Solodkyi I. Influence of phases and structure forming under spark plasma sintering conditions on properties of oxide ceramics В6О, Ce0.8Sm0.2O1.9

The aim of the present study is to develop physical and chemical principles of fabrication of dense oxide ceramics В6О, Ce0.8Sm0.2O1.9 by SPS. The influence of synthesis parameters at atmospheric pressure on phase composition and crystallite size of the В6О powder was investigated. The mixture of B2O3 and amorphous B with mole ratio 1:14 is shown to react and form B6O during SPS and conventional treatment. The optimum SPS temperature and time to obtain B6O phase are 1250 °C and 30 min, respectively. The Rietveld refinement of the XRD patterns indicates that the oxygen occupancy of B6Ox is reasonably high at x=0.89. Electron micrographs have shown that each star shape particle have five unique vertices with thickness of around 40 nm. This contribution, for the first time, reports the consolidation of hard and tough B6O bulks using the SPS technique. The influence of types of protection, i.e. inner surface of graphite die wrapped only with graphite foil, BN coating, and tantalum foil on phase compositions and properties of bulk ceramic was studied. As a result, tantalum foil was proposed to prevent carbon diffusion from the die to the sample. Thus SPS-ed specimens with densities of >=98% was obtained, while the combination of hardness of 34 GPa and fracture toughness of 4 MPa m1/2 was achieved. The influence of B4C content on phases composition of B6O-xB4C (x=0, 3, 5, 10, 20, 40 wt%) composite ceramics was elucidated in the present work. Ceramics fabricated by SPS demonstrated a significant improvement in the mechanical properties. Dense B6O-10wt%B4C composite material with hardness higher than 40GPa and fracture toughness of 4.82 MPa?m1/2 was obtained. A one-step in-situ reactive SPS for fabrication high dense B6O ceramic was designed and applied for the first time. The prepared B6O dense ceramic has Vickers hardness (36.7±1.2 GPa) and fracture toughness (K1С=4.2±0.15 MPa.m1/2) comparable with the highest values reported in the literature for the bulks obtained from already reacted B6O powders (ex situ routes). A comprehensive study of the influence of structural parameters on the electrical conductivity of the electrolyte Ce0.8Sm0.2O1.9 was carried out. The influence of spark plasma sintering parameters on microstructure and electrical conductivity of the ceramics was investigated. The effectiveness of low-temperature post-SPS annealing in air to compensate the oxygen deficient on the grain boundaries nanostructural ceramics was demonstrated. As a result, higher grain boundary conductivity of the electrolyte was achieved.