Large Graphite Flakes Research Articles

Study on the Interaction Dynamics of Magnesium and Molten Cast Iron: A Chill Casting Perspective

AbstractThis study presents a reassessment of the role of magnesium in the production process of cast iron. Through detailed experimentation involving an interrupted reaction process employing a chill casting approach in a copper mold, several reaction products that emerged during the high-temperature interaction between magnesium and cast iron were thoroughly examined. Two cast irons with different chemical compositions, notably different sulfur contents, were used to describe the contribution of magnesium during desulfurization and nodularization. The designed approach successfully revealed that rapid deoxidation and desulfurization reactions of cast iron involving liquid magnesium take place before following its ignition characteristics. Furthermore, the behavior of dissolved carbon concerning the remaining magnesium and magnesium-containing reaction products in molten iron is also empirically identified. Specifically, large graphite flakes and an indication of solidification behavior shifting are circumstantially detected in the magnesium-rich regions, which, to a certain extent, can provide additional insight into the role of magnesium during graphite nucleation.

Delamination and defects in graphene nanosheets exfoliated from 3D precursors

Delaminating Van der Waals-based three-dimensional (3D) solid precursors into ultra-thin sheets of layered materials using liquid-phase exfoliation gained global interest due to the process' simplicity and industrial applicability. The characteristics of exfoliated graphene depend on the processing technique and starting precursor materials' properties. In this work, graphene is stabilized after exfoliation in a water/surfactant medium where 3D graphite of various sizes, including large flakes (∼700 μm) to fine powders (∼8 μm) are used as initial precursors in ultrasonication (bath and probe). The graphene concentration exhibited a peak-like and flat trend in these processes. After exfoliating from different-sized graphite, the nanosheet's morphology is assessed extensively by electron microscopy, and defects are probed by Raman spectroscopy. Large graphite flakes (∼700 μm and <N > 17000) produced nanosheets with high aspect ratios (∼700) with average lateral size of ∼1 μm and <N > 4 ± 1. Surprisingly, graphite with the size of 8 μm and <N > 1235 also produced graphene with lateral size of ∼1 μm and <N > 8 ± 5. Graphite with a large particle size undergoes significant fragmentation and then exfoliation. In contrast, fragmentation is less when the small-sized graphite undergo exfoliation. This study provides insights into the role of initial 3D precursors in creating 2D materials.

Genesis of the Tianping flake graphite deposit at the western margin of Yangtze Block, SW China

The Tianping flake graphite deposit is located in the western margin of the Yangtze Block, South China. It is one of the biggest and representative flake graphite deposits with 8 million tons of flake graphite in the Kangdian Axis graphite metallogenic belt. The graphite orebodies are hosted in graphite schist and mica quartz schist. Carbon isotope data for graphite ore show light carbon values between −26.3‰ to −28.8‰ which suggests a biogenic origin. The carbonaceous materials of the Tianping graphite deposit were deposited younger than ca. 833 Ma according to U-Pb dating results of detrital zircons in graphite samples. The calculated metamorphic temperature of graphite from Raman spectra is between 414 and 436 ± 50 °C, indicating a greenschist to amphibolite facies metamorphism is responsible for the transformation of disordered carbonaceous matter into well-crystallized graphite. After carbonaceous material deposition and graphite formation by regional metamorphism, the graphite may recrystallize and form large graphite flakes in a heated environment brought by the intrusion of plutons in the region. Based on previous studies and new data, a three-stage genesis model including processes of material deposition, regional metamorphism, later magmatism, and contact metamorphism for Tianping flake graphite deposit is proposed.

Thermo-Mechanical Fatigue of Grey Cast Iron for Cylinder Heads - Effect of Niobium, Molybdenum and Solidification Time

Grey iron alloyed with molybdenum and niobium in seven different compositions has been casted using three, in industrial components viable, solidification times which resulted in 21 different samples. The samples have been investigated with respect to microstructure, static properties and thermo-mechanical fatigue performance. It was found that the solidification time is very important for both the static and thermo-mechanical performance. If the solidification time is long the properties are controlled entirely by the large graphite flakes and there is no influence of the alloying elements. On the other hand if the solidification time can be kept short the need for alloying elements may be removed. For the shorter solidification times an influence from the matrix and thus the alloying elements can be seen. It was found that molybdenum enhances TMF-life while no such effect was found for niobium. Niobium, on the other hand, has a larger effect on static strength than molybdenum and also on the cyclic stress in the thermo-mechanical fatigue experiments.

