Synthesis and ageing effect in FeO nanoparticles: Transformation to core-shell FeO/Fe3O4 and their magnetic characterization

This paper reports the magnetic properties of partially oxidized FeO nanoparticles (NPs) prepared using thermal decomposition of iron acetylacetonate at high temperature. X-ray diffraction (XRD) analysis confirmed that the resulting NPs comprise a mixture of wüstite and magnetite phases, which are subsequently confirmed using high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern. Magnetic properties were investigated using vibrating sample magnetometer (VSM), which exhibit superparamagnetic (SPM) behavior at room temperature. Alternatively, below 200 K, a large exchange bias field has been observed in field cooled mode whose magnitude increases with the decrease in measuring temperature attaining a maximum value of ∼2.3 kOe at 2 K accompanied by coercivity enhancement (∼3.4 kOe) and high field of irreversibility (>50 kOe). The results are discussed taking into account the role of interface exchange coupling on the macroscopic magnetic properties of the nanoparticles.     

Effect of Ca on the microstructure and magnetocaloric effects in the La1−xCaxFe11.5Si1.5 compounds

The effect of Ca on the microstructure and magnetocaloric effects has been investigated in the La_1−xCa_xFe_11.5Si_1.5 (x = 0, 0.1, 0.2 and 0.3) compounds. The introduction of Ca leads to the appearance of minor α-Fe and Ca-rich phases, which affects the actual compositions of the main phases for the Ca containing samples. With increasing the Ca concentration, the Curie temperature T_C increases from 183 to 208 K, and the maximum magnetic entropy changes |ΔS| at the respective T_C with a magnetic field change from 0 to 5 T are 21.3, 19.5, 16.9, and 11.2 J/kg K for x = 0, 0.1, 0.2, and 0.3, respectively. The nature of the magnetic transition changes from first-order to second-order with an increase in Ca concentration, which leads to a reduction of the hysteresis and a decrease of the magnetic entropy change. However, the relative cooling power for La_1−xCa_xFe_11.5Si_1.5 compounds remains comparable with or even larger than that of other magnetocaloric materials over a wide temperature range. The higher T_C and the smaller hysteresis in comparison with those of the parent compound suggest that the La_1−xCa_xFe_11.5Si_1.5 compounds could be suitable candidates for magnetic refrigerants in the corresponding temperature range.     

Effect of Gd doping on magnetic, electric and dielectric properties of MgGdxFe2−xO4 ferrites processed by solid state reaction technique

MgGd_xFe_2−xO₄ (x = 0.0, 0.05, 0.1 and 0.15) ferrites, with improved dc resistivity, initial permeability, saturation magnetization, and extremely low relative loss factor, have been synthesized by solid state reaction technique. The microstructures, electric, dielectric and magnetic properties have been investigated by means of X-ray diffraction, Keithley 2611 system, impedance analyzer and VSM respectively. The addition of Gadolinium in Mg ferrite has been shown to play a crucial role in enhancing the electric, dielectric and magnetic properties. The dc resistivity is increased by two orders of magnitude as compared to Mg ferrite. Saturation magnetization has been increased by two times and remnant magnetization has been increased by more than three times due to the doping of Gd³⁺ ions in Mg ferrite. The relative loss factor was found to have very low values and is of the order of 10⁻⁴-10⁻⁵ in the frequency range 0.1-30 MHz. The variations of electric, dielectric and magnetic properties of the samples have been studied as a function of frequency and Gd³⁺ ions concentration measured at room temperature. High resistivity and improved magnetic properties can be correlated with better compositional stoichiometry and the replacement of Fe³⁺ ions by Gd³⁺ ions. The mechanisms responsible to these results have been discussed in this paper.     

纳米复相永磁材料交换耦合作用的评述

对比纳米复相永磁材料一维到三维的不同交换耦合作用模型,交换耦合作用会抑制软磁相的磁化反转,不同交换耦合作用模型对矫顽力影响不同。Henkel曲线δM峰值越高表明晶粒间交换耦合作用越强,一阶翻转曲线(first order reversal curve,FORC)峰值对应交换耦合作用强弱。纳米复相永磁材料有效各向异性K_eff随晶粒尺寸减小而下降,当晶粒尺寸一定时,软磁相体积分数越高K_eff越低。为了得到最大磁能积高并且K_eff不低的纳米复相永磁材料,软磁相晶粒尺寸应在10nm左右,软磁相体积不能超过50%。     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Effect of magnetization state on the corrosion behaviour of NdFeB permanent magnets

The corrosion behaviour of uncoated NdFeB permanent magnets in the non-magnetized and magnetized state was comparatively investigated in H₂SO₄ solutions by potentiodynamic and potentiostatic polarization experiments. Depth profiles of the corroded surfaces were recorded and an effect of magnetization on the localization of the corrosion attack was identified. The anodic behaviour is discussed on the basis of previously reported results on the corrosion of neodymium and iron and on basis of the magnetic forces acting on the electrochemical system.     

