Wei-ying Sun

Study on the solid solubility of Al in the melilite systems R2Si3 − xAlxO3 + xN4 − x with R = Nd, Sm, Gd, Dy and Y

Abstract The solubility of aluminium in the nitrogen-containing melilite structure has been studied by preparation of specimens of the general formula R 2 Si 3 − x Al x O 3 + x N 4 − x with R = Nd, Sm, Gd, Dy and Y by hot pressing and pressureless sintering techniques in the temperature interval 1600–1750 °C. For elements with large ionic radii, i.e. R = Nd and Sm, up to one Si can be replaced by Al, but the solubility limits in melilite solid solutions decrease with decreasing the ionic radii. Consequently, yttrium-melilite had the lowest observed aluminium solubility (x ≈ 0.6) in rare earth elements-melilite solid solutions.

Effect of processing on the morphology of α-sialon grains

Abstract In order to explore the effect of processing on the formation of elongated α-sialon grains, a mixture of α-Si 3 N 4 and β-Si 3 N 4 (5–40 wt.%) powders combined with variant firing cycles was used for preparing an α-sialon doped with Dy 2 O 3 . The results showed that the morphology of α-sialon grains was more closely relevant to the firing conditions than to the composition of the starting powder. The specimen sintered by gas pressure sintering (GPS) or through heat treatment at 1800°C or1900°C (for hot-pressed (HP) specimen) possessed a bimodal microstructure. The mechanism of forming elongated α-sialon grains is discussed in the present work.

Mechanical properties of hot-pressed 12H ceramics

Abstract The mechanical properties of hot-pressed 12H poly-typoid phase with YAG as the sintering additive have been studied. The results showed that small amounts of YAG can greatly increase high temperature strength. At room temperature, the 12H with 2.5 wt% YAG possesses a flexural strength of 367 ±18 MPa, but its high temperature strength can reach above 500 MPa in the temperature range from 1000 to 1300 °C. The fracture mechanism has been discussed on the basis of the fracture morphology observed by SEM.

J-Phase solid solution series in the Dy-Si-Al-O-N system

Abstract Detailed studies on J-phase compositions of the type Dy 4 Si 2 − x Al x O 7 + x N 2 − x have demonstrated the existence of five different types of structure, all based on the monoclinic cuspidine arrangement characteristic of the end-members Dy 4 Si 2 O 7 N 2 and Dy 4 Al 2 O 9 . Series 1 and 3 extend into the system from the endmembers, in the ranges 0 ≤ x ≤ 0.4 and 1.6 ≤ x ≤ 2.0, respectively. Series 2 is characterised by a lower value of the b lattice parameter and occurs in the range 0.5 ≤ x ≤ 1.5, while Series 4 is observed in the range 0.8 ≤ x ≤ 1.1 alongside Series 2. Series 5 occurs in the range 1.4 ≤ x ≤ 1.5, at which point X-ray patterns are much simpler, indexing on an orthorhombic unit cell with a halved c -axis repeat. These observations are consistent with structural differences observed in naturally-occurring cuspidine minerals, and also with recent observations on phase transformations in mixed rare earth Ln 4 Al 2 O 9 compounds.

Glass-forming region in the Sm-Si-AI-O-N system

We report the glass-forming region at 1700°C for N-rich compositions of the Sm-Si-Al-O-N system

Phase formation and microstructural evolution of R-α′-AlN-polytypoid ceramics (R=Y, Y+Sm and Sm)

Abstract In the present work, Y, Sm and (Y+Sm) were separately used to explore the effects of rare-earth elements on the reaction sequence and microstructural evolution in α-sialon-AlN-polytypoid composite ceramics. The results showed that the reaction sequences in the composite ceramics are affected by rare earth elements. With the existence of AlN-polytypoid α-sialon develops into elongated morphology. The Sm–α-sialon composition exhibits the feature of fine grains with high aspect ratio and Y-composition possesses rather coarse morphology with low aspect ratio. In the composite ceramics, AlN-polytypoid was not distributed homogeneously around the α-sialon grains, especially in the Y-composition. The variation in microstructure with rare-earth elements in the Sialon materials was discussed based on their reaction sequences.

Approach for improving homogeneity of Ln-α-sialon–AlN-polytypoid composite ceramics

Abstract The approach of improving homogeneity of α-sialon–AlN-polytypoid composite ceramics was studied. The results showed that both the alkalinity of the Ln element and the AlN powder size influenced the distribution of AlN-polytypoid in the α-sialon matrix. The microstructure of Sm-α-sialon–AlN-polytypoid was rather homogeneous compared to the Y-α-sialon–AlN-polytypoid. The homogeneity of phase distribution in Sm-α-sialon–AlN-polytypoid composite ceramics can be further improved by using specially made AlN instead of the commercial AlN as starting powder.