Microstructural evolution of amorphous silica following alkali–silica reaction

Combining X-ray diffraction, NMR (CP-MAS and ²⁹Si NMR-MAS) and diffuse reflectance infra-red Fourier transformed spectroscopy (DRIFTS), the local structural changes induced during the reactivity of amorphous (a-SiO₂) under alkali–silica reaction (ASR) have been investigated. XRD patterns provide useful and new qualitative information concerning the reactivity of a-SiO₂. A shift of the maximum of the halo towards the high values of 2θ is observed when the reaction progresses. NMR shows that ASR is accompanied in a-SiO₂ by the diminution of Q₄ species and by the formation of Q₃ (and to a less extend to Q₂) species. This importance played by Q₃ species confirms the results obtained with natural alkali–silica gels. The formation of OH bonds observed by DRIFTS can be followed at ca. 950 cm^?1 and confirms NMR results. The microstructural state of silica (here represented by the medium- and short-range order (MRO) of amorphous silica) plays a crucial role in governing the kinetics of the ASR.     

Structure characterization of CVD amorphous Si3N4 by pulsed neutron total scattering

The structure factor S(Q) of chemically vapor-deposited (CVD) amorphous Si₃N₄ has been measured by pulsed neutron diffraction over the range of the scattering vector Q from 1–330 nm^?1. The oscillatory behavior in the S(Q) persists up to Q = 300 nm^?1 and there is appreciable small angle scattering intensity. The SiN bond length is l_SiN = 0.1729 nm, and its coordination numbers n_SiN and n_NSi are 3.70 and 2.78 respectively. The bond angles around a Si and a N atom are found to be 109.8 and 121°. Analysis of the small angle scattering intensity shows the existence of voids with an average diameter of about 1 nm and a volume fraction of about 4%, which may stabilize the amorphous structure of Si₃N₄ having rigid covalent bonds due to relaxing the strain energy accumulated in the matrix.     

Neutron diffraction studies of vapour-deposited ammonia-ND3

A vapour-deposited sample of ND₃ at 10 K was studied by neutron diffraction and found to consists of a mixture of crystalline and amorphous material. The Bragg peaks were in good agreement with those for the cubic crystalline form of ND₃ and could be subtracted to give the diffraction pattern for the amorphous solid. The data for the amorphous phase were transformed to give the real-space distribution function d(r) which contained significant oscillations for r-values extending to 15 Å. Comparison of the data with that of liquid ND₃ shows a structural change which is consistent with the formation of a random network of hydrogen-bonds. This behaviour resembles that observed in water and amorphous ice but has several important differences resulting from the dimensionality of the network.     

Structural investigation of amorphous and liquid Ge0.175Te0.825 by neutron scattering

The angular distribution of intensities of 1.003 Å neutrons scattered by an alloy of Ge_0.175Te_0.825 both in the amorphous and in the liquid state have been measured. The bulk amorphous sample, prepared by water quenching of the melt, has been examined at room temperature and just above the crystallization temperature T_x. The liquid sample has been examined at 400, 600 and 800°C. The analysis of the structure factors obtained from the corrected and scaled intensities indicates that the structural features of the amorphous sample are similar to those which would be expected if it were possible to cool the alloy to room temperature, while maintaining it in the liquid state. The analysis of the radical distribution functions, from Fourier inversion of the structure factors, indicates that a change of the coordination number n₁ from 2.43 to 3.25 occurs in passing from the amorphous to the liquid state at 400°C. This result has been interpreted in terms of coordination models. In the liquid state, it has been found that a model with fourfold coordinated Ge and threefold coordinated Te is consistent with n₁=3.25. The internal consistency of this model for the liquid state allows us to make a reasonable choice between the possibility of a 3 or 4 fold coordination for Ge in the glass. We conclude that a model based on four-fold coordinated Ge and twofold coordinated Te seems an appropriate representation for the coordination of the amorphous material.     

