Jiayong Zhong

SUPERHEATING OF Ag NANOPARTICLES EMBEDDED IN Ni MATRIX

Nanometer-sized Ag particles embedded in a Ni matrix were prepared by using melt spinning. The uniformly distributed Ag nanoparticles with a mean size of 30 nm exhibit a cube-cube orientation relationship with the Ni matrix, and some Ag nanoparticles are surrounded by {111} and {100} low-energy interfaces. Differential scanning calorimetry (DSc) and in situ X-ray diffraction (XRD) analysis results indicate that the Ag nanoparticles can be substantially superheated above the equilibrium melting point (TO) of the bulk Ag, as much as about 70 K above T-0 in the DSC measurement at a heating rate of 20 K/min. The superheating phenomenon is reproducible upon several heating/cooling cycles. In situ XRD results also indicate that the smaller the Ag particle, the higher the superheating. Our observations can be qualitatively interpreted with thermodynamic considerations

Plasmoid Ejection and Secondary Current Sheet Generation from Magnetic Reconnection in Laser-Plasma Interaction

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

Collisionless shock generation in high-speed counterstreaming plasma flows by a high-power laser

The experimental demonstration of the formation of a strong electrostatic (ES) collisionless shock has been carried out with high-speed counterstreaming plasmas, produced by a high-power laser irradiation, without external magnetic field. The nearly four times density jump observed in the experiment shows a high Mach-number shock. This large density jump is attributed to the compression of the downstream plasma by momentum transfer by ion reflection of the upstream plasma. Particle-in-cell (PIC) simulation shows the production of a collisionless high Mach-number ES shock with counterstreaming interaction of two plasma slabs with different temperatures and densities, as pointed out by Sorasio et al. [Phys. Rev. Lett. 96, 045005 (2006)]. It is speculated that the shock discontinuity is balanced with the momentum of incoming and reflected ions and the predominant pressure of the electrons in the downstream with PIC simulation.

Collisionless shockwaves formed by counter-streaming laser-produced plasmas

The interaction between two counter-streaming laser-produced plasmas is investigated using the high-power Shenguang II laser facility. The shockwaves observed in our experiment are believed to be excited by collisionless mechanisms. The dimensionless parameters calculated with the results suggest that it is possible to scale the observation to the supernova remnants using transformation and similarity criteria.

Aluminium doping induced enhancement of p–d coupling in ZnO

Valence-band type Auger lines in Al doped and undoped ZnO were comparatively studied with the corresponding core level x-ray photoelectron spectrography (XPS) spectra as references. Then the shift trend of energy levels in the valence band was that p and p-s-d states move upwards but e and p-d states downwards with increasing Al concentration. The decreased energy of the Zn 3d state is larger than the increased energy of the 0 2p state, indicating the lowering of total energy. This may indicate that Al doping could induce the enhancement of p-d coupling in ZnO, which originates from stronger Al-O hybridization. The shifts of these states and the mechanism were confirmed by valence band XPS spectra and 0 K-edge x-ray absorption spectrography (XAS) spectra. Finally, some previously reported phenomena are explained based on the Al doping induced enhancement of p-d coupling.

R-matrix electron-impact excitation data for astrophysically abundant sulphur ions

We present results for the electron-impact excitation of highly-charged sulphur ions (S 8+ –S 11+ ) obtained using the intermediatecoupling frame transformation R-matrix approach. A detailed comparison of the target structure has been made for the four ions to

Energy levels and radiative rates for optically allowed and forbidden transitions of Ni XXV ion

Aims. We report calculations of energy levels and radiative rates for 1300 fine-structure levels generated from 189 configurations of Be-like Nickel. Methods. The General Purpose Relativistic Atomic Structure Program (GRASP2), developed by the I P Grant group in 1989 and partly improved by our group, is adopted in the calculations of energy levels and radiative rates. Results. Our calculations are compared with those of other numerical methods and experiments so that their accuracy can be assessed. Additionally, the wavelengths, oscillator strengths, and radiative rates (for a given radiative rate and level structure, it is then easy to derive the line strength) are reported for all electric dipole (E1), electric quadrupole (E2), magnetic dipole (M1) and magnetic quadrupole (M2) transitions among these levels. Finally, the lifetimes of all excited levels are reported.

One-dimensional and multichannels multi-imaging x-ray streak camera for imploded core plasma of shell-cone target.

Recently one-dimensional and multichannels multi-imaging x-ray streak camera (1D+McMIXS) has been proposed as an ultrafast diagnostic tool for imploded core plasma of shell-cone target. This diagnostic system can provide much more information of core plasma dynamics, such as the shell trajectory before the maximum compression, two dimensional (2D) x-ray images, and 2D map of electron temperature. The 1D+McMIXS was used in an implosion experiment of a shell target with a cone for fast ignition at Gekko XII laser facility. The interaction between the core plasma and the tip of the cone was observed and revealed some detailed structures. A series of time-resolved 2D x-ray images were reconstructed and a nominal temperature evolution was also mapped.

Melting, superheating and freezing behaviour of indium interpreted using a nucleation-and-growth model

The traditional phenomenological asymmetry between melting and freezing has been studied by varying the crystal nucleation behaviour during freezing and the liquid nucleation behaviour during melting. The existence of a series of freezing peaks, whose shapes resemble normal melting peaks of bulk In, of In melt with pre-existing crystal nuclei has been demonstrated experimentally by means of accurate differential scanning calorimetry (DSC). A superheating peak has been investigated by using DSC for In nanoparticles embedded in an Al matrix. A phenomenological kinetic symmetry between melting and freezing has been demonstrated using DSC curves, which suggests that the melting and superheating behaviour of metal can be interpreted in terms of non-nucleation, heterogeneous nucleation and homogeneous nucleation models, respectively.

Formation and evolution of a pair of collisionless shocks in counter-streaming flows

A pair of collisionless shocks that propagate in the opposite directions are firstly observed in the interactions of laser-produced counter-streaming flows. The flows are generated by irradiating a pair of opposing copper foils with eight laser beams at the Shenguang-II (SG-II) laser facility. The experimental results indicate that the excited shocks are collisionless and electrostatic, in good agreement with the theoretical model of electrostatic shock. The particle-in-cell (PIC) simulations verify that a strong electrostatic field growing from the interaction region contributes to the shocks formation. The evolution is driven by the thermal pressure gradient between the upstream and the downstream. Theoretical analysis indicates that the strength of the shocks is enhanced with the decreasing density ratio during both flows interpenetration. The positive feedback can offset the shock decay process. This is probable the main reason why the electrostatic shocks can keep stable for a longer time in our experiment.