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Tungsten powders were successfully sintered and simultaneously joined to a graphite disk using an interlayer of SiC ceramic powder, which contained Al 2 O 3 and Y 2 O 3 as sintering aids for the SiC. The tungsten/graphite joints were prepared at 30 MPa and temperatures ranging from 1700 °C to 2000 °C for 5 min using spark plasma sintering (SPS). The tensile strength of the joints prepared at 1700 °C reached 21 MPa. The suggested joining mechanism of the joints prepared below 1800 °C is as follows: both the sintered W metal layer and the SiC ceramic interlayer are chemically bonded together accompanying reaction phases of WC, W 2 C and W 5 Si 3 . A physical bond is achieved between the SiC interlayer and graphite when the SiC powder fills the open pores of the graphite and is sintered there. Although a similar joining mechanism is suggested when the joining temperature exceeds 1900 °C, the SiC interlayer disappears, and the broad reaction layer consisting of WC, W 2 C and trace W 5 Si 3 is formed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Joining of AlN and graphite was achieved by SPS. [•] The effect of interlayer tapes contained either AlN and graphite or AlN was investigated. [•] Both tapes contained 5 mass%Y2O3 as sintering aid of AlN. [•] The maximum tensile strength of joints was obtained with the AlN-graphite composite. [•] AlN and graphite were joined physically with Al–Y–O phase during SPS. [Copyright &y& Elsevier]

Abstract: After placing SiC powder on an isotropic graphite substrate, the two materials were successfully joined by spark plasma sintering (SPS). The effect of an Al2O3–Y2O3 sintering aid on SiC during joining was studied. The tensile strength of the joints prepared at 1800°C under 30MPa for 5min reached 18MPa. The fracture occurred not at the interface, but at the graphite substrate. The joining mechanism of SiC/graphite is attributed to the following: after the SiC powder squeezes into the open pores of graphite during sintering, the sintered SiC produces a strong bond through the interface between SiC and graphite. [Copyright &y& Elsevier]

A dense SiC coating on carbon particles has been developed using SiO powders as the Si source and a chemical vapor reaction method. The coating process was controlled by atmosphere, coating temperature, time, and weight ratio of SiO/C. The SiC-coated carbon particles, with a mean particle size of 20 μm, were sintered at 2000°C under 40 MPa for 20 min by spark plasma sintering ( SPS). The product, called ceramic bonded carbon ( CBC), had a unique microstructure, with a 3D micro-network structure consisting of carbon particles covered with 1~5 μm thick SiC boundary layers. The sintering behavior of SiC/ CBCs was analyzed in terms of the network formation of SiC boundary layers at over 1800°C and the grain growth of SiC at 2000°C. SiC/ CBCs showed a bending strength of 150 MPa and a low shore hardness of 40. These materials could also be easily joined to SiC ceramics using SPS. [ABSTRACT FROM AUTHOR]

New composites called ceramic-bonded carbon ( CBCs), consisting of a three-dimensional structure of carbon particles bonded with thin ceramic boundaries, were developed. To fabricate light and tough CBCs and to understand their reinforcing mechanism, Si3N4 and carbon powders (25:75 in volume ratio) were gelcasted and then sintered by spark plasma sintering at temperatures of 1700-1900°C. The ceramic boundary of SiC was formed in situ at above 1700°C by the reaction of Si3N4 and C. The sintered CBCs showed a unique microstructure consisting of carbon particles and ceramic boundaries of 15 μm in size and 0.5-3 μm in thickness, respectively. With an increase in sintering temperature, physical bonding of ceramic grains to the carbon particles was enhanced as the grain growth of SiC increased. The SiC/ CBCs sintered at 1900°C were highly dense (97% theoretical density), lightweight (2.36 mg/m3), and had both relatively high bending strength and thermal conductivity (135 MPa and 140 W/mK, respectively). [ABSTRACT FROM AUTHOR]

Abstract: To obtain light and tough materials with high thermal conductivity, AlN ceramic bonded carbon (AlN/CBC) composites were fabricated at temperatures from 1600 to 1900°C in a short period of 5min by the spark plasma sintering technique. All AlN/CBCs (20vol% AlN) have unique microstructures containing carbon particles of 15μm in average size and continuous AlN boundary layers of 0.5–3μm in thickness. With an increase in sintering temperature, AlN grains grow and anchor into carbon particles, resulting in the formation of a tight bonding layer. The AlN/CBC sintered at 1900°C exhibited a light weight (2.34g/cm3), high bending strength (100MPa), and high thermal conductivity (170W/mK). [Copyright &y& Elsevier]

