Application of micromixers - Micromixers Fundamentals Design and Fabrication - page 327

three spatial dimensions. A quantum dot contains 100-100,000 atoms corresponding to a diameter of
10-50 atoms or 2-10 nanometers. Quantum dots have electronic properties between those of bulk
semiconductors and those of discrete molecules. The properties of a quantum dot depend on its size
and shape. If quantum dots are used as fluorescent dye, tuning the size from large to small results in
a color shift of the emitted light from red to blue. Colloidal quantum dots are synthesized from three
components: precursors, organic surfactants, and solvents. At a sufficiently high temperature, the
precursors decompose into monomers. Once the monomers reach the supersaturation level, the growth
of the nanoparticles starts with a nucleation process. Key parameters for the successful growth of
nanocrystals are the precise control of temperature and the concentration of monomers. The
temperature should be high enough for the rearrangement of the atoms and low enough to promote
crystal growth. The concentration of the monomers affects the size of the particle and its distribution.
Both temperature and concentration can be well controlled in a micromixer. Typical quantum dots are
made of binary alloys such as cadmium selenide (CdSe), cadmium sulfide (CdS), indium arsenide
(InAs), and indium phosphide (InP). Nakamura et al. [34] controlled the size of CdSe nanoparticles by
adjusting the flow rate of the reactants in a micromixer. A shift in particle size from 2.8 nm to 4.2 nm
was achieved. Chan et al. [35] tuned the size of CdS nanoparticles between 2.44 nm and 2.69 nm by
controlling the temperature.
Metal nanoparticles are the other type of solid particles that can be synthesized in micromixers.
Colloidal metal nanoparticles are synthesized by the reduction of metal salt or metal complex solu-
tions. For instance, gold nanoparticles can be synthesized by reduction of tetrachloroaurate with citric
acid at high temperature. Micromixers allow the fast mixing of metal salt and reducing agent as well as
the rapid change of temperature between the steps of nucleation and particle growth. Wagner et al. [36]
synthesized gold nanoparticles of 16-18 nm in a micromixer at room temperature using ascorbic acid
as the reducing agent. The size of the nanoparticles can be tuned by adjusting the flow rate of the
reactants. Higher flow rate and faster mixing result in smaller particles. Micromixers are suitable for
the synthesis of metal nanoparticles with special shapes such as nanorods. Gold nanorods were formed
in a micromixer by mixing tetrachloroauric, ascorbic acid, and CTAB [37] . The shape of the nanorods
can be tuned by the flow rate ratio and the temperature.
9.1.6 Fuel processing
The need for clean energy sources and clean fuels leads to the recent rapid development of fuel cell
technology. Hydrogen is the main fuel for fuel cells. Micromixers and microreactors are ideal plat-
forms for the conversion of hydrocarbon fuels into hydrogen for miniature fuel cells for use in portable
applications. Hydrocarbon fuels can be converted into hydrogen by catalytic partial oxidation (CPO)
or oxidative steam reforming (OSR). The improved heat-transfer capability in micromixers makes
them suitable for these fuel-reforming processes. The challenge in designing micromixers for this
application is the integration of the reforming catalyst. The catalyst can be coated on the channel wall
by sputtering or other coating techniques. However, coating the channel wall brings relatively small
catalytic surface area. A packed bed or reaction chambers with microstructures such as pillars could
increase the surface area and the catalytic activity. Furthermore, steam-reforming reaction is endo-
thermic. The reformer requires external heat supply. The integration of a microcombustor and a heat
exchanger into the system can utilize leftover hydrogen from the fuel cell for this purpose. Another
alternative is the integration of resistive microheater in the system.