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1.
《Journal of Non》2005,351(6-7):499-507
For melts within the compatibility triangle anorthite–wollastonite–gehlenite (An–Wo–Geh) (CaAl2Si2O8–CaSiO3–Ca2Al2SiO7), the relationships between the chemical composition, the melt structure, and the rheological and thermodynamic properties are studied for each of the boundary lines, i.e. the An–Wo, Wo–Geh and Geh–An join. The influences of the degree of polymerization of the structural network and the amount of Al–O–Al linkages on the fragility and the configurational entropy of the melts are studied along the boundary lines. For the An–Wo join, the degree of polymerization dominates the responses of both fragility and configurational entropy to compositional variations. For the Wo–Geh and Geh–An joins, the amount of the Al–O–Al linkages dominates those responses. It is found that the depolymerization of the structural network and the formation of Al–O–Al linkages increase the fragility of the melt and the degree of structural disordering. 相似文献
2.
For the investigation of the effect of fluorspar and alumina on the viscous flow and the crystallization behavior of the silicate melts, the viscosity of the CaO-SiO2-10 wt%MgO-CaF2 (or -Al2O3) system was measured. The addition of CaF2 decreased the crystallization temperature (TCR) of the CaO-SiO2-MgO-CaF2 system, while the TCR of the CaO-SiO2-MgO-Al2O3 (wt%CaO/wt%SiO2=1.0) system exhibited a minimum value at 10 wt% Al2O3. The activation energy was decreased in the CSMF system and vice versa in the CSMA system by addition of CaF2 or Al2O3 in each system. From the relationship between the activation energy and the composition of the glasses, the following equation could be obtained. , where the Btotal is the ratio of (wt%CaO + wt%CaF2 + wt%MgO)/(wt%SiO2 + wt%Al2O3). This means that CaO, CaF2, and MgO would behave as a network modifier, while SiO2 and Al2O3 behave as a network former in the glasses investigated. 相似文献
3.
Phase separation in sodium-aluminoborosilicate glasses was systematically studied as a function of Gd2O3 concentration with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) methods. Gadolinium-induced phase separation in the glasses can be consistently explained by proposing that Gd cations partition to the borate-rich environments and subsequent agglomeration of the Gd-borate moieties, or short-range ordered structural groups, in the glass. Agglomeration of the Gd-borate rich environments is further discussed within the context of excess metal oxides, [Na2O]ex or [Al2O3]ex=|Na2O-Al2O3|, and excess B2O3, [B2O3]ex, available for incorporating Gd cations. Results showed that agglomeration of the Gd-borate rich environments occurred at a much lower Gd2O3 concentration in the glass without [Na2O]ex or [Al2O3]ex and at a significantly higher Gd2O3 concentration in the glass with either [Na2O]ex or [Al2O3]ex. Assuming 1BO4:1Gd:2BO3 (based on literature-reported Gd-metaborate structure) as a local Gd-borate environment in glass, we introduced the saturation index of boron, SI[B]=Gd2O3/(1/3[B2O3]ex), to examine the glass susceptibility to Gd-induced phase separation for all three alkali-aluminoborosilicate systems. While our results have provided some insight to the glass structure, they also provide insight to the mechanism by which the metal oxide is dissolved into the melt. This appears to occur predominately through boron complexation of the metal oxide. 相似文献
4.
Polymeric aluminosilicate gels with Al2O3/SiO2 molar ratios of 1.3/2, 2.5/2, 3/2 and 4.7/2 were prepared by gelling a mixture of tetraethoxysilane and ethyl acetoacetate aluminium diisopropoxide. Mullite, without any other crystalline phase, directly crystallizes from the gel matrix at about 1000 °C for all investigated samples. The Al2O3 or SiO2 crystalline phase can only be detected at relatively high temperatures accompanying the modification of mullite in the lattice structure. 27Al and 29Si MAS-NMR studies indicate that segregation of Al and Si atoms occurs in all samples below 900 °C in the amorphous state, regardless of the gel composition. However, the species of segregated units are very different and strongly dependent on the composition of the starting gels. This segregation may not cause the exothermic effect in differential thermal analysis examination, but it appears to be responsible for the composition change behavior of crystalline mullite with different heat treatment. 相似文献
5.
Hexagonal mesoporous silica (MCM-41) membranes were prepared at air-water interface by means of an interfacial silica-surfactant self-assembly process. The free-standing and oriented mesoporous silica membranes with pore size ≈2.9-3.8 nm were synthesized at room temperature in acidic media and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) observations. Small-angle X-ray diffraction (SAXRD) patterns of membranes clearly indicated that as-synthesized membranes were typical of MCM-41 materials with a periodic hexagonal structure with the channels parallel to the surface. SEM images showed that the as-synthesized membrane was continuous and crack-free. In this paper, some novel findings are reported. 相似文献
6.
