Ni Segregation Research Articles

Long-Range Segregation Phenomena in Shape-Selected Bimetallic Nanoparticles: Chemical State Effects

A study of the morphological and chemical stability of shape-selected octahedral Pt0.5Ni0.5 nanoparticles (NPs) supported on highly oriented pyrolytic graphite (HOPG) is presented. Ex situ atomic force microscopy (AFM) and in situ X-ray photoelectron spectroscopy (XPS) measurements were used to monitor the mobility of Pt0.5Ni0.5 NPs and to study long-range atomic segregation and alloy formation phenomena under vacuum, H2, and O2 environments. The chemical state of the NPs was found to play a pivotal role in their surface composition after different thermal treatments. In particular, for these ex situ synthesized NPs, Ni segregation to the NP surface was observed in all environments as long as PtOx species were present. In the presence of oxygen, an enhanced Ni surface segregation was observed at all temperatures. In contrast, in hydrogen and vacuum, the Ni outward segregation occurs only at low temperature (<200-270 °C), while PtOx species are still present. At higher temperatures, the reduction of the Pt oxide species results in Pt diffusion toward the NP surface and the formation of a Ni-Pt alloy. A consistent correlation between the NP surface composition and its electrocatalytic CO oxidation activity was established.

On the Role of Ni in Cu Precipitation in Multicomponent Steels

The nature of Cu precipitation in quench-tempered multicomponent high-strength low-alloy steels is characterized by atom probe tomography. The detected nanometer sized Ni-rich clusters act as preferential nucleation sites for Cu-rich clusters, and Ni segregation at the Cu-rich precipitate/matrix heterophase interface contribute to fast growth of Cu precipitates. Molecular dynamics simulation indicates that local Ni clustering at atomic scale significantly quickens the solute diffusion. The initial Ni composition has a profound effect on the nature of Cu precipitation.

First-principles investigations of oxygen adsorption at TiNi surface and the TiO2/TiO–TiNi interface

Ab-intio study of the interaction of atomic and molecular oxygen with the TiNi(110) surface was performed using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. The oxygen adsorption energies were calculated and the preferential adsorption sites of oxygen atom on the surface were determined. Our results confirmed the formation of a Ni-rich layer at the alloy–oxide interface. Atomic and electronic structure of both TiO/TiNi(110) and TiO2/TiNi(110) interfaces were analyzed. The formation energies (Ef) of point defects at the interfacial layers as well as in bulk TiNi, monoclinic TiO, and rutile TiO2 were estimated. It was shown that Ef of Ti–Ni swap defect has a lower energy than that for the Ni antisites at the TiO2(100)/TiNi(110) interface. In the case of TiO(100)/TiNi(110) interface, the formation energies of Ti–Ni swap and Ni-antisites defects are close to each other. Our results demonstrated that Ef of Ni-defect in TiO is twice less than that in TiO2. The increase in the formation energies of defects in TiO2 reveal the increase in diffusion barriers of Ni atoms in comparison with those in oxides with a lower oxygen content, which hampers Ni segregation.

High-temperature stability of alumina containing nickel–zirconia cermets for solid oxide fuel cell anodes

The long-term stability of anode cermets for solid oxide fuel cells (SOFC) was studied for the composites NiO/scandia stabilized zironia (ScSZ) and NiO/scandia-alumina stabilized zirconia (SASZ). Heat treatment at 800 °C over 600 h resulted in gradual conductivity degradation for the NiO/ScSZ composites, while no significant changes were found for the NiO/SASZ composites. The conductivity drop was caused by an aggregation of Ni in the cermet matrix at high temperatures, and the addition of alumina to the cermet matrix protected the Ni segregation and improved the long-term stability. The effect of the alumina was examined using a sintered model anode of the NiO/ScSZ system. Although the initial conductivity decreased with increasing alumina content, an anode with an alumina content of 3 mol% showed no conductivity degradation during heating over 600 h. The initial conductivity drop was caused by a NiAl2O4 spinel formed during the initial sintering process. The spinel phase was reduced at 800 °C and transformed slowly into metastable NiAl10O16, which contributed to the long-term stability of the alumina composite anode cermets.

Cu precipitation on dislocation and interface in quench-aged steel

Abstract

Correlating Ethylene Glycol Reforming Activity with In Situ EXAFS Detection of Ni Segregation in Supported NiPt Bimetallic Catalysts

The structural changes in supported NiPt/C and NiPt/γ-Al2O3 catalysts were investigated using in situ extended X-ray absorption fine structure (EXAFS) under aqueous phase reforming (APR) of ethylene glycol conditions. Reverse Monte Carlo is introduced to analyze the EXAFS data. Parallel reactor studies of APR of ethylene glycol showed that NiPt catalysts were initially more active than monometallic Pt catalysts. The enhanced activity was correlated to changes in the catalyst structure. Under APR conditions, Ni segregated to the surface of the catalysts, resembling Ni-terminated bimetallic surfaces that were predicted to be more active than Pt from theoretical and experimental studies on model surfaces.