Phase separation in GeGeO2 glasses

De Neufville prepared homogeneous glasses ranging in composition from pure GeO₂ to GeO by quenching bulk samples from the melt and by vapor deposition. For compositions in the range of 10–20 mol % excess Ge dissolved in GeO₂, he found that phase separation into amorphous Ge rich and amorphous GeO₂ phases occurred. The results reported here on a 7.5 mol % excess Ge composition using differential scanning calorimetry have shown that a two-step phase separation mechanism is operative. A homogeneous GeGeO₂ glass phase separates at 450°C into amorphous GeO₂ and amorphous GeO. The GeO phase separates at 570°C into crystalline Ge and amorphous GeO₂. The heat measured at 570°C is equal to the sum of the heats of phase separation of GeO and crystallization of Ge. The amorphous GeO₂ crystallizes at 670°C with a heat of crystallization of 4.65 kcal/mol (± 0.5). Additional support for a two-step phase separation mechanism is provided by kinetic arguments based on the viscosity dependence on composition and on the structure of the amorphous GeO phase and its stability relative to the homogeneous GeGeO₂ glass.     

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni-Al alloys

Atomic and electronic structures of liquid and rapidly quenched Ni-Al (NiAl₃, NiAl, and Ni₃Al) alloys were studied by ab initio molecular dynamics simulation. Pair correlation function, structure factor, bond pair, electron population, and density of states were calculated. It is found that amorphous Ni-Al alloys can be prepared by rapid quenching of liquid alloys, with the former bearing similar structures to the latter, although amorphous alloys have a more ordered structure. Bond pair analysis indicates that both the liquid and amorphous Ni-Al alloys consist mainly of 1441, 1431, 1421, and 1422 pairs of tetrahedral local order. The positions of the first peaks of the Al-Ni pair correlation functions are lower than the sum of the metallic radii of Ni and Al, suggesting the occurrence of chemical bonding between Ni and Al in Ni-Al alloys. Electronic structure analysis further revealed that the interaction between the d-electrons of Ni and the p-electrons of Al is responsible for the bonding. The main peak positions of the total DOS of amorphous Ni-Al alloys become more and more positive when the content of Ni in the alloys increases.     

A thermodynamic and kinetic investigation of amorphous (1−x)As2Se3 · xSb2Se3 alloys

The heats of formation of amorphous (1?x)As₂Se₃ · xSb₂Se₃ (x = 0 to 0.4) referred to crystalline As₂Se₃ and Sb₂Se₃ were measured by liquid metal solution calorimetry. The values of heats of formation of amorphous (1?x)As₂Se₃ · xSb₂Se₃ decreased from 1.39 ± 0.03 kcal · (g-at)^?1 at x = 0 to 1.27 ± 0.04 kcal · (g-at)^?1 at x = 0.4.The glass transition temperature and the temperatures of the maximum rates of crystallization and fusion were measured by differential scanning calorimetry. The glass transition temperature increased and the temperatures of the maximum rates of crystallization and fusion decreased with increasing Sb₂Se₃ content.The relaxation process in amorphous (1?x)As₂Se₃ · xSb₂Se (x = 0.3) was investigated by measuring changes in microhardness, small-angle X-ray scattering and heat capacity with time of annealing at several temperatures ranging from room temperature to 413 K. With increasing annealing time the microhardness, the height and the temperature of the glass transition peak increased whereas the intensity of small-angle X-ray scattering decreased. These changes reflect relaxation towards a more stable structure of smaller molecular mobility. The changes in the enthalpy with annealing time and the activation energy spectra for relaxation were derived from the heat capacity data. The effects of temperature and time of annealing on the various properties are explained in terms of structural changes and relaxation kinetics.     

