Interdiffusion Of Cr Research Articles

Effect of diffusion barriers on steam oxidation properties and interface evolutions of Cr coating for zirconium alloy at 1200 °C

Under the ATF design concept, Cr-based coating is the most promising zirconium alloy cladding protective coating in the case of PWR accidents. However, the primary factor affecting the oxidation resistance of Cr coatings is the interdiffusion of Cr and Zr. In this study, Cr coatings with three refractory metal interlayers of Nb, Mo, and Ta were deposited on zirconium alloy surfaces using closed-field unbalanced magnetron sputtering. The study investigated the effects of different interlayers on the microstructure, oxidation properties, and interface evolution process of Cr coatings under steam conditions at 1200 ℃, and compared them with Cr coatings without intermediate layers. The results indicate that the refractory metal interlayer influences the orientation and growth rate of Cr coating grains. The formation of a Laves phase mixed layer in the interlayer during oxidation effectively hinders the diffusion of O and Cr to the zirconium alloy matrix, enhancing its oxidation resistance. Furthermore, the diffusion rate and vacancy concentration of Mo and Ta elements to the Zr layer exceed that of diffusion to the Cr layer. During the cooling process, a precipitate phase forms in the Zr-4 matrix, while the Nb coating forms an infinite solid solution in the β-Zr.

Effect of PEO interlayer on oxidation behavior of PEO/Cr composite coating on Zr–1 Nb alloy in 1200 °C steam

The PEO/Cr composite coating on Zr–1 Nb alloy prepared by plasma electrolytic oxidation (PEO) and filtered cathodic vacuum arc deposition (FCVAD) technologies exhibited good oxidation resistance in 1200 °C steam. The PEO interlayer was gradually dissolved by Zr substrate, and M-ZrO2 to Zr3O phase transformation occurred, but it still moderated the inward oxygen diffusion and the inter-diffusion of Cr and Zr to some extent. Moreover, the PEO interlayer suppressed the formation of ZrCr2 in Zr substrate and ZrCr2O5 secondary phases in its steam oxide layer. The PEO layer and steam oxide layer could prevent from hydrogen permeation into the Zr substrate.

Microstructure evolution and mechanical property of high temperature solid-state diffusion bonded Cr-Zry4 with and without a 316 SS interlayer

Solid-state diffusion bonding between pure Cr and Zircaloy-4 (Zry4) was studied with and without 316 stainless steel (SS) as an interlayer at temperatures ranging from 1203 to 1573 K, followed by the detailed microstructural analysis and mechanical property evaluation at the bonding interfaces. For the samples directly bonded between Cr and Zry4, two regions were confirmed at the diffusion interface. One was a continuous block of Zr2Cr intermetallic compound formed by the inter-diffusion of Cr and Zr, and the other was a mixture of Zr2Cr and Zrα phases as the consequence of Zr-Cr eutectic reaction. For the samples with SS as an interlayer, three regions were mainly observed at the SS-Zry4 interface, consisting of (FeCr)α-, Zr(Fe, Cr)2-, and Zr2(Fe, Ni)-phase dominated layers, with (FeCr)α-phase observed at Cr-SS interface. The microstructure of the phases at diffusion interfaces were closely related to the bonding temperature. Nano-indentation results confirmed a rapid increase in hardness at Cr-Zry4 and SS-Zry4 interfaces for Cr-Zry4 and Cr-SS-Zry4 bonded specimens, respectively, due to formation of the ZrCr2/Zr(Fe, Cr)2 phase. The presence of the hard brittle intermetallic compounds at the interface infers the possible brittle fracture risk of the Cr-Zry4 interface during long-term operation under normal condition and high temperature under accident conditions.

Atomic scale structure investigations of epitaxial Fe/Cr multilayers

Abstract Fe/Cr multilayers were deposited by molecular beam epitaxy on the MgO(1 0 0) substrate. Structural properties of the samples were analyzed by low energy electron diffraction, high resolution transmission electron microscopy (HRTEM), as well as by X-ray reflectivity, conversion electron Mossbauer spectroscopy (CEMS) and Auger electron spectroscopy. Investigations revealed multilayered system built of well-ordered Fe and Cr thin films with (1 0 0) orientation. A high geometrical perfection of the system, i.e. planar form of interfaces and reproducible thickness of layers, was also proven. Fe/Cr interface roughness was determined to be 2–3 atomic layers. CEMS studies allowed to analyze at atomic scale the structure of buried Fe/Cr interfaces, as well as to distinguish origin of interface roughness. Roughnesses resulting from interface corrugations and from the Fe–Cr interdiffusion at interfaces were observed. Fe/Cr multilayers showed strong antiferromagnetic coupling of Fe layers.

