Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Exceptional micromachining performance of silicon carbide ceramics by adding graphene nanoplatelets

The electrical discharge machining (EDM) performance of silicon carbide (SiC) ceramics containing graphene nanoplatelets (GNPs) is investigated for the first time. Under fine machining conditions, the material removal rate (MRR) dramatically increases up to 186% when 20 vol.% of GNPs are added to SiC ceramics, leading to reductions on the electrode wear rate of 132%. The EDMed nanocomposites exhibit surface roughness ≤ 0.8 μm. This outstanding EDM response of the graphene nanocomposites as compared to monolithic SiC is explained by their enhanced transport properties, establishing a direct dependence of MRR with the electrical conductivity. EDM performance of the nanocomposites also depends on the testing direction for materials with low GNPs connectivity (≤ 10 vol.%). Melting/evaporation are the main removal mechanisms, thermal spalling also operating for low thermal conducting materials. The employ of EDM on SiC/graphene nanocomposites allows machining microparts with a fine dimensional precision, opening new opportunities for SiC-based microcomponents.     

Effect of graphene addition on the mechanical and electrical properties of Al2O3-SiCw ceramics

This paper presents a study on graphene-reinforced Al₂O₃-SiCw ceramic composites and the relationship between graphene oxide (GO) loading and the resulting mechanical and electrical properties. Well-dispersed ceramic-GO powders were fabricated using a colloidal processing route. Dense composites were obtained via spark plasma sintering, a technique that has the ability to reduce GO to graphene in situ during the sintering process. The mechanical properties of the sintered composites were investigated. The composite with only a small amount of graphene (0.5 vol.%) showed the highest flexural strength (904 ± 56 MPa), fracture toughness (10.6 ± 0.3 MPa·m^1/2) and hardness (22 ± 0.8 GPa) with an extremely good dispersion of graphene within the ceramic matrix. In addition to these exceptional mechanical properties, the sintered composites also showed high electrical conductivity, which allows the compacts to be machined using electrical discharge machining and thus facilitates the fabrication of ceramic components with sophisticated shapes while reducing machining costs.     

Alumina/zirconia composites toughened by the addition of graphene flakes

The effect of added graphene flakes on the mechanical properties of a composite containing 20 wt% Al₂O₃ and 80 wt% ZrO₂ (stab. 3 mol% Y₂O₃) was studied. To obtain samples, a commercial ceramic powder produced by Tosoh (Japan), and graphene oxide (GO) made at the Institute of Electronic Materials Technology (Poland) were used. The obtained composites were based on aqueous mixtures of both components. After drying, they were sintered in an uniaxial pressure (HP) furnace. The composites contained from 0% to 3% of GO by weight. Results showed the influence of GO content i.e. fracture toughness has a maximum for 0.02% GO (increase by 42% in comparison to GO-free matrix) and afterwards decreased, strength decreased in the whole GO content range. Young's modulus and Vickers hardness remained constant up to 0.2% GO, and then decreased.     

Wire electrical discharge machinable SiC with GNPs and GO as the electrically conducting filler

SiC based composites filled with graphene nano-platelets (GNPs) or graphene oxide (GO) prepared by rapid hot-pressing exhibit sufficient electrical conductivity for their machinability by wire electro-discharge machining (WEDM). Composites microstructure anisotropy caused by graphene alignment as a consequence of rapid hot pressing was confirmed by measuring of electrical conductivity and thermal diffusivity. Electrical conductivity increased significantly with increased weight fraction of graphene in both measured directions. Highest value of 2031 S/m was obtained for composites with 15 wt. % of GNPs in parallel direction and only 1246 S/m in perpendicular direction to aligned GNPs. Thermal diffusivity is 63.3 mm²/s in parallel and only 23.3 mm²/s in perpendicular direction. The increase of the electrical conductivity has resulted in successful WEDM. The MRR was almost doubled when the filler concentration increased from 5 wt. % GNPs/GO to 15 wt. % GNPs. At the same time, the surface roughness decreased.     

