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microstructures should be required to satisfy a range of heat flux from the
chip. Recently, nanomaterials such as carbon nanotubes,
104
copper nano-
wires,
105,106
and silicon nanowires
107
were investigated and showed
enhanced boiling heat transfer. Lee et al., as shown in Figure 12.8,
108
applied
hierarchical ZnO nanowire forests for cooling electronics by means of pool
boiling heat transfer. Nanowire forests with branched tree-like hierarchical
structures greatly enhanced the surface area for the heat to be dissipated.
Furthermore, nanowires facilitated nucleation of bubbles when the coolant
started to boil. The testing chips were fabricated by a combination of top-
down microfabrication of the heater on one side of the chips and bottom-up
hydrothermal synthesis of ZnO nanowires on the other side of the chips.
Nanowire forest structures were grown by two methods: lengthwise growth
and branched growth. As shown in Figure 12.9, the superheat (subtraction of
d
n
3
r
4
n
g
|
8
Photoresist
SiO
2
(e)
Nanoparticle
seed
Si
Si
(a)
Photolithography
Si
Si
Si
(b)
Evaporation of Ti/Pt
.
Si
Si
Si
(c
) Lift-off
(d)
Heater
RTD
Si
Si
Lengthwise
Growth (LG)
Branched
Growth (BG)
Figure 12.8 Fabrication process of hierarchical nanowire forest pool boiling heat
transfer testing chips: (a-c) microfabrication of a platinum heater and
resistance temperature detector (RTD) on a silicon wafer, (d) optical
image of fabricated heater and detector pattern with a size of 1 cm by
1 cm, (e) synthesis of a ZnO nanowire forest. Both lengthwise (LG) and
branched growths (BG) were used in this study.
Reproduced with permission from ref. 108. Copyright 2013, The Japan
Society of Applied Physics.
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