High frequency properties of composite membrane with in-plane aligned Sendust flake prepared by infiltration method

Composite membranes (with thickness around 100–200 μm) containing highly in-plane aligned Sendust flakes embedded in polyvinyl alcohol matrix were prepared with a novel infiltration method. As compared with tape-casting method, infiltration method results in enhanced magnetic permeability, which could be caused by better alignment and less porosity. Annealing process could modify the grain size, improve saturation magnetization and coercivity of Sendust flakes. Hence, the radio and quasi-microwave frequency permeability (between 10 MHz to 3 GHz) of composites membranes with annealed Sendust flakes could be enhanced significantly as compared with that of the as-prepared flakes. Infiltration method is especially suitable for composites with high concentration of flaky fillers. The composite membranes prepared have potential applications, including electromagnetic shielding, noise reduction and wave absorption.     

Analysis of the magnetoresistance contributions in a nanocrystallized Cr-doped FINEMET alloy

The magnetoresistance (MR) was measured at 200, 250 and 300 K in magnetic fields up to B=12 T for a nanocrystallized Fe_63.5Cr₁₀Nb₃Cu₁Si_13.5B₉ alloy. Both the longitudinal (LMR) and transverse (TMR) component of the magnetoresistance decreased from B=0 to about 0.1 T. This could be ascribed to a giant MR (GMR) effect due to spin-dependent scattering of conduction electrons along their path between two Fe-Si nanograins via the non-magnetic matrix. Such a scattering may occur if the nanograin moments are not or only weakly coupled in the absence of a strong exchange coupling (due to the high Cr content in the matrix) and/or only weak dipole-dipole coupling is present (due to sufficiently large separations between the nanograins). For larger fields, the GMR saturated and a slightly nonlinear increase in MR with B was observed due to a contribution by the residual amorphous matrix. The anisotropic MR effect (AMR≡LMR−TMR) was negative for all fields and temperatures investigated. By measuring the MR of melt-quenched Fe_100−xSi_x solid solutions with x=15, 18, 20, 25 and 28, the observed AMR could be identified as originating from the Fe-Si nanograins having a D0₃ structure.     

Grain oriented NiMnSn and NiMnIn Heusler alloys ribbons produced by melt spinning: Martensitic transformation and magnetic properties

We outline the microstructural, martensitic transformation and magnetic properties of Heusler alloys with starting compositions Ni₅₀Mn₃₇Sn₁₃, Ni₅₀Mn₃₆In₁₄, and Mn₅₀Ni₄₀In₁₀, produced by melt spinning. The ribbons were obtained in argon environment at a high wheel linear speed of 48 m s⁻¹ (typical dimensions: 1.2-2.0 mm in width, 4-12 mm in length, and 7-12 μm in thickness). EDS microanalysis showed that the resulting average elemental chemical composition is slightly shifted with respect to the starting one. Ribbons are fully crystalline and tend to show a highly ordered columnar-like microstructure with grains running through the entire ribbon thickness; the larger dimension of the grains is perpendicular to the ribbon plane. As-spun alloys were single-phase with ferromagnetic bcc L2₁ austenite as high-temperature parent phase. At low temperatures austenite transforms into a structurally modulated martensite with a lattice symmetry that depends on the system (7 M orthorhombic for Ni₅₀Mn₃₇Sn₁₃, 10 M monoclinic for Ni₅₀Mn₃₆In₁₄, and 14 M monoclinic for Mn₅₀Ni₄₀In₁₀). Magnetization isotherms measured in the temperature interval where martensite thermally transforms into austenite confirmed the occurrence of field-induced reverse martensitic transition in the alloys studied.     

Magnetic viscosity in Nd60Fe30Al10 amorphous alloys

Ferromagnetic amorphous Nd₆₀Fe₃₀Al₁₀ alloys melt spun at wheel speeds between 5 and 20 m/s exhibit hard magnetic properties, which are found to be very sensitive to the cooling conditions. The magnetization reversion mechanisms leading to the rather high coercive forces found are investigated by thermally activated magnetic relaxation experiments; the mean fluctuation field and the activation volume are measured—in specimens cooled at different rates—at the critical field for extensive magnetization reversion. These preliminary results show a qualitative agreement with the predictions of a ferromagnetic cluster model.     

