The objective of this project is to achieve a complete computational and experimental description of the structure and fundamental properties of crystal/glass interfaces in a series of materials systems which reflect the underlying issues. In particular, the following properties will be investigated:

‡       Spatially-varying atomistic structure and composition

‡       Bonding and electronic structure

‡       Transport properties

‡       Dispersion forces across the interface

‡       Bulk and gradient thermodynamic quantities

The understanding gained will provide the key to solving major problems in a wide range of ceramic and nanostructure materials. The industries which will benefit include the ceramics industry and those which make use of or depend upon components which involve sintering as part of their production cycle.

The research programme is multifaceted, with the key issues (computational, characterisation) being tackled in several alternative ways in parallel. This approach brings to the problem a range of powerful experimental and computational tools in the hands of leading practitioners. It is important to emphasise the unique nature of this partnership. This is not a partnership drawn together because it sees an opportunity. It is a partnership of internationally leading researchers who have together, over several years and at diverse meetings, realised that a scientific challenge exists, and who have been searching for a way to tackle this problem in a coordinated way. If this opportunity is not seized here, it could easily disappear. Consequently, because of this coordinated commitment, the risk to progress is minimal - all the participants are keen to be involved and to contribute.

Progress will be monitored in a number of ways, both by the EU and US coordinators, but particularly by the Steering Committee. A timeline for the delivery of progress reports is given, and these reports will be distributed in a variety of ways and reviewed by the Steering Committee with input from all the individual partners. As is natural with collaborative research, there will also be many informal avenues for monitoring progress, including interactions between partners visiting each others laboratories, presentations at workshops and the exchange of pre-prints of published works. There will also be an annual meeting of the consortium at which progress and planning will be discussed in depth.

Materials to be Studied

The materials to be investigated have been selected because they can be both modelled and studied experimentally at a high level of detail using the expertise of the investigators, and because they cover the fundamental issues that need exploring. Each system also has clear technological relevance. Through the in-depth study of these chosen systems, an understanding applicable to broader classes of materials is expected to emerge. The materials are (a) silicon-based crystals Si₃N₄, SiC (b) Si-O-N and Si-O-N-(Y,La) intergranular films (c) Si-O-C and Si-O-C-Al intergranular and surface films (d) Si-O-La and Si-O-N-La surface films and (e) grain boundaries in SrTiO₃. The partners have enormous experience with these materials, and have the infrastructure and knowledge to carry out a comprehensive study.

Silicon-Based Crystals Si₃N₄, SiC and Si have in common a native siliceous (SiO₂-based) surface layer that is modified by N, C or other impurities and intentional additives. It is now well-established that a nanometre-thick disordered glass film is present at many grain boundaries in polycrystalline Si₃N₄ and SiC. The induced structure and composition of the glass film has an important role in polycrystalline fracture and creep behaviour. For silicon, the undoped surface native oxide is an indispensable component of microelectronic devices, but is expected to be replaced by higher performance oxides as length scales shrink in future technology. In the search for new gate and DRAM dielectric oxides with high permittivity and low leakage/high breakdown voltage, fundamental understanding of the structure and chemistry induced in the glass layer by the adjacent crystal and their impact on electrical properties has been largely absent.

Silicon Nitride: Si-O-N and Si-O-N-(Y,La) Intergranular Films. Pure silicon oxynitride films are the most basic that can be both experimentally fabricated and ab initio modelled in this system, and will be the starting model system. Note that Si-O-N films do not provide increased fracture toughness in polycrystalline Si₃N₄, whereas Si-O-N-(Y,La) films represent the simplest rare-earth doped films that do. The enrichment of La in the intergranular film is typically greater than that of Y, providing some advantages for experimental analysis.

         In Si₃N₄-SiO₂, the thickness of the glassy film is 1nm ± 1 Å irrespective of the amount of SiO₂ present. EELS applied to the glassy film in high purity Si₃N₄-SiO₂ ceramics has shown the chemical composition of the boundary film to be SiN_xO_y with x = 0.53 and y = 1.23 indicating that the anion concentration in the silicon oxynitride film is x/(x+y) = 30%.

