In order to predict the release rates of fission products from UO2 nuclear fuel, it is necessary to determine the energetics associated with their segregation from the bulk to surfaces. Here segregation is determined by calculating the total energy of a simulation repeat unit which includes a fission product, as a function of the distance of the fission product from the surface. We have investigated the possibility that specific fission products segregate preferentially to certain surfaces. In particular, Ce4+ and Zr4+ segregate to the (100) surface, while Ba2+ and Sr2+ segregate to the (111) surface. Two issues make these calculations more complex. First, charged defects need to be compensated by oxygen vacancies. For example, divalent Ba2+ and Sr2+ substituting on uranium sites are compensated by a single oxygen vacancy. Second, certain surfaces, such as the (100), are inherently unstable, but can be stabilized by oxygen vacancies. Arrangements of these surface defects lead to complex surface structures that affect segregation energetics.

Metal matrix composites (MMC's) are known to have wide applications in parts of transportation devices such as automobiles and aircraft. Al matrix composites using SiC particles as reinforcements are especially spotlighted because of low cost, superior specific modulus, specific strength, wear resistance and high temperature stability. However, Al4C3 formed by the interfacial reaction between Al and SiC weakens the interfacial bonding strength. It is also known to be unstable in the water-soluble atmosphere.

In this study, the passive oxidation of SiC powder is used as protective layer against the reaction between the Al matrix and the SiC particle. We investigated the changes in interfacial product of the composites and mechanical properties such as interfacial bonding strength, and tensile strength in terms of the oxidized layer thickness of the reinforcement.

Scanning electron and scanning Auger microscopy studies were performed on in-situ fractured B-doped hypostoichiometric Ni3Al alloys. The Auger measurements on the fracture surface showed a very small amount or the total absence of B. Further, B segregated to the grain boundaries during subsequent exposure to the ambient system ultra-high vacuum environment at room temperature. The B segregation appeared to be driven by a mechanism of electronic nature related to Ni enrichment and O supplied from the environment. Ni-oxidation at room temperature is in accordance with model predictions for small beam sizes (≤10 µm) based on the premise that the electron beam creates additional nucleation sites around of which oxide growth occurs. With increasing the size of the e-beam the oxidation process becomes slower and chemisorption of oxygen plays a significant role. As a result the Ni-oxide depth decreases drastically with increasing spot size and offers an alternative route for monitoring the thickness of NiO in a nanometer range.

The sensitivity of semiconductive tin dioxide (SnO2) to reducing gases is determinedby the electrical conductivity change in the material. This change in conductivity strongly dependson the thickness of the electron depletion layer near the oxide film surface. In this paper we study the effect of crystal defects and interfaces on the electrical properties and the gas sensing performance of SnO2 thin films. SnO2 thin films with the thickness varying from 15 nm to 100 nm were deposited on sapphire substrates with different surface crystallographic orientations by femtosecond pulsed laser deposition. Films grown on the (1012) sapphire (R-cut) are epitaxial, single crystal. High resolution transmission electron microscopy studies showed the existence of a large number of crystal defects including crystallographic shear planes and misfit dislocations at the film/substrate interface. Films grown on the (0001) sapphire substrates (Ccut) are nanocrystalline with (200) texture. The gas sensitivity of the films was measuredin a gas reactor at high temperature. It was found that the sensitivity to reducing gases increases with decreasing film thickness. Electrical transport properties of the SnO2 thin films were investigated byHall effect measurements. Models correlating the microstructures of thin films to electrical properties are proposed.

The (100) and (110) surface relaxations are calculated for SrTiO3 and BaTiO3 perovskite thin films by means of a semi-empirical shell model (SM) for different surface terminations. Our SM results for the (100) surface structure are in good agreement with our present ab initio Hartree-Fock calculations with electron correlation corrections, previous ab initio pseudopotential calculationsand LEED experiments. The surface energy for the Ba-, Sr-, TiO- terminated (110) surfaces is found much larger than that for the (100) one. In contrast, the surface energy for the asymmetric O-termination, where outermost O atoms are strongly on-plane displaced, is the lowest for all (110) terminations and thus the most stable.

