Phase segregation of non-stoichiometric aluminosilicate gels characterized by Al and Si MAS-NMR

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.     

F-19 NMR study of the ordering of high field strength cations at fluoride sites in silicate and aluminosilicate glasses

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⁺.     

New evidence for tetrahedral triclusters in aluminosilicate glasses

The formation of thermodynamically stable 3/2-mullite (3 Al₂OAl₃·2 SiO₂) was investigated by scanning electron microscopy using reaction couples consisting of 2/1-mullite (2 Al₂O₃·1 SiO₂) plus SiO₂ glass, or Na₂O-SiO₂ glass, respectively. The mullite substrates were partially dissolved, thus leading to Al incorporation in the siliceous phases. In both reaction couples thin layers of stoichiometric 3/2-mullite form on the 2/1-mullite substrates. However, the major mullitization steps are different: The 2/1-mullite/SiO₂ reaction couple gives rise to 3/2-mullite crystallization within the bulk of the glass, whereas epitactic growth of c-axis orientated 3/2-mullite needles on the 2/1-mullite substrate was observed in the presence of Na₂O-SiO₂ glass. The differences in mullite nucleation were attributed to the existence or non-existence of tetrahedral triclusters in the as-reacted non-crystalline Al₂O₃-SiO₂ and Na₂O-Al₂O₃-SiO₂ phases, respectively. Triclusters of (Si,Al)O₄-tetrahedra in the Al₂O₃-SiO₂ glass may act as nuclei for 3/2-mullite crystallization in the bulk of the glass since these structural units also occur in mullite. In Na₂O-Al₂O₃-SiO₂ glasses triclusters are absent, and epitactical 3/2-mullite formation on the mullite substrate becomes more favorable energetically.     

Structural analysis of bioactive glasses

In this study four series of single and mixed alkali glass systems were made and investigated using MAS NMR. Additionally the densities of the glasses were measured experimentally, as well as calculated theoretically using Doweidar’s model. MAS NMR was used to obtain a quantitative structural understanding of glasses by calculating the concentrations of bridging and non-bridging oxygens per silicon oxygen tetrahedron as a function of the alkali oxide concentration expressed as Qⁿ. ²⁹Si MAS NMR spectra exhibited a single resonance corresponding closely with Si in a Q² state. The chemical shift of the ³¹P MAS NMR peak was attributed to phosphate in an orthophosphate environment. The ²⁹Si NMR spectra are in agreement with the density data. Using Doweidar’s model the proportions of Q² and Q³ were calculated, showing that all glasses studied are predominantly Q² in structure, i.e. silica chains which readily dissolve. The changes in the chemical shifts of the Q² and Q³ species with composition have been interpreted as resulting from the preferential association of Na⁺ with Q³ and Ca²⁺ with Q².     

Study of the dynamic structure factor of strong glasses

The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm⁻¹. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q² dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

Preparation and characterization of glasses in the Ag2S + B2S3 + GeS2 system

The glass forming range of the Ag₂S + B₂S₃ + GeS₂ ternary system was investigated for the first time and a wide range of ternary glasses were obtained. The Archimedes’ method was used to determine the densities of the Ag-B-Ge glasses. The thermal properties of these thioborogermanate glasses were studied by DSC and TMA. The Raman, IR and NMR spectroscopy were used to explore the short-range order structure of the binary (Ag-B) and (Ag-Ge) and ternary (Ag-B-Ge) glasses. The results show the presence of bridging sulfur tetrahedral units, GeS_4/2 and AgBS_4/2, and trigonal units, BS_3/2, in the ternary glasses. Non-bridging sulfur units, AgSGeS_3/2 and Ag₃B₃S₃S_3/2 six membered rings, are also observed in these glasses at higher Ag₂S modification levels because the further addition of Ag₂S results in the degradation of the bridging structures to form non-bridging structures. The NMR studies show that Ag₂S goes into the GeS₂ subnetwork to form Ag₃S₃GeS_1/2 groups before going to the B₂S₃ subnetwork. In doing so, it is suggested that B₁₀S₂₀ supertetrahedra exist in Ag₂S + B₂S₃ and Ag₂S + B₂S₃ + GeS₂ glasses. Significantly B-S-Ge bonds form in the B₂S₃ + GeS₂ glasses, whereas they appear to be absent in the ternary glasses. From these observations, a structural model for these glasses has been developed and proposed.     

Structure and properties of Ge2.5PSx glasses

Ge_2.5PS_x glasses were studied with a combination of Raman spectroscopy, nuclear magnetic resonance, and neutron diffraction. From these experiments the distribution of bonding configurations was determined, and used to explain the compositional dependence of the index of refraction and the glass transition temperature. On reducing the sulfur content of these glasses below the stoichiometric amount, the sulfur deficit is accommodated by the progressive loss of the non-bridging sulfur of SPS_3/2 groups, followed by the conversion of the resultant PS_3/2 groups into species such as P₄S₃ characterized by P-P bonding. The presence of metal-metal bonds involving germanium, found in samples with the lowest sulfur content, was found to be the most important structural feature in determining the optical response.     

