Anomalous Si Research Articles

Selective depressions of surface silicic acid within cyclonic mesoscale eddies in the oligotrophic western North Pacific

To reveal spatial dynamics of silicic acid [Si(OH)4] in the poorly sampled oligotrophic western North Pacific, we investigated the surface distribution of Si(OH)4 and associated biogeochemical parameters by using an underway survey system with a highly sensitive nutrient analyzer along the 138°E transect (between 30 and 34°N) and the 155°E transect (between 10 and 35°N) during the summers of 2007 and 2008. Surface Si(OH)4 concentrations ranged from the detection limit (11nmolL−1) to 2462nmolL−1. High Si(OH)4 concentrations (>1000nmolL−1) and dynamic fluctuations were generally observed north of 23°N, while consistently stable low concentrations of 415–751nmolL−1 were observed south of 23°N. Surface nitrate+nitrite (N+N) and phosphate (PO43−) were typically depleted to <20nmolL−1, except for PO43− in the area south of 16°N. The majority of the study area was characterized by high-Si(OH)4 and low-N+N and PO43−. However, submesoscale/mesoscale depressions of Si(OH)4 were locally observed in the cyclonic eddy fields north of 23°N. Among a total of six Si(OH)4 depressions within the eddies, a complete Si(OH)4 depletion (<11nmolL−1) was observed on the cyclonic side near the Kuroshio axis (33.1°N, 138°E). This depletion was closely coupled with a diatom bloom, suggesting that Si(OH)4 was exhausted by diatoms. All of the Si(OH)4 depressions were selective and not accompanied by local depressions of N+N and PO43−. This unique phenomenon might be driven by biogeochemical processes such as selective Si export (Si pump), anomalous Si uptake associated with diatom physiology, and/or Si uptake supported by N2 fixation.

Evidence for ‘silicon’ within the cell walls of suspension‐cultured rice cells

Despite the ubiquity and beneficial role of silicon (Si) in plant biology, structural and chemical mechanisms operating at the single-cell level have not been extensively studied. To obtain insights regarding the effect of Si on individual cells, we cultivated suspended rice (Oryza sativa) cells in the absence and presence of Si and analyzed single cells using a combination of physical techniques including atomic force microscopy (AFM). Si is naturally present as a constituent of the cell walls, where it is firmly bound to the cell wall matrix rather than occurring within intra- or extracellular silica deposition, as determined by using inductively coupled plasma mass spectrometry (ICP-MS) and X-ray photoelectron spectroscopy (XPS). This species of Si, linked with the cell wall matrix, improves the structural stability of cell walls during their expansion and subsequent cell division. Maintaining cell shape is thereby enhanced, which may be crucial for the function and survival of cells. This study provides further evidence that organosilicon is present in plant cell walls, which broadens our understanding of the chemical nature of 'anomalous Si' in plant biology.

ISOCHRONS IN PRESOLAR GRAPHITE GRAINS FROM ORGUEIL

Primitive meteorites contain tiny dust grains that condensed in stellar outflows and explosions. These stardust grains can be extracted from their host meteorites and studied in detail in the laboratory. We investigated depth profiles of the Al-Mg, Ca-K, and Ti-Ca isotopic systems obtained during NanoSIMS isotopic analysis of presolar graphite grains from the CI carbonaceous meteorite Orgueil. Large 26Al/27Al, 41Ca/40Ca, and 44Ti/48Ti ratios, inferred from 26Mg, 41K, and 44Ca excesses from the decay of the short-lived radioisotopes 26Al, 41Ca, and 44Ti, indicate a supernova (SN) origin. From the depth distribution of the radiogenic isotopes and the stable isotopes of their parent elements we constructed isochron-type correlation plots. The plots indicate quantitative retention of radiogenic 26Mg, 41K, and 44Ca in most grains. Deviations from straight lines in the Al-Mg and Ca-K plots can be explained by contamination with 27Al and isotopically normal Ca, respectively. For the Ti-Ca system in some grains, the lack of parent-daughter correlation indicates either redistribution of radiogenic 44Ca or heterogeneity in the initial 44Ti/48Ti ratio. We also obtained Si isotopic depth profiles in three graphite grains with large 29Si and 30Si excesses, for which a SN origin has been proposed. In two grains no Si-rich subgrains are observed; in the third grain with an apparent Si-rich subgrain the anomalous Si isotopic ratios in the subgrain are the same as in the rest of the graphite host. Our studies show that by measuring depth profiles, information on presolar grains can be obtained that cannot be obtained by whole-grain analysis.

