[Image above] Bulk compression of single-crystal potassium tantalate oxide along the <001> direction. (A) Engineering stress–strain curve. (B1–B6) Screenshots of the in-situ bulk compression at different strains. The black arrows indicate the slip traces, and the red arrow indicates the crack formation. The scale bar in (B1) is consistent for all six subfigures. Credit: Fang & Zhang et al., Journal of the American Ceramic Society (CC BY 4.0)
With clocks now turned back to the best time for natural circadian rhythms, we have an extra hour of sleep to prepare for the upcoming winter holiday season. And for many people, those preparations include breaking out the stamps and envelopes to send an annual holiday card to family and friends.
Of the countless options for holiday cards, my favorite type growing up were musical ones that played a song when opened. Even though I now know it is not magic but science that makes these cards work, I would argue that just makes the technology even more impressive.
Inside musical greeting cards are small speakers made of piezoelectric materials, or materials that can convert mechanical stimuli into electrical signals and vice versa. Opening the card causes a circuit to close and apply voltage to the piezoelectric speaker, which leads it to vibrate and produce sound waves that match the predetermined song.
Ceramics such as lead zirconate titanate are popular piezoelectric materials because of their high polarization, which means they can be used to generate high voltages. Harnessing this potential depends on the ceramic’s ability to handle mechanical stress, however, and ceramics are traditionally known to be brittle materials.
In recent years, an upswell of research on dislocations in ceramics has shown that some ceramics may be more ductile at room temperature than previously believed. Dislocations are one-dimensional line defects that are the main carriers of plastic deformation in crystalline solids. Metallurgists regularly use dislocations to tune mechanical properties in metallic materials. The possibility of using this technique to functionalize ceramics has been known for some time, but it is only within the past decade or so that it started gaining in popularity among ceramic engineers.
To date, room-temperature plasticity in ceramics has been largely limited to the nano- and microscale. Of the ceramics that do exhibit bulk and mesoscale plasticity under ambient conditions, most of them are alkali halide crystals or simple oxides with rock-salt structure, such as magnesium oxide.
These ceramics typically have bandgaps well above 7 eV, which is much wider than the so-called wide-bandgap semiconductors, such as silicon carbide and gallium nitride (~3–4 eV). This much wider bandgap limits their application in electronic devices.
In the past 20 years, researchers have identified a few bulk ductile ceramics with desirable electrical properties, including the perovskite oxides strontium titanate (SrTiO3, 2001) and potassium niobate (KNbO3, 2016). But “the pursuit of finding more room-temperature ductile ceramics remains largely unexplored so far,” researchers write in a recent open-access paper.
The researchers come from several institutions in Germany, China, and the Czech Republic. They are led by ACerS member Xufei Fang, group leader of Dislocations in Ceramics and Hydrogen Micromechanics at Karlsruhe Institute of Technology, Germany.
Fang has led research on dislocation-tuned properties of ceramics since 2019, when he joined Jürgen Rödel’s ceramics group at the Technical University of Darmstadt in Germany as a junior group leader. He and his colleagues have now published numerous papers on procedures for mechanically generating and tuning dislocations in ceramics, including the cover story for the June/July 2023 ACerS Bulletin.
In the new paper, Fang and his colleagues turned their attention to the perovskite oxide potassium tantalate oxide (KTaO3). This ceramic has garnered a lot of attention recently due to its desirable spin and tunable ferroelectric properties, which make it an ideal choice for electronic applications. It also has a very similar crystal structure to the ductile perovskite oxides SrTiO3 and KNbO3, which suggests it may exhibit bulk ductility as well.
If KTaO3 does exhibit bulk ductility, it would “represent an additional degree of freedom that translates into all these [electronic] applications, and it is therefore likely to spark more research interest in dislocation-tuned functional properties in KTaO3,” the researchers write.
To test the potential bulk ductility of KTaO3, Fang and his colleagues first tested the surface ductile behavior of KTaO3 using their previously established experimental deformation toolbox (here and here) for mechanical tailoring of dislocations. This toolbox includes procedures for performing cyclic Brinell indentation and scratching tests on samples approximately 1×5×5 mm in size.
Based on these tests, the researchers determined that KTaO3 demonstrates a very similar plastic zone size and slip trace features to SrTiO3, the most established ductile perovskite oxide system.
“Such similarities suggest that the lattice friction stress in KTaO3 shall be sufficiently low to allow easy dislocation glide and multiplication at room temperature,” they write.
To validate this assumption, the researchers performed uniaxial bulk compression tests on KTaO3 samples approximately 3×3×6 mm in size. They used transmission electron microscopy to visualize the dislocation structures in the plastic zone.
These tests revealed that KTaO3 requires a higher level of shear stress to experience dislocation movement (~137 MPa) compared to SrTiO3 (∼90 MPa) and KNbO3 (∼30 MPa), even though these materials share similar structures.
This finding raises intriguing questions concerning the origin for such differences, particularly when compared with most other perovskite oxides that do not display bulk plasticity at room temperature. Further high-resolution microscopy characterizations, as well as computational simulations, will be needed to understand the underlying mechanisms for yielding such dislocation plasticity.
Regardless, “it is expected that our finding will serve as a fundamental building block for upcoming versatile studies in KTaO3 tuned by dislocations,” the researchers write.
The open-access paper, published in Journal of the American Ceramic Society, is “Room-temperature bulk plasticity and tunable dislocation densities in KTaO3” (DOI: 10.1111/jace.20040).
