Low-pressure Environment Research Articles

The effects of low temperature and pressure on the fracture behaviors of organosilicate thin films

Abstract

Homogeneity improvement of field emission beam from metallic nano-tip arrays by noble-gas conditioning

The authors explored control of field-emission electron beam homogeneity emitted from metallic nano-tips fabricated by molding. Improved beam homogeneity and increased maximum bunch charge were observed after DC operation of cathodes in low-pressure argon gas environment. They also observed average current per tip of 4.5 μA from a 1 × 104 emitter array device after conditioning in neon gas. Evolution of Fowler-Nordheim parameters as a result of noble-gas conditioning was discussed.

Growth and thermoelectric properties of FeSb2 films produced by pulsed laser deposition

Thermoelectric FeSb2 films were produced by pulsed laser deposition on silica substrates in a low-pressure Ar environment. The growth conditions for near phase-pure FeSb2 films were confirmed to be optimized at a substrate temperature of 425°C, an Ar pressure of 2 Pa, and deposition time of 3 h by ablating specifically prepared compound targets made of Fe and Sb powders in atomic ratio of 1:4. The thermoelectric transport properties of FeSb2 films were investigated. Pulsed laser deposition was demonstrated as a method for production of good-quality FeSb2 films.

Pulsed laser deposition growth of FeSb 2 films for thermoelectric applications

FeSb 2 films were produced in a low-pressure Ar environment by pulsed laser deposition at 355 nm. The influence of growth parameters such as substrate temperature, Ar pressure and deposition time on the growth of FeSb 2 films was studied. Nearly phase-pure FeSb 2 films with thicknesses of 100–400 nm were produced at 425 °C with an Ar pressure of 1.5–2 Pa. Thermal transport and Hall measurements were performed to explore the thermoelectric transport properties of the FeSb 2 films. A maximum thermopower of ∼120 μVK −1 at 40 K was obtained. In general it is highly important to understand the growth properties of FeSb 2 films if they are to eventually reach thermoelectric applications at cryogenic temperatures.

The electrical characterization of a multi-electrode odor detection sensor array based on the single SnO 2 nanowire

The variation of electrical properties of individual SnO 2 wedge-like nanowire along its length was characterized comparatively in a vacuum, low-pressure oxygen environment and in synthetic air mixed with various analytes. For characterization, a vapor–solid grown nanowire was indexed with multiple Pt mesoscopic contacts fabricated by e-beam induced deposition from Pt containing metalorganic precursor. The surface of the selected nanowire segments was functionalized with a catalyst via deposition of Pd nanoparticles. Detailed analysis of the electrical properties of the nanowire shows that the effective diameter of the conducting channel of the nanowire can be substantially modified along its length by the surface functionalization and nanowire width change. These variations of the electrical properties of nanowire segments are sufficient for reliable discrimination of acetone, 2-propanol, CO and hydrogen by this elemental multisensor array.

Formation of biaxially textured molybdenum thin films under the influence of recrystallization conditions

This article highlights the formation of biaxially textured body centered cubic Mo nanorods under recrystallization conditions using glancing angle deposition. The flux incidence angle has been changed (α=0°, 70° and 85° away from the surface normal) to observe its effect on the formation of biaxial texture under a constant low Ar pressure environment (0.306Pa). Only at a glancing flux incidence (α=85°), the directional diffusion overcomes the effect of recrystallization to yield a highly biaxial texture. In another study, a normal flux incidence (α=0°) was kept constant and the Ar pressure was changed (0.67, 1.33 and 2.67Pa) to see its influence on the film morphology and the resulting texture. The Ar pressure variation was aimed at attempting a zone transformation in accordance with the structure zone model. While the morphology appeared to agree with the expected zone transformation, the texture did not.

