Effects of Preform and Pyrolytic Carbon Structure on Thermophysical Properties of 2D Carbon／Carbon Composites

Four kinds of carbon/carbon (C/C) composites, including the needled carbon fiber felt/the pyrolytic carbon (two different pyrolytic carbon microstructures), the chopped carbon fiber/the resin + pyrolytic carbon (PyrC), and the carbon cloth/PyrC, named as the composites 1#, 4#, 2#, and 3#, are prepared respectively. Effects of the preform and pyrolytic carbon structure on the thermophysical properties of 2D C/C composites are studied. The C/C composites possess low coefficient of thermal expansion (CTE). In a range of some temperatures, the negative expansion emerges in x-y direction for four C/C composites. From 0 to 900℃, the CTE is small and almost linear with the temperatures. The C/C composites have high thermal conductivities (TCs). As a function of temperature, TCs of the C/C composites are varied with the structures of preform and pyrc as well as the direction of heat transfer. In x-y and z direction, TCs differ greatly and that in x-y direction (25.6-174 W/m·K) is several times larger than that in z direction(3.5-50 W/m·K).

Microwave dielectric properties of Nextel-440 fiber fabrics with pyrolytic carbon coatings in the temperature range from room temperature to 700℃

Pyrolytic carbon(PyC) coatings are deposited on the Nextel-440 fiber fabrics by chemical vapor deposition(CVD).The dielectric properties of the Nextel-440 fiber fabrics with PyC coatings(Nextel-440/PyC) are investigated in a temperature range from room temperature to 700℃ in X-band. Compared with the permittivity of the original Nextel-440 received,the complex permittivity of the Nextel-440/PyC(the real part εand the imaginary part ε), is significantly improved: εof the Nextel-440/PyC has a positive temperature coefficient, in contrast, εof the Nextel-440/PyC exhibits a negative temperature coefficient. Moreover, the reflection loss in units of d B is calculated. The results indicate that the microwave absorbing properties of the Nextel-440/PyC coatings is enhanced at 700℃ compared with that at lower temperatures.

Study on the Pyrolytic Carbon Generated by the Electric Heating CVD Method

A new method of fabricating C/C composite materials, namely electric heating CVD method, was used, which electrified the carbon fiber directly by using the conductivity of itself. Acetylene was used as the carbon source with nitrogen as dilution gas, and the pyrolytic carbon started to deposit on the carbon fiber surface when the deposition temperature was reached. The morphology of pyrolytic carbon was characterized by SEM, and the surface properties of carbon fibers before and after CVD were characterized by Raman spectroscopy. The experimental results show that the electric heating method is a novel method to fabricate C/C composite materials, which can form a dense C/C composite material in a short time. The order degree and the average crystallite size of the carbon fiber surface were decreased after the experiment.

GROWTH CHARACTERSAND MODEL OF PYROLYTIC CARBON IN CHEMICAL VAPOR INFILTRATION PROCESS

Chemical Vapor Infiltration (CVI) processes are the essential techniques for fabrication of high performance carbon carbon composites. Based on the polarized light and scanning electron analysis, the authors study the micro morphology and texture characteristics of pyrolytic carbon deposited in CVI process, as well as the growth behavior of pyrolytic carbon. The research shows that Rough Laminar (RL) texture has the hierarchical and self similar structural features, which reflects the stage growth and self similar behavior during the growth course of pyrolytic carbon. According to the two growth features, a laminated growth model of pyrolytic carbon is proposed with the concept of Cone Growth Units (CGU). The laminated growth model can provide a fine description for the growth course of RL pyrolytic carbon. The model indicates that formation, developing and combination of local high order structures (such as CGU structures) are the essential factors for the growth of RL texture. Smooth Laminar (SL) texture and ISO carbon come into being with long range orderliness and isotropy structure respectively, which no local high orderliness intermediate involves in.

Recovery Method of Pyrolytic Carbon Surface after Exposure to Chlorine Trifluoride Gas

An annealing process was developed for the practical recover of a purified py-rolytic carbon (PPyC) surface from damage, such as color change and peeling, due to fluorination during exposure to chlorine trifluoride gas at high tempera-tures for the cleaning of a silicon carbide chemical vapor deposition reactor. The PPyC surface was annealed at 900°C for 10 min in ambient nitrogen con-taining oxygen at the concentrations of 0.01% - 20%. The rainbow-like colored surface returned to the dark gray color of the original PPyC. Simultaneously, the thin peeled films disappeared. The Raman spectra showed that the PPyC surface chemical bonding returned to that of the original one. The oxygen concentration as low as 0.01% could recover the PPyC surface along with re-ducing the surface pit formation, when the combination of the exposure to chlorine trifluoride gas and the recovery was repeated.