The objective of the present work is the in-situ study of the transformation of small oxide precipitates in a metal matrix by conventional and high-resolution transmission electron microscopy (HRTEM). As an example the reduction of Mn3O4 into MnO for nano-sized oxide precipitates in a silver matrix was studied in detail. A convenient method for monitoring the reduction process is shown for a large number of precipitates simultaneously. It is based on two-beam dark-field images showing distinct Moiré patterns for the MnO and the various types of Mn3O4 precipitates embedded within an Ag matrix. A controlling factor of the transformation kinetics appeared to be the rate in which the system can relax the strains due to the accompanying volume reduction of the precipitates. Other interesting aspects of the Mn3O4 to MnO transformation scrutinized and explained were the shape change of the precipitates upon reduction and the fact that mixed Mn3O4/MnO precipitates were only detected within a small temperature/time interval. Ostwald ripening of the MnO precipitates was observed as well.

Tin oxide films were deposited on sapphire and silicon substrates using reactive femtosecond pulsed laser deposition at temperatures ranging from room temperature to 700°C. The effect of electrical discharge and background oxygen pressure on the thin film microstructure was studied. The microstructure of the films was characterized by transmission electron microscopy and x-ray diffraction. SnO2 films fabricated consist of different textures in microstructures that depend on the deposition conditions and substrate surface structures. For instance, films deposited on the (1012) sapphire (R-cut) are amorphous if deposited at room temperature, whereas films deposited at 700°C were epitaxial, single crystalline. Discharge and oxygen pressure had a strong effect on the ion/neutral ratio of the ablated plasma plume of SnO2.

The influence of the ferroelectric domain structure of BaTiO3 on the photochemical reactions that occur on its surface has been examined using atomic force microscopy. Both the photochemical reduction of aqueous silver cations and the oxidation of steric acid thin films were studied. During reduction, silver selectively deposits on the surface in patterns determined by the ferroelectric domain structure. Based on the analysis of domain polarization in single crystals, we find that the photochemical reduction reaction occurs preferentially on the positive ends of the dipoles. The most likely explanation for this phenomenon is that when the static dipolar field is oriented with the positive end of the dipole on the surface, photogenerated electrons are driven to the solid-liquid interface where they reduce metal cations. The oxidation of steric acid films, on the other hand, is not spatially selective. During oxidation, the films dissipate uniformly as they are converted to CO2 and H2O. In this case, we conclude that the oxidation occurs indirectly. Photogenerated holes create hydroxyl radicals which can migrate on the surface before reacting with the steric acid molecules.

We have examined the structure, composition and bonding at an un-doped 58° [001] tilt grain-boundary in SrTiO3 in order to investigate the control that the grain boundary exerts over the bulk properties. Room temperature and in-situ heating experiments show that there is a segregation of oxygen vacancies to the grain boundary that is increased at elevated temperatures and is independent of the cation arrangement. These measurements indicate that the widely observed electronic properties of grain boundaries may be due to an excess of mobile oxygen vacancies that cause a highly doped n-type region in the close proximity ( ≍ 1 unit cell) to the boundary. These results are shown to be consistent with both theoretical models and lower resolution chemical analysis.

Tin dioxide thin films are widely used as chemical sensors. The mechanism for gas sensitivity depends on the chemisorption characteristics of the oxide surface and on the electronic characteristics of thefilm. Chemical doping and the film microstructure can influence the electronic properties. In this paper tin dioxide thin films doped with rare earth and transition metals were deposited on the (1012 -oriented sapphire substrates by femtosecond pulsed laser deposition. The resulting films were single crystalline with a thickness of about 15 nm and 30 nm. The response of the films to reducing gases was measured in a gas reactor at high temperature. The electrical transport properties of the films were determined by Hall effect measurements within the gas reactor. The valence and ionic radius of dopants have strong influence on the mobility and concentration of electrical carriers (i.e. electrons) and thus affect the response of the thin films to reducing gases. A model correlating the dopant characteristics to the electrical properties was developed.