Abstract: To obtain light, strong materials with high thermal conductivity, a new carbon-based material, AlN ceramic bonded carbon (AlN/CBC), was fabricated by combining gelcasting and spark plasma sintering techniques. The results showed that AlN/CBC (20vol% AlN) has a unique microstructure containing carbon grains of 15μm in size and an AlN grain-boundary layer of 0.5–3μm in thickness. Continuous AlN ceramic networks bonded the carbon grains together. Compared with the conventional AlN/carbon (AlN/C) material made by a ball-milling method, AlN/CBC showed a higher strength and a higher thermal conductivity by two and four times, respectively. [Copyright &y& Elsevier]

Dense three-dimensional (3D) microdevices of ZrO2-toughened Al2O3 (ZTA) were fabricated using microstereolithography and a subsequent sintering process. Using microstereolithography, 3D green bodies could be formed from a 40 vol% ZTA ceramic–resin paste. After sintering, the fabricated 3D devices are converted into dense ceramic devices without deformation. In this study, a gear (with a tooth edge of 25 μm) and a photonic crystal (with a lattice constant of 500 μm) were designed and fabricated. The dimensional accuracy of the fabrication process is within 20 μm and the sintering shrinkage is around 26% for these microdevices. The relative density of the sintered ZTA ceramics reached 96.5% of theoretical value. The measured hardness and toughness were about 14 GPa and 11 MPa m1/2, respectively, in both the top and side surfaces. A band gap between 320 and 420 GHz was observed in the ZTA photonic crystal. The microstereolithography process can be easily applied to other ceramic materials and devices. [ABSTRACT FROM AUTHOR]

Abstract: Multi-walled carbon nanotubes (MWCNTs) were coated with a nanometer-sized SiC polycrystalline layer by the reaction of SiO(g) and CO(g). Nanometer-sized SiC powders were mixed with 1–5vol% SiC-coated MWCNTs and successfully sintered at 1800°C by means of pulsed electric current sintering. These composites showed superior microhardness of 30.6GPa and toughness of 5.4MPam1/2 compared to monolithic SiC ceramics due to the improvement of adhesion between MWCNTs and the SiC matrix by the SiC coating. The microhardness and toughness of the monolithic SiC ceramics are 25.5GPa and 4.8MPam1/2, respectively. The SiC-coated MWCNTs/SiC composites showed an interesting behavior with the elastic recovery when undergoing an indentation test. [Copyright &y& Elsevier]

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO2 ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO2 paste (before sintering) was around 15 μm. After the SiO2-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO2 ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO2 photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X〈100〉 direction, between 490 and 630 GHz in the Γ– K〈110〉 direction, and between 400 and 510 GHz in the Γ– L〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method. [ABSTRACT FROM AUTHOR]

Dense Al2O3-based composites (≥99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300°C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al2O3 powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength σb (∼800 MPa) and fracture toughness K IC (∼5 MPa·m1/2) were attained at the composition of CNF/Al2O3 = 5:95 vol%, which σb and K IC values were ∼25% and ∼5%, respectively, higher than those of monolithic Al2O3. This might be due to the small Al2O3 grains (1.6 μm) of dense sintered compacts compared with that (4.4 μm) for the pure Al2O3 ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al2O3 composites decreased rapidly from >1015 to ∼2.1 × 10−2 Ωm (5vol%CNF addition), suggesting the machinability of Al2O3-based composites by electrical discharge machining. [ABSTRACT FROM AUTHOR]

Ceramic photonic crystals with diamond structure were fabricated using stereolithography and successive sintering. The green body of an epoxy resin incorporating 10 vol% TiO2–SiO2 was formed by stereolithography and then heated in air at 1100°–1400°C for 2 h. The sintered products maintained the diamond structure with a linear shrinkage ratio of about 57% and a porosity of 38%. The ceramic photonic crystal with eight unit cells showed a photonic band gap at the center frequency of 23.5 GHz. This fabrication method of three-dimensional (3D) ceramic photonic crystals is applicable to other 3D structural ceramics and does not require any molding techniques. [ABSTRACT FROM AUTHOR]