Efficient Graphene Production by Combined Bipolar Electrochemical Intercalation and High-Shear Exfoliation.

In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the ID/IG ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale.

Superlubricity of graphene nanoribbons on gold surfaces.

The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of devices. Superlubricity, as detected with atomic force microscopy, appears when sliding large graphite flakes or gold nanoclusters across surfaces, for example. However, the origin of the behavior is poorly understood because of the lack of a controllable nanocontact. We demonstrated the superlubricity of graphene nanoribbons when sliding on gold with a joint experimental and computational approach. The atomically well-defined contact allows us to trace the origin of superlubricity, unraveling the role played by ribbon size and elasticity, as well as by surface reconstruction. Our results pave the way to the scale-up of superlubricity and thus to the realization of frictionless coatings.

Simple Preparation of Exfoliated Graphene Oxide Sheets via Simplified Hummer’s Method

Today, research on graphene and other two-dimensional sp2-hybridized carbon nanomaterials has tremendously impacted the areas of modern chemistry, physics, and materials science and engineering. The significant attraction of these materials can be attributed to the outstanding electrical, optical, electrochemical, and mechanical properties of graphene-like materials, especially in comparison to other carbon materials. In this manner, graphene oxide as a substrate for graphene-like materials reduction process is getting more and more interesting. Although early routes to these materials were challenging, significant advances in synthetic and processing methods have enabled access to high-quality exfoliated graphene oxide sheets in appreciable quantities. Herein, we introduced a simple and efficient method for the high-conversion preparation of graphene oxide using a simplified hummer’s method from large graphite flakes (an average flake size of 100 μm). One-pot chemical oxidation of graphite was carried out at room temperature for the preparation purpose. It was found that different degree of oxidation of graphite flakes could be realized by stirring graphite in a mixture of sulphuric acid and potassium permanganate under different oxidation durations, resulting in exfoliated graphene oxide sheets with large lateral dimension and area. The simplified Hummer’s method provides a facile approach for the preparation of large-area exfoliated graphene oxide sheets.

Green preparation of reduced graphene oxide using a natural reducing agent

A simple and efficient method was introduced for the high-conversion preparation of graphene oxide (GO) from large graphite flakes (average flake size=100μm) using a simplified Hummer׳s method. Natural reducing agents such as lemon juice and vinegar were compared with hydrazine (N2H4) as potential reducing agents. Graphene was prepared by chemical reduction of GO because this method was low cost and could be used for large-scale graphene production. This one-pot graphene preparation was performed at room temperature. Different degrees of oxidation of graphite flakes were obtained by stirring graphite in a mixture of sulfuric acid and potassium permanganate at different oxidation times, and highly exfoliated GO sheets were produced. GO was subsequently reduced effectively by lemon juice, a new, green, and potential reducing agent with pH 2.3. This reduced GO exhibited a high electrical conductance of 24.6μS attributed to its higher C/O ratio (≈8:2) compared with other samples.

Melting textures and microdiamonds preserved in graphite pseudomorphs from the Beni Bousera peridotite massif, Morocco

Over 30 graphite aggregates that represent pseudomorphs after diamond were manually extracted from a garnet pyroxenite layer in the Beni Bousera peridotite massif, northern Morocco. The inclusions present in the aggregates were characterized by combining scanning electron microscopy, micro-Raman spectroscopy and cathodoluminescence. Large composite clinopyroxene–orthopyroxene–garnet inclusions ( ca . 500 μm across) are common in the core of the graphite aggregates. Silicate films with a thickness of a few micrometres occur intercalated between graphite flakes throughout each aggregate. They are of basaltic composition and are interpreted as partial melts formed by in situ melting of the large composite inclusions and, possibly, of the host pyroxenite, during the Beni Bousera massif uplift. In addition, various solid inclusions composed of chlorides, sulphates and carbonates are found to be evenly distributed irrespective of the graphite aggregate texture (coarse in the core, in some instances fine-grained on the rim). Diamond crystals, 0.5–2 μm in size, were also observed in several aggregates, apparently included in large graphite flakes, and were characterized using cathodoluminescence and Raman micro-spectroscopies. They are interpreted as relics of large mantle-stage diamonds, now heavily graphitized. This finding confirms earlier propositions that the graphite aggregates in Beni Bousera and Ronda garnet pyroxenites are pseudomorphs after diamond and raises questions on the kinetics of graphitization.

