Carbon Preforms Research Articles

Thermopower coefficient of pine-wood biocarbon preforms and the biocarbon-preform/copper composites

The thermopower S(T) of the composites prepared by filling empty sap channels in high-porosity biocarbon preforms of white pine wood by melted copper in vacuum and the thermopower S(T) of these preforms have been measured in the temperature range 5–300 K. The biocarbon preforms have been obtained by pyrolysis of pine wood in an argon flow at two carbonization temperatures, 1000 and 2400°C. An analysis of the experimental data has demonstrated that the thermopower of the composites is determined by the contribution related to the copper filling the channels of the biocarbon preform and exhibits a characteristic temperature dependence with a deep minimum close to 20 K. This suggests that copper in the preform channels is essentially a Kondo alloy with the iron and manganese impurities entering into it from the carbon preform in the course of infiltration of melted copper.

Elasticity and inelasticity of biomorphic carbon, silicon carbide, and SiC/Si composite produced on the basis of medium density fiberboard

The amplitude and temperature dependences of the Young’s modulus and the internal friction (ultrasonic absorption) of biomorphic carbon, silicon carbide, and SiC/Si composite produced from medium density fiberboard (MDF) by pyrolysis (carbonization), followed by infiltration of molten silicon into the prepared carbon preform have been studied in the temperature range 100–293 K in air and under vacuum. The measurements have been performed by the acoustic resonance method with the use of a composite vibrator for longitudinal vibrations at frequencies of approximately 100 kHz. The data obtained by acoustic measurements of the amplitude dependences of the elastic modulus have been used for evaluating the microplastic properties of samples under study. It has been shown that the Young’s modulus, the decrement of elastic vibrations, and the conventional microyield strength of the MDF samples differ from the corresponding data for previously studied similar materials produced from natural eucalyptus, beech, sapele, and pine woods. In particular, the desorption of environmental molecules at small amplitudes of vibrations, which is typical of biomorphic materials based on natural wood, is almost absent for the MDF samples. The results obtained have been explained by different structures and the influence of pores and other defects, which, to a large extent, determine the mechanical characteristics of the biomaterials under investigation.

Fabrication and properties of reaction-formed SiC by infiltrating molten Si into mesocarbon microbeads-based carbon preform

Reaction-formed SiC ceramic was fabricated by infiltrating molten Si into the carbon preform derived from a mixture of mesocarbon microbeads (MCMBs) and SiC powders which content was ranged from 10 to 40 wt.%. SiC powder limited the volume shrinkage of MCMBs during sintering and the porosity of the preform can be adjusted by altering SiC powder content. Aromatic layers in the carbon spheres derived from MCMBs were split below 1300 °C due to the graphitization effect of SiC powder. Molten Si infiltrated into the interfaces between carbon spheres as well as the microcracks inside the split spheres. As a result, network SiC ceramic was formed and some unreacted carbon distributed uniformly in the matrix. The reaction-formed SiC ceramics have excellent mechanical properties (the maximum bending strength, fracture toughness and hardness of 359 MPa, 4.4 MPa m 1/2 and 2348 Hv, respectively) and electrical conductivity (the lowest electric resistivity of 18.18 μΩ m).

Microstructural and mechanical evaluation of porous biomorphic silicon carbide for high temperature filtering applications

Biomorphic SiC (bioSiC) is a low cost SiC/Si composite obtained by melt infiltration of carbon preforms obtained from the pyrolysis of cellulose precursors. The porosity and pore size distribution of bioSiC can be tailored for specific applications by adequate selection of the wood precursor. Natural and artificial industrial woods were explored as possible bioSiC precursors. Silicon was removed by chemical etching. Relevant microstructural parameters such as pore size distribution, total porosity, and permeability were characterized. Since the filtration process involves large pressure gradients along the material at high temperatures, mechanical properties of porous bioSiC from the different precursors were evaluated at room temperature and 800 °C. The feasibility of porous bioSiC as a filtration material for high temperature gasification processes is discussed in terms of these properties. MDF-bioSiC is shown to be a promising material for such applications because of its good mechanical properties, interconnected porosity, pore sizes, and permeability.

