Phase transition studies in mesogenic dimers

Schiff base liquid crystal dimers, both symmetric and non‐symmetric, in which two anisotropic groups are linked by a flexible spacer, exhibit a rich variety of smectic mesomorphism. The interest in this class of mesogens stems not only from their ability to act as model compounds for semi‐flexible mainchain liquid crystal polymers but also from their quite different properties compared to conventional low molar mass liquid crystals (monomers). We report here the phase transition studies on two examples of these schiff base liquid crystal dimers using the Differential Scanning Calorimetry and density measurements as a function of temperature. The symmetric liquid crystal dimer, α,ω‐bis (4‐n‐dodecylaniline benzylidene‐4′‐oxy)decane (10.O12O.10) exhibits a very rare Isotropic to G transition. Where as, the non‐symmetric dimer, α‐(4‐cyano biphenyl‐4′‐yloxy)‐ω‐(4‐n‐decylanilinebenzylidene‐4′‐oxy)decane (CB.O10O.10) exhibits a rare nematic to intercalated smectic A phase transition. The transitions studied, isotropic to nematic and isotropic to G exhibit a large density jump at the transition confirming their first order nature. Nematic to intercalated smectic A transition is found to be a second order transition whose behavior is similar to nematic to smectic A transition. Calculated values of pressure dependence of transition temperature and thermal expansion coefficient are also reported.     

Nematic-isotropic phase transition in polar liquid crystals. 1. Statistical theory

The theory of transition from the nematic to the isotropic phase for liquid crystals in the system of rodlike particles with large longitudinal dipoles has been developed with due regard for the equilibrium between monomers and antiparallel molecular pairs—dimers. The order parameters of monomers and dimers are determined as well as the dimer fraction. It is shown that, in accordance with the results obtained earlier, for low values of dipole moments, the temperature of the nematic-isotropic phase transition increases with an increase of the dipole moment. However, for large dipoles, the transition temperature starts decreasing with an increase of the dipole moment because of higher dimer concentration. This provides the interpretation of the recent computer simulation, which showed a destabilization of the nematic phase in the system of rigid rods with pronounced central dipoles. The temperature dependence of the dimer fraction is also studied. The qualitative relation between the sign of the jump in the dimer fraction at the transition point and the effect of dimerization on the transition temperature are established.     

Ionic jump processes and high field conduction in glasses

A theory of multiple-ion jumps is proposed to explain the large apparent jump distances which have been calculated from high field conduction in glasses. An analysis of published data on correlation coefficients suggests that multiple ion jumps up to about four ions at a time may occur in single-alkali glasses. In mixed-alkali glasses such processes are shown to be less probable. Measurements of high field conductivity were made on both single- and mixed-alkali glasses, using blown membranes between N/10 acid solutions. The onset of departures from Ohm's law occurred at fields of about 10⁷ V m^?1 from which a jump distance of about 25–30 Å is calculated. This is too large for the proposed multiple-ion jump processes suggested, and, furthermore, the high field currents followed the Poole-Frenkel (PF) law. An alternative theory is given in which the PF law is derived for ionic migration: shallow coulombic potential basins are assumed to be distributed throughout the volume of the glass, owing to non-bridging oxygens whose charge is not balanced by cations on adjacent sites.     

Incorporation of surface tension to interface energy balance in crystal growth

Effect of surface tension across a growth interface is known as the Gibbs‐Thomson effect, and the associated energy balance is widely referred to as the Stefan condition in the literature, which is derived from thermodynamics. In this article, the interface energy balance that accounts for the effect of surface tension is derived by writing the jump condition for the energy balance on a surface of discontinuity which represents in crystal growth the evolving growth interface (solidification front) between the liquid and solid phases. To the best of our knowledge, the derivation of energy balance by writing jump conditions on a surface of discontinuity (interface) is new. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theory of fluidity of liquids, glass transition, and melting

