Graphitization Temperature Research Articles

Effect of graphitization temperature on microstructure, mechanical and ablative properties of C/C composites with pitch and pyrocarbon dual-matrix

Two graphitized C/C composites with a density of 1.83 g/cm3 were fabricated by CVI, high-pressure liquid-phase carbonization, and graphitization process. After graphitization at 2500 and 3000 °C, the graphitization degree of carbon phases in both composites shows a trend of pitch carbon > pyrocarbon > PAN carbon fiber. With increasing graphitization temperature, the orderly arrangement and growth of the graphite microcrystals cause the improved graphitization degree of the composites. Therefore, the thermal conductivity increases from 68.78 to 126.09 W/(m·K) in the transverse direction and 31.09 to 75.60 W/(m·K) in the thickness direction. Meanwhile, the increase in micro-cracks and pores formed at the interface induced the declined flexural and compressive strength, with the corresponding strength reducing to 104.17 and 113.92 MPa, respectively. During the first 22 s of ablation, the improved graphitization degree and higher thermal conductivity can transfer the heat rapidly, resulting in a lower surface temperature and a higher backside temperature. After 30 s ablation, the mass ablation rate decreases from 0.0030 to 0.0018 g/s, while the linear ablation rate increases from 0.0040 to 0.0093 mm/s due to the increased open porosity and defects under higher graphitization temperature.

Cold sintering of Van Der Waals layered compounds

Van der Waals (vdW) layered materials have a wide range of applications and their bulks are commonly prepared using energy intensive pressure assisted sintering. For example, in the case of graphite or boron nitride temperatures in excess of 2000 °C are required. In this work, for the first time, we demonstrated that most of the vdW layered materials (i.e., Graphite, h-BN, Bi2Te3, MoS2, WS2 and SnS2) can be consolidated at room temperature under an uniaxial pressure ranging between 100 and 800 MPa. The consolidation is driven interplanar slipping, while inter-flake adhesion relies on the formation of vdW bonds. Large flakes exceeding 100 μm in size results in improved consolidation due to their high tap density (in the range of 70–80% of the relative density) and facilitated interplanar slipping under the applied uniaxial pressure. Despite its obvious advantages, the cold sintering of vdW layered compounds remains unexplored and this work is an initial attempt to generalize its feasibility.

Physics Evaluation of Alternative Uranium-Based Oxy-Carbide Annular Fuel Concepts for Potential Use in Compact High-Temperature Gas-Cooled Reactors

Abstract Lattice physics calculations have been carried out to evaluate the performance and safety characteristics of a modified high temperature gas-cooled reactor (HTGR) prismatic fuel block concept, based on the MHTGR-350 benchmark problem. Key changes were to replace the conventional tri-structural isotropic (TRISO)-filled fuel compacts with heterogeneous, multilayer annular fuel pellets made with UCO, ThCO, or (U,Th)CO. These fuel pellets have multiple protective cladding layers of pyrolytic carbon and silicon carbide, which will give it robust qualities. With the increased loading of U-235 in the fuel block, it was necessary to replace up to 78 fuel holes and 42 coolant holes with a hydrogen-based moderator (7LiH), in order to ensure a thermal neutron energy spectrum in the lattice. Calculation results demonstrate that the modified fuel concept has several advantages and some challenges relative to the conventional MHTGR-350 design concept. With the increased uranium loading and the reduced neutron leakage due the use of 7LiH moderator rods, higher burnup levels and lower natural uranium consumption levels can be achieved with the same level of uranium enrichment. In addition, the expected fuel residence time increased by a factor of 20 or more, making such a concept very attractive for use in small, modular, “nuclear battery” HTGRs that would only need to be fueled once. Calculation results for the current concept indicate positive graphite and hydrogen moderator temperature coefficients, and further modifications will be required to ensure a negative power coefficient of reactivity.