Effect of pyrolytic carbon interphase on mechanical properties of mini T800-C/SiC composites

The effect of pyrolytic carbon(PyC) thickness on the tensile property of mini T800 carbon fiber reinforced SiC matrix composites(C/SiC) was studied. PyC interphase was prepared by chemical vapor infiltration(CVI) process using C3H6–Ar as gas source, the PyC thickness was adjusted from 0 to 400 nm, and then the SiC matrix was prepared by CVI process using methyltrichlorosilane(MTS)–H2–Ar as precursor and gas source. The results showed that the tensile strength of mini T800-C/SiC increased first and then decreased with the increase of the PyC thickness. When the thickness of PyC was 100 nm, the average strength reached the maximum value of 393 ± 70 MPa. The Weibull modulus increased from 2.0 to 8.06 with the increase of PyC thickness, and the larger the Weibull modulus, the smaller the dispersion, which indicated that the regulation of PyC thickness was conducive to improve tensile properties.

Effect of Temperature on the Synthesis of SiC Coating on Carbon Fibers by the Reaction of SiO with the Deposited Pyrolytic Carbon Layer

The influence of reaction temperature on the preparation of SiC coating on carbon fibers by the reaction of silicon monoxide with the deposited pyrolytic carbon （PyC） layer has been discussed.With rising reaction temperature,the thickness of SiC layer increases and the SiC grain is coarsening.The apparent activation energy for the synthesis of SiC layer is about 103.3 kJ/mol.The oxidation resistance of carbon fiber can be improved by the SiC/PyC layers significantly.The initial oxidation temperature of the SiC/PyC coated carbon fiber is about 300℃ higher than that of the uncoated carbon fiber.The oxidation of the SiC/PyC coated carbon fiber is owing to the diffusion of oxygen through the cracks generated by the mismatch of thermal expansion.

Pyrolytic carbon derived from spent coffee grounds as anode for sodium-ion batteries

This paper reported the facile preparation of pyrolytic carbon derived from spent coffee grounds and the evaluation of its electrochemical performance when used as anode in sodium-ion battery.X-ray diffraction analysis,scanning electron microscope,Brunauer–Emmett–Teller were employed to characterize the structure of pyrolytic carbon.Electrochemical performances were tested by constant current charge–discharge,cyclic voltammetry and electrochemical impedance spectroscopy.Results showed that the pyrolytic carbon possess a porous structure(1–2 lm)and a specific surface area of 94.35 m2 g
1.When used as anodes in sodium-ion batteries,a reversible capacity of 154.2 mA h g
1 at a current density of 200 mA g
1 after 50 cycles was obtained.Several electrolytes were evaluated and their electrochemical performances were compared.The result indicated that this material has excellent storage capacity and good cycling stability.Our method provided a preparation of pyrolytic carbon from environmentally friendly resources.

Methane pyrolysis in preparation of pyrolytic carbon:Thermodynamic and kinetic analysis by density functional theory

The density functional theory has been successfully applied in analyzing pyrolytic carbon deposition by methane pyrolysis from the view of thermodynamics and kinetics based on a total number of 39 elementary reactions.M06-2X/def2-TZVP level was employed to optimize species structures and locate the transition states.The enthalpy changes and Gibbs free energy changes of all the reactions in the temperature range of 298.15–1800K were derived with optimized species.Results show that the reacting temperature should be above 1200 K based on the equilibrium constant analysis,which is consistent with the typical reaction temperature adopted in experiments.Potential energy surface profiles report that radical attacking reactions have lower energy barriers than those direct decomposition reactions,especially hydrogen radical attacking reactions.The energy barriers of the first steps,dehydrogenations of methane and ethylene,are 272.4 kJ·mol^(-1)and 288.9 kJ·mol^(-1)at 1200 K,which are very close to the experimental activation energy for methane pyrolysis.The most favorable decomposition reaction path is the path of hydrogen radical attacking reactions.The highest energy barrier of the path at 1200K is 185.7kJ·mol^(-1)presented by the C–H bond breaking in ethynyl attacked by hydrogen radical.

Fabrication and functional characterization of engineered features on pyrolytic carbon

Engineered features on pyrolytic carbon （PyC） have been reported to improve the functional performance of the bio-implants. This paper is focused on the functional characterization of micro-features created on the surface of PyC. Two different types of micro-features （wide channels and arrayed holes） have been created by micro-electrical discharge machining （micro-EDM）. Two other micro-fea- tures （fine channels and micro-pillars） have been created by micromilling process. Coliform bacterial strain was iso- lated from a sample of water and grown on all four tex- tured. Cell growth was carried out on an unmachined surface to see the behavior of the isolated bacterial strain on the textured/non-textured surfaces. The samples were examined under SEM before and after wash to see cell growth and cell adhesion capability of the textures. The wide channels by micro-EDM show the maximum cell growth but poor cell adhesion. 184% higher cell growth has been observed on the wide channels in comparison with unmachined surface. The fine channels by micro-milling show comparatively lower growth but the cell adhesion on this surface was found excellent. 71% cells remain unwa- shed after washing of the surface having fine channel textures. It means that the channels structure shows the maximum cell growth and adhesion independent of machining process.