High-frequency magnetic properties of soft magnetic cores based on nanocrystalline alloy powder prepared by thermal oxidation

FeSiBNbCu nanocrystalline alloy powder was thermally oxidized in an air atmosphere to enhance an oxide layer formation on the surface of the powder and subsequently toroidal shape FeSiBNbCu nanocrystalline alloy powder cores were prepared by compaction at room temperature. The phase change on the surface of FeSiBNbCu nanocrystalline alloy powder by thermal oxidation was analyzed and its effect on the high frequency magnetic properties of the compacted cores was investigated. By thermal oxidation, the formation of the oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the surface of FeSiBNbCu nanocrystalline alloy powder was enhanced and the thickness of oxide layer could be controlled by changing the thermal oxidation time. FeSiBNbCu nanocrystalline alloy powder core prepared from the powder treated by thermal oxidation exhibits a stable permeability up to high frequency range over 10 MHz. The core loss could be reduced remarkably and the dc-bias property could be improved significantly, which were due to the formation of oxide layer consisting of Fe₂O₃, CuO, and SiO₂ on the FeSiBNbCu nanocrystalline alloy powder. The improvement in high-frequency magnetic properties of the FeSiBNbCu nanocrystalline alloy powder cores could be attributed to the effective electrical insulation by oxide layer between the FeSiBNbCu nanocrystalline alloy powders.     

Mössbauer spectroscopical investigation of amorphous Fe–Y alloy ribbons prepared by melt spinning

Fe–Y amorphous alloy ribbons were prepared by the melt spinning method and characterized by X-ray diffraction, Mössbauer spectroscopy and inelastic neutron scattering. X-ray diffraction demonstrates that the Fe_0.7Y_0.3 ribbons are completely amorphous, whereas the Fe_0.3Y_0.7 ribbons contain a small fraction of crystalline Y precipitates in the amorphous Fe–Y matrix. Mössbauer spectroscopy between 4.2 to 300 K reveals the amorphous nature of the Fe–Y matrix and the Fe_0.7Y_0.3 ribbons. The preliminary neutron scattering results S(Q, ω) show excess low energy vibrational modes which gives rise to the so called “boson peak” in this amorphous material.     

Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique

A ferromagnetic shape memory alloy with nomial composition Ni_52.5Mn_24.5Ga₂₃ (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L2₁ austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite–austenite transformation spontaneously.     

3D打印激光制备多孔镍合金组织和力学性能研究

采用3D打印激光熔化技术制备了多孔镍基合金,并对其显微组织构成和压缩力学性能进行了分析。研究结果表明,采用3D打印激光熔化技术制备的多孔镍合金,孔隙率为14.68%~18.97%、抗压强度为461~535 MPa,其微观组织主要呈现γ-Ni枝晶,压缩断口为撕裂式枝晶断裂。     

Facile synthesis of nanostructured n-type SiGe alloys with enhanced thermoelectric performance using rapid solidification employing melt spinning followed by spark plasma sintering

SiGe alloy is widely used thermoelectric materials for high temperature thermoelectric generator applications. However, its high thermoelectric performance has been thus far realized only in alloys synthesized employing mechanical alloying techniques, which are time-consuming and employ several materials processing steps. In the current study, for the first time, we report an enhanced thermoelectric figure-of-merit (ZT) ∼ 1.1 at 900 °C in n-type Si₈₀Ge₂₀ nano-alloys, synthesized using a facile and up-scalable methodology consisting of rapid solidification at high optimized cooling rate ∼ 3.4 × 10⁷ K/s, employing melt spinning followed by spark plasma sintering of the resulting nano-crystalline melt-spun ribbons. This enhancement in ZT > 20% over its bulk counterpart, owes its origin to the nano-crystalline microstructure formed at high cooling rates, which results in crystallite size ∼7 nm leading to high density of grain boundaries, which scatter heat-carrying phonons. This abundant scattering resulted in a very low thermal conductivity ∼2.1 Wm⁻¹K⁻¹, which corresponds to ∼50% reduction over its bulk counterpart and is amongst the lowest reported thus far in n-type SiGe alloys. The synthesized samples were characterized using X-ray diffraction, scanning electron microscopy and transmission electron microscopy, based on which the enhancement in their thermoelectric performance has been discussed.     

