Narrow Results

The oxidation process of calcium hexaboride (CaB₆) ceramic sintered body with 28wt% nickel as a sintering aid was investigated systematically in this paper. DTA-TG was used to analyze the oxidation behavior at various temperatures. The morphologies of the oxidation surfaces and the interface between the oxide layer and the bulk material were given by EPMA and BSE, respectively. Element distribution on cross-sections was measured by an energy spectrum analyzer. It is found that CaB₆-28wt%Ni shows an excellent oxidation resistance below 1173K owing to the protection provided by calcium borate on the surface. Catastrophic fast oxidation begins at 1273K. The grain boundaries act as the diffusion channels for O₂ penetration. The existence of Ni doesn't affect the oxidation behavior of the matrix CaB₆. The mechanism of oxidation was also discussed in this paper.

Monoclinic hydroxyapatite was prepared by a wet method and followed by heating in air at 1473 K for 1 h, and its phase transition was investigated by high temperature X-ray powder diffractometry and differential scanning calorimetry. The prepared hydroxyapatite is 98% monoclinic. The Rietveld refinement results revealed that the low temperature phase has monoclinic symmetry, space group P2₁/b and the high temperature phase has hexagonal symmetry, space group P6₃/m. In monoclinic hydroxyapatite, two different sizes of oxygen triangles exist. The rotation of phosphate tetrahedra at about 6° and the change of the hydroxyl order to disorder arrangement occurred due to the phase transition. The phase transition took place reversibly at 480.5(1) K in heating process and 477.5(1) K in cooling process with the enthalpy value of 130(1) Jmol^-1. Further, a sintered monoclinic hydroxyapatite was prepared. Discontinuity of dielectric curves was observed at 483 K The discontinuity of dielectric curves was concluded to the change of hydroxyl order to disorder arrangement.