Realization of the Giant Pyroelectric Response via Modulated Polar Structures.

Among pyroelectric materials, Bi0.5 Na0.5 TiO3 (BNT)-based relaxors are particularly noteworthy due to their significant polarization fluctuation near the depolarization temperature (Td ), resulting in a large pyroelectric response. What has been overlooked is the dynamic behavior of inherent polar structures, particularly the temperature-dependent evolution of polar nanoregions (PNRs), which significantly impacts the pyroelectric behavior. Herein, based on the large pyroelectric response origination (the ferroelectric-relaxor phase transition), the mixed nonergodic and ergodic relaxor (NR+ER) critical state is constructed, which is believed to trigger the easily fluctuating polarization state with excellent pyroelectric response. Composition engineering (with Li+ , Sr2+ , and Ta5+ ) strategically controls the relaxor process and modulates the dynamic behavior of inherent polar structures by the random field effect. The pyroelectric coefficient of more than 1441 µCm-2 K-1 at room temperature (RT), more than 9221 µCm-2 K-1 (RT), and ≈107911 µCm-2 K-1 (Td ) are achieved in the Li+ -doped sample, the Sr2+ -doped sample, and the (Li+ +Ta5+ ) co-doped sample, respectively. This work earns the highest RT pyroelectric coefficient in BNT-based relaxors, which is suitable for pyroelectric applications. Furthermore, it provides a strategy for modulating the pyroelectric performance of BNT-based relaxors.

Reversible Multimode Luminescence Modulations in Photochromic-Translucent Yb3+/Eu3+ Codoped K0.5Na0.5NbO3 Ceramics.

Luminescent tunable materials have promising application potential in optical switches, optical information storage, and so on. Although europium (Eu) is a good downconversion red luminescent rare earth element, there are few studies on the upconversion luminescence and photochromism of Eu-doped potassium sodium niobate (KNN) ferroelectrics. In this paper, Eu3+ and Yb3+ codoped KNN translucent ferroelectric ceramics were synthesized and the effect of Yb3+ content on the luminescence and photochromism is studied. Both the up- and downconversion luminescence intensity and decay rate before and after photochromism can be well controlled by Yb3+ content. That is, an upconversion luminescent translucent ceramic that can be completely discolored by 405 nm light illumination for 10 s was obtained. The luminescence modulations are closely related to the evolution of oxygen vacancy and crystal field around the luminescence center, which can be verified by the illumination-induced electron paramagnetic resonance (EPR) signal and local piezoresponse switching behavior variation as well as the discovery of energy level splitting and spectral line shift. We believe that this work shows a paradigm for designing high-performance reversible multimode luminescence modulation ferroelectric ceramics.

Unveiling the Pivotal Role of dx2-y2 Electronic States in Nickel-Based Hydroxide Electrocatalysts for Methanol Oxidation.

The anodic methanol oxidation reaction (MOR) plays a crucial role in coupling with the cathodic hydrogen evolution reaction (HER) and enables the sustainable production of the high-valued formate. Nickel-based hydroxide (Ni(OH)2) as MOR electrocatalyst has attracted enormous attention. However, the key factor determining the intrinsic catalytic activity remains unknown, which significantly hinders the further development of Ni(OH)2 electrocatalyst. Here, we found that the d x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ electronic state within antibonding bands plays a decisive role in the whole MOR process. The onset potential depends on the deprotonation ability (Ni2+ to Ni3+), which was closely related to the band center of d x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital. The closer of d x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital to the Fermi level showed the stronger the deprotonation ability. Meanwhile, in the high potential region, the broadening of d x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital would facilitate the electron transfer from methanol to catalysts (Ni3+ to Ni2+), further enhancing the catalytic properties. Our work for the first time clarifies the intrinsic relationship between d x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ electronic state and the MOR activities, which adds a new layer of understanding to the methanol electrooxidation research scene.

Mechanochemical Synthesis of Aryl Fluorides by Using Ball Milling and a Piezoelectric Material as the Redox Catalyst.

