The unusual case of plastic deformation and high dislocation densities with the cold sintering of the piezoelectric ceramic K0.5Na0.5NbO3

K_0.5Na_0.5NbO₃ (KNN) can be readily densified using the cold sintering process, but despite observing high relative permittivity, the ferroelectric hysteresis is strongly suppressed along with a major suppression in the all-important piezoelectric properties. In this study, KNN is fabricated using a NaOH+KOH transient flux under a uniaxial pressure of 400 MPa and heating to 300 °C for 2 h to drive densification to 93% theoretical. It is only after a secondary heat treatment that we observe improvements of the ferroelectric hysteresis and piezoelectric properties. From a detailed structural-property-processing study using analytical transmission electron microscopy (TEM), X-ray line broadening and high field dielectric characterization methodologies we conclude that there is an unusual in-situ plastic deformation process that takes place in addition to the densification under the cold sintering process. High densities of dislocations within grains were observed that lead to multiple pinning sites that impact both the intrinsic and extrinsic contributions to the high field dielectric and piezoelectric properties. Annealing significantly reduced the dislocation density in the highly defective crystallites, observed directly from the TEM and from the sharpening of the X-ray diffraction peaks, resulting in piezoelectric and ferroelectric properties that approached those of conventionally sintered KNN.     

Enhanced electromechanical properties of CaZrO3-modified (K0.5Na0.5)NbO3-based lead-free ceramics

Pairing of large strain response and high d₃₃ with high T_c in (K_0.5Na_0.5)NbO₃-based materials is of high significance in practical applications for piezoelectric actuators. Here, we report remarkable enhancement in the electromechanical properties for (1-x)(K_0.52Na_0.48) (Nb_0.95Sb_0.05)O₃-xCaZrO₃ (KNNS-xCZ) lead-free ceramics through the construction of a rhombohedral (R)-tetragonal (T) phase boundary. We investigated the correlation between the composition-driven phase boundary and resulting ferroelectric, piezoelectric, and strain properties in KNNS-xCZ ceramics. The KNNS-xCZ ceramics with x=0.02 exhibited a large strain response of 0.23% while keeping a relatively large d₃₃ of 237pC/N, which was mainly ascribed to the coexistence of R and T phases confirmed by the XRD and dielectric results. It was found that pairing of large strain response and high d₃₃ in KNN-based materials was achieved. As a consequence, we believe that this study opens the possibility to achieve high-performance lead-free electromechanical compounds for piezoelectric actuators applications.     

Enhanced piezoelectric property in Mn-doped K0.5Na0.5NbO3 ceramics via cold sintering process and KMnO4 solution

Through mixing the KMnO₄ solution with K_0.5Na_0.5NbO₃ (KNN) powders, cold sintering process (CSP) was employed to fabricate high-density Mn-doped KNN green pellets and ceramics. The microstructure, doping effect of Mn and electrical properties of these ceramics were studied in detail. Compared with conventional sintering (CS), the CSP supports the homogeneity of dopants and then promotes grain growth and ceramic densification; thus the Mn-doped KNN ceramics prepared by CSP show the obviously higher density and larger grain size. Besides, the less alkalis volatilization and oxygen vacancies result in more Mn³⁺ but less Mn⁴⁺ in CSP ceramics compared to CS ones, which endows the pinning effect and good poling characteristics in CSP ceramics. All the previous results contribute to the high dielectric constant and remnant polarization in CSP ceramics, which support the enhanced piezoelectric coefficient and are much superior than Mn-doped KNN ceramics prepared by CS. This work reveals that CSP can be a new doping strategy to perform chemical modification of electrical properties in KNN ceramics.     

Composition,microstructure and electrical properties of K0.5Na0.5NbO3 ceramics fabricated by cold sintering assisted sintering

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K_0.5Na_0.5NbO₃ piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K₄Nb₆O₁₇ second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.     

Compositional inhomogeneity and segregation in (K0.5Na0.5)NbO3 ceramics

In this report, the effects of the calcination temperature of (K_0.5Na_0.5)NbO₃ (KNN) powder on the sintering and piezoelectric properties of KNN ceramics have been investigated. KNN powders are synthesized via the solid-state approach. Scanning electron microscopy and X-ray diffraction characterizations indicate that the incomplete reaction at 700 °C and 750 °C calcination results in the compositional inhomogeneity of the K-rich and Na-rich phases while the orthorhombic single phase is obtained after calcination at 900 °C. During the sintering, the presence of the liquid K-rich phase due to the lower melting point has a significant impact on the densification, the abnormal grain growth and the deteriorated piezoelectric properties. From the standpoint of piezoelectric properties, the optimal calcination temperature obtained for KNN ceramics calcined at this temperature is determined to be 800 °C, with piezoelectric constant d₃₃=128.3 pC/N, planar electromechanical coupling coefficient k_p=32.2%, mechanical quality factor Q_m=88, and dielectric loss tan δ=2.1%.     

