Highly thermally conductive dielectric coatings produced by Plasma Electrolytic Oxidation of aluminum

Accepted Manuscript
Highly thermally conductive dielectric coatings produced by Plasma Electro-
lytic Oxidation of aluminum
Tamires E.S. Araújo, Marcos Macias Mier, Alfredo Cruz Orea, Elidiane C
Rangel, Nilson C Cruz
PII: S2590-1508(19)30040-7
DOI: https://doi.org/10.1016/j.mlblux.2019.100016
Article Number: 100016
Reference: MLBLUX 100016
To appear in: Materials Letters: X
Received Date: 27 March 2019
Revised Date: 8 May 2019
Accepted Date: 20 May 2019
Please cite this article as: T.E.S. Araújo, M. Macias Mier, A. Cruz Orea, E.C. Rangel, N.C. Cruz, Highly thermally
conductive dielectric coatings produced by Plasma Electrolytic Oxidation of aluminum, Materials Letters: X (2019),
doi: https://doi.org/10.1016/j.mlblux.2019.100016
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Highly thermally conductive dielectric coatings produced by Plasma Electrolytic
Oxidation of aluminum
Tamires E.S Araújo1, Marcos Macias Mier2, Alfredo Cruz Orea2, Elidiane C Rangel1,
Nilson C Cruz1,*
1Laboratory of Technological Plasmas, Sao Paulo State University (Unesp), Sorocaba, SP, Brazil
2Centro de Investigación y de Estudios Avanzados – IPN, Department of Physics, Mexico D.F.
Abstract
Dielectric materials with high thermal conductivity are of great interest in applications as heat
sinks for microelectronics. In this work, an experimentally simple and economic viable technique, the
Plasma Electrolytic Oxidation, has been applied to produce dielectric coatings with high thermal
conductivity on aluminum substrates. The samples have been prepared in sodium silicate solutions using
pulsed DC voltage. Scanning electron microscopy with X-ray energy dispersive spectroscopy, X-ray
diffraction, ultraviolet-visible reflectance and impedance spectroscopies, and photoacoustic spectroscopy
have been applied to characterize the produced materials. Under certain conditions, samples with electric
resistivities as high as 4.6x1012 Ω.cm and thermal diffusivity 900% higher than that of Al2O3 or AlN have
been grown at room temperature.
Keywords: Plasma Electrolytic Oxidation, Photoacoustic Spectroscopy, Thermal Conductivity, Electrical
resistivity

1. Introduction
In microelectronics, the continuous miniaturization and the development of more and more
complex circuits have strongly increased the heat generation per square unit of chip surface. Consequently,
great attention has been devoted to the development of high thermal conductivity materials to be used as
heat sinks (HS). In this sense, based only on thermal properties, metals would be a good choice. However,
very specific problems inhibit the use of metallic materials in such applications. In power electronics, for
instance, voltage transients are capacitively transferred from the power semiconductor to the HS, which
behaves as an antenna, generating noise and interfering on the performance of the device. In other cases,
the mismatching of the thermal expansion coefficients of the semiconductor and the material of the HS may
lead to structural defects and even fractures of the device. Therefore, the requirements of candidates for
heat dissipators also include suitable electrical and mechanical properties.
Most of the requirements of such application can be fulfilled by non-metallic materials. With
thermal conductivities that can be as high as 2000 W m-1K-1 diamond and graphite would be excellent
options. However, high production costs make them unsuitable for industrial applications.
A group of particularly interesting materials are those ceramics based on aluminum, such as Al2O3
and AlN, which present excellent chemical stability and low thermal expansion coefficient, matching with
that of silicon. Owing to that, several approaches have been proposed for the development of electrically
insulating aluminum-based ceramic substrates. Kumari et al. [1] have produced carbon nanotube – alumina
nanocomposites by chemical vapor deposition and spark plasma sintering and obtained a nanocomposite
with thermal conductivity of 90.44 Wm-1K-1. However, their procedures included sintering in temperatures
as high as 1450°C. High sintering temperatures and mechanical instability are also some disadvantages of
using AlN [2,3].
A technique with excellent potential to produce oxide coatings on metallic substrates is the so-
called Plasma Electrolytic Oxidation (PEO) [4]. PEO is an environmentally friendly and economically
viable process of electro-thermochemical conversion, which results on the formation of oxide coatings on
lightweight metals. Essentially, the technique is based on the application of a DC voltage to a sample
immersed in an electrolytic solution. Under lower voltages, mainly oxygen-containing ions are attracted to
the sample, causing the formation of an insulating oxide layer on the surface. As the voltage is increased,
intense electric fields may be built up through this thin insulating layer. On those spots where the electric