Aryl fluorides are important structural motifs in many pharmaceuticals. Although the Balz-Schiemann reaction provides an entry to aryl fluorides from aryldiazonium tetrafluoroborates, it suffers from drawbacks such as long reaction time, high temperature, toxic solvent, toxic gas release, and low functional group tolerance. Here, we describe a general method for the synthesis of aryl fluorides from aryldiazonium tetrafluoroborates using a piezoelectric material as redox catalyst under ball milling conditions in the presence of Selectfluor. This approach effectively addresses the aforementioned limitations. Furthermore, the piezoelectric material can be recycled multiple times. Mechanistic investigations indicate that this fluorination reaction may proceed via a radical pathway, and Selectfluor plays a dual role as both a source of fluorine and a terminal reductant.

Origin of Surface Reconstruction in Lattice Oxygen Oxidation Mechanism Based-Transition Metal Oxides: A Spontaneous Chemical Process.

A fundamental understanding of surface reconstruction process is pivotal to developing highly efficient lattice oxygen oxidation mechanism (LOM) based electrocatalysts. Traditionally, the surface reconstruction in LOM based metal oxides is believed as an irreversible oxygen redox behavior, due to the much slower rate of OH- refilling than that of oxygen vacancy formation. Here, we found that the surface reconstruction in LOM based metal oxides is a spontaneous chemical reaction process, instead of an electrochemical reaction process. During the chemical process, the lattice oxygen atoms were attacked by adsorbed water molecules, leading to the formation of hydroxide ions (OH- ). Subsequently, the metal-site soluble atoms leached from the oxygen-deficient surface. This work also suggests that the enhancement of surface hydrophilicity could accelerate the surface reconstruction process. Hence, such a finding could add a new layer for the understanding of surface reconstruction mechanism.

Accelerated Surface Reconstruction through Regulating the Solid-Liquid Interface by Oxyanions in Perovskite Electrocatalysts for Enhanced Oxygen Evolution.

A comprehensive understanding of surface reconstruction was critical to developing high performance lattice oxygen oxidation mechanism (LOM) based perovskite electrocatalysts. Traditionally, the primary determining factor of the surface reconstruction process was believed to be the oxygen vacancy formation energy. Hence, most previous studies focused on optimizing composition to reduce the oxygen vacancy formation energy, which in turn facilitated the surface reconstruction process. Here, for the first time, we found that adding oxyanions (SO4 2- , CO3 2- , NO3 - ) into the electrolyte could effectively regulate the solid-liquid interface, significantly accelerating the surface reconstruction process and enhancing oxygen evolution reaction (OER) activities. Further studies indicated that the added oxyanions would adsorb onto the solid-liquid interface layer, disrupting the dynamic equilibrium between the adsorbed OH- ions and the OH- ions generated during surface reconstruction process. As such, the OH- ions generated during surface reconstruction process could be more readily released into the electrolyte, thereby leading to an acceleration of the surface reconstruction. Thus, it was expected that our finding would provide a new layer of understanding to the surface reconstruction process in LOM-based perovskite electrocatalysts.

Defects and Aliovalent Doping Engineering in Electroceramics.

Since the positive influences of defects on the performance of electroceramics were discovered, investigations concerning on defects and aliovalent doping routes have grown rapidly in the fields of inorganic chemistry and condensed matter physics. In this article, we summarized the types of defects in electroceramics as well as characterization tools of defects and highlighted the effects of intrinsic and extrinsic defects on the material performances with the emphasis on dielectric, ferroelectric, and piezoelectric properties. We mainly introduced defect related theoretical simulation and experimental results in several typical incipient ferroelectrics, ferroelectrics, and antiferroelectrics. Hence, the influences of defects on the crystal lattice were summed up, and then the main physical mechanisms were highlighted. Particularly, the performance enhancements of aliovalently doped electroceramics were also evaluated and reviewed. Finally, the outlook and challenges were discussed on the basis of their current developments. This article covers not only an overview of the state-of-the-art advances of defects and aliovalent doping routes in electroceramics but also the future prospects that may open another window to tune the electrical performance of electroceramics via intentionally introducing certain defects.

