H.-E. Schaefer

OXYGEN DIFFUSION IN ULTRAFINE GRAINED MONOCLINIC ZRO2

The diffusion of oxygen in ultrafine grained, undoped monoclinic ZrO2 was studied using 18O as tracer and secondary ion mass spectroscopy profiling. Samples with a relative mass density of 97%–99% and average crystallite sizes of 80 or 300 nm were prepared from Zr by sputtering, inert-gas-condensation, oxidation, in situ consolidation of nanocrystalline (n-)ZrO2 powder and subsequent pressureless sintering at 950 or 1050 °C in vacuum. Volume and interface diffusivities were directly determined from the 18O diffusion profiles in n-ZrO2 in the type B and type A regime of interface diffusion. The diffusion of 18O in interfaces in undoped n-ZrO2 is 103–104 times faster than in the bulk of the crystallites throughout the temperature range of 450 to 950 °C studied. These diffusivities are independent of the crystallite size in the range of 70–300 nm. The activation energies QV=2.29 eV and QB=1.95 eV for the volume (QV) and interface diffusion (QB) are considerably higher than the diffusion activation energies f...

Enhanced oxygen diffusivity in interfaces of nanocrystalline ZrO2⋅Y2O3

First measurements of oxygen grain boundary diffusion coefficients in nanocrystalline yttria-doped ZrO2 (n-ZrO2⋅6.9 mol % Y2O3) are presented. The 18O diffusion profiles measured by secondary ion mass spectroscopy are much deeper in the nanocrystalline specimens than in single crystals. An oxygen diffusivity, DB, in the grain boundaries can be deduced, which is ≈3 orders of magnitude higher than in single crystals. From the present data the temperature variation of the oxygen grain boundary diffusivity, DB = 2.0 × 10−5 exp (−0.91 eV/kBT) m2/s, and the oxygen surface exchange coefficient, k = 1.4 × 10−2 exp (−1.13 eV/kBT) m/s, are derived.

Magnetic properties of nanocrystalline nickel

Abstract The ferromagnetic properties of compaction-prepared nanocrystalline Ni specimens, (crystallite size 10 nm) were investigated in order to study the correlation between the disordered interfacial structure and the macroscopic properties. The magnetic moment of the atoms in the interfaces is decreased to 0.34 μB/atom (0.6 μB/atom in bulk Ni) and the Curie temperature Tci=545K of the interfacial components is lower than the value Tcb=630 K of the Ni bulk crystal. Both results are due to open structure of the interfaces and will be discussed within a band model of itinerant ferromagnetism. The temperature variation of the coercivity Hc of the nanocrystalline specimens suggest a model of weak magnetic coupling of single-domain particles with a non-spherical shape.

Shear modulus and internal friction in nanometre-sized polycrystalline palladium

Abstract Nanometre-sized or nanocrystalline materials are polycrystals with a small crystallite size (5-15 nm) and a large fraction of atoms (20-50%) in the disordered interfaces. For the study of the atomic arrangement and the relaxation processes in the interfacial structure, measurements of the shear modulus G and the internal friction Q −1 have been performed on nanocrystalline Pd (n-Pd) at torsional frequencies of about 1 Hz. The reduction in G in comparison with that for coarsegrained Pd is attributed to the increased interatomic distances in the interfaces of n-Pd. For the increase in G at about 400 K, which is ascribed to an irreversible reordering of the interfacial structure, an activation energy of 0·65 eV is found. These re-ordering phenomena are discussed together with recent annealing studies of free volumes by positron annihilation techniques, of the electrical resistivity and of the mass density. The steep increase in the internal friction in n-Pd above 400 K will be tentatively discussed ...

High-temperature atomic defect properties and diffusion processes in intermetallic compounds

Abstract Data on thermal vacancy formation in intermetallic compounds obtained from positron lifetime spectroscopy yield high effective formation enthalpies H F V in close-packed structures and low values in bcc-type structures which can be well understood theoretically. The vacancy migration enthalpy H M V could be determined at high temperatures for B2-FeAl by studying the equilibration process after temperature changes. As demonstrated here in a comparative study on B2-FeAl the thermal formation and migration of defects can also be sensitively investigated by time-differential length-change studies after temperature changes in the vicinity of the equilibration temperatures. The present vacancy data can explain the wide variation of the transition metal self-diffusivities in intermetallic compounds. For B2-FeAl it is shown that the high-temperature mechanical properties are closely linked to the formation of thermal defects as evidenced by the temperature variation of the yield stress anomaly and its time dependence after fast heating.

