J. Cizek

Characterization of point defects in yttria stabilized zirconia single crystals

Characterization of point defects in a fully stabilized ZrO2 + 9 mol.% Y2O3 single crystal with cubic structure was performed in this work. It was found that the crystal contains a high density of vacancy-like defects characterized by a lifetime of 175 ps. First principle theoretical calculations showed that this lifetime is comparable with lifetime of positrons trapped in zirconium vacancies associated with hydrogen. In particular, in the vicinity of the zirconium vacancy hydrogen forms an O-H bond with one of the nearest neighbour oxygen atoms. The calculated bond length is close to 1 A. Using nuclear reaction analysis it was found that the hydrogen concentration in the crystal is 0.3 at.%. This amount of hydrogen is sufficient to form zirconium vacancy – hydrogen complexes capable of saturated positron trapping.

Sintering of zirconia-based nanomaterials studied by variable-energy slow-positron beam

A variable-energy slow-positron beam was applied to the investigations of the tetragonal yttria-stabilised zirconia (YSZ), the YSZ co-doped with small amount of Cr2O3. The initial nanopowders exhibiting the mean particle size of ≈ 20 nm were prepared by co-precipitation technique. Prior the sintering, the nanopowders were calcined and compacted using a pressure of 500 MPa. The ordinary shape parameters of the Doppler-broadened annihilation peak and the relative positronium 3γ-fractions were determined as functions of positron energy. The results are consistent with a remarkable sintering-induced grain growth and disappearance of porosity which is driven out from the sample interior toward a thin subsurface layer.

Positron annihilation studies of zirconia doped with metal cations of different valence

New results obtained by applying positron annihilation spectroscopy to the investigation of zirconia-based nanomaterials doped with metal cations of different valence are reported. The slow-positron implantation spectroscopy combined with Doppler broadening measurements was employed to study the sintering of pressure-compacted nanopowders of tetragonal yttria-stabilised zirconia (t-YSZ) and t-YSZ with chromia additive. Positronium (Ps) formation in t-YSZ was proven by detecting 3γ-annihilations of ortho-Ps and was found to gradually decrease with increasing sintering temperature. A subsurface layer with enhanced 3γ-annihilations, compared to the deeper regions, could be identified. Addition of chromia was found to inhibit Ps formation. In addition, first results of positron lifetime measurements on nanopowders of zirconia phase-stabilised with MgO and CeO2 are presented.

Characterisation of irradiation-induced defects in ZnO single crystals

Positron annihilation spectroscopy (PAS) combined with optical methods was employed for characterisation of defects in the hydrothermally grown ZnO single crystals irradiated by 167 MeV Xe26+ ions to fluences ranged from 3×1012 to 1×1014 cm-2. The positron lifetime (LT), Doppler broadening as well as slow-positron implantation spectroscopy (SPIS) techniques were involved. The ab-initio theoretical calculations were utilised for interpretation of LT results. The optical transmission and photoluminescence measurements were conducted, too. The virgin ZnO crystal exhibited a single component LT spectrum with a lifetime of 182 ps which is attributed to saturated positron trapping in Zn vacancies associated with hydrogen atoms unintentionally introduced into the crystal during the crystal growth. The Xe ion irradiated ZnO crystals have shown an additional component with a longer lifetime of ≈ 360 ps which comes from irradiation-induced larger defects equivalent in size to clusters of ≈10 to 12 vacancies. The concentrations of these clusters were estimated on the basis of combined LT and SPIS data. The PAS data were correlated with irradiation induced changes seen in the optical spectroscopy experiments.

