Phase-transforming pyroelectric materials have emerged as promising candidates for low-grade thermal energy harvesting. However, whether first-order transformations with large pyroelectric coefficient or second-order transformations with better reversibility are preferable remains unclear. Here we report compositionally tunable phase transformations in Ba_1-xSr_xTiO_3 (x ∈ [0, 0.3]), revealing evolution from first-order to second-order character. We identify a transitional regime between Sr_0.15 and Sr_0.22 where transformation mechanism fundamentally changes. Within this regime, Sr_0.19 achieves optimal lattice compatibility, exhibiting electrical leakage suppressed by over two orders of magnitude while retaining substantial polarization response. Energy conversion demonstrations show the multilayer Sr_0.19 device delivers pyroelectric current of ∼1.6 μA at 64^∘C with an energy density of 1.6 mJ/cm^3 per cycle and 5.5% conversion efficiency. Remarkably, this composition operates stably over 10,000 full energy conversion cycles without external bias field or recharging, demonstrating that transitional regime compositions provide the optimal balance between energy density and operational durability for practical low-grade heat harvesting.

Phase-transforming ferroelectrics are widely utilized in pyroelectric devices. However, elevated temperatures lead to significantly increased electric leakage. Mitigating electric leakage near the transformation temperatures is essential for device functionality and lifetime. In this work, we tune the lattice parameters in fine-grained, donor-doped ferroelectric Eu-BTO-Zrxmaterials and discover that tuning lattice parameters effectively suppresses electric leakage. Through first-principles calculations, we investigate the influence of lattice parameters on donor energy level and discover that the donor energy level shifts significantly in Eu-BTO-Zr5 composition, coinciding with the observed leakage mitigation at tetragonal-to-cubic transformation. We analyze the lattice compatibility of developed materials and reveal that the leakage mitigation appears in materials with lattice parameters equating structural anisotropy and lattice compatibility. Our findings clarify the coupled role of donor states and lattice parameters in leakage mitigation across phase transformation, providing a theoretical framework for designing pyroelectric materials with improved thermal stability and reduced leakage.

Narrow-temperature-fluctuation waste heat is abundant in data centers. Ferroelectric materials exhibit the pyroelectric effect, which enables direct conversion of thermal energy into electricity. Power-source-free pyroelectric energy conversion is a new approach that generates electricity without any external bias or battery connection. It represents a bias-field-free thermodynamic cycle that fundamentally differs from the conventional Olsen/Ericsson cycles. The device performance is governed by a figure-of-merit that considers both the pyroelectric coefficient and the latent heat across the phase transformation. Long-cycle electricity generation is achieved by continuously driven phase transformations. Recent progress has demonstrated microampere-level electricity output over more than 7000 phase-transformation cycles, showing compelling potential for practical applications. This review summarizes the fundamental mechanisms, material design strategies, device demonstrations, and future outlook of power-source-free pyroelectric conversion. We aim to provide a comprehensive understanding of this emerging method and offer useful insights for long-cycle electricity generation from waste heat in data centers.

Enhancing the reversibility and reducing the hysteresis of antiferroelectric (AFE)-to-ferroelectric (FE) transformations is essential for improving the functionality of energy-storage devices by using antiferroelectric ceramics. In this work, we show that, in PbZr1-xTixO3 (PZT), the AFE-FE transformation occurs as a rhombohedral-to-orthorhombic transition-a symmetry-breaking process that is not conventionally expected to be reversible. Through thermal and structural analysis, we find that this transformation can become reversible when a stable rhombohedral phase mediates the transition, satisfying geometric compatibility conditions. We theorize transformation pathways, derive lattice correspondence and compatibility criteria, and identify that the Ti0.005 composition closely satisfies the conditions of compatibility and exhibits the lowest thermal hysteresis and reduced bias field for AFE-FE switching. Micropillar compression experiments confirm mechanical reversibility is enhanced at this composition, validating the theoretical predictions. These results establish phase compatibility as a design principle for achieving reversible AFE-FE switching in functional oxides.