Influence of Sb doping on thermoelectric properties of Mg2Ge materials

The Sb-doped Mg₂Ge compounds were successfully synthesized by tantalum-tube weld melting method followed by hot pressing and the thermoelectric properties were examined. The effects of Sb doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity have been investigated in the temperature range of 300–740 K. It was found that the Sb doping with sufficient Mg excess increased the electrical conductivity dramatically, leading to enhancement of the power factors. The thermal conductivity was also reduced upon Sb doping, mainly due to mass fluctuation scattering and strain field effects. Mg_2.2Ge_0.095Sb_0.005 showed a maximum thermoelectric figure of merit of ≈0.2 at 740 K.     

Effect of alloying elements on the behaviour of TiAl-based alloys

TiAl-based alloys containing Nb, Mo and Cr have been produced by induction melting both under an Ar atmosphere and in vacuum. Fracture toughness and isothermal oxidation tests at 900 °C have been performed on a number of alloys in order to evaluate the effect of alloying elements on both fracture toughness and hot corrosion. The results highlighted that there is an alloy composition range that could allow to obtain a good compromise between high temperature oxidation resistance and fracture toughness. In fact only for a selected Al/Ti ratio and a limited content of alloying elements it is possible to attain the alloy microstructural control and, for Mo free alloys, good protectiveness against oxidation.     

SA517 Gr.B焊接工艺

采用E11018-G和E12018-G两种不同强度的焊材,对SA517Gr.B高强钢进行焊接工艺试验研究,进行拉伸试验、侧弯试验和冲击弯试验。结果表明,E11018-G和E12018-G焊接材料均能够满足SA517Gr.B钢材的焊接技术要求,采用E11018-G可有效降低接头焊接应力,降低焊缝裂纹敏感性,提高焊缝质量稳定性。     

First-principles study on half-metallic properties of the CoMnZ (Z = S,Se, Te) half-Heusler compounds

The first-principles calculations based on the density functional theory have been employed to explore the electronic structure and magnetic properties of the CoMnZ (Z = S, Se, Te) half-Heusler compound. CoMnTe is predicted to be half-metallic ferromagnet with an energy gap of 1.04 eV in the minority spin and a completely spin polarization at the Fermi level. CoMnS and CoMnSe compounds are nearly half-metallic with spin polarization of 98.9 and 97.9%, respectively. All compounds have a total magnetic moment of 4 μ_B/f.u., which agrees with the Slater–Pauling rule. CoMnTe compound keeps half-metallicity within a wide range of lattice constants between 5.65 and 6.05 Å. Under tetragonal distortions, high spin polarization at the Fermi level is maintained for the CoMnTe compound.     

合金元素Ga对Al-Zn-Bi系阳极材料腐蚀机理的影响

采用恒电流法测定了添加Ga元素前后的Al-Zn-Bi系合金在人造海水中的开路电位、工作电位和电流效率,观察了该试验过程后合金的表面腐蚀形貌;利用极化曲线和电化学阻抗谱研究了该阳极合金在3.5%NaCl溶液中腐蚀过程的演变规律。结果表明：Ga能使Al-Zn-Bi系合金开路电位负移,工作电位稳定,电流效率升高,腐蚀形貌更加均匀;Ga可均匀固溶于铝合金中,并与回沉积的阳离子形成Ga基汞齐,使得阳极合金的活化控制步骤由第二相粒子优先溶解-脱落机理转变为金属阳离子的溶解-再沉积机理,促进阳极合金的均匀溶解,从而提高其综合电化学性能;等效电路RL（Cs（CpRp）（Q1Rd1）（LRa））和RL（Cs（CpRp）（Q1Rd1）（LRa）（Q2Rd2））能较好地表征两种合金的腐蚀行为和活化机理。     

Study on the Si penetration into Fe sheets using PVD method and its application in the fabrication of Fe-6.5 wt.% Si alloys

FeSi (12 wt.% Si) and Si were alternatively deposited on pure iron (Fe) substrates by direct current magnetron sputtering. Subsequent annealing in vacuum at 1150-1190 °C results in penetration of Si into the substrate. Cross-sectional microstructure and Si concentration were investigated by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The penetration mechanism is found to depend greatly on Si amount in the as-deposited films. When FeSi/Si/FeSi/Si/FeSi was deposited on the Fe substrate, the Si penetration is controlled by phase-boundary migration, while a diffusion-controlling penetration is observed in FeSi/Si/FeSi deposited samples. Fe-6.5 wt.% Si sheet with thickness of 0.35 mm is obtained through the deposition of FeSi/Si multilayer on a Fe-3 wt.% Si sheet together with subsequent annealing at 1180 °C for 2 h.     