This is a presentation of a rigorous theory of fluidity of liquids, glass transition and melting of solids in the frame of an asymmetric double well potential model. Potential wells are doubled time to time by the local density fluctuations caused by the thermal longitudinal waves. The average frequency of doubling of potential wells is equal to the frequency of the most energetic waves which obey a law similar to Wein’s displacement law in black body radiation. Based on the equilibrium thermodynamic theory of fluctuations and the displacement law, a law of linear pre-diffusion mean-square displacement of particles in a solid is derived: the mean-square displacement of molecules within their potential wells increases linearly with temperature. It is shown that when this is broken-down (where the mean-square displacement at a certain temperature rapidly changes its slope as a function of temperature) glass devitrifies and crystal melts, and all possible solid-liquid transitions of a substance occur at the same critical mean-square displacement: any solid (not only crystals) transforms into liquid when the mean-square displacement, as a fraction of the average intermolecular distance, acquires a certain universal critical value - the same for different substances. It is proved that molecules in a liquid perform specific Brownian motion. The average jump distance is a function of temperature and it is much smaller than the nearest intermolecular distances. At a certain temperature, shown to be the Kauzmann temperature, the average jump distance of Brownian motion becomes equal to zero: the super-cooled liquid undergoes glass transition. The transition was proven to be a phase transition of the fourth order: the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Molecular mobility, diffusion and viscosity are obtained as functions of temperature.     

The liquid state of large clusters with pairwise atomic interactions

Formation of the liquid state for clusters with a pair interaction between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters and from real thermodynamic parameters of the liquid states of condensed inert gases. Phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during the solid-liquid phase transitions in two parts, so that one corresponds to configuration excitation at zero temperature, and the other is a contribution from thermal vibrations of atoms. The latter part contributes from approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms up to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though this phase transition results from configurational excitation of ensembles of bound atoms. It is shown that the void concept allows us to analyze various aspects of the liquid state of clusters including the existence of the freezing temperature below which no metastable liquid state exists, and the properties of glassy states which may exist below the freezing point.     

On the internal nucleation of melting

Crystals of quartz have been superheated by some 450°C and crystals of albite by some 185°C above their respective melting points. In all cases, melting took place by the nucleation of liquid at the external surfaces (and internal boundaries as well in the case of albite). No evidence for the internal nucleation of liquid was found at any superheat for either material.The results of quartz indicate an exceptionallu large barrier to the internal nucleation of liquid. It is suggested that this large nucleation barrier is associated with the strain energy of forming a liquid nucleus within the crystalline phase.It is also indicated that the nucleation of liquid at the external surfaces of crystals at negligible superheats suggests that the free surfaces of liquids do not per se serve as preferred nucleating sites for crystallization — and that the crystal nucleation often observed at external surfaces or internal surfaces is in fact associated with condensed second-phase impurities.     

Synthesis,Characterisation and Phase Transition Studies of 50.9 and 90.5

The synthesis, characterisation of phases of the compounds N(p-n-pentyloxy benzylidene) p-n-nonylaniline, 50.9 and N(p-n-nonyloxy benzylidene) p-n-pentylaniline, 90.5 and phase transition studies using different experimental techniques are presented. The compound 50.9 exhibits a phase sequence NAB whereas 90.5 shows a phase variant ABG. A large density jump across IN transition is observed in 50.9 similar to that observed in 50.10 compound.     

Heterogeneity length scale for oxygen diffusion in glassy squalane

The kinetics of phenanthrene phosphorescence quenching by mobile oxygen molecules was used to study characteristics of oxygen diffusion on a nanometer length scale in glassy squalane. The phosphorescence decay was described using a model of glassy matrix with spatially correlated oxygen jump rates. The correlation length was determined to be 2.5 nm. The dispersion of barrier energy for oxygen jumps is 1.8 kJ/mol and the average barrier energy is 32 kJ/mol. The structural heterogeneities are deduced to be fluctuations in structure frozen in from the supercooled liquid.     

Measurement of the An isotropic Refractive Indices of Polybenzylglutamate Liquid Crystals. Molecular Factors and Dispersion

The ordinary and extraordinary refractive indices have been measured for lyotropic liquid crystal solutions of poly-γ-benzyl-L-glutamate. The onset of the liquid crystal formation is noted as an abrupt jump in the sample birefringence in the biphasic region. The concentration dependence is otherwise unremarkable. The polymer is found to add a small, anisotropic increment to the refractivity of the supporting solvent. Both n _o and n _e can thus be measured in an ordinary Abbé refractometer for the low refractive solvents common to these liquid crystal solutions. The optical birefringence does not depend significantly on polymer molecular weight. Data is also included for the wavelength dependence (dispersion) of n _o, n _e, and Δn.     