A simple process for depositing a coating of silicon carbide (SiC) crystallites ∼10 nm in size onto diamond particles has been developed. SiO powders react with diamond in a vacuum at 1350°C to form a uniform β-SiC polycrystalline layer ∼60 nm thick. The SiC coating improves the oxidation resistance of the diamond. A cemented carbide material containing 20vol%-SiC-coated diamond particles was sintered to a relative density of 99.5% by pulsed-electric-current sintering. A Vickers hardness and indentation fracture toughness of 15 GPa and 16.3 MPa·m[sup ½] respectively, were obtained. This toughness is two times higher than that of cemented carbide containing no particles. The higher toughness is attributed to deflection and blockage of crack propagation by the diamond particles. [ABSTRACT FROM AUTHOR]

Abstract: A new carbon composite called ceramic bonded carbon (CBC) is composed of separated carbon particles bonded with thin ceramic boundaries. To fabricate reliable plasma facing components, AlN ceramic bonded carbon (AlN/CBC, 20vol.% AlN) was directly cladded with tungsten (W) at 1700°C by spark plasma sintering. The cladding shows strong bonding, which is attributed to the formation of interface layer containing mixed WC/W2C and AlN ceramic. The interface between W and AlN/CBC was analyzed and the cladding mechanism was discussed. [Copyright &y& Elsevier]

Abstract: Fabrication process for three-dimensional ceramic photonic crystals with a diamond structure was investigated. A diamond structure composed of epoxy lattice including SiO2–TiO2 ceramic particles at 10vol.% was fabricated as a precursor by stereolithography. After burning off the epoxy resin in air, the diamond structure of SiO2–TiO2 was successfully sintered at 1400°C for 2.5h. The linear shrinkage ratio was 50%. Cracks were not found in the sintered diamond structure. Photonic bandgap was observed at around 19GHz. [Copyright &y& Elsevier]

Abstract: Single-phase β-sialon powders (z =2–4) have been prepared with homogeneous compositions by the combustion synthesis. The raw materials (Si, Al and SiO2) were combusted under N2 pressure of 1MPa. Without using a diluent, the reaction temperatures were very high (>2000°C) and the combustion products contained Si and Al residues. With addition of commercial β-sialon (z =1) as a diluent (up to 50wt%), both the reaction temperatures and amount of residual Si and Al significantly decreased. The combustion reactions completed at 50wt% dilution, and pure β-sialon phases were synthesized. When the combustion product itself is used instead of the commercial diluent, the phase content of desired z value increased with the repetition times until a single-phase powder is produced. The sinterability of the synthesized powders was then tested using 5wt% Y2O3 as a sintering aid by the spark plasma sintering (SPS). [Copyright &y& Elsevier]

Abstract: Carbon-matrix composites with self-assembly interconnected carbon nano-ribbon networks were fabricated using natural and inexpensive rice husks by impregnating the rice husks with transitional metal solutions and sintering. The nano-structure of the composite was characterized using transmission electron microscopy. The electromagnetic interference shielding effectiveness of the composite was evaluated in the frequency range of 30kHz–1.5GHz. The electrical conductivity was measured as a function of the sintering temperature. These characteristics were compared with those of carbon-matrix composites without nano-ribbon networks. [Copyright &y& Elsevier]

Abstract: Dense diamond/cemented carbides were fabricated at around 1300°C, 41MPa for 3min under meta-stable conditions for diamonds by pulsed-electric current sintering. Diamond particles were coated with SiC to prevent the graphitization of diamond with molten binder during sintering. No graphitization of diamond was confirmed by microstructure observation and Raman scattering analysis. The combination of SiC coating of diamond and rapid sintering was effective to fabricate diamond/cemented carbides. These new materials, composed of diamond and cemented carbide, showed a 50% larger fracture toughness and 10 times better wear resistance compared with conventional cemented carbides, and five times better machinability compared with diamond compacts. The diamond/cemented carbides are applicable to wear-resistant sliding tools such as centerless blades, work rests, and stoppers. [Copyright &y& Elsevier]