Surfaces and Interfaces of Ceramic Materials

The electronic and geometric properties of both nearly perfect and defect metal oxide surfaces have been studied by using surface sensitive electron spectroscopic techniques such as photoemission, electron energy loss spectroscopy, low energy electron diffraction, etc. and single crystal samples cleaved in ultrahigh vacuum; surface defects are created by ion or electron bombardment or thermal treatment. The use of such well characterized ceramic samples permits a determination of the physical and chemical properties of their surfaces on the atomic scale. The geometric structure of defect free oxide surfaces is nearly a termination of the bulk crystal structure, and their electronic structure is usually similar to that of the bulk. Maximal valence oxides are quite inert with respect to molecular adsorption. The interaction of sub-oxide surfaces with molecules, on the other hand, exhibits a wide range of behavior. Point defects on surfaces are the active sites for chemisorption and exhibit electronic structures very different from those of perfect surfaces.

The present understanding of internal interfaces in ceramic materials is reviewed by considering several recent studies. The emphasis of the review is experimental with illustrations, obtained using transmission electron microscopy, of interfaces in alumina, spinel and MgO/NiO. The structure of twin boundaries are discussed in some detail emphasizing the different types of interface which are commonly grouped together under this topic. Some of these interfaces do not involve any major distortion of the anion sublattice while others are more closely related to twin boundaries in metals or semiconductors. In the first group, the position of the interface is determined solely by the distribution of the cations. This feature makes these interfaces similar, in important respects, to certain heterophase boundaries, which are therefore also discussed.

The majority of polycrystalline ceramics contain phases, often noncrystalline, along their grain boundaries. These intergranular phases affect the overall electrical and high temperature mechanical properties of the ceramics. This tutorial describes the microstructure, the conditions for wetting, and dewetting, of the boundaries by the phase and the factors affecting the equilibrium thickness of the phase. Finally, how the connectivity of the intergranular phase may be exploited in post-fabrication processing is described.

At Low frequency, the conductivity of fluid saturated porous media varies with porosity as φm where m is the Archie’s exponent. At higher frequencies, the real part of the conductivity exhibits a dispersive (non Gaussian) behaviour characterized by a power law σ′[ω] α ωx. The real part of the dielectric constant which can attain very large value at low frequency varies in the dispersive region as ε′[ω] α ω-y with x + y =1. We obtained the values of these exponents from data in brine saturated porous alumina ceramics presented in previous papers.The conclusion of our analysis is that the low frequency conductivity of porous alumina ceramics saturated with saline water yields an Archie’s exponent compatible with the theory of percolation. The conductivity frequency exponents in the dispersive region are consistent with the model of anomalous conduction on fractals and the high value of the real part of the dielectric constant can be accounted for if the microgeometry of the grain-pore interface and multipole effects are incorporated into the Maxwell-Garnett theory of composite media.

Bicrystals with (100) and (110) symmetrical tilt boundaries and with (100) twist boundaries were obtained by joining two single crystals at about 2480°C without pressure. Boundary energies were measured by the thermal grooving technique at 1200 to 1600°C. It was clear that small angle boundaries lower than 10° are composed of an array of dislocations, and it was also confirmed from observation by transmission electron microscopy (TEM) in (100) twist and (110) tilt boundaries. The critical angle estimated from diffusional properties in the (100) tilt boundary was also 10°. On the other hand, the critical angle of the (100) tilt boundary following the Read-Shockley model was measured to be 20°. Up to 10° the (100) tilt boundary is composed of a straight dislocation arrangement. Above 10° boundary is composed of a crisscrossed dislocation arrangement in a kink and jog fashion. For high angle boundaries, all of the low energy boundaries observed in the (100) twist boundary and (110) tilt boundary correspond to the high density coincident site lattice (CSL) boundary. In the (100) tilt boundary, no low energy boundary was observed at higher angles because of the higher atomic density on the (100) plane than on the (110) and the introduction of dislocations with different slip systems.

