Graphite Atomizer Research Articles

Diffusion behavior of cesium in graphite matrix for a High-temperature Reactor Pebble-bed Module: a first-principles study

ABSTRACT Graphite is one of the main materials of the fuel element in High Temperature Reactor Pebble-bed Module (HTR-PM). Knowledge of the behavior of Cesium (Cs) in graphite is necessary for reliable source term analysis under normal and accidental conditions for HTR-PMs. In this study, the diffusion behavior of Cs atoms in graphite was calculated to estimate the diffusivity of Cs using the first-principles density functional theory. The results show that the energy barriers for vacancy and interstitial diffusion are significantly different (2.257 eV and 0.051 eV, respectively), suggesting that interstitial diffusion is more likely to occur in high-temperature graphite. However, experimental data in literatures indicate that the activation energy for Cs diffusion in graphite falls between 1.5 to 3.3 eV, aligning with the migration energy for vacancy diffusion identified in this study. It is hypothesized that experimental observations of diffusion may be primarily influenced by vacancies. The results may help designers to select appropriate parameters or make physical assumptions and to consider the radiation risks caused by the residual fission product (Cs) in graphite.

Hydroxymethylfurfural adsorption modulating charge separation characteristics of carbon-rich carbon nitrides for simultaneous photocatalytic hydrogen evolution and 2,5-diformylfuran production

Carbon-rich melon-based carbon nitrides (CNs), in which the sp2 N atoms and the terminal C–NH2 within the heptazine rings are partially replaced by the graphitic carbon atoms and C–H terminals, respectively, are synthesized through the condensation of 2,4,6-triaminopyrimidine-added supramolecular complexes comprising melamine and cyanuric acid. Optical characterizations reveal that increasing the C/N ratio in CNs improves their light-harvesting and charge-separation capabilities. Nevertheless, the photocatalytic activities of carbon nitrides reach an optimal level at an appropriate C/N ratio for simultaneous H2 evolution and 2,5-diformylfuran (DFF) production from 5-(hydroxymethyl)furfural (HMF) aqueous solution. Hydrogen evolution from water and DFF production by selective HMF oxidation are driven by photogenerated electrons and holes, respectively. Density functional theory calculations suggest that the adsorption of HMF on carbon-CN dramatically influences the molecular orbital contributions to the lowest singlet excited state, altering its charge separation character and, consequently, the photocatalytic activity. The improvement of charge separation in the optimized CN by HMF adsorption is further confirmed by experimental measurements.

Controlling the optoelectronic properties of nitrogen-doped carbon quantum dots using biomass-derived precursors in a continuous flow system

The synthesis of carbon quantum dots (CQDs) from high molecular weight biomass-derived precursors poses a significant challenge due to the complex molecular structures and low conversion efficiency. This work demonstrates a green, rapid, and sustainable continuous hydrothermal flow synthesis (CHFS) approach for nitrogen-doped carbon quantum dots (NCQDs) from various biomass-derived precursors, including high molecular weight polymeric sources like chitosan, lignin, and humic acid. We find that the precursor structure significantly impacts the size of the fabricated NCQDs and their optical properties. Citric acid, a low molecular weight precursor, yields NCQDs with excitation-independent emission, higher quantum yields, and low non-radiative losses, while NCQDs derived from polymeric precursors exhibit excitation-dependent, red-shifted, and lower efficiency emission. Theoretical calculations, performed to understand the configuration and distribution of nitrogen dopants within the NCQD structure, show that pyridinic and graphitic nitrogen atoms exhibit a strong preference to aggregate near the centre of the edge of the NCQD and not in the vertices nor in the graphitic core, thus affecting the HOMO and LUMO, bandgap, and light absorption and emission wavelengths. The life cycle assessment (LCA) analysis highlights the green and scalable advantages of the CHFS process for producing NCQDs compared to batch methods, making it a sustainable and economically viable approach for large-scale NCQD synthesis from high molecular weight biomass-derived precursors. Hence, the combination of experimental data and theoretical calculations provides a comprehensive understanding of the structure-property relationships in these NCQDs.