Shock degree and graphite geothermometry in ureilites NWA 6871 and NWA 3140

AbstractCarbon aggregates from two differently shocked ureilites were analyzed to gain insight into the shock transformation of graphite to diamond in ureilites, which happened when the ureilite parent body (UPB) was most likely destroyed by massive impact events. We present data for carbon aggregates from the highly shocked (U‐S6) Northwest Africa (NWA) 6871 and the medium shocked (U‐S3) NWA 3140. Both samples contain abundant carbon aggregates which were analyzed by X‐ray diffraction and micro‐Raman spectroscopy revealing the presence of close associations of (compressed) nanographite, micro‐ and nanodiamond, as well as Fe‐rich phases. Graphite and diamond in NWA 6871 show shock indicators that are absent in NWA 3140. Based on Raman geothermometry on graphite, we calculated mean temperatures of 1368 ± 120 °C and 1370 ± 120 °C for NWA 3140 and NWA 6871, respectively. For comparison, a geothermometer based on the partitioning of Cr between olivine and low‐Ca pyroxene was applied on NWA 3140, which yielded a temperature of only 1215 ± 16 °C. The graphite‐based temperatures are the highest reported for graphite in ureilites so far and exceed calculated magmatic temperatures for ureilites from silicate‐ and chromite‐based geothermometers. Graphite temperatures fall into the temperature field of catalytic diamond synthesis, which supports the hypothesis of direct transformation from graphite to diamond upon shock. Although the temperatures estimated seem to be independent of the shock degree, they can be ascribed to the shock event that destroyed the UPB.

In-Situ Doping B4C Nanoparticles in Mesophase Pitch for Preparing Carbon Fibers with High Thermal Conductivity by Boron Catalytic Graphitization.

The boron carbide (B4C) nanoparticles doping mesophase pitch (MP) was synthesized by the in-situ doping method with tetrahydrofuran solvent, and the corresponding MP−based carbon fibers (CFs) were successfully prepared through the melt−spinning, stabilization, carbonization and graphitization processes. The structural evolution and properties of boron−containing pitches and fibers in different processes were investigated for exploring the effect of B4C on mechanical, electrical and thermal properties of CFs. The results showed that the B4C was evenly dispersed in pitch fibers to provide active sites of oxygen, resulting in a homogeneous stabilization and ameliorating the split−ting microstructures of CFs. Moreover, the thermal conductivity of B1−MP−CF prepared with 1 wt.% B4C increased to 1051 W/m•K, which was much higher than that of B0−MP−CF prepared without B4C (659 W/m•K). While the tensile strength of B4C−doped CFs was lower than that of pristine CFs. In addition, a linear relationship equation between the graphite microcrystallite parameter (ID/IG) calculated from Raman spectra and the thermal conductivity (λ) calculated according to the electrical resistivity was found, which was beneficial to understand the thermal properties of CFs. Therefore, the doping B4C nanoparticles in MP did play a significant role in reducing the graphitization temperatures due to the boron catalytic graphitization but decreasing the mechanical properties due to the introduction of impurities.

Upcycling of Plastic Wastes and Biomass for Sustainable Graphitic Carbon Production: A Critical Review.

Upcycling of waste plastics diverts plastics from landfill, which helps in reducing greenhouse gas emissions. Graphitic carbon is an interesting material with a wide range of applications in electronics, energy storage, fuel cells, and even as advanced fillers for polymer composites. It is a very strong and highly conductive material consisting of weakly bound graphene layers arranged in a hexagonal structure. There are different ways of synthesizing graphitic carbons, of which the co-pyrolysis of biomass and plastic wastes is a promising approach for large-scale production. Highly graphitized carbon with surface areas in the range of 201 m2/g was produced from the co-pyrolysis of polyethylene and pinewood at 600 °C. Similarly, porous carbon having a superior discharge capacity (290 mAh/g) was developed from the co-pyrolysis of sugar cane and plastic polymers with catalysts. The addition of plastic wastes including polyethylene and high-density polyethylene to the pyrolysis of biomass tends to increase the surface area and improve the discharge capacity of the produced graphitic carbons. Likewise, temperature plays an important role in enhancing the carbon content and thereby the quality of the graphitic carbon during the co-pyrolysis process. The application of metal catalysts can reduce the graphitization temperature while at the same time improve the quality of the graphitic carbon by increasing the carbon contents. This work reports some typical graphitic carbon preparation methods from the co-pyrolysis of biomass and plastic wastes for the first time including thermochemical methods, exfoliation methods, template-based production methods, and salt-based methods. The factors affecting the graphitic char quality during the conversion processes are reviewed critically. Moreover, the current state-of-the-art characterization technologies such as Raman, scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy are discussed in detail, and finally, an overview on the applications, scalability, and future trends of graphitic-like carbons is highlighted.

