We report the temperature-and frequency-dependent dielectric spectrum of magnetite ceramic single phase samples at 77.4–300 K and 200 Hz–1 MHz.In temperature-dependent dc resistivity,the sharp transition expected in...We report the temperature-and frequency-dependent dielectric spectrum of magnetite ceramic single phase samples at 77.4–300 K and 200 Hz–1 MHz.In temperature-dependent dc resistivity,the sharp transition expected in single crystals is much suppressed.At higher temperatures,the grain boundaries contribute to the relaxation process.Below 120 K,the temperature-dependent dielectric constant reveals a weak broadened peak as cooling,from our analysis this behavior may be intrinsically correlated with the charge ordering of Fe^(3+) and Fe^(2+).Under a relatively low dc bias at 77.4 K,the polarization of the magnetite ceramic decreases,while under a much stronger electrical field,the dielectric spectrum in the lower frequency region is suppressed remarkably for the excitation of carriers bounded by grain boundaries.展开更多
The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal tran...The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min.Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm,and that the interface of the bainite leading tip is wedge shaped.X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase,with absence of carbides.The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip.Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary.The austenite grain boundary impedes the longitudinal growth of the bainite plate,i.e.,the growth of bainite plate stops at the austenite grain boundary.The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.展开更多
基金Supported by the National Innovation Experiment Program for University Students,the National Natural Science Foundation of China under Grant No 10704054the Key Project of Natural Science Foundation of Tianjin(No 09JCZDJC18500).
文摘We report the temperature-and frequency-dependent dielectric spectrum of magnetite ceramic single phase samples at 77.4–300 K and 200 Hz–1 MHz.In temperature-dependent dc resistivity,the sharp transition expected in single crystals is much suppressed.At higher temperatures,the grain boundaries contribute to the relaxation process.Below 120 K,the temperature-dependent dielectric constant reveals a weak broadened peak as cooling,from our analysis this behavior may be intrinsically correlated with the charge ordering of Fe^(3+) and Fe^(2+).Under a relatively low dc bias at 77.4 K,the polarization of the magnetite ceramic decreases,while under a much stronger electrical field,the dielectric spectrum in the lower frequency region is suppressed remarkably for the excitation of carriers bounded by grain boundaries.
基金Item Sponsored by Tianjin Momentous Technology Supporting Program Foundation of China(11ZCKFGX20500)
文摘The growth behavior at the early stage of bainitic transformation was investigated using optical microscopy,X-ray diffraction analysis and transmission electronic microscopy.The bainite was obtained by isothermal transformation at 200 ℃ only for a short time in a high carbon silicon-containing steel after austenitization at 200 ℃ only for 20 min.Transmission electronic microscopy shows that the bainite appears in the form of plates with a width of about 30 nm,and that the interface of the bainite leading tip is wedge shaped.X-ray diffraction analysis reveals that the bainite plates consist of single ferrite phase,with absence of carbides.The results confirm the occurrence of the moiré which suggests the existence of austenite grain boundaries at the bainite leading tip.Both the lateral growth and longitudinal growth of bainite have weak ability to traverse the lattice-distortion strain fields and austenite grain boundary.The austenite grain boundary impedes the longitudinal growth of the bainite plate,i.e.,the growth of bainite plate stops at the austenite grain boundary.The longitudinal growth of bainite associated with the features of shear mechanism can not completely be in accordance with that of martensitic transformation.
