Graphite Grains Research Articles

What causes the ultraviolet extinction bump at the cosmic dawn?

Abstract The enigmatic ultraviolet (UV) extinction bump at 2175 Å, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, has recently been detected at the cosmic dawn by the James Webb Space Telescope (JWST) in JADES-GS-z6-0, a distant galaxy at redshift z ≈ 6.71, corresponding to a cosmic age of just 800 million years after the Big Bang. Although small graphite grains have historically long been suggested as the carrier of the 2175 Å extinction bump and graphite grains are expected to have already been pervasive in the early Universe, in this work we demonstrate that small graphite grains are not responsible for the UV extinction bump seen at the cosmic dawn in JADES-GS-z6-0, as the extinction bump arising from small graphite grains is too broad and peaks at wavelengths that are too short to be consistent with what is seen in JADES-GS-z6-0.

Petrofabric and Raman microspectroscopy study of the Mina Afortunada gneiss dome: Mapping its thermal gradient

Black quartzites and migmatitic orthogneisses from the Mina Afortunada dome have been analysed through quartz and graphite petrofabric analysis and Raman microspectrometry on graphitized carbonaceous material. The results permit us to recognize a deformation temperature increase from the dome envelope towards its core marking the transition from basal-<a> to rhomb-<a> quartz intracrystalline slip systems at maximum T of 475 °C, and from rhomb-<a> to prism-<a> at maximum T of 515 °C. The complementary petrofabric and Raman study on graphite discloses a change from basal to prismatic <a> slip systems at maximum T of ca. 470 °C, accompanied by a strengthening of its structural order.The Raman study of large graphite grains revealed a non-random crystallinity organization in them, with domains of weaker lattice structural order (reflecting lower formation T) in core areas and higher crystallinity sectors (higher formation T) at the rims. This finding might reflect a shielding effect during prograde metamorphism, the rim material preventing grain core material from reorganization and increase in its crystallinity. This study shows also that graphite can be a good candidate for pressure-temperature-time path reconstruction in metamorphosed organic-rich rocks.

Mid-infrared echoes of ambiguous nuclear transients reveal high dust covering fractions: evidence for dusty tori

ABSTRACT Alongside the recent increase in discoveries of tidal disruption events (TDEs) have come an increasing number of ambiguous nuclear transients (ANTs). These ANTs are characterized by hot blackbody-like UV/optical spectral energy distributions (SEDs) and smooth photometric evolution, often with hard power law-like X-ray emission. ANTs are likely exotic TDEs or smooth flares originating in active galactic nuclei (AGNs). While their emission in the UV/optical and X-ray has been relatively well-explored, their infrared (IR) emission has not been studied in detail. Here, we use the NEOWISE mission and its low-cadence mapping of the entire sky to study mid-IR dust reprocessing echoes of ANTs. We study 19 ANTs, finding significant MIR flares in 18 objects for which we can estimate an IR luminosity and temperature evolution. The dust reprocessing echoes show a wide range in IR luminosities (∼1042–1045 erg s−1) with blackbody temperatures largely consistent with sublimation temperature of graphite grains. Excluding the two sources possibly associated with luminous supernovae (ASASSN-15lh and ASASSN-17jz), the dust covering fractions (fc) for detected IR flares lie between 0.05 and 0.91, with a mean of fc = 0.29 for all ANTs (including limits) and fc = 0.38 ± 0.04 for detections. These covering fractions are much higher than optically selected TDEs and similar to AGNs. We interpret the high covering fractions in ANT host galaxies as evidence for the presence of a dusty torus.

Experimental Investigation on the Impact of Graphite Electrodes Grain Size on Technological Parameters and Surface Texture of Hastelloy C-22 after Electrical Discharge Machining with Negative Polarity.

