Institute of Materials Physics

1 Introduction Yttria-stabilized zirconia (YSZ) are used in a wide range of industrial applications, e.g. as solid electrolytes in gas sensors or solid oxide fuel cells. This is because of their high oxygen conductivity due to the high concentration of oxygen vacancies for charge compensation of the sub-valent yttrium cations and their good mechanical strength .

The 95Zr cation self-diffusivity in fully-dense, nanocrystalline cubic ZrO2·9.5 mol% Y2O3 (n-YSZ, mean grain size 50 nm) has been studied by tracer self-diffusion and precision-grinding sectioning. In the temperature range of 897 °C to 937 °C the 95Zr diffusion coefficient in grain boundaries was found to be about 7 orders of magnitude higher than in single crystals of similar composition. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stable suspensions of pentacene functionalised ZrO2 nano-particles were synthesised using a microwave plasma process. The particles were dispersed in-situ in ethylene glycol. The formation of coated particles with small cores and a well defined size in the range of 3–5 nm was shown by X-ray diffraction. In difference to resublimed pure pentacene, suspensions of the coated nano-particles remained stable for weeks, as confirmed by the observation of a small aggregate size in dynamic light scattering. Thin films of the particles on Si based substrates were obtained by drop-casting. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

1 Introduction Yttria-stabilized zirconia (YSZ) are used in a wide range of industrial applications, e.g. as solid electrolytes in gas sensors or solid oxide fuel cells. This is because of their high oxygen conductivity due to the high concentration of oxygen vacancies for charge compensation of the sub-valent yttrium cations and their good mechanical strength .

The 95Zr cation self-diffusivity in fully-dense, nanocrystalline cubic ZrO2·9.5 mol% Y2O3 (n-YSZ, mean grain size 50 nm) has been studied by tracer self-diffusion and precision-grinding sectioning. In the temperature range of 897 °C to 937 °C the 95Zr diffusion coefficient in grain boundaries was found to be about 7 orders of magnitude higher than in single crystals of similar composition. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Stable suspensions of pentacene functionalised ZrO2 nano-particles were synthesised using a microwave plasma process. The particles were dispersed in-situ in ethylene glycol. The formation of coated particles with small cores and a well defined size in the range of 3–5 nm was shown by X-ray diffraction. In difference to resublimed pure pentacene, suspensions of the coated nano-particles remained stable for weeks, as confirmed by the observation of a small aggregate size in dynamic light scattering. Thin films of the particles on Si based substrates were obtained by drop-casting. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanocrystalline yttria-stabilized zirconia (YSZ) powder and compacts have been successfully synthesized by microwave plasma synthesis co-evaporating Y and Zr precursors from a single source. Both liquid and solid mixtures of chemically homologous precursors were tested. Electron micrographs and X-ray diffraction studies reveal small crystallites in the range 3–4 nm with a narrow size distribution. Crystallite sizes smaller than 25 nm are preserved even after subsequent annealing at temperatures up to 950 °C. A comparison with other gas-phase synthesis routes of nanocrystalline YSZ is given. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We studied the hyperfine interactions of 181 Ta in In 2 O 3 by means of perturbed-angular-correlation ͑PAC͒ measurements. We prepared thin films of indium sesquioxide with different degrees of initial amorphism and implanted them with 181 Hf. Chemically prepared indium-sesquioxide powder samples were also made starting from neutron-irradiated HfCl 4 , which provides the 181 Hf PAC probes. PAC experiments were performed on each sample at room temperature, after each step of annealing programs at increasing temperatures up to the full crystallization of the samples. The results indicate that the PAC probe occupies preferentially the axially symmetric cation site. Point-charge-model calculations were performed. The calculated asymmetry parameters were compared with those obtained in 181 Hf PAC experiments performed also on other binary oxides,

Electric field-induced tuning of material properties is usually restricted to nonmetals such as semiconductors and piezoelectric ceramics. We show that variations of the electrical resistance of a metal (Pt) in the range of several percent can be reversibly induced at low charging voltages making use of a nanocrystallite-electrolyte composite. The charge-induced resistance variation is analyzed taking into account the modification of the charge carrier density and scattering rate by surface charging. The contribution arising from the charge-induced variation of the lattice constant is found to be small. Materials, which change their macroscopic properties, such as the length or electrical conductivity, upon applying an electric field are the focus of basic research and applications. Field-induced tuning of material properties is usually restricted to nonmetals such as semiconductors and piezoelectric ceramics. In bulk metals, no charge-induced changes of such properties are observed. Here, screening due to the high density of conduction electrons limits the extent of space charge regions to the length scale of a few atomic layers.

The formation of thermal vacancies, which is a key issue with respect to the high-temperature properties of ordered intermetallic compounds, was studied in y-TiAl by means of positron lifetime spectroscopy between ambient temperature and 1400 K. An estimate of the atomic concentration of thermal vacancies yields a value similar as in pure fcc metals and an effective vacancy formation enthalpy of 1.41+0.06 eV in good agreement with nearest-neighbor-bond model calculations. A comparison of these data with the results of recent self-diffusion studies suggests a low mobility of vacancies as typical for ordered intermetallic compounds. No evidence for structural vacancies at ambient temperature could be observed.

