Characterization of Ceramic Raw Minerals in Uganda for Production of Electrical Porcelain Insulators
Sammanfattning: In this work an extended literature survey on ceramics and porcelains in particular, has been documented. Samples from two Ugandan deposits of each feldspar and quartz were characterised by means of X-ray diffraction, thermal analysis, chemical analysis and scanning electron microscopy and found to possess requisite properties for making electro porcelain insulators. Five porcelain bodies were formulated and samples were fabricated from materials collected from selected deposits in Uganda using different mixing proportions of clay, feldspar and quartz. The properties of the formulated bodies were studied in relation to workability, firing temperature, dielectric and bending strengths. The strengths (both mechanical and dielectric) were found to decrease as the firing temperature was increased above 1250°C. Additionally, ten formulations with components in the range of 30-60% clay, 20-45% feldspar and 20-25% sand were prepared. Specimens for mechanical and physical characterisation were made using the plastic extrusion method. The samples were characterised in terms of constituent oxide composition, flexural strength, fracture toughness, dielectric strength along with microstructural and phase properties using ICP-AES analyses, 4-point load bending strength test, Vicker’s indentation method, SEM and XRD analyses respectively. XRD studies revealed that the crystalline phases formed were mullite and quartz and their intensity was almost identical for all samples fired at 1250°C but there was a decrease inquartz content as temperature was increased above 1250°C. Samples with 20% sandcontent resulted in higher density, MOR and fracture toughness compared to thosecontaining 25% sand. Mullite content on the other hand did not change at temperatures above 1200°C but there were significant differences in the morphologies of the mullite crystals in the samples. Optimum mechanical and electrical properties were found at maximum vitrification and a microstructure showing small closely packed mullite needles. This occurred at a firing temperature of 1250°C.Three promising porcelain formulations from the above investigations were chosen and five glazes formulated to fit on biscuit fired specimens. The best-fitting glaze was adopted. The formulated specimens were investigated using dilatometry, Steger test, FEG-SEM, XRD, 4-point bending, dielectric strength and fracture toughness tests. A porcelain specimen consisting of 68% SiO2, 19% Al2O3, 4.7% K2O and a glaze of Seger formula RO:0.57Al2O3:4.86SiO2 exhibited MOR of 105MPa with Weibull modulus of 5.6 and a dielectric strength of 18kV/mm upon firing at a heating rate of 6ºC/min to 1250ºC, soaking for 2h at the top temperature and cooling down to 500°C at a rate of 6°C/min, followed by furnace cooling. The microstructure of the high strength specimen exhibited round mullite needles, quartz and glass. Holding samples for 2h at peak temperature resulted in a 22% increase in MOR compared to 1h holding. Glazing further improved strength by 67% for the best sample. Compressive stresses in glaze contributed to the strengthening effect. The dielectric and mechanical strengths obtained make the ormulated sample suitable for application in low voltage electrical insulation.
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