Papers by Author: Mamoru Omori

Abstract: Multi-walled carbon nanotube (MWCNT) reinforced alumina composites were prepared by spark plasma sintering using pristine MWCNTs and acid-treated MWCNTs. The effect of acid treatment on the structure and surface potential of the MWCNTs was examined by transmission electron microscopy (TEM) and zeta potential analyzer. It is demonstrated that with the acid treatment of the MWCNTs, we have deliberately introduced nanoscale defects and negatively charged functional groups on the surface of the MWCNTs. The average depths of the defects are typically 4.8-10.8 nm. Mechanical measurements revealed that surface modification of the MWCNTs is effective in improvement of bending strength and fracture toughness of the MWCNT/alumina composites. Only 0.9 vol.% acid-treated MWCNT addition results in 27% and 25% simultaneous increases in bending strength (689.6 MPa) and fracture toughness (5.90 MPa m1/2), respectively.

Abstract: Functionally graded materials (FGM) were fabricated for bio-medical applications, (1) implants, (2) dental core and post, (3) guided tissue regeneration (GTR) membranes. (1) FGM implants of Ti/HAP and TiN/HAP with the concentration changing gradually in the longitudinal direction of cylinder were fabricated to optimize both mechanical properties and biocompatibility in each region. Concentration gradient was formed by packing of dry powders into mold. Spark plasma sintering was effective for sintering of non-uniform composition in FGM. Brinel hardness decreased gradually from Ti part to HAP part, which contributes to stress relaxation in the implanted region of bone. In vivo tests showed that osteogenesis and maturation is more advanced in the HAP rich region. (2) FGM dental core and post made of composite resin was fabricated by laser lithography, one of the photo-curing type CAD/CAM systems. The elastic modulus changed gradually from 10.6GPa in core part to 2.9GPa at the apex of post by decreasing the filler content of ceramic powders from 64% to 0% in polymer matrix. Stress analysis using finite element method showed the stress relaxation by further 30 % in FGM, compared with the uniform composite resin. (3) Biodegradable GTR membrane composed of nano HAP/collagen reinforced with PLGA was prepared. The membrane has the three layer structure with the thickness of about 100μm for each where the composition of HAP/collagen is increased from pure PLGA. The graded membrane structure could give the different functions of the high degradation speed and Ca ion release to enhance osteoconductivity for bone remodeling in the high HAP/collagen side and the relatively low degradability to prevent the ingrowth of fibroblasts in the pure PLGA side.

Abstract: β-SiAlON nanoceramics were fabricated from β-SiAlON nano powder using the spark-plasma sintering (SPS) technique. The β-SiAlON nanopowder (Si4Al2O2N6) was synthesized from a mixture of SiO2 (QS-102, Tokuyama Co., Japan), AlOOH (Tomita, Japan) and C (Mitsubishi Chemical, Japan) using the carbothermal reduction nitridation (CRN) method. The heating rate for SPS was 50
/min. The β-SiAlON nanoceramics had high strength (500 MPa). TEM observation showed that the intergranular glassy phase was scarcely present at the grain boundary of the β-SiAlON nanoceramics. Aqueous corrosion resistance was evaluated by measuring the weight loss after soaking in 5 and 35 wt.% H2SO4aq. and 5 wt.% HNO3aq. at 80
for 100 h. It was found that β-SiAlON nanoceramics have much higher corrosion resistance than commercialized silicon nitride ceramics in acid solutions. Commercialized Si3N4 ceramics have an intergranular glassy phase created as a result of the sintering aids in them. Thus, they are easily corroded by acid solutions because the intergranular glassy phase is easily corroded under such conditions. The excellent corrosion resistance of the β-SiAlON nanoceramics stems from their glass-free grain boundaries, since the β-SiAlON nanoceramics were produced without using a sintering aid.