Diffusion Of Ce Research Articles

Enhanced reliability of yttria-stabilized zirconia for dental applications

An increasing number of dental applications based on yttria-stabilized zirconia (3Y-TZP) have been developed in recent years as a result of the advances and versatility of dry-processing and soft machining at the pre-sintered state. Nonetheless, the long-term surface stability of these materials in humid environment is still a matter of concern and may limit its application. In this work, a simple method to prevent hydrothermal degradation on the zirconia surface is studied in detail. This method involves the infiltration of pre-sintered parts with optimized solutions containing Ce salts, leaving unchanged the other processing steps, allowing the diffusion of Ce during conventional sintering. Several pre-sintering conditions, solution concentrations and sintering temperatures were studied and characterized, obtaining working parameters for the production of zirconia parts with mechanical properties similar to standard 3Y-TZP and high resistance to hydrothermal aging. This optimal combination was obtained with the 1150°C pre-sintering temperature, 50wt.% solution and sintering at 1450°C, leading to a superficial CeO2 content of about 3mol.%.

Diffusion of Zr, Ru, Ce, Y, La, Sr and Ba fission products in UO2

The diffusivity of the solid fission products (FP) Zr (Zr4+), Ru (Ru4+, Ru3+), Ce (Ce4+), Y (Y3+), La (La3+), Sr (Sr2+) and Ba (Ba2+) by a vacancy mechanism has been calculated, using a combination of density functional theory (DFT) and empirical potential (EP) calculations. The activation energies for the solid fission products are compared to the activation energy for Xe fission gas atoms calculated previously. Apart from Ru, the solid fission products all exhibit higher activation energy than Xe. For all solid FPs except Y3+, the migration of the FP has lower barrier than the migration of a neighboring U atom, making the latter the rate limiting step for direct migration. An indirect mechanism, consisting of two successive migrations around the FP, is also investigated. The calculated diffusivities show that most solid fission products diffuse with rates similar to U self-diffusion. However, Ru, Ba and Sr exhibit faster diffusion than the other solid FPs, with Ru3+ and Ru4+ diffusing even faster than Xe for T<1200K. The diffusivities correlate with the observed fission product solubility in UO2, and the tendency to form metallic and oxide second phase inclusions.

Effects of Cr on the interdiffusion between Ce and Fe–Cr alloys

Fuel cladding chemical interaction (FCCI) has been a long-standing issue for the metallic fuel with a steel cladding in a sodium-cooled fast reactor, particularly for a high burnup fuel. Although the FCCI has been largely improved by alloying the fuels with Zr or Pd elements, applying a physical diffusion barrier between fuel and cladding, and employing advanced ferritic/martensitic (F/M) claddings, there is a scientific knowledge gap in understanding the behavior of chromium and its effects on the interdiffusion between lanthanides and advanced F/M steels that contain 9–12wt.% Cr. In this paper, we systematically studied the interdiffusion between cerium and Fe–Cr model alloys with Cr contents of 6, 9 and 12wt.%. Following the thermal annealing at 560°C for up to 100h, detailed microstructural characterizations were performed to determine the interdiffusion microstructures, compositional distributions, diffusion kinetics, and phase structures in the interdiffusion zone. This study unambiguously disclosed that, as the Ce diffuses into Fe–Cr model alloys, Cr segregates and precipitates into Cr-rich σ phase consisted of Fe and Cr instead of forming a ternary phase together with Fe and Ce. The precipitation of those nano-sized σ phase particles at the Ce diffusion front would effectively slow down the interdiffusion.

Direct observation of dopant atom diffusion in a bulk semiconductor crystal enhanced by a large size mismatch.

Diffusion is one of the fundamental processes that govern the structure, processing, and properties of materials and it plays a crucial role in determining device lifetimes. However, direct observations of diffusion processes have been elusive and limited only to the surfaces of materials. Here we use an aberration-corrected electron microscope to locally excite and directly image the diffusion of single Ce and Mn dopants inside bulk wurtzite-type AlN single crystals, identifying correlated vacancy-dopant and interstitial-dopant kick-out mechanisms. Using a 200 kV electron beam to supply energy, we observe a higher frequency of dopant jumps for the larger and heavier Ce atoms than the smaller Mn atoms. These observations confirm density-functional-theory-based predictions of a decrease in diffusion barrier for large substitutional atoms. The results show that combining depth sensitive microscopy with theoretical calculations represents a new methodology to investigate diffusion mechanisms, not restricted to surface phenomena, but within bulk materials.

