Changes in the microstructure and characteristics of carbon/carbon composites with mesophase mesocarbon microbeads added during graphitization

Carbon/carbon (C/C) composites were prepared from oxidative PAN fiber felts, a resol‐type phenolic resin, and mesophase pitch derived from coal tar. In this study, the effects on mesocarbon microbeads (MCMBs), flexural strength, flexural moduli, electric conductivity, and thermal conductivity of C/C composites with a mesophase content ranging from 0 to 30 wt % were examined during pyrolysis. The results show that the C/C composite with the addition of 10–30 wt % mesophase had a higher density, greater stacking size, and higher preferred orientation than the C/C composites without any mesophase during heat treatment. These composites also exhibited an improvement in flexural strength from 19.7 to 30.3%. The flexural moduli of these composites with mesophase added increased by 15.1 to 31.3% compared to that with no mesophase added. These composites also showed improved electric conductivity, from 15.1 to 43.7%, and thermal conductivity, from 12 to 31.3%. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2178–2190, 2005     

Eugenol‐Derived Bismaleimide High Performance Resins and Composites Using Diisocyanate as Property Modifier

A new kind of high performance bismaleimide resin with good processability and improved toughness is synthesized by chemical modification of 4,4′‐bismaleimidodiphenylmethane (BMI) by eugenol (EG) and different contents of 4,4′‐diphenylmethane diisocyanate (MDI). MDI‐EG‐BMI resins exhibit good thermal stability for its 5% weight loss temperatures around 300 °C and its residue of 41.61% at 900 °C, which are much higher than those of EG‐BMI resin. Then, the carbon fiber‐reinforced MDI‐EG‐BMI composites are fabricated. The mechanical properties of the composites matrixed by MDI‐EG‐BMI resins are better than those by EG‐BMI resin. For carbon/MDI‐EG‐BMI composites, their glass transition temperatures are higher than 300 °C, and their flexural strength, moduli, and toughness are maintained at a range of 217.47–404.36 MPa, 35.12–48.49 GPa, and 1.16–2.63 MJ m^?3 respectively; with the contents increasing of MDI in the resin formulation, the flexural properties first increase then decrease; comprehensively the composite with 30 wt% MDI has the best mechanical and thermal properties.     

Influence of the carbonization temperature on the mechanical properties of thermoplastic polymer derived C/C-SiC composites

Carbon/Carbon (C/C) composites derived from the thermoplastic polymer polyetherimide (PEI) were pyrolized up to 1000 °C, subsequently carbonized in inert atmosphere up to 2200 °C and afterwards infiltrated with liquid silicon. The investigation of fibers and matrix with Raman microspectroscopy revealed, that an increased carbonization temperature leads to an increased carbon order as well as an incipient stress-induced graphitization of the carbon matrix close to the fiber surfaces at 2200 °C. The derived C/C-SiC samples show a maximum flexural strength of 180 MPa with C/C composites treated at 2000 °C and monotonically increasing Young’s moduli ranging from 49 GPa with C/C preforms treated at 1600 °C up to 59 GPa after carbonization at 2200 °C. The carbon fiber strength was evaluated with a single fiber tensile test, which showed a monotonically increased Young’s modulus and a decrease of the strength after carbonization at 2200 °C.     

Biobased composites prepared by compression molding with a novel thermoset resin from soybean oil and a natural‐fiber reinforcement

Biobased composites were manufactured with a compression‐molding technique. Novel thermoset resins from soybean oil were used as a matrix, and flax fibers were used as reinforcements. The air‐laid fibers were stacked randomly, the woven fabrics were stacked crosswise (0/90°), and impregnation was performed manually. The fiber/resin ratio was 60 : 40. The prepared biobased composites were characterized by impact and flexural testing. Scanning electron microscopy of knife‐cut cross sections of the specimens was also done to investigate the fiber–matrix interface. Thermogravimetric analysis of the composites was carried out to provide indications of thermal stability. Three resins from soybean oil [methacrylated soybean oil, methacrylic anhydride modified soybean oil (MMSO), and acetic anhydride modified soybean oil] were used as matrices. The impact strength of the composites with MMSO resin reinforced with air‐laid flax fibers was 24 kJ/m², whereas that of the MMSO resin reinforced with woven flax fabric was between 24 and 29 kJ/m². The flexural strength of the MMSO resin reinforced with air‐laid flax fibers was between 83 and 118 MPa, and the flexural modulus was between 4 and 6 GPa, whereas the flexural strength of the MMSO resin reinforced with woven fabric was between 90 and 110 MPa, and the flexural modulus was between 4.87 and 6.1 GPa. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Fabrication of electrically conductive polymer composites for bipolar plate by two‐step compression molding technique

