During sintering, ceramic powders are heated about halfway to their melting point to compress them into a denser solid. This practice is energy-intensive and it is restricted to materials that can withstand high heat.

Hanzheng Guo, Clive Randall, and colleagues, The Pennsylvania State University, University Park, USA, have developed a “cold sintering” process, which uses pressurized water to partially dissolve particle surfaces. The particles can then rearrange in a more compact way and the gaps between them can be closed when the water evaporates [1]. They modified their process for use with BaTiO₃, an important material in the electronics industry that is typically sintered at 1200–1400 °C. They maintained a high barium ion concentration in the solution phase to retard leaching of barium ions from the particle surfaces, and they added titanium ions to promote BaTiO₃ precipitation.

When the team cold-sintered pre-annealed BaTiO₃ pellets at room temperature, capillary forces and pressure drove water into the pore spaces and partially dissolved the particle surfaces. Next, they raised the temperature to 180 °C to evaporate some of the water and re-precipitate ions onto the surfaces, joining the particles together. After post-annealing at 700–900 °C, the pellets reached a relative density of about 95 %.

• Hydrothermal-Assisted Cold Sintering Process: A New Guidance for Low-Temperature Ceramic Sintering,
  Hanzheng Guo, Jing Guo, Amanda Baker, Clive A. Randall,
  ACS Appl. Mater. Interfaces 2016.
  DOI: 10.1021/acsami.6b07481

References

• [1] Protocol for Ultralow-Temperature Ceramic Sintering: An Integration of Nanotechnology and the Cold Sintering Process,
  Hanzheng Guo, Amanda Baker, Jing Guo, Clive A. Randall,
  ACS Nano 2016.
  DOI: 10.1021/acsnano.6b03800