Stable Crystalline Phase Research Articles

Nanocrystallization of the amorphous Co-B-Si alloys formed by melt spinning and mechanical alloying

X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy were used to investigate the thermal stability and peculiarities of nanocrystallization upon heating of amorphous Co65.5S18B10 M 6.5 (M = Cr, Fe) alloys obtained by melt-spinning (MS) and mechanical alloying (MA) of mixtures of elemental components. It is shown that, irrespective of the method of manufacture, the amorphous iron-containing alloys, in general, have a lower thermal stability than the chromium-containg alloys. The thermal stability of MA alloys is lower than that of the MS alloys of the same chemical composition. The processes of nanocrystallization during heating of MS and MA amorphous alloys differ. The first crystallize, when heating, through the formation of intermediate phases, and the second, with the formation of stable crystalline phases directly from the amorphous matrix. After the crystallization, the alloys have a nanocrystalline structure with a crystallite size of about 20 nm and the same phase composition of crystallization products for the alloys of the same nominal composition obtained by MA and MS: ɛ-Co, Co2B, and β-Co2Si for Co65.5Si18B10Cr6.5 and α-Co, Co2B, β-Co2Si, and (Fe,Co)3Si for Co65.5Si18B10Fe6.5. The results are discussed in terms of the excess free volume of the amorphous phase and of the value of its specific surface area.

Study of Polymorphism of 4-Hexyl-4′-isothiocyanatobiphenyl by Complementary Methods

The results of adiabatic calorimetry, mid-infrared spectroscopy, X-ray diffraction, and polarized microscopy of 4-hexyl-4'-isothiocyanatobiphenyl are presented. Apart from the well-known isotropic liquid and smectic phases, calorimetric measurements indicate the existence of two stable crystalline phases. Quantitative analysis of infrared bands enabled the determination of phase transitions, observation of crystallization kinetics, and elucidation of the glass transition of the crystal-like smectic E phase. The structural information about the glass of the smectic E obtained from the diffraction studies is complemented by textures recorded by polarized microscopy.

Phase Transformation of Sulfamerazine Using a Taylor Vortex

A Couette–Taylor (CT) crystallizer was used to demonstrate the unique fluid dynamic properties of a Taylor vortex for the phase transformation of sulfamerazine (SMZ). With a conventional Rushton mixing tank (MT) crystallizer, the phase transformation from a metastable crystalline phase to the stable crystalline phase took more than 60 h with acetonitrile (ACN) as the solvent and an agitation rate of 3000 rpm. Using a CT crystallizer, this phase transformation occurred within 3–7 h with rotation speeds in the CT crystallizer of 300–1000 rpm. Increasing the rotation speed of the CT crystallizer also significantly enhanced the phase transformation, whereas adding water to the solvent increased the solubility difference between the two polymorphs and accelerated the phase transformation in both crystallizers. The phase transformation in the CT crystallizer was always many times faster than that in the MT crystallizer. Nucleation and mass-transfer models were used to describe the nucleation induction time of t...

Liquid Crystalline Order and Magnetocrystalline Anisotropy in Magnetically Doped Semiconducting ZnO Nanowires

Controlled alignment of nanomaterials over large length scales (>1 cm) presents a challenge in the utilization of low-cost solution processing techniques in emerging nanotechnologies. Here, we report on the lyotropic liquid crystalline behavior of transition-metal-doped zinc oxide nanowires and their facile alignment over large length scales under external fields. High aspect ratio Co- and Mn-doped ZnO nanowires were prepared by solvothermal synthesis with uniform incorporation of dopant ions into the ZnO wurtzite crystal lattice. The resulting nanowires exhibited characteristic paramagnetic behavior. Suspensions of surface-functionalized doped nanowires spontaneously formed stable homogeneous nematic liquid crystalline phases in organic solvent above a critical concentration. Large-area uniaxially aligned thin films of doped nanowires were obtained from the lyotropic phase by applying mechanical shear and, in the case of Co-doped nanowires, magnetic fields. Application of shear produced thin films in which the nanowire long axes were aligned parallel to the flow direction. Conversely, the nanowires were found to orient perpendicular to the direction of the applied magnetic fields. This indicates that the doped ZnO possesses magnetocrystalline anisotropy sufficient in magnitude to overcome the parallel alignment which would be predicted based solely on the anisotropic demagnetizing field associated with the high aspect ratio of the nanowires. We use a combination of magnetic property measurements and basic magnetostatics to provide a lower-bound estimate for the magnetocrystalline anisotropy.