Electrical discharge machining (EDM) is a rapidly evolving method in modern industry that manufactures highly complex components. The physical properties of a tool electrode material are significant factors in determining the effectiveness of the process, as well as the characteristics of the machined surfaces. The current trend of implementing graphite tool electrodes in manufacturing processes is observed. Innovative material engineering solutions enable graphite production with miniaturized grain size. However, the correlation between the graphite electrode grain size and the mechanism of the process removal in the EDM is a challenge for its widespread implementation in the industry. This research introduces a new method to evaluate the impact of the graphite electrode grain size and machining parameters on the material removal effectiveness, relative tool wear rate, and surface roughness (Ra) of Hastelloy C-22 following EDM with negative polarity. The study utilized new graphite materials with a grain size of 1 µm (POCO AF-5) and 10 µm (POCO EDM-180). An assessment of the impact of the EDM process parameters on the technological parameters and the development of the surface roughness was carried out. Electrical discharge machining with fine-grained graphite electrodes increases process efficiency and reduces tool wear. Graphite grains detached from the tool electrode affect the stability of electrical discharges and the efficiency of the process. Based on the experimental results, mathematical models were developed, enabling the prediction of machining effects to advance state-of-the-art manufacturing processes. The obtained mathematical models can be implemented in modern industrial EDM machines as guidelines for selecting adequate machining parameters depending on the desired process efficiency, tool wear rate, and surface roughness for advanced materials.

Single-photon emission from silicon-vacancy color centers in polycrystalline diamond membranes

Single-color centers in thin polycrystalline diamond membranes allow the platform to be used in integrated quantum photonics, hybrid quantum systems, and other complex functional materials. While single-crystal diamond membranes are still technologically challenging to fabricate as they cannot be grown on a non-diamond substrate, free-standing polycrystalline diamond membranes can be conveniently fabricated at large-scale from nanocrystalline diamond seeds on a substrate that can be selectively etched. However, their practical application for quantum photonics is so far limited by crystallographic defects, impurities, graphitic grain boundaries, small grain sizes, scattering loss, and strain. In this paper, we report on a single-photon source based on silicon-vacancy color centers in a polycrystalline diamond membrane. We discuss the spectroscopic approach and quantify the photon statistics, obtaining a g2(0) ≈ 0.04. Our findings hold promise for introducing polycrystalline diamond to quantum photonics and hybrid quantum systems.

Impact of surface treatments on the electron affinity of nitrogen-doped ultrananocrystalline diamond

In recent years, various forms of nanocrystalline diamond (NCD) have emerged as an attractive group of diamond/graphite mixed-phase materials for a range of applications from electron emission sources to electrodes for neural interfacing. To tailor their properties for different uses, NCD surfaces can be terminated with various chemical functionalities, in particular hydrogen and oxygen, which shift the band edge positions and electron affinity values. While the band edge positions of chemically terminated single crystal diamond are well understood, the same is not true for nanocrystalline diamond, which has uncontrolled crystallographic surfaces with a variety of chemical states as well as graphitic grain boundary regions. In this work, the relative band edge positions of as-grown, hydrogen terminated, and oxygen terminated nitrogen-doped ultrananocrystalline diamond (N-UNCD) are determined using ultraviolet photoelectron spectroscopy (UPS), while the band bending is investigated using photoelectrochemical measurements. In contrast to the widely reported negative electrode affinity of hydrogen terminated single crystal diamond, our work demonstrates that hydrogen terminated N-UNCD exhibits a positive electron affinity owing to the increased surface and bulk defect densities. These findings elucidate the marked differences in electrochemical properties of hydrogen and oxygen terminated N-UNCD, such as the dramatic changes in electrochemical capacitance.

Unravelling the nuclear dust morphology of NGC 1365: a two-phase polar–RAT model for the ultraviolet to infrared spectral energy distribution

ABSTRACT We present a 3D radiative transfer model for the spectral energy distribution (SED) of NGC 1365, which is a ‘changing look’ Seyfert 1.8 type active galactic nucleus (AGN). The SED from the ultraviolet (UV) to the infrared (IR) is constructed using archival data from the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat, along with IR data from the literature. The skirt radiative transfer code is used to model the SED and derive the geometry and composition of dust in this AGN. Similar to our earlier SED model of NGC 4151, the nuclear region of NGC 1365 is assumed to contain a ring or disc-like structure concentric to the accretion disc, composed of large (0.1–1 $\mu$m) graphite grains in addition to the two-phase dusty torus made up of interstellar-medium-type grains (Ring And Torus or RAT model). We also include, for the first time, an additional component of dusty wind in the form of a bipolar cone. We carry out a detailed analysis and derive the best-fitting parameters from a χ2 test to be Rin, r = 0.03 pc, σ = 26°, and τtotal = 20 for the assumed ring–torus–polar wind geometry. Our results suggest the presence of hot dust at a temperature T ∼ 1216 K at the location of the ring that absorbs and scatters the incident UV radiation and emits in the near-IR. In the mid-IR, the major contributors are the polar cone and warm dust with T ∼ 914 K at Rin, t = 0.1 pc. Not only are our model radii in agreement with IR interferometric observations, but also our study reiterates the role of high-resolution UV observations in constraining the dust grain size distribution in the nuclear regions of AGN.

