefound to be 4, 1.3 and 1.8 for samples microwave irradi-ated at 175, 525 and 660 W, respectively. These shapechanges have been reported in conventional synthesiswith an increase in temperature [5,6,16].

In the conventional precipitation process, the crystal-lite size increases with an increase in synthesis tempera-ture in a regular fashion [5]. The size, morphology andordering of HA precipitates have been shown to be sig-nificantly affected by the temperature and maturationconditions [5]. However, in microwave synthesis of nanoHA, the crystallite size shows an oscillating trend withan increase in microwave power. This behaviour canbe explained by considering the following factors. The

0 500 600 700

power (Watt)

Figure 3. TEM bright field image of HA nanoparticles synthesized at various microwave powers: (a) 175 W, (b) 525 W and (c) 660 W.

A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178 177

HA was directly obtained under the effect of microwaveirradiation and not involving crystallographic transfor-mation as in case of the precipitation process [11]. Themicrowave energy at 2.45 GHz frequency is appreciablyabsorbed by bound water in the sphere of hydration of apolyvalent ion, whereas the absorption by the free water(a loose ice-like 3 D network of hydrogen bonded watermolecule in hexagonal rings) is minimal. The absorptionof microwaves at this frequency by the bound waterweakens the bonds between the calcium ion and itssphere of hydration, facilitating the deaquation step,which is of paramount importance for the formationof apatite in aqueous solutions. The effect of microwaveenergy on free water is merely that of heating, enhancingevaporation [11]. Dispersion of raw materials in solutionincreases with the increase in microwave power [12]. Thebound water seems to absorb microwave energy only ata certain critical temperature above room temperature.Absorption of microwaves by the free water increaseswith microwave power. So the time required by thebound water to absorb microwave energy depends onthe temperature rise due to microwave absorption ofthe free water. It has been reported that the crystallitesize parallel with the c-axis of the HA structure reachesa maximum at a temperature of 60 �C and above thistemperature growth of monocrystalline HA along c-axisis limited, resulting in more regular and circular particles[16]. This temperature was interpreted as the limit be-cause the speed of germination becomes higher thanthe speed of nucleation. It was also reported that the lar-

ger width of HA crystals was due to crystal bondingpredominantly occurring along the a–c side [5].

For microwave power ranging from 175 W to 385 W,the time required for drying was higher for lowestpower and the duration decreased due to absorptionof microwaves by free water with an increase in power.A prolonged maturation time of 75 min and lowerpower of 175 W resulted in long crystals with aneedle-like shape as shown in the TEM micrograph.The bound and free water absorption of microwavesis lower, resulting in fewer HA nuclei and growthoccurring at a slower pace at low temperature. The de-crease in crystallite size at 245 W suggests that thebound water via microwave absorption has reachedthe critical temperature, resulting in a large number ofstable nuclei. The nuclei formation may have consumedmost of the microwave energy and the poor maturationcondition results in a decrease in crystal growth evenwith 60 min of drying. At 315 W and 385 W powerthe mechanism remains the same as earlier but the tem-perature increase due to free water microwave absorp-tion favours the maturation for crystal growth. Butthe need for less time for drying (30 min) accounts forthe smaller size at 385 W than at 315 W of microwavepower, which takes about 45 min. The crystallite sizeshows a minimum at 455 W suggesting that the freewater and bound water absorption simultaneously sat-isfy the conditions for nuclei formation and crystalgrowth resulting in rapid formation of HA. The timerequired for drying at higher powers (greater than

178 A. Siddharthan et al. / Scripta Materialia 55 (2006) 175–178

385 W) is reduced to 15 min compared to 75 min at175 W, which supports the proposition that free waterabsorption of microwaves increases with power. The in-crease in crystallite size for 525 W is due to the temper-ature rise by the free water absorption of microwaveswhile a decrease in size at higher powers can be sup-ported by the limitation of crystal growth in the c-axisabove 60 �C temperature [16]. The TEM micrographsalso indicate a change in the HA morphology to acicu-lar shape for 525 W and platelet shape for 660 W. Themorphology change occurs due to the limitation ofcrystal growth in the c-axis and crystal bonding occur-ring predominantly in the a–c side. Thus selectivity ofsize and shape of HA nanosized particles seems possiblein microwave processing.

In conclusion, the size and shape depending on spe-cific requirements can be prepared with the selection of asuitable microwave power setting. This in turn dependson the microwave response of the reaction medium em-ployed for synthesis. Microwave processing of nanoma-terials thus seems to be superior in its ability to controlparticle size and shape.

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