During the last three decades, an extensive research and development effort in the aera of composite materials has taken place. Among the various materials which have been studied, oxide - oxide aligned eutectics were considered because of their anisotropic properties, high strength- to - weight ratios at high temperatures, high melting points, and also resistance to oxidation [1–2].

Fundamental definitions of the geometrical description of grain boundaries are firstly recalled. Then the particularities of the non-cubic systems are emphasized and the results obtained in a hexagonal material, tungsten carbide-cobalt composite are presented. Different techniques such as TEM, HREM, EDX were used to characterize the grain boundaries according to the orientation relationship, the nature of the grain boundary planes and the chemical composition of the grain boundaries.

Pigeonite and clinoamphibole exsolution lamellae in augite have been studied by conventional and high resolution transmission electron microscopy. The pigeonite lamellae lie close to the (100) and (001) planes of augite with a deviation of about 15°. With increasing width of the lamellae the interphase boundary becomes semicoherent. The nucleation of pigeonite occurs preferentially at dislocations. Clinoamphibole lamellae lie parallel to the (010) plane of augite. Its formation is via stacking faults with a 1/2 [101] displacement vector. The stacking faults are bounded by partial dislocations with Burgers vectors of the same type. The minimum width of the lamellae is 9Å, that is one double chain. Broadening of the lamellae is by the motion of partial dislocations (steps) along the interface. With increasing thickness, the clinoamphibole lamellae become semicoherent. Nucleation of clinoamphibole also occurs at augite-orthopyroxene interfaces. The deviation of the pigeonite exsolution lamellae from (100) and (001) can be explained in terms of the differences in lattice parameters of the matrix and the precipitate phase. The reason for the formation of amphibole lamellae along (010) planes in augite is given by the dislocation aided exsolution mechanism.

The saturation adsorption of tetracyanoethylene (TCNE) from the vapor phase onto magnesia activated at 200 and 400 C is studied. Fourier transform infrared and electron spin resonance spectroscopy are used to characterize the chemisorbed material. FT-IR spectra of authentic samples of TCNE, TCNE-1, TCNE-2, and the tricyanovinylalcoholate anion (TVA) are also reported for comparison purposes. We find that the uptake of TCNE as a function of activation temperature is most closely related to the surface hydroxyl coverage. It does not correlate well with the BET surface area. We also find that the primary chemisorption product is a TVA like species (perhaps polymeric). Radical signals are detected, presumably from TCNE-1, but correspond to less than 5% of the chemisorbed TCNE.

A high surface area cordierite aerogel was studied by Fourier Transform Infrared Spectroscopy. After activation, treatments with methanol or isotopic exchange with deuterium have evidenced that the silanol groups are all located on the surface. Methanol reaction with hydroxyl, gives typical methoxyl groups covalently grafted on the surface and very heat resistant.

The surface chemistry of SiC and Si3N4 prepared by laser-driven gas-phase reactions has been characterized by transmission/absorption FT-IR spectroscopy. Different surface sites, such as surface silanols, silyl amino- and imido- groups, alkyl groups, silane bonds, surface carbonyls and siloxanes bridges have been evidenced. Their amounts depend on surface treatments: evacuation, oxidation, surface nitridation.

The basic approaches to the computer simulation of ionic solids are briefly reviewed, leading to a strategy for the calculation of the heat of segregation of cation dopants in metal oxides. It is shown that equilibrium surface coverages of dopant cations can be derived from simulation studies if due account is taken of the variation of segregation energy with surface coverage. Isovalent dopants in MgO are considered in detail and comparison is made with experimental work on Ca/MgO(001) and Ba/MgO(001). The behaviour of isovalent dopants in Al₂O₃ is more complex owing to major surface relaxation effects in the pure material and the existence of several different low index surfaces of comparable energy. However comparison with experimental work on Y/Al₂O₃ is encouraging. The behaviour of aleovalent dopants is illustrated with respect to Ti in MgO and Mg in Al₂O₃. In both cases the simulation studies treat a neutral dopant-vacancy complex.

