Hardening Effect on the Electromechanical Properties of KNN-Based Ceramics Using a Composite Approach

The high mechanical quality factor (Q_m) of KNN-based ceramics is usually achieved by acceptor doping. However, this hardening effect has serious limitations due to the increased mobility of oxygen vacancies under large electric fields and hence is difficult to use in high-power applications. In this work, the hardening mechanism is demonstrated by the development composites of the 0.957(K_0.48Na_0.52)Nb_0.94Ta_0.06O₃-0.04(Bi_0.5Na_0.5)ZrO₃-0.003BiFeO₃ (KNNT-BNZ-BFO) matrix with the K₄CuNb₈O₂₃ (KCN) phase using the two-step ball-milling method. A decrease in remnant polarization and dielectric constant and an increase in resistivity and Q_m are observed compared to that in the KNNT-BNZ-BFO sample. A high Q_m of 160, Curie temperature, T_C, of 310 °C, and piezoelectric coefficient, d₃₃, of 330 pC/N can be obtained simultaneously in the composite with a 0.008 mole ratio of KCN. This can be explained by the mechanical clamping effect of KCN due to strain incompatibility and the domain wall pegging that traps charges at the KNNT-BNZ-BFO/KCN interface. This composite approach is considered a general hardening concept and can be extended to other KNN-based ceramic systems.

Hardening Effect in Lead-Free KNN-Based Piezoelectric Ceramics with CuO Doping

As the most promising lead-free piezoelectric ceramics to replace lead zirconate titanate (PZT) ceramics, potassium sodium niobate (KNN) ceramics have been widely studied for their application prospects in various electronic devices. Increasing Q_m while maintaining a high piezoelectric activity is quite important for piezoelectric ceramics applied in ultrasonic devices. A KNN-based ceramic with high d₃₃ and Q_m is prepared by a conventional solid-state technique to construct polycrystalline phase boundaries and induce defect dipoles. The best overall performance can reach d₃₃ = 260 pC/N, Q_m = 210, and T_C = 293 °C. The temperature dependence of the relevant parameters is tested, where Q_m increases but d₃₃ decreases with the rise of temperature accompanied by escaping ferroelectric boundary, which shows that the polarization rotation plays an important role in the two parameters. The hardening effect of KNN-based ceramics with CuO doping is further studied by first-principles calculations, demonstrating that the Cu doping strongly disturbs the ferroelectric order, but the formation of defect dipoles could stabilize the ferroelectric order. It is illustrated that defect dipoles always find their ground state at the site near the domain walls and the oriented defect dipoles hinder the polarization rotation severely, confirming the role of the defect dipoles in KNN-based materials.

Isolated-Oxygen-Vacancy Hardening in Lead-Free Piezoelectrics

Defect engineering is a well-established approach to customize the functionalities of perovskite oxides. In demanding high-power applications of piezoelectric materials, acceptor doping serves as the state-of-the-art hardening approach, but inevitably deteriorates the electromechanical properties. Here, a new hardening effect associated with isolated oxygen vacancies for achieving well-balanced performances is proposed. Guided by theoretical design, a well-balanced performance of mechanical quality factor (Q_m ) and piezoelectric coefficient (d₃₃ ) is achieved in lead-free potassium sodium niobate ceramics, where Q_m increases by over 60% while d₃₃ remains almost unchanged. By atomic-scale Z-contrast imaging, hysteresis measurement, and quantitative piezoresponse force microscopy analysis, it is revealed that the improved Q_m results from the inhibition of both extrinsic and intrinsic losses while the unchanged d₃₃ is associated with the polarization contributions being retained. More encouragingly, the hardening effect shows exceptional stability with increasing vibration velocity, offering potential in material design for practical high-power applications such as pharmaceutical extraction and ultrasonic osteotomes.

Hardening Effect Analysis by Modular Upper Bound and Finite Element Methods in Indentation of Aluminum, Steel, Titanium and Superalloys

The application of incremental processes in the manufacturing industry is having a great development in recent years. The first stage of an Incremental Forming Process can be defined as an indentation. Because of this, the indentation process is starting to be widely studied, not only as a hardening test but also as a forming process. Thus, in this work, an analysis of the indentation process under the new Modular Upper Bound perspective has been performed. The modular implementation has several advantages, including the possibility of the introduction of different parameters to extend the study, such as the friction effect, the temperature or the hardening effect studied in this paper. The main objective of the present work is to analyze the three hardening models developed depending on the material characteristics. In order to support the validation of the hardening models, finite element analyses of diverse materials under an indentation are carried out. Results obtained from the Modular Upper Bound are in concordance with the results obtained from the numerical analyses. In addition, the numerical and analytical methods are in concordance with the results previously obtained in the experimental indentation of annealed aluminum A92030. Due to the introduction of the hardening factor, the new modular distribution is a suitable option for the analysis of indentation process.