Amorphous Liquid Phase Research Articles

PCA Analysis of In Situ X-ray Powder Diffraction and Imaging Data Shedding New Light on Solid-State Transformations: The Crystallization of Low Temperature Eutectic Mixtures

Eutectic mixtures are usually studied by differential scanning calorimetry (DSC), able to identify the transition temperatures, possible hysteresis, and investigate the energetic features of transformations. However, DSC is not able to give compositional, structural, or morphological information. A new approach is proposed exploiting powder X-ray diffraction (XRPD) and imaging to overcome the issues posed to diffraction by the presence of an amorphous liquid phase. Principal component analysis (PCA) is applied blindly to in situ XRPD data from both solid and liquid phases in an approach called differential scanning diffraction (DSD), with PCA scores being the reaction coordinate of melting or crystallization steps. PCA was used in a similar way to analyze the imaging data in what was named differential scanning imaging (DSI). Exploiting this approach, the structural and morphological changes during phase transitions can be characterized by XRPD and imaging respectively, complementarily to the energetic effects probed by DSC. Melting and crystallization points can be identified together with the hysteresis between downward and upward temperature ramps, by the structural and morphological viewpoints. A three-component mixture (NaBr, KCl, and water), with wide industrial applications, was studied to describe the behavior around the eutectic composition and examine how small mixture changes can affect the transition temperature and the freezing/melting behaviors. The phase composition at the solid state was elucidated and a new phase of NaBr was identified and its lattice parameters were obtained by XRPD. DSD and DSI resulted complementary to traditional DSC data with many potential applications in solid state chemistry and materials science.

An Equation of the State and Surface Properties of Amorphous Iron

It has been shown that three singular points related to an amorphous state can be distinguished in the dependence of the first coordination number kn on packing coefficient kp in the structure of a unicomponent material. The equation of state and properties of iron for both these three amorphous structures and the crystal state are calculated on the basis of the Mie–Lennard-Jones pair interatomic potential. It is shown that the chemical potential becomes minimal at kp = 0.45556 and kn = 6.2793; that is, this packing can be assigned to a thermodynamically stable amorphous structure corresponding to a liquid phase. A point that is energetically equivalent to this phase and has the same value of kn but another value of kp (0.6237) is a thermodynamically unstable amorphous structure corresponding to a solid phase. It was shown that the specific surface energy of an amorphous solid metal is higher than that of the amorphous liquid phase but is lower than the specific surface energy of a crystalline metal. This should lead to the fact that the surface of a crystalline metal will tend to amorphization.

On the properties of the amorphous state of a single-component substance

It is shown that on the nonlinear dependence of the first coordination number (kn) versus the packing coefficient (kp) of the structure of a single-component substance, three special points corresponding to the amorphous structures can be distinguished. Based on the pairwise interatomic potential of Mie–Lennard-Jones, the state equation and properties of iron for both these three amorphous structures and the crystal state are calculated. It is shown that at kp = 0.45556 and kn = 6.2793 the minimum chemical potential is reached, i.e. this point is corresponding a thermodynamically stable amorphous structure into the liquid phase. An energetically equivalent point with the same kn value, but with kp = 0.6237, is a thermodynamically unstable amorphous structure corresponding to the solid phase. The following properties are calculated for amorphous iron: specific volume, chemical potential, entropy, Debye temperature, elastic modulus, thermal expansion coefficient, isobaric heat capacity, surface energy, and Poisson's ratio. It is shown that the specific surface energy of an amorphous solid metal is greater than that of an amorphous liquid phase, but less than that of a metal in the crystalline state. This should lead to the fact that the surface of the crystal metal should tend to amorphization.

Structural changes during crystallization and vitrification of dilute FCC-based binary alloys

Structural mechanisms causing local coordination changes during crystallization and vitrification in dilute face-centered-cubic (FCC) alloys was investigated using model Al-Sm, which also serves as prototype for lightweight Al-rare earth (RE) structural alloys. Molecular dynamics simulations were performed to study the solidification behavior of Al-1at.%Sm and Al-5at.%Sm at 1010, 1011 and 1012 K/s cooling rates. Two structural features were identified from these simulations, which are related to the molten state and the glass transition (Tg). In case of Al-1at.%Sm, we learn that, near the melting point, liquid phase manifested pockets of unique transitional structures comprising triangular arrangements in near-parallel layers that encapsulated a FCC-HCP coordinated core. We defined such a structure as the pre-critical nucleus, which is contained within an otherwise predominantly uncoordinated amorphous liquid phase. However, within the range of cooling rates employed, Al-5at.%Sm manifested only amorphous structure after solidification. The liquid structure in the transitional state contained temperature dependent icosahedron clusters that manifested as double-peak in the radial distribution function. Near the Tg Al-5at.%Sm achieves additional local ordering via the formation of inter-penetrating icosahedral frameworks. Combined, the structural mechanisms identified in Al-1at.%Sm and Al-5at.%Sm opens new alloy design pathways by (i) stabilizing pockets of pre-critical nucleus to form refined solidification microstructure, and (ii) tuning Tg in the solid amorphous phase, respectively.

