β-Si 3 N 4 rodlike seed crystallites were successfully produced by single-step heat treatment of commercial α-Si 3 N 4 powder at 1900 • C for 20 h under an N 2 gas pressure of 980 kPa. The average diameter, length, and aspect ratio of the seed crystallites were 0.73 μm, 1.37 μm, and 1.86, respectively. The α-⇀ β-Si 3 N 4 phase transformation proceeded mainly at 1900 • C, and this temperature was lower than the theoretical α-Si 3 N 4 dissociation temperature (1933 • C) under N 2 gas pressure of 980 kPa. The formation of metastable solid solution due to the dissolution of O impurity into the α-Si 3 N 4 crystal lattice was suggested as the driving force for the present oxide additive-free α-⇀ β-Si 3 N 4 phase transformation. β-Si 3 N 4 ceramics were fabricated by liquid phase sintering promoted by an additive system of 1 wt% MgO with 3 wt% Gd 2 O 3 . Starting α-Si 3 N 4 powder with 10 vol% rodlike β-Si 3 N 4 seed crystallites prepared in this study and an extended sintering time for up to 20 h at 1950 • C resulted in the formation of bimodal microstructure composed of fine matrix grains and large elongated grains originated from the seed crystallites. The β-Si 3 N 4 ceramics exhibited improved fracture toughness and thermal conductivity of 5.9 ± 0.8 MPa m −1/2 and 109.3 ± 0.4 W m −1 K −1 , respectively, retaining a high fracture strength of about 1 GPa.

The occupation sites and the occupancies of trace dopants in La/Co co-doped Sr-M-type ferrite, SrFe12O19, were quantitatively and precisely determined by beam-rocking energy-dispersive X-ray spectroscopy (EDXS) on the basis of electron-channeling effects. Because the Co atoms, in particular, should be partially substituted for the five crystallographically inequivalent sites, which could be key parameters in improving the magneto-crystalline anisotropy, it is difficult yet intriguing to discover their occupation sites and occupancies without using the methods of large-scale facilities, such as neutron diffraction and synchrotron radiation. In the present study, we tackled this problem by applying an extended statistical atom location by channeling enhanced microanalysis method, using conventional transmission electron microscopy, EDXS and dynamical electron elastic/inelastic scattering theories. The results show that the key occupation sites of Co were the 2a, 4f1 and 12k sites. The quantified occupancies of Co were consistent with those of the previous study, which involved a combination of neutron diffraction and extended X-ray absorption fine structure analysis, as well as energetics considerations based on by first-principles calculations.

No abstract

No abstract

The effect of microstructure in the vicinity of ferrite grain boundary in Ca-La-Co M-type ferrite on coercivity was investigated. According to Spherical Aberration Corrected Scanning Transmission Electron Microscopy (Cs-STEM) analysis, the Ca concentration in the vicinity of ferrite grain surface of the sintered body prepared under rapid cooling was found to be lower than that in a sintered body prepared under gradual cooling at the rate of −300 K/h. Considering that Co concentration was high in the region where Ca concentration was low as was found by Electron Probe Micro Analyzer (EPMA) analysis, it is suggested that the Co concentration was high in the vicinity of the ferrite grain surface prepared under rapid cooling, thereby enhancing crystal magnetic anisotropy in the vicinity of ferrite grain surface, which contributed to improvement of coercivity. KEY WORDSCa-La-Co M-type ferrite, rapid cooling, crystal magnetic anisotropy, FE-SEM, Cs-STEM M H cJ Ca-La-Co 2008 Sr-La-Co 1-3) 20% H a 4) H cJ H a 15~20% Kronmüller 5) Nd-Fe-B H cJ