Optimizing electro-strain via manipulating the oxygen octahedral structure in BF-BT-based ceramics.

Much attention has been paid to the electrical performance caused by doping, while the property regulation mechanism of intrinsic contributions such as symmetry and tilt of the oxygen octahedron is still deficiently understood in bismuth ferrite-barium titanate (BF-BT) ceramics. To establish the correlation between the evolution of the intrinsic structure and electro-strain, three doping systems of BF-BT-xLiNbO3/xNaNbO3/xKNbO3 are designed, in which Li+, Na+, and K+ have similar chemical properties but different ionic radii. Macro-property characterization suggests that the largest electro-strain (S ∼ 0.25%) could be achieved in the BF-BT-xNaNbO3 system when x = 0.02. Microscopic crystal structure analysis manifests that Na+ can enhance the symmetry of O-O and Fe-O bond lengths and maintain a certain degree of oxygen octahedron tilt, while smaller (Li+) and larger (K+) ionic radii can induce the asymmetry of O-O and/or Fe-O bond lengths. The real-space domain images indicate that the domain configuration of ceramics with improved strain exhibit similar miniaturized maze-like structures. Therefore, the synergic contributions, including symmetry of the bond length and appropriate oxygen octahedron tilt as well as miniaturized maze-like domain structure, were the origin of the improved electro-strain in BF-BT-0.02NaNbO3. We believe that understanding the effect of the intrinsic crystal structure on the electro-strain is meaningful for tailoring BF-BT electrical properties.

Emerging new phase boundary in potassium sodium-niobate based ceramics.

Developing eco-friendly high-performance piezoceramics without lead has become one of the most advanced frontiers in interdisciplinary research. Although potassium sodium-niobate {(K,Na)NbO3, KNN} based ceramics are believed to be one of the most promising lead-free candidates, the relatively inferior piezoelectric properties and strong temperature dependency have hindered their development for more than 50 years since being discovered in the 1950s. It was not until 2014 that our group initially proposed a new phase boundary (NPB) that simultaneously improved the piezoelectric properties and temperature stability of non-textured KNN-based ceramics to the level of partly lead-based ceramics. The NPB has been then proved by some researchers and believed to pave the way for "lead-free at last" proposed by E. Cross (Nature, 2004, 432, 24). However, the understanding of the NPB is still in its infancy, leaving many controversies, including the phase structure and physical mechanisms at the NPB as well as the essential difference when compared with other phase boundaries. In this context, we systematically summarized the origin and development of the NPB, focusing on the construction, structure and intrinsic trait of the NPB, the effects of the NPB on the performance, and the validity and related incipient devices of the NPB. Particularly, we concluded the phase structure and domain structure locating at the NPB, analyzed the physical mechanisms in depth, proposed the possible methods to further improve the performance at the NPB, and demonstrated the validity and scope of the NPB as well as the device application. Finally, we gave out our perspective on the challenges and future research of KNN-based ceramics with NPB. Therefore, we believe that this review could promote the understanding of the NPB and guide the future work of KNN-based ceramics.

Ultrahigh Performance in Lead-Free Piezoceramics Utilizing a Relaxor Slush Polar State with Multiphase Coexistence.

