Lanting Zhang

Electronic structure and transport properties of doped CoSi single crystal

The lattice parameter and electronic structures of CoSi and CoSi1−xYx (Y=Al and P, x=0.03125 and 0.125) were calculated using the full-potential linear augmented plane wave approach based on density functional theory. The calculated lattice parameter of binary CoSi is about 0.27% smaller than the experimental value. Calculated electronic structures show that CoSi is a semimetal and its density of states is very small at the Fermi level. Doping with Y on the Si site can tune the Fermi level and the effective masses as well as the density of the hole carriers or electron ones in the electronic structure of CoSi, which is a valuable way to modulate the transport properties of this compound. Based on the calculated electronic structures and our experimental results on CoSi, the intrinsic relations between the electronic structures and transport properties of doped and undoped CoSi are discussed in detail.

Dysprosium Nitride-Modified Sintered Nd–Fe–B Magnets with Increased Coercivity and Resistivity

The magnetic properties, electrical resistivity and related microstructures of (Nd,Pr)14.5Dy1.2FebalAl0.8B6 magnets modified by DyN addition have been investigated. The coercivity of the magnets increases almost linearly as a function of DyN content without a considerable reduction in remanence. Similarly, the resistivity in the transversal orientation increases from 1.6 to about 2.0 µΩ m. N dissolves in the grain boundary Nd-rich phase, leading to the formation of an N-containing amorphous phase. This amorphous phase is found to enhance the electrical resistivity. Dy tends to diffuse into the matrix phase, enriched on the matrix side of the interface between the grain boundary phase and the matrix. The significant improvement of the coercivity is ascribed to the increased anisotropy of R2Fe14B by partial substitution of Dy for Nd.

Microstructure and corrosion resistance of sintered NdFeB magnet modified by intergranular additions of MgO and ZnO

Microstructure and corrosion resistance of sintered Nd15Dy1.2Fe77Al0.8B6 and Nd22Fe71B7 magnets modified by intergranular addition of MgO and ZnO were investigated. Both the remanence and sintering density of the magnets increased slightly with intergranular additions of MgO and ZnO. There was a remarkable increase in coercivity of Nd22Fe71B7 after addition. Besides, the effects on magnetic properties and an improved corrosion resistance were observed. Compared with the native magnets without addition, corrosion potential of the magnets with MgO and ZnO additives was more positive and the current density in the anodic branch of the polarization curve was reduced. Corrosion resistance resulting from autoclave testing (2 × 105 Pa of steam pressure, 120 °C) showed that the corrosion rate of NdFeB magnets reduced with the increase of additive amount. Microstructure observation revealed that MgO and ZnO additives were incorporated into the intergranular phases in the magnets. With the introduction of MgO and ZnO, more intergranular phase with high oxygen content was formed while keeping the volume fraction of all the intergranular phases almost unchanged, which may contribute to improved corrosion resistance. Furthermore, addition of MgO and ZnO refined the grain size of Nd22Fe71B7.

Sensitivity of coercivity and squareness factor of a Nd-Fe-B sintered magnet on post-sintering annealing temperature

Abstract An N38SH-grade magnet with low oxygen content was used to study the evolution of magnetic properties upon post-sintering annealing. Phase transformation of as-sintered magnet was investigated by differential scanning calorimetry (DSC). Three low temperature eutectic transition points were detected. Little change could be found when annealed below the lowest eutectic transition point. A wide annealing temperature range (460-560 °C) between the lowest and highest eutectic transition point was available for this magnet to achieve a relatively high coercivity (∼1671 kA/m) at a relatively low Dy content (∼3 wt.%). However, squareness factor (SF) of the demagnetizing curve and its temperature stability were found to decrease after annealing above the highest eutectic transition point. This was attributed to the change of Cu content in the Nd-rich phase under different annealing temperatures.

Microstructure evolution in TiAl alloyed with Si

Titanium aluminides are among those promising candidates for high-temperature structural use. Development of these alloys requires improved toughness and creep resistance while maintaining their inherently attractive properties. Eutectic alloy consisting of two intermetallic phases is a competitive design to the costly intermetallic-matrix composites in the present research. It has been shown that silicon additions to Ti and Ti{sub 3}Al lead to dual phase alloys containing Ti{sub 5}Si{sub 3} as the second phase. Based on previous work, it might be inferred that silicon additions to TiAl may introduce Ti{sub 5}Si{sub 3} in the matrix. Moreover, by referring to the Ti-Al diagram in the vicinity of the {gamma} field, changes to the liquidus valley in this region of the Ti-Al-Si system are implied. In this paper, investigation was undertaken to extend the understanding of the titanium aluminides which were alloyed with silicon and the Ti-Al-Si system.

