Heat treatment of mesocarbon microbeads under high pressure

Mesophase spherules separated from pitches and asphalt by quinoline (mesocarbon microbeads) were heat-treated under a pressure of 5 kbar at 1300–2000°C for 1 hr. The as-separated microbeads gave a bulk density of 1.8–1.9 g/cm³ after heat treatments at high temperatures. The pre-heated microbeads gave high bulk density above 2.0 g/cm³ after high temperature heat treatment. The mesocarbon microbeads were found to have low graphitizability under pressure. The as-separated microbeads showed a heterogeneous process of graphitization, but the maximum amount of the graphitic component was only 60% even after 2000°C-treatment under 5 kbar. On the pre-heated microbeads, almost no graphitization was observed. The spherical shape of the as-separated microbeads was lost even after the heat treatments at low temperatures under 5 kbar. However, the pre-heated microbeads showed the tendency to keep the original spherical shape even after heat treatment at 1900°C.     

Magnetic susceptibility and kinetics of “graphitization” of glass-like carbons

The room temperature magnetic susceptibility (X) of glass-like carbons (GLC) from several sources has been determined as a function of heat treatment temperature (HTT) over the range 1000–3000°C. Effects of dilute ferromagnetic impurities were observed for HTT < 1500°C. The impurity (probably Fe) exists in a non-magnetic form in the carbonized GLC; transforms to a ferromagnetic state with 1000 ? HTT ? 1400°C; then disappears at higher HTT. The dependence of the diamagnetism of pure GLC on HTT is characterized by a slope decrease and inflection near 1500°C and increases smoothly thereafter. X ～- 5.2 × 10^?6emu/g and is still increasing at HTT = 3000°C, and the values for different GLCs with the same HTT differ by < 10%. The evolution of X as a function of isothermal residence time (HTt) over the range 2400–3000°C for three GLCs was analyzed by the superposition method. Very high effective activation energies, 360–420 kcal/mole (~ 1500–1750 kJ/mole) were obtained and attributed to successive-activation processes of structural development required by the microstructural constraints inherent to difficult-to-graphitize carbons. Evidence was found for a small Xincrease due to the plastic volume dilation (density decrease) processes that occur in some GLCs at high HTT.     

Cooperative accelerating effect of calcium carbonate and gaseous nitrogen on graphitization of carbon

Heat treatments of a Polyvinylchloride coke in the presence of calcium carbonate were carried out under flow of nitrogen or argon. Under the flow of nitrogen, a graphitic component appeared above 1700°C and increased rapidly with HTT. Flaky particles giving graphite diffraction pattern were frequently found under electron microscope, some of them having singular discoidal appearance. Under the flow of argon, the graphitic component appeared above 1900° C and its amount was relatively small, less than 10% at 2000° C. Heat treatments of calcium carbide were also performed under the same condition. Graphite was obtained even at 1000° C and its amount was much larger under flow of nitrogen than under argon. The results suggest that there must be a cooperative accelerating effect on the graphitization of carbon of the calcium carbonate and gaseous nitrogen, through the calcium cyanamide process.     

ESR of carbons in the transition range—I

The main purpose of this work was the clarification of the ESR behavior of carbons in the heattreatment range 900-1500°C by trying to pin down the reasons for the large broadening of the ESR line observed in this range. The quartz broadening occurring in heating samples in vavuo, and the Hennig-Smaller graphite furnace broadening were investigated and techniques developed for their avoidance. Although the broadening observed upon reheat of carbons heattreated to above 1600°C (Hennig-Smaller effect) has been practically eliminated and in spite of the purity of heating runs achieved, large broadening is still observed for raw carbon materials on first heattreatment (with a maximum of about HTT 1300°C) and for highly heattreated (HTT ? 1600°C) carbons upon anneal, if, previous to anneal, their structure has been damaged by neutron irradiation. The variation in position of the maximum broadening with HTT indicates that the broadening is connected with some changes occurring in the structure of carbons in this range. Decrease in width was obtained by chlorination at 1000°C but the post-chlorination anneals have shown that such chlorination must be a complex process, not amenable to a simple explanation.     

Graphitization and sintering of coals under pressure

Anthracite from Abernant, Wales, and sub-bituminous coal from Taiheiyo, Hokkaido, were graphitized above 1500 °C at a pressure of 0.5 MPa. The graphitization occurred abruptly in a very narrow range of temperature. In association with the graphitization, sintering of the sample powder and densification were observed.     

