Zr Surface Research Articles

Surface characteristics and corrosion behavior of Ti–Al–Zr alloy implanted with Al and Nb

Ti–Al–Zr alloys were implanted with Al and Nb to doses ranging from 1 × 10 17 to 1 × 10 18 ions cm −2. The valence states of element on the sample surfaces were analyzed by X-ray photoelectron spectroscopy (XPS). Glancing angle X-ray diffraction (GAXRD) was employed on the as-implanted specimens to understand phase formation. X-ray diffraction (XRD) measurement revealed α-Ti on Al-implanted samples and (α + β)-Ti on Nb-implanted samples. The tendency was observed in increasing corrosion resistance from α- toward (α + β)-phase. In deaerated 5 M HCl, the ion-implanted Ti–Al–Zr surface containing Nb-stabilized β-phase was spontaneously passive, while Al-implanted surface displaying an active/passive behavior. In the aerated solution with pH = 10, all the implanted surfaces are passive. Enhanced reoxidation was confirmed on implanted surfaces by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analysis. The corrosion in the solution with pH = 10 was governed by a predominantly TiO 2 surface film. The cathodic kinetics was seen to affect the corrosion behavior in 5 M HCl.

Microbial adhesion to zirconium alloys

We present data and analyses concerning the adhesion of clinically relevant Staphylococcus aureus, Staphylococcus epidermidis, and Pseudomonas aeruginosa (bacteria) and Candida albicans (yeast) to Zircaloy-2 (Zry-2) and Zircadyne-705 (Zr705) surfaces. These zirconium-based materials are similar to those now being used in total hip and knee replacements. Here we study clinical strains of microbes under shaken and stationary exposure conditions, and their ability to adhere to Zr surfaces having different oxide thicknesses. We use X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), viable counts, endotoxin assays, and statistical analysis methods, and demonstrate a predictive model for microbial adhesion based on XPS data.

Photoemission study of the (2 × 2) structure formed by H 2O adsorption on the Zr(0 0 0 1) surface

We have studied adsorption of water on the Zr(0 0 0 1) surface at sub-monolayer coverage by means of LEED and photoemission spectroscopy. An ordered (2 × 2) structure is formed after adsorption of 0.6–1.4 Langmuirs at 473 K. The sharpest LEED pattern was observed at an exposure of 1.2 L implying a coverage of 0.5 ML of oxygen. The same exposure at 293 K gives only a weak and diffuse (2 × 2) pattern. In addition, the sharp (2 × 2) pattern obtained at 473 K can be reversibly weakened by cooling to 293 K and subsequently sharpened by heating. For the sharp (2 × 2) structure, valence band spectra indicated dissociation of water and showed a peak composed mainly of O 2p derived states with two components at 6.0 eV and 6.6 eV binding energy. On cooling to 293 K, the O 2p peak became narrower and a new state appeared at 7.9 eV. Two components of the O 1s core level were resolved for the (2 × 2) structure, assigned to oxide and hydroxyl groups. The hydrogen on the surface of Zr(0 0 0 1) resulting from the dissociation of water and from bulk segregation strongly influenced the formation of the (2 × 2) structure of oxygen, and caused a temperature instability of the structure.

Dynamics of D2 released from the dissociation of D2O on a zirconium surface

Hydrogen is efficiently released during water dissociation on zirconium (Zr), while even very rapid temperature programmed heating of a hydrogen covered Zr surface predominantly leads to dissolution (approximately 99% dissolution). To help resolve these apparently contradictory observations, we have studied the dynamics of water (D2O) dissociation on a crystalline Zr surface by probing the rotational and vibrational energy distributions of the D2 produced using resonant enhanced multiphoton ionization spectroscopy. The internal-state energy distribution of the D2 product was found to be rotationally cold and vibrationally hot with respect to the temperature of the surface. The rotational distribution shows slight deviations from Boltzmann's law, with a mean rotational temperature of 426 K while the surface is at 800 K. The population of the nu"=1 vibration is at least four times higher than a 800 K temperature would allow, this corresponding to a vibrational temperature of 1100 K. Information on the translational energy of the D2 product have also been obtained by time-of-flight spectroscopy and it is found to be nearly thermally equilibrated with the surface temperature. Similar results were obtained from studies of D2 scattered from a clean Zr surface, and of D2 released by a slow thermal desorption process which involves dissolved hydrogen as the source. The reconciliation of the present results with those for thermal desorption of preadsorbed hydrogen implies a role for both surface and subsurface adsorption sites on the Zr surface and clearly demonstrates that at high temperatures, the release of D2 arises from the recombinative desorption of adsorbed hydrogen formed by the complete dissociation of D2O.

