Emerging materials for Thermal Management Al und Cu based diamond composites

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

2. The heat is on!

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

4. 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) Typical requirements on substrate or base-plate materials

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

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

7. Industrial diamond price 2005: 10’000.- down to 600.- [US$/litre] Raw material prices 2007: [US$/litre] Platinum 800’000.- Gold 380’000.- Palladium 150’000.- C-Nanotubes 12’500.- Silver 4’100.- CBN 3’000.- HC carbon fibres 2’400.- Tungsten carbide 1’300.- Tungsten 750.- Ni-Superalloys 700.- Molybdenum 680.- Titanium diboride 500.- Nickel 450.- Aluminium nitride 256.- Titanium 225.- Tin 100.- Copper 72.- Silicon carbide 50.- Alumina 40.- Aluminium 6.- Industrial diamond price 1994 (after Ashby&Jones): >1’000’000.- [US$/litre] Diamond price

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

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. 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? Matrix alloy development

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) 1145-50

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 HT @ T<700°C) and is maximum about 0.4 µΩcm after HT @ 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

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

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

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

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

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

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

29. 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. Conclusions