Papers by Author: P.S. Kwawukume

Abstract: Carbon-Carbon composite (C/C) substrate materials are prone to severe oxidation and volatilization problems. To address these issues mullite/SiC coatings were deposited on C/C composite. The coatings were applied by a two-step approach; pack cementation and silica sol based slurry-coating processes. The pack mixture composition for SiC coating comprised; 15-50 wt. % Si (75 μm), 0-25 wt. % B (38.2 μm), 0.01-3 wt. % SiO₂ (38 μm) and 40-85 wt. % SiC green (20 μm). Laboratory synthesized mullite via sol-gel (SG) process of different Al₂O₃/SiO₂ ratios (containing approx. 84-72wt. % Al₂O₃) based on stoichiometric calculation were used; these were admixed with silica sol to form a slurry. The pack mixture and the silica sol based slurry coatings were sintered at temperatures of 1800oC and 550°C respectively. The microstructure and isothermal oxidation behavior of the mullite/SiC coatings and C/C substrates during isothermal oxidation cycle at 1500oC were investigated using XRD and SEM mounted with EDS. Results indicate that optimized oxidation protection for mullite-SiC coating was achieved with higher Al₂O₃ content. The weight loss rate of sol-gel mullite with bulk composition 84 wt. % Al₂O₃ (SG84) was 0.09 gcm-²h^-1 as compared to bulk composition 72 wt. % Al₂O₃ (SG72) which had a weight loss rate of 0.20gcm-²h^-1. The oxidation protection for the synthesized sol-gel mullite/SiC coatings offered effective protection as evidenced in the cycle hour exposure time with limited degradation, chemical compatibility between the coating/substrate interface was good and the phases were quite stable after isothermal oxidation cycle.

Abstract: Physical and chemical studies were conducted on Afari and Mfensi clays in Atwima Nwabiagya District in Ashanti Region of Ghana, with the view of improving their strength properties by chemical stabilization with cocoa pod ash (CPA) and lime as chemical stabilizers. The idea was to produce stabilized clay brick without firing as an improved building material for the rural housing in areas where the materials exist and beyond. The results revealed an improvement in both dry and wet compressive strengths with the additions of 10% to 15% lime and CPA. The maximum dry compressive strength of 2.07N/mm² and 5.85N/mm² and wet compressive strength of 0.91N/mm² and 2.67N/mm² respectively were recorded for Afari and Mfensi clays. The water absorption also reduced from 100% to 27.59% and 17.78% respectively for Afari and Mfensi clays. Although there were decreases in wet compressive strength on soaking the specimens in water, the stabilized bricks did not disintegrate after 28 days of soaking. Moreover, the stabilized clay bricks showed good durability behaviour and did not disintegrate when exposed in the open air for two years.

Abstract: This paper reports work done which seeks to enhance the insulating property of locally mined clays with application to the building and insulation of medium temperature furnaces. Mixtures of locally mined clays with kaolin obtained from Teleku bokasso in the Western Region of Ghana were made. Samples were mixed in different ratios to form composite bodies in the form of bricks of dimensions 7mm×7mm×7mm cast at constant pressure and sintering temperature of 1000 °C. The clays used were characterized in terms of their particle size and chemical composition. The thermal resistivity, water absorption, weight loss, shrinkage, porosity and strength of samples were determined. The results showed that for all samples studied, an increase in Teleku bokasso clay enhanced the insulating properties of the samples from an average of 0.24% for samples containing 20% Teleku bokasso to approximately 2% for samples containing 60% of Teleku bokasso. Mixtures of Teleku bokasso with Afari clay were on the average 1% more thermally resistant than those of Mfensi. Sample A3 (with 40% Teleku bokasso with Afari) is recommended for the building of local furnaces because it had its strength more than doubled (105%) and was by 2% more resistant to heat than M3 (40% Teleku bokasso with Mfensi). All samples showed low weight loss levels, low water absorption, good linear shrinkage and compressive strength.

Abstract: The use of non-essential materials in Ghana like granites, cocoa pod ash and shells as fluxes has been studied in the composition of a local brown glaze in the Na2O-K2O-CaO-Al2O3-SiO2 systems to support electrical porcelain glaze development which hitherto relied totally on imported glazes. Chemical analyses have been performed on the various materials and revealed the presence of the right quantities of aluminous-silicate minerals necessary for combinations in glaze development using the Segar classification. It was found that the range of 0.2 - 0.4 molecular weight of the granites, 0.1 - 0.3 for the cocoa pod ash and 0.4 - 0.8 for the shells forming the R2O/RO2 group and the ratio of 1:12 for the clay and the sand was sufficient to cause excellent melt at 1200OC when 5% - 8% of local manganese and 5% local iron oxide were added to the recipe. The molecular weights of the various oxides in the R2O group were calculated from the chemical analyses of the various materials to reflect the right quantities of the oxides. The results also indicate that holding the RO/R2O group within the above limits and varying the clay to sand ratio from1:8 - 1:12, other glaze melting surfaces from matt to gloss are achievable. Physical examination of the melted glaze on the electrical porcelain bodies showed consistent rich brown colour, well vitrified with no crazing, peeling or pin holes, at pottery temperatures of 1140 - 12000C.