Deduction of the chemical state and the electronic structure of Nd2Fe14B compound from X-ray photoelectron spectroscopy core-level and valence-band spectra

Characterization of chemical state and electronic structure of the technologically important Nd2Fe14B compound is attractive for understanding the physical nature of its excellent magnetic properties. X-ray photoelectron spectroscopy (XPS) study of such rare-earth compound is important and also challenging due to the easy oxidation of surface and small photoelectron cross-sections of rare-earth 4f electrons and B 2p electrons, etc. Here, we reported an investigation based on XPS spectra of Nd2Fe14B compound as a function of Ar ion sputtering time. The chemical state of Fe and that of B in Nd2Fe14B compound can be clearly determined to be 0 and −3, respectively. The Nd in Nd2Fe14B compound is found to have the chemical state of close to +3 instead of +3 as compared with the Nd in Nd2O3. In addition, by comparing the valence-band spectrum of Nd2Fe14B compound to that of the pure Fe, the contributions from Nd, Fe, and B to the valence-band structure of Nd2Fe14B compound is made more clear. The B 2p states and B 2s states are identified to be at ∼11.2 eV and ∼24.6 eV, respectively, which is reported for the first time. The contribution from Nd 4f states can be identified both in XPS core-level spectrum and XPS valence-band spectrum. Although Nd 4f states partially hybridize with Fe 3d states, Nd 4f states are mainly localized in Nd2Fe14B compound.

Strengthening of an Al-Containing Austenitic Stainless Steel at High Temperature

Three Al-containing austenitic steels with slightly different contents of Nb, V and C in the Fe-19.95Ni-14.19Cr-2.25Al-2.46Mo-1.95Mn-0.15Si-0.01B (wt.%) system were designed to study the effect of precipitations on creep/rupture resistance. After induction melting, alloys were cast into a metal mold followed by thermo-mechanical treatment. A continuous Al-rich oxide scale was formed on the surface after exposure at 800°C for 146 hrs in air. By decreasing the C content from 0.07 to 0.04%, coarse NbC precipitates in the as-cast microstructure could be removed during annealing treatment. Thermo-mechanical treatment enabled nano-scale precipitation of NbC in the alloys containing 0.04% C. Although the yield strength of the alloy with 0.07% C was relatively high at 750°C, its creep/rupture life was 164 hrs at 700°C/150 MPa. Alloys having low carbon content formed a uniform fine MC precipitation around 10–20 nm and showed a creep/rupture life between 1002 and 1530 hrs at 700°C/150 MPa. This is comparable with that of super304H tested under the same condition. Fe2(Mo,Nb) Laves phase was found in the microstructure after creep/rupture testing. NiAl precipitated in alloys after creep/rupture testing for more than 1000 hrs. However, strengthening effect from these two phases is not obvious, indicating that nano-scale NbC precipitates are the major source of strengthening during creep/rupture at high temperature. In addition, nano-scale (Nb,V)C was found in V containing alloy corresponding to the longest creep/rupture life.Copyright

Experimental and computational analysis of the two-step demagnetization behavior of the surface grains of sintered Nd-Fe-B magnets

The two-step demagnetization behavior was observed before the coercive field of a sintered Nd-Fe-B magnet with a large specific surface (S/V). The squareness factor of the demagnetization curve and the energy product decreased with the increase in the S/V, while the remanence and the coercivity were relatively independent of the S/V. The magneto-optical Kerr effect was used to study the magnetic properties of the magnet surface. The surface magnetization switched freely, while the bulk magnetization remained unchanged under a low cyclic magnetic field. Micromagnetic simulations revealed that the anti-demagnetization abilities of grains at the polar surface perpendicular to the c-axis were larger than those at the lateral surface parallel to the c-axis. The different anti-demagnetization abilities of the surface grains induced the initial two-step demagnetization behavior.The two-step demagnetization behavior was observed before the coercive field of a sintered Nd-Fe-B magnet with a large specific surface (S/V). The squareness factor of the demagnetization curve and the energy product decreased with the increase in the S/V, while the remanence and the coercivity were relatively independent of the S/V. The magneto-optical Kerr effect was used to study the magnetic properties of the magnet surface. The surface magnetization switched freely, while the bulk magnetization remained unchanged under a low cyclic magnetic field. Micromagnetic simulations revealed that the anti-demagnetization abilities of grains at the polar surface perpendicular to the c-axis were larger than those at the lateral surface parallel to the c-axis. The different anti-demagnetization abilities of the surface grains induced the initial two-step demagnetization behavior.

Electronic structure of the (Nd1−xDyx)2Fe14B (0 ≤ x ≤ 1) system studied by X-ray photoelectron spectroscopy

Systematic characterization of electronic structures in the (Nd1−xDyx)2Fe14B system, especially the 4f behavior, provides an insight to the physical nature of the evolution of magnetic properties. A series of X-ray photoelectron spectroscopy (XPS) core-level and valence-band spectra were used to study the electronic structures. It was found that substitution of Dy for Nd in Nd2Fe14B results in a nonlinear variation in the evolution of electronic structures. Only the finite coupling between the Nd 4f states and the Fe 3d states is found at both the Nd-rich regime and the Dy-rich regime. When the Dy concentration and the Nd concentration approach to be equal, a strong coupling between the Nd 4f states and the Fe 3d states is found, which results in a bonding state between them. Additionally, the 4f components in the (Nd1−xDyx)2Fe14B system are ascribed to three parts: 1) the individual contribution of the Dy 4f states, which emerges just after the Dy-substitution; 2) the contribution of the coupling between...