Internal and External Alignment of Carbonaceous Grains within the Radiative Torque Paradigm

We study the internal and external alignment of carbonaceous grains in the interstellar medium (ISM) within the Radiative Torque (RAT) paradigm. For internal alignment (IA), we find that hydrogenated amorphous carbon (HAC) grains having nuclear paramagnetism due to hydrogen protons have efficient nuclear relaxation, whereas both HAC and graphite grains can have efficient inelastic relaxation at both low-J and high-J attractors. For external alignment, HAC and pure graphite grains can align with the radiation direction (k-RAT) at low-J attractors but cannot have stable alignment at high-J attractors due to the suppression of radiative precession. However, HAC grains can align with the magnetic field (B-RAT) at high-J attractors due to fast Larmor precession compared to gas collisions. For HAC grains drifting through the ISM, they can align along the induced electric field (E-RAT) at low-J attractors due to the fast electric precession and only small HAC grains can align at high-J attractors. Nuclear paramagnetic relaxation is inefficient for HAC due to the suppression of nuclear susceptibility. We then study the alignment of carbon dust in the envelope of a C-rich Asymptotic Giant Branch star (IRC+10216) and find that grains aligned at low-J attractors may occur via k-RAT with the wrong IA in the inner region but via B-RAT in the outer region. However, grains aligned at high-J attractors have the right IA alignment via k-RAT due to efficient inelastic relaxation. The polarization pattern observed toward IRC+10216 by SOFIA/HAWC+ can be reproduced when only grains at low-J attractors are present due to the removal of grains at high-J attractors by the RAT disruption.

Polycyclic aromatic hydrocarbon molecules and the 2175Å interstellar extinction bump

ABSTRACT The exact nature of the 2175$\mathring{\rm A}$ extinction bump, the strongest spectroscopic absorption feature superimposed on the interstellar extinction curve, remains unknown ever since its discovery in 1965. Popular candidate carriers for the extinction bump include nano-sized graphitic grains and polycyclic aromatic hydrocarbon (PAH) molecules. To quantitatively evaluate PAHs as a possible carrier, we perform quantum chemical computations for the electronic transitions of 30 compact, pericondensed PAH molecules and their cations as well as anions with a wide range of sizes from 16 to 96 C atoms, and a mean size of 43 C atoms. It is found that a mixture of such PAHs, which individually exhibit sharp absorption features, show a smooth and broad absorption band that resembles the 2175$\mathring{\rm A}$ interstellar extinction bump. Arising from π* ← π transitions, the width and intensity of the absorption bump for otherwise randomly selected and uniformly weighted PAH mixtures, do not vary much with PAH sizes and charge states, whereas the position somewhat shifts to longer wavelengths as PAH size increases. While the computed bump position, with the computational uncertainty taken into account, appears to agree with that of the interstellar extinction bump, the computed width is considerably broader than the interstellar bump if the molecules are uniformly weighted. It appears that, to account for the observed bump width, one has to resort to PAH species of specific sizes and structures.

New, 3D binder-jetted carbons with minimal periodic surface structures

This study focuses on the design, additive manufacturing, and characterization of porous carbon-based structures in the form of a triply periodic minimal surface (TPMS) with outstanding mechanical properties and oxidation resistance, combined with good electrical and thermal conductivity. Binder jetting (BJ) of graphite-carbon black powders was used to 3D print computational models of three TPMS with different topologies and geometric porosities. Infiltration and pyrolysis (PIP) with furan resin was then performed to densify the parts. Composite materials, comprising a highly disordered carbon matrix binding well-crystallized graphite grains, were obtained. The printed and pyrolyzed samples are highly porous TPMS cylinders with diameter, height and a surface thickness of ∼19 mm, ∼33 mm and 0.76 mm, respectively. The samples have a skeleton intrinsic porosity of 20%, of which 8% is open, meaning that the material could potentially be further infiltrated and densified. Nevertheless, the samples have better mechanical properties than compressed carbon-graphite composites, as well as 3D-printed carbon produced by direct ink writing and stereolithography. High-temperature tests showed that, although the amorphous carbon matrix is more prone to oxidation than the graphite grains, the overall oxidation resistance remains exceptionally high. These properties allow for applications as Joule resistors and in seasonal thermal storage.

