Sheet Growth Research Articles

Silicate Weathering and Carbon Cycle Controls on the Oligocene‐Miocene Transition Glaciation

AbstractChanges in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient “Mi‐1” glaciation event at the Oligocene‐Miocene transition (OMT; ~23 Ma). However, detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ18O, δ13C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate an ~20% increase in silicate weathering rates, which would have lowered atmospheric CO2, potentially forcing the Antarctic glaciation ~ 23 Ma. δ13C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ13C across the OMT and during the Mi‐1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ13C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi‐1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca) and enhanced respiratory dissolution of carbonates, characterized by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering‐induced CO2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT, and increased burial of organic carbon acted as a feedback that intensified cooling at this time.

Zinc-oxide nanorod array fabricated by high temperature hydrothermal method applied to gas sensor

Zinc oxide (ZnO) nanorod based gas sensor was successfully fabricated with interdigital electrode on silicon substrate. The highest sensitivity value is 5.24 at 600 ppm CO2 gas ambient. The influence of growth temperature of ZnO ranging from 90 to 190 °C by the high temperature hydrothermal method was investigated. The ZnO nanorod array were characterized by using field emission scanning electron microscopy (FE-SEM), photoluminescence spectroscopy, X-ray diffraction pattern. From FE-SEM results, the morphology of ZnO nanorods changes from hexagonal structure to sponge-like shape as the growth temperature increases. When the growth temperature is below 150 °C, hexagonal nanorod growth is observed. On the other hand, when the growth temperature is above 170 °C sheet growth is found. In the gas sensor measurements, it is found that ZnO nanorod grown at 150 °C has the best gas sensing performance due to its least intrinsic defects in this study.

The influence of laser surface treatment on the fatigue crack growth of AA 2024-T3 aluminum alloy alclad sheet

The purpose of this work is to study the effect of local heating using the laser heating process as a surface treatment to retard the crack propagation of an aluminum alloy. The heating process was applied over an aerospace grade aluminum alloy AA2024-T3 alclad sheet with 1.6mm thickness. One and two heating lines were applied by a defocused Yb:fiber laser beam that generated a laser spot diameter of 2mm, with laser speeds between 1 and 50mm/s. For crack propagation tests, modified C(T) specimens were manufactured and tested according to ASTM E647. A layer of carbon black was applied over the surface of some specimens previously to the laser heating process. The obtained results shown that the fatigue crack growth was reduced using specific parameters for the laser heating process.

Earliest accumulation history of the north polar layered deposits, Mars from SHARAD

The approximately 2km thick north polar layered deposits (NPLD) are often considered to contain the most complete and detailed stratigraphic records of recent climate of Mars. Exposures of the dense layering within troughs and scarps allowed detailed reconstructions of the latest accumulation history of these water ice deposits, but we lack knowledge of their initial emplacement. The Shallow Radar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO) penetrates the NPLD to their base and detects their internal layering, overcoming the limitation of scarce and scattered visible outcrops of the lowermost sequences.In this study, we map reflectors in SHARAD data that result from discrete stratigraphic horizons in order to delineate the three-dimensional stratigraphy of the lowermost ∼500m NPLD sequence and reconstruct their accumulation history. We confirm the large-scale lateral continuity and thickness uniformity of the deposits previously detected within the lowermost NPLD. However, stratigraphic complexity—in the form of pinch-outs and significant thickness variations—arises when we examine single radar units. We find evidence of an initially limited geographic stability of water ice within two deposits that are centered at the North Pole and present-day Gemina Lingula. A period of lateral ice sheet growth followed, interrupted only once by a retreat episode. Lower net accumulation is observed on pre-existing slopes, suggesting a reduction of water ice stability due to increased solar radiation incidence and/or transport by katabatic winds. Lateral transport of water ice by wind is also suggested by thickness undulations toward the top of the sequence, resembling cyclic steps.Water ice accumulation models based on orbital forcing predict a sequence of deposition and retreat events that is generally compatible with our reconstructed accumulation history. Therefore, we interpret the stratigraphic complexity that we observe as regional and, possibly global, climate change induced by orbital forcing. We also find that at least two units are completely buried within the NPLD and do not outcrop, and that NPLD deposition in some places was contemporaneous with deposition of the stratigraphically underlying cavi unit in other places. Both of these findings show that radar reflector mapping is a necessary complement to any stratigraphic reconstruction based on visible exposures.