Effects of expanded graphite’s structural and elemental characteristics on its oil and heavy metal sorption properties

Expanded graphite has promising potential environmental applications due to its porous structure and oleophilic nature, which allow it to absorb large quantities of oil. The material is produced by intercalating graphite and applying heat to convert the intercalant into gas to cause expansion between the layers in the graphite. Using different intercalants and temperature conditions results in varying properties of expanded graphite. This work has proven that the sorption properties of commercial expanded graphite differ significantly due to the material’s structural and elemental characteristics, which can be attributed to the intercalation method. This resulted in various degrees of exfoliation of the graphite and possible functionalisation of the graphene sheets within the structure. This affected the material's sorption capacity and its affinity for heavy metal sorption by incorporating selectivity towards the sorption of certain metals. It was found that sample EG3, which underwent a less harsh expansion, exhibited lower porosity than EG1, and thus, the sample absorbed less oil at 37.29 g/g compared to the more expanded samples EG1 and EG2 with 55.16 g/g and 48.82 g/g, respectively. However, it was able to entrap a wider variety of metal particles compared to EG1 and EG2, possibly due to its smaller cavities allowing for a capillary effect between the graphene sheets and greater Van der Waals forces. A second possibility is that ionic or coordination complexes could form with certain metals due to the possible functionalisation of the expanded graphite during the intercalation process. This would be in addition to coordination between the metals and expanded graphite carbon atoms. The findings suggest that there is evidence of functionalisation as determined by XRD and elemental analyses. However, further investigation is necessary to confirm this hypothesis. The findings in this work suggest that the first mechanism of sorption was more likely to be related to the degree of expansion of the expanded graphite. Various metals are present in used oil, and their removal can be challenging. Some metals in oil are not considered heavy since they have a relatively low density but can be associated with heavy metals in terms of toxicity.

The Formation and Transformation of Low-Dimensional Carbon Nanomaterials by Electron Irradiation.

Low-dimensional materials based on graphene or graphite show a large variety of phenomena when they are subjected to irradiation with energetic electrons. Since the 1990s, electron microscopy studies, where a certain irradiation dose is unavoidable, have witnessed unexpected structural transformations of graphitic nanoparticles. It is recognized that electron irradiation is not only detrimental but also bears considerable potential in the formation of new graphitic structures. With the availability of aberration-corrected electron microscopes and the discovery of techniques to produce monolayers of graphene, detailed insight into the atomic processes occurring during electron irradiation became possible. Threshold energies for atom displacements are determined and models of different types of lattice vacancies are confirmed experimentally. However, experimental evidence for the configuration of interstitial atoms in graphite or adatoms on graphene remained indirect, and the understanding of defect dynamics still depends on theoretical concepts. This article reviews irradiation phenomena in graphene- or graphite-based nanomaterials from the scale of single atoms to tens of nanometers. Observations from the 1990s can now be explained on the basis of new results. The evolution of the understanding during three decades of research is presented, and the remaining problems are pointed out.

REVIEW OF PROXIMATE ANALYSIS OF HEAVY METALS IN ACTION BITTER ALCOHOLIC HERBAL DRINKS CONSUMED IN NIGERIA

Consumption of alcoholic herbal products from beverages or medicinal drinks contaminated with heavy metals can cause serious consequences on human health. This is a major concern for traditional and herbal medicine. The present study was carried out to analyze and quantify the levels of seven potentially toxic heavy metals namely Magnesium, lead, cadmium, copper, iron, chromium and nickel in Action Bitter alcoholic herbal bitter. Twenty one ACTION BITTER alcoholic bitter samples were previously pretreated and homogenized were digested and analyzed to obtain a concentration of Cadmium (Cd), Chromium (Cr),Copper (Cu), Iron (Fe), Magnesium (Mg), Nickel (Ni) and lead (Pb) using atomic absorption spectrophotometer equipped with graphite tube atomizer. The concentration obtained are Cadmium (0.017 mg/l),Chromium (0.061 mg/l),Copper (0.056 mg/l), Iron (0.223 mg/l), Magnesium (1.118 mg/l), Nickel (0.112 mg/l) and lead (-0.073 mg/l). The analysis of heavy metals can be useful to evaluate the dosage of herbal drinks prepared from these plants. Therefore, it is of great importance to establish universal standards and quality requirements for hazardous elements in herbal drinks so that this natural resource can continue and expand further, to benefit health globally.