The collective dynamics of the ‘non-simple’ liquid metal Sn

The dynamic structure factor S(Q, ω) of liquid Sn was measured at 320 °C and 1000 °C in a Q-range between 2 nm⁻¹ and 30 nm⁻¹ using inelastic X-ray scattering. The obtained spectra clearly demonstrate the existence of longitudinal propagating modes in this liquid metal. At 320 °C a positive dispersion is found in the low Q-region, where the mode velocity is about 12% larger than the bulk adiabatic speed of sound. At 1000° this so-called ‘positive dispersion’ reduces to about 6%. The quasielastic lineshape contains a Gaussian contribution near the Q-positions of the first maximum in S(Q), which has not yet been reported in simple liquid metals like, e.g. the liquid alkalis. With increasing temperature, the relative Gaussian contribution increases and can be observed along a wider Q-range. This anomaly is related to a partially covalent nature of liquid Sn in analogy to other liquid non-simple metals.     

On the number of amorphous phases in n-butanol: Kinetics of free radicals oxidation by oxygen in frozen n-butanol

We report on the discovery of a new solid, presumably amorphous n-butanol at ambient pressure. According to the literature data the melting point T_m of n-butanol is 183 K and the glass transition temperature T_g is 118 K. If kept isothermally at a fixed temperature between 130 and 160 K, the supercooled liquid n-butanol undergoes remarkable phase transformations to a solid phase. The new phase converts to liquid at a temperature of about 170 K. It is presumably amorphous because foreign substances dissolved in liquid n-butanol keep the same state in this new phase of butanol. The kinetics of free radical oxidation by dissolved oxygen in both solid amorphous phases is investigated in detail.     

The neutron diffraction study of liquid GeTe and As2Te3

The radial distribution analyses for GeTe and As₂Te₃ are made at temperatures above the melting point in the range of momentum transfer between 0.7 and 10.0 Å^?1 by the neutron diffraction technique. Furthermore, the local order in amorphous GeTe is determined by analyzing the intensity data of the electron diffraction of its thin film. The result for the amorphous film indicates that the local distribution of atoms in amorphous GeTe is not characteristic of the structure of its crystalline state. The shape of the peaks of the intensity curve for liquid GeTe differs from that for the amorphous and crystalline states. However, the short bond length and the small coordination number determined from liquid RDF suggest that the covalent-like bonding between nearest-neighbor atoms remains unbroken when melting. The general form of the structure factor for liquid As₂Te₃ is similar to that for the amorphous material reported previously. The position of the first peak of RDF in the liquid state is observed to be shifted to a shorter distance than the average of nearest-neighbor atoms in crystalline As₂Te₃. The structure of GeTe differs considerably between the crystalline, amorphous and liquid states, whereas the local order in the liquid As₂Te₃ is similar to that in the amorphous state but not in the crystalline state.     

The structure analysis liquid Sb2Se3 by neutron diffraction

The radial distribution analysis of liquid Sb₂Se₃ at 670, 720 and 770°C is carried out by the neutron diffraction method. A small but apparent maximum is found at $\text{K = 1.2}\text{A}\text{?}{}^{\mathrm{?1}}$ in the structure factor. The interatomic distance and the coordination number are 2.74 Å and 2.84 respectively, at 670°C. These results suggest that intramolecular bonds in the crystalline Sb₂Se₃ remain unbroken in the liquid state. The general feature of the structure factor and the distribution function for liquid Sb₂Se₃ is similar to that for the amorphous state. However, it is clear that the liquid phase tends to have a similar atomic arrangement to that of liquid tellurium.     