Owing to growing environmental concerns, the development of lead-free piezoelectrics with comparable performance to the benchmark Pb(Zr,Ti)O3 (PZT) becomes of great urgency. However, a further enhancement of lead-free piezoelectrics based on existing strategies has reached a bottleneck. Here we achieve a slush polar state with multiphase coexistence in lead-free potassium-sodium niobate (KNN) piezoceramics, which shows a novel relaxor behavior, i.e., frequency dispersion at the transition between different ferroelectric phases. It is very different from the conventional relaxor behavior which occurs at the paraelectric-ferroelectric phase transition. We obtain an ultrahigh piezoelectric coefficient (d33) of 650 ± 20 pC/N, the largest value of nontextured KNN-based ceramics, outperforming that of the commercialized PZT-5H. Atomic-resolution polarization mapping by Z-contrast imaging from different orientations reveals the entire material to comprise polar nanoregions with multiphase coexistence, which is again very different from conventional ferroelectric relaxors which have polar domains within a nonpolar matrix. Theoretical simulations validate the significantly decreased energy barrier and polarization anisotropy, which is facilitated by the high-density domain boundaries with easy polarization rotation bridging the multiphase-coexisting nanodomains. This work demonstrates a new strategy for designing lead-free piezoelectrics with further enhanced performance, which should also be applicable to other functional materials requiring a slush (flexible) state with respect to external stimulus.

Rietveld Analysis and Electrical Properties of BiInO3 Doped KNN-Based Ceramics.

Rietveld refinement is used to investigate the crystal structure of prepared (0.965 - x)(K0.48Na0.52)NbO3- xBiInO3-0.035(Bi0.5Na0.5)ZrO3 (KNN- xBI-BNZ) ceramics. From refined results, the distortion degree of crystal structures in KNN- xBI-BNZ ceramics presents a rising trend with BiInO3 modification, which is in keeping with the results of diffuseness. The spontaneous polarization ( Ps) is also calculated using refined structural parameters. The submicron domains are observed when x = 0.004, which presents good electrical properties ( d33 = 317 pC/N, Tc = 336 °C) simultaneously. Excellent thermal stability of ceramics modified with BiInO3 is observed in a broad temperature range.

Low-Operating-Voltage Resistive Memory Based on Bi1+δFe0.95Zn0.05)O3 Films.

A resistive memory device based on the Ag/Bi1+δ(Fe0.95Zn0.05)O3/SRO/Pt/TiO2/SiO2/Si(100) structure was prepared using radio frequency magnetron sputtering. The composition of the thin film element was analyzed by X-ray photoelectron spectroscopy and the thickness of the thin film was characterized by scanning electron microscope. Through the electrical test, we found that the device exhibited low operating voltage, which included VSET of about 0.1 V, VRESET of about -0.1 V, and VF of about 0.25 V. This facilitated the perfect integration of the device with the circuit design. Testing for 10,000 s at a substrate temperature of 85 °C, the device showed excellent retention. The I-V fitting curves of the resistive devices were analyzed. The low resistance state was in line with the ohmic mechanism and the high resistance state was in accordance with the Space Charge Limited Current mechanism. The resistance change of the device was attributed to the formation of Ag conductive filaments.

Practical High Piezoelectricity in Barium Titanate Ceramics Utilizing Multiphase Convergence with Broad Structural Flexibility.

Due to growing environmental concerns on the toxicity of lead-based piezoelectric materials, lead-free alternatives are urgently required but so far have not been able to reach competitive performance. Here we employ a novel phase-boundary engineering strategy utilizing the multiphase convergence, which induces a broad structural flexibility in a wide phase-boundary zone with contiguous polymorphic phase transitions. We achieve an ultrahigh piezoelectric constant ( d33) of 700 ± 30 pC/N in BaTiO3-based ceramics, maintaining >600 pC/N over a wide composition range. Atomic resolution polarization mapping by Z-contrast imaging reveals the coexistence of three ferroelectric phases (T + O + R) at the nanoscale with nanoscale polarization rotation between them. Theoretical simulations confirm greatly reduced energy barriers facilitating polarization rotation. Our lead-free material exceeds the performance of the majority of lead-based systems (including the benchmark PZT-5H) in the temperature range of 10-40 °C, making it suitable as a lead-free replacement in practical applications. This work offers a new paradigm for designing lead-free functional materials with superior electromechanical properties.

An Alternative Way To Enhance Piezoelectricity and Temperature Stability in Lead-Free Sodium Niobate Piezoceramics.

