Single-element Metal Research Articles

Preferential composition during nucleation and growth in multi-principal element alloys

The crystallization of complex, concentrated alloys can result in atomic-level short-range order, composition gradients, and phase separation. These features govern the properties of the resulting alloy. While nucleation and growth in single-element metals are well understood, several open questions remain regarding the crystallization of multi-principal component alloys. We use molecular dynamics to model the crystallization of a five-element, equiatomic alloy modeled after CoCrCuFeNi upon cooling from the melt. Stochastic, homogeneous nucleation results in nuclei with a biased composition distribution, rich in Fe and Co. This deviation from the random sampling of the overall composition is driven by the internal energy and affects nuclei of a wide range of sizes, from tens of atoms all the way to super-critical sizes. This results in short-range order and compositional gradients at nanometer scales.

Shock-induced deformation and spallation in CoCrFeMnNi high-entropy alloys at high strain-rates

Shock-induced deformation and fracture in single-crystalline and nanocrystalline CoCrFeMnNi high-entropy alloys (HEAs) under different shock intensities are investigated systematically using large-scale molecular dynamics simulations. The strong anisotropy in the single-crystalline HEA and how the reversibility of HCP-structured atoms influences the void nucleation and growth are revealed. Specifically, shock-induced FCC to HCP structural transformations are largely reversible in the case of the [001] loading direction, leading to homogeneous void nucleation; while they are largely irreversible in the cases of the [110] and [111] loading directions, resulting in void nucleation in the intersection of HCP layers. In nanocrystalline HEA, the deformations in the grain interior are similar to those in the single crystals, while the grain boundaries serve as void nucleation sites and thus weaken the spall strength. The SRO effects on the shock response in single-crystalline and nanocrystalline CoCrFeMnNi HEA are revealed, and the chemical heterogeneity is analyzed. Our results show that SRO can only slightly increase the shear stress and the spall strength, but it can cause a reduction of ductility during the spallation. The differences in SRO effects between single-crystalline and nanocrystalline models are attributed to the grain boundaries, where Cr and Mn are favorable, while Ni is favored inside the grains in the SRO model. Moreover, the comparison between CoCrFeMnNi HEA and its subsystem as well as single-element metal indicates Mn seems to be the most influential elements in CoCrFeMnNi HEA, resulting in significant changes in strength. A relation between the spall strength and strain rate is proposed to describe the simulation results and reported experimental data. Our comprehensive and systematical studies provide deep insights into the deformation and fracture in CoCrFeMnNi HEA subjected to shock loading, which deepens the understanding on the dynamic deformations of CoCrFeMnNi HEA and benefits the rational design of new HEAs/MEAs with enhanced performance.

Multiscale nanoindentation modelling of concentrated solid solutions: A continuum plasticity model

Recently developed single-phase concentrated solid-solution alloys (CSAs) contain multiple elemental species in high concentrations with different elements randomly arranged on a crystalline lattice. These chemically disordered materials present excellent physical properties, including high-temperature thermal stability and hardness, with promising applications to industries at extreme operating environments. The aim of this paper is to present a continuum plasticity model accounting for the first time for the behaviour of a equiatomic five-element CSA, that forms a face-centred cubic lattice. The inherent disorder associated with the lattice distortions caused by an almost equiatomic distribution of atoms, is captured by a single parameter α that quantifies the relative importance of an isotropic plastic contribution to the model. This results in multiple plasticity mechanisms that go beyond crystallographic symmetry-based ones, common in the case of conventional single element metals. We perform molecular dynamics simulations of equiatomic CSAs: NiFe, NiFeCr, NiFeCrCo, and Cantor alloys to validate the proposed continuum model which is implemented in the finite element method and applied to model nanoindentation tests for three different crystallographic orientations. We obtain the representative volume element model by tracking the combined model yield surface.

Recent Research Activities of Single-crystal Growth Groups in Korea

Single crystal growth research is the first step of experimental studies in condensed matter physics. Various new phenomena have been discovered in high-quality single crystalline materials, which was otherwise hidden in their polycrystalline form. In this article, we review recent research activities of single crystal growth groups in Korea. These activities cover a wide range of materials from epitaxial crystal of single-element metal to naturally-heterostructured crystals, which exhibit facinating properties and functionalites including strong oxidation resistance, superconductivity, multiferroics, quantum magnetism, and topological properties.

