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451.
The effect of the variation in phosphate (P2O5) content on the properties of two series of bioactive glasses in the quaternary system SiO2-Na2O-CaO-P2O5 was studied. The first series (I) was a simple substitution of P2O5 for SiO2 keeping the Na2O:CaO ratio fixed (1:0.87). The second series (II) was designed to ensure charge neutrality in the orthophosphate (), therefore as P2O5 was added the Na2O and CaO content was varied to provide sufficient Na+ and Ca2+ cations to charge balance the orthophosphate present. Network connectivity’s of the glasses were calculated, and densities and thermal expansion coefficients predicted using the Appen and Doweidar models, respectively. Theoretical densities were measured using the Archimedes principle. Characteristic temperatures, namely the glass transition temperature, Tg, and crystallization temperatures, Tx, were obtained using differential analysis (DTA). Two crystallization exotherms were observed for both glass series (Txi and Txii). Both Tg and Tx decreased with P2O5 addition for both series. The working range of the glasses, Tx-Tg was shown to increase to a maximum at around 4 mol% P2O5 then decrease at higher P2O5 contents for both series. Thermal expansion coefficients were measured using dilatometry increasing with P2O5 addition and showed good agreement with the Appen values. Dilatometric softening points, Ts, were also measured, which increased with P2O5 addition. X-ray diffraction (XRD) was performed on the glasses to confirm their amorphous nature. The glass containing 9.25 mol% P2O5 from series I exhibited well-defined peaks on the XRD trace, indicating the presence of a crystalline phase. 相似文献
452.
The effect of the variation in phosphate (P2O5) content on the structure of two series of bioactive glasses in the quaternary system SiO2-Na2O-CaO-P2O5 was studied. The first series (I) was a simple substitution of P2O5 for SiO2 keeping the Na2O:CaO ratio fixed (1.00:0.87). The second series was designed to ensure charge neutrality in the orthophosphate , therefore as P2O5 was added the Na2O and CaO content was varied to provide sufficient Na+ and Ca2+ cations to charge balance the orthophosphate present. The glass network connectivity (NC) was calculated for each glass and a modification for the presence of a separate P2O5 phase was included (NC′). 31P and 29Si magic-angle-spinning nuclear magnetic resonance (MAS-NMR) spectroscopy was performed on glass series I and II to determine the structural units present and their relation to glass properties. 31P MAS-NMR spectra of series I resulted in a broad resonance around 9 ppm corresponding to orthophosphate in an amorphous environment. The 9.25 mol% P2O5 glass shown to be partially crystalline by X-ray diffraction was heat treated, and the 31P MAS-NMR spectrum showed a sharp peak around 3 ppm corresponding to calcium orthophosphate or sodium pyrophosphate and overlapping broader peaks at 8.5, 10.5 and 14 ppm possibly corresponding to two mixed calcium-sodium orthophosphate phases and amorphous sodium orthophosphate respectively. 31P MAS-NMR spectra of series II resulted in a broad resonance around 10.5 ppm corresponding to orthophosphate in an amorphous environment. 29Si MAS-NMR spectra of glasses from series I showed a shift in the resonance peak from around −78 to −86 ppm indicating an increase in Q3 species in the glass and a reduction in Q2 with phosphate addition confirming the presence of orthophosphate. The heat treated sample showed a sharp 29Si-NMR resonance at −88 ppm, indicating a crystalline Q2 six-membered combeite (Na2O · 2CaO · 3SiO2) silicate-type phase, which was confirmed by powder X-ray diffraction. 29Si MAS-NMR spectra of glasses from series II showed no shift in the resonance at around −78 ppm across the series, confirming an orthophosphate environment. 相似文献
453.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option. 相似文献
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G. Michael Blackburn Peter R. Ashton Mao-Jun Guo Michael Rogers Graham Taylor Andrzej Guranowski Donald Watts 《Heteroatom Chemistry》1991,2(1):163-170
The enzymatic cleavage of a scissile P O bond can be blocked by recourse to phosphonate analogues of biological phosphate esters. α-Fluorophosphonates have an enhanced electronegativity at the bridging carbon, which, in many cases, makes them superior to simple methylene phosphonates for the study of enzyme reactions. Thus, the β,γ-difluoro-methylene analogue of ATP is a good substrate for the interferon-induced (2→5)An synthetase, which converts it into a (2→5)A4 species having a 5′-β,γ-difluoromethylenetriphosphate. This binds strongly to RNase L but does not activate it. The unsymmetrical Ap4Aases from Artemia and Lupin are strongly inhibited by P1,P4-dithiophosphate analogues of diadenosyl-5′,5‴-P1,P4-tetraphosphate although nonregiospecific cleavage of certain P2,P3-methylene analogues can be observed. Some of these analogues are remarkably effective inhibitors of platelet aggregation and are effective inhibitors in vivo of arterial blood-clotting in rabbits. Separation of all diastereoisomers of P1,P4-dithiophosphate analogues of Ap4A is achieved cleanly using reverse-phase hplc chromatography and this provides direct access to β,γ-CHF-bridged analogues of ATP with resolved stereochemistry at the CHF center. Lastly, growing cells of Dictyostelium discoideum not only tolerate a range of substituted methylene bisphosphonates in their growth medium but actually incorporate them into nucleotide analogues of ATP and Ap4A. 相似文献