Fire detection model in Tibet based on grey-fuzzy neural network algorithm

The fire signals are much weaker in low oxygen concentration and low pressure environment such as Tibet. Fire detectors which were calibrated in correlating standard conditions cannot work well in such condition. This paper presents a synthesis method of GM(1, 1) grey prediction model and adaptive neuro-fuzzy inference system (ANFIS) in advance to detect fire and to make it work in the environment. The theoretical analysis of the algorithm and experimental evaluation in Tibet are presented. In this process, the grey GM(1, 1) predict model can anticipate the development of fire signals without any assumption, thus allowing earlier fire alarm than traditional fire detection equipments, meanwhile, ANFIS can make sure the data processing more accurate to avoid false alarms. This work will supply useful suggestions with the fire detectors design in low ambient pressure and low oxygen concentration such as Tibet, etc.

Removal of Surface Contamination and Self-Assembled Monolayers (SAMs) from Silver (Ag) Nanorod Substrates by Plasma Cleaning with Argon

Surface contamination of surface-enhanced Raman (SERS)-active metallic substrates has been a limitation to the utility of SERS as an analytical technique, potentially affecting surface coverage, spectral reproducibility, and analytical limits of detection. We have developed a simple and versatile cleaning method for SERS-active Ag nanorod arrays that consists of a short (4 min) exposure of the substrate to an Ar(+) plasma in a low-pressure environment. The findings presented here demonstrate that this cleaning procedure essentially eliminates organic background contamination. This procedure works equally well for self-assembled monolayers of thiolates that strongly adsorb onto Au and Ag surfaces. For SERS-active surfaces composed of arrays of Ag nanorods prepared by oblique-angle vapor deposition, we investigated the (1) Raman band intensities, (2) nanorod morphology via scanning electron microscopy, and (3) surface hydrophobicity via static contact angle measurements, as a function of exposure time of the Ag nanorods to the Ar(+) plasma. Short (4 min) exposure to Ar(+) plasma eliminated background contamination but decreased the observed SERS intensity for re-adsorbed analytes by approximately a factor of 2 while leaving the nanorod morphology essentially unchanged. Prolonged exposure to Ar(+) plasma (>10 min) resulted in substantial morphological changes of the Ag nanorod lattice and led to a decrease in the observed SERS intensities by a factor of 10. The results presented here suggest that Ar(+) plasma cleaning is an efficient process for removing carbonaceous and organic contamination as well as thiolate monolayers from SERS-active Ag surfaces, as long as the plasma conditions and exposure times are carefully monitored.

Testing of Greenhouse Cladding Materials for Space Environments

Bioregenerative Life Support Systems in the form of transparent, inflatable greenhouses present an economical and reliable solution for providing consumables to astronauts on long duration space missions. With NASA's goal to place human explorers on Mars, identification of cladding materials with appropriate optical and physical properties that can withstand the high ultraviolet radiation, low pressure, and low temperature Martian environment is necessary for greenhouse design. The objective of this study was to develop methods for evaluating the effects of simulated Mars environmental conditions on the transmissivity and mechanical strength of candidate greenhouse cladding materials. The University of Florida's Mars Simulation Chamber was utilized to expose material samples to simulated Mars environmental conditions. Changes in control versus exposed material sample transmissivities were measured using a spectroradiometer over a range of wavelengths from 200 to 800 nm and material peak yield stresses were determined using an Instron universal testing machine. Results indicated that, under Mars simulated environmental conditions for 42 Mars equivalent days, materials experienced a reduction in transmission in the photosynthetically active region and a decrease in material strength. In short, the methods developed in this study provide a means for evaluating the transmissivity and strength of candidate greenhouse cladding materials under Mars environmental conditions for the determination of their suitability in greenhouse design.