Experimental approaches in studies of surface and grain boundary segregation in ionic solids are considered. The experimental material on both intrinsic and extrinsic segregation is discussed from the viewpoint of predominant driving forces of segregation. The direct effect of segregation involves an increased solubility limit within the interface region and resulting formation of bidimensional structures of extraordinary properties such as transport, magnetic and optical properties. The effect of segregation on properties of materials is considered in terms of phase diagrams of fine powder ceramics and sintering mechanism of ceramics. Also the catalytical aspect of the problem is briefly discussed.Experimental data on surface and grain boundary segregation are reviewed for several oxide materials such as ZrO₂; Al₂O₃, MgO, SnO₂, NiO, CoO and their solid solutions. Possible effect of interfaces on critical parameters of high T_c oxide superconductors is also considered. Most urgent research pathways in the surface and grain boundary chemistry are discussed.

The available data on surface diffusion and matter transport driven by surface energy in ceramics are reviewed. The competing effects of matter transport by evaporation-condensation and by volume diffusion are treated. The materials considered are mainly oxides (MgO, NiO, Al₂O₃, Fe₃O₄, UO₂ etc.), but other ceramics (UC, UN) are also dealt with. The knowledge on surface energies of such ceramics is reviewed as well. A number of different techniques has been developed to measure both properties, and the effects of deviation from stoichiometry on surface diffusion or surface energy have been measured for UO_2±x and Fe_3−xO₄. The data are applied to discuss the technologically important aspect of the formation and the behavior of gas-filled bubbles in UO₂ nuclear fuel.

Diffusion at a grain boundary occurs at a different rate from that in the bulk crystalline lattice. Usually bulk lattice diffusion is relatively slow and boundary diffusion is much more rapid so that the boundary acts as an easy (or short circuit) path for transport in parallel with bulk diffusion.The experimental approaches to direct measurement of tracer diffusion along grain boundaries are well-established and, in principle, can give the grain boundary diffusion coefficient and the grain boundary width (or, in the case of an impurity atom, the product of boundary width and the segregation coefficient).Published data cover a range of materials in doped and undoped forms, and specimen types (single crystals, bicrystals, sintered powders and thin films), with host atoms or impurities as the diffusing species. In addition, atomistic simulations have been carried out using static lattice methods to help interpret the experimental observations. There is much disagreement in the detailed results in this field, but certain general conclusions are emerging.

A brief discussion is given of the factors that influence the concentration of impurities at ceramic surfaces and interfaces. The relationship between the experimentally measured temperature dependence of surface composition and the binding energy differences between surface and bulk is discussed. The simple Langmuir model of surface segregation describes the limiting case when no interactions occur among impurity ions and will generally be inadequate for real systems in which surface condensation and phase transition may occur. The use of segregation isosteres to determine heats of segregation is discussed.Model calculations of the distribution of divalent impurities near alumina surfaces, using the mean field approach involving the Poisson equation, show that impurity concentrations may be enhanced both in the surface monolayer and in the near-surface space charge region.Experimental studies of the segregation of Mg and Ca to the (0001) and (1010) surfaces of Al₂O₃ are described. These experiments demonstrate the importance of suppressing net evaporation in order to approach thermodynamic equilibrium. The application of the sine-way decay method to determine mass transport parameters for Al₂O₃ is described, with particular emphasis on the effect of segregated Mg.

The influence of grain boundaries and interfaces on critical current density in both high-temperature, cuprate superconductors and conventional superconductors is discussed. Recent measurements of the critical current density of individual grain boundaries in yttrium barium cuprate are also reviewed.

