A study on the deposition of pyrolytic carbons from hydrocarbons

A new method for forming isotropic, laminar, and columnar pyrolytic carbons is proposed. For this, a low RPM (below 2.4 rpm) tumbling bed has been used to deposit pyrolytic carbons from hydrocarbon gases. All deposits were made on graphite substrates from propane and methane at a constant temperature of 1200°C. The microstructures of the pyrolytic carbons deposited were dependent on the flow pattern of the reactant gas, the rpm of the reactor, the hydrocarbon concentration, the nature of the hydrocarbon, and the geometry of the bed. Isotropic pyrolytic carbon is formed under deposition conditions where homogeneous nucleation occurs in the gas phase and at the gas flow conditions where the gas-borne droplets can collide on the substrate. Laminar carbon is formed under deposition conditions where homogeneous nucleation does not occur in the gas phase and at gas flow conditions where the carbon species existing in the bulk of the gas phase can collide on the substrate. Columnar carbon is formed when any carbon products existing in the bulk of the gas phase cannot collide on the substrate. The suggested deposition mechanism can also be applied to pyrolytic carbons deposited in a fluidized bed or in a stationary bed. In particular, isotropic carbon can be obtained even in a stationary bed if the requirements for the deposition of the isotropic carbon described above are satisfied.     

Isochronal annealing kinetics and neutron-induced dimensional changes of titanium-doped pyrolytic carbons

A series of isochronal 1 hr anneals was conducted on a laminar pyrolytic carbon deposited at 1225°C containing 6.7 wt.% titanium, an isotropic pyrolytic carbon deposited at 2200°C containing 9.7 wt.% titanium, and titanium-free companion carbons. Changes in layer spacing, apparent crystallite height (L_c), density, electrical resistivity, and microhardness were followed. Heterogeneous recrystallization to graphite started in the titanium-doped isotropic carbon at 2300°C and was completed at 2500°C. The titanium-doped laminar carbon did not show catalytic recrystallization. The undoped isotropic carbon was not responsive to annealing. The titanium-doped and titanium-free laminar carbons showed a steady increase in L_c and density and a steady reduction in layer spacing and microhardness over a temperature range from 1400° to 2800°C, but significant reduction in electrical resistivity did not begin until ~ 1800°C. The dimensional changes induced by fast-neutron exposures up to ~ 8 × 10²¹n/cm² at ~ 600°C to 1250°C were measured and found to be lower by a factor of 3–10 in a catalytically graphitized pyrolytic carbon than in an as-deposited carbon with the same degree of preferred orientation.     

Influence of the deposition parameters on the texture of pyrolytic carbon layers deposited on planar substrates

Pyrolytic carbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at various residence times, methane pressures and deposition temperatures. The depositions were performed in a cavity oriented perpendicular to the gas flow. The small surface area/reactor volume ratio of the reactor geometry allows depositions in the growth and nucleation mechanism. Transmission electron microscopy was applied to study the texture and microstructure of the carbon layers. A texture transition from medium- to high-textured pyrolytic carbon occurs as a result of increasing residence times, methane pressures and temperatures. Improved textures are generally correlated with increasing deposition rates, which are not necessarily constant during long-term depositions. Lower textures are observed in the vicinity of the substrate interface that are attributed to the influence of the substrate morphology and microstructure.     

液相气化快速致密化工艺研究

对一种新型快速炭／炭复合材料制备工艺－－液相气化快速致密化工艺进行了初步探索。研究表明，采用该工艺，致密化效率可以得到快速提高，数小时内制得密度达1．7g/cm^3以上的炭／炭复合材料，致密速率 达到37g/h。偏光显微镜观察表明，试样中热解炭具有较高的光学活性；束内小孔隙热解炭，绝大多数为光学各向异性组织，但具体归属粗糙组织（RL）还是光滑组织（SL），很难定论；束间大孔隙内的热解炭具有明显的锥状生长结构，是较典型的RL组织；在偏光显微镜下没有观察到试样中有炭黑出现。     

Kinetics of graphitization of carbon-felt/carbon-matrix composites

The kinetics of graphitization of two carbon/carbon composites, prepared by the chemical vapor deposition (CVD) of carbon within carbon felt substrates, were investigated over a temperature range of 2300 to 2660°C and time intervals of 5 to 360 min. Each of these carbon-felt/carbon-matrix (CVD/felt) materials graphitized at a more rapid rate than a massive chemically-vapor-deposited pyrolytic carbon. The graphitization rates of the CVD/felts were related to the observed microstructures: the composite which had a rough laminar matrix graphitized at two to three times the rate of one with a banded matrix. The activation energies of the CVD/felt composites fell within the range observed for a wide variety of pyrolytic carbons and petroleum cokes.     

