Mingzhi Wang

Analysis of the nanodiamond particle fabricated by detonation

In order to comprehensively analyse the structures and the surface states of the nanodiamond particles fabricated by detonation, various apparatus were used to investigate the nanodiamond powder including a high-resolution transmission electron microscope, an energy diffraction spectrometer, an X-ray diffractometer, a Raman spectrometer, a Fourier transform infrared spectrometer and differential scanning calorimeter. The grain size of the nanodiamond particles was in the range of 2–12 nm. However, the average grain size of the nanodiamond was approximately 5 nm. Moreover, the shapes of the nanodiamond particles were spherical or elliptical. The nanodiamond as fabricated was very pure, containing almost only the element of carbon. The contents of the impure element including O, Al and S were very small, which came from the synthesis and purification processes when fabricating the nanodiamond. The surfaces of the nanodiamond particles absorbed many functional groups, such as hydroxy, carbonyl, carboxyl and ether-based resin. The initial oxidation temperature of the nanodiamond powder in the air was about 520°C, which was lower than that of the bulk diamond. However, the oxidation temperature of the nanographite existing in the nanodiamond powder was about 228°C. The graphitisation temperature of the nanodiamond powder in the Ar gas was approximately 1305°C.

Transformation of onion-like carbon from nanodiamond by annealing

Onion-like carbon (OLC) was synthesized by annealing nanodiamond in low vacuum (1 Pa) at the temperatures from 500 to 11400°C. The high-resolution transmission electron microscope images, X-ray diffraction patterns and Raman spectra showed that, when the annealing temperatures were lower than 900°C, there was no OLC fabricated. The amorphous carbon and the nanodiamond coexisted. The graphitization started from the surfaces of the nanodiamond particles. When the annealing temperatures were higher than 900°C, the OLC was fabricated. At 900°C, OLC began appearing and the size of the OLC particles was smaller than 5 nm. At the annealing temperature of 1400°C all the nanodiamond particles were transformed into OLC. The OLC particles exhibited similarity to the original nanodiamond particles in shape. Based on these results, a mechanism for the OLC synthesis by the method of annealing in vacuum was provided.

Effects of Plastic Warm Deformation on Cementite Spheroidization of a Eutectoid Steel

The warm compression tests were performed on the eutectoid steel to investigate the evolution of cementite morphology. Several processing parameters, such as temperature, strain rate and reduction, were changed to analyze the effect of each parameter on spheroidization of cementite. The results showed that the warm compression promoted the fragmentize and the spheroidization of lamellar cementites. When the specimen was compressed with reduction of 50% at 700 °C and in the strain rate of 0. 01 s−1, the excellent spheroidized cementite was obtained. The mechanism of fragmentation and spheroidization of lamellar cementites during compression was discussed by using transmission electron microscope. The formation of spheroidized cementite was related to the time of compression process. The fragmentize of lamellar cementites was due to the extension of sub-grain boundary in the cementite. The spheroidization of cementite depended on the diffusion of carbon atoms at the tip of bended and breakup cementite.

HRTEM and Raman characterisation of the onion-like carbon synthesised by annealing detonation nanodiamond at lower temperature and vacuum

Onion-like carbon (OLC) was synthesised by annealing detonation nanodiamond for 1.5 h at temperatures from 500 to 1400°C and at a vacuum of 1 Pa. The results showed that the nanodiamond was transformed into the amorphous carbon (a-C) at first and then the a-C was transformed into the OLC gradually with the increase in annealing temperature. Moreover, at the annealing temperature of 600°C, the nanodiamond started transforming into a-C from the edge of the nanodiamond particle (1 1 1) crystal plane. At the annealing temperature of 750°C, the nanodiamond was transformed into the a-C completely. At the annealing temperature of 850°C, the a-C began transforming into the OLC at the edge area. At the annealing temperature of 1000°C, the OLC particle with a size smaller than 5 nm was synthesised. However, in the centre of the OLC particle, untransformed a-C coexisted. At the annealing temperature of 1100°C, the microstructure of the OLC particle with a size smaller than 5 nm became optimised. At the annealing temperature of 1200°C, the OLC particle with a size larger than 5 nm was fabricated. There was also untransformed a-C coexisting in the centre of the OLC particle. At the annealing temperature of 1350°C, all the a-C was transformed into the OLC. The average size of the OLC was approximately 5 nm, which was the same as that of the nanodiamond. The layers of the OLC were varied from several to 12.

