Outward Diffusion Of Cu Research Articles

Electron Beam Induced Enhancement and Suppression of Oxidation in Cu Nanoparticles in Environmental Scanning Transmission Electron Microscopy.

We have investigated the effects of high-energy electron irradiation on the oxidation of copper nanoparticles in environmental scanning transmission electron microscopy (ESTEM). The hemispherically shaped particles were oxidized in 3 mbar of O2 in a temperature range 100-200 °C. The evolution of the particles was recorded with sub-nanometer spatial resolution in situ in ESTEM. The oxidation encompasses the formation of outer and inner oxide shells on the nanoparticles, arising from the concurrent diffusion of copper and oxygen out of and into the nanoparticles, respectively. Our results reveal that the electron beam actively influences the reaction and overall accelerates the oxidation of the nanoparticles when compared to particles oxidized without exposure to the electron beam. However, the extent of this electron beam-assisted acceleration of oxidation diminishes at higher temperatures. Moreover, we observe that while oxidation through the outward diffusion of Cu+ cations is enhanced, the electron beam appears to hinder oxidation through the inward diffusion of O2- anions. Our results suggest that the impact of the high-energy electrons in ESTEM oxidation of Cu nanoparticles is mostly related to kinetic energy transfer, charging, and ionization of the gas environment, and the beam can both enhance and suppress reaction rates.

High‐Responsivity Natural‐Electrolyte Undersea Photoelectrochemical Photodetector with Self‐Powered Cu@GaN Nanowires Network

AbstractUndersea optical communication (UOC) has been considered as the most potential next‐generation underwater wireless communication technology for ocean exploration. Photodetector is the essential component in UOC system, however, the harsh undersea environment like light attenuation and seawater corrosivity restricts the applications of conventional photodetectors. Herein, a novel natural‐electrolyte self‐powered photoelectrochemical (PEC) photodetector based on core‐shell structured Cu@GaN nanowires (NWs) network is demonstrated and direct utilization of seawater. High quality GaN shell is encapsulated on the Cu NWs network through Ga‐coating and high temperature nitridation processes. A Schottky junction along radial direction has formed at the Cu/GaN interface due to the outward diffusion of Cu into the GaN layer. Such a structure provides narrowed band detection on blue light as well as efficient carrier separation. A self‐powered undersea PEC photodetector is designed with a mini‐pipes connected device chamber, which allows direct indrawing of seawater and blue channel light communication (458 nm). This photodetector works stably for UOC in both shallow and deep‐sea conditions in Pacific Ocean area. It shows a high responsivity up to 5.04 mA W−1 and rapid response time of 0.68 ms. This photodetector can be easily integrated to marine equipment without waterproof packaging for the future energy‐saving UOC.

Regulating the oxidation resistance of Cu-5Ag alloy by heat treatment

Improving the oxidation resistance without sacrificing the electrical conductivity of Cu is of great importance for researchers due to its widespread application. In this study, oxidation resistance of Cu-5 wt%Ag alloy under solution treatment (ST) and aging treatment (AT) were studied systematically. It has been confirmed that AT sample demonstrates considerably higher oxidation resistance than ST sample at elevated temperatures. The aging treatment enhanced the formation of a continuous Ag network along grain boundaries, which could act as a barrier for both inward diffusion of oxygen and outward diffusion of Cu along grain boundaries during oxidation.

Influences of heat treatment on the oxidation and corrosion behavior of Cu–Ni–Fe inert anodes for aluminium electrolysis

Abstract The homogenization heat-treatments were implemented to the experimental as-cast Cux(Ni1.67Fe)100-x alloys with x = 60,52,44 wt%, which have a two-phase structure to obtain a uniform microstructure. Moreover, the oxidation kinetics of the as-cast and as-homogenized Cux(Ni1.67Fe)100-x alloys were investigated by the thermogravimetry at 850 °C under 1atm O2 atmosphere. The thermogravimetry results showed that all the samples approximately follow the parabolic law during oxidation under this condition. The homogeneous alloys have the lower oxidation rate than the as-cast alloys due to the inhibition of outward diffusion of Cu. Furthermore, the as-cast and as-homogenized 52Cu–30Ni–18Fe alloys were tested as inert anode materials in laboratory aluminium electrolysis tests. The as-homogenized 52Cu–30Ni–18Fe alloy anode performs significantly better corrosion resistance due to the formation of NiFe2O4. In the end, there is less impurity contents of Cu in the produced aluminium after electrolysis using the as-homogenized 52Cu–30Ni–18Fe alloy anode than using the as-cast 52Cu–30Ni–18Fe anode.

Local Deformation-Controlled Fast Directional Metal Outflow in Metal/Ceramic Nanolayer Sandwiches upon Low Temperature Annealing.

