Final Report Summary - INCEMS (Interfacial Materials - Computational and Experimental Multi-Scale Studies)
Composed of a team of experts, the 'Interfacial materials - computational and experimental multi-scale studies' (INCEMS) project effectively studied ceramic interfaces by combining computational modelling across all length scales, from atomic structures to real-life devices, with a strong and critical validation by experiments. The target material for INCEMS was polycrystalline SrTiO3 (STO) with varying microstructures, species and concentrations of defects and dopants. This has been realised by:
1. developing a next generation of theoretical models that will treat complex but technologically relevant impacts of multi-component chemistry, space charges and dispersion forces on the thermodynamic stability of interfaces. Modelling tools vary from ab-initio to continuum and are able to describe electronic and atomic structure (ab-initio theory, atomistic simulation), composition and stability of grain boundaries (phase field theory, self-consistent classical density functional theory - SCCDFT) as well as microstructural evolution (vertex dynamics).
2. integrating the individual models to validate and inform companion methods, and to derive self-consistent descriptions of the material behaviour across all length scales.
3. preparing materials with technologically relevant fabrication methods having decisively controlled compositions, grain boundaries and grain sizes. These materials are the basis for the characterisation work as well as for the measurement of macroscopic properties for correlations with model predictions.
4. experimental characterisation of complex interfaces that typify those in real materials.
5. iterative validation of the models by experiments.
6. delivering predictive tools for the design of materials and processes that will promote a technological paradigm shift in materials design for microstructure.
Experiments:
Further sets of polycrystalline SrTiO3 materials with varying microstructures, defect and dopant compositions were synthesised by sintering of powders under carefully controlled conditions. The microstructures and interfaces of these materials were characterised by photoelectron spectroscopy and by reflection electron energy-loss spectroscopy (REELS), by high-resolution transmission electron microscopy (HRTEM), by electron back-scattering diffraction (EBSD), and by high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM), and by transmission electron energy-loss spectroscopy (TEELS).
The most important and unexpected outcome from these experimental investigations in the first two years was that no evidence had been found for the existence of IGFs in these polycrystalline SrTiO3 materials, which were highly pure or doped in well controlled manner, and that many analysed grain boundaries were rather sharp and well ordered on the atomic scale. The major subsequent experimental effort was to vary the materials synthesis systematically and even better controllably, in order to determine processing conditions for a formation and, subsequently, a suppression of IGFs at will. The experimental challenge for the last two years was to augment interfaces in polycrystalline ceramic materials by interfaces in bicrystals. These extended the possibilities for controlled variations of interface structure and composition in the search for the formation conditions of IGFs, which still remained absent although, and for atomic-scale characterisations by microscopy, spectroscopy, and theory, which developed indeed to a very productive and insightful line of activities.
Modelling and simulation:
Computational methods and material models for all length scales were running productively: On the atomic scale, ab-initio electronic-structure calculations for stoichiometric Σ3 symmetrical tilt grain boundaries (twin interfaces), (111) stacking faults and (110) antiphase boundaries in pure SrTiO3, and for SrTiO3/Pt, BaTiO3/Pt and various other perovskite/metal interfaces were done, ab-initio calculations for non-stoichiometric Σ3 twin interfaces, as interface models with less structural order, were pursued. Also the degrees of structural order at a variety of atomic-scale models for asymmetric Σ3 grain boundaries, Σ5 twist and tilt grain boundaries were explored by empirical atomistic simulations. The self-consistent classical density functional theory (SCC-DFT) approach for thermodynamic modelling had been completed successfully for the one-component system silicon. The extension to the multicomponent system SrTiO3 had turned out to be conceptually too problematic and therefore no longer pursued. Thermodynamic phase-field modelling with meso-scale incorporation of interface properties was continued, microstructural vertex-dynamics modelling and simulation made strong progress, and calculations of IGF properties by dispersion-force theory were continued. The important output of the theoretical activities was that the needed theoretical and computational approaches on all relevant length scales got set up and operating for INCEMS.
The decision to abandon SCC-DFT after the first two years and to concentrate on atomistic simulations with empirical potentials in the last two years has turned out to be very fruitful.
By linking to first-principles calculations and to bicrystal experiments, the atomistic simulations enabled us to obtain a clear understanding of strengths and weaknesses of empirical interatomic potentials that had been originally developed for bulk SrTiO3, when applying them to SrTiO3 interfaces. The major subsequent theoretical efforts used the atomistic simulation methods for supporting the search for criteria why no IGFs were detected so far in the experimentally investigated SrTiO3 ceramics, and for elucidating thermodynamic and kinetic mechanisms at the sub-nanometre scale which could lead to the formation or suppression of IGFs at the sub-micrometre scale.
In addition to this summarised research work, continuous efforts were carried on in the last two years for the dissemination of knowledge, namely numerous invited and contributed presentations by INCEMS participants were given on international scientific conferences, and numerous papers were published or accepted for publication in refereed international journals. The board of five industrial advisors from five large European companies who are themselves experts in functional ceramics for industrial applications, continuously supported INCEMS by giving guidance to the scientific research work through their thoughtful advice.
