Besides the positive cost-efficienc

Besides the positive cost-efficiency of CSP, there are still open challenges regarding the implementation of the process at the industrial scale. First of all, (almost) only elementary geometrical shapes like discs have been produced. Up to date, just in few works researchers tried to move toward a rectangular shape [56,57] or multi-layered structures [58,59]. Fabrication of simple shapes is actually a common feature in dry/semi-dry compaction processes [4,60,61]. Since the applied pressure is not uniformly distributed due to the friction between powders and die walls, the formation of pressure gradients, and consequently density gradients, is almost unavoidable [4,60]. Another limitation to be considered in large parts production regards the geometrical aspect: thickness should never be greater than twice the diameter [4] in order to have a uniform pressure distribution and to avoid lamination and end-cap defects [60,4]. Size scaling-up showed difficulties in guaranteeing a homogeneous microstructure due to uneven evaporation of the solvent from one region to another, this causing density gradients and different grain growth rates [56]. The production of larger components by cold sintering could face technological limitations due to the applied pressure levels: hundreds of MPa require mechanical machines able to apply hundreds of tons [8]. Just to have an idea, a force of 13 kN is applied to produce a disc with Φ = 13 mm pressed at 100 MPa but to apply the same pressure to a squared specimen of L = 50 mm, a 250 kN force should be provided, which is about 20 times higher. In this sense, Bang et al. [56] tried to reduce the applied pressure down to 27−45MPa for ZnO and ZnO-based composites, when usually CSP of ZnO is carried out under 100−350MPa [19,[62], [63], [64],13,[65], [66], [67]], and they succeed in producing specimens with >88 % relative density.