Yu-Xian Liu

One step convenient synthesis of crystalline β-Si3N4

Well-crystallized β-Si3N4 was directly prepared at an initial reaction temperature of 150 °C through the reaction of SiCl4 and NaN3 in the presence of a small amount of CCl4. Characterization by X-ray diffraction, high-resolution electron microscopy, X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy indicates that the product synthesized is crystalline β-Si3N4. The yield of β-Si3N4 is about 86% based on the amount of precursor SiCl4 used at the initial reaction temperature of 150 °C, and is more than 90% at the reaction temperature of 200 °C with the product β-Si3N4 in high crystallinity. The dominant morphology of the product is short rods with the growth axis along the [001] direction. The formation mechanism of crystalline β-Si3N4 was discussed briefly, and the role of CCl4 in the formation process of β-Si3N4 was analyzed.

The sensitive magnetoimpedance effect in Fe-based soft ferromagnetic ribbons

A comprehensive analysis of magnetoimpedance (MI) phenomena in Fe-based nanocrystalline ribbons is presented in this paper. Giant MI responses have been observed in nanocrystalline samples annealed over the temperature range 490 and . More than 400% increases of the MI ratio were obtained both in and in ribbons. The sensitivity can reach a value larger than in the field range 3 - 7.2 Oe at 800 kHz for ribbons. No MI effect was observed in an as-quenched amorphous sample, because of its very low permeability. A mostly resistive feature was found in an as-quenched amorphous sample. The field dependences of the effective permeability show a typical transverse anisotropy property, and the development of the transverse anisotropy in naturally annealed Fe-based ribbons was analysed.

Sensitive field- and frequency-dependent magnetoimpedance in nanocrystalline Fe73.5CuNb3Si13.5B9 ribbons

The magnetoimpedance spectra (0 less than or equal to f less than or equal to 3 MHz) of amorphous and of nanocrystallized (550 degrees C annealing for 0.5-18 h) FeCuNbSiB ribbons were measured at room temperature using a commercial impedance analyzer as a function of applied magnetic field up to 75 Oe. The magnetoimpedance (MI), Z(f, H), shows strong dependence on frequency of the drive, ac current and magnetic field. For all the samples, the MI ratio Delta Z/Z = (Z(H-max - Z(0)\Z(H-max) first increases rapidly with frequency (f) and reaches a maximum at about 300-500 kHz: then decreases with f. The maximum value of the MI ratio Delta Z/Z, - 227%, is obtained in the sample annealed at 550 degrees C for 3 h with ac current at 300 kHz. At higher frequency, apeak in field dependence of the MI ratio (Z(H) - Z(0))/Z(H-max) was obtained for nanocrystalline samples, but not for as-quenched amorphous counterparts. The field dependence of effective permeability has been shown to be responsible for their MI effects

Microstructural changes of a CuZnAlMnNi shape memory alloy due to cooling in liquid nitrogen

The microstructures of a CuZnAlMnNi alloy immediately after quenching, as well as further cooling in liquid nitrogen, were investigated by scanning electron microscopy and transmission electron microscopy. It was found that when the as-quenched alloy was subjected to cooling in liquid nitrogen, the spear-like martensite changed into the granular microstructures, resulting in the destruction of the continuity of the martensite plates and the increment of the boundary area. Simultaneously, the stacking faults in the original martensite plates became confused and the degree of ordering decreased from 0.0929 to 0.0417 A.

Magneto-impedance effect in nanocrystalline ribbons

Magneto-impedance effects have been studied in amorphous and nanocrystalline ribbons. Large magneto-impedance responses have been obtained in nanocrystalline samples by applying a longitudinal DC field, but not in amorphous samples. It was found that the transverse field dependences of the resistance and the reactance of nanocrystalline samples show very broad peaks at relatively high fields (above 35 Oe), while the longitudinal magnetic responses show very sharp peaks at the field of 2 - 7 Oe, sharing some common features with those reported for Co-based amorphous wire and ribbon samples, and can be understood from the same mechanism. It was also found experimentally that the properties of the longitudinal and transverse field dependences of the effective permeability of nanocrystalline samples correlates with those of the impedance, showing a sharp decrease for the longitudinal applied field and a slow decrease for the transverse applied field at a corresponding field value and, the higher the effective permeability, the larger is the magneto-impedance effect at a fixed frequency. and a sensitivity of 20% at a low field (2 Oe) are obtained in this paper.

Structural change due to martensite aging of CuZnAlMnNi shape memory alloy

The microstructures of a CuZnAlMnNi shape memory alloy after one year aging in martensite phase were investigated by transmission electron microscopy. It was found that the substructure of stacking faults in the original martensite plates becomes indistinct, and the amount decreases. However, the equilibrium face centered cubic (fcc) α-phase can be observed forming at martensite plate boundaries or inside the plates. The structural variation during aging is responsible for the degradation of shape memory property and transformation temperatures when Cu-based SMA actuators are in use.