Structure and thermoelectric properties of nanocomposite bismuth telluride prepared by melt spinning or by partially alloying with IV–VI compounds

Bismuth telluride samples are compared with respect to the evolution of their thermoelectric material parameters like thermal and electrical conductivity. The Seebeck coefficient is discussed in dependence on the melt spinning fabrication technique. The melt spinner used is only able to produce small thin ribbon shaped specimens, some as thin as 10 μm. This limits melt spinning to mainly production of research specimens for alloys with high critical cooling rate, which are difficult to fabricate with other techniques. Additional parameters are alloying or doping of the base material by comparing the properties as prepared to different annealing conditions. The intrinsic p‐ and n‐doped material was alloyed with up to 0.5% lead telluride by rapidly cooling the bulk material to improve the thermoelectric properties analysed from RT up to about 600 K. A Seebeck coefficient of well above 200 µV/K could be obtained for p‐ and n‐type materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature magnetization behavior in Co36Fe36Si3Al1Nb4B20 (at%) nanostructured alloy

The investigation addresses low temperature magnetization behavior in Co₃₆Fe₃₆Si₃Al₁Nb₄B₂₀ alloy ribbons in their as-spun as well as annealed state. Optimum heat treatment at 875 K led to nanocrystallization whereby bcc-(FeCo)SiAl nanoparticles were dispersed in an amorphous matrix as evidenced from transmission electron microscopy. Low temperature magnetization studies were carried out in the range 77-300 K. Using the method of mathematical fittings, magnetization extrapolated to 0 K was obtained. The dependence of the magnetization with respect to temperature of BT^3/2 was used to determine the Bloch coefficient “B” and spin wave stiffness constant “D”. Magnetic softening revealed by lowering in the coercivity in the optimum nanostructured state was also the cause of a drop in the stiffness constant. The range of exchange interaction given by D/T_C was higher in the nanostructured state compared to the as-spun amorphous state. The effect of nanocrystallization and the resulting ferromagnetic coupling was further evidenced by low temperature magnetization studies.     

Microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated by spark plasma sintering

The microstructural,phase transformation and magnetic properties of Ni-Mn-Ga alloy fabricated using the spark plasma sintering method have been investigated. The results show that both the as-sintered and annealed sintered specimens exhibit typical martensitic transformation behaviours. The martensite of the sintered specimen after annealing exhibits a ferromagnetic nature. Moreover,study of the fracture surface indicates that the transgranular fracture contributes to the higher ductility of sintered Ni-Mn-Ga alloy. In addition,the transformation strain in sintered Ni-Mn-Ga alloy is studied for the first time.     

EXAFS and cumulant expansion studies of an amorphous Se90P10 alloy produced by mechanical alloying

We investigated the local atomic order of an amorphous Se₉₀P₁₀ alloy produced by Mechanical Alloying through EXAFS measurements on Se K edge at five temperatures followed by a cumulant expansion analysis. We obtained a lot of structural information such as average coordination numbers and interatomic distances, structural and thermal disorder, asymmetry of the pair distribution functions g_ij(r), anharmonicity of the interatomic potential, thermal expansion and Einstein and Debye temperatures for the Se-Se and Se-P pairs. We also reconstructed the g_ij(r) functions for the Se-Se and Se-P pairs at the temperatures investigated.     

Magnetic and transport properties of Cu2MnAl Heusler alloy prepared by rapidly quenched method

The Cu₂MnAl alloy was prepared by rapidly quenched (suction-casting and melt-spinning) methods with various thicknesses of 20, 40 and 1000 μm. The X-ray diffraction (XRD) patterns of the fabricated samples show a single phase of Cu₂MnAl. All the samples reveal soft magnetic behavior with coercivity below 1.6 kA/m and Curie temperature of about 600 K. Resistance of the alloy behaves as a linear function of applied magnetic field. Magnetoresistance (MR) ratio depends on the thickness of the samples and achieves ∼0.8% at the field of 240 kA/m for the sample with thickness of 20 μm. The variation of the properties of the alloy can be interpreted by the difference of energy band structure caused by defects in the alloy.     

