Immersion tube heating systems are being used in some aluminium foundries, as they are more compact and thermally efficient than conventional radiative heating. Ceramic immersion tubes have been applied to these systems. However, the tubes can fail prematurely and unpredictably. This is thought to be due to the effects of the oxidising environment in which the tubes operate, and reaction with the molten aluminium alloys. The work described in this dissertation investigates these two factors. The ceramic in use is a Silicon Nitride-Bonded Silicon Carbide, SNBSC (particles of silicon carbide, SiC, in a porous silicon nitride, Si<SUB>3</SUB>N<SUB>4</SUB>, matrix), formed by a reaction bonding process. Four grades of this material, made using different quantities of fine and coarse SiC particles and differing sizes of silicon powder, were studied. The material was characterised using X-ray diffraction and optical, scanning electron and transmission electron microscopy. Dense (sintered) silicon carbide and silicon nitride ceramics were also studied to compare their behaviour with that of SNBSC. Oxidation was carried out in dry air at 1000°C, 1200°C and 1400°C. Thermogravimetric analysis (TGA) techniques were used to determine weight gains. Oxidised samples were studied using SEM and XRD. Changes in pore structure and surface chemistry were investigated by mercury porosimetry and Fourier Transform Infrared spectroscopy respectively. Oxidation was found to occur predominantly in the porous nitride phase, giving silica (SiO<SUB>2</SUB>) as crystalline cristobalite and a glassy silicate. The weight gain results can be fitted to an asymptotic law representing a process where the pores are progressively blocked by oxide product: Δm = Δm<SUB>∞</SUB>[1 - e<SUP>-k</SUP>a<SUP>t</SUP>] The kinetic results were in accord with measured changes in the pore structure. SNBSC with fine SiC oxidised more than the standard ceramic. Grades of SNBSC with coarse SiC had better oxidation resistance. Dense Si<SUB>3</SUB>N<SUB>4</SUB> oxidised parabolically, forming tridymite and silicates arising from sintering aids. At 750°C, liquid aluminium reacted slightly with unoxidised SNBSC if the alloy's silicon content was less than 5at%. Aluminium alloys readily wetted and reacted with oxidised material, and thus penetrated the pore network and enlarged pores. Interactions between aluminium and dense ceramics were in accord with results for SNBSC. Aluminium reacted with silica, producing alumina. This followed parabolic kinetics after an incubation period of approximately 4 minutes.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:605666 |
| Date | January 1994 |
| Creators | Johnston, M. W. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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