Solid State Amorphization Reaction Research Articles

Impurity-enhanced solid-state amorphization: the Ni–Si thin film reaction altered by nitrogen

Solid-state amorphization, the growth of an amorphous phase during annealing, has been studied in a wide variety of thin film structures. Whereas research on the remarkable growth of such a metastable phase has mostly focused on strictly binary systems, far less is known about the influence of impurities on such reactions. In this paper, the influence of nitrogen, introduced via ion implantation, is studied on the solid-state amorphization reaction of thin (35 nm) Ni films with Si, using in situ x-ray diffraction (XRD), ex situ Rutherford backscattering spectrometry, XTEM, and synchrotron XRD. It is shown that due to small amounts of nitrogen (<2 at.%), an amorphous Ni–Si phase grows almost an order of magnitude thicker during annealing than for unimplanted samples. Nitrogen hinders the nucleation of the first crystalline phases, leading to a new reaction path: the formation of the metal-rich crystalline silicides is suppressed in favour of an amorphous Ni–Si alloy; during a brief temperature window between 330 and 350 °C, the entire film is converted to an amorphous phase. The first crystalline structure to grow is the orthorhombic NiSi phase. We demonstrate that this impurity-enchanced solid-state amorphization reaction occurs only under specific implantation conditions. In particular, the initial distribution of nitrogen upon implantation is crucial: sufficient nitrogen impurities must be present at the interface throughout the reaction. Introducing implantation damage without nitrogen impurities (e.g. by implanting a noble gas) does not cause the enhanced solid-state amorphization reaction. Moreover, we show that the stabilizing effect of nitrogen on amorphous Ni–Si films (with a composition ranging from 40% to 50% Si) is not restricted to thin film reactions, but is a general feature of the Ni–Si system.

Atomic resolution observation of conversion-type anode RuO2 during the first electrochemical lithiation

Transition metal oxides have attracted great interest as alternative anode materials for rechargeable lithium-ion batteries. Among them, ruthenium dioxide is considered to be a prototype material that reacts with the Li ions in the conversion type. In situ transmission electron microscopy reveals a two-step process during the initial lithiation of the RuO2 nanowire anode at atomic resolution. The first step is characterized by the formation of the intermediate phase LixRuO2 due to the Li-ion intercalation. The following step is manifested by the solid-state amorphization reaction driven by advancing the reaction front. The crystalline/amorphous interface is consisted of {011} atomic terraces, revealing the orientation-dependent mobility. In the crystalline matrix, lattice disturbance and dislocation are identified to be two major stress-induced distortions. The latter can be effective diffusion channels, facilitating transportation of the Li ions inside the bulk RuO2 crystal and further resulting in non-uniform Li-ion distribution. It is expected that the local enrichment of the Li ions may account for the homogeneous nucleation of dislocations in the bulk RuO2 crystal and the special island-like structures. These results elucidate the structural evolution and the phase transformation during electrochemical cycling, which sheds light on engineering RuO2 anode materials.

The local atomic structure and chemical bonding in sodium tin phases

To understand the electrochemically-derived Na–Sn we have reinvestigated the formation of Na–Sn alloys to identify all the phases which form when x ≥ 1 (NaxSn) and characterized the local bonding around the Sn atoms with X-ray diffraction, 119Sn Mossbauer spectroscopy, and X-ray absorption spectroscopies. The results from the well-defined crystallographic materials were compared to the spectroscopic measurements of the local Sn structures in the electrochemically prepared materials. The reinvestigation of the Na–Sn compounds yields a number of new results: (i) Na7Sn3 is a new thermodynamically-stable phase with a rhombohedral structure and Rm space group; (ii) orthorhombic Na9Sn4 (Cmcm) has relatively slow formation kinetics suggesting why it does not form at room temperature during the electrochemical reaction; (iii) orthorhombic ‘Na14.78Sn4’ (Pnma), better described as Na16−xSn4, is Na-richer than cubic Na15Sn4 (I3d). Characterization of electrochemically prepared Na–Sn alloys indicate that, with the exception of Na7Sn3 and Na15Sn4, different crystal structures than similar Na–Sn compositions prepared via classic solid state reactions are formed. These phases are composed of disordered structures characteristic of kinetic-driven solid-state amorphization reactions. In these structures, Sn coordinates in asymmetric environments, which differ significantly from the environments present in Na–Sn model compounds.