Thermal Vacancies and High-Temperature Mechanical Properties of FeAl

The formation and the kinetics of migration of thermal vacancies in Fe61Al39 have been studied by positron lifetime spectroscopy. From the temperature variation of the equilibrium vacancy concentration an effective vacancy formation enthalpy of HFV = 0.98 eV has been determined. From the equilibrium of the thermal vacancy concentration after temperature changes the vacancy migration enthalpy (HMV = 1.7 eV) could be derived. As observed recently, vacancy-type defects give rise to an internal friction maximum at 680 K (2 s—1) after quenching of FeAl. This reorientation effect as well as its recovery by long-range migration of defects is characterized by the above vacancy migration enthalpy. For the defects of the 680 K internal-friction maximum, a complex of a vacancy on the Fe sublattice and an antisite atom is suggested. A direct relationship between the high-temperature mechanical properties and atomic defects arises from the time dependence of the yield stress anomaly in Fe60Al40 measured after fast heating, which fits perfectly with the equilibration kinetics of the thermal vacancy concentration. From this we conclude that the yield stress anomaly is sensitively affected by thermal defects.

Self-diffusion in high-density nanocrystalline Fe

Abstract In order to gain insight into the characteristic behavior of grain boundaries in nanocrystalline (n-) materials, high-density n-Fe specimens are prepared by compaction of gas-condensed nanocrystallites at elevated temperatures and the self-diffusion coefficients are measured by radiotracer techniques. The self-diffusion coefficients of n-Fe (relative density higher than 91 %) determined by assuming a type-C kinetics are similar to those extrapolated from high temperature data of conventional grain boundaries, suggesting that the grain boundaries in the high-density n-Fe are similar to those in conventional polycrystalline Fe.

Magnetic properties of high purity nanocrystalline nickel

Abstract High purity nanocrystalline Ni has been prepared by means of the inert gas condensation technique with oxygen contents of less than 0.5 at% compared to 6 at% after exposure to air. In-situ magnetic measurements with the specimens sealed in vials under vacuum indicate that the saturation magnetization of high purity nanocrystalline Ni is unchanged compared to the bulk value but reduced to ∼ 80% of the bulk value in specimens which were exposed to air. The magnetization process in nanocrystalline Ni changes from a homogeneous rotation in powder specimens to domain wall movement in specimens after compaction where magnetic domains are observed by means of magneto-optic Kerr techniques.

Irradiation-induced atomic defects in SiC studied by positron annihilation

The present paper reports on positron lifetime measurements on atomic defects in SiC after low-temperature (80 K) electron irradiation of low (0.47 MeV) and high (2.5 MeV) electron energies and doses from 1.8×1017 to 1.9×1019 e/cm2 as well as after subsequent isochronal annealing up to 1900 K. For these studies the single crystals of nitrogen doped (2–3×1018 cm−3) SiC grown by a modified Lely technique with hexagonal structure (6H polytype) were used.According to the positron lifetime measurements, very different types of vacancy-like positron traps are introducted after irradiation with electrons of either low or high energy. The formation of defect agglomerates and their decay at high temperatures is studied during isochronal annealing and related to earlier studies.

Defects in semiconductors after electron irradiation or in high-temperature thermal equilibrium, as studied by positron annihilation

The authors report on positron lifetime studies which investigate vacancy-type defects in elemental and compound semiconductors after electron irradiation or in thermal equilibrium concentrations at high temperatures. In Si the upper limits of positron lifetimes in monovacancies (272 ps) and divacancies (285 ps) as well as the lifetime in phosphorus-vacancy complexes ( approximately 255 ps) are determined after low-temperature electron irradiation and annealing. In addition, the influence of doping on positron trapping and defect annealing is studied. In electron-irradiated GaAs the annealing between 200 K and 350 K, which is not observed after low-energy irradiation, is ascribed to divacancies or Ga vacancies present in defect complexes. In the annealing processes above 450 K As vacancies disappear. The detection of radiation-induced vacancies is substantially influenced by doping and by the temperature at which measurements are made. In as-grown GaSb a positron lifetime of 253 ps is found, which increases by about 12 ps on low-temperature electron irradiation. The annealing occurs in stages at 200 K and 350-500 K. In Si and Ge the temperature variation of the mean positron lifetime between ambient temperature and the melting points does not exceed a few picoseconds at most. This is in contrast to results in Si recently reported by Dannefaer and co-workers. The present high-temperature results on Si and Ge may be interpreted in terms of a thermal vacancy concentration too low to be detected by positrons. However, an insufficient interaction between positrons and high-temperature vacancies cannot be excluded.