Microstructural,structural, dielectric and piezoelectric properties of potassium sodium niobate thick films

In the framework of a systematic study, we present the influence of processing parameters – in particular the presence of a packing powder during sintering and the sintering temperature – on the microstructural and structural properties of potassium sodium niobate (K_0.5Na_0.5NbO₃ or KNN) thick films. These KNN thick films were prepared with a 1 mass% addition of potassium sodium germanate (KNG), which serves as a liquid-phase sintering aid. The sintered films exhibited preferential crystallographic orientations along [100]_pc and [10−1]_pc, the origin of which lies in the compressive stresses developed during cooling as a result of the thermal expansion mismatch between the film and the substrate. In addition, the dielectric permittivity, dielectric losses and the piezoelectric d₃₃ coefficient of the obtained films were compared with those of KNN bulk ceramics.     

Defect engineering electrical properties of lead-free potassium sodium niobate-based ceramics

Defect engineering plays an important role in property modification for piezoelectric materials. In this work, we pay much attention to the effect of Nb nonstoichiometry on structure and properties of typical 0.95(K_0.45Na_0.55)Nb_1+xO₃–0.05Bi_0.5Na_0.5HfO₃ ceramics. Large piezoelectric constant (d₃₃ ~ 425 pC/N and ~482 pm/V) together with high Curie temperature (T_C ~ 315°C) have been achieved in the ceramics with excess Nb content (x = 0.01). However, the ceramics with deficient Nb element have seriously suppressed cryogenic ε_r-T curves and deteriorated electrical properties. Multi-scale characterizations including phase structure, microstructure, defect structure and domain structure have been adopted to explain the corresponding phenomenon. Defect complex caused by deficient Nb induces clamped domain wall motion, leading to blocked polarization vector and poor electrical properties. On the contrary, the enhanced properties for the ceramics with excess Nb are attributed to easier domain switching due to the suppressed vacancies. We believe that defect engineering, for example nonstoichiometry, cannot only modulate electrical properties but also help us to understand some fundamental and critical problems about KNN-based ceramics.     

Cytotoxicity and degradation behavior of potassium sodium niobate piezoelectric ceramics

In the present study, two lead-free piezoelectric ceramics, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) and lithium-doped potassium sodium niobate (Li_0.06K_0.47Na_0.47NbO₃, LKNN), were prepared by a solid-state reaction process. The cytotoxicity evaluation indicated that the cytotoxicity of KNN is low. However, a strength decrease was noted after soaking in saline solution for 7 days. The addition of 6 mol% Li into the KNN improves its density; the strength and piezoelectric coefficient are enhanced consequently. Nevertheless, the cytotoxicity of LKNN is slightly higher than that of KNN. The higher cytotoxicity is related to the release of Li ions. The release of Li ion also induces the degradation of piezoelectric performance.     

Relaxor behavior of potassium sodium niobate ceramics by domain evolution

Relaxor behavior is proved to be responsible for high piezoelectricity in piezoelectric materials due to the promoted polarization rotation, and an ultrahigh piezoelectricity can be also induced in potassium sodium niobate [(K,Na)NbO₃, KNN]-based ceramics by dielectric relaxation at the multiphase coexistence region. However, it is still absent of the association study between domain evolution and relaxor behavior created by nanoscale multiphase coexistence. Herein, the frequency-dependent domain response, dwell-time dependence of domain radius and voltage-dependent domain switching are characterized in KNN-based ceramics with relaxor R–T phase boundary. These novel domain evolutions further illustrate the contribution of relaxor behavior on high piezoelectricity, yielding to easy polarization rotation based on low energy barrier of polar nanoregions merging into long-range ordering states. And an improved temperature stability is observed at 30?60 °C for relaxor R–T due to the unusual domain evolution. This study affords a new perspective in the mechanism of high-performance lead-free piezoelectrics.     

