Annealing Of Graphite Research Articles

Formation of composite SiC-C coatings on graphite via annealing Si-melt in CO

A method for the synthesis of silicon carbide–graphite composite coatings is proposed and described. The method is based on the two simultaneous chemical reactions involving graphite. One of these reactions consists of the chemical interaction of a silicon molten mass with carbon monoxide on the surface of the graphite at a temperature slightly exceeding the melting point of silicon. It transforms Si into gaseous silicon monoxide (SiO) and SiC. Simultaneously, a second reaction involves the SiO in the transformation of the graphite itself into SiC. A coating with a thickness exceeding 1 mm is formed during the synthesis. The resulting coating has high mechanical strength and hardness. Samples of the composite coating were investigated via scanning electron microscopy, energy-dispersive spectroscopy , Raman spectroscopy , XRD, and nanoindentation . The coating has a complicated structure. It consists of a dense overlayer of SiC and has a branched structure consisting of dendrite-like SiC crystals extending into the depth of the graphite. The dendrites are interspersed with large (up to 20 μm) single-crystalline SiC grains consisting of predominantly cubic polytypes. The coating leads to significant hardening of the material: the composite has a hardness of 28 GPa, which is ~254 times higher than the hardness of the original graphite surface. • A method is proposed for the formation of protective SiC coatings on graphite. • The method consists of annealing of graphite with Si melt in presence of CO. • Thick, up to 1 mm coatings are being formed in 20 min at low T (1450C).

Tin‐Containing Graphite for Sodium‐Ion Batteries and Hybrid Capacitors

AbstractThe limited Na‐storage capacity of graphite anodes for sodium‐ion batteries (∼110 mAh g−1) is significantly enhanced by the incorporation of nanosized Sn (17 wt%). The composite (SntGraphite), prepared by simple annealing of graphite with SnCl2, shows a specific capacity of 223 mAh g−1 (at 50 mA g−1) combined with excellent cycle life (i. e., 96 % of capacity retention after 2,200 cycles at 1 A g−1) and initial Coulomb efficiency (90 %). The combined storage of sodium in graphite (by solvent co‐intercalation) and Sn (by alloy formation) is followed by in situ X‐ray diffraction and in situ electrochemical dilatometry (ECD). While the additional tin almost doubles the electrode capacity, its contribution to the electrode expansion (∼3 %) is surprisingly small. The use of SntGraphite as anode for sodium‐ion hybrid capacitors with activated carbon as cathode provides a maximum energy and power density of ∼93 Wh kg−1 and 7.8 kW kg−1, with a capacity retention of ∼80 % after 8,000 cycles.

High-temperature annealing of graphite: A molecular dynamics study

A modified AIREBO potential was developed to simulate the effects of thermal annealing on the structure and physical properties of damaged graphite. AIREBO parameter modifications were made to reproduce Density Functional Theory interstitial results. These changes to the potential resulted in high-temperature annealing of the model, as measured by stored-energy reduction. These results show some resemblance to experimental high-temperature annealing results, and show promise that annealing effects in graphite are accessible with molecular dynamics and reactive potentials.

Nucleation and growth characteristics of graphite spheroids in bainite during graphitization annealing of a medium carbon steel

The evolution of microstructure in bainite during graphitization annealing at 680°C of Jominy-quenched bars of an Al-Si bearing medium carbon (0.4C wt%) steel has been studied and compared with that in martensite by using light, scanning and transmission electron microscopy. The results show that the graphitization process in bainite is different from that in martensite in many aspects such as the initial carbon state, the behavior of cementite, the nucleation-growth feature and kinetics of formation of graphite spheroids during graphitization annealing, and the shape, size and distribution of these graphite spheroids. The fact that the graphitization in bainite can produce more homogeneous graphite spheroids with more spherical shape and finer size in a shorter annealing time without the help of preexisting coring particles implies that bainite should be a better starting structure than martensite for making graphitic steel.

Formation of qualified epitaxial graphene on Si substrates using two-step heteroexpitaxy of C-terminated 3C-SiC(-1-1-1) on Si(110)

Formation of epitaxial graphene (EG) on 3C–SiC films heteroepitaxially grown on Si substrates, otherwise known as graphene-on-silicon (GOS) technology, has a high potential in future nanocarbon-based electronics. The EG's quality in GOS however remains mediocre due mostly to the high density of crystal defects in the 3C–SiC/Si films caused by the large (~20%) lattice-mismatch between Si and 3C–SiC crystals. Resultant Si out-diffusion along the planar defects during the high-temperature (~1525K) graphitization annealing can also account for the degradation. Here we propose a two-step growth technique that consists of seeding of rotated 3C-SiC(-1-1-1) crystallites on the Si(110) substrate, conducted in the high-temperature-low-pressure regime, followed by a rapid growth of SiC films in the low-temperature-high-pressure regime. We succeeded in forming an almost lattice-relaxed 3C-SiC(-1-1-1) film on Si(110), having a sufficient thickness (~200nm) that we believe is able to suppress the Si out-diffusion during graphitization. A graphitization annealing applied to this epi-film yields an EG, whose domain size is increased by 60% as compared to that of conventional GOS films.