In this study, we introduce a novel material descriptor and corresponding mechanical criteria to guide the development of low-fatigue shape memory alloys. Our approach synergistically combines compatibility theories, crystallographic algorithms, and micromechanical experiments to optimize materials through a two-parameter compositional tuning strategy. We demonstrate this method on a series of CuAlx1Mnx2alloys, where the atomic composition vectorx=(x1,x2)is an element of[0.17,0.22]x[0.09,0.11]. By employing a scalar-valued function to index the functional fatigue property based on cofactor conditions, we analyze the continuity and extremes with respect to compositional variables. Through just three iterative development steps, we identify the composition CuAl20.2Mn11.3, achievinga reduction in thermal hysteresis by a factor of 2 and enhancing mechanical reversibility upto 1000 cycles. This result underscores the potential of mathematical methods in designingcomplex materials with desirable mechanical properties. Ourfindings not only provide atheoretical framework for the design of shape memory alloys but also highlight the impor-tance of integrating theoretical and experimental techniques to achieve optimal materialproperties

Pyroelectric energy conversion shows potential for low-grade waste heat harvesting. In this work, we design and investigate the SrxBa1−xNb2O6 material at x = 0.6 (SBN60), focusing on the role of oxygen vacancies and their impacts on pyroelectric performance. By employing floating-zone synthesis with proper heat-treatment process, we systematically tune oxygen vacancies and reveal their dual effect: oxygen vacancies enhance the pyroelectric figure-of-merit, while removing oxygen vacancies extends the effective lifetime in one-charge pyroelectric energy conversion. We discover that the phase transition in a heat-treated single crystal SBN60 device helps achieve stable electricity generation for approximately 7000 cycles in 30 h with enhanced functional reversibility. The dual effect of oxygen vacancies on the coupling of pyroelectricity and effective lifetime provides a useful strategy to design high-performance pyroelectric materials, especially for one-charge pyroelectric energy conversion, offering a practical solution for real-world applications of pyroelectric devices.

The mechanical reversibility of stress-induced martensitic transformations is critical for the functional durability of superelastic materials, particularly at small scale. This study investigates the impact of grain boundary (GB) compatibility on the mechanical reversibility of martensitic transformations in bicrystal Cu67Al24Mn9 micropillars. By combining the geometrically nonlinear theory with high-resolution transmission electron microscopy (TEM) and nanomechanical compression experiments, we demonstrate that the crystallographic orientation and geometric coherence of GBs play a decisive role in transformation reversibility. Micropillars with GBs satisfying a necessary condition for rank-one connections between twin laminates exhibit diffuse GB morphology and recoverable transformation strains over 10,000 cycles. The findings validate recent theoretical predictions on kinematic compatibility in polycrystalline systems with non-transforming defects. The rank-one criterion underlies a design strategy for polycrystalline superelastic alloys to achieve high fatigue-resistant without altering chemical composition. The theoretical framework opens new directions for advancing microstructural design in shape memory alloys.

Shape memory alloys that can deform and then spring back to their original shape, have found a wide range of applications in the medical field, from heart valves to stents. As we push the boundaries of technology creating smaller, more precise tools for delicate surgery treatments, the behavior of these alloys at tiny scales becomes increasingly crucial. In this study, we discover that the size effect of critical stress required for stress-induced phase transformation is not universal. We propose an orientation-dependent power decay law, indicating a specific increase in critical stress for pillars smaller than 1 micrometer for the nominally soft [001] and hard [111] orientations. Additionally, we observe high transformability with 11% recoverable strain under high stress (2 GPa) through lattice frustration at 200 nm scale. This research opens new avenues for exploring the superior elastic behavior of shape memory alloys for nanodevices.

Composition-temperature phase diagrams are crucial for designing ferroelectric materials, however predicting them accurately remains challenging due to limited phase transformation data and the constraints of conventional methods. Here, we utilize natural language processing (NLP) to text-mine 41,597 research articles, compiling a dataset of 2838 phase transformations across 846 ferroelectric materials. Leveraging this dataset, we develop FerroAI, a deep learning model for phase diagram prediction. FerroAI successfully predicts phase boundaries and transformations among different crystal symmetries in Ce/Zr co-doped BaTiO3 (BT)-xBa0.7Ca0.3TiO3(BCT). It also identifies a morphotropic phase boundary in Zr/Hf co-doped BT-xBCT at x = 0.3, guiding the discovery of a new ferroelectric material with an experimentally measured dielectric constant of 11,051. These results establish FerroAI as a powerful tool for phase diagram construction, guiding the design of high-performance ferroelectric materials.

