Structure and magnetic properties of mechanically alloyed Tb0.2Pr0.8(Fe0.4Co0.6)1.93 and magnetostriction of its epoxy composite

The C15 Laves phase with composition Tb0.2Pr0.8(Fe0.4Co0.6)1.93 was synthesized by mechanical alloying (MA) and subsequent annealing process. The structure and magnetic properties of Tb0.2Pr0.8(Fe0.4Co0.6)1.93 were investigated by means of X-ray diffraction (XRD), a vibrating sample magnetometer, and a standard strain technique. The effect of annealing on the structure and magnetic properties was studied. The analysis of XRD shows that the high Pr-content Tb0.2Pr0.8(Fe0.4Co0.6)1.93 alloy with the single phase of MgCu2-type structure can be success-fully synthesized by MA method. The sample annealed at 450℃ is fotmd to have a coercivity of 196 kA/m at room temperature. An ep-oxy/Tb0.2Pr0.8(Fe0.4Co0.6)1.93 composite was produced by a cold isostatic pressing technique. A large magnetostriction of 400 ppm, at an ap-plied magnetic field of 800 kA/m, was found for the composite. The epoxy-bonded Tb0.2Pr0.8(Fe0.4Co0.6)1.93 composite combines a high mag-netostriction with a significant coereivity, which is a promising magnetostrictive material.     

Structure and magnetostriction of Tb0.4Nd0.6(Fe0.8Co0.2)1.90 alloy prepared by solid-state synthesis

Mechanical alloying (MA) and subsequent solid sintering process was used to prepare the Nd-containing magnetostrictive Tb0.4Nd0.6(Fe0.8Co0.2)1.90 alloy. The structure, thermal stability and phase transformation were investigated as functions of composition, milling process and annealing temperature. An amorphous phase was formed by high-energy ball milling for 5 h with the ball-to-powder weight ratio of 20:1, which crystallized into MgCu2-type and PuNi3-type crystalline structure with different annealing temperatures. The magnetoelastic properties were investigated by means of a standard strain technique. The high Nd-content (Tb,Nd)(Fe,Co)2 Laves phase for the composition Tb0.4Nd0.6(Fe0.8Co0.2)1.90 was synthesized by MA process plus annealing at 500 ℃ for 30 min.     

The magnetocaloric effect of Gd-Tb-Dy-Al-M (M = Fe,Co and Ni) high-entropy bulk metallic glasses

In this article, we report the formation of the high-entropy Gd₂₀Tb₂₀Dy₂₀Al₂₀M₂₀ (M = Fe, Co and Ni) bulk metallic glasses with good magnetocaloric properties. Compared with most of the rare earth based metallic glasses, these alloys are found to have the comparably large maximum magnetic entropy changes (ΔS_M), but much broader widths of the ΔS_M peaks, and hence larger refrigerant capacity (RC). This can be attributed to the combination of the spin glass behaviors and the complicated compositions in these alloys. Our work show that the high entropy bulk metallic glasses is a promising candidate material as the magnetic refrigerant.     

Magnetocaloric effect of nonstoichiometric La1−xFe11.4+xSi1.6 alloys with first-order and second-order magnetic transitions

Variation from a first-order magnetoelastic transition (FOMT) to a second-order magnetic transition (SOMT) could be controlled by finely adjusting the Fe/La ratio in the non-stoichiometric La_1−xFe_11.4+xSi_1.6 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys. Particularly, a nearly stoichiometric ratio of NaZn₁₃-type phase was obtained with an addition of excess 0.15 at% Fe. When x value increased from 0 to 0.20, the Curie temperature (T_C) increased from 198.6 to 216.6 K due to lattice shrinkage and the magnetic entropy change (−ΔS_M) decreased from 18.4 to 8.0 J/(kg·K) (at 0−2 T) and from 22.5 to 13.8 J/(kg·K) (at 0−5 T), which may be ascribed to the change from the itinerant-electron metamagnetism (IEM) to second order magnetic transition. Thus their effective refrigeration capacities declined from 347.6 to 245.9 J kg⁻¹ under 0–5 T, which values were comparable to the Gd₅Si₂Ge_1.9Fe_0.1. The Fe/La ratio in NaZn₁₃-phase was possibly dominant to induce the transition from first to second order among La_1−xFe_11.4+xSi_1.6 alloys. The thermal hysteresis decreased by 87% to 0.3 K and magnetic hysteresis losses reduced by 90% to 1.4 J kg⁻¹ under 0–5 T. Near the transition border, the alloys with x = 0.15 produced a giant magnetocaloric effect of about 11.8 J/(kg·K) under 0–2 T at T_C = 205.6 K. The magnetic hysteresis was 3.01 J kg⁻¹ and the thermal hysteresis was 1.9 K. The material near the border of transition could be suitable for future magnetic refrigeration application.     

