High temperature oxidation of TiN and CrN films was studied in air at temperature from 923 to 1173 K. Oxidation rates were calculated by the mass gain with reaction time. Formed oxide layers were analyzed by XRD, SEM, SAM, and EPMA. The oxidation of TiN films is controlled by the oxygen diffusion through the formed TiO2 layer. The oxidation of CrN, however, is controlled by the outward diffusion of Cr ion through Cr2O3 layer formed along the grain boundaries. Oxidation resistance of CrN was superior than that of TiN. The pin-hole densities of TiN and CrN films were evaluated as a function of film thickness by means of Ferroxyl test. The observed pin-hole densities of TiN and CrN were decreased with increasing film thickness. The sufficient film thickness for TiN coatings with pin-hole zero was estimated as more than 15μm for TiN. The pin-hole densities of CrN films were remarkably less than that of TiN films. Corrosion resistance in 10 % HCl solution was measured for the TiN and CrN coatings deposited under the various conditions. CrN coatings showed the high corrosion resistance.
Low-energy (0.1-10 keV) ion scattering (LEIS) is used for the determination of the atomic composition and structure of oxide surfaces. Its extreme surface sensitivity enables the selective analysis of the outermost atomic layer. It is precisely this layer that is largely responsible for many chemical and physical properties of oxides. Lowering of the surface energy provides a strong driving force for segregation to the surface. Surface segregation is very important at temperatures that are high enough to enable diffusion. Since the surface enrichment is often restricted to the outermost atomic layer, the unique capability of LEIS to analyze this layer selectively, is one of the main applications of the technique. Other applications pertain to studies of the mechanism of oxidation of metals, semiconductors and to that of the growth of oxides on itself and on other materials. For equilibrated surfaces of spinels (powders or sintered) both LEIS and chemical methods show that, while cations in octahedral sites are accessible, cations in tetrahedral sites are not. This suggests that the equilibrium surfaces of these oxides are dominated by only one or two crystallographic planes. For single crystals, blocking, shadowing and focussing effects of the incident ions or scattered ions have been used to determine the location of surfce atoms as well as the presence and annealing characteristics of surface defects. Although the number of such studies are very limited, the results show great promise for the understanding of oxide surfaces.
Zirconia-based materials combine good thermomechanical properties and oxygen ion conductivity with chemical inertness and structural stability in a range of chemical environments. Polycrystalline materials, which are used in the bulk of applications, display a rich variety of segregation behaviour due to impurities which are invariably present either in the starting powders or inadvertently incorporated during processing. The mechanisms whereby, many, though not all, of these impurities and alloying components accumulate at the external or grain boundary interfaces are examined. The accumulation of impurities or components at the grain boundary can significantly change the grain boundary composition. Consequently, many of the macroscopic properties of these materials can depend critically on the chemistry of the grain boundary. An overview from both a thermodynamic and atomistic approach, of the development of the grain boundary network and the changing chemistry in the region of the grain boundary during the sintering process is presented. A qualitative appraisal of some current segregation and sintering theories are discussed. Surface analysis, by a variety of techniques, can probe the composition of interfaces within ZrO2-based ceramics giving valuable information of the distribution of impurities and components. A review of the contribution of surface analysis, particularly X-ray photoelectron spectroscopy, to the understanding of segregation phenomena and its effects on ceramic properties, is given. The review concentrates on Y2O3-ZrO2, CeO2-Y2O3-ZrO2 and Al2O3-Y2O3-ZrO2. The contribution of surface analysis to the understanding of low temperature degradation of yttria-tetragonal zirconia polycrystal is also examined.
A continuous Tyranno (Si-C-Ti-O) fibre-reinforced barium magnesium aluminosilicate (BMAS) glass was mechanically tested at room temperature and at 1000°C. Auger electron spectroscopy, scanning electron microscopy, electron probe microanalysis and transmission electron microscopy were used to examine the fibres, the matrix and the fibre/matrix interface of this composite, both prior to and after testing at 1000°C. At room temperature the composite contained a carbon interlayer between the fibres and the matrix and behaved in a tough manner during testing. After exposure to air at 1000°C the interface region degraded, which resulted in the formation of bubbles enclosed by a viscous Si-rich oxide, and Ca- and As-containing precipitates. The reaction products bonded the fibres to the matrix and the composite exhibited brittle behaviour.
