Three‐dimensional flow in a thin annular layer of silicon melt with bidirectional temperature gradients

Bidirectional temperature gradients coexist virtually in surface tension driven flows. However, the simulations have been performed to the flow with only one temperature gradient. A series of 3 D numerical simulations are conducted to investigate the Marangoni‐thermocapillary flow of silicon melt in a thin annular layer with bidirectional temperature gradients. The temperature gradients are produced by the temperature difference ΔT between walls and the constant heat flux q on the bottom, respectively. When changing q, the melt presents different state evolutions at different ΔT. Furthermore, two critical q are found, one makes the minimum melt temperature higher than the crystallization temperature and the other makes the flow unsteady. Both of the critical heat fluxes decrease with increasing ΔT. q contributes more to the elevation of the melt temperature, while ΔT contributes more to the enhancement of the melt instability. In addition, the melt on the free surface flows mainly along the radial direction.     

Antisolvent crystallization of aqueous ammonium dihydrogen phosphate solutions by addition of acetone at different rates

Results of an investigation of in situ measurements of laser‐beam intensity I_s and I_m transmitted through aqueous ammonium dihydrogen phosphate (ADP) solutions saturated at 30 °C and water, respectively, and temperature T_s of solution and T_m of water during feeding of antisolvent acetone at different rates R_A, using an indigenously designed experimental setup, are presented and discussed. It was found that: (1) for the measurement of MSZW, defined as the maximum volume fraction of acetone content Δx_max in the solution, obtained from temperature measurements are more reliable than transmitted laser‐beam intensity measurements for solutions, (2) two minima ΔT_min1 and ΔT_min2 associated with endothermic reactions, separated by a maximum ΔT_max due to exothermic reactions appear in the plots of temperature difference ΔT = T_s−T_m against acetone feeding time t, and (3) in the ΔT(t) plots there are time intervals Δt of constant rates R_T of increase in ΔT of aqueous ADP solutions, and these values of R_T increase linearly with acetone feeding time rate R_A. The experimental data on the observed dependence of MSZW on antisolvent feeding rate R_A, the appearance of minima ΔT_min1 and ΔT_min2 and maximum ΔT_max and their dependence on R_A, and the relationship between R_T and R_A are discussed from consideration of processes of nucleation and growth of crystallites.     

Physical Property of New Liquid Crystal Materials and Mixture Design for Active Matrix LCD

Abstract

We studied the physical properties of three series of new fluorinated liquid crystal components and prepared some mixtures by using these components. Decahydronaphthalenes have low Δn with relatively high T _NI. Naphthalenes and Tetrahydronaphthalenes have large Δ? and variety range of Δn (0.08–0.21). Moreover, we have revealed that the fluoro- substituent at C-1 position for the naphthalene and the tetrahydronaphthalene ring has effects to increase T _NI and to reduce γ₁ with good solubility. Then, we have designed some LC mixtures for AM-LCD having good performance with 4V-driving, quick response, high birefringence, low birefringence, wide temperature range and low driving voltage.     

Investigation of metastable zone width of ammonium oxalate aqueous solutions

The metastable zone width of pure ammonium oxalate aqueous solutions, as represented by maximum supercooling ΔT_max, is investigated as functions of cooling rate R and saturation temperature T₀ by the polythermal method. The experimental results are discussed by using two recently advanced approaches: (1) self‐consistent Nývlt‐like approach based on a power‐law relationship between nucleation rate J and maximum supersaturation lnS_max, and (2) a novel approach based on the relationship between J and lnS_max described by the classical three‐dimensional nucleation theory. Analysis of the experimental data revealed that both approaches describe the experimental data on metastable zone width by the polythermal method reliably and provide useful information about the physical processes and parameters involved in nucleation kinetics. The values of various physical quantities predicted by both of these approaches are reasonable for a fairly‐soluble compound. A careful examination of the data on ΔT_max as a function of T₀ obtained by polythermal method and from density measurements showed that ΔT_max has a slight tendency to decrease with increasing saturation temperature T₀. The values of lnS_max at saturation temperature 303 K suggest that the metastable zone width of ammonium oxalate aqueous solutions is determined by primary nucleation in the polythermal method and by secondary nucleation during density measurements. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Thermodynamic properties of strontium tungstate crystal growth from sodium tungstate melts

Energy (E), enthalpy (ΔH_a), entropy (ΔS_a) and free-energy (ΔG_a) of activation and the pre-exponential factor (k₀) of the ordinary Arrhenius equation k_Dl = k₀e–E/RT were estimated for diffusion-controlled crystal growth of SrWO₄ from Na₂WO₄ melts. E increased slightly with increased cooling rates (R_T). k₀ was parallel to k_Dl and increased with increasing R_T. ΔH_a, ΔS_a, and ΔG_a did not change with the changes in R_T and crystallization temperature (T₀). The distance (d₁₂), between a diffusing particle and its host necessary for a successful diffusion, was estimated. Such distances slightly increased with T₀ and R_T.     

