Crystallite orientation in polycrystalline graphites made from glass-like carbons under high pressure

The preferred orientation of crystallites was investigated on polycrystalline graphites made from the glass-like carbons with spherical particles under a pressure of 5 kbar at 1300 to 2000? C. The glassy carbon spheres 40 to 70 Μm diameter which were pre-heated at 1000? C under normal pressure, gave sintered discs with a bulk density of 1.5 g cm^?3 and a relatively high degree of orientation. However, spheres of the same type pre-heated at 2000? C gave the high bulk density of 1.9 g cm^?3 and a very low degree of orientation. The carbon beads 40 to 70 Μm diameter gave a relatively low degree of orientation and pre-heating at 2000? C reduced the degree very remarkably. Carbon beads with a smaller particle size, <20 Μm, gave a lower degree of orientation. Pre-heating of the initial carbon of small particle size at temperatures as high as 2000? C resulted in a low degree of preferred orientation of the crystallites in the sintered discs, the temperature dependence being the strongest.     

Thermal expansion of siliconated pyrolytic carbon

The lattice and bulk thermal expansions perpendicular to the layer plane of siliconated pyrolytic carbon, PC(Si), produced by pyrolyzing a mixture of propane gas and silicon tetrachloride vapour at the deposition temperatures of 1440 to 2025° C, have been measured over the temperature ranges 20 to 550° C and 20 to 960° C, respectively. The expansion behaviours of PC(Si) are related to the density and the degree of preferred orientation of crystallites, as is the case for pyrolytic carbon without silicon PC. At a deposition temperature of about 1700° C, the bulk thermal expansion coefficient of PC(Si) is about three times as large as that of PC.     

Influence of grinding on the graduation graphitization and densification of coke powder

Petroleum coke powder, made by the delayed coking method at about 500° C, was ground for various times from 15 min to 44 h. Carbon solids were made from the ground coked powder compacts and heat-treated at 1000 to 2800° C without the use of binder materials. The coke particles ground for a considerable time had a spherical appearance and an amorphous structure, and showed non-graphitizability. These ground powders were easily densified and hard carbon solid could be obtained by heat-treatment. However, if the coke powder was pre-heat treated above about 600° C before grinding, no densification occurred and the powder graphitized as well as non-ground ones. The hard carbon solids made from powder ground for 44 h had a bulk density of 1.71 g cm^–3, Shore hardness of 120 and bending strength of about 700 kg cm^–2 at a heat-treatment temperature of 1000° C. These values changed with increasing heat-treatment temperature to a bulk density of 1.93 g cm^–3, Shore hardness of 90 and bending strength of 500 kg cm^–2 at a heat-treatment temperature of 2800° C.     

Characterization by electron microscopy of carbon phases (intermediate turbostratic phase and graphite) in hard carbons when heat-treated under pressure

As hard carbons are heat-treated under pressure (1 h, 5 kbar) an intermediate thermally stable carbon phase (d _00.2=3.39Å, turbostratic) appears above 1100° C. Its proportion increases up to 1600 or 1700° C depending in the initial sample (saccharose or glassycarbon). The morphology of this new phase has been determined by HREM to be similar to crumpled sets of paper sheets, it is thus highly porous. When heat-treated under pressure above 1600 or 1700° C, this phase is suddenly transformed into graphite, i.e. the porous texture flattens into lamellae.     

Effects of boron addition on some properties of hard-type carbons

The boron-containing hard-type (HT) carbons were prepared by heating the raw coke compacts with 1.6 wt% boron at temperatures ranging from 1000 to 2800° C. Some physical and mechanical properties of boron-doped HT carbons have been measured and compared with those for boron-free materials. It was confirmed that the boron enters the HT carbon at a relatively low temperature of 1400° C and enhances the densification process of compacts during heat-treatment above 1800° C. The addition of boron caused increases in Young's modulus and thermal conductivity, and decreases in hardness and electrical conductivity of HT carbons. The effects are discussed, and compared with those for graphitizable carbons.     

