Parameters determining crystallinity in beta-SiC thin films prepared by catalytic chemical vapor deposition

The effects of deposition gas pressure and H-2 dilution ratio (H-2/SiH4+CH4+H-2), generally considered two of dominant parameters determining crystallinity in beta-SiC thin films prepared by catalytic chemical vapor deposition (Cat-CVD), often called hot-wire CVD method, on the films properties have been systematically studied. As deposition gas pressure increase from 40 to 1000 Pa, the crystallinity of the films is improved. From the study of H-2 dilution ratio, it is considered that H-2 plays a role as etching gas and modulating the phases in beta-SiC thin films. On the basis of the study on the parameters, nanocrystalline beta-SiC films were successfully synthesized on Si substrate at a low temperature of 300degreesC. The Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) spectra show formation of beta-SiC. Moreover, according to Sherrer equation, the average grain size of the films estimated is in nanometer-size. (C) 2003 Elsevier B.V. All rights reserved.

Purification of single-walled carbon nanotubes synthesized by the catalytic decomposition of hydrocarbons

A procedure for purifying single-walled carbon nanotubes (SWNTs) synthesized by the catalytic decomposition of hydrocarbons has been developed. Based on the results from SEM observations, EDS analysis and Raman measurements, it was found that amorphous carbon, catalyst particles, vapor-grown carbon nanofibers and multi-walled carbon nanotubes were removed from the ropes of SWNTs without damaging the SWNT bundles, and a 40% yield of the SWNTs with a purity of about 95% was achieved after purification. (C) 2000 Elsevier Science Ltd. All rights reserved.

Hydrogenation and anisotropic expansions in R3Fe29-xTx (R = Y, Ce, Nd, Sm, Gd, Tb and Dy; T = V and Cr)

A systematic study of the phase formation, structure and magnetic properties of the R3Fe29-xTx compounds (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy; T=V and Cr) has been performed upon hydrogenation. The lattice constants and the unit cell volume of R3Fe29-xTxHy decrease with increasing R atomic number from Nd to Dy, except for Ce, reflecting the lanthanide contraction. Regular anisotropic expansions mainly along the a- and b-axis rather than along the c-axis are observed for all of the compounds upon hydrogenation. Hydrogenation leads to an increase in the Curie temperature and a corresponding increase in the saturation magnetization at room temperature for each compound. First order magnetization processes (FOMP) occur in the external magnetic fields for Nd3Fe24.5Cr4.5H5.0, Tb3Fe27.0Cr2.0H2.8, and Gd3Fe28.0Cr1.0H4.2 compounds.

Catalytic Dehydration of Ethanol to Ethylene on TiO2/4A Zeolite Composite Catalysts

TiO2/4A zeolite composite catalysts were prepared by coating TiO2 on 4A zeolite via liquid phase deposition. The TiO 2/4A zeolite composite catalysts wtih higher surface weak acidity and lower mediate strong acidity exhibit much better catalytic performance on ethanol dehydration to ethylene compared with 4A zeolite. It is suggested that the TiO2 promoter could improve the effective Lewis acidity of composite catalyst which consequently enhanced the catalytic performance.

MoNi/gamma-Al2O3 Catalyst: Preparation and Catalytic Activity for Acetic Acid Hydrotreating

MoNi/gamma-Al2O3 catalysts were prepared by the impregnation method. The catalyst samples were characterized by XRD and TPR. The effects of Mo promoter content and the catalyst reducing temperature Oil hydrotreatment activity of the catalyst were studied under 200 degrees C and 3 MPa hydrogen pressure using acetic acid as the model compound. The XRD results indicate that the addition of Mo promoter is beneficial to the uniformity of nickel species on the catalyst and decreases the Interaction between nickel species and the support Which results in the decrease the of NiAl2O4 spinel formation. The addition of Mo promoter also decreases the reducing temperature of the catalyst. After the catalyst of 0.06 MoNi/gamma-Al2O3 being reduced Under the atmosphere of H-2/N-2(5/95, V/V), nickel oxide was reduced to Ni-0. The reaction was promoted obviously upon the addition of the MoNi/gamma-Al2O3 catalyst reduced at 600 degrees C. The Mo-modified Ni/gamma-Al2O3 catalyst reduced at 600 degrees C displayed the highest activity during the reaction, the conversion of acetic acid reached the highest point of 33.2%. The products included ethyl acetate and water.

Transient flow patterns and bubble slug lengths in parallel microchannels with oxygen gas bubbles produced by catalytic chemical reactions

Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O-2) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0 kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended.

Bio-syngas production from biomass catalytic gasification

A promising application for biomass is liquid fuel synthesis, such as methanol or dimethyl ether (DME). Previous studies have studied syngas production from biomass-derived char, oil and gas. This study intends to explore the technology of syngas production from direct biomass gasification, which may be more economically viable. The ratio of H-2/CO is an important factor that affects the performance of this process. In this study, the characteristics of biomass gasification gas, such as H-2/CO and tar yield, as well as its potential for liquid fuel synthesis is explored. A fluidized bed gasifier and a downstream fixed bed are employed as the reactors. Two kinds of catalysts: dolomite and nickel based catalyst are applied, and they are used in the fluidized bed and fixed bed, respectively. The gasifying agent used is an air-steam mixture. The main variables studied are temperature and weight hourly space velocity in the fixed bed reactor. Over the ranges of operating conditions examined, the maximum H-2 content reaches 52.47 vol%, while the ratio of H-2/CO varies between 1.87 and 4.45. The results indicate that an appropriate temperature (750 degrees C for the current study) and more catalyst are favorable for getting a higher H-2/CO ratio. Using a simple first order kinetic model for the overall tar removal reaction, the apparent activation energies and pre-exponential factors are obtained for nickel based catalysts. The results indicate that biomass gasification gas has great potential for liquid fuel synthesis after further processing.

EFFECTS OF DEEP CENTERS AT PT SI INTERFACES AND HYDROGENATION

Electrical measurements were combined with surface techniques to study the Pt/Si interfaces at various silicide formation temperatures. Effects of deep centers on the Schottky barrier heights were studied. Hydrogen plasma treatment was used to passivate the impurity/defect centers at the interfaces, and the effects of hydrogenation on the Schottky barrier heights were also examined. Combining our previous study on the Pt/Si interfacial reaction, factors influencing the PtSi/Si Schottky barrier diode are discussed.