Effect of grain size on damage and failure in two-phase materials: Homogenized CuPb

It is well known that spall failure strongly depends on the microstructure of a material. There have been numerous studies to study the effect of grain size on the overall spall strength and the total amount of damage in single element metals like copper. However, such systematic studies remain rare in two-phase materials and alloys. In this work, two incipient spall experiments were performed on a Cu–1%Pb alloy to understand the effect of grain size on the damage and failure in a two-phase material. Overall, these results showed that even though the spall strength did not change as a function of grain size, there were significant differences in the total amount of damage as a function of grain size. A clear increase in the total damage present in the material was seen as the grain size was increased from 32 to either 70 or 75 μm in either of the experiments. This difference was attributed to variations in the void growth rate as the grain size was increased.

Spectroscopic and Spectrometric Approaches for Assessing the Composition of Embedded Metals in Tissues.

Many medical devices contain metals that interface with the body. Additionally, embedded metal fragments from military wounds are typically not removed, to avoid the risk of morbidity associated with invasive surgery. The long-term health consequences of many of these materials are not thoroughly understood. To this end, we have exposed rats for up to one year to implanted single-element metal pellets of any one of Al, Co, Cu, Fe, Ni, Pb, Ta, or W. Various tissues were harvested and flash frozen for analysis of their metal distribution. We discuss approaches to most thoroughly and reliably evaluate the distribution of metal in these tissues. The path to the most appropriate analytical technique took us through extensive examination of the tissues using scanning electron microscopy with energy dispersive X-ray spectroscopy (XPS), X-ray photoelectron spectroscopy (XPS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Though any one of these methods is highly relied upon in surface chemistry analysis, LA-ICP-MS alone showed presence of metal in the tissue. This information will help build robust methods to bridge the gap in our understanding of biosolubility and distribution of embedded metal throughout the body.

Microscale Surface Nanostructuration and Catalysis of Metallic Electrodes through Electrochemical Deposition and Dissolution in Ionic Liquids

Incorporation of highly active nanostructures into metallic electrode surface layers is of particular interest for a wide range of potential applications, such as electrochemical sensing and energy storage-conversion systems, because the nanostructures not only can increase the active area accessible to reactant molecules but can also improve electron mobility in the solid ligaments. However, controlling the nano-architecture of electrode surface layers faces great challenges since metals at nanoscale favor low surface areas in order to minimize the surface energy. In the past two decades, several strategies such as template synthesis, surfactant mediated synthesis and dealloying, have been developed to design and fabricate nanostructured electrodes with highly ordered networks and high surface area. We have developed a green-chemistry method based on in-situ electrochemical deposition-dissolution (ECDD) of an active metal on a target metal substrate in ionic liquids to achieve the nanostructuration of metallic electrodes.1-4 As shown in Figure 1A, during electrochemical deposition, a highly reactive metal component such as Zn is introduced via electrodeposition to interact with a less active electrode substrate such as Au to form an alloy phase. During subsequent electrochemical dissolution, the reactive component is selectively removed from the alloy phase leading to the formation of nanostructured surface layers. The nanostructured layers can grow deeper by repeating the process. The most obvious advantage of the ECDD method over other arts is that no net chemicals are consumed and it is a green-chemistry process. This method has been employed to successfully create different nanostructures on single-element metal and alloy electrodes at tens-to-hundreds micrometer scales, which are shown in Figure 1B to E. In this presentation, we will introduce the green-chemistry method based on the electrochemical alloying-dealloying in ionic liquids, the characterization and polarization behavior of resulting surface nanostructures, further catalysis of the nanostructures, and their applications to electrochemical sensing and energy storage conversion.

Impurity dominated thin film growth

Magnetron sputter deposition was applied to grow thin metal films in the presence of impurities. These impurities are ambient gas molecules and/or atoms from the residual gas present in the vacuum chamber. Seven materials were investigated: four single element metals (Al, Ag, Cu, and Cr), two widely applied alloys (Cu55Ni45 and Ni90Cr10), and one high entropy alloy (CoCrCuFeNi). The thin films were analyzed using X-ray diffraction to determine the domain size, the film texture, and the lattice parameter. The same trend for all studied materials is observed. When the ratio between the impurity and metal flux towards the substrate is low, the domain size is not affected by the presence of the impurities. In this regime, the incorporation of the impurities affects the lattice parameter. At high flux ratios, the change of the domain size can be described by a power law with the exponent equal to −1/2 for all studied materials. A kinetic Monte Carlo code is used to demonstrate this observed trend.