To further balance the relationship between piezoelectricity and temperature stability, the (0.975 - y)NaNbO3- yBaTiO3-0.025BaZrO3 ( y = 0-0.20) ceramics are developed by constructing a wide tetragonal phase region. Effects of BaTiO3 on the relationships among phase structure, electrical properties, and temperature dependence are investigated. With increasing BaTiO3 contents, the ceramics endure the structural evolutions from orthorhombic phase to tetragonal phase, and then to relaxor cubic phase. A wide tetragonal phase zone of 24-180 °C can be realized in the ceramics with y = 0.08, together with an enhanced piezoelectric coefficient d33 = 215 pC/N. Intriguingly, excellent temperature stability of unipolar strain ( Suni) and piezoelectric coefficient ( d33) are observed in the ceramics with y = 0.08 within 20-180 °C. This work provides an alternative way to enhance piezoelectricity and temperature stability in lead-free piezoceramics.

Reduced dielectric loss in new colossal permittivity (Pr, Nb)TiO2 ceramics by suppressing adverse effects of secondary phases.

Here, we develop new colossal permittivity (CP) (Pr0.5Nb0.5)xTi1-xO2 ceramics by controlling the secondary phases, and then both colossal permittivity (εr = 6-8 × 104, 1 kHz) and low dielectric loss (tan δ = 3.7-7.5%, 1 kHz) can be realized in a composition range (x = 0.5-2.5%). The ceramics with x = 1% possess a high dielectric constant (εr = 74 533), and importantly a low dielectric loss (tan δ = 3.7%) can be found, which is lower than most of the typical CP materials and chemically modified TiO2 ceramics. In addition, the εr changing rates of 143 per degree Celsius and 35 per kiloHertz indicate an excellent temperature and frequency stability of the dielectric behaviors. XRD, FE-SEM and element mapping are conducted to probe the secondary phases, and element line scanning is used to explore the elemental composition of the secondary phases. The test results indicate that optimized dopants can enhance the dielectric properties, while secondary phases induced by x > 5% dopants can cause adverse effects on the dielectric properties. XPS results further demonstrate that the defect-dipole theory may be suitable to explain the significant improvement of dielectric properties. We believe that (Pr, Nb)TiO2 ceramics are one of the most competitive candidates in the field of electronic and energy-storage devices.

Enhanced temperature stability in the R-T phase boundary with dominating intrinsic contribution.

In this study, 0.96KNNSx-0.01SZ-0.03BNZ ceramics (x = 0-0.08) were used as examples to illustrate the effects of phase boundaries on strain property and temperature stability. The addition of Sb5+ resulted in a rhombohedral-tetragonal (R-T) phase boundary at x = 0.05-0.07, as confirmed by temperature-dependent Raman spectra. In the R-T region, an improved piezoelectric constant (d33 = 390-440 pC N-1) and high unipolar strain (Suni = 0.14-0.15%) were observed due to the dominating intrinsic contribution. More importantly, a favorable temperature stability of Suni was observed in the ceramics with x = 0.05; for example, there was a slight variation of +5% to -13% when the temperature was increased from 20 °C to 180 °C. Through systematic investigations of composition and temperature-dependent strain, methods to improve Suni and consolidate its temperature stability in KNN-based ceramics were subsequently suggested. We believe that this study can promote the understanding and design of KNN-based ceramics with high strain and favorable temperature stability.

Giant Piezoelectricity and High Curie Temperature in Nanostructured Alkali Niobate Lead-Free Piezoceramics through Phase Coexistence.

