A fluctuation theory of normal crystal growth in the case of one-component metallic macrosystems

A Model of spontaneous fluctuations of the limited spectrum solid state particle concentrations in the case of one-component metallic macrosystems is considered. Within the small supercoolings' region of the melt-crystal macrosystem a normal law of crystallization is established due to the action of fluctuation mechanism. For some metallic macrosystems a comparison on the crystallization kinetics between the theory and experiments has been realized. Some quantitative restrictions of the melt-crystal system supercoolings' region within which the mechanism of spontaneous fluctuations is expected to be realized are established for a collection of one component metallic macrosystems. Two-phase transitional zone length's influence on the values of admitted supercoolings in presented.     

A fluctuation mechanism of crystallization processes for the one-component metallic melts

In given work a mechanism of spontaneous fluctuations of the „unlimited”︁ spectrum solid state particle concentrations acting in some monolayers of two-phase transitional zone within the small supercoolings' region is considered. This mechanism is applied for some one-component metallic macrosystems crystallizing from the supercooled melts. It is established that given mechanism in the case of macrosystems transforms into action of only one spontaneous fluctuation occuring in each monolayer of two-phase transitional region as a result of which the solid state particle concentration becomes equal to nearly 1/2. It is shown that in this case within the small supercoolings' region given mechanism leads to so-called normal law of crystallization connected with one-component metallic macrosystems. In the particular scheme of mechanism presented an effective kinetic coefficient is estimated. In the vicinity of melting temperature for some metallic melts a comparison of theoretical and experimental data concerning the mean crystallization velocities has been fulfilled. In the case of spontaneous fluctuations of the „unlimited”︁ spectrum solid state particle concentrations two-phase transitional zone's sizes do not have influence on the one-component metallic melts' crystallization kinetics within the small supercoolings' region.     

Fluctuation theory of the one-component and binary metallic melt's crystallization in the case of micro- and macro-systems

All types of fluctuation mechanisms resulting in a normal kinetics of the one-component and binary melt's crystallization within a small supercooling region for micro- and macro-systems are systematized. Three principal mechanisms of spontaneous fluctuations acting in the monolayers of two-phase transitional zone in the vicinity of melting temperature for one-component metallic crystals and of the liquidus temperature for binary ones are described. The formulas of kinetic coefficients figuring in the normal law of crystallization conformably to one-component and binary metallic micro- and macrosystems for all types of fluctuation mechanisms are presented. Some peculiarities of each fluctuation mechanism are presented. Some peculiarities of each fluctuation mechanism acting within a small supercooling region of the melt-crystal micro- and macrosystems are pointed out. A comparison of theoretical and experimental data concerning the one-component and binary melt's crystallization kinetics at some small supercoolings is presented.     

Effective kinetic coefficients for some binary metallic alloys within the small and finite supercooling region

The normal crystallization kineties concerning the base of In, Rb, Ga some binary metallic melts either within the small supercoolings (Δ T ≦ 1 K) or finite ones (ΔT ≦ 20 ÷ 60 K) has been considered. When crystallizing the binary alloys: In + 2.3 at.% Pb; Rb + 10 at.% K; Ga + 2 at.% Sn; Ga + (2; 4; 8) at.% In; Ga + (0.5; 3.5) at.% Zn; Ga + (5; 8; 10) at.% Zn very dilute binary solid solutions are supposed to be formed. The solid solutions have the B-component distribution coefficients which are either much less or less compared with unit, i.e. k_B ≦ 1, k_B < 1 in all the layers of diphase transition region separating the melt from the crystalline phase. By virtue of fluctuation theory of normal crystal growth within the small and finite supercoolings' region for the above-mentioned binary metallic melt some effective kinetic coefficients have been calculated. A good agreement between theoretical effective kinetic coefficients and available experimental ones in the case of given binary systems has been established.     

Investigation of the stability of the flat crystallization front of a binary melt by a variational method

The stability of the flat crystallization front of a dilute binary melt is investigated within the two-dimensional model of solidification taking into account the latent heat of fusion and the difference between thermal conductivities of the solid and liquid phases by the method of integral functionals with an unknown region of integration. The stationary problem of impurity diffusion in a crystallizing melt and heat propagation in a limited region Ω is solved within the second-order approximation in the amplitude of deviation from a flat crystallization front. An expression for the functional is obtained whose value is proportional to the energy dissipated in the region Ω owing to heat and mass transfer processes. The results obtained are compared with the literature data on the stability of the flat crystallization front of a binary melt.     

