1. CH-1015, Lausanne, Switzerland
Emerging materials for Thermal Management Al und Cu based diamond composites
L. Weber
Laboratory for Mechanical Metallurgy Ecole Polytechnique Fédérale de Lausanne (EPFL)
CH-1015, Lausanne, Switzerland

2. The heat is on!

3. The heat is on! Solution: phase change materials heat pipes Solution:
small active component
transient heating
small active component
permanent heating
large active component
permanent heating
cooling plate/circuit
cold air flow
spreading/absorbing
the heat
spreading and transfer
mostly transfer
Solution:
phase change materials
heat pipes
Solution:
High l in plane
Medium/high l through plane
Solution:
High l through plane

4. Typical requirements on substrate or base-plate materials
CTE similar to that of GaN and Si (3-5 ppm/K) (passive cycling) or slightly higher (active cycling).
High thermal conductivity, l [W/mK]
High thermal diffusivity
Sometimes: electrical conductivity
Structural properties (stiffness, strength)

5. Candidate materials Metals: CTE too high Ceramics:
“no” electrical conductivity, too brittle, CTE too low
=> obvious choice:
composites

6. Composite concepts using carbon material
Chopped Carbon short-fibres
Continuous Carbon fibres
Graphite flakes
Common forms of Carbon
Diamond (particles and fibres)
Carbon nanotubes and nanofibres

7. Diamond price Raw material prices 2007: [US$/litre]
Platinum 800’000.-
Gold 380’000.-
Palladium ’000.-
C-Nanotubes 12’500.-
Silver ’100.-
CBN ’000.-
HC carbon fibres ’400.-
Tungsten carbide ’300.-
Tungsten
Ni-Superalloys
Molybdenum
Titanium diboride
Nickel
Aluminium nitride
Titanium
Tin
Copper
Silicon carbide
Alumina
Aluminium
Industrial diamond price 1994 (after Ashby&Jones):
>1’000’000.- [US$/litre]
Industrial diamond price 2005:
10’000.- down to [US$/litre]

8. The making of diamond composites

9. Liquid metal infiltration process
Alternative routes:
hot pressing of powder mixtures
hot pressing of coated particles

10. Pressure infiltration apparatus
Cold wall vessel (250 bar, 200°C) Inner side of the wall in contact with a water cooled heat shield
Induction heating (using a graphite susceptor)
primary vacuum pump (0.1 mbar)
Crucible can be lowered on quench (directional solidification)
100 mm

11. Selected diamond grit Mono-crystalline diamond Low nitrogen level
Relatively large size (>100µm)

12. Net-shape fabrication

13. Ag-Diamond composites
Pure Ag + 60 %-vol diamonds (100µm)
Low thermal conductivity (270 W/mK)
High coefficient of thermal expansion (≈17ppm/K)
Ag-Si alloy + 60 %-vol diamonds (100µm)
High thermal conductivity (>700 W/mK)
Low coefficient of thermal expansion (≈7ppm/K)

14. Cu-Diamond composites
Pure Cu + 60 %-vol diamonds (200µm)
Low thermal conductivity (150 W/mK)
High coefficient of thermal expansion (≈16ppm/K)
Cu-B alloy + 60 %-vol diamonds (200µm)
High thermal conductivity (>600 W/mK)
Low coefficient of thermal expansion (≈7ppm/K)

15. Matrix alloy development
What is it that makes an alloying element an “active” element
How much active element do we need to get the right interface?
And what does this quantity of active element do to the matrix properties?

16. Effect of active element on CTE
Active elements are needed to form carbides at the Metal/diamond (carbon) interface

17. Ag-Si: thermal conductivity
After infiltration
L.Weber, Metall. Mater. Trans. 33A (2002)

18. Ag-Si-X: alloy requirements
The ternary alloying element X should have/generate
“no” solubility in solid Ag
some solubility in liquid Ag
reduced Si-activity in the solid state
 weak silicide-forming element
Ni Fe Mn   
  
  

19. Ag-Ni binary system Ni content limited to 0.3-0.4 at-%
Resistivity increase due to Ni<0.05µΩcm (after T<700°C) and is maximum about 0.4 µΩcm after 950°C.

20. Ag-Ni-Si: Si activity
700°C
NiSi2
NiSi
Ni3Si2

21. Ag-Ni-Si: thermal conductivity
∆r [µΩcm]
Typical situation after infiltration

22. Kinetic effects: Al-diamond
GPI SC
Thermal conductivity
660
110
CTE
10-12
17-25

23. Interface study of Al-Diamond composites
Comparison of GPI and Squeeze Casting

24. Influence of diamond volume fraction on CTE
bimodal
monomodal
Al-SiC
Interesting CTE range can be achieved with mono-modal particle size distribution
Low pressure infiltration is possible

25. Influence of diamond volume fraction electrical conductivity
Going from 60 to 75 pct vol diamond reduces the el. conductivity by a factor >2!

26. Importance of the interface transfer problem
Electrical conductivity:
High phase contrast
No effect of interface
resistance
=> no effect of phase region size and field-line distortion
Thermal conductivity:
low phase contrast
=> Effect of interface resistance

27. Effective particle properties
Effective particle thermal conductivity:
Various models (extension to finite volume fractions):

28. Indirect measurement of the ITC — size effects
Small particles:
Higher strength
Better machinability
Lower thermal cond.

29. Conclusions
Metal diamond composites are a promising material for next generation thermal management solutions.
They can exhibit twice the conductivity of pure silver, while having a coefficient of thermal expansion similar to semiconductor devices.
The interface is extremely important for both, thermal conductivity and coefficient of thermal expansion.