Nanocrystalline ZrO 2 thin films as electrode materials using in high temperature–pressure chemical sensors

The nature of the ZrO 2 thin films in the Zr/ZrO 2 electrode plays a vital role to build Zr/ZrO 2 high temperature–pressure chemical sensors, when the electrode is utilized to be in situ measuring electro-chemical properties of hydrothermal fluids. Zr metal was oxidized with a NaCO 3 melt to form a thin film of ZrO 2 on the surface of Zr, then an oxidation–reduction electrode was performed. The electro-chemical properties of the Zr/ZrO 2 electrode were tested from room temperature to high temperatures and high pressures. By using SEM, EPMA and HRTEM, analyses of topography, chemical composition and structure of the ZrO 2 thin films revealed that Zr/ZrO 2 interface is divided in to 5 zones from the outmost zone to the center: 1) prismatic and oxygen-rich ZrO 2, 2) ZrO 2; 3) oxygen-rich Zr; 4) oxygen-bearing Zr; 5) Zr metal. Especially, the outmost oxygen-rich ZrO 2 zone of the thin films is composed of nanometer-sized monoclinic crystals with high oxygen content, when Zr/ZrO 2 electrode is conducted to make a good high temperature chemical sensor.

Characteristics of Shear Bands and Fracture Surfaces of Zr<SUB>65</SUB>Al<SUB>7.5</SUB>Ni<SUB>10</SUB>Pd<SUB>17.5</SUB> Bulk Metallic Glass

In this work, the characteristics of shear bands and fracture surfaces of Zr 65 Al 7.5 Ni 10 Pd 17.5 bulk metallic glass fractured by a tensile test was investigated. Zr 65 Al 7.5 Ni 10 Pd 17.5 bulk metallic glass shows a yield stress of approximately 1.3 GPa, a fracture stress of approximately 1.5 GPa and a tensile plastic strain of 0.1-0.2% irrespective of the applied strain. Wavy, meandering shear bands were observed in the relatively wide area of specimen surfaces around the point of failure, and typical vein patterns were observed on the fracture surfaces. Shear bands and fracture surfaces were further examined by confocal microscopy to obtain more precise information on their roughness. On the other hand, evidence of viscous flow due to crack propagation was also obtained around the edge at the point of failure on specimen surfaces by confocal microscopy. The deformability of Zr 65 Al 7.5 Ni 10 Pd 17.5 bulk metallic glass is discussed on the basis of the obtained results.

Effects of electron bombardment on the thermal desorption of cyclic hydrocarbons from zirconium surfaces

In this Letter, we compare the effects of 500 eV electron bombardment on the thermal desorption of benzene (C 6H 6) and cyclohexane (C 6H 12) from Zr(0 0 0 1) surfaces. In both cases, unexpectedly high desorption temperatures above 600 K indicate strong adsorbate-substrate interaction. Two desorption states are observed for each molecule, presumably due to tilted and flat molecular orientations. Electron fluence does not significantly alter the amount of desorbing species, but for C 6H 12/Zr(0 0 0 1) results in small amounts of H 2 desorption near 310 K. Electron bombardment seems to affect the tilted species more than the flat-lying species.

