Defects in yttria‐stabilised zirconia: a positron annihilation study

Abstract

High-resolution positron lifetime and coincidence Doppler-broadening measurements on the yttriastabilised zirconia compacted nanopowders, differing in phase, stabiliser content and preparation conditions, are reported. The nature of defects observed in these materials is discussed on the basis of available experimental data and their comparison with theoretical calculations. It is concluded that positrons annihilate mainly in vacancy-like defects at grain boundaries and in larger open-volume defects associated with triple points. A fraction of positrons annihilating in mesopores of ≈ 2.5 nm diameter was also detected. 1 Introduction Zirconia-based materials made of nanometer grain size powders offer an exceptional combination of advantageous thermal and mechanical properties, which makes these materials attractive for various industrial applications like, for example, heat-resistance structural and functional ceramics, solid oxide fuel cells and oxygen sensors. However, a stabilisation of the tetragonal and cubic phases of zirconia, e.g. by an addition of trivalent yttrium oxide is needed when high-temperature applications of zirconia are required. Such an addition leads to a formation of a certain amount of defects in the ZrO 2 lattice. These defects in turn influence properties of yttria-stabilised zirconia (YSZ) and the nature and the role of defects introduced by stabilisation is in need of a detailed elucidation in order to taylor zirconia-based materials of required properties. Moreover, the role of grain boundaries (GBs) which constitute a large volume fraction in the compacted nanopowders should be taken into consideration, too. Porosity is another important property of interest in these materials. It is then obvious that positron annihilation spectroscopy (PAS) is a promising tool for microstructure studies of zirconia-based nanomaterials. Despite of several PAS investigations on YSZs published earlier, see e.g. [1][2][3][4], there is still a rather poor understanding and ambiguous explanations of the observed PAS data.In the present Contribution, high-resolution positron lifetime (PL) measurements on the YSZ compacted nanopowder materials, differing in phase, stabiliser content and conditions of preparation, are reported. These measurements are supplemented by a coincidence Doppler broadening (CDB) investigation using two HPGe detectors. The nature of defects observed in these materials is discussed on the basis of available experimental data and their comparison with the theoretical calculations of positron lifetimes and high-momentum parts of Doppler profiles, performed [5] within the framework of the atomic superposition method considering appropriate vacancy-type defect configurations.

Positron Lifetimes in Zirconia-Based Nanomaterials

“…This further supports our conclusions [6] that positrons annihilate mainly as trapped in defect structures associated with GB's in the compacted nanopowders. Present PL data also suggest that the process of sintering zirconia-based nanoceramics may be followed also via the shorter lifetime components (< 0.5 ns) in PL measurements with sufficiently high resolution.…”

“…[3] the two components arise mainly from the annihilation of positrons inside nanograins, authors [4,5] related these components to the GB's. In our paper [6], the latter interpretation was further supported by the observed I 2 : I 1 ratios vs. the mean grain size. Positrons thus annihilate most likely in the vacancy-like defects at grain surface (the τ 1 lifetime) or in a larger open-volume defects (probably triple points at GB intersections) with lifetime τ 2 .…”

“…An existence of pores is indicated by the o-Ps τ 3 and τ 4 components in the nanopowder specimens (see Table and refer to our recent paper [6] for more details). On the contrary to the nanopowder specimens, these o-Ps components were found to be absent in the sintered materials.…”

“…The three zirconia-based nanopowders [6,7] were used: (i) the pure ZrO 2 (monoclinic phase, further referred to as Z0Y), (ii) the ZrO 2 + 3 mol.% Y 2 O 3 (tetragonal, Z3Y) and (iii) the ZrO 2 + 8 mol.% Y 2 O 3 (cubic, Z8Y). Powders were pressure-compacted under 250 MPa at room temperature.…”

“…Results obtained on YSZ nanopowders have recently been reported in detail as a conference contribution [6]. The PL data observed in these materials of different complexity are compared and discussed from the point of defects involved.…”