First principles study of Al and Ni segregation to the α-Fe/Cu (1 0 0) coherent interface and their effects on the interfacial cohesion

The first-principles method is used to characterise the segregation behaviours of aluminium and nickel at the interfaces of copper precipitates in ferritic steels. We find the segregation of aluminium and nickel at interfaces of copper precipitates are both energetically favourable. The main contributor to segregation energies of aluminium and nickel are the strong bonding energy of aluminium and nickel at the interface of copper precipitate, respectively. We also find their segregation energies depend on the composition of copper precipitate. Furthermore, we investigate the influences of segregation on interfacial cohesive properties for copper precipitates in ferritic steels. The interfacial cohesion of copper precipitates is enhanced by aluminium segregation but reduced by nickel segregation. Opposite roles derive from the different symmetrical features of valence state for aluminium and nickel.

Diffusion and segregation along grain boundary at the electrolyte–anode interface in IT-SOFC

The atomic level chemical and microstructural features of grain boundaries in gadolinium-doped ceria (GDC) electrolyte thin film supported by Ni–GDC cermet anode were characterized by high resolution transmission electron microscope (HR-TEM) and scanning TEM (STEM). It was found that metallic Ni can diffuse from the anode into the thin film electrolyte along grain boundaries. In addition, Ce and Gd can also diffuse and segregate at grain boundaries between Ni grains in the anode substrate. HR-TEM observations revealed that Ni diffusion and segregation at grain boundaries between GDC grains enhanced the inhomogeneity and led to microstructural changes at grain boundary regions, i.e. the formation of superstructure. The observations also indicated that enhanced inhomogeneity at grain boundaries might play a significant role in the conductivity of GDC electrolyte film in solid oxide fuel cells.

Structure, microstructure and electrical conductivity of 8YSZ containing NiO

The effects of NiO addition on the structure and microstructure of yttria-stabilized zirconia were investigated to clarify the role of the additive in the microstructure-related electrical conductivity of the solid electrolyte. Specimens of 8 mol% yttria-stabilized zirconia with NiO contents up to 5.0 mol% were prepared using nickel oxide and trihydroxi nickel carbonate as precursors. The specimens were sintered at 1350 °C for several holding times. The evolution of the lattice parameter with NiO content was evaluated by X-ray diffraction and the microstructural features by scanning electron microscopy. Electrical conductivity was evaluated by impedance spectroscopy measurements. The solubility limit of NiO at 1350 °C was found to be 1.5 mol% by X-ray diffraction. Energy dispersive spectroscopy results revealed Ni segregation for large holding times at 1350 °C. The grain boundary conductivity was found to be influenced by Ni segregation and to decrease with increasing holding times at high temperature.

Precipitation and stability of reversed austenite in 9Ni steel

A new method was used to analyze the factors affecting the precipitation of reversed austenite during tempering. The samples were kept at various tempering temperatures for 10 min and their length changes were recorded. Then, the precipitation of reversed austenite which led to the length reduction was shown by thermal expansion curves. The results show that the effects of process parameters on the precipitation of reversed austenite can be determined more accurately by this method than by X-ray diffraction. When the quenching and tempering process is adopted, both the lower quenching temperature and higher tempering temperature can promote the precipitation of reversed austenite during tempering; and when the quenching, lamellarizing, and tempering process is used, intercritical quenching is considered beneficial to the precipitation of reversed austenite in the subsequent tempering because of Ni segregation during holding at the intercritical temperature.

Influence of the temperature and rate of generation of point defects on the processes of deformation-induced segregation in the Fe-Cr-Ni alloy

The process of nickel segregation at grain boundaries in the Fe-Cr-Ni alloy upon severe plastic deformation has been investigated depending on the temperature. The kinetics of the formation of segregates has been studied by numerically solving diffusion equations that take into account the generation and absorption of point defects and their mutual recombination. It has been shown that the Ni concentration at grain boundaries is reduced with increasing temperature and at T ≥ 600 K no segregates are formed, which agrees with the experiment. The dependence of the degree of Ni segregation on the rate of generation of point defects is nonmonotonic, with a maximum in the range of medium rates of generation, which qualitatively corresponds to the analogous dependence observed upon radiation-induced segregation of nickel in this alloy.

