Copper Segregation Research Articles

Correlative impurity distribution analysis in cadmium telluride (CdTe) thin-film solar cells by ToF-SIMS 2D imaging

Polycrystalline cadmium telluride (CdTe) thin-films are commonly annealed in an atmosphere containing oxygen and chlorine and this passivation is essential in order to establish high photovoltaic conversion efficiencies. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is employed to acquire two-dimensional (2D) chemical distributions of impurities in CdTe samples with a lateral resolution of ~150nm. A statistical methodology is developed to examine the ToF-SIMS data via a two-sample Kolmogorov-Smirnov test. This test is used to determine the degree of chemical segregation at the grain boundaries for chlorine, sulfur, oxygen and copper. Chlorine and oxygen are highly segregated at grain boundaries. Sulfur is found to slowly diffuse into the grain interior from the grain boundary and this process is accelerated with an increasing temperature during the chlorine heat treatment. The distribution of copper is found to be uniform in bi-crystalline and polycrystalline CdTe samples and, hence, it is concluded that within the sensitivities of the ToF-SIMS measurement, no segregation of copper at the grain boundary is detected.

Effect of beam oscillation on electron beam welding of copper with AISI-304 stainless steel

The quality of dissimilar weld joint between copper and stainless steel (SS) plates using electron beam welding with and without beam oscillation was evaluated through microstructural analysis, hardness, tensile, bend and impact tests. Microstructural analysis as well as quantitative elemental analyses demonstrated that improved mixing of copper with SS in the fusion zone could be achieved by an oscillating beam of optimum oscillation diameter. Although the tensile properties at room temperature did not vary much between the welds; high temperature measurements exhibited significant difference between them. Welds made with beam oscillation having an optimum oscillation diameter demonstrated significant increase in impact strength as well as percentage elongation compared to joints made without beam oscillation. Increased beam oscillation beyond optimum oscillation diameter was found to be detrimental promoting segregation of copper in the weld zone exhibiting deteriorated mechanical properties. A hypothesis has been put forward for better mixing during welding with beam oscillation.

Effect of organic stabilizers on Pt–Cu nanoparticle structure in liquid-phase syntheses: control of crystal growth and copper reoxidation

The effect of organic stabilizers on the structure of supported Pt–Cu nanoparticles was investigated for liquid-phase reduction methods. The crystallite size and copper content of Pt–Cu alloy nanoparticles decreased as the number of carboxylate groups in an employed stabilizer molecule increased (acetate > succinate > citrate). The steric and inductive effects of carboxylate groups suppress the reduction and crystal growth of platinum to increase the chance for copper to make alloy with platinum, but at the same time they destabilize metallic copper resulting in a segregation of copper as oxides. In a radiolytic synthesis method, the size and Cu content in Pt–Cu nanoparticles were similarly reduced in the presence of citrate, but the copresent reduction enhancer also had a strong influence. The final particle structure was controlled by the kinetics of reduction of metal precursors and the thermodynamics of metal–stabilizer complexes.

Microstructural investigation of galvanized coatings with prior flash coating of copper on DP steels

Abstract Advanced high strength steels have gained immense importance for typical auto body structures owing to their light weight and high performance. These steels are mostly alloyed with Mn, Cr, Al, Si, etc. that have a tendency to form oxides on the steel surface during annealing and result in uncoated spots during galvanizing. Various new coating processes and annealing atmospheres have been tried to improve the coatability of such steels. One such method is the use of a diffusion barrier on bare steel. The present investigation has been carried out with DP steel, having Mn content of 2 wt.% coated with copper following the replacement reaction of iron and copper. The annealing and subsequent galvanizing operations were performed using hot dip simulator. XRD and SEM analysis were done to examine the characteristics of deposited copper layer. GDOES analyses showed that inter diffusion of copper and iron took place during annealing. SEM investigations of annealed specimen showed copper depleted iron rich grain boundary regions. Different layers in the galvanized coating were investigated in great detail. The morphology of the inhibition layer of copper coated sample was found to be different compared to conventional galvanized steel although, no compositional variation was detected. The TEM analysis showed that the inhibition layer was lean in copper, as supported by the ThermoCalc Al–Fe–Cu ternary phase diagram showing negligible solubility of copper in Fe2Al5 crystals. The copper was found to be distributed in the overlay zinc coating. TEM elemental mapping revealed segregation of copper at different regions of overlay zinc. The bright field and dark field images confirmed the presence of nano size precipitates within zinc coating. Finally a schematic illustration of the probable mechanism for formation of inhibition layer and copper rich zinc coating in presence of a prior copper coating was formulated.