Electron spin resonance in glassy carbon

The ESR was investigated for a series of samples of glassy carbon heat-treated to various temperatures in the range 1000–3000°C using a Q and X band spectrometers. For the solid material, the width first increases slightly, then goes through a minimum at around 1400°C and increases greatly above HTT 2600°C. For a ground material, a large broadening in the transition range (~1300°C) is observed. The intensity for all samples passes through a minimum at HTT 1200°C and increases to a strong maximum at around HTT 1550°C after which it decays to a value 0·5 × 10¹⁹ spin/g. The influence of neutron irradiation and of subsequent anneal are also reported. The irradiated material shows a large broadening in the transition range (~ 1300°C) similar to the broadening observed for irradiated soft carbons. Temperature dependence measurements of the ESR signal were carried out for nonirradiated and for irradiated samples, in order to determine the contribution of the localized spin and of the conduction carriers to the total intensity.     

Evidence of catalytic effect of sulphur on graphitization between 1400 and 2000°C

The catalytic effect of the sulphur content of carbons was studied on samples of petroleum and pitch cokes with 0.4–1.7 w/o S. It is shown by in situ X-ray diffraction measurements that the irreversible contraction of interlayer distance starts during heating below 1500°C, indicating an amplified tendency towards graphitization of the main part of the sample with increasing sulphur content. Studies of the graphitization kinetics revealed a reduction of the activation energy of graphitization from 7–10 eV to about 3–4 eV between 1400 and 2000°C caused by the catalytic effect of sulphur. Besides this effect a shoulder at the high angle side of the (001)-reflections appears at the temperatures of the sudden sulphur release. This shoulder indicates the formation of a well ordered graphitic phase with an interlayer distance of 3.359 Å at temperatures above 1500°C. The content of this graphitic phase within the sample is found to be below 0.5 w/o.     

Study on the oxygen diffusion in the oxide layers of SiBCN ceramics by SIMS

SiBCN, SiC and SiC-BN ceramics/composites were prepared by mechanical alloyed combined hot-pressing sintering at 1900 °C, and the oxidation kinetics of SiBCN, SiC and SiC-BN were calculated based on the thickness of oxide layers at 1100～1500 °C. The oxide layer can be divided into outer and inner parts under 1300 °C. At 1100 °C, the oxygen molecules diffused in SiC through the gaps in lattice, while diffused in SiBCN by substituting the O in SiO₂. Moreover, BN(C) phase in SiBCN can slow down the generation rate of gases such as CO, N₂, NO₂ and B₂O₃.     

Formation of highly oriented layers of graphite in glass-like carbon heat-treated under pressure

A glass-like carbon of homogeneous structure cured up to 1000°C (GC-10) was heat-treated under pressure of 5 kbar. The heat treatment was performed in two different cell arrangements; in one arrangement quasi-hydrostatic pressure was dominant and in the other there were many contact points between the carbon investigated and small angular grains of glass-like carbon heat-treated to 2000°C. In the first arrangement, the glass-like carbon did not graphitize even at 1900°C. In the second, graphitization was observed above 1750°C. Below 1700°C, optically anisotropic areas were initiated at contact points with the angular grains where the stress concentrations occur. These areas show turbostratic structure. At a little higher temperature, they transform to graphite. The graphitized parts in cross section look like bamboo leaves and grow into the bulk of the carbon at the expense of nongraphitized parts. The graphite layers were found to align perpendicularly to the compressive stress, contrary to our previous report. The previously observed orientation was probably due to oriented regions originally present in the starting material. The mechanism of the spreading of the graphitization from the stress concentration points at the surface is discussed.     

Structure changes of MPECVD-grown carbon nanosheets under high-temperature treatment

Vertically-aligned carbon nanosheets (CNSs), which were fabricated by microwave plasma-enhanced chemical vapor deposition in Ar and CH₄ system, have been annealed at high temperatures in the range of 1200–3000 °C. The morphologies and microstructures of the treated CNSs were analyzed by scanning and transmission electron microscopes, and Raman spectroscopy. High temperature treatment process efficiently removed the amorphous carbon and some defects and improved the graphitization of the CNSs. The graphitized grains increase and the interlayer spacing decreases with increasing heat temperatures. Heat treatment of the CNSs at temperatures from 1500 to 2000 °C was found to achieve the edges consisting of many single-layer graphene sheets. Annealing at temperatures above 2100 °C, the edges of nanosheets consist of 2–5 layer graphene with many zigzag junctions. The mechanism of reconstruction for the edges in the CNSs ascribes possibly to the carbon atom vaporization at high temperatures.     

