3D printing of doped barium-titanate using robocasting - Toward new generation lead-free piezoceramic transducers

Lead-free piezoelectric ceramics, most commonly prepared via conventional process based on a solid-state synthesis, can be used as acoustical transducers or sensors in many application fields. Particularly in some applications where specific geometries are desirable, the need of complex, time consuming, and costly operations could make conventional process inappropriate. This paper proposes an alternative solution based on additive manufacturing (AM), which is of great interest to control and shape the structure in an automatic manner. A formulation of ceramic slurry, consisting of 80%wt doped barium titanate (BT) and organic mixture, was investigated. The synthesized slurry is revealed to be compatible with the robocasting technique while maintaining good mechanical strength after debinding and sintering steps. Empirical measures together with microscopic analysis confirm high densities of the printed BT ceramics (∼ 95% after sintering), which is consistent to those obtained in literatures. To better assess the material and process performances, microstructural, dielectric and piezoelectric characterizations are conducted on both 3D printed and conventional BT ceramics. The results demonstrate that the printed samples give rise to excellent dielectric and piezoelectric properties (d₃₃ coefficient of 260 pC/N and dielectric constant of 1800), which are very close to the values obtained by conventional method on the same composition. As only a few and recent works have been found in the literatures on such systems, this study highlights the considerable and growing potential of robocasting techniques in the field of piezoelectric ceramics. The obtained d₃₃ coefficient is revealed as the highest value reported for printed BT doped systems.     

Extreme High-Speed Laser Material Application-Coated Carbide-Reinforced Control Plates and Conventional Lead-Based Plates in Axial Piston Machines

Hydraulic drive train technology is widely used in both mobile and stationary systems, when high forces or torques are applied. In hydraulic components, high load levels must be supported due to increasing pressure levels of the fluid. Conventionally, leaded brass is used right against steel, where the lead not only aids the manufacturing process but also provides good tribological properties, such as low friction over a wide range of speeds and pressures. Currently promising lead-free coatings using high-speed laser material deposition (EHLA) will be implemented in a state-of-the-art axial piston pump.     

Defect Dipole-Induced Fatigue Strain Recoverability in Lead-Free Piezoelectric Ceramics

Piezoelectric ceramics have garnered extensive utilization in high-precision actuators, where the magnitude of electric field-induced strain and fatigue resistance play crucial roles in actuation applications. Herein, an innovative strategy based on defect dipoles is proposed to form a defect-engineered polymorphic phase transition and achieve a giant piezoelectric strain coefficient of 3080 pm V⁻¹ in Li/Sr-doped (K_0.5Na_0.5)NbO₃ lead-free piezoceramics. The mechanism responsible for the enhanced strain performance lies in the optimized strain compatibility between the refined stripe domains and the nanosized domains. Additionally, the results demonstrate that the material is able to recover from fatigue-induced strain depletion under the stimulation of bipolar electric fields. This property can be phenomenologically explained by the rigid ion model, in which the application of reversal electric fields can facilitate the restoration of defect dipoles, thereby greatly contributing to strain recoverability. This study establishes a close correlation between the unipolar strain properties and the inherent flexibility of defect dipoles and provides new insight into the design of high-reliability, large-stroke piezoceramics.     

Giant Electric Field-Induced Strain with High Temperature-Stability in Textured KNN-Based Piezoceramics for Actuator Applications

Large-strain (K,Na)NbO₃ (KNN) based piezoceramics are attractive for next-generation actuators because of growing environmental concerns. However, inferior performance with poor temperature stability greatly hinders their industrialized procedure. Herein, a feasible strategy is proposed by introducing ${\mathrm{V}}_{\text{K/Na}}^{{}^{\prime }}$-${\mathrm{V}}_{\stackrel{\mathrm{..}}{\mathrm{O}}}$ defect dipoles and constructing grain orientation to enhance the strain performance and temperature stability of KNN-based piezoceramics. This textured ceramics with 90.3% texture degree exhibit a giant strain (1.35%) and a large converse piezoelectric coefficient d₃₃^* (2700 pm V⁻¹), outperforming most lead-free piezoceramics and even some single crystals. Meanwhile, the strain deviation at high temperature of 100 °C–200 °C is obviously alleviated from 61% to 35% through texture engineering. From the perspective of practical applications, piezo-actuators are commonly utilized in the form of multilayer. In order to illustrate the applicability on multilayer actuators, a stack-type actuator consisted of 5 layers of 0.4 mm thick ceramics is fabricated. It can generate large field-induced displacement (11.6 µm), and the promising potential in precise positioning and optical modulation are further demonstrated. This work provides a textured KNN-based piezoceramic with temperature-stable giant strain properties, and facilitates the lead-free piezoceramic materials in actuator applications.     

(Ba,Ca)(Ti,Zr)O3–CeO2 translucent and opaque ceramics — Optical and electromechanical characteristics

This work demonstrates promising approaches to synthesize multifunctional-translucent BCZT-CeO₂ ceramic for electro-mechanical/optical conversion. Nearly fully dense BCZT-CeO₂ electroceramics were fabricated by spark plasma sintering (SPS) showing high dielectric permittivity of 5875 and low dielectric loss of 2.1%. Altogether, an extraordinary optical transmittance was attained as ～ 40% at 750 nm wavelength and ～ 46% in the infrared region. The fine grains restricted piezoelectric constant, d₃₃, to a moderate value of ～ 145 pC/N but with an enhanced Curie temperature, T_C ～ 135 °C. Post rapid pressure-less sintering on SPSed samples led to a fast grain growth within an extra hour of sintering and a major improvement of piezoelectric properties (d₃₃ = 532 pC/N and k_p = 43%) was achieved. A substantial amount of energy can be saved by both approaches owing to rapid heating/cooling rates and short dwell-times, at least when compared to conventional sintering. This success highlights ingenious avenues for further studies on BCZT-CeO₂ electro-optical ceramics.     