Effects of hydrogen on the structural and optical properties of MoSe2 grown by hot filament chemical vapor deposition

The role of reactive environment and hydrogen specifically in growth and structure of molybdenum selenide (MoSe2) nanomaterials is presently debated, and it is not clear whether hydrogen can promote the growth of MoSe2 sheets and alter their electronic properties. To find efficient, convenient methods for controlling the nucleation, growth and resultant properties of MoSe2 nanomaterials, MoSe2 nanoflakes were synthesized on silicon substrates by hot filament chemical vapor deposition using molybdenum trioxide and selenium powders in pure hydrogen, nitrogen gases and hydrogen-nitrogen mixtures. The structures and composition of synthesized MoSe2 nanoflakes were studied using the advanced characterization instruments including field emission scanning electron microscopy, micro-Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy and energy dispersive X-ray spectrometry. The analysis of the growth process indicates that hydrogen can improve the formation of MoSe2 nanoflakes and significantly alter their properties due to the high reduction capacity of hydrogen and the creation of more nucleation centers of MoSe2 nanoflakes on the silicon surface. The study of photoluminescent (PL) properties reveals that the MoSe2 nanoflakes can generate a strong PL band at about 631nm, differently from the plain MoSe2 nanoflakes. The major difference in the PL properties may be related to the edges of MoSe2 nanoflakes. These results can be used to control the growth and structure of MoSe2-based nanomaterials and contribute to the development of advanced MoSe2-based optoelectronic devices.

Glacial Inception on Baffin Island: The Role of Insolation, Meteorology, and Topography

Abstract Geologic evidence suggests that the last glacial inception (115 kya) occurred within the mountains of Baffin Island. Global climate models (GCMs) have difficulty simulating this climate transition, likely because of their coarse horizontal resolution that smooths topography and necessitates the use of cumulus parameterizations. A regional configuration of the Weather Research and Forecasting (WRF) Model is used to simulate the small-scale topographic and cloud processes neglected by GCMs, and the sensitivity of the region to Milankovitch forcing, topography, and meteorology is tested. It is found that ice growth is possible with 115-kya insolation, realistic topography, and slightly colder-than-average meteorology, represented by specific years within the past three decades. The simulation with low GCM-like topography shows a negative surface mass balance, even with the relevant orbital parameter configuration, demonstrating the criticality of realistic topography. The downslope growth of the ice sheets is studied by looking at the sensitivity of the mass balance to initial snow cover prescribed beyond that of the present day. It is found that the snow-albedo feedback, via its effects on the mass balance, allows such larger snow cover to persist. Implications for GCM studies of glacial inception are discussed.

Numerical investigation of factors affecting the shape of the crystal-melt interface in edge-defined film-fed growth of sapphire crystals

Numerical modeling is used to investigate the shape of the crystal-melt interface in edge-defined film-fed growth (EFG) of large size sapphire rods and sheets. The present analysis shows that the temperature distribution in the meniscus is significantly affected by the internal radiative exchanges in the sapphire crystal. 2D axisymmetric computations performed in the case of sapphire rods, show a concave shape of the interface for opaque crystals, and a convex shaped interface for semi-transparent crystals. The temperature gradient across the meniscus increases significantly in the case which accounts for the internal radiative effect in the crystal. Large temperature differences along the free surface of the meniscus generate intense Marangoni flow, which can influence the shape of the growth interface. In this case, the meniscus height increases, producing instabilities in the growth process. The effect of die geometry on the interface shape is analyzed by increasing the angle between the working edges of the die. Computations shows that the interface curvature decreases as this angle increases, but the solidification isotherm moves up, leading to an increased meniscus height. 3D modeling is applied to investigate the EFG growth of large size sapphire sheets. Numerical results show a non-uniform temperature distribution in the meniscus, and a complex 3D flow pattern. However, the intensity of the flow is low in this case, having no influence on the temperature field and interface shape.

