Papers by Author: Derek P. Thompson

Abstract: The preparation of high temperature ceramics simultaneously containing silicon, nitrogen and carbon has only relatively recently become an area of interest for inorganic crystal chemists, and the recent discovery of a new series of carbonitrides with the general formula MM’Si4N6C is of interest because of the good high temperature properties they appear to display. On the one hand, M and M’ can be the same trivalent metal - either rare earth or yttrium; in this case, the resulting compounds display orthorhombic (pseudo-hexagonal) structures. Alternatively the metals may be a mix of di- (Ca,Sr, Ba) and tri-valent (Y,Ln) cations, in which case the carbon is replaced by nitrogen, and the overall symmetry is hexagonal. Other quaternary nitrides of a similar type can be produced if the two metal cations remain trivalent and one of the silicon atoms is replaced by aluminium. The present study describes the preparation of powder samples of Y2Si4N6C and LaYSi4N6C starting from YH2, La, Si3N4 and carbon precursors, and summarises attempts to achieve a dense product by hot-pressing at 1700-1800oC. Some preliminary mechanical property measurements are included.

Abstract: Nicalon SiC fibre tows have excellent properties for ceramic matrix reinforcement but residual oxygen within the fibres degrades fibre properties when these are incorporated into ceramic matrices at elevated temperatures. β-SiAlON ceramics also have excellent mechanical and physical properties, especially fracture toughness. However, sintering of β-SiAlON is generally carried out at 1650-1750°C, considerably higher than the temperatures above which fibre degradation occurs (>1200°C). In the present study, the refractoriness and strength of Nicalon fibres were improved by high pressure CO heat treatment, and densification temperatures of β-SiAlON were lowered by using different kinds of sintering additives. Heat-treatment of the fibres under 45 bar CO pressure at 1500-1650°C led to an increase in fibre strength and to the formation of a thin carbon layer on the surface of the fibres. These improvements in the Nicalon SiC fibres allowed them to be incorporated successfully into β-SiAlON matrices. The as-received and heat-treated fibres were infiltrated with β-SiAlON starting powder mixes and hot-pressed with low temperature sintering additives at 1600-1700°C for 30 min. Bending strength and fracture toughness measurements showed that samples containing heat-treated fibres provided a significant strength and fracture toughness increase compared with similar samples prepared using as-received fibres, and massive pull-out was observed because of the weak interface resulting from the surface carbon coating on the fibres.

Abstract: Silicon nitride samples were pressureless sintered with up to 5 w/o MgO to give densities in the range 98-99% of theoretical. After pressureless sintering, selected samples were placed in a vacuum heat treatment furnace surrounded by a carbon bed in a carbon crucible at a pressure of less than 4x10-4 mbar, and vacuum heat treated at different temperatures and times to remove grainboundary glass. The results showed that this was substantially achieved at 1575oC for 3h and that increasing the time to 5 hours gave still further improvement. SEM images, EDX analysis and oxidation tests provided additional evidence for the removal of Mg from the samples.

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: 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: The discovery of sialons in 1971 was a significant step in the history of nitrogen ceramics, because it broadened the field into a wider range of chemistry, and simultaneously increased the flexibility to modify microstructure and properties. During the last 35 years this has resulted in the development of a spectrum of materials, mainly based on the α- and β- structural forms. However, the subject has remained broadly within the scope of structural ceramics. During the 1990s, a range of new sialon derivatives were prepared with a more varied starting chemistry, and the corresponding final materials demonstrated a correspondingly wider variety of structural complexities. In response, many sialon researchers have started to broaden their interests beyond the limiting horizon of structural applications, and considerable progress has been made in the development of transparent and coloured materials, and also derivatives with useful electronic properties.This enlargement of the sialons field is still in its infancy, but promises to generate a much wider spectrum of materials, which can be tailored to meet the increasingly multifunctional requirements of modern day engineering applications.