Modelling of the Transport Processes in Bridgman grown Gallium Antimonide

In this work the thermal, velocity and species fields in the melt during the crystal growth by the vertical Bridgman method, has been studied. The simulations were focused on the special case of GaSb, which is a semiconductor of high technological importance. The simulations have been carried out both in 2 and 3‐D. In both cases the momentum (Navier‐Stockes), energy and mass transport equations were solved. The wall‐to‐wall radiation has also been included. In the two‐dimensional case an axisymmetric global model was developed taking into account the different elements present inside the real Bridgman growth system. In order to study the transport processes in the whole system during a complete growth process, the time dependence has also been considered. In the three‐dimensional case, the mathematical domain is restricted to the melt. These simulations were developed in order to study the influence of the ampoule tilting on the dopant distribution in the melt.     

Helical impurity distribution produced by a hydrodynamic instability

Temperature oscillations have been recorded in the liquidn phase during vertical solidarification of salt solutions and dilute aqueous suspensions. The oscillations produce a characteristic solidification pattern consisting of a helical segragation of impurities. The pitch of the helix is correlated with the temperature period of the oscillation. A model is proposed based on unstable thermal convection. It involves the rotation of a convective roll around its own axis. It is shown that the solidification technique provides a new method for the determination of the characteristics of the convective behaviour.     

Chemical Segregation in Vertical Bridgman Growth of GaInSb Alloys

A set of experiments on the solidification of Ga _1-x In _x Sb alloys with a large variation of the sample diameter (from 1mm to 10mm), of the growth rate (from 0.7 to 7 μm/s) and of the concentration (from x=0.01 to x= 0.1) is described. The associated radial and longitudinal segregation of the In have been analysed by SIMS or electron microprobe. Numerical simulation of the experiments, taking into account thermal, hydrodynamic and chemical behaviour has been carried out with the help of FIDAP. It is shown from these numerical results that a plateau of concentration can be reached even if a convective loop is present close to the interface, provided that the convection does not extend into the bulk of the liquid. This is in full agreement with the experimental results obtained. Supporting this analysis, in some experiments, a defect in term of verticality of the crucible led to complex 3-D convection involving the whole liquid, and in that case no plateau was obtained. For the radial segregation ΔC_R, three regimes of transport are found, characterised by the convective level: - A diffusive one, with a low, constant, ΔC_R related to the interface curvature. - A quasi-diffusive one (weak convection) in which ΔC_R increases with convection. - A convective one in which it decreases. Measured radial segregations are in good agreement with predictions from the numerical simulations.     

Molecular dynamics study of UV-laser-induced densification of fused silica. II. Effects of laser pulse duration,pressure, and temperature,and comparison with pressure-induced densification

Numerical simulations on UV-laser-induced densification of fused silica have been performed using classical molecular dynamics. The effect of laser irradiation is modeled by the energy transfer from the absorbed laser photons to the bonded silicon and oxygen atoms, i.e., the thermal effects of laser-irradiation on silica glass. Results from simulations at various laser pulse duration, pressure, and temperature conditions show that longer laser pulse duration, higher pressure, and higher temperature cause larger densification. We have also compared the microstructural and elastic properties of fused silica densified by UV-laser and hydrostatic pressure, respectively. Similar changes are observed in both cases; several notable differences are noticed, too, and include Si–O bond length change, number of over-coordinated atoms, and ring distributions.     

Structural and transport properties of the AgI–AgBr system in its superionic and molten phases by computer simulation

Molecular dynamics simulations were performed to investigate the structural properties of AgI–AgBr system in its superionic and molten phases. Parrinelo, Rahman and Vashishta (RVP) type potentials are used for the simulation. The obtained pair distribution functions are in good agreement with the neutron diffraction experimental data. The transport coefficients in these two phases are also calculated from the equilibrium and non-equilibrium molecular dynamics simulation method. In molten phase, the ratios of partial conductivities on constant concentration are found to be constant at different temperatures.     

Solute segregation in a lid driven cavity: Effect of the flow on the boundary layer thickness and solute segregation

Our objective in the present work is to study the effect of convective flows, ranging from laminar to fully turbulent, on solute segregation in directional solidification configurations. To do so, numerical simulations performed in a model 2D lid driven cavity; the problem parameters, apart from the species molecular diffusion coefficient, are the lid and growth velocities. Purely diffusive to fully convective mass transport conditions are modelled in our parametric study. In parallel, a scaling analysis aiming at the determination of the solute boundary layer thickness is proposed. The results show that a single non-dimensional number, based on the interface stress, is able to capture the physics of the solute transport phenomena.     

