Elastocaloric effect and mechanical behavior for NiTi shape memory alloys

The NiTi shape memory alloy exhibits excellent superelastic property and elastocaloric effect. The large temperature change(DT) value of 30 K upon loading and-19 K upon unloading are obtained at room temperature, which are higher than those of the other NiTi-based materials and among the highest values reported in the elastocaloric materials. The asymmetry of the measured DT values between the loading and unloading process is ascribed to the friction dissipation.The large temperature change originates from the large entropy change during the stress-induced martensite transformation(MT) and the reverse MT. A large coefficient-of-performance of the material is obtained to be 11.7 at ε= 1%, which decreases with increasing the applied strain. These results are very attractive in the present solid-state cooling, which potentially could replace the vapor compression refrigeration technologies.     

Magnetism and giant magnetocaloric effect in rare-earth-based compounds R_3BWO_9(R = Gd,Dy, Ho)

The magnetism and magnetocaloric effect(MCE) of rare-earth-based tungstate compounds R_3 BWO_9(R=Gd,Dy,Ho) have been studied by magnetic susceptibility,isothermal magnetization,and specific heat measurements.No obvious long-range magnetic ordering can be found down to 2 K.The Curie-Weiss fitting and magnetic susceptibilities under different applied fields reveal the existence of weak short-range antiferromagnetic couplings at low temperature in these systems.The calculations of isothermal magnetization exhibit a giant MCE with the maximum changes of magnetic entropy being 54.80 J/kg-K at 2 K for Gd_3 BWO_9,28.5 J/kg-K at 6 K for Dy_3 BWO_9,and 29.76 J/kg-K at 4 K for Ho_3 BWO_9,respectively,under a field change of 0-7 T.Especially for Gd_3 BWO_9,the maximum value of magnetic entropy change(-ΔS_M~(max)) and adiabatic temperature change(-ΔT_(ad)~(max)) are 36.75 J/kg·K and 5.56 K for a low field change of 0-3 T,indicating a promising application for low temperature magnetic refrigeration.     

Magnetic properties and magnetocaloric effects in HoPd intermetallic

A large reversible magnetocaloric effect accompanied by a second order magnetic phase transition from PM to FM is observed in the Ho Pd compound. Under the magnetic field change of 0–5 T, the magnetic entropy change-ΔS max M and the refrigerant capacity RC for the compound are evaluated to be 20 J/(kg·K) and 342 J/kg, respectively. In particular,large-ΔS max M(11.3 J/(kg·K)) and RC(142 J/kg) are achieved under a low magnetic field change of 0–2 T with no thermal hysteresis and magnetic hysteresis loss. The large reversible magnetocaloric effect(both the large-ΔS M and the high RC)indicates that Ho Pd is a promising material for magnetic refrigeration at low temperature.     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

Magnetic transition and large reversible magnetocaloric effect in EuCu1.75P2 compound

The magnetocaloric effect(MCE) in EuCu1.75P2 compound is studied by the magnetization and heat capacity measurements.Magnetization and modified Arrott plots indicate that the compound undergoes a second-order phase transition at TC ～ 51 K.A large reversible MCE is observed around TC.The values of maximum magnetic entropy change(-△SxMma) reach 5.6 J·kg-1·K-1 and 13.3 J·kg-1·K-1 for the field change of 2 T and 7 T,respectively,with no obvious hysteresis loss in the vicinity of Curie temperature.The corresponding maximum adiabatic temperature changes(△Tadmax) are evaluated to be 2.1 K and 5.0 K.The magnetic transition and the origin of large MCE in EuCu1.75P2 are also discussed.     

Table-Like Large Magnetocaloric Effect in the Misch Metal RSi Compound

Magnetic properties and the magnetocaloric effect(MCE)of the RSi(R=Ce,Pr,Nd)compounds made of Misch metal(MM)are investigated.Two transitions are found at 12K and 38K.Field variation generated large MCE and two peaks are found in the magnetic entropy change(△S)curves,which correspond to the two transition temperatures.The maximum values of the magnetic entropy changes(△S)are found to be-5.1 J/(kg·K)and-9.3 J/(kg·K)for the field ranges of 0-2 T and 0-5 T,respectively.The large AS as well as ultra-low price of MM make(MM)Si a competitive magnetic refrigerant candidate for low temperature in Eriksson cycle.     

Magnetism and magnetic entropy change in LaFe11Al2Cx compounds around room temperature

Magnetism and magnetic entropy changes in LaFe11A12Cx(x=0.0, 0.2 and 0.5) compounds have been investigated.The Curie temperature TC is conveniently controlled from 200K to room temperature by varying the carbon concentration.Large magnetic entropy change is obtained over a wide temperature range due to the high magnetization and the drastic decrease in the magnetization around TC.The large magnetic entropy change in wide temperature range,low cost and the convenience of controlling TC suggest that the LaFe11Al2Cx compounds are promising candidates for magnetic refrigerants in the corresponding temperature range.     

