SUMMARYResults are presented from six micrometeorological studies conducted over a grass turf at Davis, California, in 1066 and 1967. Highly reliable surface drag and evaporation data from very sensitive lysimeters of 6.1 m diameter afforded excellent opportunity to evaluate several parameters important to aerodynamicprediction equations.For the six studies the mean von Kirman constant, k, ranged from 0-40 to 0 4 4 , strongly supporting continued acceptance of k at around 0.42.The Monin-Obukhov (1954) universal 4 '~ function was found to vary as IRij-*/3 under near-free convection, indicating significantly greater diabatic profile effects than suggested in the form of the KEYPS profile as given by Sellers (1965).Empirical relationships providing excellent fit to experimental data for the range -3.5 < Ri < 0.3, were 4~ = (1-16 Ri)-1'3 and $M = (1+16 Ri)1/3 respectively for unstable and stable conditions. For +W corresponding expressions were 4w = .885 (1-22 Ri)-.40 and $w = .885 ( l f 3 4 Ri).40.The ratio Kw/KM showed a systematic drop from 1.13 at neutral to a value around 0.75 under strongly

High temperature superconductors (HTS), especially YBa 2 Cu 3 O 7-x (YBCO), are among the most promising materials for electronic and magnetic applications involving electronic devices and wire technologies. The main challenge in optimizing the critical current density of the HTS is the immobilization of the vortices by means of artificial flux pinning centers. Various methods such as neutron [1] and ion irradiation [2] and mechanical or thermal techniques [3] have been used to introduce effective artificial pinning centers. Chemical doping is also a common way of enhancing critical current density giving rise to formation of nanometer-size second phase particles acting as flux pinning centers throughout the superconductor matrix [4]. For effective flux pinning, the size of the pinning centers should be on the order of the coherence length of the superconductor with a uniform size and spatial distribution. Therefore, the size, spatial distribution and shape of the flux pinning centers have a major role in determining the critical current density of superconductor materials.Previous studies showed that the addition of rare earths (Dy, Ho) increases the critical current density for perpendicular magnetic field application [5][6][7]. In this work, the structural effects of the addition of Dy to a YBCO coated conductor have been studied using a Scanning Transmission Electron Microscope (STEM) to ascertain the reasons for the increase in critical current density. The studied sample consists of a 1 µm thick YBCO layer doped with Dy (50% in molar basis) which is grown by metal-organic deposition process on cube textured NiW(5%) alloy substrate buffered with oxide layers with the structure of CeO 2 /YSZ/Y 2 O 3 /NiW. Energy Dispersive X-ray Spectroscopy analysis showed that the added Dy atoms substitute the Y atoms giving rise to a compound (Y 0.5 Dy 0.5 )Ba 2 Cu 3 O 7 as the matrix phase and Y 1.32 Dy 0.66 Cu 2 O 5 as a second phase particles which can be seen in Fig. 1.In order to understand the enhanced flux pinning, STEM tomography experiments were performed to determine the 3-D morphology and distribution of the particles by collecting the high angle scattered electrons with a dark field detector. Utilization of a high angle annular dark field detector (HAADF) in STEM mode minimizes the diffraction effects resulting in a more accurate tomographic reconstruction compared to conventional TEM. Due to minimal contrast between the matrix and particles image processing techniques were applied to improve the tomographic reconstruction. Tomography studies showed that 25% of the material volume is occupied by 71 particles ranging between 13 nm and 135 nm. It was observed the spatial and size distributions of the particles are not uniform. The number density of particles within the regions with small particles is high, whereas it is low in the regions around the large particles. Additionally, the large particles have

Most industrial catalysts are high-surface-area solids such as amorphous or crystalline oxides, onto which an active component (often clusters of a metal, metal oxide, or metal sulfide, often including high-Z contrast elements) is dispersed in the form of very small clusters or particles [1,2]. So far, metal clusters on support have been restricted almost entirely to group 8 metals but recently supported early transition metal catalysts exemplified by tantalum clusters on SiO 2 have been reported [3]. Catalyst performance is sensitive to cluster size, because the surface structure, electronic properties and cluster-support interactions depend on this size. The location of metal particles and their orientation with respect to the support material are also important in determining catalytic properties. For example, partial coverage of the metal by an amorphous oxide can influence the catalytic activity and selectivity [2].In this investigation, a silica-supported tantalum catalyst was investigated to determine the detailed 3D morphology of the nanoparticles, the degree of encapsulation of the metal cluster by the support, the location of the clusters on the support material, and the size and distribution of the clusters. For this purpose, tomography based on Scanning Transmission Electron Microscopy (STEM) was performed on a 200kV JEOL 2500SE TEM/STEM microscope. Because of the sensitivity of the image to the atomic number, the Z-contrast technique of STEM provides images of the nanoclusters on the oxide support with a high resolution (figure 1). To provide a 3D reconstructed volume, a tilt range of images from -70° to +70° with a 2° increment processed and analyzed with the Composer and Visualizer software package (JEOL Ltd.) [4].Previously reported results characterizing the sample by extended X-ray absorption fine structure (EXAFS) spectroscopy showed an average Ta-Ta coordination number of 4.8 for the clusters that had been treated only in inert atmospheres, with an average Ta-Ta distance corresponding to a chemical bond between the Ta atoms. Accordingly, a preliminary model of the tantalum clusters was suggested to be a single layer (raft) of approximately 40 Ta atoms.However, when the sample was treated in air and characterized by STEM, the tomography results showed that the shapes of the clusters tended to be almost spherical rather than raft-like. Tomography reconstruction results shown in figures 2A and B also show that the nanoclusters are not evenly distributed within the SiO 2 support. Thus, being partially encapsulated, the tantalum nanoparticles would not be expected to show the same catalytic activity as such clusters positioned on the support [5].

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