Structural and magnetic relaxations of mechanically alloyed Fe-Mo

The Fe–Mo sample mechanically alloyed for 250 h under air atmosphere was exposed to a series of isothermal and isochronal treatments with the aim to follow changes in the structure and magnetic properties regarding relaxations of strains and defects and stability of chemical composition. For this purpose x-ray diffraction, positron annihilation, scanning and transmission electron microscopy, and Mossbauer spectrometry were applied and supplemented by magnetic measurements. The temperatures for the magnetic studies were selected from the thermomagnetic curve of the as-prepared sample. The time interval of isothermal treatments was chosen from 0–300 min. The Mo content in the bcc-Fe(Mo) phase has substantially exceeded the equilibrium solubility limit but it has been found to decrease under the thermal treatment which was reflected by decreasing lattice parameters. The small crystallite size of approximately 10 nm in the initial state starts to grow only after a certain amount of strains induced by severe deformation, due to mechanical alloying being released. This was also reflected in the magnetic parameters. From their time dependences at selected temperatures the characteristic relaxation times were obtained and used for a calculation of the activation enthalpy of relaxation processes.

Hydrogen-induced surface modifications in ZnO single crystals

Surface changes in ZnO single crystals electrochemically doped with hydrogen were investigated in this work using slow positron implantation spectroscopy (SPIS) combined with atomic force microscopy (AFM) and optical microscopy. It was found that hexagonally shaped pyramids were formed on the surface of hydrogen-loaded crystals. The formation of these pyramids can be explained by hydrogen-induced plastic deformation realized by a slip in the [0001] direction. Such a picture is supported (i) by AFM where steps of a height comparable with the c-lattice parameter were found at the base of the pyramids, and (ii) by SPIS which revealed a defected subsurface layer, formed by the hydrogen-induced plastic deformation and exhibiting an enhanced concentration of open-volume defects in hydrogen-loaded crystals.

Quenched-in vacancies in Fe-Al alloys

Quenched-in vacancies in Fe-Al alloys with Al content ranging from 24 to 49 at.% were investigated employing two complementary techniques of positron annihilation: slow positron implantation spectroscopy and positron lifetime measurements. It was found that quenched alloys exhibit a very high concentration of vacancies. Although the free positron component cannot be resolved in positron lifetime spectrum in majority of samples, the concentration of quenched-in vacancies can be still determined from the positron diffusion length measured by a variable energy positron beam. The lowest concentration of vacancies was found in a stoichiometric (SM) Fe3Al alloy. The concentration of defects increases with increasing degree of non-stoichiometry with respect to Fe3Al, i.e. in alloys with under-SM and over-SM Al concentration. However, the increase in concentration of quenched-in defects is more pronounced in Al-rich alloys, i.e. alloys containing more than 25 at.% of Al.

Defect in Ultra-Fine Grained Mg-Based Alloys Deformed by High-Pressure Torsion

Applications of Mg-based alloys at elevated temperatures are limited by the low melting point of Mg. This difficulty can be overcome by an addition of rare earth elements. A number of novel promising Mg-based hardenable alloys with high creep resistance at elevated temperatures have been developed, e.g. Mg-Gd, Mg-Mn-Sc etc. Despite the favorable strength and thermal stability, a disadvantage of these alloys consists in a low ductility, which is not sufficient for industrial applications. Grain refinement is known as a way how to improve ductility. It has been demonstrated that an extreme grain size reduction can be achieved by methods based on severe plastic deformation (SPD). In the present work we used high pressure torsion (HPT), which is the most efficient in grain size reduction among the SPD-based techniques, for preparation of selected Mg-based alloys with ultra fine grained (UFG) structure. Microstructure investigations and defect studies of HPT deformed UFG Mg-based alloys are presented in this paper. The extraordinary properties of UFG materials are closely related with defects (grain boundaries, dislocations) introduced by HPT. Positron lifetime (PL) spectroscopy [1] is a well-established non-destructive technique with high sensitivity to open volume defects. It enables identification of the defect types present in the material studied and determination of defect densities. Thus, PL spectroscopy represents an ideal tool for defect studies of UFG materials. In the present work PL spectroscopy was combined with X-ray diffraction (XRD), microhardness measurements, and direct observations of microstructure by TEM.