High Field Environment for EPM at the LNCMI

The Laboratoire National des Champs Magn(?)tiques Intenses(LNCMI) is the CNRS French National High Magnetic Field Facility associated with three universities:Joseph Fourier in Grenoble,Paul Sabatier and INSA in Toulouse.It delivers to researchers direct current high magnetic field up to 35 teslas in 34 mm in Grenoble and pulsed field up to 80 T in Toulouse.Developments of new high field magnets and associated instrumentations are user oriented. These developments(highly homogeneous magnets,magnets for diffraction studies,high field gradient magnet...) are impulsed by different user communities.Since a few years,magneto-science is attracting new users for the study of high field effects on physical,chemical and biological processes such as electrochemistry,metallurgy,nanostructuration, implying new methodologies and new experimental developments such as high temperature environment up to 1600 K.A survey of these developments is given.The laboratory is supported by the European Commission which contributes for the 'Transnational Access' to the facility.

金属熔体中的液−液结构转变现象及其对凝固的影响

液态合金结构和性质的研究一直是凝聚态物理和金属材料科学领域的重要研究方向之一。液态合金并非理想的均质液体,在温度和压力等作用下会发生不连续转变。液−液结构转变广泛存在于金属和合金中,并且对最终凝固组织和性能有着很大的影响。系统阐释金属熔体中液−液结构转变现象,总结液−液结构转变对最终凝固组织和宏观性能的影响等方面的主要研究进展。结果表明,利用熔体液−液结构转变可以有效调控金属及合金的组织,改善其宏观性能。

Characterization of the stability and dynamics of a laser-produced plasma expanding across a strong magnetic field

Magnetized laser-produced plasmas are central to many studies in laboratory astrophysics,in inertial confinement fusion,and in industrial applications.Here,we present the results of large-scale three-dimensional magnetohydrodynamic simulations of the dynamics of a laser-produced plasma expanding into a transverse magnetic field with a strength of tens of teslas.The simulations show the plasma being confined by the strong magnetic field into a slender slab structured by the magnetized Rayleigh–Taylor instability that develops at the plasma–vacuum interface.We find that when the initial velocity of the plume is perturbed,the slab can develop kink-like motions that disrupt its propagation.

Highly-collimated, high-charge and broadband MeV electron beams produced by magnetizing solids irradiated by high-intensity lasers

Laser irradiation of solid targets can drive short and high-charge relativistic electron bunches over micron-scale acceleration gradients.However,for a long time,this technique was not considered a viable means of electron acceleration due to the large intrinsic divergence(∼50°half-angle)of the electrons.Recently,a reduction in this divergence to 10°–20°half-angle has been obtained,using plasma-based magnetic fields or very high contrast laser pulses to extract the electrons into the vacuum.Here we show that we can further improve the electron beam collimation,down to∼1.5°half-angle,of a high-charge(6 nC)beam,and in a highly reproducible manner,while using standard stand-alone 100 TW-class laser pulses.This is obtained by embedding the laser-target interaction in an external,large-scale(cm),homogeneous,extremely stable,and high-strength(20 T)magnetic field that is independent of the laser.With upcoming multi-PW,high repetition-rate lasers,this technique opens the door to achieving even higher charges(>100 nC).

Ab Initio Study of the Electronic and Vibrational Properties of 1-nm-Diameter Single-Walled Nanotubes

The electronic structure, band gap, density of states of the (8,8), (14,0) and (12,3) single-walled carbon nanotubes by the SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) method in the framework density-functional theory (DFT) with the generalized gradients approximation (GGA) were studied. Also, we studied the vibrational properties of the (8,8) and (14,0) nanotubes. Only the calculated relaxed geometry for (12,3) nanotube show significant deviations from the ideal rolled graphene sheet configuration. The electronic transition energies of van Hove singularities were studied and compared with previous results. The calculated band structures, density of states and dispersion curves for all tubes were in good agreement with theoretical and experimental results.

Magnetic field intensity dependent microstructure evolution and recrystallization behavior in a Co-B eutectic alloy

Systematic understanding on the magnetic field intensity dependent microstructure evolution and re-crystallization behavior in a Co-B eutectic alloy under a constant undercooling(ΔT≈100 K)were carried out.Absent of the magnetic field,the comparable size of divorced FCC-Co and Co_(3)B eutectic ellipsoidal grains coexist with a few regular lamellas.When the magnetic field is less than 15 T,the elongated primary FCC-Co dendrites parallel to the magnetic field with the dispersed FCC-Co nano-particles em-bedded within the Co_(3)B matrix occupy the inter-dendrite regions.Once the magnetic field increases to 20 T,the FCC-Co/Co_(2)B anomalous eutectic colonies dominate.The formation mechanism of Co_(2)B phase is discussed from several aspects of the competitive nucleation,the chemical redistribution induced by the thermomagnetic-induced convection and magnetic dipole interaction,and the strain-induced trans-formation.Furthermore,the application of magnetic field is found to promote recrystallization,proved by the lower density of misorientation,the appearance of FCC-Co annealed twins and more Co_(3)B sub-grains.This work could further enrich our knowledge about the magnetic-dependent microstructure evolution and recrystallization process in the undercooled Co-B system and provide guidance for controlling the microstructures and properties under extreme conditions.