The reaction mechanism of SnSb and Sb thin film anodes for Na-ion batteries studied by X-ray diffraction, 119Sn and 121Sb Mössbauer spectroscopies

The electrochemical reaction of Sb and SnSb anodes with Na results in the formation of amorphous materials. To understand the resulting phases and electrochemical capacities we studied the local order using 119Sn and 121Sb Mössbauer spectroscopies in conjunction with measurements performed on model powder compounds of Na–Sn and Na–Sb to further clarify the reactions steps. For pure Sb the sodiation starts with the formation of an amorphous phase composed of atomic environments similar to those found in NaSb, and proceeds further by the formation of crystalline Na3Sb. The reversible reaction takes place during a large portion of the charge process. At full charge the anode material still contains a substantial fraction of Na, explaining the lack of recrystallization into crystalline Sb. The reaction of SnSb yields Na3Sb at full discharge at higher temperatures (65 and 95 °C) while the RT reaction yields amorphous compounds. The electrochemically-driven, solid-state amorphization reaction occurring at RT is governed by the simultaneous formation of Na-coordinated Sn and Sb environments, as monitored by the decrease (increase) of the 119Sn (121Sb) Mössbauer isomer shifts. Overall, the monitoring of the hyperfine parameters enables to correlate changes in Na content to the local chemical environments.

Macroscale and microscale evolutions of mechanically alloyed Fe [sbnd]Zn systems

The present work aims to correlate, in time, macroscale and microscale phenomenological evolutions of the microstructure of Fe and Fe Zn alloys processed by mechanical milling (MM) and alloying (MA), respectively. Powders were characterized for particle size distribution (PSD), particle morphology (optical microscopy, OM, scanning electron microscopy, SEM), microhardness, crystallite size, differential scanning calometry (DSC) and transmission electron microscopy (TEM). Two macroscopic regimes of PSD behavior were distinguished: the first one dominated by the cold welding process; and, the other where both fracture and agglomeration play a significant role. Solid solubilization of Zn on bcc Fe was found to reduce the final microhardness as well as increase the lattice parameter and is very well predicted by Miedema's thermodynamical approach. Microhardness and solid solution formation kinetics were correlated in time and both could be precisely described by a logistic function. After 5 h of planetary milling, microhardness and the lattice parameter become stable as well as the PSD and particle morphology, indicating that the system has already reached steady state. Indeed, this condition can be monitored by both macroscopic and microscopic parameters. Prior to an homogeneous powder, DSC results suggest an endothermic solid-state amorphization reaction for samples processed for up to 1 h as a result of the formation of clean Fe/Zn interfaces during MA.

Interfacial reactions in Sn/Ni–7 wt.%V couple

In the early stage of the Sn/Ni–7 wt.%V reaction at 200 °C, solid state amorphization reaction occurs and an amorphous T phase layer is formed. After 48 h of reaction, a Ni3Sn4 phase layer is formed between Sn and the T phase, and the reaction path is Ni–V/T/Ni3Sn4/Sn. Periodic layers, Ni–V/T/Ni3Sn4/T/Ni3Sn4/Sn, are found in the couples after 72 h of reaction. The reliability assessment of the flip chip product using Ni–V barrier needs to be evaluated based on the results in the Sn/Ni–7 wt.%V couples.

Amorphization in Gd–Co alloys and multilayers

A semiempirical model is used to analyse the results of published experimentsreporting on the solid-state amorphization reactions in bilayers and multilayersformed by Gd and Co. The role of the interfacial effects in raising the free energyof the initial arrangement in a multilayered configuration, and in promoting theamorphization reaction, is studied in detail. The model explains the observation ofamorphous alloys over a broad composition range in the bilayer experiments. Thepreferred composition obtained in the multilayer experiments is discussedcritically and the model prediction of a preferred composition Gd0.46Co0.54 is ingood agreement with the compositions observed in recent experiments.

