Huaping Sheng

Rapid doubling of the critical current of YBa2Cu3O7-δ coated conductors for viable high-speed industrial processing

We demonstrate that 3.5-MeV oxygen irradiation can markedly enhance the in-field critical current of commercial second generation superconducting tapes with an exposure time of just 1 s per 0.8 cm2. The speed demonstrated here is now at the level required for an industrial reel-to-reel post-processing. The irradiation is made on production line samples through the protective silver coating and does not require any modification of the growth process. From TEM imaging, we identify small clusters as the main source of increased vortex pinning.

Interactions of titania based nanoparticles with silica and green-tea: Photo-degradation and -luminescence

An effective way to modify the photocatalytic activity of both anatase and rutile TiO2 nanoparticles by coating the surface with either an inorganic (SiO2/silica) or organic (green-tea) layer using a chemical approach is demonstrated. Tetraethyl orthosilicate (TEOS) was used to cover the surface of TiO2 with silica which facilitates the inhibition of photocatalytic activity, ensuring its application in sunscreens by blocking the reactive oxygen species (ROS). Green-tea extract, rich in epigallocatechin gallate (EGCG), was used to coat/stabilize nano-sized TiO2. The morphology of these coatings was revealed by transmission electron microscopy (TEM) and by energy dispersive spectroscopy (EDS) mapping. These studies showed good coverage for both types of coating, but with somewhat better uniformity of the coating layer on rutile TiO2 compared to anatase due to its more uniform particle geometry. The effectiveness of each coating was evaluated by photodegradation of an organic dye (methyl orange). These studies showed rutile_polyphenol exhibits the highest photocatalytic activity among rutile forms which validates its feasibility to be used in photodegradation.

Decoupling and tuning competing effects of different types of defects on flux creep in irradiated YBa2Cu3O7-δ coated conductors

YBa2Cu3O7−δ coated conductors (CCs) have achieved high critical current densities (J c) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase J c and reduce creep. CCs grown by metal organic deposition show non-monotonic changes in the temperature-dependent creep rate, S(T). Notably, in low fields, there is a conspicuous dip to low S as the temperature (T) increases from ~20 to ~65 K. Oxygen-, proton-, and Au-irradiation substantially increase S in this temperature range. Focusing on an oxygen-irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study S(T) as we tune the relative density of point defects to larger defects by annealing both an as-grown and an irradiated CC in O2 at temperatures T A = 250 °C–600 °C. We observe a steady decrease in S(T > 20 K) with increasing T A, unveiling the role of pre-existing nanoparticle precipitates in creating the dip in S(T) and point defects and clusters in increasing S at intermediate temperatures.

Fractal growth of platinum electrodeposits revealed by in situ electron microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth – either dense branching or ramified islands – are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.

Microstructural Evolution in Transition-metal-oxide Cathode Materials for Lithium-Ion Batteries

Layered transition metal oxides are promising materials for high-energy lithium-ion battery cathodes. These materials offer high capacity and rate capability, good safety, and relatively low cost compared to many alternative materials [1]. Mixed oxides such as NCA (LiNi0.8Co0.15Al0.05O2) exhibit high capacity but suffer from a significant capacity fade with cycling [2]. The composition of alternatives such as NMC (LiNi1-x-yMnxCoyO2) can be tuned to show less capacity fade, but generally at the cost of lower capacity. Consequently, there is great interest in cycling these materials to higher potentials for more energy, but then cycling performance decreases.

Dynamic Nanobubbles in Graphene Liquid Cell under Electron Beam Irradiation

The use of graphene windows in liquid cells for in situ electron microscopy has become popular because the single atom thickness, extraordinary mechanical strength and high conductivity of graphene allows the study of liquid in the confined environment with atomic resolution. Such liquid cells are commonly used for in situ observation of nanoparticle growth in liquid at the atomic scale using transmission electron microscopy (TEM). These studies improve our understanding of the initial growth mechanisms and future design of nanomaterials. However, the electron beam generates local heating or irradiation and can strongly influence the growth process [1]. At higher dose rates, electron beam irradiation can lead to liquid decomposition, ionization, vapor generation and even nanobubble formation that can influence the reliability of the in situ observation [2]. In addition, the hydrophic surface of graphene may influence these processes.

Modulating the Redox Equilibrium of Silver Using Electron Beams

1. Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439 2. School of Physics and Technology, Center for Electron Microscopy, MOE Key Laboratory of Artificial Microand Nano-structures, and Institute for Advanced Studies, Wuhan University, Wuhan 430072, China 3. Science and Technology on High Strength Structural Materials Laboratory, Central South University, Changsha 410083, China

Atomic Surface Structures of Oxide Nanoparticles with Well-defined Shapes

Recent studies have shown that catalytic activities can be tuned by controlling the shape of nanoparticles such as SrTiO3, CeO2, and Co3O4 [1]. Therefore, determination of surface structure is very important to understand structure-property relationships for these oxide nanoparticles. The Argonne Chromatic-corrected TEM (ACAT) has an image corrector that corrects both spherical (Cs) and chromatic aberration (Cc). Cc correction allows the correction of Cs towards zero to improve resolution without compromising contrast. Using this unique feature, we correct both Cs and Cc to small values to achieve direct structure interpretable HREM images including oxygen atomic columns. In this study, atomic surface structures of SrTiO3, CeO2, Co3O4 nanocubes are observed by using aberration-corrected HREM.

Fractal Growth of Platinum Electrodeposits Revealed by in situ Electron Microscopy

Fractals are commonly observed in nature and elucidating the mechanisms of fractal-related growth is a compelling issue for both fundamental science and technology. Here we report an in situ electron microscopy study of dynamic fractal growth of platinum during electrodeposition in a miniaturized electrochemical cell at varying growth conditions. Highly dendritic growth - either dense branching or ramified islands - are formed at the solid-electrolyte interface. We show how the diffusion length of ions in the electrolyte influences morphology selection and how instability induced by initial surface roughness, combined with local enhancement of electric field, gives rise to non-uniform branched deposition as a result of nucleation/growth at preferred locations. Comparing the growth behavior under these different conditions provides new insight into the fundamental mechanisms of platinum nucleation.