Copper-zirconium tungstate composites exhibiting low and negative thermal expansion influenced by reinforcement phase tr
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Copper-Zirconium Tungstate Composites Exhibiting Low and Negative Thermal Expansion Influenced by Reinforcement Phase Transformations DORIAN K. BALCH and DAVID C. DUNAND A fully-dense Cu-75 vol pct ZrW2O8 metal matrix composite was fabricated by hot isostatic pressing of Cu-coated ZrW2O8 particles. A small amount of the high-pressure -ZrW2O8 phase was created during the cooldown and depressurization following densification; near complete transformation to -ZrW2O8 was achieved by subsequent cold isostatic pressing. The thermal expansion behavior of the composite between 25 °C and 325 °C was altered by the cold isostatic pressing treatment, and also depended on the length of time that had passed between thermal cycles. The measured thermal expansion coefficients within specific temperature ranges varied from 6 106 K1 to far above the thermal expansion coefficient of the copper matrix. The complex temperature-dependent expansion/ contraction behavior could be justified by considering the evolution of phase transformations taking place in the ZrW2O8 phase, which were observed by in-situ synchrotron X-ray diffraction measurements.
I. INTRODUCTION
ZIRCONIUM tungstate (ZrW2O8) is a ceramic with a strongly negative coefficient of thermal expansion (CTE).[1–4] In contrast to most other ceramics exhibiting negative CTE,[4,5] the CTE of ZrW2O8 is isotropic and has a large negative magnitude (average CTE of 7.2 106 K1) over a wide range of temperature (273 °C to 777 °C).[4] These unusual properties suggest the incorporation of ZrW2O8 into a metallic matrix, to create a metal matrix composite with very low CTE, which could be tailored to specific engineering applications. Such a composite could be used in electronic packaging applications where high thermal conductivities coupled with low CTE values matching that of silicon (4.1 106 K1) or alumina (6.7 106 K1) are desired.[6,7] Alternatively, a ZrW2O8base composite could be designed to have a CTE of zero, i.e., constant dimensions over a specified temperature range, which would be advantageous in metrology, precision optics, and space structures.[8] At lower metal content, a composite with negative CTE could be achieved, with applications for temperature compensation, e.g., for optical Bragg gratings.[4] Also, ZrW2O8 additions have been shown to reduce the CTE of cement-sand mixtures, with potential application in temperature-compensated concrete.[9] Recently, it was demonstrated that hot-pressing could be used to create Cu-ZrW2O8 composites, provided processing temperature and time were maintained within a process window where chemical reaction between the two phases does not take place.[10] The thermal expansion behavior of ZrW2O8 is complicated, however, by the fact that it undergoes two phase transformations from the -phase stable at ambient temperature and pressure to (a) a high-temperature -phase, formed at 155 °C and ambient pressure;[2] and (b) a high-pressure -phase, formed at ambient temperature and hydrostatic p
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