Thermal Plasma Synthesis of Nano-sized Powders

Thermal plasmas, normally generated by DC (direct current) arc or inductively coupled RF (Radio Frequency) discharge, can be described as a high enthalpy flame with extremely high temperature fields (1,000~20,000 K) and a wide range of velocity fields from several m/s [1-5] to supersonic values [6-7]. Since the resultant huge enthalpy can be realized for various kinds of plasma forming gases and easily controlled by electricity, thermal plasmas have been expected to facilitate not only fast chemical reactions but also rapid heat transfer in a variety of synthetic routes for nano-sized materials [8-11]. For example, reactive gases, such as, nitrogen and oxygen, can be chemically activated in the form of various radicals dissociated or ionized in their thermal plasma state. As reported in many other papers [8-10], these radicals can be directly used in the formation of nitrides and oxides with very small sizes. Moreover, many kinds of micron-sized solid precursors can be immediately heated up to their evaporation points when they are injected into thermal plasma flames. In this case, nano-sized particles can also be synthesized by quenching those vapors as shown in Fig. 1. These features of thermal plasmas available for the synthesis of nano-sized materials, however, vary depending on the employed plasma torches [9,11]. For example, non-transferred DC torches with hot cathodes normally produce a hot ionized flame with temperatures in the range of 8,000~16,000 K at the torch exit [2]. The velocities of this flame can reach to hundreds m/s or to several thousand m/s depending on the torch nozzle structures at a given gas flow rate [2,12-14]. On the other hand, RF plasma torches produce relatively large flame of 5,000~10,000 K moving at mild velocities of up to several tens m/s [2,5,15-17]. In addition, the absence of electrodes may be favorable for the formation of reactive thermal plasma. Hence, actual synthesis systems have adopted plasma torches, maximizing their unique features, such as, flow fields, flame sizes, and torch structure itself, to the targeted nano materials. Among the torches adopted in this way, three kinds of plasma torches are illustrated in Fig. 2 as the typical plasma sources available for synthesis of nano-sized materials. Since the characteristics of these torches have been intensively studied with the main features of the generated thermal plasmas [1~5], we will address a brief review on their application results to various materials in this paper, such as, metals, ceramics, glasses, carbonaceous materials and other functional composites like metal-oxide catalysts and core-shell structured nano-materials.