Peculiarities of the initial transient process of binary melt crystallization

The time dependence of the crystallization rate V(t) in the transient process that occurs during crystal pulling into a cold zone at a constant rate W has been investigated within the one-dimensional time-dependent model of binary melt solidification using numerical and analytical approaches. It is shown that the character of the dependence V(t) in this process is mainly determined by the parameter B _w = V _c /W, where V _c is the crystallization rate at which the constitutional supercooling zone is formed in the melt. At B _w ≥ 1, the function V(t) monotonically approaches its steady-state value W, while at B _w < 1 V(t) is generally nonmonotonic. The nonmonotonic character of V(t) can manifest itself both as aperiodic damping and in the form of damping oscillations. At B _w ∼ 0.1, the vibrational amplitude of the crystallization rate in the initial transient process may reach several tens of a percent of W.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Effect of Neutron Irradiation and Annealing on the Intensity of the Copper Related 1.01 eV Emission Band in n-type GaAs

Effect of neutron irradiation (E = 2 MeV, ϕ ≤ 10¹⁵ n/cm²) and subsequent annealing (T ≤ 700 °C, t = 30 min) on the intensity of the copper-related peaked at hvm =1.01 eV emission band in n-type GaAs (n₀ = 2 × 10¹⁸ cm⁻³) is studied. A strong irradiation-induced increase of the above emission intensity was observed testifying about the irradiation-stimulated growth in the concentration of copper-related 1.01 eV radiative centres (Cu_GaV_As pairs). A model is presented to explain this effect.     

Induction periods and mechanism and kinetics of nucleation of barium tungstate crystal growth from sodium tungstate melts in platinum crucibles

Induction periods (t) and mechanism and kinetics of nucleation in barium tungstate crystallization from sodium tungstate melts in platinum crucibles were studied. A theoretical relation has been developed to express the dependence of t on the cooling rates (R_T) and the rate (R_c) of development of excess solute concentration. At any crystallization temperature the average rate-constant (k_n) for heterogeneous nucleation was related to R_c by (k_n/p)^1/(p+1) = 1/(tR_c^γ), where γ is a constant and p is the average number of particles in the critical nuclei. The critical temperature (T), critical supersaturation (S) and γ values were estimated.     

Non-Parallel Stacking of a One-Dimensional Rod and Loop Chain Cd(II) Complex With the Semi-Rigid 3,3′-Bipyridyl Ligand

Abstract  

A new Cd(II) complex [Cd(L)_1.5(NO₃)₂]_n·2nCHCl₃ (1) with a semi-rigid 3,3′-bipyridyl ligand [L = N,N′-bis(3-pyridylmethyl)pyromellitic diimide], has been synthesized and structurally characterized by X-ray diffraction analysis. It consists of one-dimensional rod and loop chains featuring the combination of Z _c -(anti, cis) and Z _t -(anti, trans) mode conformations for the coordinated ligands. The one-dimensional chains are assembled in parallel and non-parallel directions into three-dimensional networks by complicated supramolecular interactions. The solid photoluminescence of 1 was also investigated at room temperature.     

Studies on the Smectic Phase of Some Schiff's Bases With a Terminal Trifluoromethyl Group

Some terminally substituted trifluoromethylbenzylideneanilines have been synthesized with a view to investigate an unidentified smectic phase which exhibits an unusual texture as observed under the polarizing microscope. This phase was first reported by Griffin et al.² for the compound N-(4-n-decyloxybenzylidene)-4'-trifluoromethylaniline. In a comparable compound but with reversed imine linkage the nature of this smectic phase remains unchanged. Also if the terminal n-decyloxy group is replaced by an n-decyl group the nature of the smectic phase remains the same except that it is observed at a considerably lower temperature. We observe that this smectic phase has uniaxial symmetry when studied with a polarizing microscope as well as with NMR techniques; the latter showing an order parameter S = 0.8 for this phase. X-ray diffraction patterns show this phase to be crystal smectic B.     

Formation of a Banded Texture in Solutions of Liquid Crystalline Polymers: 1. Hydroxypropylcellulose in H2O

Abstract

The formation of a banded texture was investigated for sheared anisotropic solutions of hydroxypropylcellulose (HPC) in H₂O. A cone and plate rheometer made of highly polished glass allowed the exploration of the shear rate (γ) range between 0.1 and 150 sec^?1 and the study of the effect of shear time t_s. Texture, monitored with a video-camera, appeared after a time t_b following cessation of shear. t_b is usually a decreasing function of γ and t_s. In most cases a precursor banded texture precedes the banded texture, and t_b attains zero at high enough γ or t_s. However, when a particularly good orientation of the flowing solution is achieved, the precursor texture is not observed, t_b is finite, and the bands are particularly well formed. We speculate that bands are the normal relaxation mode for well oriented solution. For less oriented solutions flow instability may lead to bands even during flow.     

