Yoku Inoue

Interfacial Oxygen Stabilizes Composite Silicon Anodes

Silicon can store Li(+) at a capacity 10 times that of graphite anodes. However, to harness this remarkable potential for electrical energy storage, one has to address the multifaceted challenge of volume change inherent to high capacity electrode materials. Here, we show that, solely by chemical tailoring of Si-carbon interface with atomic oxygen, the cycle life of Si/carbon matrix-composite electrodes can be substantially improved, by 300%, even at high mass loadings. The interface tailored electrodes simultaneously attain high areal capacity (3.86 mAh/cm(2)), high specific capacity (922 mAh/g based on the mass of the entire electrode), and excellent cyclability (80% retention of capacity after 160 cycles), which are among the highest reported. Even at a high rate of 1C, the areal capacity approaches 1.61 mAh/cm(2) at the 500th cycle. This remarkable electrochemical performance is directly correlated with significantly improved structural and electrical interconnections throughout the entire electrode due to chemical tailoring of the Si-carbon interface with atomic oxygen. Our results demonstrate that interfacial bonding, a new dimension that has yet to be explored, can play an unexpectedly important role in addressing the multifaceted challenge of Si anodes.

Seebeck effect in PbTe films and EuTe/PbTe superlattices

Theoretical calculations of the Seebeck coefficients of bulk PbTe and PbTe based superlattices were described in the framework of Boltzmann equation, taking into account temperature dependent band gaps, nonparabolicity, and anisotropy of effective masses. It is shown that the temperature gradient along the superlattice layer works more effectively on the enhancement of the thermoelectric figure of merit than the temperature gradient normal to the superlattice layer. Calculated Seebeck coefficients were compared to the experimental values for n-type PbTe, p-type PbTe, and EuTe/PbTe superlattices. The Seebeck coefficient of p-type PbTe was higher than that of n-type PbTe. The relatively high Seebeck coefficient is explained by the contribution from other extrema in the valence band. The EuTe/PbTe [001] superlattice shows higher Seebeck coefficients than PbTe bulk owing to the large density of states.

Electrical and thermoelectrical properties of SnTe-based films and superlattices

SnTe-based films and superlattices (SLs) were prepared and their electrical properties were measured. A EuTe/SnTe SL exhibited a hole mobility of 2720 cm2/V s, which is the highest value reported for any semiconductor material at room temperature. The SnEuTe film also exhibited high hole mobility in contrast to the PbEuTe system. These properties are explained in terms of the band offsets of EuTe/SnTe heterojunction and a decrease in the number of Sn vacancies. In addition, SnTe/PbSe and SnTe/PbS SLs with thin SnTe layers displayed n-type conduction with Seebeck coefficients comparable to those for PbSe and PbS. These properties reflect the type II heterostructures.

Study of Carbon-Nanotube Web Thermoacoustic Loud Speakers

Thermoacoustic carbon nanotube (CNT) speakers were fabricated using CNT webs spun from a multiwalled carbon nanotube (MWNT) array of 0.8–1.6 mm height. The generated sound pressure level (SPL) showed a linear relationship with frequency over a wide range from 10 Hz to 40 kHz with a slope of 6 dB/octave. In addition to this, significantly broad and flat SPLs were obtained in the ultrasonic region, ranging from 40 to 100 kHz. The distance from the speaker to the microphone was 0.5 m. The high SPL is due to the good heat radiation property of the MWNT web. Herein, we showed an acoustical property for the MWNT web thermoacoustic speaker from the viewpoint of the structural web morphology. The role of heat radiation behavior and the effects of the length of individual MWNTs are discussed.

Mechanical property enhancement of aligned multi-walled carbon nanotube sheets and composites through press-drawing process

A solid-state drawing and winding process was done to create thin aligned carbon nanotube (CNT) sheets from CNT arrays. However, waviness and poor packing of CNTs in the sheets are two main weaknesses restricting their reinforcing efficiency in composites. This report proposes a simple press-drawing technique to reduce wavy CNTs and to enhance dense packing of CNTs in the sheets. Non-pressed and pressed CNT/epoxy composites were developed using prepreg processing with a vacuum-assisted system. Effects of pressing on the mechanical properties of the aligned CNT sheets and CNT/epoxy composites were examined. Pressing with distributed loads of 147, 221, and 294 N/m showed a substantial increase in the tensile strength and the elastic modulus of the aligned CNT sheets and their composites. The CNT sheets under a press load of 221 N/m exhibited the best mechanical properties found in this study. With a press load of 221 N/m, the pressed CNT sheet and its composite, respectively, enhanced the tensile strength by 139.1 and 141.9%, and the elastic modulus by 489 and 77.6% when compared with non-pressed ones. The pressed CNT/epoxy composites achieved high tensile strength (526.2 MPa) and elastic modulus (100.2 GPa). Results show that press-drawing is an important step to produce superior CNT sheets for development of high-performance CNT composites.

