Papers by Keyword: MAS-NMR

Abstract: Mineral polymer is a new category of inorganic non-mental materials, which was manufactured from metakaolin in this paper. Changes of metakaolin in the polymerization could be described as: (1) the glassy parts of the metakaolin were dissolved in the high molar alkaline solution and some SiO, AlO bonds of them broke; (2) at the presence of OH-and Na+, these broken bonds formed aluminosilicate gel and then transformed into zeolitic precursors with changing compositions and concentrations of the solution; (3) by dehydration, these zeolitic precursors changed into the amorphous phase materials. NMR analysis indicated that in the geopolymeric products cured for 28 days, Q⁴(2Al) was the principal form of Si element and the content of four coordinate Al was increasing with the reaction. The experimental results provided a good basis for understanding the reactive mechanism of polymerization during processing of geopolymer products.

Abstract: Mineral polymer is a new category of inorganic non-mental materials, which was manufactured from fly ash in this paper. Changes of fly ash in the polymerization can be described as: (1) Q4(0Al) is the main form of Si element in fly ash, with 67.5%content.During the solidification of mineral polymer, the content of Q4(0Al) decrease from 38.7%(3d) to 28.2%(7d) and 26%(14d), then increased to 37.8% in 28 d product; (2) the intensity of Q2(0Al) and Q3(1Al) decrease at first stage, then increase finally during the solidification of mineral polymer; (3) content of Q4(2Al)increase from 14.6%(3d), 39.2%(7d), 40.6% (14d) and decreased to 23.1% in product curing for 28 d product;(4)octahedral aluminum were the main form of Al element in fly ash, which decreased during the polymerization, simultaneously, tetrahedral aluminum increased. The experimental results provided a good basis for understanding the reactive mechanism of polymerization.

Abstract: Viscosities of molten RE-Mg-Si-O-N (RE=Y, Gd, Nd and La) glasses have been measured using rotating bob viscometer with a gas tight furnace at elevated temperature (~1873 K). Moreover, structural characterizations of these quenched vitreous samples have been investigated using solid state 29Si MAS-NMR, which would resolve the relationship between the viscosity of high temperature melts and network structure of RE-Mg-Si-O-N systems. The viscosities of molten RE-Mg-Si-O-N glasses exponentially increased with nitrogen content. 29Si MAS-NMR spectra of RE-Mg-Si-O-N (RE=Y and La) glasses revealed that content of silicon-oxynitride species, like SiO3N, increased with nitrogen content, which indicates that nitrogen clearly modifies the glass network structure. Depending on cationic radius of rare-earth elements, Y was found to be more effective in silicon-oxynitride species formation than La, which are consistent with the results of viscosity measurement of molten RE-Mg-Si-O-N glasses at elevated temperature (~1873 K).

Abstract: Fluorine containing calcium aluminosilicate glasses are widely used for a number of technological applications including dental cements, mould fluxes in steel making and in a variety of glass-ceramic systems. Despite of their importance these systems remain quite poorly understood with respect to their composition. To address this question a glass composition corresponding to the equimolar binary system anorthite−fluorite (Ca2Al2Si2O8−CaF2) was chosen as a base point for two series of compositions. One of the series is designed on the anorthite stoichiometry and considered as classically charge balanced. Another series starts from the fluorine free composition of the anorthite−lime (Ca2Al2Si2O8−CaO) stoichiometry and, therefore, is characterized by a disrupted network with at least one non-bridging oxygen (NBO) attached to silicon. A multinuclear 19F, 27Al, 29Si solid state NMR study of the glasses was undertaken. It is shown that in both series fluorine is predominantly coordinated by calcium, F−Ca(n), and in addition interacts with aluminium forming Al−F−Ca(n) complexes, where n denotes the number of first neighbouring calcium cations. Small amounts of high coordinated aluminium grows with increasing fluoride content in both glass series. However, the high coordinated aluminium may not be solely due to the formation of the Al−F−Ca(n) complexes. Glasses of the first series displayed systematic upfield shift of 29Si NMR resonance while substituting fluoride for oxide, starting from the fluorine free composition. This upfield shift is interpreted as the lack of cations in the network, due to formation of the F−Ca(n), which drives silicon network to polymerize toward a higher Qn structure. Contrary to the first series, the 29Si NMR resonance remains constant for fluorine containing compositions of the second series but differs downfield from the initial anorthite glass. The latter is explained by the excess of cations in the network due to addition of the fluorite resulting in formation of NBO on the silicon. Binding of fluorine with silicon is considered negligible in these systems. Thus, fluorine and calcium both define the degree of network polymerization and are considered as a cause for the changes in silicon and aluminium networks.

Abstract: The glass based on a 1.5SiO2-Al2O3-0.5P2O5-CaO-0.67CaF2 composition was produced and substituted gradually by barium. The structure of the glasses was studied by multinuclear Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) and Fourier Transform Infrared Spectroscopy (FTIR). It was indicated by 29Si and 31P MAS-NMR spectra that silicon was present as Q4 (4Al) and Q3 (3Al) species and phosphorus was in a Q1 pyrophosphate environment. 29Al MAS-NMR spectra showed that four fold coordinated aluminum Al (IV) was the dominant species with a second peak assigned to octahedral aluminum Al (VI). The 19F spectra suggested that the barium addition caused the formation of Al-F-Ba(n) and F-Ba(n) species. Furthermore, a distribution of silicate network including Si-O-Si stretching (Q4 and Q3) and Si-O-[NBO] (Q3) per SiO4 was reflected by the FTIR study.

Abstract: Strontium is one of the most common substituents in apatite crystals. The presence and behavior of Sr in apatite-group phases are of considerable significance in biology. The present paper investigates the substitution of strontium for calcium in a glass-ceramic of the following composition 4.5SiO23Al2O31.5P2O54CaO1CaF2. The glasses were characterized using Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), neutron diffraction (ND) and 19F Resonance Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR). The all calcium glass crystallized to calcium fluorapatite (Ca5(PO4)3F). Substituting strontium partially for calcium resulted in the formation of a mixed strontium/calcium fluorapatites. In contrast complete substitution resulted in the formation of strontium fluorapatite. MAS-NMR showed the the F to be present as F-Ca(3) representing a fluoride ion surrounded by three Ca2+ ions in the all calcium glass and was present as F-Sr(3) in the all strontium glass. In the mixed glasses fluorine was present as FCa( 3), F-Ca(2)Sr, F-CaSr(2) and F-Sr(3). Ca had a higher tendency to occupy the F-M(3) sites than Sr which may reflect the higher charge to size ratio of Ca2+ relative to Sr2+ and its greater affinity for F- ions.