Songmin Zhang

Femtosecond laser induced space-selective precipitation of nonlinear optical crystals in rare-earth-doped glasses

We report on space-selective precipitation of nonlinear optical crystals in an Er(3+)-doped BaO-TiO(2)-SiO(2) glass by using a focused femtosecond laser with 800 nm, 250 kHz and 150 fs. An intense green emission due to upconversion luminescence is observed around the focal point of the femtosecond laser beam at the initial stage of the laser irradiation. A blue emission due to second harmonic generation begins to emerge from the irradiation region after 40 s irradiation. Micro-Raman spectra indicate that nonlinear optical crystals (Ba(2)TiSi(2)O(8)) are precipitated after the laser irradiation. The irradiation time for crystallization in Er(3+)-doped BaO-TiO(2)-SiO(2) glasses is longer than that in BaO-TiO(2)-SiO(2) glasses under the same irradiation conditions. The mechanisms responsible for the observed phenomena are discussed.

Influence of the material processing on the electrochemical properties of cobalt-free Ml(NiMnAlFe)5 alloy

Abstract Co-free MlNi 4.1 Mn 0.35 Al 0.3 Fe 0.25 (Ml: La-rich mischmetal) alloy samples were prepared in three different ways. The original as-cast alloy was prepared by vacuum levitation melting in a cold crucible. One-third of the as-cast ingot was annealed at 1273 K for 10 h, and another one-third was remelted and rapidly solidified by melt-spinning. The influence of the alloy preparation methods on their microstructure and electrochemical properties was studied. XRD and SEM results revealed that all of the three differently prepared alloys were of the single CaCu 5 -type structure phase, but their microstructure and electrochemical properties were changed markedly. The as-cast alloy had a typical dendrite structure with noticeable composition segregation and rather poor cycling endurance. While the annealed and melt-spun alloy were of an equiaxed structure and a very fine cellular structure, respectively, and had a more homogeneous composition and dramatically improved cycling endurance, although their activation rate and high-rate dischargeability were lowered somewhat. In this study, the melt-spun Co-free alloy showed the best cycling stability ( S 500 =69.2%) and a reasonably high capacity (305 mA h g −1 ) and 1C rate dischargeability (85.6%), which is attributed to its lower degree of pulverization and more uniform composition.

Enhanced upconversion luminescence of transparent Eu3+-doped glass-ceramics containing nonlinear optical microcrystals.

We report on near-infrared to visible upconversion luminescence in transparent Eu3+-doped SrO-TiO2-SiO2 glass-ceramics. X-ray diffraction analysis indicates that Sr2TiSi2O8 crystallites are precipitated in the glass after heat treatment at 850 degrees C for 2 h. The Eu3+-doped glass-ceramics show strongly enhanced upconversion luminescence and greatly improved damage threshold compared with the as-prepared glass. The mechanisms of the observed phenomena are discussed.

Multi-photon absorption upconversion luminescence of a Tb 3+ -doped glass excited by an infrared femtosecond laser

In this paper we presents the near infrared to visible upconversion luminescence in a Tb(3+)-doped ZnO-B(2)O(3)-SiO(2) glass excited with 800nm femtosecond laser irradiation. The upconversion luminescence is attributed to (5)D(4) to (7)F(j) (j=3, 4, 5, 6) transitions of Tb(3+).The relationship between upconversion luminescence intensity and the pump power indicates that a three-photon simultaneous absorption process is dominant in this upconversion luminescence. The calculated value of the three-photon absorption cross section sigma(3) of the glass is 1.832x10(-81)cm(6)6s(2). Also, three-dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence for the first time.

The effect of solidification rate on the microstructure and electrochemical properties of Co-free Ml(NiMnAlFe)5 alloys

Abstract In this work, Fe-containing Co-free Ml(NiMnAlFe) 5 alloy samples were prepared by both arc-melting and melt-spinning processes at different solidification rates, and tested for electrochemical performance. The electrochemical tests indicated that higher solidification rates led to a better cycling stability, but lower discharge capacity and high-rate dischargeability. The X-ray diffraction results indicated that the as-cast alloy and the melt-spun alloys were both of the CaCu 5 -type structure single phase, but the lattice constants of the alloys were varying. The scanning electron microscope and EPMA results indicated that the microstructure of melt-spun alloy formed at the relative lower solidification rate (5 m / s ) was of very fine dendritic structure and that for the alloy formed at a relative higher solidification rate (15 m / s ) was of cellular structure, and both had a more homogeneous composition than the as-cast alloy, which had a typical dendrite structure with noticeable composition segregation. The Co-free MlNi 4.1 Mn 0.35 Al 0.3 Fe 0.25 prepared by melt-spinning at 15 m / s rate is a promising candidate for low-cost electrode alloy.

