Toshiaki Sogabe

Anode property of boron-doped graphite materials for rechargeable lithium-ion batteries

Abstract Anode properties of boron-doped graphites were investigated by means of electrochemical measurements. A discharge capacity of about 315 mAh/g was obtained for 3.8 mass% boron-doped pitch coke-derived graphite in galvanostatic measurements. Increased discharge capacity for boron-doped graphite compared with boron-free one was considered to be mainly due to the enhanced graphitization by boron. Also, in this measurements, a shoulder was observed at 1.3 V (vs. Li/Li+) in boron-doped graphites. This peculiar reaction was confirmed to be the diffusion control and reversible process by the cyclic-voltammogram measurements, and it occurred before intercalation of lithium-ion into graphite-layers in charge process and after deintercalate of lithium-ion from graphite layers in discharge process. These phenomena are inferred to be explained as that the lithium atom can be trapped easily to boron solid-solution phase due to the valence band hole created by boron. This capacity existed at high potential is probably worthwhile for the utilization as the signal of the ending of discharge process to prevent over-discharging in rechargeable lithium-ion batteries.

Behavior of plasma-sprayed tungsten coatings on CFC and graphite under high heat load

Tungsten coatings of 0.5 and 1 mm thickness were successfully deposited by the vacuum plasma spraying technique on carbon/carbon fiber composite (CFC), CX-2002U, and isotropic fine grained graphite, IG-430U. High heat flux experiments by irradiation of electron beam with uniform profile were performed on the coated samples in order to prove the suitability and load limit of such coating materials. Heat load properties, gases emission, surface modification and structure changes of cross-section of the samples were investigated. Cracks on the surface and exfoliation between the joint interface of the samples were not formed below the melting point. These results indicated that the thermal and adhesion properties between the substrate and coatings were good under high heat flux. Microstructure of the joint interface of the sample was changed in the case of a peak temperature at about 2800°C. Many cracks and traces of melted tungsten flow were observed on the surface after melting and solidification. Large cavities were also formed inside the resolidified tungsten layer.

Effects of titanium impregnation on the thermal conductivity of carbon/copper composite materials

Abstract Carbon/copper-based materials with high thermal conductivity and good stability at high temperatures were developed by adding a small amount of titanium. The isotropic fine-grained nuclear grade graphite and felt type C/C composite, which were impregnated by copper (10–18 vol.%) and titanium (0.5–0.8 vol.%), provided ∼1.3 times higher thermal conductivity of 110 and 200 W/mK at 1200 K than the original carbon materials. Microstructural analyses showed that the increase of thermal conductivity is due to the formation of titanium compounds at the carbon/copper interface, and that the thermal energy would pass through both the carbon and copper. The present study indicates that addition of a small amount of a third element with a low enthalphy of alloy formation with carbon and copper will increase the thermal conductivity and the stability of carbon/copper-based materials. These carbon-based materials could be one of candidate materials for the plasma facing components of the fusion devices.

Improvement in properties and air oxidation resistance of carbon materials by boron oxide impregnation

Abstract Boron oxide (B 2 O 3 ) has been impregnated into a high-density isotropic graphite block and a felt carbon fiber reinforced carbon composite (CC composite). Impregnation was carried out at 1200 °C under a pressure of 15 MPa for 1 hour. The bulk density of the original graphite and CC composite increased by 12% and 20% respectively as a result of impregnation, the mechanical properties of these materials were improved significantly, and the thermal conductivity slightly increased. Gas permeability was reduced by B 2 O 3 impregnation to the order of 10 −5 cm 2 /s from 10 −1 cm 2 /s of the original graphite, and to 10 −2 cm 2 /s from 10 cm 2 /s of the original CC composite. Oxidation loss by air of both B 2 O 3 -impregnated materials appeared to be almost suppressed at temperatures below 800 °C. Serious oxidation loss was observed in both the impregnated materials by exposing to air for several hours at 850 °C because of loss of B 2 O 3 from the specimen.