Factors Affecting the Electrochemical Behaviors of Graphene-Like Graphite as a Positive Electrode of a Dual-Ion Battery.

Charge-discharge behaviors of various graphene-like graphite (GLG) samples have been investigated, and factors affecting them were discussed. By changing the oxidation method of graphite to obtain the precursor material of graphite oxide and heat treatment temperature of it, GLG samples with various structural parameters were successfully prepared. The onset potential of intercalation changed mainly depending on the interlayer spacing and decreased with its increase. The oxygen content was also important especially for GLG with smaller interlayer spacings. The influence of oxygen became apparent when the intercalation of bis(fluorosulfonyl)amide ions was performed. The onset potential considerably decreased for GLG with larger oxygen contents. The capacity of GLG increased with the increase in oxygen content to reach a maximum value of 149 mAh g-1 and then slightly decreased.

Effect of stress state on graphitization behavior of diamond under high pressure and high temperature

Pure diamond powder and mixed powder of diamond and sodium chloride were treated at high pressures (5 GPa and 10 GPa) and high temperatures (1000–2700 °C), and then the graphitization behavior of diamond powder under non-hydrostatic (pure diamond powder) and quasi-hydrostatic (mixed powder of diamond and sodium chloride) compression was quantitatively studied by analyzing the X-ray diffraction peaks' area of treated samples. It is found that the graphitization under non-hydrostatic compression is much more easily triggered by heating due to the heterogeneous stress distribution within the diamond grains. Under the quasi-hydrostatic compression of 5 GPa, the starting graphitization temperature is about 1600 °C, which is 300 °C higher than that under non-hydrostatic compression. Under a pressure of 10 GPa, graphitization could be observed after pure diamond powder was treated at 1400 °C, but no graphitization occurred in the mixed powder even after a high temperature treatment at 2300 °C. Optimizing the stress distribution among diamond grains by the mixing of grain-size can significantly reduce the graphitization at high temperature even under non-hydrostatic compression.

Research on the normal emissivity of graphite between 150 and 500 °C by an infrared camera for nuclear fusion devices

• An emissivity measurement equipment by using an infrared camera is constructed and calibrated by a blackbody furnace with a suitable emissivity model. • By using an infrared camera with no filter (band wavelength range of 7.5 μm–13 μm), a 10.2 ± 0.1 μm filter, and a 10.8 ± 0.19 μm filter, the relationship between the temperature and emissivity of graphite and the residual between data and model are obtained respectively and compared. • The emissivity of graphite at different roughness is studied, and uncertainty quantification is used to provide an error estimate for the resulting curves. Due to the advantages of high-temperature resistance, low thermal expansion coefficient, and thermal shock resistance, etc., graphite is regarded as one of the materials for the first wall of nuclear fusion devices. The real-time monitoring of temperature distribution of the first wall based on an infrared camera is widely used to guarantee the stable operation of devices. To accurately measure the temperature of the first wall, it is important to measure the normal emissivity of graphite. However, the emissivity of graphite is affected by many factors. For example, the effect of the surface layers, like carbon-hydro layers-layers, will influence the measurement of the emissivity. The ambition of this work is to study the relationship between graphite emissivity and infrared camera with filters of different wavelengths and the roughness of graphite, and then give the proposed emissivity model to improve the accuracy of graphite surface temperature measurement. The experiment is carried out with a FLIR infrared camera and calibrated by using a blackbody furnace. The emissivity at given temperatures is systematically investigated. By using an infrared camera with no filter (band wavelength range of 7.5–13 μm), a 10.2 ± 0.1 μm filter, and a 10.8 ± 0.19 μm filter, this paper studies the relationship between the temperature and emissivity of graphite from 150 °C to 500 °C and calculates the relative error of temperature measurement respectively. A suitable fitting model which fits emissivity curves is selected. The measurement results show the use of a 10.2 μm filter to measure the graphite surface temperature has the smallest relative error, the range is 0.90%∼1.17%. The emissivity of graphite with different roughness is also studied. At the same temperature, the greater the roughness of the graphite surface, the higher the emissivity value. The uncertainty of the emissivity measurement is analyzed, and its value is 0.0099–0.0211. The graphite surface temperature measurement method based on the emissivity model proposed in this paper provides useful references for the non-contact temperature measurement of the first wall in nuclear fusion devices. The research results can be used as a reference for plasma experiments in some fusion devices and other similar applications.