(Bi0.5Na0.5)TiO3 ferroelectric ceramics: Achieving high depolarization temperature and improved piezoelectric properties

(Bi_1/2Na_1/2)TiO₃-based materials have received much attention due to large electro-strain and high piezoelectric constant (d₃₃), but the tough issue is that the existence of inherent depolarization temperature (T_d) limits the temperature stability and application temperature range. Previously, reports about the formation of BNT/oxide (i.e., ZnO, Al₂O₃) composites thought that T_d can be deferred to a higher temperature and then thermal depolarization improves. However, the deferred T_d of BNT/oxide composites is limited, accompanied by a low d₃₃. Here, we design the {[Bi_0.5(Na_0.8K_0.2)_0.5]_1-xPb_x}TiO₃ ceramics, leading to a big shift of T_d from 77 ℃ to 390 ℃. Large d₃₃ (140 pC/N) and high T_d (∼263 ℃) can be simultaneously achieved for the sample with Pb=0.05, and T_d could be further deferred higher (390 ℃) for Pb=0.20. The off-centre displacement of Pb induced by Pb-O hybridization in the PbO₁₂ polyhedron and ferroelectric order stabilized by the addition of Pb can provide the driving force to strengthen the ferroelectric order, and then promote the thermal stability.     

铜掺杂对(Na0.5K0.5)NbO3无铅压电陶瓷性能的影响

采用传统固相反应法对(Na0.5K0.5)NbO3压电陶瓷进行铜掺杂改性研究。使用SEM、XRD并结合常规压电陶瓷性能测试手段对该体系的显微结构、压电性能等进行表征。研究结果表明:CuO的掺入使材料出现“硬化”现象,即材料的压电系数d33、平面机电耦合系数Kp和介电损耗tanδ下降了,机械品质因子Qm大大提高;CuO掺入量在1%mol时各项性能最佳。另外,从SEM图片中可以看出:(Na0.5K0.5)NbO3压电陶瓷材料的平均晶粒尺寸随着CuO掺入量的增加明显变大。这表明CuO有烧结助熔作用,能降低烧成温度。     

Microstructures,piezoelectric properties and energy harvesting performance of undoped (K0.5Na0.5)NbO3 lead-free ceramics fabricated via two-step sintering

Piezoelectric energy harvesters have become increasingly popular in the field of green energy because of the ability to convert low-frequency environmental vibrations into usable electricity. To fabricate high-performance energy harvesters, the key requirements are piezoelectric ceramics with a small grain size, of near-full density, the intended stoichiometric ratio and a high transduction coefficient. In this work, the effects of two-step sintering on the sinterability, microstructure, piezoelectric properties and energy harvesting performance of (K_0.5Na_0.5)NbO₃ were systematically investigated. Compared with conventional single-step sintering, two-step sintering samples were of higher density, increasing from 91 % to 95 % of theoretical, reduced mean grain size, down from 17 μm to 7.5 μm, and decreased evaporation of the alkali metals. This translated into an improved piezoelectric performance (d₃₃ ∼122 pC/N, k_p ∼36 % and Q_m ∼76), a higher transduction coefficient and energy conversion efficiency as well as a higher open-circuit voltage and power density. This demonstrates the potential of two-step sintering as a high through-put sintering technique for moderate-performance, pure KNN ceramics.     

Defect structure,ferroelectricity and piezoelectricity in Fe/Mn/Cu-doped K0.5Na0.5NbO3 lead-free piezoelectric ceramics

For perovskite Pb-based ceramics, outstanding hardening piezoelectric properties can be easily induced by acceptor dopings of Fe, Mn or Cu, but in this work, completely different hardening effects are observed in Fe/Mn/Cu-doped K_0.5Na_0.5NbO₃ ceramics. Pure K_0.5Na_0.5NbO₃ exhibits a well-saturated single P-E loop, giving low Q_m of 72. Fe₂O₃-doped ceramic exhibits the combined effects of dominant donor and slight acceptor, giving a slightly slanted single P-E loop and relatively low Q_m of 156. For MnO₂-doped ceramic, moderate hardening properties with a slightly pinched P-E loop and relatively high Q_m of 370 are exhibited. Unlike Fe₂O₃ and MnO₂-doped ceramics, a double P-E loop and superhigh Q_m of 1965 are obtained in CuO-doped ceramic. The defect structure and corresponding microscopic mechanisms in the ceramics have been systematically investigated. Our study shows that defect characteristics should be responsible for distinct hardening properties in Fe, Mn and Cu-doped K_0.5Na_0.5NbO₃ materials.     

