Longsha Wei

Magnetostructural coupling and magnetocaloric effect in Ni-Mn-Ga-Cu microwires

Ni-Mn-Ga-X microwires were produced by melt-extraction technique on a large scale. Their shape memory effect, superelasticity, and damping capacity have been demonstrated. Here, the excellent magnetocaloric effect was revealed in Ni-Mn-Ga-Cu microwires produced by melt-extraction and subsequent annealing. The overlap of the martensitic and magnetic transformations, i.e., magnetostructural coupling, was achieved in the annealed microwires. The magnetostructural coupling and wide martensitic transformation temperature range contribute to a large magnetic entropy change of −8.3 J/kg K with a wide working temperature interval of ∼13 K under a magnetic field of 50 kOe. Accordingly, a high refrigeration capacity of ∼78 J/kg was produced in the annealed microwires.

Gate length related transfer characteristics of GaN-based high electron mobility transistors

The measured drain current of GaN high electron mobility transistor (HEMT) shows a saturated characteristic as gate bias increases. HEMTs with short gate length have drain currents saturate more readily as compared to long gate length. In this paper, the relationship between drain currents saturation and gate bias was analyzed by using a simple series resistance model and was also validated by two-dimensional device simulation. The effects of these saturated transfer characteristics on electron mobility extraction of GaN HEMT were evaluated. Furthermore, the possible extension of GaN HEMT to logic inverter by utilizing the feature of saturation was also discussed.

Orientation dependent cyclic stability of the elastocaloric effect in textured Ni-Mn-Ga alloys

High-performance elastocaloric materials require a large reversible elastocaloric effect and long life cyclic stability. Here, we fabricated textured polycrystalline Ni50.4Mn27.3Ga22.3 alloys by cost-effective casting method to create a texture. A strong correlation between the cyclic stability and the crystal orientation was demonstrated. A large reversible adiabatic temperature change ΔT ∼6 K was obtained when the external stress was applied parallel to direction. However, the ΔT decreased rapidly after 50 cycles, showing an unstable elastocaloric effect (eCE). On the other hand, when the external stress was applied perpendicular to , the adiabatic ΔT was smaller ∼4 K, but was stable over 100 cycles. This significantly enhanced eCE stability was related to the high yield strength, low transformation strain and much higher crack initiation-propagation resistances perpendicular to direction. This study provides a feasible strategy for optimizing the eCE property by creation of the texture structure in polycrystalline Ni-Mn-Ga and Ni-Mn-X (X= In, Sn, Sb) alloys.High-performance elastocaloric materials require a large reversible elastocaloric effect and long life cyclic stability. Here, we fabricated textured polycrystalline Ni50.4Mn27.3Ga22.3 alloys by cost-effective casting method to create a texture. A strong correlation between the cyclic stability and the crystal orientation was demonstrated. A large reversible adiabatic temperature change ΔT ∼6 K was obtained when the external stress was applied parallel to direction. However, the ΔT decreased rapidly after 50 cycles, showing an unstable elastocaloric effect (eCE). On the other hand, when the external stress was applied perpendicular to , the adiabatic ΔT was smaller ∼4 K, but was stable over 100 cycles. This significantly enhanced eCE stability was related to the high yield strength, low transformation strain and much higher crack initiation-propagation resistances perpendicular to direction. This study provides a feasible strategy for optimizing the eCE property by creation of the t...