Evolution mechanism of cyclized structure of PAN-based pre-oxidized fiber during low temperature carbonization process

The low temperature carbonization process is an important stage to realize the structural transition from the organic cyclized structure of PAN based pre-oxidized fiber to the inorganic pseudo-graphite structure of the ultimate carbon fiber. In the present paper, the evolution mechanism of cyclized structure and aggregation structure of PAN stabilized fiber during low temperature carbonization was studied by means of TGA, 13C-NMR, XRD, XPS and Raman. The results indicated that when the heat-treated temperature was lower than 450 °C, the mainly chemical reactions were the dehydrogenation and pyrolysis reactions in acyclic linear molecular chain or partial cyclized structure. At this stage, the growth of cyclized structure was not obvious. While the original ordered structure was destroyed gradually and the internal stress increased significantly. It induced the cyclized structure to be further oriented. When the temperature was higher than 450 °C, the polycondensation and reconstruction in aromatic heterocyclic structure was more important. The early aromatic heterocycles had many different structural scales, poor homogeneity and many defects in the heterocycles. At this stage, a new pseudo-graphite crystalline structure gradually formed and the d-spacing of graphite layer decreased slightly and crystallites size increased slowly with the increase of heat-treated temperature. When the temperature was higher than 550 °C, the pseudo-graphite base structure gradually formed. The d-spacing were further reduced slightly, and the crystallites size increased slowly. A new ordered basis structure was gradually developed into carbon fiber.

Metal-Organic Frameworks Derived Catalyst for High-Performance Vanadium Redox Flow Batteries

Vanadium redox flow battery (VRFB) is one of the most promising technologies for grid-scale energy storage applications because of its numerous attractive features. In this study, metal-organic frameworks (MOF)-derived catalysts (MDC) are fabricated using carbonization techniques at different sintering temperatures. Zirconium-based MOF-derived catalyst annealed at 900 °C exhibits the best electrochemical activity toward VO2+/VO2+ redox couple among all samples. Furthermore, the charge-discharge test confirms that the energy efficiency (EE) of the VRFB assembled with MOF-derived catalyst modified graphite felt (MDC-GF-900) is 3.9% more efficient than the VRFB using the pristine graphite felt at 100 mA cm−2. Moreover, MDC-GF-900 reveals 31% and 107% higher capacity than the pristine GF at 80 and 100 mA cm−2, respectively. The excellent performance of MDC-GF-900 results from the existence of oxygen-containing groups active sites, graphite structure with high conductivity embedded with zirconium oxide, and high specific surface area, which are critical points for promoting the vanadium redox reactions. Because of these advantages, MDC-GF-900 also possesses superior stability performance, which shows no decline of EE even after 100 cycles at 100 mA cm−2.

Experimental Investigations on Microwave-Current Assisted Sintering Process and Oxidation of Graphite Die at High Temperature

In the current-assisted sintering technique, graphite is mainly used to fabricate die and other components (such as electrodes and spacers) because of its excellent thermoelectric properties, high melting point and high ratio of the tensile strength to the compressive strength. As widely known, graphite is one of the brittle materials, and the failure is difficult to be anticipated before it happens. Besides, there is a lack of information about the effects of sintering process, environment and impurity on the graphite structure of the furnace, especially the die, which is the weakest part of the graphite structure. Therefore, the effects of electrical field and oxidation on the graphite die of microwave-current assisted sintering apparatus were investigated at a high temperature of 600-1900 °C based on physical characteristics and mechanical strength. In this article, the spark discharge phenomenon was experimentally proved during the sintering process of nonconductive material. The tensile strength of the upper punch after the sintering process was 20.2% higher than the pristine one because of the transforming of micro-graphite to carbon nanotubes which increased with increasing the temperature. On the other hand, the tensile strengths of graphite lower punch and sleeve were slightly dropped. While, the oxidation of GW-6S graphite in the air caused a mass loss that led to the reduction in tensile and compressive strengths.

Polyimide-Derived Carbon-Coated Li4Ti5O12 as High-Rate Anode Materials for Lithium Ion Batteries

Carbon-coated Li4Ti5O12 (LTO) has been prepared using polyimide (PI) as a carbon source via the thermal imidization of polyamic acid (PAA) followed by a carbonization process. In this study, the PI with different structures based on pyromellitic dianhydride (PMDA), 4,4′-oxydianiline (ODA), and p-phenylenediamine (p-PDA) moieties have been synthesized. The effect of the PI structure on the electrochemical performance of the carbon-coated LTO has been investigated. The results indicate that the molecular arrangement of PI can be improved when the rigid p-PDA units are introduced into the PI backbone. The carbons derived from the p-PDA-based PI show a more regular graphite structure with fewer defects and higher conductivity. As a result, the carbon-coated LTO exhibits a better rate performance with a discharge capacity of 137.5 mAh/g at 20 C, which is almost 1.5 times larger than that of bare LTO (94.4 mAh/g).