Because of growing environmental concerns, the development of lead-free piezoelectric materials with enhanced properties has become of great interest. Here, we report a giant piezoelectric coefficient (d33) of 550 pC/N and a high Curie temperature (TC) of 237 °C in (1-x-y)K1-wNawNb1-zSbzO3-xBiFeO3-yBi0.5Na0.5ZrO3 (KNwNSz-xBF-yBNZ) ceramics by optimizing x, y, z, and w. Atomic-resolution polarization mapping by Z-contrast imaging reveals the intimate coexistence of rhombohedral (R) and tetragonal (T) phases inside nanodomains, that is, a structural origin for the R-T phase boundary in the present KNN system. Hence, the physical origin of high piezoelectric performance can be attributed to a nearly vanishing polarization anisotropy and thus low domain wall energy, facilitating easy polarization rotation between different states under an external field.

Giant piezoelectricity in potassium-sodium niobate lead-free ceramics.

Environment protection and human health concern is the driving force to eliminate the lead from commercial piezoelectric materials. In 2004, Saito et al. [ Saito et al., Nature , 2004 , 432 , 84 . ] developed an alkali niobate-based perovskite solid solution with a peak piezoelectric constant d33 of 416 pC/N when prepared in the textured polycrystalline form, intriguing the enthusiasm of developing high-performance lead-free piezoceramics. Although much attention has been paid on the alkali niobate-based system in the past ten years, no significant breakthrough in its d33 has yet been attained. Here, we report an alkali niobate-based lead-free piezoceramic with the largest d33 of ∼490 pC/N ever reported so far using conventional solid-state method. In addition, this material system also exhibits excellent integrated performance with d33∼390-490 pC/N and TC∼217-304 °C by optimizing the compositions. This giant d33 of the alkali niobate-based lead-free piezoceramics is ascribed to not only the construction of a new rhombohedral-tetragonal phase boundary but also enhanced dielectric and ferroelectric properties. Our finding may pave the way for "lead-free at last".

Flexible, Wearable Wireless-Charging Power System Incorporating Piezo-Ultrasonic Arrays and MXene-Based Solid-State Supercapacitors.

With the continuous development of wearable electronics, higher requirements are put forward for flexible, detachable, stable output, and long service life power modules. Given the limited capacity of energy storage devices, the integration of energy capture and storage is a viable approach. Here, we present a flexible, wearable, wireless-charging power system that integrates a piezoelectric ultrasonic array harvester (PUAH) with MXene-based solid-state supercapacitors (MSSSs) in a soft wristband format for sustainable applications. The MSSS as the energy storage module is developed by using Ti3C2Tx nanosheet-loaded inserted finger-like carbon cloth skeletons as electrodes and poly(vinyl alcohol)/H3PO4 gel as electrolytes, with high energy density (58.74 Wh kg-1) and long cycle life (99.37%, 10,000 cycles). A two-dimensional stretchable piezoelectric array as a wireless-charging module hybridizes high-performance 1-3 composite units with serpentine electrodes, which allows wireless power via ultrasonic waves, with a maximum power density of 1.56 W cm-2 and an output voltage of 20.75 V. The overall PUAH-MSSS wireless energy supply system is 2 mm thick and offers excellent energy conversion/storage performance, cyclic stability, and mechanical flexibility. The results of this project will lay the foundation for the development of next-generation wearable electronics.

Lead-free dual-frequency ultrasound implants for wireless, biphasic deep brain stimulation.

Ultrasound-driven bioelectronics could offer a wireless scheme with sustainable power supply; however, current ultrasound implantable systems present critical challenges in biocompatibility and harvesting performance related to lead/lead-free piezoelectric materials and devices. Here, we report a lead-free dual-frequency ultrasound implants for wireless, biphasic deep brain stimulation, which integrates two developed lead-free sandwich porous 1-3-type piezoelectric composite elements with enhanced harvesting performance in a flexible printed circuit board. The implant is ultrasonically powered through a portable external dual-frequency transducer and generates programmable biphasic stimulus pulses in clinically relevant frequencies. Furthermore, we demonstrate ultrasound-driven implants for long-term biosafety therapy in deep brain stimulation through an epileptic rodent model. With biocompatibility and improved electrical performance, the lead-free materials and devices presented here could provide a promising platform for developing implantable ultrasonic electronics in the future.