Hongxian Shen

Combined current-modulation annealing induced enhancement of giant magnetoimpedance effect of Co-rich amorphous microwires

We report on a combined current-modulation annealing (CCMA) method, which integrates the optimized pulsed current (PC) and DC annealing techniques, for improving the giant magnetoimpedance (GMI) effect and its field sensitivity of Co-rich amorphous microwires. Relative to an as-prepared Co68.2Fe4.3B15Si12.5 wire, CCMA is shown to remarkably improve the GMI response of the wire. At 10 MHz, the maximum GMI ratio and its field sensitivity of the as-prepared wire were, respectively, increased by 3.5 and 2.28 times when subjected to CCMA. CCMA increased atomic order orientation and circumferential permeability of the wire by the co-action of high-density pulsed magnetic field energy and thermal activation energy at a PC annealing stage, as well as the formation of uniform circular magnetic domains by a stable DC magnetic field at a DC annealing stage. The magnetic moment can overcome eddy-current damping or nail-sticked action in rotational magnetization, giving rise to a double-peak feature and wider working field range (up to ±2 Oe) at relatively higher frequency (f ≥ 1 MHz).

Enhanced refrigerant capacity in Gd-Al-Co microwires with a biphase nanocrystalline/amorphous structure

A class of biphase nanocrystalline/amorphous Gd(50+5x)Al(30−5x)Co20 (x = 0, 1, 2) microwires fabricated directly by melt-extraction is reported. High resolution transmission electron microscopy and Fourier function transform based analysis indicate the presence of a volume fraction (∼20%) of ∼10 nm sized nanocrystallities uniformly embedded in an amorphous matrix. The microwires possess excellent magnetocaloric properties, with large values of the isothermal entropy change (−ΔSM ∼ 9.7 J kg−1 K−1), the adiabatic temperature change (ΔTad ∼ 5.2 K), and the refrigerant capacity (RC ∼ 654 J kg−1) for a field change of 5 T. The addition of Gd significantly alters TC while preserving large values of the ΔSM and RC. The nanocrystallites allow for enhanced RC as well as a broader operating temperature span of a magnetic bed for energy-efficient magnetic refrigeration.

Cryogenic Joule annealing induced large magnetic field response of Co-based microwires for giant magneto-impedance sensor applications

We have presented herein the results of microstructure, surface magnetic domains (SMDs), and giant magneto-impedance (GMI) effect of melt-extracted Co68.15Fe4.35Si12.25B11.25Nb2Cu2 amorphous wires for the first time employed by using a cryogenic Joule annealing (CJA) technique with large DC current amplitude. Compared with the conventional JA method, experimental results indicate that the maximum GMI ratio [ΔZ/Z0]max achieves up to 425% at 8.1 MHz with monotonic increase of the axial magnetic field Hex up to 6.5 Oe for 300 mA (equal to around 1.06 × 106 A/dm−2) CJA-ed wire, which is about 75% larger than the [ΔZ/Z0]max for the 100 mA (nearly 3.53 × 105 A/dm−2) JA-ed microwires. The remarkable features of large and linearly sensitive response field (2.5 ∼ 6.5 Oe) and the sensitivity of 99.4%/Oe with higher GMI ratio simultaneously make the CJA tailored melt-extracted microwires promising candidate materials for miniaturized GMI sensors. Another interesting result of GMI profiles of 200 mA (appropriately eq...

A soft ferromagnetic multiwire-based inductance coil sensor for sensing applications

We present an effective approach to improve the sensitivity of inductance coil sensors by designing a sensor core that consists of multiple soft ferromagnetic microwires. A systematic study of the longitudinally excited magneto-inductive (LEMI) effect has been performed in a non-magnetic copper wire coil with a filler composed of multiple Co-rich amorphous microwires. Melt-extracted microwires of Co68.2Fe4.3B15Si12.5 and glass-coated microwires of Co68B15Si10Mn7 with excellent soft magnetic properties were used for this study. We have shown that the LEMI ratio and field sensitivity of an inductive coil depend strongly upon the filler-to-air ratio inside the coil, the magnetic softness, and the anisotropy axis distribution of the microwire. Relative to a single-microwire based sensor, the LEMI ratio and field sensitivity of a multi-microwire based sensor are enhanced by three to four times, when varying the number of microwires inside the inductive coil. The sensitivity of the sensor using four glass-coate...

Relating surface roughness and magnetic domain structure to giant magneto-impedance of Co-rich melt-extracted microwires

Understanding the relationship between the surface conditions and giant magneto-impedance (GMI) in Co-rich melt-extracted microwires is key to optimizing their magnetic responses for magnetic sensor applications. The surface magnetic domain structure (SMDS) parameters of ~45 μm diameter Co69.25Fe4.25Si13B13.5-xZrx (x = 0, 1, 2, 3) microwires, including the magnetic domain period (d) and surface roughness (Rq) as extracted from the magnetic force microscopy (MFM) images, have been correlated with GMI in the range 1–1000 MHz. It was found that substitution of B with 1 at. % Zr increased d of the base alloy from 729 to 740 nm while retaining Rq from ~1 nm to ~3 nm. A tremendous impact on the GMI ratio was found, increasing the ratio from ~360% to ~490% at an operating frequency of 40 MHz. Further substitution with Zr decreased the high frequency GMI ratio, which can be understood by the significant increase in surface roughness evident by force microscopy. This study demonstrates the application of the domain period and surface roughness found by force microscopy to the interpretation of the GMI in Co-rich microwires.

