Hiroaki Wakayama

Structural change of graphite subjected to mechanical milling

The synthesis of disordered carbon was attempted by a method of mechanical milling. Hexagonal graphite was employed as a starting material. X-ray diffraction patterns indicate that the hexagonal graphite, composed of layers, was transformed into amorphous-like carbon by mechanical milling. This transformation indicates that milling breaks down the layer structure of graphite. However, the atomic arrangement within 3 A in the radial distribution function, RDF(r), observed by neutron diffraction indicates no change in environment. On the contrary, the coordination number of the first nearest neighbour gradually decreased with increasing milling time. These results indicate that the mean size of the graphite crystallites becomes smaller on milling and approaches about 30 A diameter in the c axis direction after 36 h of milling.

Structural defects in mechanically ground graphite

Abstract Highly ordered synthetic graphite was mechanically ground in a planetary ball mill. The interlayer spacing d002 was found to increase in the grinding process with the reduction of the crystalline size La, Lc. The specific surface area increased to over 600 m2/g at the maximum. The result of Raman spectroscopy shows that the ground graphite yields a smaller value of full width at half maximum (FWHM) of the 1350 cm−1 peak than the heat-treated carbon for a given value of La. This indicates the difference in the type of structural defects involved between the heat-treated carbon and ground graphite. A further suggestion of this result is that the mechanically ground graphite has more uniform boundary and a less-defective structure of hexagonal graphite lattice within the layer planes than heat-treated carbon.

Fabrication of CaCO3–biopolymer thin films using supercritical carbon dioxide

The preparation of useful products based on the integration of carbon dioxide fixation and biomass utilization is important for the future development of environmentally harmonized materials technologies. Here we report the use of supercritical carbon dioxide and spin-coated cellulose/chitosan matrices to produce biopolymer films infiltrated with densely packed calcium carbonate (vaterite) particles. Composite films of high uniformity are prepared specifically in the presence of low concentrations of polyacrylic acid.

A novel method for tailoring porous structures of nanoporous materials using supercritical solvents

Supercritical fluids (SCFs) were used for tailoring the structures of micro- and mesoporous silica with uniform pore sizes, CnFSM-16 (n = 8, 10, 12 and 16), where the value n denotes the carbon atom number in the alkyl chain of the template surfactant. Silica precursors, tetraethylorthosilicate (TEOS) dissolved in supercritical CO2 or in liquid ethanol, were impregnated onto FSM-16. The pore size of all the FSM-16 series decreased after coating in the supercritical solvent. On the other hand, silica was not introduced into pores smaller than that of the C12FSM-16 by coating in the liquid solvent. This is elucidated in terms of the penetration difference into the nanoporous structures of supercritical solvents and liquid solvents. These results are compared with those of activated carbon with various pore sizes coated in supercritical solvents or in liquid solvents.

Simple and scalable preparation of master mold for nanoimprint lithography

Nanoimprint lithography (NIL) is one of the most prominent bottom-up techniques for duplicating nanostructures with a high throughput. However, fabrication of starting master mold commonly requires expensive equipment of top-down techniques, or additional steps to transfer the fabricated patterns from bottom-up methods. Here we demonstrate that a SiO2 nanostructure manufactured from a self-assembled block copolymer, polystyrene-b-polydimethylsiloxane (PS-b-PDMS), directly serves as a master mold for NIL without further modification. A hexagonally aligned pattern over the entire substrate is established using a simple technique; solvent annealing and etching. Etching also plays an important role in endowing fluorine on the surface of SiO2, thus promoting smooth demolding upon imprinting. The obtained pattern of the SiO2 nanostructure is transferred to a polymer surface using UV nanoimprint. Identical patterns of the SiO2 nanostructure are elaborately reproduced on Ni and Cu nanodot arrays via electroplating on the polymer transcript, which was verified by morphological observations. The uniformity of the replicated Ni nanodot array is evaluated using spectroscopic ellipsometry. The measured optical response of the Ni nanodot is validated by electromagnetically simulated results, indicating that the pattern transfer is not limited to a small local area. In addition, the durability of the SiO2 mold pattern is corroborated after the imprinting process, thus guaranteeing the reusability of the fabricated nanostructure as a master mold. The proposed approach does not require any high-end lithographic techniques; this may result in significant cost and time reductions in future nanofabrication.

