Effect of stoichiometry and milling processes in the synthesis and the piezoelectric properties of modified KNN nanoparticles by solid state reaction

Nanoparticles in the system (K,Na,Li)(Nb,Ta,Sb)O₃, modified KNN, were synthesized following a solid state reaction procedure. Milling of the individual carbonate and oxide raw materials was carried out before mixing of the components to optimize particle size. These mixtures were calcined at 700 °C for 2 h, obtaining nanoparticles with size ranging between 50 and 200 nm. The optimization of the raw materials particle size and the particle refinement of the carbonates during their decomposition play a key role in the formation of the modified KNN nanoparticles by solid state route. The obtained nanoparticles show tetragonal and orthorhombic phases coexistence that could be attributed in part to the lack of homogeneity of cations distribution confirmed by EDS analysis. The K⁺ cation excess on the modified KNN system produces a displacement of Li⁺ cations from the perovskite structure that is the origin of the stabilization of the orthorhombic symmetry. These nanoparticles are used to sintered ceramics with good piezoelectric properties without needing of anisotropic preparation methods. The sintered ceramics show resistance to hygroscopicity and deliquescence.     

Structural and electrical characterization of (K0.48Na0.52)0.96Li0.04Nb0.85Ta0.15O3 synthesized by spray drying

The synthesis of lead-free ferroelectric materials with composition (K_0.48Na_0.52)_0.96Li_0.04Nb_0.85Ta_0.15O₃ has been achieved by spray drying technique. Pure perovskite phase was obtained after calcination of as-prepared powders at 800 °C for 1 h. Crystallized powders have a particle size average of 100 nm. Well sintered samples were obtained at 1120 and 1130 °C for 2 h in air with densities of 4.58 and 4.49 g/cm³, respectively. We attributed this improvement as the result of sintering process and the very small particle size in powders. Sintered samples have promising piezoelectric parameters, k_p 0.40–0.41, d₃₁ 50–55 pC/N and dielectric losses around 1%.     

High piezoelectricity in CuO-modified Ba(Ti0.90Sn0.10)O3 lead-free ceramics with modulated phase structure

High piezoelectricity was achieved in Ba(Ti_0.90Sn_0.10)O₃ lead-free ceramics by optimizing CuO addition and sintering temperature. The phase structure of 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramic is coexisting rhombohedral and tetragonal phases as sintered at 1300 °C. The coexistence of rhombohedral, tetragonal and orthorhombic phases appears in 1.0 mol% CuO-doped Ba(Ti_0.90Sn_0.10)O₃ ceramics as sintered at 1350–1450 °C, which leads to highly enhanced d₃₃ up to 650pC/N. This work demonstrates that high piezoelectric property (d₃₃ = 650pC/N) can be obtained in BaTiO₃-based lead-free piezoceramics with a simple composition modification by modulating phase structures, which also indicates that Ba(Ti,Sn)O₃ is a promising candidate to replace the lead-based piezoceramics.     

Dielectric and piezoelectric properties of (1−x)(Bi,Na)TiO3–x(Bi,K)TiO3 lead-free ceramics for piezoelectric energy harvesters

The piezoelectric and dielectric properties of the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0.12, 0.14, 0.18, 0.20 and 0.30) lead-free ceramics were investigated. Specimens were prepared by the conventional mixed oxide method and sintered at 1170 °C in air. Scanning electron microscopy indicated that increasing x from 0.12 to 0.30 causes a decrease in the grain size. The (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ ceramics shows a homogeneous microstructure and excellent dielectric and piezoelectric properties. Specimens with optimum composition showed a piezoelectric charge constant d₃₃ of 166 pC/N, an electromechanical coupling factor k_p of 0.5, a dielectric constant ε_r of 1591.32 at 1 kHz and generated power output of 37.49 nW/cm².     

Enhanced dielectric and piezoelectric properties in Li/Sb-modified (Na,K)NbO3 ceramics by optimizing sintering temperature

In this paper, lead-free (Na_0.474K_0.474Li_0.052)(Nb_0.948Sb_0.052)O₃ ceramics were synthesized by a conventional solid-state reaction route. The effects of sintering temperature on the crystal structure, microstructure, densification, dielectric properties, and ferroelectric properties of the KNNLS ceramics were addressed. X-ray diffraction patterns and Raman spectrum indicated a transition from orthorhombic to tetragonal phase during the sintering temperature region. This transition is attributed to the migration of Li between the matrix grain and grain boundary. Scanning electron microscopy study revealed increased grain size and enhanced densification with increasing sintering temperature. The density of the ceramics sintered at 1080 °C reached a maximum value of 4.22 g/cm³. KNNLS ceramics sintered at an optimum temperature of 1080 °C exhibited high piezoelectric properties, that is 242 pC/N for d₃₃, 0.42 for k_p and 18.2 μC/cm² for P_r.     