Organic molecules on zirconium surfaces

In this work, the behavior of Zr(0001) exposed to benzene (C6H6) and deuterated methanol (CD3OD) is discussed. Relatively simple surface kinetics for the C6H6/Zr(0001) and CD3OD/Zr(0001) systems are observed, with only molecular desorption detected in thermal desorption experiments. We attribute this to the presence of a passivating layer of carbon formed by the initial decomposition of these species on the reactive Zr(0001) surface. Following adsorption at 150 K, benzene desorbs at about 715 K, whereas methanol desorbs near 600 K. An increase in adsorption temperature of 20 K for C6H6 and of 10 K for CD3OD lowers desorption yields significantly. An increase in surface oxygen is observed by Auger electron spectroscopy (AES) after exposure to benzene, possibly due to surface-carbon/subsurface-oxygen exchange. AES indicates that carbon stays in the near-surface region after thermal desorption, but that oxygen diffuses into the bulk in both cases.

Surface Structure of Hydroxylated and Sulfated Zirconia. A Periodic Density-Functional Study

The surface structure of sulfated zirconia (SZ) is examined by density-functional theory (DFT) with periodic boundary conditions. Adsorption of H2O and SO3 (or H2SO4) on the (101) surface of tetragonal zirconia is studied for different loadings up to H2SO4·3H2O and 2H2SO4·2H2O per two surface unit cells (four Zr surface sites). The considered surface species include H2O, [H+,OH-], SO3, [H+,HSO4-], [2H+,SO42-], [H+,HS2O7-], and [2H+,S2O72-]. Statistical thermodynamics is used to evaluate the relative stability of different surface structures for different temperatures and pressures of H2O and SO3 (or H2SO4). The simulated surface phase diagrams show a strong dependency on the considered sulfur species (H2SO4 or SO3) as well as on pressure and temperature. Monosulfates and pyrosulfates may occur, but higher condensated sulfates are not observed. In agreement with infrared experiments, we predict transformation of water-rich structures, [SO42-,2H+,3H2O], into pyrosulfate structures, [S2O72-,2H+,H2O], during calcination. Further increase of the temperature yields adsorbed SO3 before the clean surface is reached. Water adsorbed on the t-ZrO2(101) surface leaves in three steps upon heating from 250 to 730 K at 0.01 bar pressure: physisorbed water below room temperature, the first chemisorbed water at about 440 K and the last water at about 730 K.

Depth profiling of ZrO 2/SiO 2/Si stacks—a TOF-SIMS and computer simulation study

This study is dedicated to a better understanding of the processes occurring under ion bombardment of ultra-thin ZrO 2/SiO 2/Si gate dielectric stacks. Complex-shaped depth profiles were obtained by using TOF-SIMS with dual beam (500 eV for sputtering and 10 keV for analysis) Ar + ions. The SIMS intensities of all the elements depend critically on the amount of oxygen at any moment of the sputtering process. Increased intensity is observed at the surface and at the ZrO 2/SiO 2 interface. A long tail of the Zr signal is present in the Si substrate, even after the second (SiO 2/Si) interface, and a double bump structure in the 90 Zr and ZrO dimer is observed, which is more pronounced with increasing thickness of the interfacial SiO 2 layer. Computer simulations using the dynamic Monte Carlo code (TRIDYN) are performed in order to distinguish the ion-bombardment-induced effects from changes in the ionization degree. The original code is extended with simple models for the ionization mechanism and for the molecular yield during sputtering. Oxygen preferential sputtering at the surface and ballistic transport of Zr towards and through the interface are clearly demonstrated, but there is also evidence that due to recoil implantation oxygen gets piled-up near the ZrO 2/SiO 2 interface.