Synthesis of cementite-dispersed carbons by pressure pyrolysis of organoiron copolymers

Cementite-dispersed carbons were synthesized by pressure pyrolysis of divinylbenzene-vinylferrocene and styrene-vinylferrocene copolymer at temperatures below 600° C and the pressure of 125 MPa. The pyrolysis process of both copolymers was analysed by infrared spectra and magnetization of the pyrolysed substances. The absorption band of iron-carbon bond of divinylbenzene-vinylferrocene copolymer decreased on increasing its pyrolysis temperature from 300 to 450° C and finally disappeared at 500° C. The carbonization of divinylbenzene-vinylferrocene proceeded more rapidly than styrene-vinylferrocene at temperatures between 450 and 500° C. Styrene-vinylferrocene was heat-treated at 250° C for 2 h under 100 MPa affording a paramagnetic product, whereas the paramagnetic character of divinylbenzene-vinylferrocene was revealed after heat-treatment at 380° C. The saturation magnetization of cementite-dispersed carbon synthesized from both kinds of copolymers was comparable when the pressure pyrolysis was carried out at temperatures between 520 to 600° C at 125 MPa. The saturation magnetization of cementite-dispersed carbon formed at 550° C under 125 MPa was correlated linearly with the iron content in carbon. Threedimensional cross-linked divinylbenzene-vinylferrocene copolymer gave the highly dispersed cementite particles less than 50 nm with the coercive force of 950 Oe. On the other hand, the larger particle size of cementite up to 120 nm and the lower coercive force about 400 Oe were obtained in carbon matrix prepared by the pressure pyrolysis of styrene-vinylferrocene copolymer.     

Formation of non-graphitizable isotropic spherulitic carbon from poly-divinylbenzene by pressure pyrolysis

Isotropic spherulites of carbon stable at 2000° C were synthesized by the pressure pyrolysis of divinylbenzene polymer sealed in a capsule. The morphology of the synthesized carbon was pressure and temperature dependent being influenced by the state of polymerization of the starting polymer. Using a polymer prepared at atmospheric pressure and 150° C without catalyst, isolated spherulitic carbon was formed at 700° C and pressures of 1000 to 1250 kg cm^–2. These spherulitic carbons were optically isotropic, hard and non-graphitizable after heat treatment at 2000° C. Such carbons originate in the co-existence of higher and lower molecular weight products of pressure pyrolysis and the survival cross-linkages in the original polymer.     

Compatibility studies between pyrolytic carbon and nickel

It is well known that carbon is graphitized by foreign substances. In previous work, graphitization of carbon fibre by nickel was found at temperatures as low as 700° C, but it was difficult to establish the graphitization rate because of the fibre's round cross-section and the variable orientation of the crystallites. In this work, pyrolytic carbon was used instead of the carbon fibre because of its flat surfaces and the preferred orientation of the carbon networks. The pyrolytic carbon which was electro-deposited with nickel, was held at high temperatures. A nickel layer was observed in the pyrolytic carbon, and the carbon area through which the nickel layer passed became graphitized. The graphitization rates were constant at each temperature. And activation energy of 39.7 kcal mol^–1 was obtained from the graphitization rates, which agrees well with the activation energy for diffusion of carbon atoms in nickel. No obvious difference of the graphitization rates was recognized between the directions parallel and normal to the carbon networks.     

Effect of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers

The influence of hot stretching graphitization on the structure and mechanical properties of rayon-based carbon fibers was studied. It was observed that the Young’s modulus of the treated fibers increased with heat treatment temperature (HTT) and hot stretching stress, to 173 GPa by 158.2 % through hot stretching at 2700 °C under stress of 270 MPa compared to that of the as-received carbon fiber. Meanwhile the tensile strength increased to 1.75 GPa by 73.3 % through hot stretching at 2700 °C under 252 MPa. The field emission scanning electron images showed markedly increased roughness on the external surface and bigger and more compacted granular morphologies on the cross section of the treated fibers with increasing HTT. The preferred orientation of graphitic layers was improved by hot stretching, and the higher the HTT, the stronger the effectiveness of the hot stretching. The crystallite sizes grew and the crystallite interlayer spacing decreased obviously with increasing HTT but changed just slightly with increasing stretching stress. The analysis based on uniform stress model and shear fracture theory proposed that the improvement of tensile strength and Young’s modulus for rayon-based carbon fiber was mainly due to the increased preferred orientation and nearly unchanged shear modulus between planes with increasing HTT during hot stretching graphitization, which was much different from polyacrylonitrile-based carbon fibers.     