Nanometer scale chemistry and microstructure of CrN/AlN multilayer films

Nanometerscale multilayer CrN/AlN thin films were deposited by pulsed DC magnetron sputtering. Nanoscale resolution chemistry and microstructure of CrN/AlN films with modulation periods (Λ) of 4nm and 18nm were studied using advanced microscopy and diffraction. At the nanometer level, the interface between the CrN and AlN nanolayers contains a concentration gradient of Al and Cr. Inter-diffusion of Cr and Al atoms into the AlN and CrN layers was identified. Microstructural characterization revealed the CrN nanolayers epitaxially stabilize the AlN layers (2nm) to a cubic structure for the Λ=4nm film. The rapid repetition of the interfacial energy across the AlN nanolayers is critical for the coherent epitaxial growth. The film at Λ=4nm exhibited a superhardness of 42GPa. In contrast, the AlN layers (12.5nm) exhibited a hexagonal structure in the Λ=18nm film as the bulk energy becomes dominating, in which misorientated and incoherent interfaces were found. The hardness of the film dropped down to 23GPa.

TEM studies of microstructure, interfaces, and intermixing of FePt/MgO/FePt/Pt/Cr(Ru) films

We have investigated the microstructure, interfaces, and intermixing in sputter-deposited FePt/MgO/FePt/Pt/Cr(Ru)/SiN/Si magnetic tunnel junctions by high-resolution transmission electron microscopy (HRTEM) and correlated these results to the magnetic properties. Cr90Ru10 is used as a texturing layer for the growth of perpendicular magnetic anisotropy (PMA) films. HRTEM confirmed the formation of the desired [001] texture in cubic Cr90Ru10 as well as in the adjacent L10-FePt layer using our growth methods. A 1.5 nm layer of MgO was deposited as a tunnel barrier for magnetic tunnel junction in an attempt to fabricate fully perpendicular tunnel junctions of these materials. The HRTEM observations, however, show that the MgO layer was partially crystalline but randomly oriented. The top FePt layer was also found to grow as discrete islands with random orientations and is mostly fcc crystal structure reducing the utility of this multilayer in PMA tunnel junction devices. The fine probe energy-dispersive X-ray spectroscopy showed the interdiffusion of Cr to Pt/FePt layers. The magnetic properties show out-of-plane coercivity larger than that of the in-plane values but with a loss of squareness, consistent with the loss of [001] texture and of L10 structure in the top magnetic layer.

Vector magnetisation measurements on thermally evaporated CoCr multilayers and solid solutions for spintronic applications

Structure, morphology and magnetic properties of thermally evaporated Co/Cr multilayers and of the solid solution obtained by isothermal treatment of the layered samples have been studied to evaluate their potential application as high-coercivity materials for hard spin injectors in spintronic devices such as magnetic tunneling junctions. The thermal treatment was performed in a partial pressure of inert gas to allow interdiffusion of Cr and Co. Structure and composition of the materials, as well as the effects of the annealing treatment, have been investigated by field emission scanning electron microscopy and energy-dispersive X-ray analysis. The magnetic configuration of these films was investigated by magnetic force microscopy. Simultaneous measurements of two mutually perpendicular magnetisation components were performed by means of a vector vibrating sample magnetometer, and showed that the in-plane coercivity can be increased by proper thermal treatment from less than 10 Oe to about 240 Oe. However, annealing may result in a loss of compositional and morphologic homogeneity of the films.

The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-beam Epitaxy

ABSTRACTEpitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is -C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer.