鉱物結晶学の多様化：新たな広がりと挑戦 ３．宇宙鉱物（地球外物質）地球外試料中角閃石の結晶化学と形成過程について

Although amphibole is one of the most common rock-forming minerals on the earth, it rarely occurs in meteorites. This is due to anhydrous and low-pressure environment of most meteorite parent bodies. In this paper we report two different occurrences of amphibole from extraterrestrial materials. One is hornblende from an R chondrite, and the other is kaersutite from Martian meteorites. Both amphiboles bear unique crystal chemistry different from terrestrial ones and provide important information about their thermal history especially related to the water in extraterrestrial environment.

Experimental Study of Electron Emission Characteristics of a Surface Flashover Trigger in a Low Pressure Environment

Characteristics of electron emission induced by a surface flashover trigger device in a low-pressure trigger switch were investigated. A test method to measure the emitted charges from the trigger device was developed, and the factors affecting the emitted charges were analyzed. The results indicated that the major emitted charges from the trigger device were induced by surface plasma generated by surface flashover occurring on the trigger dielectric material. The emitted charges and the peak emission current increased linearly with the change in the trigger voltage and bias voltage. The emitted charges collected from the anode were affected by the gap distance. However, the emitted charges were less affected by the anode diameter. Furthermore, the emitted charges and the peak emission current decreased rapidly with the increase in gas pressure in a range from 0 Pa to 100 Pa, and then remained stable or changed slightly when the increase in gas pressure up to 2400 Pa.

Growth of Ge Nanowires from Au−Cu Alloy Nanoparticle Catalysts Synthesized from Aqueous Solution

Elemental metal nanoparticles are widely used to catalyze semiconductor nanowire growth, but size-controlled alloy nanoparticles have not been explored in this context. We present a simple aqueous synthesis of Au−Cu2O core−shell nanoparticles to produce Au−Cu alloy nanoparticles of controlled size and composition by vacuum annealing. Colloidal Au nanoparticles were used as size-controlled seeds, and fine control of the Cu2O shell thickness enabled tuning of the average Au−Cu alloy composition. The alloy nanoparticles were found to catalyze Ge nanowire growth in a low-pressure chemical vapor deposition environment. The nanowire growth rate for Au−Cu nanoparticles was intermediate to that of Cu (slowest) and Au (fastest) nanoparticles under identical conditions, suggesting a vapor−solid−solid growth process. These catalysts provide a useful platform to explore the influence of catalyst phase and chemistry on nanowire growth mechanisms that determine important variables including the doping rate and junction...

Exploring the Low-Pressure Growth Limit: Evolution of Bacillus subtilis in the Laboratory to Enhanced Growth at 5 Kilopascals

Growth of Bacillus subtilis cells, normally adapted at Earth-normal atmospheric pressure (∼101.3 kPa), was progressively inhibited by lowering of pressure in liquid LB medium until growth essentially ceased at 2.5 kPa. Growth inhibition was immediately reversible upon return to 101.3 kPa, albeit at a slower rate. A population of B. subtilis cells was cultivated at the near-inhibitory pressure of 5 kPa for 1,000 generations, where a stepwise increase in growth was observed, as measured by the turbidity of 24-h cultures. An isolate from the 1,000-generation population was obtained that showed an increase in fitness at 5 kPa when compared to the ancestral strain or a strain obtained from a parallel population that evolved for 1,000 generations at 101.3 kPa. The results from this preliminary study have implications for understanding the ability of terrestrial microbes to grow in low-pressure environments such as Mars.

Modeling of laser induced breakdown spectroscopy for very low-pressure conditions

Laser Induced Breakdown Spectroscopy (LIBS) can be considered as a prominent technology for compositional analysis of materials in low-pressure space applications. In space applications, usually LIBS is conducted in a low-pressure environment and proper understanding of the plasma parameters is significant for any improvement in the system. A model is developed to describe the heating and subsequent melting, vaporization and ionization of a target material during LIBS process. A numerical model based on one-dimensional thermal conductivity equation is being used to simulate the target evaporation and a hydrodynamic model is used to simulate plume expansion. Further, an experimental approach of measuring spectral emission from the ablation plume using emission spectroscopy and estimating the plasma state, such as the ionization species, and average plasma temperature, is investigated. An important result of this work is that for different ambient conditions, laser ablation plume dynamics can be estimated.