High mechanical quality factor and piezoelectricity in potassium sodium niobate ceramics

Plenty of works have done to enhance the piezoelectricity of potassium-sodium niobate (KNN), aiming to replace lead-zirconate titanate (PZT) in the consideration of eco-friendly requirement. However so far, KNN ceramics with high piezoelectric performances tend to have a low mechanical quality factor (Q_m), which could result in excessive dielectric loss, especially when working in high frequencies. Thus, increasing Q_m is a crucial task in KNN-based ceramics. By constructing phase boundaries together with inducing oxygen vacancies, a new KNN ceramic system is built by using conventional solid-state method with high Q_m (>250), high piezoelectric performance as well as outstanding temperature stability. Optimum overall properties of KNN-based ceramics can be as large as d₃₃ = 231 pC/N, Q_m = 355, T_C = 366 °C. This work provides a deeper insight to KNN-based ceramics with high mechanical quality factor and makes a progress on the high frequency application of lead-free ceramics.     

Electrophoretic deposition and properties of strontium-doped sodium potassium niobate thick films

We have processed strontium-modified potassium sodium niobate (KNNSr) piezoelectric thick films on metalized alumina by electrophoretic deposition and sintering for energy-harvesting applications. The deposition yield, the electric field in the close vicinity of the planar working electrode (calculated with a finite-element method) and the thickness profiles of the as-deposited layers were examined for ethanol-based suspensions with different conductivities, deposition times, inter-electrode distances and sizes of the counter electrode. Uniformly thick and defect-free, as-deposited KNNSr layers were processed from a suspension with a conductivity of 21.2 μS/cm, deposited for 90 s, for an inter-electrode distance of 3 mm and a diameter of the counter electrode similar to the sample’s dimensions. KNNSr layers sintered at 1100 °C with a thickness of 30 μm and a relative density of 75% exhibited a dielectric permittivity of 325, dielectric losses of 0.04 at 10 kHz and room temperature, and a d₃₃ coefficient of 60 pC/N.     

Optical transmittance and energy storage properties of potassium sodium niobate glass-ceramics

In this work, 0.8(K₂O-Na₂O-2Nb₂O₅)?0.2((1-x)B₂O₃-xP₂O₅) (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) glass-ceramics have been fabricated. The effects of P₂O₅ on the microstructure and properties of the glass-ceramics were comprehensively studied. The addition of P₂O₅ promotes the transition of the glass network structure from a negatively charged [B?₄]^- tetrahedron to an electrically neutral [BP?₄] tetrahedron. With the increase of P₂O₅ content, the formation of K₂B₄O₇ is inhibited, with major phase of Na_0.9K_0.1NbO₃ and minor phase of K₂B₄O₇. It is found that the band gap width of the glass-ceramics increases from 3.34 eV to 3.52 eV firstly and then decreases to 3.43 eV. The grain size of the glass-ceramics decreases from 150 nm to 50 nm. High optical transmittance (63%), large discharge energy density (4.58 J/cm³) and large energy storage efficiency (98%) have been simultaneously obtained for K₂O-Na₂O-Nb₂O₅-B₂O₃-P₂O₅ glass-ceramics, which are potential for the applications of the transparent pulse capacitors.     

Sharpening polycrystalline phase boundary for potassium sodium niobate ceramics with MnF2 modification

The influence of anion on structure and performance is unclear in potassium sodium niobate ((K,Na)NbO₃; KNN)-based ceramics, while cation doping has been widely researched. Here, the phase structure and electrical properties are explored in MnF₂-doped KNN ceramics. Significantly, sharp rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) phase boundary as well as reduced diffusion degree is exhibited in the ceramics along with little changed phase transition temperatures due to the optimized F⁻ content at O site, which is different from that of cation replacement for A and B sites. Notably, the domain wall motion is facilitated due to the increased A vacancy and decreased O vacancy along with strengthened polarity, originating from the higher valence state and electronegativity of F⁻ with respect to O²⁻. And then, enhanced ferroelectricity is realized via MnF₂ modification, the piezoelectricity is elevated in turn. This work presents a new idea of anion doping for controlling structure and properties in perovskite materials.     

