Growth Of Intermediate Phases Research Articles

Choice and ambiguity of diffusion paths during formation and growth of intermediate phases and two-phase zones in ternary systems

Choice and ambiguity of diffusion paths during formation and growth of intermediate phases and two-phase zones in ternary systems

Metal-induced crystallization of amorphous semiconductor films: Nucleation phenomena in Ag-Ge films

The initial stages of interfacial interactions between Ag nanoparticles and amorphous Ge (a-Ge) films, in particular the mechanism and kinetics of metal-induced crystallization (MIC) of a-Ge films are investigated herein. Unsupported Ag-Ge films formed by PVD were used as a model. In situ high-resolution (S)TEM and low-loss monochromated STEM-EELS techniques were used for real-time observation of the nucleation and growth of crystalline Ge and intermediate phases, as well as mapping of chemical elements at the reaction zone. We show that crystallization is activated at the metal–semiconductor interface around ≈200 °C, followed by lateral crystallization of a-Ge film at temperatures above ≈400 °C. It was revealed that the MIC process occurs through intermediate phases. Metastable Ag0.8Ge0.2 hcp phase was identified at the reaction front at elevated temperatures, thus revealing the non-equilibrium melting of a eutectic alloy as the most probable mechanism of Ag-induced crystallization of a-Ge film. We show that the MIC process is activated only if the size of Ag nanoparticles exceeds a critical value, which is ≈8 nm in our case and equal to the thickness of the a-Ge film.

Alternative algorithms for simultaneous modeling of ordering and intermediate compound growth during reactive diffusion

Standard Monte Carlo algorithms with constant pair interactions within the first coordination shell are usually quite effective for the description of ordering, tracer diffusion, chemical diffusion (but without Kirkendall shift effect) within any already existing ordered intermediate phase. Yet, one encounters big problems if one tries to simulate the formation and growth of ordered intermediate phase or, especially, simultaneous formation and growth of several intermediate phases during interdiffusion. Two algorithms providing solution of this problem, are discussed. The first algorithm is known (but not widely used for reactive growth) – account of interactions in the two coordination shells with negative mixing energy in the first coordination shell and positive mixing energy in the second one. The second algorithm is new: interactions only within the first coordination shell of each atom but depending on the local composition within the cluster around interacting atoms. Both algorithms are shown to provide formation and growth of the ordered intermediate phases with rather narrow concentration ranges, growing with time (after some initial period) according to parabolic law.

Simulated Crystal Growth of Intermediate Phases Formed during Li and O Extraction of Nano-Li2MnO3 Cathode Material

Li2MnO3 is considered as a potential candidate for high capacity (theoretical capacity of 458 mAh/g) lithium ion batteries applicable in electric vehicles owing to its larger number of exploitable Li ions. However, it is vulnerable to degradation originating from complex phenomena such as the oxygen release, phase transformation and Li+ /H+ exchange reaction, but the detailed mechanisms governing these degradation behaviors have not been fully understood yet. In the current work, spontaneous crystal growth of the nano-Li2MnO3 structures via the amorphisation and crystallisation technique is carried out with systematic removal of oxygen and lithium contents in a quest to explore the formation of different intermediate structures/phases. The simulated nano-architectures crystallised into multi-grained (except for Li1.75MnO2.75) and highly defective structures. To study the formation possible phases, the XRD patterns were measured for all intermediate structures and reveal emergence of new Mn3O4 peak at 2Ө~ 29⁰ and strong cubic spinel LiMn2O4 at 2Ө~ 37⁰. Microstructural analysis revealed the formation of layered and spinel-type structures for low lithium and oxygen content nanostructures. These findings shed light towards mechanisms that take place during the cycling of nanostructured high capacity oxide electrodes during lithium and oxygen extraction that will help guide the optimisation of new Li-rich materials for rechargeable lithium batteries.