         The constancy of the film thickness and composition is truly remarkable. The fact that the thickness and composition of the film depend only on the composition of the sintering aid, and not on the amount of the sintering aid, indicates that the film exists as a consequence of chemical equilibrium. In other words, in Si₃N₄-SiO₂ ceramics the film is a consequence of equilibrium segregation of oxygen to grain boundaries. The observation that the film thickness and composition are nearly independent of the boundary plane and misorientation indicates that these geometric variables do not strongly affect the equilibrium condition. There are then only 3 primary thermodynamic degrees of freedom governing the equilibrium of the glassy film. For example, once the temperature, pressure and chemical potential of oxygen (i.e. partial pressure of O₂) have been specified then the film thickness and composition are uniquely determined in Si₃N₄-SiO₂ ceramics.

Silicon Carbide: Si-O-C and Si-O-C-Al Intergranular and Surface Films. Silicon carbide grain boundaries and surfaces with oxycarbide films will be studied as the exemplar for amorphous films which can be crystallized upon heat treatment (e.g., in the Al-doped case). The modelling of oxycarbide glass films will be similar to that described above for oxynitride films, and will involve the same investigators. The electron microscopy and spectroscopy techniques applied to silicon nitride grain boundaries will be applied here as well.

         In addition, the availability of large single SiC crystals (e.g., from Cree Inc.) allows the study of controlled-misorientation bicrystals. Bulk and thick-film (100 nm) silicon oxycarbide glasses have been synthesized from polymer precursors, and are stable at compositions/temperatures clearly not allowed by bulk phase equilibria. Thus this system is also well-suited to the study of amorphous films of kinetically-stabilized, nonequilibrium compositions. P9 and colleagues will prepare sol-gel films on single crystal SiC of equilibrium and nonequilibrium Si-O-C and Si-O-C-Al compositions. These typically have thicknesses well beyond the range of interest for a grain boundary. However, by lithographically grooving the single crystal surface to provide local reservoirs for excess glass, a bicrystal sandwich of controlled misorientation can be brought to the nanometre-scale separations representative of intergranular glass films in polycrystals. This experimental configuration will be developed by P9 and colleagues. In addition, thinner films of oxycarbide glasses can be deposited by PVD techniques (magnetron sputtering, laser ablation) from SiC/SiO₂ targets if necessary, and bicrystals prepared by pressure-welding.

Silicon: Si-O-La and Si-O-N-La Surface Films. Recent studies of alternative gate oxides to SiO₂ have focused on systems such as Si-O-Hf and Si-O-Zr, selected with the expectation that a high Z constituent of fixed valence will raise permittivity without introducing interface states (contrary to d or f transition metals). Thermodynamic compatibility with silicon at processing temperatures is also necessary. Since crystallization of the gate oxide is generally thought to introduce undesirable interface states as well, the compositions studied have been highly siliceous, for example, Zr₄Si₃₁O₆₅. These compositional selection criteria, combined with the fact that sub-2 nm thicknesses are ultimately desired, creates an opportunity for the development of new gate oxides based on the concept of equilibrium-thickness, disordered surface films. We note that Zr is a classical nucleating agent in silicate glasses, while lanthanide additives satisfy dielectric and thermodynamic stability criteria for gate oxides while being more benign from the crystallization viewpoint. High permittivity amorphous films are also of interest for DRAM technology. While much recent work has focused on high permittivity perovskites such as (Ba,Sr)TiO₃, reactivity with silicon at the higher processing temperature necessary in fabrication are limiting their introduction. A stable, high permittivity siliceous film would be of much interest in this technology as well.

Grain Boundaries in SrTiO₃. Strontium titanate is of interest as a non-silicate system that also represents a broad range of complex perovskites of interest for their ferroelectric, dielectric, magnetoresistive, and superconducting properties. Perhaps the most compelling reasons for its inclusion here as a model system are the observations of a wetting transition in polycrystals sintered with titania excess producing eutectic liquid, the transition temperature furthermore being dopant-sensitive, and the existence of amorphous intergranular films in silicate-free boundaries. These pieces of data suggest that a systematic progression from dry to moist (multilayer-adsorbed) grain boundaries to boundaries fully-wetted by a titania-rich eutectic liquid occurs in strontium titanate. The atomic structure and composition of various special boundaries in SrTiO₃, with and without dopants, have recently been experimentally characterized and modelled at several laboratories. Here our goal will be to identify whether and where multilayer adsorption and wetting transitions occur, and to understand the criteria for such transitions.

