Nb芯增强MgB2多芯超导线材的制备及性能研究

以Nb/Cu复合管作为外包套材料,Nb棒作为中心增强体,通过原位法粉末装管工艺(in-situ PIT)制备了不同芯丝结构的TiC 粉末掺杂MgB2/NbCu多芯线材.在高纯流动氩气保护条件下、700℃烧结2h进行成相反应.对烧结后的线材分别进行了应力-应变分析、微观结构以及超导电性等分析检测.拉伸实验分析表明,Nb芯...     

Aerodynamic levitation and laser heating:

Aerodynamic levitation is an effective way to suspend samples which can be heated with CO₂ lasers. The advantages of this containerless technique are the simplicity and compactness of the device, making it possible to integrate it easily in different kinds of experiments. In addition, all types of materials can be used, including metals and oxides. The integration of aerodynamic levitation at synchrotron and neutron sources provides powerful tools to study the structure and dynamics of molten materials. We present here an overview of the existing techniques and of the developments made at the CEMHTI in Orléans, as well as a few examples of experimental results already obtained.     

Inverse problem approaches for stationary Fourier transform spectrometers

A design of a miniaturized stationary Fourier transform IR spectrometer has been developed that produces a two-dimensional interferogram. The latter is disturbed by effects like parasitic interferences or disparities in the cutoff wavelength of the pixels. Thus, a simple Fourier transform cannot be used to estimate the spectrum of the scene. However, as these defects are deterministic, they can be measured and taken into account by inversion methods. A regularization term can also be added. The first experimental results prove the efficiency of this processing methodology.     

Coherent combining of two Nd:YAG lasers in a Vernier–Michelson-type cavity

The coherent combining of two Nd:YAG lasers using a Vernier–Michelson-type cavity has been demonstrated. The spectral behaviour and the energetic performance are reported. We firstly show that the combining efficiency is not strongly spoilt by the gain imbalance between the two amplifying media, and secondly, that despite the interferometric nature of the cavity, the Vernier–Michelson laser can withstand environmental perturbations without alteration in output power. Received: 25 April 2002 / Revised version: 18 May 2002 / Published online: 25 September 2002 RID="*" ID="*"Corresponding author. Fax: +33-5/5545-7253, E-mail: kermene@ircom.unilim.fr     

Three-dimensional microfabrication by two-photon-initiated polymerization with a low-cost microlaser

Fabrication of submicrometer structures by two-photon-initiated polymerization is performed with an inexpensive and low-power microlaser. This is made possible by the design of photoinitiators with strong two-photon absorption cross sections. We analyze the influence of both material properties and irradiation conditions on the two-photon polymerization rate and show that resins based on our highly sensitive two-photon photoinitiator can be solidified with microlaser excitation, whereas commercial UV photoresins require ultrashort and intense laser pulses.     

Stability of metal vicinal surfaces revisited

The stability of metal vicinal surfaces with respect to faceting is investigated using empirical potentials as well as electronic structure calculations. It is proven that for a wide class of empirical potentials all vicinal surfaces between (100) and (111) are unstable at 0 K when the role of third and farther nearest neighbors is negligible. However, electronic structure calculations reveal that the answer concerning the stability of vicinal surfaces is not so clear-cut. Finally, it is shown that surface vibrations at finite temperatures have little effect on the stability of vicinal surfaces.     

Raman study of orbital mediated multiphonons in RMnO3 (R = Pr,Sm,Eu,Tb,Y)

We have studied RMnO₃ manganites (R = Pr, Sm, Eu, Tb, Y) Raman excitations in the 200–2800 cm^-1 range as a function of temperature. Combinations of phonon energies are observed up to the fourth order, indicating the presence of electron-phonon coupling. In comparison to Γ-point phonon combinations, double phonon excitations appear to be blue shifted in large size rare earth ion compounds. The phonon combination intensities decrease rapidly with their increasing order, confirming other studies which conclude that the electron-phonon coupling is not as strong as supposed in the localized limit. Moreover, different intensity order dependences are observed between the phonon combination and the so-called Jahn-Teller mode. These effects are better described in the orbiton-phonon coupling scheme.     

A high-order finite volume method for systems of conservation laws—Multi-dimensional Optimal Order Detection (MOOD)

In this paper, we investigate an original way to deal with the problems generated by the limitation process of high-order finite volume methods based on polynomial reconstructions. Multi-dimensional Optimal Order Detection (MOOD) breaks away from classical limitations employed in high-order methods. The proposed method consists of detecting problematic situations after each time update of the solution and of reducing the local polynomial degree before recomputing the solution. As multi-dimensional MUSCL methods, the concept is simple and independent of mesh structure. Moreover MOOD is able to take physical constraints such as density and pressure positivity into account through an “a posteriori” detection. Numerical results on classical and demanding test cases for advection and Euler system are presented on quadrangular meshes to support the promising potential of this approach.     

A Hamiltonian Model for Linear Friction in a Homogeneous Medium

 We introduce and study rigorously a Hamiltonian model of a classical particle moving through a homogeneous dissipative medium at zero temperature in such a way that it experiences an effective linear friction force proportional to its velocity (at small speeds). The medium consists at each point in a space of a vibration field modelling an obstacle with which the particle exchanges energy and momentum in such a way that total energy and momentum are conserved. We show that in the presence of a constant (not too large) external force, the particle reaches an asymptotic velocity proportional to this force. In a potential well, on the other hand, the particle comes exponentially fast to rest in the bottom of the well. The exponential rate is in both cases an explicit function of the model parameters and independent of the potential. Received: 18 July 2001 / Accepted: 20 April 2002 Published online: 12 August 2002