Investigation into nanoscratching mechanical response of AlCrCuFeNi high-entropy alloys using atomic simulations

The mechanical behaviors and deformation mechanisms of scratched AlCrCuFeNi high entropy alloys (HEAs) have been studied by molecular dynamics (MD) simulations, in terms of the scratching forces, atomic strain, atomic displacement, microstructural evolution and dislocation density. The results show that the larger tangential and normal forces and higher friction coefficient take place in AlCrCuFeNi HEA due to its outstanding strength and hardness, and high adhesion and fracture toughness over the pure metal materials. Moreover, the stacking fault energy (SFE) in HEA increases the probability to initiate dislocation and twinning, which is conducive to the formation of complex deformation modes. Compared to the single element metal workpieces, the segregation potency of solutes into twinning boundary (TB) is raised due to the decreasing segregation energy of TB, resulting in the stronger solute effects on improving twinning properties for HEA workpiece. The higher dislocation density and the more activated slipping planes lead to the outstanding plasticity of AlCrCuFeNi HEA. The solute atoms as barriers to hinder the motion of dislocation and the severe lattice distortion to suppress the free slipping of dislocation are significantly stronger obstacles to strengthen HEA. The excellent comprehensive scratching properties of the bulk AlCrCuFeNi HEAs are associated with the combined effects of multiple strengthening mechanisms, such as dislocation strengthening, deformation twinning strengthening as well as solute strengthening. This work provides a basis for further understanding and tailoring SFE in mechanical properties and deformation mechanism of HEAs, which maybe facilitate the design and preparation of new HEAs with high performance.

Deformation Twinning of Molecular Dynamics Simulation in a Ternary Titanium Alloy under Nanoindentation

Nanotwinned (nt) metals exhibit excellent mechanical, electrical and thermal properties, and therefore attract much attentions. To fabricate large area nt surface, the fundamental mechanisms of deformation twinning induced by molecular dynamics (MD) are necessary to be explored. Nevertheless, MD of nt metals currently focus mainly on nt copper (Cu) and other single element metals with face-centered cubic (fcc) structure. In addition, MD simulations are usually performed on a built nt model, rather than from a single crystal, due to the difficulty of forming nanotwins. In this study, a single crystal is constructed in a ternary titanium (Ti) alloy with hexagonal closed-packed (hcp) lattice cell. Deformation twinning of MD simulation is performed in a ternary Ti alloy under nanoindentation from the built single crystal. Zonal structure is found during loading under nanoindentation, and nanograins transforms into nanotwins. Deformation twinning is significant to understanding the formation of nanotwins, as well as fabricating large area nt surface on a Ti alloy.

