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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.”     

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.     

A composition design rule for crystal growth of centimeter scale by normal sintering process in modified potassium sodium niobate ceramics

It has been reported that single crystals could be grown by normal sintering process, without the addition of a seed, in (K,Na)NbO₃ (KNN)-based ceramics. The growth of huge grains (approximately 5–30 mm) is due to the donor effect on abnormal grain growth (AGG) in KNN-based ceramics. In this study, a composition design rule is suggested to obtain the large single crystal without the seed addition in KNN-based ceramics. In addition, it is also identified by the microstructure observation that the huge grains can be due to the donor effect on the abnormal grain growth which is found in perovskite materials. The donor ratio must be designed to be located at the range between 0.7% and 0.9% compared to that of Na⁺ ions, in order to obtain the large single crystals in KNN-based ceramics. This range of the donor ratio is narrower than that in BaTiO₃ (or SrTiO₃) ceramics.     

Thermoelectric properties of oxide ceramics

Thermoelectric properties of several oxides were investigated. Al₂O₃-, TiO₂- or ZrO₂-doped ZnO showed large power factor over the wide temperature range. Fe₂O₃. showed large power factor by doping of TiO₂. BaO-doped RuO₂, BaRuOi₃ and CaRuO₃ showed large power factor at high temperature. Improvement of electrical conductivity by doping was effective to increase power factor in these oxide materials.     

Electric properties of alkali metal doped potassium niobate crystals

Alkali metal (Na, Rb or Cs) doped KNbO₃ single crystals are grown using an original pulling down method by means of co-doping with Na (small ionic size), Rb (large ionic sizes) or Cs (large ionic sizes) into KNbO₃. Single-phase crystals are successfully grown with orthorhombic system at room temperature, for all the pure and doped KNbO₃. Their electric properties, such as the dielectric constant and the impedance, are found to be changed according to the co-doping elements.     

Effect of microwave sintering on the properties of copper oxide doped Y-TZP ceramics

The effect of various amounts of copper oxide (CuO) up to 1?wt% on the densification behaviour and mechanical properties of 3?mol% yttria-tetragonal zirconia polycrystal (Y-TZP) were studied by using microwave (MW) sintering method. The MW sintering was performed at temperatures between 1100?°C and 1400?°C, with a heating rate of 30?°C/min. and holding time of 5?min. The beneficial effect of MW in enhancing densification was also compared for the undoped and 0.2?wt% CuO-doped Y-TZP when subjected to conventional sintering (CS) method. The results showed that significant enhancement in the relative density and Vickers hardness were observed for the undoped Y-TZP when MW-sintered between 1100?°C and 1250?°C. It was revealed that the 0.2?wt% CuO-doped Y-TZP and MW sintered at 1250–1300?°C could attain ≥?99.8% of theoretical density, Vickers hardness of about 14.4?GPa, fracture toughness of 7.8 MPam^1/2 and exhibited fine equiaxed tetragonal grain size of below 0.25?µm. In contrast, the addition of 1?wt% CuO was detrimental and the samples exhibited about 50% monoclinic phase upon sintering coupled with poor bulk density and mechanical properties. The study also revealed that the addition of 0.2?wt% CuO and subjected to conventional sintering produced similar densification as that obtained for microwave sintering, thus indicating that the dopant played a more significant role than the sintering method.     

Densification behaviour and two stage master sintering curve in lithium sodium niobate ceramics

The Master Sintering Curve (MSC) has received much attention in recent years due to its ability to predict sintering behaviour of a given powder and green body process regardless of its thermal history. In this paper MSC, based on the combined stage sintering model is constructed for one of the most important lead-free piezoelectric viz. lithium sodium niobate, Na_1-xLi_xNbO₃ (x=0.12, LNN-12), ceramic using shrinkage data. The present study has been carried out to understand and control the densification behaviour during pressureless sintering. Two distinct stages of densification have been observed en route to the upper limit to sintering temperature. The activation energies of densification for the two temperature ranges viz. 800–1150 °C and 1150–1300°C were found to be 365 kJ/mol and 2530 kJ/mol, respectively, through the construction of MSC. The MSC should be useful in predicting the densification behaviour and the final density and for designing a reproducible fabrication schedule for the LNN-12 ceramics.     

Composition‐induced phase transitions and enhanced electrical properties in bismuth sodium titanate ceramics

Here, the composition‐induced phase transition and enhanced electrical properties were investigated in terms of lead‐free {[Bi_0.5(Na_0.82?xK_0.18Li_x)_0.5]_1?ySr_y}TiO₃ (BNKLST‐x/y, x=0‐0.175 and y=0‐0.125) ceramics. The rhombohedral and tetragonal phase boundary can be constructed, and then the enhancement of piezoelectric properties (d₃₃~271 pC/N and k_p~0.38) can be observed for x=0.10 and y=0.05, which is superior to most reported results in polycrystalline BNT‐based ceramics. In particular, a fatigue‐free behavior after 10⁶ polarization switching cycles was shown in the BNKLST‐0.10/0.05 ceramics due to the reversible field‐induced phase transition, and a slight decrease in d₃₃ (~4.5%) was also shown. More importantly, the general model of electric field, temperature, and composition‐induced phase transition was employed to explain the enhancement of piezoelectric and fatigue properties. We believe that the composition design of this system can promote the development of bismuth sodium titanate lead‐free ceramics.     

Structural and electrical properties of sintered zinc-titanate ceramics

The aim of this work was an investigation of structural and electrical properties of sintered zinc-titanate ceramics obtained by mechanical activation. Mixtures of ZnO and TiO₂ were mechanically activated in a planetary ball mill up to 90 min and sintered isothermally in air for 120 min at 1100 °C. The phase composition in the ZnO–TiO₂ system after milling and sintering was analyzed using the XRD method. Microstructure analyses were performed using SEM. The results of electric resistivity, capacitance and loss tangent of the sintered samples were obtained. The existence of zinc-titanate as a dielectric was proved (?_r = 12.5, Q = 386.1, tgδ = 0.0026, ρ = 1.02 Ωm).     

Microstructural,structural and electrical properties of La3+-modified Bi4Ti3O12 ferroelectric ceramics

Bi_4?xLa_xTi₃O₁₂ (BLT) ceramics were prepared and studied in this work in terms of La³⁺-modified microstructure and phase development as well as electrical response. According to the results processed from X-ray diffraction and electrical measurements, the solubility limit (x_L) of La³⁺ into the Bi₄Ti₃O₁₂ (BIT) matrix was here found to locate slightly above x = 1.5. Further, La³⁺ had the effect of reducing the material grain size, while changing its morphology from the plate-like form, typical of BIT ceramics, to a spherical-like one. The electrical results presented and discussed here also include the behavior of the temperature of the ferroelectric–paraelectric phase transition as well as the normal or diffuse and/or relaxor nature of this transition depending on the La³⁺ content.