Pitch Impregnation Research Articles

The Microstructure and Thermal Conductive Behavior of Three-Dimensional Carbon/Carbon Composites with Ultrahigh Thermal Conductivity.

Carbon-based composite materials, denoted as C/C composites and possessing high thermal conductivity, were synthesized utilizing a three-dimensional (3D) preform methodology. This involved the orthogonal weaving of mesophase pitch-based fibers in an X (Y) direction derived from low-temperature carbonization, and commercial PAN-based carbon fibers in a Z direction. The 3D preforms were saturated with mesophase pitch in their raw state through a hot-pressing process, which was executed under relatively low pressure at a predetermined temperature. Further densification was achieved by successive stages of mesophase pitch impregnation (MPI), followed by impregnation with coal pitch under high pressure (IPI). The microstructure and thermal conductivity of the C/C composites were systematically examined using a suite of analytical techniques, including Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and PLM, amongst others. The findings suggest that the volumetric fraction of fibers and the directional alignment of the mesophase pitch molecules can be enhanced via hot pressing. The high graphitization degree of the mesophase pitch matrix results in an increased microcrystalline size and thus improved thermal conductivity of the C/C composite. Conversely, the orientation of the medium-temperature coal pitch matrix is relatively low, which compensates for the structural inadequacies of the composite material, albeit contributing minimally to the thermal conductivity of the resultant C/C composites. Following several stages of impregnation with mesophase pitch and subsequent impregnation with medium-temperature coal pitch, the 3D C/C composites yielded a density of 1.83 and 2.02 g/cm3. The thermal conductivity in the X (Y) direction was found to be 358 and 400 W/(m·K), respectively.

Bi-Continuous Si/C Anode Materials Derived from Silica Aerogels for Lithium-Ion Batteries

Poor cycling performance caused by massive volume expansion of silicon (Si) has always hindered the widespread application of silicon-based anode materials. Herein, bi-continuous silicon/carbon (Si/C) anode materials are prepared via magnesiothermic reduction of silica aerogels followed by pitch impregnation and carbonization. To fabricate the expected bi-continuous structure, mesoporous silica aerogel is selected as the raw material for magnesiothermic reduction. It is successfully reduced to mesoporous Si under the protection of NaCl. The as-obtained mesoporous Si is then injected with molten pitch via vacuuming, and the pitch is subsequently converted into carbon at a high temperature. The innovative point of this strategy is the construction of a bi-continuous structure, which features both Si and carbon with a cross-linked structure, which provides an area to accommodate the colossal volume change of Si. The pitch-derived carbon facilitates fast lithium ion transfer, thereby increasing the conductivity of the Si/C anode. It can also diminish direct contact between Si and the electrolyte, minimizing side reactions between them. The obtained bi-continuous Si/C anodes exhibit excellent electrochemical performance with a high initial discharge capacity of 1481.7 mAh g−1 at a current density of 300 mA g−1 and retaining as 813.5 mAh g−1 after 200 cycles and an improved initial Coulombic efficiency of 82%. The as-prepared bi-continuous Si/C anode may have great potential applications in high-performance lithium-ion batteries.

Synthesis and evaluation of petroleum-based impregnating pitch for production of high-performance artificial graphite

The pitch impregnation process enhances the properties of synthetic graphite by optimizing their physical structure of channels and pores generated during heat treatment. Synthesizing a well-balanced molecular structure of impregnation pitch (IP) is essential for facilitating impregnation and obtaining a high carbonization yield. However, achieving this delicate balance is exceptionally challenging due to the inherent trade-off relationship between these factors. To resolve this issue, we propose a stepwise pitch synthesis recipe and the molecular weight distribution (MWD) tailoring, specifically applied to the petroleum IP from pyrolysis fuel oil (PFO). The stepwise recipe is composed of a pressurized pretreatment step applied to increase production yield and coking value, followed by a thermal polymerization step. The tailoring process is systematically designed to obtain the best performing petroleum IP by effectively removing impregnation inhibitor. Our study highlights the potential benefits of using petroleum-based pitch for impregnation processes in manufacturing of artificial graphite. The impregnation performance was enhanced by 6.7% and carbonation yield was notably improved by 16.2% relative to the coal-tar derived commercial IP. Our results show that tailoring the molecular weight distribution (MWD) of the impregnating pitch enhances impregnation performance by 1.88% compared to the raw IP.

Correlation between Pitch Impregnation Pressure and Pore Sizes of Graphite Block.