Shape‐ and Size‐Controlled Synthesis of Uniform Anatase TiO2 Nanocuboids Enclosed by Active {100} and {001} Facets

AbstractUniform anatase TiO2 nanocuboids enclosed by active {100} and {001} facets over a wide size range (60–830 nm in length) with controllable aspect ratios were solvothermally synthesized through hydrolysis of titanium tetraisopropoxide (TTIP) using acetic acid (HAc) as the solvent and the ionic liquid 1‐butyl‐3‐methylimidazolium tetrafluoroborate ([bmim][BF4]) as the capping agent. The size and aspect ratio of the anatase TiO2 nanocuboids can be readily adjusted by changing the composition parameters including the contents of [bmim][BF4], water, and HAc in the quaternary solution system. It was revealed that [bmim][BF4] played an important role in stabilizing both the {100} and {001} facets of the anatase TiO2 nanocuboids. On the one hand, [bmim][BF4] acted as a fluoride source to release F− ions for stabilizing the {001} facets; on the other hand, the [bmim]+ ions acted as effective capping ions to preferentially stabilize the {100} facets. The obtained near‐monodisperse anatase TiO2 nanocuboids exhibited an interesting self‐assembly behavior during deposition. These single‐crystalline anatase nanocuboids showed extremely high crystalline phase stability, retaining the pure phase of anatase as well as the morphology even after being calcined at 900 °C. Moreover, the anatase nanocuboids exhibited considerably enhanced photocatalytic activity owing the wholly exposed active {100} and {001} facets.

Thermodynamic model and physical properties of selected zirconia containing silicate glasses

The compositional dependence of density, molar volume, thermal expansion, glass transition temperature, refractive index, and molar refractivity of glasses with composition xNa2O·(15 − x)K2O·yCaO·(10 − y)ZnO·zZrO2·(75 − z)SiO2 (x = 0, 7.5, 15; y = 0, 5, 10; z = 0, 1, 3, 5, 7) was analyzed. The studied glasses were described by the thermodynamic model of Shakhmatkin and Vedishcheva considering the glass as an equilibrium ideal solution of species with stoichiometry given by the composition of stable crystalline phases of respective glass forming system. Property–composition relationships were described by the regression approach considering the particular physical quantity as multilinear function of molar amounts of system components. The classical approach where the mole fractions of individual oxides are considered as independent variables was compared with the thermodynamic model. On the basis of statistical analysis there was proved that the thermodynamic model is able to describe the composition–property relationships with higher reliability. Moreover, due its better physical justification, thermodynamic model can be even used for predictive purposes.

Restructuring mechanism of NbO 6 octahedrons in the crystallization of KNbO 3 in supercritical water

Fine particles of KNbO 3 can be synthesized on a time scale of seconds in supercritical water with a flow-type reactor. The restructuring of NbO 6 octahedrons was found to be a primary factor in the mechanism of KNbO 3 crystallization in high-temperature aqueous environments. Two different crystallization routes exist depending on the conditions of water. The ion product of water affects whether the restructuring of edge-sharing NbO 6 octahedrons to corner-sharing NbO 6 octahedrons proceeds directly or stepwise according to the degree of dehydration. The direct crystallization route provides polygonal-shaped KNbO 3 particles via direct restructuring, namely, homogenous nucleation from supercritical water that is strongly correlated with a low ion product. The stepwise crystallization route provides irregularly shaped KNbO 3 particles via restructuring in stages to produce an intermediate K 4Nb 6O 17 phase that is correlated with a high ion product. As the ion product of water decreases in high-temperature water, the stability of crystalline phase is in order of KNbO 3 > K 4Nb 6O 17 > K 4Nb 6O 17·3H 2O > defect pyrochlore due to the suppression of dehydration. Water acts to not only dominate the restructuring mechanism of NbO 6 octahedrons but also to provide efficient transport of K + and OH − ions that allows rapid restructuring of NbO 6 octahedrons during KNbO 3 crystallization.

