Clay being the most common minerals is a composite of several minerals such as iron oxides, alumina, silica etc which have many useful properties, including high specific surface area and excellent adsorptive capacity. Nevertheless, due... more
Clay being the most common minerals is a composite of several minerals such as iron oxides, alumina, silica etc which have many useful properties, including high specific surface area and excellent adsorptive capacity. Nevertheless, due to the low purity level of the material in nature, its usefulness at high temperatures that exceed 1200 o C has been hampered. Attempt is made in this study to investigate the possibility to utilize the various composite minerals in the clay as an advantage to synthesize mullite fibers and zircon ceramic based composites through powder metallurgy technique, for high temperature applications. Powder zirconia was mixed in a turbula mixer with varying compositions of clay of known mineralogical composition and mechanically milled in a planetary ball mill. The raw clay and blended powders/clay were compacted into standard sample dimensions, and finally fired at 1200 o C and 1300 o C for one hour, also at 1400 o C and 1500 o C for one, two and three hours. The fired samples were characterized using ultra-high resolution field emission scanning electron microscope (UHR-FEGSEM) equipped with energy dispersive spectroscopy (EDX), and X-ray diffractometry (XRD). Various mechanical properties of the samples were also determined with the intention to select a recipe of optimum performance. Zircon phase was discovered in the samples of powder/clay mixtures from temperature of 1200 o C. Also mullite fibers were discovered in those at 1400 o C. The addition of the zirconia greatly improved on the mechanical properties of the samples when compared with those without the additives. It was concluded that the optimum performance was obtained from the sample with composition 30% (vol) ZrO2 and 70% (vol) clay fired at 1400 o C and held at the temperature for an hour.
Understanding the phase transformations/developments that result due to varying the production parameter of engineering materials is vital to development of new materials. The effects of yttria and niobium oxide on the phase changes and... more
Understanding the phase transformations/developments that result due to varying the production parameter of engineering materials is vital to development of new materials. The effects of yttria and niobium oxide on the phase changes and mechanical properties of mullite fiber reinforced zircon-zirconia ceramic composites produced by in-situ sintering of raw materials was investigated. Varied amounts of powder zirconia, yttria and niobium oxide were mixed in a turbula mixer with a fixed composition of clay (70% volume) of known mineralogical composition and mechanically milled in a planetary ball mill. The blended powders/clay were compacted into standard sample dimensions, and finally fired at 1400°C and held at varied time of one, two and three hours. The fired samples were characterized using ultra-high resolution field emission scanning electron microscope (UHR-FEGSEM) equipped with energy dispersive spectroscopy (EDX), and X-ray diffractometry (XRD). Various mechanical properties of the sintered samples were also investigated. It was observed that the investigated mechanical properties (with the exception of shrinkages) improved with the amount of raw zirconia initially used in the samples. It was also observed that addition of niobium oxide favours the formation polymignite phase, while the presence of both yttria and niobium oxide in the raw materials resulted in the formation of fergusonite phase in the samples. Both additives favour the phase transformation of zirconia from monoclinic to tetragonal and cubic phases even at temperature as low as 1400°C. It was concluded that the improved mechanical properties of the samples was due to strengthening by both mullite fibers reinforcement and phase transformation strengthening.
