Showing papers by "Kenneth J. D. MacKenzie published in 1996"

MAS NMR Evidence for the Presence of Silicon in the Alumina Spinel from Thermally Transformed Kaolinite

Abstract: Georgia kaolinite dehydroxylated at 950°C for 24 h, then leached with KOH at 90°C for 1 h, shows a well-resolved 29 Si MAS NMR resonance at -77.5 ppm, within 2 ppm of the calculated position for Si incorporated in γ-alumina spinel. From a knowledge of the total SiO 2 in the leached sample and its partitioning between the spinel and residual grain boundary phases, a maximum of 3.9 wt% is estimated to be present in the spinel.

Thermal reactions of alkali-leached aluminosilicates studied by XRD and solid-state 27Al, 29Si and 23Na MAS NMR

Abstract: The interactions of dehydroxylated Zettlitz kaolinite with NaOH solution have been studied using X-ray powder diffraction and solid-state MAS NMR to characterise the crystalline and amorphous products, respectively, and to provide further information about the reaction sequence of kaolinite. Under the present conditions, leaching for very short times (ca. 10 min) preferentially removes uncombined amorphous silica (but not the quartz impurity), and also introduces Na into the remaining amorphous aluminosilicate phase. Leaching for longer times continues to remove Si from the amorphous phase, with a concomitant increase in Na incorporation and an increase in the proportion of available Al. On reheating to 1140 and 1300°C, the resulting crystalline phases reflect the increase in AI and Na content with increased leaching time, forming α-alumina and, in samples leached for > 60 min, a nepheline solid solution at the expense of mullite, residual quartz and cubic spinel-type phase.

Additive-assisted pressureless sintering of carbothermal β′-sialon: an X-ray and solid-state MAS NMR study

Abstract: β-Sialon pellets (Si6 –zAlzOzN8 –z; z= 2.45) prepared from carbothermally synthesised sialon powder were sintered in N2 at 1400–1700 °C with additions of 15 mol% each of Al2O3, MgO and Y2O3, and 10 mol% each of various combinations of these oxides. The most effective densifying agent is Y2O3, both alone and in combination, followed by MgO and Al2O3. XRD and multinuclear MAS NMR (27Al, 92Si, 25Mg and 89Y) studies revealed traces of X-phase sialon, SiC and an X-ray amorphous glass-like phase in the starting material, and were used to monitor phase changes during sintering. Y2O3 by itself and with other additives forms Y3Al5O12(YAG) with β′-sialon; at higher temperatures, an aluminium-rich polytypoid sialon and yttrium-containing glass are formed, to the detriment of the strength of the sintered body. MgO initially forms MgAl2O4, lowering the sialon z-value. At higher temperatures, this further reacts to form a glass containing magnesium in both tetrahedral and octahedral sites. A12O3 reacts with β′-sialon, increasing its z-value. The NMR spectra confirm the XRD phase analyses and provide information about phases not detectable by XRD, and about the possible constitution of the glasses formed.

Volatile products formed by carboreduction and nitridation of clay mixtures with silica and elemental silicon

Abstract: Simultaneous carboreduction and nitridation (CRN) of clays provides a useful route for the preparation of low-cost β-sialon powders. During the course of the CRN reaction at 1400–1450 °C, mass losses of ca. 10% in excess of the theoretical amounts are typically found. This discrepancy is mainly due to transport of gaseous SiO from the reactants into the N2–CO gas stream, followed by condensation in the cooler parts of the furnace system. In this work, the condensed material lost from the reacting system was collected under controlled conditions, and its constitution studied by 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) and complementary X-ray powder diffraction (XRD). Predictions from thermodynamic calculations of the various possible gas-phase reactions are compared with the observed results.

Thermal reactions of alkali-leached aluminosilicates studied by XRD and solid-state Al-27, Si-29 and Na-23 MAS NMR

Abstract: The interactions of dehydroxylated Zettlitz kaolinite with NaOH solution have been studied using X-ray powder diffraction and solid-state MAS NMR to characterise the crystalline and amorphous products, respectively, and to provide further information about the reaction sequence of kaolinite. Under the present conditions, leaching for very short times (ca. 10 min) preferentially removes uncombined amorphous silica (but not the quartz impurity), and also introduces Na into the remaining amorphous aluminosilicate phase. Leaching for longer times continues to remove Si from the amorphous phase, with a concomitant increase in Na incorporation and an increase in the proportion of available Al. On reheating to 1140 and 1300 degrees C, the resulting crystalline phases reflect the increase in Al and Na content with increased leaching time, forming a-alumina and, in samples leached for > 60 min, a nepheline solid solution at the expense of mullite, residual quartz and cubic spinel-type phase.