The effect of charged additives on the conductivity and the thermopower in liquid selenium

The simultaneous measurements of conductivoty σ and thermopower S for liquid Se and its dilute mixtures containing Na, Tl and Cl have been performed in the temperature and pressure range up to 1500°C and 2200 bar. The addition of Na, Tl and Cl give rise to a substantial increase in σ at low temperatures. The value of S of liquid Se is large positive at low temperature. The sign of S is changed to be negative by addition of Na and Tl. By the addition of Cl the sign of S remains positive and the value increases. The effect of charged additives on the transport properties of liquid Se is discussed in connection with the change of the bonding configuration.     

Fast and slow dynamics in single and mixed alkali silicate glasses

A molecular dynamics (MD) simulation for single and mixed alkali metasilicates has been performed. The mixed alkali effect (MAE) in the diffusion coefficients is reproduced. The motion of lithium ions in Li₂SiO₃ can be divided into slow (type A) and fast (type B) categories clearly in the glassy state. It turns out that the slow dynamics are caused by localized jump motions with a long tail of the waiting time distribution. On the other hand, fast dynamics of the lithium ions are caused by successive cooperative jump motions and are identified as Lévy flights. The MAE in LiKSiO₃ occurs due to mutual interception of jump paths of both kinds of mobile ions. In such paths, the fast component is considerably suppressed. Namely, the cooperative forward-correlated jump process is sensitive to the blockage of the jump paths. Propagation of the immobilization in the MAE is discussed in relation with the mechanism of the cooperative jump motion. The ratio of the fast and slow dynamics plays an important role to determine the transport properties in the glassy state.     

Supercooled Lennard-Jones liquids and glasses: a kinetic Monte Carlo approach

A kinetic Monte Carlo (KMC) method is used to study the structural properties and dynamics of a supercooled binary Lennard-Jones liquid around the glass transition temperature. This technique permits us to explore the potential energy surface without suffering an exponential slowing down at low temperature. We find a transition temperature that separates two distinct regimes around the dynamical transition temperature of mode-coupling theory. Below this temperature the number of different local minima visited by the system for the same number of KMC steps decreases by more than an order of magnitude. The mean number of atoms involved in each jump between local minima and the average distance they move also decreases significantly, and new features appear in the partial structure factor. At higher temperature the probability distribution for the magnitude of the atomic displacement per KMC step exhibits an exponential decay, which is only weakly temperature dependent.     

Numerical optimization of thermal field in CdTe crystal growth by travelling heater method

Cadmium telluride (CdTe) and his compounds play a leading role in X‐ray and γ‐ray detector technology. One of the most used methods for growing these crystals is the travelling heater method (THM). The ingots obtained by using this technique show excellent composition uniformity, but the structural quality is affected by the presence of large grains which appear because of large curvatures of the solid‐liquid interface during the solidification process. This numerical work investigate the thermal field and melt convection in CdTe processing by THM in order to elucidate the mechanism of growing these crystals. The influence of the furnace geometry on the interface shape and temperature gradient in liquid is analyzed for samples with small (1 cm) and large (5 cm) diameters. The computations include flow effects on thermal field in the melted zone. The thermal conditions are optimized for THM growth of CdTe crystals at high solidification temperatures. A new multi‐zone furnace configuration for growing crystals of large diameter and flattened interface is proposed in this work.     

The estimation of concentration fluctuations in liquid Ag–Si and Au–Si alloys

The specific electrical resistivity was measured for liquid states of Ag–Si and Au–Si with deep eutectic points for which the ‘electron atom ratio’ at the eutectic composition is 1.33 and 1.56, respectively. On lowering the temperature, the specific electrical resistivity deviates from the straight line, which was obtained by extrapolating its behavior from high to low temperatures. The concentration dependence of this deviation at the liquidus shows the largest value at the eutectic composition. The analysis based on the effective medium theory tells us that the volume fraction of concentration fluctuation in the homogeneous liquid phase is largest at the eutectic composition. The degree of concentration fluctuation was summarized on one systematic trend among metal–semiconductor eutectic systems. It is concluded that the large concentration fluctuation develops in the homogeneous liquid phase of eutectic systems. The possibility of the poor supercooling tendency of homogeneous liquids at the eutectic composition was revealed with relation to this concentration fluctuation.     

