Duanwei He

Nanocrystallization of ZrTiCuNiBeC bulk metallic glass under high pressure

Nanocrystallization of Zr41Ti14Cu12.5Ni9Be22.5C1 bulk metallic glass (BMG) under high pressure is investigated. It is found that the nanocrystallization is pressure assisted, and the primary nanocrystallization temperature decreases as the applied pressure increases. Pressure annealing of the BMG in the supercooled liquid region produces a composite with dispersion of very fine nanocrystallites in the amorphous matrix. A fully nanocrystallization is obtained by pressure annealing under 6 GPa at 723 K. The pressure also controls the phase selection during the crystallization. The mechanism for the pressure-assisted nanocrystallization is discussed.

Synthesis of carbon nitride crystals at high pressures and temperatures

Carbon nitride crystals have been synthesized from C3N4H4 in the presence of a nickel-based alloy or cobalt as a catalyst at high pressure of 7 GPa and temperature of about 1400 degrees C. Scanning electron microscopy showed rod-like, well-faceted crystals with size of several micrometers, and the N content in these carbon nitride crystals was 47-62%, X-ray diffraction indicated the crystals were composed of alpha-C3N4 and beta-C3N4. The experimental lattice constants of alpha-C3N4 (a = 6.425 Angstrom, c = 4.715 Angstrom) and beta-C3N4 (a = 6.419 Angstrom, c = 2.425 Angstrom) agree with ab initio calculations very well.

Reversible phase transition between amorphous and crystalline in Zr41.2Ti13.8Cu12.5Ni10Be22.5 under high pressure at room temperature

A reversible phase transition between amorphous and crystalline in bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 has been investigated under high pressure at room temperature. The BMG displayed a structure memory under high pressure as detected by in situ synchrotron radiation of x-ray diffraction and resistance measurement in a diamond anvil cell. Direct experimental observations found that the crystallization of the BMG occurred at 24 GPa on uploading and the crystalline phase reverted back to the amorphous state during downloading. This unusual phenomenon was discussed thermodynamically

Effects of buoyancy convection on phase morphology during solidification of Pd40Ni40P20 alloy

Samples of Pd40Ni40P20 alloy were solidified both on the ground and on board a Chinese Retrievable Satellite in order to study the effects of gravity-induced convection on the phase morphology. In comparison to the secondary dendritic spacing, the effective mass transport coefficient under normal gravity condition was estimated to be 1.95 times as high as that under microgravity conditions at the cooling rate of 0.056 K s(-1). The higher mass transport coefficient value due to the existence of buoyancy convection on the ground led to the formation of coarser dendrites of the primary phase Ni5P2 in the ground-based sample

Macroscopic synthesis of onion-like graphitic particles

High-resolution transmission electron microscopy (HRTEM) study of some low-dimensional nanostructures, such as carbon nanotubules and silicon-based blue-light emitting beta-SiC nanoparticles will be reported. We have shown that nested hollow tubules formed by successive cylindrical graphite sheets are found, frequently, to be polyhedral or elliptical in cross-section, which are perpendicular to the tube-axis. Varied spacing between adjacent tube sheets is observed and edge-type dislocations can be distinguished in some tubules. These abnormal structural features are related to accommodations of various strains taking place simultaneously in tube sheets. Blue-light emitting porous beta-SiC formed by C+ implantation of a single crystal silicon wafer have also been investigated. Buried layer structures with different beta-SiC concentration were formed in the implanted silicon substrate. These beta-SiC nano-particles are 2-8 nm in size and formed epitaxially from the upper and lower damaged layers, respectively, but randomly in the middle layer. The structural characteristics of the layered structure may be responsible for the blue-light emitting effect of the porous beta-SiC material

Solidification characteristics of Pd40Ni40P20 alloy under microgravity condition

The microstructure and solute distribution of Pd40Ni40P20 alloy solidified both on board a Chinese retrievable satellite (μg) and on the earth (1g) were studied. It was found that the dendritic primary phase formed under microgravity condition was finer and shorter. In the central area of the sample some asteroidal patterns of the primary phase were present in the microstructure. The primary spacing of the dendrites at the cooling rate of 0. 056 K/s was smaller than that measured in the ground-based experiments at the same cooling rate, but almost the same as that cooled at 0.67 K/s on the ground. With these experimental results, mass transport coefficients both in space and on the earth were evaluated.

Evaluation of effective mass transport coefficients through comparison of solidification on the ground and on board a satellite

Mass transport caused by buoyancy convection in front of the solid–liquid interface was evaluated in terms of measurements of primary dendritic spacing combining with separation of the effective (or integral) mass transport coefficient DL. It was shown that DL in normal gravity (1g) condition was 1.64 times as high as that in microgravity (μg) condition at the cooling rate (v) of 0.056 K/s for Pd40Ni40P20 alloy. The higher DL value is due to the contribution of buoyancy convection on the ground.

Influence of buoyancy convection on solute distribution in Pd40Ni40P20 alloy

The differences in the solute distribution in microstructure of Pd40Ni40P20 alloy solidified on board a Chinese retrievable satellite and on the ground were studied. In comparison with those crystallized under normal gravity conditions (1 g), it was found that the P content was lower, but the Pd content was higher in the primary phase in microgravity conditions (μ g). In the eutectic region the P content, however, was increased but the Pd content was decreased. The differences in solute distribution crystallized under 1 g and μ g conditions show the influence of buoyancy convection on the mass transport coefficient in liquids under normal gravity conditions.

SYNTHESIS OF C3N4 CRYSTALS UNDER HIGH PRESSURE AND HIGH TEMPERATURE

C3N4 crystals with the size of several micrometers have been synthesized from C3N4H4 in the presence of nickel-based alloy or cobalt as catalyst under high pressure of 7 GPa and temperature of about 1400°C for 10 min. Scanning electron microscopy, energy-disperse X-ray analysis, and X-ray diffraction were used to examine the grown crystals. The general rule on selecting the starting materials for synthesis of carbon nitride crystals at high pressures and high temperatures is suggested.

Solidification of Undercooled Ge73.7Ni26.3 Alloy Subjected to Sputtering-Deposition of Ni Clusters

Undercooled melt of Ge73.7Ni26.3 alloy sample floated on B2O3 melt was subjected to sputtering-deposition of Ni clusters on cooling to induce solidification. The initial temperature of solidification was 975 K. Its undercooling Delta T was 134 K with respect to the melting temperature of the primary phase Ge. On the cooling curve existed dual recalescences representing the transformation of the primary phase (Ge) and the eutectics (Ge+GeNi) respectively But for the sample unsputtered, only a single exothermic peak presented on the cooling curve with the initial solidification temperature of 904 K and undercooling of 205 K. Careful observation of the microstructure proved that the Ge phase primarily formed in the sputtering experiment was now formed together with the formation of the eutectics in the experiment without sputtering.