Neutron detection using boron gallium nitride semiconductor material

In this study, we developed a new neutron-detection device using a boron gallium nitride (BGaN) semiconductor in which the B atom acts as a neutron converter. BGaN and gallium nitride (GaN) samples were grown by metal organic vapor phase epitaxy, and their radiation detection properties were evaluated. GaN exhibited good sensitivity to α-rays but poor sensitivity to γ-rays. Moreover, we confirmed that electrons were generated in the depletion layer under neutron irradiation. This resulted in a neutron-detection signal after α-rays were generated by the capture of neutrons by the B atoms. These results prove that BGaN is useful as a neutron-detecting semiconductor material.

Mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg

This study examined the processing and mechanical properties of cross-ply and quasi-isotropic composite laminates processed using aligned multi-walled carbon nanotube/epoxy prepreg sheets. Three kinds of CNT/epoxy laminates, ([0°/90°]s, [60°/0°/−60°]s, [0°/45°/90°/−45°]s) were successfully fabricated using aligned CNT/epoxy prepreg sheets. The CNT volume fraction was approximately 10%. No visible void or delamination was observed in composite laminates, and the thickness of each layer was almost equal to that of the prepreg. To evaluate the elastic moduli, E11, E22, and G12, of each ply in the laminates, on-axis and off-axis tensile tests (0°, 45°, 90°) were conducted of aligned CNT/epoxy lamina specimens. The Young’s modulus of CNT/epoxy cross-ply and quasi-isotropic laminates agreed with the theoretical values, which were calculated using classical laminate theory and elastic moduli of CNT/epoxy lamina. The respective failure strains of [0°/90°]s, [60°/0°/−60°]s, and [0°/45°/90°/−45°]s laminates are 0.65, 0.92, 0.63%, which are higher than that of 0° composite lamina (0.5%). Results suggest that the failure strain of 0° layer in composite laminates is improved because of the other layers.

Direct Dry Spinning of Millimeter-long Carbon Nanotube Arrays for Aligned Sheet and Yarn

Abstract Ultralong multiwalled carbon nanotube arrays (forests) were grown by chloride-mediated chemical vapor deposition, in which iron chloride was used as a catalyst precursor. Highly spinnable millimeter-long arrays were grown with a very rapid growth rate of 100 μm/min. By stacking long-lasting carbon nanotube (CNT) webs, unidirectionally aligned CNT sheets were fabricated. The sheet was highly anisotropic in electrical and thermal properties and due to high alignment of the CNTs in the sheets. CNT yarns were fabricated using the millimeter-long CNTs and a detailed analysis of various postspin processes, including postspin twisting and multiply twisting, and their effect on CNT yarns were studied. Mechanical properties clearly depended on the dimensions of CNTs, where thinner and longer CNTs led to strong and stiff yarns. Large contacting surface areas in the yarns, brought by closer packing with high-aspect-ratio CNTs, were effective for higher van der Waals interaction leading to higher tensile properties. Growth of millimeter-long highly spinnable CNT arrays and the material properties of tailored large-scale CNT structures, including unidirectionally aligned sheets and spun yarns, are described.

Double-Polarity Selective Area Growth of GaN Metal Organic Vapor Phase Epitaxy by Using Carbon Mask Layers

For nonlinear optical applications using gallium nitride (GaN), periodic inversion of crystallographic orientation (polarity) is required for quasi-phase matching. We developed a novel procedure for designing polarity patterns in GaN using metal organic vapor phase epitaxy (MOVPE), and we used this to fabricate periodic polarity-inverted GaN films. By using a carbon mask for the formation of the selective area, substrate nitriding and mask removal of the selective area were carried out in the GaN epitaxial growth process. In this report, double-polarity selective area growth (DP-SAG) was realized by optimizing the nitriding and mask removal conditions. The interface of the Ga-polarity/N-polarity region became sharp by controlling the V/III ratio at 4700.

Cross-linking multiwall carbon nanotubes using PFPA to build robust, flexible and highly aligned large-scale sheets and yarns.

Multi-walled carbon nanotube (CNT) structures, including unidirectionally aligned sheets and spun yarns, were fabricated by direct dry-spinning methods from spinnable CNT arrays. We improved the mechanical properties of the CNT structures. CNTs were tailored in sheets and yarns using perfluorophenyl azide (PFPA) as a binding agent. The azide group of PFPA bonds to graphene crystal surfaces under UV radiation exposed for 1 h. For the CNT sheet, Youngs modulus increased from 1.6 to 32.9 GPa and tensile strength increased from 35.9 MPa to 144.5 MPa. For the CNT yarns Youngs modulus increased from 29.5 to 78.0 GPa and tensile strength increased from 639.1 to 675.6 MPa. With this treatment, the CNT sheets became more robust and more flexible materials. Since cross-linking of CNTs by PFPA is a simple and rapid process, it is suitable for fabrication of enhanced CNT materials.