Surface modification of titanium oxide in pulse-modulated induction thermal plasma

Recently, it was reported that RF induction plasma of a pulse-modulated operating mode had been successfully generated, for the first time, at a sufficiently high electric power level for materials processing. The unique conditions in the plasma, such as a non-equilibrium situation at the instance of pulse-on and -off and the increase of chemically reactive radical species, as well as the time-dependent change of plasma temperature, is expected to offer the unique physico–chemical conditions necessary for materials processing. As the first step of the work directed to materials processing, the thermal and chemical interactions between plasma and solid materials were examined for both cases of pulse-modulated and continuous modes. Green compacts of titanium dioxide (TiO₂), whose characteristics strongly depend on the formation of lattice defects and the incorporation of hydrogen, were treated in Ar---H₂ plasma of continuous and pulse-modulated generation modes. The oxide disks were placed at the downstream of plasma flow. The sample position and the plasma generation pressure were changed as processing parameters. The plasma-treated specimens were characterized by X-ray diffractometry and the hydrogen content was measured. The plasma treatment gave a thermal effect, rather than a chemical one, on the surface of oxide specimens. The oxides were thermally reduced by the plasma treatment and showed a change of color at the surface, while XRD did not show a change of crystal structure. Comparing specimens treated in the pulse-modulated plasma with those treated in the continuous plasma, the disk specimens had less of a thermal effect on the plasma.