Solid–state diffusion–controlled growth of the intermediate phases from room temperature to an elevated temperature in the Cu–Sn and the Ni–Sn systems

The solid–state growth of intermediate phases in the Cu–Sn and Ni–Sn systems is investigated in the temperature range of room temperature to an elevated temperature. In the Cu/Sn diffusion couples, Cu6Sn5 and Cu3Sn grow at temperatures ≥ 100 °C and only the Cu6Sn5 phase is found at lower temperatures. The integrated interdiffusion coefficients are estimated in the temperature range of 125–215 °C because of the observation of parabolic growth. The activation energies are estimated to be 64±4 and 75±6 kJ/mol for Cu3Sn and Cu6Sn5, respectively. In the Ni/Sn diffusion couples, only Ni3Sn4 grows at higher temperatures. At 50–100 °C, metastable NiSn4 phase is found along with an equilibrium Ni3Sn4 phase. At room temperature, only the metastable NiSn4 phase is found. The activation energy for the integrated interdiffusion coefficient of Ni3Sn4 is estimated as 71.6±5.3 kJ/mol, which is similar to the values estimated in the Cu–Sn system. The growth rate of the product phases, for electroplated couples, is found to be a little higher in the Ni–Sn system as compared to the Cu–Sn system. During storage at room temperature and service at elevated temperatures, the phases in the Ni–Sn system have comparable growth rate when compared to the same in Cu–Sn system.

Criteria of kinetic suppression of lateral growth of intermediate phases

In many cases, the formation of intermediate phases in solid-state reaction starts from nucleation, proceeds at first by lateral spreading along interface in the form of narrow layer and after that continues slow growth in normal direction. So far, the lateral growth stage was considered as something almost instant. Yet, if the emerging phase is not the first one and the already growing phase layer has high diffusivity, the lateral growth can be also kinetically suppressed due to ‘sucking out’ of atoms from the moving interface into the growing first phase layer. Criteria of such suppression are formulated and examples of real systems are discussed.

Effect of Ag addition to Zn–12Al alloy on kinetics of growth of intermediate phases on Cu substrate

Abstract Zn–Al–Ag solders based on eutectic Zn–12Al alloy containing 0.5–1.5 at.% Ag are developed for ultra high temperature applications. Wettability tests carried out on Ag-doped Zn–Al alloys on Cu substrates showed the formation of Cu–Zn phases at the interface. Wettability studies were performed with the use of flux at 500 °C for 15, 30, 60, 180, 240, 900, 1800 and 3600 s, and for 240 s at 460, 480, 520, 550 °C, respectively. Experiment was designed this way in order to demonstrate the effect of Ag addition on the kinetics of formation and growth of CuZn, Cu5Zn8, CuZn4 phases, which were identified with EDS analysis. In addition to improving spreadability of Zn–Al–Ag alloys, Ag increases their melting temperature, electrical resistivity, and coefficient of thermal expansion.

The Stressing Effect of Electromigration from the Maxwell Stress and a Preliminary Mean-Time-to-Failure Analysis

As temperature increases, it is suggested that atoms on lattice sites serve as dynamic defects and cause a much more homogeneous distribution of the Maxwell stress throughout the crystal lattice compared with that caused by static defects. Though this stressing effect mostly leads to Joule heating, it also results in distortion of the crystal lattice, which leads to a decrease in the activation energy for atomic diffusion and causes enhancements in the phase growth rates at both interfaces of diffusion couples. Due to this stressing effect, the decrease in the activation energy is proportional to a square term of the current density J. A mean-time-to-failure analysis is performed for failure caused by excessive growth of intermediate phases, and a mean-time-to-failure (MTTF) equation is found. This equation appears similar to Black’s equation but with an extra exponential term arising from the stressing effect of the crystal lattice.

The Growth Kinetics of Nickel Aluminide Coatings by the Bi‐velocity Method

AbstractIn this work we solve the problem of the simultaneous growth and competition of intermediate phases. The crystal lattice as the frame of reference for diffusion is defined, based on the volume continuity equation. It permits the computation of the material velocity in a system in which reactions at several moving interfaces occur. All reactions lead to the lattice shift due to the difference of diffusivities and molar volumes. We simulate the CVD process followed by annealing in a non‐oxidizing atmosphere. The computed kinetics and concentration profiles are compared with experimental results.