Effect of ultra-high temperature treatment on microstructures and mechanical properties of TRISO particles

Tristructural-isotropic (TRISO) fuel particles have long been widely used in high temperature gas-cooled reactors (HTGR), where their high temperature mechanical integrity and ability to retain fission products even in the accident conditions are the critical safety issues. In the present work, TRISO particles were subjected to ultra-high temperature thermal treatments at 1900, 2000, and 2200 °C, respectively, which were much higher than the projected accident temperature (1620 °C). Phase compositions and microstructures of the samples before and after thermal treatments were studied by X-ray diffraction (XRD), Raman spectroscopy, and Scanning Electron Microscopy (SEM). Nano hardness and Young's modulus of the SiC layer in TRISO particles were studied by nano indentation technique, and crushing strength of the TRISO particles was evaluated by mechanical crush tests. The results show that, there were no obvious temperature effects on phase composition and microstructure, except for SiC layer in the sample thermal treated at 2200 °C, in which many pores caused by SiC decomposition were observed. The nano hardness and Young's modulus values of SiC layer in the as-received and thermally treated TRISO particles were similar within the experimental error. On the other hand, the crushing strength of the thermally treated TRISO particles decreased greatly as compared with the as-received particles, but the crack propagation mode changed from through-the-particle to deflection inside the buffer PyC layer or between buffer PyC and inner PyC (IPyC) layers. The change of crack propagation mode could probably due to the shrinkage of PyC layers due to anisotropy at high temperature and ordering of graphitization of IPyC layers, which probably weakened the interfacial bonding between the IPyC and the buffer PyC layers. The results of this study provide a database reference for the performance of TRISO particles under ultra-high temperature operation and even accident conditions.

Graphitization of flax shive cellulose modified by citric acid: properties of magnetic composites and their dye adsorption capacity

Magnetic composites were synthesized in an inert medium within the temperature range T = 500–1000 °C by one-stage pyrolysis of microcrystalline flax shive cellulose modified by citric acid (MCCA) and homogeneously impregnated with Fe(NO3)3. Catalytic graphitization was studied by the thermogravimetric analysis method. The crystal structure, pore development, morphological structure and surface chemistry of carbon composites were characterized by the N2 adsorption-desorption, XRD, SEM and FTIR methods. The onset temperature of MCCA graphitization was found to be equal to 600 °C. The magnetic carbon materials were tested as adsorbents for removal of anionic methyl orange (MO) and cationic methylene blue (MB) dyes. The highest equilibrium adsorption capacity in MB and MO was found in the magnetic composites synthesized at 700 and 800 °C.

Influence of thermal treatment atmosphere on N-doped carbon spheres for wastewater treatment and supercapacitor applications

Carbon-based materials have emerged as some of the most proficient materials for diverse applications including wastewater treatment and energy storage devices. In the present report, N-doped carbon spheres synthesized from soy flour and glucose were thermally treated at 900 °C in argon (Ar) or nitrogen (N2) atmospheres. Apart from the morphological changes the samples also undergo graphitization as well as conversion of the doped-N functionalities to more stable functionalities. The thermal treatment under inert gas for both the cases resulted in porous samples with micropores. Ar atmosphere resulted in larger diameter micropores (>1 nm) as compared with N2 (0.8 nm). Pore formation mechanism has been proposed for differences in the micropore size and distribution as well as graphitization temperature for the two gases. The adsorption and photocatalytic performance of carbons were studied using neutral (p-nitrophenol, PNP), cationic (methylene blue, MB), and anionic (eriochrome black-T, EBT) model pollutants. The sample heat-treated under Ar showed maximum %decolorization under dark as well as UV irradiation. The kinetics of the adsorption as well as adsorption mechanisms of model pollutants onto the surface of the heat-treated samples has been studied. The supercapacitve performance of all the samples was studied using cyclic voltametery and galvanostatic charge−discharge curves. Ar heat-treated sample showed the best supercapacitve performance with a total capacitance of 59.06 F/g.

