Chemical Framework to Design Linear-like Relaxors toward Capacitive Energy Storage.

ABO3-type perovskite relaxor ferroelectrics (RFEs) have emerged as the preferred option for dielectric capacitive energy storage. However, the compositional design of RFEs with high energy density and efficiency poses significant challenges owing to the vast compositional space and the absence of general rules. Here, we present an atomic-level chemical framework that captures inherent characteristics in terms of radius and ferroelectric activity of ions. By categorizing A/B-site ions as host framework, rattling, ferroelectrically active, and blocking ions and assembling these four types of ions with specific criteria, linear-like relaxors with weak locally correlated and highly extendable unit-cell polarization vectors can be constructed. As example, we demonstrate two new compositions of Bi0.5K0.5TiO3-based and BaTiO3-based relaxors, showing extremely high recoverable energy densities of 17.3 and 12.1 J cm-3, respectively, both with a high efficiency of about 90%. Further, the role of different types of ions in forming heterogeneous polar structures is identified through element-specific local structure analysis using neutron total scattering combined with reverse Monte Carlo modeling. Our work not only opens up new avenues toward rational compositional design of high energy storage performance lead-free RFEs but also sheds light on atomic-level manipulation of functional properties in compositionally complex ferroelectrics.

Surface Conditions after LASER Shock Peening of Steel and Aluminum Alloys Using Ultrafast Laser Pulses.

Laser shock peening (LSP) is a mechanical surface treatment process to modify near-surface material properties. Compared to conventional shot peening (SP) the process parameters can be finely adjusted with greater precision and a higher penetration depth of compressive residual stresses could be reached. However, high process times of LSP leads to high production costs. In this study, ultrafast LSP (U-LSP) with an ultrafast laser source (pulse time in the picosecond range) was applied on specimens made of X5CrNiCu15-5 and AlZnMgCu1.5. The surface characteristics (surface roughness) and surface-near properties (microstructure, residual stresses, and phase composition) were compared to the as-delivered condition, to conventional laser shock peening (C-LSP), and to SP, whereas metallographic analyses and X-ray and synchrotron radiation techniques were used. The process time was significantly lower via U-LSP compared to C-LSP. For X5CrNiCu15-5, no significant compressive residual stresses were induced via U-LSP. However, for AlZnMgCu1.5, similar compressive residual stresses were reached via C-LSP and U-LSP; however, with a lower penetration depth. A change in the phase portions in the surface layer of X5CrNiCu15-5 after C-LSP compared to SP were determined.

In situ neutron diffraction for analysing complex coarse-grained functional materials.

Complex functional materials play a crucial role in a broad range of energy-related applications and in general for materials science. Revealing the structural mechanisms is challenging due to highly correlated coexisting phases and microstructures, especially for in situ or operando investigations. Since the grain sizes influence the properties, these microstructural features further complicate investigations at synchrotrons due to the limitations of illuminated sample volumes. In this study, it is demonstrated that such complex functional materials with highly correlated coexisting phases can be investigated under in situ conditions with neutron diffraction. For large grain sizes, these experiments are valuable methods to reveal the structural mechanisms. For an example of in situ experiments on barium titanate with an applied electric field, details of the electric-field-induced phase transformation depending on grain size and frequency are revealed. The results uncover the strain mechanisms in barium titanate and elucidate the complex interplay of stresses in relation to grain sizes as well as domain-wall densities and mobilities.

Deciphering the phase transition-induced ultrahigh piezoresponse in (K,Na)NbO3-based piezoceramics.

Here, we introduce phase change mechanisms in lead-free piezoceramics as a strategy to utilize attendant volume change for harvesting large electrostrain. In the newly developed (K,Na)NbO3 solid-solution at the polymorphic phase boundary we combine atomic mapping of the local polar vector with in situ synchrotron X-ray diffraction and density functional theory to uncover the phase change and interpret its underlying nature. We demonstrate that an electric field-induced phase transition between orthorhombic and tetragonal phases triggers a dramatic volume change and contributes to a huge effective piezoelectric coefficient of 1250 pm V-1 along specific crystallographic directions. The existence of the phase transition is validated by a significant volume change evidenced by the simultaneous recording of macroscopic longitudinal and transverse strain. The principle of using phase transition to promote electrostrain provides broader design flexibility in the development of high-performance piezoelectric materials and opens the door for the discovery of high-performance future functional oxides.

