Pitch-based ribbon-shaped carbon-fiber-reinforced one-dimensional carbon/carbon composites with ultrahigh thermal conductivity

Ribbon-shaped carbon fibers have been prepared from mesophase pitch by melt-spinning, oxidative stabilization and further heat treatment. The internal graphitic layers of ribbon-shaped carbon fibers graphitized at 2800 °C show a highly preferred orientation along the longitudinal direction. Parallel stretched and unidirectional arranged ribbon-shaped carbon fibers treated at about 450 °C were sprayed with a mesophase pitch powder grout, and then hot-pressed at 500 °C and subsequently carbonized and graphitized at various temperatures to produce one-dimensional carbon/carbon (C/C) composite blocks. The shape and microstructural orientation of ribbon fibers have been maintained in the process of hot-pressing and subsequent heat treatments and the main planes of the ribbon fibers are orderly accumulated along the hot-pressing direction. Microstructural analyses indicate that the C/C composite blocks have a typical structural anisotropy derived from the unidirectional arrangement of the highly oriented wide ribbon-shaped fibers in the composite block. The thermal conductivities of the C/C composites along the longitudinal direction of ribbon fibers increase with heat-treatment temperatures. The longitudinal thermal conductivity and thermal diffusivity at room temperature of the C/C composite blocks graphitized at 3100 °C are 896 W/m K and 642 mm²/s, respectively.     

Microstructure and thermal/mechanical properties of short carbon fiber-reinforced natural graphite flake composites with mesophase pitch as the binder

Short carbon fiber reinforced graphite blocks (SFGs) were fabricated from a mixture of mesophase pitch, natural graphite flakes and short carbon fibers by hot-pressing at 2773 K. The effect of fiber content on the structure and thermal/mechanical properties of the SFGs was investigated. It was found that introducing the fibers lowered the densification, and also changed the pore structure and pore size distribution. Compared with the pristine block, all the SFGs earned improved in-plane thermal conductivity and mechanical strength. The formation of a heat flow network and the increase of crystalline sizes made a synergistic effect on the promotion of in-plane thermal conductivity. In-plane thermal conductivity reached the maximum when the fiber content was 6 wt.%. The increase of mechanical strength was mainly attributed to the pull-out of fibers from the matrix. The bend and compressive strength in the direction perpendicular to graphite layers reached the maximum values of 39.6 MPa and 65.5 MPa for fiber content of 8 wt.%, respectively.     

Preparation of erythritol–graphite foam phase change composite with enhanced thermal conductivity for thermal energy storage applications

Three-dimensional interconnected graphite composite foam as a heat conductive matrix was fabricated by using low cost polymeric precursors and polyurethane (PU) foam as carbon source and sacrificial macroporous template, respectively. Erythritol–graphite foam as a stable composite phase change material (PCM) was obtained by incipient wetness impregnation method. The thermophysical properties such as thermal diffusivity, specific heat, thermal conductivity and latent heat of the erythritol–graphite composite foam were measured. From the results, it was found that the thermal conductivity of the erythritol–graphite composite foam (3.77 W/mK) was enhanced 5 times as compared with that of pristine erythritol (0.72 W/mK). This enhancement can significantly reduce the charging and discharging times of the PCM storage system. There is no chemical reaction between erythritol and graphite as confirmed by X-ray diffractometer (XRD). The PCM/foam composite has a melting point of 118 °C and latent heat of 251 J/g which corresponds to the mass percentage (75 wt.%) of the erythritol within the composite foam. The obtained results confirmed the feasibility of using erythritol–graphite foam as a new phase change composite for thermal energy storage (TES) applications, thus it can contribute to the efficient utilization and recovery of solar heat or industrial waste heat.     

Preparation by tape casting and hot pressing of copper carbon composites films

During this last decade, the use of metal matrix composites (MMCs) such as AlSiC or CuW for heat dissipation in microelectronic devices has been leading to the improvement of the reliability of electronic power modules. Today, the continuous increasing complexity, miniaturization and density of components in modern devices requires new heat dissipating films with high thermal conductivity, low coefficient of thermal expansion (CTE), and good machinability. This article presents the original use of copper carbon composites, made by tape casting and hot pressing, as heat dissipation materials. The tape casting process and the sintering have been adapted and optimised to obtain near fully dense, flat and homogeneous Cu/C composites.A good electrical contact between carbon fibres and copper matrix and a low porosity at matrix/copper interfaces allow obtaining a low electrical resistivity of 3.8 μΩ cm⁻¹ for 35 vol.% carbon fibre (electrical resistivity of copper = 1.7 μΩ cm⁻¹). The CTE and the thermal conductivity are strongly anisotropic due to the preferential orientation of carbon fibres in the plan of laminated sheets. Values in the parallel plan are, respectively, 9 × 10⁻⁶ °C⁻¹ and 160–210 W m⁻¹ K⁻¹ for 40 vol.% fibres. These CTE and thermal conductivity values are in agreement with the thermo-elastic Kerner's model and with the Hashin and Shtrikman model, respectively.     

