Page 21 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin

Introduction - Plasma and Microplasma-assisted Nanofabrication
purely inductive, purely capacitive, or hybrid power-coupling mode, with the frequency varies from low (e.g. ~ 20 kHz) to high (e.g., 450 MHz) range. Plasma gases (Ar, N2, He, H2 or several mixtures thereof) are flown through the capillary/tube vessel, with characteristic flow rate of 50-100 sccm. Due to the flexibility of the configuration, the precursors can either be transported through the tube or be placed outside the tube in direct contact with the microplasma jet, allowing for a wide range of nanoarchitectures to be produced.
For microplasma jets with external electrodes, there is no direct interaction between plasma and electrode materials, thus eliminating possible contaminations. Meanwhile, the process has larger operational space, since a wider range of processing parameters (e.g. precursor’s ratio, power coupling mode, voltage frequency, and precursor residence time) can be set and tuned. In terms of precursors, this type of plasma has more flexibility. In principle, precursors can be gases, liquids or even solids. Another distinct advantage is the possibility to apply microplasma jet, installed on the positioning stage, in the fields where local/on-site “dry- process” production of well-defined nanostructures is required i.e. printing-like technologies. However, disadvantages also exist for this plasma system. In comparison to the hollow- electrode microdischarges, in jet configuration with external electrodes the plasma typically occupies larger volume, therefore, the nanoparticles tend to grow larger and have broader size distributions. Moreover, the RF electronics, used in the majority of configurations with external electrodes, are relatively expensive, complex and need matching networks, leading to increased overall costs. Furthermore, for long time operation precursor needs to be somehow injected downstreatm plasma plume to avoid the block of capillaries or tubes.
3) Microplasma jets with consumable electrodes
There is also a type of microplasma jet using consumable metal wires as electrodes to produce metallic nanomaterials. In this configuration ultra-thin metal wires with a typical diameter size of 50-100 μm are inserted into the tubes and act as sacrificial precursors. Plasmas are formed at the end of the electrode, and are partially or totally confined in quartz or alumina tubes. High frequency power is coupled via external electrodes to sustain the plasmas. Inert
gases such as argon or helium flow into the tube as the plasma gas. By feeding oxygen,
Figure 1.3 Schematic diagram of the microplasma jets with external electrodes. Reprinted with
permission from [79], copyright 2009 IEEE
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