###### Abstract  
In order for ceramic additive manufacturing (AM) to achieve its full potential, it is increasingly important to develop a more rigorous understanding of fundamental phenomena that govern the kinetics and thermodynamics of ceramic AM processes. In the case of additive build processes, such as direct ink write and ceramic extrusion, methods for densifying the resulting green-body product need to be considered to complement the efficiencies of ceramics AM, itself. One densification route, at least for monolithic components, built layer-by-layer, is offered by the recently developed cold sintering process, whereby high-density final product is achieved through addition of a small amount of liquid solvent and application of modest uniaxial compressive stress at relatively low temperature. In situ small-angle X-ray scattering methods and X-ray diffraction have been applied to characterize and quantify the pore morphology evolution during cold sintering for a model system: potassium di-phosphate, KH\({}_{2}\)PO\({}_{4}\) (KDP). It is shown that both temperature and applied stress affect the densification rate, but stress has a stronger effect on the evolving morphology. A regime with an approximate linear densification rate can be identified, yielding an effective densification activation energy of \(\approx\)90 kJ/mol. additive manufacturing; cold sintering process; ceramic densification; microstructure characterization; X-ray scattering; materials and process characterization tools +
Footnote †: journal: Applied  
In CSP, water, an alkaline or acidic aqueous solution, or a non-aqueous organic solvent, is used to facilitate densification of a ceramic green body under uniaxial stress. In contrast to traditional high-temperature ceramic sintering processes (mass-diffusion controlled processes requiring temperatures that can approach the melting point), CSP temperatures offer reduced energy consumption hence reduced carbon emissions. Like ceramics AM in general, CSP can play a role in advanced manufacturing of ceramic materials if developed as a post-process densification step. Although originally demonstrated for metal oxides, CSP can now be applied to a wide range of ceramic systems, including non-oxides and has been applied to ceramic materials for applications that encompass composites, solid-state battery electrolytes, dielectrics, and various structural ceramics [6; 7; 8; 9; 10]. In this connection, it should be noted that while the application of uniaxial stress suggests finished components with planar or layered geometries, these can include components with gradient microstructures or composites with quite complex 3D morphologies, or layered structures with different melting temperatures. Despite these successes, fundamental aspects of the cold sintering mechanisms remain unclear. While incongruuous dissolution and reprecipitation of the ceramic phase in the presence of added liquid solvent is considered the dominant mechanism, other sintering modes such as viscous sintering may also play a role [11]. Furthermore, while a low-density high-defect phase has been shown to form between the ceramic grains during CSP both in potassium di-phosphate (studied here) [6] and in ZnO [12], it is not clear whether this phase densifies to full theoretical density during the later stages of CSP. In many cases, the onset and time scale for expulsion/evaporation of excess liquid solvent during CSP also remain uncertain [13]. Finally, the coexistence of multiple phenomena associated with densification can obfuscate the physical meaning of activation energy values extracted from [PERSON] analyses of the CSP temperature dependence at constant stress, based on a master sintering curve approach [14]. However, it is shown below that the temperature dependence of the densification rate during part of the CSP regime can be used to extract an effective activation energy with potential predictive capabilities at least for some aspects of CSP during densification of potassium di-phosphate.