Precise nanoindentation on AlN/Cu/AlN nanolayer sandwiches has been conducted by using an atomic force microscope to promote fast and directional metal (Cu) outflow upon heating at low temperatures. Local plastic deformation during indentation results in the generation of high defect densities and stress gradients, which not only effectively reduce the activation energies for fast in-plane diffusion but also direct the in-plane transport of confined Cu to the indent location. In addition, a steep chemical potential gradient of O will be established across the AlN barrier upon exposure to air, which drives fast outward diffusion of Cu along defective pathways in the top AlN layer at the indent location. Selective and fast Cu metal outflow can thus be achieved at the indent locations upon annealing at a relatively low temperature of 350 °C for 5 min in air. The microstructures and phase boundaries of the AlN barrier and confined Cu nanolayers are unperturbed outside the plastically deformed region and remain metastable after annealing at 350 °C. By changing the surface processing modes, patterned nanoparticles and isolated nanowire structures can be fabricated straightforwardly. Such local deformation-controlled directional mass transport phenomena can be utilized to manipulate materials down to the atomic scale for designing functional nanoarchitectures for nanophotonic and nanoelectronic applications.

Dewetting behavior of Ag in Ag-coated Cu particle with thick Ag shell

With the aim of application as either a conductive filler or a sinter-bonding material with enhanced antioxidation properties, Ag-coated Cu (Cu@Ag) particles with thick Ag shells were fabricated and heated at 220 °C for 10 min in air to observe the dewetting behavior of the thick Ag shell. The fabricated Cu@Ag particles were spherical with the average size of 2 μm, and the Ag shell was 250–400 nm in thickness, comprising a mixed Ag-Cu layer and a dense Ag layer. As the heat-treatment time increased, the Cu@Ag particles were aggregated by forming numerous Ag nodules covering the surface of the particle. Dewetting in the thick Ag shell occurred from 4 min and only in the outer layer of the Ag shell even after 10 min of heat-treatment. In addition, the thickness of the dewetting layer and the mixed Ag-Cu layer were increased because the outward diffusion of Cu and Ag were increased proportionally with the time. However, because the dense Ag layer remained even after the dewetting of the Ag shell and the formation of the mixed Ag-Cu layer, the Cu@Ag particles were not oxidized after heating at 220 °C for 10 min in air.

The Effect of La on the Oxidation and Corrosion Resistance of $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ Cu 52 Ni 30 Fe 18 Alloy Inert Anode for Aluminum Electrolysis

The effect of La on the high-temperature oxidation and corrosion performance of ternary $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy inert anode for aluminum electrolysis in low-temperature KF–NaF– $$\hbox {AlF}_{3}$$ electrolyte was studied. The results indicate that the oxidation kinetics of ( $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18})_{1{-}x}\hbox {La}_{x}$$ (x = 0, 0.5, 1, 2 wt%) alloys at $$850\,^{\circ }\hbox {C}$$ under 1 atm oxygen atmosphere follow the parabolic law. The oxidation scales are stratified state, which contains the Cu oxide as the external layer. While the internal layer of the oxidation products are the mixture of Fe oxide, Ni oxide and nickel ferrite. The Cu outward diffusion oxidation is the dominant factor for the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy during high temperature oxidation. After adding La to the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy, the oxidation mechanism has become the combined action with outward diffusion of Cu and internal diffusion of O to form the Ni/Fe oxide underneath the outmost CuO layer. The corrosion resistance of $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18}$$ alloy anode during the aluminum electrolysis is further improved by adding 0.5 wt% La. Compared with $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18 }$$ alloy, the corrosion rate of the $$\hbox {Cu}_{52}\hbox {Ni}_{30}\hbox {Fe}_{18 }$$ alloy with 0.5 wt% addition of La has reduced from 1.9 to 1.8 cm/a.

Electrochemical profiling of multi-clad aluminium sheets used in automotive heat exchangers

A combination of glow discharge optical emission spectroscopy sputtering and local electrochemical measurements was used to determine electrochemical changes upon brazing in a multi-layered Aluminium sheet (AA4343/AA3xxx/AA4343) with an additional low-Cu (AA3xxx) interlayer. Ecorr values from potentiodynamic polarization, galvanic corrosion behaviour by ZRA, microstructure and composition by SEM and TEM were investigated and compared to those obtained for sheet without the interlayer. Inward diffusion of Si from clad, and outward diffusion of Cu from core are found to degrade the corrosion properties of conventional sheet, whereas presence of interlayer reduced outward diffusion of Cu and hence improved corrosion protection.