Iron Oxide-Coupled Graphite/Fe–Si Steel Structure for Analog Computing from Recycling Principle

Analog computing from recycling principle for next circular economy scenario has been studied with an iron oxide-coupled graphite/Fe–Si steel structure which was built using recycled waste materials, such as lead pencil and 3% Si steel (Fe–Si steel) foils. Proximity phenomena, such as disordered structure of iron oxide and magnetostriction-induced conduction, inside graphite lattice resulted in functional properties to advance analog architectures. Thermal oxidation was the synthesis route to produce iron oxide as coating film on Fe–Si steel foil, whose structure properties were validated by Raman spectroscopy where phase formation of hematite, α-Fe2O3, resulted as iron oxide thin-film. Three graphite layers with different compositions were also analyzed by Raman spectroscopy and used for studying electrical conduction in Fe–Si steel/α-Fe2O3/graphite structure from current–voltage plots at room temperature.

PRODUCTION OF CARBON NANOFIBERS BASED ON COAL TAR AND POLYACRYONITRILE BY ELECTROSPINNING METHOD

The article presents experiments on obtaining composite fibers based on Shubarkol coal tar (CT) and polycarlonitrile (PAN) by electrospinning in a laboratory setup. As a result of energy dispersive X-ray spectroscopy and SEM microscopy, the elemental composition (C-85.83%) and the diameter of the carbon fiber were determined, which ranged from 89.0 nm to 449.8 nm. The resulting CNF was subjected to oxidation in air at 300 °C, the holding time was 1 hour, after which the carbonization process was carried out at 800 °C, followed by cooling to room temperature. Raman spectra were recorded to study the degree of graphitization. The results of Raman scattering of light (RS) showed the degree of graphitization - 15.98%. Ratio I (D) / I (G) = 0.99, I (G) / I (D) = 1. The broad bands D (disordered part) and G (ordered graphite structure) suggest that CNFs contain partially graphitized carbon along with amorphous carbon. The ID / IG ratio represents the conversion of disordered carbon to graphite carbon during carbonization. The resistance of this material is 70-200 ohms. The results obtained confirm the semiconductor nature of the conductivity. On the basis of SEM drawings of CNFs from CT and PAN, it was found that the structure of CNFs after oxidation and carbonization retains the original fibrous structure. It was also found that the diameter of nanofibers decreases from 320.5 - 625.7 nm to 89-449.8 nm. Thus, the proposed method of obtaining CNF is built on the basis of the electrospinning method, which is the most promising method of industrial production.

Graphite Dendrites in Cast Iron and Their Fundamental Role in the Control of Morphology to Obtain Aero-Eutectic Graphite

This work analyzes the growth of graphite in the eutectic system of gray cast iron, focusing on laminar type A and undercooled type D morphology, and a modified morphology, such as vermicular or compact graphite. The objective of the study is to find an optimal graphite structure, from which a new class of lightweight materials results that has been called aero-eutectic graphite (AEG). The method to obtain AEG consists of dissolving the gray iron ferrous matrix by means of a chemical attack. From experiences of unidirectional solidification, it has been found that laminar graphite grows in a non-faceted way, coupled to austenite, while in vermicular the growth is through foliated dendrites. This characteristic allows vermicular graphite to have a higher specific intrinsic surface area. According to the Brunauer-Emmett-Teller (BET) analysis, the surface of the vermicular was 106.27 m2 g−1, while those corresponding to type A and D were 83.390 m2 g−1 and 89.670 m2 g−1, respectively. AEG with graphite type D was used as a cathode in Li-O2 batteries with satisfactory results, reaching more than 70 charge and discharge cycles, and 150 cycles at this time and still cycling, using Ru(bpy)3(ClO4)2 as redox mediator.