Morphology Evolution and Structural Transformation of Solution-Processed Methanofullerene Thin Film under Thermal Annealing

The film morphology and nanostructure of the soluble fullerene, [6,6]-phenyl-C(61) butyric acid methyl ester (PCBM), are crucial for its applications in organic thin film devices, such as organic solar cells and organic thin film transistors. In this work, the morphology, structural transformation, and crystallite orientation of PCBM film under thermal annealing as a function of annealing temperature, processing solvents, and solution concentrations are systematically investigated. Crystalline PCBM films with needle-like crystallites, axialitic aggregates, and faceted slices are formed in the annealing process. The axialites, made up of needle-like aggregates, are proposed to be partially developed spherulites frozen at the early growth stage formed through low-angle branching. The faceted slices are found to be PCBM single crystallites with hexagonal packing in the film plane. The film undergoes both amorphous-to-crystalline and crystalline-to-crystalline phase transformations as the annealing temperature is increased. The former transformation, corresponding to the self-organization of disordered PCBM molecules in the kinetically frozen films, occurs at a relative low temperature once the motion of these molecules is thermally activated, whereas the later one, corresponding to the transformation between two thermally stable crystalline phases, occurs when further increasing the annealing temperature. The PCBM crystallites composing these films are found to have an orientation preference normal to the film surface, which can be attributed to the confinement of film thickness for PCBM crystallite growth.

Preparation and Transformation of True Nifedipine Polymorphs: Investigated with Differential Scanning Calorimetry and X-Ray Diffraction Pattern Fitting Methods

The amorphous → metastable and metastable → stable crystalline phase transitions of nifedipine and their relationship with polymorph composition during storage at controlled temperature/humidity conditions were investigated. Metastable form C was produced from both differential scanning calorimetry (DSC) thermal treatment and storage [22 °C/0% and 75% relative humidity (RH)] of the amorphous form. Amorphous conversion rate accelerated with storage temperature up to 40 °C, but a further 8 °C increase to 48 °C (3 °C above the glass transition) resulted in a more than 12-fold decrease in amorphous conversion rate. DSC and X-Ray diffraction (XRD) analysis revealed a faster amorphous conversion rate relative to the metastable crystal transformation with 75% RH having a greater accelerative effect on the former. Relative phase quantification from XRD pattern fitting included the use of integrated peak intensities of the crystalline phases, Rietveld and the Rietveld-based partial or no known crystal structures method. Kinetic analysis with Johnson-Mehl-Avrami equation indicated that the accelerated amorphous conversion in 75% RH was associated with a 10-fold increase in rate constant with dimensional growth little affected. The smaller rate increase for metastable crystal conversion was associated with an increased dimensional growth while the rate constant was little affected.

The effect of lithium addition on aluminum-reinforced α-LiAlO 2 matrices for molten carbonate fuel cells

The effect of lithium hydroxide (LiOH) addition as a lithium source is discussed as a way to prevent Li-ion shortages in aluminum-based α-LiAlO 2 matrices of molten carbonate fuel cells. Our results show that the use of LiOH as a lithium source to prevent a Li-ion shortage caused by a lithiated Al-reaction during the operation of the cell allows for more stable performance and greater durability than when lithium carbonate (Li 2CO 3) is used as the lithium source. The behavior of high-lithium content mixtures is attributed to the presence of reactive aluminum particles, which promote the formation of lithium aluminate (LiAlO 2) phases at 650 °C. The incorporation of low-melting-point lithium and an efficient pathway to the aluminum in a reinforced matrix has improved the in-situ mechanical strength via the lithiated Al-reaction, and they do not lead to any noticeable loss in cell performance, even after 4000 h of operation. From the post-test results, the cell with LiOH stored in the cathode channel shows effective formation of the stable crystalline phase of α-LiAlO 2 and enhancement of the mechanical strength during cell operation.