Thermal conductivity of the pine-biocarbon-preform/copper composite

The thermal conductivity of composites of a new type prepared by infiltration under vacuum of melted copper into empty sap channels (aligned with the sample length) of high-porosity biocarbon preforms of white pine tree wood has been studied in the temperature range 5–300 K. The biocarbon preforms have been prepared by pyrolysis of tree wood in an argon flow at two carbonization temperatures of 1000 and 2400°C. From the experimental values of the composite thermal conductivities, the fraction due to the thermal conductivity of the embedded copper is isolated and found to be substantially lower than that of the original copper used in preparation of the composites. The decrease in the thermal conductivity of copper in the composite is assigned to defects in its structure, namely, breaks in the copper filling the sap channels, as well as the radial ones, also filled by copper. A possibility of decreasing the thermal conductivity of copper in a composite due to its doping by the impurities present in the carbon preform is discussed.

Synthesis and Characterization of Laminated Si/SiC Composites

Abstract:Laminated Si/SiC ceramics were synthesized from porous preforms of biogenous carbon impregnated with Si slurry at a temperature of 1500oC for 2 hours. Due to the capillarity infiltration with Si, both intrinsic micro- and macrostructure in the carbon preform were retained within the final ceramics. The SEM micrographs indicate that the final material exhibits a distinguished laminar structure with successive Si/SiC layers. The produced composites show weight gain of ≈ 5% after heat treatment in air at 1300 oC for 50 h. The produced bodies could be used as high temperature gas filters as indicated from the permeability results.

Dimensional stability of multiaxis 3D-woven carbon preforms

Multiaxis 3D-woven carbon preforms are fabricated using prototyped multiaxis weaving. The fabricated preform has structural instability at thickness. For this reason, the structural parameter and processing parameters were evaluated to make uniform preform and to understand the preform-process relations. The important process parameters are identified and described to enhance the preform and unit cell architecture. Also, preform structural parameters are analyzed in terms of fiber cross-section and fiber tow size, bias angle, and fiber waviness. The useful recommendation is also to make uniform multiaxis 3D-woven preform for composites.

Manufacture of Biomorphic SiC Components with Homogeneous Properties from Sawdust by Reactive Infiltration with Liquid Silicon

Biomorphic SiC components with homogeneous properties were manufactured from sawdust using a novel method to produce preforms, without addition of any extra binder, but with enough mechanical strength to be carbonized up to 1400°C without deformation. Reactive infiltration of carbon preforms of an adequate open porosity with liquid silicon has been successfully used to prepare biomorphic components. Moreover, the modification of bioSiC properties induced by the structural rearrangement of carbon preforms when they are further heat treated at 2500°C was additionally studied. BioSiC components showed a maximum in bending strength when the material is processed from carbon preforms exhibiting around 40% of open porosity, which seems to be the optimum value for carbon preforms treated at 1400° and 2500°C. However, the heat treatment of the carbon preforms at 2500°C produced bioSiC components with a finer and more homogeneous microstructure than those obtained from carbon preforms treated at 1400°C, improving their bending strength up to 22%.

Characterization of 3D morphology and microcracks in composites reinforced by multi-axial multi-ply stitched preforms

The micro-structure of polymer matrix composites reinforced by multi-axial multi-ply stitched carbon preforms and manufactured by liquid resin infusion is analyzed. The stitching induces deviations in fibre placement and creates openings which become resin-rich regions after the resin infusion. Characterization of the size and shape of the resin-rich regions of composites with different stitching yarn size and tightness and various stacking sequences has been performed by 2D metallographic micrography and X-ray microtomography. The resin-rich region volume was estimated at roughly 3.0 ± 0.5% of the material volume. The resin-rich regions constitute about 9% of the resin in the entire composite, whose fibre volume fraction is close to 65%. X-ray microtomography was successfully used to characterize the 3D microcracks created by hygrothermal fatigue.

A wax-mold route to fabricate polymer-derived SiC scaffold

A wax-mold route was developed to fabricate phenolic resin-derived SiC scaffold. Firstly, a wax-mold with the desired structure of the scaffold was obtained. Then resin mixtures were poured into the mold. After curing and pyrolyzing, porous carbon preform was obtained. The wax-mold was removed through being melted during curing, and the pattern material could be recycled. Finally, the SiC scaffold was fabricated by infiltrating liquid silicon into the preform. The dimension shrinkage of SiC scaffold was between 1.3 and 3 % before/after infiltration, which was affected by the infiltrating temperature and the starting components in resin mixtures, and there is no distortion. When the preform with apparent porosity of 45.5% was employed, the flexural strength and the density of SiC scaffold were 445 MPa and 3.07 g/cm 3, respectively.