Influence of B-site compositional homogeneity on properties of (K0.44Na0.52Li0.04)(Nb0.86Ta0.10Sb0.04)O3-based piezoelectric ceramics

The effect of B-site compositional homogeneity on microstructure, piezoelectric properties and dielectric behaviour of lead-free piezoelectric ceramics, (K_0.44Na_0.52Li_0.04) (Nb_0.86Ta_0.10Sb_0.04)O₃, is investigated. The B-site compositional homogeneity is evaluated by using an intermediate precursor obtained by solid state reaction between adequate amounts of Nb₂O₅, Ta₂O₅ and Sb₂O₅, calcined at 1350 °C and attrition milled. The B-site precursor powder is mixed with alkaline carbonates to synthesize perovskite powders and, finally, sinter piezoceramics. X-ray diffraction and Raman spectroscopy reveal the formation of a perovskite phase, although tetragonal tungsten-bronze structure is detected as minor secondary phase. Ceramics processed by using B-site precursor show different crystalline structure as a function of sintering conditions or K/Na ratio. The B-site precursor route produces thus lower piezoelectric properties, but the control of alkali volatilization by using sintering powder bed resulted in a relevant decrease of dielectric losses that favours the d₃₃ enhancement.     

Understanding the piezoelectric properties in potassium-sodium niobate-based lead-free piezoceramics: Interrelationship between intrinsic and extrinsic factors

Lead zirconate titanate (PZT) based ceramics are currently enjoying a wide use in piezoelectric devices despite lead toxicity. Due to growing environmental concerns, the attention on piezoelectric ceramics has been moving to lead-free materials, in particular to (K,Na)NbO₃-based ceramics. Here we report a systematic evaluation of the effects of the compositional modifications on [(K_0.44Na_0.52Li_0.04)[(Nb_0.86Ta_0.10Sb_0.04)_1-xZr_5x/4]O₃ lead–free piezoceramics. We show that an interrelationship between the intrinsic and extrinsic factors is the linchpin for the development of good piezoelectric properties. Hence, the stabilization of the tetragonal symmetry on the orthorhombic-tetragonal polymorphic phase boundary facilities the poling process of the system, thereby enhancing the piezoelectric response. Additionally, the microstructure appears to be related to the piezoelectric properties; i.e., the improved piezoelectric properties correlate to the increase in grain size. The results of this work could help to understand the origin of piezoelectricity in potassium–sodium niobate-based ceramics.     

Low-temperature synthesis of K0.5Na0.5NbO3 ceramics in a wide temperature window via cold-sintering assisted sintering method and enhanced electrical properties

High temperatures (≥ 1100 °C) and narrow temperature window (～ 20 °C) for sintering dense K_0.5Na_0.5NbO₃ ceramics always deteriorate their electrical properties. Here, via cold-sintering assisted sintering method, dense K_0.5Na_0.5NbO₃ ceramics were obtained in a wide temperature span between 800 °C and 1000 °C. An aqueous solution of NaOH and KOH mixture was used as transient liquid. Effects of liquid content (LC), molar concentration (MC) of liquid, cold-sintering temperature (T_CS), and post-annealing temperature (T_AN) on densification and electrical properties of the ceramics were investigated in detail. The ceramics prepared using LC = 10 wt%, MC = 10 mol/L, T_CS = 350 °C, and T_AN = 900 °C exhibit excellent electrical properties with d₃₃ = 123 pC/N, ε_r = 609, tanδ = 0.021, P_r = 28.0 μC/cm², P_m = 39.2 μC/cm², and E_c = 20.3 kV/cm. Compared to the ceramics with same or similar compositions via conventional solid-state sintering, the present K_0.5Na_0.5NbO₃ ceramics exhibit excellent electrical properties. The study endows the cold-sintering assisted sintering the successful method to prepare K_0.5Na_0.5NbO₃ ceramics at low temperatures and in a wide temperature window.     