The disparate impact of two types of GMI effect definition on DC Joule-heating annealed Co-based microwires

Based on a comprehensive study of the effect of progressive DC Joule-heating annealing (DJA) on giant magneto-impedance (GMI) properties of melt-extracted amorphous microwires, we systematically analysed the different mechanisms for two types of GMI effect definition. Experimental results show that DJA can improve GMI response characteristics and magnetic field sensitivity (MFS) effectively for both definitions, but ΔZ/Z0 is enhanced much more than ΔZ/Zmax of the as-cast wires. At 20 MHz, the maximum GMI ratios, as denoted by ΔZ/Z0 and ΔZ/Zmax, of DJA microwires are enhanced to 582.59% and 639.13%, while the values for the as-cast wires are 69.09% and 520.48% respectively. Meanwhile, the MFS (ξmax) and equivalent magnetic anisotropy field (Hk) increase to 1346.4%/Oe (ΔZ/Z0), 2927.9%/Oe (ΔZ/Zmax) and 1.1 Oe, respectively. These significant effects of GMI properties as denoted by Z0 are mainly attributed to the change of resistivity (ρdc) for amorphous microwires induced by DJA. Revealing the mechanism of different effects can also result in the development of a micromagnetic field sensor, especially for geomagnetic sensor applications (∼±1.0 Oe).

Tensile properties and fracture reliability of a glass-coated Co-based amorphous microwire

Co68.15Fe4.35Si12.25B15.25 (at%) amorphous microwires with a smooth surface and a circular cross-section were fabricated by the glass-coated melt spinning method. Their mechanical properties were evaluated through tensile tests of the glass-coated amorphous microwires, and their fracture reliability was estimated using two- and three-parameter Weibull analysis. X-ray diffraction and transmission electron microscopy results showed that these glass-coated Co-based microwires were mostly amorphous. The coated Co-based microwires exhibit a tensile strength of 1145 to 2457 MPa, with a mean value of 1727 MPa and a variance of 445 MPa. Weibull statistical analysis showed that the tensile two-parameter Weibull modulus of the amorphous microwires is 4.16 and the three-parameter Weibull modulus is 1.61 with a threshold value as high as 942 MPa. These results indicate that the fabricated microwires exhibit good tensile properties and fracture reliability, and thus appear to be good candidates for electronics reliability engineering applications.

Magnetocaloric Effect in Uncoated Gd55Al20Co25 Amorphous Wires

The Gd55Al20Co25 amorphous wires exhibit a relatively strong magnetocaloric effect (MCE). These melt-extracted amorphous wires display second-order magnetic transition (SOMT) and the value of maximal magnetic entropy (–ΔSm) for the melt-extracted wires is calculated to be ~9.7 J·kg–1·K–1 around the Curie point (TC) of ~110 K with applied field of 5 T. Moreover, the melt-extracted amorphous wires show a high refrigerating efficiency with a relatively large cooling power (RCP, ~804 J·kg–1) and refrigeration capacity (RC, ~580 J·kg–1) under an applied magnetic field of 5T due to the broad paramagnetic-ferromagnetic (PM-FM) region associated with amorphous alloys. These favorable properties make melt-extracted Gd-based amorphous wires to be the potential refrigerant for magnetic refrigeration (MR) of liquid oxygen.

Tensile Properties and Fracture Reliability of Melt-extracted Gd-rich Amorphous Wires

Gd60Al20Co20 amorphous wires with smooth surfaces and circular cross-sections were fabricated by a melt-extraction technique. The mechanical properties of the extracted microwires were evaluated by tensile tests and their fracture reliability was estimated by using Lognormal and two- or three- parameter Weibull analysis, respectively. The microwires exhibit a tensile fracture strength ranging from ~788 to ~1196 MPa, with a mean value of 1008 MPa and a standard variance of 121 MPa. Lognormal method of statistical analysis presents that the average stress of microwires is ~1012 MPa. Weibull statistical analysis indicates that the two-parameter tensile Weibull modulus is 8.5 and the three-parameter Weibull modulus is 5.3 with a threshold value ~365 MPa for as-extracted amorphous microwires. Our results show that the extracted Gd-based wires possess excellent tensile properties and high fracture reliability, with a high potential for applications in and magnetic refrigeration.

Formation Mechanism of Surface Crack in Low Pressure Casting of A360 Alloy

A surface crack defect is normally found in low pressure castings of Al alloy with a sudden contraction structure. To further understand the formation mechanism of the defect, the mold filling process is simulated by a two-phase flow model. The experimental results indicate that the main reason for the defect deformation is the mismatching between the height of liquid surface in the mold and pressure in the crucible. In the case of filling, a sudden contraction structure with an area ratio smaller than 0.5 is obtained, and the velocity of the liquid front increases dramatically with the influence of inertia. Meanwhile, the pressurizing speed in the crucible remains unchanged, resulting in the pressure not being able to support the height of the liquid level. Then the liquid metal flows back to the crucible and forms a relatively thin layer solidification shell on the mold wall. With the increasing pressure in the crucible, the liquid level rises again, engulfing the shell and leading to a surface crack. As the filling velocity is characterized by the damping oscillations, surface cracks will form at different heights. The results shed light on designing a suitable pressurizing speed for the low pressure casting process.