Influence of Cu2S precipitates dissolution on ferrite grain growth during heat treatment in the non-oriented electrical steel sheet

The factor to deduce grain growth of non-oriented electrical steel sheet during heat treatment was researched in this paper. Scanning electron microscope equipped with electron backscatter diffraction (EBSD), X-ray diffraction and transmission electron microscope were used to characterize the microstructure, dislocation density and precipitate, respectively. The EBSD results indicated that the grain size increased from 18.7 μm to 56 μm after heat treatment. Meanwhile, the characterization of grain size distribution, dislocation density and precipitates revealed that the dissolution of Cu2S precipitates, which act as inhibitor, may lead to the grain growth. In addition, the pinning force of grain boundary and the driving force of grain growth were calculated. Based on experiments results and theoretical calculations, the Cu2S precipitates with diameter of less than 39 nm and volume fraction of 1.74 × 10-4 would be sufficient to inhibit the ferrite grain growth. Heat treatment provides an efficient way to prompt the grain growth.

Asset growth anomaly in Europe: Do profits and losses matter?

This paper investigates whether the well-documented asset growth effect on stock returns exists across both profit and loss firms in European capital markets. We find that the asset growth anomaly is more pronounced across loss firms and is significantly dampened by the inclusion of profit firms in the sample. The raw and abnormal returns earned from a hedge strategy on balance sheet growth for loss firms are almost two times higher than the respective returns for profit firms. Our evidence casts doubt on a risk-based explanation, thereby lending credence to the suggestion that the asset-growth effect is attributable to mispricing.

Asset growth and the cross-section of stock returns: evidence from Greek listed firms

PurposeThe purpose of this paper is to examine whether firm-level asset investment effects in returns found for US firms occur within the Greek stock market.Design/methodology/approachThe paper utilizes portfolio-level tests and cross-sectional regressions.FindingsThe authors find that growth in total assets is strongly negatively related to future stock returns of Greek firms. In fact, the relation remains statistically significant, even when the authors control for other strong predictors of future returns (i.e. market capitalization and book-to-market ratio). Furthermore, the authors find that a hedge trading strategy on asset growth rate consisting of a long (short) position in firms with low (high) balance sheet growth generates positive returns, confirming that investment growth has significant predictive power for future returns of Greek listed firms.Originality/valueThe paper adds to the literature on the generalization of asset pricing regularities attributable to accounting figures in an emerging market.

Theoretical investigation on the effect of nitrogen doping on the growth and field emission properties of the plasma-grown graphene sheet

A theoretical model to study the impact of nitrogen-doping on the plasma-assisted growth and field emission properties of the graphene sheet (GS) has been developed. The model incorporates the charging rate of the GS, kinetics and energy balance of all plasma species i.e., electrons, positively charged ions and neutral atoms along with the nitrogen doping species, and growth rate of the GS. Numerical calculations on the effect of nitrogen doping on the thickness of the GS have been carried out for typical glow discharge parameters. It is found that the thickness of the GS decreases with nitrogen doping. The ramifications of nitrogen doping on the field enhancement factor of GS have likewise been examined on the premise of the above result. It is observed that the nitrogen doped GSexhibits better field emission as compared to undoped GS. Some of the results of the present investigation are in compliance with the experimental observations.

Growth of graphene sheets under an oxyacetylene flame without a catalyst

High quality graphene sheets that had a low ID/IG and a high 2D intensity in Raman spectra were prepared by a catalyst-free acetylene flame method. The sheets were grown vertically on the surface of carbon particles to form a petal-like morphology. A high temperature, high pressure and short residence time of the flame intensified the decomposition and cyclization reactions of acetylene, leading to the formation of graphene sheets in the gas phase. The turbulent flame and the gases released during the formation of the graphene sheets from carbon nuclei could be responsible for their petal-like morphology instead of an onion-like structure.