The phase structure and electrochemical properties of the melt-spun alloy Zr0.7Ti0.3Mn0.4V0.4Ni1.2

Abstract The crystal structure, phase abundance, microstructure and electrochemical properties of the AB 2 -type Laves phase hydrogen storage alloy Zr 0.7 Ti 0.3 Mn 0.4 V 0.4 Ni 1.2 were studied, prepared both by arc-melting and melt-spinning. The XRD patterns have revealed that two Laves phases, C14 and C15 are formed in the as-cast alloy. In the melt-spun alloy, besides the C14 and C15 phase in very fine crystal grains, there appears a new crystalline C14 phase with very fine crystallites, which is named the nanocrystallite C14 in this paper. SEM and EDS analyses have indicated that the microstructure of melt-spun alloy is a very fine dendritic structure (10×3 μm), while the as-cast alloy is a much coarser dendritic structure (300×50 μm) with noticeable composition segregation. Electrochemical tests have indicated that the melt-spun alloy has a higher discharge capacity of 385 mA h g −1 in comparison with 371 mA h g −1 for the as-cast alloy and the melt-spun alloy has very good cycling stability. After 500 cycles, 80.7% of the initial discharge capacity is retained, much higher than 63.1% of the as-cast alloy. It is thus inferred that the electrochemical capacity of the nanocrystallite C14 phase is higher than that of the conventional C14 phase. Nevertheless, the activation property and the high-rate dischargeability of the melt-spun alloy were both found to decrease noticeably, most probably due to the higher resistance against decrepitation and cracking of the alloy with a more refined grain structure.

Micro-crystalline C14 Laves phase in melt-spun AB2 type Zr-based alloy

Abstract The single phase of micro-crystalline C14 Laves of Zr 0.7 Ti 0.3 (NiVMnCr) 2.1 alloy was successfully prepared by melt-spinning processing with a certain cooling rate. The investigation with XRD Rietveld analysis and TEM showed that micro-crystalline C14 Laves phase had a unique microstructure, the grain was composed of staked c -axis textured thin plates which contained a lot of crystallites with the mean size of about 3.1×3.1×1 nm 3 . The electrochemical measurement showed that it had longer cycle life, more activation cycles and lower discharge capacity than those of conventional mixture C14+C15 Laves phase.

Effect of Cr and Co additives on microstructure and electrochemical performance of Zr(NiVMn)2M0.1 alloys

Abstract Zr-based Laves alloys of Zr(NiVMn) 2 M 0.1 (M=Cr or Co) were prepared by an arc melting process. The effect of the Cr or Co additive on the microstructure and electrochemical performance of the Zr(NiVMn) 2 M 0.1 Laves phase hydride electrode alloys was investigated. Microstructure observation showed that the morphology of the Zr(NiVMn) 2 Cr 0.1 alloy was dendritic, while the morphology of the Zr(NiVMn) 2 Co 0.1 alloy was of equiaxial grains. The XRD results showed that both alloys consisted of the predominant C14 and C15 Laves phases, and the minor Zr 7 Ni 10 , Zr 9 Ni 11 non-Laves phases. There is no big difference in the amount of Laves phase in these two alloys as they were prepared under the same solidification conditions. The effect of the Cr or Co additive on phase composition and phase abundance of the obtained Zr(NiVMn) 2 M 0.1 alloys was quite similar. The electrochemical properties of the Zr(NiVMn) 2 M 0.1 alloys were dependent on the microstructures obtained. The electrode prepared from the alloy with equiaxial grain showed less activation cycles, higher discharge capacity and poorer cycle stability than those prepared from the alloy with dendrite structure. The microstructure-dependent electrochemical properties are also discussed.

A comparative study on the electrochemical performance of rapidly solidified and conventionally cast hydride electrode alloy Zr(NiMnM)2.1

Abstract The crystalline structure, phase abundance, microstructure and electrochemical properties of hydrogen storage electrode alloy Zr(NiMnM) 2.1 (M=V, Cr) prepared by rapid solidification (melt-spinning) and conventional cast methods have been investigated and compared. Results indicate that conventional cast (CC) alloy is composed of Laves phase, non-Laves phases and a little ZrO 2 , while rapid solidification (RS) alloy are completely composed of Laves phase only. The microscopic structure of RS alloy is completely different from that of the CC alloy. The microstructure of RS alloys is fine grain, while the microscopic structure of CC alloy is coarse dendrite. Obvious differences in electrochemical properties exist in electrodes made of these two differently prepared alloys due to the difference of their phase abundance, composition distribution and microstructure. The discharge capacity of RS alloy is 56 mAh g −1 higher than that CC alloy, it reaches 356 mAh g −1 and its cycle life is much longer than that CC alloy too. But RS alloy are fully only activated after 14 cycles, more difficulty than that CC alloy, which is activated around 9 cycles. As a whole RS (melt-spinning) process improves greatly the comprehensive electrochemical properties of the alloy.

Orange‐red upconversion luminescence of Sm3+‐doped ZnO‐B2O3‐SiO2 glass by infrared femtosecond laser irradiation

— Near-infrared-to-visible upconversion luminescence was observed in Sm3+-doped ZnO-B2O3-SiO2 glass under femtosecond laser irradiation. The luminescence spectra show that the upconversion luminescence originates from 4G5/2 to 6Hj/2 (j = 5, 7, 9) transition of Sm3+. The dependence of the fluorescence intensity of Sm3+ on the pump power indicates that a two-photon absorption process is dominant in the conversion of infrared radiation to the visible luminescence. The analysis of the upconversion mechanism reveals that the simultaneous absorption of two infrared photons produces the population of upper excited states, which leads to the characteristic orange-red emission of Sm3+. A three-dimensional display is demonstrated based on the multiphoton absorption upconversion luminescence.