Reassessing cold sintering in the framework of pressure solution theory

Abstract

Cold sintering densification and coarsening mechanisms are considered from the perspective of the non-equilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolution-precipitation creep). This is an important mechanism of densification and deformation in many geological rock formations in the Earth's upper crust, and although very slow in nature, it is of direct relevance to the cold sintering process. In cold sintering, we select particulate materials and identify experimental processing parameters to significantly accelerate the kinetics of dissolution-precipitation phenomena, with appropriate consideration of chemistry, applied stress, particle size and temperatures. In the theory of pressure solution, pressure-driven densification is considered to involve the consecutive stages of dissolution at grain contact points, then diffusive transport along the grain boundaries towards open pore surfaces, and then precipitation, all driven by chemical potential gradients. In this study, it is shown that cold sintering of BaTiO3, ZnO and KH2PO4 (KDP) ceramic materials proceeds by the same type of serial process, with the pressure solution creep rate being controlled by the slowest kinetic step. This is demonstrated by the values of activation energy (Ea) for densification, which are in good agreement with the existing literature on dissolution, precipitation, or coarsening. The influence of pressure on the morphology of ZnO grains also supports the pressure solution mechanism. Other characteristics that can be understood qualitatively in terms of pressure solution are observed in the in systems such as BaTiO3 and KDP. We further consider activation energies for grain growth with respect to the precipitation process, as well as evidence for coalescence and Ostwald ripening during cold sintering. For completeness we also consider materials that show significant plastic deformation under compression. Our findings point the way for new advances in densification, microstructural control, and reductions in cold sintering pressure, via the use of appropriate transient solvents in metals and hybrid organic-inorganic systems, such as the Methylammonium lead bromide (MAPBr) perovskite.