• PDF / 1,483,720 Bytes
• 8 Pages / 576 x 792 pts Page_size
• 34 Downloads / 260 Views

DOWNLOAD

REPORT

---

Hydroxyapatite ceramics have been fabricated by microwave sintering in a 500 W microwave oven. Circular-plate specimens of various green densities were sintered in the oven at 1200 and 1300 °C, for 5, 10, and 20 min, respectively. Ceramics with density up to 97% of the theoretical were obtained. Density, grain size, microstructure, and strength of the ceramics sintered by microwave and by conventional methods were compared. The results show that microwave sintering of hydroxyapatite is not only highly efficient in saving time and energy, but can also improve the microstructure and thus enhance mechanical strength of the ceramics.

I. INTRODUCTION That the microwave-transparent ceramics such as A12O3 and SiO 2 could be melted and sintered by a microwave field was a surprising finding. Using gels of such materials, Roy, Komarneni, and Yang1-2 first showed that extraordinary rates of temperature increase (hundreds of degrees per min) could be attained by microwave heating of such materials. Since then, microwave processing has become a new technique to sinter ceramics. In microwave sintering, heat is generated within the materials instead of being transferred from outside as in conventional sintering. Sutton3 has shown how microwave sintering is a volumetric heating process and, thus, makes it possible to reduce or minimize thermal gradients inside the materials being processed. This fundamental feature of microwave processing offers one a unique technique to process a variety of materials (especially those which are difficult to process by conventional methods), to save energy and time, and to obtain improved physical or mechanical properties. Microwave sintering is very promising and seems far more powerful than previously expected. However, this technique is still in an early or exploratory stage of development. In this case, we have studied a new composition involving P2O5, i.e., hydroxyapatite (HAp) [Ca 10 (PO 4 ) 6 (OH) 2 ], a material that has been investigated for years for biomedical applications.

followed by firing at 500 °C for 24 h before pellet compaction. The BET surface area of this powder was found to be 47.4 m 2 /g. Powder x-ray diffraction (XRD) by a Scintag diffractometer (Scintag Inc., Sunnyvale, CA) confirmed that the material was pure HAp of low crystallinity. With no binder, pellets of 12.7 mm (0.5 in.) diameter were made at uniaxial pressures from 35 to 351 MPa to get various green densities. Two 19.05 mm (3/4 in.) diameter pellets, on which the patterns of a dime were pressed, were used to test the uniformity of shrinkage during microwave sintering. B. Setup A 500 W, 2.45 GHz commercial microwave oven (Model JE43, GE Co.) was used for the sintering experiments. The arrangements inside the oven are schematically shown in Fig. 1. Samples were stacked in layers at the center of a vertically placed porous zirconia cylinder (Zircar Products, Inc., Florida, NY) (28 mm ID, 51 mm OD, and 30 mm in height), which was placed on a thick Fiberfrax (Fiberfrax, Harbinson-Carborundum

II. EXPERIMENTAL A. Mat