KTa_1-xNb_xO₃ crystal doped with 0.15% (in mole fraction) Fe was prepared by a top-seeded solution growth method. The dielectric properties, electric-field-induced strain distribution, component distribution and domain structure of Fe∶KTN crystal were characterized. The results show that digital holography is a promising method to measure the spatial distribution of strain, and the electric-field-induced strain properties of KTN crystal present a uneven distribution. At room temperature, the maximum strain of crystal is 0.05% when the applied electric field is 435 V/mm, while the strain value is near 0.02% 3/4 area of crystal. There is a relationship among strain distribution, component distribution and domain structure of Fe∶KTN crystal. The domain structure of crystal in the region with less Nb content is smaller and the electric-field-induced strain is greater when the room temperature is lower than the Curie temperature.

The ceramics of 0.96K_0.48Na_0.52Nb_(1-x)Ta_xO₃ 0.04BaZrO₃ 0.3%MnCO₃ were prepared via solid-state reaction and subsequent sintering in air and reducing atmospheres. The effects of Ta doping and sintering atmosphere on the microstructure and piezoelectric properties of ceramics were investigated. The results demonstrate that all the ceramics sintered in different atmospheres exhibit a pure perovskite structure. The phase structure of KNN-based ceramics is the coexistence of rhombohedral and orthorhombic phases. Also, the generation of oxygen vacancies is suppressed as Ta doping content is increased. When x = 0.1, the ceramics sintered in a reducing atmosphere exhibit the optimum electrical properties (i.e., d₃₃=172 pC/N, , k_p=24.9, , and tanδ=0.021), which are superior to the counterparts sintered in air atmosphere (i.e., d₃₃=142 pC/N, pm/V, k_p=21.1, , and tanδ=0.022). It is indicated that sintering in a reducing atmosphere can suppress the grain growth, thus facilitating the densification of ceramics and improving the piezoelectric properties.

High-performance environmentally-benign (K,Na)NbO₃-based (KNN) lead-free piezoceramics has been the spotlight in the search for functional oxides. The KNN-based piezoceramics with the multiscale structures have the superior piezoelectricity, thermal stability, fatigue resistance, mechanical properties, and reproducibility, for the practical applications. In this review, recent development on the multiscale structural engineering of piezoelectricity, thermal stability, fatigue resistance, mechanical properties, and reproducibility, and the emerging applications of KNN-based lead-free piezoceramics was summarized. In addition, the future perspective for the development of KNN-based piezoceramics was also presented.

For crystalline materials, atomistic local structures are deviations from their long-range average crystal structures. With the recent development of experimental techniques and in-depth investigation methods for probing local structures, it has been realized and understood that local structures have profound influences on the physical properties of functional materials, and the understanding of local structures provides theoretical instructions for the improvements of materials performances as well as development of novel applications. This review summarized the well-established X-ray and neutron scattering techniques, data analysis and model refinements mothods in the local-structure investigation researches; in addition, examples of local structural investigations for several classical ferroelectric perovskites are reviewed.

Alumina ceramics were joined with Ag-10CuO-10B₂O₃ filler via reactiver air brazing for the formation of whiskers (Cu₂Al₆B₄O₁₇) in the joints. The effects of brazing temperature and holding time on the microstructure, properties of the joints and the whisker structure were investigated. The results show that Al₂O₃ reacts with CuO and B₂O₃ to form Cu₂Al₆B₄O₁₇ whiskers in the brazing process, thus obtaining a reliable joint with the superior performance. At the holding time of 60 min and the brazing temperature of > 950 ℃, an effective connection of Al₂O₃ ceramic joint can be realized. The center area of the brazing joint is mainly Ag, while the interface at the both sides of the joint is composed of CuO、Cu₃B₂O₆ phase and Cu₂Al₆B₄O₁₇ whiskers. The shear strength of the joint increases with the increase of the brazing temperature. The whiskers become thick and the adhesion occurs when the brazing temperature is > 1 050 ℃, thus reducing the strengthening effect on the joint and the shear strength of the joint. The whiskers decompose into some lamellar products, and the shear strength of the joint decreases when the brazing temperature is >1 100 ℃. The shear strength of the joint prepared under optimum process parameters (i.e., at 1 000 ℃ for 60 min) can reach 86.7 MPa, and the fracture occurs in the weld nearby alumina ceramic base material.