含硅杂质对铝酸钠溶液分解过程的影响及相互作用机理

研究了二氧化硅对铝酸钠溶液分解过程的分解率、产品粒度及形貌的影响。根据实验结果用基于密度泛函的Dmol3程序计算SiO32-以各种不同方式位于氢氧化铝(001)和(100)面时的总能量和电子结构等,用晶体生长习性计算程序Morphology分析各体系的平衡形态及生长习性。实验结果表明:在一定的分解时间范围内,硅能抑制铝酸钠溶液种分分解,对分解产物氢氧化铝的晶体形貌有影响,其(100)面的显露有所增加。理论计算结果表明:SiO32-的存在明显改变氢氧化铝(001)和(100)表面的电子结构,SiO32-在氢氧化铝(001)和(100)表面上时,靠近(001)面的O原子多的体系稳定性低。平衡形态及生长习性计算结果表明:SiO32-在氢氧化铝(001)表面上时,各体系长大晶粒的体积均小于无SiO32-体系中(001)的晶粒体积;而SiO32-在氢氧化铝(100)表面上时,各体系长大晶粒的体积比均大于SiO32-体系中(100)的晶粒体积。     

Effect of Fe substitution on magnetic properties of antiferromagnetic Heusler alloy Ru2MnGe

The present study reveals that the correlation between the magnetic properties with composition x in the polycrystalline Ru₂Mn_1−xFe_xGe system. Hard ferromagnetic properties appeared at the intermediate composition between antiferromagnetic Ru₂MnGe with soft ferromagnetic Ru₂FeGe. Ru₂Mn_0.6Fe_0.4Ge shows an anisotropic and negative magnetoresistance (MR) effect more than 8% although the both end materials are small MR effect less than 1%. These experimental results correlate the variation in MR ratio with that of coercive field as a function of x. Moreover, the correlation is shown at the boundary of magnetism between antiferromagnetic and ferromagnetic. Therefore, the present experimental results imply that the coexistence of antiferromagnetic and soft ferromagnetic leads to induce the MR effect in this system.     

Effect of nanocrystallization on giant magnetoimpedance effect of Fe-based microwires

We studied giant magnetoimpedance (GMI) effect and magnetic properties of Fe_70.8Cu₁Nb_3.1Si_14.5B_10.6 and Fe_71.8Cu₁Nb_3.1Si₁₅B_9.1 Finemet microwires. We observed that GMI effect and magnetic softness of glass-coated microwires produced by the Taylor–Ulitovski technique can be tailored either controlling magnetoelastic anisotropy of as-cast FeCuNbSiB microwires, and/or controlling their structure by heat treatment or by changing the fabrication conditions. High GMI effect has been observed in as-prepared Fe-rich and heat treated microwires with nanocrystalline structure.     

Effect of AC on stress corrosion cracking behavior and mechanism of X80 pipeline steel in carbonate/bicarbonate solution

The influence of various AC current densities on stress corrosion cracking behavior and mechanism of X80 pipeline steel was investigated in carbonate/bicarbonate solution by polarization curves and slow strain rate tensile tests. With the increasing AC current density, the SCC susceptibility of the steel increases, especially at high AC current density. A significant difference in the SCC behavior and mechanism is found for the steels with or without AC application. In the absence of AC, the fracture mode is intergranular and the mechanism is attributed to anodic dissolution. Under AC application, the cracks propagation is transgranular, and the mechanism is mixed controlled by both anodic dissolution and hydrogen embrittlement.     

Effect of superheat,mold, and casting materials on the metal/mold interfacial heat transfer during solidification in graphite-lined permanent molds

Heat transfer during the solidification of an Al-Cu-Si alloy (LM4) and commercial pure tin in single steel, graphite, and graphite-lined metallic (composite) molds was investigated. Experiments were carried out at three different superheats. In the case of composite molds, the effect of the thickness of the graphite lining and the outer wall on heat transfer was studied. Temperatures at known locations inside the mold and casting were used to solve the Fourier heat conduction equation inversely to yield the casting/mold interfacial heat flux transients. Increased melt superheats and higher thermal conductivity of the mold material led to an increase in the peak heat flux at the metal/mold interface. Factorial experiments indicated that the mold material had a significant effect on the peak heat flux at the 5% level of significance. The ratio of graphite lining to outer steel wall and superheat had a significant effect on the peak heat flux in significance range varying between 5 and 25%. A heat flux model was proposed to estimate the maximum heat flux transients at different superheat levels of 25 to 75 °C for any metal/mold combinations having a thermal diffusivity ratio (α _R) varying between 0.25 and 6.96. The heat flow models could be used to estimate interfacial heat flux transients from the thermophysical properties of the mold and cast materials and the melt superheat. Metallographic analysis indicated finer microstructures for castings poured at increased melt superheats and cast in high-thermal diffusivity molds.