Heat Capacity and Thermodynamic Properties of p'-Substituted p-n-Hexyloxybenzylideneaniline. I. p-n-Hexyloxybenzylideneamno-p'-Benzonitrile (HBAB)

Abstract

The heat capacity of the nematogenic liquid crystal, HBAB, has been measured between 15 K and 385 K by using an adiabatic calorimeter. The crystal-crystal phase transition has been discovered at 27 K below the crystal-nematic phase transition temperature. The transition temperatures, the enthalpies and the entropies of the three phase transitions have been determined: T ₁ = 306.98 K, ΔH _t = 5.11 kJ mol^?1, ΔS _t = 16.7 JK^?l; T _m = 334.05 K, ΔH _m = 23.77 kJ mol^?1, ΔS_m = 71.2 J K^?l mol^?1; and T _c = 375.10 K, ΔH _c = 1.75 kJ mol^?1, ΔS _c = 3.2 J K^?1 mol^?1, respectively. The thermodynamic functions of HBAB from 0 K to 385 K have been determined from the heat capacity data and the enthalpies of the transitions. Two crystal modifications, one yellow and granular form and the other white and needle-like form, have been obtained during the course of the preparation of the sample. It turned out that the yellow form was the stable crystal and the white the metastable modification. The crystal-crystal phase transition has been discussed as an onset of partial melting from the entropy consideration. In this connection the total entropies of the transitions, 91.1 J K^?1 mol^?1 has been proposed to be an important measure of melting.     

Convective Temperature Fluctuations in Liquid Gallium in Dependence on Static and Rotating Magnetic Fields

The influence of static axial, static transversal, and rotating magnetic fields on convective temperature fluctuations in liquid gallium (temperature range around 800 °C, Prandtl number 2.5 · 10⁻³) has been investigated. For a Rayleigh number of 6.3 · 10⁻⁵ (ΔT = 10°C, h = 50 mm), convective temperature fluctuations with peak to peak values of 3 °C have been measured. Depending on the strength, the frequency, and the configuration of the field, they could be eliminated to ΔT < 0.01 °C (static axial field of 182 mT), damped to a nearly periodic state with 0.1 °C amplitudes (static transversal field of 45 mT), or reduced to 0.03 °C oscillations by applying a rotating field.     

Conditions for single-crystal formation during bridgman growth

The microstructure of IV–VI crystals grown by varying the shape of the conical bottom of a ampoule, the melt superheating ΔT⁺ with respect to the liquidus temperature of a blend, the axial temperature gradient in a furnace ΔT, and the pulling rate of a ampoule v were studied. It was found that the nucleation of a single-crystal seed is most likely under the following conditions: ΔT ≤ 20 K/cm, ΔT⁺ < 30 K, and v < 0.4 mm/h. The conical bottom of a ampoule should have walls of constant thickness. The principle of geometric selection of a single-crystal seed is not efficient. The obtained results are discussed with regard to data on the structure and behavior of IV–VI melts upon heating and cooling.     

Thermodynamic characteristics of Ti-based glass-forming alloys

Based on thermodynamic characteristics of the stable metallic liquid at melting temperature and the supercooled liquid, the present work calculated the mixing enthalpy ΔH^mix, the mixing entropy ΔS^mix and the Gibbs free energy difference between the supercooled liquid and the resulting crystalline phases ΔG of typical Ti-based amorphous alloys. The results show that for the case of larger ΔS^mix, moderate ΔH^mix for the stable liquid and smaller ΔG for the supercooled liquid, Ti-based alloys tend to achieve high glass-forming ability (GFA). A new parameter, β, defined as (T_g ? T_k)/(T_l ? T_g), has been introduced to evaluate the GFA of Ti-based bulk amorphous alloys (wherein T_g, T_l, and T_k represent the glass transition temperature, the liquidus temperature, and the Kauzmann temperature, respectively). Experimental data imply that the larger the β, the better the GFA for Ti-based amorphous alloys.     