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si.

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni3C/NiCx)/graphene/Ni/Ni silicides (Ni2Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene.

Oxygen-Induced Reduction of the Graphitization Temperature of SiC Surface

In the solid–vapor phase equilibria between SiC and O2 system, there exists a region where the reaction (2+x)SiC+O2→(2+x)Si↑+ 2CO↑+ xC↓ takes place [Y. W. Song and F. W. Smith: J. Am. Ceram. Soc. 88 (2005) 1864]. By tuning the temperature and the oxygen pressure used in the graphitization annealing into this region, we have succeeded in the growth of epitaxial graphene on SiC crystals at 1000 °C, which is lower, by 250 °C or more, than the conventional epitaxial graphene method. The method is especially useful to formation of epitaxial graphene on silicon (GOS), which requires a lower graphitization temperature because of the Si substrate as well as of its mission to attain compatibility with Si technology.

An electron microscopic study of spheroidal graphite nodules formed in a medium-carbon steel by annealing

The detailed evolution of the graphitized microstructure of carbon steel during annealing has been studied by thin-foil transmission electron microscopy, enabled by the refined dispersion of small graphite nodules produced. The graphitization annealing of 0.38 wt.% carbon steel in the quenched martensitic state resulted in the formation of graphite nodules, of the order of 2–5 μm in diameter, after 2–4 h. Two types of graphite nodules were observed to form: one possesses an irregular morphology and exhibits a coring particle (either a nitride or an oxide), whilst the other is more regularly spheroidal and apparently without a nucleating particle at its core. Analytical electron microscopy has been employed to characterize the microstructure of the graphite nodules formed, in order to provide a better understanding of their nucleation and growth, in particular, the mechanism of formation of the spheroidal graphite nodules without an apparent nucleating particle. For these more spherical nodules, the results showed that the outer mantles had a strong graphitic character, whilst the core regions exhibited a more amorphous carbon structure. It is deduced from the available experimental evidence that the nucleation sites most likely to result in this structure are dissolving cementite particles in the martensite.

New Nanocarbons: Rod Milling and Annealing of Graphite in the Presence of Yttrium

This work investigates the mechanisms of ordering and disordering graphite in the presence of 1% and 10% yttrium through mechanical activation and subsequent thermal annealing at 1350 °C for 6 h. Some work was done using other metals. Structural information was obtained by characterizing the milled and annealed samples by laser Raman spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) spectroscopy. In the initial stages (less than 72 h milling), yttrium assists in reducing the effects of rod milling on crystallite size but not in later equilibration between broken and reformed crystallites. Examination by TEM of all annealed samples with iron, cobalt, yttrium, or nickel revealed the formation of graphite, iron, cobalt, or nickel nanoparticles encapsulated with carbon strips but not for yttrium. No evidence of carbon nanotubes was found under the milling conditions for periods of 0−240 h and annealing temperatures up to 1350 °C for 6 h.

Interaction of oxygen with a-C:H, a-C films and other carbon materials

Magnetron sputtered films of a-C:H, a-C, as well as of a-CO x , and glassy carbon and graphite were annealed in air and vacuum at different temperatures (up to 770 °C). The a-CO x films (new type of amorphous graphite-like films) contain at most 18–22% of atomic oxygen depending on the way of their preparation. Annealing in air results in the removal of H from a-C:H films and in the incorporation of O in both a-C:H and a-C films as evidenced by IR spectra. Raman spectra show the up-shift of the G band (to approx. 1605 cm −1) in a-C and a-C:H films. Annealing of a-CO x films at 770 °C in vacuum shifts the G band down to ∼1595 cm −1 and decreases oxygen content sharply down to 5 at.%. Annealing of graphite and glassy carbon in air at 600 °C does not change their Raman spectra. The electron diffraction patterns for the a-CO x films are almost the same as for the a-C films, implying that oxygen occupies mainly the positions along the edges of graphite layers in every cluster and acts as an etchant. Its action decreases the intrinsic stress and leads to short-range order increase.