Molecular dynamics simulation of radiation damage in glasses

Molecular dynamics simulations of the ballistic effects arising from displacement cascades in glasses have been investigated in silica and in a SiO₂-B₂O₃-Na₂O glass. In both glasses the T-O-T′ angle (where T and T′ are network formers) diminishes, despite radiation causes opposite effects: while the ternary glass swells and silica becomes denser. We show that radiation-induced modifications of macroscopic glass properties result from structural change at medium/range, reflecting an increasing disorder and internal energy of the system. A local thermal quenching model is proposed to account for the effects of ballistic collisions. The core of a displacement cascade is heated by the passage of the projectile, then rapidly quenched, leading to a process that mimics a local thermal quenching. The observed changes in both the mechanical and structural properties of glasses eventually reach saturation at 2 10¹⁸ α/g as the accumulated energy increases. The passage of a single projectile is sufficient to reach the maximum degree of damage, confirming the hypothesis postulated in the swelling model proposed by J.A.C. Marples.     

Vibrational control of crystal growth from liquid phase

Modeling and numerical simulations of the convective flows induced by the vibration of the monocrystal during crystal growth have been performed for two configurations simulating the Cz and FZ methods. This permitted to emphasize the role of different vibrational mechanisms in the formation of the average flows. It is shown that an appropriate combination of these mechanisms can be used to counteract the usual convective flows (buoyancy- and/or thermocapillary-driven) inherent to crystal growth processes from the liquid phase. While vibrational convection is rather complex due to these identified mechanisms, the new modeling used in the present paper opens up very promising perspectives to efficiently control heat and mass transfer during real industrial applications of crystal growth from the liquid phase.     

Protein crystallization under microgravity conditions. Analysis of the results of Russian experiments performed on the International Space Station in 2005−2015

Conditions of mass transport to growing crystals have a considerable effect on the crystal size and quality. The reduction of convective transport can help improve the quality of crystals for X-ray crystallography. One approach to minimizing convective transport is crystallization in a microgravity environment, in particular, in space. The data obtained by our research team in protein crystallization experiments on the International Space Station are surveyed and analyzed.     

Sensitivity Analysis on Indium Phosphide Liquid Encapsulated Czochralski Growth

The optimization of the InP liquid encapsulated Czochralski systems is usually difficult, time consuming and very expensive. Here, the relative importance of the different growth parameters (e.g., pull rate, system temperatures and geometry) on the growth interface deflection, the temperature gradients within the melt and the crystal and the dislocation density has been investigated through a sensitivity analysis. The sensitivity coefficients have been calculated by means of a mathematical model, previously validated, based on the thermoelastic theory for the dislocation formation and on the thermal capillary theory for the temperature field within the system. The crucible temperature profile has been selected as the more important parameter to control the crystal quality during the growth.     

Metastability effects in hydrogenated microcrystalline silicon thin films investigated by the dual beam photoconductivity method

Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark conductivity, σ_D, photoconductivity, σ_ph, and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O₂ gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si:H, films, other results were unaffected from the aging and annealing processes applied.     

A New Approach to Estimation of the Thermal Conductivity of Titanium Alloys

Crystallography Reports - The thermal conductivities of seven titanium alloys (VT1-0, VT18U, VT20, IMI834, Ti6242S, Ti6Al4V, and Ti6Al7Nb) have been measured. According to the data obtained, the...     

掺Nd钒酸盐晶体热导率的拉曼光谱研究

运用晶格动力学观点推导了热导率与积分拉曼散射强度的关系,测量了Nd:YVO4 (简称NYV)和Nd:GdVO4 (简称NGV)不同配置下的高温拉曼光谱和其a、c向的热导率,理论与实验非常吻合.     

An analytical model for combined radiative and conductive heat transfer in fiber-loaded silica aerogels

An improved analytical model for the total thermal conductivity of fiber-loaded silica aerogels was developed based on the complex refractive index, size, orientation, volume fraction and morphology of the fibers and silica aerogel. A cubic array of spherical porous secondary nanoparticles and a modified parallel-series model were proposed to model the combined solid and gaseous thermal conductivities. An anomalous diffraction theory (ADT) was used to predict the fiber extinction coefficient. Five common fiber types in the composites were studied including amorphous SiO₂ glass, silicon glass, common float glass, soda lime silica glass and borosilicate glass. The results show that the total extinction coefficient of the silica aerogel system is largest by loading with the common float glass fiber and lowest by loading with the soda lime silica glass among the five fiber types. The model provides theoretic guidelines for material designs with optimum parameters, such as the type, inclination angle, volume fraction and diameter of the fibers as well as the aerogel nanoparticle and pore sizes. The optimum fiber for improved thermal insulation should have a large spectral complex refractive index throughout the infrared region.     