Modeling of Reactive Diffusion: Mechanism and Kinetics of the Intermetallics Growth in Ag/Ag Interconnections

The phenomenological model describing the growth of intermetallic phases in multi-component systems is presented. Full time-dynamics approach is applied without the often-used simplifications such as flux constancy. General form of the species flux is considered, which consists of chemical potential gradient as a driving force for diffusion with additional drift term. Stefan-type (moving) boundary conditions are applied. In the present form, the model assumes local equilibrium at each interface and that the process of growth of intermediate phases is controlled by diffusion of reagents through the layers and/or chemical reactions at the boundaries. The model is solved in its full generality. Numerical method for the solution of the problem has been developed. Specially selected change of dependent variables transforms the moving boundary problem into an equivalent fixed boundary problem. Such problem has been treated using the method of lines which converts partial differential equations into a system of ordinary differential equations, which is subsequently solved numerically. The obtained solution was tested and compared with analytic ones available in special cases, showing satisfactory agreement. The growth of intermetallic phases in Ag/Sn/Ag system has been modeled and compared with experimental results.

Chemical interdiffusion in binary systems; interface barriers and phase competition

The problem of simultaneous growth and competition of intermediate phases during reactive diffusion is formulated and solved. In this paper, we compare existing models of steady state reaction diffusion and introduce the new one basing on the bi-velocity method. We extend old problem and propose method based on material (lattice) fixed frame of reference. It allows computing the material velocity in the reacting system in which reactions at several moving interfaces occur. All reactions lead to the lattice shift due to the difference of intrinsic diffusivities and different molar volumes. The following peculiarities are taken into account: (1) the deviation from local equilibrium at all interfaces; (2) the mobilities of the components in the bulk and interphase zone; and (3) the molar volumes of the components. We show the kinetic of the reactions, the non parabolic regime, the multiphase scale growth and present the practical application of the method.

Phase formation in contact of dissimilar metals

Formation and growth of intermediate phases in contact of the crystalline samples forming a two-component eutectic system is considered. It is shown that during the competition to a growing liquid phase the intermediate solid phases cannot grow by diffusion. The alternative is formation of metastable areas of a liquid phase.Measurements of liquid layers extent in Pb-Bi and In-Bi systems have allowed to define the composition of liquid on interface where formation of metastable liquid is possible. The results show that the concentration interval of a liquid layer corresponds to a stable constitution diagram. In order to explain the experimental results the hypothesis according to which the intermediate solid phases are formed as a result of precipitation from metastable melt is considered.The experimental confirmation of formation and crystallization of a metastable liquid is the fact that intergrowth of the samples forming system with an intermetallic phase at temperatures below the temperature of fusion of the most low-melting eutectic is observed.The possibility of the processes concerned with the occurrence of metastable areas of a liquid is showed by means of computer imitation.

Interfacial Reaction Between Cu Substrates and Zn-Al Base High-Temperature Pb-Free Solders

Chemical reactions between Cu substrates and Zn-Al high-temperature solder alloys, Zn-4Al and Zn-4Al-1Cu (mass%), at temperatures ranging from 420°C to 530°C were experimentally investigated by a scanning electron microscope using backscattered electrons (SEM-BSE) and an electron probe microanalyzer (EPMA). Intermediate phases (IMPs), β(A2) or β′(B2), γ(D82), and e(A3) phases formed and grew during the soldering and aging treatments. The consumption rate of the IMP for Cu substrates is described by the square root of t in both the alloys, while the additional Cu in the molten Zn-Al alloy slightly suppresses the consumption of Cu substrates. The growth of IMPs during soldering treatment is controlled by the volume diffusion of constituent elements, and its activation energy increases in the order of Q e < Q γ < Q β. In view of the aging process, the growth of IMPs is considered to be controlled by the volume diffusion. In particular, the layer thickness of γ rapidly grows over 200°C, although the thickness of the β layer grows very slowly.

