arXiv subjects

Torsten Granzow

Publications and source records attributed to Torsten Granzow.

4 recordsLinked to original sources

arXiv2025

Solution-Based Fabrication of High-Performance K$_{0.5}$Na$_{0.5}$NbO$_3$ Thin Films for Surface Haptics

K$_{0.5}$Na$_{0.5}$NbO$_3$ is among the most promising lead-free piezoelectrics. While its sputtered films match the performance of the champion piezoelectric Pb(Zr,Ti)O$_3$, reproducible processing of high-quality and time-stable solution-processed K$_{0.5}$Na$_{0.5}$NbO$_3$ films remains challenging. Here, we report 1 $\mu$m-thick Mn-doped K$_{0.5}$Na$_{0.5}$NbO$_3$ films prepared through a chemical solution deposition process, which have perfectly dense microstructure and uniform composition across their thickness. The films exhibit a high transverse piezoelectric coefficient ($e_{31,f} = -15.4$ C/m$^2$), high dielectric permittivity ($\varepsilon_r \approx 920$), low dielectric losses ($\tan\delta = 0.05$) and can withstand electric fields up to at least 1 MV/cm. The functional properties show excellent stability over time, and the synthesis process is reproducible. Furthermore, a surface acoustic haptic device is demonstrated by using K$_{0.5}$Na$_{0.5}$NbO$_3$ thin-film actuators. The results demonstrate the high potential of Mn-doped K$_{0.5}$Na$_{0.5}$NbO$_3$ films to become a replacement for lead-based Pb(Zr,Ti)O$_3$ films in piezoelectric applications.

cond-mat.mtrl-sci↗

arXiv2025

Tunable electrocaloric effect in lead scandium tantalate through calcium doping

State-of-the-art electrocaloric cooling prototypes rely on the conventional electrocaloric effect of ferroelectric lead scandium tantalate (PbSc0.5Ta0.5O3, PST), which peaks near room temperature. Here, we demonstrate that A-site calcium doping in highly ordered PST modifies its phase transitions and enables precise tuning of the electrocaloric response. The transition temperature shifts down to 258 K and up to 319 K, depending on Ca concentration. Calorimetry under electric field, electrical polarization loops, and piezoresponse force microscopy reveal the emergence of an intermediate antiferroelectric phase stabilized for Ca $\geq$ 2\%. These results are supported by first-principles calculations. We observe a conventional electrocaloric effect for Ca $\leq$ 2\% and an inverse electrocaloric effect at higher doping ($\geq$ 2\%). Under an applied field of 110 kV cm$^{-1}$, Ca-doped PST exhibits an adiabatic temperature change of 2 K over a range from 263 K to 353 K. Such Ca-doped PST compounds could be used to expand the temperature range of PST below the freezing point of water. Our results offer a pathway to cascaded electrocaloric cooling devices with extended operating spans.

cond-mat.mtrl-sci↗

arXiv2024

Ferroelectric HfO$_2$-ZrO$_2$ multilayers with reduced wake-up

Since the discovery of ferroelectricity in HfO$_2$ thin films, significant research has focused on Zr-doped HfO$_2$ and solid solution (Hf,Zr)O$_2$ thin films. Functional properties can be further tuned via multilayering, however, this approach has not yet been fully explored in HfO$_2$-ZrO$_2$ films. This work demonstrates ferroelectricity in a 50 nm thick, solution-processed HfO$_2$-ZrO$_2$ multilayer film, marking it as the thickest multilayer film to date exhibiting ferroelectric properties. The multilayer structure was confirmed through transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy, with high-resolution TEM revealing grain continuity across multiple layers. This finding indicates that a polar phase in the originally paraelectric ZrO$_2$ layer, can be stabilized by the HfO$_2$ layer. The film attains a remanent polarization of 9 uC/cm$^2$ and exhibits accelerated wake-up behavior, attributed to its higher breakdown strength resulting from the incorporation of multiple interfaces. These results offer a faster wake-up mechanism for thick ferroelectric hafnia films.

cond-mat.mtrl-sci↗

arXiv2022

Birefringence induced by antiferroelectric switching in transparent polycrystalline $PbZr_{0.95}Ti_{0.05}O_{3}$ film

The most characteristic functional property of antiferroelectric materials is the possibility to induce a phase transition from a non-polar to a polar phase by an electric field. Here, we investigate the effect of this field-induced phase transition on the birefringence change of $PbZr_{0.95}Ti_{0.05}O_{3}$. We use a transparent polycrystalline $PbZr_{0.95}Ti_{0.05}O_{3}$ film grown on $PbTiO_{3}/HfO_{2}/SiO_{2}$ with interdigitated electrodes to directly investigate changes in birefringence in a simple transmission geometry. In spite of the polycrystalline nature of the film and its moderate thickness, the field-induced transition produces a sizeable effect observable under a polarized microscope. The film in its polar phase is found to behave like a homogeneous birefringent medium. The time evolution of this field-induced birefringence provides information about irreversibilities in the antiferroelectric switching process and its slow dynamics. The change in birefringence has two main contributions, one that responds briskly (~ 0.5 s), and a slower one that rises and saturates over a period of as long as 30 minutes. Possible origins for this long saturation and relaxation times are discussed.

cond-mat.mtrl-sci↗