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

The heat is on!

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

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)

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

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

Diamond price 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] Industrial diamond price 2005: 10’000.- down to 600.- [US$/litre]

The making of diamond composites

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

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

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

Net-shape fabrication

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)

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)

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?

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

Ag-Si: thermal conductivity After infiltration L.Weber, Metall. Mater. Trans. 33A (2002) 1145-50

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         

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.

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

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

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

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

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

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

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

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

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

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.