The intensive and constructive scientific interaction of the eight groups of experimentalists and theoreticians at the six European locations, through the semi-annual project meetings, mutual discussion visits, semi-automated data-exchange procedures, during the four project years was working so well and synergistically that the final goal of INCEMS, a predictive multi-scale modelling of interface-controlled functional ceramic materials was achieved in by far most of the addressed aspects.
1. developing a next generation of theoretical models that will treat complex but technologically relevant impacts of multi-component chemistry, space charges and dispersion forces on the thermodynamic stability of interfaces. Modelling tools vary from ab-initio to continuum and are able to describe electronic and atomic structure (ab-initio theory, atomistic simulation), composition and stability of grain boundaries (phase field theory, self-consistent classical density functional theory - SCCDFT) as well as microstructural evolution (vertex dynamics).
2. integrating the individual models to validate and inform companion methods, and to derive self-consistent descriptions of the material behaviour across all length scales.
3. preparing materials with technologically relevant fabrication methods having decisively controlled compositions, grain boundaries and grain sizes. These materials are the basis for the characterisation work as well as for the measurement of macroscopic properties for correlations with model predictions.
4. experimental characterisation of complex interfaces that typify those in real materials.
5. iterative validation of the models by experiments.
6. delivering predictive tools for the design of materials and processes that will promote a technological paradigm shift in materials design for microstructure.
Experiments:
Further sets of polycrystalline SrTiO3 materials with varying microstructures, defect and dopant compositions were synthesised by sintering of powders under carefully controlled conditions. The microstructures and interfaces of these materials were characterised by photoelectron spectroscopy and by reflection electron energy-loss spectroscopy (REELS), by high-resolution transmission electron microscopy (HRTEM), by electron back-scattering diffraction (EBSD), and by high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM), and by transmission electron energy-loss spectroscopy (TEELS).
The most important and unexpected outcome from these experimental investigations in the first two years was that no evidence had been found for the existence of IGFs in these polycrystalline SrTiO3 materials, which were highly pure or doped in well controlled manner, and that many analysed grain boundaries were rather sharp and well ordered on the atomic scale. The major subsequent experimental effort was to vary the materials synthesis systematically and even better controllably, in order to determine processing conditions for a formation and, subsequently, a suppression of IGFs at will. The experimental challenge for the last two years was to augment interfaces in polycrystalline ceramic materials by interfaces in bicrystals. These extended the possibilities for controlled variations of interface structure and composition in the search for the formation conditions of IGFs, which still remained absent although, and for atomic-scale characterisations by microscopy, spectroscopy, and theory, which developed indeed to a very productive and insightful line of activities.
Modelling and simulation:
Computational methods and material models for all length scales were running productively: On the atomic scale, ab-initio electronic-structure calculations for stoichiometric Σ3 symmetrical tilt grain boundaries (twin interfaces), (111) stacking faults and (110) antiphase boundaries in pure SrTiO3, and for SrTiO3/Pt, BaTiO3/Pt and various other perovskite/metal interfaces were done, ab-initio calculations for non-stoichiometric Σ3 twin interfaces, as interface models with less structural order, were pursued. Also the degrees of structural order at a variety of atomic-scale models for asymmetric Σ3 grain boundaries, Σ5 twist and tilt grain boundaries were explored by empirical atomistic simulations. The self-consistent classical density functional theory (SCC-DFT) approach for thermodynamic modelling had been completed successfully for the one-component system silicon. The extension to the multicomponent system SrTiO3 had turned out to be conceptually too problematic and therefore no longer pursued. Thermodynamic phase-field modelling with meso-scale incorporation of interface properties was continued, microstructural vertex-dynamics modelling and simulation made strong progress, and calculations of IGF properties by dispersion-force theory were continued. The important output of the theoretical activities was that the needed theoretical and computational approaches on all relevant length scales got set up and operating for INCEMS.
The decision to abandon SCC-DFT after the first two years and to concentrate on atomistic simulations with empirical potentials in the last two years has turned out to be very fruitful.
By linking to first-principles calculations and to bicrystal experiments, the atomistic simulations enabled us to obtain a clear understanding of strengths and weaknesses of empirical interatomic potentials that had been originally developed for bulk SrTiO3, when applying them to SrTiO3 interfaces. The major subsequent theoretical efforts used the atomistic simulation methods for supporting the search for criteria why no IGFs were detected so far in the experimentally investigated SrTiO3 ceramics, and for elucidating thermodynamic and kinetic mechanisms at the sub-nanometre scale which could lead to the formation or suppression of IGFs at the sub-micrometre scale.
In addition to this summarised research work, continuous efforts were carried on in the last two years for the dissemination of knowledge, namely numerous invited and contributed presentations by INCEMS participants were given on international scientific conferences, and numerous papers were published or accepted for publication in refereed international journals. The board of five industrial advisors from five large European companies who are themselves experts in functional ceramics for industrial applications, continuously supported INCEMS by giving guidance to the scientific research work through their thoughtful advice.
The intensive and constructive scientific interaction of the eight groups of experimentalists and theoreticians at the six European locations, through the semi-annual project meetings, mutual discussion visits, semi-automated data-exchange procedures, during the four project years was working so well and synergistically that the final goal of INCEMS, a predictive multi-scale modelling of interface-controlled functional ceramic materials was achieved in by far most of the addressed aspects.