EXAFS studies of glassy and liquid ZnCl2: A comparison with vitreous GeO2

The Zn-EXAFS (extended X-ray absorption fine structure) above its K-absorption edge has been measured for glassy ZnCl₂ at low temperature, through T_g (375 K), and into the supercooled and normal liquid state (mp = 593 K) at 673 K. By Fourier filtering and fitting the normalized glass spectra using α-ZnCl₂ as a model compound, the Zn²⁺Cl^? distance was determined to be (2.34 ± 0.01) Å and the average coordination number about the Zn²⁺ was found to be 5.1 ± 0.8. The latter value agrees with the value of 4.7 nearest neighbors obtained by the molecular dynamics computer simulation study of Woodcock et al., for liquid ZnCl₂ just above its melting point. The agreement is supportive of the notion that short-range order in the glass is reflective of that of the corresponding liquid from which it was quenched. Spectral measurement as a function of temperature indicates that there is considerable dynamic decoupling of the Zn²⁺ from its nearest Cl^? neighbor even below T_g. This is contrasted with similar data in glassy GeO₂ which show that the motion of Ge is strongly coupled with its four oxygen neighbors all the way to T_g. This difference in dynamic disorder substantiates the notion that glassy ZnCl₂, as well as vitreous BeF₂, provides a further weakened structural analog for glassy GeO₂ and SiO₂.     

Crystallization of Li2OSiO2 glass studied by positron annihilation

The crystallization of 34 Li₂O65 SiO₂1 P₂O₅ glass has been studied by positron annihilation. The changes in the positron lifetime spectra have been correlated to the magnitude of the amorphous X-ray scattering. A linear relationship has been found for heat-treatment temperatures less than 700°C, above which non-linear phenomena in the lifetime spectra set in. The volume crystallinity reaches a saturation value, which varies from 70% at the heat-treatment temperature of 535°C to 80% at 700°C because of the phase separation. The dimensionality of the crystal growth in this glass is found to be n = 1.5 ± 0.1. The effect of the phase separation on the rate and dimensionality of the crystallization is discussed. The results show that positron annihilation is a sensitive and accurate method to follow crystallization in vitreous materials.     

Structural investigation of amorphous FeZr,CoZr and NiZr alloys with low zirconium concentration

A structural investigation has been made of alloy glasses a low concentration of zirconium: compositions M_100?xZr_x with M = Fe, Co and Ni, and x = 9 at.% using X-ray diffraction. The characteristic second peak splitting in the radial distribution function is found for all samples presently investigated. The partial radial distribution functions of amorphous FeZr and CoZr alloys were derived from the measured total distribution function data by applying the concentration method with the anomalous scattering technique. The amplitude in the oscillation of the radial distribution function for M-M and M-Zr pairs is more enhanced in comparison with that of glasses with high Zr content, x = 40–45 at.%. The estimated coordination number of nearest neighbor Fe atoms (11.6 ± 0.5) for iron in the Fe₈₄Zr₁₆ glass is larger than that (10.7 ± 0.5) found in the Fe₈₃B₁₇ glass. This is consistent with the measurements of magnetic properties of these glasses with low zirconium concentration. The possible structural features of intertransition metal alloy glasses with low zirconium concentration is also discussed based on the present experimental data.     

Preparation of Cu2ZnSnS4 single crystals from Sn solutions

We investigated the phase diagrams of the Cu₂ZnSnS₄ (CZTS)–Sn pseudobinary system in order to obtain knowledge useful for the growth of high-quality CZTS single crystals using a solution-based method. For Sn solutions saturated with less than ～60 mol% CZTS, the solutes are separated into two phases (CZTS phase+SnS_x phase+liquid phase). On the other hand, for solutions with more than 60 mol% CZTS, the solutes are single phase (CZTS phase+liquid phase). The CZTS single crystals were obtained from a 70 mol% CZTS solution (liquid temperature 850 °C) at 900 °C. The powder X-ray diffraction (XRD) pattern of the CZTS single crystal shows preferred orientations of (112), (220) and (312) planes, confirming the Kesterite structure of CZTS. The Raman spectrum shows three peaks at 287, 338, 371 cm^?1, which corresponded to CZTS peaks. The composition of the CZTS single crystal along the growth direction is found to be slightly Cu-poor, Zn-rich and S-rich. Therefore, it is assumed that the Cu vacancy is the dominant p-type conduction mechanism.     

