The combined inelastic neutron scattering (INS) and solid-state dft study of hydrogen-atoms dynamics in kaolinite-dimethylsulfoxide intercalate

Vibrational spectra of two kaolinite-dimethylsulfoxide intercalates, obtained using inelastic neutron scattering (INS), were analyzed with a view to understanding the dynamics of the hydrogen atoms in the structure. The main focus was on the spectral region 0-1700 cm, which is difficult to analyze using optical spectroscopy. The experimental vibrational spectra of kaolinite: dimethylsulfoxide and kaolinite:d6-dimethylsulfoxide collected using two different spectrometers were interpreted by means of the solid-state DFT calculations. Calculated spectra were obtained by both normal-mode analysis and molecular dynamics going beyond the harmonic approximation. The Al-O-H bending modes were found to be spread over the large interval 100 - 1200 cm, with the dominant contributions located between 800 and 1200 cm. The shape of the individual hydrogen spectrum depends on whether or not the respective hydrogen atom is involved in an O-H- • O hydrogen bond and on its strength. The modes corresponding to the in-plane movements of the inner-surface hydrogen atoms are well defined and always appear at the top of the intervals of energy transfer. In contrast, the modes generated by the out-of-plane movements of the hydrogen atoms are spread over large energy intervals extending down to the region of external (lattice) modes. The C-H modes are concentrated mainly in the three regions 1200 - 1450 cm, 800-1100 cm, and 0-400 cm. While the first two regions are typical of the various deformational modes of methyl groups, the low-energy region is populated by the modes corresponding to the movements of the whole dimethylsulfoxide molecule

Crystallization kinetics of binary Yb2O3-Al2O3 glass

The ytterbium aluminum garnet composition YbAG (62.5 mol.% Al2O3, 37.5 mol.% Yb2O3) was prepared in the form of glass microspheres by flame synthesis. Precursor powder for flame synthesis with high homogeneity was prepared by modified sol-gel Pechini method. XRD pattern of prepared glass microspheres indicated predominantly amorphous nature of the sample. Detailed study of morphology of the microspheres by scanning electron microscopy revealed the presence of a small fraction of partially or fully crystallized microspheres. The high-temperature X-ray powder diffraction analysis (HT XRD) was carried out in the temperature interval 750-1450 degrees C: The temperature dependence of phase composition was determined. Crystallization of Yb3Al5O12-ytterbium aluminum garnet phase-was observed in the temperature range 900-1200 degrees C. The DSC analysis with heating rates 2, 4, 6, 8, 10 degrees C min(-1)in temperature interval 25-1200 degrees C was performed in N(2)atmosphere to study thermal behavior and crystallization kinetics of prepared glass microspheres. The two exothermic effects at 918 and 939 degrees C were observed, which were attributed to Yb(3)Al(5)O(12)crystallization. The crystallization kinetics of prepared sample was examined with the use of JMAK model, and the kinetic triplet-frequency factorA = (1.8 +/- 2.2) 10(+28)min(-1)(for the first peak),A = (1.2 +/- 1.6) 10(+55)min(-1)(for the second peak), apparent activation energyE(app) = (6.4 +/- 0.1) 10(+02) kJ mol(-1)(for the first peak),E-app = (1.3 +/- 0.1) 10(+03)kJ mol(-1)(for the second peak) and the Avrami coefficientm = 3 (for the first peak) andm = 2 (for the second peak)-was determined using RSS,Radj2, AIC andW(AIC)criteria

Y3Al5O12-α-Al2O3 composites with fine-grained microstructure by hot pressing of Al2O3-Y2O3 glass microspheres

Yttrium aluminate glass microspheres with the eutectic composition 76.8 mol. % Al2O3 and 23.2 mol. % Y2O3 were prepared by combining the sol-gel Pechini method with flame synthesis. The sol-gel method was applied to achieve the desired composition homogeneity of the prepared glass and hence, improve the microstructure homogeneity and mechanical properties of bulk polycrystalline materials. The latter were prepared by hot pressing, more specifically pressure assisted sintering, at 1050 degrees C, 1300 degrees C and 1600 degrees C using pressures of 30 MPa and 80 MPa and holding times between 0 and 30 min. This also led to the crystallization of the glass. A composite with the Vickers hardness 18.0 +/- 0.7 GPa and an indentation fracture toughness 4.9 +/- 0.3 MPa.m(1/2) was obtained by sintering at 1600 degrees C, at the pressure of 80 MPa and with 30 min isothermal heating at the maximum temperature. Improved mechanical properties were observed when increasing the temperature of sintering and the holding time. This can be attributed to the formation of a unique microstructure consisting of alpha-Al2O3 grains in the mu m-scale embedded in a YAG (yttrium-aluminium garnet) matrix in the hot-pressed samples