Understanding the grain morphology, orientation distribution and crystal structure of nanocrystals is essential for optimizing the mechanical and physical properties of functional materials. Synchrotron X-ray Laue microdiffraction is a powerful technique for characterizing crystal structures and orientation mapping using focused X-rays. However, when the grain sizes are smaller than the beam size, mixed peaks in the Laue pattern from neighboring grains limit the resolution of grain morphology mapping. We propose a physics-informed machine learning (PIML) approach that combines a convolutional neural network feature extractor with a physics-informed filtering algorithm to overcome the spatial resolution limits of X-rays, achieving nanoscale resolution for grain mapping. Our PIML method successfully resolves the grain size, orientation distribution and morphology of Au nanocrystals through synchrotron microdiffraction scans, showing good agreement with electron backscatter diffraction results. This PIML-assisted synchrotron microdiffraction analysis can be generalized to other diffraction-based probes, enabling the characterization of nanosized structures with micrometre-sized probes.

Superelastic alloys used for stents, biomedical implants, and solid-state cooling devices rely on their reversible stress -induced martensitic transformations. These applications require the alloy to sustain high deformability over millions of cycles without failure. Here, we report an alloy capable of enduring 10 x 107 tensile stress -induced phase transformations while still exhibiting over 2% recoverable elastic strains. After millions of cycles, the alloy is highly reversible with zero stress hysteresis. We show that the major martensite variant is reversible even after multimillions of cycles under tensile loadings with a highly coherent (11 over bar 0)A interface. This discovery provides new insights into martensitic transformation, and may guide the development of superelastic alloys for multimillion cycling applications.

Phase-transforming ferroelectric materials have attracted significant attention due to their potential for energy conversion from waste heat. Here, we explore the impact of grain size and lattice compatibility on the energy conversion figure-of-merit (FOM) of a phase-transforming ferroelectric system Ba0.95Ca0.05Ce0.005ZrxTi0.995−xO3 with Zr content ranging from 0.004 to 0.03. The results demonstrate that tuning grain size and lattice compatibility can significantly increase the FOM. The optimal composition Zr0.006 exhibits the highest FOM among its neighboring compositions, with a corresponding peak pyroelectric current density of 5.6 μA/cm2 generated from a temperature fluctuation of 30 °C at a temperature rate of 5 °C/s. This work provides a rational understanding of the effect of grain morphology and crystal structure on the pyroelectric properties for energy conversion.

Phase-transforming ferroelectric materials are widely used in energy harvesting and conversion devices. However, the functionality of these devices is significantly impeded by electrical leakage at high temperatures. In this study, we fundamentally study the mechanism of electrical leakage suppression due to phase transformation in a series of donor-doped ferroelectric oxides,Ba0.955Eu0.03Ti(1-x)ZrxO3 with 0<= x<= 0.15. Our experiments clearly demonstrate that the symmetry-breaking phase transformations result in the reduction in electrical conductivity of the donor-doped ferroelectric oxides. The DFT calculation suggests that the donor energy level undergoes a shallow-to-deep transition at the phase transformation temperature. By analyzing the constitutive model of the leakage current density function, we propose a leakage suppression coefficient that rationalizes the development of ferroelectrics with low electrical leakage at elevated temperatures.

Ferroelectric materials are widely used in energy applications due to their field-driven multiferroic properties. The stress-induced phase transformation plays an important role in the functionality over repeated and consecutive operation cycles, especially at the micro/nanoscales. Here we report a systematic in-situ uniaxial compression tests on cuboidal Barium titanate (BaTiO3) 3 ) nanopillars with size varying from 100 nm to 3000 nm, by which we explore the stress-induced transformation and its interplay with plastic deformation. We confirm the superelasticity achieved in pillars by martensitic phase transformation from tetragonal to orthorhombic. There exists a critical size, 330 nm, for the yield stress. Above 330 nm, martensitic phase transformation aids slip along the plane with a low Schmid factor, in turn, the pseudo-compatible twins form within the shear band. The scaling exponent of size-dependent yield strength is found to be exactly 1. For nanopillars smaller than 330 nm, no twins form, only slips with large Schmid factors are activated, and size effect vanishes. All pillars with sizes from 100 nm to 300 nm achieve the theoretical yield limit around 9 GPa. Our experimental results uncover the interplay between twins and slips in BaTiO3 3 nanopillars, which pave the way for the optimization of microstructure design of ferroelectric materials for microelectronic applications at small scales.