Heterogeneous growth of single crystals on polycrystals

This work discloses a surprising, previously unknown heterogeneous growth mode. Namely, large-area, thin sheets of single-crystalline Ge were observed to grow laterally on top of a polycrystalline Al substrate, covering as many as tens of differently oriented Al grains at low temperatures. The observation of the Ge crystal-growth process by in situ heating transmission electron microscopy demonstrates an intriguing type of ``faceted'' growth: the growth of single-crystalline Ge thin sheets proceeding Al-grain by Al-grain on top of the polycrystalline Al substrate. The crystalline Ge growth front tends to align along the lines of intersection of the Al grain boundaries with the Al surface. Such an unusual heterogeneous growth mode has been shown to be a consequence of the strong anisotropy of the energy of the crystalline/crystalline (here: c-Ge/c-Al) interfaces.

Synthesis of electrocatalytically functional carbon honeycombs through cooking with molecule precursors

The development of efficient, economical electrocatalysts for the oxygen reduction reaction (ORR) is highly desirable for clean energy applications, including fuel cells and metal–air batteries. Here, we have developed a synthetic method for obtaining nitrogen doped carbon honeycombs through cooking oleic acid, zinc acetate, and benzimidazole. The presence of zinc acetate leads to the growth of graphitic carbon sheets interconnected in a highly porous carbon honeycomb. More importantly, the morphology and the nitrogen content can be facilely controlled by changing the precursor ratios. When tested, the as-prepared electrocatalyst exhibited efficient catalytic activity towards the ORR via a four-electron pathway, with good stability and high resistance to the methanol crossover effect, which is attributable to the nitrogen doping and the porous structure with large surface area. The good performance is promising for potential applications in fuel cells and elsewhere.

A Scheme for the Growth of Graphene Sheets Embedded with Nanocones

Based on the monolayer growth mode of graphene sheets (2D crystal) by chemical vapor deposition (CVD) on a Cu surface, it should be possible to grow the 2D crystal embedded with single wall carbon nanocones (SWCNC) if nano-conical pits are pre-fabricated on the surface. However, a previous experiment showed that the growing graphene sheet can cross grain boundaries without bending, which seems to invalidate this route for growing SWCNCs. The criterion of Gibbs free energy was applied in the present work to address this issue, showing that the sheet can grow into the valley of a boundary if the boundary has a slope instead of a quarter-turn shape, and SWCNCs can be obtained by this route as long as the lower diameter of the pre-fabricated pit is larger than 1.6 nm and the deposition temperature is higher than 750 K.

Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic

The prevailing view of western Atlantic hydrography during the Last Glacial Maximum (LGM) calls for transport and intermixing of deep southern and intermediate northern end members. However, δ13C and Δ14C results on foraminifera from a sediment core at 5.0 km in the northern subtropics show that there may have also been a northern source of relatively young, very dense, nutrient-depleted water during the LGM (18 ky to 21 ky ago). These results, when integrated with data from other western North Atlantic locations, indicate that the ocean was poorly ventilated at 4.2 km, with better ventilation above and below that depth. If this is a signal of water mass source and not nutrient storage, it would indicate that a previously unrecognized deep water end member originated along the western margin of the Labrador Sea, analogous to dense water formation today around Antarctica and in the Okhotsk Sea.

Enhanced weathering and CO2 drawdown caused by latest Eocene strengthening of the Atlantic meridional overturning circulation

On timescales significantly greater than 105 years, atmospheric pCO2 is controlled by the rate of mantle outgassing relative to the set-point of the silicate weathering feedback. The weathering set-point has been shown to depend on the distribution and characteristics of rocks exposed at the Earth’s surface, vegetation types and topography. Here we argue that large-scale climate impacts caused by changes in ocean circulation can also modify the weathering set-point and show evidence suggesting that this played a role in the establishment of the Antarctic ice sheet at the Eocene–Oligocene boundary. In our simulations, tectonic deepening of the Drake Passage causes freshening and stratification of the Southern Ocean, strengthening the Atlantic meridional overturning circulation and consequently raising temperatures and intensifying rainfall over land. These simulated changes are consistent with late Eocene tectonic reconstructions that show Drake Passage deepening, and with sediment records that reveal Southern Ocean stratification, the emergence of North Atlantic Deep Water, and a hemispherically asymmetric temperature change. These factors would have driven intensified silicate weathering and can thereby explain the drawdown of carbon dioxide that has been linked with Antarctic ice sheet growth. We suggest that this mechanism illustrates another way in which ocean–atmosphere climate dynamics can introduce nonlinear threshold behaviour through interaction with the geologic carbon cycle. During the latest Eocene, declining atmospheric CO2 levels led to the inception of the Antarctic ice sheet. Simulations suggest that the deepening of the Drake Passage caused climate changes that enhanced continental weathering and CO2 drawdown.

