SVI-6

超臨界流体プラズマの基礎と応用－自然界が創る気液ナノ混相場でのプラズマ－
Basics and Applications of Supercritical Fluid Plasmas- Presents from the Nature ;Plasmas in the multiphase field of gas and liquid on the nanoscale 寺嶋和夫、シュタウス スベン、宗岡 均
Kazuo Terashima, Sven Stauss, and Hitoshi Muneoka
東京大学 大学院新領域創成科学研究科
The Univ Tokyo, Graduate School of Frontier Sciences

Supercritical fluids (SCFs) (Fig. 1) are
media in a state of temperature and pressure
above their respective critical values, T; Tcrit, p;
pcrit. From a macroscopic point of view, SCFs
are in an intermediate state between liquid andgas. SCFs possess high density, high diffusivity,
high solubility, and they have been used in awide range of applications. These excellent
reaction process characteristics derive from themicroscopic fluid structure in SCFs —molecularclustering. In the vicinity of the critical point,
because of balancing of van der Waals’ forces
and thermal motion, molecules aggregate anddisperse in the order of pico-seconds, so that the Fig.1 Supercritical fluid (SCF) & SCF Plasma
density fluctuates greatly. Around the critical
The phase diagram of pure substance and illustration
point, the compressibility is greatly enhanced.
of species present in SCF and gaseous plasmas.
This compressibility region is not limited to theregion where T > Tcrit and p > pcrit, but reaches also in the liquid domain. In addition, it has beenfound that passed the critical point, the supercritical phase above show either gas-like or liquid-likebehaviour. In the case of the isothermal compressibility, this line is called Widom line and it has
been shown that clusters above or below the Widom line show either liquid-like or gas- likebehavior and structure. Clusters in SCF solutions are different from those in solids or liquids. Theinteractions between molecules in a cluster is governed by relatively weak forces, such as Van derWaals force. On the time scale, the average lifetime of a cluster is much shorter than in solids orliquids of the order of picoseconds (ps, 10−12 s). In addition, in the vicinity of the critical point (Tcrit,pcrit), where the thermal conductivity and specific heat attain their maximum values, it is expectedthat new types of reactions involving clusters can be used and that by changing the conditions of theSCF medium, the selectivity of reactions can be adjusted. For example, it has been shown that thereaction rate in supercritical fluids is enhanced near the critical point.
Therefore, plasmas generated in supercritical fluids (Fig. 1) contain excited species such asradical and ion clusters, in addition to electrons, ions and radicals present normally in conventional
gaseous plasmas. In combination with the advantageous properties of supercritical fluids (i.e. highdensity, high diffusivity, high thermal conductivity, low surface tension, molecular clustering anddensity fluctuation near the critical point), this allows to realize new reaction fields which can beused for the synthesis of nanomaterials.

So far, plasmas generated insupercritical fluids have permitted
thesynthesis
ofcarbonnanomaterials such as carbon
nanotubes, under more moderateconditions of temperature andpressure compared to supercriticalfluid processing without plasma,
while high-density dielectric barrierdischarge and pulsed laser plasmas
have allowed the synthesis ofvarious
moleculardiamonds,
so-called diamondoids (Fig. 2),
including novel ones.
Fig.2
Carbon nanomaterials. (a)sp2 materials (C60,Diamondoids are sometimes
carbon nanotube and grapheme),referred to as the fourth carbon nano(b)sp3 materials (diamondoid and diamond).
material following(1) fullerenes, (2) carbon nanotubes, and (3) graphenes. They are expected to play an increasingly
important role in many fields such as pharmaceutics, electronics and display technology.
In this talk, we highlight the research on plasmas in supercritical fluids, their generation,either by electric discharges or lasers, and their main characteristics. In addition their differences incomparison to gaseous plasmas is discussed. Finally, we discuss further opportunities of plasmas
generated in supercritical fluids and their relation with other ionized states of matter .

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