Atomic force microscopy and electron microscopy were used to study surface and structural properties in SnO2:F thin films deposited by spray pyrolysis on glass at a constant substrate temperature. The influence of high levels of fluorine in the starting solution on the surface topology and morphology of thin films and correlations with electrical properties are presented in this work.

We have performed ab initio calculations to determine the bonding character of the Al-terminated Al(111)/α-Al2O3(0001) interface. By using an optimized model in which all atomic coordinates were relaxed to their minimum energy positions, we have determined that Al-O bonds constitute the primary interfacial bonding interaction. Our electron localization, Mayer bond order, and Mul- liken population analyses reveal that these bonds are very similar to the cation-anion bonds found in the bulk oxide, and are therefore mainly ionic, with a smaller amount of covalent character. However, there is also evidence of metal-cation bonding across the interface, a result which could be significant to understanding bonding at interfaces with other corundum-like oxides.

BaRuO3 thin films were grown on (111) SrTiO3substrate by 90° off-axis rf-sputtering. Transmission electron microscopy studies revealed that the films consist of the metastable 4H hexagonal polymorph of BaRuO3 along with few defects. The films are c-axis oriented, single crystalline with the in-plane orientation relationship with respect to the SrTiO3substrate of [112 0] BaRuO3 // [110] SrTiO3. High-resolution transmission electron microscopy (HRTEM) studies of the film-substrate interface revealed the existence of a thin intermediate layer of the 9R hexagonal polymorph of BaRuO3 between the (111) SrTiO3 substrate and the 4H film. The formation mechanism for the intermediate layer is not fully understood though. Through the combination of HRTEM and quantitative image simulations, the atomic structure of the interface between the 9R intermediate layer and the underneath (111) SrTiO3 was constructed. Three initial growth modes were observed, each of them adopting the local continuity of the oxygen octahedral sublattice across the interface.

This work reports the influence of microstructure on the electrochemical efficiency of Al- alloys to be used as sacrificial anodes. The microstructure of as-cast Al-ingots was characterized by X-ray diffractometry and scanning electron microscope (equipped with WDS microanalysis facilities). The electrochemical behavior was investigated in 3% NaCl solution simulating sea water at room temperature. Regarding microstructure, X-ray diffractometry detected the presence of the α-Al phase, and the τ-Al2Mg3Zn3, Mg7Zn3 and δ-AlLi precipitates. SEM observations confirmed the presence of α-Al dendrites with τ-Al2Mg3Zn3 and Mg7Zn3 particles in matrix. In interdendritic regions, it was observed the presence of the eutectic α-Al + τ-Al2Mg3Zn3 plus some primary Mg7Zn3 particles. δ-AlLi particles were identified at this step only by X-ray diffractometry. Finally, it was detected that an uniform distribution of precipitates in the α-Al dendrites together with an uniform distribution of a fine eutectic gave as a result in as-cast ingots an electrochemical efficiency up to 61.4 %.

The microscopic structure and energetics of a SrTio3 (001) surface covered with thin layers of pd were investigated by means of ab-initio electronic-structure calculations. A mixed-basis pseudopotential technique based on the local density functional theory was employed. Supercells containing srTio3 sunstrates, with either SrO or Tio2 surface terminations, and pd films of varying thicknedd were used to model the (001) surfaces and the (001) heterophase interfaces. The Tio2 terminated subtrate is energetically favoured for the adhesion of Pd films, with the Pd atoms bonded on top of the O atoms. The film adhesion is strongest for one (001) layer of fcc Pd and becomes weaker with increasing film thickness.