Ag precipitation at the free interface of multilayer NiCuZn ferrites/LTCC components

The diffusion and precipitation of silver and copper in low-dielectric-constant low temperature cofired ceramics (LTCC) for multilayer NiCuZn ferrites/LTCC components are investigated in this study. Ag precipitation is formed at the free interface between NiCuZn ferrites and LTCC for multilayer NiCuZn ferrites/LTCC components after cofiring. During the heating stage Cu+ will segregate into the grain boundary from the NiCuZn ferrites due to the low oxygen partial pressure inside the sample and then dissolve into LTCC near the interface between NiCuZn ferrites and LTCC. Cu+ in glass would diffuse from the interior toward the surface and become oxidized into Cu2+ or precipitated as the CuO phase at the surface. To maintain charge neutrality this process requires an outward diffusion of Cu+ and Ag+ from the interior toward the free surface. The reduction of Ag+ into Ag on the free surface is accelerated by Cu+ oxidation.

Effect of Ag content on thermal stability and crystallization behavior of Zr–Cu–Ni–Al–Ag bulk metallic glass

Thermal stability and crystallization behavior of Zr70−xCu12.5Ni10Al7.5Agx (x=0–16) bulk metallic glasses (BMGs) have been investigated. It is revealed that glass transition temperature (Tg), crystallization temperature (Tx) shift towards higher temperature with the increase of Ag content. The supercooled liquid region ΔTx increases from 72K for the Ag-free BMG to 110K for the BMG with x=16. Two or three stages of crystallization process with primary precipitation of icosahedral phase (I-phase) occurs in the BMGs with x=0–10. However, single stage of crystallization mode with the formation of Zr2Cu and/or Zr2Ni crystalline phase takes place in the BMGs with x=12–16. The formation of I-phase involves inward diffusion of Zr and outward diffusion of Cu and Ni. Partial substitution of Zr with Ag stabilizes the supercooled liquid by suppressing the redistribution of constitutional elements to form I-phase or crystalline phase, leading to the enhancement of thermal stability of Zr-based BMGs.

Anodic behavior of mechanically alloyed Cu–Ni–Fe and Cu–Ni–Fe–O electrodes for aluminum electrolysis in low-temperature KF-AlF3 electrolyte

A comparative study on the anodic behavior of Cu65Ni20Fe15 and (Cu65Ni20Fe15)98.6O1.4 materials during the electrolysis of aluminum was conducted. Both materials were prepared in powder form by ball milling and subsequently consolidated to form dense pellets that were used as anodes. The electrochemical characterization was performed at 700°C in a potassium cryolite-based electrolyte, and the composition-morphology of the oxide scales formed on both anodes were determined by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction measurements. On Cu65Ni20Fe15, a thick (170μm) and porous oxide scale is formed after 15min of electrolysis that readily dissolves (or spalls) before a denser oxide layer is formed after a longer electrolysis time (1 and 5h). In comparison, a thin (2μm) and dense oxide layer mainly composed of NiFe2O4 is observed on a (Cu65Ni20Fe15)98.6O1.4 electrode after 15min of electrolysis. The thickness of this oxide layer increases to 10 and 30μm after 1h and 5h of electrolysis. However, the outward diffusion of Cu to form CuOx at the surface of the electrode is not totally hampered by the presence of NiFe2O4 and a porous Cu-depleted region is formed at the oxide/alloy interface. As a result, electrolyte penetration occurs in the scale, which favors the progressive formation of an iron fluoride layer at the oxide/alloy interface.

Joining Copper Oxide Nanotube Arrays Driven by the Nanoscale Kirkendall Effect

Various annealing conditions (environment, temperature, and duration) are applied to study the nanoscale Kirkendall effect of copper (Cu) nanowire (NW) arrays on a Si substrate. The results show that an appropriate amount of oxygen supply is crucial for uniform transformation from Cu NWs (average diameter ∼50 nm) into Cu oxide nanotube arrays. An annealing duration of 30 min at 200 °C in a low vacuum environment reveals that the voids are not uniformly distributed at the Cu/Cu oxide interface. This suggests that void growth is due to surface diffusion of Cu along void surfaces. Annealing above 200 °C for 60 min resulted in complete transformation from Cu NWs into Cu oxide nanotubes. X-ray photoelectron spectroscopy characterization indicates that the Cu oxides formed at 200 °C and 300 °C are Cu₂O and CuO, respectively. It is demonstrated that the transformation from Cu NW arrays into Cu oxide nanotube arrays can be combined with the joining of stacked Si chips in a single-process step with reasonable joint shear strength. Transmission electron microscopy-electron energy loss spectroscopy elemental mapping analysis reveals that the joint interface is Cu oxide. The outward diffusion of Cu driven by the nanoscale Kirkendall effect is believed to enhance the joining process. By controlling the environment, temperature, and duration, joined Cu₂O or CuO nanotube stacked chips can be achieved, which serve as a platform for the further development of nanostructured, stacked devices.