fields exceed the dielectric strength of the coating, it occurs the formation of tiny electric discharges, also
known as microarcs. The energies deposited on the surface by the microarcs can increase the temperature
to values high enough to cause the local melting of the coating and the substrate. Such molten material is
quenched by the electrolyte, resulting on the formation of complex structures involving species on the
substrate and the coating, and in the electrolyte, as well. In general, the coatings have excellent adhesion
and resistance to corrosion and wear [5-9] and its properties can be tailored through the modification of
treatment parameters.
Although it has been reported the modification of the thermal conductivity of aluminum and its
alloys after PEO processing [5,7-10], most of the published papers was devoted to the production of thermal
barriers. In this work, it is reported the formation of coatings with high thermal conductivity and electrically
insulating by plasma electrolytic oxidation of aluminum.
2. Materials and Methods
Pieces measuring 30x25x1.5 mm3 were cut from a commercially pure 1230 aluminum plate. The
samples were sonicated in a detergent solution, rinsed in deionized water, and sonicated in isopropyl alcohol
for 480 s. The treatments have been performed in a water-cooled stainless-steel cell fully described
elsewhere [11], using pulsed DC voltage (350 V, duty cycle 60%) in potentiostatic mode. The electrolytic
solution (pH 13.6) was prepared diluting 40 g of sodium silicate in 1 liter of deionized water. It has been
investigated the influence of treatment time (15 and 30 min) and excitation frequency (that ranged from
200 to 320 Hz) on the properties of the samples.
The morphology of randomly chosen regions has been evaluated by Scanning Electron
Microscopy (SEM), while the chemical composition of the samples has been assessed by X-ray Energy
Dispersive Spectroscopy (EDS). The crystallographic structures of the coatings have been determined by
X-ray diffractometry using CuKα radiation (40 kV, 40 mA) in θ-2θ mode (from 20° to 120°, 1 s/step, 0.05°
steps). Diffuse ultraviolet-visible reflectance spectroscopy (UV-Vis), has been used to evaluate the
reflectance of the surfaces, while their electrical resistivities have been measured by impedance
spectroscopy.
The thermal diffusivity α is a dynamic property that characterizes the conduction of heat through
a material under non-stationary conditions [12]. It is a function of the thermal conductivity (κ) and the
product of the specific heat at constant pressure (Cp) by the density (ρ) of the material, that is ./ Pk C 

In this work, the thermal properties (α and κ) of the samples have been evaluated by two-beam
photoacoustic spectroscopy, as fully described elsewhere [13]. Briefly, both sides of a sample enclosed in
a cell with air are alternately heated by a pulsed light beam. The periodic heat transfer from the sample to
the surroundings causes oscillations of the pressure in the air within the cell, the photoacoustic (PA) signal,
which are detected by a microphone. Therefore, the PA signal is proportional to the oscillation of the
temperature on the surface of the sample. The thermal diffusivity is numerically equal to the slope of the
plot of the phase differences of the PA signals in rear and front incidence as a function of ω, the angular
frequency of light modulation. From the analysis is also possible to evaluate the product Cp ρ, and,
consequently, the thermal conductivity κ. All experiments have been performed using a laser (150 mW,
650 nm) as light source.
3. Results and discussions
Coatings nearly 10 μm-thick were grown after the treatments. Figure 1 presents SEM micrographs
of pristine and treated samples. Only scars of the mechanical processing to obtain the samples can be
observed on the micrograph of the as-received sample. Diversely, the treatments resulted on the formation
of porous layers on all the samples. On the samples treated for 15 min, it is possible to identify pores roughly
5 µm in diameter, surrounded by swollen plateau-like regions and pores less than 1 µm in diameter scattered
on the flat surface of the coating. While the pores are formed on those spots where the microarcs were
intense enough to melt the coating, the plateaus result from the re-deposition of the molten material, which
can combine with species in the electrolyte, quenched by the liquid. When higher frequencies are used, the
voltage supply is interrupted a higher number of times per unit time, reducing the density of microarcs and,
consequently, the density of pores. When the treatment time was increased to 30 min, a smoother coating
was grown, once again, as a consequence of the reduction of the density of microarcs as the coating gets
thicker and more insulating.