High frequency dynamics of liquid bismuth

The dynamic structure factor S(Q, ω) of liquid Bi was measured at 580 K in the Q range from 0.15 to 0.6 Å⁻¹ using inelastic neutron scattering. The obtained spectra clearly demonstrate the existence of well defined longitudinal propagating modes. A positive dispersion is found in the low Q region, where the mode velocity undergoes a transition between the hydrodynamic value and a high frequency value 20% larger. The damping of the excitations does not follow the hydrodynamic Q² trend and is stronger than in any metallic liquid investigated so far. The quasielastic lineshape contains a broad Q-independent Lorentzian contribution, other than a small sharp peak, which has yet been observed in liquid Hg and Ga.     

Formation and stability of nickel-zirconium glasses

Glasses may be formed in two composition ranges: 33–42 and 60–76 at.% Zr. The crystallisation temperatures (T_x), activation energies (Q_x), crystallisation morphologies, densities, Young's moduli and peak in the X-ray diffraction pattern (Q_p) are reported as a function of composition. T_x, Q_x and Q_p do not interpolate smoothly between the two composition ranges. Maximum thermal stability occurs at 36 at.% Zr, well away from the value predicted by the free electron approach of Nagel and Tauc. The glasses are denser than would be expected from a simple dense random packing of hard spheres. Some implications for the structure of all-metal glasses are discussed.     

Structural and magnetic properties of nanocrystalline particles in an amorphous Fe73.5Nb3CuSi13.5B9 matrix

We report structural and magnetic properties of fine particles embedded in an amorphous magnetic matrix. As-quenched amorphous Fe_73.5Nb₃CuSi_13.5B₉ ribbons (FINEMET) were submitted to the thermal treatments of several times (1 ? t ? 240 min) at 570 °C using a conventional furnace. The analyses of the X-ray diffraction patterns at room temperature reveal that our samples consist of single phase Fe₃Si nanocrystals embedded in a residual amorphous phase. Magnetic measurements show that the saturation moment at T = 450 °C increases as a function of annealing time. This behavior is attributed to an increase of the fraction of nanocrystallites in the residual amorphous phase.     

Dispersion of acoustic-like and optic-like vibrational excitations in Ni2B metallic glass

The dynamic structure factor S(Q, E) of Ni₂B metallic glass was studied by inelastic neutron scattering in the ranges of momentum transfers 0.7 < ?Q/Å^?1 < 7, and energy, 2 < E/meV < 70, transfers. The measurements were performed with the IN4 spectrometer (Institut Laue-Langevin (ILL), Grenoble, France) on three samples of the same chemical composition but with different contents of Ni and B isotopes, as a result of which partial contributions were determined. The cuts through S(Q, E) for different values of E and Q demonstrated a characteristic behavior indicating that vibrational excitations, similar to acoustic and optical phonon excitations in crystals, propagate in an amorphous medium in a certain energy and momentum range.     

Solid–liquid interfacial energy of neopentylglycol solid solution in equilibrium with neopentylglycol–(D) camphor eutectic liquid

The grain boundary groove shapes for equilibrated solid neopentylglycol (NPG) solution (NPG–3 mol% D-camphor) in equilibrium with the NPG–DC eutectic liquid (NPG–36.1 mol% D-camphor) have been directly observed using a horizontal linear temperature gradient apparatus. From the observed grain boundary groove shapes, the Gibbs–Thomson coefficient (Г), solid–liquid interfacial energy (σ_SL) of NPG solid solution have been determined to be (7.5±0.7)×10^?8 K m and (8.1±1.2)×10^?3 J m^?2, respectively. The Gibbs–Thomson coefficient versus T_mΩ^1/3, where Ω is the volume per atom was also plotted by linear regression for some organic transparent materials and the average value of coefficient (τ) for nonmetallic materials was obtained to be 0.32 from graph of the Gibbs–Thomson coefficient versus T_mΩ^1/3. The grain boundary energy of solid NPG solution phase has been determined to be (14.6±2.3)×10^?3 J m^?2 from the observed grain boundary groove shapes. The ratio of thermal conductivity of equilibrated eutectic liquid to thermal conductivity of solid NPG solution was also measured to be 0.80.