The diffusion of oxygen in nanocrystalline, undoped monoclinic ZrO2 was studied using "0 tracer and SIMS profiling. Samples with a mass density of 97 to 99 % and average crystallite sizes of 80 nm and 300 nm were prepared from Zr-metal by DC sputtering, crystallite condensation in an inert-gas atmosphere, oxidation, in-situ consolidation of the n-Zr02 powder and subsequent pressureless sintering at 950 or 1050 'C in vacuum. Both volume (V) and interface (B) d'ff a usivities were determined from the depth profiles in the type B regime. The activation energies Qv = 2.29 eV and QB = 1.95 eV are considerably higher than the values found in Ca-or Y-stabilized ZrOa. No influence of the crystallize size was observed. 01999 Acta Metallurgica Inc.

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Oxide nanoparticles were used as carrier for organic semiconductor materials. Stable suspensions of ZrO 2 nanoparticles coated with anthracene, pentacene, or para-hexaphenyl were obtained by microwave plasma synthesis of ZrO 2 cores, subsequent in situ coating with organic compounds, and in situ dispersion in ethylene glycol. Powders of coated oxide nanoparticles were synthesized for comparison. The successful coating and a small uniform size distribution of the ZrO 2 cores were confirmed by comprehensive characterization including photoluminescence, absorption spectroscopy, electron microscopy, electron energy loss spectroscopy, mass spectrometry, and X-ray diffraction. Powder compacts of anthracenecoated ZrO 2 particles showed good air stability and a significant blue shift accompanied by an attenuation of the emission lines at higher vibronic orders in comparison to samples of pure anthracene as received. For para-hexaphenyl-coated nanoparticles, the same photoluminescence characteristics are observed as for pure para-hexaphenyl. In the case of pentacene-coated nanoparticles indication for degradation is found.

Surface effects in zirconium dioxide (ZrO 2) coated manganese ferrite (MnFe 2 O 4) nanoparticles have been studied by using dc and ac magnetization. The average crystallite size of MnFe 2 O 4 and ZrO 2 phase was about 9 and 4 nm, respectively as determined by Debye-Scherrer's formula. TEM images revealed that the nanoparticles are spherical in shape with less agglomeration. Selected area electron diffraction analysis shows two different crystalline species such as nanoparticle's core MnFe 2 O 4 and coating ZrO 2 , which was in agreement with the XRD analysis. Effective anisotropy constant (K eff = 3 × 10 04 erg/cm 3) as deduced from simulated ZFC/FC curves is close to bulk value (K bulk = 2.5 × 10 04 erg/cm 3) which is due to weak contribution of surface anisotropy. Saturation magnetization showed an increasing trend at low temperatures (more pronounced below 50 K) which is again due to reduced surface spins disorder. Temperature dependent coercivity revealed a sharp increase at 5 K, which is due to the surface spins freezing. The Arrhenius law fit to frequency shift of T B in ac susceptibility revealed weak interparticle interactions. The nanoparticles showed slow spin relaxation in ZFC protocol which signify the presence of disorder in our nanoparticles, however the value of shape parameter β lies outside the spin-glass regime. In summary, non-magnetic ZrO 2 coating on these fine MnFe 2 O 4 nanoparticles reduces their surface energy, magnetic interparticle interactions and surface effects, which are not sufficient to establish a spin-glass behaviour.

Effect of surface spins in chromium oxide (Cr 2 O 3) coated maghemite (γ-Fe 2 O 3) nanoparticles (13 nm) as prepared by microwave plasma technique have been studied in detail. The temperature dependent zero field cooled/field cooled (ZFC/FC) measurements revealed the blocking temperature at T B = 75 K. Simulated ZFC/FC curves exhibited large value of effective anisotropy of Cr 2 O 3 coated γ-Fe 2 O 3 nanoparticles as compared to bulk γ-Fe 2 O 3 but less than bare γ-Fe 2 O 3 nanoparticles. Bloch's law was fitted on M S-T data and revealed the values of Bloch's constant B = 3.523 × 10 −4 K −b and Bloch's exponent b = 1.10. The higher value of B than in bulk is due to weaker exchange coupling J (B ̴ 1/J) on the surface of nanoparticle due to disorder surface spins, while lower value of b is due to no spin wave excitation in presence of large energy band gap at nanoscale. Kneller's law fit on H C-T data deviated in all temperature range which is due to strong surface anisotropy, core-shell interactions and superparamagnetism. Interparticle interactions and spin glass behavior were investigated by using different physical laws for f-dependent ac susceptibility and they confirmed the presence of spin glass behavior which is due to disordered frozen surface spins and random interparticle interactions.

In this work we report the microstructural characterization and the magnetic properties of neodymium ferrites (NdFe 2 O 4) nanorods prepared by well controlled co-precipitation method. The effect of annealing temperature on the structure, morphology and magnetic properties of NdFe 2 O 4 has been investigated. The transmission electron microscopy (TEM) observations revealed that the as-prepared nanoparticles have rods-like shape with the average diameter ranging from 5 to 14 nm and uniform length. The magnetic measurements show that the as-synthesized nanorods have a superparamagnetic behavior at room temperature, with a blocking temperature of 360 K and magnetic anisotropy constant of 2.8 Â 10 5 ergs/ cm 3. The magnetization and coercitivity at room temperature are increased from 26 to 34 emu/g and from 151 to 171 Oe with increasing annealing temperature from 400 to 600°C, respectively.