Barbara Szpunar

Electrical resistivity of bulk nanocrystalline nickel

The effect of grain boundaries on the electrical resistivity of bulk nickel with a nanocrystalline structure has been investigated. For this study, nanocrystalline nickel was produced using a pulse plating technique that yields equiaxed nanostructures with negligible porosity. Using the four probe technique, resistivity measurements were performed at various temperatures ranging from 77 K to room temperature. The values of the resistivity were found to increase with decreasing grain size. This is mainly due to the increased volume fraction of interfaces at smaller grain sizes and the associated electron scattering events at the grain boundaries. The temperature coefficient of resistivity was found to decrease with decreasing grain size.

The magnetocrystalline anisotropy of cobalt

Abstract Recent high precision measurements of the uniaxial anisotropy constants K 1 and K 2 are reported taken from three samples grown by different techniques. The results vindicate the early work of Succksmith and Thompson and agree well with the recent measurements of Ono. While some variation was observed between results from the three samples we conclude that these are insufficient to account for the spread of results in the literature and suggest inaccurate shearing corrections as the most probable cause of discrepancy. A very good fit to the K 1 data was obtained using a single ion model and the c/a ratio as an adjustable parameter. These c/a values gave a satisfactory fit to K 2 and also lay within the scatter of the experimentally measured c/a values. However, a definitive test of the single ion theory was not possible due to these experimental uncertainties in c/a . Data for the temperature variation of the basal plane anisotropy constant K 4 are reported for the first time. The single ion model predictions are not in very good agreement with the K 4 data.

Application of the recursion method to calculation of the local moments in Nd2Fe14B

The recursion method has been used to calculate the local magnetic moment on eight different sites in Nd2Fe14B. The origin of the ferromagnetic arrangement between Nd and Fe moments has been explained. The results are discussed in the light of previous calculations for intermetallic compounds.

A new approach to Preisach diagrams

Magnetization processes are complex. A simple Preisach diagram can describe only the irreversible component of magnetization. An extended Preisach model is introduced, including a reversible magnetization component given by a Langevin function which describes low-field experimental hysteresis loops very well. A family of anhysteretic curves are shown that are dependent on the amplitude and form of the ac used to generate them and the turning points on the M-H plane.

Electrodeposited vs. consolidated nanocrystals: differences and similarities

Abstract Differences and similarities of structure-property relationships observed in nanostructured materials prepared by electrodeposition and consolidation of nanocrystalline precursor powders will be discussed. Directionally similar properties observed in both classes of materials, such as initial hardness, strength and electrical resistivity, can be explained in terms of enhanced volume fractions of grain boundaries and triple junctions which are common to both types of structures. However, for properties that show considerable differences such as saturation magnetization, thermal expansion, heat capacity or Youngs modulus, other microstructural characteristics must be considered.

Fuel oxidation and thermal conductivity model for operating defective fuel rods

Abstract A model has been developed to describe the fuel oxidation behaviour, and its influence on the fuel thermal conductivity, in operating defective nuclear fuel rods. The fuel-oxidation model is derived from adsorption theory and considers the influence of the high-pressure environment that results from coolant entry into the fuel-to-clad gap. This model is in agreement with the fuel-oxidation kinetics observed in high-temperature annealing experiments conducted at 1473–1623 K in steam over a range of pressure from 0.001 to 0.1 MPa. Using a Freundlich adsorption isotherm, the current model is also consistent with recent experiments conducted at a higher pressure of 7 MPa. The model also considers radiolytic effects as a consequence of fission fragment bombardment in the fuel-to-clad gap. This treatment suggests that radiolysis-assisted oxidation is insignificant in operating defective rods (as compared to thermal effects), as supported by limited in-reactor data. The effects of diffusion of the interstitial oxygen ions in the solid in the operating rod is further discussed.

Magnetism in nanostructured Ni–P and Co–W alloys

Abstract Spin polarized, linear muffin-tin orbital (LMTO), atomic sphere approximation (ASA) method calculations have been performed to examine the magnetic properties of Ni–P and Co–W alloys. It was found that the average moment decreases linearly with increasing concentrations of the alloying elements (P, W), suggesting that a simple formula derived from the rigid band filling model can explain the observed behavior. Detailed calculations showed that the local moments behave in a more complicated manner when alloying. In particular, the W alloying element has significant negative induced moment. In contrast to rigid band filling model assumptions, a negligible charge transfer was calculated. A comparison with experimental data has shown that the theory adequately describes the effect of chemical disorder (introduced by alloying) on the saturation magnetization of nanocrystalline Ni–P and Co–W alloys. It is further shown that structural disorder (introduced by grain boundaries) in these nanostructured alloys has little effect on their saturation magnetization.

Effect of porosity on the elastic response of brittle materials: An embedded-atom method approach

Abstract The values for Youngs modulus of porous single-crystal Ni in the [100] and [111] directions are computed using the embedded-atom method (EAM). Both pore volume fraction and pore size (defined by ratio S/R of the flaw size to pore radius) are varied. A reduction in Youngs modulus with increasing pore volume fraction and with increasing S/R ratio is observed in the EAM simulations, in good agreement with a recent theoretical model proposed by Krstic and Erickson. A porous Σ = 5, [100] grain boundary also demonstrates a marked reduction in Youngs modulus compared with the pore-free Σ = 5, [100] grain boundary. These results suggest that recent literature values demonstrating greatly reduced Youngs modulus for some nanocrystalline materials (compared with conventional polycrystalline materials) may be a consequence of residual porosity in the material. Poissons ratio is calculated for aligned pores with stress applied in the [100] direction. The crack-opening displacement is qualitatively and qu...

On the origin of the large magnetic anisotropy of rare earth-cobalt compounds

Experimental data on the magnetocrystalline anisotropy in Co, YCo5, GdCo5, SmCo5 and Y2Co17 is analysed using a single-ion crystal field and isotropic exchange interaction. The large magnetic anisotropy at high temperatures in the alloys is due to significant deviations in the alloy lattices of the ratio c/a from the ideal square root (8/3) ratio.

Electronic structure of antiferromagnetic YBa2Cu3O6

Abstract The first spin-polarized local spin density functional calculations have been performed for the antiferromagnetic semiconductor YBa 2 Cu 3 O 6 by means of the linear muffin-tin-orbital atomic sphere approximation method (LMTO-ASA). Our calculations show that for antiferromagnetic ordering of copper magnetic moments in the Cu2-O plane, a Peierls-type gap opens up around the Fermi energy and YBa 2 Cu 3 O 6 becomes semiconducting as observed experimentally. The local moment on the Cu2 atoms is equal to 0.38 μ B while the Cu1 atoms are nonmagnetic.