Process of graphitization in gray cast iron under pressure

1. Eutectic graphite of different morphologies is observed in cast hypoeutectic iron depending on the cooling rate. After hardening at atmospheric pressure we distinguished rosette, “twisted” flake, and point graphite, and after hardening under pressure we observed pseudoeutectic and fine-flake graphite. 2. Pressure promotes considerable deceleration of segregation of graphite independently of the cooling rate. The appearance of cementite and needle ledeburite in the structure of hypoeutectic cast iron under the action of pressure is connected with superposition of the stable and metastable transformation ranges and a change in the thermodynamic equilibrium between graphite and cementite in the metal. 3. Pressure applied to the hardening casting increases the proportion of bound carbon that segregates in the form of compact graphite inclusions in graphitization annealing, thus promoting an increase in the mechanical properties of the metal.

Ripening of subsurface amorphous C clusters formed by low energy He ion bombardment of graphite

Surfaces of highly oriented pyrolytic graphite (HOPG) were bombarded with 100 eV and 500 eV He ions at ion doses of a few 10 15 cm 2 and temperatures ranging from 300 K to 800 K. AFM images were recorded to investigate the topography of the surfaces after ion bombardment. Supplementary electron energy loss (EEL) and thermal desorption (TD) spectra were measured to determine the C sp 2 fraction of the bombarded surfaces and the amount of trapped He. The temperature at which He ion bombardment was performed had a drastic effect on the surface structure and topography of the targets on the angstrom-scale and micrometer-scale as well. At 300 K, limited defect atom transport revealed an amorphous but relatively flat HOPG surface. Bombardment at 400 K leads to a granular structure of small protrusions in micrometer-scale AFM images, however, without crystalline order on the surface. The protrusions are due to the formation of subsurface clusters of carbon formed by atoms displaced by ion irradiation. Towards higher temperatures during bombardment the clusters agglomerate and cause the surface layers to bend upwards in dome-like shapes. Simultaneously, the microscopic order of the graphite lattice recovers. At 800 K large areas of the top layer retain their order during bombardment, however, a small number of domes indicate that there still exist some subsurface C clusters. The cluster–cluster distance deduced from the dome distribution indicates that the clusters grow through a ripening process. Annealing of graphite at high temperatures subsequent to ion bombardment at low temperatures is much less effective for recovering the surface crystallinity than ion bombardment at high temperature.

過共析鋼における析出黒鉛の形態に関する研究

In the solid phase of the Fe-C alloy system, C atoms precipitate as graphite into defects such as voids in alloys. Therefore, the morphorogy and distribution of graphite may depend on those of the defects. In order to verify this assumption that had been previously presented by us and other investigators hyper-eutectoid Fe-C-Si alloys were heated to the austenite temperature range and held isothermally. Thereafter, the alloys were quenched in water or cooled to the graphitization temperature in a rate 0.17 K/s prior to graphitization.Results obtained are as follows:(1) In specimens containing Mg, spherical cavities less than 5 μm in size were observed and their number was decreased by the sweeping effect mainly due to migration of grain boundaries during the isothermal treatment. Micro-cracks were also observed in all quenched specimens.(2) After the graphitization annealing, graphites in these specimens had a shape and distribution similar to the above-mentioned defects.(3) It was suggested that the formation mechanism of graphites, which might be precipitated into spherical cavities and micro-cracks, would be substantially the same in these two cases, because characteristics of crystals resembled each the other in spite of much difference in their shapes in the observation of micro-structure of graphites.(4) These experimental facts would directly prove the above-mentioned assumption on the precipitation mechanism of graphite.

High-sulfur cast iron with high strength

1. Under certain conditions sulfur is a suitable alloying element for cast iron that increases the strength and ductility due to the fact that it slows down primary graphitization during solidification of the ingot and imparts a compact form to the graphite inclusions after graphitization annealing or self-annealing. 2. By a combination of simple technological measures it is possible to obtain castings which are cheaper than those of gray cast iron and with operating characteristics superior to those of malleable iron and high-strength cast iron.

Changes in the properties and composition of cementite in the process of graphitization

1. Cementite is enriched with manganese during graphitization annealing of white cast iron. 2. The change in the manganese content of cementite had no effect on the microhardness and is not accompanied by phase reconstruction. 3. The graphitization process during annealing of cast iron results from the dissociation of cementite directly, without formation of any intermediate carbide.

黒鉛鋼の研究（第1報）黒鉛鋼に及ぼすSiの影響

This paper deals with the graphitizing condition of graphitic steel (Graph-Sil), containing 1.1∼1.5% C and 0.7∼1.5% Si at elevated temperature. The experiments were carried out on forgeability, graphitization, change of hardness and microstructure of this steel by graphitization annealing. As is the case in gray cast iron, the silicon content of graphitic steel is the main factor controlling the precipitation of free graphite. However, to forge graphitic steel its silicon content must be held within definite limits. As above, the standard composition of graphitic steel is determined as 1.3∼1.5% C and 1.0∼1.5% Si and the suitable content of graphite is about 0.6%.