Effect of SiC whiskers and graphene nanosheets on the mechanical properties of ZrB2-SiCw-Graphene ceramic composites

Ultrahigh temperature ZrB₂-SiC_w-Graphene ceramic composites are fabricated by hot pressing ZrB₂-SiC_w-Graphene oxide powders at 1950 °C and 30 MPa for 1 h. The microstructures of the composites are characterized by Scanning electron microscopy, Raman spectroscopy and X-ray diffraction. The results show that multilayer graphene nanosheets are achieved by thermal reduction of graphene oxide during sintering process. Compared with monolithic ZrB₂ materials, flexural strength and fracture toughness are both improved due to the synergistic effect of SiC whisker and graphene nanosheets. The toughening mechanisms mainly are the combination of SiC whisker and graphene nanosheets crack bridging, pulling out.     

Graphene nanoplatelet/silicon nitride composites with high electrical conductivity

Silicon nitride (Si₃N₄) processed with up to 25 vol.% of graphene nanoplatelets (GNPs) gives conductive composites with the highest electrical conductivity (40 Scm^?1) reported for these ceramics with added conductive particles. During compaction and pressure-assisted densification of the composites in the spark plasma sintering (SPS), a preferred orientation of GNPs occurs. Consequently, the electrical conductivity measured along the direction perpendicular to the SPS pressing axis is more than one order of magnitude higher than the one measured along the parallel direction.Percolation in the composites is observed for 7–9 vol.% of GNPs, depending on the measuring direction, perpendicular or parallel to the pressing axis. Different conduction mechanisms are apparent for the two orthogonal orientations. Charge transport along the direction defined by the graphene ab-plane (perpendicular direction) may be explained by a two dimensional variable range hopping mechanism, whereas conduction in the parallel direction shows a more complex behavior, with a metallic-type transition (dσ/dT < 0) for high GNP contents. A thin amorphous layer was identified at the Si₃N₄/GNPs interface that may affect the conduction for the parallel configuration.     

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9 wt.% of Y₂O₃) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200 S m⁻¹) for graphene contents above 5 vol.%.     

Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites

The Graphene/Mn_0.7Zn_0.3Fe₂O₄ composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343–973 K. The effect of graphene on the thermoelectrical properties of Mn_0.7Zn_0.3Fe₂O₄ was investigated. The dispersion of 2 wt% graphene in Mn_0.7Zn_0.3Fe₂O₄ effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973 K was obtained in the composite with 2 wt% graphene.     

Mechanical properties and sliding wear behaviour of Al2O3-SiC nanocomposites with 3–20 vol% SiC

Al₂O₃/SiC composites containing different volume fractions (3, 5, 10, 15, and 20 vol%) of SiC particles were produced by conventional mixing of alumina and silicon carbide powders, followed by hot pressing at 1740 °C for 1 h under the pressure of 30 MPa in the atmosphere of Ar. The influence of the volume fraction and size of SiC particles (two different powders with the mean size of SiC particles 40 and 200 nm were used), and final microstructure on mechanical properties and dry sliding wear behaviour in ball-on-disc arrangement were evaluated. The properties of the composites were related to a monolithic Al₂O₃ reference. Microstructure of the composites was significantly affected by the volume fraction of added SiC, with the mean size of alumina matrix grains decreasing with increasing content of SiC particles. The addition of SiC moderately improved the Vickers hardness. Fracture toughness was lower with respect to monolithic Al₂O₃, irrespective of the volume fraction and size of SiC particles. Al₂O₃/SiC nanocomposites conferred significant benefits in terms of wear behaviour under the conditions of mild dry sliding wear. Wear resistance of the alumina reference was poor, especially at the applied load of 50 N. The wear rates of composites markedly decreased with increasing volume fraction of SiC. Wear of the composites was also influenced by the material of counterparts, especially their hardness, with softer counterparts resulting in lower wear rates. All composites wore by a combination of grain pull-out with plastic deformation associated with grooving and small contribution of mechanical wear (micro-fracture). No influence of SiC particle size on wear rate or mechanism of wear was observed in the materials with identical volume fractions of SiC.     