Tailoring mechanical and magnetic properties of AlCoCrFeNi high-entropy alloy via phase transformation

Phase constitutions,either changed by alloying or by phase transformation,are the key factors to determine the magnetic and mechanical performances of high-entropy alloys(HEAs).Using the AlCoCrFeNi HEA as a candidate alloy,this paper demonstrates the effect of phase transformation on both the mechanical and magnetic properties in the multi-phase system.With increasing heat treatment temperature,the sigma(σ)and face-centered-cubic(FCC)phases disappeared at 1000℃and 1200℃,respectively.Such volume fraction changes ofσ,FCC and body-centered-cubic(BCC)phases have divergent effects on mechanical and magnetic properties.The excellent strength-ductility combination will be achieved as the disappearance ofσphase and formation of FCC phase.As for the magnetic properties,the volume fraction of BCC phase plays a major role in determining its saturation magnetization.When the volume fraction change of BCC phase is not evident,the higher volume fraction of FCC phase will influence its magnetization at 2 T.Our present work might provide insights into analyzing the evolution of both mechanical and magnetic properties of HEAs caused by complex phase transformation.

Effects of an ultra-high magnetic field up to 25 T on the phase transformations of undercooled Co-B eutectic alloy

While there have been multiple recent reports in the literature focusing on the effects of magnetic field on the phase transformation behaviors,the research conducted with an ultra-high magnetic field greater than 20 T is still preliminary.In the current study,the structure evolution of Co-B alloys are experimentally studied with undercooling.The effects of a 25 T magnetic field on the solidification behavior and the subsequent solid-state phase transformation behavior have been investigated.The 25 T magnetic field is confirmed to have little effect on the homogeneous nucleation,but have some influence on the heterogeneous nucleation of Co_(3) B and Co_(23)B6 phases by modifying the wetting angleθ.The decomposition of Co_(23)B6 phase in the subsequent cooling process can be effectively suppressed by applying the 25 T magnetic field.The present work might be helpful for not only theoretically understanding the influence of ultra-high magnetic field on the phase transformation behaviors but a potential technology of field-manipulation of magnetic materials.

Liquid state dependent solidification of a Co-B eutectic alloy under a high magnetic field

The structure transition inside the Co-81.5at.%B alloy liquid has been studied by an in-situ magnetization measurement.A crossover was observed on the 1/M-T curve during the overheating process,indicating that a liquid-liquid structure transition(LLST)took place in the melt.Based on this information,the effects of LLST on the solidification behavior,microstructure and tribology property were investigated experimentally.The sample solidified with the LLST exhibits significantly different solidification behaviors,i.e.,the nucleation undercooling and the recalescence extent are conspicuously enlarged,and the solidification time is shortened.As a result,the microstructure is effectively refined and homogenized,and the hardness and wear resistance are significantly enhanced.The present work might be helpful for not only theoretically understanding the influence of LLST on the solidification behavior but also providing an alternative approach to tailor the microstructure and properties.

Solidification of Hyper-Monotectic Zinc-Bismuth Alloy in a Strong Magnetic Field

In order to obtain homogenous Zn-Bi hyper-monotectic alloys and investigate what mechanism the magnetic field functions,we carried out the solidification experiments of Zn-4wt%Bi,5wt%Bi,6wt%Bi,7wt%Bi, 15wt%Bi and 30wt%Bi alloys under a 18T static magnetic field which was set up in LNCMI.Water quenching was also chosen to further damp the segregation caused by Stokes convection and Marangoni movement.The results indicated that when the content of Bismuth is 5-7wt%,the superimposition of 18Tesla magnetic field can damp the segregation remarkably,and the size of the second phase particles also is decreased.Furthermore,to Zn-4wt%Bi alloy solidified in 18T magnetic field,no spherical bismuth particles are found even magnifying 1000 times by microscope, which hints that the 18T magnetic field may change its solidifying character.To Zn-15wt%Bi and Zn-30wt%Bi alloy, due to their strong segregation trend,the 18T magnetic field still cannot damp the Stokes settlement thoroughly even by quenching way,however,no layered bismuth and zinc appears when compared to 0T,large Bismuth block are formed in the lower half part of the sample.