Уравнение состояния и поверхностные свойства аморфного железа

It is shown that on the nonlinear dependence of the first coordination number (kn) versus the packing coefficient (kp) of the structure of a mono-component substance, three special points corresponding to the amorphous structure can be distinguished. Based on the pairwise interatomic potential of Mie–Lennard-Jones, the state equation and properties of iron for both these three amorphous structures and the crystal state are calculated. It is shown that at kp = 0.45556 and kn = 6.2793 the minimum chemical potential is reached, i.e. this point is corresponding a thermodynamically stable amorphous structure into the liquid phase. An energetically equivalent point with the same kn value, but with kp = 0.6237, is a thermodynamically unstable amorphous structure corresponding to the solid phase. It is shown that the specific surface energy of an amorphous solid metal is greater than that of an amorphous liquid phase, but less than that of a metal in the crystalline state. This should lead to the fact that the surface of the crystal metal should tend to amorphize.

Amorphous liquid phase induced synthesis of boron nitride nanospheres for improving sintering property of h-BN/ZrO2 composites

Hexagonal boron nitride (BN) nanospheres with large quantities and high crystallinity have been successfully synthesized from BxCyNzOkHl precursors without any catalyst. The uniform size of the obtained BN nanospheres is about 20 nm. The structure, morphology and surface chemical property of the products were characterized using XRD, FTIR, SEM, HRTEM and XPS. The results show that BN nanospheres with good crystallinity are prepared by heat treatment of amorphous BxCyNzOkHl precursor at 1100 °C in flowing N2. The urea content has great influence on the crystallinity and morphology of BN nanospheres. The diameter and morphology of BN nanospheres dramatically vary at different temperature. Based on the amorphous liquid phase encapsulated induce model, the reaction mechanism and morphological evolution mechanism were described. The viscous liquid containing B and N amorphous phase plays a crucial role in the formation of BN nanospheres. The BN nanospheres improved the density of h-BN/ZrO2 composites from 94.43% to 97.02%.

High-temperature ionic and electronic resistivity of MgO- and Ta2O5- doped aluminum nitride

In this work, using high-temperature impedance spectroscopy and microstructure analysis, we investigated the ionic and the electronic transport properties of aluminum nitride materials doped with MgO and Ta2O5 at temperatures up to 773 K. The electronic conductivity, due to the electron carrier, was greatly inhibited by addition of MgO, which might be due to the decreased electron carrier concentration via electronic compensation of MgO in the AlN matrix. The ionic conductivity due to grains of MgO-doped AlN increased by several orders of magnitude due to ionic defects generated by MgO substitution, whereas the ionic conductivity of the grain boundary of MgO-doped AlN decreased by one order of magnitude as a result of the formation of Mg′Al defects in the grain boundary, which elevated the Schottky barrier. The microstructural analysis showed that MgO addition promoted formation of an amorphous liquid phase including Mg, which is evidence of the selective precipitation of Mg in the grain boundary. Ta2O5-doped AlN also exhibited a decreased ionic conductivity of the grain boundary, which might have been due to the formation of an ionic pair of (ON·−V‴Al) caused by the dissolution of Ta in the AlN matrix.

Analysis of the transformation behaviors of a Chinese coal ash using in‐/ex‐situ XRD and SEM‐EXD

AbstractCrystal and phase transitions of a Chinese Datong coal ash were evaluated using room temperature X‐ray diffraction (RT‐XRD), in‐situ high temperature (HT)‐XRD, and scanning electron microscope combining energy dispersive X‐ray analysis (SEM‐EDX) as temperature varied from 300 °C to 1600 °C. The transforming behaviors of ash were related to its melting properties. Results revealed that the transforming behavior of minerals from in‐situ HT‐XRD was different with that from routine RT‐XRD. Low‐temperature Datong ash contained quartz and clays (such as kaolinite); the changes in minerals at low‐temperature region (300–1000 °C) were mainly due to the minerals decomposition and partial phase transition. Significant transition of quartz to cristobalite occurred at ca. 1100 °C; the continuous transition and reaction of silica and alumina at ca. 1200 °C accelerated the softening of the ash (ST ≈ 1290 °C). An X‐ray amorphous liquid phase was formed mainly by the eutectic solution of silica and mullite at ca. 1400 °C, which obviously affected the hemisphere temperature (HT ≈ 1490 °C) of ash; meanwhile, ultra fine alumina solid particles, which were probably derived from kaolinite, were found in the melted ash. Virtually, the fusing (FT > 1500 °C) of Datong ash was considered to be determined by the formation of solid particles and eutectic solution. Eventually, the semi‐quantitative evaluation of transformations of ash was successfully obtained through the combined methods. © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

Transmission Electron Microscopy Microstructure of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 Ceramics

Microstructural characterizations using transmission electron microscopy on 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics sintered at 1030°–1150°C for 2 h were carried out. The liquid phase was found at the triple junction of the grains in all specimens and abnormal grain growth occurred in the presence of the liquid phase. Abnormally grown grains whose shapes were cuboidal were well developed. Anisotropically faceted amorphous liquid phase pockets were observed inside the grain in a specimen sintered at 1060°C for 2 h. The interface between the grain and the liquid matrix was flat and some were identified to be {100} planes of the grains. A certain amount of liquid at the sintering temperature of 1060°C enhanced the abnormal grain growth and contributed to the improvement of the piezoelectric properties.

Liquid Crystallinity in Relation to Composition and Temperature in Amphiphilic Systems

Abstract The purpose of this paper is to give a broad review of the “fused” type of liquid crystalline phases formed by amphiphilic compounds and to examine the constitutional relationships of these phases to one another and to the amorphous liquid phase. Further, an account will be given of how, on changes in composition and temperature, the various phases undergo reversible interconversions through first-order phase transitions. It will be shown how these transitions follow a regular pattern which, qualitatively, can be satisfactorily interpreted on a molecular basis.