The effect of heat treatment on tensile properties of 2D C/SiC composites

Two-dimensional (2D) carbon fiber reinforced silicon carbide (C/SiC) composites with different initial strength were prepared by chemical vapor infiltration (CVI). After tensile property testing, results exhibited that as the heat-treatment temperature (HTT) increases to 1900°C, the tensile strength and toughness of the low strength specimen (LSS) increased by 110% and 530%, while the high strength specimen (HSS) increased by 5.4% and 550%, respectively. As observed from morphologies, the heat treatment increases the graphitization of the amorphous PyC interphase, which leads to the weakening of interfacial bonding strength (IBS). Meanwhile, the defects arising from heat treatment cause thermal residual stress relaxation. Therefore, the tensile strength and toughness of LSS with relatively high initial IBS increase significantly as HTT increases. For HSS with moderate initial IBS, the heat treatment slightly improves the tensile strength, but significantly improves the toughness. Consequently, the post-heat-treatment tensile properties of 2D C/SiC composites can be regulated by varying HTTs and different initial strength.     

Rapid sintering of nano-diamond compacts

Diamond compacts were sintered from nano-size diamond crystals at high pressure, 8 GPa, and temperature above 1500 °C for very short times ranging from 5 to 11 s. Structure and mechanical properties of the compacts have been characterized. Although we have not completely avoided graphitization of diamonds, the amount of graphite produced was low, less than 2%, and despite relatively high porosity, the compacts were characterized by high hardness, bulk and Young moduli.     

Thermal stress relaxation creep and microstructural evolutions of nanostructured SiC ceramics by liquid phase sintering

We explored the thermal relaxation creep characteristics of nanostructured SiC ceramics by bend stress relaxation (BSR) method. The effects of the differences in microstructure and secondary phases by liquid phase sintering at 1800 or 1900 °C were especially discussed, based on microstructural evolutions during the creep. The creep was characterized by the BSR ratio (m) of ～0.80 up to 1200 °C, and the proportion of amorphous phase as a secondary phase was related to the creep resistance at 1300 °C. The microstructural evolutions during the creep consisted firstly in the re-distribution of amorphous phase, probably as a consequence of its viscous flow, and secondly in an extensive nucleation and growth of cavities. Furthermore, the former enhanced inter-diffusion of Al–Y among intergranular areas above the ternary eutectic temperature, which caused the significantly reduced creep resistance, and the latter reflected the crystalline YAG decomposition as another secondary phase near 1500 °C.     

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650?°C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200?°C to 1400?°C, the % age porosity was around 80%. At 1500?°C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7?µm, while an individual whisker had a diameter of about 100?nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100?°C, while at 1200?°C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300?°C to form aggregates with smooth surfaces. At 1400?°C, these aggregates started joining with each other by neck formation and at 1500?°C, a three-dimensional network was formed. For sintering temperatures of up to 1400?°C, pores with sizes around 260?nm were very stable. At 1500?°C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260?nm decreased and those with sizes around 10?µm, 1?µm and 5?nm increased. The compression strength of samples sintered at 1100?°C to 1300?°C was in the range of 3.4–4.3?MPa. At 1400?°C, the strength increased to 5.2?MPa, while at 1500?°C, it jumped to 10.8?MPa due to the formation of three-dimensional network.     

Ozone Production in a High Frequency Dielectric Barrier Discharge Generator

Factors that affect the performance of an expanded-mesh dielectric barrier discharge ozone cell were investigated. A gas feed pf 94% O₂, 4% Ar and 1% N₂ was used. An improvement in the productivity (g ozone/kWh) of about 20 % was achieved by doubling the gas flow rate through the cell. Decreasing the cell operating frequency (in the range 72 kHz to 19 kHz) increased the productivity of the ozone generator at constant power. The ozone production increased approximately in proportion to the input power; however productivity did not vary significantly with power above a minimum level. As the cell voltage was increased the dependence of productivity on power or frequency was reduced. Changing the feed gas temperature between ? 5^°C and + 42^°C had no effect on productivity. Finer meshes drew more power than coarser ones for a given voltage. Using a thinner mesh for the centre electrode increased productivity. The best results were obtained with a 6 × 3 × 1.86 mm titanium mesh giving a productivity of 110 g ozone/kWhr at 30–60 W, 1500–1900V and 23 KHz.     