Electrically conductive polymer composites for bipolar plate were fabricated by two‐step compression molding technique. Raw materials consisted of natural graphite flakes (G), expanded graphite (EG), carbon black (CB), and phenol resin (PF). The G/EG/CB/PF composites were first compressed at a temperature lower than curing point (100°C) and then cured at a high temperature above curing point (150°C) and high pressure (10 MPa). Results showed that G and EG are oriented in the direction parallel to the composite plate surface. CB is dispersed not only in the phenol resin matrix but also in the packing and porous space of G and EG. The addition of EG and CB significantly increases number of the electrical channels and thus enhances the electrical conductivity of the composite. Under optimal conditions, electrical conductivity and flexural strength of the composite were 2.80 × 10⁴ S/m and 55 MPa, respectively, suggesting that the dipolar plates prepared by two‐step compression molding technique are adequate to meet the requirement of proton exchange membrane fuel cells. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2296–2302, 2013     

The effect of pyrolysis on the mechanical properties and microstructure of carbon fiber-reinforced and stabilized fiber-reinforced phenolic resins for carbon/carbon composites

Carbon/carbon composites were prepared with phenol-formaldehyde resin, one kind of commercial carbon fiber, and a stabilized fiber that was developed in our laboratory. The effect of pyrolysis on the microstructure, fracture behavior, and flexural strength of the composites during the carbonization process was studied. During the pyrolysis of the composites a chemical reaction at the fiber/resin interface apparently took place. A thermogravimetry (TG) study indicated that the use of stabilized fiber reinforced composites inhibited decomposition reactions and thermal fragmentation in the matrix resin, and reduced the weight loss of the final composites. The X-ray reflection of the resin and the two composites showed a reflection appearing at 2θ ≈ 12° when the samples were carbonized above 600°C. The intensity of this reflection in the composites made with stabilized fiber was higher than that of the composite made with carbon fiber. Because of the formation of strong bonding in the fiber-matrix interface, the composites made with stabilized fiber showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with an increase in the carbonization temperatures, even if the fracture behaviors showed catastrophic failure. The flexural strength of the composites made with carbon fiber showed pseudo-plastic patterns and debonding with very little fiber pullout. Above 800°C, these composites showed a catastrophic failure and smooth failure surfaces. During pyrolysis the flexural strength decreased with an increase in the carbonization temperature.     

The effect of pyrolysis on the properties of stabilized PAN fabric reinforced phenolic resins for 2D carbon/carbon composites

Two-dimensional (2D) carbon/carbon composites were prepared with phenol-formaldehyde resin and a commercial stabilized PAN fabric. The effect of pyrolysis on the microstructure and flexural strength of the composites during the carbonization process was studied. The interaction between fabric and matrix inhibited the decomposition and the thermal fragmentation, leading to a higher carbon yield for the final composition. Because of the formation of strong bonding in the fiber/matrix interface, the composites made with stabilized PAN fabric showed catastrophic failure and low flexural strength below carbonization temperatures of 600°C. Above 600°C, the flexural strength of the composites increased with the increase in the carbonization temperatures, even when the fracture behavior showed catastrophic failure.     

Microstructure evolution and phase transformation of TiB2/SiC/B4C composites synthesized from Ti‐SiC‐B4C ternary system

Bulk TiB₂/SiC/B₄C composites have been synthesized from Ti‐SiC‐B₄C ternary system with different Ti weight percentages via reactive hot pressing at 1800°C under an applied pressure of 30 MPa for 1.5 h. By Ti amount increasing, the flexural strength curve exhibited an “M‐like” tendency reaching the maximum value of 512.36 MPa for 30 wt.% Ti. Microstructural evolution of the composites from conterminously large matrix grains to finely clear‐edged particles was observed by scanning electron microscopy. The phase transformation and element diffusion were analyzed by XRD and Energy Disperse Spectroscopy. A hybrid reinforcing mechanism of fracture and crack deflection is proposed to illustrate the change in flexural strength.     

