Cu-Bi Alloys Research Articles

Investigation of spin-orbit torque efficiency in CuBi alloys

Investigation of spin-orbit torque efficiency in CuBi alloys

The modulation of perpendicular magnetic anisotropy and spin–orbit toque in Pt/Co/Pt multilayers with interfacial decoration by insertion of a Bi layer

Bismuth is a well-known semimetal material with strong spin–orbit coupling and has been confirmed to exhibit a larger spin Hall angle in CuBi and PtBi alloys with low Bi-doping concentration. In this study, we investigated perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) in Pt/Co/Pt multilayers by inserting a Bi layer with a thickness of 2 nm at the Co/Pt interface. Two types of PMA multilayers, Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm), with different spin accumulations, were designed and prepared by varying the top Pt thickness. A significant enhancement of PMA and SOT efficiency is observed in the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (5 nm) multilayer via a Bi-layer interfacial decoration. However, for the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (1 nm) multilayers, both PMA and SOT efficiency decrease with decoration of the Bi layer at the Co/Pt interface. Meanwhile, the sign of SOTs changes in the Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm) multilayers when introducing a Bi layer. This completely opposite behavior illustrates that the Bi/Pt interface plays an important role in modulating the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers. Optimizing the alloying of Bi/Pt could be an effective approach to increase the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers.

Understanding solidification and wear behaviour of Al-3Cu-Bi alloys

Considering that Bi is able to decrease friction between contacting surfaces, wear damage is expected to decrease as well. Bi-containing Al alloys have emerged as a choice for creating self-lubricating parts. The current work compares the effects of two distinct Bi alloy contents on the microstructure, Bi morphology/spacing, hardness, and wear resistance of Al-3.0wt.%Cu-Bi alloys. The novelty is in determining the microstructure constituting the Al-Cu based alloys with different Bi additions. Also, functional relationships between the wear resistance and the Bi droplets are derived. Increasing the Bi content from 2.0 to 3.2 wt.% changed the size and spacing of Bi particles as well as decreased hardness and increased the wear resistance. Even though the Al-3.0wt%Cu-3.2wt%Bi alloy has a lower hardness, the greater Bi fraction seems to be the key factor enhancing wear resistance. The bigger Bi areas spreading over the Al-rich phase under wear allowed higher wear resistance.

Creep of Cu-Bi Alloys with High Bi Content Near and Above Melting Temperature of Bi

Cu-Bi alloy with high Bi content can be used for thermal surge protection and energy storage. For these applications, creep at high temperatures, including temperatures above the melting temperature of Bi, Tm,Bi, becomes important. Accordingly, the creep behavior of Cu-Bi alloys, comprising 30 and 40 vol pct Bi, was studied under compression at temperatures above and below Tm,Bi. At 200 °C, which is below Tm,Bi, Cu-Bi showed a stress exponent of ~ 4 at high stresses and ~ 1 at low stresses. Finite element analysis revealed that the creep behavior of Cu-Bi at 200 °C was predominantly governed by Bi. On the other hand, at temperatures higher than Tm,Bi, Cu-Bi showed a short transient stage at high stresses, followed by sudden failure of the material. However, at low stresses, the sample first continued to expand and then started to accumulate compressive strain. A qualitative model based on interaction between liquid Bi and Cu is developed to explain the observed creep behavior at temperatures higher than Tm,Bi. The results obtained here shed light on the creep behavior of alloys with constituents having significantly different creep behavior and containing a non-reacting liquid phase.