A novel mechanism of B4C-TiB2-graphite composites with high flexural strength by spark plasma sintering

The B4C-TiB2-graphite composites with a high flexural strength of 915.4 ± 41.7 MPa was prepared by spark plasma sintering (SPS) at 1900 °C under 60 MPa for 10 min. The duplex structure reinforcement was formed with lamellar graphite and submicron TiB2 grains inter-embedded in each other. Likewise, the flexural strength and fracture toughness of the composites were significantly strengthened. Synthesis reaction mechanism studies showed that the sintering densification is promoted by the fast thermal diffusion of boron and carbon in SPS. The effect of the amount of TiB2 and graphite on the flexural strength of the composites was investigated, and the B4C-TiB2-graphite composites exhibit excellent combined mechanical properties at room temperature, (flexural strength of 915.4 ± 41.7 MPa, fracture toughness of 5.18 ± 0.40 MPa·m1/2, Vickers hardness of 32.14 ± 1.72 GPa). In addition, the strengthening mechanism revealed that strong internal stress and excellent dissipation ability of crack propagation energy of TiB2-graphite duplex structure made the composite with high flexural strength and fracture toughness.

Evidence for abundant organic matter in a Neoarchean banded iron formation

Abstract Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supracrustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic bio-mass, based on its co-occurrence with apatite rosettes and negative bulk rock δ13Corganic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ13Ccarb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

Microstructure and defect engineering of graphite anodes by pulsed laser annealing for enhanced performance of lithium-ion batteries

Nanosecond pulsed laser annealing significantly improves cyclability and current carrying capacity of lithium-ion batteries (LIBs). This improvement is achieved by engineering of microstructure and defect contents present in graphite in a controlled way by using pulsed laser annealing (PLA) to increase the number density of Li+ ion trapping sites. The PLA treatment causes the following changes: (1) creates surface steps and grooves between the grains to improve Li+ ion charging and intercalation rates; (2) removes inactive polyvinylidene difluoride (PVDF) binder from the top of graphite grains and between the grains which otherwise tends to block the Li+ migration; and (3) produces carbon vacancies in (0001) planes which can provide Li+ charging sites. From X-ray diffraction data, we find upshift in diffraction peak or reduction in planar spacing, from which vacancy concentration was estimated to be about 1.0%, which is higher than the thermodynamic equilibrium concentration of vacancies. The laser treatment creates single and multiple C vacancies which provide sites for Li+ ions, and it also produces steps and grooves for Li+ ions to enter the intercalating sites. It is envisaged that the formation of these sites enhances Li+ ion absorption during charge and discharge cycles. The current capacity increases from an average 360 mAh/g to 430 mAh/g, and C–V shows significant reduction in SEI layer formation after the laser treatment. If the vacancy concentration is too high and charge-discharge cycles are long, then trapping of electrons by Li+ may occur, which can lead to Li0 formation and Li plating causing reduction in current capacity.

Carbon as a key driver of super-reduced explosive volcanism on Mercury: Evidence from graphite-melt smelting experiments

Here we present the results of experiments designed to reproduce the interaction between super-solidus mercurian magmas and graphite at high temperatures (ramped up from ambient temperature to 1195–1390°C) and low pressure (10 mbar). The compositions of resultant gases were measured in situ with a thermal gravimeter/differential scanning calorimeter connected to a mass spectrometer configured to operate under low pressures and reducing conditions. Solid run products were analyzed by electron microprobe and Raman spectroscopy. Three magma starting compositions were based on the composition of the Borealis Planitia region (termed NVP for the Northern Volcanic Plains) on Mercury ± alkali metals, sulfur, and transition metal oxides. Smelting between FeOmelt and graphite was observed above 1100°C, evidenced by the generation of CO and CO2 gas and the formation of Fe-Si metal alloys, which were found in contact with residual graphite grains. Experiments with transition metal oxide-free starting compositions did not produce metal alloys and showed no significant gas production. In all runs that produced gas, C-O-H±S species dominated the degassing vapor. Our results suggest that the consideration of graphite smelting processes can significantly increase calculated eruption velocities and that gas produced by smelting alone can account for >75% of the pyroclastic deposits identified on Mercury. A combination of S-H-degassing and CO-CO2 production from smelting can explain all but the single largest pyroclastic deposit on Mercury.

