In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of ²⁷Al NMR spectra and spin–lattice relaxation time constants T₁ have been studied for a calcium aluminosilicate (43.1CaO–12.5Al₂O₃–44.4SiO₂) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T₁ drastically increases near the glass transition temperature T_g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T₁ minimum (≈1200 °C), NMR correlation times obtained from T₁ are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.     

Atomic order in magnetic Mn inclusions in Si crystals: XAS and TEM studies

A Si (1 0 0) crystal implanted with Mn⁺ ions, exhibiting ferromagnetic properties, was studied with XAS and TEM to examine the local atomic order around Mn atoms. The advantage of the XAS technique was its elemental selectivity, which allowed extracting information on the atomic surroundings of Mn atoms even at very low concentrations of this element, incapable of producing a signal in X-ray diffraction. It is very important to find out what is responsible for the ferromagnetism of this new class of materials. The knowledge of the location of Mn atoms in the Si crystal lattice is crucial in developing models of ferromagnetic interactions. The performed studies have proven beyond doubt that Mn atoms are not located randomly in the Si crystalline matrix but form Mn–Si clusters immersed in a strained Si matrix. Assuming the atomic order and dimensions of the cluster found through EXAFS and HRTEM, we have reproduced the electronic structure of Mn atoms by modeling the XANES spectrum in agreement with the experimental one.     

Ex situ XRD,TEM, IR,Raman and NMR spectroscopy of crystallization of lithium disilicate glass at high pressure

The structure of Li₂O · 2SiO₂ (LS₂) glass was investigated as a function of pressure and temperature up to 6 GPa and 750 °C, respectively, using XRD, TEM, IR, Raman and NMR spectroscopy. Glass densified at 6 GPa has an average Si–O–Si bond angle ∼7° lower than that found in glass processed at 4.5 GPa. At 4.5 GPa, lithium disilicate crystallizes from the glass, while at 6 GPa new high pressure form of lithium metasilicate crystallizes. This new phase, while having lithium metasilicate crystal symmetry, contains at least four different Si sites. NMR results for 6 GPa indicate the presence of Q⁴ species with (Q⁴)Si–O–Si(Q⁴) bond angles of ∼157°. This is the first reported occurrence of Q⁴ species with such large bond angles in alumina free alkali silicate glass. No five- or six-coordinated Si are found.     

On the anomalies in density,Young's modulus and glass temperature of PdSi glasses

The densities, Young's moduli and glass transition temperatures have been measured for binary PdSi glassy alloys. These glassy alloys exhibit a negative deviation from Vegard's law an enhancement in Young's modulus and a tendency to lower the glass transition temperature in the region of the eutectic composition. The observed nonlinearity in these physical properties is shown to be better agreement with an alloy mixing effect rather than the structural ordering model.     

Friction measurement in precision glass molding: An experimental study

Understanding the frictional behavior between glass and metals at elevated temperatures is necessary for accurate modeling and simulation of the state of the art applications such as the PGM process, pressing of micro lens arrays, hot embossing, and extrusion or drawing of glass fiber. In this research, the frictional behavior of N-BK7, an oxide glass, at elevated temperatures in contact with polished and coated tungsten carbide (WC) material and under conditions similar to the PGM process has been studied.The experimental results show that the friction coefficient between a polished and coated WC mold and N-BK7, which is a typical material for glass molding, and in conditions similar to those used in the PGM process ramps up to 0.7 and then levels off around 0.6 with a smooth transition, meaning that there is not a specific point to differentiate between static and dynamic friction. The friction coefficient of 0.6 is reported in literature for best fitting in micro lens molding simulation. Moreover, increasing the temperature in the transition regime in the mold/glass interface causes the friction force to increase which is in good agreement with the data published for the friction between soft polymeric material and metal in the transition regime.     

Excitation energy dependence of the photoluminescence band at 2.7 and 4.3 eV in silica glass at low temperature

In this paper, we report the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (20 K). The increase or decrease of the PL intensity at low temperatures is different for different exciting light wavelengths. The PL intensity tended to decrease with low temperatures when the excitation was near the upper and lower end of the excited level. The peak energy of the excitation spectrum increases with cooling. These results indicate that the change in excitation level with cooling is associated with the low-temperature dependence of light emission. Thermal motion is suppressed, when the sample temperature is lowered, and the energy-width of the excited level decreases, i.e., the light emission probability decreases (the emission intensity decreases), when near the upper and lower end of the excitation level. These phenomena were observed in the low-temperature dependence of the 4.3 eV emission intensity.     