The formation of intermetallic compounds during interdiffusion of Mg–Al/Mg–Ce diffusion couples

The formation of Al–Ce intermetallic compounds (IMCs) during interdiffusion of Mg–Al/Mg–Ce diffusion couples prepared by solid–liquid contact method was investigated at 623K, 648K and 673K for 24h, 48h and 72h, respectively. During the whole diffusion process, Al was the dominant diffusing species, and it substituted for Mg of the Mg–Ce substrate. Five Al–Ce IMCs of Al4Ce, Al11Ce3, Al3Ce, Al2Ce and AlCe were formed via the reaction of Al and Ce. The formation of Al4Ce as the first kind of IMC was rationalized on the basis of an effective Gibbs free energy model. The activation energy for the growth of the total diffusion reaction layer was 36.6kJ/mol.

Ceria Diffusion and Electrical Properties on Lanthanum Doped Strontium Titanate

The Ce component was coated on the as-prepared La0.3Sr0.7TiO3 (LST) particles. The lattice of LST did not change with the amount of Ce-coating after 1300-1400°C sintering in air but distortion occurred for the Ce-LST sintered at the temperature of 1500°C. The Ce diffusion into LST lattice to affect the lattice distortion was also observed for sintering at 1400°C in activated carbon atmosphere. The 1.5 and 3.0 mol% Ce coatings exhibited slightly lower conductivities than LST without Ce-coating under the 95%Ar+5%H2 atmosphere. The conductivity decreased significantly when the Ce-coating was too high such as 6 mol%. The Ce-LST anode increased the power performance of half-cell compared with the LST anode without Ce-coating. A suitable Ce-coating could behave high power density of the half-cell 3.0 mol% Ce-LST/LSBC. Thus, the Ce coating on LST could enhance the electrical properties of LST/LSBC cell due to the effect of Ce species diffusion into LST structure.

Cage effect in the thermal decomposition of solid-state azobisisobutyronitrile

The method of cross-recombination of isotopically labeled NC(CH3)2C and NC(CD3)2C radicals with equal free-valence reactivity is applied to measure the cage effect, F = krec/(krec + kdif), in the thermal decomposition of azobis(isobutironitrile) in isomorphic crystals consisting by half of fully deuterated molecules of the initial substance. It is shown that the decomposition occurs at crystal lattice defects. The products are analyzed by chromatography-mass spectrometry at the degree of conversion of 1% at 70°C. The CE effect and diffusion coefficient at 70°C have been found to be F = 0.87 and D = 7.7 · 10−7 cm2/s. These results are indicative of a high mobility of molecules on the inner surface of the crystal.

Diffusion barriers for CeFe4Sb12/Cu thermoelectric devices

ABSTRACTThe efficiency of a tantalum nitride interlayer as a diffusion barrier for CeFe4Sb12 thermoelectric material against electrode copper material has been investigated. The thermal stability of CeFe4Sb12/TaN/Cu stackings has been investigated after annealing at 600°C from a microstructural study. CeFe4Sb12 and Cu appear to chemically react through the formation of CeCu2 and Cu2Sb phases whereas no reaction is observed for CeFe4Sb12 with TaN. This study showed that the TaN interlayer cannot inhibit the diffusion of Sb from the skutterudite substrate to the copper electrode but prevents the diffusion of Ce and consequently the formation of the CeCu2 phase.