Modelling equilibrium grain boundary segregation, grain boundary energy and grain boundary segregation transition by the extended Butler equation

The Butler equation is extended to model equilibrium grain boundary (GB) energy and the equilibrium GB composition of a polycrystal, as a function of the following state parameters: bulk composition, temperature, pressure and the five degrees of freedom of the GB. In the simplest case of an ideal solution and equal atomic sizes of the components, the Butler equation reduces back to the well-known McLean equation of GB segregation. When the components repulse each other in the solid solution, grain boundary segregation transition (GBST) appears below the critical temperature of the bulk solid miscibility gap. The GBST line is a new equilibrium line in equilibrium phase diagrams. This new model is demonstrated for copper (Cu) segregation to the GBs in nickel (Ni) and for the phosphorous (P) segregation to the GBs in bcc iron (Fe). The GBST line appears in the Ni-rich (Fe-rich) corner of the Ni–Cu (Fe–P) phase diagram in coordinates of bulk Cu (P) mole fraction vs temperature at fixed pressure. The mole fraction of the solute (Cu or P), corresponding to the GBST line steadily increases with temperature. At a lower solute content (Cu or P), or at a higher temperature compared to the GBST line, the GB is composed mostly of the solvent atoms (Ni or Fe). Contrariwise, at a higher solute content (Cu or P), or at a lower temperature compared to the GBST line, the GB is composed mostly of the solute atoms (Cu or P). These low-segregation and high-segregation states of the GB are transformed into each other via a reversible first-order GBST. This latter process takes place when the GBST line is crossed by changing the bulk composition or the temperature. The results, theoretically estimated, are in agreement with earlier experimental results.

Transient liquid assisted nucleation mechanism of YBa2Cu3O7−δ in coated conductor films derived by BaF2 process**Project supported by the Science and Technology Commission of Shanghai Municipality, China (Grant Nos. 13111102300 and 11dz1100302), the National Natural Science Foundation of China (Grant Nos. 11174193 and 51202141), the National Basic Research Program of China (Grant Nos. 2011CBA00105), and the Science and Technology Commission of

It is significant for low-cost preparation of YBa2Cu3O7−δ (YBCO) coated conductors to make clear the mechanism of orientation, copper segregation, and nucleation density in BaF2-derived YBCO crystallization. In the present work, a distinct nucleation mechanism was proposed based on a transient liquid phase induced by the size effect as well as near-equilibrium assumption. With this scheme the nucleation of YBCO prepared by metal–organic deposition (MOD) or the physical vapor deposition BaF2 process was semi-quantitatively analyzed, revealing that the direct driving force for nucleation is YBCO supersaturation in the liquid phase. The theoretical analysis on the nucleation orientation portion is evidenced by the experimental result.

Segregation of copper in an Fe–Cu alloy under pulsed electric current

Effect of electric current on the segregation of copper precipitates in the Fe–13.6Cu alloy is evaluated. Results of this approach present two stages of segregation, namely, grain-boundary segregation during the solidification and interphase-boundary segregation during the decomposition of a solid solution. The segregation becomes apparent not only because the thermodynamic barrier for segregation is decreased, but also because the diffusion is greatly enhanced. Based on the thermodynamic and kinetic aspects, the segregation process under electric current would be of great interest and of physical importance because this kind of electric current-induced segregation was much stronger than the thermal diffusion segregation.

Investigation of the Copper Gettering Mechanism of Oxide Precipitates in Silicon

One of the reasons why the principal gettering mechanism of copper at oxide precipitates is not yet clarified is that it was not possible to identify the presence and measure the copper concentration in the vicinity of oxide precipitates. To overcome the problem we used a 14.5 nm thick thermal oxide layer as a model system for an oxide precipitate to localize the place where the copper is collected. We also analyzed a plate-like oxide precipitate by EDX and EELS and compared the results with the analysis carried out on the oxide layer. It is demonstrated that both the interface between the oxide precipitate being SiO2 and the silicon matrix and the interface between the thermal oxide and silicon consist of a 2–3 nm thick SiO layer. As the results of these experiments also show that copper segregates at the SiO interface layer of the thermal oxide it is concluded that gettering of copper by oxide precipitates is based on segregation of copper to the SiO interface layer.