Glass forming ability and crystallization of Zr–Cu–Ag–Al–Be bulk metallic glasses

The glass forming ability of Zr 46Cu 37.64− x Ag 8.36Al 8Be x ( x = 0, 6 and 10 at.%) bulk metallic glasses (BMGs) were significantly improved by Be addition. The critical size of amorphous rods can be over 35 mm diameter. The high GFA achieved is mainly due to the decrease of melting point and liquidus temperature, and suppression of the formation of crystalline phases during solidification from liquid state. The high stabilization with supercooled liquid regime of 115 K was found for the BMG with x = 10 at.%. Two independent exothermic events happen in x = 0 and 6 at.% BMGs, corresponding to the formation of primary crystalline phases Cu 10Zr 7 and AgZr, then transforming to final stable crystalline phases Zr 2Cu and AlCu 2Zr. However, in the x = 10 at.% BMG, the precipitation of primary phases and transformation to final stable phases are within the first exothermic event and the AlCu 2Zr phase is totally suppressed.

The structure and melting transition of two-dimensional colloidal alloys

We study theoretically the structure and melting transition of two-dimensional (2D) binary mixtures of colloidal particles interacting via a dipole–dipole potential. Using a lattice sum method, we find that at zero temperature (T = 0) the system forms a rich variety of stable crystalline phases whose structure depends on the composition and dipole moment ratio. Using Monte Carlo (MC) simulations, we also find that the melting temperature of the different T = 0 structures is a very strong and non-monotonic function of composition. For example, from a direct analysis of the radial distribution function vs.temperature, we find that the melting temperature of hexagonal AB2 and AB6 phases is three orders of magnitude higher than that of hexagonal AB5. Finally the melting transition for our binary colloidal system is found to proceed via at least two stages for hexagonal AB2 and AB6 and at least three stages for hexagonal AB5 and is thus much richer compared to the melting transition of 2D one component colloidal systems.

Atomic structure of nanoscale quasicrystal-forming Zr–noble metal binary metallic glasses

We report the results of the local structural evaluation and mechanism of QC formation in the Zr 70Pd 30 and Zr 80Pt 20 glassy alloys. Voronoi analysis indicates the difference of local environment between two alloys. The perfect icosahedron frequently exists around Zr atom and major polyhedra have prism-like structure around Pd in Zr 70Pd 30. In contrast, icosahedral-like distorted polyhedra formation is favorable around Pt as well as Zr in Zr 80Pt 20. It is therefore, concluded that the quasicrystallization originates from the medium-range order based on the Zr-centered perfect icosahedron and the Pd-centered prism-like ones remain during the QC phase formation in Zr 70Pd 30. Icosahedral-like local structure around Zr and Pt might contribute together to the nucleation of QC phase in Zr 80Pt 20. This feature with a different mechanism of QC formation in the two alloys may correlate to the difference of solute concentration and the structure of stable crystalline phase after the decomposition of QC phase.

Self‐Assembly of a Low‐Symmetry Monodendron Containing Two Asymmetrically Linked Molecular Wedges

Herein, we describe the synthesis of a low-symmetry monodendron, 3,4-bis(dodecyloxy)-5-[3,4,5-tris(dodecyloxy)benzyloxy]benzoic acid, following a simple route which starts from gallic acid ethyl ester and does not require any protecting groups. The self-assembled structures formed by the compound in 3D and 2D were investigated by synchrotron X-ray scattering and scanning force microscopy (SFM). In 3D, the compound forms a stable crystalline phase with an orthorhombic lattice in which the alkyl chains connected to different benzene rings form crystalline and amorphous domains. Upon cooling from the isotropic melt the compound exhibits a monotropic smectic mesophase. In 100-nm-thick films on a neutral substrate the structure loses its biaxiality, adopting a hexagonal columnar structure with the columns oriented parallel to the substrate. By contrast, in ultrathin films on graphite the SFM likely reveals two crystal orientations, which can develop due to the epitaxial adsorption on the substrate of the alkyl chains pertinent to different benzene rings.

