Vapor-phase Sources Research Articles

Synthesis mechanism of α-Si3N4 whiskers via SiO vapor in reaction bonded Si3N4-SiC composite

An indirect nitridation mechanism was designed to realize a high conversion rate to α-Si3N4 whiskers in reaction bonded Si3N4-SiC composite via VS mechanism, in which the dominant vapor-phase source of reactants is SiO vapor produced by the active oxidation of Si. The role played by the temperature-time program and addition of ultra-fine Si powder on the formation of SiO vapor and the resulting Si3N4 whiskers was clarified based on the experimental results and the thermodynamic calculations. The result shows that during sintering, in the presence of trace O2, ultra-fine Si particles preferentially undergo active oxidation and volatilized completely in the form of SiO(g). While an equilibrium of SiO(g) and Si(s) is formed on the surface of the coarse Si particle meanwhile. By stepped holding temperature further, this equilibrium is constantly upgraded, so the coarse Si particles is evaporated as SiO(g) shell layer by layer. Massive α-Si3N4 whiskers is formed by SiO(g) nitridation while the stepped holding temperature sintering. A reasonable model of the controllable indirect nitridation mechanism was established.

Methods and Devices for the Preparation of Standard Gas Mixtures

A review of methods for the preparation of standard gas mixtures (SGMs) with the known concentrations of organic and inorganic gases and vapors is presented. The classification of methods is proposed and their advantages and disadvantages are compared. Along with the widely known thermal diffusion and gasextraction methods and devices for the preparation of SGMs functioning on their basis, i.e., sources of gas and vapor microflows and vapor-phase sources of gas mixtures, the review considers and discusses metrological possibilities proposed by Russian researchers of chromatomembrane and chromato-desorption methods of SGM preparation and corresponding instrumentation.

Growth of aluminum oxide nanorods using sandwich structures composed of Al and SiO x layers

In this work, we report a simple method for growing aluminum oxide nanorods based on low-temperature annealing of a sandwich structure composed of a thin Al film sandwiched between two silicon-rich oxide (SiOx: 0 < x < 2) layers produced by plasma-enhanced chemical vapor deposition. Aluminum oxide nanorods produced using a typical sandwich structure of SiO1.4(20 nm)/Al(2 nm)/SiO1.4(20 nm)/Si substrate exhibited the features of the mean diameter and length of the nanorods of about 0.3 and 1.7 μm, respectively. A possible growth mechanism is discussed on the basis of the vapor–solid process. The nanorod growth is proposed to be mediated by a vapor–solid mechanism in which the dominant vapor-phase source of reactants is Al2O/AlO produced by a phase separation of SiOx.

Gas-chromatographic headspace analysis: Metrological aspects

Various methods are studied for preparing stable gas mixtures with trace amounts of volatile substances based on a buffer effect of heterogeneous systems and implemented into headspace analysis, that is, static and dynamic versions of gas extraction. Information is provided on certified procedures for preparing such mixtures, that is, vapor-phase sources of gas mixtures included into the State Register of Measuring Instruments.

Metal-catalyzed graphitization in Ni-C alloys and amorphous-C/Ni bilayers

ABSTRACTMetal-catalyzed graphitization from vapor phase sources of carbon is now an established technique for producing few-layer graphene, a candidate material of interest for post-silicon electronics. Here we describe two alternative metal-catalyzed graphene formation processes utilizing solid phase sources of carbon. In the first, carbon is introduced as part of a cosputtered Ni-C alloy; in the second, carbon is introduced as one of the layers in an amorphous carbon (a-C)/Ni bilayer stack. We examine the quality and characteristics of the resulting graphene as a function of starting film thicknesses, Ni-C alloy composition or a-C deposition method (physical or chemical vapor deposition), and annealing conditions. We then discuss some of the competing processes playing a role in graphitic carbon formation and review recent evidence showing that the graphitic carbon in the a-C/Ni system initially forms by a metal-induced crystallization mechanism (analogous to what is seen with Al-induced crystallization of amorphous Si) rather than by the dissolution-upon-heating/precipitation-upon-cooling mechanism seen when graphene is grown by metal-catalyzed chemical vapor deposition methods.