Computer simulation of the growth of intermediate phases in a complex metallic system

The technique and results of computational experiment on the contact melting in a two-component system with an intermetallic compound have been considered. The possibility of existence of metastable states in the liquid interlayer has been assumed. The conditions under which the parabolic law of growth of the liquid-interlayer extension is obeyed and the liquid-interlayer concentration field fits the equilibrium phase diagram have been clarified. Results of the simulation and real experiment have been compared.

Intermediate Phase Growth of Mg-Al Diffusion Couple under a Strong Static Magnetic Field

Intermediate phase growth in Mg-Al diffusion couples were studied with different intensity of a strong static magnetic field from 0 to 10 Tesla. Thickness measurement of the intermediate phases (Mg17Al12 and Al3Mg2) shows that with the increasing of magnetic field intensity, the growth rate of both intermediate phases is retarded. The decrease of the phase growth rate is ascribed to the suppressed Al, Mg atom interdiffusion in the diffusion couple under the static magnetic field. It is also found that the orientation relationship between couple interface and magnetic field direction has no influence on the growth of intermediate phases.

Physico-Chemical Analysis of Compound Growth in Binary Interdiffusion Systems

A diffusion-controlled growth of intermetallic phases and the role of the Kirkendall effect in morphological evolution of the product phase layers can be described in terms of an alternative theory considering chemical reactions at the interphase interfaces. Application of this “physicochemical” treatment to diffusional growth of intermediate phases with fairly wide homogeneity ranges is illustrated by the example of interaction in the Ag-Zn system. The model is purely phenomenological, and its use is convenient, since no explicit assumption of the underlying diffusion mechanism is required.

An effective approach to describe growth of binary intermediate phases with narrow ranges of homogeneity

An effective approach to describe growth of binary intermediate phases with narrow ranges of homogeneity

Dissolution Rates of Pure Nonionic Surfactants

Small drops of the pure linear alcohol ethoxylates C12E5 and C12E6 were injected into water at temperatures below their cloud points, and the times required for their dissolution were measured using videomicroscopy. Separately the rates of growth of the various liquid crystalline intermediate phases formed during penetration experiments with these surfactants in vertical linear cells were measured using videomicroscopy. It was found from both types of experiments that the dissolution process was controlled by diffusion, not by kinetics of phase transformation at interfaces. Effective diffusivities of the various phases were calculated from the data obtained and were found to be of order 10-10 m2/s. Finally, interferometry was used to measure concentration distributions as a function of time during dissolution of the lamellar phase of C12E5. Diffusivity in the micellar solution was found to increase with increasing surfactant concentration with a further increase occurring at the concentration where the la...

Metal-ceramic solid state bonding: Mechanisms and mechanics

Most applications of ceramics in devices and structures require some type of ceramic-metal joining. Various processes are available for joining. Two most frequently used techniques are liquid phase joining and solid state bonding. This paper describes the mechanisms leading to the formation of high strength bonds and the mechanical behavior of ceramic-metal multimaterials made by solid state bonding. In this process, bonding is achieved through two stages: (a) morphological adaptation of the surfaces to be joined and (b) subsequent formation of bonds: van de Waals interactions, chemical bonds by simple recovery of the molecular orbitals or growth of intermediate phases. From the mechanical point of view, the study of ceramic-metal interfaces has to consider other factors like: interfacial fracture mechanics, residual stresses, modifications, damages and embrittlement of materials near the interface, as well as many physico-chemical aspects. Thus, a complete understanding of ceramic-metal structures requires an overall analysis of properties of bulk materials and interfaces in the joining area.

Reactive diffusion in thin films

Interdiffusions in thin films and in bulk specimens are not basically different, contrarily to a frequent statement. The kinetics of the whole process results from a number of elemental processes, i.e. diffusion of chemical species and point defects, chemical reactions at interfaces leading to the nucleation and growth of intermediate phases, point defect production and/or annihilation at interfaces, grain boundary diffusion,… The interplay of these various elemental processes, either in parallel or in series, makes the resulting story rather complicated, and very sensitive to the geometry of the specimen, the microstructure and the preparation of the material. The general features of the so-called “reactive diffusion” will be discussed in this article on the basis of simple arguments. Some rules — or eventually absence of rule — will be set forth, that should serve as guide lines for the people who have to tackle such problems.