Papers by Author: Daisuke Hiratsuka

Abstract: AlN ceramics used in electronic substrates and packages are fabricated by the densification of green bodies with sintering aids such as rare-earth oxides. The homogeneous dispersion of the sintering aids, achieved with the help of the mechanochemical bonding of several types of fine powders, is the key process for obtaining a good sinterability and high performance. In this study, we fabricated the AlN ceramics using AlN and nano-Y2O3 composite particles prepared by mechanical treatment. The AlN powder and nano-Y2O3 powder were ball-milled with Al2O3 balls and a dispersant in ethanol in a plastic pot. The powder mixture of AlN and Y2O3 was composited by mechanical treatment. The composite powder was granulated and pressed to obtain a green body. After dewaxing, the AlN green body was fired at 1800°C in 0.6 MPa N2. The sintered body possessed a fracture toughness of 3.6 Pa•m1/2, higher than that, 3.1 Pa•m1/2, of the AlN ceramics fabricated without the mechanical treatment. An observation of the fractured surface revealed that grain boundary reinforcement enhances the fracture toughness of the AlN ceramics made of composite particles.

Abstract: The high-temperature deformation of dense nano-sized Ca-α SiAlON and submicron- sized β-SiAlON polycrystals were investigated by compression tests. These polycrystals were fabricated from Ca-α SiAlON and β-SiAlON nano powders respectively, and contained almost no intergranular glass phase. Both SiAlONs exhibited large plastic deformations with the stress exponent n ~1 in the higher stress region, and n ~2 in the lower stress region. Shear-thickening creep (n < 1), which has been reported in liquid contained SiAlONs, was not observed.

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.