Torus and polar dust dependence on active galactic nucleus properties

We present a statistical analysis of the properties of the obscuring material around active galactic nuclei (AGN). This study represents the first of its kind for an ultra-hard X-ray (14–195 keV; Swift/BAT), volume-limited (DL &lt; 40 Mpc) sample of 24 Seyfert (Sy) galaxies (BCS40 sample) using high angular resolution infrared data and various torus models: smooth, clumpy, and two-phase torus models and clumpy disc+wind models. We find that torus models (i.e. without including the polar dusty wind component) and disc+wind models provide the best fits for a comparable number of galaxies, 8 out of 24 (33.3%) and 9 out of 24 (37.5%), respectively. We find that the best-fit models depend on the hydrogen column density (NHX−ray), which is related to the X-ray (unobscured or obscured) and optical (Sy1/Sy2) classification. In particular, smooth, clumpy, and two-phase torus models are best at reproducing the infrared (IR) emission of AGN with relatively high hydrogen column density (median value of log (NHX−ray cm−2) = 23.5 ± 0.8; i.e. Sy2). However, clumpy disc+wind models provide the best fits to the nuclear IR spectral energy distributions (SEDs) of Sy1/1.8/1.9 (median value of log (NHX−ray cm−2) = 21.0 ± 1.0) – specifically, in the near-infrared (NIR) range. The success of the disc+wind models in fitting the NIR emission of Sy1 galaxies is due to the combination of adding large graphite grains to the dust composition and self-obscuration effects caused by the wind at intermediate inclinations. In general, we find that the Seyfert galaxies having unfavourable (favourable) conditions, namely, nuclear hydrogen column density and Eddington ratio, for launching IR dusty polar outflows are best-fitted with smooth, clumpy, and two-phase torus (disc+wind) models, confirming the predictions from simulations. Therefore, our results indicate that the nature of the inner dusty structure in AGN depends on the intrinsic AGN properties.

Effect of grain size of graphite powder in carbon paper on the performance of proton exchange membrane fuel cell

This work focuses on the effect of graphite powder grain size on the properties of carbon paper and proton exchange membrane fuel cell. In this paper, graphite powders of different grain sizes (0.5–30 μm) are added to carbon paper for proton exchange membrane fuel cell. Furthermore, the carbon paper is subjected to hydrophobic treatment at polytetrafluoroethylene (PTFE). The research results show that with the increase in graphite grain size, the cell performance increases at dry condition, the carbon paper containing 10 wt% PTFE and graphite powder grain size of 30 μm has the best performance, which power density of 892 mW cm−2 and limit current density of 2.40 A cm−2. With the increase in graphite grain size, the cell performance decrease at wet condition, the carbon paper containg 20 wt% PTFE and graphite powder grain size of 0.5 μm has the best performance, which power density of 524 mW cm−2 and limit current density of 1.84 A cm−2. The paper illustrates that reducing the grain size of graphite powder is beneficial to enhancing the water management ability of gas diffuse layer and reduces the performance loss at wet condition, while, increasing the gas transmission resistance and the performance loss at dry condition.

Effect of Inoculation Treatment on Number of Primary Austenite Grains in Hypoeutectic Chromium Cast Iron: EBSD Imaging and Mathematical Structure Prediction

This study proved the influence of an inoculation substance on the primary structure of chromium-cast iron. The inoculation procedure has developed very well in the field of grey cast iron production and mainly concerns the crystallisation of graphite eutectic grains in this material. However, in chromium cast iron, the inoculation problem is not well-recognised due to the formation of chromium carbides in white cast iron. One can easily increase the number of carbides in the cast iron’s structure, but this procedure will not always bring the expected benefits in terms of increasing the overall mechanical properties. In the research included in this publication, an experiment was carried out with the use of ferrotitanium as an inoculant for chromium-cast iron. As a result of using the EBSD (electron backscatter diffraction analysis) imaging method, it was proven that the Fe–Ti interaction significantly influenced the primary structure of chromium cast iron that was formed by austenite grains. The paper presents the growth laws of primary austenite grains in modified hypoeutectic chromium cast iron depending on the degree of supercooling, ΔT, and the amount of the Fe–Ti inoculant M. The results of the research made it possible to predict the structure of hypoeutectic chromium cast iron after Fe–Ti inoculation treatment. The article proves that the use of the inoculant can change the primary structure of chromium cast iron, increasing its impact strength by more than three times.

