Scanning Tunneling Microscope Imaging of Cu on Cu(111) Yields Estimates of Surface-Mediated Interaction Potentials
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P.N. Kumta, professor of materials science and engineering at CMU, said, “We chose TiN because of its electrical conductivity and mechanical strength, combined with electrochemical inertness to Li and chemical inertness to both Li and Si. It seemed to be a very promising material, but of course we first had to find a suitable synthesis process for composites.” His group explored high-energy mechanical milling of Si and TiN, as this technique can generate amorphous, metastable, and nanophase structures. Preliminary experiments showed that a 1:2 molar ratio of Si:TiN exhibited the best electrochemical properties. X-ray diffraction in combination with energydispersive x-ray analysis demonstrated that the composites consisted of nanocrystalline TiN with amorphous Si nanoparticles homogeneously distributed throughout the matrix after as little as 6 h of milling time. Increased milling times led to a decrease in the initial capacity of the composites. “We are not sure yet what causes this
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decrease,” said Il-seok Kim of CMU. “It could be because the Si nanoparticles are embedded or enclosed by TiN during milling, which would prevent their reaction with Li.” Composites obtained after milling for 12 h exhibited stable capacities of 300 mAh/g, indicating that a large fraction of Si is inactive (~44%). Although the gravimetric capacity is lower than that of conventional carbon, this value corresponds to a 30% higher volumetric capacity, reflecting the promising nature of the composites. Cycling data showed an irreversible capacity loss (~30%) during the first cycle. “We are still trying to understand the exact reason for this loss. It could be due to the formation of a Li-containing passivation layer or surface oxidation,” said G.E. Blomgren. No cracks were present on the electrode surfaces after 30 cycles, and no changes in morphology were observed, indicating good microstructural stability. Kumta said, “We have shown that Si/TiN nanocomposites are promising candidates as anodes for Li-ion batteries.
But there is still a lot of optimization that needs to be done. We are currently working on detailed structural and electrochemical studies.” CORA LIND
Scanning Tunneling Microscope Imaging of Cu on Cu(111) Yields Estimates of Surface-Mediated Interaction Potentials A quantitative study of the long-range interaction between single copper adatoms on Cu(111) reveals significant changes in the growth of Cu/Cu(111) at low temperatures. As reported in the October 2 issue of Physical Review Letters, an international research team from Berlin University and the Paul Drude Institute for Solid-State Electronics in Berlin, Germany, and Chalmers University of Technology and Göteborg University in Sweden have experimented with a long-range interaction mediated by a two-dimensional nearly free electron gas. Their experiments were performed with a scanning tunneling microscope (STM) operated at low temper-
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ature (9–21 K) using electrochemically etched tungsten wires for analyzing tips
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