Heat capacity and thermopower coefficient of the carbon preform of sapele wood

This paper reports on measurements of the heat capacity at constant pressure Cp in the 80–300-K temperature interval and the thermopower coefficient S at 5–300 K of the carbon preform of sapele wood, which was prepared at the carbonization temperature of 1000°C. Measurements of Cp(T), our previous data on the phonon thermal conductivity, and literature information on the sound velocity have been used to calculate the phonon mean free path l(T) for this material. It has been shown that within the temperature interval 200–300 K, l is constant and equal to 11 A, a figure matching the size of the nanocrystallites (“graphite fragments”) making up the carbon framework of the sapele carbon preform. The high-temperature parts of S(T) have been found to follow a linear course characteristic of diffusive thermopower for the degenerate state of charge carriers, with only one type of charge carriers present. The anisotropy of the thermopower coefficient has been estimated.

Heat capacity of the white pine biocarbon preform and the related biocarbon/copper composite

This paper reports on measurements in the 80–300-K temperature interval of the heat capacity at constant pressure C_(p)(T) of high-porosity amorphous white pine carbon preforms (biocarbon) prepared by pyrolysis (carbonization) at T_(carb) = 1000 and 2400°C in an argon flow. The dependences C_(p)(T) for biocarbon/copper composites based on the carbon preforms obtained have also been determined. It is shown that the mixture rule holds for the composites, i.e., that C_(p)(T) of the composite is a sum of the heat capacities of the constituent materials taken in the corresponding ratios. Phonon mean free paths for the white pine carbon preforms prepared at T_(carb) = 1000 and 2400°C have been calculated and used to estimate the size of the nanocrystallites contributing to formation of the carbon frameworks of these preforms.

Microstructural evolution during silicon carbide (SiC) formation by liquid silicon infiltration using optical microscopy

Abstract Optical microscopy and quantitative digital image analysis were used to examine the formation of fully dense, net shape silicon carbide by liquid silicon infiltration (LSI) of porous carbon preforms. By examining the phase distribution and structural changes during the reaction, we identified six reaction stages (I–VI) that describe reaction mechanisms and their time scales. The initial stages (0–15 min) of the LSI reaction include (I) liquid silicon infiltration of the carbon preform, (II) dissolution of carbon, and (III) formation of silicon carbide at the liquid–solid interfacial regions. These initial stages occur simultaneously and very rapidly, and culminate in (IV) the completion of a continuous silicon carbide layer of about 10 μm at every liquid–solid interface. Further reaction can only be achieved by (V) carbon diffusion through this layer. The reaction is essentially complete after ∼120 min. Longer reaction times should be avoided because over-reacting causes (VI) long, thin silicon-filled cracks to develop within the continuous silicon carbide matrix.

Rapid manufacture of net-shape SiC components

Stereolithography was introduced into the net-shape SiC components fabrication process to produce the molds for the preparation of porous carbon preforms. The mixed resin was cast into the molds and then pyrolyzed to produce the porous carbon preforms which had a high porosity of 40.93% and were infiltrated and reacted with molten silicon to get reaction-formed silicon carbide (RFSC) components. To realize the complete infiltration of the thick wall parts, pipelines as the channels for the molten silicon were added into the components. The hierarchical structures of the porous carbon had been attained to realize the controllability of the microstructure and properties of RFSC. The samples had a high linear and volume shrinkage, 24.7% and 57.3%, respectively, during the pyrolysis process. Phase composition had been investigated by optical microscopy and X-ray diffraction analysis. The results indicate that no residual carbon was available in the final RFSC. The net-shape fabrication process for the RFSC components could promote the industry applications where SiC components with complex surface and inner structure were needed.

Development of titanium-doped carbon–carbon composites

The development of titanium-doped carbon matrix–carbon fibre reinforced composites (CCCs) via liquid impregnation of carbon fibre preforms using mesophase pitch is studied. Two different approaches for introducing the dopant into the carbon material are investigated. One consists of doping the matrix precursor followed by the densification of the preform with the doped precursor. The second approach consists of doping the porous preform prior to densification with the undoped mesophase pitch. Titanium-doped CCCs with a very fine distribution of dopant (in the nanometric scale) are obtained by adding TiC nanoparticles to the matrix precursor. Thermal decomposition of titanium butoxide on the carbon preform prior to densification yields doped CCCs with higher titanium content, although with larger dopant size. The combination of these two methods shows the best results in terms of dopant content.