Stark splitting of the ground state of Er in fluorozirconate glass at low temperature

Optical properties of Er³⁺-doped ZBLAN glass matrix have been studied by luminescence spectroscopy under 488 nm excitation. The spectrum of the ⁴S_3/2⁴I_15/2 transition, carried out at temperature T = 2 K, shows a new line in the lowest energy region. This new line, centered at 17 996 cm⁻¹, was attributed to the lower transition between the Stark components of the ⁴S_3/2⁴I_15/2 transition. Measurements from T = 2 K to room temperature show the disappearance of this new line. From the results we estimate the splitting of 415 cm⁻¹ for the ground state and 100 cm⁻¹ for the ⁴S_3/2 excited multiplet. The experimental result allows us to assign the positions of the eight Stark components of the ground state multiplet of the Er³⁺ in the ZBLAN glass matrix.     

The effect of fictive temperature on the structure of E-glass: A high resolution,multinuclear NMR study

The variation of the structure of E-glass with fictive temperature has been examined with high resolution, multinuclear (¹¹B, ²⁷Al, ²⁹Si, ¹⁷O, and ¹⁹F) MAS NMR. The previously observed decrease in 4-coordinated boron with increasing fictive temperature is confirmed and, for the first time, is directly correlated with an increase in non-bridging oxygens. An increase in 5-coordinated Al is also observed, and is linked to an increase in association of F with Al at higher fictive temperature.     

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature T_c, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below T_c the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above T_c the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below T_c. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near T_c in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.     

Correlation between the low temperature phonon mean free path and glass transition temperature in amorphous solids

An interpretation is offered for the increased chemical resistance of glass surfaces upon treatment with SiF₄/H₂O gaseous mixtures. Scanning electron microscopy examinations of the morphology and evolution of the surface structure, the surface sodium concentration profile as revealed by a nuclear reaction and ion release experiments support the formation of a very adherent amorphous silica layer - still transparent at the earlier stages of treatment - which fits well all experimental findings.     

Molecular dynamics study of the structure and dynamic behavior at the surface of a silicate glass

Molecular dynamics calculations of a complex SiO₂+B₂O₃+Al₂O₃+ZrO₂+Na₂O+CaO glass with surfaces revealed the formation of outer layers having different structural properties than the bulk glass. Alkali cations tend to migrate toward the surface, lowering the coordination number of the trivalent elements in the subsurface layer. The outer layer is also enriched in oxygen. Depolymerization leads to the formation of slightly larger rings on average. Atom mobility is enhanced in the lower density layers, as shown by the application of an external electric field and during simulated displacement cascades. Although these results are consistent with previously published findings, it is difficult to assess the actual scope of these surface effects (estimated by molecular dynamics to extend to a depth of about 10 Å) because this approach does not take into account the effects of long-range diffusion or of the interface with the surrounding environment.     

An inelastic X-ray scattering study on the collective dynamics in the ‘non-simple’ liquid metal Si

The dynamic structure factor S(Q,ω) of liquid silicon has been measured using inelastic X-ray scattering. The data show that distinct longitudinal collective modes exist in this ‘non-simple’ liquid metal similar as in the so-called ‘simple’ metallic fluids as, e.g. the liquid alkalis. However, the Si-modes are considerably more strongly damped and in S(Q,ω) the corresponding excitation peaks cease to exist for momentum transfers of about half the structure factor maximum position. The degree of positive dispersion, in the Q-ω-relation of the modes is, however, similar as in the liquid alkalis, indicating that the microscopic viscosity is similar in l-Si and that the stronger damping may rather be related to the higher thermal diffusivity. Additionally, it is found that the shape of the quasielastic line close to the structure factor maximum deviates considerably from the shape found in simple liquid metals. This observation is related to the onset of short time correlations between colliding particles on the sub picosecond scale which is in accord with findings from ab initio computer simulations under similar conditions.     

Hall and giant magnetoimpedance effects in the Co70Fe5Si15B10 metallic glass

The transversal giant magnetoimpedance and the Hall effects of a high magnetic permeability metallic glass, Co₇₀Fe₅Si₁₅B₁₀, were measured as a function of the frequency (f) and amplitude of ac electrical current (I_ac). The room temperature data were obtained for 0.01 ⩽ f ⩽ 100 kHz, 0.5 ⩽ I_ac ⩽ 45 mA and for applied magnetic field (H) varying in the range ±10 kOe. For low frequencies, the H-dependence of the Hall voltage is similar to those measured using dc currents. However, the Hall voltage develops spectra made of two strongly f-dependent symmetrical peaks centered around ±1.6 kOe when the frequency is increased. The anomaly in the Hall voltage was identified as mainly due to a planar Hall effect.     

