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Ceramics have traditionally been processed at high temperatures, often exceeding 1000 ’C, using relatively slow heating rates. Despite continuous technological advances in the world, the fundamental techniques of sintering have remained largely unchanged for decades. A major shift began with the emergence of flash sintering in 2010, initially observed as an experimental anomaly, showing that densification could occur within seconds under the application of an electric field. This unexpected behavior challenged the conventional time–temperature paradigm and opened the way for a new generation of non-conventional sintering approaches.

Within this broader context, cold sintering has gained attention in 2016 as a low-temperature alternative, enabling densification below 400 °C through solution-assisted mechanisms and external uniaxial pressure. Although similar concepts were explored approximately half a century ago within hydrothermal processing, its simple and modern development has created new opportunities, particularly for materials capable of dissolution and reprecipitation. At the same time, this also defines one of its inherent limitations, as not all ceramic systems respond equally to such conditions. My PhD began within this evolving landscape, focusing on cold sintering during its rapid development as a research field. This approach is particularly effective for material systems where dissolution-assisted mechanisms can be activated, such as silica-based compositions or ZnO, where densification can be easily achieved at relatively low temperatures.

Through the support of the JECS Trust mobility program, I later continued non-conventional sintering techniques at the University of Trento, shifting towards processing strategies based on high heating rates. Fast firing represents one of the most accessible ancient approaches in this direction. By rapidly exposing materials to high temperatures, it becomes possible to reduce the time spent in temperature ranges where grain coarsening dominates, promoting densification instead. In potassium sodium niobate, fast firing leads to higher density compared to conventional sintering, while the final properties remain dependent on the processing conditions. A similar effect can be observed in more complex systems, where high heating rates favor sintering over crystallization and directly influence the final microstructure and mechanical response.

In 2020, ultrafast high-temperature sintering (UHS) further extends this concept by enabling densification within seconds. Under such conditions, the role of the processing atmosphere becomes important. For example, yttria-stabilized zirconia processed under nitrogen shows clear differences in phase evolution and grain boundary behavior compared to inert argon atmospheres, indicating that atmosphere is not a simple background parameter but an active component of the process. At very high heating rates, another aspect becomes relevant. The system does not always have enough time to reach equilibrium. As a result, grain boundaries can remain in metastable states, which can alter diffusion behavior and densification kinetics. This shifts the focus from simply achieving high density towards controlling interfaces and defects during processing.

All these observations point to a simple conclusion. Non-conventional sintering techniques depend on key processing parameters that differ across methods and define both their advantages and limitations. In practice, this means that these approaches are not universal solutions, but complementary tools, each effective only within its own sintering window.

“There is no universal sintering route for all ceramics”

Levent Karacasulu 

Glass & Ceramics Lab, Department of Industrial Engineering, University of Trento, Italy

E-mail: leventkaracasulu@gmail.com levent.karacasulu@unitn.it

LinkedIn Profile: https://www.linkedin.com/in/levent-karacasulu-13b13168

Further details can be found in recent publications:

https://doi.org/10.1016/j.cossms.2020.100807

https://doi.org/10.1016/j.icarus.2022.115270

https://doi.org/10.1016/j.oceram.2024.100541

https://doi.org/10.1016/j.jeurceramsoc.2024.116879

https://doi.org/10.1146/annurev-matsci-080323-042441

https://doi.org/10.1016/j.cossms.2026.101258