Polar and chemical nanocrystalline doma:in textures have been observed directly, using high-resolution transmission electron microscopy (HRTEM), for lead scandium tantalate (PST). This is a relaxor-type electronic or functional ceramic, derived from the perovskite structure type. Atomic structure images having 0.17nm resolution revealed directly the Pb, Ta and Sc atom positions for <101> zone axis projections. Careful comparison of experimental and computer-simulated images allowed the Pb atom shifts to be measured with respect to those of Sc and Ta atoms, yielding 0.035nm, directed along one of the eight <111> pseudocubic directions. The local structure is trigonal, space group R3. Local variations of polarization vector were mapped directly off the HRTEM images in favourable cases, where the crystal tapered to a very thin edge.High-resolution (0.4nm) dark-field images were used to locate polar domain fluctuations occurring with length scales of 1-5nm and time scales of 0.04-1s. The images were recorded using video techniques. Thus it was possible to establish that spontaneous polar domain fluctuations occurred on the same nanometer scales as the smallest of the chemical domains for disordered PST, which has short-range order due to Sc and Ta segregation onto alternate (111) planes. The chemical domain textures have been modelled using Monte Carlo (MCS) and next-nearest-neighbour Ising (NNNI) models. These results provide a statistical physics framework for developing analytical theories for the dielectric response of relaxor-type ceramics. Thus the frequency and temperature variation of the permittivity are due essentially to dipolar-type fluctuations on nanometre scales. These simulations also help us understand and quantify the domain textures and domain wall configurations which may occur in complex perovskite-type oxides, as they appear in HRTEM images.
For fission reactor fuels, UO2 or (U, Pu)O2, grain boundaries play important roles. During sintering, motion of grain boundaries causes grain growth and microstru~tural development. Grain boundaries are a path for the fission gas release. As methods to enhance grain growth, oxidative and doping sinterings are useful. Increase in grain size is benefical for depressing fission gas release, while adverse from the view point of the pellet cladding interaction due to increment in the creep strength. As for fusion reactor plasma-facing materials, surface impurities play an important role to control the transport of hydrogen isostope particles in the material. A new concept of superpermeability may supply an effective method to separate hydrogen isostopes from the helium exhaust. In the case of fusion reactor breeder materials, surface and grain boundaries play vital roles in effective recovering of tritium from the ceramic breeder materials.
Several aspects of Synroc which fall into the broad class of interface phenomena are discussed. These are radiation damage processes which give rise to interfaces between damage tracks and neighbouring unirradiated material, intergranular films which have deleterious effects on chemical durability, and aqueous leaching of Synroc which takes place primarily at the interface between the solid and groundwater.
The principle of direct energy conversion was evaluated and the conditions to create a spontaneous emf on electrodes that were exposed to the same gas mixture are discussed. The electrochemical sensor was made of yttria-stabilized zirconia and had a thick-film-multilayer structure. Each sensor had electrodes made of platinum. No reference gas (air) was used for the electrodes. Two ceramic heaters were used in each sensor to maintain the sensor temperature. The emf demonstrations were carried out on CH4O2N2, C2H6O2N2, N2OO2N2, and NOO2N2 gas mixtures. In general, for gas mixtures containing CH4 and C2H6. Spontaneous emfs were observed for gas concentrations near or beyond the stoichiometric points. For NO or N2O gas mixtures, the sensors gave emf outputs which were nonlinearly proportional to the concentrations of the gas
Thermodynamic and kinetic conditions, under which grain boundaries can be described as thick, are considered. It is shown that in many cases such thick grain boundaries consist of thin films of a phase different from that in the adjacent grains. Such films can be liquid, amorphous or can have a crystalline structure different from that in adjacent grains. A variety of materials are considered, from pure metals to high-temperature superconducting ceramics. Phase diagrams for grain boundaries are constructed. The lines on such diagrams are constructed from the points of phase transitions on grain boundaries, when the layer of another phase appears on them. In complete analogy with the bulk phase diagrams such lines can be treated as grain boundary solidus or grain boundary solvus. The influence of the phase state of grain boundaries on the properties of polycrystalline materials (such as mechanical properties and grain growth behaviour) is considered.
The synthesis and in situ characterization of ultrathin films of copper and nickel on single-crystalline surfaces of metal oxides, and in particular the development in the epitaxial and electronic structures of the metal particles and films during atomic layer epitaxial growth, are reviewed. We consider recent results for low-index surfaces of α-Al 2 O 3 , CaO 1 α-Fe 2 O 3 , LaAlO 3 , MgO, NiO, SrTiO 3 , TiO 2 , ZnO and yttria-stabilized ZrO 2 , using low-energy electron diffraction and electron spectroscopies, and discuss the problem of electron-impact induced surface charging. Analysis by combined electron spectroscopy and photodesorption methods of the reactivity of the metal deposits is briefly discussed for the cases of CO exposure to Ni-deposited TiO 2 and to Cu-deposited ZnO crystal surfaces.
