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Abstract

This thesis describes an investigation of electrically conductive composites based on Si3N4 with dispersed TiN or ZrN particles fabricated by an in-situ composite method. The influence of the amount of conductive phase of the Si3N4/TiN and Si3N4/ZrN composites on the density, the flexural strength and the electrical conductivity were evaluated.
The first part of my work was focused on the formation of TiN and ZrN nanoparticles on the Si3N4 powder surface by an in-situ gel-precipitation of Ti- and Zr(OH)4 and a subsequent chemical reaction at an elevated temperature. The results showed that the conversion of TiO2 to TiN with a narrow particle size distribution, ranging from 4 nm to 10 nm can be completed after a thermal treatment at 900 °C for 6 h in a NH3 gas flow. The in-situ formation of nano-sized ZrN on the Si3N4 particles is more challenging, due to the higher reaction temperature between the ZrO2 and Si3N4 and due to the change in the SiO partial pressure. The complete conversion of ZrO2 to ZrN with an average particle size of 40 nm can be attained at 1600 °C for 3 h in flowing nitrogen at low pressure.
In the second part of my research, relatively dense and homogeneous Si3N4/TiN and Si3N4/ZrN composites were fabricated by the pressureless sintering of compacted TiN- or ZrN-coated Si3N4 powders with Y2O3 and Al2O3 sintering additives at 1850 °C for 2 h in N2. The results of the flexural strength and the electrical resistivity measurements indicate that with an increasing amount of TiN and ZrN in the composites, the electrical conductivity increases, while at the same time the flexural strength decreases, making these ceramics containing 24 vol. % of TiN and 47 vol. % of ZrN suitable for the production of heating elements. The percolation threshold for the electrical conductivity of these composites was two times lower than in the case of the composites prepared with the conventional composite method using mechanically mixed powders. In addition, a simple theoretical percolation-threshold model was developed by considering the various aspect ratios of the insulating ellipsoids as well as the different diameters of the conductive particles. The results revealed that this model provides a good interpretation of the obtained experimental results for the electrical conductivity of both composites.
In the third part an attempt was made to fabricate the Si3N4/ZrN composites by the direct sintering of ZrO2-coated Si3N4 powders together with yttria and alumina additives. It was expected that the reaction of ZrO2 with Si3N4 would take place during the heating stage. The results demonstrated that when the composite contained 20 vol. % of ZrO2 in the starting composition, the reaction leading to the formation of ZrN was not completed. Irrespective of the density gradient from the core to the outer surface of the sample, the composite exhibited a high flexural strength and its surface was electrically conductive, while the internal part remained as an insulator. We found that these results are promising for the production of heating elements. As a result, a one-step manufacturing process for a composite ceramic heater was patented.