Simple room-temperature preparation of high-yield large-area graphene oxide

Graphene has attracted much attention from researchers due to its interesting mechanical, electrochemical, and electronic properties. It has many potential applications such as polymer filler, sensor, energy conversion, and energy storage devices. Graphene-based nanocomposites are under an intense spotlight amongst researchers. A large amount of graphene is required for preparation of such samples. Lately, graphene-based materials have been the target for fundamental life science investigations. Despite graphene being a much sought-after raw material, the drawbacks in the preparation of graphene are that it is a challenge amongst researchers to produce this material in a scalable quantity and that there is a concern about its safety. Thus, a simple and efficient method for the preparation of graphene oxide (GO) is greatly desired to address these problems. In this work, one-pot chemical oxidation of graphite was carried out at room temperature for the preparation of large-area GO with ~100% conversion. This high-conversion preparation of large-area GO was achieved using a simplified Hummer’s method from large graphite flakes (an average flake size of 500 μm). It was found that a high degree of oxidation of graphite could be realized by stirring graphite in a mixture of acids and potassium permanganate, resulting in GO with large lateral dimension and area, which could reach up to 120 μm and ~8000 μm2, respectively. The simplified Hummer’s method provides a facile approach for the preparation of large-area GO.

Mechanical properties of pyrolytic graphite flakes

Large graphite flakes (5–50 μm thick × 1–10 cm in length × ~ 1.5 cm in width) can be prepared via the decomposition of aromatic and heteroaromatic hydrocarbons. This work describes the dependence of the mechanical properties of benzene (B)- and pyridine (PR)-derived graphite flakes on heat-treatment temperature (HTT) and nature of precursor. The tensile strength and modulus of the flakes increase with increasing HTT, and are in the range 10–80 MPa for tensile strength, 5–10 GPa for Young's modulus, and l%–2% for strain at failure. The flexural strength of the graphite flakes rapidly increases from ~0.3 GPa for the 800°C product to 20 GPa at an HTT of 2600°C. At low HTT, the tensile strength and strain to failure of PR-derived flakes are greater while at high HTT B-derived flakes are generally stronger than those derived from PR.

鑄鐵製電氣抵抗體の研究

Electrical properties of iron grid materials containing 3% C and 3-6% Si were discussed metallographically, and the following results were obtained.1) Assuming the materials as Fe-C-Si ternary alloys, the specific electrical resistance was calculated theoretically in the case of two phase alloys consisting of both silico-ferrite and graphite or cementite, and it was clarified that the specific resistance is influenced significantly by the amount, form and distribution of graphite even in cast iron of the same chemical composition.2) Ferrite enhances the electric resistance as the concentration of silicon in the solid solution increases. As the electric resistance of graphite is very high as compared with that of ferrite, irons high in both carbon and silicon are suitable so far as the electric resistance is concerned.3) The higher the concentration of silicon in ferrite, the less becomes its temperature coefficient of the electric resistance. Besides, the value of the coefficient in graphite has negative sign. The temperature coefficient of irons, therefore, is less in the case of high silicon and high carbon irons having large graphite flakes.4) While the values of specific resistance become high, their temperature coefficients become low in slowly solidified irons containing much silicon, because of the existence of flaky graphite and high silicon ferrite.5) By the graphitization treatment of cementite-containing irons in the as-cast condition, the amount of graphite increases and the concentration of silicon in ferrite decreases. The increasing amount of graphite enhances the electric resistance, while lowers its temperature coefficient. On the contrary, the lowering of silicon concentration in ferrite causes to decrease electric resistance and to increase its temperature coefficient. As the result of the above mentioned opposite actions, the graphitization of irons generally causes both specific resistance and its temperature coefficient to increase.