Atomic Structure and Binding of Carbon Atoms

Many studies discuss carbon-based materials because of the versatility of carbon. These studies include different ideas and discuss them within scientific scope and application. Depending on the processing conditions of carbon precursors, carbon exists in various allotropic forms. The electron transfer mechanism is responsible for converting the gaseous carbon atom into various states – graphite, nanotube, fullerene, diamond, lonsdaleite and graphene states. A typical energy shaped like parabola trajectory enables the transfer of the electron in carbon atom by preserving its equilibrium state. In the conversion of carbon atom from one state to other state, the trajectory of energy links to suitable filled and unfilled states of the east side, and the other trajectory of energy links to suitable filled and unfilled states of the west side. In this way, filled state electrons instantaneously and simultaneously transfer to unfilled states through the paths of involved typical energy trajectories. The involved typical energy remains partially conserved. Thus, the forces exerted to the electrons at the instant of transferring also remain partially conserved. Carbon atoms, in graphite, nanotube and fullerene states, partially evolve and partially develop the structures. Atoms form structures of one dimension, two dimensions and four dimensions, respectively. In the formation of such structures, binding atoms involve the typical energy shaped like parabola, where partially conserved forces also engage at the electron level. The graphite structure under only attained dynamics of atoms is also formed, but in the order of two dimensions and amorphous carbon. The binding energy among graphite atoms is due to the small difference of east force and west force. The structural formations in diamond, lonsdaleite and graphene atoms involve a different shaped typical energy to control the orientation of electrons undertaking one more clamp of the energy knot. The involved typical energy has a form like golf-stick, which is half of the parabola shaped trajectory. To undertake double clamping of energy knot, all four targeted electrons of the outer ring (of depositing diamond atom) aligned along the south pole, and all four unfilled energy knots of the outer ring (of deposited diamond atom) positioned along the east-west poles. Thus, the growth of diamond is found to be south to ground. The depositing diamond atom binds to the deposited diamond atom from ground to south. Thus, diamond atoms form the tetra-electron topological structure. Graphene atoms can form structure oppositely to diamond atoms. Binding of lonsdaleite atoms can be from ground to a bit south. To nucleate the structure of glassy carbon, three layers of carbon atoms having different state for each layer, i.e., gaseous, graphite and lonsdaleite, bind in successive manner. Mohs hardness of carbon nanostructures and microstructures is also sketched.

Genesis of Graphite in Metapelites in the North-Western Border of the Lypniazhka Massif (the Inhul Domain of the Ukrainian Shield)

This study presents results of an investigation of metamorphic rocks of the Inhul-Inhulets series located in the northwestern border of the Lypniazhka granite-migmatite massif (Inhul domain, the Ukrainian Shield). The rocks were studied petrographically and mineralogically and carbon isotope, Raman spectroscopic and microprobe measurements were made. Graphite and calcite were given special attention. Metapelites and quartz-rich graphite-biotite-garnet rocks were investigated. The former consist of biotite, graphite-biotite, amphibole-bearing graphite-biotite gneisses. Graphite in them is evenly distributed through the rock groundmass. The δС13 values of graphite lie between -39.4‰ and -33.6‰ (relative to PDB). The graphite is considered to be of biogenic origin. Quartz-rich graphite-biotite-garnet rocks are less common, but they also contain graphite. The latter occurs as inclusions in the major minerals either forming clusters. Its δС13 values fall between -28.45‰ and -22.2‰ (relative to PDB). Based on the Raman spectra, carbon from the gneisses has an ordered graphite structure. The temperature of graphite crystallization was estimated to be between 554 and 630°С and corresponds to the amphibolite facies.

Characterization of the Heterogeneous Evolution of the Nanostructure of Coal-Based Graphite

The graphitization of coal is complicated due to multiple factors, such as magmatic intrusions, tectonic stresses and the catalysis of minerals. Heterogeneous graphitization was found based on the nanostructural characterization of anthracite and coal-based graphite. It was determined that the graphitization of coal is not only the rearrangement of carbon layers but also the extinction of structural defects, as revealed by the evolution of XRD and Raman spectra and structural parameters (i.e., the interlayer spacing d002 and R2). Based on a comprehensive analysis of the nonstructural evolution of coal, the graphitization of coal could be divided into four stages at the nanoscale. The first stage (d002 > 0.344 nm and R2 < 0.7) is the transition process from coalification to graphitization, the second (0.337 nm < d002 ≤ 0.344 nm and R2 > 0.65) is the crystallization of carbon layers, the third stage (0.337 nm < d002 ≤ 0.344 nm and R2 ≤ 0.65) is characterized by the elimination of structural defects and straightness of carbon layers, and the fourth stage (d002 ≤ 0.337 nm and R2 ≤ 0.6) shows that the locally ordered graphite structure expanded to the whole sample.