Characterization of porous silicon nitride/silicon oxynitride composite ceramics produced by sol infiltration

Porous silicon nitride/silicon oxynitride composite ceramics were fabricated by silica sol infiltration of aqueous gelcasting prefabricated Si3N4 green compact. Silica was introduced by infiltration to increase the green density of specimens, so suitable properties with low shrinkage of ceramics were achieved during sintering at low temperature. Si2N2O was formed through reaction between Si3N4 and silica sol at a temperature above 1550 degrees C. Si3N4/Si2N2O composite ceramics with a low linear shrinkage of 1.3-5.7%, a superior strength of 95-180 MPa and a moderate dielectric constant of 4.0-5.0 (at 21-39 GHz) were obtained by varying infiltration cycle and sintering temperature. (C) 2010 Published by Elsevier B.V.

Magnetic and optical properties of cobalt nanowires fabricated in polycarbonate ion-track templates

Submitted by CAS-IR

Preparation and characterization of CdS nanotubes and nanowires by electrochemical synthesis in ion-track templates

CdS nanotubes and nanowires have been synthesized with controlled dimensions by means of template-electrodeposition method in etched ion-track membranes. The diameters of nanotubes and nanowires are between 20 and I 10 nm, and the lengths are up to tens of micrometers. X-ray diffraction (XRD) and selected area electron diffraction (SAED) pattern investigations demonstrate that CdS nanotubes and nanowires are polycrystalline in nature. The UV-vis absorption spectra of CdS nanotubes and nanowires embedded in polycarbonate (PC) membranes show that the absorption edges of PC films shift towards the shorter wavelength, with decreasing diameters of the deposited nanostructures. The results indicate that nanowires are formed from nanotubes by nanotube-stuffing-growth mechanism.

Preparation of silver nanowires in ion-track membranes

Polycarbonate (PC) membranes were irradiated with swift heavy ions and latent tracks were created along the ions' trajectories. Nanopores, diameters between 100 and 500 nm, were obtained after illuminating the membranes with UV light and etching in NaOH solution. Silver nanowires were produced in the etched ion-track membranes by electrochemical deposition. The morphology and crystallinity of the silver nanowires were studied by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). Under certain conditions (deposition voltage 25 mV, current density 1-2 mA.cm(-2), temperature 50 degrees C, electrolyte 0.1 mol.L-1 AgNO3), single-crystalline silver nanowires with preferred orientation along the [111] direction can be synthesized.

Electrochemical fabrication of single-crystalline and polycrystalline Au nanowires: the influence of deposition parameters

We report the electrochemical growth of gold nanowires with controlled dimensions and crystallinity. By systematically varying the deposition conditions, both polycrystalline and single-crystalline wires with diameters between 20 and 100 nm are successfully synthesized in etched ion-track membranes. The nanowires are characterized using scanning electron microscopy, high resolution transmission electron microscopy, scanning tunnelling microscopy and x-ray diffraction. The influence of the deposition parameters, especially those of the electrolyte, on the nanowire structure is investigated. Gold sulfite electrolytes lead to polycrystalline structure at the temperatures and voltages employed. In contrast, gold cyanide solution favours the growth of single crystals at temperatures between 50 and 65 degrees C under both direct current and reverse pulse current deposition conditions. The single-crystalline wires possess a [110] preferred orientation.

Electrochemical polymerization and characterization of polypyrrole nanowires and nanotubules

Polypyrrole nanostructure arrays, including simultaneously large quantities of nanowires and small quantities of partially filled nanotubules have been electrochemically synthesized in home-made etched ion-track polycarbonate (PC) templates. Diameter of the prepared nanostructures varies from 45 to 320 nm with their lengths up to 30 microns. Morphological studies of these nanostructures were performed by field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. While optical absorption properties were studied by ultraviolet-visible-near infrared spectrophotometry (UV-vis-NIR). It has been observed that the absorption maximum of polypyrrole shifts to the longer wavelength side as the diameter of these nanostructures (nanowires and nanotubules) increases. (C) 2010 Elsevier B.V. All rights reserved.

Effect of Crystallographic Texture on Magnetic Characteristics of Cobalt Nanowires

Cobalt nanowires with controlled diameters have been synthesized using electrochemical deposition in etched ion-track polycarbonate membranes. Structural characterization of these nanowires with diameter 70, 90, 120 nm and length 30 mu m was performed by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction techniques. The as-prepared wires show uniform diameter along the whole length and X-ray diffraction analysis reveals that [002] texture of these wires become more pronounced as diameter is reduced. Magnetic characterization of the nanowires shows a clear difference of squareness and coercivity between parallel and perpendicular orientations of the wires with respect to the applied field direction. In case of parallel applied field, the coercivity has been found to be decreasing with increasing diameter of the wires while in perpendicular case; the coercivity observes lower values for larger diameter. The results are explained by taking into account the magnetocrystalline and shape anisotropies with respect to the applied field and domain transformation mechanism when single domain limit is surpassed.