Effects of sintering aid and atmosphere powder on the growth of (K0.5Na0.5)NbO3 single crystals fabricated by solid-state crystal growth method

(K_0.5Na_0.5)NbO₃ (KNN) + x (= 1, 0.5, 0.05, and 0) wt%Co₃O₄ single crystals were fabricated by a solid-state crystal growth method with a KTaO₃ seed crystal and a KNN atmosphere powder, and the effects of the sintering aid content x and the addition of Co₃O₄ to the atmosphere powder on the growth of the single crystals were investigated. The formation of pores in the single crystals was suppressed by a decrease of x, which, however, decreased the crystal growth length. On the other hand, dense and large KNN single crystals could be fabricated by sintering with a KNN + 5 wt%Co₃O₄ atmosphere at x = 0. The dielectric, ferroelectric, and piezoelectric properties of the KNN single crystals were comparable to those of reported (K,Na)NbO₃ single crystals. These results would be useful for fabricating dense and large single crystals by the solid-state crystal growth method.     

Effects of Bi0.5Na0.5HfO3 addition on the phase structure and piezoelectric properties of (K,Na)NbO3‐based ceramics

Lead‐free perovskite (1‐x)(K_0.48Na_0.48Li_0.04)Nb_0.95Sb_0.05O₃‐x(Bi_0.5Na_0.5)HfO₃ piezoelectric ceramics were prepared by a traditional ceramic fabrication method. An investigation was conducted to assess the effects of (Bi_0.5Na_0.5)HfO₃ content on the crystal structure, microstructure, phase‐transition temperatures, and piezoelectric properties of the ceramics. The X‐ray diffraction results, combined with the temperature dependence of dielectric properties, revealed that the ceramics experienced a structural transition from an orthorhombic phase to a tetragonal phase with the addition of (Bi_0.5Na_0.5)HfO₃, and a coexistence of orthorhombic and tetragonal phases was identified in the composition range of 0.005≤x≤0.015. An obviously improved piezoelectric activity was obtained for the ceramics with compositions near the orthorhombic‐tetragonal phase boundary, among which the composition x=0.005 exhibited the maximum values of piezoelectric constant d₃₃, and planar and thickness electromechanical coupling coefficients (k_p and k_t) of 246 pC/N, 0.435, and 0.554, respectively. Furthermore, the Curie temperature of the ceramics was found decreasing with the increase in (Bi_0.5Na_0.5)HfO₃ content, but still maintaining above 300°C for the phase boundary compositions. These results indicate that the ceramics are promising lead‐free candidate materials for piezoelectric applications.     

Microstructure evolution and electromechanical properties of (K,Na) NbO3-based thick films

This study investigated an unconventional method of electrophoretic deposition (EPD) for the processing of environmentally benign (K_0.5Na_0.5)_0.99Sr_0.005NbO₃ (KNNSr) thick films on Pt/alumina substrate. EPD allows rapid, economical, and low-waste processing of thick films and thus offers an integration advantage for electronics manufacturing. To understand the functional response of the KNNSr thick films, the effect of the sintering temperature and atmosphere on their structure, microstructure, and electromechanical properties was investigated. KNNSr thick films densify in constrained conditions in a very narrow temperature range only a few 10°C below the melting temperature of 1140°C. Up to 1100°C the relative density increases to 80%, upon further heating to 1110°C we observed only the grain growth and pore coalescence. The densification is not affected significantly by the atmosphere. The local domain structure of 25-33 μm thick KNNSr films was similar, while the dielectric and electromechanical properties increased with the increasing sintering temperature. KNNSr thick film sintered at 1100°C has a thickness-coupling factor k_t of 0.4, comparable to that of bulk. The results reveal that the EPD enables the economic processing of high-performance thick films on complex-shape substrates that are difficult to fabricate using conventional thick-film methods.     

Cold sintering assisted processing of Mn-Zn ferrites

Typically, commercial Mn-Zn ferrites are sintered at high temperatures with prolong times. In this work, commercial Fe₂O₃-rich ferrite powders with the composition of 0.21Mn_0.8Zn_0.2Fe₂O₄-0.79Fe₂O₃ (wt%) are densified by cold sintering at 300 °C with the assistance of organic salts, including MnC₂O₄·2H₂O, FeC₂O₄·2H₂O, and Zn(C₂H₃O₂)₂·2H₂O. Excessive Fe₂O₃ enters into spinel structure forming a solid solution through annealing in low pO₂ at 1350 °C. The sintering behaviors, microstructures, magnetic properties and impedances are investigated. The dehydration of organic salts provides mediate liquid phase to trigger the dissolution-precipitation process, which assists the densification of ceramics. The grains grow from 0.15 µm to 0.52 µm and 7.67 µm after cold sintering at 300 °C and annealing at 1350 °C, respectively. The initial permeability of cold sintered sample is improved to 11000 with a Curie temperature of 125 °C. This work provides a feasible route for cold sintering assisted processing of commercial soft magnetic ferrites.     