A survey is presented of growth and mass transport in ceramic type protective scales on metals and alloys at high temperatures by lattice and short-circuit diffusion of metal and oxygen through low resistance diffusion paths in oxide boundaries. A methodology based on correlating phenomenological theory of oxidation with reactant short-circuit diffusion across reaction product layers is discussed in some detail for Cr₂O₃, Al₂O₃ and SiO₂ scales on the pure metals and on iron and nickel alloys because independent measurements of diffusivities by isotopes are available to confirm the oxidation models. Isotope studies using sequential oxidation in O16 and O18 have been definitive in describing oxide growth within protective Cr₂O₃ and Al₂O₃ scales. Mechanisms relating loss of scale adherence to the metal substrate with the formation of new oxide within these protective scales are discussed and they are shown to remain open to speculation.

Reactivity and demixing tendency of multicomponent oxides are discussed on the basis of a formal treatment of matter transport under oxygen potential gradients. The present study shows how the dynamic segregation of cations near interfaces takes place in p-type semi-conducting oxides. This effect has been considered for different cases encountered depending on the range of stability of initially homogeneous multicomponent oxides. Experimental examples illustrate the importance of these phenomena.

Surfaces of oxygen-deficient yttrium oxide, pure or Zr-doped, have been studied by means of X-ray photoelectron spectroscopy and scanning electron microscopy.The bulk local geometric structure of these non-stoichiometric compounds was previously determined around the Y atom by an EXAFS (Extended X-ray absorption fine structure) study.The local electronic structure around both Y and 0, at the surface, was investigated by X-ray photoelectron spectroscopy.The partial transfer of the electronic distribution between the anion and the cation was probed using the Auger parameter.Coupling of these experiments with microscopic observations show that :In the pure oxygen-deficient sample, the concentration of oxygen vacancies appears to be increased at the grain boundaries.The Auger parameter shows upon reduction an evolution of the Y-0 bond towards a more covalent one, this evolution being modulated with the presence of ZrO₂.

The composition of grain boundaries and surfaces often control phenomena like sintering, grain growth, wear resistance and corrosion behaviour. In the present study the surface of zirconia ceramics doped with various amounts of Yttria was examined in order to find an explanation for observed differences in grain growth. This was done by using AES (Auger Electron Spectroscopy) and XPS (X-ray Photoelectron Spectroscopy). Specimen heat treated at 873K had a surface composition similar to their bulk composition. Specimen heat treated at 1273K showed a surface composition containing 30-34 at% Yttria independent of the bulkcomposition. This results in a larger solute drag for zirconia with low amounts of yttria which probably causes a slower grain growth by means of an impurity drag mechanism. It could be shown that the relative increase of the 76eV Augerline was not due to Silicon segregation but solely to the segragation of Yttrium.

Numerous studies have been done about the influence of foreign ions in solid solutions on the structure and reduction of wustite because some of these ions are present in the iron ores and wustire remains a reference material in many fundamental studies. The present study is one among the series of our investigations devoted to the role of Ca on the structure and the reduction process of (Fe_1-x-yCa_y)O wustite (1). Calcium has relatively high ionic size (r_Ca2+=0.99 Å) as compared to Fe (r_Fe3+=0.564 Å); this causes expansion of the crystal lattice and therefore may change the defect structure of wustite and furthermore influence the reduction process.

In this tutorial on metal-ceramic interaction the fundamentals of metal-ceramic bonding will be considered from the equilibrium thermodynamics point of view. This includes the wetting of a solid ceramic by a liquid or solid metal or vice versa, the effect of the gas atmosphere on the wetting behaviour and on the chemical reactions that can occur at a metal-ceramic interface. From this approach the conditions for a metal-ceramic reaction can be predicted and it also gives indications to the chemical stability of the metal-ceramic bond. Additional to this the effect of impurities or second phases present in the ceramic, such as segregation of impurities or melt formation, on the formation of a metal-ceramic bond is discussed. Finally the formation of interfacial phases and the effect of the interface morphology is briefly considered.

