Electrochemical Polishing Technique Yields Apparatus for Manipulation of Microto Nanometer-Sized Magnetic Beads
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First, the investigators patterned fuel channels of 80–90 µm depth by photolithography and wet etching a highly doped Si wafer. An n-type Si wafer was chosen because of its higher wet-etching rate with low resistivity to ensure it worked as a current collector. A 100 nm copper layer was sputtered on the Si wafer to supply current from a potentiostat. On the opposite side, anodization of the Si wafer in a 46% HFethanol solution using a current density of 100 mA/cm2 resulted in a porous Si layer that grew up to the base of the fuel channels at a rate of 45 nm/s. These were the optimal conditions found for formation of a uniform porous Si layer. Anodizing typically lasted for 5–6 min, as observed experimentally. A catalyst layer was later deposited on the porous Si layer by using a plating bath of 1.0 M H2SO4 + 10 mM H2PtCl6 + 5 mM K2RuCl5 + 50 mM HF at 293 K. The hydrofluoric acid added to the bath removed any silica present at any stage and resulted in electroless deposition of Ru and Pt. Thus, combining the HF addition to the bath with the use of a pulse plating technique assured the deposition of catalyst metal ions inside the pores of the catalyst layer. Otherwise, it was not possible to obtain coverage inside the pores since the catalyst metal ions were not able to reach the pores by electrodeposition means exclusively. The pulse plating technique utilized consisted of applying a current of 5 mA/cm2 at a frequency of 1 Hz for 0.2 s followed by a halt of 0.8 s. This cycle was repeated for 5 min, obtaining coverage toward the porous Si layer to a depth of 10 µm. The Si wafer was then immersed in a 40% FeCl2 solution at 313 K for 3–5 min to remove the copper layer. Two Si wafers thus processed were hot pressed with their catalyst sides facing top and bottom of a Nafion 112 piece, which worked as a polymer electrolyte membrane, using Nafion 5% solution as an adhesive. Hot pressing was accomplished at 0.05 MPa and 443 K for 30 min. The total thickness of the miniature fuel cell thus assembled was 250 µm. Testing the full assembly with hydrogen gas gave a peak power of 1.5 mW/cm2 at 353 K, and the porous Si layer indeed functioned as a current collector, reported the researchers. They said that the parabolic shape of the polarization curves show that the controlling mechanism is the catalyst performance. Power-generation capabilities of this miniature fuel cell are better than those of similar structure not using activated carbon as a catalyst layer, they said. SIARI SOSA
MRS BULLETIN/NOVEMBER 2004
Electrochemical Polishing Technique Yields Apparatus for Manipulation of Microto Nanometer-Sized Magnetic Beads To date, the application of miniature electromagnets for molecular and cellular manipulation has been limited by weak magnetic field gradients and resultant weak magnetic forces that are produced by these devices. A further complication is resistive heating of the electromagnet that may damage living cells and lead to expansion of the material used for the electromagnet core. This thermally induced expansion dim
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