Development and application of triglyceride-based polymers and composites

Triglyceride oils derived from plants have been used to synthesize several different monomers for use in structural applications. These monomers have been found to form polymers with a wide range of physical properties. They exhibit tensile moduli in the 1–2 GPa range and glass transition temperatures in the range 70–120 °C, depending on the particular monomer and the resin composition. Composite materials were manufactured utilizing these resins and produced a variety of durable and strong materials. At low glass fiber content (35 wt %), composites produced from acrylated epoxidized soybean oil by resin transfer molding displayed a tensile modulus of 5.2 GPa, a flexural modulus of 9 GPa, a tensile strength of 129 MPa, and flexural strength of 206 MPa. At higher fiber contents (50 wt %) composites produced from acrylated epoxidized soybean oil displayed tensile and compression moduli of 24.8 GPa each, and tensile and compressive strengths of 463.2 and 302.6 MPa, respectively. In addition to glass fibers, natural fibers such as flax and hemp were used. Hemp composites of 20% fiber content displayed a tensile strength of 35 MPa and a tensile modulus of 4.4 GPa. The flexural modulus was ∼2.6 GPa and the flexural strength was in the range 35.7–51.3 MPa, depending on the test conditions. The flax composite materials had tensile and flexural strengths in the ranges 20–30 and 45–65 MPa, respectively. The properties exhibited by both the natural- and synthetic fiber-reinforced composites can be combined through the production of “hybrid” composites. These materials combine the low cost of natural fibers with the high performance of synthetic fibers. Their properties lie between those displayed by the all-glass and all-natural composites. Characterization of the polymer properties also presents opportunities for improvement through genetic engineering technology. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 703–723, 2001     

Effect of quasi‐carbonization processing parameters on the mechanical properties of quasi‐carbon/phenolic composites

In this work, quasi‐carbon fabrics were produced by quasi‐carbonization processes conducted at and below 1200°C. Stabilized polyacrylonitrile (PAN) fabrics and quasi‐carbon fabrics were used as reinforcements of phenolic composites with a 50 wt %/50 wt % ratio of the fabric to the phenolic resin. The effect of the quasi‐carbonization process on the flexural properties, interfacial strength, and dynamic mechanical properties of quasi‐carbon/phenolic composites was investigated in terms of the flexural strength and modulus, interlaminar shear strength, and storage modulus. The results were also compared with those of a stabilized PAN fabric/phenolic composite. The flexural, interlaminar, and dynamic mechanical results were quite consistent with one another. On the basis of all the results, the quasi‐static and dynamic mechanical properties of quasi‐carbon/phenolic composites increased with the applied external tension and heat‐treatment temperature increasing and with the heating rate decreasing for the quasi‐carbonization process. This study shows that control of the processing parameters strongly influences not only the mechanical properties of quasi‐carbon/phenolic composites but also the interlaminar shear strength between the fibers and the matrix resin. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Catalytic effect of iron oxide on carbon/carbon composites during graphitization

Coal tar pitch matrix was modified by addition of iron oxide in different proportions i.,e. 0, 1, 3 and 5% by weight. The matrix was used to develop carbon fibre reinforced composites heat treated to 1000 and 2500 °C, respectively. The catalytic effect of iron oxide was ascertained by measuring the physical properties viz. the inter layer spacing, thermal conductivity and flexural strength of the composites. Low concentrations of the catalyst resulted in improvement in the thermal conductivity of composites from 68 × 10⁻² W/m K for 0% to 127 × 10⁻² W/m K for 1% iron oxide concentration. The flexural strength of graphitized composites, however, showed a remarkable increase from 325 MPa for 0% to 450 MPa for 5% iron oxide concentrations. The increase in flexural strength was probably due to the development of large numbers of grain boundaries whereas the increase in the thermal conductivity was most likely due to larger crystallite size i.e. decreases in the interlayer spacing (d₀₀₂) of the graphitized composites.     

Preparation and properties of an oxide fiber‐reinforced silica matrix composite

Oxide (Nextel? 440) fiber‐reinforced silica composites, with the density and porosity of 1.97 g/cm³ and 21.8%, were prepared through sol‐gel. Their average flexure strength, elastic modulus, shear strength, and fracture toughness at room temperature were 119.7 MPa, 25.6 GPa, 10.8 MPa, and 4.0 MPa·m^1/2, respectively. The composites showed typical toughened fracture behavior, and distinct pullout fibers were observed at the fracture surface. Their mechanical properties were performant up to 1000°C, with the maximum flexural strength of 132.2 MPa at 900°C. Moreover, the composites showed good thermal stability, even after thermal aging and thermal shock at elevated temperatures.     