Axial temperature distribution in Silicon-Germanium grown by the RF-heated float zone technique

The RF-heated float zone method has been used to study the segregation behaviour of silicongermanium solid solutions in the concentration range from 0 to 25 at% germanium. Completely dislocation free crystals up to 8 at% germanium could be obtained. Since constitutional supercooling is the main reason preventing single crystalline growth, pyrometric temperature measurements have been performed to reveal the temperature slope at the interface and to determine the critical growth rate. The temperature gradient ΔT/Δz was found to increase with increasing germanium concentration.     

A novel self‐consistent Nývlt‐like equation for metastable zone width determined by the polythermal method

Using a power‐law relation between three‐dimensional nucleation rate J and dimensionless supersaturation ratio S, and the theory of regular solutions to describe the temperature dependence of solubility, a novel Nývlt‐like equation of metastable zone width of solution relating maximum supercooling ΔT_max with cooling rate R is proposed in the form: ln(ΔT_max/T₀) = Φ + β lnR, with intercept Φ = {(1–m)/m }ln(ΔH_s/R_GT_lim) + (1/m)ln(f/KT₀) and slope β = 1/m. Here T₀ is the initial saturation temperature of solution in a cooling experiment, ΔH_s is the heat of dissolution, R_G is the gas constant, T_lim is the temperature of appearance of first nuclei, m is the nucleation order, and K is a new nucleation constant connected with the factor f defined as the number of particles per unit volume. It was found that the value of the term Φ for a system at saturation temperature T₀ is essentially determined by the constant m and the factor f. The value of the factor f for a solute–solvent system at initial saturation temperature T₀ is determined by solute concentration c₀. Analysis of the experiment data for four different solute‐water systems according to the above equation revealed that: (1) the values of Φ and m for a system at a given temperature depend on the method of detection of metstable zone width, and (2) the value of slope β = 1/m for a system is practically a temperature‐independent constant characteristic of the system, but the value of Φ increases with an increase in saturation temperature T₀, following an Arrhenius‐type equation with an activation energy E_sat. The results showed, among others, that solubility of a solute is an important factor that determines the value of the nucleation order m and the activation energy E_sat for diffusion. In general, the lower the solubility of a solute in a given solvent, the higher is the value of m and lower is the value of E_sat. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical Description of Strongly Nonlinear Pressure Dependence of Phase Transition Temperatures

In order to describe mathematically the pressure dependence of phase transition temperatures we have derived an equation using the following assumption: i. The transition enthalpy Δ_tH = const. ii. The transition volume Δ_tV depends on the pressure p according to: The modified Simon-Glatzel-equation T = T₀(1 + p/b)^a exp (cp) allows to calculate the transition temperatures T also in cases of strongly non-linear relations.     

Thermodynamic analysis and technology of preparation of high-Tc superconducting system YBa2Cu3O7−x

By modeling the equilibrium state of the phase formation reaction of High-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) and using the results of P-T−x correlation investigation of some authors, the variation of Gibbs free energy (ΔGm) versus temperature has been calculated. On the basis of the variation of ΔGm, the technology procedure of preparation of phase 1:2:3 with desired x has been established to meet requirement of basic investigation and application of High-T_c superconductors YBa₂Cu₃O_7−x.     

AlGaAsSb and AlGaInAsSb Growth from Sb-rich Solutions

This work is related to the use of Sb as a solvent for the growth of antimony-based solid solutions by LPE. We have developed the growth technology of AlGaAsSb on GaSb substrates. A GaSb solid phase is eroded or melted back in contact with a saturated Al-Ga-As-Sb liquid due to the high non equilibrium degree on this system and the erosion increases with Al concentration. One of the techniques to diminish the erosion consists in increasing the initial supercooling but in this system, in the investigated area of compositions, it is impossible because of the low critical supercooling (ΔT_cr) of the liquid phase. We have conceived and developed a method to control ΔT_cr by adding In to the liquid phase. It was found that when the In concentration increased the ΔT_cr also increased. So the transition from the quaternary AlGaAsSb to the pentanary AlGaInAsSb allowed us to decrease the erosion process. This technique permits to grow high quality multilayer heterostructures based on antimonides.     

