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J. F. Duncan

Bio: J. F. Duncan is an academic researcher from Victoria University of Wellington. The author has contributed to research in topics: Kaolinite & Mullite. The author has an hindex of 3, co-authored 5 publications receiving 177 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, the phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry, with particular attention paid to the controversial intermediate Al-containing phases.
Abstract: The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2O3 and SiO2, before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline “primary” mullite, is γ-Al203 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2O3, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2O3/amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite.

144 citations

Journal ArticleDOI
TL;DR: In this paper, the kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X-ray diffraction analysis, and the evidence in both cases was consistent with a nucleation-controlled model.
Abstract: The kinetics of mullite growth from kaolinite and halloysite at high temperatures were studied by quantitative X-ray diffraction analysis. The evidence in both cases is consistent with a nucleation-controlled model.

27 citations

Journal ArticleDOI
TL;DR: In this paper, the iron-nucleated devitrification of a lead silicate glass was studied; it produces a glass-ceramic consisting of sodium metasilicate crystals in a lead-silicate glass matrix.
Abstract: The iron-nucleated devitrification of a sodium lead silicate glass was studied; it produces a glass-ceramic consisting of sodium metasilicate crystals in a lead silicate glass matrix. The nucleating agent is not incorporated in the crystal phase, Anisotropic devitrification of the glass in a thermal gradient produces an aligned microstructure with needles of sodium metasilicate oriented with the c axis parallel to the direction of crystallization.

13 citations

Journal ArticleDOI
TL;DR: In this paper, the iron-nucleated devitrification of a lead silicate glass was studied; it produces a glass-ceramic consisting of sodium metasilicate crystals in a lead-silicate glass matrix.
Abstract: The iron-nucleated devitrification of a sodium lead silicate glass was studied; it produces a glass-ceramic consisting of sodium metasilicate crystals in a lead silicate glass matrix. The nucleating agent is not incorporated in the crystal phase, Anisotropic devitrification of the glass in a thermal gradient produces an aligned microstructure with needles of sodium metasilicate oriented with the c axis parallel to the direction of crystallization.
Journal ArticleDOI
TL;DR: In this article, the phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry, with particular attention paid to the controversial intermediate Al-containing phases.
Abstract: The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al2O3 and SiO2, before the “950°C” exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO2 and some poorly crystalline “primary” mullite, is γ-Al203 (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al2O3, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al2O3/amorphous SiO2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO2 eventually crystallizes as cristobalite.

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Journal ArticleDOI
TL;DR: Two kinds of xerogels were prepared by slow and rapid hydrolyses of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol as discussed by the authors.
Abstract: Two kinds of xerogels were prepared by the slow and rapid hydrolyses of tetraethoxysilane and aluminum nitrate nonahydrate dissolved in ethanol. Xerogels prepared by slow hydrolysis crystallized mullite directly from the amorphous state on firing whereas those formed by rapid hydrolysis crystallized a spinel phase before mullite formation. The composition of the spinel phase was characterized by various methods to be near SiO2·6Al2O3. The process of mullite formation is discussed in relation to the structures of the starting materials.

282 citations

Journal ArticleDOI
TL;DR: In this article, the authors used micromorphology, Fourier transform infrared spectrometry (FTIR), X-ray powder diffractometry (XRD), and XRF spectrometers to characterize natural sediments sampled on and in the proximity of the tell and monitor their transformations due to exposure to high temperatures in an oven and in open fires, focusing on the transformations of the clay mineral components of mud-brick materials.

229 citations

Journal ArticleDOI
TL;DR: The 27Al and 29Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C as discussed by the authors.
Abstract: The 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS-NMR) study of the kaolinite-mullite transformation has shown the presence of Al in tetra- and pentacoordination in dehydroxylated kaolinite. The 29Si NMR signal analysis of samples heated above 400°C demonstrates that the tetrahedral sheet of kaolinite begins to break down near 600°C and continues to do so to 900°C. From the 27Al NMR signal evolution, it can be deduced that the exothermic peak at 980°C in DTA curves is associated with the modification of the coordination of Al, which changes from the tetra- or pentacoordination to the more stable octahedral coordination. Heating the sample at 880°C for 36 h produces the same transformation in the coordination of Al ions and the elimination of the exothermic peak at 980°C in the DTA diagram. After this transformation, all spectra show two tetrahedral lines characteristic of mullite, indicating that nuclei of mullite with low crystallinity are generated during the exothermic process which are not detected by XRD. At higher temperatures tetrahedral NMR peaks increase in intensity, yielding, at 1200°C, the 3:2 mullite NMR spectrum.

216 citations

Journal ArticleDOI
01 Mar 1990
TL;DR: The 27Al and 29Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; they discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH as discussed by the authors.
Abstract: Thermal transformations of kaolinite of different degree of crystallinity have been monitored by 27Al and 29Si high-resolution NMR with magic-angle spinning (MAS NMR), X-ray diffraction, Fourier transform infrared, atomic absorption spectrophotometry and thermogravimetric analysis. NMR shows differences in the dehydroxylation process of kaolinites with different degree of crystallinity and reveals the presence of short-range order in metakaolinite. 29Si NMR spectra acquired with a 30 s recycle delay of poorly and highly crystalline samples heated at 480 and 500° C, respectively, contain three distinct signals; we discuss their assignment in the light of experiments involving leaching of the samples with aqueous KOH. Ca. 40% of Si sites retain their original Q 3 symmetry just above the onset of dehydroxylation and the Q 4 environment is present showing that a small amount of amorphous silica has already segregated. The spectrum of samples treated at 1000° C contains a signal at -110ppm (from Q 4 silicons) and a faint resonance, from mullite, at ca. -87 ppm. 29Si NMR also shows that cristobalite germs are already present at 950–1000° C. The 27Al MAS NMR spectra of metakaolinite reveal the presence of 4-, 5-and 6-coordinated Al. Changes in the three Al populations as a function of temperature have been monitored quantitatively. Below 800° C, 4-and 5-coordinated Al appears at the expense of 6-coordinated Al, but above 800° C the amount of 6-coordinated Al increases again. We suggest a dehydroxylation scheme which accounts for the presence of 4-and 5 coordinated Al. Above 900–950° C the latter signal is no longer present in the 27Al NMR spectra and new 4-and 6-coordinated Al species (mullite and γ-alumina) appear. We propose new ideas for the structure of metakaolinite.

200 citations

Journal ArticleDOI
TL;DR: In this article, it was shown that γ-Al2O3 type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series.
Abstract: With the use of differential thermal analysis, X-ray diffraction, and transmission electron microscopic techniques, we showed that γ-Al2O3 type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series. Transmission electron microscopic characterization indicated that the spinel formation is preceded by a phase separation in the amorphous dehydroxylated kaolinite matrix. Chemical analysis of the spinel phase by energy dispersive X-ray spectroscopy revealed a nearly pure Al3O2 composition.

174 citations