Under most experimental conditions, the wetting of Al2O3 by liquid Al is a non-equilibrium phenomenon characterised by the continuous oxidation of Al. While the effect of oxygen at the liquid Al-vapour interface appears to be well understood, less is known about the influence of oxygen at the liquid Al-sapphire interface, due to a lack of microstructural evidence. Sessile drop experiments of liquid Al on sapphire (α-Al2O3) were conducted under a low pressure (10-3Torr) controlled Ar atmosphere as a function of oxygen partial pressure, temperature and/or time. Microstructural investigations of the samples from the wetting experiments indicated that two different dominant processes occur at the liquid Al-sapphire interface: epitaxial growth of new α-Al2O3 layers on the sapphire substrate at temperatures below ≍1100°C and dissolution of the sapphire substrate at temperatures above ≍1100°C.

The microstructural evidence indicates possible mechanisms by which oxygen is involved in the non-equilibrium wetting and adhesion of liquid Al on sapphire. The non-wetting to wetting transition in this system may be explained by the formation of an oxygen-rich interphase at the liquid Al-sapphire interface.

The presence of a liquid film at grain boundaries affects the mechanical properties of the material. One of the ways to eliminate the liquid film is to exude it from the grain boundaries to the free surface of the material. The present paper presents results on the exudation of a liquid silicate from grain boundaries in polycrystalline alumina. The results are compared to exudation behavior from bicrystals of alumina. This approach is very useful in understanding the mechanism of the exudation process.

An attempt has been made to study the film-substrate interface by using a sensitive, non- conventional tool. Because of the prospective use of gate oxide in MOSFET devices, we have chosen to study alumina films grown on silicon. Film-substrate interface of alumina grown by MOCVD on Si(100) was studied systematically using spectroscopic ellipsometry in the range 1.5-5.0 eV, supported by cross-sectional SEM, and SIMS. The (ε1,ε2) versus energy data obtained for films grown at 600°C, 700°C, and 750°C were modeled to fit a substrate/interface/film “sandwich”. The experimental results reveal (as may be expected) that the nature of the substrate -film interface depends strongly on the growth temperature. The simulated (ε1,ε2) patterns are in excellent agreement with observed ellipsometric data. The MOCVD precursors results the presence of carbon in the films. Theoretical simulation was able to account for the ellipsometry data by invoking the presence of “free” carbon in the alumina films.

Two sapphire (Al2O3, r-cut) wafers were bonded at room temperature. The bonding energy increased during heating up to 1100 °C. The interface was investigated by plan-view and cross-section TEM. The bonding strength after heating to 1100 °C is similar to the bulk bonding strength. A high density of voids at the bonded interface was observed. The voids are elongated along the interface in a pancake shape. The surfaces are low index planes. The volume inside the voids is higher than expected from a model, where a hydrophobic initial bonding is assumed. The strength of initial bonding and the free volume at the interface can be understood, if the sapphire surfaces are hydrophilic and covered by several monolayers of water during initial bonding. During annealing the water diffuses out along the bonded interface and a free volume remains.

La2Zr2O7-based pyrochlore (LZO) islands were grown on (001)-oriented Y2O3-stabilized ZrO2 (YSZ) single crystal surfaces by the reaction between the crystals and a La2O3 vapour. The islands are epitaxial with a small tilt. On a flat substrate surface each pyramidal island consists of four domains which are slightly tilted around different <110> axes with respect to the YSZ lattice. On a YSZ surface with pits of high density, more interestingly, each larger island formed at a pit has a central hole and contains two to four <100>-tilted stripe domains in addition to the four regular <110>-tilted domains. A square network of interfacial edge dislocations having Bur- gers vectors as/2 [101] (as being the lattice parameter of YSZ) was observed in the regular domains at the LZO/YSZ reaction front of positive lattice mismatch (+5%). In stripe domains two sets of orthogonal misfit dislocations are present with different Burgers vectors of type b = as/2 [101] andas/2 [010]. The perpendicular components of the Burgers vectors give rise to the observed tilts of the LZO lattices. Possible models are proposed to explain the generation of dislocation half-loops, the preferred nucleation of the islands near YSZ surface pits as well as their growth kinetics and the formation of stripe domains.