Giant strain response with low hysteresis in potassium sodium niobate based lead-free ceramics

In this work, the relationships between the composition-driven phase boundary, ferroelectricity and strain properties of the (1-x)(K_0.48Na_0.52)(Nb_1-ySb_y)O₃-xBi_0.5(Na_0.82K_0.18)_0.5ZrO₃ (abbreviated as (1-x)KNN_1-yS_y-xBNKZ) ceramics were investigated. A giant electric field-induced strain of 0.3% (d₃₃¹ = 750 p.m./V) and a low hysteresis (16.4%) were obtained in the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics. The giant strain is attributed to the enhanced piezoelectricity induced by the appearance of the O-T phase boundary and the electric-field-induced phase transition from the relaxor phase to the ferroelectric phase. Furthermore, the 0.97KNN_0.98S_0.02-0.03BNKZ ceramics exhibit good thermal stability in the temperature range from 25 °C to 150 °C. Hence, this work can promote the practical applications of KNN-based lead-free piezoelectric ceramics in highly sensitive and precise piezoelectric actuators.     

Induced anisotropic behavior and enhanced electrical properties on hot-pressed strontium barium niobate ceramics

Sr_0.30Ba_0.70Nb₂O₆ (SBN30) ferroelectric ceramics were fabricated by conventional sintering (CS) and by hot-pressing sintering (HP) and their crystal structure, microstructure, dielectric, ferroelectric and pyroelectric properties were studied and compared. Preferred orientation of the HP ceramics was detected through X-ray diffraction. Dense microstructure virtually free of pores has been achieved in HP samples. Moreover, the HP samples manifested prominent anisotropy in electrical properties. Besides, the relative permittivity (ε_r), saturated polarization (P_s), pyroelectric coefficient (p) in the direction perpendicular to the pressing axis were much higher than those of the randomly oriented CS samples. The HP samples sintered under 200 MPa show excellent pyroelectric properties in the direction perpendicular to the pressing axis, with pyroelectric coefficient of 2.38×10⁻⁸C/cm²K and pyroelectric figure of merit of F_i=1.13 pm/V, F_v=1.89 m²/C and F_d=0.63 μPa^−1/2,which roughly triple the values obtained in CS samples. These results indicate that hot-pressing is a viable option for accessing single-crystal-like anisotropy as well as enhanced electrical properties in polycrystalline ceramics, thus unveiling the distinctive potential of HP SBN30 ceramics for infrared detector applications.     

Anodically formed oxide films on niobium: Microstructural and electrical properties

The electrical and structural properties of nanoscale niobium pentoxide (Nb₂O₅) dielectric layers in niobium-based solid electrolyte capacitors were studied. The Nb₂O₅ layers are formed by anodic oxidation of Nb-powder compacts. Capacitance measurements show a strong bias-voltage dependence of the capacitance after anodization. Heat treatments at temperatures up to 320 °C, which are applied in the capacitor-production process, lead to an increase of the capacitance and a reduction of the bias dependence. Based on the electrical and structural properties, which are characterized by electron microscopic techniques, a model is presented which explains the behavior of the specific capacitance after the various processing steps.     

Defect engineering of high‐performance potassium sodium niobate piezoelectric ceramics sintered in reducing atmosphere

Reducing atmosphere fired (K_0.5Na_0.5)NbO₃‐based ceramics open up an important opportunity for manufacturing lead‐free multilayer piezoelectric devices cofiring with the base metal inner electrode. In this study, the effects of sintering atmosphere on the piezoelectric, dielectric, and insulating properties in Sn‐doped (Na_0.52K_0.44Li_0.04)NbO₃ (KNN) were investigated. To establish the relationship between the defect structure and electrical properties, the thermally stimulated depolarization current (TSDC) technique was implemented. The electrical properties degrade severely when the pure KNN ceramics are sintered in reducing atmosphere, compared with the ceramics sintered in air. The reason is that the oxygen vacancy concentration in reduced fired ceramics is much higher than that in air fired ceramics. However, the 0.6 mol% Sn‐doped KNN ceramics sintered in reducing atmosphere exhibit comparable electrical properties to the air fired ceramics. From the TSDC analysis, the reducing atmosphere fired ceramics have approximately the same oxygen vacancy concentration as the air fired ceramics because B‐site substituted Sn works as an acceptor.     

