Deposition with alternating current

Deposition Utilizing Alternating Current. Hydrogen and boron tribromide vapors were used to obtain a coating of boron on 1-mil tungsten wire in a corona developed with 60-c.p.s. alternating current. At... 

Just before he died, Ralph K. Iler (personal communication, 1985) was working on a device for electrophoretically depositing particles with a controlled degree of order. In addition to the direct current field for deposition, he imposed an alternating current component. At low frequencies, the ac component tended to unsnarl packing defects, whereas at high frequencies it apparently created a dipolar interaction between particles. 

A simple conductivity cell is shown in Fig. 31.2. Two platinum electrodes are sealed in the ends of the cell. These are usually coated with a deposit of finely divided platinum, platinum black, to eliminate some of the effects of electrolysis. The cell is filled with the solution, and the resistance is measured by placing the cell in one arm of the alternating current version of a Wheatstone bridge. The frequency ordinarily used is about 1000 Hz. 

In Figure 12.17A a simple double pulse program is shown. In Figure 12.17B a third pulse that has zero current or even reversed current is added between the two deposition pulses. This pulse is applied to stop the non-noble metal deposition. In the low current pulse only the noble component will be deposited at higher currents and at more cathodic potential the non-noble component will be deposited together with the noble component. The concentration of the noble component must be so low that the amount of noble metal co-deposited with the non-noble component is negligible. This shows that the plating conditions are very restrictive. For this reason the dual bath technique could be an alternative for electrochemical deposition of a special metal combination. 

An alternative method is quartz crystal microbalance (QCM), which may be used to measure the deposition rate during film deposition (King, 1964). The collecting surface consists of a quartz crystal typically coated with gold or silver electrodes. The crystal is then biased with alternating current and driven at its resonance frequency. The adsorption (or removal) of mass to the surface is then measured as a shift in the resonance frequency. The mass resolution of a QCM is usually below 10 pg/m per second, which makes it suitable for very thin films. It should be noted that QCM also measures deposited mass, not film thickness directly. Limitations include sensitivity to temperature changes and the fact that the electrode is gold or silver, which may not be a suitable proxy for the test substrate. 

Eiectroformation. For electroformation (132), a thin film is deposited on adjacent electrodes (Pt, Au, ITO-coated glass) and the budding process is induced by an alternating current at typically 10 Hz (10 V) (7). Tbe electric field decreases the membrane tension and thereby facilitates vesicle formation. This method 5uelds the largest vesicles (giant vesicles) with diameters up to 100 /um. 

Krupke et al. (2003) have shown that the bulk separation of various CNT types could be carried out using alternating current dielectrophoresis. It was established that, when submitted to an electric field, the SWNTs have an induced dipole moment which can be used to move these tubes selectively. Of course, the bundles must be separated previously in individual tubes using an ultrasonication treatment of the nanotubes with surfactants such as the SDS (sodium dodecyl sulfate). The dielectrophoresis is then applied to this solution. The metallic tubes are deposited onto the electrodes, while the semiconducting ones remain in suspension. 

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