Conformational changes in novel thermotropic liquid crystalline polymer without conventional mesogens: A Raman spectroscopic investigation

Raman spectroscopy was applied to study the conformational changes of poly[di(alkyl) vinylterephthalate] (alkyl = n-butyl and sec-butyl). Spectral regions assigned to C–H, C–C and ester carbonyl stretching modes were utilized to provide information on the molecular motion, the relative content of trans and gauche conformers, and the specific interactions existed in the mesophase formation process of these novel thermotropic liquid crystalline polymers. Both the experimental results and the conformational analysis suggested that the aliphatic side groups do have a significant impact on the formation of stable liquid crystalline phase. Based on the van’t Hoff relation, the thermodynamic parameters in the phase transition of poly[di(n-butyl) vinylterephthalate] were estimated (ΔH = 5.52 kJ mol−1, ΔS = 14.02 J mol−1 K−1), which were much smaller than the typical values generally obtained in first-order phase transition. Based on generalized two-dimensional correlation Raman analysis, specific interaction among the phenyl-ring in the mesophase development process was elucidated.

Optical applications of glass-ceramics

Glass-ceramics are usually formed through passing several stages of phase transformations (nucleation, crystallization of metastable and stable crystalline phases). The properties of glass-ceramics depend on at what stage the process was stopped. The phase assemblage of glass-ceramics is controlled by the composition of the initial glass and heat-treatment conditions. Raman and optical spectroscopy, in combination with X-ray diffraction and small angle X-ray scattering techniques are used to study the behavior of the nucleation and crystallization of lithium, magnesium and zinc aluminosilicate glasses. Specific heat-treatments has given the opportunity to achieve a desired phase assemblage ensuring unique properties of glass-ceramics by interrupting the process at a certain stage. Examples of such glass-ceramic developments include thermal shock resistant color filters, passive Q-switchers for eye-safe Er:glass lasers, as well as transparent materials based on rare-earth titanates with pyrochlore structure with luminescence in the range of 1.5 μm. The approach to developing the dual active–passive media for the eye-safe spectral region is demonstrated for the first time.

Polymorphism, crystal nucleation and growth in the phase-field crystal model in 2D and3D

We apply a simple dynamical density functional theory, the phase-field crystal (PFC) modelof overdamped conservative dynamics, to address polymorphism, crystal nucleation, andcrystal growth in the diffusion-controlled limit. We refine the phase diagram for 3D, anddetermine the line free energy in 2D and the height of the nucleation barrier in 2Dand 3D for homogeneous and heterogeneous nucleation by solving the respectiveEuler–Lagrange (EL) equations. We demonstrate that, in the PFC model, thebody-centered cubic (bcc), the face-centered cubic (fcc), and the hexagonal close-packedstructures (hcp) compete, while the simple cubic structure is unstable, and that phasepreference can be tuned by changing the model parameters: close to the critical point thebcc structure is stable, while far from the critical point the fcc prevails, with anhcp stability domain in between. We note that with increasing distance fromthe critical point the equilibrium shapes vary from the sphere to specific facetedshapes: rhombic dodecahedron (bcc), truncated octahedron (fcc), and hexagonalprism (hcp). Solving the equation of motion of the PFC model supplied withconserved noise, solidification starts with the nucleation of an amorphous precursorphase, into which the stable crystalline phase nucleates. The growth rate is foundto be time dependent and anisotropic; this anisotropy depends on the drivingforce. We show that due to the diffusion-controlled growth mechanism, which isespecially relevant for crystal aggregation in colloidal systems, dendritic growthstructures evolve in large-scale isothermal single-component PFC simulations. Anoscillatory effective pair potential resembling those for model glass formers has beenevaluated from structural data of the amorphous phase obtained by instantaneousquenching. Finally, we present results for eutectic solidification in a binary PFC model.

Links between Carbonatite and Kimberlite Melts in Chloride–Carbonate–Silicate Systems: Experiments and Application to Natural Assemblages

