Xuexi Zhang

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 −8.3u2009J/kgu2009K with a wide working temperature interval of ∼13u2009K under a magnetic field of 50u2009kOe. Accordingly, a high refrigeration capacity of ∼78u2009J/kg was produced in the annealed microwires.

Electron heating and mode transition in dual frequency atmospheric pressure argon dielectric barrier discharge

Plasma ionization, excitation, mode transitions and associated electron heating mechanisms in atmospheric pressure dielectric barrier discharges (DBD) driven by dual radio frequency sources are investigated in this paper. The electrons are found to be heated mainly by the high frequency component in the plasma bulk when discharged in α mode. On the contrary, the low frequency component is primarily responsible for heating in the sheath which is caused by intense motion in the sheath. It was also found that variation of the lower frequency component ratio could effectively modulate the electron energy distribution as determined from time averaged EEDF. The results above have demonstrated that the independent control of plasma parameters via non-linear synergistic effect between the dual frequency sources can be achieved through reasonable selection of processing parameters.Plasma ionization, excitation, mode transitions and associated electron heating mechanisms in atmospheric pressure dielectric barrier discharges (DBD) driven by dual radio frequency sources are investigated in this paper. The electrons are found to be heated mainly by the high frequency component in the plasma bulk when discharged in α mode. On the contrary, the low frequency component is primarily responsible for heating in the sheath which is caused by intense motion in the sheath. It was also found that variation of the lower frequency component ratio could effectively modulate the electron energy distribution as determined from time averaged EEDF. The results above have demonstrated that the independent control of plasma parameters via non-linear synergistic effect between the dual frequency sources can be achieved through reasonable selection of processing parameters.

Enhanced magnetocaloric effect in Ni-Mn-Sn-Co alloys with two successive magnetostructural transformations

High magnetocaloric refrigeration performance requires large magnetic entropy change ΔSM and broad working temperature span ΔTFWHM. A fourth element doping of Co in ternary Ni-Mn-Sn alloy may significantly enhance the saturation magnetization of the alloy and thus enhance the ΔSM. Here, the effects of Co-doping on the martensite transformation, magnetic properties and magnetocaloric effects (MCE) of quaternary Ni47−xMn43Sn10Cox (xu2009=u20090, 6, 11) alloys were investigated. The martensite transformation temperatures decrease while austenite Curie point increases with Co content increasing to xu2009=u20096 and 11, thus broadening the temperature window for a high magnetization austenite (13.5, 91.7 and 109.1u2009A·m2/kg for xu2009=u20090, 6 and 11, respectively). Two successive magnetostructural transformations (Au2009→u200910u2009M and Au2009→u200910u2009Mu2009+u20096u2009M) occur in the alloy xu2009=u20096, which are responsible for the giant magnetic entropy change ΔSMu2009=u200929.5u2009J/kg·K, wide working temperature span ΔTFWHMu2009=u200914u2009K and large effective refrigeration capacity RCeffu2009=u2009232u2009J/kg under a magnetic field of 5.0u2009T. These results suggest that Ni40.6Mn43.3Sn10.0Co6.1 alloy may act as a potential solid-state magnetic refrigerant working at room temperature.

Ionization asymmetry effects on the properties modulation of atmospheric pressure dielectric barrier discharge sustained by tailored voltage waveforms

The dielectric barrier discharge (DBD) is a promising technology to generate high density and uniform cold plasmas in atmospheric pressure gases. The effective independent tuning of key plasma parameters is quite important for both application-focused and fundamental studies. In this paper, based on a one-dimensional fluid model with semi-kinetics treatment, numerical studies of ionization asymmetry effects on the properties modulation of atmospheric DBD sustained by tailored voltage waveforms are reported. The driving voltage waveform is characterized by an asymmetric-slope fundamental sinusoidal radio frequency signal superimposing one or more harmonics, and the effects of the number of harmonics, phase shift, as well as the fluctuation of harmonics on the sheath dynamics, impact ionization of electrons and key plasma parameters are investigated. The results have shown that the electron density can exhibit a substantial increase due to the effective electron heating by a spatially asymmetric sheath structure. The strategic modulation of harmonics number and phase shift is capable of raising the electron density significantly (e.g., nearly three times in this case), but without a significant increase in the gas temperature. Moreover, by tailoring the fluctuation of harmonics with a steeper slope, a more profound efficiency in electron impact ionization can be achieved, and thus enhancing the electron density effectively. This method then enables a novel alternative approach to realize the independent control of the key plasma parameters under atmospheric pressure.The dielectric barrier discharge (DBD) is a promising technology to generate high density and uniform cold plasmas in atmospheric pressure gases. The effective independent tuning of key plasma parameters is quite important for both application-focused and fundamental studies. In this paper, based on a one-dimensional fluid model with semi-kinetics treatment, numerical studies of ionization asymmetry effects on the properties modulation of atmospheric DBD sustained by tailored voltage waveforms are reported. The driving voltage waveform is characterized by an asymmetric-slope fundamental sinusoidal radio frequency signal superimposing one or more harmonics, and the effects of the number of harmonics, phase shift, as well as the fluctuation of harmonics on the sheath dynamics, impact ionization of electrons and key plasma parameters are investigated. The results have shown that the electron density can exhibit a substantial increase due to the effective electron heating by a spatially asymmetric sheath stru...