不同硼含量的硼掺杂各向同性热解炭制备及微观结构研究

以CH4、H2和BCl3为原料,利用化学气相沉积方法制备了硼掺杂各向同性热解炭材料。研究了制备工艺及硼元素掺杂对热解炭微观结构和抗氧化性能的影响。结果表明,硼元素能够均匀地掺入热解炭中,其含量随着BCl3流量的增大而增加;硼掺杂并未改变热解炭的各向同性性质,但随着BCl3流量的增加,硼掺杂各向同性热解炭的结构由球形颗粒状结构逐渐向卷曲片层状结构转变,硼在热解炭中的存在形式则由单一形式(取代)逐渐过渡到混合形式(取代硼和碳化硼);硼元素的掺杂能有效抑制热解炭的氧化,大幅提高材料的起始氧化温度,显著降低氧化损失。     

限域变温强制流动CVI工艺制备C／C复合材料的组织及力学性能特点研究

分析了采用限域变温强制流动CVI工艺制备C/C复合材料的组织及力学性能的特点,结合C/C复合材料的组织形成规律和组织对性能的影响规律,详细研究了在同一工艺条件下所获得的具有不同组织和不同力学性能的C/C复合材料的力学性能及组织分布规律,并从微观组织结构的角度对力学性能的变化规律给予解释。     

A study of the properties of pyrolytic carbons deposited from propane in a tumbling and stationary bed between 900 and 1230°C

In order to obtain low-temperature-isotropic (LTI) pyrolytic carbons, a new “Tumbling Bed” reactor has been developed. The characteristics of the pyrolytic carbons deposited in this tumbling bed have been studied by varying the gas composition, the total gas flow rate, the weight of bed particles and the rotational speed (RPM) of the reaction tube in the temperature range of 900–1230°C. It was found that all pyrolytic carbons deposited were isotropic in the temperature range of 1050–1230°C. The density of the Isotropie carbon increased slightly with temperature, but it was independent of the other variables with values of 1.9–2.0 g/cm³. The apparent crystallite size, L_e, of the isotropic carbon was about 30 Å regardless of coating conditions. The deposition rate increased with temperature, propane concentration, and total flow rate, showed a minimum with increasing RPM of reaction tube, and decreased with the weight of bed particles. The deposition mechanism of the isotropic pyrolytic carbon was suggested from the results. Additionally, a few experiments were carried out in a stationary bed in order to study the role of the rotating action in the tumbling bed reactor. Columnar, sooty and filamentous carbons were obtained in the stationary bed. From scanning electron micrographs of fracture surfaces of the filamentous carbons, it appeared that they are constructed by spiral growth of carbon flakes.     

Mechanical properties of isotropic pyrolytic carbon deposited in a tumbling bed

The relation between the coating conditions and mechanical properties of the isotropic pyrolytic carbons deposited in a tumbling bed have been investigated. At a constant total flow rate of mixed gases and a constant revolution rate of the reaction tube, pure and Si-alloyed pyrolytic carbons were deposited using a wide range of deposition conditions.The structures of the pyrolytic carbons were characterized by density and crystallite size. The mechanical properties (Young's modulus, fracture strength and strain energy to fracture) were measured by three point bend tests with a knife edge span of 0.35 in.In spite of similar density and crystallite size, all of the mechanical properties of pyrolytic carbons deposited in this process were lower than those of carbons deposited in a fluidized bed. These differences were caused by soot inclusions which had a non-load bearing character and behaved as cracks under load, which were not present in carbons deposited in a fluidized bed.By alloying silicon with the carbon, the number of soot inclusions was not changed, and the fracture strength increased linearly with the SiC content.     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

Studies on pyrolytic carbons obtained by acetylene pyrolysis at 1273 K—I: Types,properties and structures of carbons deposited

Various types of pyrolytic carbon, i.e. compact, feathery, brittle, spongy and soot-like, were obtained by acetylene pyrolysis in a flow reactor at 1273 K. The effects of the process variables (acetylene concentration in argon and gas flow rate) on both the solids yield and the relative proportions of the various types of carbon were estimated. These carbons, except the soot-like, showed similar elementary composition and structure parameters, but were distinguishable by their microstructures. In general, the compact carbon showed various forms of conical texture; the feathery, brittle and spongy carbons had anisotropic spherulitic textures with characteristic sphere size and packing for each type; and the soot-like carbon had an isotropic texture.     

Dependence of pyrocarbon microstructure on the substrate and annealing during the initial stage of chemical vapor deposition

Electron microscopic and electron spectroscopic techniques were applied to study interface properties, microstructure and texture of pyrolytic carbon obtained after short-time chemical vapor deposition (CVD) on planar Si substrates. The pyrolytic carbon was obtained in a hot-wall reactor from methane at a total pressure of 20 kPa and temperature of 1100 °C. Only short depositions between 2.5 and 240 min were performed. The carbon deposition starts with the nucleation of isolated islands. The increase of residence and deposition time leads to the formation of a continuous layer by larger island sizes and higher island densities, a transition from rough to smooth surfaces and formation of pores on smooth surfaces. An increased deposition rate during the first 15 min is observed which is correlated with a granular morphology of the carbon layer. Using BN-covered Si wafers with a surface roughness on a 100 nm scale reduces the texture degree in the vicinity of the interface and strengthens adhesion of the pyrolytic carbon compared to the smooth Si substrate. The texture of high-textured pyrolytic carbon is improved significantly by annealing at 1100 °C.     