Crystallization studies on AgI-Ag2O-MoO3 superionic system synthesized by melt quenching and mechanical milling

Crystallization in the melt-quenched (MQ) and mechanically milled (MM) superionic systems has been thoroughly investigated using differential scanning calorimetry, X-ray diffraction and electrical conductivity measurements. It is observed that the two systems obey different crystallization processes. The conventionally melt-quenched samples exhibit only one crystallization peak near 112 °C, whereas, the mechanochemically synthesized samples show two well-separated crystallization peaks at T_cl∼75-97 °C and T_c2∼132±2 °C. The higher value of electrical conductivity in the mechanochemically synthesized samples (∼10⁻² Ω⁻¹ cm⁻¹ at 300 K) than the melt-quenched samples is attributed to the higher value of disorder (entropy) in the former.     

Microstructure of a composite with a quasicrystalline phase fraction obtained by directional solidification of Al61Cu27Fe12 alloy

The microstructure of a composite containing a quasicrystal phase, i.e. so-called crystal–quasicrystal (CQ) composite, was studied. The CQ composite was obtained by the Bridgman method via solidification of Al₆₁Cu₂₇Fe₁₂ alloy (numbers indicate at%). The process was conducted at a pull out rate of v = 0.07 mm/min. The average temperature gradient in the heating zone was 43 K/cm. The composite matrix consisted of cubic β phase Al(Fe, Cu), with reinforcement of λ-phase rod-shaped fibres surrounded by a quasicrystal icosahedral ψ phase, which also existed in the fibre core. The fibres were rhomboidal in cross-section. The composite was studied using X-ray and electron diffraction, light-optical and scanning electron microscopy (SEM), X-ray topography and Laue diffraction.     

Free volumes in bulk nanocrystalline metals studied by the complementary techniques of positron annihilation and dilatometry

Free-volume type defects, such as vacancies, vacancy-agglomerates, dislocations, and grain boundaries represent a key parameter in the properties of ultrafine-grained and nanocrystalline materials. Such free-volume type defects are introduced in high excess concentration during the processes of structural refinement by severe plastic deformation. The direct method of time-differential dilatometry is applied in the present work to determine the total amount and the kinetics of free volume by measuring the irreversible length change upon annealing of bulk nanocrystalline metals (Fe, Cu, Ni) prepared by high-pressure torsion (HPT). In the case of HPT-deformed Ni and Cu, distinct substages of the length change upon linear heating occur due to the loss of grain boundaries in the wake of crystallite growth. The data on dilatometric length change can be directly related to the fast annealing of free-volume type defects studied by in situ Doppler broadening measurements performed at the high-intensity positron beam of the FRM II (Garching, Munich, Germany).     

Magnetocaloric properties in a FeNiGaMnSi high entropy alloy

We investigated a new Fe_26.7Ni_26.7Ga_15.6Mn₂₀Si₁₁ high entropy alloy (HEA) without the rare earth element. The structural, magnetic and magnetocaloric properties of the resulting materials are presented. The HEAs successfully is produced by the arc melting with suction casting method. The crystal structures are characterised through multiphase Rietveld refinement of X-ray diffraction data. The structure of the HEAs was found to be the body centred cubic (bcc). In the magnetic measurements, the ferromagnetic to paramagnetic transition was obtained in the range of 300–400 K. With the employed suction casting method; the Fe_26.7Ni_26.7Ga_15.6Mn₂₀Si₁₁ HEA shows the best magnetocaloric properties as 1.59 Jkg⁻¹K⁻¹ maximum magnetic entropy change (0–2 T) and 75.68 Jkg^-1 refrigeration capacity after the annealing process.     

Measurements of fractal dimensionality in 0.1Cs2O-0.9TeO2 glass-forming tellurite system by Raman spectroscopy

Low-frequency Raman spectroscopy was utilized in order to estimate the fractal dimensionality for tellurite 0.1Cs₂O-0.9TeO₂ binary glass-forming system in a temperature range, which includes the glassy and supercooled liquid state. A variation in fractal dimensionality was observed and calculated directly from the Raman spectroscopic data. This variation can be interpreted as an indication of structural alterations caused by temperature variation and correlated with existing structural models concerning tellurite network. Low-frequency Raman spectroscopy has proved to be a valuable tool for studying the fractal dimensionality and structural transformation in amorphous phases. The estimation of the localization degree of lattice vibration in disordered materials is important and discussed in the context of current phenomenological status of the field.