THERMAL PLASMA PROCESSING

A review is made of the fundamental aspects involved in material processing using thermal plasma technology. The description of plasma-generating devices covers dc plasma torches, dc transferred arcs, radio-frequency (RF) inductively coupled plasma torches, and hybrid combinations of them. Emphasis is given to the identification of the basic energy-coupling mechanism in each case and the principal characteristics of the flow and temperature fields in the plasma. Materials-processing techniques using thermal plasmas are grouped in two broad categories, depending on the role played by the plasma in the process. Only typical examples are given in this review of each type of processes. The simplest and most widely used processes such as spheroidization, melting, deposition, and spray-coating make use of the plasma only as a high-temperature energy source. Thermal plasma technology is also used in applications involving chemical synthesis in which the plasma acts as a source of chemically active species. Examples of such applications are: hydrocarbon processing for the production of acetylene or acetylene/ethylene mixtures from natural gas, the synthesis of titanium dioxide pigment, high-purity synthetic silica, and a large number of high-purity ultrafine ceramic powders such as Al2O3, SiC, Si3N4, TiN, and TiB2. Recently, attention has also been given to the use of thermal plasma technology for the deposition of thin films and the destruction of toxic wastes. These are not covered in this review.