Electrochemical Reduction of Carbon Dioxide at Alloy Systems: Cu-In and Cu-Bi

The ability to maintain high efficiencies while simultaneously tuning the selectivity of the electrochemical reduction of CO2 (ERC) using low cost electrodes has proven to be one of the greatest obstacles to the widespread commercialization of this technology. In this study, we prepare dendritic copper-indium and copper-bismuth alloys of various compositions and investigate their catalytic activity and selectivity towards the reduction of CO2. The Cu-In electrocatalysts are increasingly dendritic with higher In fraction and, depending on composition, consist of mixed phases of Cu, In and Cu-In intermetallic phases. ERC at these electrodes produces formate at high efficiencies (up to 62% with a 80 at% alloy, -1 V)) while also tuning the CO/H2 ratio to achieve an ideal syngas composition with a 40 at% In alloy, -1 V. The observed product distribution as a function of alloy composition and applied potential is rationalized in terms of the relative adsorption strengths of CO and COOH intermediates at Cu and In sites, and their distinct variation with applied potential may be induced by the differences in electronic structure. We also studied dendritic Cu, Bi, and Cu-Bi alloys with various compositions. The dendritic structures exhibit a high density of defect sites such as grain boundaries and twins, with these defects being increased at alloy dendrites. While the pure metals exhibited high H2 production and relatively high formate formation, the alloys exhibit a lower overall current density and a much higher selectivity towards formate, up to about 90%. Most importantly we show in this work that a combination of a d metal (Cu) with a p-block metal (In, Bi) with intimate contact between the two components is capable to tune the relative adsorption strength of the intermediates, thus steering the product distribution towards formate. The relatively purity of this product would be of interest for the direct feed of a direct formate fuel cell. Differences between In and Bi will be discussed in terms of the affinity for oxygen. This study highlights the opportunities of using alloys to enhance control over the product distribution and suggests that suitable alloys could be promising catalysts for the inexpensive and efficient production of fuels.

Skew Scattering from Correlated Systems: Impurities and Collective Excitations in the Spin Hall Effect

The spin Hall effect is affected by the Coulomb interaction as well as spin–spin correlations in metals. Here we examine the enhancement in the effect caused by resonant skew scattering induced by electron correlations. For single-impurity scattering, local Coulomb correlations may significantly change the observed spin Hall angle. There may be additional effects because of the special atomic environment close to a surface — extra degeneracies compared to the bulk, enhanced correlations that move the relative d- or f-levels, and interference effects coming from the lower local dimension. Our results may explain the very large spin Hall angle observed in CuBi alloys. We discuss the impact on the spin Hall effect from cooperative effects, firstly in an itinerant ferromagnet where there is an anomaly near the Curie temperature originating from high-order spin fluctuations. The second case considered is a metallic spin glass, where exchange via slowly fluctuating magnetic moments may lead to the precession of...

Cu-Bi alloys with high volume fraction of Bi: A material potentially suitable for thermal surge protection and energy storage

Thermal energy storage and surge protection materials store surplus heat energy and inhibit the increase in the temperature of materials in case of excess energy generation or surge. This study explores usage of Cu-Bi alloys comprising high volume fraction of Bi for energy storage and surge protection. Cu-Bi alloys, comprising 20, 40 and 60vol% of Bi, are prepared using liquid phase sintering by heating a mixture of Cu and Bi powders above the melting temperature of Bi, Tm, Bi. Compression testing of Cu-Bi alloys is conducted at temperatures above and below Tm, Bi. It is observed that the flow stress, yield stress, stress exponent and strain hardening exponent drastically decrease above Tm, Bi. In addition, compression creep testing is performed for Cu-40vol% Bi at temperatures above and below Tm, Bi. The experimental results establish that this alloy can sustain moderate level of stresses at temperatures above Tm, Bi for long periods. Moreover, differential scanning calorimetry is employed to evaluate the heat storage capacity of the alloy with different compositions and effect of cyclic heating and cooling on the heat storage capacity. It is observed that the heat storage capacity of Cu-Bi alloy remains almost constant for several melting-solidification cycles. Finally, it is suggested that Cu-Bi alloy containing high volume fraction of Bi can be used for energy storage and thermal surge protection.