数值模拟自然对流对直拉单晶硅的影响

在直拉单晶硅生长的过程中,自然对流对晶体界面的形状、温度场及应力分布影响很大。本文采用二维模型对熔体内自然对流对单晶硅的影响作了数值模拟,在低雷诺数时采用层流模型,高雷诺数时采用紊流模型,Gr的变化范围从3×106到3×1010,这样涵盖了从小尺寸到大尺寸的直拉单晶硅生长系统。数值结果表明熔体的流动状态不仅与熔体的Gr有关,还与熔体高度和坩埚半径的比值密切相关。当Gr>108时,熔体内确实存在紊流现象,层流模型不再适合,随着Gr的增大,紊流现象加剧,轴心处的等温线变得更为陡峭,不利于晶体生长。     

Structure,chemical durability and crystallization behavior of incinerator-based glassy systems

Vitrification by melting is being proposed as a convenient method to solidify different kinds of silicate and other oxide-based inorganic wastes. Incinerator bottom and fly ashes have been mixed with glass cullet, feldspar and clay by-products as melting fluxing agents. Washing, drying, and grinding pre-treatments followed by melting at 1450 °C lead to the formation of glasses and glass-ceramics, depending on the starting materials composition and thermal treatment. The obtained glasses have been studied by SEM, chemical durability tests in aqueous and alkaline environment, leaching test (UNI 10802), and by differential thermal analysis. The glass-ceramics morphology was investigated by XRD and SEM. The results were explained by the structure of the glasses caused by the presence of different amount of modifiers in the glassy lattice. The obtained glasses show good chemical resistance, in particular in alkaline environment and thermal characterization highlighted that the materials are also suitable to obtain glass-ceramics.     

Some physicochemical studies on organic eutectics

The phase diagrams of phenothiazine with each of m-nitrobenzoic acid (m-NBA) and m-dinitrobenzene (m-DNB) have been studied by thaw-melt method. These materials have been characterized by X-ray diffraction. Growth behavior of the parent components, eutectic and charge transfer complex (CTC) studied by measuring the rate of movement of the growth front in a capillary suggests the applicability of Hillig–Turnbull equation for the system. Microstructure and electrical conductivities of congruent melting complexes and eutectics have been determined. The low electrical conductivities of these materials have been due to weak interaction and mixed stacking of donor and acceptor. Excess thermodynamics functions of the charge-transfer (CT) materials and eutectics have been determined.     

γ-氨基丁酸晶体成核动力学、晶体生长及热力学性质研究

本文研究了γ-氨基丁酸的热力学及成核动力学性质.对γ-氨酪酸晶体的热力学性质进行研究,γ-氨基丁酸的差示扫描量热研究表明:γ-氨基丁酸的熔点为190.6 ℃,熔化焓为-343.4 J/g.测定了不同温度下γ-氨基丁酸在水中的溶解度及不同过饱和度下的诱导期.结果表明:γ-氨基丁酸的诱导期随着过饱和度的增加而降低.通过经典成核理论计算了固-液表面张力、成核自由能和临界成核半径.     

Directional Solidification and Melting of Eutectic GaIn

Eutectic gallium-indium is studied in a horizontal Bridgman furnace geometry. Differential temperature gradients are applied to solidify and melt the alloy while observing in-situ the interface morphology and the chemical segregation in the melt and in the solid as well. Upon cooling, a wedge-type indium-rich mushy zone develops at the cold wall. The melt is initially stirred by convective flow. After solidification starts the roll cell recedes to be replaced by a chemically layered conductive melt that eventually solidifies with rather uniform eutectic structure. Upon re-melting, the morphology of the interface adopts a profile that is predetermined by the original solid structure. Those patterns, as well as the flow, are different from single element solid melting experiments and have yet to be modeled. Under high thermal gradient the convective flow mixes the binary melt and the visualized density pattern eventually becomes that of a homogeneous melt.     

Phase diagrams and optical properties of binary liquid crystalline systems of cobalt (II) caprylate with lithium and lead caplylates

The concentration and temperature ranges of liquid crystal and glass formation in binary systems of cobalt (II) caprylate with either lithium or lead caplylates have been studied by differential thermal analysis and optical polarization microscopy. Absorption spectra of Co(II) cations in mesophases and glasses of these binary caprylate systems have been analyzed as a function of temperature and composition to get information on the coordination of Co(II) ions.For the binary system of lead–cobalt caprylate, we found that the Co(II) ions are octahedrally coordinated over the range of temperatures and compositions studied, whereas in the binary lithium–lead caprylate system, the Co(II) ions can exist in both octahedral and tetrahedral coordination depending on composition and temperature. Thermochromic behavior was observed for all compositions studied here, from blue to violet and pink, depending on the composition and temperature of the system studied. An unusual increase of optical density with decreased Co(II) ion concentration has been observed for the Co–Li caprylate binary system up to 50 mol%. An increasingly ionic Co(II) coordination environment as Li ion concentration increases facilitates a concomitant increase in the proportion of more optically dense tetrahedrally coordinated Co(II) ions.