Controlled crystallinity and crystallographic orientation of Cu nanowires fabricated in ion-track templates

The hallmark of materials science is the ability to tailor the structures of a given material to provide a desired response. In this work, the structures involving crystallinity and crystallographic orientation of Cu nanowires electrochemically fabricated in ion-track templates have been investigated as a function of fabrication condition. Both single crystalline and polycrystalline nanowires were obtained by adjusting applied voltages and temperatures of electrochemical deposition. The anti-Hall-Petch effect was experimentally evidenced in the polycrystalline nanowires. The dominant crystallographic orientations of wires along [111], [100], or [110] directions were obtained by selecting electrochemical deposition conditions, i.e., H2SO4 concentration in electrolyte, applied voltage, and electrodeposition temperature.

Segmented All-Platinum Nanowires with Controlled Morphology through Manipulation of the Local Electrolyte Distribution in Fluidic Nanochannels during Electrodeposition

Synthesis of segmented all-Pt nanowires is achieved by a template-assisted method. The combination of a suitably chosen electrolyte/template system with pulse-reverse electrodeposition allows the formation of well-defined segments linked to nanowires. Manipulation of the morphology is obtained by controlling the electrokinetie effects on the local electrolyte distribution inside the nanochannels during the nanowire growth process, allowing a deviation from the continuously cylindrical geometry given by the nanoporous template. The length of the segments can be adjusted as a function of the cathodic pulse duration. Applying constant pulses leads to segments with homogeneous shape and dimensions along most of the total wire length. X-ray diffraction demonstrates that the preferred crystallite orientation of the polycrystalline wires varies with the average segment length. The results are explained considering transitions in texture formation with increasing thickness of the electrodeposit. A mechanism of segment formation is proposed based on structural characterizations. Nanowires with controlled segmented morphology are of great technological importance, because of the possibility to precisely control their substructure as a means of tuning their electrical, thermal, and optical properties. The concept we present in this work for electrodeposited platinum and track-etched polycarbonate membranes can be applied to other selected materials as well as templates and constitutes a general method to controlled nanostructuring and synthesis of shape controlled nanostructures.

The reaction route and active site of catalytic decomposition of hydrazine over molybdenum nitride catalyst

The catalytic properties of the passivated, reduced passivated, and fresh bulk molybdenum nitride for hydrazine decomposition were evaluated in a microreactor. The reaction route of hydrazine decomposition over molybdenum nitride catalysts seems to be the same as that of Ir/gamma-Al2O3 catalysts. Below 673 K, the hydrazine decomposes into N-2 and NH3. Above 673 K, the hydrazine decomposes into N-2 and NH3 first, and then the produced NH3 further dissociates into N-2 and H-2. From the in situ FT-IR spectroscopy, hydrazine is adsorbed and decomposes mainly on the Mo site of the Mo2N/gamma-Al2O3 catalyst. (C) 2004 Elsevier Inc. All rights reserved.

In situ IR spectroscopic studies on molybdenum nitride catalysts: active sites and surface reactions

Recent IR spectroscopic studies on the surface properties of fresh Mo2N/gamma-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/gamma-Al2O3, both Modelta+ (0<δ<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm(-1), due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/gamma-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/gammaAl(2)O(3) catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/gammaAl(2)O(3) catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites.

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo(2)N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo(2)N catalysts on three different surface sites: 4-fold vacancies, Mo(delta+) ( 0 < delta < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo(2)N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo(2)N phase shows lower electron density than pure Mo(2)N. On reduced passivated Mo(2)N catalysts the CO was found to adsorb mainly on Mo(4+) sites, although some slightly different surface Mo(delta+) d (0 < delta < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo(2)N/gAl(2)O(3) were similar to those on reduced MoO(3)/gamma-Al(2)O(3).

Catalytic decomposition of hydrazine over supported molybdenum nitride catalysts in a monopropellant thruster

The catalytic decomposition of hydrazine over a series of MoNx/gamma-Al2O3 catalysts with different Mo loadings was investigated in a monopropellant thruster (10 N). When the Mo loading is equal to or higher than the monolayer coverage of MoO3 on gamma-Al2O3, the catalytic performance of the supported molybdenum nitride catalyst is close to that of the conventionally used Ir/gamma-Al2O3 catalyst. The MoNx/gamma-Al2O3 catalyst with a loading of about 23wt% Mo (1.5 monolayers) shows the highest activity for hydrazine decomposition. There is an activation process for the MoNx/gamma-Al2O3 catalysts at the early stage of hydrazine decomposition, which is probably due to the reduction of the oxide layer formed in the passivation procedure.

Controllable synthesis of gold nanowires

In this work, we demonstrate a new and simple method for preparing Au nanowires by ethanol reduction of hydrogen tetrachloroaurate (HAuCl4) without additional capping agents by a thermal process. The resulting products were investigated by UV-vis spectroscopy. transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results indicated that the size and shape of the Au nanocrystals could be controlled by systematic variation of the experimental parameters including the concentration of HAuCl4 in the reaction solution, heating temperature and reaction time.