In-situ formation of Cu–Al2O3 nano-scale composites by chemical routes and studies on their microstructures

Copper–alumina composite briquettes have been prepared by hydrogen (pH2=0.1 atm) reduction of a homogeneous mixture of finely divided CuO and Al2O3 formed by two chemical processes. In these processes, the Al2O3 of the desired pct. has been formed in-situ by (1) addition of Al(NO3)3 solution to CuO; or (2) by addition of stoichiometric amount of ammonia to a slurry of CuO in Al(NO3)3 solution, followed by filtration, and then heating the mixture in both the cases, at 850°C for 2 h. The Cu–Al2O3 composites thus formed, are sintered at 975°C for 2 h in H2 atmosphere (composites C-1 and C-2). Composite samples have also been prepared by H2 reduction of the briquette, made out of; (3) the mixture of Al2O3 and CuO powders; or (4) by direct addition of Al2O3 to Cu powder, followed by sintering at 975°C for 2 h (composites C-3 and C-4), in order to compare their homogeneity and microstructures with those (C-1 and C-2), made by the above two chemical processes. The characterization of the composite samples has been done by metallography, SEM and TEM techniques. The distribution of Al2O3 particles in the Cu matrix, is found to be improved when the Cu is produced in-situ by reduction of CuO by H2 and very significant improvements in this regard, are noticed in case of composites prepared by the chemical methods, particularly the one through Al(NO3)3 decomposition. In this case (C-1), besides the grain refinement of Cu (50–250 nm) and homogeneous distribution of Al2O3 in it (about 10 nm in size), an appreciable amount of a third phase is found to appear. The existence of the third phase has been indicated by SEM, TEM and EDS analyses. The third phase is suggested to be possibly CuAlO2.