Biomedical Engineering Reference
In-Depth Information
grafting functional groups at the CNT surface will be presented and
discussed.
14.3 Plasma treatment of carbon nanotubes
14.3.1 Introduction to plasma: the fourth state of matter
Plasma is physically defined as an ionized gas with an equal number of
positively and negatively charged particles. It consists of free electrons, ions,
radicals, UV radiation and various highly excited neutral and charged
species (Chapman, 1980). The entire plasma is electrically neutral; however,
the displacement of the species is controlled by electric and magnetic fields.
According to the gas temperature, the plasma can be classified into thermal
plasma, which is characterized by being fully ionized (gas temperature T g =
electron temperature T e ), or non-thermal (or cold) plasma where the gas is
only partially ionized (T g is much lower than T e ).
Thermal plasma implies that the temperature of all active species
(electrons, ions and neutral species) is the same. This is true for stars, for
example, as well as for fusion plasma. High temperatures are required to
form these equilibrium plasmas, typically ranging from 4000K to 20 000K
(Bogaerts et al., 2002).
In non-thermal or cold plasmas, the temperature of neutral and positively
charged species is low, with the electrons having a much higher temperature
than the other particles because they are light and easily accelerated by the
applied electromagnetic field. As a result, the plasma is in the non-
equilibrium state and the reactions may proceed at low temperature. Indeed,
the electrons can reach temperatures of 10 4 -10 5 K (1-9 eV) while the
temperature of the gas can be as low as room temperature (Grill, 1994).
Figure 14.5 shows an image of a cold plasma discharge.
￿ ￿ ￿ ￿ ￿ ￿
14.3.2 Characteristics and principal applications of plasma
To reach the plasma state of atoms and molecules, energy for the ionization
must be input into the atoms and the molecules from an external energy
source. Ionization occurs when an atom or a molecule gains enough energy
from the excitation source or via collisions with other particles. There are
many kinds of plasma sources differing greatly from one another. The
electrically induced discharge in gas is the most common method for plasma
ignition because of handling convenience. Direct current (DC) discharges,
pulsed DC discharges, radio frequency (RF) discharges (13.56 MHz) and
microwave (
-wave) discharges (2.45 GHz) represent the plasma categor-
ization based on electric apparatuses (Denes and Manolache, 2004). These
are the common standard frequencies used for RF, and
μ
μ
-wave plasmas are
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