Microwave sintering of nanosized hydroxyapatite powder compacts

Microwave sintering of nanosized hydroxyapatite powder compacts

November 2002 Materials Letters 56 (2002) 827 – 831 www.elsevier.com/locate/matlet Microwave sintering of nanosized hydroxyapatite powder compacts S...

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November 2002

Materials Letters 56 (2002) 827 – 831 www.elsevier.com/locate/matlet

Microwave sintering of nanosized hydroxyapatite powder compacts S. Vijayan, Harikrishna Varma * Sree Chitra Tirunal Institute for Medical Sciences and Technology, Biomedical Technology Wing, Poojapura, Thiruvananthapuram 695012, India Received 3 June 2001; accepted 6 August 2001

Abstract Gel cast sample of nanocrystalline hydroxy apatite powder has been sintered to high densities by short-time microwave processing and is characterised by X-ray diffractometry (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SCM), density measurements and Vickers microhardness (Hv) measurements. The basic crystallite size of the precipitated HAP was found to be around 35 nm by XRD and TEM studies. The gel cast samples of the above powder could be sintered to above 95% by exposing to microwave radiation for a period of 5 min. The sintered grainsize of the sample was 200 – 300 nm having microhardness in the range of 5.25 GPa and the effect of microwave exposure time on the densification and grainsize is reported. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydroxy apatite; Gel casting; Microwave sintering; Microstructure; Grain size

1. Introduction Microwave heating of inorganic materials has been studied for the last two decades as a speedy processing route for the sintering of various ceramic systems [1]. Microwave couples effectively with certain materials generating heat within the substance depending on their dielectric constant and dielectric loss factor properties. As a unique processing method, microwave sintering not only offers shorter time of processing but also is able to impart better physical and mechanical properties to the final sintered ceramic. The volumetric nature of microwave coupling has


Corresponding author. E-mail address: [email protected] (H. Varma).

been well understood over these years [2– 4]. HAP is very familiar in biomedical implant science which is used as a hard tissue replacement material by virtue of its chemical and structural similarity to that of the mineral phase of bone and teeth. It is used in various physical forms such as granules, rods, and coating over metallic implants [5]. Even though the biocompatibility of HAP is excellent, their poor mechanical property limits the applications in the area of load bearing prostheses [6]. Many attempts are being made to improve the mechanical properties by controlling microstructural features. Microwave sintering has been utilised to prepare porous as well as dense HAP ceramic starting from powders synthesised by hydrothermal methods [7]. It was found that the nature of powders plays an important role in the microwave-assisted sintering of HAP. The present

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work describes a method of microwave sintering of dense green compact of nanoparticles of HAP resulting into a high density sintered ceramic composed of submicron grains.

2. Experimental Ammonium dihydrogen orthophosphate [NH4H2 (PO4)] and calcium nitrate tetra hydrate [Ca(NO3)2 4H2O] were taken in stoichiometric ratio for Ca10 (PO4)62H2O. Aqueous solutions of the above chemicals at (0.5 N) have been prepared. The pH of these solutions was made above 10 by adding 25% ammonia solution. The solutions were placed in a deep freezer to bring down the temperature to 0 jC. The cooled calcium nitrate solution was taken in a freezing mixture bath and thoroughly stirred. Ammonium dihydrogen orthophosphate solution was dripped into it from a separating funnel. Once the precipitation was completed, the contents were kept in the freezing mixture under stirring for one more hour. Then the precipitate

was aged for 24 h in the mother liquor. Hydroxy apatite precipitate was then washed with distilled water using a high-speed centrifuge. The washed HAP mass was made in a cake form by filtering through a Buchner funnel. The HAP cakes were dried very slowly in a humidity controlled area at 25 jC for 30 days before subjecting to microwave sintering at different time periods. Dried samples were placed in a rectangular cavity made out of silicone carbide of size 25 mm20 mm10 mm covered by ceramic wool. The arrangement was placed in a household microwave oven (power conception 980 W, power output 490 W, microwave frequency 2.45 GHz) and subjected to microwave radiation for a period of 5 to 15 min. The density of the green samples was obtained by measuring the dimensions and weight, and that of sintered samples by Archimedes method. X-ray diffraction patterns were taken in a SIEMENS D5005 Xray diffractometer using Ka radiation. The transmission electron images of the powders were taken in a Hitachi H 600 TEM after dispersing the powders in

Fig. 1. XRD pattern of the calcined HAP powder.

S. Vijayan, H. Varma / Materials Letters 56 (2002) 827–831


Fig. 2. TEM micrograph of HAP prepared at 0 jC (bar—100 nm).

acetone and spread over carbon-coated copper grids. The morphology of the sintered surface of the samples was obtained using a Hitachi 2400 scanning electron microscope. Microhardness of the sintered and polished samples was recorded using a Shimazdzu HMV 2000 microhardness tester using Vickers’ indenter.

density increased further to 98% by exposing for 15 min. Fig. 4a and b shows the SEM micrograph of the polished surface of a sample sintered for 5 and 15 min. The microstructure of the former is composed of sintered grainsize in the range of 200 – 300 nm. There were few inter-granular porosities in the sample. The

3. Result Fig. 1 shows the XRD pattern of the calcined HAP powder prepared at 0 jC. The pattern matches with the standard HAP peaks ( JCPDS 9-432 ). This pattern was used to calculate the crystallite size of the basic precipitate using Scherrer equation [8]. It was found that the basic crystallite size of the 0 jC derived sample was in the range of 35 nm. The TEM study also showed that crystallite size is in the same range (Fig. 2). The green density of the dried samples of HAP cake was 38% of the theoretical density. Fig. 3 shows the plot of the sintered density of the above sample subjected to microwave sintering for various time periods. The sample was densified to 95% by exposing to microwave for a period for 5 min and the

Fig. 3. Microwave sintering time vs. density.


S. Vijayan, H. Varma / Materials Letters 56 (2002) 827–831

Fig. 4. (a) SEM micrograph of polished surface of HAP sintered for 5 min. (b) SEM Micrograph of polished surface of HAP sintered for 15 min.

grain size was further increased to 500 – 600 nm with an increase in microwave exposure time for 15 min. The microhardness (Hv) of the sample exposed for 5 min was 5.62F0.6 GPa while that of 15 min was 5.91F0.8.

4. Discussion Microwave heating behaviour of hydroxyapatite ceramics has been extensively studied by Roy et al. [2]. The method offers advantage over normal furnace

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heating with respect to energy saving and time. This study shows the high microwave sinterability of nanosized HAP crystallites synthesised at 0j. In order to get the similar densification in the gel cast samples, temperatures above 1000j were required when ordinary furnace sintering was employed [9,10]. There is a slight increase in the microhardness value of the sintered samples with increase in microwave exposure time. Further increase in microwave exposure is expected to affect deleteriously with respect to hardness property due to the excessive grain growth and liquid phase formation at the grain boundaries. The major problem associated with the microwave densification is in getting large samples without microcracks. This is believed to be due to the nonuniform packing of crystallites in the green compact as well as due to the nonuniform heating profile occurring inside the microwave cavity. Further work is in progress to address these problems.

5. Conclusion The sintering of nanosized hydroxyapatite was carried out using microwave heating. The microwave couples with low temperature synthesized nanocrystalline compound very effectively and sintered density as high as 95% was obtained by subjecting to microwave sintering for a period of 5 min. The microwave sintering route is a promising method for faster


processing of HAP ceramics having better microstructural features.

Acknowledgements The authors wish to thank the Director of SCTIMST for giving permission to publish this work. The authors would also like to thank the Department of Science and Technology for the financial support under the SERCYS scheme.

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