Processing and mechanical behavior of Cu-Bi alloys with high volume fraction of Bi: Suitability for high temperature soldering application

This work evaluates potential of Cu-Bi alloys comprising high volume fraction of Bi for high temperature soldering application. Cu-Bi alloys with 20, 40 and 60vol% Bi are processed using liquid phase sintering (LPS). Effects of various sintering parameters, such as sintering time, temperature and relative green density, are studied to establish experimental conditions for preparing Cu-Bi alloys with optimum microstructure. Compression tests at different temperatures, ranging from RT to 260°C, and strain rates, ranging from 5×10−4 to 1×10−2s−1, are conducted to assess effects of temperature, strain rate and Bi content on the overall mechanical response of the material. The sintered density increases with increase in the green density and, with some exception, also with the sintering period and the sintering temperature. On the other hand, overall densification decreases with green density. Yield strength and strain-hardening exponent of Cu-Bi alloys decrease with increase in both the mechanical testing temperature and the volume fraction of Bi in the alloy. In addition, a set value of strain rate sensitivity can also be attained by optimizing the Bi content in Cu-Bi alloy and the test temperature. Microstructurally aware finite element analysis is performed to gain insights into deformation processes. Finally, a rationale for using Cu-Bi alloys as high temperature solders is presented.

Enhanced spin Hall effect by electron correlations in CuBi alloys

A recent experiment in CuBi alloys obtained a large spin Hall angle (SHA) of −0.24 (Niimi et al., Phys. Rev. Lett. 109, 156602 (2012)). We find that the SHA can be dramatically enhanced by Bi impurities close to the Cu surface. The mechanisms of this enhancement are two-fold. One is that the localized impurity state on surface has a decreased hybridization and combined with Coulomb correlation effect. The other comes from the low-dimensional state of conduction electrons on surface, which results in a further enhancement of skew scattering by impurities. Furthermore, we note that a discrepancy in sign of SHA between the experiment and previous theories is simply caused by different definitions of SHA. This re-establishes skew scattering as the essential mechanism underlying the spin Hall effect in CuBi alloys.

Extrinsic spin Hall effects measured with lateral spin valve structures

The spin Hall effect (SHE), induced by spin-orbit interaction in nonmagnetic materials, is one of the promising phenomena for conversion between charge and spin currents in spintronic devices. The spin Hall (SH) angle is the characteristic parameter of this conversion. We have performed experiments of the conversion from spin into charge currents by the SHE in lateral spin valve structures. We present experimental results on the extrinsic SHEs induced by doping nonmagnetic metals, Cu or Ag, with impurities having a large spin-orbit coupling, Bi or Pb, as well as results on the intrinsic SHE of Au. The SH angle induced by Bi in Cu or Ag is negative and particularly large for Bi in Cu, 10 times larger than the intrinsic SH angle in Au. We also observed a large SH angle for CuPb, but the SHE signal disappeared in a few days. Such an aging effect could be related to a fast mobility of Pb in Cu and has not been observed in CuBi alloys.

Spin Hall effect induced by Bi impurities in Cu: Skew scattering and side-jump

The spin Hall effect (SHE) has recently turned out to be an interesting tool for the conversion between charge and spin currents, the conversion factor being characterized by the spin Hall angle ${\ensuremath{\Phi}}_{H}$. Large spin Hall angles have been now measured in heavy metals like W (${\ensuremath{\Phi}}_{H}=\ensuremath{-}0.33$) and Cu doped with Bi impurities (${\ensuremath{\Phi}}_{H}=\ensuremath{-}0.24$). In this article we express the contributions to the SHE induced by skew scattering and scattering with side-jump from Bi impurities in Cu, and we use ab initio calculations of the electronic structure of CuBi alloys to estimate the values of these two contributions. The predominant effect comes from skew scattering; the spin Hall angle is negative in agreement with experiments, but the calculated amplitude is smaller.

Giant Spin Hall Effect Induced by Skew Scattering from Bismuth Impurities inside Thin Film CuBi Alloys

We demonstrate that a giant spin Hall effect (SHE) can be induced by introducing a small amount of Bi impurities in Cu. Our analysis, based on a new three-dimensional finite element treatment of spin transport, shows that the sign of the SHE induced by the Bi impurities is negative and its spin Hall (SH) angle amounts to -0.24. Such a negative large SH angle in CuBi alloys can be explained by applying the resonant scattering model proposed by Fert and Levy [Phys. Rev. Lett. 106, 157208 (2011)] to 6p impurities.