Captured Orientational Order in Polymer Network Assemblies

Abstract

The “marriage” between low molecular weight liquid crystals and polymers burgeoned in the 1980s with the idea of dispersing liquid crystal droplets in a rigid polymer matrix to create an electrically controllable light scattering medium. The orientation of the liquid crystal droplets, and hence the refractive index match and scattering, can be systematically controlled with an electric field. Today, dispersions of liquid crystals and polymers are found in many forms depending on the concentration of polymer, which can be as large as 70% or as small as 1%. The systems most understood are those of larger concentration where the liquid crystal is segregated out in the form of droplets randomly distributed throughout the surrounding polymer (see page 2). Dispersions of liquid crystals and polymers differ from macroscopic bulk liquid crystals because of the large surface-to-volume ratio and symmetry breaking non-planar geometry imposed by the polymer. Their composite nature profoundly affects the ordering of the liquid crystal, and their susceptibility to external fields makes them suitable for many new electro-optic applications, as well as intellectually challenging to study from the basic science perspective.     

A review of measurement of thermophysical properties of silicon melt

Measurements of thermophysical properties of Si melt and supplementary study of X-ray scattering/diffraction by the authors' group were reviewed. The values obtained differed variously from those of literature. Density was 2–3% larger, surface tension 20–30% smaller, viscosity up to 40% larger, electrical conductivity 8% smaller, spectral emissivity more or less in good agreement with literature values, and thermal diffusivity a few percent larger. An anomalous density jump was found near the melting point. Surface tension and viscosity also showed anomaly. A strange time-dependent change of density was observed over 3 h after melting. X-ray analyses suggested a slight change in local atom ordering, but showed no sign of cluster formation. An addition of 0.1 at% gallium caused the density jump to disappear, while that of boron caused no change. An EXAFS study of the former melt indicated a strong interaction between Ga and Si atoms as if molecules of GaSi₃ existed. The implications of the measured properties are a possibility of soft-turbulence in an Si melt in a relatively large crucible, a more complicated manner of intake of oxygen depleted molten Si from the free surface region to underneath the growing crystal, and a relaxation of the melt after melting arising from trapped gas species.     

Amplitude of spatial density fluctuations in glassy propylene carbonate probed by oxygen diffusion

Oxygen translational mobility is very sensitive to structural heterogeneities in molecular glass formers. The heterogeneities manifest themselves as the regions, within which the oxygen jump rates have either the same or similar values. The length of spatial correlation for the oxygen jump rates in glassy propylene carbonate has been determined to be about 1.5 nm [Syutkin et al., J. Chem. Phys. 133 (2010) 074501]. The heterogeneities detected by oxygen diffusion are likely the static density fluctuations associated with a spatially nonuniform distribution of free volume. In the present paper the isothermal dependence of oxygen diffusion coefficient on glass density was measured in propylene carbonate using the technique of phenanthrene phosphorescence quenching by oxygen molecules. Glasses of different densities were produced by annealing samples to the equilibrium state at various temperatures. The amplitude of spatial density fluctuations was calculated assuming that (i) a heterogeneous diffusion is due to density fluctuations, and (ii) the dependence of jump rates on region density is the same as that of oxygen diffusion coefficient on the average glass density. The value for was found to be 0.008 ± 0.003.     

In situ diffraction studies of the phase selection in undercooled Ti–Fe–Si–O melts

A time-resolved in situ determination of the phase formation sequence in undercooled Ti–Fe–Si–O melts was performed using the combination of electromagnetic levitation and energy-dispersive X-ray diffraction of synchrotron radiation. Ti–Fe–Si–O alloys are known to form a large variety of stable and metastable phases, including quasicrystals and approximant phases. Different solidification pathways are found as a function of undercooling, with the primary nucleation of the 1/1 crystal approximant phase from the liquid occurring at the largest undercoolings. The results on phase selection are discussed with respect to the influence of the short-range order in the undercooled liquid on the nucleation behavior of solid phases.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.