On survival of dust grains in the sublimation zone of cold white dwarfs

ABSTRACTWe consider a mechanism for the deposition of dust grains on to the surface of cold white dwarfs (WDs). Calculations show that grains can fall on to a cold WD directly, without reaching the phase of complete evaporation, if the parent bodies and the grains orbit on elongated, close to parabolic, orbits. To this end, we calculated the dynamics of evaporating silicate and graphite dust grains moving in circular and parabolic orbits around the white dwarf WD J1644−0449 with Teff ≈ 3830 K and M⋆ = 0.45 M⊙. The calculations accounted for the influence of radiation pressure and Poynting–Robertson drag on the grain dynamics. The results show that silicate grains of all sizes considered that leave the parent bodies on circular orbits evaporate completely at a distance of ∼3 stellar radii (R⋆) from the star. The boundary of the dust-free zone for graphite grains is closer to the star, ∼1.5R⋆, and is represented confidently only for larger grains with radius &amp;gt; 0.5 ${\mu m}$. We determined the lower limits of the radius for grains capable of reaching the stellar surface. For comparison, we analysed the dependences of lower size limits for infalling silicate grains for a set of WDs within the temperature range 3000–5000 K. We conclude that silicate grains with an initial size ≥ 300 ${\mu m}$ can reach the surface of WD J1644−0449. For stars with temperatures in the range 3000–5000 K, the corresponding grain size range is 0.016 μm–5 cm.

Less energy-intensive synthesis of mesoporous multi-oriented graphite microspheres with low defect concentration for advanced potassium-ion battery anodes

Potassium-ion batteries (KIBs) have emerged as prospective alternatives to lithium-ion batteries (LIBs) given the wide availability of K resources and the similar operating voltage. As in the case of LIBs, graphite is regarded as the most promising anode material for KIBs due to high energy density attributed to its low potential plateau and resource abundance. However, the practical use of graphite in KIBs is hindered by poor stability due to huge volume changes during K+ ion insertion-deinsertion. Here, a facile and scalable synthesis method of mesoporous multi-oriented graphite microspheres via pilot-scale spray drying process and less energy-intensive graphitization step is introduced. A high degree of graphitization is achieved even at 1100 °C by virtue of in situ formed Fe nanocrystals within the microsphere that serve as graphitization catalyst. The degree of graphitization can be further improved by selective removal of amorphous carbon by oxidation at 450 °C, which greatly increases the initial Coulombic efficiency and reversible capacity of the graphite microspheres. The synthesized microspheres with the multi-oriented graphite grains and the mesoporous network enable rapid K+ ion insertion-deinsertion, and alleviate stress from volume changes during cycling, thereby exhibiting markedly improved cyclability (247 mA h g−1 after 400 cycles at 0.5C) and rate performance (58 mA h g−1 at 3C) that outperform those of commercial artificial graphite. This study provides a new perspective for designing reliable nanostructured graphite materials for advanced KIB anodes.

Hydrogen Burning of 29Si and Its Impact on Presolar Stardust Grains from Classical Novae

Abstract Presolar stardust grains found in primitive meteorites are believed to retain the isotopic composition of stellar outflows at the time of grain condensation. Therefore, laboratory measurements of their isotopic ratios represent sensitive probes for investigating open questions related to stellar evolution, stellar explosions, nucleosynthesis, mixing mechanisms, dust formation, and galactic chemical evolution. For a few selected presolar grains, classical novae have been discussed as a potential source. For SiC, silicate, and graphite presolar grains, the association is based on the observation of small N(12C)/N(13C) and N(14N)/N(15N) number abundance ratios compared to solar values, and abundance excesses in 30Si relative to 29Si, as previously predicted by models of classical novae. We report on a direct measurement of the 29Si(p,γ)30P reaction, which strongly impacts simulated δ 29Si values from classical novae. Our new experimental 29Si(p,γ)30P thermonuclear reaction rate differs from previous results by up to 50% in the classical nova temperature range (T = 100–400 MK), while the rate uncertainty is reduced by up to a factor of 3. Using our new reaction rate in Monte Carlo reaction network and hydrodynamic simulations of classical novae, we estimate δ 29Si values with much reduced uncertainties. Our results establish δ 29Si values measured in presolar grains as a sensitive probe for assessing their classical nova paternity. We also demonstrate that δ 30Si values from nova simulations are currently not a useful diagnostic tool unless the large uncertainty of the 30P(p,γ)31S reaction rate can be significantly reduced.