Materials Science Forum Vol. 554

Abstract: Research in nitrides and oxynitrides has experienced a significant downturn during the last 10 years – perhaps because there have been fewer new developments in the R. & D. field compared with the regular new initiatives reported in the 1980s and early 1990s, but also because the public perception is now very much that research issues associated with commercial nitride materials have been “worked out”. With the limited market (both in volume and price) for structural materials, funding bodies are tending to direct their resources into other research areas where it is perceived there are more opportunities and a larger market place. lnfluenced by these trends, new researchers at the start of their careers are tending to focus their research on other areas of ceramics or materials science, rather than striving to find new developments in what may be seen as a “worked-out” subject area. This is not to say that there is not still a considerable amount of research that can be done in the nitrides/oxynitrides field, but radical new ideas are needed, focused on topical and appreciably-sized market applications, in which the portfolio of properties offered by oxynitrides demonstrates clear advantages over cheaper and more easily prepared alternatives.

Abstract: Oxynitride glasses are found at grain boundaries, i.e. triple point junctions and intergranular films, in silicon nitride based materials as a result of cooling of liquid phases formed by reaction of sintering additives with silicon nitride and silica present on the nitride surface during the densification of the ceramics. The glass chemistry, particularly the content of modifying cation, usually Y or a rare earth (RE) ion, and the volume fractions of these oxynitride glass phases within the ceramic affect the properties of silicon nitride such as fracture toughness and creep at high temperature. As nitrogen substitutes for oxygen in silicate and alumino-silicate glasses, increases are observed in glass transition and softening temperatures, viscosities (by two to three orders of magnitude), elastic moduli and microhardness. If changes are made to the RE:Si:Al ratios or as the size of the rare earth cation decreases, properties such as viscosity can be increased by a further two to three orders of magnitude. These effects have a strong impact on the mechanical properties of silicon nitride based ceramics, especially creep resistance. This paper provides an overview of previous work on oxynitride glasses and outlines the effect of glass composition on their properties and discusses the implications for high temperature behaviour of Si3N4 ceramics.

Abstract: Ca-Sialon glasses have been known for some time [1] and they are effectively calciumalumino- silicate glasses containing nitrogen which improves their mechanical properties. Calciumalumino- silicate glasses containing fluorine are known to have useful characteristics as potential bioactive materials [2]. Therefore, the combination of both nitrogen and fluorine additions to these glasses may give useful bioglasses with enhanced mechanical stability.Addition of fluorine to oxynitride glasses was not reported previously and this paper gives the first report of the glass forming regions (and evaluation of some properties) in the Ca-Si-Al-O-N-F system. Within the previously defined [1] glass forming region in the Ca-Si-Al-O-N system, homogeneous, dense glasses are formed. Addition of fluorine extends the glass forming region but also increases the reactivity of the glass melts. One major problem is fluorine loss as SiF4, but also loss of nitrogen, which affects the final composition and results in porous samples. To suppress the fluorine loss and CaF2 precipitation, consideration of the ratio of cations to fluorine and the coordination number of Al atoms is important. Discussion of the role of cations in these oxyfluoronitride glasses is presented.

Abstract: Rapid cooling rates and quenching have traditionally been associated with glass formation. Hampshire et al. [1] investigated oxynitride glasses cooled in a tungsten resistance furnace at approximately 200oC/min and found that fast cooling rates were only important near the limits of the glass-forming region. In the current work on various M-Si-Al-O-N (M=Y, La, Yb, Nd) systems, it was found that even at a relatively slow cooling rate glass formation was still possible for a wide range of compositions. Different cooling rates were investigated to determine the minimum cooling rate at which a glass will form. Quantitative X-ray analysis of melted compositions indicated the relative amounts of amorphous phase and crystalline phase.

Abstract: The preparation of bulk glasses in Ca-Si-Al-O-N-F system with the composition in equivalent % of 28e/oCa:56e/oSi:16e/oAl:100-X-Ye/oO:Xe/oF:Ye/oN are reported. The glass formation behaviour and properties of this new range of glasses are examined in detail. Fluorine decreases the glass transition temperature, the density and the mechanical properties of the glasses while nitrogen increases them. Therefore, it appears that fluorine acts as a network modifier while, on the contrary, nitrogen acts as a network former even in presence of fluorine.