Figure 1. SEM micrographs of aluminum samples as-received and after treatment by PEO at 350
V with different treatment parameters.
From EDS analysis it was possible to observe that the samples treated for 15 min contained, in
average, 58% O, 22% Al, and 16% Si with traces of sodium and carbon. The proportion of oxygen and
silicon increased to 64% and 26%, respectively, and only 6% of aluminum has been detected in the sample
Al

treated for 30 min. It is worth noting that the incorporation of silicon helps to match coating and
semiconductor thermal expansion coefficients, as required for HS.
Impedance spectroscopy revealed that the electric resistivity of the coatings, very similar to values
reported to alumina and AlN, ranged from 8.4x1011 Ω.cm to 4.6x1012 Ω.cm as the treatment time was
increased. Therefore, the samples we have produced also fulfill a second major requirement for heat sinks.
X-ray diffractometry revealed that the treatments resulted on the formation of γ-Al2O3, with main
peaks indexed to (200) and (302) planes and no significative differences in phase composition after the
various treatments. Therefore, silicon, detected by EDS, has been incorporated to the coatings as a vitreous
amorphous form or in crystalline proportions below the detection limit.
UV-Vis reflectance spectra of the samples are presented in Figure 2. It is possible to observe that
pristine aluminum is an excellent heat reflector, as its reflectance is as high as 93% over the entire infrared
region. Moreover, all the treatments reduced the reflectance of the samples in comparison to the as-received
aluminum. In addition to incorporation of new chemical states able to absorb radiation, the reduction of
reflectance may also be attributed to the porosity, which confers a matte, non-metallic aspect to the surfaces.
200 400 600 800 1000 1200
0.0
0.2
0.4
0.6
0.8
1.0
Reflectance(%)
Wavelength (nm)
Aluminum
15 min, 200 Hz
15 min, 300 Hz
15 min, 320 Hz
30 min, 320 Hz
Figure 2. UV-Vis reflectance spectra of samples as-received and after the treatments with PEO at 350 V
and different parameters.

Figure 3 illustrates a plot of phase difference of PA signal as a function of the frequency of light
modulation in front and rear illumination experiments used to determine the thermal properties of the
samples.
9 12 15 18
1.0
1.1
1.2
1.3
1.4
1.5
1.6
tan  =  = (1.35 ± 0.09) x 10
-4
m
2
/s
F
R
(rad)
 (Hz)
)
Figure 3. Phases differences of front and rear incidence of light on the photoacoustic cell as a function of
the angular frequency of light modulation.
The results of the fittings for all the samples, that are summarized in Table 1, can be better
appreciated if compared with values, also presented in Table 1, for aluminum [14], Al2O3 [15] and AlN
[16], which have been considered some of the most promising candidates for semiconductor heat sinks. As
it can be concluded, the treatments increased very impressively the thermal response of the substrates if
compared to the reference materials. The treatment during 15 min with 320 Hz increased more than 900%
both evaluated parameters. Furthermore, the values of k obtained in this work is more than three orders of
magnitude higher than that achieved by Curran et al. [17] and two orders of magnitude higher than the
values observed by Dudareva et al. [18]. It is important to emphasize that the treatments providing such
excellent results have been performed at room temperature using an experimentally simple and
economically viable technique, without any pre- or post-treatment, such as, for instance, sintering in
temperatures as high as 1600°C for several hours to obtain good Al2O3 and AlN substrates.

Table 1: Thermal diffusivity and conductivity of samples as-received and after treatment with
PEO. For comparison are also included values for Al2O3 [15] and AlN [16].
Sample α (x10-4 m2/s) κ (Wm-1 K-1)
Al 0.97 237.6
Al2O3 0.15 25–30
AlN 0.14 17–285
15 min, 200 Hz 1.23 242.3
15 min, 300 Hz 1.22 275.5
15 min, 320 Hz 1.38 287.0
30 min, 320 Hz 1.35 288.9
It is interesting to note that, the sample with best performance (15 min, 320 Hz) presented the
smallest reflectance in the whole range investigated in this work.
Conclusions
Layers with high thermal conductivity and elevated electric resistivity have been grown at room
temperature by plasma electrolytic oxidation of aluminum. The coatings, whose main crystalline phase was
γ-Al2O3, also contained silicon and presented elevated electric resistivity. The treatments decreased the
infrared reflectance in comparison with pristine aluminum, which contributed to improve both the thermal
diffusivity and thermal conductivity of the substrates. Under certain conditions, it has been produced
coatings with electric resistivities as high as 4.6x1012 Ω.cm and thermal diffusivity 900% higher than Al2O3
or AlN, suggesting that the samples can be considered as good candidates in applications as heat sinks for
the microelectronics industry.
Acknowledgments

The authors acknowledge the financial support of CNPq and FAPESP.
References
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Growth of highly thermal conductive dielectric coatings at room temperature
No pre- or post-treatments required
Thermal diffusivity 900% higher than Al2O3 and AlN

Highly thermally conductive dielectric coatings produced by Plasma Electrolytic Oxidation of aluminum

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