Rainer Birringer

Grain size-dependent electrical conductivity of polycrystalline cerium oxide. I. Experiments

The electrical conductivity of polycrystalline cerium oxide was investigated in the nanometer and micrometer size range. Nanocrystalline samples of different grain size were prepared by uniaxial hot-pressing of nanocrystalline powder at various temperatures and pressures. Additional annealing at high temperatures was employed in order to obtain microcrystalline samples. An equivalent-circuit analysis of ac-impedance spectra based on the brick-layer model was performed and the apparent bulk conductivity determined. The effect of a variation in temperature or oxygen partial pressure revealed the rather different nature of the electrical transport properties in the nano- and microcrystalline materials. Nanocrystalline cerium oxide exhibited electronic conductivity under conditions at which microcrystalline samples showed impurity-controlled ionic conductivity. The electronic conductivity of nanocrystalline samples was larger than the intrinsic electronic conductivity of pure single crystalline cerium oxide and was increasing with decreasing grain size. The experimental results were analyzed according to the defect chemistry of cerium oxide and consequences of a space charge effect on the partial electronic and ionic conductivity in polycrystalline cerium oxide will be discussed.

Grain Size Dependence of Electrical Conductivity in Polycrystalline Cerium Oxide

The magnitude and activation energy of electrical conductivity in nanocrystalline cerium oxide exhibit a clear grain size dependence. Experimental results compiled from the literature were analyzed using a space charge model, which takes into account the deviation of point defect concentrations from their bulk values in the vicinity of grain boundaries. The consequences on conductivity arising from such space charge layers were calculated using the brick-layer model (BLM) for grain sizes L large compared to the screening length λ. The obtained results were supplemented by the calculated conductivity in the flat-band limit for L ≪ λ. This combination allowed for a quantitative comparison with experimental values, which were obtained in the mesoscopic regime of grain sizes from 10–40 nm. The analysis yielded a value for the space charge potential in cerium oxide of 0.55 V. This space charge potential is caused by a reduced standard chemical potential of oxygen vacancies in the grain boundary core as compared to the bulk phase.

Nanometre-sized solids, their structure and properties☆

Abstract Nanometre-sized or nanocrystalline materials are solids with very small grain size (5–10 nm) and, therefore, a considerable fraction of atoms located in the disordered interfacial structure, giving rise to novel physical properties and potential technological applications. X-ray diffraction, Mossbauer spectroscopy and positron lifetime measurements indicate a wide interatomic distance distribution in the interfacial structure. The thermal, elastic and magnetic properties, the internal friction behaviour, as well as the high diffusivities and reactivities are discussed in terms of the present results on nanocrystalline metals, semiconductors and metal oxides. Some future prospects are outlined.

Rotational diffusion of magnetic nickel nanorods in colloidal dispersions

Colloidal dispersions of Ni nanorods were synthesized by pulsed electrodeposition of Ni into nanoporous aluminum oxide layers followed by dissolution of the templates. Geometrical characterization of the nanorods by transmission electron microscopy and scanning electron microscopy allowed us to determine the average length (100-250 nm) and diameter (20-40 nm) of the rods and to estimate the thickness of the polyvinylpyrrolidone surfactant layer. Due to their acicular shape, nanorods of the given size are uniaxial ferromagnetic single domain particles and exhibit a distinct anisotropic polarizability. These two characteristic properties are the physical basis for magnetic field-dependent optical transmission and allow us to investigate the rotational diffusion of the nanorods in liquid dispersion. In the present study, we employed AC magnetization measurements, dynamical light scattering and optical transmission measurements in a rotating magnetic field to determine the rotational diffusion coefficient. The results from all three methods were consistent and agree with theory within a factor of 2.

Magnetic-field-dependent optical transmission of nickel nanorod colloidal dispersions

Aqueous dispersions of nickel nanorods, ≈13 nm in diameter and 40–160 nm in length, were synthesized using ac electrodeposition into porous alumina templates. The nanorods in suspension can be aligned by modest magnetic fields, which leads to a change in the optical transmittance of the dispersion. Optical transmission measurements with polarized and unpolarized light as a function of magnetic field were performed on suspensions of different particle concentration and varying aspect ratio of the nanoparticles. The experimental results were compared with a theoretical model in which the optical absorption of the nanorods is calculated from the polarizability of prolate ellipsoids in the quasistatic approximation. The magnetic field dependence is introduced in terms of the static orientational distribution function of magnetic moments in an external field. In addition, the relaxation dynamics of the optical transmission was studied, which allowed us to determine the rotational diffusion coefficient of the n...