Crystal structure,electrical, and magnetic properties of the Sm2Cu0.8Ge3 compound

The crystal structure, electrical, and magnetic properties of the new ternary compound Sm₂Cu_0.8Ge₃ have been investigated using powder X-ray diffraction, electrical resistivity, and magnetic susceptibility measurements. X-ray diffraction patterns reveal that this compound crystallizes in a tetragonal α-ThSi₂ structure (space group I4₁/amd) with lattice parameters a = 0.413(1) nm and c = 1.420(3) nm. The irreversibility of the zero field cooled and field cooled dc magnetization data reveals the occurrence of spin glass transition with the spin freezing temperature T_f ∼ 8.5 K in this compound. Ac susceptibility and isothermal remanent magnetization data also confirm the existence of the spin glass phase. In addition, a broad maximum is observed in the ρ(T) curve near T_f. The spin glass phase is ascribed to the formation of random interaction between the Sm ions due to the irregular distribution of Cu and Ge atoms as well as the deficiency of the occupancy on the 8e crystallographic sites of the sample.     

Magnetism and magnetocaloric effect in the ternary equiatomic REFeAl (RE = Er and Ho) compounds

Magnetic properties and magnetocaloric effect (MCE) in ErFeAl and HoFeAl intermetallic compounds have been studied systematically. Both compounds undergo a second order magnetic phase transition from paramagnetic to ferromagnetic state together with a probable spin reorientation transition at low temperature. A considerable reversible magnetocaloric effect was observed around its own Curie temperatures T_C ∼55 K and 80 K for ErFeAl and HoFeAl, respectively. For a magnetic field change of 0–7 T, the maximum values of magnetic entropy change (−ΔS_M^max) are found to be 8.3 and 9.9 J/kg K for RE = Er and Ho, respectively. The corresponding values of refrigerant capacity (RC) are evaluated to be 384 and 647 J/kg.     

Magnetic properties and magnetocaloric effect in the rare earth-rich phases RE4PtMg (RE = Ho and Er)

The magnetic and magnetocaloric properties of Ho₄PtMg and Er₄PtMg with cubic Gd₄RhIn-type structure have been investigated by magnetization and heat capacity measurements. The compounds undergo a second order magnetic phase transition from a paramagnetic to a ferromagnetic state at Curie temperatures of T_C ∼ 28 and 16 K for Ho₄PtMg and Er₄PdMg, respectively. A large reversible MCE was observed around T_C for both compounds. For a field change of 0–7 T, the maximum values of magnetic entropy change (−ΔS_M^max) are 16.9 and 22.5 J/kg K for Ho₄PtMg and Er₄PtMg, respectively. The corresponding maximum values of adiabatic temperature change (ΔT_ad^max) are 5.3 and 6.5 K.     

Giant magnetocaloric effect in a Heusler Mn50Ni40In10 unidirectional crystal

A modified high-pressure optical zone-melting technique was used to grow a Mn-rich Heusler Mn₅₀Ni₄₀In₁₀ unidirectional crystal. Experimental results showed that the produced unidirectional crystal underwent a magnetic transition in austenite, followed with a martensitic transformation from a ferromagnetic austenite to a ferromagnetic martensite upon cooling. Under a magnetic field change of 30 kOe, the total effective refrigeration capacities (RC_total) reached as high as 231.58 J/kg when the magnetic field was applied along parallel to the crystal growth direction, or 246.79 J/kg when the magnetic field was applied along perpendicular to the crystal growth direction. It was suggested that this unidirectional crystal growing technique may provide an effective approach to enhance the magnetocaloric effect of Mn-rich Heusler materials.     