Pyrolytic carbon layers—an electron spin resonance analysis

Chemical vapour infiltration is simulated by deposition of pyrolytic carbon on planar boron nitride substrates and carbon fibers in a hot-wall tubular reactor at about 1100 °C for varied pressure and flow-velocity of methane. The degree of orientation of the deposited graphite-like domains can be monitored via orientation and temperature dependence of the electron spin resonance parameters (g-tensor, linewidth). The electronic structure of the localized defects and conduction electrons is accessible by a quantitative modelling of these parameters. The interaction of the built-in hydrogen atoms with the unpaired electron spins is analysed by electron-proton spin double resonance technique (Overhauser shift)     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes: Part 1. Synthesis and morphology

The chemical vapor deposition of the pyrocarbon from a CH₄+H₂ mixture is investigated using nanofilamentous substrates. The process consists of growing carbon nanotubes via a catalytic process, which then are thickened by pyrolytic carbon deposition to reach diameters in the nanometer to micrometer range. A key characteristic of the experimental reactor used was the long length of its isothermal zone, preceded (and followed) by a low thermal gradient zone. This allowed us to investigate the role of the variation of the local gas phase composition, which depends on the post-cracking secondary reactions, and on the quantity and quality of the deposited carbon. The ‘time of flight’ of the reactive species was found to be a leading parameter in the pyrolytic carbon deposition process. Various nanometric and micrometric morphologies, several of which are new, were synthesised and found constituted with an association of different sub-morphologies. The various morphologies, that can be sorted following a factor of morphological complexity, were investigated by scanning electron microscopy.     

Deposition rate and temperature of carbon films during laser-induced CVD on a moving substrate

The feasibility of using pyrolytic laser-induced chemical vapor deposition (LCVD) to deposit carbon coatings on moving fused quartz substrates is investigated. This LCVD system uses a CO₂ laser to locally heat a substrate in open air to create a hot spot. Pyrolysis of hydrocarbon species occurs and subsequently deposits a layer of carbon film onto the substrate surface. The results of this study indicate that growth kinetics and the geometry of uniform carbon stripes deposited by pyrolytic LCVD strongly related to the laser power, the traverse velocity of the substrate, the type of hydrocarbon species used in deposition, and the diameter of the substrate. The deposition rate of carbon film increases exponentially with the laser power, while an increase in traverse velocity of the substrate will also increase the deposition rate until a maximum deposition rate is reached; further increases in the traverse velocity will decrease the deposition rate. We suspect that this optimal deposition rate is caused by substrate motion, which affects the substrate surface temperature, and consequently the effective surface area available for film deposition. The substrate temperature is observed to behave linearly with the deposition parameters considered in this study.     

Metal-free synthesis of carbon nanotubes filled with calcium silicate

A new metal-free catalyst CaSiO₃ was developed to grow carbon nanotubes (CNTs) on a pyrolytic graphite paper tape by an ethanol catalytic chemical vapor deposition method at 1200–1400 °C. The prepared CNTs with a droplet tip had a multi-walled structure and were filled with amorphous CaSiO₃. Temperature, determining the melting of CaSiO₃, was critical for the growth of the CNTs. This new catalyst is suggested to follow the similar roles of transition metals in the growth of CNTs by a molten state to absorb carbon and form CNTs after reaching saturation.     

Hydrogen diffusion and solubility in pyrolytic carbon

Tritium diffusion coefficients and deuterium solubilities have been measured for laminar pyrolytic carbon in the temperature range 900–1500°C. The tritium diffusion coefficients were much lower than those for metals at equivalent temperatures, but the activation energy for the diffusion was much higher (～-100 kcal/mole). Tritium diffusion coefficients measured for silicon-doped pyrolytic carbon were over an order of magnitude higher than the values for the undoped laminar pyrolytic carbon. The solubility of deuterium in laminar pyrolytic carbon was found to decrease with increasing temperature and exhibited a pressure dependence of $\text{p.}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$     

Templated pyrolytic carbon: the effect of poly(ethylene glycol) molecular weight on the pore size distribution of poly(furfuryl alcohol)-derived carbon

Poly(ethylene glycol), which has a negligible carbon yield upon pyrolysis, was used as a template to study the controlled formation of mesoporosity in pyrolytic carbons. A series of carbons was produced from mixtures of poly(ethylene glycol) and poly(furfuryl alcohol) with 25, 50 and 75% composition by weight and an M_n of 300 to18?500 g/mol of template. Polydisperse dextran adsorption reveals a maximum in uptake for 8000 g/mol and 50% templated carbons, while materials from 75% mixtures or those from less than 2000 g/mol template yielded negligible dextran uptake. These results correlated well with the intensity ratio of a broad peak between 7 and 11° 2θ in the X-ray diffraction spectrum and the 002 diffraction peak and also qualitatively with micrographs of the internal microstructure of the carbons. The results suggest a templating process dominated by both the molecular size of the template and the rate of expulsion of decomposed template material during the formation of the solid.