Self-diffusion near symmetrical tilt grain boundaries in UO2 matrix: A molecular dynamics simulation study

Molecular dynamics simulations have been carried out to study the influence of grain boundaries in stoichiometric UO₂ on uranium and oxygen self-diffusions over a large range of temperature varying from 300 K to 2100 K. The study was carried out on two symmetrical tilt grain boundaries, Σ5 and Σ41, which have respectively two different atomic structures. Firstly, the study of the temperature effect on the grain boundary core structure is presented. With the raise of temperature, the grain boundary core grows with an increase of disorder. Secondly, self-diffusion near both grain boundaries is studied. It has been found that grain boundaries accelerate the uranium and oxygen self-diffusion rates over several nanometres from the grain boundary interface. Uranium and oxygen self-diffusion are anisotropic, with a high acceleration along the grain boundary interface. Using the self-Van Hove correlation functions, hopping mechanisms were identified for Σ41 in all directions while for Σ5 hopping mechanism takes place along the grain boundary interface and random diffusion appears in the perpendicular direction of the grain boundary plane.     

Multiscale modelling of bi-crystal grain boundaries in bcc iron

Atomistic simulations have provided much insight into grain boundary (GB) structures and mechanisms which are important in understanding the properties of materials. In this paper, the ∑3{1 1 2}, ∑3{1 1 1} and ∑5{0 1 3} (coincidence site lattice) GBs of bcc iron are investigated using molecular statics (MS) simulations, ab initio DFT calculations and the simulated HRTEM method. For the MS calculations, four empirical potentials, the Ackland potential (1997), Mendelev potentials 2 and 4 and the Dudarev-Derlet potential have been used. The MS results for all three symmetrical grain boundaries show the results to be independent of the empirical potential implemented. After relaxation, the symmetrical structures of the GBs remain, in agreement with ab initio calculation results.     

Quantum Degenerate Fermi Gases of Ytterbium Atoms

We performed evaporative cooling for dilute gases of ytterbium (Yb) isotopes in a crossed optical dipole trap and successfully cooled two fermionic and two bosonic species down to quantum degenerate regime, following the previous realization of Bose-Einstein condensation (BEC) in ¹⁷⁴Yb. The elastic collision rate of fermionic ¹⁷³Yb atoms with 6 spin components was found to be large enough to carry out efficient evaporation, which enables us to cool the atoms down to 0.6 T _F , where T _F is the Fermi temperature. In this regime, a plunge of evaporation efficiency was observed as an effect of the Fermi degeneracy. The other fermionic isotope ¹⁷¹Yb was cooled down to the temperature below T _F by sympathetic cooling with bosonic ¹⁷⁴Yb atoms. The sympathetic cooling technique has also been applied to ¹⁷⁴Yb-¹⁷⁶Yb Bose-Bose mixture. We have observed almost pure BEC of ¹⁷⁴Yb and the bimodal distribution of ¹⁷⁶Yb, showing the formation of BEC-BEC mixture. Moreover, we performed evaporative cooling of ¹⁷⁰Yb atoms and realized the BEC.     

Correlating structure with ferromagnetism in melt-spun Gd100−xFex

X-ray diffraction and transmission electron microscopy measurements of melt-spun Gd_100−xFe_x (0 ≤ x ≤ 40) and inert-gas condensed/compacted samples (3.8 ≤ x ≤ 12.7) reveal a structure of crystalline hcp-Gd grains surrounded by a non-crystalline Gd1−xeffFexeff phase, where x_eff > x is the effective iron concentration within the amorphous region. The two-phase structure is responsible for an unusual dependence of the coercivity on temperature in which non-zero coercivity is observed above the hcp-Gd T_c with a peak near 320 K. The coercivity decreases as the hcp-Gd grains order, then increases with decreasing temperature. This behavior is explained by the presence of magnetically correlated Fe-rich regions.     