Quantitative Evaluation of Elastic Properties of Nano-Crystalline Nickel Using Atomic Force Acoustic Microscopy

Atomic force acoustic microscopy (AFAM) is a near-field technique, where the vibration behavior of a micro-fabricated elastic cantilever beam in contact with a sample surface is sensitive to its local elastic properties. The resolution of this technique is given by the contact radius ac of the atomic force microscope sensor-tip on the sample surface. Taking into account only the Hertzian forces, ac depends on the static load applied by the cantilever, on the elastic constants of the tip and the sample and on the geometry of the contacting bodies. The shape of the sensor tip used in atomic force acoustic microscopy is between a sphere and a flat punch. Hence ac extends from just below 10nm to a few tens of nanometers. In this review, we give an overview of the AFAM technique, present data on the indentation moduli of nanocrystalline nickel, and discuss some of the error sources in quantitative AFAM. The AFAM indentation moduli measured are comparable to the values obtained by nanoindentation and lower than the indentation moduli calculated from ultrasonic velocity measurements. There seems to be a decrease of the indentation modulus with decreasing grain size for grain sizes below 30nm. The data are discussed taking into account X-ray diffraction and electron back-scattering data revealing some texture and macro-strain due to internal stresses in the samples investigated.

Elastic and plastic behavior of submicrometer-sized polycrystalline NiAl

The elastic properties and flow behavior of Ni-48.4Al have been characterized by means of the miniaturized disk-bend test (MDBT), Vickers indentation and measurements of the longitudinal and transverse velocities of sound. Youngs modulus, shear modulus, yield stress and fracture stress have been determined as a function of grain size ranging from 30 nm to 2 μm. It has been found that nanometer-sized polycrystalline Ni-48.4Al is essentially brittle at room temperature and 700 K. Elastic moduli obtained suggest no elastic weakness of interfaces. Nanometer-sized Ni-48.4Al samples appear to behave as ceramic material.

Formation of nanocrystalline B2-structured (Ru,Ni)Al in the ternary Ru–Al–Ni system by mechanical alloying and its thermal stability

Abstract B2-structured single-phase (Ru,Ni)Al is synthesized directly from the three components by mechanical alloying. Complete solubility between pseudo-binary RuAl–NiAl is realized in a range between 10 and 25 at.% Ni, while the range for Al remains at 50 at.%. B2-structured (Ru,Ni)Al is formed by an abrupt reaction at the beginning of milling and a certain quantity of Ru was found to remain in all three compositions after the reaction. Further alloying and reaction between Ru and Al sticking on the surfaces of milling tools require a much longer time and finally result in single-phase B2-structured (Ru,Ni)Al. The alloying element Ni is found to improve the sluggish reactivity of Ru by promoting the abrupt reaction. No decomposition process has been observed for the as-milled single-phase (Ru,Ni)Al either after high-temperature XRD or after isothermal annealing (1273 K), demonstrating the complete mutual solubility between pseudo-binary RuAl and NiAl. The structural evolution of as-milled materials includes reordering, strain relaxation and grain growth. The grain size of the as-milled (Ru,Ni)Al after exposure to elevated temperatures (up to 1273 K) turns out to be less than 35 nm and reflects its strong thermal stability.

Magnetization reversal in Nd-Fe-B based nanocomposites as seen by magnetic small-angle neutron scattering

We have studied the magnetization-reversal process of a Nd2Fe14B/Fe3B nanocomposite using small-angle neutron scattering. Based on the computation of the autocorrelation function of the spin misalignment, we have estimated the characteristic size lC of spin inhomogeneities around the Nd2Fe14B nanoparticles. The quantity lC approaches a constant value of about 12.5 nm (∼average Nd2Fe14B particle radius) at 14 T and takes on a maximum value of about 18.5 nm at the coercive field of −0.55 T. The field dependence of lC can be described by a model that takes into account the convolution relationship between the nuclear and the magnetic microstructure.

Synthesis of nano-RuAl by mechanical alloying

Abstract Single phase RuAl has been synthesised directly by an abrupt reaction during mechanical alloying (MA). The structural evolution during MA and subsequent thermal stability of as- milled powders have been analysed by thermal analysis (DSC) and isothermal annealing. The results indicate that there are two stages of alloying and reaction between Ru and Al in MA before single phase RuAl is obtained under the present milling conditions, and that the as-milled single phase RuAl undergoes reordering, strain relaxation and grain growth at high temperatures. The grain size of 20–40 nm after annealing at 1073 K for 0.5 h shows a strong stability of the nano-grained RuAl.