High-temperature properties and interface evolution of C/SiBCN composites prepared by precursor infiltration and pyrolysis

C/SiBCN composites with a density of 1.64 g/cm³ were prepared via precursor infiltration and pyrolysis and the bending strength and modulus at room temperature was 305 MPa and 53.5 GPa. The precursor derived SiBCN ceramics showed good thermal stability at 1600 °C and the SiC and Si₃N₄ crystals appeared above 1700 °C. The bending strength of the composites was 180 MPa after heat treatment at 1500 °C, and maintained at 40 MPa-50 MPa after heat treatment for 2 h at 1600 °C–1900 °C. In C/SiBCN composites, SiBCN matrix could retain amorphous up to 1500 °C and SiC grains appeared at 1600 °C but without Si₃N₄. The reason for no detection of Si₃N₄ was that the carbon fiber reacted with Si₃N₄ to form an interface layer (composed of SiC and unreacted C) and a polycrystalline transition layer (composed of B and C elements), leading to the degradation of the mechanical properties.     

Potassium-graphite intercalation compounds prepared from carbon materials of different heat-treatment temperature and their reaction with water vapor

By means of the gravimetric technique, the reaction of potassium-graphite intercalation compounds (K-GICS) with water vapor has been investigated in relation to the heat-treatment temperature (HTT) of pristine carbon materials. K-GICs with the saturated composition of KC_9.6 prepared from the material whose HTT = 1500°C was remarkably reactive as compared with other K-GICs from those of HTT = 1000, 2000 and 2600°C. Through the X-ray diffraction measurement, a resultant product with an apparent composition of (K-GIC) · (H₂O)_1.6 was found to be a mixture of KOH · H₂O and KOH · 2H₂O with a residue compound of K-GIC. Even if the residue was washed with water after the reaction, potassium still remained in the matrix; the amount varied in accordance with the HTT of the pristine material from 20% for 1000°C up to 40% for 2600°C. The washed residues give X-ray diffraction patterns different from those of the original materials. It was also found that KC₂₄ reacts more rapidly with water vapor than KC₈ does, where the steep weight increase takes place in the initial stage of the reaction, followed by a characteristic weight loss not observed for KC₈.     

Catalytic graphitization of graphitizable carbon by chromium,manganese and molybdenum oxides

Catalytic graphitization of a graphitizable carbon from Kureha pitch is investigated in the temperature range 1200–2800 K, using chromia and other chromium compounds. Two distinct stages of catalytic graphitization are detected at temperatures of 1500–1800 K and above 2000 K. Values of L_c(002) and C₀(002) are 23 nm and 673.0 pm at 1800 K and 80 nm and 670.9 pm at 2800 K. Consideration of the phase diagram of chromium-carbon system suggests that graphitization at 2800 K may proceed via a partial dissolution-precipitation process of carbon. The known catalytic graphitization of non-graphitizable carbons at 2800 K supports this mechanism. Graphitization at 1500–1800 K occurs only with a graphitizable carbon, suggesting that graphitization may proceed via the elimination of structural defects in the carbon although catalytic chemical or physical transformations may also be possible. The catalytic activity of other oxides was investigated and the high activity of manganese and molybdenum oxides is noted.     

Preparation and properties of phosphorus-containing pyrocarbon — part II. Effect of heat-treatment on phosphorus-containing pyrocarbon

Heat-treatment of pyrocarbon samples containing 3·3–4·1 w/o of phosphorus was performed in vacuum, at various HTT ranging from 1220°C to 2035°C. With increasing HTT, apparent density of the samples decreases and preferred orientation is deteriorated, but their phosphorus content remains unchanged, except at the highest HTT. Such a behaviour is caused by a strong vapour pressure of the elementary phosphorus remaining in the closed porosity. The drop in phosphorus concentration occurring at the highest HTT is also followed by an abrupt change of certain properties; in particular, the interlayer spacing which at lower HTT remains larger than that of the pure pyrocarbon, at this HTT approaches to the pure pyrocarbon value. Such a behaviour may be due to phosphorus incorporated in the pyrocarbon lattice by substitution. This remains stable up to 1800°C, but heat-treatment to 2035°C removes it from the lattice.