Giant Functional Properties in Porous Electroceramics through Additive Manufacturing of Capillary Suspensions.

Dedicated hierarchical structuring of functional ceramics can be used to shift the limits of functionality. This work presents the manufacturing of highly open porous, hierarchically structured barium titanate ceramics with 3-3 connectivity via direct ink writing of capillary suspension-type inks. The pore size of the printed struts (∼1 µm) is combined with a printed mesostructure (∼100 µm). The self-organized particle network, driven by strong capillary forces in the ternary solid/fluid/fluid ink, results in a high strut porosity, and the distinct flow properties of the ink allow for printing high strut size to pore size ratios, resulting in total porosities >60%. These unique and highly porous additive manufactured log-pile structures with closed bottom and top layers enable tailored dielectric and electromechanical coupling, resulting in an energy harvesting figure of merit FOM33 more than four times higher than any documented data for barium titanate. This clearly demonstrates that combining additive manufacturing of capillary suspensions in combination with appropriate sintering allows for creation of complex architected 3D structures with unprecedented properties. This opens up opportunities in a broad variety of applications, including electromechanical energy harvesting, electrode materials for batteries or fuel cells, thermoelectrics, or bone tissue engineering with piezoelectrically stimulated cell growth.

Dielectric Relaxation and Magnetic Structure of A-Site-Ordered Perovskite Oxide Semiconductor CaCu3Fe2Ta2O12.

A new perovskite oxide semiconductor, CaCu3Fe2Ta2O12, was synthesized through a high-pressure and high-temperature approach. The compound possesses an Im3̅ space group, where it crystallizes to an A-site-ordered but B-site partial ordered quadruple perovskite structure. Spin ordering occurs around 150 K owing to the antiferromagnetic coupling between Fe3+ spins and ferromagnetic coupling between Cu2+ spins. The room-temperature dielectric permittivity of CaCu3Fe2Ta2O12 was measured to be approximately 2500 at 1 kHz. More importantly, isothermal frequency-dielectric spectroscopy demonstrates the existence of two dielectric relaxations. Debye-like relaxation is attributed to charge carriers trapped among the oxygen vacancies at low temperatures and Maxwell-Wagner polarization relaxation at high temperatures. CaCu3Fe2Ta2O12 is a new magnetic semiconductor, where A-site ordering is intercorrelated with second-order Jahn-Teller distortion. These findings offer opportunities to design novel perovskite oxides with attractive magnetic and dielectric properties.

Multi-analyser detector (MAD) for high-resolution and high-energy powder X-ray diffraction.

For high-resolution powder diffraction in material science, high photon energies are necessary, especially for in situ and in operando experiments. For this purpose, a multi-analyser detector (MAD) was developed for the high-energy beamline P02.1 at PETRA III of the Deutsches Elektronen-Synchrotron (DESY). In order to be able to adjust the detector for the high photon energies of 60 keV, an individually adjustable analyser-crystal setup was designed. The adjustment is performed via piezo stepper motors for each of the ten channels. The detector shows a low and flat background as well as a high signal-to-noise ratio. A range of standard materials were measured for characterizing the performance. Two exemplary experiments were performed to demonstrate the potential for sophisticated structural analysis with the MAD: (i) the structure of a complex material based on strontium niobate titanate and strontium niobate zirconate was determined and (ii) an in situ stroboscopy experiment with an applied electric field on a highly absorbing piezoceramic was performed. These experiments demonstrate the capabilities of the new MAD, which advances the frontiers of the structural characterization of materials.

Electric-Field-Induced Phase Transformation and Frequency-Dependent Behavior of Bismuth Sodium Titanate-Barium Titanate.

The electric field response of the lead-free solid solution (1-x)Bi0.53Na0.47TiO3-xBaTiO3 (BNT-BT) in the higher BT composition range with x = 0.12 was investigated using in situ synchrotron X-ray powder diffraction. An introduced Bi-excess non-stoichiometry caused an extended morphotropic phase boundary, leading to an unexpected fully reversible relaxor to ferroelectric (R-FE) phase transformation behavior. By varying the field frequency in a broad range from 10-4 up to 102 Hz, BNT-12BT showed a frequency-dependent gradual suppression of the field induced ferroelectric phase transformation in favor of the relaxor state. A frequency triggered self-heating within the sample was found and the temperature increase exponentially correlated with the field frequency. The effects of a lowered phase transformation temperature TR-FE, caused by the non-stoichiometric composition, were observed in the experimental setup of the freestanding sample. This frequency-dependent investigation of an R-FE phase transformation is unlike previous macroscopic studies, in which heat dissipating metal contacts are used.

