Biomedical Engineering Reference
In-Depth Information
of a hydrogen/nitrogen mixture (with a mole component ratio of 7.0/93.0 or
99.999% high purity nitrogen at flow rates from 100 to 1000 cm
3
/min).
176
For the ex situ process, the catalyst particles were formed in a HWG posi-
tioned outside the furnace that were mixed with the carbon source and the
promoter before entry into the reactor.
176
The in situ process involved a small
ceramic tube (external and internal diameters of 13 and 9 mm, respectively, and
25-cm long) that was inserted inside the reactor.
176
The particles formed were
mixed in the reactor at about 400 °C with an outer flow at 400 cm
3
/min contain-
ing a carbon source. The carbon sources used were alcohols such as ethanol
(Primalco Oy, 99.5%) or 1-octanol (J.T. Baker, 99%)
176
that were introduced
by bubbling a carrier gas (CO or N
2
) through the liquid carbon source at RT
in the presence of a promoter such as thiophene (0.5%), to increase the yield
of CNTs.
177
To lower the carbon source and promoter vapor pressures in the
reactor the partial vapor pressures were calculated from the equilibrium vapor
pressures of the constituents and their mole fractions in solution. The products
were collected from the gas phase with an electrostatic precipitator (combina-
tion electrostatic precipitator, InTox Products) on carbon-coated copper grids
(SPI Lacey Carbon Grid).
The ex situ experimental setup showed disadvantages that included the fol-
lowing: (1) the size of the particles cannot be controlled well due to the rela-
tively long time between primary particle formation and their introduction into
the reactor; (2) significant particle losses before introduction to the reactor; (3)
the multiwalled CNTs formed via the gas-phase process do not posses well-
graphitized wall structures. The presence of a promoter (thiophene) in the sys-
tem increased harvest in the synthesis of multiwalled CNTs, while minimal
increase was found for the single-walled CNT production.
176
2.4.2 Fullerenes
Fullerenes, C60 being the most common form, is another class of carbon NMs
that were predicted to exist in 1970 and officially publicized in 1985.
178,179
To
date, fullerenes have led to the synthesis of fullerene derivatives, such as C61-
butylic acid and other fullerene variations, such as C70, C20 (the smallest mem-
ber), CNTs (elongated, tube-structured fullerene), carbon nano-onions, and
nano-buds.
180-182
One of the most unique properties of fullerenes is the ability
to assume different forms and to engage compounds.
176
Their unique physical,
chemical, electrical, and optical properties allowed their application as added
components of new or improved devices and materials leading to advancements
in science, engineering, and industry.
183-186
Fullerenes with well-defined prop-
erties and containing polymers and amphiphilic systems that are sensitive to
stimuli can be easily synthesized using the azido coupling and atom transfer
radical addition process.
187
Research continues into ways to increase the solu-
bility of fullerenes and to investigate the toxicity of fullerenes and their derived
compounds.
188