PZT Capacitors on Top of CrTiN/TiN Double Barrier Layers

In this paper we investigated the thermal stability of a novel CrTiN/TiN double barrier, compared to those of TiN and CrTiN single layer barriers. The CrTiN/TiN double layer was stable against heat treatments at higher than 700°C in oxygen ambient for 30 min. However, the double layer showed a severe Pt and Cr interdiffusion issue after crystallization annealing for PZT. We found that pre-annealing treatments prior to deposition of Pt significantly reduced the interdiffusion. In addition, we also demonstrated ferroelectric characteristics of PZT capacitors on top of Pt/CrTiN/TiO2/Si bottom electrode system for ultra-high density memory applications.

Characterization of iron-based alloy interconnects for reduced temperature solid oxide fuel cells

The oxidation kinetics and electrical properties of oxide scales thermally grown on the surface of a commercial ferritic alloy have been investigated on the un-oxidized and pre-oxidized alloys as functions of temperature and time under oxidizing atmospheres with four different electrodes. Oxidation kinetic studies with the un-oxidized alloys show a nearly parabolic dependence on time of oxide-scale growth rate, but a significantly increased growth rate with a coating of LSCo (La 0.6Sr 0.4CoO 3− δ ) compared to those without and with the coatings of LSM (La 0.85Sr 0.15MnO 3)+LSGM (La 0.8Sr 0.2Ga 0.83Mg 0.17O 2.815) and platinum. Short-term resistance measurements in stagnant air as a function of temperature with pre-oxidized alloys indicate that the oxide scale has a semiconducting transport property. The overall activation energy includes a term from small-polaron hopping inside the oxide scale Δ H m and terms Δ H i and Δ H j from charge transfers at the electrode/oxide-scale and alloy/oxide-scale interfaces, respectively. For the LSCo electrode, long-term resistance measurements as a function of time with un-oxidized alloys reveal a secondary oxidation mechanism related to the formation of an insulating spinel phase in addition to a primary oxidation mechanism associated with the formation of Cr 2O 3. SEM observations show that oxidation of the un-oxidized alloy in the presence of an oxide electrode results in considerable interdiffusion of Cr and the electrode cations, especially Co, across the interfaces. Since the ASR values of the oxide scale measured with oxide electrodes quickly approach the permitted limit of a practical SOFC, highly recommended for prevention of a secondary electrochemical oxidation of iron-based alloy interconnects is (1) the use of an oxide coating having purely electronic conductivity and/or (2) prior-to-use conditioning of the alloys via pre-oxidation.

Growth mode of Cr on Fe(001) described by ab-initio interactions

Abstract The Fe-Cr interface has been widely studied since it provides a rich model system that allows an experimental confrontation of theoretical approaches. In particular, the growth mode of Cr on Fe(001) has revealed unexpected behaviour; strong interdiffusion of Cr into Fe and a Cr-Cr nearest-neighbour repulsion have been reported. Simple arguments based on phenomenological schemes cannot be invoked and thus a more detailed study is necessary. In this work a solid-on-solid model is used to simulate the growth of Cr on Fe(001) from first principles. The key ingredients in the lattice Hamiltonian are the effective cluster interactions, which are obtained by a Korringa-Kohn-Rostoker Green's function method in the dilute limit. These parameters are then used in a Monte Carlo simulation of deposition and diffusion. Rapid migration of Cr into the bulk is observed in agreement with experiments. Snapshots for different deposition conditions are used to elucidate the various structures. The consequences for t...

Resistivity and adhesive strength of thin film metallizations on single crystal quartz

Resistivity and adhesive strength were measured for the thin films 450 A Cr-1800 A Au, 450 A Cr-1000 A Mo-1800 A Au, 450 A Cr-1000 A Ni-1800 A Au, 450 A Mo-1800 A Au, 1800 A Au, and 2000 A Al on z-and AT-oriented single crystal quartz substrates in the as-deposited condition as well as after thermal annealing at 380 degrees C and 450 degrees C for 30 min in air or vacuum. The Cr-Au films exhibited significant resistivity increases after thermal annealing which were caused by the interdiffusion of Cr and Au. Barrier layers of Mo or Ni limited such increases after heat treatment. The Mo-Au, Au, and Al films exhibited resistivity decreases following thermal annealing. The mean adhesive strengths of the Cr-Au, Cr-Mo-Au, and Cr-Ni-Au films were excellent in the as-deposited and annealed conditions, ranging from 41 MPa to 70 MPa. The Mo-Au and Au films maintained relatively poor adhesion under all circumstances. Heat treatment improved the poor adhesive strength of the as-deposited Al films to values exceeding 63 MPa. Resistivity and adhesive strengths did not differ significantly between the z- and AT-oriented substrates.

