Masayoshi Ohashi

Effect of additives on some properties of silicon oxynitride ceramics

Silicon oxynitride ceramics are formed by reaction sintering of silicon nitride and silica with certain metal oxide additives. The reaction rate during sintering and the subsequent properties of silicon oxynitride are affected by the quantity and kinds of additives. The reaction rate increased for addition of equal molar amounts of ZrO2, ZrO2 (+2.8 mol % Y2O3), AlO1.5, LnO1.5, CeO2, MgO, in that order (where Ln=Nd, Sm, Gd, Dy, Er, Yb and Y). The lanthanide oxide (1.5 mol %)-doped silicon oxynitride ceramics had a high fracture toughness, because crack deflection occurred due to the precipitation of an intergranular crystalline phase with a high thermal expansion coefficient compared with silicon oxynitride. The oxidation rate was higher with an increasing quantity of additive. In samples containing an intergranular crystalline phase, stability of the crystalline phase is an important factor and could impair the oxidation resistance of silicon oxynitride ceramics.

Factors affecting mechanical properties of silicon oxynitride ceramics

Abstract The effect of additives and impurities on the mechanical properties of silicon oxynitride ceramics was investigated. The toughening of the ceramics was affected by three factors: (1) the thermal tensile stress in the intergranular glassy phase and the compressive stress in Si2N2O grains, developed by a thermal expansion mismatch between Si2N2O grains and the intergranular glassy phase; (2) the large grain size of Si2N2O; and (3) the concentration of impurities in the intergranular glassy phase. The degradation of high-temperature strength was primarily dependent on the basic chemical composition (the kind of additives (MeOx) and Me Si ratio) of the intergranular phase and impurities.

Preparation of translucent mullite ceramics

Des ceramiques de mullite hautement translucides ont ete preparees par filtrage sans pression ni additif, avec elimination des phases secondaires et agrandissement approprie de la taille des grains de mullite

COMPARISON OF CAPILLARY AND SKIMMER INTERFACES IN EVOLVED GAS ANALYSIS-MASS SPECTROMETRY (EGA-MS) WITH REGARD TO IMPURITIES IN CERAMIC RAW MATERIALS

Capillary and skimmer interfaces in EGA-MS were compared by analysis of the carbon substances remaining in the hydrolysis products of aluminum alkoxides. Peaks with higher intensities were detected in the skimmer interface mode than those in the capillary interface mode. It was confirmed in particular that the skimmer interface is effective in increasing the sensitivity for species with higher mass, e.g. the evolved butyl group, as carbon substances remaining in the hydrolysis product of aluminum sec-butoxide.

Pressure Dependence of Laser-Induced Fluorescence of Sm3+ in Al2O3—Y2O3 System Compounds

The piezospectroscopic (PS) coefficients relating frequency shift to pressure are evaluated for Sm3+ in the Al2O3–Y2O3 system compounds, which include Al2O3, Y3Al5O12, YAlO3, Y4Al2O9, and Y2O3. The fluorescence peaks are inherently narrow for all the synthesized compounds, on the basis of the f–f transition nature of Sm3+ ion. Furthermore, the PS coefficients are ranged from −2.2 to −8.2 cm−1/GPa, which are similar to the reported value for the R1 line of ruby and suitable for the PS stress evaluation. PS stress distribution measurements are demonstrated with sintered Y3Al5O12 ceramics as a sample, with 50 μm intervals, under a microscope.

Effect of fluorine impurity on the oxidation of silicon oxynitride ceramics doped with gadolinium oxide

Impurities in raw Si3N4 powders remain in intergranular glassy phases in Si3N4 and Si2N2O ceramics and degrade their high-temperature properties. Fluorine is one of the typical impurities in the raw powders. The oxidation rate of Si2N2O ceramics doped with Gd2O3 greatly varied with a difference in impurity contents (especially F) of the raw Si3N4 powders used. When a high concentration of impurity existed in the intergranular glassy phase, the rate of oxidation was controlled by O2− diffusion through the glassy phase in the partly oxidized scale and unoxidized body; outward diffusion of Gd3+ occurred concurrently. On the other hand, when the impurity contents in the intergranular glassy phase was very low, the diffusion rate of ions (Gd3+, O2−, etc.) in the glassy phase became very low (substantially zero in the oxidation at 1300°C). Only cristobalite (SiO2) was formed on the surface. The rate of oxidation was controlled by O2 diffusion through the cristobalite layer, and was very low.