Search

The effect of MgO on the microstructures and macroscopic properties of SiO2‐poor/rich MgO‐CaO‐SiO2 melts was investigated by in situ high temperature time‐gated Raman spectroscopy coupled with aerodynamic levitation heating of molten samples. Results of density functional theory calculations of Mg2SiO4, γ‐Ca2SiO4, and MgCaSiO4 crystals and quantum chemistry ab initio simulations of Q0 species (Q0 refers to tetrahedral Si‐O units without bridging oxygen) of SiO2‐poor series MgO‐CaO‐SiO2 molten samples certified that with the substitution of CaO by MgO, the variation of Raman intensity at 900–1100 cm−1 of xMgO‐(2‐x)CaO‐SiO2 melts was caused by the antisymmetric stretching vibration modes of distorted silicon‐oxygen tetrahedra. The experimental result of SiO2‐rich series of MgO‐CaO‐SiO2 molten samples could identify various Qi species quantitatively and demonstrated that with the substitution of CaO by MgO, the abundance of Q3 species dramatically decreased, but Q4 species increased slightly, whereas Q1 and Q2 species maintain insignificant change. A positive correlation between viscosity and the abundance ratio of Q3 to Q4 was assumed and illustrated. [ABSTRACT FROM AUTHOR]

Comparing with the crystalline tungstate compounds, little work has been carried out and reported on the relation among the microstructure, W–O bond lengths, and characteristic Raman‐active vibration wavenumber of the molten tungstates, which allows us to diagnose and determine the structure of unknown clusters existing in and further predict the physicochemical properties of molten binary alkali tungstates. Raman spectra of an ensemble of eight model clusters were simulated in the present work by density functional theory (DFT) to establish the effect of the fine structures and W–Onb (non‐bridging oxygen) bond lengths of W–O complexes on the characteristic wavenumber of W–Onb Raman‐active vibration modes of the molten tungstates. Results show that the characteristic wavenumbers of the symmetric stretching vibration modes of W–Onb bonds increase almost linearly with the decreasing bond length. The characteristic wavenumbers of W–Onb symmetric stretching vibration modes generally follow [WO4]2− > [WO5]4− > [WO6]6− present in the melt simultaneously. The characteristic wavenumbers were also found to increase with the number of bridging oxygen for the same W–O complex. In situ Raman spectra of molten A2WnO3n + 1 (A = Li, Na, K; n = 1, 2, 3) were then measured in order to verify the correlation observed among the microstructure, W–O bond lengths, and characteristic Raman‐active vibration mode wavenumbers. The correlation was successfully applied to deconvolute the in situ Raman spectrum of the molten Na2W3O10. [ABSTRACT FROM AUTHOR]

In order to help predict the properties of molten salts electrolytes and aluminium brazing flux used in the Hall–Héroult process and in aluminium brazing, the microstructure of binary KF‐AlF3 system from 298 K to its molten states was explored by in situ high‐temperature Raman spectroscopy in argon atmosphere. Comparing with experimental Raman spectra in molten system, quantum chemistry ab initio simulations were used to calculate the symmetric stretching vibrational modes of probable clusters in the melts. The molten KF‐AlF3 system was found to comprise monomer [AlF6]3−, [AlF5]2−, [AlF4]−, and dimer [Al2F7]− clusters. With the increase of KF content in the melts, the major species changes from [AlF4]− tetrahedra to [AlF6]3− octahedra. From the deconvoluted measured Raman spectra, the mole fractions of varied clusters in the system were also calculated. As the KF:AlF3 ratio was increased from 1 to 3, the content of [AlF6]3− in the melt increased from 22.30 to 98.83 mol%, whereas the [AlF4]− content decreased from 68.92 to 0.33 mol%. [AlF5]2− and [Al2F7]− clusters were also found in small amounts in the melts. [ABSTRACT FROM AUTHOR]

Featured Application: Temperature dependent in-situ Raman spectroscopy was applied to study the structure of coals at high temperature. Raman spectroscopy can be used to record the characteristic spectra of carbonaceous materials. The D and G bands are the most popular and most important spectral characteristics when discussing carbonaceous materials. In this paper, a Raman spectroscopic study of different coals was first carried out using a 355 nm wavelength laser beam as an excitation source. The spectral parameters of the resultant spectra were evaluated and analyzed. Raman spectral characteristics of different kinds of coals were explored. The high temperature-dependent Raman spectra of the coals were further collected in a temperature range from 298 to 1473 K in order to investigate the transformations of the internal structure of the coals during the pyrolysis process. An abnormal blue shift of the G band occurred at moderate temperature (600–900 K), and the intensity of the G band became weaker at high temperatures, indicating pyrolysis and graphitization of the sample at moderate and high temperature, respectively. [ABSTRACT FROM AUTHOR]