Three interfaces in ceramic substrates for electronic applications are reviewed. The substrates discussed here are aluminum nitraide A1N, glass/ceramic composite and magnesia. Calcium added A1N shows very tight and highly purified grain boundaries after 2000°C firing. The lattice constant of A1N changed by diffusion of oxygen in A1N lattice. This decreases the thermal conductivity. Alumina dispersed in B 2 O 3 -SiO 2 glass react with silica chain structure which prevent crystallization of glass/ceramic composite. This cause a steep change in thermal expansion curve. B i-Sr-Ca-Cu-O compound has been deposited on MgO single crystal. The Bi superconducting film grows with c-axis perpendicular and parallel to MgO surface.
The science of ceramic interfaces overlaps several existing disciplines such as solid state chemistry and physics, solid state electrochemistry, surface science, metallurgy and catalysis. Papers representing these disciplines constitute the present volume. This paper considers current research trends in the field of the science of ceramic interfaces and related applied aspects in the development of advanced ceramic materials.
In the first part of the paper recent experimental results on diffusion-induced grain boundary migration in ceramics are reviewed. It is demonstrated that all types of diffusion-driven phenomena known from the study of metallic systems occur also in oxide ceramics. Amorphous films on grain boundaries in ceramics play an important role in migration processes. It is shown that none of the existing theories of diffusion-induced grain boundary migration can describe satisfactorily the whole set of experimental data. In the second part of the paper, a new approach to the phenomenon is proposed, based on the gradient term in the expression for the free energy of binary alloys. The developed theory describes satisfactorily the stationary grain boundary migration and initial stages of diffusion-induced grain boundary migration. A new vacancy theory of the phenomenon is also reviewed.
The composition, structure, orientation and properties of the interfaces play a crucial role in determining the properties of the HTSC materials. The interfaces act as Josephson weak links, and in effect control the Jc in random oriented polycrystalline materials. Chemical examination on the grain boundaries of the HTSC shows that the composition even in the clean boundaries deviates from the interior of the grains. Some high angle boundaries show a flux pinning characteristic. Studies on the grain boundary structures reveal that the lattice matches can be described by a coincident site lattice (CSL) model or a constrained CSL model for YBa2C3O7−x(YBCO). Most of the grain boundaries in the textured Bi-Pb-Sr-Ca-Cu-O(BPSCCO) are parallel to the basal plane, while they are perpendicular to the basal plane in YBCO. The impressive high critical current densities Jc obtained for the Ag-clad Bi-based superconducting tapes are of great interest for high current and high field applications, and hence has attracted more attention to the grain boundary studies in these materials. A strong correlation between the high Jc and textured platelike grain morphology has suggested the "brickwall" model. Most of the grain boundaries in the textured BSCCO are twist boundaries in (001) with c-axis as rotation axis. Due to the mica-like layered structure, most of the boundaries are in low-angle boundary region. Grain boundary dislocations are commonly observed in these twist boundaries with high density. The presence of such a high density of the dislocations is probably due to the deviation from misorientation between grains in order for the interface to retain the low energy configuration over most of its area. Recently, studies on the transport and magnetisation Jc, and the irreversibility behaviour of the textured Ag-clad Bi-based tapes show no evidence of grain boundaries for being the weak links and the overall Jc is controlled by the intragrain pinning at high temperatures. Weak link behaviour becomes evident at low temperatures. The interfaces between the HTSC oxides and the silver sheath exhibit unusual mechanical properties that are responsible for the improved strength and flexibility of the Ag/HTSC composites.
The interfaces play a critical role in establishing both electrical and electrochemical properties of zirconia based electrolyte cells. Various types of interfaces which exist in such systems have been described. Apart from gas/electrode and electrode/electrolyte interfaces, the role of interfaces within the grains (coherent growth of precipitates, compositional variations within grains and second phase inclusions) and at grain boundaries (intermediate phase formation from the matrix, intermediate phase of the impurity type, and inclusions and pores) of polycrystalline electrolyte materials has been elucidated. The effect of interfaces on electrical and electrochemical properties has been discussed.