Practical high-performance lead-free piezoelectrics: structural flexibility beyond utilizing multiphase coexistence.

Due to growing concern for the environment and human health, searching for high-performance lead-free piezoceramics has been a hot topic of scientific and industrial research. Despite the significant progress achieved toward enhancing piezoelectricity, further efforts should be devoted to the synergistic improvement of piezoelectricity and its thermal stability. This study provides new insight into these topics. A new KNN-based lead-free ceramic material is presented, which features a large piezoelectric coefficient (d 33) exceeding 500 pC/N and a high Curie temperature (T c) of ∼200°C. The superior piezoelectric response strongly relies on the increased composition-induced structural flexibility due to lattice softening and decreased unit cell distortion. In contrast to piezoelectricity anomalies induced via polymorphic transition, this piezoelectricity enhancement is effective within a broad temperature range rather than a specific small range. In particular, a hierarchical domain architecture composed of nano-sized domains along the submicron domains was detected in this material system, which further contributes to the high piezoelectricity.

Coordination Polymer to Atomically Thin, Holey, Metal-Oxide Nanosheets for Tuning Band Alignment.

Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal-based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two-unit cells. The process involves rapid exfoliation of bulk-layered, MCPs (Ce-, Ti-, Zr-based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high-yield, room-temperature, and template-free approach to synthesize ultrathin holey nanosheets with high-level functionalities.

Piezoelectricity and rotostriction through polar and non-polar coupled instabilities in bismuth-based piezoceramics.

Coupling of order parameters provides a means to tune functionality in advanced materials including multiferroics, superconductors, and ionic conductors. We demonstrate that the response of a frustrated ferroelectric state leads to coupling between order parameters under electric field depending on grain orientation. The strain of grains oriented along a specific crystallographic direction, 〈h00〉, is caused by converse piezoelectricity originating from a ferrodistortive tetragonal phase. For 〈hhh〉 oriented grains, the strain results from converse piezoelectricity and rotostriction, as indicated by an antiferrodistortive instability that promotes octahedral tilting in a rhombohedral phase. Both strain mechanisms combined lead to a colossal local strain of (2.4 ± 0.1) % and indicate coupling between oxygen octahedral tilting and polarization, here termed "rotopolarization". These findings were confirmed with electromechanical experiments, in situ neutron diffraction, and in situ transmission electron microscopy in 0.75Bi1/2Na1/2TiO3-0.25SrTiO3. This work demonstrates that polar and non-polar instabilities can cooperate to provide colossal functional responses.

A sample cell for in situ electric-field-dependent structural characterization and macroscopic strain measurements.

When studying electro-mechanical materials, observing the structural changes during the actuation process is necessary for gaining a complete picture of the structure-property relationship as certain mechanisms may be meta-stable during actuation. In situ diffraction methods offer a powerful and direct means of quantifying the structural contributions to the macroscopic strain of these materials. Here, a sample cell is demonstrated capable of measuring the structural variations of electro-mechanical materials under applied electric potentials up to 10 kV. The cell is designed for use with X-ray scattering techniques in reflection geometry, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The capabilities of the cell have been successfully demonstrated at the Powder Diffraction beamline of the Australian Synchrotron and the potential implementation of this cell with laboratory X-ray diffraction instrumentation is also discussed.

Design of zeolite by inverse sigma transformation.

Although the search for new zeolites has traditionally been based on trial and error, more rational methods are now available. The theoretical concept of inverse σ transformation of a zeolite framework to generate a new structure by removal of a layer of framework atoms and contraction has for the first time been achieved experimentally. The reactivity of framework germanium atoms in strong mineral acid was exploited to selectively remove germanium-containing four-ring units from an UTL type germanosilicate zeolite. Annealing of the leached framework through calcination led to the new all-silica COK-14 zeolite with intersecting 12- and 10-membered ring channel systems. An intermediate stage of this inverse σ transformation with dislodged germanate four-rings still residing in the pores could be demonstrated. Inverse σ transformation involving elimination of germanium-containing structural units opens perspectives for the synthesis of many more zeolites.