The thermal-mechanical properties of 60Si20Mg20RE (RE: rare earth) oxynitride glasses can be tailored by the substitution of rare earths of decreasing ionic size. The Young’s modulus, hardness, glass transition temperature and viscosity all increase with the substitution La by Gd and Gd by Lu while the thermal expansion coefficients decrease. Compared to the 55Si25Al20RE oxynitride glasses, replacement of Al by Mg lowers the glass transition temperatures and viscosities and raises the thermal expansion coefficients substantially. On the other hand, the Young’s moduli are higher in the Mg-bearing glasses. These behaviors are seen to be a result in changes in the nature of the bonding in the glass structure. 相似文献
7.
Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by 29Si, 27Al and 19F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO2 · Al2O3 · 2CaO (ART1) and the mineral anorthite 2SiO2 · Al2O3 · CaO (ART2). The observed chemical shifts for 29Si and 27Al agreed with previous studies. The fluorine containing glasses were based on 2SiO2 · Al2O3 · (2−X)CaO · XCaF2. The 29Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The 27Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0-0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The 19F spectra indicated the presence of F-Ca(n) in low fluorine content glasses and both F-Ca(n) and Al-F-Ca(n) in high fluorine content glasses. We speculate here that the Al-F-Ca(n) species are oxyfluorides [AlOxFy]n−, where x=1-6, y=1-6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca2+ to F-Ca(n). 相似文献
8.
Several glasses were synthesized to explore cation ordering at fluorine sites in silicate and aluminosilicate glasses. Utilizing 19F NMR, we found a significant (at least ∼30% of the total intensity) amount of the fluorine signal to be due to F-Mg(n) (fluorine with an unknown number of exclusively Mg2+ nearest neighbors) bonding in the Mg-aluminosilicates. By combining this with previous data on Ca- and Ba-aluminosilicate glasses, we demonstrated a clear trend of an increasing amount of F-M(n) type bonding with increasing field strength of the network modifying cation. This indicates that the higher field strength cations can more effectively compete with Al3+ for fluorine bonds. The mixed-modifier (Na,La)-silicate glasses have primarily F-M(n) type bonding, with a pronounced preference for bonding to the higher field strength La3+ over Na+. In addition, Si-F bonding was found in the (Na,La)-silicate glasses at a level (∼2% of the total intensity) comparable to that found in other silicate glasses, suggesting that Si-F bonding is consistently present in silicate glasses. The (Na,La)-silicate glasses also had unusually short spin-spin relaxation times, suggesting short (similar to crystalline fluorides) fluorine-fluorine distances in environments associated with La3+. 相似文献
9.
M KarabulutG.K Marasinghe C.S RayD.E Day G.D WaddillC.H Booth P.G AllenJ.J Bucher D.L CaulderD.K Shuh 《Journal of Non》2002,306(2):182-192
The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe2O3-60P2O5 (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mössbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mössbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron-oxygen coordination is in the 4-5 range indicating that iron ions have mixed tetrahedral-octahedral coordination. 相似文献
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11.
A review of immiscibility data in binary borate and germanate systems was performed in order to compare miscibility gap consolute temperatures with ionic potentials and radii of their associated cations. The trends obtained demonstrate that a selective solution mechanism similar to the one identified for the binary silicate systems is present in the borate and germanate binaries. More importantly, the borate and germanate immiscibility data permitted the identification of a new group of cations depicting an immiscibility behaviour different from the ones identified in binary silicate systems. The new group involves highly polarizable cations possessing a lone pair of electrons. This lone pair of electrons together with oxygen bonded by strong covalent bonds to modifier cations provides efficient shielding to the cations' nuclei which considerably reduces the coulombic repulsions and produces miscibility gaps with very low consolute temperatures. A new group of cations having an homogenizing effect on melts (i.e. a capacity to make immiscible melts single phase) is thus reported. Experimental and spectroscopic data suggest that miscibility gaps associated with cations having a lone pair of electrons exist in binary silicate systems such as TlO1/2-SiO2, PbO-SiO2, SnO-SiO2 and Bi2O3-SiO2. The consolute temperature of their miscibility gaps is expected to be relatively low and metastable. 相似文献
12.
An exhaustive review of compositional and thermal extents of miscibility gaps in 41 binary silicate systems permits identification of three groups of cations exhibiting different immiscibility behaviours. The first group comprises network-modifier cations with an ionic radius larger than about 87.2 pm. They have coordination numbers equal to, or higher than, 5 and their miscibility gap size increases linearly with increasing ionic potential. The second group involves cations with an ionic radius larger than 26 pm and smaller than about 87.2 pm (in octahedral coordination). They have at least two coordination numbers: the first one is always 4 and the other 5 (or more). For this reason they are called amphoteric. Their miscibility gap sizes do not increase linearly with an increase of the ionic potential, but follow curves. The third group includes cations with variable crystal field stabilization energies. They are characterized by larger miscibility gap sizes than expected when they are compared with cations with similar ionic radii despite the fact that some of them (e.g. Cr3+) may behave as an amphoteric element because their ionic radii in octahedral coordination are smaller than about 87.2 pm. The origin of phase separation in binary silicate systems is due to coulombic repulsions between poorly screened cations bounded by bridging oxygen strongly polarized towards the silicon, and by non-bridging oxygen. 相似文献
13.
