Dan S. Perera

High aspect ratio β-Si3N4 grain growth

Abstract It has been shown by others that high aspect ratio β-Si 3 N 4 grains formed in-situ on sintering lead to high fracture toughness in Si 3 N 4 ceramics. In this work α-Si 3 N 4 powder was sintered at 1750°C for varying times with Y 2 O 3 , La 2 O 3 , SiO 2 , and CaO additives. The microstructure of the materials was examined by electron microscopy and microanalysis. The β-Si 3 N 4 grain growth rate was controlled by diffusion through the liquid phase. A maximum aspect ratio of about 20 was observed for β-Si 3 N 4 grains heated for 6 h and was dependent on the chemistry of the liquid phase present. The formation of high aspect ratio Si 3 N 4 at a relatively low temperature and short sintering times is discussed.

Geopolymers for the Immobilization of Radioactive Waste

Geopolymers are made by adding aluminosilicates to concentrated alkali solutions for dissolution and subsequent polymerization to form a solid. They are amorphous to semicrystalline three dimensional aluminosilicate networks. Although they have been used in several applications their widespread use is restricted due to lack of long term durability studies and detailed scientific understanding. Three important tools for the study of geopolymers are transmission electron microscopy (TEM), solid state magic angle spinning nuclear magnetic resonance (MAS NMR) and infra red (IR) spectroscopy. Cs and Sr are two of the most difficult radionuclides to immobilize and are therefore suitable elements to study in assessing geopolymers as matrices for immobilization of radioactive wastes. In this study Cs or Sr was added to geopolymer samples prepared using fly ash precursors. A commercial metakaolinite geopolymer was studied for comparison. The geopolymers were mainly amorphous as shown by TEM, whether they were made from fly ash or metakaolinite. In the fly ash geopolymer, Cs preferentially inhabited the amorphous phase over the minor crystalline phases, whereas Sr was shared in both. The MAS NMR showed that Cs is held mostly in the geopolymer structure for both fly ash and metakaolinite geopolymers. The IR spectra showed a slight shift in antisymmetric Si-O-Al stretch band to a lower wavenumber for the fly ash geopolymer, which implies that more Al is incorporated in this geopolymer structure than in the metakaolinite geopolymer.

Characterisation of Geopolymers for the Immobilisation of Intermediate Level Waste

Geopolymers are made by adding aluminosilicates to concentrated alkali solutions for dissolution and subsequent polymerisation to take place. Their physical behaviour is similar to that of Portland cement and they have been considered as a possible improvement on cement for several applications including as media for the encapsulation of hazardous or low/intermediate level radioactive waste. We studied in detail a commercial geopolymer to try to get a better understanding of geopolymers in order to enhance its leach resistance for immobilisation of intermediate level radioactive waste. We also briefly investigated two types of experimental geopolymers, one made with a metakaolinite and another from fly ash as the aluminosilicate source. The commercial geopolymer paste had an apparent porosity of 26% and it was possible to reduce it to 17% by adding ∼ 30 weight % foundry sand. The apparent porosities of the geopolymer made from metakaolinite and from fly ash were 13% and 26% respectively. X-ray powder diffraction showed in the three geopolymers, an amorphous phase (deduced by the presence of a very broad diffuse peak centred at a d-spacing of ∼ 0.32 nm), quartz and other minor phases. The energy dispersive spectroscopic analysis under the scanning electron microscope confirmed these. Magic angle spinning nuclear magnetic resonance data from the samples showed Al to be mainly in 4-fold coordination and Si sites varying from Q0 to Q4 coordination as also found by other researchers. 23 Na spectra indicated that the Na was mainly in the pore water. The 133 Cs spectra showed a strong possibility of Cs being mainly bound in the structure while a small amount could still be in the pore water. The initial leach tests showed alkalis were leached out at rates of several orders of magnitude more than the Al and Si network ions. The most likely reason is that a significant alkali inventory is in the pore water. To remove pore water and incorporate simulated radionuclides such as Cs in the network the commercial geopolymer was heated up to 1200°C. Differential thermal/thermogravimetric analysis showed the loss of water occurs in three stages and most had been lost by 700°C. These results are in broad agreement with the Infra red spectra obtained for samples heated over the temperature range 30–900°C. The broad water band intensity in the range 2600-30-900°C. The broad water band intensity in the range 2600–3800 cm −1 decreased steadily with temperature although a small fraction remained even after heating to 500°C. The silanol band had disappeared at 800°C, and the 3619 cm−1 band (due to OH) virtually disappeared by 900°C.Copyright

Study of Different Sintering Techniques for A Ti-A1-O-N Spinel Composite

A composite in the Ti-A1-O-N system consisting of a spinel matrix with a dispersion of TiN and A1N phases was fabricated from a mixture of TiO2, A1O3 and A1N powders by several sintering methods at 1800 °C. The composite formed from the starting composition Ti0.6A13.2O3.0N2.0 was pressureless sintered, hot pressed, sinter + HIPed and HIPed. The density, porosity and microstructure were studied with respect to the sintering method. It was possible to produce a dense material with no open porosity and free of grain boundary glass. This spinel composite has the potential to be developed as a high temperature engineering ceramic.