快淬工艺对低钴AB5型贮氢合金电化学性能的影响

研究了快淬工艺对稀土基低钴AB5型LaxMm1-x（NiMnSiAlFe）4．7Co0．2（x=0、01）贮氢合金的电化学性能及其微观结构的影响。研究结果表明,快淬改善了两种合金的循环稳定性,其主要原因是快淬使合金的晶胞体积增大,且使合金的晶粒细化;快淬对La含量x=0合金的循环寿命的增加幅度尤其明显,主要是因为快淬使该合金的晶粒更为细小。La替代Mm使合金的放电容量提高,其原因是La替代Mm抑制了第二相Ce2Ni7相的形成,并使合金的晶胞体积增大。     

Chemical modification of jute fibers for the production of green-composites

Natural fiber reinforced composites is an emerging area in polymer science. Fibers derived from annual plants are considered a potential substitute for non-renewable synthetic fibers like glass and carbon fibers. The hydrophilic nature of natural fibers affects negatively its adhesion to hydrophobic polymeric matrices. To improve the compatibility between both components a surface modification has been proposed. The aim of the study is the chemical modification of jute fibers using a fatty acid derivate (oleoyl chloride) to confer hydrophobicity and resistance to biofibers. This reaction was applied in swelling and non-swelling solvents, pyridine and dichloromethane, respectively. The formation of ester groups, resulting from the reaction of oleoyl chloride with hydroxyl group of cellulose were studied by elemental analysis (EA) and Fourier Transform infrared spectroscopy (FTIR). The characterization methods applied has proved the chemical interaction between the cellulosic material and the coupling agent. The extent of the reactions evaluated by elemental analysis was calculated using two ratios. Finally electron microscopy was applied to evaluate the surface changes of cellulose fibers after modification process.     

AC conductivity, density related and magnetic properties of Ni1 − xZnxFe2O4 ferrites with the variation of zinc concentration

AC conductivity, density related and magnetic properties are reported for Ni_1 − xZn_xFe₂O₄ ferrites with the variation of zinc concentration prepared by the standard solid state reaction technique. X-ray powder diffraction patterns confirmed the spinel structure of the prepared compounds. AC conductivity (lnσ_ac) increases from − 10.045 (S/m) to − 3.781 (S/m) with the increase in zinc concentration from 0.0 to 1.0 at the frequency of 1 kHz. Lattice parameters, sintered density and grain size increase whereas X-ray density and porosity decrease with the increase in zinc concentration. Saturation magnetization increases with the increase in zinc concentration up to x = 0.4 and after that it decreases with the increase in zinc concentration. Remanence magnetization and magnetic moment almost have the similar trend as that of saturation magnetization. Yafet-Kittel angles increase with the increase in zinc concentration. The possible reasons responsible for these changes are undertaken.     

快凝Al—Fe—V—Si—Mm合金中稀土作用的正电子湮没研究

测量了几种不同混合稀土（Ｍｍ：Ｍｉｓｈｃ Ｍｅｔａｌ）含量的快凝Ａｌ－Ｆｅ－Ｖ－Ｓｉ－Ｍｍ合金薄带的正电子寿命谱,并分析了这些快凝合金薄带中稀土含量对正电子寿命的影响。     

A novel surface texture for magnetic fluid lubrication

Magnetic fluid has many advantages when serving as lubricant. With an appropriate magnetic field this lubricant can prevent leakage and increase the load capacity of lubricant film. It can also be fixed at the friction zone by applying an external magnetic field. A novel design of magnetic fluid lubrication with magnetic surface texture was proposed in this paper. A micro-scale dimple pattern was firstly fabricated on the surface of tribo-pair and then a permanent magnet material was electrodeposited into these dimples, so that there are both geometric surface texture and periodic distribution of magnetic field on the surface (magnetic surface texture). In this paper, CoNiMnP permanent magnetic film (about 25 μm thickness) was electrodeposited into micro-dimples (500 μm in diameter) on the surface of 316 stainless steel. The impact of magnetic surface texture on lubrication was investigated using a pin-on-disk test rig. The test results showed that magnetic surface texture was conducive to form effective lubrication at low sliding velocity when lubricated by magnetic fluids.     

Analyzing the effect of displacement rate on radiation-induced segregation in 304 and 316 stainless steels by examining irradiated EBR-II components and samples irradiated with protons

Recent studies have indicated that, at temperatures relevant to fast reactors and light water reactors, void swelling in austenitic alloys progresses more rapidly when the radiation dose rate is lower. A similar dependency between radiation-induced segregation (RIS) and dose rate is theoretically predicted for pure materials and might also be true in complex engineering alloys. Radiation-induced segregation was measured on 304 and 316 stainless steel, irradiated in the EBR-II reactor at temperatures near 375 °C, to determine if the segregation is a strong function of damage rate. The data taken from samples irradiated in EBR-II is also compared to RIS data generated using proton radiation. Although the operational histories of the reactor irradiated samples are complex, making definitive conclusions difficult, the preponderance of the evidence indicates that radiation-induced segregation in 304 and 316 stainless steels is greater at lower displacement rate.