Chemical stability and electrical properties of Nb doped BaCe0.9Y0.1O3−δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe_0.9?xNb_xY_0.1O_3?δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550–750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe_0.87Nb_0.03Y_0.1O_3?δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10^?2 S cm^?1 in wet hydrogen at 650 °C compared to 1.01×10^?2 S cm^?1 for undoped electrolyte.     

Dependence of dielectric properties on structural characteristics of (Zn1/3A2/3)0.5 (Ti1−xBx)0.5O2 (A = Nb5+, Ta5+, B = Ge4+, Sn4+) ceramics at microwave frequencies

The effects of structural characteristics on the dielectric properties of (Zn_1/3A_2/3)_0.5(Ti_1?xB_x)_0.5O₂ (A = Nb⁵⁺, Ta⁵⁺, B = Ge⁴⁺, Sn⁴⁺) (0.1 ≤ x ≤ 0.3) ceramics were investigated at microwave frequency. The sintered specimens showed solid solutions with a tetragonal rutile structure within the solid solution range of compositions. With an increase of BO₂, the temperature coefficient of resonant frequency (TCF) and dielectric constant (K) decreased with a decrease of oxygen octahedral distortion and dielectric polarizabilites, respectively. However, the quality factor (Qf) of the sintered specimens was increased with BO₂ due to the reduction of Ti⁴⁺ ions. The Qf value of the specimens with A = Ta was higher than that of the specimens with A = Nb.     

Enhanced transport critical current density of (Bi,Pb)-2223/Ag superconductor tapes added with nano-sized Bi2O3

Ag sheathed superconductor tapes with starting composition (Bi, Pb)-2223(Bi₂O₃)_0.01 were prepared. Bi₂O₃ with average size 150 nm was used in this work. The Bi₂O₃ amount was chosen based on our initial study on nano-sized Bi₂O₃ added pellets which showed an optimal superconducting property for 0.01 wt% addition. Non-added tapes were also prepared for comparison. The tapes were investigated by X-ray diffraction method, scanning electron microscopy and transport critical current density, J_c measurements (30 K to 77 K). The influence of different sintering times (50, 100, and 150 h) on J_c under applied magnetic field (0–0.75 T at 77 K) parallel and perpendicular to the surface of the tapes was also investigated. J_c of added tapes was found to increase significantly as compared with the non-added tapes. The Bi₂O₃ added tapes sintered for 150 h exhibited the highest J_c at 30 K of 57,900 A/cm² as compared with 19,400 A/cm² for the non-added tapes sintered for 100 h. The improvements in flux pinning and connectivity between grains due to nano Bi₂O₃ addition led to the enhancement of J_c.     

Synthesis,structure and electronic calculation of alkali metals borate Li2Na[B5O8(OH)2]

A new alkali metals borate complex, Li₂Na[B₅O₈(OH)₂], has been successfully synthesized by a facile hydrothermal method. Single-crystal X-ray diffraction analysis reveals that it crystallizes in orthorhombic space group Pbcn with a = 8.919(3) Å, b = 9.181(3) Å, c = 8.416(2) Å, Z = 4. The crystal structure is constructed of two dimensional (2D) [(B₅O₈)(OH)₂]_∞ layers, while stacking along b axis and then connected by Li⁺ and Na⁺ cations to extend to 3D framework Li₂Na[B₅O₈(OH)₂]. UV-vis-NIR spectrum shows that Li₂Na[B₅O₈(OH)₂] possesses a wide range of transparency and a UV cut-off edge below 190 nm which indicates that it may be applied in the deep ultraviolet region. The calculated band structures and the density of states indicate that Li₂Na[B₅O₈(OH)₂] is a direct band gap compound with a band gap of 5.68 eV. In addition, IR spectroscopy, thermal stability and theoretical calculations of Li₂Na[B₅O₈(OH)₂] are also reported in this work.     

Low-temperature sintering and electrical properties of (K,Na)NbO3 based lead-free ceramics with high Curie temperature

Lead-free ceramics (1 ? x)(K_0.48Na_0.52)NbO₃–(x/5.15)K_2.9Li_1.95Nb_5.15O_15.3 (x = 0.3–0.6, KNN–KLN100x) were prepared by conventional sintering technique at a low temperature of 960 °C. The effects of KLN contents on microstructure, dielectric, and piezoelectric properties were investigated. After the addition of KLN, the sintering performance and Curie temperature of the ceramics were markedly improved. The ceramics with x = 0.3 exhibited very good piezoelectric properties: d₃₃ = 138 pC/N, k_p = 45.03%, T_c = 495 °C, the dielectric constant at room temperature ?_r (RT) = 478 and the maximum dielectric constant ?_r (max) = 5067. These results indicated that the KNN–KLN100x lead-free ceramics sintered at low temperatures are promising for high temperature piezoelectric applications.     