Transparent and translucent SnO2 aerogels with high specific surface area (>300 m2/g) have been prepared by sol-gel process using tetra(n-butoxy)tin(IV) as a starting compound, and supercritical drying technique for solvent extraction. Light scattering measurements reveal that the polymeric cluster size distribution in sol system is gradually broadened during sol-gel transition. SEM images show that the aerogels are made up of the cottonlike oxide agglomerates with a large number of pores. TEM images show that these aerogels seem to be self-similar at different magnifications. Their pore size distribution is pretty wide ranging from mesopore to macropore especially for that of translucent aerogel. 相似文献
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15.
The density, surface tension and viscosity of the 50RO-50P2O5 (R: Mg, Ca, Sr, Ba and Zn) glass melts have been measured over the range, 1073-1623 K. The effects of R cations on these properties have been investigated. The density of the melt was found to increase in the order, R: Mg<Ca<Zn<Sr<Ba, with increasing molar weight of cation. The surface tension in the temperature range of 1373-1473 K increased approximately with cation in the order, R: Zn<Ba<Mg<Ca<Sr. The viscosity and the negative, temperature coefficient of surface tension increased in the order, R: Ba<Sr<Ca<Zn<Mg. All melts exhibited negative temperature coefficients of surface tension. The effect of Mg and Zn cations on the properties were different to that for Ba, Sr, and Ca cations and this is discussed using bulk glass data published in a previous report. The features of Mg and Zn metaphosphate glass melts, that is high values of viscosity, and temperature coefficient of surface tension, are related to the small Oxygen Coordination Number of the cations (=4) when compared with those of Ca, Sr, Ba metaphosphate glass melts. 相似文献
16.
Carbon aerogels with high BET surface area were developed by sol-gel polycondensation of resorcinol and furfural in isopropanol using hexamethylenetetramine (HMTA) as a catalyst, and then directly drying the organic gels under isopropanol supercritical conditions, followed by carbonization under a nitrogen atmosphere. The preparation conditions of carbon aerogels were explored by changing the mole ratio of resorcinol to basic catalyst HMTA (R/C), the ratio of resorcinol to isopropanol (R/I), and the mole ratio of resorcinol to furfural (R/F). The effect of these preparation conditions on the porous structure of the carbon aerogels obtained was studied by nitrogen adsorption isotherms. According to the characterizations of TEM, SEM and nitrogen adsorption, the carbon aerogels obtained have a three-dimensional network that consists of carbon nano-particles with size from 20 to 30 nm, which define numerous micropores, mesopores and macropores. HMTA reacts not only as a catalyst but also as a reagent in the gelation polymerization. XRD characterization indicates that carbon aerogels have disordered nanocrystalline structures similar to activated carbon. 相似文献
17.
Nickel-alumina composite aerogels with large nickel loading were synthesized using a novel complexing agent-assisted sol-gel procedure to obtain materials displaying very large and open internal surface area, easily accessible for large molecules and demonstrating excellent catalytic properties. They were tested in the following reactions: (i) hydrogenation of propene and 1-butene, (ii) methanization of carbon dioxide, (iii) isometization of 1-butene, (iv) dehydrogenation of cyclohexane, (v) conversion of equimolar mixture of CO and NO to CO2 and N2. Full conversions and 100% selectivities were obtained in reactions (i), (ii), (iv) and (v) and an equilibrium composition in reaction (iii). These catalysts showed no pyrophoric activity on exposing to air even at Ni content as large as 50 wt%. 相似文献
18.
Glasses with the basic compositions 10Na2O · 10CaO · xAl2O3 · (80 − x)SiO2 (x=0, 5, 15, 25) and 16Na2O · 10CaO · xAl2O3 · (74 − x)SiO2 (x=0, 5, 10, 15, 20) doped with 0.25-0.5 mol% SnO2 were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na2O concentration the peak potentials get more negative. Mössbauer spectra showed two signals attributed to Sn2+ and Sn4+. Increasing alumina concentrations did not affect the isomer shift of Sn2+; however, they led to increasing quadrupole splitting, while in the case of Sn4+ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mössbauer parameters. 相似文献
19.
Owing to the lack of available thermodynamic data based on experimental measurements of heat capacity, decomposition reaction or vapour pressure measurements, the problem of BAs stability is considered. We propose a new set of thermodynamic data for enthalpy of formation, entropy and Gibbs free energy of Bas compound. By using thermodynamic database, our approach is based on the semi-empirical trends and analogy in the variation of those quantities for several binary series in different III–V systems like arsenides, nitrides and phosphides. Thus, the values for BAs were derived by extrapolation from Al to boron-based compounds (BAs, BP and, BN). For pure BAs(s), we predict a low enthalpy of formation in the standard state of at 300 K and a Gibbs free energy of indicating a lower stability of this compound than GaAs. Those values are contradictory discussed with trends in the cohesive energy of several III–V systems. A cohesive energy of 900 kJ/mol (9.4 eV) is proposed in agreement with Philips's rule. 相似文献
20.
This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels. 相似文献