The Mechanism of Bi Extraction Along Dislocation Lines in Dilute Cu-Bi Alloys

The Mechanism of Bi Extraction Along Dislocation Lines in Dilute Cu-Bi Alloys

Nanostructured Cu-Bi alloys prepared by co-evaporation in a continuous gas flow

Abstract A new preparation process for nanostructured alloys utilizing co-evaporation in a continuous gas flow has been developed. Inert gas is injected into a preparation chamber through a nozzle in order to establish a convectional flow that carries crystallites of one element above an evaporation source of a second element so that the atoms of the second source condense at the crystallite's surfaces in the form of a thin coating. In this manner Cu crystallites were coated with Bi, resulting in Bi-doped grain boundaries after consolidation of the Bi-coated Cu crystallites. The maximum overall concentration of Bi without formation of crystalline Bi was 10 at.%, corresponding to one monolayer of Bi atoms distributed along the grain boundaries. The average grain size in the consolidated samples was found to depend on the Bi concentration. Annealing experiments showed that the onset of grain growth was shifted to higher temperature for Bi-doped samples.

Annealing of electrolytic Cu-Bi, Ni-Bi, and Co-Bi alloys

The influence of annealing on the structure and some properties of electrodeposited supersaturated solid solutions of bismuth in copper, nickel, or cobalt was studied. The decomposition of supersaturated solid solutions begins with the formation of dissolved metal atom clusters in the lattice of base metals. During the process of annealing, the lattice of the clusters, originally coinciding with that of the base metals is transformed into the lattice of the second phase. The alloys formed in the process of annealing are solid solutions with precipitates of the second phase.

固体銅と液体Cu-BiおよびCu-Pb合金間の界面張力

固体銅と液体Cu-BiおよびCu-Pb合金間の界面張力

Equilibria between silica-saturated iron silicate slags and molten Cu-As, Cu-Sb, and Cu-Bi Alloys

The solubilities of copper, arsenic, antimony, and bismuth in silica-saturated iron silicate slag, equilibrated with molten copper which included the corresponding element, were measured at temperatures 1473 and 1523 K under oxygen pressures ranging from 10−1 to 10−7 atm. The results confirm that copper is dissolved as CuO0.5 in silica-saturated fayalite slag. Dissolution of As, Sb, and Bi was found to be dependent upon the oxygen potential, suggesting oxidic rather than atomic dissolution. The data obtained also support models in which these elements exist in the slag mainly as two different types of oxides, but occasionally these oxides coexist with neutral atoms. Based on these models, equations were obtained that related the solubilities of these elements in the slags to the oxygen potential in them. The knowledge obtained in this investigation will be helpful in eliminating deleterious minor elements in copper smelting.

Activity of Oxygen in Liquid Cu-Bi Alloys

Activity of Oxygen in Liquid Cu-Bi Alloys

Grain boundary faceting in Cu-Bi alloys

Observations on an embrittled Cu 0.0115 wt% Bi alloy in the electron microscope have shown that grain boundaries in this material exhibit very marked faceting. This faceting cannot be explained by the geometrical requirements of the coincidence site lattice. Boundaries in embrittled Cu- Sb alloys do not show such faceting. A theoretical model is given to investigate the conditions for facet stability in the presence of segregated atoms. A study has also been made of the nature of fracture paths in these alloys. There are two possible modes of fracture: the first is transgranular and is accompanied by significant plastic deformation: in the second mode, fracture accurately follows the facets of the boundary and there is no evidence of accompanying plastic deformation.

A study of grain boundary segregation in Cu-Bi alloys using STEM

Abstract A V.G. Microscopes' HB5 STEM has been used to study a segregated copper-bismuth alloy. It is shown that the Z contrast mechanism due to Crewe (1970 a, b) is not valid for such crystalline materials. Attempts to use the energy loss spectrometer, with which the microscope is fitted, to detect the bismuth spectrum from the grain boundary region were not successful. However, it was found possible to image regions of bismuth on or near to the surface of the foil using a technique analogous to tilted dark field work in a CTEM.