Preparation of Na0.5Bi0.5TiO3 ceramics by hydrothermal-assisted cold sintering

A recently proposed novel technique, termed “cold sintering process” (CSP), can provide dense ceramic solids at remarkably low temperatures around 180?°C. In a recent work, we successfully obtained dense Na_0.5Bi_0.5TiO₃ ceramics by this method. Bismuth titanate sodium nanoparticles were prepared as the raw material powder by the hydrothermal synthesis route. A hydrothermal precursor solution was used as the transient solvent for cold sintering. Under the combined action of pressure and temperature, the Na_0.5Bi_0.5TiO₃ green body was densified by dissolution-precipitation, and a preliminary densified ceramic sheet was obtained. The amorphous phase in the ceramic sheet was then transformed into a crystalline phase by annealing. Finally, densified Na_0.5Bi_0.5TiO₃ ceramic sheets were obtained, with density of up to 99%, relative permittivity of 681, and dielectric loss of 0.08 at 10?kHz and room temperature. The piezoelectric coefficient d₃₃ of the sample was 52.5?pC/N. The properties of the prepared ceramics were comparable to those of the conventional sintered ceramics.     

Cycling- and heating-induced evolution of piezoelectric and ferroelectric properties of CuO-doped K0.5Na0.5NbO3 ceramic

For accepter-doped perovskite piezoelectric ceramics, macroscopic properties of the materials (eg, hardening, fatigue, and aging) are closely related to microscopic characteristics (eg, oxygen vacancies and defect dipoles). In this work, the relationship of macroscopic and microscopic characteristics in CuO-doped K_0.5Na_0.5NbO₃ (KNN) ceramic has been studied by subjecting the material to electric field cycling, quenching, heating, and consequently aging. The introduction of CuO in KNN generates and . The defect dipoles exhibit obviously the pinning effect on ferroelectric domains and thus induce a completely pinched/double P-E loop and excellent hardening piezoelectricity of high Q_m of 2235. With the destruction of short-range symmetry uniformity between defect dipoles and ferroelectric dipoles induced by electric field cycling, quenching and heating, the ceramic can be depinned and softened. As a result, the depinned ceramic possesses an opened single ferroelectric hysteresis loop and the significantly decreasing Q_m. A distinctive aging is observed in the depinned ceramic. This study provides deep insights into the evolution of electrical properties of accepter-modified alkali niobate perovskite ceramics under electric field cycling, quenching, and heating.     

Composition segregation and grain growth in (K0.5Na0.5)NbO3 piezoceramics prepared by two-step cold sintering process

Cold sintering process (CSP) is a new method to prepare ceramics under quite low temperature. In this work, two-step CSP under different pressures was employed to prepare (K_0.5Na_0.5)NbO₃ (KNN) ceramics. The density of KNN green pellets can be raised by enhancing the pressure of second-step CSP. Energy-dispersive spectroscopy reveals the composition segregation of A-site cations in large grains. The dissolution rate of K⁺ in an aqueous medium is faster than Na⁺, and high pressure can accelerate K⁺ dissolution, resulting in more Na⁺ in some grains. Besides, the diffusion rate of Na⁺ in grains is better than K⁺, which promote the grains growth. Finally, the piezoelectric property is improved even with low ceramic density due to the larger grains, which possess the higher performance composition. This result demonstrates that the pressure and inhomogeneous dissolution of alkali metal ions among CSP play an important role in grain growth and piezoelectric enhancement.     

Microstructure and piezoelectric properties of CuO-doped 0.95(K0.5Na0.5)NbO3-0.05Li(Nb0.5Sb0.5)O3 lead-free ceramics

The effects of sintering temperature and the addition of CuO on the microstructure and piezoelectric properties of 0.95(K_0.5Na_0.5)NbO₃-0.05Li(Nb_0.5Sb_0.5)O₃ were investigated. The KNN-5LNS ceramics doped with CuO were well sintered even at 940 °C. A small amount of Cu²⁺ was incorporated into the KNN-5LNS matrix ceramics and XRD patterns suggested that the Cu²⁺ ion could enter the A or B site of the perovskite unit cell and replace the Nb⁵⁺ or Li⁺ simultaneously. The study also showed that the introduction of CuO effectively reduced the sintering temperature and improved the electrical properties of KNN-5LNS. The high piezoelectric properties of d₃₃ = 263 pC/N, k_p = 0.42, Q_m = 143 and tan δ = 0.024 were obtained from the 0.4 mol% CuO doped KNN-5LNS ceramics sintered at 980 °C for 2 h.