Hydrophilic property of titanium oxide aiming for bio-applications

Surface functionalization of titanium metal is very attractive for bio- and environmental applications. This is because titanium metal is very stable and has a good biocompatibility. In this case, surface roughness and crystalline structure are important factors for obtaining effective characteristics. Titanium metal is usually covered with a surface passive film of thermodynamically stable rutile-TiO₂ that grows as the heat treatment temperature in air increases. On the other hand, to obtain an anatase-TiO₂ surface layer on titanium metal, we must employ specific treatments such as our previous method, which uses a silica-coexisting heat-treatment process. In this paper, the relationship between the fine structure formed on the titanium metal and the surface hydrophilic property was clarified, and the potential for the bio-application was discussed. The formed anatase-TiO₂ coexisting with silica exhibited improved biocompatibility with good apatite formation.     

Characterization of spherical silica–titania mixed oxide particles synthesized by using a dry process

Silica–titania mixed oxides have excellent properties, such as a low thermal expansion coefficient and a refractive index that can be adjusted by changing the Ti content. However, when the Ti content increases, silica and titania phases in silica–titania mixed oxides can separate. This phase separation leads to the precipitation of the titania component as rutile or anatase crystals. When silica–titania mixed oxides undergo phase separation, their properties become unstable; for example, the refractive index of the particles becomes non-uniform. Therefore, it is preferable to synthesize silica–titania mixed oxides in an amorphous state without causing phase separation. Based on our previous studies on particle size control in silica synthesis, we employed a dry process using organosilicon compounds to synthesize silica–titania mixed oxides. In this study, spherical amorphous silica–titania mixed oxide particles were obtained via flame synthesis using organosilicon and organotitanium compounds. The purpose of this study was to characterize the obtained powder and explore the possibility of controlling particle size during synthesis. By studying the dry process synthesis of spherical silica–titania mixed oxide particles, we confirmed the relationships: between the Si/Ti molar ratio and the obtained crystal structure and between the adiabatic flame temperature and the particle size.     

Application of h-BN particles using Ca-assisted carbothermal reduction nitridation to high-thermal-conductive resin/h-BN composites

Hexagonal boron nitride (h-BN) particles exhibit high thermal conductivity and are promising fillers for resin filling. However, h-BN particles are plate-like particles with thermal anisotropy in the planar and thickness directions. Therefore, their applications are limited due to low thermal conductivity in the direction of the thickness of a resin sheet filled with h-BN particles. In this study, we control the size and thickness of h-BN particles using carbothermal reduction nitridation (CRN), which involves the carbothermic reduction of boric oxide in an N₂ gas atmosphere and develop them into resin sheets. In CRN using a CaO promoter, a novel method is developed to control the shape, size, and thickness of h-BN particles. Using h-BN particles grown in the thickness direction, we have successfully provided resin sheets with high thermal conductivity.     

Wet milling synthesis of ammonium cobalt phosphate hydrate platelets for LCP-olivine cathodes of LIBs using a bead mill

In this study, NH₄CoPO₄·H₂O (ACP) platelets were synthesized from NH₄H₂PO₄ and Co(OH)₂ by wet milling process using a bead mill. ACP is one of the precursors of LiCoPO₄ (LCP) cathode materials for Li-ion batteries. In the wet milling, ACP large platelets were synthesized in a short time at the beginning, and the large platelets were gradually cleaved into the smaller platelets. The lateral sizes of the ACP platelets were several 10 μm at 30 min and about 10 μm at 5 h. It was found that the ACP platelets were synthesized efficiently by using the bead mill as a promising machine for the scale-up in manufacturing. The ACP platelets were converted into LCP by mixing with Li₂CO₃ and heating, whereas platelet shapes were retained. The initial discharge capacities of the cathodes using the LCP platelets converted from the ACP of 30 min and 5 h milling were 70 and 95 mAh/g at 0.05 C, respectively. It is because the specific surface area of the latter sample (3.3 m²/g) was larger than that of the former (2.2 m²/g). The larger specific surface area possibly led to the increase of contacts with electron conductive additives (carbon) and electrolytes.     

Rapid synthesis of YAG phosphor by facile mechanical method

In this study, synthesis of yttrium aluminum garnet (YAG):Ce³⁺ phosphor powders for white light emitting diodes was investigated by mechanical method using the attrition-type mill with no external heating and no flux in dry phase. High mechanical energy input to the starting powder mixture of Y₂O₃, Al₂O₃ and CeO₂ achieved the synthesis of YAG:Ce³⁺ without any flux materials. X-ray diffraction patterns of the processed powders after 5 min processing revealed the peaks of YAG were clearly identified. The maximum temperature of the mill chamber during the processing was 240℃. The YAG phosphor obtained by the mechanical method revealed the internal quantum yield of 65% in the case of the sample mechanically processed under a reducing atmosphere. The synthesized powder showed granule structure consisting of submicron size of YAG particles, which is better handling for the fabrication of light emitting diode devices.     

Easy control of surface morphology through a natural phenomenon

This paper provides an innovative controlling process of surface morphology. The contact area between a liquid and a solid is strongly affected by the critical surface tension (cft) of both materials. Using this phenomenon, a wide range of morphologies, from flat surface layers to sea-grape-like surface structures, were created. Due to the bleed-out phenomenon, low-molecular-mass additive (liquid) oozed out of a precursor polymer (solid), leading to different surface morphologies depending on the cft values of the liquid (Mcft) and solid (Pcft). When Mcft < < Pcft, a flat surface layer was obtained; however, in the case of Pcft < < Mcft, the formation of a sea-grape-like surface layer was created. Tetra-butoxy-titanium and polydimethylsiloxane were used as low-molecular-mass additive and precursor polymer, respectively. After coating on titanium metal and calcination, sea-grape-like materials composed of titanium oxide and silicon oxide were obtained. Furthermore, unique characteristics (bioactivity, photocatalytic activity, and prevention of atomic diffusion) were observed. A remarkable increase in the wettability, bioactivity, and catalytic activity of materials was achieved using our simple process to create unique surface morphologies. Our proposed process is applicable to a wide range of materials and morphologies, and can be used in catalysts, biomaterials, and environmental barrier coatings.