铁铬合金团簇点阵中与成分相关的磁性耦合

本文采用气相团簇束流沉积法制备了不同铁铬比的铁铬合金密集团簇点阵,研究了团簇点阵中复杂的多相结构和各种耦合效应.当合金中铬含量较大时,在团簇中能够观测到一种晶格失配的类四方结构,这种结构的出现导致了不同铁铬比的合金团簇中交换偏置效应的不同.随着铬的含量增加,合金团簇的交换偏置场减小,而团簇间的偶极相互作用增强.在合金团簇的铁磁-超顺磁转变温度以上,能明显观察到在类四方结构团簇中有更大的残存矫顽力与剩磁.在场冷条件下,对矫顽力和偏置场随温度变化的研究表明含有更多类四方结构的合金团簇有更好的热稳定性和更大的各向异性.     

Mesoporous Rh Emerging from Nanophase‐separated Rh‐Y Alloy

Mesoporous precious metals with abundant active sites and high surface area have been widely recognized as high‐performance catalytic materials. However, the templated synthesis is complex and costly. Herein, we report a mesoporous rhodium (m‐Rh) that can be readily synthesized from entangled nanofibres of Rh and Y₂O₃ without templates. The entangled nanofibres, prepared from uniform Rh‐Y alloys under redox atmosphere, were the key precursor in the synthesis processes. Moreover, the m‐Rh efficiently catalyzed carbon dioxide reforming of methane (DRM) at a low reaction temperature of 683 K. Further, electrochemical methods of CO electro‐oxidation were innovatively used to demonstrate the stability of CO and oxygen species for the DRM reaction.     

Thermal stability of microstructures and mechanical properties in a Fe-based Fe-Cr-Mn-Cu-Mo multi-component alloy

A Fe-based multi-component alloy, 60Fe-12Cr-10Mn-15Cu-3Mo, was designed and fabricated for nuclear applications in the present study. The crystal structure of the alloy is a ～85% body – and 15% face–centered cubic. A detailed microstructure analysis indicated that Cu segregated in the alloy to form Cu-rich precipitates, and the Cu precipitates grew during high temperature annealing at up to 773?K. The high temperature tensile test of the alloys showed that both the yield stress and tensile strength of the 60Fe-12Cr-10Mn-15Cu-3Mo were greater than those of typical austenitic and ferritic stainless steels at 773?K. The Vickers microhardness of the designed alloy did not change after high temperature annealing for 1?h at up to 1073?K. The results indicated that the designed alloy has the potential to be used for high temperature applications.     

Analysis of precipitates in ultrafine-grained metallic materials

In the present work, new possibilities of the X-ray diffraction for investigations of the phase composition, size, shape and volume fraction of particles in ultrafine-grained ferritic/martensitic steel and the Cu-Cr-Zr alloy have been demonstrated. For the first time, the method of X-ray diffraction in the transmission mode was used to identify particles with a volume fraction of less than one percent in the ultrafine-grained materials studied. The small-angle X-ray scattering method was used to estimate the size and shape of dispersed particles. Specific features of the ultrafine-grained structure determined by the X-ray diffraction and transmission electron microscopy studies were used for estimation of dispersion hardening in the samples obtained by equal-channel angular pressing in comparison with the samples subjected to standard treatment.     

Mass and thermal transport in liquid Cu-Ag alloys

In this paper, the diffusion, thermodynamic and thermotransport properties in Cu–Ag liquid alloys are extensively investigated with molecular dynamics over a wide composition and temperature range. The simulations are performed with the most reliable EAM potential. The Green-Kubo formalism is employed for calculating transport properties. It is found that the reduced heat of transport in Cu–Ag is very small (about 0.10?eV in absolute value) and almost temperature independent. Further it is found that the interdiffusion coefficient together with both self-diffusion coefficients are almost composition independent. In Cu–Ag, the thermodynamic factor is found to be less than unity whereas the Manning factor is greater than unity (with significant composition and temperature dependence) and their product is very close to 1.     

