The formation of copper aluminate by solid-state reaction
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Solid-state reactions between bulk samples of copper oxide and alumina have been studied using scanning electron microscopy and electron microprobe analysis. Both CuAl 2 O 4 and CuA102 were found to form during reactions in air at 1100 °C between CuO powder and single-crystal alumina substrates. The relative position of the CuAl 2 O 4 and CuA102 layers was observed to depend on the crystallographic orientation of the surface of the alumina substrate: CuAl 2 O 4 formed in contact with (0001) alumina substrates while CuA102 formed when the alumina substrate surface was (1120). Faceted Cu-amminate/alumina phase boundaries were observed to develop when single-crystal alumina rods were reacted with CuO, although the interfaces invariably tended to be wavy.
I. INTRODUCTION Solid-state reactions1 are an important step in the processing of many ceramic materials. The phase relations that develop during these reactions are traditionally examined from bulk diffusion couples that have been formed using polycrystalline materials. Many previous solid-state reaction experiments have been conducted on "model" oxide systems, such as NiO/alumina, where only one phase forms during the reaction and where each phase in the reaction couple possesses a relatively simple crystal structure. In the NiO/alumina portion of the N i - A l - 0 system, for example, the spinel NiAl 2 O 4 is the only stable reaction product and shares a face-centered cubic oxygen sublattice with the NiO and a related pseudo-hexagonal close packed oxygen sublattice with alumina. Crystallographic influences on the reaction between NiO and alumina have been observed previously.2 In the present study of the C u - A l - 0 system, the chemical and structural relations are more complex. Two oxides of copper can form in air: CuO (melaconite or tenorite) is the stable room-temperature oxide where the Cu ion is in the +2 valence state. Cu2O (cuprite) where the Cu ion is in the +1 valence state is thermodynamically stable above 1020 °C. There are also two stable copper aluminates that can form in the C u - A l - 0 system: CU(II)A1 2 O 4 (spinel structure) and Cu(i)A102 (delafossite structure3). The Cu—Al-0 ternary system is of considerable interest in electronic packaging where thin Cu wires are bonded to alumina substrates. In this application, the substrates act as electrical insulators and, in addition, dissipate the heat generated by the active electronic
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'Present address: Analytical Sciences Laboratory, The Dow Chemical Company, Midland, Michigan 48667. b )Address correspondence to this author.
1958 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 9, Sep 1991
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components.4 It has been suggested5 that a ternary bonding environment is required at the Cu/alumina interface in order to optimize the adhesion properties of Cu on alumina. Furthermore, it has been shown that small Cu particles can adopt special orientation relations to (0001) and (1120) alumina substrates.6 The question then arises as to whether the copper oxides and Cu-a
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