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External electric field controls grain boundaries in electroceramics

By Aditi Risbud October 12, 2018
High-angle annular dark-field scanning transmission electron microscopy images of SrTiO3 bicrystals with (a) no electric field applied during diffusion bonding and (b) an electric field (30 V bias) applied during diffusion bonding. Lower half crystal is in <100> orientation. Energy-loss near edge fine structures of the O K-edges at the grain boundary core and bulk of SrTiO3 bicrystals with (c) no electric field applied during diffusion bonding and (d) an electric field (30 V bias) applied during diffusion bonding, showing the potential to tailor specific grain boundary configurations without adding dopant elements. Credit: Applied Physics Letters

Nanocrystalline ceramics naturally possess a large number of grain boundaries that play a crucial role in determining their properties and performance. Since the behavior of defects, vacancies, and dopants at grain boundary interfaces profoundly alter macroscopic physical properties, mechanisms to control grain boundary structures could provide a pathway to designing functional oxide microstructures for a wide range of applications, such as high voltage capacitors, photocatalysts, and oxygen sensors.

Traditionally, the functional properties of oxide materials are controlled by the addition of dopants that segregate to grain boundaries. These dopants can be used to manipulate densification and grain growth, but it is a challenge to form stable grain boundary networks while minimizing impurities.

In a study reported recently in Applied Physics Letters, researchers at the University of California, Davis used externally applied electric fields to alter the atomic and electronic structures within (100) twist grain boundaries of the ceramic oxide strontium titanate (SrTiO3); specifically, the locations of interfacial atoms and their bonding configurations were changed by the applied field. Interestingly, the external control system reported in this study allows scientists to alter physical properties (such as the dielectric constant) of SrTiO3 without adding dopants.

Researchers Lauren Hughes and Klaus van Benthem applied an electric field during the diffusion bonding of undoped SrTiO3 bicrystals, which resulted in different grain boundary core structures compared to bicrystals bonded in the absence of an electric field. Applying electric fields during diffusion bonding modifies the core structures of grain boundaries without changing misorientation angles. The applied field not only changed atomic and electronic interfacial structures, but also changed dielectric properties by altering the concentration of local oxygen vacancies. Across an applied electric field frequency range of 10 Hz to 100 MHz, decreases in oxygen vacancy concentration increased resistivity, which improved overall dielectric properties.

“Our findings of this study open a door to characterize, in intimate detail, the effects of electric field strength and direction on grain boundary formation,” van Benthem says. “We can now assess whether the thermodynamic stability of specific grain boundary structures can be changed by applying electric fields during manufacturing.”

The research team, including co-author M. Marple, was inspired by earlier studies that showed that electric fields enhanced densification of ceramic powder compacts as well as affected grain growth. Using scanning transmission electron microscopy to study atomic-scale defect structures, the team determined that grain boundary core structures are significantly changed by applying electrostatic fields during grain boundary formation.

“The authors show that applying an electric field during sintering of a ceramic material can alter the atomic structure of grain boundaries in the material,” says Susanne Stemmer of the University of California, Santa Barbara, who was not involved in this research. “This is an important discovery because electroceramic materials such as ionic conductors or capacitor materials are often dominated by the properties of their grain boundaries, which depend on their atomic structure. This work potentially opens up a new way to control these grain boundaries during synthesis.”

Along with the variables of time, temperature, and applied pressure, the research group hopes to leverage applied electric field strength as an additional processing parameter for future ceramic manufacturing efforts. Future studies will investigate oxygen vacancy distribution and dopant segregation in the presence of electric fields.

“If the choice of electric field application can change the grain boundary network and, hence, the resulting physical properties of functional and structural ceramics, we will be able to design new microstructures with unprecedented properties from ceramic powder compacts that we have been using in the past. This will open up a new dimension in ceramic manufacturing,” van Benthem says.

Read the article in Applied Physics Letters.