Intergranular Regions inside nanocrystalline Ceramics

New interfaces between nanoparticles form when related powders are subjected to compression and sintering to produce nanocrystalline ceramics. These interfaces can exhibit functional properties that modify or add to the intrinsic metal oxide nanoparticle characteristics but can also give rise to undesired ones. Many innovation efforts related to the development of functional nanocrystalline ceramics suffer from process-related loss of the targeted properties. For this reason, knowledge-based interface engineering strategies are needed to design and prepare composition and defect structure inside the intergranular region. This project concentrates on intergranular regions inside nanocrystalline metal oxide ceramics and aims at the understanding of how formation and gradual chemical transformation of the intergranular region affect the integral optical properties and electrical conductivity of the resulting ceramic. We will employ MgO cube powders with different size distributions in the range between 1 nm and 10 m as model systems. Starting with dry powders that lack any orientational order between the individual particles we propose to apply hydration induced particle organization effects to generate different mesoscale precursor motifs for tailored micro- structures. Surface functionalized cubes will be embedded into dense macroscopic objects that should either exhibit strong and robust photoluminescence emission properties or electronic conductivity. For optical ceramics with down conversion properties, alkaline earth oxides in form of segregates or highly dispersed inclusions will be incorporated into the dense nanocrystalline MgO phase. For the generation of novel nanocrystalline ceramics with semiconducting intergranular regions we plan to explore nanocube based composites of the In2O3 - MgO system. The compositional heterogeneity will be varied between compressed core-shell particle systems to consolidated homogeneous MgIn2O4 nanoparticle networks. Intermediate materials states will be isolated at different stages of consolidation and sintering and structural, morphological and spectroscopic changes will reveal the origin of novel optical and semiconducting properties specific to the intergranular region. Supporting the development and manufacture of nanocrystalline ceramics, this project will also advance the translation of metal oxide nanoparticles into products via their implementation into macroscopic electronic and optical devices.

Functional ceramics represent an irreplaceable class of materials spanning a wide application range from microelectronics, materials components for energy conversion and catalysis up to inorganic phosphors. The functional properties are controlled by the constituent grains and, equally important, by the intergranular region in between them. To advance the functionality of the ceramics but also to render the manufacturing process environmentally more benign requires an understanding of structure and composition evolution of the intergranular regions. This research project has started with gas phase synthesized and magnesium oxide based nanoparticle systems with reproducibly introduced impurities, which later on control processing and functional properties during the performance of the ceramic. We identified for the first time tribochemical changes and charge separation effects during metal oxide nanoparticle powder compaction, explored the critical effect of thin films of adsorbed water on sintering and microstructure evolution and employed segregation engineering to decorate the intergranular part of the sintered ceramic bodies with a semiconducting network or with ferrimagnetic inclusions. The key insights obtained open a way to employ composite metal oxide nanoparticles that are composed of earth abundant and noncritical components and that were grown under non-equilibrium conditions for ceramics with higher energy efficiencies during processing as compared to traditional source materials and processes.