Interactions between components of SrTi0.98Nb0.02O3−δ–YSZ and SrTi0.98Nb0.02O3−δ–CeO2 composites

AbstractComposites consisting of SrTiO3‐based perovskite and either yttria‐stabilized zirconia or ceria were investigated. The mechanical compatibility and possible inter‐diffusion between phases were characterized by scanning electron microscopy, X‐ray diffraction, and energy‐dispersive X‐ray spectroscopy. A gradual disappearance of Ce‐containing phases with an increase in the temperature of reduction in hydrogen was noticed. Moreover, a diffusion of Sr, Ti, and Ce was observed between the composites and the yttria‐stabilized zirconia electrolyte support in the conditions of high temperature (1300–1400 °C) reduction in H2.

Anisotropic ternary Ce13Fe80B7 powders prepared by hydrogenation–disproportionation–desorption–recombination process and the diffusion of Ce–Cu eutectic alloys

Anisotropic Ce13Fe80B7 powders were obtained for the first time by a modified hydrogenation– disproportionation–desorption–recombination (HDDR) process. It was found that the formation of anisotropy (the crystal texture) in the HDDR treated powders is related to a rod-like disproportionation microstructure. In order to further improve the coercivity, the HDDR treated powders were mixed with Ce72Cu28 powders and then were heated. As a result, the coercivity of the HDDR powders was enhanced from 400Oe to 1450Oe.

TiN-based coatings on fuel cladding tubes for advanced nuclear reactors

Titanium nitride (TiN) thin films are coated on HT-9 and MA957 fuel cladding tubes and bars to explore their mechanical strength, thermal stability, diffusion barrier properties, and thermal conductivity properties. The ultimate goal is to implement TiN as an effective diffusion barrier to prevent the inter-diffusion between the nuclear fuel and the cladding material, and thus lead to a longer lifetime of the cladding tubes. Mechanical tests including hardness and scratch tests for the samples before and after thermal cycle tests show that the films have a high hardness of 28GPa and excellent adhesion properties despite the thermal treatment. Thermal conductivity measurements demonstrate that the thin TiN films have very minimal impact on the overall thermal conductivity of the MA957 and HT-9 substrates, i.e., the thermal conductivity of the uncoated HT-9 and MA957 substrates was 26.25 and 28.44Wm−1K−1, and that of the coated ones was 26.21 and 28.38Wm−1K−1, respectively. A preliminary Ce diffusion test on the couple of Ce/TiN/HT-9 suggests that TiN has excellent material compatibility and good diffusion barrier properties.

Electrochemical behavior of Ce(III) in LiF-BaF2 melts

The electrochemical behavior of Ce(III) was investigated in the molten LiF-BaF2 (81 mol.%−19 mol.%) on a molybdenum electrode in the temperature range of 1098–1188 K using cyclic voltammetry and chronopotentiometry. It was observed that CeF3 could be reduced into cerium metal in a reversible one-step process exchanging three electrons (Ce(III)+3e−→Ce(0)) at the operating temperatures on a molybdenum cathode. The electrochemical reduction process was controlled by the diffusion of Ce(III) in the solution. The Ce(III) diffusion coefficients were calculated at different temperatures and the values obeyed the Arrhenius law with an activation energy of 87.5 kJ/mol.

Study on the intermetallic phases in the Mg–Ce system: Part II. Diffusion couple investigation

Mg–Ce diffusion couple annealed at 400 °C was investigated via electron probe microanalysis (EPMA), and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). The results validated the compositions of the intermetallic phases experimentally determined by Zhang et al. [1] on the Mg-rich side of the binary phase diagram and refined the phase diagram suggested by Zhang et al. [2] for the composition range of 38–70 wt% Ce. A revised equilibrium phase diagram was proposed for the Mg–Ce system up to 50 at% Ce.