Effect of boron-doping on structure and some properties of carbon-carbon composite

Boron-doped carbon-carbon composites with boron concentration around 11–15 mass % were prepared from a carbon fibre felt with dispersed boron carbide powder by infiltration of pyrolytic carbon. The composite was heat treated at several different temperatures from 2000–2800 °C. The highest bending strength was obtained for the composite at a heat treatment temperature (HTT) of 2200 °C. Carbon fibre began to be destroyed after heat treatment at 2400 °C and the structure of the composite was drastically changed above 2600 °C where the anisotropy of the composite originally existing in the thermal expansion coefficient and the thermal conductivity has been faded away. X-ray diffraction measurement indicated that graphitization of the composite was enhanced by boron doping. At HTTs above 2400 °C, the composite became graphitic, the crystallite sizes of which were more than 100 nm in L_c (004) and L_a (110). It was shown that boron was uniformly distributed in the composite at an HTT of 2400 °C and also that heat treatment at higher temperatures, such as 2600 °C, incurred condensation of boron. Air-oxidation loss at 800 °C appeared to be the lowest for the composite with an HTT of 2400 °C and the rate of oxidation loss was 22 times lower than that of the non-boron-doped composite.     

Binderless moulding of green cokes delivered from solvent refined coal and ethylene tar pitch

Green cokes, derived from the co-carbonization of Solvent Refined Coals with ethylene tar pitch, have been moulded into discs without using a binder. Cokes with a range of size of optical texture have been prepared by control of the ratios of the two components of the carbonization blend. The appearance of the discs was assessed by optical and scanning microscopy after calcination to 1200° C. The most acceptable disc was prepared by moulding carbon of a heat treatment temperature (HTT) of 440° C. With cokes of HTT< 440° C excessive dilation adversely decreased the density of the disc. With cokes of HTT > 440° C, dimished cohesion of the coke grains prevented the development of a strong disc on calcination. It is considered that the presence of benzene-soluble (BS) and benzene-insoluble/quinoline-soluble (BI/QS) fractions in the pitch systems contribute to cohesion of coke particles having a HTT of 440° C.     

Effects of crystallinity of starting carbons on diamond formation in presence of nickel under high pressure and high temperature condition

The processes of graphitization and diamond formation of several carbons in the presence of nickel were investigated under 8 GPa at temperatures up to 1800° C. Diamond was formed easily from graphitized pitch coke which had a well-developed graphitic structure and in less amount from glassy carbon preheated at about 3000° C which was partly graphitized. On the other hand, pitch coke and glassy carbon, preheated at about 2000° C and not graphitized, did not transform to diamond but remained graphitized even in the diamond stable region. Diamond from graphitized pitch coke and glassy carbon preheated at about 3000° C grew to form by direct bonding.     

Heat treatment temperature effects on structural and electrochemical properties of PVDC-based disordered carbons

X-ray diffraction (XRD), BET surface area, Raman spectroscopy and electrochemical measurement were used to investigate the morphological behaviors of poly(vinylidene chloride) (PVDC)-based carbons in the heat treatment temperature range 700 to 3000°C. X-ray diffraction and Raman scattering results proposed that PVDC-based carbons heat treated up to 3000°C behaved as typically non-graphitizable carbons. The structural parameters were correlated with discharge/charge characteristics of Li ions into PVDC-based carbons. It was realized that the Li ion storage capacity of PVDC-based carbons during the first cycle tended to decrease with increasing the heat-treatment temperature due to the reduction of hydrogen content and the reduction of the specific surface area from pores.     

Effects of substrate temperature on the microstructure and photocatalytic reactivity of TiO2 films

Transmission electronic microscopy is used to study the structure, morphology and orientation of thin TiO2 films prepared by reactive magnetron sputtering on glass slides at different substrate temperatures (100 to 400 °C). The TiO2 films are used to purify a dye in waste water. The microstructure and photocatalytic reactivity of TiO2 films have been shown to be functions of deposition temperature. In the temperature range examined, all film samples have a porous nanostructure and the dimension of particles grown with increasing deposition temperature. Films are amorphous at temperatures of 100 °C and only anatase phase forms at 200 °C and above. Films deposited between 200 to 300 °C show a preferred orientation, while films at 400 °C change into complete random orientation. Deposition at 250 °C yields high efficiency in photocatalytic degradation owing to the high degree of preferred orientation and nanocrystalline/nanoporous anatase phase. © 1998 Kluwer Academic Publishers     

