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The sintering behaviour of powders of two calcium phosphates, namely hydroxyapatite (HA) and dicalcium phosphate (DCP), were studied at various temperatures and in various environments. The density, flexural strength and Knoop hardness of HA sintered in air for 4 h initially increased with the sintering temperature, reaching maxima at around 1150°C, and then decreased due to decomposition of HA into tri-(TCP) and tetracalcium phosphates. Sintering in vacuum caused decomposition of HA at lower temperatures, and consequently the mechanical properties were poorer than those of HA sintered in air. The densification and mechanical properties of DCP sintered in air and vacuum showed similar behaviour to those of HA. In air DCP underwent phase transformation from 7-to [3-and to 0~-phases. In vacuum DCP started to decompose into tricalcium phosphate at 1000°C. To reduce dehydroxylation, HA powder was sintered in moisture at various temperatures up to 1350°C and X-ray diffraction study did not indicate any decomposition at the highest sintering temperature. The density, flexural strength and hardness of HA sintered in moisture increased with the sintering temperature and eventually reached plateaux at about 1300°C, but below 1200°C they were lower than those of HA sintered in air at corresponding temperatures. Thus, it is seen that dehydroxylation did not hinder sintering of HA. On the other hand, decomposition obstructed sintering of both HA and DCP.

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ABSTRACT:Conductive rubber-based composites were derived from EVA, EPDM, and 50/50 EVA/EPDM blend filled with carbon black and short carbon fiber (SCF). The electromagnetic interference (EMI) shielding effectiveness and return loss were studied. The measurements of shielding effectiveness (SE) of the composites were carried out in two different frequency ranges: 100 -2,000 MHz and 8 -12 GHz (X-band). It has been observed that the shielding effectiveness of the composites is frequency dependent and increases with increasing frequency. The increasing of filler loading also enhances the SE of the composites. It is also found that 50/50 EVA/EPDM blend systems have higher shielding effectiveness than pure EVA and EPDM SCF-filled composites. The ADVANCES IN POLYMER TECHNOLOGY 227 EMI SHIELDING EFFECTIVENESS OF CARBON COMPOSITEScorrelation between shielding effectiveness and bulk conductivity of various composites is also discussed.

The variation of electrical resistivity of carbon black and short carbon ®bre (SCF) ®lled rubber composites was studied against the degree of strain at constant strain rate. It was found that both the degree of strain and strain rate affect the electrical resistivity of the composites. The change in resistivity against the strain and strain rate depends both on the concentration and the type of conductive ®ller. The incorporation of short carbon ®bres (SCF) imparts higher conductivity to the composite than carbon black at the same level of loading. Composites ®lled with carbon black exhibit better mechanical properties than SCF ®lled composites. Electrical setting, ie a permanent change in electrical resistivity, was observed during extension±retraction cycles. A good correlation was found between the mechanical response and the electrical response towards strain sensitivity. The results of different experiments are discussed in the light of breakdown and formation of conductive networks in the ®lled rubber composites.

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Perovskite barium titanate (BaTiO 3 ) multipods were prepared via high temperature solid state reaction. The crystal structure and morphology of BaTiO 3 particles were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and scanning probe microscopy (SPM). The XRD analysis of the crystal structure revealed that a single-phase compound was formed having tetragonal crystal structure. Calorimetric study (DSC) over room to high temperature was used to find the energy involved in different steps of synthesis especially during the initiation and the termination process for the formation of BaTiO 3 . These multipods have high average aspect ratio ($10, where average diameter $300 nm and average length $3 mm) as seen from FESEM. UV-Vis spectroscopy reveals that the prepared material is UV active. The bulk and surface chemical composition of these BaTiO 3 particles as investigated by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra reveals that in the prepared BaTiO 3 , the titanium ions exist in two different oxidation states, namely Ti 3+ and Ti 4+ . The BaTiO 3 multipod exhibits high permittivity with relatively low dielectric loss. From impedance analysis of the material, the dual resistivity characteristics, one for grain and the other for grain-boundary can be distinguished. An equivalent circuit has been proposed through analysis of the complex impedance plot (Nyquist plot) for BaTiO 3 multipods. This material has perfect capacitative nature as seen from the Bode plot, and can be used for charge storage devices and other electronic applications. From temperature dependent dielectric analysis, the Curie temperature of BaTiO 3 multipods is found to be $85 C.