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Cold sintering of ceramics instead of high-temperature firing

Friday, August 19, 2016
Microstructures of a–d) NaCl, e,h) Li2MoO4, f,i) Na2Mo2O7, and g,j) K2Mo2O7. NaCl sintered at a) room temperature and 75 % relative humidity (RH) for 24 h, b) room temperature and 85 % relative humidity for 10 h, c) 600 °C for 50 min, and d) 700 °C for 10 min. Cross-section of e) Li2MoO4, f) Na2Mo2O7, and g) K2Mo2O7 ceramics cold-sintered at 120 °C and 350 MPa for 15 min. Prepared h) Li2MoO4, i) Na2Mo2O7, and j) K2Mo2O7 powders.

Both hobbyists' pottery and engineered high-performance ceramics are only useable after they are fired for hours at high temperatures, usually above 1000 °C. The sintering process that takes place causes the individual particles to "bake" together, making the material more compact and giving it the required properties, like mechanical strength.

In the journal Angewandte Chemie, American researchers have now demonstrated that sintering can also take place at significantly lower temperatures. This cold sintering process is based on the addition of small amounts of water to aid the key transport processes that densify the material.

"Since the stone age, ceramics have been fabricated by sintering at high temperatures," reports Clive A. Randall from Pennsylvania State University (USA). "This includes the Venus of Doli Vestonice, one of the oldest ceramic objects." The traditional firing process may now become unnecessary for many ceramic materials, because a broad spectrum of inorganic materials and composites can also be sintered between room temperature and 200 °C.

In conventional high-temperature sintering processes, individual ceramic powder particles densify into a hard, dense object. The driving force for this process is the reduction of the high surface free energy of the powder by material diffusion—a process that only occurs at high temperatures. "In contrast, cold sintering relies on interfacial solution effects in water for the densification of the material; a process that occurs at low temperatures and over much shorter time frames, minutes instead of hours, when pressure is applied," says Randall.

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