Role and activity of iron and indium impurities on coarsening and functional properties in MgO nanoparticle derived ceramics

Nanoscale grain boundary design is an emerging field in materials science. It addresses the knowledge based transformation of well-defined nanocrystalline starting materials into consolidated networks of nanograins, with designed particle interfaces and grain boundaries. Moreover, controlled impurity segregation into solid-solid interfaces during sintering can be a strategy to induce functional properties that originate from interparticular, impurity-rich crystalline phases. [1,2]

In this study the flexibility of flame spray pyrolysis was used to synthesize MgO based mixed metal oxide nanopowders from the gas phase, with high control over composition up to 20 at% of Fe3+ and In3+ admixtures. Functional oxide ceramics were obtained by dry uniaxial pressing of as-synthesized nanoparticle powders followed by a pressureless sintering step up to 1373 K and/ or 1673 K, respectively.

Comprehensive structural characterization of the porous ceramics (X-ray diffraction and electron microscopy) revealed both phase separation and impurity segregation. Moreover, we tracked that impurity concentration sensitively influences the intergranular wetting behaviour, thus either favouring the formation of thin intergranular films or the formation of triple- and/or multiple grain junctions. Whereas, sintering at 1373 K of Fe-Mg-O was found to be sufficient to form the magnetic magnesioferrite phase (VSM PPMS) in case of In-Mg-O sintering at 1673 K triggered the formation of an intergranular MgIn2O4 percolation path, decreasing the ceramics’ grain boundary resistivity by orders of magnitude (4 Point Probe Resistivity).

Literature: [1] M. Niedermaier, T. Schwab et al., ChemNanoMat 2019, 5, 634. [2] M. Niedermaier, T. Schwab et al., J. Phys. Chem. C 2019, 42, 25991