Study on Structural Evolution of Synthetic Graphite Derived from Lignite Prepared by High Temperature–High Pressure Method

Abundant and cheap lignite is regarded as inferior fuel due to its disadvantages such as low heat and high moisture. In order to realize the high value and clean, non-fuel utilization of lignite, we used lignite as precursor to prepare high-quality synthetic graphite through graphitization treatment using the high temperature–high pressure (HTHP) method, and afterwards characterized the samples by XRD, Raman, FTIR, SEM-EDS, and HRTEM, and systematically analyzed the effect of P-T conditions on the structure evolution of lignite. Our results demonstrated that temperature dominates the graphitization of lignite, and high pressure accelerates the graphitization process of lignite. Under HTHP treatment, the aromatic layer of lignite accelerates directional rearrangement; the ordered domain increases rapidly, and the ductility and stacking degree of the carbon sheets of the lignite sample are greatly enhanced. Compared with the traditional high-temperature graphitization method, the HTHP method greatly reduces the graphitization temperature and shortens the graphitization time. Remarkably, the as-prepared graphite with a graphitization degree of 91.87% superior to graphite fabricated by the conventional high-temperature processing were rapidly prepared from lignite at 6 GPa and 1300 °C in 20 min. This study demonstrates that the HTHP method is a feasible and effective method to realize the green, efficient, and high value utilization of lignite.

Thermo-mechanical limits of a magnetically driven fast-ion loss detector in the ASDEX Upgrade tokamak

A real-time control system is being developed for a magnetically driven Fast-Ion Loss Detector (FILD) at the ASDEX Upgrade tokamak. The insertion of the diagnostic head will be adjusted in real-time to react to changes in the graphite head temperature, plasma position and appearance of MHD instabilities. The graphite probe head of the detector is exposed to an intense heat flux (located ∼3–5 cm from separatrix). The control algorithm performance is constrained by: the graphite head sublimation temperature, the ultimate stress limit, the reaction time of the controller and the retraction time. In this work, the temperature and thermal induced stress distribution on the probe head are assessed to determine what temperature-related magnitude is the limiting factor. The heat flux at the probe head has been estimated using the time-averaged parallel heat flux measured at the divertor target via infrared thermography. A field line tracing algorithm determines which regions of the probe head receives a weighted heat flux due to shadowing (self-induced or from other structures) and the incidence angle of the field lines. A finite element model is used to simulate the temporal evolution of the graphite head temperature and to obtain the induced thermal stress. The temperature spatial distribution at the probe head is validated against measurements of the probe head for different FILD systems showing a good agreement. These measurements have been obtained from visible cameras with an infrared filter. The model concludes that the maximum stress (∼100 MPa) does not overcome the graphite mechanical limit (170 MPa) and that the probe head is not affected by fatigue. Therefore, the graphite sublimation temperature (2000 °C) is set as the limiting factor of the new control system.

Ultrahigh thermal and electric conductive graphite films prepared by g-C3N4 catalyzed graphitization of polyimide films

• High-quality graphite film with high thermal and electric conductivity was obtained. • Using g-C 3 N 4 as catalyst can reduce the graphitization temperature and save energy. • Using g-C 3 N 4 as catalyst can reduce the graphitization temperature. • The new processes of the graphitization can save 20.5% energy. High-efficiency heat-dissipation materials are in urgent need in microelectronics industry in 5G era. However, large-scale, low-cost, and high-efficiency preparation technologies still have great challenges. Here, ultrahigh thermal and electric conductive graphite films prepared by graphite-phase carbon nitride (g-C 3 N 4 ) catalyzed graphitization of polyimide film (PI) were reported for the first time. The graphite film obtained from 0.25 wt% g-C 3 N 4 catalyzed polyimide graphitization (G0.25) has homogeneous and perfect graphene structure and state-of-the-art properties. The d -spacing of the graphite layers is about 0.335 nm, which is similar to the ideal layer thickness of graphene (0.334 nm). The thermal conductivity is 1781 W·m −1 ·K −1 and the conductivity is 1.63 × 10 6 S·m −1 , which is 1.49 and 2.18 times of graphite film from pure PI film, 1.94 and 2.34 times of graphite film from commercialized Kapton film (I-film), respectively. The thermal conductivity of G0.25 film is 4.52 times of Cu and 7.46 times of Al. More importantly, the addition of g-C 3 N 4 can reduce the graphitization temperature from 2800 to 2500 ℃, which can reduce the energy consumption by 20.5% in the whole polyimide graphitization process. The developed preparation technology has important guiding significance and very attractive industrial application value for the preparation of large-scale, low cost and high-quality heat dissipation materials.