Solid Phase Studies of the n-Eicosane: n-Docosane System by Positron Annihilation

Positron lifetime spectra have been obtained as a function of temperature for various solid mixtures of n-eicosane (C₂₀H₄₂) and n-docosane (C₂₂H₄₆). These compounds, when mixed in various proportions, are known¹ to form at least six distinct solid phases. Positron lifetime parameters T ₂ and I ₂ have been measured in each of these phases and are found to exhibit both a temperature and a phase dependence. Comparison with crystallographic data reveals that t ₂ variations in the n-eicosane: n-docosane system depend more strongly on the lattice dimensions a and b than on the dimension c.     

Kinetic investigations within the transition region from polyhedral to skeletal growth mode (III). Growth kinetics of the basal face of zinc single crystals and its edge under diffusion conditions

The growth kinetics of the basal face and its edge of zinc single crystals grown from zinc vapours in the presence of argon at pressures 5 – 250 Torr has been investigated simultaneously. The alterations of the crystal size R and the length of the basal edge a with time t have been measured at constant temperature and supersaturation. It is shown that the R²(t) and a²(t) dependences change their initial shape into linear at one and the same moment. It is established that the transition from a kinetic to a diffusion regime can be determined both by the kinetic critical size R_k and by the kinetic critical size a_k. It is found also that in the kinetic regime the lateral growth rate of the basal edge is higher than the normal growth rate of the {0001} face. During the growth in a diffusion regime the R²(t) and a²(t) curves of a given single crystal become parallel, due to the isotropic character of the transport processes of the material. The rate determining role of the diffusion in the parent phase on the growth of the basal face and its edge is demonstrated.     

Suitable Stress Waveforms for the Deformation-Induced Electronic Excitation in Crystals

A quantum mechanical approach is made to the deformation-induced electronic excitation in crystals. The ML intensity is assumed to be related to the transition probability P(m, n) from n^th to m^th state, and it may be expressed as I_ML = βP(m, n) where P(m, n) = 4π²/hα² |σ_mn (ω_mn)|² ·( n · μ )_mn·². It is shown that the ML excitation can take place only by the time-dependent applied stress. The ML excitation probability is analysed for different waveforms of the applied stress such as constant stress, δ-function, rectangular stress, single sided and double sided exponential stress, unit step function, external sinusoidal signals cos ω₀t and sin ω₀t, external exponential function, a periodic function and the stress of sequence of equidistant impulses of unit strength separated by a particular time. It is found that when the stress is a δ-function, a rectangular waveform, sinusoidal singals cos ω₀t and sin ω₀t or a sequence of equidistant impulses of unit strength and separated by a particular time, then there is a considerable probability of electronic excitation by the resonance transfer mechanism between the Fourier transform of the stress waveform and the frequency of the emitting electronic system i.e. (E₂ – E₁)/h.     

Some correlations in liquid phase growth (I)

In this work we consider some circumstances of the growth process of an n-component solid phase from an n-component liquid phase and provide some general relationships, which must be observed in this growth process. In more detail we treat the two extreme cases: growth when (∂C/∂t = 0) — the stationary case; growth when (∂C/∂x = 0) — no concentration gradients in the liquid phase. In our opinion these theoretical considerations will simplify the investigations of multicomponent systems.     

Simulation of metastable zone width and induction time for a seeded aqueous solution of potassium sulfate

The metastable zone width (MSZW, ΔT_m) and induction time (t_ind) were determined with computer simulation for seeded batch crystallization of potassium sulfate from aqueous solution. The MSZW and induction time determined with simulation showed the same behavior as experimental values reported in the literature; log (ΔT_m) increased linearly with an increase in log R (R: cooling rate) and t_ind decreases in proportion to (ΔT)⁻ⁿ (ΔT: supercooling, n: nucleation order in the secondary rate expression of B=k_n(ΔT)ⁿ). The secondary nucleation parameters (k_n and n) were deduced both from the simulated MSZW and induction times by using the previously proposed model [J. Cryst. Growth, 2010, 312, 548–554]. The secondary nucleation rate calculated with the deduced parameters was in agreement with that calculated with the parameters input for simulation.     

Ein eindimensionales Modell zur Beschreibung von Transporteigenschaften in offenen Systemen

A simple one-dimensional model is dealt with describing the material transport by means of linear gas flow and diffusion. In the GaAs AsCl₃ H₂ system the dependence of the partial pressure on the x-coordinate is calculated, and the role of the temperature gradient near the growing interface is discussed in view of the constitutional supercooling of the gas phase.     

