Page 20 - Synthesis of Functional Nanoparticles Using an Atmospheric Pressure Microplasma Process - LiangLiang Lin
P. 20

Chapter 1
 Figure 1.2 Schematic diagram of a hollow-electrode microdischarge for nanomaterial synthesis.
 Reprinted with permission from [78], copyright 2011 Wiley-VCH
 Hollow-electrode microdischarges are mostly operated at atmospheric pressure, with the characteristic interelectrode distance of few mm. For this arrangement the typical plasma voltage and current is at the level of hundred V and several mA. Therefore, nanomaterials can be obtained at very low power consumption. In addition, due to the extremely small reaction zone, the residence time of precursor vapors in the plasma is very short, rendering it possible to generate ultra-fine nanoparticles with narrow size distributions. On the other hand, several issues still need to be solved: 1) In such configuration, products are easily accumulated on the electrode mesh. Thus long time operation can lead to unstable conditions and unreproducible products. 2) Due to the electrode material erosion and sputtering, metal contaminations may exist. 3) Owing to the small inner diameter of the capillaries, it is only allowed to use the gaseous precursors to avoid the blocking problem. Thus the carried precursor quantities are rather limited, leading to quite low throughput of each processing.
2) Microplasma jets with external electrodes
Microplasma jet with external electrodes is another commonly used configuration for nanomaterial synthesis. In this configuration the plasmas are totally or partially confined in dielectric (e.g., quartz) capillaries or tubes, and are mostly sustained by radio frequency (RF) powers inductively or capacitively coupled by external electrodes outside the capillaries or tubes. The external electrode implies AC or pulsed power coupling, while the frequency of applied voltage in principle may vary in a very broad range from tens of Hz to GHz region. Precursors are directly injected or carried by gas flows into the plasmas, either inside or outside the capillaries (tubes). The obtained nanoparticles can be collected by depositing onto substrates downstream the gas flows or by flowing through proper solvents.
Figure 1.3 shows several examples of microplasma jets with external electrodes for nanomaterial synthesis.79 In general, the capillary/tube is made of quartz for diagnostic purposes, with a typical internal diameter in the range of 0.3 to 0.7 mm to confine the plasma into a submillimeter-sized spatial scale. The microplasma can be generated and sustained by 8




























































































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