High integrity ceramic-metal interfaces are required for several areas of advancing technology but are difficult to produce because of the markedly differing lattice bonding characteristics of ceramics and metals. If the required properties are to be achieved, the chemistries of interfaces must be changed and this paper considers to what extent simple chemical concepts can account for the behaviour of brazed and diffusion bonded ceramic-metal systems. Particular attention is paid to the role of Ti promoting the wetting of ceramics through the formation of metal rich, hypostoichrometric, reaction product and to the beneficial effects of incorporating such metals as Sn and In in Ti containing brazes. The need to control the extent as well as the nature of the reactions if strong bonds are to be produced is high lighted, and comparisons are made between the fabrication processes and properties of brazed and diffusion bonded interfaces.

This paper is aimed at presenting some recent ideas and experimental results on characterization and properties of small particles and thin films of metals on oxide surfaces. Analysis is restricted to metallic films on well-defined surfaces, mainly monocrystalline surfaces and to ceramic oxides having a marked ionic character. The properties of metallic films obtained by chemical reduction of oxides and those of deposits are compared. Nucleation and growth modes are described. Interfacial properties are analysed in terms of size or thickness effect and in the general framework of the ceramic-metal bonding.

The growth of cation-diffusing scales on pure metals is described from a modelling of metal-scale interface in terms of intrinsic dislocations for an epitaxial scale; such a model is consistent with the experimental observations. It is proposed that the annihilation of cationic vacancies occurs at the metal-scale interface by the climb into the metal of some fraction of the intrinsic misfit interface dislocations, a process which generates tensile stress in the metal and compression in the scale. Above a critical interfacial strain, the glide of dislocations in the metal, in combination with dislocation glide in the scale, recreates the interface dislocations. These processes provide plastic deformation in both phases near the interface and maintain metal-scale epitaxy during oxide growth. The model may explain the origin of stresses arising during the growth of cation-diffusing scales on an extensive flat surface.

The recent development of experimentation in space, under microgravity conditions, makes it possible to measure the interfacial tension (σ) between immiscible fluids (organic compounds, liquid alloys…) by a straigthforward application of the Laplace equation in its simplest form.

A review of the fundamentals required to 1) produce narrow size distribution ceramic powders 2) make suspensions of ceramic powders and 3) make green bodies with a uniform packing of particles is presented. In all cases, the interfaces that ceramic powders present to their environment is very important in controlling the properties of the powders during processing. For this reason, methods of surface characterization are presented.

A non exhaustive selection of open questions arising when characterizing the structures of polycrystals and modelling their changes during grain growth is presented. Geometrical parameters describing size, shape and their distributions, both static and as they evolve with time are first discussed theoretically. Then an account is given of those computer simulation studies on two-dimensional structures undergoing grain growth which have been published recently. Finally, recently observed complexities of grain boundary migration in ceramic systems are mentioned briefly.

The preparation of ceramic materials involves a large number of independent steps. The path from the ores to the final product is influenced by the presence of surfaces and by their chemical and physical constitution. Since ceramic manufacturing is based on the consolidation of powder particles which contact each other via their surfaces, the control of processing requires control of the surfaces. The importance of surfaces for technical manufacturing and some associated difficulties will be reported. Powder preparation (ZrO₂), shaping processes (Si₃N₄) and thermo-electrical properties (ZrO₂) will be used as examples.

Anybody agrees with the idea that particle size plays an important role in the behaviour of a polycrystalline material but very few studies are giving precise information about this role. In this paper some experimental results are presented succintly to illustrate the importance of this physico-chemical factor. They are related to barium titanate, the main component of the dielectric material in the type II ceramic capacitors.Particle size is playing a physical role. For example, the crystalline structure of the BaTiO₃ grains and, therefore, the dielectric properties of this material, are directly dependent on the grain size distribution.However, particle size is also playing a chemical role. The reactivity of BaTiO₃ and, consequently, the dielectric properties of the resulting material can be apparently modified according to the grain size distribution.These results prove a careful control of BaTiO₃ granulometry is needed. The chemical routes enable such a control. This control can be also obtained by the more traditional solid way synthesis. As an example, it is showed how granulometry of BaTiO₃ can be controlled by the granulometry of TiO₂ one of the two raw materials in this solid state synthesis.