Crystal structure and electrical properties of bismuth sodium titanate zirconate ceramics

Lead-free bismuth sodium titanate zirconate (Bi_0.5Na_0.5Ti_1-xZr_xO₃ where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) [BNTZ] ceramics were successfully prepared using the conventional mixed-oxide method. The samples were sintered for 2 h at temperatures lower than 1,000°C. The density of the BNTZ samples was at least 95% of the theoretical values. The scanning electron microscopy micrographs showed that small grains were embedded between large grains, causing a relatively wide grain size distribution. The density and grain size increased with increasing Zr concentration. A peak shift in X-ray diffraction patterns as well as the disappearance of several hkl reflections indicated some significant crystal-structure changes in these materials. Preliminary crystal-structure analysis indicated the existence of phase transition from a rhombohedral to an orthorhombic structure. The dielectric and ferroelectric properties were also found to correlate well with the observed phase transition.     

Insights into the correlation between strain and electrostrictive coefficient of potassium sodium niobate based ceramics from relaxor structure

Eco-friendly (K, Na)NbO₃ (KNN)-based electrostrictive materials have attracted increasing attention as potential candidates for high-precision displacement actuators. Although a series of breakthroughs have increased the electrostrictive coefficient of KNN-based materials with relaxor behaviour (Q₃₃ > 0.0450 m⁴/C²), the electrostrictive strain is still low (<0.1%), making the improvement of the electrostrictive strain a crucial next step. Here, a KNN-based relaxor ceramic of 0.96K_0.48Na_0.52Nb_1-xSb_xO₃-0.04Bi_0.5Na_0.5ZrO₃-0.3%Fe₂O₃ (KNNS_x-BNZ) was designed to simultaneously achieve high electrostrictive strain and Q₃₃. The phase structure transformed from the T phase to the C phase with increasing Sb concentration, which also introduced fine grains and domains. A high electrostrictive strain (～0.102%) and Q₃₃ (～0.0461 m⁴/C²) were obtained at x = 0.09 through a small adjustment of the structure of the relaxor, while an electrostrictive strain with low hysteresis (<10.5%) and an outstanding temperature stability (≥95%) were achieved in the broadened temperature range of 20–180 °C, representing properties superior to those of previous KNN-based and typical PZT-based materials. Our results will help researchers understand how to balance the strain and electrostrictive coefficient in lead-free materials, and thereby contribute toward accelerating the application of KNN-based electrostrictive materials in actuators.     

铌酸钾的合成及其光催化性能

以草酸铌和醋酸钾为原料,加水研磨干燥后得到前驱体,前驱体分别在500℃和800℃焙烧3 h合成高纯相和高结晶度的铌酸钾粉末。红外光谱分析表明,草酸铌和醋酸钾发生离子交换反应生成前驱体K[NbO(C_2O_4)_2]·xH_2O。500℃合成的铌酸钾颗粒粒径约50 nm,800℃合成的颗粒粒径在100 nm以上。以制备的铌酸钾粉末为光催化剂,在高压汞灯照射下,对亚甲基蓝溶液进行光催化降解。实验表明,500℃焙烧合成的铌酸钾粉末具有光催化性能,而800℃焙烧合成的不具有催化性能。催化动力学分析表明,亚甲基蓝溶液的光催化降解反应符合一级动力学方程。     

Preparation and characterization of potassium sodium niobate nanofibers by electrospinning