Amorphous silica-coated graphite particles for thermally conductive and electrically insulating resins

A thermally conductive and electrically insulating composite filler was produced by surfactant assisted sol–gel coating of amorphous silica on flake graphite. Amorphous silica-coated graphite (a-Si coated grp) obtained using a cationic surfactant showed the best enhancement of the insulating coating. The resulting a-Si coated grp/boehmite/polybutylene terephthalate polyester resin composite exhibited a high volume resistivity, exceeding 1.0 × 10¹⁴ Ω cm at an applied voltage of 500 V, and a thermal conductivity of 3.3 W/m K at 22.9 vol.% a-Si coated grp loading. The heat releasing performance of the developed resin composite in actual light-emitting diodes bulb housings was compared with conventionally used thermally and electrically conductive resin. This comparison revealed that the new composite released heat more effectively. This innovative technology, which may solve the trade-off between material properties and cost, will be available for a broad range of thermally conductive resin applications that simultaneously require thermal conduction and electrical insulation.     

Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

Microstructure and thermophysical properties of B4C/graphite composites containing substitutional boron

B₄C/graphite composites (BGC) containing substitutional boron were fabricated by pressureless sintering of powder mixtures of petroleum coke, coal tar pitch and B₄C. After sintering at 900 °C and graphitizing at 2200 °C, the microstructure of BGC was characterized by SEM, TEM, XRD, Raman spectroscopy and optical microscopy. XPS measurements revealed the formation of BC₃, and the matrix carbon contained around 6 wt.% substitutional boron. The thermal conductivity of the BGC at room temperature is 52.7 W/m K and the flexural strength is up to 35.1 MPa. The bulk density and electrical resistivity are 1.72 g/cm³ and 13.4 μΩ m, respectively. The correlation between microstructure and properties was investigated. The results showed that the microstructure improvement of the BGC has obvious effect on the thermal conductivity, flexural strength, and electrical resistivity.     

Low cost preparation of high thermal conductivity carbon blocks with ultra-high anisotropy from a commercial graphite paper

A carbon block with ultra-high anisotropy was produced from a commercial graphite paper as the thermal reinforcement and a thermosetting phenolic resin as the binder. Hot-pressing at a maximum temperature of 200 °C was used to densify and integrate the graphite paper stacks. It has been found that the graphite paper blocks have high thermal conductivities in the paper direction and low ones perpendicular. An anisotropy of 98.8% and a thermal conductivity of 197.8 W m^?1 K^?1 in the paper direction were achieved when the density was 1.1 g cm^?3. The thermal conductivity increased to 284.8 W m^?1 K^?1 with a decrease of anisotropy to 98.3% with a density of 1.56 g cm^?3.     

Electric field-assisted sintering of tin dioxide with manganese dioxide addition

Green compacts of pure SnO₂ and with addition of 0.25, 0.5 and 1.0 wt.% MnO₂ were sintered by applying 100 V cm⁻¹ at 1 kHz and limiting the current to 5 A during 5 min at 1100 and 1200 °C. The shrinkage was monitored precisely during the electric current pulses. The role played by the additive was clearly seen in the shrinkage data: the higher is the additive content the lower is the onset of the shrinkage and the higher is the attained final shrinkage level. Sintering experiments on cylindrical samples with 0.5 wt.% MnO₂ with different thickness-to-diameter ratio show that the lower is that ratio, the higher is the shrinkage level, showing that the imparted Joule heating play a key role in the mechanisms responsible for sintering. The total electrical resistivity, evaluated by impedance spectroscopy, depends on the maximum attained shrinkage level, due to pore elimination upon sintering.     

Preparation and properties of mullite-bonded porous fibrous mullite ceramics by an epoxy resin gel-casting process

Porous fibrous mullite ceramics with a narrow range of pore size distribution have been successfully prepared utilizing a near net-shape epoxy resin gel-casting process by using mullite fibers, Al₂O₃ and SiC as raw materials. The effects of sintering temperatures, different amounts of fibers and Y₂O₃ additive on the phase compositions, linear shrinkage, apparent porosity, bulk density, microstructure, compressive strength and thermal conductivity were investigated. The results indicated that mullite-bonded among fibers were formed in the porous fibrous mullite ceramics with a bird nest pore structure. After determining the sintering temperatures and the amount of fibers, the tailored porous fibrous mullite ceramics had a low linear shrinkage (1.36–3.08%), a high apparent porosity (61.1–71.7%), a relatively high compressive strength (4.4–7.6 MPa), a low thermal conductivity (0.378–0.467 W/m K) and a narrow range of pore size distribution (around 5 µm). The excellent properties will enable the porous ceramics as a promising candidate for the applications of hot gas filters, thermal insulation materials at high temperatures.     