Promoting χ-Fe5C2(100)0.25 with copper – a DFT study

The role of copper in iron based Fischer–Tropsch catalysts was investigated using DFT with χ-Fe5C2(100)0.25 as a model surface. The presence of atomic copper on the iron-rich χ-Fe5C2(100)0.25-surface is more favorable than its presence in surface. Nevertheless, the segregation of copper from the surface yielding fcc-Cu remains an exergonic process. Carbon monoxide at a coverage of 2.2 CO per nm2 stabilizes atomic copper on this surface. The presence of copper results in the redshift in the stretching frequency of adsorbed CO. The mobility of copper atoms was investigated on χ-Fe5C2(100)0.25 in the presence of CO. The hopping frequency is reduced due to the presence of CO, although never enough to avoid formation of fcc-Cu on a shorter time scale than typically required for the formation of hydrocarbons in the Fischer–Tropsch synthesis.

Controlled activation of ZnTe:Cu contacted CdTe solar cells using rapid thermal processing

Back contacts can significantly limit CdTe solar cell performance, reducing both open circuit voltage (Voc) and fill factor (FF). Copper is an essential component of effective back contacts, but its presence in the CdTe absorber creates detrimental recombination centers. Rapid thermal processing (RTP) is demonstrated as a highly effective approach for reducing back contact barriers in CdTe solar cells contacted with ZnTe:Cu buffer layers, substantially improving both FF (>73%) and Voc (>850mV). Current density and quantum efficiency remain essentially unchanged, but a five-fold increase in minority carrier lifetime is observed which is attributed to passivation of recombination sites in the back contact region. Quantitative analysis of secondary ion mass spectrometry shows that the majority of Cu segregates to the Au metallization layer and that the ZnTe buffer appears to inhibit the Cu diffusion into CdTe. 3D imaging of the back contact region using atom probe tomography shows that optimized devices are characterized by preferential segregation of copper to both the Au|ZnTe and CdTe|ZnTe interfaces, perhaps in the form of CuxTe. With its low thermal budget the RTP process has been successfully applied to multiple device architectures.

Selective hydrogenation of butadiene over TiO2 supported copper, gold and gold-copper catalysts prepared by deposition-precipitation.

Oxide supported copper and gold catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons but their low activity compared to palladium catalysts and the deactivation of copper catalysts limit their use. There are only a very limited number of studies concerned with the use of bimetallic Au-Cu catalysts for selective hydrogenation reactions and the aim of this work was to prepare TiO2-supported monometallic Au and Cu and bimetallic AuCu (Cu/Au atomic ratio of 1 and 3) catalysts and to evaluate their catalytic performance in the selective hydrogenation of butadiene. Small gold, copper and gold-copper nanoparticles (average particle size < 2 nm) were obtained on TiO2 using the preparation method of deposition-precipitation with urea followed by reduction under H2 at 300 °C. Very small clusters were observed for Cu/TiO2 (∼1 nm) which might result from O2 induced copper redispersion, as also supported by the XPS analyses. The alloying of copper with gold was found to inhibit its redispersion and also limits its reoxidation, as attested by XPS. The bimetallic character of the AuCu nanoparticles was confirmed by XPS and EDX-HAADF. Cu/TiO2 was initially more active than Au/TiO2 in the selective hydrogenation of butadiene at 75 °C but it deactivated rapidly during the first hours of reaction whereas the gold catalyst was very stable up to 20 hours of reaction. The bimetallic AuCu/TiO2 catalysts displayed an activation period during the first hours of the reaction, which was very pronounced for the sample containing a higher Cu/Au atomic ratio. This initial gain in activity was tentatively assigned to copper segregation at the surface of the bimetallic nanoparticles, induced by the reactants. When the AuCu/TiO2 catalysts were pre-exposed to air at 75 °C before butadiene hydrogenation, surface copper segregation occurred, leading to higher initial activity and the suppression of the activation period. Under the same conditions, Cu/TiO2 totally lost its activity, probably due to irreversible copper oxidation.

Microstructural investigation of controlled short circuiting gas metal arc welding deposited aluminium–lithium alloy