Effect of Martensite Morphology on Impact Toughness of Ultra-High Strength 25CrMo48V Steel Seamless Tube Quenched at Different Temperatures

A 25CrMo48V steel for ultra-deep oil/gas weil casings was quenched at 900–1200 °C and tempered at 650 °C. The lath martensitic structures were characterized by optical microscope (OM), field emission scanning electron microscopy (FESEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the trans verse impact energy at 0 °C was measured from the as-quenched and tempered specimens. The results show that with the quenching temperature decreased, the prior austenite grain, martensitic packet and block are refined, while the lath width seems to remain unchanged. The enhancement of impact toughness with the decreasing quenching temperature can be attributed to refinement of the martensitic structure with high-angle boundaries, and the block is the minimum structure unit controlling impact toughness. The trans verse impact energy [ECVN (0 °C) ≥ 100 J] required for seamless casings with ultra-high strength (Rp0.2 ≥ 932 MPa) has been finally achieved with the experimental steel quenched at 900–1000 °C and tempered at 650 °C.

Onion-like carbon synthesis by annealing nanodiamond at lower temperature and vacuum

The onion-like carbon (OLC) was synthesised by annealing the nanodiamond fabricated by detonation for 1 h at the temperature of 1150°C in the low vacuum of 2 Pa. The OLC particles were characterised using a high-resolution transmission electron microscope (HRTEM) for observing its microstructure, an X-ray diffractometer (XRD) for determining its crystal structure and component, and a Raman spectrometer for confirming its content. The results showed that the OLC particles exhibited similar shape to that of the original nanodiamond particles. The average size of the OLC was found to be approximately 5 nm. The transformation mechanism of the OLC from the nanodiamond by annealing at lower temperature and lower vacuum was also discussed.

Adsorption and conversion of hydrogen sulfide over PAN-based ACF

In this study, we investigated the adsorption and conversion behavior of H 2 S over PAN-based activated carbon fiber (ACF) in the absence or in the presence of oxygen.

Generation of microplasma using multiwall carbon nanotubes cathode

Microplasma was produced in argon gas in a scanning electron microscope at near-atmospheric pressure using a multi-wall carbon nanotube (CNT) film as a cathode. It is demonstrated that with the CNT film used as a cathode, the breakdown voltage was much lower than the breakdown voltage when the conventional cathode made of flat metal film was used and the discharge was highly reproducible. These features of the gas discharge are defined by the field emission from the CNT cathode.

Field emission from carbon nanotubes in air

The properties of the field emission (FE) from multi-walled carbon nanotubes (CNTs) in air used to generate the microplasma at near-atmospheric pressure were investigated in a removable gas cell built into a scanning electron microscope. The gaps between the electrodes were adjusted from 5 to 100 μm and the pressure was changed from 0 to 100 kPa. The obtained results have shown that the FE properties of the CNTs at 10 kPa and lower pressures were the same as those in vacuum. At a pressure more than 10 kPa, the FE threshold voltage in air was higher than those in vacuum, and increased with increasing atmospheric pressure. When the FE threshold voltage became higher than that of the gas breakdown, the microplasma was ignited before the FE initiation. Thus, the FE properties of the CNTs in air were stable when the FE potential was lower than the voltage of conventional gas discharge with CNT cathode.

Influence of Initial Microstructure on Warm Deformation Processability and Microstructure of an Ultrahigh Carbon Steel

Various isothermal compression tests are carried out on an ultrahigh carbon steel (1. 2% C in mass percent), initially quenched or spheroidized, using a Gleeble-3500 system. The true stress is observed to decrease with increasing temperature and decreasing strain rate. The true stress of the initially quenched steel is lower than that of the initially spheroidized steel at high deformation temperature (700 °C) and low deformation strain rate (0. 001 s−1). The value of the deformation activation energy (Q) of the initially quenched steel (331. 56 kJ/mol) is higher than that of the initially spheroidized steel (297. 94 kJ/mol). The initially quenched steel has lower efficiency of power dissipation and better processability than the initially spheroidized steel. The warm compression promotes the fragmentation and the spheroidization of lamellar cementites in the initially quenched steel. The fragmentation of lamellar cementites is the spheroidizing mechanism of the cementites in the initially quenched steel. Results of transmission electron microscope investigation showed that fine grains with high angle boundaries are obtained by deformation of the initially quenched steel.