Measuring the thermal conductivity of residue-free suspended graphene bridge using null point scanning thermal microscopy

Despite the importance of the accurate measurement of the thermal conductivity of graphene, deviations in previous data are still quite large due to the low signal-to-noise ratio in the measurement of graphene temperature, the uncertainties in the measurement of the heat dissipation, and the influence of the polymeric residues. Herein, we improve signal-to-noise ratio by using null point scanning thermal microscopy, which profiles temperature quantitatively with nanoscale spatial resolution (∼50 nm), independently of both the heat flux through the air and the variation of the sample surface properties. Also, we control and monitor the heat generation rate accurately by heating the suspended graphene bridge electrically. Furthermore, we prevent the disturbance of the thermal conductivity caused by the polymeric residues by using polydimethylsiloxane stamping method, which leaves much less residue than using polymethylmethacrylate. The thermal conductivity values of graphene, whose length and width are 3.6 and 5.52 μm, respectively, were measured as 2430 ± 190, 2150 ± 170, and 2100 ± 160 W/mK at the peak temperatures of 335, 361, and 366 K, respectively, with much smaller error range compared to the previously reported values. The measured values exceed the highest value (∼2000 W/mK at room temperature) obtained for graphite.     

Preparation of nanoporous alumina superinsulator with ultra-low thermal conductivity and improved heat resistance up to 1200 °C

Nanoporous alumina superinsulator (NanoASI) with ultra-low thermal conductivity and excellent thermal stability has been prepared by a low-cost and simple dry pressing method. The thermal conductivity of the NanoASI is as low as 0.11 W/m K at 1200 °C and the linear shrinkage is less than 2% after heating at 1200 °C for 1 h. These values are superior to that of previous reported nanoporous insulation materials. Thermal conductivities of this material in the temperature range of 25–1200 °C and pressure range of 10–10⁵ Pa were firstly measured by the transient hot-plane method. The mechanism that improves the heat resistance of the NanoASI is discussed and found that the stabilization of the alumina nanoparticles contributes significantly to the thermal stability of the NanoASI.     

Infiltration mechanism of diamond/SiC composites fabricated by Si-vapor vacuum reactive infiltration process

Dense diamond/SiC composites were fabricated by Si vapor vacuum reactive infiltration of carbon-containing diamond porous preform at 1600 °C for 1 h. The microstructural evolution of the composites was investigated. The infiltration mechanisms during reactive infiltration were discussed. The composite consists of diamond, β-SiC and a small amount of Si. Epitaxial growth of nano-sized SiC on diamond and graphite surfaces occurred due to the diffusion-reaction mechanism in the initial stage of infiltration. Growth of micron-sized SiC with no preferential orientation was controlled by solution-precipitation mechanism in the final stage. The infiltration process was determined both by molecular diffusion and capillary effects. Explosive evaporation of molten Si, volume expansion of the solids and heat release during the reaction were the key factors contributing to the rapid densification of diamond/SiC composites. High thermal conductivity (580 W m^?1 K^?1) and low density (3.33 g cm^?3) of the composites were beneficial to thermal management applications.     

Electromagnetic characteristics and microstructure stability of Nextel 610 fiber after heat treatment

The thermal and microstructure stability of Nextel 610 fibers has great influence on high-temperature application of Nextel 610 fiber-reinforced ceramic matrix composites. In this work, Nextel 610 fibers were heat treated at 500–700 °C in vacuum and 800–1100 °C in Ar atmosphere, respectively. The sizing agent on Nextel 610 fiber surface could be decomposed into pyrolytic carbon, SiC and gaseous little molecules at lower temperatures, otherwise it was decomposed mainly in the form of gaseous little molecules at higher temperatures, so that the complex permittivity firstly increased and then decreased with the increasing of temperatures. The results showed that the annealed Nextel 610 fiber (T>900 °C) could be regarded as electromagnetic wave transparent fibers, while the tensile strength had declined by half when the temperature increased to 1100 °C. Therefore, Nextel 610 fibers after being annealed at higher temperatures could be further used as reinforcement to prepare high temperature ceramic matrix composites for electromagnetic wave absorption and transparent applications.     