Magnetic properties and magnetocaloric effects of Mn5Ge3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1,0.2,0.3,0.4,0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically.The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9).The average Mn magnetic moments increases with increasing Ga content,reaching a maximum value at x=0.6.The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation.The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3.One is that the magnitude of the magnetic entropy change decreases,the other is that the MCE peak becomes broadened.     

Abnormal breakdown of Stokes–Einstein relation in liquid aluminium

We present the results of systematic molecular dynamics simulations of pure aluminium melt with a well-accepted embedded atom potential. The structure and dynamics were calculated over a wide temperature range, and the calculated results(including the pair correlation function, self-diffusion coefficient, and viscosity) agree well with the available experimental observations. The calculated data were used to examine the Stokes–Einstein relation(SER). The results indicate that the SER begins to break down at a temperature T_x(~1090 K) which is well above the equilibrium melting point(912.5 K).This high-temperature breakdown is confirmed by the evolution of dynamics heterogeneity, which is characterised by the non-Gaussian parameter α_2(t). The maximum value of α 2(t), α_(2,max), increases at an accelerating rate as the temperature falls below Tx. The development of α_(2,max) was found to be related to the liquid structure change evidenced by local fivefold symmetry. Accordingly, we suggest that this high-temperature breakdown of SER has a structural origin. The results of this study are expected to make researchers reconsider the applicability of SER and promote greater understanding of the relationship between dynamics and structure.     

Magnetic properties and magnetocaloric effect in compound PrFe12B6

Magnetic properties and magnetocaloric effect of compound PrFe 12 B 6 are investigated.The coexistence of hard phase PrFe 12 B 6 and soft phase α-Fe causes interesting phenomena on the curves for the temperature dependence of magnetization.PrFe 12 B 6 experiences a first order phase transition at the Curie temperature 200 K,accompanied by an obvious lattice contraction,which in turn results in a large magnetic entropy change.The Maxwell relation fails to give the correct information about magnetic entropy change due to the first order phase transition nature.The large magnetic entropy changes of PrFe 12.3 B 4.7 obtained from heat capacity method are 11.7 and 16.2 J/kg.K for magnetic field changes of 0-2 T and 0-5 T respectively.     

Multicaloric effect in a piezoelectric layer

Changes in the temperatures and entropy of a piezoelectric layer under the electric field and stresses applied to its boundaries (a multicaloric effect) have been investigated. It has been shown that these changes are composed of three terms, which describe the electrocaloric, elastocaloric, and piezoelectrocaloric effects, respectively. If the influence of the strain gradient on the polarization is taken into account, one more term associated with the flexoelectrocaloric effect arises in the entropy change. The influence of the multicaloric effect on losses and motion of domain walls in ferroelectrics has been discussed.     

Magnetic and magnetocaloric effect in a stuffed honeycomb polycrystalline antiferromagnet GdInO3

The magnetic and magnetocaloric properties were studied in a stuffed honeycomb polycrystalline antiferromagnet GdInO₃. The onset temperature of antiferromagnetic ordering was observed at ～ 2.1 K. Negligible thermal and magnetic hystereses suggest a reversible magnetocaloric effect (MCE) in the GdInO₃ compound. In the magnetic field changes of 0 kOe-50 kOe and 0 kOe-70 kOe, the maximum magnetic entropy change values are 9.65 J/kg· K and 18.37 J/kg· K, respectively, near the liquid helium temperature, with the corresponding relative cooling power values of 115.01 J/kg and 211.31 J/kg. The MCE investigation of the polycrystalline GdInO₃ serves to illuminate more exotic properties in this frustrated stuffed honeycomb magnetic system.     

Elastocaloric effect in rubber on exposure to a periodic tensile force

The elastocaloric effect (ECE) is experimentally studied in latex rubber subjected to the periodic action of a tensile force. The ECE is measured as a function of the relative elongation and the cycle frequency up to 4 Hz. Three cycle frequencies in which different thermodynamic processes, namely, elastocaloric cooling, self-heating, and a decrease in the ECE, occur are revealed. The maximum measured value of ECE in latex rubber reaches 14 K at a relative strain of 700% in the frequency range 0.1–0.3 Hz. The possibility of practical application of latex rubber for elastocaloric cooling is discussed.     

Studying the elastocaloric effect in a fast-quenched Ti<Subscript>2</Subscript>NiCu ribbon with the shape memory effect

The elastocaloric effect (ECE) in fast-quenched Ti₂NiCu ribbon under the periodic action of tensile force is studied experimentally. The ECE is measured as a function of relative elongation and the frequency of cycles in the range of 0.2 to 4 Hz. The maximum measured ECE in the alloy is 9.4 K under a mechanical load of 300 MPa at a relative deformation of 1%, frequencies of 0.2 to 0.5 Hz, and a temperature of 67°C. The specific power of the ribbon is estimated; it reaches its maximum at a frequency of f = 4 Hz, corresponding to W = 10 W g^?1. The possible practical use of Ti₂NiCu alloy for elastocaloric cooling is discussed.     