Recent data suggest an active role for chloride-bearing alkali carbonatitic melts in the formation and evolution of kimberlites, whereas experiments indicate that chlorides could be responsible for liquid immiscibility during melting of carbonated mantle rocks. This study considers melting trends in kimberlite-related chloride- carbonate-silicate systems at a pressure of 5.5 GPa.The direction of these trends largely depends on the chlorine concentration in the system. All trends start with a Cl-rich carbonatitic liquid coexisting with crystalline phases. Melting of a peridotite-carbonate-chloride system containing 4.4 wt % Cl results in a gradual transition from a carbonatitic melt (5-7 wt % SiO2, MgO/CaO= 0.5-0.6, ~2 wt%Cl) at 1000-1100 oC through a Cl-rich carbonate-silicate melt (12-15 wt % SiO2, MgO/CaO =0.6, and up to 14 wt % Cl) at 1360-1400 oC towards a Cl-bearing ultrabasic carbonate- silicate melt (UCSM), i.e. kimberlite-like (26-29 wt % SiO2, MgO/CaO=1.5-2.8, and 6-4 wt % Cl), at 1500-1600 oC.This trend results from the specific behavior of chlorides, which are found to be stable crystalline phases up to about 300 oC above the solidus. In addition, the trend touches a miscibility gap at about 1450 oC, although immiscibility does not significantly influence the melt evolution. In contrast, the melting trend of a peridotite system with about 1.3 wt%Cl does not intersect the miscibility gap and proceeds from a carbonatitic melt (~5 wt % SiO2, MgO/CaO= 0.4-0.5, ~2wt % Cl) at 1100-1150 oC toward the UCSM (~25 wt % SiO2, MgO/CaO= 1.7-1.8, and 0.6-1•0 wt % Cl) at 1500- 1600 oC. Chloride-carbonate-silicate systems containing 17 wt % Cl show an abrupt transition from the chloride-carbonate liquid toward the UCSM because of the immiscibility gap between carbonate- silicate and chloride-carbonate melts. Melting relations in all studied chloride-carbonate-silicate systems are exclusively regulated by peritectic reactions between the silicate phases and carbonate constituents (mostly CaCO3) of the melts. Chlorides decrease the solidus temperatures of carbonated peridotites, but do not reduce the temperature interval for the transition from carbonate- to silicate-dominated melts. The experimental results suggest that Cl-rich carbonatite liquids preserved in diamonds and the Cl-rich kimberlites of the Udachnaya-East kimberlite pipe (Yakutia) could represent a linked system of chloride-rich liquids that evolved over a wide temperature interval during incipient melting and evolution of kimberlite magma from a carbonated mantle source containing 3-4 wt % Cl.

Anthraquinone-Based Discotic Liquid Crystals

The syntheses of six room-temperature discotic liquid crystals based on an alkoxy-anthraquinone (AQ) framework is described. Differential scanning calorimetry, X-ray diffraction (XRD), and cross-polarized microscopy were used to identify phases and confirm phase-transition temperatures. Cross-polarized microscopy results suggest columnar discotic structures at room temperature. However, the AQ derivatives also undergo mesophase-to-mesophase transitions, which are attributed to rectangular- to hexagonal-columnar discotic transitions based on XRD analysis. Furthermore, some of the compounds display remarkable liquid crystalline phase stability that spans from -50 to 150 degrees C, a useful temperature range for organic materials applications. The different AQ derivatives did not exhibit electronic perturbations, as all compounds have absorption onsets of approximately 400 nm. Finally, solution cyclic voltammetry of the AQ derivatives was carried out to determine the redox potentials, diffusion coefficients, and electron transfer rate constants. All AQ derivatives had E(1/2) values that ranged between -1.52 and -1.70 V vs Fc/Fc(+). Diffusion coefficients and electron transfer rates for all AQ derivatives ranged between 0.4 and 7.1 x 10(-6) cm(2) x s(-1) and 0.9 and 5.2 x 10(-3) cm x s(-1), respectively.

Statistical theory of thermodynamic stability of crystalline phases

The equation of state of a face-centered cubic phase has been quantitatively analyzed in terms of the statistical theory of crystals. It has been shown that, for xenon at room temperature, the pressure equal to 1.5 GPa determines the instability point where the condition of the positive bulk modulus of a face-centered cubic crystal is violated. A “universal line” bounding the thermodynamic stability region of the face-centered cubic phase of van der Waals crystals has been constructed. An analysis of the data available in the literature allows the conclusion that the revealed transition of face-centered cubic xenon to the martensitic phase at a pressure of 1.5 GPa and a temperature of 300 K can be considered a manifestation of the predicted instability. In this respect, it is important to perform detailed experiments on polymorphic transformations of real xenon (and also krypton). Another aspect of the proposed theory is that it provides a means for quantitatively predicting the characteristics of the so-called “cold” (at negative pressures) melting, which recently has become accessible for experimental observation.