Light‐weight thermal insulating fly ash cenosphere ceramics

Light weight fly ash cenosphere (FAC) ceramic composites were developed by simple slip casting method. Thermal properties, Bulk density, Microstructure, flexural strength, and phase analysis of the FAC ceramic composites were measured. The results proved that the FAC have ability to lower bulk density and thermal conductivity effectively. The lowest thermal conductivity achieved for FAC ceramic composites (0.27 W/m.K) was further reduced 0.21 W/m.K by adding combustible additives ie activated charcoal and corn starch. The flexural strength, bulk density and thermal conductivity of FAC ceramic composites reduced consistently with an increase in FAC content. The maximum flexural strength of 13.45 MPa was achieved with 50% FAC and the minimum flexural strength of 4.07 MPa was obtained with 80% FAC. The open porosity increased from 35.51% to 43.76% and 38.19% with an addition of 15% activated charcoal and corn starch, respectively, when compared to no additives. The bulk density of 699, 619, and 675 kg/m³ was achieved with 80% FAC, 80% FAC with the addition of 15% activated charcoal and corn starch, respectively. The 80% FAC ceramic composite shows low thermal expansion coefficient 6.54 × 10^?6/°C at the temperature of 50°C then it varies between 3.7 and 5 × 10^?6/°C in the temperature range above 100°C. These results prove that the developed light weight FAC ceramic are excellent low‐cost thermal insulating materials.     

Preparation of C/C‐SiC Composites from All‐Cellulose Precursors

All‐cellulose composites (ACCs) are manufactured from high‐performance cellulose fibers and a cellulose‐containing ionic liquid (IL) as matrix‐forming dope via wet‐winding processes, using different concentrations of cellulose in the IL. ACCs are carbonized at 1650 °C and then infiltrated with liquid silicon. Application of a carbonization aid (ammonium dihydrogenphosphate, ADHP) substantially improves the carbon yield after carbonization but also results in the depletion of the mechanical properties of the final carbon/carbon silicon carbide (C/C‐SiC) material. The microstructure of the porous carbon/carbon preforms strongly depends on both the concentration of cellulose in the IL and the concentration of ADHP. A C/C‐SiC composite manufactured from 6 wt% cellulose in the matrix‐forming dope, in the absence of ADHP, has a maximum flexural strength of 60 MPa. New C/C‐SiC composites with different shapes including Z‐profiles and tubes are successfully manufactured from pre‐shaped ACC precursors. These composites keep their shape during carbonization and the final siliconization process step.     

Effect of PyC interphase thickness on mechanical behaviors of SiBC matrix modified C/SiC composites fabricated by reactive melt infiltration

A dense carbon fiber reinforced silicon carbide matrix composites modified by SiBC matrix (C/SiC-SiBC) was prepared by a joint process of chemical vapor infiltration, slurry infiltration and liquid silicon infiltration. The effects of pyrolytic carbon (PyC) interphase thickness on mechanical properties and oxidation behaviors of C/SiC-SiBC composites were evaluated. The results showed that C/SiC-SiBC composites with an optimal PyC interphase thickness of 450 nm exhibited flexural strength of 412 MPa and fracture toughness of 24 MPa m^1/2, which obtained 235% and 300% improvement compared with the one with 50 nm-thick PyC interphase. The enhanced mechanical properties of C/SiC-SiBC composites with the increase of interphase thickness was due to the weakened interfacial bonding strength and the decrease of matrix micro-crack amount associated with the reduction of thermal residual stress. With the decrease in matrix porosity and micro-crack density, C/SiC-SiBC composites with 450 nm-thick interphase exhibited excellent oxidation resistance. The residual flexural strength after oxidized at 800, 1000 and 1200 °C in air for 10 h was 490, 500 and 480 MPa, which increased by 206%, 130% and 108% compared with those of C/SiC composites.     

Carbon‐Polymer Composite Bipolar Plate for HT‐PEMFC

Graphene reinforced carbon‐polymer composite bipolar plate is developed using resole phenol formaldehyde resin, and conductive reinforcements (natural graphite, carbon black, and carbon fiber) using compression molding technique. Graphene is reinforced into the composite to alter various properties of the composite bipolar plate. The developed composite bipolar plate is characterized and the effect of temperature on mechanical and electrical properties is investigated with an overall aim to achieve benchmark given by US‐DOE and Plug Power Inc. The flexural strength and electrical conductivity of the composites was almost stable with the increase in temperature upto 175 °C. The composite bipolar plate maintained high in‐plane and through‐plane electrical conductivities, which is about 409.23 and 98 S cm^–1, respectively, at 175 °C. The flexural strength and shore hardness of the developed composite was around 56.42 MPa and 60, respectively, at 175 °C, and on further increase in the temperature the mechanical strengths deceases sharply. The electrical and mechanical properties of the composite bipolar plates are within the US‐DoE target. However, the various properties of the composite bipolar plate could not be sustained above 175 °C.     

