Jiheng Li

Self-consistent simulation of Stark shift of intersubband transition in modulation-doped step quantum wells

Self‐consistent simulations, including free carrier screening and many body effects, have been carried out for Stark shifts of intersubband transitions in step quantum wells. Unlike carrier screening in single quantum wells, where the charge distribution screens the applied field and reduces the Stark shift, our numerical results show that the asymmetric nature of the step quantum well may yield to an enhancement in the Stark shift. Discrepancies between experimental data and theoretical simulations are also addressed.

Variable stiffness Fe82Ga13.5Al4.5 spring based on magnetoelastic effect

The elastic modulus of magnetostriction materials can be controlled in real time by changing the magnetic field. Accordingly, spring with variable stiffness coefficients is available. In this work, Fe82Ga13.5Al4.5 wires of 3.5 mm diameter were prepared by heat forging, rolling, and drawing. Holding at 600 °C for 30 min after annealing at 1100 °C significantly improved the magnetostriction and soft magnetic properties of drawn wires. The tensile test shows two types of relative strain at the initial stage of tension: the elastic modulus under the condition of unsaturated magnetized (Ea) and purely mechanical elastic modulus (Es). The modulus defect ((Es−Ea)/Es × 100%) due to the additional magnetoelastic strain is up to 41.6%. Then, Fe82Ga13.5Al4.5 spring and the loading way of the magnetic field were designed and processed. By tuning the magnetic field strength, the stiffness coefficient of the spring is controlled accordingly. An apparent change in the stiffness coefficient of 10.86% is observed by furth...

Electromagnetic induced voltage signal to magnetic variation through torquing textured Fe81Ga19 alloy

The results of a study on the suitability of Fe-Ga alloys for torque sensor applications are presented. A Fe81Ga19 rod with a ⟨100⟩ preferred orientation along the length direction is prepared for the torque shaft and as the electromagnetic induction sensitive element, which is wound with three coils for signal excitation, signal pickup, and applied bias magnetic field, respectively. An apparent decrease in the induced voltage signal (peak voltage) of 3.88 mV is observed as the torque loading is 50 N m in the presence of a sine excitation signal (10 V, 1 kHz) and a bias current of 0.5 A. Meanwhile, a good repeatability and stress sensitivity are obtained, especially in the low torque range. These behaviors stem from the stress induced decrease in the magnetic permeability and the rotation of the arranged magnetic moment. Here, we use the Fe81Ga19 alloy as the shaft material; nevertheless, in practical use, the same effect can be achieved by forming a Fe-Ga layer with large magnetostriction on the surface ...

Tailoring magnetostriction with various directions for directional solidification Fe82Ga15Al3 alloy by magnetic field heat treatment

The magnetostriction of the Fe82Ga15Al3 alloy, along the length and width, can be tailored by applying a magnetic field heat treatment. In this work, the Fe82Ga15Al3 sheet was cut from the directional solidified Fe82Ga15Al3 alloy with the ⟨100⟩ preferred orientation and was annealed at 720 °C for 30 min under a magnetic field of 800 Oe along the length direction with a heating and cooling rate of 100 °C/min. The magnetostrictive properties along the length and width directions were modified to λ// = 7 ppm and λ⊥ = −210 ppm from λ// = 210 ppm and λ⊥ = −10 ppm for the initial sample prior to the magnetic field heat treatment. The cellular-like magnetic domain structure was composed of parallel 180° stripe domains and vertical 90° domains observed using a magnetic-force microscope. The change in magnetostriction along parallel and perpendicular directions was mainly resulted from the rotation of the magnetic domain units.

Influence of Al on the magnetostriction of Fe-Ga polycrystal alloys under compressive stress

Fe80Ga20−xAlx (x = 0, 6, 9, 14) ingots were prepared from high purity elements using a vacuum induction system. X-ray diffraction patterns show that the alloys are A2 disordered structures. The influence of the partial substitution of Ga in Fe-Ga alloys with Al on their magnetostrictive properties was investigated, and the effects of different heat treatment conditions on the magnetostriction and microstructure of the alloy rods were also examined. The saturation magnetostriction value of Fe80Ga20 can reach to 240 · 10−6 under a compressive stress of 20 MPa. The Fe80Ga11Al9 alloy has many good properties, such as low hysteresis, high linearity of the magnetostriction curve, and low saturated magnetic field, which make it a potential candidate for magnetostrictive actuator and transducer applications. It is found that subgrains have little influence on the magnetostriction of Fe-Ga alloys.