Mechanically driven solid state amorphization reaction of ball-milled Nb 50Zr 10Al 10Ni 10Cu 20 powders and the effect of annealing

A single amorphous phase of Nb 50Zr 10Al 10Ni 10Cu 20 alloy powders was synthesized by ball milling the elemental powders at room temperature. During the first stage of milling, the atoms of Zr, Al, Ni and Cu migrated and diffused into the Nb (base material) lattice to form a bcc supersaturated solid solution that transformed into an amorphous phase with the same composition upon post-annealing at 725 K. As the milling time increased, the powders were subjected to continuous lattice imperfections that lead to gradual changes in the free energy, so that the solid solution phase is transformed to an amorphous phase. At the final stage of milling (720 ks) the crystallization temperature and the enthalpy change of crystallization for the obtained amorphous alloy were 963 K and −8.44 kJ mol −1, respectively.

Critical Solid Solubility of the Ni-Ti System Determined by Molecular Dynamics Simulation and Ion Mixing

From a realistic n-body potential of the Ni–Ti system, the critical concentrations of the Ni- and Ti-rich solid solutions were determined by molecular dynamics (MD) simulation to be 38 at% Ti and 15 at% Ni, respectively, beyond which a disordered atomic configuration was more stable than the respective crystalline solid solutions. It follows that the central composition range bounded by the critical solubilities, i.e. within 38–85 at% of Ti, can be considered as the glass-forming range of the system, which was confirmed by room temperature 200 keV xenon ion mixing of alternately deposited Ni–Ti multilayered films. Moreover, MD simulation of a Ni–Ti bilayer revealed that during the solid-state amorphization reaction, the growth of the amorphous interlayer followed exactly a t1/2 law and grew faster towards the Ti lattice than to the Ni side. The physical origin of such an asymmetric behaviour was found to be due to a difference in critical solid solubilty of the constituent metals.

Epitaxial growth of CoSi2 film by Co/a-Si/Ti/Si(100) multilayer solid state reaction

Epitaxial growth of CoSi2 by solid state reaction of Co/a-Si/Ti/Si(100) is investigated. A Ti/a-Si composite interlayer is used to modify the diffusion barrier and influence the epitaxial growth process. The epitaxial quality of the CoSi2 is improved compared to the film grown by Co/Ti/Si reaction. A multielement amorphous layer is formed by a solid-state amorphization reaction at the initial stage of the multilayer reaction. This layer acts as a diffusion barrier, which controls the atomic interdiffusion of Co and Si while limiting the supply of Co atoms. CoSi2 grows as the first phase and the growth interface of the epitaxial CoSi2 is at both the CoSi2/Si and CoSi2/CoSi interfaces. Investigation of the growth kinetics shows that the activation energy of CoSi2 formation is larger than that without an amorphous Si layer.

Effect of elastic stresses on solid-state amorphization of Zr/Co multilayers

We have examined the effect of externally applied stresses on the solid-state amorphization reaction of Zr/Co multilayers prepared by electron-beam evaporation. Three identically deposited samples were attached onto a three-point bending apparatus to impose different elastic strains on individual samples and subsequently annealed at temperatures of 483, 503 and 543 K. The imposed bending strain, estimated to be as small as 0.08%, had a significant influence on the amorphization kinetics. Cross-sectional transmission electron microscopy revealed clearly that changing the applied bending strain from tensile to compressive enhanced the amorphization reaction. This enhancement was accompanied by an extensive amorphization along grain boundaries in the Zr layers. According to the present observations, it is suggested that the applied bending stresses play an important role in the amorphization reaction by influencing the interface reaction barrier associated with the actual transformation of the terminal solution phase of Zr into the amorphous phase.