The proposal presented here is innovative in that it brings together international experts with expertise in three different areas - modelling, growth and characterisation - in a coordinated program of research to solve a materials problem. The rationale for specific experiments is discussed later with respect to each materials system. Particularly innovative is the cooperation of computational experts across the length scales of computational modelling. At this point, it will assist the reader to give an overview of the collaborative modelling approach. Every intergranular film has several length scales. Modelling the behaviour of thin films requires cooperation and communication between modellers whose expertise lies in a particular length scale. P7 and P2 will produce ab initio energy calculations in silicate-based and strontium titanate films. These calculations will be compared to experimental ELNES data by P4 and P11 and to experimental REELS (reflection electron energy loss spectroscopy) data by P3 and to electron diffraction structural determinations by P1. P7 will produce calculations of nanometre and interfacially resolved electronic excitations that can be compared directly to ELNES. P7 and P2 will investigate modelling predictions of thermal conductivity and optical excitation that can be used to guide or interpret experiments in specific systems. The ab initio calculations will provide input potentials for molecular dynamic potentials by P8. The P8 results should determine whether a stable amorphous film can be simulated from the input potentials and whether any additional structure can be inferred in stabilized films. From stable amorphous film results, data for composition profiles and induced structural order from the abutting crystalline phases can be obtained. Such structural and compositional order will be reduced to bulk thermodynamic quantities by P6 and then modelled as a continuum thermodynamic system as a phase field model. The phase field models will give predictions of stable film behaviour on a relatively small set of physical parameters and thus allow general predictions to be made and tested against specific experimental results.

Project workplan

This research programme has a workplan with 11 workpackages (see Table 1) which run simultaneously. Because of the complexity of the topic, leading international researchers are essential and the workpackage leaders come from the EU and the US (and so the application is under the EU-US Agreement).

Each of the leaders is an international expert in their field, and each comes with a team of colleagues who will provide intellectual support to each workpackage. The details of the workpackages and their defined objectives and verifiable deliverables are given below in dot point form, and a flow diagram showing their interactions is shown in Figure 1. This is the most succinct way to explain the interactions.

Work Package Details

W1 Structural Characterisation of Interfaces by HREM and ED

Objectives:

(a)   Characterize the structure of amorphous phases of ZnO- Bi₂O_3, SrTiO₃ and silicon-based systems (Si₃N₄, SiC) in the studied materials

(b)  Characterize the structure of the interfaces in the studied systems using HREM techniques

(c)   Use the structures derived to refine against the model systems developed in the computational parts of the workplan

Verifiable deliverables

(a)   Publication of the structures in (a)

(b)  Agreement of the experimental data with model outcomes

W2 Ab initio Computations of Interface Structures

Objectives:
            (a) Grand Canonical Simulations of Silicon Oxynitride Films

            (b) Development of bond order potentials for silicon oxynitride systems

Verifiable Deliverables

            (a) Paper submitted for publication

            (b) Comparison with experimental data

W3 Surface Analysis

Objectives

(a)    Use REELS (Reflection Electron Energy Loss Spectroscopy), XPS and UPS (X-ray and UV photoelectron spectroscopy) to derive the electronic structure of very thin films of interest in the proposal, such as Si-La-O and SrTiO3, using currently available samples.

(b)    Characterize the chemical bonding in the very thin Si-La-O amorphous films, compared with pure SiO2 and bulk glass phases.

(c)    Determine the surface electron energy loss function of thin amorphous layers by REELS.

Verifiable deliverables

(a)    publication of results

(b)    comparison with VUV optical measurements and EELS experiments, as well as electronic structure calculations.

W4 Electron Energy Loss Spectroscopy and Fine Structure

Objectives

(a)    Measurement of electron energy loss spectra with high spatial and energy resolution

(b)    Measurement of fine structure of electron loss edges of different elements (ELNES investigation)

(c)    Calculate the fine structure by appropriate theoretical models

(d)    Characterize the structure of interfaces by high-resolution transmission electron microscopy
Verifiable Deliverables

(a)    Papers submitted for publication

(b)    Determination of agreement between results from theoretical studies and experimental investigations

W5 Materials Growth and Testing

Objectives

(a)             To process Si-N ceramics as in (a) below.