The use of combined impedance and modulus spectroscopy to analyse a.c. impedance data for doped polycrystalline BaTiO₃ shows the presence of two component impedances which are attributed to bulk and grain boundary effects. This assignment is based on the temperature dependence of their associated capacitances: the grain boundary capacitance does not change significantly with temperature, whereas the bulk capacitance passes through a maximum at the Curie point and shows Curie-Weiss behaviour at higher temperatures. The PTCR effect, characterised by a large increase in resistance as the temperature is raised through the Curie point, is shown by both components. The influence of processing conditions on the PTCR effect, especially the rate at which the sample is cooled to room temperature after sintering, is discussed. At the shorter rates, the PTCR response is dominated by the bulk impedance, whereas at longer times, the grain boundary impedance dominates.

The use of a sol-gel processed cordierite precursor sinterable about 900 °C allows cosintering of the copper and the ceramic. A strong bonding between the copper film and the cordierite substrate can be achieved through an eutectic bonding technique. Three types of interfaces have been studied by EPMA, ESCA and SEM: (a) cosintered in hydrogen, leading to poor metal adhesion, (b) cosintered in wet argon with slow heating rate (500°C/h) leading to strong copper diffusion, (c) cosintered in wet argon with fast heating rate (800°C/h) leading to the best macroscopic properties, i.e. no deformation, limited diffusion and strong adhesion.

The linear Frenkel law of sintering is briefly reviewed. Sintering of pure and Cr3+ doped ZnO and NiO was studied by using computer assisted dilatometry. The linear shrinkage law was observed during sintering of both oxides for low values of shrinkage. Its extent is changed by doping with Cr₂O₃. The sintering kinetics is discussed on the grounds of the MO oxides grain boundary.

Study of the sintering of Nd₂O₃ made possible to determine the best conditions for its high densification. Unfortunately, the compacts show a very poor resistance to humidity. This is mainly due to the A hexagonal structure. Stabilization against hydration was attempted by coating the Nd₂O₃ grains, to avoid the existence of an interface oxide-OH-. Sintering of samples doped with TiO₂ leads to the hexagonal oxide which appears as Nd₂O₃ particles surrounded by an intergranular phase. The later was identified as Nd₂TiO₅ by use of microprobe analysis. The resistance to hydration of A-Nd₂O₃ was then four times improved.

We describe the preparation and the microtexture of three classes of precursors for the synthesis of ceramic powders or films of the SiMON family (M=Al, Mg, Y). (i) simple mechanical mixtures of a non-swelling clay of the kaolinite family with charcoal, in which the two phases are separated at micronic or slightly submicronic level. The interface area, S, is of the order of a few m2/g. (ii) colloidal microcomposites obtained by a sol-gel method from mixed colloidal suspensions of a swelling clay (montmorillonite or hectorite) and graphitic oxide. This method, which permits the casting of films a few microns thick, leads to a material in which the two phases are dispersed as lamellar stacks a few nanometers thick. S is of the order of 100 m2/g. (iii) layered clay-carbon nanocomposites. The method involves the intercalation of acrylonitrile vapor in a swelling clay, followed by polymerisation and carbonisation of the organic molecules in the interlayer space. It leads to a material in which the carbon and silicate phases are organized as alternate layers of subnanometric thickness. S is larger than 800 m2/g.

A sol-gel process for producing microspheres of alumina with 10% by weight of zirconia has been developed. A fluid and concentrated sol has been obtained by peptization, in suitable conditions, of a gel of Al and Zr hydroxides. From this sol a powder of homogeneous chemical composition and characterized by well-defined geometric parameters (spherical shape, controllable size) has been produced in a pilot plant. The microspheres calcined at 1200°C were not agglomerated, were dense and uncracked, their average size was 20 microns and the particle size distribution was close to the average value. All the steps of this process (gel precipitation, washings, peptization, sol concentration, microspheres formation, heat treatment) have analitically been followed, above all by means of: X-Ray diffraction, thermogravimetric analysis, scanning electron microscopy, laser granulometry and microgra nulometry, rotational viscosimetry, B.E.T. technique.