Catalytic graphene formation in coal tar pitch- derived carbon structure in the presence of SiO2 nanoparticles

A simple and effective way to manufacture graphene from a coal tar pitch (CTP) is demonstrated. Silica (SiO₂) nanoparticles were used to modify the CTP as carbon precursor. A silica nanofiller introduced into the CTP matrix underwent carboreduction during heat treatment to 2000 °C, resulting in the formation of silicon carbide. Surfaces of SiC grains were sites for graphene formation. The influence of SiO₂ on the structure and microstructure of CTP- based carbon matrix, after annealing up to 2800 °C, was studied. Carbon samples were analyzed using X- ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Raman Spectroscopy. Crystallite sizes (L_a, L_c) and interplanar distance (d₀₀₂) were determined. The presence of SiO₂ in CTP carbon precursor favored the crystallites’ growth in the ‘a′ crystallographic graphite direction, and inhibited their growth on the ‘c′ axis. The crystallites composing of graphene layers, were characterized by an elongated dimension in the ‘a′ axis direction. Above 2000 °C silicon carbide decomposed, followed by the sublimation of silicon from the carbon matrix.     

Fracture characteristics of SiC/graphene platelet composites

Silicon carbide/graphene platelet (SiC/GPLs) composites were prepared using different weight percent of GPLs filler by hot pressing (HP) technology at 2100 °C in argon. The influence of the GPLs addition on bending strength, fracture toughness and related fracture characteristics was investigated. Both the bending strength and fracture toughness increased with increasing GPLs additives. The main fracture origins – strength degrading defects were pores at the low content of platelets and combination of pores and GPLs or clusters of GPLs particles in systems with a higher content of platelets. The fracture toughness increased due to the activated toughening mechanisms mainly in the form of crack bridging and crack branching, while the crack deflection was limited. The highest fracture toughness of 4.4 MPa m^1/2 was achieved at 6 wt.% of GPLs addition, which was ∼30% higher than the K_IC value of the reference material.     

Microstructure and mechanical properties of hot pressed Al2O3/SiC nanocomposites

Al₂O₃/SiC micro/nano composites containing different volume fractions (5, 10, 15, and 20 vol.%) of SiC were prepared by mixing a sub-micron alumina powder with respective amounts of either micro- or nano-sized silicon carbide powders. The powder mixtures were hot pressed 1 h at 1740 °C and 30 MPa in the atmosphere of Ar. The effect of SiC addition on the microstructure and mechanical properties, i.e. hardness, fracture toughness, and room temperature flexural strength were investigated. The flexural strength increased with increasing volume fraction of silicon carbide particles. The maximum flexural strength (655 ± 90 MPa) was achieved for the composite containing 20 vol.% of coarse-grained SiC, which is more than twice as high as in the Al₂O₃ reference. Hardness and fracture toughness were also moderately improved. The observed improvement of mechanical properties is mainly attributed to alumina matrix grain refinement and grain boundary reinforcement.     

Reactive hot pressing of SiC-ZrC composites at low temperature

SiC-ZrC composites with relative density in excess of 99% were prepared by reactive hot pressing (RHP) of SiC and ZrH₂ at 1800 °C for 1 h. The reaction between SiC and ZrH₂ resulted in the formation of ZrC_1-x. The formation process and densification behavior during RHP process were investigated. Low temperature densification of SiC-ZrC composites is attributed to the formed nonstoichiometric ZrC_1-x and the removal of SiO₂ impurity on the surface of SiC particles. As reinforced phase, ZrC_1-x has inhibiting effect on the abnormal grain growth of SiC, resulting in homogeneous microstructure of fine SiC grains. Adding 10 wt% ZrH₂ to SiC, the formed SiC-4.62 vol% ZrC composite exhibited better mechanical properties (Vickers hardness of 27.6 ± 0.7 GPa, flexure strength of 448 ± 38 MPa, fracture toughness of 6.0± 0.3 MPa·m^1/2, respectively) than monolithic SiC ceramic.     