Thermal and electrical conduction in the compaction direction of exfoliated graphite and their relation to the structure

The effects of the compaction and graphite layer preferred orientation on the thermal and electrical conductions in the compaction direction of graphite-flake-based exfoliated graphite have been decoupled. The compact’s electrical and thermal conductivities decrease with increasing compaction (density increasing from 0.047 to 0.67 g/cm³, solid content increasing from 2.1 to 30 vol.%) and preferred orientation. The essentially linear correlation between electrical and thermal conductivities (Wiedemann–Franz Law) is because both conductions are governed by the preferred orientation. With increasing compaction, the fraction (f) of conduction path that is the graphite a-axis decreases from 0.997 to 0.937 and from 0.994 to 0.798 for thermal and electrical conductions respectively. For the solid-part thermal and electrical conductivities to exceed 140 W/(m K) and 60 kS/m respectively, f must exceed 0.95; the highest solid-part conductivities are 550 W/(m K) and 230 kS/m. The compaction-related variation in the solid-part conductivities is large [21–550 W/(m K) and 10–230 kS/m], due to the preferred orientation variation. The through-thickness Lorentz number (7.3 × 10⁻⁶ W Ω/K²) is similar to the in-plane value, being independent of the preferred orientation. At 2–7 vol.% solid, conductivities of 7 W/(m K) and 3 kS/m are obtained for the compact – toward the targets for fuel cell biopolar plates.     

Synthesis of silica-coated graphite by enolization of polyvinylpyrrolidone and its thermal and electrical conductivity in polymer composites

Silica-coated graphite flakes, which have electrical insulating property and high thermal conductivity, were synthesized by a polyvinylpyrrolidone (PVP)-assisted sol–gel reaction. The critical role of keto-enol tautomerism of PVP in base-catalyzed silica sol–gel reaction was elucidated. The degree of silica coating on graphite was controlled by the amount of PVP and silica precursor, tetraethyl orthosilicate. The silica-coated graphite was used as a filler in thermoplastic polyester elastomer (TPEE). The in-plane (Λ) and through-plane (Λ_⊥) thermal conductivity values of silica-coated graphite/TPEE composites are 67.5% and 86.6% of those of raw graphite/TPEE at 80 phr loading. Even after a severe mixing process under high shear at elevated temperature, silica-coated graphite/TPEE composites retain the perfectly insulating surface resistivity of >10¹³ Ω/sq up to high filler contents.     

Thermal conductivity of individual carbon nanofibers

In the present paper, we present results of thermal conductivity measurements in commercially-available, chemical vapor deposition grown, heat-treated and non-heat-treated individual carbon nanofibers (CNFs). The thermal conductivity measurements are made using the T-type probe experimental configuration using a Wollaston wire probe inside a high resolution scanning electron microscope. The results show a significant increase in the thermal conductivity of CNFs that are annealed at 2800 °C for 20 h when compared with the non-heat-treated CNF samples. When adjusted for thermal contact resistance, the highest measured thermal conductivity is 449 ± 39 W/m-K. The average thermal conductivity of the heat-treated samples is 163 W/m-K, while the average thermal conductivity of the non-heat-treated samples is 4.6 W/m-K. The results demonstrate the importance of the quality of the CNFs, in particular their heat treatment (high temperature annealing), in controlling their thermal conductivity for thermal management applications.     

Electrical and thermal response of silicon oxycarbide materials obtained by spark plasma sintering

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO₂ glassy matrix reinforced with SiC nanowires grown in situ, graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10⁵ Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.     

Low temperature graphitization of interphase polyacrylonitrile (PAN)

Ordered polyacrylonitrile (PAN) interphase structures were formed in solution-cast PAN/carbon nanotube (CNT) composite films by enhancing polymer crystallization conditions and processing parameters for five types of CNTs. All film samples were heat-treated using similar stabilization and carbonization (up to 1100 °C) processes. Both the precursor and carbonized materials were characterized by electron microscopy and X-ray spectroscopy. Highly ordered graphitic structure was formed predominantly in the carbonized materials at 1100 °C (i.e., ∼1500 °C lower than the temperature used in a commercial graphitization process). The ordering of the graphite structure formed at 1100 °C was further improved by heat treatment up to 2100 °C. Multiple characterization results indicate that the early onset of PAN conversion to graphite is directly related to the polymer interphase formation as well as the CNT type. Based on the stabilization and carbonization parameters used in this study, PAN/single-wall carbon nanotube (SWNT) samples showed more prevalent graphite formation at 1100 °C. This work demonstrates the influence of CNT type regarding interfacial confinement toward this low-temperature polymer-to-graphite conversion process.     

Thermal and electrical conductivity of array-spun multi-walled carbon nanotube yarns

The electrical resistivity of CNT yarns of diameters 10–34 μm, spun from multi-walled carbon nanotube arrays, have been determined from 2 to 300 K in magnetic fields up to 9 T. The magnetoresistance is large and negative at low temperatures. The thermal conductivity also has been determined, by parallel thermal conductance, from 5 to 300 K. The room-temperature thermal conductivity of the 10 μm yarn is (60 ± 20) W m^?1 K^?1, the highest measured result for a CNT yarn to date. The thermal and electrical conductivities both decrease with increasing yarn diameter, which is attributed to structural differences that vary with the yarn diameter.