Tetsuro Tojo

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.

Water Vapour Adsorption of Microporous Carbon Films Prepared from Fluorinated Aromatic Polyimides

The adsorption of water vapour onto microporous carbon films derived from fluorinated aromatic polyimides was studied at 25°C over the relative pressure range 0–0.95. The carbon films prepared were microporous with a high micropore volume ranging up to 0.44 ml/g and a sharp pore-width distribution at ca. 0.6 nm. The adsorption/desorption isotherm of water vapour onto the carbon films was of type V with slight hysteresis. The carbon films derived from fluorinated polyimides were found to exhibit a much higher water vapour adsorption at low relative pressure regions relative to commercially available activated carbon fibres, with their saturated adsorption capacities being dependent on their micropore volumes. The microporous carbon derived from the polyimide with the highest fluorine content (31.3 mass%) showed the highest saturated adsorption capacity of ca. 465 mg/g, with 90 mass% of the water vapour being adsorbed at a relative pressure below 0.46. Both the high micropore volume and the high water vapour adsorption were maintained even after heat treatment at 1300°C, although the steep uptake of water vapour was shifted to higher relative pressures to some extent.

Reduction of Stacking Faults in Fast Epitaxial Growth of 4H-SiC and its Impacts on High-Voltage Schottky Diodes

Generation of stacking faults (SFs) in fast epitaxial growth of 4H-SiC(0001) has been reduced in vertical hot-wall chemical vapor deposition (CVD). 52 µm-thick epilayers with and without SFs are used to investigate impacts of SFs on the performance of Schottky barrier diodes (SBDs). The density, shape and structure of stacking faults have been characterized by cathodeluminescence (CL), photoluminescence (PL) and high-resolution transmission electron microscopy (HR-TEM). These analyses indicate that most (> 75 %) SFs with an 8H structure are generated near the epilayer/substrate interface during CVD. It is also revealed that the SFs cause the lowering of Schottky barrier height as well as the decrease of breakdown voltage.

Fast Epitaxial Growth of Thick 4H-SiC with Specular Surface by Chimney-Type Vertical Hot-Wall Chemical Vapor Deposition

Fast epitaxial growth of 4H-SiC epilayers at high temperatures has been investigated in a chimney-type vertical hot-wall chemical vapor deposition (CVD), which has enabled a high growth rate up to 44 μm/h with mirror-like surface morphology. The C/Si ratio dependencies of micropipe closing ratio, doping concentration and photoluminescence (PL) spectra have been investigated. High micropipe closing ratio of 94 ∼ 100 % and low doping concentration of 6.7×10 cm have been achieved at a high growth rate of 19 μm/h. A 51 μm-thick epilayer grown at 36 μm/h led to produce high-voltage Ni SBDs (Schottky Barrier Diode) with a small ideality factor of 1.1 and a breakdown voltage of 3420 V. Introduction Recent progress in silicon carbide (SiC) crystal growth technology has supported the fabrication of high-power devices. One of the major remaining issues in SiC epitaxial growth is its relatively low growth rate, typically 2 ∼ 6 μm/h. Although a few attempts of fast epitaxial growth with growth rates from 10 to 25 μm/h at high temperatures have been reported [1–3], investigation and understanding of material properties is still limited. In this paper, the authors describe fast epitaxial growth of 4H-SiC at high temperatures in a chimney-type reactor. The growth rate, uniformity and quality of epilayers are presented. Experiments The epitaxial growth was carried out with an originally-designed vertical hot-wall chimney-type CVD reactor in a SiH4-C3H8-H2 system at a growth temperature of 1835 °C [3]. The flow rate of H2 carrier gas was fixed at 5 slm. The flow rates of SiH4 and C3H8 as source gases were varied in the range of 10 ∼ 24 sccm and 2 ∼ 6 sccm, respectively. A C/Si ratio of 0.7 was typically used at a pressure of 100 Torr. 4H-SiC (0001) wafers with 8° off-oriented toward the [11-20] direction were mounted on one of the inner channel walls of reactor. Results & Discussion Figure 1 shows the SiH4 flow rate dependence of growth rate and surface roughness, which was obtained for growth at 1835 °C with a C/Si ratio of 0.7 for 1 hour. The growth rate proportionally increased with the SiH4 flow rate, although the extrapolated growth rate is negative at a low SiH4 flow rate due to significant H2 etching at the high temperature. High growth rates from 14 to 44 μm/h could Materials Science Forum Online: 2004-06-15 ISSN: 1662-9752, Vols. 457-460, pp 205-208 doi:10.4028/www.scientific.net/MSF.457-460.205