The paper considers several aspects of nonstoichiometry and related charge neutrality conditions in the bulk phase and in the interface region of metal oxides. The effects of segregation on local properties of the interface layer of ceramic materials are discussed involving the development of concentration gradients, related electric fields and structural deformations. Applied aspects of ceramic interfaces are also briefly discussed. Several questions have been formulated with respect to the effect of interfaces on processing and properties of ceramic materials.
Radiation damage. Factors affecting radiation damage of steels. Recovery from radiation damage. Radiation embrittlement of low-alloy steels for reactors. Experimental determination of the mechanism of the recovery of 15Kh2NMFA steel after radiation strengthening. Radiation damage to zirconium and its alloys. Changes in the properties of Zr alloys as a result of irradiation. Corrosion of Zr alloys in water and steam.
Electrical properties and microstructures of holmium-doped (Ba,Ca,Mg)(Ti,Zr)O 3 system were studied. Addition of Ho 2 O 3 had little effect in preventing the dielectrics from reducing at high temperature, but the resistivity at low temperature of Ho-doped samples increased with increasing amount of Ho 2 O 3 when they were treated in oxidizing atmosphere at the cooling stage. The newly developed dielectric materials exhibited high dielectric constant 18000 for Y5V specification, 3200 for X7R specification. Using these dielectric materials, Ni-electrode multilayer ceramic capacitors with high specific capacitance (Y5V 4.7μF, X7R 1μF in EIA 1206 size) were developed. They showed highly reliable electrical characteristics.
Surface analytical and high resolution microscopic techniques have been used to study the surface modification of oxide and sulphide materials resulting in major alteration of their surface reactivity.These surface modifications include restructuring of the first few unit cells of the surface layers, new surface "phases", and the effect of impurity atoms on oxidation site initiation. Ionic oxides (e.g. MgO) have been found to restructure on exposure to water vapour or solution to produce {100}-based pits and protusions with dimensions of a few unit cells.Some semiconducting oxides (e.g. C 0 O) also restructure in AFM images but the scale (10–20nm) is larger and the restructured regions are rounded and not obviously {100}-based. These oxides show initial dissolution kinetics in which the dissolution rate increases with increasing pH during the restructuring process. Low temperature plasma reactions have been used to produce silicate structures in the first few oxide layers on nickel metal. The silicate layers are strongly passivating against hydrolysis and acid attack. Non-stoichiometric iron sulphide surfaces (i.e. Fe 1−x S) oxidise in air and solution by the loss of iron ions from the sulphide lattice to form hydroxide overlayers. The underlying sulphide lattice has been shown to form a crystalline, defective tetragonal Fe 2 S 3 surface phase in which linear change of Sn atoms have a S-S distance similar to elemental sulphur. Oxidation and reaction of PbS surfaces in air and in solution has been extensively studied using the STM technique. Impurity sites in the cleaved (100) surfaces initiate oxidation in preference to low-coordination sites at edges and corners of steps and ledges.Synthetic, pure PbS shows much slower oxidation and preference for the low-coordination sites. The implications of these surface modifications will be discussed in relation to dissolution and leaching kinetics, flotation separation of sulphide minerals and weathering of oxide and sulphide surfaces.
This paper presents a new approach for the development of better ceramics. Important roles of interfacial phenomena are emphasized. The following subjects will be discussed with the particular emphasis on the problems and their origin which are encountered in the ceramic production process of powder compaction route. o 1. Liquid immersion method — Opening a new approach This is a novel and very powerful method for characterizing processing defects in ceramic green body. Green bodies can be made transparent by an immersion liquid which suppresses the reflection at the interface of particle and surroundings. Subsequent examination with an optical microscope provides us detailed informations which have never been obtained before. New informations on the micro- and macro structure of green body open a new methodology in the research of ceramic processing. 2. Systematic understanding on features from granules to ceramics Against the implicit expectation, a few very large macro-defects of processing origin were found above in even the best ceramic green bodies formed with the granules by the powder compaction process. They grow during densification with the interfacial energy as the driving force and can not be removed by sintering. Importance in the achievement of defect- free green body is emphasized for the ceramics with better properties. 3. Binder segregation during processing of ceramics Binder segregation occurs during the drying process by the flow of binder solution to the drying surface. The segregation of binder is a major source of defect in green body. A detailed model is proposed to analyze the segregation process. It considers the flow of binder to the surface and back diffusion which is driven by the concentration gradient established by the segregation. Adsorption of binder plays an important role. The model can explain the result of experiment accurately.