Using Activated Biochar from Caryocar brasiliense Pequi Almonds for Removing Methylene Blue Dye in an Aqueous Solution

Water pollution remains a global problem that urges researchers to develop new technologies aimed at environmental restoration. Here, this study aimed at obtaining an activated biochar from pequi almonds for dye removal. Before and after adsorption, the materials underwent characterization using techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Raman spectroscopy, and thermogravimetric analysis (TG). The biochar from the pequi almond was called BA, and the activated biochar from the pequi almond was called ABA. The influence of the pH, contact time, and adsorbate concentration on adsorption was investigated using the dye methylene blue. The morphological assessment revealed higher cracks and pores in the ABA than in the BA. The TG showed that the BA lost approximately 19% more mass than the ABA, indicating that activation occurred. The activation contributed to the decrease in the degree of disorder in the BA because of the increased number of graphitic carbon atoms (ordered) in the ABA, as observed via Raman. The adsorption kinetics followed a pseudo-second-order model, while the adsorption isotherms followed the Langmuir model. The BA adsorption capacity was 500.00 mg g−1, constituting a robust solution for dye removal from aqueous environments. Therefore, this implies the success of the process.

The influence of Compton electrons on the rearrangement of carbon atoms in graphite: new evidence from the analysis of 2D Raman band

The influence of Compton electrons on the rearrangement of carbon atoms in graphite: new evidence from the analysis of 2D Raman band

Defective Potassium Poly(Heptazine Imide) Preventing Spin Delocalization and Hole Transfer Deactivation for Efficient Solar Energy Conversion and Storage.

Anti-site defective potassium poly(heptazine imide) (KPHI) with the central nitrogen atoms partially replaced by graphitic carbon atoms in the flawed heptazine rings is prepared by direct ionothermal treatment of the rationally designed supramolecular complex in KSCN salt molten. Compared to the KPHIs without the anti-site defect, the anti-site defective KPHI demonstrates significantly improved photocatalytic and dark photocatalytic performances for H2 evolution reaction (HER). In the presence of the hole scavenger, the anti-site defective KPHI exhibits superior photocatalytic stability for HER lasting 20h, whereas the deactivation is observed from the ordinary KHPIs after 3h HER. Moreover, the H2 yield in the dark by the stored photoelectrons in the anti-site defective KPHI increases by more than an order of magnitude. Density functional theory calculations reveal that the anti-site defective unit in KPHI not only prevents spin delocalization but also inhibits the deactivation of hole transfer, which are beneficial to photoelectron storage and photocatalytic activity. The findings in this study provide insight into the photophysical and catalytic properties of KPHI, which conclude a strategy to improve the performances for solar energy conversion and storage by incorporating intrinsic anti-site defects in KPHI.

Effects of charge rearrangement on interfacial contact resistance of TiO2/graphite from first-principles calculations

Titanium bipolar plates have been widely employed in fuel cells, due to the high resistance achieved by the oxide film (TiO2). However, that also results in high interfacial contact resistance between the titanium and the gas diffusion layer of graphite, reducing the cells’ efficiency significantly. To improve the conductivity properties, the effects of thirty-five metallic dopants on the contact resistance and electrical conductivity were studied based on the first-principles merged with the Schottky-Mott theory and Boltzmann transport equation. The results show that the contact resistance depends on the charge distribution induced by graphite. The contact resistance depends on the conductivity when the charge depletion and accumulation layer are distributed in the space-charge region. If the charge rearrangement occurs in the heterojunction as a whole, the contact resistance is determined by the electronic injection. In addition, the single conduction path between titanium, oxygen and graphite atoms disappears in the TiO2 doped with Zn, Cd, etc., because those dopants have stronger electron affinity in the heterojunction. While a stronger built-in electric field is formed that can further enhance the electronic conductivity.