Electro-Regeneration of Ce(IV) in Simulated Spent Cr-Etching Solutions Containing Abundant Ce(III)

This study investigated the effects of the initial Ce(III) concentration, nitric acid concentration, anion impurity, and cell device on Ce(IV) regeneration in simulated spent Cr-etching solutions from thin-film transistor liquid crystal display manufacture. Increasing (electrolyte) concentration (0.5–4 M) favored 0.2 M Ce(III) electro-oxidation, Ce(III) diffusion , and internal cell resistance reduction. The amount of Ce(IV) regenerated from Ce(III) electro-oxidation and the current efficiency (CE) at electrolysis on Pt increased with increasing initial Ce(III) concentration in 4 M without impurities in a divided cell with an anion-exchange (AMI-7001) separator. Both Ce(IV) yield and CE were lowered in the copresence of multiple impurities, and the adverse influence of a single anion impurity on the Ce(IV) yield and CE was in the order . The apparent rate constant of Ce(III) oxidation without impurity was approximately fourfold that with multiple impurities; nevertheless, the two cases exhibited similar Ce(IV) yields or CEs at electrolysis for 300 min. For the Ce(IV) regeneration, the divided cell was superior to an undivided cell in the absence/presence of multiple impurities, and the AMI-7001 was better than a cation-exchange separator (CMI-7000).

Annealing-temperature Dependence of Compositional Depth Profiles and Chemical Bonding States of CeOx/LaOx/Si and LaOx/CeOx/Si Structure

We have investigated the influence of post deposition annealing (PDA) on compositional depth profiles and chemical structures of CeOx/LaOx/Si(100) and LaOx/CeOx/Si(100) interfaces by angle-resolved X-ray photoemission spectroscopy. Analyses of Ce 3d and O 1s spectra show that a Ce oxidation state changes from Ce4+ to Ce3+ by PDA in N2 at and above 500{degree sign}C and changes from Ce3+ to Ce4+ by PDA in O2 at 300{degree sign}C. This implies that a Ce oxidation state can be controlled by changing the condition of PDA. The diffusion of Ce and La atom occurs at CeOx/LaOx interface by PDA above 500{degree sign}C.

Influence of the oxidative/reductive treatments on Pt/CeO 2 catalyst for hydrogen iodide decomposition in sulfur–iodine cycle

A Pt/CeO 2 catalyst was prepared by sol–gel method. The as-received sample was successively oxidized, reduced and re-oxidized. The samples were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and hydrogen iodide (HI) catalytic decomposition activity evaluation for the sulfur–iodine cycle. The oxidative/reductive atmosphere affected the structure and performance of the catalyst by the strong metal-support interaction (SMSI). It was suggested that a migration of Ce 4+ from the bulk to the surface occurred during the reductive treatment. The diffusion process was reversed when the atmosphere was changed to an oxidative one. The decoration or encapsulation of Pt by ceria support changed with the atmosphere, and affected the activity of the catalyst. At temperature below 400 °C, the reduced sample exhibited the best activity. After then the activity of the reduced and re-oxidized samples tends to be the similar, but still better than the as-received and oxidized samples. The surface and interfacial Pt 0 sites were both considered as the effective factors. Models were constructed to describe the diffusion of Ce 4+ and oxygen vacancies as well as the possible shell-core structure of Pt crystallites and the decoration/encapsulation by ceria support.

Impurity diffusion of cerium and gadolinium in single- and polycrystalline yttria-stabilized zirconia

Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive material for solid oxide fuel cells or for nuclear applications such as inert matrix for the destruction of excess plutonium and host material for nuclear waste storage. Long-term properties such as phase stability depend on cation diffusion. Therefore, the present work is focused on the diffusion of Ce and Gd in YSZ single crystals and high-density polycrystals. A thin film of Ce or Gd was deposited either by the spin-coating method or by physical vapour deposition on the surface of polished samples. Diffusion experiments were performed between 1173 and 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. Analysis of the penetration profiles led to the determination of bulk diffusion coefficient (D b) in single crystals and the effective diffusion coefficient (D eff), bulk and grain boundary diffusion coefficients (D gb) were determined in polycrystals. The dependence of diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with results in the literature.

