HIGH-TEMPERATURE REACTIVITY IN THE ZrW,O,-Cu SYSTEM

Zirconium tungstate (ZrW,OJ exhibits the unusual property of a negative coefficient of thermal expansion (CTB) over a Iwide range of temperature, from 0.3 to 1050 K. This property was reported about thirty years ago (l), but only recently were the structure of this compound solved, the low-temperature CTE measurement performed and a physical explanation for the negative CTE proposed (2). Zirconium tungstate, which is stable between 1105°C and 1257°C (3), is metastable at room temperature and decomposes into the base oxides ZrO, and WOs when heated above 750°C in air (4). Beside the fundamental physical interest of a negative CTE, the material is of great technological interest for low thermal expansion applications. However its complicated synthesis, which includes many days of heating at temperature above 12OO”C, has prevented its large-scale use in engineering applications. The recent discovery of a more rapid precursor approach to the synthesis (2) is likely to change this situation. As a reinforcement in a composite, zirconium tungstate can reduce the overall thermal expansion of the composite much more effectively than a ceramic with positive CTB. If a metallic matrix is used, the resulting composite will also exhibit high thermal conductivity, with applications such as heat sink for microelectronics devices (CTE matching that of silicon or alumina) or high precision optical elements subjected to thermal fluctuation (zero CTE) (5). As a matrix, copper is prime candidate, because it has the second highest thermal conductivity of all metals after silver, and because it can be easily processed within the temperature window imposed by the metastability of ZrW,O,. Moreover, copper is already widely used in electronics industry and can be easily soldered. In the present work, we investigate the stability of ZrW,O, when hot-isostatically pressed with copper, in order to explore the processing feasibility of a low thermal expansion, high conductivity ZrW,O,-Cu composite.