NiTi Production via the FFC Cambridge Process: Refinement of Process Parameters

The electrochemical FFC Cambridge process is well suited for NiTi production as it avoids the Ni segregation associated with conventional manufacturing routes. A homogeneous low oxygen product can be produced through the direct electro-deoxidation of the mixed metal oxides in a molten salt bath. This work builds on previous literature concerning the reduction of NiTi [ J. Electrochem. Soc. , 155 , E171 (2008) ; Chin. Sci. Bull. , 51 , 2535 (2006) ] through the investigation of the effects of reduction temperature and the materials of construction on the phases formed. Previous work has shown that the use of a Ti cathodic current collector gave Ti enrichment, forming monoclinic monoclinic NiTi at ambient temperature [ J. Electrochem. Soc. , 155 , E171 (2008) ]. The use of a Ni current collector caused Ni enrichment in the product, stabilizing the high temperature B2 cubic form at ambient temperature. This was true for all reductions carried out from 1173 to 1273 K, but for 1123 K and below, monoclinic NiTi was formed. A NiTi current collector yielded Ni enrichment producing B2 cubic NiTi at ambient temperature. Mechanisms for the formation of these phases were proposed. The work also detailed the importance of good cell design and how sealing quality can dramatically affect the homogeneity of the products.

Adsorption of (S)-glutamic acid induces segregation of Ni at bimetallic Au/Ni surfaces

Abstract The adsorption of the catalytically important chiral modifier, (S)-glutamic acid, was investigated on bimetallic Ni/Au surfaces created by annealing thin Ni films on Au{1 1 1}. Reflection absorption infrared spectroscopy revealed that adsorption of (S)-glutamic acid at room temperature resulted in the immediate formation of pyroglutamate species on Ni-rich surfaces and the non-zwitterionic glutamic acid species on Au-rich bimetallic surfaces. For Au-rich bimetallic surfaces, TPD revealed that pyroglutamate is also formed on annealing. Medium energy ion scattering studies showed that significant adsorbate-induced Ni segregation was observed on Au-rich bimetallic surfaces – the driving force for segregation is likely to be the formation of nickel pyroglutamate. The implications of our findings for enantioselective catalysis are discussed.

Optimization of the FFC Cambridge Process for NiTi Production

The FFC Cambridge process is ideal for NiTi production, as it avoids the Ni segregation associated with pyrometallurgical routes to manufacture NiTi, and is able to produce a homogenous low-oxygen product. The process involves the direct electrodeoxidation of NiTiO3 (first stable oxide to form upon mixing and sintering of NiO and TiO2) to NiTi in a molten CaCl2 salt bath. This work builds on previous literature that has elucidated the reduction pathway by investigating the effect of reduction temperature and current collector material on the microstructure of the final reduction product. It was found that the use of a Ni current collector caused Ni-enrichment at the surface of the product, stabilizing the high temperature B2 cubic form to room temperature. Ni-rich phases Ni4Ti3 and Ni3Ti were observed after 24h reductions. The former is favoured at higher reduction temperatures, and the latter at lower temperatures. Mechanisms for formation are proposed.

Irradiation-Induced Grain-Boundary Solute Segregation and Its Effect on Ductile-to-Brittle Transition Temperature in Reactor Pressure Vessel Steels

Abstract A study on grain-boundary segregation and embrittlement in terms of the Charpy ductile-to-brittle transition temperature (DBTT) has been performed for the neutron-irradiated A533B steels with typical contents of impurities of Japanese reactor pressure vessel ones. The neutron irradiation was conducted at 563 K to a fluence of 1.3× 1024n/m2 (E&amp;gt;1 MeV) using material testing reactors. The neutron irradiation induced the P and Ni segregation and the reduction in C in some cases at grain-boundaries. The increase in the P segregation at high fluence (&amp;gt;5×10 23n/m2, E&amp;gt;1 MeV) was less than 0.1 in monolayer coverage for the steels with the bulk content of P not exceeding 0.02 wt%. The hardening more strongly affected the DBTT shift than the P segregation for those steels. The reduction in segregated C that enhances the grain-boundary cohesion by neutron fluence is not large enough to cause the DBTT shift.

Control of grain position in Ni-mediated crystallization of amorphous silicon

We report the control of nucleation sites for Ni-mediated crystallization of a-Si. It was shown that the self-organization of Ni atoms at the pressure-induced indentation sites on a-Si can be possible during thermal annealing without selective Ni contact or patterning. We have succeeded in forming Ni segregation sites (SSs) on a-Si by pressing a SiO 2 coated steel tip-array, where the nucleation starts. Any mechanical damage, such as peeling off and crack, was not found in the a-Si by atomic force microscope (AFM) inspection, when the tip-pressing pressures are in the range of 31–94 MPa. By controlling nucleation sites, the poly-Si with well-aligned rectangular grains of 20 μm×40 μm have been achieved for the first time.

