Biomedical Engineering Reference
In-Depth Information
8.1
Depiction of ALD vs. CVD process windows for particle coating.
The cost effectiveness of the process also requires that both exposure and
purge steps are rapidly completed.
A flow-type reactor with moving substrate particles is a good solution for
such requirements. One major effort for ALD coating on particles consisted
of using fluidized bed reactors (FBRs). The first success with this process
was demonstrated by Wank et al. (2004a, 2004b). This unit operation was
also used to deposit Al
2
O
3
on micron-sized BN (
10
m), as well as Ni
~
μ
(
m) particles. In the design of an FBR as shown in Fig. 8.2 (King
et al., 2007), a porous metal disc is used as a gas distributor and a porous
metal filter is used at the inside top of the reactor column to retain the
particles in the reactor at all times. Mechanical vibration has proven to be
an effective means for improving the fluidization of cohesive particles
(Wank et al., 2001; Hakim et al., 2005c; Xu and Zhu, 2006). Vibration can
break up the large agglomerates generated by cohesive interparticle forces,
thereby achieving stable fluidization. The reactor itself is maintained at
reduced pressure using a vacuum pump. After these successes, the size scales
of the substrate particles were pushed further down to nanoscale using an
FBR (Hakim et al., 2005a, 2005b, 2006). For example, ultra-thin Al
2
O
3
films
were deposited on TiO
2
nanoparticles with a primary particle size of 21 nm
(Hakim et al., 2006). A rotary reactor is another kind of reactor that has
been demonstrated for ALD coating on particles. Al
2
O
3
ALD on
nanoparticles in a rotary reactor was reported by McCormick et al.
(2007). In the design of a rotary reactor, a cylindrical drum with porous
150
~
μ