Abstract: Nitridation of amorphous aluminium phosphate (AlPO4) and mixed aluminium gallium phosphate (Al0.5Ga0.5PO4) under ammonia flow allows preparing “AlPON” and “AlGaPON” bifunctional acid-base heterogeneous catalysts. Their acid-base bi-functional character allows their use as selective catalysts for the production of jasminaldehyde through the cross-condensation of heptanal and benzaldehyde, followed by the dehydration of the aldol intermediate product. Three parameters enabling to maximize the jasminaldehyde yields in a batch reactor operated at 125°C were investigated: the catalyst composition (Al/Ga and O/N ratio), the reactant concentration and the amount of catalyst in the reactor. Maximum jasminaldehyde selectivities were obtained for intermediate nitrogen contents (7-10 wt.%). Maximum reaction rates were obtained using pure reactants and a catalysts weight equal to 10% of the total reagent weight.

Abstract: Oxygen/nitrogen (O/N) configuration in RE-J-phase, RE4Si2O7N2 (RE = rare earth element), was simulated by the Monte Carlo method applied to O/N distribution models. Proportion of local structures of Si2(O,N)7 ditetrahedra and Si(O,N)4 tetrahedra in the J-phase was quantitatively assessed. For the Lu-J-phase model with the bridging site between two Si atoms being occupied by nitrogen atom, the Si2(O,N)7 ditetrahedra composed of O3≡Si–N–Si≡O2N (> 40 %), O3≡Si–N–Si≡O3 (c.a. 30 %), and O2N≡Si–N–Si≡O2N(c.a. 15 %). Tetrahedra of SiO3N and SiO2N2 were dominant and small amount of SiON3 tetrahedra coexisted. For La-J-phase model with the O/N occupancy of 0.1/0.9 at the bridging site, configurations of O3≡Si–O–Si≡O2N (c.a. 5%), O3≡Si–O–Si≡O3 (c.a. 3%), and O2N≡Si–O–Si≡O2N (c.a. 2%) were demonstrated in addition to the three configurations of ditretrahedra recognized in the Lu-J-phase. In La-J-phase coexistence of SiO4 tetrahedra was presented.

Abstract: A frequent criticism of nitride materials during the last 30 years, and especially those designed for structural applications has been that the cost is too high by a factor of (say) 10. In the competition with cheaper materials (albeit with poorer properties and shorter lifetimes), users have generally preferred to go for the cheaper option, rather than the more expensive nitrides. Despite many attempts to address this issue, the cost of nitride processing has remained high – due to the high price of starting materials, the high temperatures needed for firing, and also the finishing costs (often involving diamond machining), and this has been a major factor limiting the market share enjoyed by these materials. A number of studies have been reported recently using the technique of mechanochemical synthesis, in which nitrogen is incorporated (usually via ammonia) into the starting powders during a high-energy milling process (at room temperature). In the subsequent firing, considerably lower temperatures are needed to produce the resulting final nitride product(s). In this presentation, the technique of mechanochemical synthesis is discussed, the range of materials that have been produced are reviewed, and the potential of this technique for reducing the cost of bulk nitride production is reviewed.

Abstract: Nano-structured β-sialon precursor powders were obtained as a result of milling in a planetary mill with a high acceleration. Various mixtures of initial nitrides/oxides have been prepared as β-sialon precursor with low (0.4-1.0z) substitution and were milled in a planetary mill of acceleration 28g (TTD, Russia) with zirconia or silicon nitride balls for a various times (30-45 min). The resultant powder showed various extents of crystalline lattice deformation: smaller crystallites, dislocations, bumpy surface. The degree of lattice destruction was influenced by the milling time and grinding media, however various mixture components showed diverse susceptibility to deformation.

Abstract: The synthesis and deposition behavior of tungsten nitrides on a Si(400) or quartz plate were studied using a vertical hot-wall tube reactor. The preparation of the tungsten nitride by chemical vapor deposition (CVD) is predicted by the sticking probability of tungsten nitride by calculating the step coverage on the Si(400) engraved with a microtrench of different aspect ratios. The CVD deposition was performed at temperatures of 556–1063 K for deposition times up to 45 min in a gas mixture of WF6–NH3–H2 in Ar and at a total pressures of 5 and 13 Pa. From the XRD analysis, amorphous crystallites were observed at 556 and 673 K but β–W2N (111) was obtained at 790 K. The film thickness of the tungsten nitride linearly increased with the increasing deposition time at 673 and 790 K without any orientation despite the film thickness. The sticking probabilities, η, are 0.00044–0.00123 for Si(400) with different aspect ratios under the conditions of 5–13 Pa and 10–20 min.