Xiangyi Zhang

Synthesis of diamond from carbon nanotubes under high pressure and high temperature

The investigation on elemental carbon has long been of nanotubes to diamond at 4.5 GPa and 13008C using a considerable interest because of its great importance in six-anvil high pressure apparatus with the existence of both science and technology. One of the most outstanding NiMnCo catalyst. The detailed characterization conducted achievements which occurred in carbon science was the shows that carbon nanotubes transform to quasi-spherical synthesis of diamond under high-pressure–high-temperaonion-like structures first and then to diamond crystals. It ture (HPHT) conditions [1,2]. Now, man-made diamond is is different from the phase transformation behavior of commercially available and plays an indispensable role in graphite to diamond under high pressure and high temperamodern industry for abrasives, tool coatings, microelecture conditions. tronics, optics and other applications. Another important For the experiments presented here, multiwalled carbon advance in carbon science was the discovery of fullerenes nanotubes with diameters of 20–50 nm produced by and carbon nanotubes [3,4]. These novel carbon phases catalytic chemical vapor deposition (CCVD) were used as have gained high visibility because they have the potential starting material (Fig. 1). A 63600 ton six-anvil high to exhibit unique structural variety and extraordinary pressure apparatus with an electric current heating device optical, mechanical, and electronic properties [5]. Recentwas employed for the high pressure experiments. In each ly, a number of studies relating to the behaviors of experiment, about 50 mg of carbon nanotubes placed fullerenes under high pressure have been reported which between two NiMnCo alloy flakes were put into the high were focused on probing the cage structure stability and pressure cavity and a cubic body with a cylinder sample looking for new novel structures [6–8]. Moreover, it has chamber of pyrophyllite was used as pressure seal and been demonstrated that fullerenes can convert to diamond transmitting medium. Before and after high pressure runs, by applying high pressure at either room temperature [9] or the samples were monitored by transmission electron high temperature [10]. However, only a few works have microscopy (TEM; H8100), scanning electron microscopy been so far focused on the structural stability and phase (SEM; S-4200) and Raman spectroscopy (T64000). Hightransformation of carbon nanotubes at high pressure resolution transmission electron microscopy (HRTEM) [11,12]. As we know, the transformation of graphite or investigations were carried out using a JEOL2010 micrographite-like materials to diamond is of great technological scope operating at 200 kV. importance and therefore remains an exciting field in both Fig. 1 shows the representative morphology of carbon experimental and theoretical studies. Therefore, a detailed nanotubes in the starting material. The carbon nanotubes study of the behaviors of carbon nanotubes at high exhibit typical concentric graphitic shell structure with a pressure is very necessary for further understanding their hollow core. The diameters of most carbon nanotubes in structures and properties and comparing with that of our sample are around 30–40 nm uniformly. No other graphite. forms of carbon can be detected in the sample by TEM In the present work we report the conversion of carbon study.

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.

Growth and morphology of beta-FeSi2 single crystal with chemical vapor transport

Single crystals of β-FeSi2 have been grown from vapor by employing chemical vapor transport technique and using iodine as transport agent in a closed ampoule. In the experiment, β-FeSi2 single crystals with well-developed faces and edges have been observed on the silicon substrate, but no needle-like ones have been found. The changes in the size and the geometric shape of the growth ampoule and temperature gradient are primary factors which result in the variation on β-FeSi2 single crystal morphology.

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.

Differences in microstructure of Pd77.5Au6Si16.5 alloy solidified under microgravity and gravity conditions

The microstructure of Pd77.5Au6Si16.5alloy solidified both on board a Chinese Retrievable Satellite and on the earth is studied. Postmortem analyses of microstructure presented that the same types of phases, primary phase (Pd3Si) and eutectics (Pd3Si + Pd solid solution) were formed in both cases. But the phase morphologies were quite different. It was dendritic for the primary phase and lamellar for the eutectics under normal gravity condition. However, under microgravity condition the primary phase was granular and the eutectic was peculiar network. Detailed analysis showed that the differences in morphologies of the microstructure were due to the existence of gravity-induced buoyancy convection on the earth which increased the mass transport abilities and decreased the thickness of the solute boundary in front of the solid-liquid interface during solidification under normal gravity condition.

Solid/liquid interface of Ag/Sn/Ag trilayers by in situ resistivity measurement

The in situ four-point probe resistivity measurement was used as a main method to study the solid/liquid interfacial characteristics in Ag/Sn/Ag trilayers at temperatures ranging from 150 to 305°C. It is found from the variation of resisitivity that three processes take place on annealing: the dissolution of silver atoms, the diffusion of silver atoms, and the formation of Ag3Sn in liquid tin layer. The first one plays the leading role in the variation of resistivity during annealing process. The apparent diffusivity of silver in liquid tin at 305°C is determined to be 7.3 × 10-17 cm2/s.

FORMATION OF NANOCRYSTALLINE FE73.5CU1NB3SI13.5B9 ALLOY UNDER HIGH PRESSURE

The formation of nanocrystalline Fe73.5 Cu1Nb3Si13.5 B9 alloy by annealing an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy at a temperature of 823 K under pressures in the range of 1–5 GPa is investigated by using X-ray diffraction, electron diffraction, and transmission electron microscopy. The high pressure experiments are carried out in belt-type pressure apparatus. Experimental results show that the initial crystalline phase in these annealed alloys is a-Fe solid solution (named a-Fe phase below), and high pressure has a great influence on the crystallization process of the a-Fe phase. The grain size of the a-Fe phase decreases with the increase of pressure (P). The volume fraction of the a-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (Pδ2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy is discussed