Tseng-Ying Tien

Determination of phase equilibria in the system Si-Al-Zr/N-O by experiment and thermodynamic calculation

The phase relations in the system Si-Al-Zr/N-O are studied by experiment and by thermodynamic calculations. The results show the conditions under which such ceramics can exist and thereby explain contradictions between earlier results. The knowledge of these phase equilibria makes it possible to choose suitable material compositions in order to apply the toughening effects of ZrO2 on a SiAlON matrix.

Subsolidus phase relationships in the systems ReAlON (Where Re = Rare Earth Elements)

Abstract Subsolidus phase relationships in the systems RAlON (where Re = Ce, Pr, Nd, and Sm) were determined. A family of lanthanum aluminium oxynitrides, ReAl12O18N, with magnetoplumbite structure occurs, where the rare earth elements have large ionic radii ranging from La to Eu. The lattice parameters of these compounds were determined, and the results indicated that all these compounds have nearly the same value of a = 5.564 A , c = 22.00 A , and c a = 3.96 A .

Mechanical properties and thermal expansion behaviour in leucite containing materials

The effect of a change in sodium content and thermal history on a leucite composition material produced by a coprecipitation process was studied. Five materials with formulae of (K(1−x),Nax)2O-Al2O3-4SiO2 (x= 0.0, 0.2, 0.4, 0.6, 0.8) were investigated for differences in phases, thermal expansion, and strength. Strengths of up to 175 MPa were obtained for a leucite composition material (x = 0.0). Sodium was effective in lowering the thermal expansion coefficients of these materials. Leucite was linked to higher flexural strengths, but was present only in those specimens which were sintered at 1200 °C. Leucite was not present in those specimens sintered at 1100 °C or lower. Porosity was present in all specimens sintered at 1000 °C or higher.

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part I Compositions and properties

X-ray diffraction was utilized to follow the transformation from β-SiC (3C) to the various α-SiC polytypes in the presence of AlN and Al2O3 additives after hot pressing from 1700 to 2100°C. The 2H- and 6H-polytypes of α-SiC were the predominate polytypes with additions of only AlN or Al2O3, respectively. The amount of 2H- and 6H-polytypes, and subsequently the microstructural morphology of the SiC materials, were found to be controlled by varying the amount of AlN and Al2O3. Improvements in fracture toughness to ∼9 MPa-√m were achieved with flexural strengths ranging from 600 to 900 MPa. These results suggest that accurate control of the polytypic make-up of SiC-based materials, along with their mechanical properties, can be achieved through AlN and Al2O3 additions.

High-temperature electrical conductivity of aluminium nitride

The electrical conductivity of hot-pressed polycrystalline aluminium nitride doped with oxygen and beryllium was measured as a function of temperature from 800 to 1200° C and over a range of nitrogen partial pressure from 102 to 105 Pa. The effect of beryllium dopant, the independence of conductivity from nitrogen partial pressure, and the observed activation energy suggested extrinsic electronic species or aluminium vacancies as charge carriers. Polarization measurements made with one electrode blocking to ionic species indicated that the aluminium nitride with oxygen impurity was an extrinsic electronic conductor.

Effect of AlN and Al2O3 additions on the phase relationships and morphology of SiC Part II: Microstructural observations

Additions of AlN and Al2O3 to β-SiC hot pressed at 2100°C strongly effect the β- to α-SiC phase transformation and the resultant α-SiC polytypes which are formed. Scanning and transmission electron microscopy were utilized to investigate the microstructural changes occurring in SiC due to these additions and to correlate these observations to their mechanical properties. The results suggest that Al2O3 additions stabilize the formation of the 6H-polytype of α-SiC which grows rapidly into an elongated plate-like morphology, while AlN additions stabilize the 2H-polytype of α-SiC resulting in fine equiaxed 2H-SiC: AlN solid solution grains. It is speculated that the elongated growth of 6H-SiC with Al2O3 additions can be controlled through the simultaneous addition of AlN. The formation of 2H-SiC : AlN solid solution grains inhibits the growth of the 6H-SiC grains since AlN(2H) will not go into solid solution in the SiC(6H) structure, effectively pinning the growth of the 6H-SiC grains.

The effect of GPS parameters on mechanical properties of Y-α-SiAlON ceramics

A dense, oxygen-rich Y—α-sialon ceramic, with a small amount of YAG as a grain boundary phase, has been fabricated through GPS (gas pressure sintering). The effect of nitrogen pressure during GPS on densities and mechanical properties of the materials has been studied, which was observed to be relevant to the YAG content. The α-sialon with 2.5 wt% YAG, GPSed under 3 MPa and at 1800 °C, possesses a flexural strength of 500 MPa, hardness (Vickers) of 1816 kg/ mm2 and fracture toughness (K1c) of 3.4 MPa · m12.

Phase Equilibrium Studies in Si3N4 — Metal Oxides Systems

This paper is not intended to be a review of phase equilibrium studies in Si3N4 — metal oxides systems. This presentation includes only phase diagrams established at The University of Michigan and the Max-Planck-Institute fur Metallforschung. The systems to be discussed are: Si, Al/N,0; Si, Be/N,0; Si, Mg/N,0; Si, Y/N,0; Si, Ca/N,0; Si, Th/N,0; Si, Zr/N,0; Si, A1, Be/N,0; Si, A1, Mg/N,0; Si, Al, Y/N,0; Si, A1, Ca/N,0; Si, Al, Zr/N,0 and the investigators involved in these two institutes are, in alphabetical order: L. J. Gauckler, H. G. Hohnke, I. C. Huseby, H. L. Lukas, R. Muller, I. K. Naik, S. D. Nunn, G. Petzow, W. Y. Sun, T. Y. Tien, and J. Weiss.

Solid‐Liquid Reactions in the System Si3N4–β‐SiAlON–Y3Al5O12

Solid-liquid equilibria in the system Si, Al, Y/N, O were determined for the compatibility triangle bounded by the [beta]-SiAlON solid-solution line and the compound Y[sub 3]Al[sub 5]O[sub 12]. X-ray diffraction was used to determine the crystalline phases present in the equilibrated, rapidly cooled specimens. The liquid phase was quantified with volume fraction measurements performed on scanning electron micrographs. The solid-liquid tie lines at 1,650 and 1,750 C were established from lattice parameters of the [beta]-SiAlON phase and from the amount of liquid phase in equilibrium with the crystalline solid. The liquid phase was crystallized to verify the location of the starting composition.

Small angle X-ray scattering (SAXS) studies of sol to gel transition in K2O-Al2O3-SiO2 system

The structural evolution during a sol to gel transition reaction in the K2O-Al2O3-SiO2 system was investigated by using in situ small angle X-ray scattering (SAXS) technique. The results are interpreted as evidence of the presence of at least two different gel phases. The analysis shows that the primary phase is composed of small particles with characteristic size of ∼3.0 nm, and considerable size and shape variation. The primary phase is identified as a random phase in the Debye sense. The primary particles aggregate into larger formations with spherical symmetry. The size of larger aggregates (second phase) increases continuously in the course of gelation and levels off after reaching ∼15.0 nm. The volume fraction of the second phase keeps increasing until the end of the measurement which is far after the gelation (3-fold of the gelation time).