Effect of homogenization treatment on the fracture behaviour of silicon nitride/graphene nanoplatelets composites

Si₃N₄ based composites with 7 wt.% of graphene nanoplatelets (GNPs) were prepared using different homogenization methods. Si₃N₄/GNPs powder mixtures were dispersed in isopropanol and homogenized by attritor milling, ball milling or planetary ball milling. The ball milling technique was also used for the homogenization of Si₃N₄/GNPs mixture in dry state. Fractography analysis was carried out in order to assess the individual homogenization treatment. Depending on the homogenization methods, the size of the processing flaws varied from 20 μm up to 400 μm. The agglomeration of the GNPs and the residual porosity were found as the most frequently observed types of the critical flaws. The planetary ball milling with previous ultrasonication of GNPs in isopropanol was found to be the most promising homogenization technique, resulting in the composites with the highest bending strength (average value is 740 MPa) and the lowest average size of the processing flaws (around 20 μm).     

Al2O3–SiC composites prepared by warm pressing and sintering of an organosilicon polymer-coated alumina powder

Al₂O₃/SiC micro/nano composites were prepared by axial pressing of poly(allyl)carbosilane-coated submicrometre alumina powder at elevated temperature (called also warm pressing, or plastic forming) with subsequent pressureless sintering in the temperature interval between 1700 and 1850 °C. Warm pressing at 350 °C and 50 MPa resulted in green bodies with high mechanical strength and with markedly higher density than in green bodies prepared by cold isostatic pressing of the same powder at 1000 MPa. The sintering of warm pressed specimens moreover yielded the composites with higher final density (less than 4% of residual porosity) with the microstructure composed of micrometer-sized alumina grains (D₅₀ < 2 μm) with inter- and intragranular SiC precipitates. High sintering temperatures (>1800 °C) promoted the formation of intergranular platelets identified by TEM as 6H polytype of α-SiC. The maximum hardness (19.4 ± 0.5 GPa) and fracture toughness (4.8 ± 0.1 MPa m^1/2) were achieved in the composites containing 8 vol.% of SiC, and sintered for 3 h at 1850 °C. These values are within the limits reported for nanocomposites Al₂O₃/SiC by other authors and do not represent any significant improvement in comparison to monolithic alumina.     

Graphene for tough and electroconductive alumina ceramics

A simple, fast and upscalable method is described to produce graphene/alumina (G/Al₂O₃) composites by spark plasma sintering (SPS) with a significant improvement on both mechanical and electrical properties of monolithic Al₂O₃. Graphene oxide (GO) was mixed with Al₂O₃ using a colloidal method obtaining an excellent dispersion of GO in the alumina matrix. The material was consolidated by SPS that allowed, in one-step, the in situ reduction of the GO during the sintering process. A detailed Raman analysis was found to be very useful to study the orientation of the graphene in the composite and to evaluate and optimise its thermal reduction. Graphene platelets acted as elastic bridges avoiding crack propagation and providing this material with a crack bridging reinforcement mechanism. A very low graphene loading (0.22 wt%) led to a 50% improvement on the mechanical properties of the alumina and to an increase of the electrical conductivity up to eight orders of magnitude.     

Sintering behaviour of silicon carbide matrix composites reinforced with multilayer graphene

The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1], [2], [3], [4], [5], [6], [7], [8], [9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.     

Surface modification of graphene oxide nanoplatelets and its influence on mechanical properties of alumina matrix composites

This paper discusses the effect of modified graphene oxide nanoplatelets (RGO-Al₂O₃) and unmodified graphene oxide nanoplatelets (GO) addition on the microstructure and mechanical properties of alumina matrix composites. The sinters were prepared by powder metallurgy processing using Spark Plasma Sintering to consolidate the powder mixtures. Moreover, the influence of applied reinforcing phase on the fracture mechanism was also investigated. Significant improvement of the fracture toughness (60%) for the composites with 0.5 wt.% RGO-Al₂O₃ compared to the reference sample was observed. Moreover, 20% higher K_IC was noticed for RGO-Al₂O₃ reinforced composites than for Al₂O₃-GO.     

Comparison of the homemade and commercial graphene in heightening mechanical properties of Al2O3 ceramic

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al₂O₃ powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m^1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al₂O₃ composites. Compared to the blank Al₂O₃ sintered samples, the graphene/Al₂O₃ specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al₂O₃ ceramic.