Superelastic Hard Carbon Nanofiber Aerogels

Superelastic carbon aerogels have been widely explored by graphitic carbons and soft carbons. These soft aerogels usually have delicate microstructures with good fatigue resistance but ultralow strength. Hard carbon aerogels show great advantages in mechanical strength and structural stability due to the sp³‐C‐induced turbostratic “house‐of‐cards” structure. However, it is still a challenge to fabricate superelastic hard carbon‐based aerogels. Through rational nanofibrous structural design, the traditional rigid phenolic resin can be converted into superelastic hard carbon aerogels. The hard carbon nanofibers and abundant welded junctions endow the hard carbon aerogels with robust and stable mechanical performance, including superelasticity, high strength, extremely fast recovery speed (860 mm s^?1), low energy‐loss coefficient (<0.16), long cycle lifespan, and heat/cold‐endurance. These emerging hard carbon nanofiber aerogels hold a great promise in the application of piezoresistive stress sensors with high stability and wide detection range (50 kPa), as well as stretchable or bendable conductors.     

Morphological instability of ZrO2 crystallites formed by CVD technique operated under atmospheric pressure

Zirconium oxide (ZrO₂) crystallites were deposited using a chemical-vapor-deposition apparatus operated under atmospheric pressure on a silicon substrate. The X-ray diffraction pattern reveals that the crystallites have a preferred crystallographic orientation along the ZrO₂ <200> direction and ascertains the presence of the monoclinic structure. The sample morphology indicates the existence of the aggregation of the whisker with a growth rate of 0.5 nm/s, 0.7 nm/s and 0.9 nm/s corresponding to the substrate temperature of 650°C, 680°C and 700°C, respectively. High temperatures introduce morphological instability in the whiskers. The appearance of a dendrite structure is an example of such morphological instability. Furthermore, a noticeable cathodoluminescence emission is observed in the U.V. and blue regions at room temperature.     

A gradual annealing of amorphous sputtered indium tin oxide: Crystalline structure and electrical characteristics

Thin films of amorphous indium tin oxide were deposited by soft sputtering. The film was gradually annealed in air at temperatures from 110 °C to 150 °C. Its structural and electrical properties were monitored in order to get a better understanding of the annealing process. Firstly, carrier density decreases by oxygen intake. Crystallization speeds up at 150 °C, with a 2.5 D growth of crystallites. The preferred orientations come from sputtering induced seeds. Then, the carrier density increases again due to tin activation. Meanwhile, the carrier mobility is more damaged by the low temperature annealing in air than by a standard annealing in a reducing atmosphere. Thus, tin oxide segregation is suspected at grain boundaries.     

Formation of layered structure on bismuth-borate glass surface

X-ray diffraction analysis shows that the transformation during heat treatment from amorphous to crystalline phases of bismuth-borate glass samples takes place in sequences. After a short heat treatment, 5 min at 550 °C, a layered structure with a preferred orientation of crystallites on the surface is observed. After a long heat treatment, 8 h at the same temperature, normal polycrystalline bulk samples are obtained.     

Effect of high-pressure pre-treatment of starting carbon on diamond formation

Four starting carbons differing in crystallinity and grain size were pre-treated with or without nickel at 3 GPa and 1800° C or at 6 GPa and 1700° C. Diamond synthesis from carbons pre-treated and then further treated in vacuum was carried out at 8 GPa and 1700° C. Pre-treated carbons with or without nickel, which were fully or partly graphitized, changed a little or did not convert to diamond at 8 GPa and 1700° C. Diamond did form from the pre-treated carbons after treatment in a vacuum at 1000° C. Diamond formation, even from the graphitized carbons, was found to be inhibited mainly by gases adsorbed on the treated carbon during the pre-treatment under high pressure.     

Correlation between chemical vapor deposited diamond and carbon fibers substrates

Diamond film formation has been studied on carbon felts (CF) substrates produced from polyacrylonitrile precursor, at different heat treatment temperatures (HTT). Scanning electron microscopy images have revealed a polycrystalline and preferential (111) diamond film covering the whole CF surface, even for deeper planes. The average grain size increased from 3.0 up to 6.0 μm for films grown on CF treated between 1000 and 2000 °C. This behavior may be attributed to different contributions associated to the facility to extract carbons atoms from CF substrate. For CF treated at lower HTT, higher carbon atoms amount will promote higher nucleation density on diamond films and consequently a smaller grain size. Raman spectroscopy indicated good quality diamond films and the lower amount of graphitic phase was observed for diamond grown on CF obtained at 2000 °C HTT. The microstructural properties of the CF were obtained by X-ray diffraction (XRD) and show a strong dependence with HTT.