Fluctuation mechanism of normal crystal growth during solidification of binary metallic melts

A recently published theory on the solidification of a one-component melt has been extended to the more complex case of binary systems. The theory is based on the model of a two-phase transitional zone existing between the crystalline phase and the melt. The concentration of solid state atoms within each mono-atomic layer of the transitional zone are assumed to fluctuate due to thermal fluctuations. A crystal growth law has been derived expressing the crystallization velocity in terms of probability functions describing these concentrations fluctuations. When certain restricting conditions concerning the atomic interaction energies within the transitional zone and the distribution of the atoms among the solid and liquid phases at supercooling are fulfilled the crystal growth law attains a simple form predicting for small supercoolings a growth rate proportional to supercooling (linear growth law), roughly proportional to physical parameter θ_AA, and with a weak dependence on another parameter Δ.     

The order-disorder transformation at supercooled melt/crystal transition regions of binary melts (II). The steady-state solution

In a preceding paper a kinetic master equation has been derived describing the kinetics of atomic exchange processes that occur within the transition region separating a growing binary crystal and its nonsolid surrounding. For simple cubic structures, with the lattice sites being occupied by equal particle numbers of two components (in the case of perfect order corresponding to an NaCl-type lattice) solutions for the steady-state conditions have been derived. According to the solutions the long-range order parameter is related to the atomic interaction energies, the arrival rate of particles at the crystal surface from the melt, and to temperature. A critical supercooling is predicted for the transition from a partly crdered structure to a disordered structure of the crystals growing from the melt. At the critical supercooling the temperature dependence of the crystallization rate reveals a characteristic change.     

Thermodynamic restrictions of LPE of bismuth-containing А3В5 solid solutions

A heterophase equilibrium estimation procedure is proposed for systems InBiAsSb and GaInBiAsSb using the simple solution model and a ‘virtual’ GaBi compound. Thermodynamic characteristics and crystal-chemical parameters of the GaBi compound have been determined for the calculation with the linear interpolation method. Phase equilibrium in In–Bi–As–Sb and Ga–In–Bi–As–Sb systems has been analyzed for the 650–780 K temperature range. The existence limits for solid solutions have been set and the thermodynamic restrictions characterizing the capacity for synthesis have been ascertained. The data obtained may be useful in optimization of LPE methods.     

On normal kinetics in the case of some refractory and transition metals' crystallization

In given work the crystal growth kinetics connected with some refractory and transition metals is considered. For melts of Fe, Co, Ni, Cr, Zr, Ti, Ag, Cu, and Pb in the vicinity of melting temperatures the solid phase growth is described by means of a fluctuation theory applied for the macro-systems. By virtue of the “unlimited” and limited spectrum solid state particle concentration fluctuations' theory some effective kinetic coefficients have been estimated in the work. The effective kinetic coefficients are specific in the case of normal crystal growth kinetics. In the case of the crystallization of Fe, Co, Ni, Cr, Zr, Ti, Ag, Cu, and Pb-melts a comparison between two effective kinetic coefficients (based on the fluctuation theory and so-called equilibrium growth points density) has been realized. In most cases a satisfactory agreement between the effective kinetic coefficients has been observed.     

Binary Ni–P bulk amorphous glass prepared by electrodeposition method

Compact and uniform bulk Ni–P binary amorphous alloys (BAAs) with thickness of up to 1 mm were prepared using electrodeposition method. This is the first finding that amorphous alloy can be obtained up to ‘bulk’ size not by rapid solidification but by electrochemical method. By improving the electrodeposition techniques, selecting proper plating solution, electrodeposition is probably a new method for preparing bulk amorphous alloys. The formation mechanism of the bulk amorphous was discussed within the concept of ‘disorder solid’.     

