1 FUNCTIONAL MATERIALS Powder Technology Center - PTC Copper based composites reinforced with carbon nanofibers René Nagel, Erich Neubauer ARC Seibersdorf research GmbH Tel.: +43 50550 3378 Fax: +43 50550 2724 E-Mail: [email protected]www.materials-technology.at
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FUNCTIONAL MATERIALSPowder Technology Center - PTC 1 Copper based composites reinforced with carbon nanofibers René Nagel, Erich Neubauer ARC Seibersdorf.
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FUNCTIONAL MATERIALS Powder Technology Center - PTC
Copper based composites reinforced with carbon nanofibers
Selection of suitable raw materials (different suppliers & qualities)
Data sheet of different carbon fibers are available, thermal, mechanical properties are available from measurements
Characterisation of CNF properties is not easy, only rare experimental data are available, there are different suppliers, to get reproducible quality is not easy (thermal properties!)
Separation and dispersion of short fibers in the matrix material
Optimisation of conventional blending techniques is sufficient, coating of fibers provides an advanced solution, fiber breakage has to be taken into account
To coat the CNFs seems to be the most appropriate way to get a homogenous distribution, mechanical milling (damage of fibers?), dispersion techniques with surfactants
Alignment/ Orientation/Anisotropy
Fiber aspect ratio of 1:10 to 1:100 results in an orientation of the fibers during PM processing => anisotropy of properties
Alignment of CNFs during PM processing is not confirmed yet, extrusion results in a prefered alignment of CNFs
Densification of the composite
Optimisation of processing conditions with regard to densification and interfacial reactions
Remaining porosity is higher, interfacial reactions must be controlled with high precision, analysis of interface difficult; low CNF loading can be realized by PM process using CNFs, high loading of CNF only achievable via liquid phase infiltration of pre-forms
Interface Interface plays an important role for mechanical and thermophysical properties
Interface plays an essential role to exploit the potential of the reinforcement => using of alloying elements and/or coated CNFs
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FUNCTIONAL MATERIALS
Copper – Carbon Nanofiber Composites:
Cu coated CNF+
Cr or Ti powder
PVD (Cr, Mo) coated CNF+
Cu powder
Route A: Electro chemical coating of Cu on CNF Admixing of „Active elements“
Route B: „Active element“ directly deposited on CNF Admixing of Cu powder
Dispersion of CNF in copper matrix: =>using of chemical coating techniques to deposit the matrix material (copper)
on the CNF
Reduction of Thermal Contact Resistance between Copper and CNF =>Interface „design“ between copper matrix and carbon nanofiber necessary
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FUNCTIONAL MATERIALS
Processing: Powder Metallurgical (PM) process
Hydraulic pressure
Vacuum chamber
Graphite Heater
Graphite die
Graphit Punch
Powder
Graphite Punch
Chemical Coating/“decoration“ of CNF with Cu
1. Coating/Mixing
2. Hot Pressing 3. Composite
PVD coating (Mo) on CNF
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FUNCTIONAL MATERIALS
Results (I): Comparison of microstructure
ROUTE A ROUTE B
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FUNCTIONAL MATERIALS
Results (II): Comparison of microstructure
ROUTE A ROUTE B
„Perfect CNF distribution for route A Clusters of CNF observed in route B resulting in porosity
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FUNCTIONAL MATERIALS
Thermal Properties:
pca
0 20 40 60 80 100 1200
20
40
60
80
100
120
140
160
180
200
+5%+14%
Th
erm
al C
on
du
ctiv
ity [W
/mK
]
PVD "nominal" coating thickness [nm]
Cr Mo
0 1 2 30
50
100
150
200
250
300
350
+20% +22%
Th
erm
al C
on
du
ctiv
ity [W
/mK
]
Alloying Content [wt.%]
Ti Cr
ROUTE A ROUTE B
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FUNCTIONAL MATERIALS
CTE (@50°C)=12.8 ppm/KCTE (@250°C)=14 ppm/K
Coefficient of thermal expansion (CTE)
Reduction of CTE by addition of CNF was achieved (12.8 ppm/K) Further reduction expected by increase of the CNF volume content High temperature applications (>300°C) might lead to a degradation of
the interface (optimisation necessary)
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FUNCTIONAL MATERIALS
Conclusion (I): PM processing „perfect“ CNF distribution is necessary and can be realised by the copper
coating on the CNF Both concepts: PVD coating of fibers with „active“ element and alloying of
„active“ elements showed significant improvements compared to pure Cu-CNF composites (up to 20 % in thermal conductivity)
Further improvements (up to 100%) are expected from: Use of better quality of CNF material (lower impurity, high temperature treated CNFs) Increasing of CNF content from approx. 20 vol% to 40 vol.% Optimisation of the content of the „active“ element Optimisation of processing/sintering conditions
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FUNCTIONAL MATERIALS
Liquid Phase Infiltration of CNF felts: concept Infiltration process would allow to realise composite materials with a
higher CNF loading (40vol% or higher) => higher thermal conductivity and lower CTE
Larger Parts can be manufactured (compared to PM) Main Problem: No wetting between carbon/CNF by copper and high
Thermal Contact Resistance between Copper and CNF =>
Approach: using of „designed“ copper alloys which promote wetting and form a good thermal and mechanical interface in combination with proper selected processing conditions
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FUNCTIONAL MATERIALS
First results (I): Wetting of CNF by Cu and Cu alloys
Cu alloy
CNF Foam
Non wetting between CNF and pure Cu Wetting of Cu alloy
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FUNCTIONAL MATERIALS
Increasing reaction between Cu-Alloy and CNF
First results (II): Infiltration of CNF with Cu alloy
Metal
CNT pre-form
Pressure
Alloying Content/Contact Time between melt and CNF requires a careful optimisation to avoid gradients and reaction products (total consumption of CNF) due to severe reaction between CNF and melt
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FUNCTIONAL MATERIALS
Conclusion (II): Infiltration processing The use of Cu alloys results in wetting of the CNF felt First infiltration tests have shown that infiltration is possibleBUT Further optimisation (of alloy composition and process) is necessary to
allow a complete infiltration and to avoid too severe reactions between the alloying elements and the CNF felt
„Quality“ of CNF felt (its high thermal conductivity) is not confirmed yet Thermal analysis of composite materials will be necessary to assess