Crystallization kinetics of Ni-doped Ca2Al2SiO7 glass microspheres

The Ni-doped Ca(2)Al(2)SiO(7)glass systems were prepared by flame synthesis. Solid-state reaction was used to prepare the powder precursors. The concentration of Ni was 0.5, 1 and 3 mol%. Polydisperse systems were prepared with diameters between 5 and 140 mu m. Detailed examination of morphology of the glass microbeads by SEM revealed no features indicating the presence of crystalline phases. However, X-ray diffraction analysis showed that the samples GNi0.5 (0.5 mol% of Ni) and GNi1.0 (1.0 mol% of Ni) contained traces of crystalline gehlenite. HT-XRD was used to determine the temperature dependence of phase composition. For all prepared compositions, only one crystalline phase (Ca2Al2SiO7) was observed. DSC measurements in the temperature range 30-1200 degrees C at five different heating rates were carried out to study the thermal behavior. The DSC curves of all glasses contained one exothermic peak, which was attributed to crystallization of the gehlenite. The maximum of the peak decreased with increasing Ni content in the microspheres. The kinetic parameters (frequency factorA, apparent activation energyE(app)and the Avrami coefficientm) of the crystallization were determined using the Johnson-Mehl-Avrami-Kolgomorov model. In case of GNi0.5 and GNi1.0 glasses, the nucleation's rate had linear temperature dependence, the crystal growth interface is controlled by chemical boundary and the crystal growth is one-dimensional. The rate of nucleation is linear, the crystal interface growth is controlled by diffusion and one-dimensional crystal growth prevails in crystallization of the GNi3.0 (3.0 mol% of Ni)

Structure, thermal properties and crystallization behavior of binary Y2O3–Al2O3 glasses with high alumina content

Five compositions in the system Al2O3–Y2O3 with high level of homogeneity were prepared in the form of glass microspheres by flame synthesis. The amorphous nature of prepared glasses with highly disordered structure was confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman and nuclear magnetic resonance (NMR) spectroscopy. In the NMR spectra, typical signals with chemical shifts of 75, 42 and 12 ppm were observed, which were attributed to the presence of AlO4, AlO5 and AlO6 motifs in the glass structure. The ratio of individual motifs in glass samples did not change significantly with the composition. The crystallization of yttrium-aluminium garnet (YAG) phase was observed as a major process in the glasses thermally treated up to 1450 °C, with slow crystallization of θ- and α-Al2O3 phases detected in the temperature interval 980–1450 °C. IR and Raman spectra of the microspheres crystallized at 998, 1300 and 1500 °C for 4 h contained typical bands, that were assigned to the vibrations of AlO4 and AlO6 groups in YAG and Al2O3 structures. The comparison of 27Al and 89Y magic angle spinning (MAS) NMR spectra showed the presence of only YAG and α-Al2O3 phase in the samples crystallized at 1500 °C and the presence of a trace amount of θ-Al2O3 in the sample crystallized at 998 and 1300 °C. The yttrium aluminium perovskite (YAP) and yttrium aluminium monoclinic (YAM) phases, expected in this system, were no detected

Thermal behaviour of yttrium aluminate glasses studied by DSC, high-temperature X-ray diffraction, SEM and SEM–EDS

DSC, SEM-EDS, XRD and high-temperature XRD analysis was used to study thermal and crystallization behaviour of yttrium aluminate glasses prepared in the form of microspheres. The glasses YA-E (eutectic composition from the pseudo-binary system Al2O3-Y3Al5O12) and YA-G (a composition identical to the stoichiometric Y3Al5O12 (YAG) phase) were prepared by combination of the Pechini method with flame synthesis. The resulting microspheres were largely amorphous, but contained traces of yttrium-aluminium garnet as the main crystalline phase embedded in the yttrium aluminate glass matrix. Crystallization of the YAG phase was observed as the dominant exothermic process on DSC curves. From the DSC records, the basic thermal characteristics of the matrix glass, i.e. T (g) (glass transition temperature), T (x) (onset of crystallization peak temperature), T (f) (temperature of the inflection point of the crystallization peak) and T (p) (maximum of crystallization peak temperature), were determined. HT XRD experiments in the temperature interval 750-1200 A degrees C and isothermal HT XRD experiments at 932, 998 and 1200 A degrees C with 6-h holding time were also performed. Crystallization experiments at lower temperatures 932 A degrees C (YA-E) and 915 A degrees C (YA-G) were conducted to study phase development in a low-temperature region. Crystallization experiments at higher temperatures (1000, 1300 and 1500 A degrees C) with maximum holding time of 6 h were performed to study crystallization of alpha-Al2O3 in the eutectic system. The SEM and SEM-EDS examination of polished cross sections of crystallized microspheres revealed slow volume crystallization of the YAG phase in the AY-E glass. Eventually, polycrystalline microspheres with fine-grained microstructure were prepared after 6-h treatment at 1500 A degrees C