In‐situ studies of the vibrational characteristics and microstructure evolution of the Na2WO4·2H2O (sodium tungstate dihydrate) crystal during the temperature‐induced solid‐state phase transformation and melting process were carried out using high‐temperature Raman spectroscopic technique. Results showed that the thermal decomposition process of the Na2WO4·2H2O crystal takes place mainly within the temperature range of 348–383 K, along with the structure transforming from the orthorhombic to cubic symmetry. As the sample temperature increased further, another solid‐state phase transformation from the cubic to orthorhombic structure was observed approximately at 893 K before melting occurred at 1023 K. Although the isolated [WO4]2− tetrahedron was preserved within the entire temperature range from room temperature to 1023 K, subtle changes were observed with the mean bond length of W–O bonds in the tetrahedron unit. Furthermore, Raman active vibrational modes of Na2WO4·2H2O, two Na2WO4 crystal phases, and corresponding melt were assigned based on the density functional theory simulation and compared with the literature data. Finally, four‐molecule cluster arranged as Td symmetry is considered to be the most likely configuration in the molten state according to density functional theory simulation based on the different multimolecular clusters proposed. In‐situ Raman spectra of the polycrystalline Na2WO4·2H2O from room temperature to 1023 K were investigated systematically. The vibrational modes of Na2WO4·2H2O, two Na2WO4 crystal phases, and corresponding melt were assigned based on density functional theory simulation. Microstructure evolution of [WO4]2− during dehydration, solid‐state phase transformation, and melting process was evaluated. Several multimolecular cluster models were constructed, and their Raman active vibrational modes were simulated in order to determine the most likely arrangement of [WO4]2− tetrahedra in the molten state. [ABSTRACT FROM AUTHOR]

In situ high-temperature Raman spectra of polycrystalline KBi(MoO4)2 were recorded from room temperature to 1073 K. Thermal stability of the monoclinic KBi(MoO4)2 was examined by temperature-dependent XRD. The monoclinic phase transformed into the scheelite tetragonal structure at 833 K, and then to the monoclinic phase at 773 K. Quantum chemistry ab initio calculation was performed to simulate the Raman spectra of the structure of KBi(MoO4)2 high-temperature melt. The experimental Raman band at 1023 K was deconvoluted into seven Gaussian peaks, and the calculated results were in good agreement with the experimental data. Therefore, the vibrational modes of Raman peaks of molten KBi(MoO4)2 were assigned. It was confirmed that the isolated structure of [Bi(MoO4)2]− monomer, consisting of Mo6+ centers and Bi3+ sub-centers connected by edge-sharing, mainly exists in the melt of KBi(MoO4)2. [ABSTRACT FROM AUTHOR]

In situ high temperature X-ray diffraction and Raman spectroscopy were used to investigate the temperature dependent micro-structure of KAlF4. Density functional theory was applied to simulate the structure of crystalline KAlF4 while a quantum chemistry ab initio simulation was performed to explore the structure of molten KAlF4. Two crystal polymorphs demonstrated to be present in solid KAlF4. At the temperature below 673 K, it belongs to the tetragonal crystal system within the P4/mbm space group, while the high temperature phase is attributed to the monoclinic crystal system within the P21/m space group. Both polymorph KAlF4 phases are characterized by a layered structure consisting of K+ and [AlF6]3− octahedra, each of the [AlF6]3− octahedra equivalently shares four corners with other four [AlF6]3− octahedra along the layer. The layered structure became unstable at higher temperatures and crashed when the temperature exceeded the melting point. It demonstrated that the molten KAlF4 consisted of predominant [AlF4]− and a small amount of [AlF6]3−. The Raman spectrum of molten KAlF4 simulated by using a quantum chemistry ab initio method agreed well with the experimental Raman spectrum. [ABSTRACT FROM AUTHOR]

To investigate the structural behaviors of Al3+ in aluminosilicate systems, the microstructural characteristics of CaO–SiO 2 -based glassy samples with various Al 2 O 3 contents were examined quantitatively by Raman spectroscopy and 27Al MAS NMR. A sequence of multiple model clusters of aluminosilicate systems modified with Ca2+ and Na ​+ ​cations was designed. Then quantum chemistry (QC) ab initio calculations were performed for geometric optimization, and Raman spectral simulations were carried out. The functional relationship between the Raman scattering cross section (RSCS) and stress index of silicon-oxygen tetrahedron (SIT) for aluminosilicates was established, which was successfully applied to calibrate experimental Raman spectra. Some fivefold coordinated aluminum (AlⅤ, approximately 5%) and less than 2% sixfold coordinated aluminum (AlⅥ) were detected by 27Al MAS NMR, while most aluminum remained in tetrahedral sites (AlⅣ). The ever-finer quantitative results of Raman spectroscopy and NMR showed a gradual production of AlⅣ with the addition of Al 2 O 3 , along with a significant adjustment in Q i species distribution. Specifically, Q 1 , Q 2 decreased, fully polymerized Q 4 increased and Q 3 showed a nonmonotonic variation and obtained the maximum at Al 2 O 3 ​= ​12 ​mol%. Furthermore, the effects of aluminum on bridging oxygen bond types (T-O b , T ​= ​Si, Al) and the degree of polymerization were discussed in detail. These structural features related to composition are essential theoretic foundations for understanding the properties of these systems. [ABSTRACT FROM AUTHOR]

