Yvonne Menke

Effect of rare-earth cations on properties of sialon glasses

Abstract The preparation of bulk glasses in Ln–Si–Al–O–N systems (Ln = Y, Ce, La, Eu, Gd, Dy, Er and Yb) with the composition in equivalent % of 35e/oLn:45e/oSi:20e/oAl:83e/oO:17e/oN are reported. The properties of these glasses are examined in detail. Changes observed in density, Youngs modulus, thermal expansion and glass transition temperature are represented. It is found that the mechanical properties vary linearly with cationic field strength (CFS) or ionic radius of the rare-earth modifier ( E min =92 GPa for Eu 2+ and E max =160 GPa for Er 3+ ). The thermal properties do not show a linear behaviour. The glass transition temperature increases linearly through Gd followed by a linear decrease. The thermal expansion coefficient shows a linear decrease with a discontinuity at Gd as substituting cation.

The crystallisation of the yttrium–sialon glass: Y15.2Si14.7Al8.7O54.1N7.4

Abstract The development of microstructure during crystallisation of a glass with composition Y15.2Si14.7Al8.7O54.1N7.4 has been studied by analytical and high resolution transmission electron microscopy. Crystal nucleation at temperatures in the range 965–1050°C occurs by the heterogeneous nucleation of lenticular-shaped yttrium, silicon and aluminium containing crystals on silicon-rich clusters that formed during glass preparation. The lenticular crystals have a wide range of composition after heat treatment at 1050°C; the yttrium cation percentage varies around that of the expected B-phase composition Y2SiAlO5N but the aluminium content is lower and the silicon content generally significantly higher than that. The crystals display the hexagonal crystal structure of B-phase, although the results from EDX analysis imply that the atomic arrangement of the lattice is not the previously proposed B-phase structure. Crystal growth during prolonged heat treatment at 1050°C occurs to a significant extent by coalescence.

On the glass transition domain in some M-SiAlON (M = Y or Ln) oxynitride glasses

The behaviour in the glass transition domain of some oxynitride glasses has been studied by thermoanalytical methods (dilatometry and differential thermal analysis) and mechanical techniques (creep and ultrasonic measurements of Youngs modulus). The thermoanalytical data are in good agreement with the glass transition domain defined from viscosity data. The sharp decrease of Youngs modulus, that starts at the temperature of the strain point, is compared to results obtained from mechanical spectroscopy by other authors. The difference in the apparent activation energies for viscous flow above and below the temperature of the strain point is used to separate the contribution of the thermal and structural components. The high temperature apparent activation energy of viscosity is in fair agreement with that of the α-relaxation peak described by the formalism of hierarchically correlated molecular mobility.

The Crystallisation of B-Phase Glass-Ceramics

Five-component B-phase may be readily formed through the nucleation and crystallisation heat treatment of nitrogen-rich parent glasses with composition (e/o) 35R:45Si:20Al:83O:17N. This paper is focussed on the B-phase structure where R stands for ytterbium, erbium or yttrium. Fine probe EDX analysis in the TEM has shown that the lenticular B-phase crystals take up a substantial range of composition and that the element R is always clearly anti-correlated with silicon. A larger R3+ cation radius moves the B-phase composition range to lower R contents, and as a consequence of the anti-correlation with silicon, the silicon solid solution range goes to higher values. The EDX results lend support to a B-phase structure consisting of two-dimensional network of randomly linked (Si,Al)(O,N)4 tetrahedra between layers of R3+ cations. It is suggested that, in addition to the random substitution of silicon by aluminium in the (Si,Al)(O,N)4 tetrahedra, a locally increased density in the bi-dimensional network of randomly oriented tetrahedra is associated with an increased density of vacancies in the R3+ cation lattice.

The Formation of B and Iw Phases in the YSiAlON System: Influence of Nitrogen Content

YSiAlON glasses have been produced using a cation ratio of Y:Si:Al=3.5:3.38:2 with a nitrogen content of 17 e / o and Y:Si:Al=3.45:3:2 with varying nitrogen contents (N=8, 10, 12, 14, 17, 20 e / o ). Crystal phase formation occurred after heat treatment of the glasses under nitrogen atmosphere for 10 hours in the temperature range 1050-1150°C. Glass-ceramics were characterised by X-ray diffraction (XRD) and certain mechanical properties measured. Depending upon heat treatment temperature and/or nitrogen content, either a single crystalline phase glass-ceramic or a multi-phase glass-ceramic containing B and I w was produced. Low nitrogen contents (8 e / o N) favour the crystallisation of I w -phase over B-phase, whereas B-phase is formed more readily for higher nitrogen contents (17 e / o N).

SiAlON B-Phase Glass-Ceramic Microstructures

This paper is focussed on the development of microstructure during crystallisation heat treatment of B-phase parent glasses with composition (e/o) 35R:45Si:20Al:83O:17N, where R = Er, Yb, Y or a mixture of Y and Yb. Extensive high resolution analytical transmission electron microscopy has shown that the lenticular B-phase crystals take up a substantial range of composition. The element R is always clearly anti-correlated with the Si, and a larger R3+ cation radius moves the composition range to lower R contents. It is suggested that a locally increased density in the bi-dimensional network of randomly oriented (Si,Al)(O,N)4 tetrahedra is associated with an increased density of vacancies in the R3+ cation lattice.

The Intergranular Oxynitride Microstructure in Silicon Nitride Based Ceramics

The intergranular microstructure in a liquid phase sintered silicon nitride based ceramic may be viewed as an oxynitride glass-ceramic. This work is concerned with the incorporation of yttrium B-phase, which is a five-component phase, into the intergranular regions of silicon nitride ceramics. The silicon nitride materials were fabricated with the addition of a powdered B-phase parent glass with composition (e/o) 35Y:45Si:20Al:83O:17N, or the addition of a mixture of Y2O3, SiO2 and Al2O3 with cation composition (e/o) 35Y:45Si:20Al. The starting powder mixtures contained 10 wt% of sintering additives. Sintering for 2 h at 1800°C was followed by a two-step post-densification heat treatment in order to promote nucleation and growth of yttrium B-phase. Detailed imaging and elemental analysis of the intergranular regions was carried out by EDX in a FEGTEM.