A free energy study of carbon clusters on Ir(111): Precursors to graphene growth

It is widely accepted that the nucleation of graphene on transition metals is related to the formation of carbon clusters of various sizes and shapes on the surface. Assuming a low concentration of carbon atoms on a crystal surface, we derive a thermodynamic expression for the grand potential of the cluster of N carbon atoms, relative to a single carbon atom on the surface (the cluster work of formation). This is derived taking into account both the energetic and entropic contributions, including structural and rotational components, and is explicitly dependent on the temperature. Then, using ab initio density functional theory, we calculate the work of formation of carbon clusters CN on the Ir(111) surface as a function of temperature considering clusters with up to N = 16 C atoms. We consider five types of clusters (chains, rings, arches, top-hollow, and domes), and find, in agreement with previous zero temperature studies, that at elevated temperatures the structure most favoured depends on N, with chains and arches being the most likely at N<10 and the hexagonal domes becoming the most favourable at all temperatures for N>10. Our calculations reveal the work of formation to have a much more complex character as a function of the cluster size than one would expect from classical nucleation theory: for typical conditions, the work of formation displays not one but two nucleation barriers, at around N = 4-5 and N = 9-11. This suggests, in agreement with existing LEEM data, that five atom carbon clusters, along with C monomers, must play a pivotal role in the nucleation and growth of graphene sheets, whereby the formation of large clusters is achieved from the coalescence of smaller clusters (Smoluchowski ripening). Although the main emphasis of our study is on thermodynamic aspects of nucleation, the pivotal role of kinetics of transitions between different cluster types during the nucleation process is also discussed for a few cases as illustrative examples.

Midlatitude land surface temperature impacts the timing and structure of glacial maxima

AbstractOrbitally modulated insolation is thought to be among the key mechanisms driving ice sheet change. Yet, late Pleistocene ice sheets attained maximum marginal extents prior to summer insolation minima, suggesting that additional mechanisms influenced ice growth and decay. Continuous orbital‐scale terrestrial temperature records can help define the role that temperature near ice sheet margins plays on the timing and structure of glacial maxima. We hypothesize, based on a 360 kyr long air temperature record from the Chinese Loess Plateau and shorter records from North America and Europe, that midlatitude terrestrial temperature influenced the rate of ice sheet growth prior to and during glacial maxima; cold conditions prior to glacial maxima enhanced ice sheet growth, while warming during glacial maxima inhibited further growth, despite low summer insolation. Thus, the midlatitude surface energy budget may be an important component of understanding and modeling ice volume, particularly for intervals prior to and during glacial maxima, when ice sheet margins reached midlatitudes.

Direct observation of grain boundaries in chemical vapor deposited graphene

Graphene has received great attention owing to its superior physical properties, making graphene suitable for multiple applications. Numerous graphene growth techniques have been developed in the past decade to provide scalable high quality graphene. Among these techniques, chemical vapor deposition (CVD) on catalytic metal films holds great promises for a large-scale graphene growth. Even though extensive efforts have been devoted to synthesize high quality graphene, formation of defects. In particular, grain boundaries (GBs) have a dominant effect on properties, motivating extensive efforts to tune the CVD growth process to minimize GB. Rapid imaging of GBs will significantly aid in studies of CVD graphene grain structure. Here we report a straightforward technique to optically observe GBs in CVD-grown graphene via optical microscopy, allowing rapid assessment of graphene quality as well as the number of layers. The local oxidation of copper through the damaged GBs induces an optically discernable color change in the underlying copper due to different extend of oxidation between the two copper regions under grains and GBs. Our observation technique for GBs of graphene paves a path for understanding fundamental mechanisms of graphene growth and efficient quality evaluation of large-scale graphene sheet for mass production.