Pd(111) 박막의 자성: 제일원리계산

Pd은 단일 원소 금속 중에서 자기 감수율이 가장 크고 외부 환경에 따라 강자성 특성을 보이기도 한다. 본 논문은 자성 연구에 가장 적합한 FLAPW 방법을 사용하여 5-ML과 9-ML Pd(111) 박막의 자성을 계산하였다. 전자간 교환-상관 상호작용을 고려하기 위해 LDA와 GGA를 도입하여 두 계산 결과를 비교하였다. LDA 계산은 덩치 Pd이 상자성 상태가 바닥 상태임을 예측하여 실험과 일치하는 반면, GGA 계산은 덩치 Pd이 강자성을 갖는 것으로 잘못 예측하여 Pd 자성에 대해서는 LDA 계산이 더 신뢰할 수 있음을 알았다. 5-ML Pd(111) 박막에 대한 LDA 계산 결과에 의하면 상자성 상태가 바닥인 덩치 Pd과는 다르게 강자성 상태가 더 안정적이었다. 박막의 중앙에 위치한 Pd이 가장 큰 자기모멘트(<TEX>$0.273{\mu}_B$</TEX>)를 가지며 중앙 Pd 층의 |m| = 1 상태가 덩치의 그것과 큰 차이가 있어 5-ML Pd(111) 박막의 강자성을 유발하는 데 주요 역할을 함을 알 수 있었다. 9-ML Pd(111) 박막은 상자성 상태가 강자성 상태에 비해 안정하였으나 두 상태 사이의 총에너지 차이가 거의 없고 유한한 자기모멘트를 안정적으로 가지는 것으로 계산되어 강자성 상태가 준 안정적임을 보였다. Pd has the highest magnetic susceptibility among single element metals and often shows ferromagnetism under some special environments. In this paper, we report magnetism of 5- and 9-monolayers (ML) calculated by using full-potential linearized augmented plane wave method. Exchange-correlation interaction is taken into account in local density approximation (LDA) and generalized gradient approximation (GGA) and calculational results in LDA and GGA are compared with each other. It is found that calculations by LDA are more reliable compared to those by GGA because LDA prediction of paramagnetism of bulk Pd is consistent with experiments, whereas GGA predicts wrongly ferromagnetim of bulk Pd. Calculational results in LDA on a 5-ML Pd(111) thin film shows a ferromagnetic ground state unlike a paramagnetic ground state of bulk Pd. The center Pd layer of the 5-ML Pd(111) thin film has the largest magnetic moment (<TEX>$0.273{\mu}_B$</TEX>) among the layers and |m| = 1 orbital states play a dominant role in stabilizing the ferromagnetism of the 5-ML Pd(111) thin film. A 9-ML Pd(111) thin film in a ferromagnetic state has almost the same total energy as in a paramagnetic state. Since the magnetization of the 9-ML Pd(111) thin film is stable, the ferromagnetic state may be meta-stable.

Atomic layer deposition of tungsten using sequential surface chemistry with a sacrificial stripping reaction

Tungsten (W) films were grown with atomic layer control using a novel sequence of self-limiting surface reactions. The tungsten film growth was achieved by dividing the binary reaction WF 6+Si 2H 6→W+2SiHF 3+2H 2 into two separate half-reactions. Alternating exposures to WF 6 and Si 2H 6 in an ABAB… sequence produced tungsten deposition at temperatures between 425 and 600 K. The Si 2H 6 reactant served only a sacrificial role to strip fluorine from tungsten without incorporating into the film. FTIR spectroscopic investigations demonstrated that the WF 6 and Si 2H 6 half-reactions were complete and self-limiting at T>400 K. In situ spectroscopic ellipsometry measurements determined a tungsten growth rate of 2.5 Å/AB cycle with WF 6 and Si 2H 6 reactant exposures sufficient for complete half-reactions. The surface topography of the deposited tungsten films was flat indicating smooth film growth. The tungsten films were either amorphous or composed of very small crystalline grains and contained no measurable silicon or fluorine. These results represent the first demonstration of atomic layer deposition of smooth single-element metal films using sequential surface chemistry.

Optical Properties of Multi-Component Antimony-Silver Nanoclusters Formed in Silica by Sequential Ion Implantation

AbstractThe linear and nonlinear optical properties of nanometer dimension metal colloids embedded in a dielectric depend explicitly on the electronic structure of the metal nanoclusters. The ability to control the electronic structure of the nanoclusters may make it possible to tailor the optical properties for enhanced performance. By sequential implantation of different metal ion species multi-component nanoclusters can be formed with significantly different optical properties than single element metal nanoclusters. We report the formation of multi-component Sb/Ag nanoclusters in silica by sequential implantation of Sb and Ag. Samples were implanted with relative ratios of Sb to Ag of 1:1 and 3:1. A second set of samples was made by single element implantations of Ag and Sb at the same energies and doses used to make the sequentially implanted samples. All samples were characterized using RBS and both linear and nonlinear optical measurements. The presence of both ions significantly modifies the optical properties of the composites compared to the single element nanocluster glass composites. In the sequentially implanted samples the optical density is lower, and the strong surface plasmon resonance absorption observed in the Ag implanted samples is not present. At the same time the nonlinear response of the these samples is larger than for the samples implanted with Sb alone, suggesting that the addition of Ag can increase the nonlinear response of the Sb particles formed. The results are consistent with the formation of multi-component Sb/Ag colloids.

