Biomedical Engineering Reference
In-Depth Information
generated by means of both continuous pumping and gas inlet via
mass low controller. A major advantage over convectional gas low
is the improved control of the particle sizes. Depending on the low
rate of the He-gas, particle sizes are reduced by 80% and standard
deviations by 18%.
Evaporation can be done from refractory metal crucibles (W, Ta,
or Mo). If metals with high melting points or metals which react with
the crucibles, are to be prepared, sputtering, i.e. for W and Zr, or laser
or electron beam evaporation has to be used. Synthesis of alloys or
intermetallic compounds by thermal evaporation can only be done
in the exceptional cases that the vapor pressures of the constituents'
elements are similar. As an alternative, sputtering from an alloy or
mixed target can be employed.
However, control of the composition of the elements has been
dificult and reproducibility is poor. Nanocrystalline oxide powders
are formed by controlled postoxidation of primary nanoparticles
of a pure metal (e.g., Ti to TiO
2
) or a suboxide (e.g., ZrO to ZrO
2
).
Although the gas condensation method including the variations has
been widely employed to prepare a variety of metallic and ceramic
materials, quantities have so far been limited to a laboratory
scale. Recently, it should be mentioned that the scale-up of the gas
condensation method for industrial production of nanocrystalline
oxides by a company called nanophase technologies has been
successful.
Laser ablation has been extensively used for the preparation
of nanoparticles and particulate ilms. In this process, a laser
beam is used as the primary excitation source of ablation for
generating clusters directly from a solid sample in a wide variety
of applications. Laser vaporization cluster beams were introduced
by Smalley and coworkers to overcome the limitations of oven
sources [15]. In this method, a high energy pulsed laser with an
intensity lux exceeding 10
7
W/cm
3
is focused on a target containing
the material to be made into clusters. The resulting plasma causes
highly eficient vaporization since with current, pulsed lasers one
can easily generate temperatures at the target material greater than
10
4
K. This high temperature vaporizes all known substances so
quickly that the rest of the source can operate at room temperature.
Typical yields are 10
14
-10
15
atoms from a surface area of 0.01 cm
2
in
a 10
-8
s pulse. The local atomic vapor density can exceed 10
18
atom/
cm
3
(equivalent to 100 Torr pressure) in the microseconds following