Controlled short circuiting gas metal arc welding (CSC-GMAW) was investigated as a potential solid freeform fabrication (SFF) process for AA2199. The low heat input of the CSC-GMAW process resulted in a cooling rate on the order of 840–3500°C s−1 being realised during deposition. The solidification time was then calculated to range between 2–5 ms depending on the location within the weldment. The deposited material displayed a fine (4·3±1 μm) cellular structure, comparable to that previously reported for electron beam welding. Through comparison with the Kurz–Giovanola–Trivedi (KGT) model for microstructural development during solidification, the solidification front velocity (SFV) of the CSC-GMAW process was estimated to be ∼2–4·5×10−4 m s−1. Chemical analysis revealed lateral segregation of copper to the cell walls. TOF-SIMS revealed a homogeneous lateral lithium distribution, however depth profiling displayed some extent of lithium enrichment at the surface of the deposited material.On a examiné le soudage à l’arc sous protection gazeuse à court-circuit contrôlé (CSC-GMAW) comme processus potentiel de fabrication en forme libre solide (SFF) pour l’AA2199. Le faible débit de chaleur du processus de CSC-GMAW produisait une vitesse de refroidissement de l’ordre de 840 à 3500°C/s lors du dépôt. On a ensuite calculé que le temps de solidification variait de 2 à 5 ms, dépendant de l’emplacement dans la soudure. Le dépôt montrait une structure cellulaire fine (4·3±1 μm), comparable à la structure rapportée antérieurement pour le soudage par faisceau d’électrons. Par comparaison avec le modèle de Kurz–Giavanola–Trivedi (KGT) de développement de la microstructure lors de la solidification, on a estimé que la vélocité du front de solidification (SFV) du processus de CSC-GMAW était de ∼2 à 4·5×10−4 m s−1. L’analyse chimique a révélé une ségrégation latérale du cuivre sur les parois de la cellule. TOF-SIMS a révélé une distribution latérale homogène du lithium, cependant le profilage en profondeur a exposé une certaine zone d’enrichissement en lithium à la surface du matériel déposé.

Design of Large Size, Force Flow Superconductors for DEMO TF Coils

The DEMO plant will demonstrate by mid-century the feasibility of electric power generation by nuclear fusion. In the scope of design studies coordinated by the European Commission, CRPP and ENEA are developing two force flow conductor layouts as candidates for the toroidal field (TF) coils of DEMO. The operating conditions include 82 kA operating current and 13.5 T peak field, placing the DEMO TF conductor at substantially higher performance compared to the ITER TF (68 kA/11.5 T). The innovative winding layout is graded, layer wound with Nb3Sn/NbTi hybridization for both conductor designs, aiming at minimizing the size and the cost of the superconductor: one of the conductors is a wind and react cable-in-conduit conductor with reduced void fraction and has a rectangular shape. The other conductor is a react and wind flat cable with copper segregation and thick conduit assembled by longitudinal weld. The conductor designs were first drafted in 2012 and reviewed in 2013 based on a first round of assessments and on an updated requirements catalog. The manufacture of full-size prototypes is planned in 2014.

A Novel Concept of High Cu, Low Mn, Environmentally-Friendly Weathering Steels

A novel concept of hot shortness control on copper containing weathering steels is proposed. The weathering effect of copper addition in steels and its improvement in the mechanical and impact properties by age hardening are well known. Again, it is also very well know the problem of cracks on hot processing of those steels, caused by liquid copper segregation, the “hot shortness”. Hot shortness is the main problem of commercial producing of copper containing steels because of the increase of cost production caused by cracks the Ni addition to prevent them. Recent research found that copper segregates surrounding MnS inclusions in the steel, worsening the copper segregation caused by the preferential oxidation of iron on the hot processing of copper containing steels, which is the traditional cause of the “hot shortness”. The same results also show that copper additionally precipitates as CuS in the bulk of the steel. This work points that the “hot shortness” can then be prevented by reducing the Mn in the steel, rather than by the expensive Ni addition. Reducing Mn means reduction of inclusions of MnS sites for copper segregation in the steel. Residual sulphur can therefore be trapped by the copper as finer CuS precipitates, so copper is then dissolved in the bulk of the steel rather than segregated. This paper introduces an alternative solution for the problem of the “hot shortness” by partially or fully replacing the Mn by Cu in those steels. It is then proposed a novel concept of low Mn, or Mn free, Cu containing weathering steels in which Cu replaces Mn in trapping the S. That new steel opens the possibility to a cheaper and easier controls of the “hot shortness” so the copper contamination from scraps become a technological benefit because of its age hardening and weathering effect on steels. The recycling of highly copper contaminated scraps is another important environmental advantage. The conclusion of this work is that the current results introduces a new technology of cheaper copper containing low Mn, or Mn free, high strength, environmentally-friendly weathering steels with high potential market for different welded on shore and offshore specifications.