The novel and facile electrolysis method for removing the cobalt binder phase from large diameter polycrystalline diamond compacts

The thermal stability, wear resistance and impact toughness of polycrystalline diamond compact (PDC) are the primary determining variables in determining the performance. The cobalt binder contributes significantly to the PDC's thermal stability and wear resistance. To increase the thermal stability of the PDC, this paper used electrolysis to remove the cobalt binder from the polycrystalline diamond (PCD) layer of the PDC with a diameter of 62 mm. The optimal process parameters for cobalt removal via electrolysis were determined by examining the electrolyte concentration, the electrolytic voltage, and the electrolytic time. The cobalt removal impact was evaluated using a scanning electron microscope (SEM), an energy dispersive spectrometer (EDS), an X-ray diffraction (XRD), and thermogravimetric analysis-differential thermal analysis (TG-DTA). In addition, the abrasion ratio of the PDC samples were tested. The experimental results indicated that the optimal electrolysis parameters for cobalt removal were obtained when the concentration of cobalt sulphate was 4 g/100 mL, the electrolysis voltage was 2.5 V, and the electrolysis time was 10 h; in this case, the cobalt removal rate from the PCD layer exceeded 80%, and the cobalt removal depth was 372 μm. TG-DTA analysis revealed that the thermal stability of PDC was significantly enhanced as a result of the initial graphitization temperature of the cobalt removal sample being increased from 1071 °C to 1113 °C, but the abrasion ratio rose by more than 20%. It was discovered that electrolysis may efficiently remove the cobalt binder phase from the PCD layer, therefore improving the thermal stability and wear resistance of PDC.

XRD-based crystallinity degrees and formation temperature of graphite in some regional-metamorphic rocks from the Central and Eastern Rhodopes, Bulgaria: a test of the new graphitization degree GD(0–30) scale

Graphitization degrees and temperatures of the regional metamorphism in the Central and Eastern Rhododpes have been determined by the values of the structural parameter d002 (Å) of graphite in graphite-bearing marbles and shists, from Vacha and Madan-Erma Reka areas (Madan lithotectonic unit), Ardino–Nedelino area (Startsevo lithotectonic unit), and Chernichevo–Boturche area (Byala Reka lithotectonic unit). Equations of different authors, formulated between 1951 and 2021, were used. They are integrated in one system, allowing a conversion of the results and deriving new quantitative correlations between the temperature of metamorphism, the structural parameter d002 (Å) and the degree of crystallinity order of semi-graphite and graphite. The comparison of the data allows the creation of a new scheme, GD(0–30), based on the values of the parameter d002 (Å) of the natural carbon matter. The validity range of this geothermometer is large, from 84 °C to 804 °C (from zeolite to granulite facies of the regional metamorphism). The temperature peak of metamorphism in the studied areas is 660 °C (i.e., the upper limit of the amphibolite facies), and the lowest temperature is 468 °C, which is characteristic for the lower part of the greenschist facies P-T field. The temperature range and graphitization degrees of the regressive metamorphism in the Central and Eastern Rhodopes are: 660 °C (GD(0–30) = 24, well-crystallized graphite); 588 °C (GD(0–30) = 21, graphite); 564 °C (GD(0–30) = 20, graphite); 516 °C (GD(0–30) = 18, graphite); 492 °C (GD(0–30) = 17, graphite); and 468 °C (GD(0–30) = 16, graphite). The calculated temperatures of metamorphism in the Central and Eastern Rhodopes, by equations of all authors, correspond to the conditions of metamorphism established by other methods without using of the structural parameter d002 (Å) of the graphite. The GD(0–30) system for determination of graphitization degree of the natural carbon is simple, informative and functional. It is more convenient than earlier schemes showing correlation: metamorphic temperature − d002 (Å) − graphitization degree. In addition to the influence of the temperature, the values of the degrees of graphitization also include the effect of all secondary factors on the graphitization processes.