(b)             Analyse growth kinetics and influence of film composition on growth morphology.

(c)             Determine macroscopic properties as in (c) below.

Verifiable Deliverables

(a) Samples with different intergranular films for the electron microscopy groups.
(b) Experimental data of the influence of the intergranular film composition on growth morphology and growth kinetics and agreement with the model outcomes.
(c) Agreement of experimentally determined macroscopic properties, measurements of interfacial bonding with the electronic structure modelling.

W6 Phase Field Modelling of Stable Interfacial Films

Objectives:
(a) To develop a continuum thermodynamic framework for the prediction and catagorization of stable thin interfacial films.
(b) To develop a method of coarse-graining direct structural measurements or molecular simulation to obtain self-consistent input parameters of phase field models.
(c) To classify thin film behavior on the basis of relatively few thermodynamic parameters.
Verifiable Deliverables:
(a) Publication of a quantitative theory for interface phase structure and composition.
(b)  Demonstrated and articulated technique for obtaining parameters from data that is self consistent.

W7 Electronic Structure and Spectroscopic Properties

Objectives

1. Electronic structure calculation and materials properties evaluation based on ab-initio technique.
2. Detailed studies on crystalline, amorphous and interface model structures.
3. Development of predictive methods for materials characterization.

Verifiable Deliverables

1.     Publications of the work described above.

2.     Provision of useful parameters from ab-initio calculations for MD simulation and continuum-scale modelling.

3.     Provision of theoretical database of band structures, ELNES spectra (fingerprints), optical properties etc. for materials characterization and experimental interpretations.

4.     Development of new and robust methods in computational materials theory.

W8 Molecular Dynamics Simulations of Multicomponent Systems

Objectives:

(a) MD simulations of the surface and interface structure of the amorphous oxide films on Si3N4, SiC, and SrTiO3 and the effect of additives in the amorphous films on structure.

(b) Simulations of transport properties of ions in the amorphous phase as a function of composition.

(c) Simulations of contact between dissimilar crystal orientations in contact with the same intergranular film.

(d) Comparison of all results with ab-initio calculations of local structures and available experimental data for refinement of the potentials.

Verifiable Deliverables:

(a)    Publication of data

(b)    Refinement of interatomic potentials

W9 Preparation and Characterisation of Surface Films

Objectives

(a)    Prepare films of oxycarbide and oxynitride compositions and heat treat.

(b)    Determine temperature and composition dependence of above on film thickness.

(c)    Characterise film morphology, composition and structure

(d)    Determine electrical properties

Verifiable Deliverables

(a)    Publication of results as in 1-4 of WP9.

(b)    Supply of samples to other WPs.

W10 Interparticle Forces and Wetting

Objectives

(a)               Determine surface electronic structure of thin films, by using REELS, XPS and UPS.

(b)               Characterize the chemical bonding in the very thin amorphous films (e.g. Si-La-O), compared with pure SiO2 and bulk glass phases.

Verifiable Deliverables

(a)    Compositional characterization of surface films

(b)    REELS data on surface films

W11 Spatially resolved Measurements of Electronic Structure and Dispersion Forces

Objectives

a)               Determine electronic structure of the bulk silicon based systems Si, SiC an Si₃N₄ using VUV spectroscopy and Ellipsometry, using currently available samples.

b)              Determine electronic structure of the bulk strontium titanate using VUV spectroscopy and Ellipsometry using currently available samples.

c)               Determine electronic structure of the bulk silicon based systems Si, SiC an Si₃N₄ using VEELS microscopy, using currently available samples.

d)              Determine electronic structure of the bulk strontium titanate using VEELS microscopy using currently available samples.

e)               Survey current literature data on optical properties and electronic structure and where necessary update results of reference materials to permit accurate calculations of full spectral Hamaker constants.

f)               Determine spatially resolved electronic structure and optical properties for intregranular and surface films in the silicon based materials system for the determination of full spectral Hamaker constants for the London dispersion forces.

g)              Determine spatially resolved electronic structure and optical properties for intergranular and surface films in the strontium titanate based materials system for the calculation of full spectral Hamaker constants for the London dispersion forces.

h)              Develop quantitative analysis of surface reflection valence electron energy loss spectroscopy of silicon or strontium titanate bulk system.