Phase Transition of 1,16-Hexadecanediol

It was found that 1,16-hexadecanediol exhibited a solid-solid phase transition. X-ray diffraction experiments and dielectric constant measurements were carried out in order to investigate the structure of the high-temperature phase. A model of molecular arrangement in the high-temperature phase was presented. The high-dielectric constant values were observed in the high-temperature phase. It is considered that the high-temperature phase of 1,16-hexadecanediol is a rotator phase in analogy with n-higher alcohols.     

Densities,refractive indices,and order parameters of a binary liquid-crystalline system with an induced smectic A phase

For the binary mixture of 4′-Nitrophenyl 4-n-octyloxybenzoate and 4-n-Octyloxyphenyl 4′-pentyloxybenzoate with an induced smectic A phase the refractive indices (n_c and n_o) and the density were measured in dependence on the temperature. The orientational order parameter and the density were calculated using a mean field fitting method. In the induced S_A phase the refractive indices and the calculated orientational order parameters are showing only a weak dependence on concentration. The calculated and measured densities are in the same order of magnitude.     

A new one-dimensional oxalato- and acetato-bridged Mn(II) polymer with 1,10-phenanthroline as terminal ligand: [Mn2(C12N2H8)2(μ-C2O4)(μ-CH3COO)2] n ·4nH2O

A new oxalato- and acetato-bridged Mn(II) polymer, [Mn₂(C₁₂N₂H₈)₂(μ-C₂O₄)(μ-CH₃COO)₂] _n ·4nH₂O was synthesized and characterized by X-ray diffraction analysis. The compound exhibits a one-dimensional chain-like structure, in which the Mn atom is coordinated by two nitrogen atoms and four carboxyl oxygen atoms with octahedral coordination geometry. As far as we are aware, this type of coordination polymer involving the simultaneously bridging oxalate and acetate ligands has not been studied so far.     

Doping process control in silicon epitaxy (I). System identification

The transition behaviour of the phosphorus incorporation in silicon epitaxial layers grown in a CVD reactor has been investigated, considering the reactor as a linear control system with u = lg p⁰_PH3 (t) as the input and y = lg N(t) as the output. The response of system to both upward and downward step inputs has been studied experimentally, using SiH₄ and PH₃ sources. The dopant system of a horizontal silicon epitaxial reactor has been identified and a mathematical model relating to the transient behaviour of the system has been found. The parameters of the model have been estimated from layer growth experiments. The step response functions found can be approximated by an exponential function relating to n time constants T, all equal to each other. It was found for the system investigated that the second order model is of sufficient accuracy for the optimal control calculations described in the next part of this series.     

Liquid Crystalline Phases in 4-Acetyl-4′-n-Alkanoyloxy-Azobenzenes

Abstract

The preparation and melting behaviour of the homologous series of 4-acetyl-4′-n-alkanoylox-azobenzenes, CH₃-CO-C₆H₄-N?N-C₆H₄-O-CO-(CH₂) _n -CH₃, (with n ranging from zero to 16) is reported. The melting behaviour has been investigated by means of thermal microscopy and differential scanning calorimetry. The experimental results indicate for almost all the compounds the presence of solid state polymorphism and a type A smectic; for one compound a type B smectic phase is also present. Some homologs present, in addition, a nematic phase.     

The Precipitation of Sparingly-Soluble Alkaline-Earth Metal and Lead Salts with Slow Development of Supersaturation (Slow Anion and Hydroxyl Ion Addition): Kinetics of Nucleation and Induction Periods

Barium and lead sulphate and chromate precipitations were studied at 20° to 95°C at precipitation rates varying from 10⁻³ to 10⁻⁷ ion l⁻¹ sec⁻¹: the supersaturation was developed by slow direct addition of anion to metal nitrate solution and by neutralisation of equivalent metal salt in excess acid solution. Slow heterogeneous nucleation occurred onto particles dispersed within the aqueous solution. The nucleation rate at any time was Where k_n is the rate constant for heterogeneous nucleation, N_∞ is the maximum number of potential nuclei, n_t is the number of nuclei after time t, I. P_t is the ionic product (C_mt)(C_mt) and 2π is the number of metal salt ions in the critical nucleus, generally eight. Crystal growth started after induction periods (t̄) at times just after the times for maximum rate of formation of nuclei. The induction periods (t̄) for precipitations from solutions of initial cation concentration ^cM₀ varied with precipitation rate (R) according to the relation, where γ = π/(π + 1) and k₁ (the unit reciprocal induction period) = . Nucleation rate constants for different precipitations were estimated from the k₁ values and are tabulated. For slow precipitations by direct anion addition, the rate constants were lower for precipitation from solution of the salt of greater solubility. The rate constants for slow precipitation of metal chromates from acid solution were far lower than those for slow precipitation by direct chromate addition. Rate constants decreased somewhat with rise in precipitation temperature.