Ti3(ZnxAl1-x)C2固溶体热学、电学和力学性质的理论研究

采用第一性原理的密度泛函理论平面波赝势法, 通过投影缀加波(PAW)和广义梯度近似(GGA)系统地研究了Ti₃(Zn_xAl_1-x)C₂的结构、能量、声子性质、电子性质和弹性性质。对MAX相Ti₃AlC₂晶体中A位置的Al元素用Zn元素进行替换掺杂,构建出Ti₃(Zn_xAl_1-x)C₂(x=0,0.25,0.5,0.75,1)固溶体结构模型。计算分析表明:在所研究的掺杂浓度范围内Ti₃(Zn_xAl_1-x)C₂均是热力学、动力学和力学稳定的脆性材料;此外,Ti₃(Zn_xAl_1-x)C₂(x=0,0.25,0.5,0.75,1)均呈现金属性,在费米能级处的电子态密度主要贡献来自Ti-3d态,同时具有离子键、共价键和金属键的综合性质。随着Zn原子掺杂浓度的增加,在一定程度上其导电性和塑性均增强。     

The order-disorder transformation at supercooled melt/crystal transition region of binary melts (I) the master equation

Binary melts crystallizing with almost perfectly ordered structures if exposed to conditions close to thermodynamic equilibrium exhibit an increasing tendency to form metastable crystals with increasing disorder when these alloys crystallize on the growth conditions of supercooling. Based on the model of a two-phase transition zone separating a growing crystal from its non-solid surrounding a kinetic master equation is formulated describing the kinetics of competitive atomic exchange processes within the transition region. The theory considers the case of simple cubic structure with equal particle numbers of two components that form a NaCl-type lattice at the state of perfect order. In a subsequent paper solutions of the master equation obtained for steady-state conditions are discussed.     

‘Star‐like’ defects in Cd‐annealed CdZnTe crystals—an experimental study of their origin and formation mechanism

We conducted low‐temperature annealing experiments at temperatures slightly above and below the melting point of Te to clarify the effects of the state of Te inclusions (solid or liquid) upon the formation of ‘star‐like’ defects in Cd‐annealed CdZnTe (CZT). We also carried out post‐growth annealing experiments with and without using Cd vapor to clarify the mechanism of formation of such defects. We demonstrated that these ‘star‐like’ defects are due to the reaction between in‐diffused Cd atoms and the molten Te inclusions, but we found no observable ‘one‐to‐one’ correlation between ‘star‐like’ defects and Te inclusions. The non‐uniform distribution of Te inclusions in the CZT matrix could account for this phenomenon since the punching distance of the dislocations depends on the volume fraction of inclusions within the matrix.     

Synthesis by co-sputtering of Au–Cu alloy nanoclusters in silica

In this work we investigated the Au–Cu binary nanophase diagram by means of structural and compositional characterization of alloy nanoclusters synthesized in silica by means of rf-cosputtering technique. In order to obtain the formation of metallic nanoclusters, as deposited samples have been annealed in reducing atmosphere at 900 °C. A good agreement with the bulk alloy phase diagram has been found: in particular the fcc cubic alloy solid solution is present in all the investigated compositions from pure Au to pure Cu, with a lattice parameter following the modified Vegard’s law typical of the bulk alloy. Three ordered phases are also present corresponding to the Au:Cu atomic ratios of 3:1, 1:1 and 1:3. The linear and non-linear optical properties of the nanocomposites have been measured: the surface plasma resonance red-shifts with increasing the Cu content in the alloy and it spans the entire region from that of pure Au (530 nm) to that of pure Cu (570 nm).     

Investigation of the phase diagram KCl?BaCl2 in the region of solid solutions with the base of KCl

This paper is concerned with the determination of the lines of maximum solubility of the phase diagram for the system KCl BaCl₂ in the region of solid solutions with the base of KCl by the method of light scattering. The results are discussed in terms of the theory of dilute solutions of solid electrolytes. The enthalpy of solution Ψ = 0.64 eV and the change of vibrational entropy ΔS/K = 1.9 are defined also.     

Pseudopotential in the study of superconducting state parameters of metallic glasses

A well recognized model potential is used to study the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ¹, transition temperature T_C, isotope effect exponent α and effective interaction strength N₀V of some binary metallic glasses of simple, non-simple and transition metals. Five local field correction functions proposed by Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used in the present investigation to study the screening influence on the aforesaid properties. Instead of using Vegard’s law, pseudo-alloy-atom model (PAA) in the present investigation is proposed and found successful. The present results of the SSP are found in qualitative agreement with other such earlier reported data wherever exist, which confirms the superconducting phase in the metallic glasses.     

