US technique could rapidly create high-density ceramics

Dr Jay Narayan’s new technique, selective-melt sintering, reportedly allows sintering of yttria-stabilized zirconia at 800^oC instead of the conventional 1450^oC.

In addition, using the selective-melt sintering technique, it is possible to sinter zirconia at 800^oC in less than a second, and create a material with no porosity. In contrast, traditional sintering techniques take four to five hours at 1450^oC.

‘This technique allows you to achieve ‘theoretical density,’ meaning it eliminates all of the porosity in the material,’ said Narayan, who is the John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State. ‘This increases the strength of the ceramic, as well as improving its optical, magnetic and other properties.’

According to a statement, the key to Narayan’s approach is the application of an electric field, at approximately 100 volts per centimetre, to the material. When this field is applied, it creates subtle changes in the material’s grain boundaries.

Namely, the field draws ‘defects’ to the grain boundary. These defects consist of missing atoms (vacancies) which can carry charges. The defects are negatively charged and draw current from the electric field to the area, which raises the temperature along the grain boundary.

Raising the temperature along the grain boundary means that the material can be sintered at a much lower temperature, because sintering is done by selectively melting the grain boundaries to fuse the crystals together.

The work is described in two papers published in Scripta Materialia. The papers are ‘Grain growth model for electric field-assisted processing and flash sintering of materials,’ and an invited viewpoint paper, ‘New mechanism for field-assisted processing and flash sintering of materials.’ Narayan is the sole author.