Thermoanalytic studies carried out on hydrate phases Na2[SiO2(OH)2]·nH2O (n = 4, 5, 7, 8) reveal two phase transitions at elevated temperatures under open-system and non-isothermal conditions Congruent melting at temperatures Tmp is followed by condensation at temperatures Tc > 380 K with subsequent formation of solid Na2[SiO3]. Foamingof the melts upon condensation is observed through heating stage microscopy. In situ 29Si NMR experiments proved oligomeric or endless chain-type anions, [Si1+nO2+3n(OH)2](2+2n)?, to be formed in the melts under open-system conditions. From DTA double peak configurations at temperatures Tc the thermal decomposition of the melts is shown to occur in a two-step reaction, uniformly with all four hydrate phases.Isothermal studies below melting temperatures show distinct decomposition of crystalline Na2[SiO2(OH)2]·8H2O to crystalline Na2[SiO2(OH)2]·4H2O whereas all the other crystallinephases within this hydrate series dehydrate to amorphous waterglass-type materials under identical experimental conditions in long-term annealing experiments. This exceptional phase relation between the octa- and the tetrahydrate has also been proved by X-ray powder diffraction heating experiments. 相似文献
ZrW2O7(OH,Cl)2·2H2O and its isomorphs were prepared by Acidic Steam Hydrothermal (ASH) method and the pure cubic ZrW2O8 was synthesized by dehydration of ZrW2O7(OH,Cl)2·2H2O. The control factors of ASH method, such as the type, concentration and volume of the acidic steam source, temperature and time of the reaction, were studied in detail. The results showed that HCl was a better volatile acidic steam source and the concentration of HCl determined the product. At lower concentration of HCl, the main product was Na2W2O7·H2O rather than ZrW2O7(OH,Cl)2·2H2O. When the concentration was higher than 6 mol·L-1, the products in turn became sodium tungstates with the structure types of pyrochlore, hexagonal tungsten bronze and finally were transformed to ZrW2O7(OH,Cl)2·2H2O precursors. 相似文献
Chirai sodium zincophosphate nanocrystalline has been prepared and characterized. The standard molar enthaipy of the following reaction 12Na3PO4·12H2O（s）＋ 12ZnSO4·7H2O（s）= Na12（Zn12P12O48）·12H2O（s）＋ 12Na2SO4（s）＋216H2O（1） was determined by solution reaction calorimetric at 298.15 K, and calculated to be 33.666±0.195 kl/mol. From the results and other auxiliary quantities, the standard enthalpy of formation for sodium zincophosphate nanocrystalline was derived to be △fHm^⊙ [Na12（Zn12P12O48）·12H2O（s）, 298.15 K] =- 24268.494 ± 0.815 kJ/mol. 相似文献
The steps associated with the thermal decomposition of Zn5(OH)8(NO3)2·2H2O and ZnOHNO3·H2O are re-examined. Previous reports have suggested that Zn5(OH)8(NO3)2·2H2O decomposes to ZnO via two intermediates, Zn5(OH)8(NO3)2 and Zn3(OH)4(NO3)2 whereas ZnOHNO3·H2O has been reported to decompose to ZnO via a Zn3(OH)4(NO3)2 intermediate. In this study, we demonstrate using TG, mass spectral analysis of evolved gases and in situ variable temperature powder X-ray diffraction analysis that, in fact, in the decomposition of Zn5(OH)8(NO3)2·2H2O an anhydrous zinc nitrate intermediate is also involved. We, additionally, show that the decomposition of ZnOHNO3·H2O to ZnO also involves the formation of an anhydrous zinc nitrate intermediate. The anhydrous zinc nitrate formed in both cases is poorly crystallised and this observation may explain why this phase could not be observed by PXRD analysis in the previous studies. 相似文献