Magnetic Domain Motion and Magnetostriction in the Fe–Ga Sheets

The magnetic domain motion with varying magnetic field is investigated by magneto-optic Kerr microscopy on a slightly Goss misoriented Fe-Ga rolled sheet, and a close relationship between the magnetostriction and the domain motion is reported during the magnetization process. Along the rolling direction (RD), a high magnetostriction value λ// of 281 ppm is observed under no prestress for the sheet. The magnetic domains on the ~12° misoriented Fe-Ga (110) surface appear in the lancet shape and align as lancet comb structures. The lancet domain motion on the surface well explains the magnetostrictive performance λ// with the consideration of 90° domains within the lancet structure. The rapid increase of magnetostriction is attributed to the quick motion of 90° domains within the lancet structure under the field below 70~100 Oe. Under the higher magnetic field, these lancet domains gradually disappear with the field increasing, accompanied with the approach to the saturation magnetostriction along the RD.

Microstructure and magnetostrictive performance of NbC-doped oriented Fe-Ga alloys

The <100> oriented Fe83Ga17 alloy rods with various NbC contents less than 1at% were prepared by the directional solidification method at a growth rate of 720 mmh−1. Low NbC-content was found to affect the oriented grain growth and slightly improve the <100> orientation. Flat grain boundaries in the alloys with low NbC contents less than 0.2at% became greatly curved at higher NbC contents, and a large amount of Nb-rich precipitates were observed in the alloys with high NbC contents. Small amounts of NbC, less than 0.2at%, resulted in an increase in magnetostrictive strain due to the improvement of the <100> orientation, and a high magnetostrictive strain value of 335 10−6 under a pre-stress of 15 MPa was obtained in the 0.1at% NbC-doped alloys. The magnetostrictive performance obviously decreased with the NbC addition higher than 0.5at%, and the strain sensitivity under no pre-stress was lower than that in the binary Fe-Ga alloy.

Influence of yttrium on the structure and magnetostriction of Fe83Ga17 alloy

Polycrystalline Fe83Ga17 alloy rods with various amounts of yttrium were prepared by high vacuum induction melting. It is found that yttrium addition has a significant effect on the structure and magnetostriction of Fe83Ga17 alloy. The small addition of yttrium alters the solidification character and the grain shape of Fe83Ga17 alloy, and as a result, columnar grains with the 〈100〉 preferential direction are produced. Yttrium addition improves the magnetostrictive performance of the as-cast Fe83Ga17 alloy. The magnetostriction values of the as-cast alloy with 0.32at% and 0.64at% yttrium addition go up to 119×10−6 and 137×10−6 under 15 MPa compressive stress, respectively. The energy dispersive spectroscopy (EDS) result shows that almost all of the yttrium atoms exist in the Y2Fe17−xGax phase. A small amount of this kind of secondary phase cannot obviously increase the saturate magnetic field.

Temperature and magnetic field dependencies of the Young's modulus in magnetostrictive Fe-Ga alloys

Anomalous temperature dependences of Youngs modulus were observed in the as-cast and directionally solidified Fe100-xGax alloys (x = 20, 22, 24, 27). However, the absence of the anomalous temperature dependence of Youngs modulus in the DO19 and L12 ordered Fe73Ga27 alloys revealed that the anomalous temperature dependences of Youngs modulus of Fe-Ga alloys with different structures were only related with the structure transformation during the heating process. The relationship between the variations of Youngs modulus and structural transformation in the Fe78Ga22 alloy indicated that the lattice shrinkage owing to the structural transformations of A2 → DO3 and BCC(A2/DO3) → L12 was offset by the lattice expansion of the A2 phase caused by the thermal effect, which lead to a near constant coefficient of Youngs modulus in the temperature range from 500 °C to 560 °C. In addition, the variability of Youngs modulus as high as 12.8% was obtained in the as-directional solidified Fe73Ga27 alloy.

Enhancement of ductility and improvement of abnormal Goss grain growth of magnetostrictive Fe–Ga rolled alloys

The influences of initial microstructures on the mechanical properties and the recrystallization texture of magnetostrictive 0.1at% NbC-doped Fe83Ga17 alloys were investigated. The directionally solidified columnar-grained structure substantially enhanced the tensile elongation at intermediate temperatures by suppressing fracture along the transverse boundaries. Compared with tensile elongations of 1.0% at 300°C and 12.0% at 500°C of the hot-forged equiaxed-grained alloys, the columnar-grained alloys exhibited substantially increased tensile elongations of 21.6% at 300°C and 46.6% at 500°C. In the slabs for rolling, the introduction of <001>-oriented columnar grains also promotes the secondary recrystallization of Goss grains in the finally annealed sheets, resulting in an improvement of the saturation magnetostriction. For the columnar-grained specimens, the inhomogeneous microstructure and disadvantage in number and size of Goss grains are improved in the primarily annealed sheets, which is beneficial to the abnormal growth of Goss grains during the final annealing process.