Data-driven Machine Learning (ML) has been widely used in materials performance optimization and novel materials design due to its ability to quickly fit potential data patterns and achieve accurate prediction. However, the results of data-driven ML are often inconsistent with the materials basic theory or principle, which results mainly from the lack of the guidance of materials domain knowledge, e.g., the correlation among descriptors and the driving mechanism associated with the properties. Herein, by analyzing the characteristics of materials data and the modeling principle of data-driven ML methods, we clarify the three main contradictions occurring to the application of ML in materials science, i.e., the contradictions between high dimension and small sample, complexity and accuracy of models, learning results and domain knowledge. Following this, we propose the ML method embedded with materials domain knowledge to reconcile these three contradictions. Further, surrounding the whole ML process including target definition, data collection and preprocessing, feature engineering, model construction and application, we explore some key techniques to realize domain knowledge embedding by summarizing the related basic and exploratory efforts. Finally, opportunities and challenges facing the ML method embedded with domain knowledge are also discussed.

Dielectric energy storage materials as "the blood of modern industry" are the key components of various pulse power electronic systems. These materials are attracted extensive attentions because of the high dielectric constant, low loss, high power density, fast charge/discharge speed and excellent reliability. However, dielectric materials for multi-fields applications still have some problems, such as low energy storage density, poor efficiency. Therefore, improving the energy storage performances of dielectric materials becomes one of the research hotspots in recent years. This review represented the performance advantages of inorganic dielectric energy storage materials, summarized the energy storage principle and the main parameters of energy storage performance, and analyzed the energy storage performance of linear dielectrics, relax or ferroelectrics and antiferroelectrics from the viewpoint of component design and multiple material forms (i.e., ceramics, films and multilayer ceramic capacitors). The performance control methods and enhancement mechanisms from the aspects of material composition, structure and preparation technology were discussed. Finally, the opportunities and challenges for inorganic dielectric energy storage materials were given, and the development trend for pulse power capacitors in the future was prospected.

Electrocaloric effect, i.e., the entropy and temperature changes arising from phase transition and dipole orientation induced in electric fields, can realize heat transport and refrigeration. The electrocaloric cooling eliminates the use of environmentally harmful coolants, possesses high cooling efficiency, small size and low weight as a promising environmental-friendly and high-efficiency cooling. One key point for electrocaloric cooling toward practical cooling is to enhance the performance of the electrocaloric effect of ferroelectrics. Ferroelectric ceramics have attracted much attention due to their high polarization, rich phase structures and variety of regulation methods. In this review, we introduced the electrocaloric effect of ferroelectric ceramic thin films, bulks and multilayer thick films with various compositions, and discussed the internal relations among electrocaloric effect, compositions, phase transition behaviors and microstructures. Furthermore, we concluded the modulation approaches of the electrocaloric effect of ferroelectric ceramics, and gave the future development of electrocaloric materials.

The influences of pH value, Fe³⁺ mass concentration, and stirring rate and their interactions on the ammonium sulfate crystallization were investigated via cooling crystallization and sieving based on a response surface methodology. An average particle size quadratic orthogonal model and a variation coefficient 2FI orthogonal model were proposed. The interactions among the various factors were analyzed. The results show that pH value (A) > stirring rate (C) > Fe³⁺ ions concentration (B) is a decreased order for the parameter effects on the average particle size and variation coefficient. Their interactions have a decreased effect order (i.e., AC >BC > AB) on the average particle size and variation coefficient. When pH value is 5.5, stirring rate is 150 r/min, and Fe³⁺ ions concentration is 0.3 g/L, the average particle size and variation coefficient of the ideal crystals is 0.985 mm and 793.719, respectively.