Showing papers by "Kenneth J. D. MacKenzie published in 2001"
TL;DR: In this paper, the influence of the leaching time on the porous properties and structure of the silica was studied by XRD, XRF, FTIR and BET adsorption methods.
Abstract: Mesoporous silica has been prepared by leaching of the Al2O3 component from mechanically amorphized kaolinite. The kaolinite was amorphized by grinding in a planetary ball mill for 1 h. After grinding the amorphized kaolinite was chemically treated with dilute sulfuric acid at 90°C for varying times. The influence of the leaching time on the porous properties and structure of the silica was studied by XRD, XRF, FTIR and BET adsorption methods. The specific surface areas of the leached samples were found to vary from 312 m2/g to 284 m2/g. The pore size distribution, calculated by the BJH method based on N2 gas isotherms, showed a unimodal pore size distribution with an average pore size of about 3.8 nm. The total pore volume of the porous silica varied from 0.28 ml/g to 0.312 ml/g, with a uniform pore size distribution in the mesopore regions. New applications exploiting the characteristic pore size of this material are to be expected.
67 citations
TL;DR: In this paper, the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al2O3·2SiO2) but of widely differing synthetic origin, were extracted using two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) spectroscopy.
Abstract: Two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR spectroscopy is used to extract the isotropic chemical shifts and quadrupolar parameters of five amorphous aluminosilicates, all of approximately mullite composition (3Al2O3·2SiO2) but of widely differing synthetic origin. Three principal types of Al site are apparent in each sample: two of these are conventionally assigned to 4- and 6-coordinate Al, while the nature of the third site, observed at a shift of δ≈30 ppm , remains a subject of debate. In some of the more anhydrous samples, two 6-coordinate Al sites are observed. Significant distributions of isotropic chemical shifts and quadrupolar parameters are evident in each of the Al sites resolved in the two-dimensional spectra and lineshape fitting is used to estimate the means and widths of these. Additional data are obtained from 27Al{1H} CPMAS NMR experiments and suggest that the protons in the samples are most closely associated with particular 6-coordinate Al sites. The NMR results from the five samples are compiled and compared with those reported for other amorphous and crystalline aluminosilicates and the possible nature of the δ≈30 ppm site is discussed.
60 citations
TL;DR: In this paper, the structure of leached products from mechanically amorphized kaolinite was investigated by various techniques and the 29Si MAS NMR spectrum of the leached product showed peaks at −102 and −111 ppm, assigned to the silanol groups and the three-dimensional silica framework, respectively.
47 citations
TL;DR: In this article, two-dimensional 27Al multiple-quantum magic angle spinning (MQMAS) NMR experiments are used to study mixtures of bayerite (alpha-Al(OH)3) with either silicic acid (SiO2.nH2O) or silica gel (Si2) that have been ground together for varying lengths of time.
17 citations
TL;DR: In this article, 27Al NMR was shown to contain aluminium in one octahedral and two tetrahedral sites, a distribution consistent with expectations based on a recent Rietveld structure determination.
Abstract: 23 Na and 27Al MAS and DOR NMR spectra are reported for the mullite-type crystalline compound NaAl9O14 and its amorphous precursor using a range of applied magnetic fields (8.45–16.9 T). The crystalline compound is shown by 27Al NMR to contain aluminium in one octahedral and two tetrahedral sites, a distribution consistent with expectations based on a recent Rietveld structure determination. By contrast, two approximately equally populated non-identical Na sites can be distinguished by 23Na NMR. Spectral simulation indicates these sites to be of slightly different distortion but of similar isotropic chemical shift. This result is not consistent with the average Pbam structure deduced from Rietveld X-ray analysis, in which the two possible Na sites are symmetrically equivalent. These measurements illustrate the ability of NMR techniques to distinguish small variations in atomic
environment which are not resolved in the averaged X-ray structure.
11 citations
TL;DR: Wet oxidation of α-Si 3 N 4 and β-sialon (Si 6 z Al z O z N 8z, z = 1, 2, and 3) powders was carried out at 1000-1300°C in atmospheres containing 0-20 kPa water vapor as discussed by the authors.
Abstract: Wet oxidation of α-Si 3 N 4 and β-sialon (Si 6 z Al z O z N 8z , z = 1, 2, and 3) powders was carried out at 1000-1300°C in atmospheres containing 0-20 kPa water vapor. The oxidation was monitored by XRD, 29 Si and 17 O magic-angle spinning-nuclear magnetic resonance (MAS-NMR), transmission electron microscopy, and thermogravimetric analysis. The kinetic results were compared with those of our previous dry oxidation studies in Ar/O 2 (80/20 kPa). The oxide product phase was composed of very fine acicular mullite grains and amorphous SiO 2 . The early-stage oxidation is described by a two-stage linear kinetic law, the first stage operating at 0-5% reaction, the second at 5-20% reaction. From 20-80% reaction, the kinetics follow a parabolic law represented by the Ginstling-Brounshtein equation. The diffusion rates for wet oxidation were 2-11 times greater than for dry oxidation. The dependence of the diffusion rate on water vapor pressure over the range 0-20 kPa was approximately parabolic, indicating the diffusion of OH . , When the z = 3 sialon was oxidized in either Ar/H 2 17 O or Ar/O 2 /H 2 17 O atmospheres, 17 O was detected by MAS-NMR in the SiO 2 and mullite oxidation products, indicating that the oxygen atoms from the water vapor contribute in nearly equal amounts to the formation of mullite and SiO 2 .
8 citations
TL;DR: In this article, an attempt was made to prepare various F-doped β-, O-, X-, and α-SiAlONs from a mixture of Si 3 N 4, SiO 2, Al 2 O 3, AlN, or Y 2 O3 using AlF 3 or topaz as the fluorine source.
Abstract: An attempt was made to prepare various F-doped β-, O-, X-, and α-SiAlONs from a mixture of Si 3 N 4 , SiO 2 , Al 2 O 3 , AlN, or Y 2 O 3 using AlF 3 or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z , x , and m/n values determined for β-, O-, and α-SiAlONs by X-ray diffraction. When AlF 3 was used, a single phase ceramic (O-SiAlON) was produced from a mixture of α-Si 3 N 4 and SiO 2 at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of α-Si 3 N 4 , AlN, and Y 2 O 3 with AlF 3 produced β-/Y-α-SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of α-Si 3 N 4 and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from α-Si 3 N 4 and SiO 2 at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y-α-SiAlON mixture showed the growth of O- and β-SiAlON and Y-α-SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.
7 citations
3 citations