Special Issue on Heat Transfer, February 2011
Manuscript received and revised January 2011, accepted February 2011 Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved
321
Influence of Manufacturing Method and Interface Activators on the Anisotropic Thermal Behaviour of Copper
Carbon Nanofibre Composites
M. Kitzmantel
1,2
, E. Neubauer
1
, V. Brueser
3
, M. Chirtoc
4
, M. Attard
5
Abstract – Producing copper carbon nanofibre composites, one is generally facing three challenges: nanofibre dispersion, fibre alignment and the bonding interface between fibres and
matrix. The first challenge of a homogeneous dispersion can be addressed by coating the reinforcement with the matrix metal followed by hot pressing compaction. This consolidation
method does also bring along a certain fibre alignment, which can be decisively improved by powder pretreatment steps. In this study a preprocessing technique for a high fibre alignment in
two dimensions is shown. The third challenge addressed is the design of an interlayer between the matrix and the nanofibres. For improved thermal properties of the composite an engineered
interface is essential. Physical vapor deposition of carbide forming elements like chromium on the naked nanofibres guarantees the controlled formation of an engineered interlayer. The planar
alignment of the nanofibres and the resulting anisotropic thermal properties of the composite were confirmed by microstructural analysis and a novel method of photothermal measurements.
Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved. Keywords:
PVD Coating, Carbon Nanofibres, Hot Pressing, MMC, Thermal Conductivity
I. Introduction
As the building elements of micorelectronics get smaller and have to cope with growing power density,
more efficient cooling is a hot topic. Suitability for cycling temperatures as well as long-term stability is also
crucial for future materials. However, it is difficult to fabricate carbonmetal composites because of the poor
wettability between carbon and molten metal alloys [1]. Miscellaneous investigations with graphite flakes and
regular carbon fibers have been reported to show promising results [2]. Due to the lack of chemical
interactions between carbon and the matrix metal, the manipulation and investigation of the interface plays an
essential role. Several potentially interesting matrices degrade the fibers [3] at high temperatures and therefore
development has concentrated on matrices chemically inert towards carbon, such as copper.
The non-wetting behavior of the CCu system [4], leads to several problems to be addressed in the
production of MMCs for example by modifying the carbon surface with carbide forming elements like
chromium. Besides the latter difficulties, the fibre dispersion and alignment is also important for certain
applications. The presented study shows the evidence of possible alignment effects by powder metallurgical
production routes on the thermal behaviour of the final composite.
II. Experimental
Carbon nanofibres of type VGCF were acquired from Showa Denko diameter: ~100-150nm, length: ~5-
20µm. The as received nanofibre material was coated by chromium in a specially developed magnetron sputtering
system, described elsewhere [5]. In general this system consists of a rotating sample dish in a vacuum chamber
diameter: 320 mm height: 300 mm, turbo molecular and scroll pumps and a magnetron sputtering source with
a chromium target diameter: 150 mm. The reactor figure 1 can be freely tilted by various angles.
Additionally to the rotating dish, beating vibrations can be applied on the axle to prevent sticking of the fibres.
The sample dish has a flat bottom to allow the nanofibres to spread over the whole area diameter: 180mm. Per
coating batch approximately 20g of carbon nanofibres CNF were coated with chromium.
The chromium coated carbon nanofibres were then subjected to electroless chemical copper deposition.
Without an additional cleaning procedure the fibres were sensitized and activated in a SnCl2 and a PdCl2 solution
for 15 minutes respectively. Ultrasonification was used for dispersing the fibres in the solution to guarantee
homogeneous pretreatment. Subsequently the pretreated fibres were filtered and immersed into the electroless
copper plating bath comprising a defined amount CuSO4 for reaching a certain thickness of the copper layer on the
carbon nanofibres. A detailed description of the copper
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