Li0.5Bi2.5Nb2O9: An alkali-metal bismuth layer structured ferroelectric with high TC and small orthorhombic distortion

Bismuth layer structured ferroelectrics (BLSFs) have drawn much attention due to their great potential for high-temperature applications. It is generally accepted that BLSFs with small orthorhombic distortion should have low T_C. In this paper, we report an anomalous example Li_0.5Bi_2.5Nb₂O₉ (LBN), which has small orthorhombic distortion but very high T_C. LBN shows the highest T_C (834°C) among currently reported prototype alkali-metal BLSFs, which is attributed to its small tolerance factor. However, the refinement result reveals that LBN is orthorhombic with the space group of A2₁am, and the orthorhombic distortion of LBN a/b is merely 1.0019. The measurement of piezoelectric coefficient (d₃₃ ∼ 6 pC/N) and observation of striped domain structure confirm the ferroelectricity nature of LBN. This work deepens our understanding of BLSFs and may lead to the finding of more novel BLSFs with high performance.     

Enhanced photostrictive properties in ternary (1-x)(0.7BiFe1-yMnyO3-0.3BaTiO3)-xKTaO3 ceramics

A novel ternary compound (1-x)(0.7BiFe_1-yMn_yO₃-0.3BaTiO₃)-xKTaO₃ (BFMBT-xKT) is fabricated by solid-state synthesis technique. A systematic investigation of the crystal symmetry, ferroelectric properties, light absorption, and photostrictive properties was carried out with various Mn and KTaO₃ doping content. The phase transition behavior induced by the addition of KTaO₃ is investigated using X-ray diffraction and Rietveld refinement. The addition of KTaO₃ leads to the appearance of the tetragonal phase and the coexistence of pseudo-cubic and tetragonal phases. Furthermore, a high fraction of the tetragonal phase is obtained due to the increased KTaO₃ content. The solid solution BFMBT-0.02 KT exhibits strong ferroelectricity (P_r ≈ 42 μC/cm²) as well as a narrow bandgap (E_g ≈ 1.91 eV). Moreover, the largest photostriction of 1.68 × 10^?3 has been achieved in BFMBT-0.02 KT under 405 nm laser irradiation. These improvements are strongly associated with the contribution of KTaO₃-induced phase transition in ceramics. The power-dependent Raman spectroscopy reveals a redshift of phonon modes associated with B–O bond vibrations, indicating microscopic lattice expansion under illumination. As a result, the ternary BFMBT-xKT solid solutions with excellent photostriction will be a promising candidate for wireless optomechanical applications.     

Giant strains of 0.70% achieved via a field-induced multiple phase transition in BNT-based relaxor antiferroelectric ceramics

Bismuth sodium titanate (BNT)-based lead-free ceramics have attracted a great deal of attention due to their large electrostrains. In this work, a remarkably symmetric strain of 0.7% together with excellent temperature (0.5–0.7% from 25 to 100 °C)/frequency (ΔS<4% from 1 to 20 Hz) stability was observed in the 0.91(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃-0.03NaNbO₃ (BNT-6BT-3NN) AFE P4bm ceramic through constructing R3c/P4mm/P4bm triple-phase coexistence phase boundary. Compared with other two compositions near double-phase coexistence ferroelectric (FE)-antiferroelectric (AFE) phase boundaries, the BNT-6BT-3NN ceramic exhibits a unique field-induced multiple phase transition from the initial AFE P4bm phase to the metastable FE P4mm phase and finally into the FE R3c phase. In-situ structural analysis evidenced a significantly enhanced lattice strain but a comparable strain value from domain switching in BNT-6BT-3NN compared with other compositions. The present study provides a novel strategy for designing high-performance large-strain ceramics in BNT-based relaxor AFE systems.     

Enhanced piezoelectric properties in BF-BT based lead-free ferroelectric ceramics for high-temperature devices

Due to the high Curie temperature (T_C), BiFeO₃–BaTiO₃ (BF-BT) ceramics have been broadly investigated in high-temperature piezoelectric devices. The piezoelectric constant is one of the most significant factors in determining the sensitivity and reliability of piezoelectric functional components. However, the poor piezoelectric constant (d₃₃) of BF-BT ceramic has prevented the practical application of the material. In this work, we innovatively introduce the 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ (0.93NBT-0.07BT) component to 0.7BF-0.3BT ceramic, to build a morphotropic phase boundary (MPB) for enhancing d₃₃. The XRD analysis shows that the (0.7BF-0.3BT)-x(0.93NBT-0.07BT) ceramics are still in the MPB region with R–PC phases coexistence, and exhibits a homogeneous solid solution. Moreover, the introduction of 0.93NBT-0.07BT ceramic suppresses the generation of defects and facilitates grain growth, thus enhancing piezoelectric property. In consequence, an optimum piezoelectricity d₃₃ = 213 pC/N along with T_c～450 °C was obtained in (0.7BF-0.3BT)-0.01(0.93NBT-0.07BT). This research provides a new idea for the application of BF-BT ceramics in high-temperature piezoelectric devices.