This study aimed to investigate the effect of impregnation pressure on the decrease in porosity of impregnated bulk graphite. The correlation between pitch impregnation behavior and the pore sizes of the bulk graphite block was studied to determine the optimal impregnation pressure. The densities and porosities of the bulk graphite before and after pitch impregnation under various pressures between 10 and 50 bar were evaluated based on the Archimedes method and a mercury porosimeter. The density increased rates increased by 1.93–2.44%, whereas the impregnation rate calculated from the rate of open porosity decreased by 15.15–24.48%. The density increase rate and impregnation rate were significantly high when the impregnation pressures were 40 and 50 bar. Compared with impregnation pressures of 10, 20, and 30 bar, the minimum impregnatable pore sizes with impregnation pressures of 40 and 50 bar were 30–39 and 24–31 nm, respectively. The mercury intrusion porosimeter analysis results demonstrated that the pressure-sensitive pore sizes of the graphite blocks were in the range of 100–4500 nm. Furthermore, the ink-bottle-type pores in this range contributed predominantly to the effect of impregnation under pressure, given that the pitch-impregnated-into-ink-bottle-type pores were difficult to elute during carbonization.

Experimental investigation on Mode-I fracture toughness of Carbon-Carbon composites fabricated by preformed yarn method

Carbon-Carbon composites are one such material that gives designers significant importance for advanced applications over conventional materials. They are applied in applications at very high temperatures (up to 3000 °C), and under extreme conditions. They have a density which is much less than metals and ceramics and thus, make low part weight a significant factor for aerospace applications. Fracturing toughness is a measurable way to express a substance's resistance to fracture in the case of a break.This paper describes the experimental studies done to investigate the Mode-1 fracture toughness of carbon–carbon composites which was fabricated via the preformed yarn method. Fracture toughness was determined for four different (a/w) ratios i.e. 0.45, 0.47, 0.50 and 0.52 respectively. The results showed that with only two cycles of pitch impregnation, HIP and graphitizing, carbon–carbon composites were successfully produced. The specimen having an (a/w) ratio of 0.45 had a higher fracture toughness value in comparison with all values. As the (a/w) ratio was increasing, the fracture toughness value decreased and the fractured surface clearly shows a brittle fracture behavior.

Effect of carbon fabric type on properties of 2D carbon–carbon composites processed by liquid pitch impregnation–pyrolysis

The present study reports on the microstructural, physical, thermal and mechanical properties of two types (A and B) of carbon–carbon composites processed by an economical route. Skeleton composites were first made by pyrolysing laminated carbon fibre-reinforced phenolic composites and subsequently densified by liquid pitch impregnation–pyrolysis process. Both types of composite employed polyacrylonitrile-based 8-harness satin woven carbon fabrics, Type A being woven with tows of continuous fibres, whereas Type B used yarns of discontinuous fibres. Experimental results indicated that the Type A composite had better flexural and tensile modulus and strength values. However, the Type B composite had better interlaminar shear strength and through-thickness thermal conductivity, despite being less dense. These results are discussed and explained from microstructural and fractographic analysis using optical and scanning electron microscopes. Finally, the results were compared with those intended for similar applications.

Characterization and Damage Evaluation of Coal Tar Pitch Carbon Matrix Used in Carbon/Carbon Composites

Flexure, compressive, and shear properties of the carbon matrix in carbon/carbon (C/C) composites made via a pitch impregnation method have been determined. The pitch carbon matrix was made using the same densification cycle used in making the C/C composite. Cyclic compression tests were performed on the matrix specimens. While unloading, a reduction in modulus was observed and residual strains were observed on complete unloading. These features were attributed to the presence of damage and plasticity in the densified matrix. A J 2 plasticity model with damage was used to simulate this behavior numerically. The parameters required for plasticity and damage model were evaluated iteratively by comparing the results in experiments with simulation.

Sublimation and oxidation zone ablation behavior of carbon/carbon composites

Three-dimensional carbon/carbon (C/C) composites comprising four reinforcement directions (4D) were fabricated using intermediate modulus carbon fibers and densified using a hybrid process. This consists of a pre-densification step using a thermal-gradient chemical vapor infiltration process followed by a high-pressure pitch impregnation and carbonization process. The specimens machined along Z-axis of the preform architecture were tested in an arc plasma heater for studying its ablation behavior at different temperatures. Regimes from ultra-high temperature (4750K) sublimation to high-temperature (2467K) oxidation zones were created by varying the mass flow rate of secondary air in the heater. The ablation rate showed a progressive increase as the environment changed from oxygen-lean sublimation to oxygen-rich oxidation conditions while the back-face temperature showed a similar temperature profile during the plasma exposure period. The thermal diffusivity value decreased with the rise in temperature till 1173K and later on became fairly flat till 1523K and onwards. In the compression test, 4750K exposed specimen showed toughening in the plasma-affected zone and crushed with shear mode from the opposite face while the 2467K exposed specimen showed end brushing in the plasma heat-affected face with a lower residual strength and Young’s modulus.