Zr_(0.8)Ti_(0.2)(Ni_(0.6)Mn_(0.2)V_(0.2)Cr_(0.05))_x贮氢合金结构和电化学性能的研究

研究了Zr0.8Ti0.2(Ni0.6Mn0.2V0.2Cr0.05)x(x=1.8～2.4)贮氢合金中化学计量x对晶体结构和电化学性能的影响。结果表明:随着x值的增大,合金中C14相含量逐渐减少,C15相含量逐渐增加,C14和C15相的晶格常数均线性减小;随着x值的增大,合金电极的活化性能提高,高倍率放电性能和放电容量均先升高,至x=2.2时达到最大值(最大放电容量为370mAh/g);超化学计量合金电极的循环寿命随x值的增大而降低,但当x<2.2时,经充放电循环500次以后容量保持率仍在80%左右。当化学计量值x等于2.2时,合金电极的综合电化学性能最好。     

Thermal and transport properties of as-grown Ni-rich TiNi shape memory alloys

Electrical resistivity, Seebeck coefficient, specific heat and thermal conductivity measurements on the Ti_50−xNi_50+x (x = 0.0–1.6 at.%) shape memory alloys are performed to investigate their thermal and transport properties. In this study, anomalous features are observed in both cooling and heating cycles in all measured physical properties of the slightly Ni-rich TiNi alloys (x ≤ 1.0), corresponds to the transformation between the B19′ martensite and B2 austenite phases. Besides, the transition temperature is found to decrease gradually with increasing Ni content, and the driving force for the transition is also found to diminish slowly with the addition of excess Ni, as revealed by specific heat measurements. While the signature of martensitic transformation vanishes for the Ni-rich TiNi alloys with x ≥ 1.3, the characteristics of strain glass transition start to appear. The Seebeck coefficients of these TiNi alloys were found to be positive, suggesting the hole-type carriers dominate the thermoelectric transport. From the high-temperature Seebeck coefficients, the estimated value of Fermi energy ranges from ∼1.5 eV (Ti_48.4Ni_51.6) to ∼2.1 eV (Ti₅₀Ni₅₀), indicating the metallic nature of these alloys. In addition, the thermal conductivity of the slightly Ni-rich TiNi alloys with x ≤ 1.0 shows a distinct anomalous feature at the B19′ → B2 transition, likely due to the variation in lattice thermal conductivity.     

La0.8-xCexMg0.2Ni3.5(x=0~0.20)贮氢合金电极的低温放电性能

用冷坩埚磁悬浮熔炼炉制各La0.8-xCexMg0.2Ni3.5(x=0.0.05,0.10,0.15,0.20)贮氢电极合金,采用X射线衍射、三电极体系系统研究合金的微观结构和电化学性能.研究表明:合金为多相结构,主相均为Ce2Ni7型六方相,还包括Cacu5型六方相、PuNi3型菱方相;P-C-T曲线显示,随着Ce含最的增加,合金放氢平台区域变窄,平台压力升高.合金中各组成相单胞体积的减小是其主要原因.随着Ce含量的增加,合金常温最大放电容量逐渐减小并且循环稳定性有一定改善;低温最大放电容量则先增大后减小,合金的低温高倍率放电性能以及交换电流密度均随Ce含量的增加而增加,但氢扩散系数随着Ce含量的增加而减小.     

Structural stability of some CsCl structure HfTM (TM = Co, Rh, Ru, Fe) compounds

In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed.     