Facile pyrolytic synthesis of silicon nanowires

One-dimensional nanostructures such as silicon nanowires (SiNW) are attractive candidates for low power density electronic and optoelectronic devices including sensors. A new simple method for SiNW bulk synthesis [1,2] is demonstrated in this work, which is inexpensive and uses low toxicity materials, thereby offering a safe, energy efficient and green approach. The method uses low flammability liquid phenylsilanes, offering a safer avenue for SiNW growth compared with using silane gas. A novel, duo-chamber glass vessel is used to create a low-pressure environment where SiNWs are grown through vapor–liquid–solid mechanism using gold nanoparticles as a catalyst. The catalyst decomposes silicon precursor vapors of diphenylsilane and triphenylsilane and precipitates single crystal SiNWs, which appear to grow parallel to the substrate surface. This opens up possibilities for synthesizing nano-junctions amongst wires which is important for the grid architecture of nanoelectronics proposed by Likharev [3]. Even bulk synthesis of SiNW is feasible using sacrificial substrates such as CaCO 3 that can be dissolved post-synthesis. Furthermore, by dissolving appropriate dopants in liquid diphenylsilane, a controlled doping of the nanowires is realized without the use of toxic gases and expensive mass flow controllers. Upon boron doping, we observe a characteristic red shift in photoluminescence spectra. In summary, an inexpensive and versatile method for SiNW is presented that makes these exotic materials available to any lab at low cost.

METHYL BROMIDE ALTERNATIVES FOR POSTHARVEST INSECT DISINFESTATION OF CALIFORNIA WALNUTS

Before shipment, California inshell walnuts destined for the valuable export market must be disinfested of both field pests (codling moth and navel orangeworm) and common storage pests (Indianmeal moth and red flour beetle). Until recently fumigation with methyl bromide has been the most common disinfestation method, but access to methyl bromide is now greatly restricted in developed countries, causing walnut processors to seek alternative methods. In addition to alternative chemical fumigants, non-chemical physical treatments are being developed. One treatment uses radio frequency energy to rapidly heat walnuts to an average temperature of 60°C for 5 minutes. Large-scale tests using an industrial radio frequency heater successfully disinfested walnuts of navel orangeworm while maintaining good product quality. A second alternative treatment is the use of cold storage (0-5°C) to either disinfest product or prevent reinfestation of clean product. At temperatures of 0 and 5°C, 95% mortality of test insects was achieved at 10 and 18 days, respectively. Storage at 10°C prevented reproduction, product damage and reinfestation, but 127 day exposures were required to kill 95% of Indianmeal moth larvae. A third alternative treatment exposes product to a low pressure (vacuum) environment of 50 mm Hg in flexible polyvinyl chloride containers. Temperature and moisture content of the product strongly affects treatment efficacy, with complete mortality of test insects achieved at treatment exposures of 48 hours or less under optimal conditions.

The combined effect of pressure and oxygen concentration on piloted ignition of a solid combustible

There are a number of situations when fires may occur at low pressures and oxygen concentrations that are different than standard atmospheric conditions, such as in buildings at high elevation, airplanes, and spacecraft. The flammability of materials may be affected by these environmental conditions. Since ignition delay is a measure of material flammability and directly influences whether a fire will occur, experiments were conducted to assess the variation of the ignition delay of PMMA in sub-atmospheric pressures and elevated oxygen concentrations. Three sets of experiments were performed at different pressures and in air, in an atmosphere having 30% oxygen/70% nitrogen by volume, and in a “normoxic” atmosphere (constant oxygen partial pressure). It was observed that as the pressure is reduced, the ignition time decreased, reached a minimum, and then increased until ignition did not occur. Several mechanisms were considered to explain the “U-shaped” dependence of ignition time on pressure, and three regimes were identified each having a different controlling mechanism: the transport regime where the ignition delay is controlled by changes in convection heat losses and critical mass flux for ignition; the chemical kinetic regime where the ignition delay is controlled by gas-phase chemical kinetics; and an overlap region where both the transport and chemistry effects are seen. The results provide further insight about the effect of the environmental conditions on the flammability of materials, and guidance about fire safety in low pressure environments.