O nuclear magnetic resonance study of Na2O-P2O5 glasses

The preparation and structural investigation of ¹⁷O-enriched xNa₂O-(100−x)P₂O₅ glasses (46.5?x?62.8) by nuclear magnetic resonance (NMR) is described. Enriched phosphoric acid was prepared by hydrolysis of PCl₅ with ¹⁷O-enriched water and neutralized with sodium carbonate. The sodium metaphosphate was then melted at 800 °C for 15 h and quenched. Polyphosphate and ultraphosphate glass compositions were prepared by remelting the metaphosphate with sodium carbonate and phosphorus pentoxide, respectively. ³¹P magic angle sample spinning (MAS) NMR was used to determine the Na₂O/P₂O₅ content in the glasses. ¹⁷O NMR spectra (quadrupole echo for non-rotating samples and multiple-quantum excitation for rotating samples (MQMAS)) show two oxygen sites in the samples with large quadrupolar coupling constants (4.7 and 7.7 MHz), in accordance with the high phosphorus electronegativity. According to the correlation of ¹⁷O quadrupolar constants with bond ionicity, these two components are attributed to bridging P-O-P and non-bridging P-O?Na oxygens. The average P-O-P bond angle is estimated with the quadrupolar asymmetry derived from the fit of the static echo spectra. The MQMAS spectrum shows a distribution of non-bridging oxygen chemical shifts, attributed to a variation of bond length and angle.     

Characterization of a mould flux glass

A simplified mould flux glass composition used for the continuous casting of steel was synthesized and then characterized using X-ray powder diffraction (XRD) differential thermal analysis (DTA) and ¹⁹F and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). DTA showed the glass to have a low glass transition temperature and to crystallize readily at 600 °C. XRD of the heat-treated glass showed it to crystallize to cuspidine. ¹⁹F MAS-NMR showed the principal fluorine species to be F-Ca(n) with no evidence of Si-F or Al-F species. Fluoride ions therefore, complex calcium in this glass, rather than forming non-bridging fluorines. The network connectivity of the glass was calculated on this basis and found to be 2.07 this would be expected to correspond to a Q² Si species which was supported by the ²⁹Si data that gave a chemical shift of −78 ppm corresponding to Q² Si.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

On the structure of phosphosilicate glasses

Vibrational spectra of phosphosilicate glasses with P₂O₅ concentrations up to 15 mol% are investigated by the methods of Raman spectroscopy and quantum-chemical modeling. We have found that the Raman band at 1320 cm⁻¹ characteristic for such glasses is not simple and may be decomposed into two components with frequencies at ≈1317 and ≈1330 cm⁻¹ caused in our opinion by single phosphorus centers (OPO₃ tetrahedra surrounded by SiO₄ ones) and by double phosphorus centers (pairs of OPO₃ tetrahedra bonded by a common oxygen atom). In the investigated phosphosilicate glasses manufactured by MCVD and SPCVD methods the ratio of concentrations of single and double centers varies from 1:5 to 1:2. A novel interpretation of the Raman bands distinct from the traditional one is suggested. The approach to the Raman spectra analysis developed in this article can be applied for control and optimization of manufacturing process of phosphosilicate and similar glasses as well as optical fibers.     

Preparation and structure of organic-inorganic hybrid precursors for new type low-melting glasses

Organic-inorganic hybrid precursors for new type low-melting glasses without pollution elements such as Pb and F have been synthesized through a non-aqueous acid-base reaction process. The hybrid compounds consist of -Si-O-P- framework, in which some of the bridging oxygens of a Si tetrahedron are substituted by organic functional groups. Terminating the oxide framework by the organic functional groups lowers the network dimension. Orthophosphoric acid (H₃PO₄) and dialkyldichlorosilane (Me₂SiCl₂ and Et₂SiCl₂) were employed as starting materials. The formation of P-O-Si linkage was confirmed by IR spectra. The larger viscosity increase at higher Si concentrations is correlated to the formation of the linkage. It is proposed that an acid-base polycondensation reaction of P-OH+Si-Cl→P-O-Si+HCl↑ dominates the network formation. When dimethylsilane SiMe₂ is substituted by divalent Sn, an organic-inorganic hybrid low-melting glass in the system of SnO-Me₂SiO-P₂O₅ can be derived, the glass transition temperature of which is about 29 °C. So the present acid-base reactions provide a new process by which precursors for new type low-melting glass are synthesized.     