Etching of photon energy into binding energy in depositing carbon films at different chamber pressures

A hot filament chemical vapor deposition is an attractive technique to deposit carbon films of different applications. In this technique, it is also feasible to study the influence of chamber pressure in the deposition of carbon films. In the deposition chamber, having dissociated from the methane precursor, gaseous carbon atoms first convert into the graphite state atoms and then into the diamond state atoms. An increase in the chamber pressure changes the morphology and structure of the deposited carbon films. The growth rate of the deposited carbon film increases by increasing the chamber pressure from 3.3 kPa to 8.6 kPa. The rate of converting gaseous carbon atoms into diamond atoms also increases. At 11.3 kPa and 14 kPa chamber pressure, gaseous carbon atoms convert into graphite state atoms at a high rate. The gas activation and gas collision processes vary broadly at varying chamber pressure. The morphology and structure of carbon films got deposited at different growth rates. The dissociation of molecular hydrogen into atomic hydrogen varies by varying the chamber pressure. Atomic hydrogen etched the photons released from the hot filaments. Thus, bits of differently shaped energy result. Gaseous carbon atoms convert into graphite and diamond state atoms under suitably shaped bits of energy. Graphite state atoms bind under the same involved bits, which is not the case when the diamond state atoms bind. The study sets a new trend in depositing, characterizing, and analyzing carbon films.

Solvent-mediated oxidative polymerization to atomically dispersed iron sites for oxygen reduction

Fe single-atom catalysts open up broad prospects for oxygen reduction reaction (ORR), but the chemical state evolution of Fe species before forming isolated sites is rarely understood. Herein, mechanistic investigation on the formation of isolated Fe sites is presented through solvent-mediated oxidative pyrrole polymerization strategy. The slow reaction kinetics of oxidative Fe3+ ions with the predesigned methanol solvent molecules can endow highly dispersed Fe sites in polypyrrole and thus Fe single-atom catalysts after pyrolysis. The Fe single-atom catalyst (Fe-SA/PNC) performs superior ORR activity with a half-wave potential of 0.90 V versus RHE and 12.8 times higher turnover frequency than that of commercial Pt/C. When assembled into Zn-air batteries, the Fe-SA/PNC cathode delivers a 1.68 V open circuit potential and ultra-long cycling stability over 9000 cycles, superior to the most reported catalysts so far. Experimental and theoretical results reveal that the rich adjacent non-coordinated graphitic nitrogen atoms can enhance electronic conductivity and promote O2 adsorption and OH- desorption, thus enabling high oxygen reduction and battery performances.

Structural analyses of carbon films deposited at different total mass rates in a hot-filament CVD system

XRR and XRIR analyses of the deposited carbon films showing different peaks related to the diamond and graphite state atoms.

Nanostructured recrystallization of iron‑carbon alloys

Recrystallization of iron-carbon alloys has been shown to be a nanostructured process. Microcrystals of secondary cementite of steels and cast iron are formed from elementary nanocrystals of iron and graphite, free atoms of graphite and iron-carbon complexes. Microcrystals of primary α-ferrita steels are formed from elementary nanocrystals of iron and graphite, free iron atoms. Microcrystals of cast iron secondary graphite are formed from elementary nanocrystals and free graphite atoms. Eutectoid microcrystals are formed from elementary nanocrystals of iron and carbon, free atoms of iron and carbon, iron-carbon complexes.

Nanostructural crystallization of cast iron

Crystallization of cast iron has been shown to be a nanostructured process. Austenitic-graphite eutectic is formed from iron and graphite nanocrystals, free iron and graphite atoms. Austenitic-cementite eutectic is formed from iron and graphite nanocrystals, free graphite atoms and iron-carbon complexes. Primary austenite microcrystals are formed from iron nanocrystals, graphite and iron-carbon complexes.

The Effect of the Process Temperature on the Bondability in Diffusion Bonding of AISI 430 with Nodular Cast Iron

Abstract The effect of process temperature on the bond-ability of stainless steel AISI 430 and nodular cast iron was studied. The diffusion bonding was investigated experimentally under a protective atmosphere at various temperatures and a prescribed constant pressure blow which would cause macro-deformation. Microstructure examinations were carried out with the help of optical microscopy, SEM and EDS. Hardness values were measured by HV scale under a load of 200 g. At the bond interface cross section, decarburized and dechromised regions were observed on the ductile cast iron side and the stainless steel side respectively. Best mechanical and metallurgical properties were observed for the specimen bonded at 1100 °C process temperature at 15 MPa, for 20 min. As a function of the process temperature elevation, an increase of graphite atoms, migrating towards the interface, and a reduction of graphite atoms at the nodular graphite diameters was noticed.