Ni–Sm 0.2Ce 0.8O 1.9 anode-supported YSZ electrolyte film and its application in solid oxide fuel cells

Ni–Sm 0.2Ce 0.8O 1.9 anode-supported YSZ electrolyte films were fabricated and a fuel cell based on this film was tested in this paper. The anode/YSZ film interface was investigated by means of SEM–EDX. The SEM results showed that the YSZ film was almost fully dense and crack-free. The analyses of EDX revealed that Ce and Sm cations preferential migrated from the NiO–SDC anode into the YSZ film for less than 3 μm while no significant diffusion occurred for Y and Zr. A cell of Ni–SDC/YSZ(14 μm)/LSM–YSZ was tested from 700 to 850 °C using humidified hydrogen as fuel and ambient air as oxidant. The cell provided maximum power densities of 304, 625, 980 and 1270 mW/cm 2 at 700, 750, 800 and 850 °C, respectively. Polarization analysis based on the impedance spectra showed that the anode polarization only accounted for 8% of the total electrode polarization while this ratio for cathode was 92%. A negligible anode polarization suggested that the diffusion of Ce and Sm and its negative effect on the performance of the cell were suppressed to an acceptable level in this study.

Ce-containing Mordenites: Synthesis, structure and reactivity towards NO and CO gases

Aqueous solutions of cerium nitrate of increasing concentrations (2.5, 5 and 7.5 wt.% Ce) have been contacted with the components forming Mordenite zeolite; during forming the gel under hydrothermal conditions, for allowing the accessibility of Ce ions to proceed into compensating positions in Mordenite structure. These materials were characterized by the methods of FTIR, XRD, N 2 adsorption and UV–vis diffuse reflectance spectroscopy. The interaction of NO and CO adsorptions; at room temperature, on thermally pre-treated (300 °C, 10 −5 Torr, 3 h) as well as pre-reduced (50 Torr, 500 °C, 1 h) samples were studied by in situ Fourier transform infrared spectroscopy. XRD and FTIR results indicate that the Ce atoms are mostly present in internal surfaces in Mordenites for 2.5 and 5CeMOR samples whereas for 7.5CeMOR, a decrease in diffusion of Ce to be in compensating positions is perceived; as conceived from lowering the lattice volume, pointing to the presence of discrete amounts of CeO 2 (582 cm −1) and cerium silicate (Si–O–Ce; 797 cm −1) species. All the samples indicate intra-crystalline mesopores as depicted from V l– t plots particularly the 7.5CeMOR sample that showed the highest wide-pore volume (0.073 cm 3/g), lowest pore radius (21 Å) and thus, revealed the highest S BET between all samples (363 m 2/g). UV–vis characterization of 7.5CeMOR sample shows octahedral Ce species (345, 360 and 390 nm) in small clusters inside zeolite channels and most probably originated from cerium silicates having different coordination with NaMOR along with discrete amounts of CeO 2 (420 nm) species. CO readily adsorbs on the Ce 3+ sites of the pre-reduced 7.5CeMOR catalyst, rather than those on Ce 4+, to display minor amounts of carboxylate and dominant amounts of monodentate carbonate that were amenable to decompose to produce CO 2 gas (2335 cm −1). On the other hand, the in situ interaction of nitric oxide (NO) gas on the 7.5CeMOR catalyst led to the formation of a series of nitrosyl species: N 2O (2240 cm −1), NO (1908 cm −1), N 2O 3 (1880 cm −1) and (NO) 2s,as (1844, 1734–1720 cm −1). Such nitrosyl complexes were favorably formed on Ce 3+ in 7.5CeMOR those exchanged Na ones.

Electrical conductivity and stability of Gd-doped ceria/Y-doped zirconia ceramics and thin films

The interaction between Ce 0.90Gd 0.10O 1.95 (CGO) and Y 0.16Zr 0.84O 1.92 (YSZ) and the electrical conductivity and thermal expansion for solid solutions of CGO and YSZ have been studied. Thin films of CGO interacted with YSZ substrates around 1000 °C, with diffusion of Zr into CGO. The reaction between CGO and YSZ powder started around 1200 °C. The diffusion of Ce into YSZ was the main reaction. The solid solutions of CGO x –YSZ 1− x were single phase and possessed a lower conductivity than that of either CGO or YSZ. The electrical conductivity for the solid solutions increased with decreasing oxygen activity, whereas the pO 2 independent ionic conductivity region was not observed. The dimensional expansion due to reduction (chemical expansion) was larger for the solid solutions compared to that for of CGO and YSZ.