The effect of the LiCoO2/Li7La3Zr2O12 ratio on the structure and electrochemical properties of nanocomposite cathodes for all-solid-state lithium batteries

Using a self-assembled block copolymer (BCP) structure as a template, an inorganic nanocomposite consisting of a cathode active material (LiCoO2) and an electrolyte (Li7La3Zr2O12) was synthesized as a candidate electrode material for all-solid state lithium batteries. The precursors of the cathode active material and those of the electrolyte were introduced into separate polymer blocks of the BCP, and then calcination was used to remove the BCP template and crystallize the cathode active material and the electrolyte. The LiCoO2/Li7La3Zr2O12 ratio that produced the optimum electrode structure was determined: the ratio of 90% resulted in the maximum capacity per active material, 135 mA h g−1, which is 98% of the theoretical value for LiCoO2. The obtained capacity was substantially larger than the values reported for other cathodes used in all-solid-state lithium batteries. The implication is that at the optimum LiCoO2/Li7La3Zr2O12 ratio, nanoscale three-dimensional conducting paths and an electrochemically effective interface between the active material and the electrolyte are formed as a result of templating by the self-assembled BCP structure.

CaCO3–Polymer Nanocomposite Prepared with Supercritical CO2

A novel process for generation of a CaCO3–polymer nanocomposite with a controlled three-dimensional shape was developed. Specifically, a nanocomposite with a high CaCO3 content was produced by introducing supercritical CO2 into a polymer matrix containing Ca ions. A mixture of poly(vinyl alcohol), Ca acetate, and poly(acrylic acid) was poured into a mold, the mold was placed in an autoclave, and CO2 was introduced to precipitate CaCO3 within the polymer matrix. Laser Raman spectroscopy and transmission electron microscopy showed that this process produced a nanocomposite containing highly dispersed CaCO3 (aragonite) nanoparticles. The flexural strength of the nanocomposite was larger than the flexural strengths of limestone and CaCO3 produced by hydrothermal hot pressing. The use of supercritical CO2 facilitated CO2 dissolution, which resulted in rapid precipitation of CaCO3 in the polymer matrix. The above-described process has potential utility for fixation of CO2.

Block copolymer-based nanocomposites with exotic self-assembled structures induced by a magnetic field

Structural control of polymer nanocomposites is important for their applications in organic semiconductors, lithographic nanopatterning, separation membranes, and nanofabrication templates. However, manufacturing nanocomposite materials with novel structures in a highly efficient yet precise manner remains a great challenge. To create nanocomposite structures, we combined self-assembly processing of block copolymer (BCP)-metal complex nanocomposites with an applied magnetic field. Here, we describe in detail the mechanism of magnetic alignment of block copolymers doped with metal complexes; specifically, we investigated the effect of the applied magnetic field on the phase behavior of the assembled block copolymer-metal complex nanocomposites with various molecular weights and with different molecular structures. We show that our combination of self-assembly processing and application of a magnetic field yielded lamellar structures of alternating multilayers with different layer thicknesses. This self-assembled structure is not included in phase diagrams of BCPs. The influence of the block copolymers’ molecular structures on the nanocomposites’ phase transformation behavior is also discussed. Our results provide a route to manufacturing nanocomposite materials in a highly efficient yet precise manner, which could lead to improvement in the material properties of nanocomposites.

Synthesis of a High-Coercivity FePt–Ag Nanocomposite Magnet via Block Copolymer-Templated Self-Assembly

Magnetic recording media are composed of magnetic thin films consisting of magnetically isolated crystallites. For practical use of magnetic particles as recording media, it will be necessary to realize high coercivity by fabricating nanocrystalline grains and forming grain boundaries with the nonmagnetic phase. In this study, a high-coercivity FePt–Ag nanocomposite magnet was synthesized by means of block copolymer-templated self-assembly. Precursors of Fe, Pt, and Ag were introduced into a polymer block, and the resulting material was oxidized and then reduced to form a nanocomposite consisting of FePt nanoparticles surrounded by a matrix of Ag. X-ray diffraction analysis revealed that the introduction of Ag did not significantly affect the crystalline ordering of the FePt. The addition of Ag increased the coercivity by 53% (from 11.1 to 17.0 kOe). Our results suggest that the grain boundaries of the nonmagnetic Ag metal acted as pinning sites, disrupting magnetic coupling between individual FePt nanocrystallites and hindering domain wall motion at an external magnetic field.