Phase quantification of ß-Si3N4/ß-SiC mixtures by X-ray powder diffraction analysis

X-ray powder diffraction methods of phase quantification were adapted and compared to mixtures of -Si3N4 and -SiC. Multiline mean-normalized-intensity methods and whole pattern analysis (Rietveld) both have advantages and disadvantages over each other. Satisfactory results (less than 3% absolute deviation) can be achieved in minimal time using intensity normalization methods. Phase quantification using the Rietveld method requires significantly longer measuring time, evaluation time and expertise to obtain the same results

In situ generated homogeneous and functionally graded ceramic materials derived from polysilazane

The pyrolysis of cross-linked poly(hydridomethyl)silazane pellets via transient isothermal ammonia gas treatment yields amorphous layered ceramic bulk Si/N-Si/N/C-materials with graded carbon content. By this process the C content in the material can be adjusted with high accuracy in the range between 0 and 14 wt. %. The influence of (1) temperature of reactive ammonia treatment (2) time of reactive isothermal ammonia treatment (3) isothermal holding time under inert atmosphere (Ar) before application of ammonia (4) degree of cross-linking of the polycarbosilazane (5) porosity of green compact (6) volume ratio of NH3 in the reactive atmosphere was examined. The temperature of 525ºC and the reactive atmosphere containing 10 vol. % NH3 were found to be optimum for carbon content adjustment. Higher ammonia contents did not allow suitable control of the process, while higher temperature of transient heat treatment caused crack formation in the specimen due to excessive pressure of gaseous reaction products. High degree of cross-linking as well as the annealing of cross-linked green bodies at transient temperature in inert atmosphere decrease the efficiency of the reactive treatment and increase the ratio of C built in the pre-ceramic continuous random network (CRN). Next to graded materials, samples with bulk homogeneous carbon distribution were generated. However, these require more sophisticated heating schedules and combination of reactive treatment with pre-annealing in inert gas

Functional ceramics via precursor chemistry coupled with forming methods

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In-situ carbon content adjustment in polysilazane derived amorphous SiCN bulk ceramics

The paper is concerned with in-situ carbon content adjustment in amorphous bulk silicon carbonitride ceramic matrices prepared by thermal polymer to ceramic conversion of crosslinked and compacted poly(hydridomethyl)silazane powders (NCP200). Heat treatment under inert (Ar) or reactive (Ar/NH3, NH3) atmosphere was applied for carbon content adjustment. Isothermal annealing steps at various intermediate temperatures were included into the schedule. Optimal annealing temperatures were found to be in the range from 500 - 550°C. Results are presented. Different possible chemical reactions are considered

Dense silicon carbide and silicon nitride-based ceramics with high electrical conductivity via precursor chemistry

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