Yuji Hotta

Thermally conductive composite films of hexagonal boron nitride and polyimide with affinity-enhanced interfaces

Thermally conductive ceramic/plastic composite materials are needed in various industries for thermal management. The present work aimed at creating composite films of hexagonal boron nitride (h-BN) particles and polyimide. A thermal conductivity of 7 W m−1 K−1 was achieved at solids loading of 60 vol% with flexibility maintained.

Enhanced boundary-scattering of electrons and phonons in nanograined zinc oxide

Nanoscale constituents in bulk materials can promote enhanced boundary-scattering in the transport of phonons as well as electrons, which is considered a key design factor for enhancing thermoelectric properties. Here, we demonstrate a method for synthesizing zinc oxide bulk materials from nanoparticles without significant grain growth by means of pressure-induced deformation at 200 °C. This allows us to comprehensively analyze the grain size dependence of thermoelectric properties in the nanoscale range above 30 nm, the size of a nanoparticle. Grain size was found to largely influence thermal conductivity as well as electrical conductivity. The observed thermal conductivity agreed with the Callaway model, indicating that enhanced phonon boundary-scattering was responsible for the variation. On the contrary, Seebeck coefficient was mostly governed by effective mass and carrier concentration, and was independent of the grain size. The dimensionless figure of merit systematically increased with grain size, ...

Coated polystyrene particles as templates for ordered macroporous silica structures with controlled wall thickness

Polystyrene (PS) colloidal particles have been used as templates to produce ordered macroporous silica structures. The silica films were deposited from ethanol solution containing acidic water and tetraethyl orthosilicate. The silica-coated PS spheres were characterized using transmission electron microscopy, and the film thickness determined by scanning electron microscopy and calculated from the relative weight of silica remaining after calcination. We found that the thickness of the silica film increased rapidly with time and reached a maximum that varied from 40 to 15 nm at pH 1.5 and 3, respectively. The data could be fitted to a simple first-order equation and the reaction rate and maximum thickness were related to the hydrolysis and condensation rate, respectively. Ordered macroporous structures were formed by centrifuging silica-coated PS spheres. Calcination of the close-packed spheres yielded a continuous silica matrix consisting of a three-dimensional well-ordered network of monodisperse pores.

Colloidal processing, surface characterization, and sintering of nano ZrO 2 powders

Colloidally stable suspensions are required to fabricate dense samples with uniform microstructure by colloidal processing methods, which necessitate dispersion of ceramic powders in a liquid medium. Aqueous nano ZrO 2 suspensions were prepared using polyethylenimine (PEI) as a dispersant. PEI adsorption on nano ZrO 2 surfaces was promoted with increasing initial PEI content and suspension pH. Isoelectric point was shifted from pH 7 at 0 wt% PEI to pH 10.4 at 3 wt% PEI. Stable suspensions had mean particle sizes in the range of 100 to 150 nm and sedimentation rates less than0.4 mm/h, as compared to 2–5.5 μm and 10–50 mm/h for unstable suspensions. Samples with 98% relative density were fabricated after sintering at 1300 °C for 4 h from colloidally stable suspensions.

Structural and Tissue Reaction Properties of Novel Hydroxyapatite Ceramics with Unidirectional Pores

Porous ceramics of hydroxyapatite was fabricated utilizing the crystal growth of thin ice columns parallel to one another in gelatin gel containing hydroxyapatite nanoparticles. The obtained ceramics possessed unidirectional pore channels with a porosity of around 75% and showed compressive strength of up to 13.1 MPa. As control materials, porous hydroxyapatite ceramics with a directionless pore structure were also fabricated by isotropic freezing and compared with the unidirectional samples regarding compressive strength and tissue reaction in vivo. Although the porosity and pore size distribution were similar, the compressive strength and new bone formation ability of the unidirectional samples were significantly greater than those of the random structured porous ceramics.

Physical properties of slip casting of high pure Al2O3 slurry using porous Al2O3-glass mold

Physical properties of advanced ceramics are influenced by impurities produced in the forming process. The forming compacts produced by slip casting using gypsum molds contain calcium and sulfur in green bodies. Therefore, a porous Al2O3-glass mold was produced and slip casting was performed in the present study. Porous Al2O3 ceramics as casting molds were examined in comparison with gypsum mold from viewpoints of free energy for wettability and rate of filter cake buildup. The sintered compact of Al2O3 produced by slip casting using the porous Al2O3-glass mold was compared with those using the gypsum mold. Transmittance of the sintered Al2O3 compacts using the porous Al2O3-glass molds was increased in comparison with that using the gypsum mold.

Fluidity of methyl cellulose-contained suspensions and pastes prepared from differently milled Al2O3 powder.

