Review of electric discharge microplasmas generated in highly fluctuating fluids: Characteristics and application to nanomaterials synthesis

Abstract

Plasma-based fabrication of novel nanomaterials and nanostructures is indispensible for the development of next-generation electronic devices and for green energy applications. In particular, controlling the interactions between plasmas and materials interfaces, and the plasma fluctuations, is crucial for further development of plasma-based processes and bottom-up growth of nanomaterials. Electric discharge microplasmas generated in supercritical fluids represent a special class of high-pressure plasmas, where fluctuations on the molecular scale influence the discharge properties and the possible bottom-up growth of nanomaterials. This review discusses an anomaly observed for direct current microplasmas generated near the critical point, a local decrease in the breakdown voltage. This anomalous behavior is suggested to be caused by the concomitant decrease of the ionization potential due to the formation of clusters near the critical point, and the formation of extended electron mean free paths caused by the high-density fluctuation near the critical point. It is also shown that in the case of dielectric barrier microdischarges generated close to the critical point, the high-density fluctuation of the supercritical fluid persists. The final part of the review discusses the application of discharges generated in supercritical fluids to synthesis of nanomaterials, in particular, molecular diamond—so-called diamondoids—by microplasmas generated inside conventional batch-type and continuous flow microreactors.

The 2017 Plasma Roadmap: Low temperature plasma science and technology

“…Among these are plasmas produced directly in SCFs, or the hybrid SCFplasma [16]. Both are of considerable interest because, in both cases, a plasma exists in a high-density medium that has not been previously explored.…”

“…Although still in its infancy, the field of SCF plasmas has already proven to be advantageous relative to both conventional SCF synthesis and plasma processes. It is hoped that recent results and examples of materials synthesis [16] will stimulate further developments, and serve to establish this approach as a viable alternative to other materials processing methods.…”

“…As the cavity dimensions approach the wavelength of light, for example, the modification of spontaneous emission rates by the cavity (known as the Purcell effect) should be observable. Furthermore, moving deeper into the 'Meso-Exotic' plasma region defined in the first Roadmap will require electron number densities above 10 16 -10 17 cm −3 . Attaining such densities with a duty cycle above 1% will undoubtedly require new microcavity designs, novel approaches to delivering power to the confined plasma through the cavity wall, and synergistic wall-plasma interactions.…”

Dusty Plasmas in Supercritical Carbon Dioxide

“…The discharge plasmas in SCFs have been successfully generated by employing electrodes with a gap on the order of micrometers [20]. The possibilities of the plasmas in SCF for application to carbon nanomaterial syntheses and unique phenomena, such as a large decrease of breakdown voltage near the critical point, were shown [21]. For application to the generation of dusty plasmas, surface dielectric barrier discharge (DBD) in the field-emitting regime was employed [22].…”

Quasi-equilibrium phase separation in supercritical fluids

“…We find that the clustering in the medium plays a significant role in the surface transport at the interface between the LL droplet and GL fluid. It is well known that the degree of intrinsic clustering in SCF, which occurs via a density fluctuation, reaches the maximum at the critical point [3] and diminishes above the critical point as manifested by the decrease of the intrinsic density fluctuations. Our study shows that this conventional knowledge is incomplete and confirms that a considerable clustering due to external perturbations can occur in the SCF region across the critical pressure but well above the critical temperature.…”