Climate Based Performance of Carbon-Carbon Disc Brake for High Speed Aircraft Braking System

Carbon composite brake discs are lighter, economical, and have excellent high energy friction characteristics. These have twice thermal capability compared to steel, remain unaffected by thermal shocks and mechanical fatigue. These are highly useful in emergency breaking situations. Prior to this work, two dimensional (2D) reinforced carbon composite laminates were prepared through pitch impregnation process. In an effort to protect the exposed non frictional surface from high temperature service degradations, multilayered ceramic coating systems were developed on 2D composite. Oxidation studies have been carried out on these systems viz., C-SiC, C-SiC-MoSi2-Al2O3 and C-SiC-B4C. These were performed both in dynamic and static conditions up to 1200 °C in 60 per cent humid climate. The hardness, surface topography, developed phases and integrity of layers on the samples at various stages of the experiment have been characterized and analyzed. It was observed that C-SiC-B4C system performs well in the oxidizing environment. Defence Science Journal, 2013, 63(5), pp.531 -538 , DOI:http://dx.doi.org/10.14429/dsj.63.3932

Tensile properties of 4D braided carbon/carbon composites produced from carbon fiber bundles and carbon rods

4D preforms woven with carbon fiber bundles and carbon rods were densified by repeated pitch impregnation, carbonization and high temperature treatment to obtain 4D-C/C composites. The tensile properties were investigated at room temperature when tension was applied in the carbon rod direction ( Z direction). The strain and stress under tensile load were measured, and the surface morphologies of specimens after tensile failure were observed by scanning electron microscopy. Results indicate that the tensile strengths of the composite were controlled by interfacial shear strength between the carbon rod and the carbon matrix. The carbon rods were pulled out entirely from matrix after tensile failure, indicating that the material was not sufficiently reinforced by the carbon rods. Numerous micro-cracks were found on the surface of specimens after tensile failure. These were perpendicular to the load direction and equally spaced by a distance of about 2 mm. The damage model of 4D-C/C is similar to that of 1D-C/C. [New Carbon Materials 2013, 28(4): 300–306]

Development and characterization of carbon-carbon composite for aircraft brake pad using preformed yarn method

This paper presents the synthesizing of carbon-carbon (CC) composites by preformed yarn (PY) method, by varying the percentage of carbon fiber volume. The PY used is carbon fiber bundle surrounded by coke and pitch which is enclosed in nylon-6. Three types of samples with fiber weight fractions of 30%, 40% and 50% respectively, are fabricated and tested. In each case, the PY is chopped and filled into a die of required shape and hot pressed at 600°C to get the carbonized composite. To obtain the graphitic structure, the specimen is heat treated at 1800°C followed by soaking for two hours. Further, one cycle pitch impregnation is done by hot isostatic pressing, to eliminate the voids. The characteristics such as hardness, compressive strength, creep, density and oxidation resistance are studied. It is observed that, as the carbon fiber percentage increases the properties also improved, provided sintering is done at fairly higher temperatures. The superiority of the new class of CC composites made by the proposed PY technique over those obtained by the conventional methods is also demonstrated.

Microstructure and mechanical properties of carbon foams and fibers reinforced carbon composites densified by CLVI and pitch impregnation

Carbon foams and fibers reinforced carbon composites were prepared using chemical liquid-vaporized infiltration and pitch impregnation. The microstructure and mechanical properties of the composites were investigated. The results showed that the final density of these samples was in a range of 1.34–1.45 g/cm 3. The pores in carbon foams were filled with pyrocarbon and pitch carbon. The flexural and tensile strength of the composites increased gradually with increasing the content of carbon fibers, whereas the compressive stress went up to a maximum value of 35.9 MPa at the fiber content of 7%.

Change of properties after oxidation of IG-11 graphite by air and CO 2 gas

Artificial graphite is typically manufactured by carbonization of a shaped body of a kneaded mixture using granular cokes as a filler and pitch as a binder. It undergoes a pitch impregnation process if necessary and finally applying graphitization heat treatment. The effect of thermal oxidation in air or a CO 2 atmosphere on IG-11 graphite samples is investigated in this study. The results show a localized oxidation process that progressively reveals the large coke particles with increasing level of overall weight loss in air. The surface of the graphite was peeled off and no change was found in the specific gravity after air oxidation. However, the specific gravity of graphite was continuously decreased by CO 2 oxidation. The decrease in the specific gravity by CO 2 oxidation was due to CO 2 gas that progressed from the surface to the interior. The pore shape after CO 2 oxidation differed from that under air oxidation.