Preparation of stable gas mixtures with microconcentrations of volatile substances in vapor-phase sources at elevated pressures

Tools are proposed for the preparation of stable vapor-gas mixtures with microconcentrations of volatile components (vapor-phase sources of gas mixtures). These work at elevated pressures and ensure the generation of gas flows with component concentrations differing within an order of magnitude from one and the same source.

Active-oxidation of Si as the source of vapor-phase reactants in the growth of SiOx nanowires on Si

Gold-coated silicon wafers were annealed at temperatures in the range from 800–1100 °C in a N2 ambient containing a low (3–10 ppm) residual O2 concentration. A dense network of amorphous silica nanowires was only observed on samples annealed at temperatures above 1000 °C and was correlated with the development of faceted etch-pits in the Si surface. Comparison with known thermodynamic data for the oxidation of Si and vapor-pressures of reactants shows that nanowire growth is mediated by a vapor-liquid-solid mechanism in which the dominant vapor-phase source of reactants is SiO produced by the active oxidation of Si.

Use of vapor-phase sources of gas mixtures for calibration and verification of analytical equipment in measuring the content of volatile substance impurity content

New devices are developed for preparing stable gas mixtures with a microconcentration of volatile substance vapors, i.e., vapor-phase sources of gas mixtures that have passed state certification and registration in the State Register for Means of Measurement. The main characteristics of these sources, methods and procedures for their use, and also an assortment of prepared gas mixtures are provided.

Photoelectric Properties and Electroluminescence of p–i–n Diodes Based on GeSi∕Si Heterostructures with Self-Assembled Nanoclusters

This paper reports on the results of investigations into the photoelectric properties and electroluminescence of p-i-n diodes based on GeSi/Si heterostructures with GeSi self-assembled nanoclusters embedded in the i region. The p-i-n diodes are grown through sublimation molecular-beam epitaxy using a vapor-phase source of germanium. The photovoltage spectra of the p-i-n diodes measured at a temperature of 300 K exhibit a photosensitivity band attributed to interband optical transitions in the GeSi nanoclusters. A new approach to analyzing the photosensitivity spectra of the heterostructures containing GeSi thin layers is proposed, and the energy at the edge of the photosensitivity bands assigned to these layers is determined. The electroluminescence observed in the p-i-n diodes at 77 K is associated with the radiative interband optical transitions in GeSi nanoclusters.

Biodegradation of BTEX vapors in a silicone membrane bioreactor system.

The biotreatment of complex mixtures of volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene, and xylene isomers (BTEX) has been investigated by many workers. However, the majority of the work has dealt with the treatment of aqueous or soil phase contamination. The biological treatment of gas and vapor phase sources of VOC wastes has recently received attention with increased usage of biofilters and bioscrubbers. Although these systems are relatively inexpensive, performance problems associated with biomass plugging, gas channeling, and support media acidification have limited their adoption. In this report we describe the development and evaluation of an alternative biotreatment system that allows rapid diffusion of both BTEX and oxygen through a silicone membrane to an active biofilm. The bioreactor system has a rapid liquid recycle, which facilitates nutrient medium mixing over the biofilm and allows for removal of sloughing cell mass. The system removed BTEX at rates up to 30 microg h(-1) cm(-2) of membrane area. BTEX removal efficiencies ranged from 75% to 99% depending on the BTEX concentration and vapor flowrate. Consequently, the system can be used for continuous removal and destruction of BTEX and other potential target VOCs in vapor phase streams.

Vapor saturation and accumulation in magmas of the 1989–1990 eruption of Redoubt Volcano, Alaska