Defect structure classification of neutron-irradiated graphite using supervised machine learning

Molecular dynamics simulations were performed to predict the behavior of graphite atoms under neutron irradiation using large-scale atomic/molecular massively parallel simulator (LAMMPS) package with adaptive intermolecular reactive empirical bond order (AIREBOM) potential. Defect structures of graphite were compared with results from previous studies by means of density functional theory (DFT) calculations. The quantitative relation between primary knock-on atom (PKA) energy and irradiation damage on graphite was calculated.and the effect of PKA direction on the amount of defects is estimated by counting displaced atoms. Defects are classified into four groups: structural defects, energy defects, vacancies, and near-defect structures, where a structural defect is further subdivided into six types by decision tree method which is one of the supervised machine learning techniques.

N, O-coupling towards the selectively electrochemical production of H2O2

Hydrogen peroxide (H2O2) synthesis generally involves the energy-intensive anthraquinone process. Alternatively, electrochemical synthesis provides a green, economical, and environmentally friendly route to prepare H2O2 via the two-electron oxygen reduction reaction, but this process requires efficient catalysts with high activity and selectivity simultaneously. Here, we report an N, O co-doped carbon xerogel-based electrocatalyst (NO-CX) prepared by a simple and economical method. The NO-CX catalyst exhibits a high H2O2 selectivity over 90% in a potential range of 0.2–0.6 V and a high H2O2 production rate of 1410 mmol gcat−1 h−1. The density functional theory calculations demonstrate that the coupling effect between N and O can effectively induce the redistribution of surface charge and the edge carbon atom adjacent to an ether group and a graphite nitrogen atom is the active site. This work provides a straightforward and low-cost process to produce highly selective H2O2 catalysts, which is in place for the expansion of electrocatalytic synthesis of useful chemicals.

Boosting graphene electrocatalytic efficiency in oxygen reduction reaction by mechanochemically induced low-temperature nitrogen doping

Mechanochemically induced low-temperature preparation of nitrogen-doped few-layer graphenes (N-Gr) and their application as electrocatalysts of oxygen reduction reaction (ORR) in alkaline medium was shown. The proposed approach allowed achieving a high proportion of graphitic nitrogen atoms. Such doping of N-Gr caused healing of vacancies in its structure, and also lead to the increased charge carrier density and high reduction ability that support advanced electrocatalytic characteristics in ORR (the onset and half-wave potentials equal to 0.98 and 0.77 V vs RHE, the average electron transfer number close to four, the Tafel slope about 60 mV/dec), the absolute values of which are close to those of the platinum-based catalyst. A model of N-Gr-based zinc-oxygen cell could provide a discharge current density of ∼40 mA/cm2 at a potential of 1 V and peak power density about 100 mW/cm2, as well as high cathode stability during cycling.

Investigating the effect of graphite pretreatment and contribution of the oxidizer in the synthesis of graphite oxide by hummers approach

Graphite oxide (GrO), the precursor of graphene oxide (GO) has been prepared by pre-treating graphite in a mixture of nitric and sulfuric acid, followed by chemical oxidation by a modified Hummers approach. The effect of pretreatment, KMnO4, and sulfuric acid on the chemical oxidation reaction was observed by characterizing GrO specimens on the bulk scale. KMnO4 oxidizes the carbon atoms by withdrawing electrons, instead of penetrating between the stacked carbon layers and directly interacting with the carbon atoms. Sulfuric acid and water molecules penetrate the interlayers of carbon planes and form the functionalities by nucleophilic attack on the charged carbon atoms. The pretreatment of graphite induces a small number of moieties, distributed evenly across the entire domain, serving as the primary reaction site of chemical oxidation. The inclusion of a small amount of functionalities marginally increases the lattice spacing and mildly intercalates the stacked carbon layers, facilitating the diffusion of acid and water molecules in the later step. The prepared GrO possesses only 24% graphitic carbon atoms and 11-layer of carbon stacking, confirming significant oxidation and intercalation.