Influence of monomodal K2CO3 and bimodal K2CO3 + NaCl as space holders on microstructure and mechanical properties of porous copper

Porous copper is potentially very attractive for numerous advanced applications like heat exchangers, heat pumps, rechargeable batteries, fuel cells, catalytic substrates for chemical reactions, EMI absorbers, purification of drinking water, etc. Monomodal K2CO3 has often been used as space holders for the fabrication of porous copper and their mechanical properties have been well studied. Within the scope of this work, porous copper samples have been fabricated for the first time using a bimodal mixture of K2CO3 + NaCl. The particle size of the NaCl powders used was considerably larger than the K2CO3 particles and correspondingly, the pore structure of the bimodal samples consisted of both large pores and fine network pores. A detailed comparison of the microstructure, elastic properties, compressive stress-strain behavior, and energy absorption characteristics between monomodal and bimodal porous copper samples has been performed. The results show that due to the presence of a larger amount of liquid phase in the bimodal samples during sintering from the melting of lower melting point NaCl particles, improved bonding between the Cu particles was achieved in these samples. Correspondingly, for similar content of space holder particles, the compressive strength and stiffness of the bimodal samples were better than the monomodal samples. While for bimodal samples the energy absorbed until densification increased monotonically with increasing porosity, for monomodal samples, the energy absorbed first increased up to a critical porosity and then decreased. The densification behavior during compression of the two types was also different - while for the bimodal samples the densification resulted in enhanced stiffening, in monomodal samples, even after densification the pores played a major role in reducing the stiffness.