Preferential Outward Diffusion of Cu during Unconventional Galvanic Replacement Reactions between HAuCl4 and Surface-Limited Cu Nanocrystals

Metal diffusion in nanoscale materials is of great interest, yet the detailed kinetic behavior of such diffusion remains elusive. We observe direction-controlled Cu diffusion during unconventional galvanic replacement reactions between surface-limited Cu nanocrystals (NCs) and HAuCl(4). Using presynthesized Cu-Cu(1.81)S hetero-oligomer and Cu-Cu(2)S heterodimer NCs as templates and reactants, the controlled addition of HAuCl(4) leads to the preferential outward diffusion of Cu, visualized by the formation of single-crystalline and straight, or polycrystalline and kinked, CuAu nanowires, respectively. The time-dependent growth of these nanowires enables determination of nanoscale diffusion coefficients of Cu during these processes, for the first time.

Structure and optical properties of pulsed sputter deposited CrxOy∕Cr∕Cr2O3 solar selective coatings

Spectrally selective CrxOy∕Cr∕Cr2O3 multilayer absorber coatings were deposited on copper (Cu) substrates using a pulsed sputtering system. The Cr targets were sputtered using asymmetric bipolar-pulsed dc generators in Ar+O2 and Ar plasmas to deposit a CrxOy (bottomlayer)∕Cr∕Cr2O3 (top layer) coating. The compositions and thicknesses of the individual component layers have been optimized to achieve high absorptance (0.899–0.912) and low emittance (0.05–0.06). The x-ray diffraction data in thin film mode showed that the CrxOy∕Cr∕Cr2O3 coating consists of an amorphous phase; the Raman data of the coating, however, showed the presence of A1g and Eg modes, characteristic of Cr2O3. The x-ray photoelectron spectroscopy (XPS) data from near-surface region of the absorber suggested that the chemical state of Cr was in the form of Cr3+ and no phases of CrO2 and CrO3 were present. The experimental spectroscopic ellipsometric data have been fitted with theoretical models to derive the dispersion of the optical constants (n and k). The optical constants of the three layers indicate that the bottom two layers are the main absorber layers and the top Cr2O3 layer, which has higher oxygen content, acts as an antireflection coating. In order to study the thermal stability of the CrxOy∕Cr∕Cr2O3 coatings, they were subjected to heat treatment (in air and vacuum) at different temperatures and durations. The coating deposited on Cu substrates exhibited high solar selectivity (α∕ε) of 0.895∕0.06 even after heat treatment in air up to 300°C for 2h. At higher temperatures, the solar selectivity decreased significantly (e.g., α∕ε=0.855∕0.24 at 350°C in air), which is attributed to oxidation of Cr crystallites, increased surface roughness, and formation of CuO. The formation of CuO and the increase in Cr3+ vacancies due to the outward diffusion of Cr at higher annealing temperatures were confirmed by XPS. In the case of vacuum annealing, for temperatures greater than 500°C the outward diffusion of Cu was the dominating degradation mechanism. The microstructural stability of the absorber coatings heat treated in air (up to 325°C) and vacuum (up to 600°C) was confirmed by micro-Raman spectroscopy measurements. Studies on the accelerated aging tests indicated that the absorber coatings on Cu were stable in air up to 250h at 250°C with a solar selectivity of 0.898∕0.11.

Oxidation of Zr55Cu30Al10Ni5 bulk metallic glass in the glassy state and the supercooled liquid state

The oxidation behavior of Zr55Cu30Al10Ni5 bulk metallic glass in air in the glassy state and the supercooled liquid state was studied using a thermogravimetric analyzer, X-ray diffraction and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. It was found that the isothermal oxidation kinetics of the glass in both states follows a two-step parabolic law. The oxidation process is governed by the inward diffusion of oxygen and the outward diffusion of Cu, with the first being dominant. The faster diffusion of atoms in the supercooled liquid state led to a network precipitation of crystalline Cu, and the crystallization that occurred in this state in the later stage of oxidation caused a reduction in the rate of oxidation. Two types of Zr oxides, i.e. t-ZrO2 and m-ZrO2, were formed in the oxidation process in both the glassy and supercooled liquid states. t-ZrO2 mainly formed in the outer layer of the oxide scale, while m-ZrO2 tended to form in the inner layer. The formation of m-ZrO2 is possibly activated by the crystallization of the glass near the interface of the oxide scale and the substrate alloy. In addition, oxidation also has a substantial effect on the formation of crystallized phases. The formation of a Cu-rich phase of Cu10Zr7 occurred in the oxidizing atmosphere. However, the formation of a Zr-rich phase of Zr2(Ni, Cu) mainly took place in a vacuum environment.