Biomedical Engineering Reference
In-Depth Information
than its surface chemistry, the reader is referred to different views on the
development of ALD and applications over the years by Goodman and
Pessa (1986), Suntola (1989, 1992), George et al. (1996), Malygin et al.
(1996), Leskela and Ritala (1999, 2002), Niinisto et al. (1996) and George
(2010). It is not the intent of this chapter to give an extensive review of the
subject of thin film coating, but the focus will be on the topic of ceramic
ultra-thin film coatings on particles by ALD in gas-phase and related
applications.
8.2
Ultra-thin ceramic films coated on ceramic
particles by atomic layer deposition (ALD)
8.2.1 Particle ALD in a fluidized bed reactor
Fine particles have gained increased interest in a variety of fields for
different applications (Bruchez et al., 1998; Panyam and Labhasetwar, 2003;
Daniel and Astruc, 2004). The ability to modify the surface of particles with
thin films, which can add new surface functionalities and keep the bulk
properties of the particles largely the same, has an increasing number of new
applications as new systems are studied. Additional functionality in thin
films can be achieved by depositing multiple layers of different materials.
For example, nanocoatings on fine capsules can modify the drug release
characteristics and provide physical and chemical protection for the drugs
(Theobald et al., 2003; Bose and Bogner, 2007); the coating of magnetic iron
nanoparticles with photoactive TiO
2
films can be used for the degradation of
aqueous contaminants, while the magnetic moment of the substrates keeps
the composite recoverable (Zhou et al., 2010).
ALD is an ideal technique to deposit conformal, pinhole-free films on
particle substrate surfaces with precision thickness control (Leskela and
Ritala, 2002). CVD processes cannot be adequately performed on
submicron-sized particles due to the tendency for these particles to
aggregate. The typical process windows for ALD vs. CVD processes for
particle coating are shown in Fig. 8.1. Film deposition on particles using
ALD was first reported in 2000 for the deposition of Al
2
O
3
on BN particles
(Ferguson et al., 2000). High surface area (
40m
2
/g) BN particles were
pressed into a tungsten grid using a polished stainless steel die and a manual
press, and were coated in a viscous flow reactor using the alumina ALD
chemistry (Dillon et al., 1995; Ott et al., 1996). Conformal ALD coating on
a whole particle surface by such methods is difficult to impossible, since
there are inherently some contact points between the particle and the
tungsten grid. The contact points will not be coated, though ALD is
independent of line of sight. Equipment that has good gas-solid contacting
properties is needed to apply conformal ALD coating on a whole particle.
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