No abstract

The effects of reaction temperature (873-1223 K), carbon sources (CH 4 , C 2 H 2 , C 2 H 4 , and C 2 H 6 ), and the amount of catalyst (2.5-20 g) on the physical properties (tube diameter, conversion, volume expansion, intensity ratio of the D-and G-band peaks (I D /I G )) of multiwalled carbon nanotubes (MWCNTs) in a gas-solid fluidized bed reactor (with an inner diameter (id) of 0.056 m and a height of 1.0 m) have been determined. The MWCNTs synthesized by the catalytic decomposition of methane produce the smallest tube diameter and the highest intensity ratio (I D /I G ) among the carbon sources (acetylene, ethylene, and ethane). Although the tube diameter of MWCNTs that have been synthesized from the decomposition of methane and ethane at 1073 K are similar, the volume expansion of the carbon nanotubes (CNTs) agglomerate from ethane is higher than that from methane. Both the tube diameter and the I D /I G ratio of the MWCNTs synthesized from the decomposition of methane decrease as the reaction temperature increases (in the temperature range of 1073-1223 K). The amount of catalyst does not affect the mean tube diameter of the synthesized CNTs; however, CNTs with a bamboo structure are synthesized when the carbon decomposition rate is higher than the CNT growth rate.

Human bone mineral is different in composition from stoichiometric hydroxyapatite (Ca10(PO4)6(OH)2) in that it contains additional ions, of which CO3 2- is the most abundant species. Two macroporous carbonate-substituted hydroxyapatite (CHA) scaffolds were prepared by treating a macroporous CaCO3 column (porosity, 88%; pore and hole size, 160 and 71 μm), prepared in this study, hydrothermally at 120 °C in 1 M phosphate solutions of (NH4)2HPO4 and K2HPO4. Especially, it was found that the CaCO3 was transformed into CHA after hydrothermal treatment (HT) for 24 h irrespective of the type of phosphate solution. Under HT for 24 h, the crystallite size and crystal shape of CHA prepared in K2HPO4 solution (KCHA) and those of CHA prepared in (NH4)2HPO4 (NH4CHA) were quite different from each other. Moreover, NH4CHA contained 2.09 wt % A-type CO3 2- out of 6.05 wt % CO3 2-, but KCHA contained a small amount of A-type CO3 2- (0.08 wt % out of 8.24 wt % CO3 2-). The chemical formulas were Ca9.23(NH4)0.12(PO4)5.36(CO3)0.64(OH)0.54(CO3)0.34 for NH4CHA and Ca8.18K0.5(PO4)4.72(CO3)1.28(OH)0.1(CO3)0.02 for KCHA. Therefore, the properties, such as carbonate type and content, and crystal shape, of CHA are significantly affected by the type of phosphate solution.