Determination of Anisotropic Factors of Pyrolytic Carbon by Electron Diffraction

The preferred orientation of pyrolytic carbon coating is an important performance parameter for the safe use of artificial mechanical heart valve.In this paper,the selected area electron diffraction(SAED)analysis of pyrolytic carbon coating samples of artificial heart valve was carried out by transmission electron microscopy(TEM),and the experimental method of characterizing the preferred orientation of pyrolytic carbon by electron diffraction spectrum,namely bacon anisotropy factor(BAF),was described,and the orientation angle and BAF corresponding to the electron diffraction spectrum were measured.The results show that the BAF value can directly reflect the anisotropy of pyrolytic carbon in the selected area.The BAF value range is from 1 to infinity,and the higher the preferred orientation is,the greater the value is.The BAF value decreases exponentially with the increase of orientation angle.When the pyrolytic carbon tends to be isotropic,the orientation angle tends to 180 and the BAF value tends to 1.

Silicon-alloyed Isotropic Pyrolytic Carbon Prepared by Steady-state Fluidized Bed Chemical Vapor Deposition

Silicon-alloyed isotropic pyrolytic carbon(Si-IPyC)was prepared by steady-state fluidized bed chemical vapor deposition(SFBCVD).The effects of deposition temperature and propane concentration on the microstructure and deposition process of Si-IPyC were investigated using X-ray diffraction(XRD),X-ray energy dispersive spectrometer(EDS)and scanning electron microscope(SEM).The results show that the Si-IPyC with different silicon content is composed of both spherical granular and laminar structures,and silicon element is uniformly dispersed in pyrolytic carbon in the form of β-SiC.The deposition temperature and propane concentration have a great influence on the microstructure of Si-IPyC,with the increase of deposition temperature or propane concentration,the laminar structure in Si-IPy C decreases gradually,spherical granular morphology becomes increasingly obvious and the density decreases with the deposition model changing from surficial growth mechanism to gaseous nucleation mechanism.For getting a faster average coating rate,deposition temperature and propane concentration should not be too high.

Synthesis and Growth Mechanism of Carbon Filaments by Chemical Vapor Deposition without Catalyst

Carbon filaments with diameter from several to hundreds deposition of methane without catalyst. The morphology micrometers were synthesized by chemical vapor microstructure and mechanical properties of the carbon filament were investigated by scanning electronic microscopy, optical microscopy, X-ray diffraction and mechanical testing. The results show that the carbon filament is inverted cone shape and grows up along the gas flow direction. The stem of it is formed of annular carbon layers arranged in a tree ring structure while the head is made up of concentrical layers. The tensile strength of the carbon filament is increased after graphitization for the restructuring and growing large of graphene. The growth mechanism of carbon filament was proposed according to the results of two series of experiments with different deposition time and intermittent deposition cycles.

Carbonization of Ni-doped Phenol Resin Under Various Conditions

Ni-doped phenol resin was prepared with 1∶100 mass ratio of Ni（ NO_3）_2·6H_2O to thermosetting phenol resin to optimize the structure and properties of pyrolytic carbon derived from phenol resin and increase its carbon yield. The specimens were cured at 200 ℃ and carbonized under different atmospheres（ carbon-embedded atmosphere and Ar atmosphere） and at different temperatures（ 600,800,1000 and 1200 ℃） for3 h,respectively. The carbon yield was measured. Thermal decomposition characteristics of Ni-doped phenol resin,and the oxidation resistance,phase composition and microstructure of pyrolytic carbon were characterized by differential scanning calorimetry,X-ray diffraction,energy dispersive spectroscopy, scanning electron microscopy and transmission electron microscopy. The results show that the carbon yield of Ni-doped phenol resin carbonized at800 or 1 000 ℃ is increased significantly,compared with that without any dopants. The graphitization degree of pyrolytic carbon structure derived from Ni-doped phenol resin increases with the increase of carbonization temperature. The massive multi-wall carbon nanotubes of 50-100 nm in diameter and of micrometre scale in length are generated at 1000 ℃. Compared with the carbonembedded atmosphere,carbon nanotubes can be more easily generated in Ar atmosphere,resulting in higher carbon yield and degree of crystallinity of the pyrolyticcarbon derived from Ni-doped phenol resin. The oxidation resistance of the pyrolytic carbon derived from Ni-doped phenol resin at 1200 ℃ is improved significantly and its highest oxidation temperature is increased by about 84℃,compared with that from Ni free phenol resin.