Overhauser Shift of the Conduction Electron Spin Resonance in (FA)2PF6 Single Crystals

The shift ΔB _ov of the ESR line due to saturation of the NMR of the hyperfine coupled nuclei (Overhauser shift) was measured for single crystals of the organic conductor (FA)₂PF₆. ΔB_ov , is proportional to [Abar] _tt , being the average hyperfine interaction between the conduction electrons and the protons in resonance, and the dynamic nuclear spin polarization (DNP), respectively. The DNP enhancement factor V was determined for the two orientations of the static magnetic field B _o , perpendicular (V _∥ = 525 ± 40) and parallel (V _∥ = 280 ± 25) to the needle axis a, respectively. The absolute value of the average hyperfine coupling is [Abar]^zz = –(1.16 ± 0.05) Gauss · g_e μ _B . Both, the temperature dependence and the anisotropy of the proton spin relaxation times T ₁ ^p and T ₁ ^p were measured from the time dependence of the Overhauser shift, ΔB_ov (t) after rf-pulses or after switching “on” and “off” the ESR saturation. Within the metallic phase of the crystal the proton relaxation is governed by a Korringa law. The experiments definitely show, that the electron spins, showing up in the ESR are those of the conduction electrons.     

Spherulitic crystallization of β‐In2Te3 by physical vapour deposition

Different morphologies of indium telluride (In₂Te₃) including novel spherulites were crystallized using the physical vapour deposition (PVD) method, by varying the difference in the growth and source zone temperature (ΔT) of a dual zone horizontal furnace assembled indigenously. Whiskers and kinked needles of In₂Te₃were grown at ΔT = 250 K and 300 K respectively, maintaining the growth zone at 500 °C. At high supersaturation (Δ T = 400 K), spherulitic crystals were obtained. The stoichiometric composition of these crystals has been confirmed using energy dispersive analysis by x‐rays (EDAX). The structure of β‐In₂Te₃ spherulitic crystals is identified as zinc blende with lattice parameter a = 6.159 Å, from x‐ray diffraction (XRD) studies. The scanning electron microscope (SEM) images revealed the radial structure of the grown spherulites. The growth mechanism for the spherulitic crystallization of β‐In₂Te₃ crystals has been discussed based on the theoretical models. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of TM and RE elements on glass formation of the ternary Al-TM-RE systems

The effects of TM and RE elements on glass formation ability (GFA) of the Al-TM-RE systems are studied systematically. The TM elements show distinct differences: critical sizes of the Al-TM-Ce systems are in the order of Ni > Co > Fe > Cu. However, the RE elements show similar effect on glass formation. These results are discussed in terms of the GFA-related factors, i.e., reduced glass transition temperature (T_rg = T_g (T_x)/T_L, where T_g, T_x and T_L are the glass transition temperature, onset crystallization temperature and liquidus temperature, respectively) and the mixing enthalpy ΔH_Al-TM. A new parameter of T_L − T_g (T_x) is potentially proposed and used to evaluate the GFA of the Al-based alloys, as well as bulk glass-formers.     

Formation Volume of Vacancies in Elements and the Defect Formation Mechanism of Melting

The formation volume V_v of vacancies is given by V_v = (h_v/L) ΔV_f with h_v = 8L (formation enthalpy h_v of vacancies and heat L of fusion given in same units; ΔV_f = change of volume due to melting). If there are phase transitions within the solid, L and ΔV_f must be replaced by (L + Δ H_t) and by (ΔV_f + ΔV_t), respectively (Δ H_t and Δ V_t refer to the heat (s) of transition (s) and to the volume change(s) due to transition(s), resp.). The pressure dependence of the melting point is dT_m/dp = (T_mV_v)/h_v. Independent of the sign of V_v any increase of the vacancy concentration above the maximum concentration possible within the solid decreases the melting point thus resulting in the observed surface melting. The melting point is fixed by the characteristics of vacancy formation (h_v, S_v, V_v) and by the bulk modulus of the solid (S_v = formation entropy of vacancies).     

Precision measurements of plastic deformation of β-CuZn at helium temperatures

By attaching condenser plates directly to the tensile specimen, creep rates of β-CuZn single crystals have been measured by the capacitance change in the strain-rate range of 10⁻⁷ to 10⁻⁵ s⁻¹ and at temperatures between 0.7 and 17 K using a ³He cryostat. Slope of the temperature variation of the stress to give a constant strain-rate decreases below 5 K and seems to level off at 0 K. The strain-rate sensitivity becomes almost constant below 2 K. The conventional activation analysis showed that an ARRHENIUS type strain-rate equation breaks down drastically below 5 K. For an effective temperature T^* which gives 2 K at 0 K and asympototically coincides with the testing temperature at 5 K, the experimental results can be fitted to the ARRHENIUS strain-rate equation with an activation enthalpy ΔH(τ) for the PEIERLS mechanism. The above result is interpreted in terms of the quantum-mechanical oscillation of the dislocations.