Premature failure of a vacuum pump impeller rotor recovered from a pitch impregnation plant

A vacuum pump that was integral to a pitch impregnation system had rapidly deteriorated, leading to compromise in quality of products being manufactured. It was found that a particular vacuum impeller rotor group had worn, with unacceptable vacuum levels being the result. A replacement rotor was installed whilst cause of premature failure was investigated. However, in a matter of just months of service, the replacement impeller group also deteriorated. Finding the underlying mechanism of failure and developing a cost effective solution to the problem became paramount in terms of both product quality and rapidly increasing production cost. It was determined that the root cause of failure was stress corrosion cracking (SCC). The leaded red brass vacuum pump vane rotors had become sensitised as a result of ammonia liberated directly from the coal tar pitch impregnation medium. It was determined that changing the material of construction to a stainless steel alternative would not provide a cost or process effective solution. Spray coating the brass vane rotors with an epoxy coating presented a quick simple and cost effective solution to the problem.

원자로용급 흑연인 IG-110의 파괴특성

Artificial graphite generally manufactured by carbonization sintering of shape-body of kneaded mixture using granular cokes as filler and pitch as binder, going through pitch impregnation process if necessary and finally applying graphitization heat treatment. Graphite materials are used for core internal structural components of the High-Temperature Gas-cooled Reactors (HTGR) because of their excellent heat resistibillty and resistance of crack progress. The HTGR has a core consisting of an array of stacked graphite fuel blocks are machined from IG-110, a high-strength, fine-grained isotropic graphite. In this study, crack stabilization and micro-structures were measured by bend strength and fracture toughness of isotropic graphite grade IG-110. It is important to the reactor designer as they may govern the life of the graphite components and hence the life of the reactor. It was resulted crack propagation, bend strength, compressive strength and micro-structures of IG-110 graphite by scanning electron microscope and universal test machine.

Densification of two-dimensional carbon/carbon composites by pitch impregnation

Densification of two-dimensional phenolic resin-based C/C composites was performed by melted petroleum pitch impregnation under a uniaxial mechanical pressure followed by recarbonization at atmospheric pressure. With increasing number of densification cycle, decrease in open porosity and increase in bulk density were obtained. However, densification efficiency was found to decline with the densification cycles. In the later stage of the densification cycles, voids closure resulted in a decrease in the apparent density, and much lower densification efficiency was obtained. Results of flexural test indicated that both strength and modulus increased with increasing number of densification cycle. Significant improvement in mechanical performance was found when an intermediate high temperature heat treatment of 2000°C was used between densification cycles.

Effect of Densification on the Thermal Cracking of Graphite Materials

The effect of densification (pitch impregnation followed by firing) on the length of thermal cracks was analyzed for two kinds of flaked polycrystalline graphite materials using 300- to 1100-K resistivity ratios before and after densification. The decrease in crack length upon densification (1–3% of the grain size) was found to increase with lamella size.

00/03293 The efficiency and mechanism of densification of 2-D C/C composites by coal-tar pitch impregnation

00/03293 The efficiency and mechanism of densification of 2-D C/C composites by coal-tar pitch impregnation

Influence of porosity on the apparent interlaminar shear strength of pitch-based unidirectional C–C composites

Changes in the open porosity, bulk density and void structure through five steps of densification by coal tar pitch impregnation and recarbonization, were studied with four different unidirectional C–C composites. The changes produced in the composite microstructure were related to the apparent interlaminar shear strength and the failure behaviour of the composites. The strength increases with the number of densification steps as well as the brittle character of the failure. Relationships between microstructure and brittle behaviour of composites were investigated.

A study on the ultrasonic nondestructive evaluation of carbon/carbon composite disks

It is desirable to perform nondestructive evaluation (NDE) to assess material properties and part homogeneity because the manufacturing of carbon/carbon brake disks requires complicated and costly processes. In this work several ultrasonic techniques were applied to carbon/carbon brake disks (322mmod, 135mmid) for the evaluation of spatial variations in material properties that are attributable to the manufacturing process. In a large carbon/carbon disk manufactured by chemical vapor infiltration (CVI) method, the spatial variation of ultrasonic velocity was measured and found to be consistent with the densification behavior in CVI process. Low frequency (e.g., 1-5MHz) through-transmission scans based on both amplitude and time-of-flight of the ultrasonic pulse were used for mapping out the material property inhomogeneity. Images based on both the amplitude and the time-of-flight of the transmitted ultrasonic pulse showed significant variation in the radial direction. The radial variations in ultrasonic velocity and attenuation were attributed to a density variation caused by the more efficient densification of pitch impregnation near theid andod and by the less efficient densification away from exposed edged of the disk. Ultrasonic velocities in the edges of the disk. Ultrasonic velocities in the thickness direction were also measured as a function of location using dry-coupling transducers; the results were consistent with the densification behavior. However, velocities in the in-plane directions (circumferential and radial) seemed to be affected more by the relative contents of fabric and chopped fiber, and less by the void content.