Pre-conceptual studies and R&D for DEMO superconducting magnets

The DEMO plant will demonstrate by mid century the feasibility of electric power generation by nuclear fusion. Since 2011, conceptual design studies are coordinated by the EFDA Power Plant Physics and Technology (PPPT) Division, with the aim of identifying requirements, propose design approaches and start R&D for the magnet system of DEMO. The input and generic boundary conditions are given by the system codes: the major radius of the tokamak is about 9m. The proposed operating current at 13.6T peak field is 82kA, placing the DEMO TF conductor at substantially higher performance compared to ITER TF (68kA/11.5T). The innovative winding layout is a graded, layer wound with Nb3Sn/NbTi hybridization, aiming at minimizing the size and the cost of the superconductor. Two options are considered for the Nb3Sn conductor: one a “wind&react” cable-in-conduit (CICC) with reduced void fraction and rectangular shape. The other conductor is a “react&wind” flat cable with copper segregation and thick steel conduit assembled by longitudinal weld. The conductor designs were first drafted in 2012 and updated in 2013 based on a first round of assessments, which includes electromagnetic, thermal-hydraulic and mechanical analysis. The manufacture of full size prototype conductors is planned in 2014.The technical requirement of the DEMO superconducting magnets is highlighted in comparison to ITER and other fusion devices. The large size of the DEMO tokamak is the main challenge for the demonstration of the feasibility of power generation by fusion. Together with the technical issues, the cost of the superconducting magnets will be eventually the crucial aspect to promote the establishment of nuclear fusion as a primary energy source in the coming centuries.

Contribution to Analysis of Co/Cu Substituted Ni-Zn Ferrites

In this communication, Co/Cu substituted Ni-Zn ferrites processed through sol-gel synthesis using polyethylene glycol (PEG) as a chelating agent are studied, intending to aid in understanding and choosing the optimum ferrite material for high frequency applications. Lattice constant and average crystallite size have been estimated from FWHM of the X-ray diffraction peaks, and these parameters are understood by considering the ionic radii of the substituted as well as the replacing ions. Observed variations in saturation magnetization and initial permeability for these ferrites have been explained on the basis of anisotropy contribution for cobalt and segregation of copper at grain boundaries evident from scanning electron micrographs.

Influence of copper diffusion on the shape of whiskers grown on bright tin layers

Whisker growth on electroplated bright (grain size<0.5μm) tin layers is examined and discussed in terms of the variety of shapes formed. Whiskers may be categorized by three basic morphological types of crystal structure which are usually observed on tin-based finishes (hillocks, nodules and filaments). In this paper, analysis of different whisker shapes and the reason for their development are defined. Constant elevated temperature in ambient humidity (105°C/15% relative humidity) tests were used to accelerate the growth of very large whiskers (>50μm nodule/hillock base) through enhanced diffusion, and to minimize the effect of corrosion on the samples. Cross sections from the layers and whiskers were etched and observed using a Focused Ion Beam (FIB). Transmission Electron Microscopy (TEM) was used to analyze the developing whiskers and to provide insight on the development of hillocks, nodules and filaments. Results indicate that fragmentation of a hillock/nodule, and later mass migration and segregation of copper, are critical steps in the evolution of these large whiskers. Copper impurities segregating at the whisker grain boundaries are responsible for the development of various whisker shapes.

Impact of growth rate on impurities segregation at grain boundaries in silicon during Bridgman growth

Bi-crystal silicon ingots with low angle grain boundaries have been grown at three different pulling rates, namely 3µm/s, 13µm/s and 40µm/s, in a small scale Bridgman type furnace. The melts have been contaminated with 50ppma of copper or indium. Selected samples were investigated by secondary ion mass spectrometry (SIMS) chemical analysis. The results show an influence of growth rate on copper segregation. Indium segregation has also been investigated. Relative impacts of solid state diffusion and solute microsegregation during solidification on the grain boundary concentration have been investigated for each growth rate.

Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles

Core-shell segregation of copper and silver in self-assembled, free nanoparticles is established by means of photoelectron spectroscopy in a wide range of relative Cu-Ag concentrations. These conclusions are based on the analysis of the photon-energy-dependent changes of the Cu 3d and Ag 4d photoelectron spectra. The nanoparticles are formed from mixed Cu-Ag atomic vapor created by magnetron sputtering of a bimetallic sample in a gas-aggregation cluster source. Even at similar Cu and Ag fractions in the primary vapor the surface of the nanoparticles is dominated by silver. Only at low Ag concentration copper appears on the surface of nanoparticles. For the latter case, a threefold decrease in the Ag 4d spin-orbit splitting has been detected. The specific component distribution and electronic structure changes are discussed in connection with the earlier results on Cu-Ag macroscopic and surface alloys.

First-principles study of oxygen-induced copper segregation in Cu3Au(1 1 1)

Using ab initio calculations based on density-functional theory, we study oxygen-induced copper segregation at the Cu3Au(111) surface. We find that high-oxygen concentrations lead to complete segregation of copper at the topmost surface plane. This result is explained by the larger oxygen affinity of copper with respect to gold. Depending on concentration, oxygen forms two-dimensional hexagonal or rectangular lattices at the surface.