Capillary infiltration studies of liquids into 3D-stitched C–C preforms: Part A: Internal pore characterization by solvent infiltration, mercury porosimetry, and permeability studies

Mathematical modeling of silicon infiltration in porous carbon–carbon (C–C) preforms is the key to fabricate liquid silicon infiltration based carbon–silicon carbide (C–SiC) composite components. Existing models for silicon infiltration are based on straight capillaries. For interconnected capillary systems, e.g. as in 3D-stitched C–C preforms these show large deviations when compared with experimental observations. The aim of the present study is to develop a mathematical model suitable for silicon infiltration in 3D-stitched C–C preforms. The work is being presented in two parts: A and B. This part (Part A) describes the experimental details pertaining to the fabrication of the C–C preforms and their pore structure characterization by mercury porosimetry, infiltration of solvents by capillary rise, and by permeability studies. A two-pore capillary infiltration model termed as modified Washburn equation has been proposed. It has been validated by experimental data of solvent infiltration. The same model correlates silicon infiltration observations as well (Part B).

The combined effect of porosity and reactivity of the carbon preforms on the properties of SiC produced by reactive infiltration with liquid Si

This study is focused on the effect of the porosity and chemical reactivity with silicon of the carbon preforms on the properties of SiC pieces produced by reactive infiltration. Petroleum residues were selected as carbon precursor because of their unique combination of properties. On the one hand, semicoke powders with optimum self-sinterability can be obtained from them to develop a wide range of porous carbon preforms without addition of binder to carry out the porosity study. On the other hand, reactivity of the carbon material with silicon can be modified by employing semicokes with different mesophase microstructure obtained from petroleum residues with different chemical composition. SiC yield was strongly affected by the degree of anisotropy of the carbon material, the most ordered carbon being more reactive. SiC pieces made from the optimum carbon preforms (made up by combining the most graphitic carbon and 40% of open porosity) achieved bending strength values of around 260 MPa. Chemical reactivity of carbon substrates was also modified by graphitization treatment. An improvement in the mechanical properties of SiC pieces up to 66% was achieved due to the structural modification caused by graphitization of the carbon preforms. Bending strengths twice the value of commercially available ones were achieved.

The effect of carbon nanotube growing on carbon fibers on the microstructure of the pyrolytic carbon and the thermal conductivity of carbon/carbon composites

Carbon nanotubes were prepared on the surface of carbon fibers in a unidirectional carbon preform and then chemical vapor deposition was used to densify the preform to get a modified carbon/carbon composite. The effects of carbon nanotubes on the microstructure of the pyrolytic carbon and the thermal conductivity of the composite were investigated. Results show that carbon nanotubes on the surface of carbon fibers induce the deposition of ordered pyrolytic carbon around the carbon fibers and adjust the bonding between carbon fibers and pyrolytic carbon. Due to the modification of carbon nanotubes, the thermal conductivity of the composites with nanotubes is greater than that of composites without nanotubes, especially the thermal conductivity perpendicular to the fiber axis.

Reaction forming of silicon carbide ceramic using phenolic resin derived porous carbon preform

SiC ceramics were prepared with porous carbon preforms derived from phenolic resin by a reaction-forming method. The effects of the structure of the preform pores and the infiltration process on the properties of SiC ceramics were investigated, and components with complex shapes were fabricated by combining this process with stereolithography (SLA). Dense SiC ceramics were obtained from carbon preforms with high apparent porosities, but SiC ceramics with many macrodefects resulted from a carbon preform with an apparent porosity of 39%. The infiltration of molten silicon into the preform pore channel was accelerated under vacuum pressure, resulting in an increase in the depth of the Si infiltration. The growth of SiC was predominantly controlled by carbon diffusion at the last stage of the reaction. The extended grain growth caused the SiC grains to coalesce and some free Si was enveloped in the SiC grains. SiC components with complex geometries were fabricated by combining reaction forming with SLA. The geometry was controlled by SLA.

Properties of Si/SiC ceramic composite subjected to chemical vapour infiltration

Si/SiC ceramic composite was prepared by infiltration of liquid silicon into carbon preforms that was made from cotton fabric and phenolic resin. This composite was subjected to the chemical vapour infiltration (CVI), using methyltrichlorosilane as a precursor gas. The effect of infiltration time on densification and mechanical properties was studied. Results show a significant improvement in density by pore closure. Flexural strength increases with increasing infiltration time. However, beyond 60 h of infiltration, the strength improvement was insignificant. The high temperature oxidation resistance of the above ceramics was also studied. The CVI treated samples show considerable resistance to oxidation compared to untreated samples. Thermogravimetric analysis also confirmed the better oxidation resistance of the CVI treated samples.