Defect-enhanced solid-state amorphization in Zr100−xAlx/Ni thin-film diffusion couples

The interdiffusion reaction between supersaturated nanocrystalline Zr100−xAlx and crystalline Ni films leads to the formation of a planar amorphous Zr–Al–Ni interlayer. This solid-state amorphization reaction has been investigated by in situ measurement of electrical conductance and by transmission electron microscopy. With increasing defect density in the Zr100−xAlx layers, the diffusivity of Ni in the amorphous interlayer increases and the activation energy decreases. These results are discussed with respect to Stephenson’s analysis of interdiffusion and stress formation in systems with components of differing mobilities.

Interdependence of stress and interdiffusion during solid-state amorphization in Ni–Hf

The evolution of stress in a Ni–Hf diffusion couple during solid-state amorphization reaction has been monitored by substrate curvature measurements and x-ray diffraction. The latter technique allowed an independent determination of the contribution of changes in stress-free lattice parameter to the stress in the crystalline layers. The results indicate that the amorphous phase forms under a large tensile stress, which relaxes as the reaction progresses. This stress in the amorphous phase is consistent with the volume change associated with the reaction. Stresses in the crystalline, elemental phases are considerably smaller and not affected by the reaction. Low-temperature Ni ion irradiation increases the tensile stress in the diffusion couple. The large observed stress variations are not accompanied by variations in the effective interdiffusion coefficient.

Amorphous phase formation during cold rolling of Al–Sm and Zr–Cu–Ni–Al multilayer structures

The early stages of the mechanically activated solid state amorphization reaction (SSAR) were investigated in multilayered samples prepared by cold rolling of elemental foils of Al92Sm8 and the bulk glass forming alloy Zr68Cu18Ni9Al5. The microstructural development in the cold rolled samples during the mechanical alloying process was measured by X-ray diffraction, scanning, and transmission electron microscopy at different deformation levels to compare the amorphization reaction for these different glass forming systems. The mechanical alloying process first yields the formation of a multilayered structure with layer thicknesses below 100 nm. During continued rolling, the elemental grain size decreases (to approximately 50 nm for Zr68Cu18Ni9Al5 and 20–30 nm for Al92Sm8) and the formation of amorphous phases starts in localized regions of the samples. In the final stage of the rolling process the Zr68Cu18Ni9Al5 sample transforms completely to the amorphous state.

Solid-state amorphization reaction in mechanically deformed AlxHf100−x multilayered composite powders and the effect of annealing

Single phase amorphous AlxHf100−x alloys with a wide amorphization range (33≤x≤75) were synthesized by the solid-state interdiffusion of pure polycrystalline Al and Hf powders at room temperature using a rod-milling technique. The mechanisms of metallic glass formation and competing crystallization processes in the mechanically deformed composite powders were investigated by means of X-ray diffraction, differential thermal analysis, scanning electron microscopy and transmission electron microscopy. The numerous intimate layered composite particles of the diffusion couples that formed during the first and intermediate stages of milling (0–173 ks) are intermixed to form amorphous phase(s) upon heating to about 980 K by so-called thermally assisted solid-state amorphization (TASSA). The amorphization heat formation for the binary AlxHf100−x system via TASSA, ΔHTASSAa, was measured directly as a function of the milling time. Homogeneous amorphous alloys were also fabricated directly without heating the composite multilayered particles after milling the particles for a longer milling time (360–720 ks). This amorphization reaction is attributed to mechanical driven solid-state amorphization (MDSSA). The maximum heat formation of amorphization for the binary AlxHf100−x system via MDSSA, ΔHMDSSAa, was estimated. The crystallization characteristics indexed by the crystallization temperature, Tx, and the enthalpy change of crystallization, ΔHx, were measured for the amorphous alloys formed either by the TASSA (TTASSAx and ΔHTASSAx) or the MDSSA (TMDSSAx and ΔHMDSSAx) processes. The roles of amorphization and crystallization in each process are discussed.

