Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
322 coating process for carbon nanofibres can be found in [6]
and referred literature. The chromium and copper coated nanofibres were
then filtered in a vacuum assisted filtering system and pre-dried on the filter paper. In order to keep the planar
alignment of the fibres, which was formed during filtration, the humid filter cake was directly transferred
into the graphite die for hot pressing. The amount of composite powder was calculated for a thin plate of
250µm. Consolidation under 30 MPa for 60 minutes at approximately 1000°C was performed under hydrogen
atmosphere to reduce the plated copper in the likely case of excessive oxygen.
The hot pressed samples were cleaned from remaining graphite and investigated measured by modulated
photothermal radiometry PTR in front-detection configuration. The used laser beam diameter is 0.85 mm
at 1e
2
and the used detector size is 1x1 mm
2
. Results with the beam expanded by an objective to 2.8 mm at
1e
2
are not shown here but confirm the present interpretation.
For the measurement the samples were mounted on a sample holder provided with a hole at the location of
laser irradiation, which means the samples were suspended in air. The amplitude and phase signals were
normalized to a file composed of two data sets: 1 the electro-optic transfer function of the PTR set up
measured with a photodiode at low frequency and 2 the PTR spectrum of a polished, opaque and
homogeneous sample i.e., a thick Ti6V metallic alloy at high frequency. Thus non-linearities inherent with any
PTR reference measurement were eliminated. A correct normalized sample signal in the range 1 Hz - 100 kHz
was obtained.
The measured values were fit with a two-dimensional 2-D model for two layers suspended in air, allowing
interpretation for sample anisotropy with different in- plane and out-of-plane thermal conductivity.
Fig. 1. Sputtering reactor for coating the carbon nanofibres with chromium
III. Results
After sputter deposition the carbon nanofibres were not completely homogeneously coated with chromium,
but from previous studies it is known that evenly distributed spots of chromium coated carbon nanofibres
already show a significant improvement on the thermal conductivity of the later on hot pressed composite. High
resolution scanning electron microscope HR-SEM images in back scattered detection show a typical result
after Cr deposition via sputtering Figure 2. The final chromium-CNF composite did typically contain 11wt
of chromium. Figure 2 does also show a typical SEM picture of the powder after copper deposition. For a
homogeneous fibre distribution within the final composite a homogeneous copper coating is essential.
Otherwise a strong clustering tendency of the CNF can be observed. After copper plating the composite powder
comprises ~ 40 vol. nanofibres, ~ 1.3 vol. Cr and ~ 58.7 vol. of copper.
Fig. 2. Left: typical SEM picture of Cr coated CNF, right: typical SEM image after copper plating
After sample consolidation by conventional hot pressing under hydrogen the samples were investigated
by PTR. The results show frequency behaviours presented in Figures 3 and 4 below. The dashed lines are
fits with a two-dimensional model for one layer suspended in air, allowing interpretation of thermal
anisotropy. The results were impossible to be explained without anisotropic parameters. In-plane and out-of-
plane thermal conductivities were significantly different which is for example also known for pyrolytic graphite
see e.g. goodfellow catalogue.
Figures 3 and 4 show the results of sample MCNF1, which had a measured thickness of 250 µm and a
diameter of 12.5 mm. Due to roughness on one side this thickness was corrected to 230 µm before interpretation.
The following values were determined for the composite sample ±15: From the fit of the cross-over
frequency situated beween 1 Hz and 10 kHz the in-plane thermal diffusivity resulted in a|| = 6.0x10
-4
m
2
s. The perpendicular to the surface out-of-plane thermal
diffusivity a
⊥
=2.0x10
-8
m
2
s resulted from the fit of the frequency spectrum in the range 1 - 100 Hz. The
respective thermal conductivities k were then calculated using k =
ρ c a with the theoretical specific heat c of the composite of 433 JkgK and density
ρ of 1400 kgm
3
, determined by weighing the measured disk and corrected
for roughness.
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
323
Fig. 3. Photothermal radiomentry: amplitude over frequency values measured circles for sample MCNF1 in comparison to 2D modeling
dashed line
Fig. 4. Photothermal radiometry: phase over frequency values measured for sample MCNF1 circles vs. 2D modeling dashed line
The fitted measured values result in an in-plane thermal conductivity of k||
≈ 360 WmK and an out-of- plane thermal conductivity of k
⊥
≈ 1.2 WmK. This results in a huge anisotropy ratio of k|| k
⊥
=300.
IV. Conclusion and Outlook
