Porosity is a characteristic present in most sintered materials, full densification only being achieved in special cases. For some sintered materials, porosity is indeed a desired characteristic, serving for the intended application of the material. In any case, the porosity present in materials can have a strong effect on some of their properties, both structural and functional. In this paper, some of the expressions proposed to describe the influence of the total porosity on the effective properties of sintered materials are examined. Moreover, a universal expression (with two fitting parameters) valid to satisfactorily represent all the analysed behaviours is proposed. One of these parameters can be assimilated to the tap porosity of the powders used to manufacture the material. The properties examined were elastic moduli, ultimate strength, thermal and electrical conductivities, magnetic characteristics, and other properties directly related to these ones. The study is valid for sintered materials, both metallic and ceramic, with a homogeneous and non-texturised microstructure.

This work presents the development of the Electrical Resistance Sintering (ERS) process for the fabrication of hard metals. The compositions of the materials produced were WC with 6 and 10 wt% of Co. In addition to the specific characteristics of the technology, the characterization of the produced parts is presented and compared to materials obtained by conventional processes.\ud \ud The parts produced by ERS present densities comparable to the ones obtained by conventional methods. The microstructural comparison shows a considerable grain size reduction in the ERS materials which consequently brings a hardness increase. ERS materials show similar fracture toughness to conventional ones.\ud \ud The very fast sintering allows performing the process without any protective atmosphere, therefore making this process very attractive for the production of materials that need to be sintered under non-oxidising environments. The total duration of the cycle, including heating, holding time and cooling is few seconds.\ud \ud Finally, some considerations about the scale up and possible industrialization of the technology are explained.This work is financially supported by the Seventh Framework Program\ud of the Commission of the European Communities under project\ud EFFIPRO contract no. NMP2-SL-2013-608729

A simple new equation for calculating thermal and electrical conductivities of powder sintered compacts is proposed. In this equation, the effective conductivity of the sintered compact is a function of the solid material conductivity, the degree of compact porosity and the packing porosity of the starting powder. The latter parameter determines to a great extent the compact pore structure, since it depends on powder particle size, shape and distribution. The new equation is physically applicable from zero porosity to the packing porosity. The conductivity equation has been evaluated using sintered compacts of iron, nickel, titanium and aluminium. The compacts were prepared by cold pressing and sintering. Results are in very good agreement with theoretical predictions.