APPLIED PHYSICS LETTERS 100, 053120 (2012)
Bo Feng,a) Keith Weaver, and G. P. Petersonb)
George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta,
Georgia 30332, USA
(Received 30 December 2011; accepted 16 January 2012; published online 3 February 2012)
Alumina nano coatings on platinum (Pt) micro wires were fabricated using atomic layer deposition.
During the pool boiling heat transfer, the critical heat flux (CHF) of Pt/Alumina in de-ionized
water was found to have a two-fold enhancement compared to that of the same Pt bare wire. The
CHF was shown to increase with coating thickness of alumina up to a thickness of 20 nm. Coating
thicknesses in excess of 20 nm had no additional influence on the CHF. The enhancement of the
CHF is the result of the superwetting property of the amorphous alumina coatings, which
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significantly increases the liquid film thickness, enhancing the rewetting of the “hot spot.” 2012
Nucleate boiling is an efficient heat transfer mode that
enables the achievement of an unusually high heat flux,
while still maintaining a relatively small superheat on solid
surfaces. As a result, it plays an important role in thermal
management of high-power-density electronics, phase-
change heat exchangers, and the cooling of nuclear reactors.
It is well understood that there exists a critical heat flux
(CHF) beyond which the temperature of the solid wall will
sharply increase, resulting in a significant decrease in the
surface heat flux. As the heat flux increases, the bubbles coa-
lesce to form “bubble mushrooms,” which prevent the liquid
from flowing back onto and rewetting the heated surface.
The result is a thin vapor blanket that completely envelops
the hot-spot.1 In many situations, this results in a significant
issue, in that a lower CHF will easily lead to the failure of
nucleate boiling and the resulting very rapid burn-out of the
heat transfer devices.1 A recent analysis has demonstrated
that a 32% increase in CHF, could result in a 20% increase
in the power density in pressurized water reactors, which
would both improve safety and effectively reduce the cost
for electricity generation.2
light has to be used to achieve the superhydrophilic
coatings.20
Atomic layer deposition (ALD) is a self-limiting chemi-
cal reaction, which has been shown to be an attractive tool
for depositing very thin layers on diverse range of materials
at relatively low temperatures.21 Compared to conventional
chemical vapor deposition, ALD is performed using multi-
pulsed precursors in series to accurately control the thickness
and achieve excellent uniformity of the coatings being
applied. In the current investigation, atomic layer deposited
alumina is utilized and is shown to significantly enhance the
CHF in pool boiling heat transfer.
The pool boiling experiment was conducted using plati-
num (Pt) wires (127 lm, purity 99.95%, Surepure Chemetals
Inc.) immersed in saturated de-ionized (DI) water at atmos-
pheric pressure, as described in Ref. 18. In brief, the Pt wires
were suspended across two copper electrodes with the two
ends of the Pt wires fixed at the copper electrodes by set
screws. By slowly increasing the power applied to the Pt
wires, the temperature history could be obtained using the
precalibrated resistance-temperature relationship and meas-
ured resistance, while the heating power was measured by
the voltage and current passing through the Pt wires. The
temperature of the DI water was maintained at a temperature
between 99.0 and 99.8 ꢀC by an external heater. The super-
heat was obtained by subtracting the saturated temperature
from the measured temperature of wires. The experimental
uncertainty is 63.83% for heat flux and 67.51% for
temperature.
Boiling occurs at the solid/liquid interface, resulting in
the formation of vapor. A wide variety of chemical and phys-
ical modifications of the solid surface, have previously been
utilized to promote the onset of the CHF. More recently, a
variety of micro and nanoscale surface features including
vertical nanowire and nanotube arrays,3–6 deposited nanopar-
ticle layers,7–16 micro/nano porous coatings,17 layer-by-layer
deposition techniques,18 and hybrid wettability treatments19
have been proposed. In these cases, the enhancement was
primarily attributed to the improved surface wettability and
enhanced capillary wetting, increase in the number and size
distribution of the nano/microscale cavities, increased effec-
tive effusivity, or a combination of these factors. All of the
techniques mentioned above, impact the hydrophilicity of
the solid surface. Takata et al.20 proposed to delay the CHF
using titanium oxide coatings. In this method, the ultraviolet
Following the initial testing, the Pt wires were coated
with Al2O3 grown in a thermal ALD system (Savannah
S100, Cambridge NanoTech Inc.), which consisted of a hot-
walled reactor, trimethylaluminum (TMA, Al(CH3)3, 97%,
Sigma Aldrich) and DI water vapor. The temperature of the
reactor was fixed at 373 K. Nitrogen (N2) gas flowed through
the reactor at 20 sccm at a pressure of approximately 1 Torr.
A typical cycle included a 40 ms pulse of TMA, a 10 s N2 gas
purge, a 100 ms pulse of water vapor, and another 10 s N2
gas purge. This process resulted in a growth rate of approxi-
mately 0.1 nm per cycle. The coatings were simultaneously
deposited on the Pt wire to be tested and a silicon chip in the
a)Electronic mail: bo.feng@me.gatech.edu.
b)Electronic mail: bud.peterson@gatech.edu.
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0003-6951/2012/100(5)/053120/3/$30.00
100, 053120-1
2012 American Institute of Physics
158.121.247.60 On: Sat, 08 Nov 2014 10:56:02
Feng, Bo