Verifiable Deliverables

(a)    Publication of bulk optical properties and electronic structure of the reference silicon and of strontium titanate systems. Objectives a,b,c, or d

(b)    Publication of the optical properties and electronic structure of an intergranular films in the silicon nitride or strontium titanate system. Objective f or g

W12 Management and Exploitation

Objectives

(a)    To manage the EU partnership and to coordinate with US coordinator.

(b)    To seek exploitation opportunities

Verifiables Deliverables

(a)    Successful outcome of programme.

(b)    Reports to EU.

Theory Workplan

P2 and colleagues will conduct ab initio modelling of the thickness and composition of silicon oxynitride films sandwiched between 2 misoriented Si₃N₄ grains, beginning with Si-O-N compositions. In previous work P7 and P4 fitted effective interatomic potentials of ionic forms to ab initio calculations of the ground state total energies of a- and b-Si₃N₄ crystals. These potentials used ionic charges obtained from the ab initio calculations, and were therefore not transferable to other chemical environments where different ionic charges may be expected. The 4 crystalline phases of Si₃N₄ were modelled with ab initio methods recently by P7 and coworkers. A number of investigators carried out discrete variational X-a calculations on Si-O-N clusters to interpret features that appear in the ELNES spectra from the silicon oxynitride boundary films. They proposed an atomic model for the film, in which there were no dangling bonds. However, to date no simulations of the boundary film in Si₃N₄-SiO₂ ceramics have treated the problem as one involving the establishment of chemical equilibrium through segregation of oxygen to the grain boundary. Since experiments indicate that the oxynitride films are a result of thermodynamic equilibrium we believe that any credible model must guarantee this explicitly.

A variety of ab initio codes are available to P2 including in-house localised-orbital DFT code that is ideally suited to this system. P2 have unique expertise in the construction of bond-order potentials. P2 also has a long track-record in simulating interfaces.

Simulated structures will be compared with the determination of local structure in the silicon oxynitride films by P1 and coworkers using novel electron diffraction techniques which give the pair correlation function. Experiments to examine differences in higher order correlation functions will be carried out. High resolution EM of the interface (crystalline component) will be performed by P1 and coworkers and P4 coworkers. Once the atomic structure is well-established, P7 will use the orthogonalized linear combination of atomic orbitals (OLCAO) method based on density functional theory to calculate the electronic structure. Comparisons of the calculated electronic structure with experiment will be through electron-energy loss near edge spectra (ELNES) spectra. Recent results using supercells and including core-hole effects have shown the predictive power of ELNES calculations. Here, ELNES calculations of the oxynitride films will be compared with experimental measurements on model grain boundaries by P4 and P11.

P8 will carry out molecular dynamics (MD) simulations using variable charge and fixed charge multibody interatomic potentials. Large scale atomistic systems relevant to intergranular films will be simulated in conjunction with the electronic structure calculations and the continuum-level phase field calculations. Two of the major benefits of current MD simulations are: a) the ability to study relatively large system sizes such that relatively large coherence lengths can be included, and b) the ability to incorporate experimentally important variables such as time, temperature, and pressure effects into the simulations such that thermodynamic and kinetic behavior can be determined.

The MD simulations will be used to create intergranular films and interface structures of the oxides on each of the aforementioned crystals as a function of crystal orientation and film compositions in a manner similar to that used experimentally. That is, high temperatures can be used to create a melt state from which the system is cooled to create the resultant crystal/glassy film/crystal interfaces. Kinetic effects on the film structure and chemistry may thereby be revealed. Local structures generated from the MD simulations can be used in electronic structure calculations in order to evaluate the simulation results or be used as a feedback for optimizing the interatomic potentials used in the simulations. The link between theory and experiment will again be through ELNES calculations by P7, applied to the MD simulated atomistic configurations and compared to experimentally determined ELNES data. Interfacial energies will also be obtained from the simulations and compared to sufficiently large size DFT calculations (where system size can affect such energies) and used in the diffuse interface calculations by P6.

Another important aspect of the MD simulations is the study of transport properties. The migration of species in the molten films can be evaluated in the simulations. The effect of interface trapping or channeling by the ordered interface structure can be evaluated as a function of crystal orientation and composition in the simulations.