Depth profiling of ZrO2/SiO2/Si stacks?a TOF-SIMS and computer simulation study

This study is dedicated to a better understanding of the processes occurring under ion bombardment of ultra-thin ZrO 2 /SiO 2 /Si gate dielectric stacks. Complex-shaped depth profiles were obtained by using TOF-SIMS with dual beam (500 eV for sputtering and 10 keV for analysis) Ar + ions. The SIMS intensities of all the elements depend critically on the amount of oxygen at any moment of the sputtering process. Increased intensity is observed at the surface and at the ZrO 2 /SiO 2 interface. A long tail of the Zr signal is present in the Si substrate, even after the second (SiO 2 /Si) interface, and a double bump structure in the 90 Zr and ZrO dimer is observed, which is more pronounced with increasing thickness of the interfacial SiO 2 layer. Computer simulations using the dynamic Monte Carlo code (TRIDYN) are performed in order to distinguish the ion-bombardment-induced effects from changes in the ionization degree. The original code is extended with simple models for the ionization mechanism and for the molecular yield during sputtering. Oxygen preferential sputtering at the surface and ballistic transport of Zr towards and through the interface are clearly demonstrated, but there is also evidence that due to recoil implantation oxygen gets piled-up near the ZrO 2 /SiO 2 interface.

Nanoscale oxidation of zirconium surfaces: Kinetics and mechanisms

We show that atomic force microscope-induced oxide features can be formed reproducibly on both Zr and ZrN surfaces, and that the growth rate decreases rapidly with increasing time. There is an increase in oxide-feature height with humidity for both systems, and an approximately linear dependence of the height of the structures on the applied voltage for all films for short exposure times. As the anodization time increases, only the thinnest (6 nm) films show a large enhancement in oxide-feature height, demonstrating the role of the film/substrate interface. Under the same conditions, the height of features grown on ZrN films is greater than for those grown on Zr films, indicating that nitrogen plays a role in the oxidation process.

Electron bombardment of water adsorbed on Zr(0001) surfaces

A study of the effects of electron bombardment on water adsorbed on Zr(0001) isreported. Zirconium surfaces are dosed with isotopic water mixtures at 160 Kfollowed by electron bombardment (485 eV). The system is then probed by lowenergy electron diffraction, temperature programmed desorption (TPD) andAuger electron spectroscopy (AES). No evidence is found that would indicatepreferential mixing of hydrogen from the bulk with isotopic water dissociationproducts during TPD. However, electron bombardment results in the sharpeningof a hydrogen/deuterium desorption peak near 320 K and the production ofwater near 730 K at low water exposures. In addition, although waterdoes not oxidize Zr(0001) thermally, electron bombardment of adsorbedwater induces a shift of about 2 eV in the Zr AES features indicatingthat the surface is partially oxidized by electron bombardment.

Electronic structure of CaF 2-type ZrO 2 surface and bulk

Clean Zr-terminated and O-terminated surface systems of the CaF 2-type ZrO 2 were calculated by ab initio ‘mixed-basis+norm conserving non-local pseudopotential’ method. New gap states of Zr-terminated surface system appear below the bottom of conduction band of bulk ZrO 2, which derive from the split of the d state due to the lower symmetry of crystal field around the Zr surface atoms. New gap states of O-terminated surface system appear above the top of the valence band, which may derive from the energy increase of surface atoms. The difference of density of states between the Zr-terminated and O-terminated surface systems suggests that the Zr-terminated surface system is more stable than the O-terminated surface system.

Oxide microstructure and deuterium ingress in Zr-2.5Nb CANDU® pressure tubes

The microstructures of oxides grown on the inside surface (water-side) of Zr–2.5Nb pressure tubes removed from CANDU® reactors were characterised by TEM and correlated with deuterium ingress. Oxide cross-sections consisted of two structurally distinct regions: a columnar oxide region next to the metal/oxide interface, and an outer coarse equiaxed oxide region. Near the metal/oxide interface, the microstructure consisted of columnar grains without overt porosity. Away from the interface, the oxide consisted of coarsened equiaxed grains with (100)m twins, grain-boundary cracks and nanopores. The oxide microstructures on various pressure tubes differ in the proportion of equiaxed grains. Electron micrographs suggest that a larger proportion of equiaxed grains is associated with higher deuterium uptake. The predominance of grain-boundary cracks in equiaxed oxides indicates that they are likely more permeable to water than columnar oxides.Energy dispersive X-ray analyses revealed substantial amounts of Fe–Kα (and Mn–Kα) in the equiaxed oxide grains at the outermost surface. Energy Dispersive X-ray mapping of Fe–Kα and detection of the Mn-Kα (produced by neutron activation of 54Fe and subsequent decay of 55Fe) in the absence of external excitation, unequivocally demonstrated that the iron had accumulated in the oxide during reactor operation. The Fe concentration was highest near the outermost region, and decreased inwards towards the metal/oxide interface. These results are consistent with permeable equiaxed oxides picking up considerable amounts of Fe at the outermost region from the heavy water coolant.

