Low-temperature growth processes with helicon radio-frequency plasmas. TiO2 films and Pt/C nano-clusters
Résumé
Low-field helicon radiofrequency plasmas at low pressure can generate large ion fluxes which can be used in a variety of processes (direct or assisted reactive sputtering, carbon nanofibre growth, catalytic cluster deposition). Applications include plasma-induced crystallisation of TiO2 thin films1, enhanced catalytic activity of hydrogen fuel cell electrodes2 and ion beam formation for electrode-less plasma thrusters3.
Large ion densities (over 7x1016 m-3) lead to the formation of nanocrystalline (~15 nm) rutile phase TiO2. The post-annealing treatment of the films in air at 300 °C induces the complete crystallization of the amorphous films to nanocrystals of anatase (~40 nm) while the rutile films shows no significant change meaning that they are already fully crystallized by the plasma process.
Low temperature processes combining the growth of carbon nanofibres in CH4/H2 plasmas subsequently coated with 4 nm size platinum nanoclusters improve hydrogen fuel cell efficiency for constant catalytic loading. More recently, efficient fuel cell electrodes containing monoatomic, bimetallic and trimetallic Pt-based nanoclusters have been synthesized by magnetron co-deposition and a gas-condensation technique.
Finally, large area high velocity ion beams can also be generated in helicon plasmas using an extended range of operating gases. The thrust components generated by such helicon plasma device have been measured and a space thruster prototype (including rf sub-systems) has been developed. The sub-systems are focused on miniaturisation and impedance matching efficiency.
References :
1) Sarra-Bournet C. Charles C. Boswell R., Surface & Coatings Technol., 205, 3939-3946 (2011).
2) Brault P. Josserand C. Bauchire J-M. Caillard C. Charles C. Boswell R., Phys. Rev. Lett., 102, 045901 (2009).
3) Takahashi K. Lafleur T. Charles C. Alexander P. Boswell R., Phys. Rev. Lett., 107, 235000 (2011).