[Image above] Schematic representation of the cold sintering process. In this example, zinc particles are initially covered by a passivation layer consisting primarily of zinc oxide and Zn(OH)2. Soaking the zinc particles in a solution of acetic acid dissolves the passivation layer and chelates the resulting Zn2+ ions. Applying external pressure facilitates the formation of contact interfaces between the zinc particles, which are sintered via the self-exchange between Zn/Zn2+. The solution is evaporated and a new passivation layer forms over the surface of the sintered electrode. Credit: Jayasayee et al., Processes (CC BY 4.0)
Traditional ceramic sintering (hot sintering, for our purposes) is a carbon-intensive process requiring very high temperatures. It typically occurs at temperatures of 1,000°C to 2,000°C for long periods of time, and so requires significant energy inputs to achieve.
To decarbonize ceramic manufacturing, alternatives to the traditional sintering process must be found. Ceramic manufacturers have considered using hydrogen or electricity to power the kilns rather than fossil fuels, but these methods still require longer processing times. Another approach that reduces both temperature and time requirements is the novel processing technique of cold sintering.
Cold sintering is a method for densifying ceramic materials at temperatures less than 400°C. The process was developed in 2016 by researchers at The Pennsylvania State University.
This CTT looks at this potentially revolutionary process, discusses its benefits and drawbacks in relation to hot sintering, and examines some of the potential innovations of this lower temperature ceramic manufacturing approach.
What is cold sintering?
As stated above, cold sintering densifies ceramic materials at relatively low temperatures compared to hot sintering, and in much less time as well (as fast as 15 minutes). It achieves this feat by using a liquid phase and external pressure to compress and densify ceramic powders.
A small quantity of liquid (as a solvent or binder) is added to ceramic powder to lower the activation energy required for particle rearrangement, grain boundary development, and densification. A thin film of liquid forms around the ceramic particles, which lowers friction and allows the particles to be pressed closer together. The liquid may cause viscous particle flow, allowing particles to deform and fill voids, thereby increasing the density.
After initial densification, solid-state diffusion can occur as the material is held at slightly elevated temperatures. This process involves the movement of atoms within the solid matrix, leading to further densification and enhanced microstructural properties.
The principles behind cold sintering can be traced to the early 1970s, when Della M. Roy et al. first used a solvent and uniaxial pressure to densify cement paste. Since then, the elements of the cold sintering process (a solvent, pressure, lower heat, and faster processing time) have steadily come together.
Benefits of cold sintering
Cold sintering achieves high-density materials without the extreme heat typically required in conventional sintering. As such, it requires less energy to power the process, which translates to reduced greenhouse gas emissions.
In addition to the environmental benefits, cold sintering presents the possibility of creating advanced multifunctional materials. For example, because of the low-temperature conditions, cold sintering can be used with metastable materials, such as those required to make solid-state batteries for electric vehicles and other machinery. It also allows the co-sintering of ceramic–polymer composites, which can be difficult to process due to the different thermal properties of the ceramic and polymer components. These capabilities make cold sintering not only an avenue to decarbonizing ceramic production, but also a facilitator of decarbonizing many other processes in our daily lives.
In addition, cold sintering improves the mechanical and thermal properties of materials by creating finer microstructures. The low temperatures reduce the risk of unwanted phase transitions. The ability to control the microstructure opens the possibility of creating additional types of conductive ceramics.
Finally, cold sintering could increase plastic recycling rates by taking waste previously destined for the landfill and combining it with ceramic material to make new composites that can be recycled many times. The abilities to create novel composites and to work with a wider range of materials gives cold sintering a much broader range of applications.
Applications and implications of cold sintering
The many potential structural and chemical modifications that cold sintering enables present intriguing possibilities for materials science. In nature, long-term geologic processes produce dense materials through pressure solution creep. (I am having flashbacks to my structural geology class now!) Studying the interactions between solvents, pressure, and activation energies could someday allow sintering at both low temperature and low pressure.
The applications of cold sintering range from the electronic to the biomedical. Combining cold sintering with conventional sintering can produce denser, finer grained pyrochlore ceramics for the long-term storage of high-level nuclear waste. Given the many potential applications and environmental benefits of the process, what’s the bad news? Well….
Challenges and limitations of cold sintering
Cold sintering has some obvious limitations compared to traditional sintering methods. Because the process is relatively new and introduces variables such as solvent choice and pressure conditions, the mechanics are not yet well understood. Perhaps the biggest challenge is in scaling up to commercial production.
Although cold sintering works well in small-scale production or prototyping, industrial-scale applications have been difficult to achieve. So far, uniformity and consistency have been sticking points in larger batches.
The choices of starting powders, solvents (and their quantities), and pressure/temperature conditions all affect the cold sintering process in ways that are still being investigated. Not all ceramic materials are suitable for cold sintering because the material must be highly soluble in a solvent. For example, silicon carbide has low solubility and is insoluble in water.
Additionally, cold sintered materials may lack the smooth surfaces of those produced by hot sintering, making them undesirable for the many aesthetic applications of traditional ceramics. Porcelain production, for example, requires high-temperature sintering, although efforts to reduce the energy-intensiveness of this process are ongoing.
Conclusion
Cold sintering of ceramics leverages the principles of liquid-assisted densification and solid-state diffusion to create high-performance ceramics at lower temperatures. Its potential for energy savings and innovative applications makes it an exciting area of research and development in materials science.
Although cold sintering offers numerous advantages, including energy efficiency and the ability to process complex shapes or create novel composite materials, its limitations highlight the need for ongoing research and development. Understanding these challenges is crucial for advancing this technology and expanding its applications in the ceramics industry.
Further reading
Grasso, S. et al. “A review of cold sintering processes,” Advances in Applied Ceramics: Structural, Functional and Bioceramics 2020, 119(3): 115–143.
Guo, J. et al. “Cold sintering: Progress, challenges, and future opportunities,” Annual Review of Materials Research 2019, 49: 275–295.
Vakifahmetoglu, C., and Karacasulu, L. “Cold sintering of ceramics and glasses: A review,” Current Opinion in Solid State and Materials Science 2020, 24(1): 100807.