In Situ Processing of Graphene/Leucite Nanocomposite Through Graphene Oxide/Geopolymer

Graphene/leucite nanocomposites (rGO/leucite) were prepared through in situ reduction of graphene oxide/geopolymer (rGO/KGP) composites. The effects of rGO on the microstructure and mechanical properties with respect to the geopolymer matrix after the high‐temperature treatment were investigated systematically. The results show that GO is first partially reduced in the geopolymeric solution and then completely under the post high‐temperature treatment. The rGO sheets undergo no interfacial reactions with the matrix even after thermal treatment. The rGO/geopolymer composites fully transform to rGO/leucite composites after being treated at 1000°C for 30 min in an argon atmosphere. Significant improvements in mechanical properties were achieved through rGO reinforcement giving flexural strength, elastic modulus, and fracture toughness of 91.1 MPa, 60.5 GPa, and 2.04 MPa·m^1/2, increased by 120%, 8%, and 1.5%, respectively, compared with the leucite matrix alone.     

Effect of heat treatment on the microstructure and strength of yttrium silicate matrix-modified SiCf/SiC composites

Yttrium silicate was introduced into the matrix of SiC_f/SiC composites via the slurry impregnation and reactive chemical vapor infiltration (RCVI) methods to improve the water and oxygen corrosion resistance of the modified composite materials. The effects of heat treatment on the modified matrix and strength of the composites were systematically investigated. The results showed that the modified matrix was composed of a mixture of yttrium monosilicate, yttrium disilicate, and silicon carbide. The modified yttrium silicate matrix (named Y-Si-O matrix) and the silicon carbide matrix were laminated and well combined. After heat treatment, the amount of Y-Si-O in the mixed matrix increased. The modified composites with yttrium silicate had a similar flexural strength as SiC_f/SiC composites (∼400 MPa). After treated at 1000 °C – 1300 °C, the strength of the modified composites increased by 17 %–26 %. The highest strength was measured for composites treated at 1200 °C.     

The influence of post‐cure on properties of carbon/phenolic resin cured composites and their final carbon/carbon composites

Two‐dimensional (2D) carbon/carbon (C/C) composites were prepared with phenol‐formaldehyde resin and graphite fabric. After curing, polymer composites were post‐cured in air at 160°C and 230°C for several hours and then all polymer composites were carbonized up to 1500°C. The effect of post‐cure on the microstructure and fracture behavior of the resultant carbon/carbon composites was studied. The post‐cure process was characterized by weight loss. This process promoted the crosslinking and condensation reactions and led to the formation of long‐chain, cross‐linked polymeric structures in the matrix. The post‐cured composites had a greater density than the unpost‐cured composite. This study indicates that a longer post‐curing time and higher post‐curing temperature would limit the shrinkage for the post‐cured composites during carbonization. The improvement in linear shrinkage was 22% to 44%. This process also limited the formation of open pores and decreased the weight loss of the resultant C/C composites. The resultant C/C composites developed from post‐cured composites had a greater flexural strength by 7 to 26% over that developed from unpost‐cured composite.     

Bio-based hydroxymethylated eugenol modified bismaleimide resin and its high-temperature composites

Hydroxymethylated eugenol (MEG) and poly (hydroxymethylated eugenol) (PMEG) were synthesized by the condensation reaction of eugenol (EG) with formaldehyde. The different contents of MEG and PMEG were used to modify 4,4′-bismaleimidediphenylmethane (BMI). The cured MEG-BMI resins exhibit good thermal stability evidenced by its 5% weight loss temperatures above 407°C and its residue above 39.4% at 800°C under nitrogen. For carbon/MEG-BMI composites, their glass transition temperatures were around 400°C; their flexural strength and moduli were maintained at a range of 488.87–575.47 MPa and 48.84–60.26 GPa, respectively. With the increasing content of BMI in the resin formulation, the flexural properties decreased; comprehensively the composite with the eugenol/maleimide unit ratio (1:0.3 mol) had the best mechanical and thermal properties, meanwhile its renewable carbon content was as high as 57.80%. As a new candidate of high temperature thermosetting resin, MEG would find promising applications for advanced composites' matrice.