J. T. Wang

Wettability and spreading kinetics of liquid aluminium on boron nitride

The wettability and spreading kinetics of liquid AI on CVD-BN were investigated by the sessile drop method in a vacuum of about 1.1×10−3 Pa at 1070 to 1430 K. The wettability on the Al-BN system was different from that of the Al-SiC system reported in the literature. The wetting angle of Al-BN linearly decreased with an increase of temperature in high temperature range, and tended towards 15° at 1430 K. Complete wetting could be obtained at about 1470 K. The interface energy between liquid Al and the reaction layer and the surface energy of CVD-BN were calculated by means of Warren thermodynamics analysis. The surface of CVD-BN was examined by XRD. The results show that the surface of CVD-BN was chiefly composed of those low energy planes parallel to (001). According to the relationship of the spreading kinetics from the experiments, the apparent activation energy of the liquid Al spreading on the surface of CVD-BN was calculated. The result shows that the activation energy of interfacial reaction accounts for about 54% of the spread activation energy. The spread of liquid Al on CVD-BN was affected by the interfacial reaction, diffusion and liquid movement on the surface.

Crystallization kinetics of Ni-P glass — activation energies for nucleation and growth of nuclei

A pre-anneal treatment was introduced to a melt-spun Ni-P alloy. By crystallizing the asquenched and pre-annealed samples in a differential scanning calorimeter at different heating rates and isothermal process, the apparent activation energies for the total crystallization of the Ni-P samples were obtained by using the Kissinger technique and the Arrhenius relation. It was found that the apparent activation energy, Ec, decreased and tended to a certain value of 180 kJ mol−1, with increase of pre-anneal time. According to the classical phase transformation theory, the activation energies for nucleation and growth of nuclei in the Ni-P glasses were deduced to be 324 and 180 kJ mol−1, respectively, which are very close to the values previously reported by our laboratory. The weight parameters of nucleation, a, and growth, b, decrease monotonically and increase and tend to certain values with an increase of pre-anneal time, respectively.

AMORPHOUS PHASE-TRANSITION MECHANISM BY THE MECHANICAL ALLOYING OF THE FE-W SYSTEM

The Fe–W binary system exhibits a zero heat of mixing in the amorphous state. Thus, there is no chemical driving force for the crystal‐to‐amorphous transformation. However, the crystal‐to‐amorphous transition was observed in a mechanically alloyed mixture of pure Fe and W powders. A structural study by x‐ray diffraction showed that supersaturated solid solutions of W in Fe [Fe(W) SSS] and Fe in W [W(Fe) SSS] were produced during the early stage of the mechanical alloying. Iron atoms were already totally involved in the solid‐state amorphization reaction after 24 h of milling. The end products were W(Fe) SSS and amorphous Fe–W alloy. The amorphization by mechanical alloying of the Fe–W system is attributed to a solid‐state amorphization reaction in which lattice distortion induced by supersaturation of W in Fe and a refinement of grain size may raise the free energy of Fe(W) SSS above that of the amorphous phase and make the Fe(W) SSS destabilize.

UV upconversion luminescence of Nd3+ and Er3+ ions in fluorozirconate glasses

Abstract Upconversion luminescence in the near UV region has been obtained in Nd 3+ and Er 3+ doped fluorozirconate glasses, under argon ion laser excitation for the first time. The intensity of the upconversion luminescence increased with an increase in pumping intensity and dopant concentration. The upconversion transition processes are described, and their mechanisms are also examined.

Solid state amorphization reactions in Ni/Ti multilayer composites prepared by cold rolling

Solid state amorphizations in mechanically deformed Ni/Ti multilayer composites have been investigated by differential scanning calorimetry and X-ray diffraction. The growth of amorphous alloy in our Ni/Ti composites was facilitated by the large number of grain boundaries and relatively large degree of disorder induced in the metal layers by the cold rolling. The formation of different products of solid-state reaction in the Ni/Ti composites has been elucidated in terms of magnetic and enthalpy analysis. It is thought that the solid state amorphization occurs first during heating, followed by the formation of intermetallic compounds through direct solid-state reaction of elemental nickel and titanium and by the crystallization of the amorphous alloy already formed by the solid state amorphization reaction. The values of the activation energy for interdiffusion and interdiffusion coefficient for the formation of amorphous alloy in the Ni/Ti composites have also been calculated.

STRUCTURE AND PROPERTY OF POLYCRYSTALLINE (FE0.99MO0.01)78SI9B13 ALLOYS

Polycrystalline Fe‐Mo‐Si‐B alloys with grain sizes of 15–200 nm were synthesized via crystallization of the amorphous alloy. Positron lifetime results show that there are two existing types of interfacial defects in nanocrystalline Fe‐Mo‐Si‐B alloys, namely free‐volume‐sized defects and nanovoids. The free‐volume‐sized defect shows almost no change in size or density during grain growth, and has a mean lifetime smaller than that of its amorphous counterpart as a result of structural relaxation in the process of crystallization. However, abnormal changes in size and density of nanovoids with grain growth were noticed. It is of great interest that the variations of intermediate lifetime τ2 and intensity ratio I1/I2 with the average grain size D are exactly compatible with those of microhardness. A lower density of nanovoids corresponds to a larger microhardness in the case of the present alloy system.

Solid state amorphization transformations induced by mechanical alloying

The solid state amorphization transformations induced in the Fe-W, (Al or Ni)-(amorphous Fe-Si-B) and Ni-Ti systems were investigated by mechanical alloying. The structure and characteristics of the mechanically alloyed materials were studied by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, differential thermal analysis, and magnetic measurement. Amorphization by mechanical alloying of the Fe-W system is attributed to a solid state amorphization reaction in which lattice distortion induced by supersaturation of tungsten in the iron crystal and a refinement of grain size may raise the free energy of the iron crystal supersaturated by tungsten above that of the amorphous phase. Amorphization by mechanical alloying of elemental aluminum or nickel metal and amorphous Fe-Si-B alloy can also be obtained within a composition range of the elemental metal. Different amorphization transformation paths are found in the Ni-Ti binary system mechanically alloyed in argon, nitrogen and oxygen atmospheres respectively.

Amorphization induced by mechanical alloying and cold rolling

Solid state amorphous phase change is studied in the Ni-Ti system. It is found that the crystalline to amorphous transition can be obtained by the mechanical alloying, or by annealing of the multilayers produced by mechanical alloying and/or cold rolling. Different products are found after the annealing of the multilayers. The activation energies and interdiffusion coefficients are obtained for the solid state amorphous reaction.

The emission characteristics of Nd3+-doped fluoride glass fiber

Abstract The emission spectra of Nd 3+ -doped ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF (ZBLAN) glass fiber at different pump power levels have been measured and studied. A shift of laser oscillation wavelength is observed in Nd 3+ -doped ZBLAN glass fiber and it can be considered as a competition among transverse modes. The measured spectra demonstrate the splitting of fluorescence with increase of pump power at 514.5 nm, regarded as the site competition of Nd 3+ ions in the excited state. The superfluorescence emission and a unified process from fluorescence to laser oscillation pumped by a Ti:sapphire laser have been obtained.