We theorize the surface step characterization by reflected incoherent-light differential interference microscopy with consideration of the optical diffraction effect. With the integration of localization analysis, we develop a quantitative differential interference optical system, by which we demonstrate that the axial resolution of measuring surface height variation is sensitive to the shear distance between the two spatially differentiated beams. We fabricate three nanometer-size steps by photolithography, and successfully characterize their 1D height variations with 0.13 nm / H z axial precision. Our result suggests that the optical differential interference microscopy can be used for real-time characterization of surface structure with a subnanometer accuracy and a large field of view, which is greatly beneficial to the surface characterization of micro/nano-electromechanical systems.

The ferroelectric material usually exhibits temperature dependent spontaneous polarization, known as pyroelectricity, which can be used to directly convert thermal energy to electricity from ambient low-grade waste heat. When utilizing the structural phase transformations of the material, the conversion capability can be magnified, consequently the device performance can be strongly boosted by orders of magnitude. However, common ferroelectric oxides suffer the mechanical fatigue and functional degradation over cyclic phase transformations, hindering widespread applications of the energy conversion device. In this paper, we investigate the mechanical and functional reversibility of the material by lattice tuning and grain coarsening. We discover the lead-free compound Ba(Ce_0.005Zr_0.005)Ti_0.99O3-0.10(Ba_0.7Ca_0.3)TiO_3 (BCZT-0.10BCT) satisfying the compatibility condition among all present phases by its lattice parameters, making the phase transformations highly reversible. We demonstrated that the energy conversion device with the equiaxial coarse grains exhibits exceptional fatigue-resistance, with stable pyroelectric current output at 4μA/cm^2 over 3,000 energy conversion cycles. Our work opens a new way to fabricate high-performance material that advances the pyroelectric energy conversion for practical application in engineering.

The in situ micromechanical tensile tests are conducted to characterize the superelastic behaviors for [223], [1214], [325] and [205] oriented Cu67Al24Mn9 micro-slats. The stress-induced martensitic transformations are captured in all textures corresponding to strong crystallographic anisotropy. We propose a one-dimensional constitutive model considering the directional anisotropy of elastic modulus for cubic symmetry and the crystallographic compatibility of twinned martensite. The modeled mechanical behaviors agree with the micromechanical tensile tests well, which suggests that the formation of compatible twins is the primary deformation mechanism. Particularly in the [205] texture, we observed formation of nano-cavities (<100nm) on the lateral surface of the micro-slat. Based on the analysis of compatible martensite twin laminates and fcc slip systems, we theorize that the massive normal elongation and little lateral shear cause the formation, stretching and growth of nano-cavities at nano scales to accommodate the external loads. As a result, the structural and functional fatigue resistance is improved compared to other textures. The experimental and theoretical results in this paper are potentially useful to guide the texture design of Cu-based shape memory alloy for high transformation strain and low functional fatigue.

We establish an experimental platform to combine the differential scanning calorimetry (DSC) with an in situ observation of microstructure by differential interference contrast microscope (DInM). We carry out the experiment on a phase-transforming alloy Au31Cu24Zn45 that closely satisfies the cofactor conditions - the strongest crystallographic compatibility constraints between phases. We confirm that the phase formation events observed by DInM agree well with the heat exchange profile characterized by DSC. We also observe different morphology of twins evolves differently, depending on the twin spacing. Through the quantitative analysis of various morphological domains in the temporal space, we discover that the scaling law between the twin evolution time and the fineness is linear, by which the characteristic transformation time for single variant martensite can be determined for this compatible martensite alloy as 0.636s.