High‐ResolutionChandraHETGS andRossi X‐Ray Timing ExplorerObservations of GRS 1915+105: A Hot Disk Atmosphere and Cold Gas Enriched in Iron and Silicon

The time-averaged 30 ks Chandra AO1 High Energy Transmission Grating Spectrometer observation of the microquasar GRS 1915+105 in the low hard state reveals for the first time in this source neutral K absorption edges from iron, silicon, magnesium, and sulphur. Ionized resonance (r) absorption from H- and He-like species of Fe (XXV, XXVI) and possibly Ca XX are also seen as well as possible emission from neutral Fe K? and ionized Fe XXV (forbidden or the resonance emission component of a P Cygni profile). We report the tentative detection of the first astrophysical signature of X-ray absorption fine structure in the photoelectric edge of Si (and possibly Fe and Mg) attributed to material in grains. The large column densities measured from the neutral edges reveal anomalous Si and Fe abundances and illustrate the importance of high-resolution X-ray measurements for assessing material that surrounds bright X-ray sources, especially if depletion onto grains plays a prominent role. Scenarios for which the anomalous abundances can be attributed to surrounding cold material associated with GRS 1915+105 and/or for which the enrichment may signify either a highly unusual supernova/hypernova or external supernova activity local to the binary are discussed. We attribute the ionized features to a hot disk, disk wind, or corona environment. Based on H- and He-like Fe (XXV, XXVI), we estimate constraints on the ionization parameter (log ? 4.15), temperature (T > 2.4 ? 106 K), and hydrogen equivalent number density (n 1012 cm-3) for this region. Variability studies with the simultaneous RXTE data show that the light-curve count rate tracks the changes in the disk blackbody as well as the power-law flux, with the most significant variations observed in the former. The Chandra data show spectral changes that also track the behavior of the light curve and may point to changes in both the ionizing flux and density of the absorber. A 3.69 Hz quasi-periodic oscillation and weak first harmonic are seen in the RXTE data.

ChemInform Abstract: First‐Principles Electronic Structure Calculations of Ba5Si2N6 with Anomalous Si2N6 Dimeric Units.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Thermal Studies on Stress-Induced Void-Like Defects in Epitaxial-CoSi2 Formation

AbstractWe report experimental findings on the thermal-induced stress-voiding observed during epitaxial CoSi2 formation on patterned Si(100) wafers associated with the Co/Ti-interposing layer scheme [1–6]. The first rapid thermal process (RTP) in a typical SALICIDE process was studied by varying the temperature and the RTP ramp-up rate. Based on ex-situ cross-sectional transmission electron microscopy (XTEM), crystal defects were observed at shallow trench isolation (STI)/Si and gate spacer/Si edges after annealing at temperatures greater than 520°C. In addition, isothermal anneals for various time periods resulted in increased encroachment of an ultra-thin CoSi2-phase at the STI/Si edge. Higher temperature anneals also resulted in a corresponding increase in void size. In addition, higher ramp-up rates led to a thicker CoSi2 “pocket” film at the tensile-stressed STI/Si edge, aligned epitaxially along the {111}-habit planes. Our attempts to bypass the CoSi formation in order to alleviate the anomalous Si diffusion was found to be irrelevant to the voiding issue, due to the inherent nature of this film stack system to nucleate CoSi2 as the first phase. By comparing our experimental observations of voiding with Hu's analysis of the trench isolation stress fields, we found good agreement with the predicted stress field distribution in the active Si and STI/Si edge with the locations of void nucleation. The anomalously large stress induced is still not known, but can be attributed to the encroaching CoSi2 film which tends to realign epitaxially along the STI/Si interface.

Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains from Meteorites

We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are $^{29}$Si and $^{30}$Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of different mass move the silicon isotopic composition? (4) Why do present calculations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley \& Weaver (1995) supernova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains.

Initial Growth Stages of Ge on Si(111).

Initial growth stages of Ge on Si (111) are studied from the viewpoint of intermixing of Si into the epitaxial Ge layer. Defect formation in the Ge islands was characterized by using moire patterns taken by transmission electron microscopy (TEM). Ge islands are grown by MBE on Si(111) surfaces with an SPE-grown buffer layer, which is expected to prevent intermixing between the epitaxial Ge layer and the Si substrate. Anomalous Si incorporation into the Ge islands is analyzed by TEM moire fringe analysis. Dislocations are visualized by means of extra 'half-lines' of the moire fringes, and thus Burgers vectors are determined. Stacking faults are observed by a displacement of the fringe spacings Coalescence of the Ge islands and dislocation generation from the contact part of the islands are detected.

TEM Moiré Pattern and Scanning Auger Electron Microscope Analysis of Anomalous Si Incorporation into MBE-grown Ge on Si(111)

Ge islands are grown by molecular beam epitaxy (MBE) on Si(111) surfaces with an SPE-grown buffer layer, which was expected to prevent intermixing between the epitaxial Ge layer and the Si substrate. The epilayer composition analyzed from the spacing of the Moiré fringe indicates that the Ge islands should be a Si-Ge alloy rather than pure Ge and that the alloying of Ge with Si is greater near the edges of the island. The alloying phenomenon was verified by the compositional depth profile analysis using the scanning Auger electron microscope (SAM) technique. The Si incorporation into the Ge islands observed here cannot be explained by a bulk diffusion mechanism and is a seriously anomalous phenomenon since Si diffusion into Ge would be negligible at the growth temperature and time used ill this experiment.

Growth structures in aluminium-silicon alloys II. The influence of strontium

Directional solidification studies using a constant temperature gradient of 125±5°C cm −1 in the liquid form the main part of this study. They are used to define the effect of Sr in the concentration range 0–0.1 wt% on the structure of Al-Si alloys containing 11–20 wt% Si and solidified in the velocity range 1–1500 μ ms −1. The high temperature gradient suppresses the nucleation of Si so the studies were complemented with observations of primary Si phase solidification under a low temperature gradient. Very low Sr concentrations induce structural modifications of the Si phase that depend upon the Si concentration and the growth velocity for a fixed temperature gradient. The action of Sr is attributed to the poisoning of growth sites rather than the prevention of nucleation. Twin-plane re-entrant (TPRE) growth sites in primary Si are readily poisoned, whereas the effect on layer growth sites is more gradual producing holes, branching and dendritic structures and dispersed equiaxed structures as the Sr concentration increases. Growth twins were observed in these structures but did not activate the TPRE growth mechanism. Sr broadens the coupled zone and modifies the anomalous Si flake eutectic to a [100] fibrous structure. Multiple growth twins were observed in some of the fibres and they play an important role in their growth. Some of the fibres develop from angular Si by splitting or branching in the plane of the [100] side plates. These fibres do not show twinning. Transitional eutectic structures were observed consisting of modified complex regular phases and modified angular Si. These structures confer a [100] texture and their occurrence depends on the Si and Sr concentration, growth velocity and temperature gradient.