Microstructure and martensitic transformation of TiNiNbB shape memory alloys

In present work, microstructure, martensitic transformation and mechanical properties of Ti₄₄Ni_47−xNb₉B_x (x = 0, 0.5, 1, 5 at.%) alloys were investigated as a function of B content. The results show that the addition of B significantly influences the microstructure of the alloys. The microstructure of Ti₄₄Ni₄₇Nb₉ alloy consists of B2 parent phase matrix and β-Nb phase. When the B content is 0.5 at.%, Nb₃B₂ phase presents. With further increasing B content to above 1 at.%, TiB and NbB phases present instead of Nb₃B₂ phase. With increasing B content, the transformation temperatures increase due to the reduced Ni/Ti ratio and Nb content in the matrix. The mechanical properties can be optimized by the addition of 1 at.% B.     

Magnetic and magnetocaloric properties of partially disordered RFeAl (R = Gd,Tb) intermetallic

Magnetic and magnetocaloric properties of TbFeAl and GdFeAl were studied in a wide temperature region in magnetic field up to 10 T. They order magnetically below T_c = 196 K and 259 K for TbFeAl and GdFeAl, respectively. The temperature change of ΔT = 1.4 K and 1.6 K was observed in GdFeAl and TbFeAl respectively for a field change of 4 T. Interestingly wide temperature region of significant magnetocaloric effect was observed in both compounds which give rise to relative cooling power of 350 J kg⁻¹ for TbFeAl and 348 J kg⁻¹ for GdFeAl for 4 T field span.     

Structural,half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb,As and Si) and IrMnAs from first-principles calculations

The structural, half-metallic and elastic properties of the half-Heusler compounds NiMnM (M = Sb, As and Si) and IrMnAs were investigated using first-principles calculations within the generalized gradient approximation (GGA) based on density function theory (DFT). The most stable lattice configurations about site occupancy are (Ni)_4a(Mn)_4c(Sb)_4d, (Ni)_4a(Mn)_4c(As)_4d, (Ni)_4a(Mn)_4c(Si)_4d and (Ir)_4a(Mn)_4c(As)_4d, respectively, and the exchange of elements in Wyckoff position 4c and 4d results in an identical (symmetry-related) phase. The half-Heusler compounds show half-metallic ferromagnetism with a half-metallic gap of 0.168 eV, 0.298 eV, 0.302 eV and 0.109 eV, respectively, and the total magnetic moments (M_tot) are 4.00 μ_B, 4.00 μ_B, 3.00 μ_B and 3.00 μ_B per formula unit, respectively, which agree well with the Slater–Pauling rule based on the relationship of valence electrons. The compound (Ir)_4a(Mn)_4c(As)_4d with half-metallic ferromagnetic character was reported for the first time. The individual elastic constants, shear modulus, Young's moduli, ratio B/G and Poisson's ratio were also calculated. The compounds are ductile based on the ratio B/G. The Debye temperatures derived from the average sound velocity (ν_m) are 327 K, 332 K, 434 K and 255 K, respectively. The predicted Debye temperature for NiMnSb agrees well with the available experimental value, and the Debye temperatures for the rest three compounds were reported for the first time.     

Role of precipitation in strength and ductility of FeCo-2V-Cr alloy with low coercivity

A good combination of high tensile strength as well as large elongation and low coercivity is achieved by optimal precipitation for FeCo-2V-Cr alloy. The precipitate is characterized by bcc structure with plate-like contrast and is disordered phase due to its enrichment of V and Cr in contrast to the partial ordering observed in matrix phase. The Cr addition is significant in such precipitation that leads to the improvement of both tensile strength and ductility without deteriorating soft magnetic property.     

Ternary RPt4B (R=La, Ce, Pr, Nd) compounds; structural and physical characterisation

The compounds RPt₄B, with (R=La, Ce, Pr, Nd), were synthesized and their crystal structure was studied either by single crystal X-ray diffraction and/or by conventional and synchrotron X-ray powder diffraction. All four compounds of this family are isostructural and belong to the CeCo₄B structure type. AC-susceptibility and magnetization studies show that: there is no magnetic ordering of the La compound down to 1.7 K; the Ce compound presents an antiferromagnetic-type-transition at 2.4 K; and both Pr and Nd compounds present a ferromagnetic-type transition at 4.2 and 4.9 K, respectively. Electrical resistivity studies show metallic behaviour for all compounds, the temperature dependence for the La compound being described by the Bloch Gruneisen relation. Thermopower studies as a function of temperature show that the thermopower is positive and small for these compounds, which is consistent with hole dominated metallic behaviour.     