The 1989–1990 eruption of Redoubt Volcano, Alaska, provided an opportunity to compare petrologic estimates of SO 2 and Cl emissions with estimates of SO 2 emissions based on remote sensing data and estimates of Cl emissions based on plume sampling. In this study, we measure the sulfur and chlorine contents of melt inclusions and matrix glasses in the eruption products to determine petrologic estimates of SO 2 and Cl emissions. We compare the results with emission estimates based on COSPEC and TOMS data for SO 2 and data for Cl/SO 2 in plume samples. For the explosive vent clearing period (December 14–22, 1989), the petrologic estimate for SO 2 emission is 21,000 tons, or ~12% of a TOMS estimate of 175,000 tons. For the dome growth period (December 22, 1989 to mid-June 1990), the petrologic estimate for SO 2 emission is 18,000 tons, or ~3% of COSPEC-based estimates of 572,000–680,000 tons. The petrologic estimates give a total SO 2 emission of only 39,000 tons compared to an integrated TOMS/COSPEC emission estimate of ~1,000,000 tons for the whole eruption, including quiescent degassing after mid-June 1990. Petrologic estimates also appear to underestimate Cl emissions, but apparent HCl scavenging in the plume complicates Cl emission comparisons. Several potential sources of ‘excess sulfur’ often invoked to explain petrologic SO 2 deficits are concluded to be unlikely for the 1989–1990 Redoubt eruption — e.g., breakdown of sulfides, breakdown of anhydrite, release of SO 2 from a hydrothermal system, degassing of commingled infusions of basalt in the magma chamber, and syn-eruptive degassing of sulfur from melt present in non-erupted magma. Leakage and/or diffusion of sulfur from melt inclusions do not provide convincing explanations for the petrologic SO 2 deficits either. The main cause of low petrologic estimates for SO 2 is that melt inclusions do not represent the total sulfur content of the Redoubt magmas, which were vapor-saturated magmas carrying most of their sulfur in an accumulated vapor phase. Almost all the sulfur of the SO 2 emissions was present prior to emission as accumulated magmatic vapor at 6–10 km depth in the magma that supplied the eruption; whole-rock normalized concentrations of gaseous excess S in these magmas remained at ~0.2 wt.% throughout the eruption, equivalent to ~0.7 vol.% at depth. Data for CO 2 emissions during the eruption indicate that CO 2 at whole-rock concentrations of ~0.6 wt.% in the erupted magma was a key factor in creating the vapor saturation and accumulation condition making a vapor phase source of excess sulfur possible at depth. When explosive volcanism involves magma with accumulated vapor, melt inclusions do not provide a sufficient basis for predicting SO 2 emissions. Thus, petrologic estimates made for SO 2 emissions during explosive eruptions of the past may be too low and may significantly underestimate impacts on climate and the chemistry of the atmosphere.

Vapor phase and particulate selenium in the marine atmosphere

Particulate and vapor phase selenium sampling was conducted in urban and coastal Rhode Island, at a coastal site in American Samoa, aboard ship in an upwelling area near the Peru coast, and at two sites in the Hawaiian Islands as part of the SEAREX program. Approximately 25% of the atmospheric selenium was found in the vapor phase at all locations with the exception of Peru, where ∼l0% of the total selenium was in the vapor phase, and above the marine boundary layer in Hawaii, where ∼45% of the selenium was in the vapor phase. Vapor phase selenium appears to be produced naturally as well as anthropogenically. An examination of representative worldwide particulate selenium concentrations reveals the remarkably uniform distribution of atmospheric selenium. Concentrations decrease by only about a factor of 100 from urban to remote marine and continental locations. This may be due to the uniform nature of one important vapor phase source of selenium, the ocean. Vapor to particle conversion of this naturally produced oceanic selenium may explain most of the particulate selenium enrichment observed in remote marine areas.

Solid‐State Anodization of Aluminum by Vapor Infusion

It is found possible to grow an oxide on aluminum by a solid‐state anodization technique with a vapor phase source of oxygen. Thin film junctions of the form Al‐Al oxide‐M are used where the overlay metal film M is either Pb or Au with less than 1000Å thickness and the initial thickness of the oxide is about 15Å. A positive bias is applied to the Al and the structure is exposed to argon or oxygen at about 95% relative humidity at room temperature. Each time the voltage is increased, the initial current decreases with time while the resistance increases. The presence of the final oxide layer is verified by the systematic increase in resistance, a measured decrease in capacitance, and an optical interference experiment which visually shows the presence of the oxide layer. The capacitance data indicate an oxide thickness proportional to voltage with a growth rate of about 15Å per volt in agreement with the results of liquid anodization. The effect is reproducible and is expected to work with other metal‐oxide combinations. Diffusion of adsorbed water through grain boundaries or pores in the overlay film is believed to be the source of the oxygen. Processes similar to liquid anodization must occur in the interfacial region between the metal electrodes and the oxide layer. These experiments emphasize the important role played by water vapor in the solid‐state anodization process.