Electrically conductive SiC–(Nb,Ti)ss–(Nb,Ti)Css cermet

The present paper deals with the processing method of SiC–(Nb,Ti)_(ss)–(Ti,Nb)C_(ss) composites. The electrically conductive phases were formed by in situ reaction: NbC_(s) + Ti_(s) → (Nb,Ti)_(ss) + (Ti,Nb)C_(ss) that takes place during the reaction sintering by hot pressing. Prepared composites exhibit good compromise between electrical and mechanical properties and the present approach allows preparing a wide variety of compositions. For example composites containing 80 wt.% SiC and 20 wt.% {(Nb,Ti)_(ss)–(Ti,Nb)C_(ss)} have an electrical resistivity 2.6 × 10⁻⁴ Ω m, hardness 21.6 GPa and fracture toughness 6.3 MPa m^1/2.     

(Na,K)NbO3–CaCu3Ti4O12 perovskite composites for supercapacitor based piezoelectric devices

The supercapacitor based piezoelectric material composite (Na,K)NbO₃–CaCu₃Ti₄O₁₂ (NKN–CCTO) is investigated for possible application in piezoelectric devices. (1−x)NKN–xCCTO (0.015≤x≤0.06) with different sintering conditions is researched for supercapacitor based piezoelectric applications. The 0.94NKN–0.06CCTO composite sintered at 975 °C shows the highest dielectric permittivity of 796. Clear SEM images of (1−x)NKN–xCCTO reveal that these compositions have high density well-crystallized structures. The composition and sintering temperature dependence of dielectric permittivities and piezoelectric coefficients, plotted in three dimensions, show that the 0.985NKN–0.015CCTO composite sintered at 1025 °C has a moderate dielectric permittivity of 405 and a piezoelectric constant of 98 pC/N.     

Enhanced piezoelectric properties and temperature-insensitive strain behavior of <001>-textured KNN-based ceramics

In this study, <001>-textured 0.99(K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃−0.01CaZrO₃ [abbreviated as 0.99KNLNS-0.01CZ] lead-free ceramics were prepared by templated grain growth (TGG) using plate-like NaNbO₃ templates and sintered by a two-step sintering process with different soaking time. All textured samples with high Lotgering factor (f >85%) presented orthorhombic and tetragonal coexisting phase, and the proportion of orthorhombic phase was varied with prolonged soaking time. A large piezoelectric constant d₃₃ (~ 310 pC/N) was obtained in the textured samples with a 12 h soaking time, which was almost twice larger compared to the randomly oriented one. Furthermore, the field-induced piezoelectric strain coefficient d₃₃^*(~ 440 pm/V) of the textured ceramics with 6 h soaking time was larger than the value of randomly oriented one (~ 298 pm/V) at room-temperature. Enhanced piezoelectric response and good temperature stability prove that <001>-textured 0.99KNLNS-0.01CZ ceramics are promising candidates in the field of lead-free piezoelectric materials.     

Optimized piezoelectric and structural properties of (Bi,Na)TiO3–(Bi,K)TiO3 ceramics for energy harvester applications

We investigated (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ (x=0, 0.14, 0.16, 0.18, 0.20, and 0.22) compositions of lead-free piezoelectric ceramics for potential energy harvester applications. Composition and sintering temperature of (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ were varied to extract the optimized processing temperature with each composition. We compared and analyzed sintering temperature-dependent surface morphologies and electrical properties. Maximum piezoelectric charge constant of 180 pC/N were obtained from the 0.8(Bi,Na)TiO₃–0.2(Bi,K)TiO₃composition at the sintering temperature of 1180 °C. Temperature dependent dielectric permittivity was measured to know the phase transition. We corresponded two different anomaly peaks, observed at 84 and 290 °C, as the rhombohedral-tetragonal and tetragonal-cubic phase transitions, respectively. Due to these phase transitions, different shapes of polarization-electric field loops (P-E loops) were measured and compared. Finally, output power of 42.39 nW/cm² were obtained for the (1−x)(Bi,Na)TiO₃–x(Bi,K)TiO₃ lead free piezoelectric ceramics.     

Effect of washing of barium titanate powders synthesized by hydrothermal method on their sinterability and piezoelectric properties

BaTiO₃ (BT) powders were synthesized by the hydrothermal method for fabricating lead-free barium titanate piezoelectric ceramics. The obtained powders were washed by distilled water and 0.01N acetic acid solutions separately, and utilized to obtain piezoelectric ceramics by traditional sintering. BT ceramics with the highest piezoelectric properties (d₃₃ value is over 190 pC/N) was obtained from the BT powder synthesized at 250 °C and washed by acetic acid solution. The influence of washing method and sintering temperature on the piezoelectric properties of BT ceramics were studied. The reasons were investigated by comparing the properties of BT powders and their compacts sintered at different temperature.     

