1. Microplasma Parameters and Applications

2. Electron Temperature and Energy Distribution Functions

3. References

micro-discharge is a kind of plasma, which we known as forth state of matter. it is called micro-discharge because it's size. usually micro discharges are small and fast.^[1] sparks are example of micro-discharge plasma.Plasma technology has been widely applied in the ozone production, material modification, gas/water cleaning, etc. Various nanomaterials were produced by thermal plasma technology. However, the high temperature process and low uniformity products limit their application for the high value added chemicals synthesis, for example the functional materials or the temperature sensitive materials. Microplasma has attracted significant attentions from various fields owing to its unique characteristics, like the high-pressure operation, non-equilibrium chemistry, continuous-flow, microscale geometry and self-organization phenomenon. Its application on the functional nanomaterial synthesis was elaborately discussed in this review paper. Firstly, the main physical parameters were reviewed, which include the electron temperature, electron energy distribution function, electron density and the gas temperature. Then four representative microplasma configurations were categorized, and the proper selection of configuration was summarized in light of different conditions. Finally the synthesis, mechanism and application of some typical nanomaterials were introduced.^[2]

Microplasma Parameters and Applications

According to the Paschen’s law, the breakdown voltage of a discharge for a certain gas is a function of the pressure and the gap length between two electrodes. Microplasma is a type of plasma which is generated in a micro scale discharge gap (at submillimeter level) under atmospheric pressure. Except the advantages of short residence time and narrow RTD, another significant feature of microplasma is its non-equilibrium state, in which the gas temperature T_g is much lower than the electron temperature T_e. There are mainly two reasons. Firstly, the electrons exchange energy via collisions with the other radicals such as ions or neutrals. When plasma is confined in a micro zone, the collision rate increases significantly, as well as the average energy exchanged. Therefore, reducing plasma size at constant pressure leads to an increase of electron temperature. Secondly, due to the high surface-to-volume ratio, the heat coupled from the power supply dissipates immediately and doesn’t accumulate. The non-equilibrium state provides new pathways for nanomaterial fabrication that cannot be achieved by conventional ways and especially favors the synthesis of temperature sensitive materials as well as the use of temperature sensitive precursors.In order to get insights into the intricate features of microplasma, systematic diagnostic studies of different types of microplasma have already been carried out. The parameters characterizing microplasma feature such as the electron temperature T_e, electron energy distribution functions (EEDFs), the electron density n_e and the gas temperature T_g have been measured and calculated by various established techniques.

Electron Temperature and Energy Distribution Functions

The electron temperature, T_e, determines the energy of electrons in microplasma, while the EEDFs reflect the distribution of energetic electrons. Both of them have a deep impact on the process in nanomaterial synthesis. The high energy electrons enable efficient, non-thermal dissociation of vapor precursors and other molecular gases to form reactive radical species. The EEDFs allow the production of high concentrations of those species in a certain T_e range. The higher electron temperature always leads to a rapid increase of “effective” collision among the radicals and results in an enhancement of average energy of them. As a consequence, more radicals for nanofabrication can be obtained during ionization processes.

According to the researches on microplasma characterization, there are mainly two factors affecting the non-thermal state of plasma: the non-transient effect and the micro dimension, which can be adjusted by applying the pulsed discharges or by changing the electrodes’ distance. As mentioned above, the difference between the electron temperature and gas temperature determines the plasma’s non-thermal state. In this section the measurement of the electron temperature are provided, along with a brief summary (Table 2) and some representative examples on the two factors and the corresponding electron temperatures^[3][4][5], followed by a general statement of EEDFs in microplasma.

References

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