The boundary factors of transition from a methyl cellulose (MC)-contained Al(2)O(3) suspension to a paste were investigated from the view points of adsorption affinity and correlation between average surface to surface separation distance between particles (SDP) and polymer size. Non-damaged and damaged particle surfaces were prepared by wet-jet milling and ball milling, respectively. The amount of MC adsorbed on the wet-jet milled Al(2)O(3) particle surface was decreased by half compared to the ball-milled one. By increasing the solids loading from 10.0 to 12.8 vol%, the MC-contained Al(2)O(3) suspension prepared from ball milling was turned into a paste. On the other hand, it prepared from wet-jet milling was not turned into a paste even if the solid loading was increased from 10.0 to 12.8 vol%. From the creep-recovery measurements, the fluidity of the wet-jet milled sample with 12.8 vol% of solids loading was estimated to be 4 times as large as that of ball-milled one. Therefore, it was found that the state of particle surface had a strong effect on the fluidity of a suspension. The polymer size estimated from the molecular weight and Flory exponent relationship was calculated to be 266 nm. The SDP of suspensions with 10.0 and 12.8 vol% solids loading was calculated to be 215 and 165 nm, respectively. With the decreasing of SDP, the effect of adsorption affinity between the particle surface and the polymer became more striking because the contact area between them was increased. Hence, there are two factors that control the transition between a suspension and a paste; one is the relationship between SDP and polymer size, and the other is the adsorption affinity between the particle surface and the polymer.

Characteristics of translucent alumina produced by slip casting method using gypsum mold

Translucent Al2O3 ceramics were successfully produced by slip casting using a gypsum mold, provided that CaSO4 impurities, which had penetrated into the green bodies from the gypsum mold, were removed by the wash of HCl aqueous solution. Some of the calcined Al2O3 compacts were washed with HCl aqueous solution before sintering the compacts and the others were not washed with HCl aqueous solution. The relative densities of the sintered Al2O3 ceramics with HCl treatment were higher than those of the untreated samples. Grains in the HCl-treated samples, which sintered at 1350°C, grew homogeneously with about 1 μm in diameter. When the sintering temperature was higher, the grains grew homogeneously. The sintered Al2O3 ceramics with the HCl treatment were translucent. The transmittance value increased from 0 to 12% with increasing wavelength from 300 to 900 nm. The Al2O3 ceramics with the HCl treatment did not have the transmittance when the solid contents of slurry were low. The transmittance was influenced by the solid contents of slurry. On the other hand, grains in the HCl-untreated samples, which sintered at 1350°C, grew heterogeneously with the range from 0.2 to 2 μm. The Al2O3 ceramics did not have the transmittance.

Quantitative evaluation of interfacial adhesion between fiber and resin in carbon fiber/epoxy composite cured by semiconductor microwave device

Abstract Interfacial adhesion between carbon fiber (CF) and epoxy resin in carbon fiber-reinforced epoxy composite, which was prepared by different heating process such as semiconductor microwave (MW) device and conventional electric oven, has been evaluated quantitatively. The interfacial shear strength (IFSS) between CF and epoxy resin, which was an indicator of adhesion on the interface, was measured by a single fiber fragmentation test. The single fiber fragmentation test showed that the IFSSs of the prepared specimens were different by heating methods. In the case of MW process, the curing reaction of epoxy resin on the CF interface would be progressed preferentially due to the selective heating of CF, resulting that the IFSSs of specimens prepared by MW irradiation were increased by enhancing the output power of MW. However, the IFSSs of the specimens were decreased by excessively high output power because the matrix resin on the CF interface was thermally degraded. As results, by optimizing the MW conditions of output power and irradiation time, the IFSS of the sample cured by MW was increased by 21% as compared to oven-heated one. It was found that the interfacial adhesion between CF and epoxy resin would be improved by the MW-assisted curing reaction on the surface of CF.

Carbon fiber/epoxy composite materials cured thermally and with microwave irradiation

In this paper, composite materials of short carbon fibers (CFs) and a thermosetting epoxy were prepared in three different ways: without curing, thermal curing, and thermal curing followed by microwave irradiation. Mechanical properties of the three kinds of CF reinforced plastic (CFRP) composites were studied to explore the effect of microwave irradiation. Microscopic study with the aid of a scanning electron microscope (SEM) was performed on fractured composite surfaces to identify the principle features of failure. Degree of polymerization of the epoxy resin in the three CFRP composites was evaluated by infrared (IR) spectroscopy. The microwave irradiated CFRP exhibited mechanically ductile behavior even though its highest degree of polymerization. Use of microwaves and resultant stronger physico-chemical linkage at the interface between CF and epoxy resin are the main feature of this study.