Thermodynamics, kinetics and mechanisms of gas-solid interactions are exemplified for oxides. The thermodynamics of oxide reduction and of oxide evaporation are demonstrated. The solubility of extraneous nonmetal atoms in oxides is discussed. Concerning kinetics and mechanisms there are not many studies with ceramic oxides, their treatment has to be based mainly on results for more reactive oxides such as FeO, Fe₃O₄, Cr₂O₃. General rate equations are presented for the oxygen transfer to and from oxides in CO₂-CO and H₂-H₂O mixtures. Examples are given for the kinetics and mechanisms of oxide reduction and of oxide conversion to carbides and oxide reduction to intermetallic compounds. It is stated and exemplified that in many solid-solid interactions gaseous intermediates play a decisive role.

Ceramics can be obtained by many techniques and mainly by powder processes such as sintering. They can also be grown from vapor phase with the Chemical Vapor Deposition (CVD) process. Numerous attempts have been made to obtain a better understanding of the mechanisms which involve complex fluid dynamics and surface phenomena but direct evidence is still lacking. Many reviews have dealt with this subject [1–5] and general trends used in the comprehension of the various steps leading to the formation of a ceramic from a gas phase are first presented. Recent developments of the process leading to more complex coatings such as ceramic composites are then discussed. The interest of the thermodynamic calculation used as a predictive tool is finally shown with a few examples dealing with two-phased materials or non-stoichiometric compounds.

Traditional ceramic oxides are in general very resistant to attack by water and aqueous solutions; new materials however are not always impervious to this attack and the ensuing degradation places a limit to their durability. In this review both the thermodynamics and the kinetics of the interaction of simple metal oxides with aqueous solutions are described. Massive dissolution, as well as surface reactions of various types are discussed: acid-base, complexation and redox reactions starting at the surface may all give rise to further massive transformations, in some cases with formation of other solid phases, when the conditions are set up outside the stability field of the original phase in the corresponding phase diagram. Potential/pH diagrams are used to discuss the stability of oxides in contact with water, and the kinetic aspects of the transformations are analyzed in terms of the current models of the structure of the interfaces.

The mechanisms which control the dissolution of simple oxides in aqueous solution are not fully understood, despite substantial research; those for ceramics are even less certain, and may involve “leaching” (preferential release of some species from the solid into solution) rather than “dissolution” (all species released at equal rates). In this paper we consider the reactivity in aqueous solutions of two materials — MgO and CaTiO₃ — to illustrate current attempts to establish the detailed mechanisms and rate-determining steps. The importance of using surface analytical and electron microscopy techniques to characterize aspects of the surfaces before and after attack is emphasized. In the case of MgO, there is an initial stage of dissolution related to the restructuring of the initial oxide surface; the dissolution process is controlled by the second protonation reaction. For the perovskite CaTiO₃, ion—exchange of the Ca2+ from surface sites is accompanied by base—catalyzed hydrolysis of the titanate lattice: there is no significant leaching of the Ca2+ from the intact solid unless hydroxylation has occurred. However, crystal defects due to deformation and microstructural characteristics of the material can have major effects on the reactivity of polycrystalline perovskite.

Manganese ferrite samples with and without minor (<1 Wt%) additions of SiO₂ and CaCO₃ quenched in air from the sintering temperature yielded a single spinel phase. Thermogravimetric analysis in a magnetic field was used to determine Curie temperatures of magnetic phases present. The Curie temperature provides an indication of differences in bulk composition and/or cation distribution between samples. Samples with and without minor additions were subjected to an anneal at temperatures up to 600°C in argon and air. There was little or no change in the Curie temperature for samples annealed in argon. However, samples without additions annealed in air showed a significant change in Curie temperature indicating decomposition of the spinel. This decomposition was suppressed by <1 Wt% SiO₂ and CaCO₃ additions indicating that the thermal stability of manganese ferrite is enhanced by the suppression of grain boundary diffusion by minor dopants concentrated at the grain boundaries.