In order to study the effect of MgO on the microstructure and macroscopic thermos-physical properties of calcium silicate melt, quantitative analysis of the various silicon-oxygen Q i species in magnesium-calcium silicate glasses and melts at a constant alkalinity has been performed by in-situ high temperature Raman spectroscopy, 29Si MAS NMR and DFT (Density Functional Theory) calculation. Several cluster models, probably existed in magnesium-calcium silicate glasses and melts, modified by Ca2+ or Mg2+ have been designed, their Raman vibrational modes and scattering activities were thus simulated and calculated. A function correlating the wavenumbers of Si-O nb (non-bridge oxygen) of various SiOTs (silicon-oxygen tetrahedrons) with their RSCS (Raman scattering cross sections) was constructed and used to calibrate the experimental Raman spectra. The abundance of each primary Q i species of various magnesium-calcium silicate glasses can be composed by all the deconvoluted hyperfine peaks within the same Q i species. The quantitative description for glasses was proven to be consistent with the result from 29Si MAS NMR. Thus, Q i species distributions for melts were also quantitatively derived. It demonstrated that the abundances of Q 1 and Q 3 species increased while Q 2 species decreased with the increasing replacing amount of MgO. The relationship between the viscosity and Q i species distributions as well as different cations has been investigated and correlated. Q 3 species was predominantly responsible for the contribution of viscosity of melt. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of aluminum-oxygen-fluorine molecular model clusters were proposed in the molten Al 2 O 3 -Na 3 AlF 6 system. • The microstructure evolution of various Al-O-F species (AlF x 3-x, Al 2 OF y 4-y, and Al 2 O 2 F z 2-z) were quantitatively studied. • The correlation between microstructure and viscosity was investigated. • DFT was applied to simulate the spectra of solid crystalline α-Al 2 O 3 and Na 3 AlF 6. The microstructure of the molten alumina-cryolite system is an important aspect in the dissolution behavior of alumina (α-Al 2 O 3) in cryolite (Na 3 AlF 6) molten salt and impacts the physical properties of the molten system. In this work, the symmetric stretching vibrational wavenumbers of aluminum nonbridging fluorine (AlF x 3-x) and aluminum-oxygen-fluorine (Al 2 OF y 4-y, and Al 2 O 2 F z 2-z) bonds in the high wavenumber range, as well as their corresponding Raman scattering cross sections (RSCS), were first analyzed and determined by quantum chemistry (QC) ab initio calculations. The species in binary Al 2 O 3 -Na 3 AlF 6 melts with varying Al 2 O 3 contents from 0 to 6.0 wt% were then qualitatively and quantitatively analyzed using in situ high-temperature Raman spectroscopy in conjunction with first principles calculation from room temperature to melt. The species of AlF 4 -, AlF 5 2- and AlF 6 3- were found to be present and predominant in the melts with Al 2 O 3 contents ranging from 0 to 6.0 wt%. The content of Al 2 OF 6 2- species tended to increase with Al 2 O 3 content from 0 to 3.0 wt%, while Al 2 O 2 F 4 2- species only existed in the melts with an Al 2 O 3 concentration of more than 3.0 wt%. Finally, the abundance of various species in the molten Al 2 O 3 -Na 3 AlF 6 system was further correlated to the melt viscosity. The contribution to viscosity was primarily determined by the distribution of AlF x 3-x (x = 4, 5, and 6) and Al 2 OF 6 2- species, and the contribution ability of corresponding species to viscosity was Al 2 OF 6 2-, AlF 6 3-, AlF 5 2- and AlF 4 - from largest to smallest. [ABSTRACT FROM AUTHOR]