Experimental Workplan

In the experimental program, P5 will fabricate reference-standard quality high purity Si₃N₄-SiO₂ polycrystals, thermally equilibrated at high temperatures, for electron microscopy and spectroscopy by P1 and colleagues, P11 and P4. Each of the experiments will provide results which can be directly compared with the calculations.

P1 and colleagues, and P4 will characterize the following crystallographic key-quantities as accurately as possible in a series of grain boundaries for collective study:

Orientation relationship of the adjacent crystals; Orientation relationship and faceting statistics of the interfaces between volume crystals and interfacial phase (internal surfaces with respect to the crystal); High-precision assessment of the thickness of the interfacial phase; Atomistic structure of the terminating crystal planes next to the amorphous films, including possible steps, reconstructions, self-similarities of apparently random fluctuations along the interfaces; Sub-nm localized examination of the terminating layers of amorphous materials, with respect to crystal-surface induced extra ordering mechanisms; Full structural solution including all (relaxed) atomic coordinates and identification of the interface core structure for reference segments of grain boundaries without (or with partially absent) amorphous films;

They will make use of several new problem-adapted techniques, which have not yet been available for major previous HREM studies of interfacial films in ceramic materials, anisotropic Fourier spectral analysis of sub-nm sized interfacial segments; geometric phase holography for the sub-0.1nm tracking of displacements to be converted into 2D-strain maps; detection/rejection of suspected partial order in apparently amorphous films; focal series evaluation to obtain the aberration-free projected electron wave at the bottom side of the specimen, which reveals more reliably the atomic structure; iterative structure refinement to detect relaxations out of a (first guess) reference structure by P7 simulated to experimental images heading for the best fitting structure;.

P1 will use primarily energy-selected electron diffraction to attempt a detailed structural determination of the intergranular glassy phases within these boundaries. Energy selected electron diffraction is now well-established as a means for studying the structure of small volumes of amorphous material. The technique gives the pair correlation function to high accuracy (0.02A first nearest neighbour), which provides a fingerprint for differentiating between possible structural models. However, for studies of intergranular phases, further refinement of the technique will be required, because, to obtain the small size of electron probe necessary to analyse material of the dimension of the intergranular phase, a highly convergent probe will be needed. Pilot studies have shown that deconvolution of the probe function from the diffraction pattern can be achieved satisfactorily.

EXELFS can be used to give the pair correlation function for specified atomic species, and this technique will be used to give results for comparison with the energy selected diffraction results described above.

P4 will obtain ELNES data from the intergranular films for comparison with calculations and P11 will carry out spatially-resolved dispersion force measurements. P10 and colleagues have previously conducted low and high temperature crack growth and creep experiments, and fracture toughness and indentation-based crack deflection measurements on polycrystalline silicon nitride. These measurements will be carried out in parallel and will compare the model materials of this study with each other and with other (generally more complex) doped systems.

A goal in this project will be to determine whether disordered Si-O-La and Si-O-N-La surface films can be stabilized on silicon, the extent of order (if any) in such films, and the correlation between order and electrical properties. These film compositions are both interesting as alternative gate oxides, and will be studied in relation to silicon nitride. Both equilibrium and nonequilibrium compositions are of clear interest given the low processing temperatures typical in silicon microelectronics; nitrogen is of interest as a dopant since it may suppress crystallization by raising viscosity. Amorphous SiO₂ is known to dewet silicon grain boundaries, and it may be that native SiO₂ surface films are largely kinetically-stabilized. However, as in the case of surface melting and stabilization of surface amorphous films in binary oxides, crystallization may be suppressed if the resulting crystalline heterointerface has high energy. At present, it is not known if equilibrium-thickness amorphous films can be stabilized at silicon free-surfaces. In the first stage of this effort, P9 and colleagues will prepare thin (1-5 nm) Si-O-La and Si-O-N-La films on silicon using magnetron sputtering or laser ablation facilities available to us, and use TEM to characterize wetting/dewetting and crystallization as a function of film composition, thickness, and temperature. Stable films and those exhibiting varying degrees of order will be further studied. The permittivity, breakdown voltage, and leakage current will be measured in order to correlate film structure and composition with electrical parameters. More detailed study of specific stable films will follow by other team members, using the theoretical and experimental techniques discussed earlier. In particular, these oxide films will be studied by P3. First, the composition will be measured by XPS. In addition, the modifications of electronic structure induced by the presence of La, Zr compared to SiO2, will be studied by XPS and REELS, with a special emphasis on the changes of the ionic charges, induced gap states etc. The experimental REELS data will be compared with the VUV data obtained by P11, so as to separate surface/bulk effects. The experimental electronic structure will be compared with the electronic structure calculations of P7.