Influence of graphitization temperature on microstructure and mechanical property of C/C-SiC composites with highly textured pyrolytic carbon

C/C-SiC composites with highly textured pyrolytic carbon (HT PyC) were prepared by a combining chemical vapor infiltration and liquid silicon infiltration. The effect of HT PyC graphitization before and after 2327 and 2723 K on C/C-SiC composites was investigated. The mechanical properties decreased with increasing graphitization temperature, but graphitization treatment changed the fracture behavior from brittle like to pseudo-ductile. The decrease in bending strength from 306.21 to 243.69 MPa resulted from the weak interfacial bonding between HT PyC and fiber, and the good orientation of graphite layers. The crack at border of fiber bundle and longitudinal crack in HT PyC shortened the path of crack propagation, resulting in fracture toughness decrease from 21.11 to 14.72 MPa·m1/2. A more pseudo-ductile behavior was due to the longer pull-out of fibers, the better orientation of graphite layers, the sliding of sublayers, and the deflection and propagation caused by the transverse cracks.

Increasing the tribological properties of the sur-face layer of cast iron by treatment in the envi-ronment of overheated steam of aqueous solution

The formation of the surface layer of cast iron during the processing of cast iron in the environment of superheated steam of an aqueous solution of salts is considered. An ecologically cleaner method of chemical-thermal treatment is offered. The article considers the surface layer obtained in the environment of superheated steam of an aqueous solution of water-soluble salts of ammonium molybdic acid. The layer is formed on the surface of the matrix and around the graphite inclusions. The impact of temperature on the formation of a multilayer coating by increasing the temperature in contact with the metal surface is the dissociation of chemical compounds in solution, with the formation of atomic oxygen, sulfur, nitrogen, molybdenum. The formation of the surface layer occurred not only on a metal base, but also on the boundaries of graphite, both lamellar and globular, coming to the surface of the metal without breaking the solid layer. It is established that as a result of interaction of technological parameters of temperature, holding time and concentration of saturated medium graphite in the surface layer changes its shape, becoming spherical, although in the matrix it had a lamellar shape. It is multiphase and contains soft and hard structural components. Comparative studies have shown that the wear rate of untreated cast iron is much higher than that of treated cast iron, and reaches 0.8 ∙ 10-4 μm/km at a load of 400 N. This increases the wear resistance of cast iron and at the same time improves the workability reducing the coefficient of friction. The effect of the coating on the workability show that the stabilization of the friction moment occurs faster in samples with a multilayer coating. Thus, for gray cast iron with lamellar graphite, the stabilization time of the friction moment in a multilayer coating is 0.6 h compared with 2 h for a single layer coating; in high-strength cast iron it is, respectively, 5.2 and 6.3 hours.

Preparation of SiC coated graphite flake with much improved performance via a molten salt shielded method

AbstractOxide‐based castables have been used in high‐temperature furnace linings, especially for steel and iron processing, however, the poor thermal shock resistance and corrosion resistance limited the further enhancement of their service life. Carbon possesses excellent performances including low wettability to slag, low thermal expansion coefficient, which can greatly improve thermal shock resistance and slag resistance of oxide‐based castables. However, it has poor water‐wettability and oxidation resistance, which limited its application in the castables. Many efforts have been attempted to improve water‐wettability of graphite flake used for carbon‐containing castables, but it is still a huge challenge. In this work, a facile method for fabricating silicon carbide (SiC) coated graphite flakes has been firstly proposed by using graphite flake and silicon powder as the precursors via a molten salt shielded synthesis technique in air atmosphere, KCl was chosen to be a salt encapsulation providing a liquid reaction medium and a shield against oxidation of air. Effects of firing temperature, holding time, and molar ratio of Si and graphite flake on the formation of SiC coatings were investigated. The results show that the as‐prepared SiC coated graphite powders were loose and non‐agglomerated, and the coatings with ∼5 μm in thickness were crack‐free and firmly grown on the surface of graphite flake, composed of SiC nanosized particles. Compared to uncoated graphite flake, the SiC coated graphite possessed significantly improved water‐wettability, dispersibility, and oxidation resistance, making it to be a prime carbon form for fabricating carbon‐containing castables.