Rutile bears some unique features in regard to oxygen transfer with the outer atmosphere, at high temperatures. It possesses very high chemical diffusivity. This diffusivity becomes apparent while performing reequilibration experiments in a final atmosphere of pure oxygen. Conversely, the oxidizing-reducing reaction with CO/CO₂ mixtures at the rutile surface is always a slow process.

Several types of materials with a nanoscale structure will be discussed. The most important synthesis methods are briefly reviewed with a certain focus on sol-gel methods.Ceramic membranes with a thickness of 1-10μm and a mean pore size which can be varied between 2,5 and 50 nm shall be treated into some detail. A focus will be given to techniques which modify the internal pore structure to decrease the effective pore size or to change the chemical character. The potential for their use in separation processes and in chemical processes is assessed.Ion implantation as a technique to modify ceramic surfaces will be briefly reviewed and an overview of the changes in surface properties is given.

It is normally assumed that glass surfaces are inert; this is why glass is frequently considered as the container material of choice. However, anyone who has ever worked with a vacuum line or looked at the chemical or biological activity of glasses knows otherwise. Furthermore, when treated in specific ways, glass surfaces can be rendered extremely reactive. This presentation will deal with particular treatments that have been shown to greatly modify the chemical activity and mechanical properties of glasses.

The role of interfaces in the strength of composites is discussed through a comparison of fracture processes in elastically homogeneoeus materials versus bimaterials. Following this the fracture strength of fibrous composite interfaces are briefly reviewed. Experimental results for the temperature variation of interfacial shear strength in a fibrous composite are presented. A microstructural investigation of the regions where failure occurs is made by scanning and transmission electron microscopy. Based on the microstructural study, a rationale for the failure of the composite under study is offered.

The literature on precipitation, precipitation toughening, and precipitation hardening in Y₂O₃ partially-stabilized ZrO₂ (Y-PSZ) single crystals is reviewed. Very strong crystals can be obtained at both ambient and elevated temperatures. At room temperature, “classical” transformation toughening appears to be operating, while at elevated temperatures (e.g. 1400°C), potent precipitation hardening results from the difficulty matrix dislocations have in cutting particles; the Burgers vectors of these matrix dislocations are not lattice vectors in the precipitates and the sheared particles are believed to be faulted. Understanding plastic deformation at intermediate temperatures (e.g. 700°C) in crystals deformed in compression remains a task for future work.

Hip prosthesis fixation and stabilisation have been improved by biological anchorage. An alumina porous ceramic coating allows bone tissue ingrowth, thus creating strong mechanical bonds.

A composite W-TiN-SiC material, as well as the technology for the manufacture of this material, are being developed at the Centre for Technical Ceramics in co-operation with partners in Italy and the Netherlands. The trilayer material will serve as a multifunctional “hot shell” in a combustion heated Thermionic Energy Converter. This device will operate at 1400°C. The composite material simultaneously serves as a protective confinement of the converter, as an emitter electrode and as an electrical lead. The multitude of requirements induced by these three functions of the hot shell are discussed. A brief outline of the design of the thermionic energy converter is given. The way in which the W-TiN-SiC material is being produced as well as some preliminary results on the evaluation of this material are presented. The properties determined include thermal fatigue and thermoshock resistance, Young’s modulus and thermal conductivity. Although the evaluation is still in progress, the results are promising.

A method is described for an organic surface modification of α-alumina particles. The modified particles combine to rapidly settling aggregates in water, whereas sedimentation in ethanol is much slower. Suspensions are filtered with a porous mold. A filtration model is introduced for simultaneous filtration and sedimentation, which explains qualitatively the observed filtration kinetics.