Phosphoric acid purification by suspension melt crystallization: Parametric study of the crystallization and sweating steps

In order to purify phosphoric acid, the suspension melt crystallization process was studied. The suspension crystallization experiments were carried out with 80, 84 and 88 wt% phosphoric acid melt at the cooling rates of 0.05, 0.1 and 0.2 K/min, respectively. Sweating experiments were executed for various crystals obtained in suspension crystallization step. The purification effects of the sweating parameters including sweating time, initial inclusion amount and initial impurity content were studied. The inclusion fraction increases with the increase in cooling rate. The inclusion fraction of the crystals which were formed with feed concentration of 84 wt% phosphoric acid melt is lowest among the three feed concentrations. Different impurities have different purification performances during sweating. High inclusion amount and low impurity concentration favor the purification of H₃PO₄·0.5H₂O crystals during sweating.     

Room,low, and high temperature dehydration and phase transitions of kernite in vacuum and in air

Kernite Na₂B₄O₆(OH)₂·3H₂O dehydration in air at high temperature and in vacuum at room temperature has been studied. It was found that kernite easily dehydrates forming a new phase‐I both on heating and in vacuum. The chemical formula Na₂B₄O₆(OH)₂·1.5H₂O of the new phase‐I has been estimated on the basis of thermogravity analysis. It is triclinic with the unit cell parameters a = 7.047(8), b = 8.76(1), c = 13.08(2) Å, α = 93.40(9), β = 95.32(9), γ = 90.28(9)° changing slightly on pressure reduction. Due to the relatively low temperature (353 K) and reversibility of the kernite ⟷ phase‐I transition an anion of the new phase‐I likely consists of the same chains [B₄O₆(OH)₂]^2– like in kernite structure. The high anisotropy of kernite thermal expansion was explained by approaching of NaO chains due to the initial removing of water molecules from kernite crystal structure. The behaviour of the new phase‐I at low temperatures in vacuum was also investigated. A formation of an additional new phase II has been detected at the temperature of 93 K. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Secondary dynamics in glass formers: Relation with the structural dynamics and the glass transition

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari–Goldstein process, originating by a local motion of the whole molecule.     

Sea water desalination by dynamic layer melt crystallization: Parametric study of the freezing and sweating steps

This work aims at developing a dynamic layer crystallizer operated batchwise, for freezing desalination of sea water. The experiments were performed with water/NaCl solutions and with samples of sea water from Nice, Rabat and Marseille. The pilot crystallizer consists of a cooled tube immersed in a cylindrical double jacketed tank. The solution is poured into the tank and the crystallization takes place on the external surface of the tube, by applying a cooling ramp in the tube. The solution is agitated by air bubbling. The whole process involves the freezing step, leading to the crystallization of the ice layer and the sweating step, which consists of purifying in depth the ice layer by melting the impure zones. A parametric study on the effect of the operating parameters has allowed quantifying the role of the different key parameters of the freezing and sweating steps. Three experiments allowed reaching salinities lower than 0.5 g/kg, satisfying the standards of drinking water. The duration of the whole process dropped to only 8 h (5 h for freezing and 3 h for sweating), with a yield of sweating equal to about 50%, provided severe conditions were applied for sweating. Higher yields required longer times. Overall, the results show the feasibility of the technique.     

The correlation between the pressure sensitivity and the fragility/glass transition temperature in bulk metallic glasses

The correlations between the pressure sensitivity and the fragility/glass transition temperature have been addressed in various bulk metallic glasses in the present work. The results demonstrate that the pressure sensitivity of bulk metallic glasses is closely related to both the fragility index (m) and the glass transition temperature (T_g). The physical origin of the correlations has been discussed from their disordered structure, which is determined by the glass transition behavior and the glass transition temperature.     

Crystallization of the polymorphs of succinic acid via sublimation at different temperatures in the presence or absence of water and isopropanol vapor

α Succinic acid was prepared from the gas phase via the sublimation of the β form. Two sublimation methods including sublimation with carrier gas and sublimation in the presence of foreign molecules were employed. The influence of the sublimation temperature and the crystallization temperature on the polymorph and morphology are presented and discussed. At high supersaturation, the α polymorph predominated, and at low supersaturation the β polymorph was obtained. The influences of water and isopropanol in the gas phase on the morphology of β succinic acid grown via the sublimation are presented and compared with morphologies of crystals grown from solution.     

Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone,evaluation of optical properties and structure of the glass

A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl₂F₂ as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl₂F₂/SiCl₄ ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10^?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl₂F₂ as a dopant precursor with suitable flow of SiCl₄ vapor along with O₂ through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of ¹⁹F MAS spectra done by high-resolution ¹⁹F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO_1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO₂F polyhedral which become increased with increasing the fluorine content.