Phase transformation and thermal expansion of Cu/ZrW 2 O 8 metal matrix composites
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Phase transformation and thermal expansion of CuyyZrW2 O8 metal matrix composites Hermann Holzera) and David C. Dunandb) Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 5 February 1998; accepted 15 July 1998)
Powder metallurgy was used to fabricate fully dense, unreacted composites consisting of a copper matrix containing 50 –60 vol% ZrW2 O8 particles with negative thermal expansion. Upon cycling between 25 and 300 ±C, the composites showed coefficients of thermal expansion varying rapidly with temperature and significantly larger than predicted from theory. The anomalously large expansion on heating and contraction on cooling are attributed to the volume change associated with the allotropic transformation of ZrW2 O8 between its high-pressure g-phase and its low-pressure a- or b-phases. Based on calorimetry and diffraction experiments and on simple stress estimations, this allotropic transformation is shown to result from the hydrostatic thermal stresses in the particles due to the thermal expansion mismatch between matrix and reinforcement.
I. INTRODUCTION
Metal matrix composites are attractive materials for applications where the high thermal conductivity of metals and the low thermal expansion of ceramics are simultaneously needed, e.g., in electronics heat sinks with high heat dissipation and low thermal expansion mismatch with the silicon chip or its alumina substrate.1 Because increasing the ceramic content of a composite decreases both its thermal expansion and its thermal conductivity, ceramics with as low a thermal expansion as possible are desirable to maximize the conductivityexpansion ratio of the composite.2,3 A composite such as CuyZrW2 O8 ,4 consisting of a high-conductivity metallic matrix and ceramic phase with a strongly negative coefficient of thermal expansion (CTE), is thus ideally suited for electronics thermal management applications, and may also exhibit an isotropic zero CTE at high ceramic fractions, for applications in metrology, precision optics, and space structures. The negative CTE of ZrW2 O8 was first measured between 50 and 650 ±C by dilatometry and x-ray diffraction by Martinek and Hummel,5 and was recently confirmed to exist in the temperature range 2273 to 777 ±C by dilatometry and neutron diffraction by Sleight and coworkers.6 – 8 The physical explanation for this unusual behavior is based on a steric contraction on heating due to polyhedra tilting which overweighs the usual chemical bond thermal expansion.6 –10 Compared to other
a)
Currently with Electrovac GesmbH, Aufeldgasse 37-39, A-3400 Klosterneuburg, Austria. b) Currently with Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208. 780
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 3, Mar 1999
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ceramics with negative CTE,6,11 ZrW2 O8 exhibits a CTE with a unique combination of properties: (i) spatial isotropy d
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