Dust ejection from (pre-)planetary bodies by temperature gradients: radiative and heat transfer

It has recently been shown that the illumination of dust beds in a low-pressure gaseous environment can generate massive ejections of dust if the light source is turned off. This effect might explain dust entrainment into the atmosphere of Mars and might destroy dusty planetesimals in protoplanetary discs. As ejection mechanism we consider compression of gas by thermal creep through the pores of the dust bed (Knudsen compressor). To approach this problem quantitatively we combine theoretical calculations of radiative and heat transfer with laboratory experiments. In this work we focus on the theoretical aspect and numerical modelling of irradiated surfaces. Temperature gradients calculated and pressure differences established by Knudsen compression are in general agreement to the threshold necessary to eject particles.

Fabrication of Dot-like Nano-protrusions on Silicon Surfaces Using Nanosecond Pulse Nd:YAG Laser Irradiation

An array of Nano-protrusions (NPs) was fabricated on silicon surfaces in air and a low pressure environment using pulsed laser irradiation. The uniformly sized NPs were linearly aligned at a low laser energy density, which is typically a few kilojoules per square meter. The linear array of NPs appeared perpendicular to the laser polarization vector E, and the interval of the NPs lines was almost equal to the wavelength of the incident laser. The primary NPs grew epitaxially and were covered with an oxide film layer, which was determined by means of a structural and elemental analysis using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDS). We propose a nano-patterning fabrication method using the laser-induced NPs alignment based on a microscopic structural analysis.

Mineralogical and geochemical evidence for multi-stage origin of mineral veins hosted by teschenites at Tichá, Outer Western Carpathians, Czech Republic

Numerous mineral veins are hosted in a body of teschenite which is situated within the Lower Cretaceous flysch siliciclastics of the Silesian Unit at Tichá. Mineralogy, fluid inclusions, stable isotopes and trace elements have been studied in order to assess the origin of this mineralization. Three stages of vein cementation have been recognized, each of them being characterized by distinct mineral composition and genetic conditions. The first stage is composed of titanite, aegirine-augite to aegirine, annite, analcime and strontian apatite. These minerals originated from NaCl-rich, CaCl 2-poor magmatic brine (total fluid salinities range between 47 and 57 wt%), leaving after crystallization of host teschenite in low-pressure (<1 kbar) environment. Crystallization temperatures reached ∼390–510 °C for early phases, titanite and aegirine-augite. The second stage is formed by calcite, chlorite, dolomite, siderite, strontianite, quartz, pyrite and sphalerite. The parent fluids were low-salinity (0.5–4.5 wt% NaCl eq.) aqueous solutions with low content of strong REE-complexing ligands, that were progressively cooled during mineral precipitation (up to ∼190 °C at the beginning, ∼90 °C at the end of crystallization). These fluids are interpreted to be predominantly of external origin, derived from surrounding sedimentary sequences during diagenetic dewatering of clay minerals. The highly positive δ 18O and near-zero δ 13C values indicate an interaction of fluids with sedimentary carbonates. The third stage is formed by a dense net of calcite veinlets, which probably originated during tectonic deformations connected with orogenetic movements during the Tertiary. The source of strontium for first stage mineralization was probably related to the special conditions of magmatic evolution of the host teschenite, whereas strontium for second stage minerals could have been remobilized during hydrothermal alteration from earlier teschenite-hosted mineral phases and/or limestone.