X-ray absorption studies of the local strontium environments in borosilicate waste glasses

X-ray absorption spectra were collected and analyzed to characterize the strontium environments in borosilicate glass formulations developed for the immobilization of nuclear wastes. Sr can become a major constituent in some radioactive wastes because of the use of Sr-compounds in waste pretreatment processes. Data are presented for 13 borosilicate glasses that are used to simulate the anticipated waste glass compositions, where SrO concentrations range from approximately 2 to 30 wt%. The XANES data are statistically identical for all glasses investigated, and indicate divalent strontium. The EXAFS data for both crystalline standards and glasses appear to provide information only about the inner four to six coordinating oxygens that surround Sr. The data and analyses for all glasses show that the inner coordination environments around Sr have average Sr-O distances near 2.53 Å, which are statistically invariant with respect to glass composition and to various synthesis conditions. Sr-O distances determined for the glasses are within the shorter end of the range of individual Sr-O distances in the SrCO₃ and haradaite (SrVSi₂O₇) crystalline standards.     

Properties and structure of cesium phosphate glasses

A series of Cs-phosphate glasses, xCs₂O(1−x)P₂O₅, where 0?x?0.60, were prepared. The glass transition temperature (T_g) decreases with the initial addition of Cs₂O to P₂O₅, from 637 K at x=0 to 472 K at x=0.16. There is little change in T_g with further additions of Cs₂O up to x=0.60. The ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) spectra show that Cs₂O additions systematically convert the cross-linked ultraphosphate network of ν-P₂O₅ to a chain-like metaphosphate structure as x approaches 0.50. The ¹³³Cs MAS NMR spectra show a 90 ppm increase in isotropic chemical shift (δ_iso) with increasing Cs₂O content, which indicates a decrease in the average electron density around the Cs⁺ ions, more covalent Cs-O bonding, and a shorter average Cs-O bond length. The physical properties and spectroscopic results are interpreted using a structural model that considers the effects of composition on the average coordination environment of Cs⁺ ions.     

Effect of Co substitution for Ni on the microstructure and magnetic properties of (Fe, Ni)-based amorphous alloys produced by melt spinning

Transmission electron microscopy with selected area electron diffraction, and X-ray diffraction were applied to study the effect of Co substitution for Ni in (Fe, Ni)-based amorphous alloys. The investigation was performed to obtain information on correlations between microstructure and magnetic properties of the (Fe, Ni, Co)-based amorphous alloys. The examination of the microstructure reveals that there are small crystallized free surface regions because the actual quenching rate is distributed inhomogeneously over the cross-section of the ribbon. Since in the free surface region, the solidification rate is lower, a spontaneous annealing process occurs at the top surface of the ribbon. The crystallization degree of the free surface region is higher for alloy ribbons that contain Co up to a 15 at.% concentration. Magnetic domains pattern are sensitive to the surface crystallization and Co content of the (Fe, Ni)-based alloy ribbons. Fine-scaled stripe domains were evidenced on the free ribbon surface while on contact surface stress domain pattern appeared. With the increase of the Co content, the domain width became small and long stripes appeared. The striped domains are responsible for an increased coercivity of the ribbons. However, there is a critical Co content (x_Co = 10) for which spontaneous narrow stripe domains are no longer more energetically favourable for ribbons with specific magnetic applications.     

Structural implications of water and boron dissolution in albite glass

A combined nuclear magnetic resonance, infrared and Raman spectroscopic study on the effect of water dissolution on the structure of B-bearing aluminosilicate glasses is presented. The base composition was albite (NaAlSi₃O₈) to which different amounts of B₂O₃ (4.8, 9.1, 16.7 wt%) were added. Hydrous glasses containing 4.4 ± 0.1 wt% water were synthesized at pressures of 2000 bar. The results show that B dissolves in both dry and hydrous glasses by forming predominantly trigonal BO₃ groups although some tetrahedral BO₄ is also present. In anhydrous glasses prepared at high pressures (above 10 kbar) the fraction of BO₄ increased. The hydrous glasses contain more BO₄ groups compared to the dry counterparts, suggesting that this species is stabilized by water. The Raman and NMR (¹⁷O, ²⁷Al, ²⁹Si) spectra show that B interacts with the aluminosilicate network by formation of Si-O-B and probably Al-O-B units. In the hydrous glasses the water speciation changes significantly towards higher hydroxyl concentrations with increasing B-content. The NIR peaks, which are related to OH groups and molecular H₂O, develop additional shoulders, suggesting that possibly B-OH complexes are formed.     

Formation of complex structural units and structure of some chalco-halide glasses

Raman spectra, thermal expansion and glass transition temperature of glasses and evolution of Raman spectra of CsGaS_1.5Cl glasses with complete substitution of S by Se and partial substitution of Ga by Al (or In) and of Cl by Br (or I) were studied. The structure of these glasses was discussed considering the ability of A(III) metals to form complex tetrahedral structural units [A(III)B(VI or VII)₄]¹⁻, which can be connected by sharing either the common corner of tetrahedra (glasses of systems at x>0.1 and CsInS_1.5Cl glass) or the common edge of tetrahedra (CsGaS_1.5Cl, CsAlS_1.5Cl, CsGaSe_1.5Cl, CsGaS_1.5Br, CsGaS_1.5I glasses).