Ultrasound for the Preparation of Graphene - Hielscher Ultrasonics0 pages

Ultrasonically assisted preparation of graphene
by Kathrin Hielscher, Hielscher Ultrasonics GmbH; kathrin@hielscher.com

1. Graphene
Graphite is composed of two dimensional sheets of sp2-hybridized, hexagonally arranged
carbon atoms — the graphene — that are regularly stacked. The graphene’s atom-thin
sheets, which form graphite by non-bonding interactions, are characterized by an extreme
larger surface area. Graphene shows an extraordinary strength and firmness along its basal
levels that reaches with approx. 1020 GPa almost the strength value of diamond.
Graphene is the basic structural element of some allotropes including, besides graphite, also
carbon nanotubes and fullerenes. Used as additive, graphene can dramatically enhance the
electrical, physical, mechanical, and barrier properties of polymer composites at extremely
low loadings. (Xu, Suslick 2011)
By its properties, graphene is a material of superlatives and thereby promising for industries
that produce composites, coatings or microelectronics. Geim (2009) describes graphene as
supermaterial concisely in the following paragraph:
“It is the thinnest material in the universe and the strongest ever measured. Its charge
carriers exhibit giant intrinsic mobility, have the smallest effective mass (it is zero) and can
travel micrometer-long distances without scattering at room temperature. Graphene can
sustain current densities 6 orders higher than copper, shows record thermal conductivity
and stiffness, is impermeable to gases and reconciles such conflicting qualities as brittleness
and ductility. Electron transport in graphene is described by a Dirac-like equation, which
allows the investigation of relativistic quantum phenomena in a bench-top experiment.”
Due to these outstanding material’s characteristics, graphene is one of the most promising
materials and stands in the focus of nanomaterial research.
2. Ultrasound
When sonicating liquids at high intensities, the sound waves that propagate into the liquid
media result in alternating high-pressure (compression) and low-pressure (rarefaction)
cycles, with rates depending on the frequency. During the low-pressure cycle, high-intensity
ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles
attain a volume at which they can no longer absorb energy, they collapse violently during a
high-pressure cycle. This phenomenon is termed cavitation. During the implosion very high
temperatures (approx. 5,000K) and pressures (approx. 2,000atm) are reached locally. The
implosion of the cavitation bubble also results in liquid jets of up to 280m/s velocity. (Suslick
1998) The ultrasonically generated cavitation causes chemical and physical effects, which
can be applied to processes
Cavitation-induced sonochemistry provides a unique interaction between energy and
matter, with hot spots inside the bubbles of ~5000 K, pressures of ~1000 bar, heating and
cooling rates of >1010K s-1; these extraordinary conditions permit access to a range of

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