Correlation of Void Formation with the Reduction of Carrier Activation and Anomalous Dopant Diffusion in Si-Implanted GaAs

AbstractWe report the study of high-dose Si-implanted GaAs containing doses ranging from 1×1014 to 1×1015 cm-2 and with subsequent anneals at 850°C for 1 hour. At doses ≥ 3×1014 cm-2, a severe reduction of carrier concentration and anomalous Si diffusion are observed in the near-surface region. In the same region, small, near-spherical voids are found by transmission electron microscopy. In contrast, for samples implanted with doses ≤ 1×1014 cm-2, voids are not found, and both normal carrier activation and Si diffusion profiles are observed. The concurrent onset of these three phenomena in the same region in high-dose samples leads us to conclude that the severe reduction of carrier concentration and anomalous Si diffusion are attributable to the formation of voids.

Geology of skarn and high-grade gold in the Carr Fork Mine, Utah

High-grade gold mineralization occurs as an overprint to copper-gold skarns in the Carr Fork mine. Geological and geochemical studies and geostatistical comparison of the distribution of gold in the copper-gold skarns and in the high-grade gold mineralization show that gold was deposited in two separate and distinguishable events: deposition with copper in skarn ores, under multiple controls, and high-grade gold mineralization, which formed a shoot in skarns hosted by the Parnell limestone. The gold content of material produced at this stage is much higher than that of the skarn ores.High-grade gold accompanies distinct alteration characterized by clay pyrite and quartz-pyrite(-tennantite), anomalous Au, As, Sb, Hg, Si, S, and Tl, and depletion of Pd, Ca, and Se. The high-grade gold mineralization comprises at least three episodes determined from petrographic and microprobe studies; it preceded the quartz molybdenum veins.The statistical characteristics of the high-grade gold are separable from the copper-gold mineralization in histograms, scattergrams, and variograms. The mean of the high-grade gold composites is 0.3 troy oz/short ton. Directional variograms of skarn ore show that a pervasive N 20 degrees E fault set and the distance to the Bingham stock contact controlled the copper and gold grades. Elevation seems to have been an additional control on gold grades in skarn.The shoot of high-grade gold mineralization in the Parnell limestone comprises a diluted drilled resource of 1.2 million tons. Evidence suggests that it continues some distance beyond present drilling.

Anomalous decomposition profile of the excited triplet SiH3F: A prediction by a b i n i t i o molecular orbital calculations

In search of new starting materials for producing amorphous silicon by plasma chemical vapor deposition, decomposition profiles through triplet states have been studied for fluorosilanes SiHmF4−m (m=0–4) by energy-gradient ab initio molecular orbital calculations with the 6–21G** basis set. Anomalous Si–F bond cleavage at the first stage was concluded for triplet SiH3F, while all the other fluorosilanes decompose through the Si–H bond cleavage. The anomaly observed for triplet SiH3F is due to the positive charge on F and to the negative overlap population of the Si–F bond. Thus, SiH3F is expected to be a unique source for amorphous silicon films.

Wind models for ζ Orionis

Several models for the winds of O stars have been proposed to explain the unexpected presence of high ionization potential ions such as N+4 and O+5. Lamers and Snow (1978) proposed that the winds of stars showing N V and O VI lines have elevated temperatures near 4±2 × 105K while cooler stars with anomalous Si IV lines have Te ≈ 7±3 × 104K. Alternately, Cassinelli and Olson (1978, CO) and Olson (1978) have explained the presence of these ions by showing that a thin corona at the base of a cool wind (Te ≲ Teff) can produce these ions by the Auger photoionization process where a single x-ray photon causes the ejection of two electrons. A third possibility is that the winds are at only slightly elevated temperatures (40 000 to 60 000K) and photoionization in an optically thick wind produces the unexpected ions. The problem of determining the local radiation field in such a wind is discussed by Klein and Castor (1978). The purpose of the present analysis is to test the ability of these three wind models to fit the observations of ζ Orionis A 09.7 Ib.