The higher conductivity of Si relative to Si₃N₄ facilitates use of methods such as XPS and grazing-angle synchrotron X-ray scattering (to observe ordering and gradients in order. Especially in the Si-O-N-(La,Y) case, structural order in the surface film may be induced by heat treatment.

P10 will conduct grain boundary penetration experiments in which bicrystals of known misorientation (available as substrates for high Tc films) will be heat treated in contact with pre-equilibrated titania rich melts at various temperatures. PbO and Bi₂O₃ dopants will be studied; the former is known to lower the wetting transition temperature. Silicate additions will be used to affirm that the transitions are not due to impurities. The bicrystal specimens will be characterized by P4 using TEM and STEM in order to determine whether adsorption and wetting transitions do occur, and their dependence on misorientation, temperature, and doping. It is expected that a clear experimental determination of the composition and structure of the Ti-enriched intergranular film will provide the impetus for modelling and additional experiments amongst the collaborators, as in the case of the other materials systems.

A complementary set of experiments will explore the colloidal behavior of Si₃N₄ particles suspended in appropriately doped silicon oxynitride liquids, and of SrTiO₃ particles suspended in variously doped liquids of SrTiO₃-TiO₂ eutectic. These experiments reveal the degree to which particles are attracted or not and provide a complement to the observations of wetting versus multilayer adsorption from polycrystalline samples. Composition or temperature induced changes of behavior should reveal wetting transitions for the grain boundaries. Samples will be provided for quantitative TEM to P4 or P1.

WP5 is divided into three parts. In the first part P5 will prepare silicon nitride ceramics by liquid phase sintering with different rare earth oxides. Densification will be achieved by a sinter-HIP-process which offers the possibility to use only small quantities the of rare earth additives (2-4 wt.%). The sintering additives are selected with regard to the ionic radius and the electronic structure. Dense microstructures could be further modified by a variation of sintering temperature and time. A special technique developed in the laboratory of P5 enables the shrinkage to be followed and the densification mechanisms to be analysed. By using this technique P5 can optimize the sintering program for a complete densification of silicon nitride with only 2 wt.% Y₂O₃ and obtain one of the most creep resistant silicon nitride ceramics. The prepared samples will be distributed to WP1 and WP4.

In the second part, P5 will study the influence of the sintering additives on the growth morphology of the silicon nitride grains. The grain impingement during the growth in a ceramic microstructure requires the design of model experiments. P5 will dissolve a small quantity of silicon nitride grains in an oxynitride glass with a similar composition as the grain boundary phase in a ceramic. However the processing conditions of these supersaturated oxynitride glasses require the use of more than one additive. Beside the analysis of the growth kinetics and morphology P5 will also determine with these types of samples the interfacial energy between the glass and a silicon nitride grain. A detailed analysis of the interface between the glass and silicon nitride will be performed by the microscopy groups.

The third part of the workpackage WP5 is the microscopic and macroscopic characterization of the ceramics. P5 will characterize the grain size and grain morphology of the ceramic material by using scanning electron microscopy. The mechanical properties will be characterized on larger specimens including the strength at room as well as at high temperatures. Fracture toughness will be measured by the SEVNB method. Creep behaviour will be measured. The data of the mechanical properties will be then correlated with the results of the microscopy workpackages.

Table 2 shows the deliverables in terms of reports.

An anonymous tabular overview of the consortium composition is shown in Table 3, as required.

A manpower bar chart is included in Table 4, as required.

An overview of Project Milestones with decision criteria is included as Table 5, as required

Conclusion
This application brings together a team of international EU and US researchers with expertise in modelling, characterization and growth to tackle a problem of fundamental importance to both the scientific community and materials technology. Each of the participants has extensive experience in the field, access to a wide range of infrastructure and vibrant research groups. Such a wealth of experience and resources is difficult to bring together, and it is hoped that this opportunity to contribute to the programme on Competitive and Sustainable Growth will not be missed.

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