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu₅₀Ti₅₀ and Ti₅₀Ni₃₀Cu₂₀, are reported. The activation energies for crystallization, E_c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E_c obtained using these methods are consistent for both the metallic glasses and it is found that E_c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.     

Theoretical calculation of dense random packings of equal and non-equal sized hard spheres applications to amorphous metallic alloys

A model describing the structure of amorphous metallic alloys is proposed using a packing of non-equal-sized hard spheres. Models containing up to 1000 spheres were generated by computation. Hypotheses guiding the calculation are chosen in order to obtain a model as compact as possible and to take into account affinity between metal and metalloids: therefore two small spheres are not allowed to be in hard contact. This method of calculation is first tested by building up heaps of equal-sized spheres. Both the radial distribution function and the interference function are calculated for each model and the variations of these curves with the number of spheres are studied. In the case of two sizes of spheres heaps calculations are devoted to the study of the so-called ‘shoulder interference function’ which characterizes many amorphous alloys such as NiP, PdSi, etc. It is shown that the existence of such a shoulder depends on two parameters: the ratio of sphere diameters and the relative concentration of small and large spheres. These results are in good agreement with some recent experimental observations. This shoulder is well-marked when the diameter ratio reaches 10% and when the concentration in small spheres lies in the range 10–15%. In such a structure (e.g. amorphous NiP and PdSi alloys) it is observed that small spheres are always surrounded by nine large spheres such as P or Si atoms in crystalline Ni₃P or Pd₃Si. Large spheres, i.e. metallic atoms, form distorted icosahedra, the distortion of which varies from one to the other. The short-range order has a five-fold symmetry which is not compatible with any long-range order.     

On the phase diagram of polymorphic ethanol: Thermodynamic and structural studies

It is well known that ethanol exhibits a very interesting polymorphism presenting different solid phases: a fully-ordered (monoclinic) crystal, a (bcc) plastic crystal, which by quenching becomes an orientationally-disordered crystal with glassy properties (hence sometimes named ‘glassy crystal’), and the ordinary amorphous glass. We have carried out calorimetric, X-ray diffraction, and Brillouin-scattering experiments above liquid-nitrogen temperatures and have found several new features that shed more light on the rich and interesting phase diagram of ethanol. Firstly, we have identified up to four different varieties of the monoclinic crystalline phase depending on the thermal history. We also present new specific-heat data of these glassy and crystalline phases below the glass transition temperature up to the melting temperature. Furthermore, we have unexpectedly found that the amorphous phase can also be obtained by the unusual route of a very slow cooling of the liquid in some particular experimental set-ups, evidencing the heterogeneous character of the crystallization kinetics of these molecular glass-formers.     

New Transport Mechanisms for Conducting Polymers: The Role of Disorder

Abstract

A unified view of the metallic state in polyaniline and heavily-doped polacetylene is presented. We first consider a single randomly-protonated strand of the emeraldine form of polyaniline. We show explicitly that the disorder inherent in this system is described by the random-dimer model of Dunlap, Wu and Phillips¹. The random dimer model is simply a tight-binding model for a binary alloy in which pairs of lattice sites are assigned one of two values at random. The random dimer model is shown to possess a narrow band of conducting states that can ultimately lead to dramatic increases in the conductivity if the Fermi level is appropriately tuned. It is demonstrated explicitly that the location in the energy band of the conducting states of the random-dimer model for polyaniline coincides with recent calculations of the location of the Fermi level in the protonated form of the polymer^2,3. We argue then that the random-dimer model is capable of explaining the insulator-metal transition in polyaniline. In the context of polyacetylene, we propose a ‘dirty metal’ picture for the metallic state which is consistent with both the closing of the band gap and the existence of a band of extended states. Our model is based on the observation that a random distribution of solitons closes the band gap at ～5-6% doping and is formally equivalent to the random dimer model. The location of the extended states in the disordered soliton lattice is computed. It is shown that because these states lie in the vicinity of the band edge, they are capable of explaining the sudden onset of the experimentally-observed Pauli susceptibility and the subsequent depinning of the solitons in the metallic state. We close by noting that because the random dimer model applies to any lattice in which the defects are extended and possess a plane of symmetry, it quite generally describes any polymeric system in which the stable defects are solitonic or bi-polaronic in nature