Improvement of the piezoelectric and ferroelectric properties of (K,Na)0.5NbO3 ceramics via two-step calcination–milling route

A second calcination–milling step was introduced in the conventional processing of (K, Na)_0.5NbO₃ (KNN) ceramics (sintered in air) to further homogenenize the particle size distribution of the pre-sintered powders. The ceramic derived from the powders prepared by the two-step route possesses grains with better uniformity and is more compact. The relative density of the bulk ceramic reached 96.9%. Excellent properties are obtained in as-prepared KNN ceramics with k_p=44%, d₃₃=111 pC/N, tanδ=0.85%, ε₃₃^T/ε_o=311, Q_m=193, P_r=25.4 μC/cm², d₃₃^⁎=251 pm/V, which are superior to those of the ceramics derived from the powders calcined once as used in the traditional processing. These results indicate that twice-calcination–milling route is shown to be a facile and effective way to simultaneously improve the piezoelectric and ferroelectric properties of KNN ceramics without sintering aids.     

Thermolytic conversion of a bis(alkoxy)tris(siloxy)tantalum(V) single-source molecular precursor to catalytic tantala–silica materials

The new complex (ⁱPrO)₂Ta[OSi(O ^tBu)₃]₃ (1) was prepared via silanolysis of Ta(O ⁱPr)₅ with (^tBuO)₃SiOH and is a useful structural and spectroscopic (NMR, FTIR) model of Ta(V) on silica. The complex was also used to prepare tantalum-containing silica materials, via the thermolytic molecular precursor method (yielding Ta₂O₅ ⋅ 6SiO₂ and Ta₂O₅ ⋅ 18SiO₂) or by grafting 1 onto mesoporous SBA-15 silica (yielding a surface-supported tantala species, TaSBA-15). The solution phase thermolysis of 1 in nonpolar media afforded homogeneous, high-surface-area (ca. 450 m² g⁻¹) xerogels (Ta₂O₅ ⋅ 6SiO₂) that are amorphous up to approximately 1100 °C. A more silica-rich tantala–silica material (Ta₂O₅ ⋅ 18SiO₂) was prepared via a solution-phase co-thermolytic route with 1 and HOSi(O ^tBu)₃, to yield a material with a Si/Ta ratio of 9/1. It was demonstrated that tantala–silica materials are active as catalysts for cyclohexene oxidation.     

(K,Na)NbO3-based ceramics with excess alkali oxide for piezoelectric energy harvester

(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ (KNLNTS) ceramics were prepared by a solid-state reaction. The effect of excess (K,Na)₂O alkali oxide on the densification, phase evolution, microstructure development, and piezoelectric properties was investigated. The figure of merit (FOM) (d₃₃·g₃₃) for piezoelectric energy harvesting applications was also compared between the samples with and without excess alkali oxide. The addition of the excess alkali oxide changed the tetragonal crystal structure to orthorhombic and decreased the sintering temperature by about 100 °C. The dielectric constant of the orthorhombic phase is much lower than that of the tetragonal phase. The orthorhombic sample with excess alkali oxide sintered at 1020 °C demonstrated higher FOM in spite of having a smaller piezoelectric constant (d₃₃) than the stoichiometric sample sintered at 1100 °C. This result indicates that a KNN ceramic with an orthorhombic composition near the MPB with a moderate piezoelectric constant and smaller permittivity is more advantageous for an energy harvesting application than that with a morphotropic phase boundary (MPB) or a tetragonal composition.     

Phase transition behavior and electrical properties of (1 − x)Bi0.5Na0.5TiO3–x(Na0.53K0.44Li0.04)(Nb0.88Sb0.08Ta0.04)O3 lead-free ceramics

(1 ? x)Bi_0.5Na_0.5TiO₃–x(Na_0.53K_0.44Li_0.04)(Nb_0.88Sb_0.08Ta_0.04)O₃ (BNT–xNKLNST) with x = 0–0.10 lead-free piezoelectric ceramics were prepared by a solid state method, and the structure and electrical properties were investigated in this study. It is found that a morphotropic phase boundary (MPB) of rhombohedral (R) and tetragonal (T) phase exists in the range of 0.03 ≤ x ≤ 0.05 and the structure changes to paraelectric phase when x > 0.07. The samples with x = 0.05 exhibit improved electrical properties owing to the formation of MPB, which are as follows: piezoelectric constant d₃₃ = 120 pC/N, remnant polarization P_r = 39.4 μC/cm² and coercive field E_c = 3.6 kV/mm. These results indicate that the enhanced piezoelectric properties for BNT can be achieved by forming the coexistence of R and T phase.