• A series of Al-F molecular model clusters were proposed in the molten NaF-AlF 3 system. • The quantitative distribution of various species in the NaF-AlF 3 molten salt with different CR has been investigated. • The relationship between the viscosity and microstructure has been investigated and corrected. • DFT was applied to simulate the spectra of solid crystalline Na 5 Al 3 F 14 and Na 3 AlF 6. The quantitative distribution of different types of clusters in a binary NaF-AlF 3 molten salt system with a series of cryolite ratios (CR, NaF/AlF 3 molar ratio) was explored by in situ high-temperature Raman spectroscopy in conjunction with quantum chemistry (QC) ab initio calculations. Density functional theory (DFT) was used to simulate the Raman spectra of solid crystalline Na 5 Al 3 F 14 (CR = 1.67) and Na 3 AlF 6 (CR = 3.00). Aluminum-fluorine molecular model clusters containing species with typical characteristic microstructures were built in a molten NaF-AlF 3 system. Deconvolution of the major stretching vibrational bands of Al-F-Al and Al-F bonds was carried out by using the Voigt function. The anionic fractions of different species present in the molten NaF-AlF 3 system were quantitatively analyzed by a function of the microstructure dependent on Raman scattering cross sections (RSCS). AlF 6 3-, AlF 5 2-, and AlF 4 - were the dominant species in the aluminum-fluorine molten salt system. As the NaF in the melt increased, the content of AlF 5 2- reached the maximum concentration when CR = 2.00. Apart from the recognized existence of both AlF 4 - and AlF 6 3-, the aluminum bridging fluorine species of Al 2 F 7 - has been proven to exist in molten systems when CR ≤ 1.00. The relationship between the viscosity and various (n-3)Na+·AlF n (n-3)- species distributions has been investigated. The results showed that the contribution of different (n-3)Na+·AlF n (n-3)- to viscosity was essentially the same, and simply depended on the number of different species. [ABSTRACT FROM AUTHOR]

In-situ high temperature Raman spectroscopic (HTRS) technique in combination with density functional theory (DFT) analysis has been adopted to investigate the micro-structure of solid and molten A 2 W 2 O 7 (A = Li, Na, K). The [WO 6 ] octahedra were found to be connected to each other by corner and edge sharing in the crystalline Li 2 W 2 O 7 and K 2 W 2 O 7 compounds. In the crystal lattice of Na 2 W 2 O 7 , on the other hand, the [WO 4 ] tetrahedra and [WO 6 ] octahedra were found to coexist and paired by corner sharing. Although the structural diversity has clearly led to distinct Raman spectra of the crystalline A 2 W 2 O 7 compounds, the spectra of their melts tended to be analogous, showing the typical vibration modes of (W 2 O 7 ) 2 − dimer. A mechanism was then proposed to explain the structure evolution occurring during the melting process of A 2 W 2 O 7 . The effect of A + cation on the Raman bands of (W 2 O 7 ) 2 − dimer in molten A 2 W 2 O 7 has also been investigated. Both the wavenumber and full width at half-height (FWHH) of the characteristic band assigned to the symmetrical stretching vibration mode of W O nb (non-bridging oxygen) in (W 2 O 7 ) 2 − were found to decrease in the sequence of Li + , Na + and K + , indicating the cation effect on the mean bond length and its distribution range of W O nb . In addition, the relative intensity of this band was also influenced by the cation and it was increased in the order of Li 2 W 2 O 7 , Na 2 W 2 O 7 and K 2 W 2 O 7 , which has been explained by the charge transfer process and confirmed by Mulliken overlap population analysis. [ABSTRACT FROM AUTHOR]

• In-situ high temperature raman spectra of binary calcium silicate melts at the temperature of 1873 K have been measured. The corresponding quenched glasses have also been determined for comparison. • A set of clusters existing in binary calcium silicate glasses and melts were optimized and calculated by quantum chemistry ab initio calculations to obtain the raman scattering cross sections of characteristic vibrations of clusters. • A method for quantitatively analyze the microstructure of binary calcium silicate glasses and melts by raman spectroscopy was proposed and verified by 29Si MAS NMR. The quantitative distribution of different Q i species in binary calcium silicate glasses and their melts has been investigated by in-situ high temperature Raman spectroscopy, 29Si MAS NMR and quantum chemistry (QC) ab initio calculations. The structural units and several cluster structure models have been designed, optimized and calculated by ab initio calculations. The attribution of their vibrational modes, vibrational wavenumbers of non-bridging silicon-oxygen (Si-O nb) and Raman scattering cross sections (SCS) have then been worked out and analyzed. The functional relationship between the SCS and stress index of silicon-oxygen tetrahedron (SIT) was established for the model clusters of delicate Q i. Quantitative distributions of Q i in calcium silicate glasses and melts are finally obtained which are in good agreement with the experimental results of 29Si MAS NMR. Although the primary structure present in calcium silicate glasses and melts is the same, the relative distribution of Q i is different. On the basis of the present work, it will be helpful to predict the physicochemical properties of silicate slag. [ABSTRACT FROM AUTHOR]