Photochromic and Electric Field-Regulating Luminescence in High-Transparent (K,Na)NbO3-Based Ferroelectric Ceramics with Two-Phase Coexistence

Rare earth Tb³⁺ doped (K_0.465Na_0.465Li_0.07)(Nb_0.93Bi_0.07)O₃ (KNNLB-x%Tb) lead-free transparent ferroelectric ceramics were designed and prepared. The effects of Tb³⁺ on phase structure, microstructure, optical transmittance, photoluminescence, and photochromic behaviors were studied. Although two ferroelectric phases coexist, the optical transmittance can reach the high value of 74% in the visible light region because of the fine grains, dense ceramic microstructure, large optical energy band gap, and relatively high symmetry of coexisting ferroelectric phases. In addition, Tb³⁺ works as a luminescent center, and the reversible photochromic modulation is achieved by alternate stimulation of illumination and heat treatment. Meanwhile, the luminescence contrast can be improved under in situ electric field stimulation due to the easy change of lattice symmetry in coexisting ferroelectric phases. The generation of color centers after illumination and the local crystal field around the luminescent center caused by in situ electric field contributes to above phenomena. These ceramics exhibit the great potential in optical data storage and anticounterfeiting.

Refreshing Piezoelectrics: Distinctive Role of Manganese in Lead-Free Perovskites

Driven by an ever-growing demand for environmentally compatible materials, the past two decades have witnessed the booming development in the field of piezoelectrics. To maximally explore the potential of lead-free piezoelectrics, chemical doping could be an effective approach, referenced from tactics adopted in lead-based piezoelectrics. Herein, we reveal the distinct role of manganese in a promising lead-free perovskite (K, Na)NbO₃ (denoted by KNN) in comparison to that in market-dominating lead-based counterparts [Pb(Zr, Ti)O₃, PZT]. In contrast to the scenario in PZT, manganese doping in KNN results in tremendously improved piezoelectric coefficient d₃₃ by nearly 200%, whereas the same doping species in PZT deteriorates the d₃₃ down to less than 30% of its original value. The result is rationalized from macroscopic and local electrical characterizations down to atomic-scale visualization. This study demonstrates that there is enormous space to further enhance piezoelectricity in lead-free systems because the chemical doping effect may completely differ in lead-containing and lead-free perovskites.

Superior Piezoelectric Properties in Potassium-Sodium Niobate Lead-Free Ceramics

A superior piezoelectric coefficient (d₃₃ = 570 ± 10 pC N^"1 ), the highest value reported to date in potassium-sodium niobate-based ceramics, is obtained in (1-x-y)K_1-w Na_w Nb_1-z Sb_z O_3-y BaZrO_3-x - Bi_0.5 K_0.5 HfO₃ ceramics. This high d₃₃ value can be ascribed to the co-existence of "nano-scale strain domains" (1-2 nm) and a high density of ferroelectric domain boundaries. Therefore, ternary KNN-based ceramics demonstrate the potential for applications.

Potassium Sodium Niobate-Based Lead-Free Piezoelectric Multilayer Ceramics Co-Fired with Nickel Electrodes

Although lead-free piezoelectric ceramics have been extensively studied, many problems must still be overcome before they are suitable for practical use. One of the main problems is fabricating a multilayer structure, and one solution attracting growing interest is the use of lead-free multilayer piezoelectric ceramics. The paper reviews work that has been done by the authors on lead-free alkali niobate-based multilayer piezoelectric ceramics co-fired with nickel inner electrodes. Nickel inner electrodes have many advantages, such as high electromigration resistance, high interfacial strength with ceramics, and greater cost effectiveness than silver palladium inner electrodes. However, widely used lead zirconate titanate-based ceramics cannot be co-fired with nickel inner electrodes, and silver palladium inner electrodes are usually used for lead zirconate titanate-based piezoelectric ceramics. A possible alternative is lead-free ceramics co-fired with nickel inner electrodes. We have thus been developing lead-free alkali niobate-based multilayer ceramics co-fired with nickel inner electrodes. The normalized electric-field-induced thickness strain (S_max/E_max) of a representative alkali niobate-based multilayer ceramic structure with nickel inner electrodes was 360 pm/V, where S_max denotes the maximum strain and E_max denotes the maximum electric field. This value is about half that for the lead zirconate titanate-based ceramics that are widely used. However, a comparable value can be obtained by stacking more ceramic layers with smaller thicknesses. In the paper, the compositional design and process used to co-fire lead-free ceramics with nickel inner electrodes are introduced, and their piezoelectric properties and reliabilities are shown. Recent advances are introduced, and future development is discussed.

(K,Na)NbO3 nanofiber-based self-powered sensors for accurate detection of dynamic strain

A self-powered active strain sensor based on well-aligned (K,Na)NbO3 piezoelectric nanofibers is successfully fabricated through the electrospinning and polymer packaging process. The device exhibits a fast, active response to dynamic strain by generating impulsive voltage signal that is dependent on the amplitude of the dynamic strains and the vibration frequency. When the frequency is fixed at 1 Hz, the peak to peak value of the voltage increases from ∼1 to ∼40 mV, and the strain changes from 1 to 6%. Furthermore, the output voltage is linearly increased by an order of magnitude with the frequency changing from 0.2 to 5 Hz under the same strain amplitude. The influence of frequency on the output voltage can be further enhanced at higher strain amplitude. This phenomenon is attributed to the increased generating rate of piezoelectric charges under higher strain rate of the nanofibers. By counting the pulse separation of the voltage peaks, the vibration frequency is synchronously measured during the sensing process. The accuracy of the sensing results can be improved by calibration according to the frequency-dependent sensing behavior.

High strain in (K,Na)NbO3-based lead-free piezoceramics

A high strain is important for practical applications of piezoelectric actuators. Here we reported a high strain in the (K,Na)NbO3 -based ceramics by doping alkaline earths or transition metals. The ceramics possess a high strain (∼0.29%) as well as a large converse piezoelectric coefficient (d33*) up to 688 pm/V, which almost matches that of PZT4 ceramics. The obtained d33* is high for nontextured (K,Na)NbO3-based ceramics. In addition, a higher d33 value (340-407 pC/N) was also attained in the ceramics. Enhanced d33 and d33* values of this work should be attributed to the multiphase coexistence's effect induced by alkaline earths or transition metals. We believe that our research can benefit the developments of (K,Na)NbO3 ceramics and widen their applications range.