Ultrahigh dielectric permittivity in oxide ceramics by hydrogenation

Abstract

Boosting dielectric permittivity representing electrical polarizability of dielectric materials has been considered a keystone for achieving scientific breakthroughs as well as technological advances in various multifunctional devices. Here, we demonstrate sizable enhancements of low-frequency dielectric responses in oxygen-deficient oxide ceramics through specific treatments under humid environments. Ultrahigh dielectric permittivity (~5.2 × 10⁶ at 1 Hz) is achieved by hydrogenation, when Ni-substituted BaTiO₃ ceramics are exposed to high humidity. Intriguingly, thermal annealing can restore the dielectric on-state (exhibiting huge polarizability in the treated ceramics) to the initial dielectric off-state (displaying low polarizability of ~10³ in the pristine ceramics after sintering). The conversion between these two dielectric states via the ambient environment-mediated treatments and the successive application of external stimuli allows us to realize reversible control of dielectric relaxation characteristics in oxide ceramics. Conceptually, our findings are of practical interest for applications to highly efficient dielectric-based humidity sensors.

Fig. 1.. Crystal and electronic structures in the as-sintered Ni-substituted BaTiO₃ ceramics.

(A) XRD pattern of the as-sintered Ni-substituted BaTiO₃ ceramics at a doping concentration x = 0.175. a.u., arbitrary units. (B) The normalized O K-edge x-ray absorption spectra of the Ni-substituted BaTiO₃ ceramics. Peaks A and B were respectively attributed to the transition from O 1s to t_2g and e_g hybridized states between O 2p–Ti 3d. (C) Schematic diagram of the hexagonal BaTiO₃ structure viewed along the [11$\overline{2}$0] direction. (D) ABF-STEM images of the as-sintered Ni-substituted BaTiO₃ ceramic along [11$\overline{2}$0] zone axis. The projected lattice structure of the hexagonal phase was superimposed in (D). The green, blue, and red balls represented the Ba, Ti, and O atoms, respectively. (E) The corresponding fast Fourier transform (FFT) pattern of the Ni-substituted BaTiO₃ ceramic along the [11$\overline{2}$0] zone axis.

Fig. 2.. The change of dielectric responses in Ni-substituted BaTiO₃ ceramics under the ambient air environment.

(A) A schematic diagram of the experimental sequence. Red, green, and blue circles represented frequency-dependent dielectric constant at room temperature, polarization-electric field hysteresis loop, and temperature-dependent dielectric constant measurements, respectively. (B) The frequency dependence of permittivity, (C) the P-E loop, and (D) the temperature-dependent dielectric constant in the as-sintered Ni-substituted BaTiO₃ ceramics. We exposed the as-sintered ceramics to the ambient air and then remeasured the dielectric properties after a time duration of 6 weeks. (E) The frequency-dependent dielectric constant, (F) the hysteresis loop, and (G) the temperature dependence of dielectric permittivity in the treated ceramic samples.

Fig. 3.. The reversible change of the dielectric responses in BaTiO₃ ceramics through thermal annealing and air exposure.

(A) A schematic figure of the experimental sequence. The treated Ni-substituted BaTiO₃ ceramics by air exposure were annealed at 350°C for 1 hour. (B) The frequency-dependent dielectric constant in treated ceramics after thermal annealing. (C) The hysteresis loop of annealed ceramic samples. (D) The temperature dependence of dielectric constant of the annealed Ni-substituted BaTiO₃ ceramics. The annealed ceramics were treated via air exposure for 6 weeks. (E) The frequency-dependent dielectric constant, (F) the P-E hysteresis loop, and (G) the temperature-dependent dielectric behavior in retreated ceramics by ambient air environment.

Fig. 4.. The evolution of dielectric responses in Ni-substituted BaTiO₃ ceramics under various ambient environments.

(A) Change of frequency-dependent dielectric constant in Ni-substituted BaTiO₃ ceramics under air, vacuum, nitrogen (N₂), carbon dioxide (CO₂), and high-humidity environments. (B) Corresponding dielectric loss in ceramics before and after chemical treatments.

Fig. 5.. The reversible change of dielectric responses in Ni-substituted BaTiO₃ ceramics.

(A) The schematic of the experimental sequence to test recyclability of the dielectric on- and off-states. (B) The evolution of the frequency-dependent dielectric constant and loss of Ni-substituted BaTiO₃ ceramics under the repetitive cycles of a high-humidity treatment and thermal annealing. (C) The number of cycle dependence of dielectric constants at 1 Hz for the dielectric on- and off-states induced by the humidity treatment and the following thermal annealing, respectively. The red and blue circles represented the dielectric permittivity of ceramic samples after the thermal annealing (i.e., off-state) and after the treatment under a humid environment (i.e., on-state), respectively.

Fig. 6.. Sensing performance experiments.

(A) Schematic diagram of a humidity sensing experiment. (B) Resistive responses of Ni-substituted BaTiO₃ ceramics to the change in RH. A stepwise decrease in the electrical resistance of the sensors was observed with the increasing humidity levels. The electrical resistance of ceramics gradually recovered to the initial value when the RH changed from 80 to 0% again.

Fig. 7.. Impedance analyses of Ni-substituted BaTiO₃ ceramics at two dielectric states (i.e., off- and on-states).

(A) Experimental setup of impedance analysis. (B) Schematic diagram of a brick-layer model for a polycrystalline ceramic. (C) The equivalent electric circuit. The impedance complex plane plots in the Ni-substituted BaTiO₃ ceramics for the (D) off- and (E) on-states at room temperature.

Fig. 8.. XPS and time-of-flight secondary ion mass spectrometry measurements.

XPS spectra at the O 1s edge of Ni-substituted BaTiO₃ ceramics (A) at the as-sintered state and (B) after 4 weeks in ambient air. The red, blue, and black solid lines in (A) and (B) represented the fitted curves of the oxygen in a lattice, the oxygen vacancy, and the chemisorbed oxygen on the surface of the ceramics. The volume fraction of the chemisorbed oxygen peak substantially increased after air exposure. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) 3D rendering maps of H⁺ signals in the (C) pure and (D) Ni-substituted BaTiO₃ ceramics that were treated by air exposure for 6 weeks (scale, 100 μm by 100 μm; sputter time, 0 to 3700 s).