Characterisation of magnetic resonance imaging (MRI) contrast agents using NMR relaxometry

ABSTRACT

This contribution describes the use of Fast Field-cycling relaxometry (FFC-NMR) for the characterisation of Gd(III)- and Mn(II)-based contrast agents for MRI. Through a series of selected examples, we analyse the role of different structural and dynamic parameters on ¹H relaxivity and on the shape of the ¹H Nuclear Magnetic Relaxation Dispersion (NMRD) profiles. The amplitude and shape of the profiles is affected by the number of water molecules coordinated to the metal ion, the water exchange rate, the rotational correlation time of the complex and the relaxation of the electron spin. As a result, ¹H NMRD profiles represent a powerful tool for the understanding of the properties of MRI contrast agent candidates at the molecular level.     

硫原子在镍基合金825(001)面吸附的第一性原理研究

元素硫在镍基合金表面吸附产生严重的电化学腐蚀,为从原子尺度研究硫腐蚀机理,采用第一性原理方法,构建并优化了镍基合金825的晶胞结构模型,计算分析了S原子在镍基合金825耐蚀性较差面(001)晶面的吸附及电子转移情况.结果表明:Ni原子占据顶角, Cr原子和Fe原子对称占据面心是镍基合金825稳定的晶胞结构;原子S在镍基合金825(001)面上最稳定的吸附位为四重穴位,吸附能为-6.51 eV; S吸附前后的态密度(DOS)和二维电荷差分密度图(DCD)对比发现,镍基合金825中Fe与S之间电荷偏移明显,形成离子键,易生成腐蚀产物Fe_xS_y. S的吸附对镍基合金825中Cr原子的电子分布影响不大,且合金中Cr和Ni抑制了合金中Fe与S之间的相互作用,从而提高了合金耐蚀性.     

压强对Zr_(67)Ni_(33)非晶合金结构和动力学性能的影响

采用分子动力学的方法模拟Zr_(67)Ni_(33)合金液体在不同压强下快速凝固过程,通过结构分析方法如对分布函数、配位数、Honeycutt-Andersen(HA)键型指数法,以及动力学参数如均方位移、自散射关联函数、非高斯参数研究压强对Zr_(67)Ni_(33)非晶合金局域原子结构和其过冷液体动力学性能的影响.研究结果表明:压强越大,Zr_(67)Ni_(33)非晶合金中Zr-Ni原子间的相互作用越强,体系结构有序性越强,过冷液体中动力学减慢和动力学不均匀现象越显著.     

Cu含量对Ni_(55)Fe_(18)Ga_(27)合金结构和磁性的影响

系统研究了Cu分别替代Fe和Ni对Ni_(55)Fe_(18)Ga_(27)结构和磁性的影响.结果表明:熔炼Ni_(55)Fe_(18-x)Cu_xGa_(27 )(x=1, 2, 3, 4)虽仍为奥氏体相结构,但伴有γ相出现;居里温度随Cu含量增加而降低,这是由于Cu掺杂引起过渡金属近邻原子间相互交换作用减弱所致;交流磁化率随Cu含量增加而降低,原因在于Fe是磁性的主要贡献者,Cu替代Fe会削弱Fe的磁矩,从而导致合金磁性降低.熔炼、退火和甩带Ni_(51)Cu_4Fe_(18)Ga_(27)均存在马氏体相变.熔炼样品马氏体相变温度最高,退火和甩带样品基本相同.这一特点表明热处理方式能够改变原子排列的有序度,因此可以通过改变热处理方式来调控马氏体相变温度.     

热粘弹波在变温非均匀合金熔体中的传播

机械波在金属凝固过程中传播的定量计算一直是一个难题,主要原因就是在这个过程中的熔体结构非常复杂.本研究考虑到熔体的变温、非均匀和粘弹性的特点,采用Kelvin粘弹性介质模型,建立了具有粘热损失特性的热粘弹性波动方程,通过隐式有限差分方法对波动方程进行求解,并以ZL203A合金熔体为研究对象,探究了热粘弹波在变温非均匀介质中的传播规律.结果表明:热粘弹波从合金熔体的低温区向高温区传播时,非均匀的温度场对波的传播有较大影响;相反,当波从合金熔体的高温区向低温区传播时,非均匀的温度场对波的传播几乎没有影响.热粘弹波在合金熔体中的衰减系数随频率的增大呈线性增大,而随温度的升高先增大后减小,在熔体的枝晶搭接温度附近达到最大值.