Segregation and structural transformations at Σ=3 grain boundaries in NiAl: A Monte-Carlo study

Grain boundary structure and grain boundary segregation in stoichiometric and Ni-rich compositions of NiAl are studied by molecular statics and grand-canonical Monte-Carlo simulations in conjunction with an embedded-atom potential. Significant Ni segregation is found at the Σ = 3(1 1 1) and Σ = 3(2 1 1) grain boundaries, with segregation isotherms showing a saturation effect. The amount of Ni segregation increases and the excess free energy of the grain boundaries decreases with increased Ni bulk concentration. In strongly Ni-rich bulk compositions, both boundaries undergo transformations to new structures capable of accommodating more excess Ni atoms than the initial structures. The structural transformation is clearly manifested by a non-monotonic behavior of the segregation isotherm. In the Σ = 3(2 1 1) grain boundary, the transformation is accompanied by a relative tangential translation of the grains.

Effect of surface stress on Ni segregation in (110) NiAl thin films

Molecular statics and Monte Carlo simulations in conjunction with an embedded-atom potential are applied to study surface stresses and surface segregation in (110) NiAl free-standing thin films with different thicknesses. Ni always segregates to the surface for both stoichiometric and Ni-rich bulk compositions. The amount of segregation and the lateral deformation of the film both vary in inverse proportion to the film thickness. The size effect of segregation is explained by the elastic deformation of the film in response to the surface stress. It is predicted that the surface free energy decreases and the surface stress increases as the film becomes thinner. These effects are not specific to free-standing films but are also relevant to films on substrates, multilayers, new phase nuclei, and similar situations.

Adsorbate-Induced Segregation in the Ni{111}/Au/(R,R)-Tartaric Acid System

The adsorption of the catalytically important chiral modifier, (R,R)-tartaric acid, was investigated on bimetallic Ni/Au surfaces created by annealing thin Au films on Ni{111}. Reflection adsorption infrared spectroscopy measurements revealed that the adsorption mode of (R,R)-tartaric acid depends very strongly on the surface composition. Deprotonation to produce an adsorbed bitartrate species was achieved only when small Ni clusters became available in the bimetallic surface. Medium-energy ion-scattering studies showed that, once the bitartrate species was able to be produced, significant adsorbate-induced Ni segregation was observed. Scanning tunneling microscopy (STM) investigations suggest that segregation is particularly facile in the proximity of step edges. In addition, STM revealed that the ordered (9 × 9) structure previously reported by Jacobsen et al.1 is stable even with relatively high concentrations of Ni in the surface layer.

Magnetization of the unsegregated and segregated (100) surface of MoV binary alloys

The present thesis concerns applications of first principles electronic structure calculations in conjunction with methods of statistical mechanics for simulations of transition metal alloys both in the bulk and at surfaces.A fully relativistic generalization of the exact muffin-tin orbitals (EMTO) method has been developed. The method accurately takes into account spin-orbit coupling and allows one to calculate orbital polarization and magneto-crystalline anisotropy in magnetic systems as well as increasing the range of applicability of the EMTO method to heavy elements. A new direct-exchange Monte Carlo (DEMC) method has been proposed, which is capable to tackling effectively statistical simulations of surface segregations in disordered and ordered alloys.The applications of relativistic methods include calculations of spin and orbital magnetization in iron-cobalt disordered and partially ordered alloys, as well as computation of the core-level shifts (CLS) in transition metal alloys. It has been found, that relativistic corrections are important for CLS calculations in 5-d metal alloys. Properties of a Ni monolayer deposited on a Cu surface have been studied. The monolayer is found to be unstable in the top layer, and its magnetization depends greatly on the surface orientation. Two distinct energy levels have been found to exist Co/Cu/Ni trilayers deposited on the (100) Cu surface, which correspond to a completely paramagnetic trilayer and the case when only Ni is paramagnetic.Vacancy ordering in substoichometric titanium carbides TiCx have been simulated. Existence of three ordered phases in the range of carbon concentration x=0.5 ÷1.0 has been revealed and a theoretical phase diagram has been proposed. Surface segregations have been calculated in disordered Ni50Pt50 and Ni50Pd50 as well as in ordered NiPt alloys. Segregation reversal has been observed in the Ni50Pt50 alloy with Pt segregation at the (111) surface and Ni segregation at the (110). In the ordered NiPt alloys segregation behaviour is found to be affected greatly by small deviations from the exact stoichiometric composition in bulk. Surface magnetization in PdV and MoV bcc alloys have been studied. It has been found, that in PdV alloys surface segregations suppress magnetic order at the surface, while in MoV alloys magnetization is substantially enhanced due to the segregation.