Reduced Annealing Time and Enhanced Magnetocaloric Effect of La(Fe,Al)13 Alloy by La-nonstoichiometry and Si-doping

LaFe_13-xM_x (M = Si, Al) alloys are promising for use in magnetic refrigeration. However, they require long annealing time (30 days) in order to optimize the magnetocaloric properties. Research has shown that the addition of extra La in off-stoichiometric alloys can greatly shorten the annealing time. Therefore, the purpose of this study is to investigate the influence of the extra addition of La on the annealing properties of a new off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x (x = 0, 0.1, 0.4) alloys. It was demonstrated that after a 36h annealing time, a large volume fraction of 1:13 magnetocaloric phase was obtained for all alloys. Further microstructural analysis of the off-stoichiometric La_1.7Fe_11.6Al_1.4-xSi_x alloys revealed a facet-like grain morphology. The La_1.7Fe_11.6Al_1.4 and La_1.7Fe_11.6Al₁Si_0.4 alloys were shown to contain large 1:13 phase precipitates separated in a La-rich matrix, while the La_1.7Fe_11.6Al_1.3Si_0.1 alloy had a continuous 1:13 phase matrix with a fine dispersion of La-rich precipitates throughout. When the magnetic field varied between 0 and 2 T, the corresponding magnetic entropy change and relative cooling capacity for the La_1.7Fe_11.6Al_1.3Si_0.1 specimen were determined as 4.58 J/kg K and 173.6 J/kg, respectively. More importantly, the La_1.7Fe_11.6Al_1.3Si_0.1 alloy displayed only a slight volume change when the meta-magnetic phase transition occurred, which is promising for cyclic use.     

X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds

The magnetic properties of CeCo₇Mn₅ and CeCo₈Mn₄ compounds have been investigated by combining X-ray photoelectron spectroscopy (XPS) and magnetic measurements in a wide temperature range (4–550) K and magnetic field up to 12 T. X-ray powder diffraction (XRD) measurements showed that CeCo₇Mn₅ and CeCo₈Mn₄ compounds are isostructural and crystallize in the ThMn₁₂ structure type. XPS spectra pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure of CeCo₇Mn₅ and CeCo₈Mn₄ is determined by the competition between the ferromagnetic (Co–Co pairs) and antiferromagnetic (Co–Mn and Mn–Mn pairs) interactions. Two different ordering temperatures T_N and T_C correlated to antiferromagnetic and ferromagnetic coupling of 3d magnetic moments, respectively, are evidenced. Magnetic moments of about 1.6 μ_B/Co and 3.2 μ_B/Mn atoms were determined by correlating the magnetic data of the two compounds, in good agreement with the exchange splitting of XPS Co 3s and Mn 3s core levels.     

Synthesis and Properties of Monodisperse Superparamagnetic Mg_（0.8）Mn_（0.2）Fe_2O_4 Nanoparticles Using Polyol Reflux Method

Superparamagnetic monodisperse Mg0.8Mn0.2Fe2O4 nanoparticles have been successfully synthesized in liquid polyol at elevated temperature of 200 °C. Diethylene glycol(DEG) used here plays dual role in synthesis as it acts as reducing agent and alternatively coats the surface of nanoparticles while synthesis and thereby maintaining uniform size and dispersibility. Powder X-ray diffraction(XRD) and magnetic measurements showed that the sample is cubic spinel and superparamagnetic at room temperature. Raman spectra confirmed the formation of the Mg0.8Mn0.2Fe2O4 nanoparticles.The nanoparticles exhibit very good stability in water due to in situ coating with DEG molecules.