Transport coefficients and flux motion in Bi2Sr2CaCu2Ox single crystals

We present measurements of the electrical resistivity, thermal conductivity, and Hall, Nernst, and Seebeck effects in the mixed state of single crystalline Bi₂Sr₂CaCu₂O_x. It is shown that the sign of the Hall voltage changes twice as temperature decreases below T_c. From the Nernst effect we estimate the transport entropy S_φ to be about 10⁻¹⁰ erg/K cm. S_φ is equal to zero in the normal state, increases and passes through a maximum at the mixed state as expected. The temperature dependences of the thermoelectric power in magnetic fields are analogous to the resistive transition curves. These phenomena are discussed in terms of flux flow. The contribution of the flux flow to the thermal conductivity is estimated to be negligible. Lowering of the thermal conductivity at temperatures below T_c by a magnetic field is attributed to phonon scattering by the vortex lines.     

Magnetic entropy change and magnetic properties of LaFe_(11.5)Si_(1.5) after controlling the Curie temperature by partial substitution of Mn and hydrogenation

Magnetic properties and magnetic entropy changes of La(Fe_(1-x)Mn_x)_(11.5)Si_(1.5)H_y compounds are investigated. Their Curie temperatures are adjusted to room temperature by partial Mn substitution for Fe and hydrogen absorption in 1-atm(1 atm = 1.01325×10~5Pa) hydrogen gas. Under a field change from 0 T to 2 T, the maximum magnetic entropy change for La(Fe_(0.99)Mn_(0.01))_(11.5)Si_(1.5)H_(1.61)is-11.5 J/kg. The suitable Curie temperature and large value of ?S_m make it an attractive potential candidate for the room temperature magnetic refrigeration application.     

应力作用下EuTiO3铁电薄膜电热效应的唯象理论研究

基于Laudau-Devonshire的热动力学模型, 计算了EuTiO₃铁电薄膜材料的电热效应. 结果显示在外加应力的调控下, 电极化、电热系数以及绝热温差都会随之变化. 外加垂直于表面的张应力加大, 薄膜的相变温度升高, 绝热温差增加, 最大绝热温差所对应的工作温度向高温区移动. 对于二维平面失配应变u_m =-0.005的薄膜, 当外加张应力σ₃ = 5 GPa时, 其最大电热系数为1.75×10^-3 C/m²·K, 电场变化200 MV/m 时室温下绝热温差ΔT 的最大值可达到14 K 以上, 绝热温差ΔT ≥13 K 的工作温区超过120 K, 表明可以通过调控外部应力来获取室温时较大的绝热温差. 此结果预示着铁电EuTiO₃ 薄膜在室温固态制冷方面可能具有较好的应用前景.     

Large magnetocaloric effect in van der Waals crystal CrBr3

We study the magnetocaloric effect (MCE) in van der Waals (vdW) crystal CrBr₃. Bulk CrBr₃ exhibits a second-order paramagnetic-ferromagnetic phase transition with T_C = 33 K. The maximum magnetic entropy change −ΔS_M near T_C is about 7.2 J·kg⁻¹·K⁻¹ with the maximum adiabatic temperature change ΔT^max_ad = 2.37 K and the relative cooling power RCP= 191.5 J·kg⁻¹ at μ₀H = 5 T, all of which are remarkably larger than those in CrI₃. These results suggest that the vdW crystal CrBr₃ is a promising candidate for the low-dimensional magnetic refrigeration in low temperature region.     

Ni47Mn32Ga21多晶合金的磁熵变和磁感生应变

对定向凝固方法制备的Ni₄₇Mn₃₂Ga₂₁多晶合金,通过扫描电镜、金相、电子能谱等手段研究其组份和组织形貌,通过对合金磁化强度与温度关系、等温磁化曲线及磁感生应变曲线等的测量分析,研究了合金结构相变和磁相变过程中的磁熵变及不同压力下的磁感生应变. 研究结果表明:合金组份与设计组份基本一致,室温下合金大部分为马氏体相. 升温过程中合金的磁熵变在居里温度(365 K)附近有最大值,并有较大的磁熵变峰值半高宽,747 kA/m的磁场下该磁熵变最大值为-1.45 J/kg ·K,磁熵变峰值的半高宽为21 K. 合金在室温(298 K)下有较好的双向可恢复磁感生应变,480 kA/m磁场下,无压力时合金的磁感生应变值达到-670×10^-6,并趋饱和;而在与磁场方向平行的27.3 MPa外压力作用下合金的磁感生应变值增大到-1300×10^-6,且未饱和. 关键词： Ni-Mn-Ga 铁磁形状记忆合金 磁熵变 磁感生应变     

Low temperature specific heat of dense kondo ferromagnets and intermediate valence CeNixPtt-x compounds

Specific heat measurements performed in the temperature range 1.5 to 30 K in the compounds CeNi_xPt_{1 - x} with x = 0.5, 0.8 and 0.95 are presented. The analysis of the magnetic entropy enhances the results obtained previously by magnetic measurements: i) the reduction of the cerium moment due to the Kondo effect; ii) the increase of the importance of the Kondo effect in competition with the RKKY interactions when Pt is replaced by Ni. The variation of γ along the series indicates a maximum around x = 0.9 which corresponds to the magnetic instability. It is worth noting that below T_c in the ferromagnetic compounds, C_mag follows a T law, characteristic of magnon dispersion.