In this work, we describe the electrospinning of (K,Na)NbO₃ fibers and the effect of calcination temperature on the final phase composition. The envisaged application is for the fabrication of ferroelectric sensor hybrid materials. A solution of potassium acetate, sodium methoxide, and niobium ethoxide dissolved in methanol, acetylacetone, and acetic acid was mixed with polyvinylpyrrolidone (PVP) dissolved in methanol, producing a viscous solution for electrospinning. Confirmation that the proposed equation on the average diameter of fibers produced from high viscosity solutions was larger than that of a lower viscosity solution was made. A scanning electron microscopy (SEM) study showed the fibers to be cylindrical, smooth with diameters of around 400 nm and an aspect ratio >1000. The electrospun fibers were calcined from 700°C to 1050°C observing the fiber morphology. With increasing calcining temperature, the grain size increased. The calcined (K,Na)NbO₃ nanofibers were brittle and generally found to display the “necklace effect.”     

Trap-induced self-recoverable photochromism of rare-earth doped sodium niobate translucent ceramics

Most photochromic ceramics can back up post-irradiation to achieve color state transition. Unfortunately, color recovery usually requires exposure of material to external physical fields (such as light and heat), which severely limits the application as a convenient energy building material such as smart windows. Here, we report a kind of sodium niobate translucent ceramics whose photochromic effect can be completely self-recoverable without any external physical field stimulation. Based on the analysis of time-response transmittance, up-conversion emission spectra and thermoluminescence spectra, the behavior of carrier migration and trap capture/release in the process of self-recovery photochromic reaction has been proposed. It is believed that the emergence of intermediate trap level is an important factor to produce this spontaneous behavior. Moreover, by controlling the amount of rare earth doping and designing two types of materials with different carrier behaviors, Sm/Er-codoped NaNbO₃ ceramics are regarded as an appropriate candidate for tracing the time of light irradiation and intelligent regulation of smart building materials. This work can promote the fundamental understanding and practical applications of self-recoverable photochromic bulk materials.     

An easy approach to obtain large piezoelectric constant in high-quality transparent ceramics by normal sintering process in modified potassium sodium niobate ceramics

It is well-known that the chemical modification of (K,Na)NbO₃ (KNN)-based ceramics (KNNs) by the addition of Bi₂O₃, or other aliovalent ions could decrease grain size and increases the crystal isotropic nature, namely, the pseudo-cubic phase since the sintered specimens could be transparent. However, due to the small grain size and pseudo-cubic phase, the transparent specimens have low piezoelectric properties. For obtaining better transparent and piezoelectric properties simultaneously, the starting KNNs composition must be selected with small grain size and high piezoelectric properties for Bi₂O₃ modification. In this study, a high transmittance 52 % was measured at the wavelength of 800 nm, simultaneously with large piezoelectric constant (d₃₃ = 125 pC/N) by normal sintering method in KNLTN + 1.5 mol.% Bi₂O₃ composition. The origin of high transmittance simultaneously with large piezoelectric constant might be due to the small reduction in grain size, owing to the small grain size of starting KNLTN composition.     

Dielectric properties of strontium iron holmium niobate ceramics

Strontium iron holmium niobate (Sr(Fe_1?xHo_x)_0.5Nb_0.5O₃) ceramics were synthesized via a solid-state reaction technique. The undoped ceramic showed an orthorhombic phase, but it transformed to a pseudocubic phase for higher Ho concentrations. A low solubility limit of Ho in SFN caused a formation of second phase for the x=0.15 ceramic. Dielectric behavior of undoped ceramic exhibited high dielectric constant over a wide temperature range. However, the doping shifted this region to a higher temperature. The doping also shifted the peak of dielectric loss to a higher temperature. Activation energy of dielectric relaxation increased with increasing Ho concentration. In addition, complex impedance analysis was applied to determine the behaviors of grain boundary and grain after doping.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.