High heat load properties of tungsten coated carbon materials

Tungsten coatings of 0.5 and 1.0 mm thickness were successfully deposited by the vacuum plasma spraying technique (VPS) on carbon/carbon fiber composite, CX-2002U, and isotropic fine grained graphite, IG-430U. High heat flux experiments were performed on the coated and non-coated samples in order to prove the suitability and load limit of such coating materials. The electron beam irradiation experiments showed that there was little difference in temperature increases among CX-2002U and the coated materials below surface temperature of 2200°C. These results indicated that thermal and adhesion properties between the substrate and coatings were good under high heat flux. A few cracks with a width of 15 μm were formed from the surface to the bottom side of the all coated samples, but plastic deformation and microcracks due to grain growth by recrycrallzation were not observed below a surface temperature of about 2200°C. The cracks are expected to be formed due to local thermal stress produced by spot-like beams.

Changes of composition and microstructure of joint interface of tungsten coated carbon by high heat flux

Abstract Tungsten coatings of 0.5 and 1 mm thickness were successfully deposited by the vacuum plasma spraying (VPS) technique on carbon/carbon fiber composite (CFC), CX-2002U and isotropic fine grained graphite, IG-430U. High heat flux experiments by irradiation of electron beam with uniform profile were performed on the coated samples in order to prove the suitability and load limit of such coating materials. The cross-sectional composition and structure of the interface of VPS–W and carbon material samples were investigated. Compositional analyses showed that the Re/W multi-layer acts as diffusion barrier for carbon and suppresses tungsten carbide formation in the VPS–W layer at high temperature about 1300°C. Microstructure of the joint interface of the sample changed in the case of a peak temperature of about 2800°C. The multi-layer structure completely disappeared and compositional distribution was almost uniform in the interface of the sample after melting and resolidification. The diffusion barrier for carbon is not expected to act in this stage.

High heat flux test of actively cooled tungsten-coated carbon divertor mock-ups

Abstract CX-2002U and IG-43OU coated by VPS-W were developed to be as a light high-Z plasma-facing material. After brazing them on OFHC blocks using a Ti foil, their thermal response and thermal fatigue properties were examined with active cooling. No cracks and no exfoliation occurred on the W surface and the braze interface even after 160 cycles of heat load for 20 s at 10 MW/m2. This result indicates that the Ti-brazing is a possible alternative to Ag-brazing for joining carbon to Cu. Heat load resistance of the VPS-W coated CX-2002U/OFHC was much better than the VPS-W coated IG-430U/OFHC due to the excellent thermal conductivity of CX-2002U. VPS-W coated CFC/OFHC is a potential candidate for a high heat resistance armor material on plasma facing components.

Fabrication and High Heat Flux Tests of Plasma Sprayed Tungsten Coated Carbon and TZM

High density tungsten is coated by vacuum plasma spraying technique (VPS) on tiles, 20mn x 20mn x 10mm. Substrate materials are carbon/carbon composite (CX-2002U), isotropic fine-grained graphite(IG-430U). Thickness of the tungsten-coating layer is 0.5 mm. The tungsten-coated tiles are jointed by Ti brazing on the OFHC surface with a cooling tube. In addition, VPS-W coated TZM also has been brazed. Thermal response and thermal fatigue lifetime tests using an electron beam facility have been carried out on the tungsten coated mock-ups under the active cooling condition. Heat flux is changed from 1 to 10 MW/m^2. The heat flux experiments have been carried out under the condition that the water flow velocity, pressure and temperature are 15.0 m/s, 0.5 MPa and 293 K, respectively. The use of high density VPS-W improves the performance of mock-ups under steady state heat flux condition. In addition, in the case of high density W coated CX-2002U on OFHC and W coated TZM on OFHC, it is demonstrated that the mock-up successfully withstood 100 cycles of heat loads at 10 MW/m^2 at steady state.