IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Long Term Stability of VariousAluminium Graphite Composites

7th European Advanced Technology Workshop on

Micropackaging and Thermal Management

La Rochelle, France

February 2011

K. Höll, K. Reiser, W. ScheckHoffmann & Co Elektrokohle AG, Bad Goisern, Austria

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Hoffmann Elektrokohle

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Hoffmann Elektrokohle

Products

Carbon Sliding Strips Ground Contacts

Brush Holder Assemblies Carbon Brushes

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Management Materials

Aluminium Graphite (ALG)

- Metal Matrix Composites (MMC)- adapted Coefficient of Thermal Expansion (CTE)- high Thermal Conductivities (TC)- lightweight- excellent machinability customized parts- coating possibilities- variety of possible applications

Al (liquid)

Graphite ALG

Manufacturing Process:

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Management Materials

Review: 6th ATW on Micropackaging and Thermal Management (2011)

- The properties of Aluminium Graphite Compositescan be tailored by using different graphite preforms

- Different graphites can be produced be usingdifferent raw materials

- Isotropic or anisotropic properties are carriedfrom the graphite preform to the final compositematerial

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Wiedemann-Franz law:

→ valid for metals, not for ALG materials!→ graphite has a strong phonon contribution to the thermal conductivity

Basics

TL ee

e

L

Thermophysical Properties

e

T

… thermal conductivity (electrons)

… electrical conductivity (electrons)

… Lorenz number (2,45 x 10-8 WΩ/K)

… Temperature

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Fatigue

Problem

Literature: Composite materials may be susceptible to thermal fatigue [1], [2]

CTE mismatch of monolithic phasesCTE: Aluminium 22-24 10-6/K

Graphite 1-7 10-6/K residual stresses in composite materials after production

Aluminium: tensileGraphite: compressive

Expected thermal fatique damage if thermally treated afterwards Influence on material‘s properties

- higher electrical resistance- lower CTE- but: no change in thermal conductivity

[1] T. Etter et al.: Physical properties of graphite/aluminium composites produced by gas pressureinfiltration method, Carbon 41 (2003), 1021-1022

[2] T. Etter: Material physical description of interpenetrating aluminium/graphite composites producedby liquid metal infiltration, Diss. ETH No. 16096 (2005), 97 ff

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Test Conditions

Experimental

Thermal cycling: in air

Cycles: 250, 500, 1000, 2000

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Investigations

- Electrical conductivity- Thermal conductivity- CTE

Measurements before/after tests

Test Conditions/Thermophysical Properties

Experimental

High temperature storage: 180°C in air

- 2.000 hours- 4.000 hours- 8.000 hours- (16.000 hours not completed yet)

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Materials

0,50,80,41,4µΩ.mSpec. elec. resistance

352595120MPaFlexural strength

450500>220>140W/m.KThermal conductivity

Microstructure

738810-6/KCTE

2,262,202,282,19g/cm³Apparent density

BUY

anisotropic

BUM

anisotropic

BUJ

anisotropic

AAC

isotropic

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Materials

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

High Temperature Storage

Specific Electrical Resistance

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

High Temperature Storage

Thermal Conductivity

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

High Temperature Storage

Coefficient of Thermal Expansion (CTE)

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Cycling

Specific Electrical Resistance

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Cycling

Thermal Conductivity

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thermal Cycling

Coefficient of Thermal Expansion (CTE)

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Conclusions

Summary

- Aluminium Graphite Compositesare materials for thermal management.

- The thermophysical properties of threedifferent ALG materials were measuredafter high temperature storage and afterthermal cycling.

- It was shown that both treatments have a negligible effect on the material‘sproperties.

- In contrast to specific literature, no influence of thermal fatigue wasobserved.

- Due to the proofed long term stabilty, Aluminium Graphite Composites aresuited for many different power electronics applications.

IMAPS 7th ATW on Micropackaging and Thermal Management – 1st of February 2012/HL

Thank you for your attention!

Contact:Klaus HöllTel.: +43/6135 400 351Mail: klaus.hoell@hoffmann.at