A Novel Modification Approach for Natural Graphite Anode of Li-ion Batteries

To improve the rate capability and cyclability of natural graphite anode for Li-ion batteries,a novel modification approach was developed.The modification approach included two steps:(a)high-energy ball milling in a rotary autoclave containing alumina balls,H_3PO_4 and ethanol;(b)coating with pyrolytic carbon from phenlic resin.The treated graphite shows obvious improvement compared with the original natural graphite in electrochemical properties such as cyclability and rate capability,especially at high current density.The primary reasons leading to the improvement in rate capability and cyclability are that the diffusion impedance of Li^+ in graphite is reduced due to the fact that P filtered into graphite layers can mildly increase interlayer distances,and the fact that the structural stability of graphite surface is enhanced since the coated pyrolytic carbon can depress the co-intercalation of solvated lithium ion.

Top priority current path between SiC particles during ultra-high temperature flash sintering: Presence of PyC “bridges”

Flash sintering(FS)is a novel technique for rapidly densifying silicon carbide(SiC)ceramics.This work achieved a rapid sintering of SiC ceramics by the utilization of ultra-high temperature flash sintering within 60 s.Pyrolysis carbon(PyC)“bridges”were constructed between SiC particles through the carbonisation of phenolic resin,providing a large number of current channels.The incubation time of the flash sintering process was significantly reduced,and the sintering difference between the centre and the edge regions of the ceramics was minimized,with an average particle size of the centre region and edge region being 12.31 and 9.02μm,respectively.The results showed that the porosity of the SiC ceramics after the flash sintering was reduced to 14.79% with PyC“bridges”introduced,and the Vickers hardness reached 19.62 GPa.PyC“bridges”gradually evolved from amorphous eddy current carbon to oriented graphite carbon,indicating that the ultra-high temperature environment in which the sample was located during the flash sintering was successfully constructed.Ultra-high temperature flash sintering of SiC is expected to be applied to the local repair of matrix damage in SiC ceramic matrix composites.

Using PyC modified 3D carbon fiber to reinforce UHTC under low temperature sintering without pressure

Finding the optimum balance between strength and toughness,as well as acquiring reliable thermal shock resistance and oxidation resistance,has always been the most concerned topic in the discussion of ultra-high temperature ceramic composites.Herein,PyC modified 3D carbon fiber is used to reinforce ultra-high temperature ceramic(UHTC).The macroscopic block composite with large size is successfully fabricated through low temperature sintering at 1300℃without pressure.The prepared PyC modified 3D C_(f)/ZrC-SiC composites simultaneously possess excellent physical and chemical stability under the synergistic effect of PyC interface layer and low temperature 1/2 sintering without pressure.The fracture toughness is increased in magnitude to 13.05±1.72 MPa·m^(1/2)accompanied by reliable flexural strength of 251±27 MPa.After rapid thermal shock spanning from room temperature(RT)to 1200℃,there are no visible surface penetrating cracks,spalling,or structural fragmentation.The maximum critical temperature difference reaches 875℃,which is nearly three times higher than that of traditional monolithic ceramics.The haunting puzzle of intrinsic brittleness and low damage tolerance are resolved fundamentally.Under the protection of PyC interface layer,the carbon fibers around oxide layer and matrix remain structure intact after static oxidation at 1500℃for 30 min.The oxide layer has reliable physical and chemical stability and resists the erosion from fierce oxidizing atmosphere,ensuring the excellent oxidation resistance of the composites.In a sense,the present work provides promising universality in designability and achievement of 3D carbon fiber reinforced ceramic composites.

Microstructures and mechanical behaviors of CVI-based C/C composites containing h-BN powdered additives

Hexagonal boron nitride(h-BN) powders were introduced into carbon fiber preform by powder addition and subsequent combined with chemical vapor infiltration(CVI) for densification to prepare carbon fiber reinforced/carbon and boron nitride dual matrix composites(C/C-BN). Microstructures and mechanical properties of C/C composites with three different volume contents of h-BN powders were investigated in comparison to pure C/C composites. Results indicated that the introduction of h-BN powders into C/C composites significantly reduced the size of Py C and the anisotropy of thermal contraction in matrix,leading to a gradual disappearance of ring defects as the h-BN content increased. In addition, an enhanced interfacial bonding between fiber and matrix obtained due to higher-textured Py C and rougher fiber surface. Thereby, the flexural strengths and modulus of as-prepared composites decreased firstly and then increased, while the impact toughness presented a decreasing tendency as the content of BN powders increased. Furthermore, with the increasing of h-BN content, anisotropies of compressive properties were weakened, and the compressive strength of C/C-BN composites were always higher than that of pure C/C composit. However, when C/C composites modified by 13.5 vol% content of h-BN, excessive loose BN aggregates appeared in C/C-BN composites, leading to a relatively slight reduction of compressive strength.