Influence of Mo and N on the Interdiffusion of Cr and Ni in CrNi-Steels

Influence of Mo and N on the Interdiffusion of Cr and Ni in CrNi-Steels

Metallizations of InP based on transition metals

Single element Ni and Cr metallizations to low doped n- and InP have been investigated to clarify how they affect electrical performance of advanced multilayer metallizations for ohmic contact and Schottky diode applications. It was found that the Ni and Cr contacts to n-InP show unstable ohmic behaviour at all annealing temperatures (300-500 °C), and that Ni and Cr metallizations to p-InP show diode behaviour. The electrical behaviour of the contacts were compared to phase formation paths of Ni-In-P and Cr-In-P ternary systems. From this it was found that amorphous ternary phases form first by interdiffusion of Cr and Ni into the InP crystal. Subsequently, at higher annealing temperatures, crystallization and eventually phase separation determines the electrical properties of the contacts. After annealing at 500 °C the Cr diodes to p-InP are almost ideal, as deduced from barrier height measurements using a combination of I-V and C-V methods. This indicates that the metallization forms a two-layer structure with Cr-P phases lying above pure In. The Ni diodes to p-InP annealed at 500 °C, on the other hand, are not ideal, indicating that Ni-In and Ni-(In)-P phases in parallel are determining the electrical properties of the diode.

Correlation between resistivity and diffusion in thin films

Empirical equations have been derived that relate resistivity to diffusivity and activation energies for thin films. Results for the Cr/Au structure agree with published data on the interdiffusion of Cr and Au thin films at temperatures ranging from 350 to 450 °C. The following relationships were determined: Δρ/ρ0C=λ exp(−Q/kT); Δρ/ρ0C=nDt/χ2; Δρ/ρ0C=ψ exp(−χ2/4Dt) and D=Nχ2 /t exp(−Q/kT), where Nχ2/t=D0(cm2/s) is the frequency factor; Δρ is the change in resistivity; ρ0 is the room temperature resistivity; χ is film thickness; T is temperature; t is time; C is concentration; k is Boltzman’s constant; D is diffusion coefficient; and n, ψ, N, and λ are constants. Analysis of the Cr/Au resistivity data using these equations gives D0=1.2×10−7 cm2/s and Q=1.00 eV for Cr diffusion in Au; and D0=1.0×10−7 cm2/s and Q=1.06 eV for Au diffusion in Cr. These values were verified by analysis using Rutherford backscattering spectroscopy and, they are consistent with those reported for grain boundary diffusion.

Modulation of atomic interdiffusion at the Si(111)–Au interface

We attained a wide modulation of the Si(111)–Au interface reaction by deposition of controlled amounts of Cr on the semiconductor surface before in situ junction formation. Synchrotron radiation photoemission studies show that Cr deposition gives rise to a Si/Cr reacted interlayer that dramatically affects the subsequent Si–Au interdiffusion. Negligible interdiffusion of Cr and Au atoms was found in all cases so that the Si–Au intermixing depends upon an exchange reaction at the Si/Cr–Au interface in which Si is the only moving species. Large reduction of the interdiffusion occurs sharply above a critical value of Cr deposition (10 monolayers) that corresponds to a fully reacted Si(111)–Cr interface.

Interdiffusion of Cr, Mo, and W in Iron

AbstractThe diffusion rates of chromium, molybdenum, and tungsten in α- and γ-iron have been determined by electron-probe microanalysis. Alloy concentrations up to 28·3 at.-% Cr, 5·05 at.-% Mo, 2·9 at.-% W have been investigated over the range 675 to 1485° C. At any temperature in either ferrite or austenite, D Mo > D Cr > D w. Interdiffusion in each alloy system was found to be independent of concentration for the composition ranges studied. Data from the present investigation have been combined with existing reliable data so that diffusion in these alloy systems may be described by the following values of frequency factor (Do ) and activation energy (Q), with errors given at the 95% confidence limits.