High-quality single crystals of ZnO in the as-grown and ${\mathrm{N}}^{+}$ ion-implanted states have been investigated using a combination of three experimental techniques—namely, positron lifetime/slow positron implantation spectroscopy accompanied by theoretical calculations of the positron lifetime for selected defects, temperature-dependent Hall (TDH) measurements, and deep level transient spectroscopy (DLTS). The positron lifetime in bulk ZnO is measured to be $(151\pm 2)\mathrm{ps}$ and that for positrons trapped in defects $(257\pm 2)\mathrm{ps}$. On the basis of theoretical calculations the latter is attributed to $\mathrm{Zn}+\mathrm{O}$ divacancies, existing in the sample in neutral charge state, and not to the Zn vacancy proposed in previous experimental work. Their concentration is estimated to be $3.7\times {10}^{17}{\mathrm{cm}}^{-3}$. From TDH measurements the existence of negatively charged intrinsic defects acting as compensating acceptors is concluded which are invisible to positrons—maybe interstitial oxygen. This view is supported from TDH results in combination with DLTS which revealed the creation of the defect $E1$, and an increase in concentration of the defect $E3$ after ${\mathrm{N}}^{+}$ ion implantation, and peculiarities in the observation of the defect $E4$.

A systematic study of various, nominally undoped ZnO single crystals, either hydrothermally grown (HTG) or melt grown (MG), has been performed. The crystal quality has been assessed by x-ray diffraction, and a comprehensive estimation of the detailed impurity and hydrogen contents by inductively coupled plasma mass spectrometry and nuclear reaction analysis, respectively, has been made also. High precision positron lifetime experiments show that a single positron lifetime is observed in all crystals investigated, which clusters at 180–182 ps and 165–167 ps for HTG and MG crystals, respectively. Furthermore, hydrogen is detected in all crystals in a bound state with a high concentration (at least $0.3\text{at}\text{.}\%$), whereas the concentrations of other impurities are very small. From ab initio calculations it is suggested that the existence of Zn-vacancy–hydrogen complexes is the most natural explanation for the given experimental facts at present. Furthermore, the distribution of H at a metal/ZnO interface of a MG crystal, and the H content of a HTG crystal upon annealing and time afterward has been monitored, as this is most probably related to the properties of electrical contacts made at ZnO and the instability in $p$-type conductivity observed at ZnO nanorods in literature. All experimental findings and presented theoretical considerations support the conclusion that various types of Zn-vacancy–hydrogen complexes exist in ZnO and need to be taken into account in future studies, especially for HTG materials.

Thermal stability of ultrafine grained (mean grain size 150 nm) copper prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy correlated with transmission electron microscopy. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high number of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Calculations of the lifetime of a positron trapped at a microvoid as a function of its size were performed to obtain information about sizes of the microvoids. Abnormal grain growth, when isolated recrystallized grains grow inside the deformed matrix, takes place from $160°\mathrm{C}.$ From 280 to $400°\mathrm{C}$ recrystallization occurs. Strongly inhomogeneous spatial distribution of defects does not allow application of the simple trapping model. Therefore a model of positron behavior in the ultrafine grained materials was developed in the present work. The model takes into account inhomogeneous spatial distribution of defects and allows for determination of dislocation density, concentration of microvoids, linear size of coherent domains, and volume fraction of the distorted regions. Moreover using this model it was possible to determine the activation energy of the recrystallization.

First-principles electronic structure and positron-state calculations for perfect and defected 3C- and 6H-SiC polytypes of SiC have been performed. Monovacancies and divacancies have been treated; the influence of lattice position and nitrogen impurities have been considered in the former case. Positron affinities and binding energies have been calculated; trends are discussed, and the results compared with recent atomic superposition method calculations. Experimental determination of the electron and positron work functions of the same 6H-SiC allows an assessment of the accuracy of the present first-principles calculations, and to suggest further improvements. © 1996 The American Physical Society.

Refractory high entropy alloys (HEA) are promising materials for high temperature applications. This work presents investigations of the room temperature tensile mechanical properties of selected 3 and 4 elements medium entropy alloys (MEA) derived from the HfNbTaTiZr system. Tensile testing was combined with fractographic and microstructure analysis, using scanning electron microscope (SEM), wavelength dispersive spectroscope (WDS) and X-Ray powder diffraction (XRD). The 5 element HEA alloy HfNbTaTiZr exhibits the best combination of strength and elongation while 4 and 3 element MEAs have lower strength. Some of them are ductile, some of them brittle, depending on microstructure. Simultaneous presence of Ta and Zr in the alloy resulted in a significant reduction of ductility caused by reduction of the BCC phase content. Precipitation of Ta rich particles on grain boundaries reduces further the maximum elongation to failure down to zero values.

The main objective of the present paper is to communicate a study of defects behavior in zirconia-based nanomaterials\char22{}pressure-compacted yttria-stabilized zirconia (YSZ) nanopowders with different contents of Y2O3 and ceramics obtained by sintering the YZS nanopowders. In addition, YZS single crystals were also investigated. Positron annihilation techniques including positron lifetime and coincidence Doppler broadening with a conventional positron source and Doppler broadening experiments on a monoenergetic positron beam were involved in this study as the principal tools. These techniques were supplemented with transmission electron microscopy and x-ray diffraction observations. In order to get better support of the experimental data interpretation, the state-of-art theoretical calculations of positron parameters were performed for the perfect ZrO2 lattice and selected defect configurations in the YSZ. Theoretical calculations have indicated that neither the oxygen vacancies nor their neutral complexes with substitutional yttrium atoms are capable of positron trapping. On the other hand, the zirconium vacancies are deep positron traps and obviously are responsible for the saturated positron trapping observed in the YSZ single crystals. In the compacted YSZ nanopowders, a majority of positrons is trapped either in the vacancylike defects situated in the negative space-charge layers along grain boundaries ($\tau${}1$\approx${}185$\hskip0.3em${}ps) or in vacancy clusters at intersections of grain boundaries ($\tau${}2$\approx${}370$\hskip0.3em${}ps). The intensity ratio I2/I1 was found to be correlated with the mean grain size d as I2/I1$\sim${}d$-${}2. A small fraction of positrons ($\approx${}10%) form positronium in large pores ($\tau${}3$\approx${}2$\hskip0.3em${}ns,$\hskip0.3em${}$\tau${}4$\approx${}30$\hskip0.3em${}ns). A significant grain growth during sintering of the YSZ nanopowders above 1000\,\ifmmode^\circ\else\textdegree\fi{}C was observed.