Takuji Takahashi

Preparation of Polyimide Films from A-BPDA/ODA by One-Pot and Two-Step Imidization Methods

Polyimide films were prepared from 2, 3, 3′, 4′-biphenyl tetracarboxylic dianhydride (a-BPDA) and 4, 4′-oxydianiline (ODA) by a two-step method (thermal and chemical imidization) and a one-pot method. In the case of the polyimide films prepared from polyamic acid solution by the two-step method, the molecular weight of the films decreased before achieving its final value. In the case of polyimide films prepared by the one-pot method, the molecular weight increased smoothly to its final molecular weight during cure. The inflection point in the viscosity curve was observed between 215 and 250 °C in the polyimide prepared by the two-step method, and the range of the inflection temperature was very different from the T g of the polyimide (340 °C). So, this inflection of molecular weight was considered to be independent of the T g of the polyimide. The films prepared from three methods had almost the same molecular weight and properties in spite of the different preparation processes. For this reason it is thought that the polyimide from a-BPDA/ODA has less crystallinity and less phase orientation.

Effect of HfO2 Addition on Mechanical Properties of SiAlON-Based ceramics

α/β-SiAlON-based ceramics were fabricated by sintering a mixture of Si3N4 and Ln-α-SiAlON powder prepared from imide-derived amorphous Si3N4 (where Ln=Y, Er and Yb). The effect of HfO2 addition on the microstructure and thermomechanical properties of Ln-SiAlON-based ceramics was studied. A small amount of HfO2 addition had a substantial influence on the development of inhomogeneous microstructure including grain growth and broadening of grain size distribution, which resulted in the increase of bending strength and fracture toughness, KIC. Yb-SiAlON-based ceramics doped with a small amount of HfO2 exhibited excellent hightemperature strength and oxidation resistance which were controlled by the ionic radii of Ln-elements.