NOVEL REACTIVE PLASMA PROCESSING FOR TRANSFORMING SURFACES OF METALS AND INTERMETALLICS TO CERAMICS

The surface of Ti, Zr, and TiAl was directly transformed to ceramics with a graded composition by carburizing, nitriding, and oxidizing using novel reactive plasma processing that has been developed by the present authors. Thick modified layers which consist of TiC, TiN, ZrC, ZrN (Zr2N), ZrO2, and Ti2AlN were successfully synthesized by this plasma process. The ceramic layers exhibited higher hardness and elastic modulus than the substrates. The formation behavior of these ceramic layers was analyzed with ab initio molecular orbital simulation and thermodynamic calculation. The chemical interaction of the interstitial atoms with the lattice of substrate materials was found to govern the diffusion and ordering processes to form the ceramic layers.

Kinetic effects of subsurface species on Zr( [formula omitted]) surface chemistry

Zirconium alloys are widely used in chemical and nuclear engineering applications because of their high corrosion resistance and low neutron cross-sections. However, fundamental studies of relevant model systems are sparse, leaving open-questions regarding the surface chemistry of this class of materials. In an effort to answer some of these questions, we have been studying the kinetics and reaction mechanisms of small molecules (water, ammonia, nitric oxide) adsorbed on Zr(0 0 0 1) surfaces under ultra-high vacuum conditions. The data from these systems indicate that kinetic surface/subsurface exchange plays a major role in the resulting surface chemistry and show strong adsorption temperature effects. In this paper we summarize the results of our studies and present a conceptual model which accounts for our experimental findings.

The influence of subsurface species on desorption kinetics : [formula omitted]/Zr(0001)

Isotopic oxygen ( 18 O 2 ) adsorption on Zr(0001) surfaces at 185 K is investigated with Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). The surface region of zirconium is saturated by low exposures of oxygen as determined by AES and LEED. However, as the exposure increases, thermal desorption features of H 2 18 O (20 amu) and H 2 16 O (18 amu) grow, with more H 2 16 O than H 2 18 O desorbing after isotopic oxygen exposure. These results indicate that significant kinetic mixing of surface and subsurface oxygen species as well as hydrogen transport from the bulk is involved in water production by this surface during heating.

Optical and structural studies of films grown thermally on zirconium surfaces

Variable angle IR reflection spectroscopy and atomic force microscopy are used to determine the thickness and morphology of films grown thermally on Zr surfaces in air. The density and homogeneity of these films increases with temperature in the range studied (773–873 K) and growth at the highest temperature follows cubic rate law kinetics. We demonstrate a structure-property relationship for these thermally grown films and suggest the application of IR reflectivity as an inspection method during the growth of environmentally passive films on industrial Zr components.

Investigation of nitric oxide adsorption on Zr( [formula omitted

Nitric oxide (NO, 15 N 18 O ) adsorption on Zr(0 0 0 1) surfaces is studied by Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). The results of our TPD experiments imply that subsurface oxygen and hydrogen are involved in surface reactions during heating, resulting in water and ammonia evolution. NO exposure shifts the Zr(MNV) AES feature by 2 eV indicating a change in oxidation state of +1 after adsorption. A superstructure (1×1) LEED pattern is observed after annealing, and is attributed to residual nitrogen at or near the surface.