Miniature Fuel Cell Fabricated Using Microelectronic Techniques Displayed a Porous Si Layer as a Catalyst Support Layer
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Figure 1. Integrated circuit for cavity quantum electrodynamics: false color electron micrograph of a Cooper pair box (blue) fabricated onto the Si substrate (green) into the gap between the center conductor (top) and the ground plane (bottom) of a resonator (beige) using electron beam lithography and double angle evaporation of aluminum. The Josephson tunnel junctions are formed at the overlap between the long thin island parallel to the center conductor and the fingers extending from the much larger reservoir coupled to the ground plane. Reprinted with permission from Nature 431 (September 9, 2004) p. 163. ©2004 Nature Publishing Group.
long and 50 nN with minimal heating. As described in the October 4 issue of Applied Physics Letters (p. 2968), Matthews and co-workers have developed a novel electropolishing technique to create micromagnetic pole tips for controlled manipulation, probing, and positioning of magnetic particles. Their apparatus consists of multiple loops of insulated electromagnetic wire coiled around a permalloy magnetic core (1 mm diameter). Copper wire (50 µm diameter, 44 gauge) was wound around the magnetic core. Typical electromagnets in this study had 2000 turns of wire, a resistance of 16 Ohms, an inductance of 1.4 mH, and a capacitance of less than 2 pF. The core and electromagnet wires were housed within a temperature-regulated water flow chamber. Two 1-mm diameter plastic shields were fitted over the ends of the core, with an exposed section of the wire between them. The exposed end of the rod was initially electropolished in an 8:7:5 phosphoric acid, sulfuric acid, and water solution with an applied potential of 6 V. After the core diameter was reduced by 50%, the plastic shield was removed from the distal end of the rod and electropolishing continued at a 4 V applied potential until the distal end broke off. The final tip geometry was determined by the initial surface area exposed between the two plastic sleeves. Optical micrographs show that the technique is reproducible. The researchers concluded that the magnitude of the magnetic field gradient generated by the EMN is primarily a function of the needle tip. EMNs with large tip
radii (20 µm) are capable of interacting with multiple beads, they said. Electropolishing to smaller radii (0.1–6 µm) allows selective capturing of single magnetic beads. The researchers demonstrated removal of a single 4.5 µm superparamagnetic bead from a group of similar ones, less than 10 µm from each other. It is then possible, they said, to relocate the bead by moving the needle and simply shutting off the current. The researchers demonstrated that 50 nN forces could be applied to 4.5 µm diameter beads using an EMN with a pole tip radius of 20 µm, while more than 1 nN could be applied to 250 nm diameter beads using an EMN with a pole tip radius of 100 nm. JEREMIAH T. ABIADE
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