Cold sintering densification and coarsening mechanisms are considered from the perspective of the nonequilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolutionprecipitation 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.
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Cold Sintering: Progress, Challenges, and Future Opportunities
Cold sintering is an unusually low-temperature process that uses a transient transport phase, which is most often liquid, and an applied uniaxial force to assist in densification of a powder compact. By using this approach, many ceramic powders can be transformed to high-density monoliths at temperatures far below the melting point. In this article, we present a summary of cold sintering accomplishments and the current working models that describe the operative mechanisms in the context of other strategies for low-temperature ceramic densification. Current observations in several systems suggest a multiple-stage densification process that bears similarity to models that describe liquid phase sintering. We find that grain growth trends are consistent with classical behavior, but with activation energy values that are lower than observed for thermally driven processes. Densification behavior in these low-temperature systems is rich, and there is much to be investigated regarding mass transport within and across the liquid-solid interfaces that populate these ceramics during densification. Irrespective of mechanisms, these low temperatures create a new opportunity spectrum to design grain boundaries and create new types of nanocomposites among material combinations that previously had incompatible processing windows. Future directions are discussed in terms of both the fundamental science and engineering of cold sintering.
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- Award ID(s):
- 1728634
- NSF-PAR ID:
- 10149608
- Date Published:
- Journal Name:
- Annual Review of Materials Research
- Volume:
- 49
- Issue:
- 1
- ISSN:
- 1531-7331
- Page Range / eLocation ID:
- 275 to 295
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1146/annurev-matsci-070218-010041
