PhD Student Sinan Faouri briefly gives us an overview of Cold Sintering Processing, a novel technique for the processing of functional materials that addresses the energy cost of manufacturing these materials.
Sintering is a thermal treatment process of forming a dense bulk solid material by heat or pressure before reaching its melting point [1,3]. The conventional sintering process is a high temperature sintering method where powders are heated between 50-75% of melting temperature to > 95% theoretical density . Heating the starting materials to high temperatures facilitates the motion of atoms, enabling the homogenisation of the bulk solid .
Cold sintering processing (CSP) is a novel technique developed recently to achieve dense ceramic solids at extremely low temperatures ( < 180 ℃ ). across a vast variety of elements and composites . The process includes using aqueous-based solutions (eg: water) as transient solvents to aid densification by a non-equilibrium mediated dissolution–precipitation process .
In order to provide a good environment and conditions for precipitation and recrystallization in hydrothermal reactions, a convenient aqueous solution should be chosen carefully which will be also significant in reducing sintering temperature .
To achieve densified bulk materials, some additional characteristics to the pre-dominance diagrams are used when studying CSP effective factors as shown in figure (1) .
Figure (1) shows a flowchart for the possible roots of CSP from particle rearrangement to densification. The internal characteristics that affect CSP are the material’s composition, crystal structure, particle size and solubility in water . Some of the physical variables determining the kinetic processes for mass transport are the hot press pressure, sintering temperature, time of sintering, the rate of heating, and the atmosphere pressure . Preparing a convenient aqueous solution is also critical as some other significant factors to CSP include the nature of the solute, the concentration of the solute and pH value. The latter can be studied via pre-dominance diagrams .
The dissolution nature plays a significant role in determining whether densification will eventually occur or not. If the materials dissolve equally with ease, a direct and simple CSP is possible, since the surface of material can be easily dissolved in water with a homogenous chemical stoichiometry . However, no densification will occur if one material dissolves more easily that the other. This is because one material will form a passive surface . Negligible dissolution can work well in CSP if a saturated solution is added that targets the chemical compounds of the material to be densified .
The next steps in CSP are evaporation of the liquid, the hydrothermal crystal growth (in the case of water as a solvent) or formation of a glass/intermediate phase, and eventually grain growth and densification or recrystallization of a glass where it gets ejected as shown in CSP flowchart in figure (1) .