FeSi2合金在高压下的凝固

研究了压力对FeSi2合金凝固组织的影响.与常压下凝固的典型共晶组织不同，高压条件下的凝固组织为初生ε相树枝晶加离异共晶.高压下凝固组织的变化主要是与压力对相图的影响和对凝固过程中溶质原子扩散的影响有关.通过引入压力参量，推导了高压凝固过程中的成分过冷判据，并应用该判据分析了高压凝固树枝晶组织的形成机理. 关键词： 高压 凝固 FeSi2     

稳恒磁场对Fe-Fe50 wt.%Si扩散偶中间相生长的影响

本文考察了Fe-Fe50 wt.%Si扩散偶在1200℃ 无磁场以及稳恒磁场下扩散层生长规律. 利用真空浇注强制冷却技术制备Fe-Fe50 wt.%Si扩散偶, 将制备的扩散偶进行1200℃不同磁感应强度下的热处理. 对获得热处理后试样进行SEM与EDS线扫描分析, 结果表明, 无论无磁场还是稳恒磁场下Fe-Fe50 wt.%Si扩散偶均生成两个扩散层, 即FeSi相层和Fe-Si固溶体层, 并且发现0.8 T下的两个扩散层宽度均小于0 T磁场下试样. 按照抛物线规律, 计算了扩散偶中间扩散层的互扩散系数, 发现0.8 T磁场下FeSi相层和Fe-Si固溶体层的互扩散系数较无磁场下 分别降低了26.7%与34.1%. 通过对磁吉布斯自由能的计算, 发现0.8 T磁场对扩散激活能Q的影响不足以影响扩散过程. 但扩散过程中原子振动频率ν会受到磁场的影响, 进而影响扩散常数D₀, 磁场对原子振动频率的影响可以用拉莫尔旋进理论进行解释. 关键词： Fe-Fe50wt.%Si扩散偶 稳恒磁场 FeSi相 Fe-Si固溶体     

Low-field microwave absorption in pulse laser deposited FeSi thin film

Low field microwave absorption (LFMA) measurements at 9.4 GHz (X-band), were carried out on pulse laser deposited (PLD) polycrystalline B20 cubic structure FeSi thin film grown on Si (111) substrate. The LFMA properties of the films were investigated as a function of DC field, temperature, microwave power and the orientation of DC field with respect to the film surface. The LFMA signal is very strong when the DC field is parallel to the film surface and vanishes at higher angles. The LFMA signal strength increases as the microwave power is increased. The LFMA signal disappears around 340 K, which can be attributed to the disappearance of ferromagnetic state well above room temperature in these films. We believe that domain structure evolution in low fields, which in turn modifies the low field permeability as well as the anisotropy, could be the origin of the LFMA observed in these films. The observation of LFMA opens the possibility of the FeSi films to be used as low magnetic field sensors in the microwave and rf frequency regions.     

Mechanism for structure conversion of FeSi under pressures

The atomistic mechanism for the structure conversion of FeSi from B20 to B2 phase at high pressures is a long-standing problem in Fe-Si system. Here we explore the atomistic mechanism about this intriguing structure conversion by first-principle calculations. We find the structure conversion from B20 to B2 is inhibited by the high kinetic barriers and the weak driving forces from low relative enthalpy under pressures. We also identify an orthorhombic structure with Pbcm symmetry as an intermediate meta-stable phase along the minimum energy path, which is possibly the precursory state of the B2 phase. These results can help to explain experimental findings and explore the kinetic origin of this intriguing structure conversion of FeSi at high pressure and temperature.     

β-FeSi2的高压研究

 对β-FeSi₂晶体进行了原位X射线衍射高压研究。利用同步辐射X射线衍射原位研究了β-FeSi₂的高压相演化，发现压力在4.3 GPa时出现相变，在25.8 GPa时相变完成。指标化结果表明：经高压处理后得到的产物具有四方结构，其晶格常数为：a=b=1.004 9 nm，c=0.339 4 nm。     

Electronic and thermodynamic properties of B2-FeSi from first principles

The electronic and thermodynamic properties of B2-FeSi have been investigated using the first-principles method based on the plane-wave basis set. The calculated equilibrium lattice constant is in good agreement with available experimental and theoretical data. Our results have shown that B2-FeSi was a narrow gap semiconductor of above 0.055 eV and exhibited metallic characteristics. The density of states (DOS) can also describe orbital mixing. Using the quasi-harmonic Debye model, the thermodynamic properties of B2-FeSi have been analyzed. Variations of the Debye temperature Θ_D, thermal expansion α, heat capacity C_v, entropy S and the Grüneisen parameter γ on temperature T and pressure P were obtained successfully in the ranges of 0-2400 K and 0-140 GPa.     

Isochronal annealing studies of deformed FeSi alloys using positron annihilation spectroscopy

A series of FeSi samples were deformed to a thickness reduction of 16%. They were isochronally annealed for one hour at different temperatures and characterized by the Doppler broadening of the annihilation radiation (DBAR) measured at room temperature. Optical microscopy (OM) is used to investigate the microstructure of the deformed samples before and after annealing. The S parameter data show a decrease with the increase of the annealing temperature. At 973 K a significant decrease sets in. The microstructures of the alloys, investigated by OM, show that recrystallization is completed at 1173 K.     

Specific heat studies on Ru substituted FeSi Kondo Insulator

The temperature dependent specific heat studies of a Ru substituted Fe_1−xRu_xSi Kondo Insulating system has been carried out in the 77 to 300 K range. The specific heat has been analyzed based on contributions from an electronic part and a lattice part. For the electronic part a Gaussian density of states model, which incorporates the effect of correlation, band structure and disorder in its defining parameters is used. The lattice part of specific heat is described in the Debye model. Parameters that define the electronic density of states and the Debye temperature are extracted from fits of the temperature dependent specific heat data and are compared with earlier resistivity measurements that employed a similar analysis.     

Hysteresis loss reduction in self-bias FeSi/SrFe12O19soft magnetic composites

The magnetic field provided by magnetized SrFe₁₂O₁₉particles in FeSi/SrFe₁₂O₁₉composites is used to replace the applied transverse magnetic field,which successfully reduces the magnetic loss of the composites with minor reduction of permeability.This magnetic loss reduction mainly comes from the decrease in hysteresis loss,while the eddy current loss is basically unaffected.The hysteresis loss reduction in magnetized composites is believed to be due to the decrease in domain wall displacement caused by the increase in the average magnetic domain size in a DC magnetic field.This is an effective method for reducing the magnetic loss of soft magnetic composites with wide application potential,and there is no problem of increasing the cost and the volume of the magnetic cores.     

FeSi掺杂的MnNiO热敏材料的研究

本文采用ＦｅＳｉ掺杂的方法，在材料常数Ｂ值改变较小的情况下，明显的提高了热敏材料的电阻率，为获得具有较大实用价值的热敏电阻提供了非常有效的方法。通过分析表明：ＦｅＳｉｒ和ＭｎＮｉＯ发生反应，有利于尖晶石结构的形成。