Effect of Co-Doping on the Microstructure, Martensitic Transformation Behavior, and Magnetocaloric Effect of Ni-Mn-Sb-Si Ferromagnetic Shape Memory Alloys

We have studied the effect of Co-doping on the microstructure, martensitic transformation, and magnetocaloric effect (MCE) of a Ni50Mn38Sb9Si3 alloy. Here, we further study the effect of Co-doping on the microstructure, martensitic transformation (MT), and MCE of Ni50Mn38Sb9Si3 alloy. The results show that phases α, β, and γ coexist in Ni50−xCoxMn38Sb9Si3 (xu2009=u20096, 12) alloys. The MT temperatures remain almost unchanged with the addition of Co element. Above the room temperature (RT), phase α exhibits MT under external magnetic fields up to 50xa0kOe, and possesses a great enhancement of ΔM from 0.008xa0emu/g in Ni50Mn38Sb9Si3 alloy to 21.4 emu/g in Ni38Co12Mn38Sb9Si3 alloy as well as an improvement of ∂M/∂T (obtained from M-T curves) that benefit ΔSM. The MT temperature intervals of M-T curves are enlarged from 3xa0K in the un-doped alloy to 11xa0K in the water-quenched Ni38Co12Mn38Sb9Si3 alloy attributed to the reduced coupling between magnetic and structural transition. This contributes to a magnetic entropy change (ΔSM) of 4.62xa0J/kgxa0K and net refrigeration capacity (RCnet) of 18.5xa0J/kg under a magnetic field of 50xa0kOe, which is superior to that of the ferromagnetic compounds containing secondary precipitates.

Dataset on enhanced magnetic refrigeration capacity in Ni–Mn–Ga micro-particles

The dataset presented in this paper is supporting the research article “Enhanced magnetic refrigeration capacity in Ni-Mn-Ga micro-particles” (Qian, et al., 2018) [1]. The martensite transformation temperature (Ms) and the Curie point (Tc) of the annealed alloys with nominal composition Ni55-xMn20+xGa25 (x = 0, 0.25, 0.5, 1, atomic percent, labeled as A1, A2, A3 and A4, respectively) varied with x, yielding a temperature difference Tc-Mc of 7.2u202fK at x = 0.5 (A3). The magnetization difference (ΔM) between the austenite and martensite, the field dependence of transformation temperature (ΔT/ΔH) and the thermal hysteresis loss of A3 and the according stress relief annealing (SRA) particles were demonstrated. The isothermal magnetization curves of A3 and the SRA particles were measured in order to determine the magnetocaloric effect.

Dataset on the microstructure Ni50Mn38Sb9Si3 alloy and compositions of Ni50Mn38Sb12−xSix (x=2.5, 3) ferromagnetic shape memory alloys

The data presented in this article is the supplementary data of Zhang et al. (2018) [1]. The Ni50Mn38Sb9Si3 alloy is annealed at 1223u202fK for 24u202fh and then quenched into ice water; while the Ni50Mn38Sb9.5Si2.5 alloy is annealed at 1173u202fK for 24u202fh and then quenched into ice water. The microstructure of the Ni50Mn38Sb9Si3 alloy indicates that a higher heat treatment temperature cannot prevent the formation of secondary phases. Furthermore, the composition of α phase is similar to the nominal composition of the alloy. On the other hand, the nominal concentration of Si atoms and heat-treatment temperature do not affect the compositions of the β and γ phases. For example, the compositions of the β and γ phases in the Ni50Mn38Sb9Si3 alloy are similar when annealed at 1223u202fK for 24u202fh and 1173u202fK for 24u202fh

Elastocaloric effects in ultra-fine grained NiTi microwires processed by cold-drawing

Efficient elastocaloric cooling in shape memory alloys requires a stable superelastic behavior in which high yield strength is needed. Here Ni50.4Ti49.6 microwires with diameter 130 μm and ultra-fine grains ∼30 nm were prepared by multi-step cold-drawing and low-temperature annealing. Enhanced cyclic stability of the elastocaloric effects induced by the superelastic training was demonstrated. The pre-trained microwire showed a stable ΔSe 43 J/(kg K) with a broad working temperature range ΔT ∼ 70 K. The superelastic trained microwire, with giant and stable ΔSe over a wide working temperature window, may act as a promising elastocaloric cooling material for minor-sized devices.

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...

Superelasticity in Polycrystalline Ni-Mn-Ga-Fe Microwires Fabricated by Melt-extraction

Ni48Mn26.4Ga19.7Fe5.9 microwires with grain size of 1-3 micron were successfully fabricated by melt-extraction. The superelastic effects in the microwires under various temperatures and loads were systematically demonstrated. The as-extracted microwires displayed partial superelasticity when attended at relatively high temperature. The critical stress for stress-induced martensite formation increases linearly with temperature and follows the Clausius-Clapeyron relationship. The temperature dependence of the as-extracted polycrystalline Ni48Mn26.4Ga19.7Fe5.9 microwires is 16.4 MPa/K, which is higher compared with Ni-Mn-Ga single crystals. In addition, the as-extracted microwires display excellent shape memory behavior with the recovery strain and recovery ratio of 1.26% and 86%, respectively, when the total strain reaches 1.47% at 310 K.