High Tc Superconducting Thin Films

While high Tc superconductivity was first discovered in bulk material, it was apparent that thin films of these materials, particularly the compound YBa2Cu3O7-δ, would be of great interest to both science and technology. In this sense the development of these materials parallels a similar history in the low Tc materials. Initially, most of the low Tc materials of interest were single element metals such as Nb, Pb and Al in bulk form. Later work, mostly in magnets, led to the development of compounds or alloys of such metals as Nb-Sn, Nb-Ti, and many others. However, many physical and technological investigations required thin films with thicknesses in the range of 0.1-10μm. Microwave, infrared, and critical current studies are examples of some of the scientific uses of thin films. A few examples of the applications would include josephson junction-based digital computer circuits, SQUID (Superconducting Quantum Interference Devices), transmission lines, and interconnects and rf mixers. These studies are also of great interest in the high Tc materials. It is readily apparent that scientific and technological developments in superconductivity are closely interwoven.The high level of interest in thin films can be appreciated by observing that it was barely a few months after the announcement of superconductivity above 77 K that the first films of these complex, multi-element materials, superconducting at about 86 K were announced. These early efforts at thin film fabrication were generally accomplished using multi-element deposition techniques but subsequent development has seen many varieties of film fabrication techniques used quite successfully to fabricate high-quality films.

Digital x-ray mapping of nanometer-size supported bimetallic catalysts

Understanding the role of metal cluster composition in determining catalytic selectivity and activity is of major interest in heterogeneous catalysis. The electron microscope is well established as a powerful tool for ultrastructural and compositional characterization of support and catalyst. Because the spatial resolution of x-ray microanalysis is defined by the smallest beam diameter into which the required number of electrons can be focused, the dedicated STEM with FEG is the instrument of choice. The main sources of errors in energy dispersive x-ray analysis (EDS) are: (1) beam-induced changes in specimen composition, (2) specimen drift, (3) instrumental factors which produce background radiation, and (4) basic statistical limitations which result in the detection of a finite number of x-ray photons. Digital beam techniques have been described for supported single-element metal clusters with spatial resolutions of about 10 nm. However, the detection of spurious characteristic x-rays away from catalyst particles produced images requiring several image processing steps.

Impregnated-electrode-type liquid metal ion source

We have developed an impregnated-electrode-type liquid metal ion source whose tip is a sintered-porous structure made of a refractory metal such as tungsten. By this structure the ratio of the liquid metal surface area facing the vacuum to the volume is low, which decreases useless metal evaporation from the surface. The maximum vapour pressure of the metal in operation for this ion source is 10 −1−10 0 Torr, which is 2–3 orders of magnitude higher than that for the needle type. Therefore, useful metal ions such as Ga +, Au +, Ag +, In +, Si 2+, Ge 2+, and Sb 2+ can be extracted from single element metals or alloys. The porous structure of the tip has also an effect on the positive control of the liquid metal flow rate to the tip head. Thus, a stable operation with a high current of a few hundreds of μA can be obtained together with a low current high brightness ion beam. Therefore, this ion source is suitable not only for microfocusing but also for a general use as a metal ion source.

Transient thermoreflectance from thin metal films

This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one-dimensional heat flow equation using a two-parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

Thermal and chemical stability of Schottky metallization on GaAs

The high-temperature stability of Schottky barriers on GaAs has been correlated with the thermodynamic driving force for chemical reaction between the metallic contacts and the substrate. The chemical stability of a gate metallurgy can result in the stability of the electrical characteristics of the contact after high-temperature anneal. Since single element metal contacts on GaAs are chemically unstable, thermally stable Schottky barriers are not expected from these systems. Alternatively, most of the common metal silicides are chemically stable on GaAs and hence are more likely to form Schottky barriers which are stable against high-temperature annealing. The chemically stable silicides of Ni and Co, which exhibit low electrical resistivities, are suggested as improved gate metallurgies in self-aligned metal-semiconductor field-effect transistor technologies.

Evaporated single element metal film resistors : D. D. Zimmerman, Proc. IEEE Symposium on “Molecular Concepts in Microelectronics”, St. Louis, May 24, 25, 26 (1965), p. 2C-1

Evaporated single element metal film resistors : D. D. Zimmerman, Proc. IEEE Symposium on “Molecular Concepts in Microelectronics”, St. Louis, May 24, 25, 26 (1965), p. 2C-1