Process and Mechanism of CoSi2/Si Solid Phase Epitaxy by Multilayer Reaction

AbstractA multilayer structure of Co/a-Si/Ti/Si(100) together with Co/Ti/Si(100) is applied to investigate the process and mechanism of CoSi2 epitaxial growth on a Si(100) substrate. The experimental results show that by adding an amorphous Si layer with a certain thickness, the epitaxial quality of CoSi2 is significantly improved. A multi-element amorphous layer is formed by a solid state amorphization reaction at the initial stage of the multilayer reaction. This layer acts as a diffusion barrier, which controls the atomic interdiffusion of Co and Si and limits the supply of Co atoms. It has a vital effect on the multilayer reaction kinetics, and the epitaxial growth of CoSi2 on Si. The kinetics of the CoSi2 growth process from multilayer reactions is investigated.

I. Electronic structure and thermodynamics of alloys: Alloy Metastability During Nucleation‐Controlled Reactions

AbstractThe initial stage of a first order transformation reactions is controlled by a nucleation process. In fact, the kinetic constraints provided by nucleation and growth difficulties of the new phase provide the necessary condition to expose metastable reaction pathways. The resulting hierarchy of metastability is viewed in terms of the severity of the imposed kinetic constraints. The identification of nucleation control has been extended to cover solid state reactions such as those involved in solid state amorphization reactions (SSAR) of so‐called driven systems. Some of the fundamental characteristics of alloy metastability under kinetic constraints are illustrated by experimental observations and model analyses that highlight the current understanding.

AMORPHIZATION REACTION PROCESS IN Ni/AMORPHOUS Si MULTILAYER

Solid state amorphization reaction process in Ni/amorphous Si multilayers has been quanlitatively studied by using in situ X-ray diffraction. An amorphous formation and growth model is suggested for elucidating the solid state reaction in the Ni/amorphous Si multilayers. Thermodynamic and kinetic interpretations for the amorphization reaction at grain boundaries in Ni sublayers are presented.

Heterogeneous Nucleation of the Amorphous Phase and Dissolution of Nanocrystalline Grains in Bilayer Al-Ge Thin Films

Solid State Amorphization Reaction (SSAR) was first reported in thin film couples of Au-La by Schwarz et al. Since then, many other systems have been shown to undergo SSAR. Various issues involved in SSAR have been extensively investigated and reviewed. The existence of a large negative heat of mixing, anomalous fast diffusion of one component, the requirement of heterogeneous nucleation sites such as grain boundaries are found to be some of the key features of solid state amorphization. The authors present in this paper evidence of heterogeneous nucleation and growth of the amorphous phase followed by the precipitation of germanium from the amorphous phase in Al-Ge bilayer films.

Early Stages of Solid-State Amorphization Reaction During Mechanical Alloying of a Multicomponent Zr-Powder Mixture

The synthesis of amorphous alloys by non-equilibrium techniques such as rapid quenching or ball milling has become an active subject during the past decade. Recently, a new group of Zr-based multicomponent metallic alloys with large glass forming ability have been found and are characterized by an extended supercooled liquid region and a glass transition before crystallization. Due to their low critical cooling rates, these alloys also offer the possibility to produce bulk metallic glasses by mechanical alloying and subsequent warm consolidation of the obtained amorphous powders above the glass transition temperature. Although amorphous phase formation by ball milling of elemental or intermetallic powders was studied extensively on several binary alloys systems such as Zr combined with late transition metals, the driving forces and the mechanisms of amorphization are still the subject of many investigations and discussions. In this regard, in order to allow for a meaningful comparison of the thermal stability and compositional homogeneity of mechanical alloyed amorphous powders with that of metallic glasses of the same average composition prepared by rapid liquid quenching, it is essential to develop a clear understanding of the microstructural evolution and phase formation during ball milling. This issue is addressed in the present workmore » by showing detailed structural investigations on the early stages of phase formation during mechanical alloying of an elemental Zr{sub 60}Al{sub 10}Ni{sub 9}Cu{sub 18}Co{sub 3} powder mixture. The results reveal that formation of strongly supersaturated solid solutions and fast grain refinement are the prior steps before the onset of amorphization.« less