Effects of K/Na ratio on the phase structure and electrical properties of 0.98(KxNa1−x)NbO3–0.02BiScO3 lead-free ceramics

Lead-free piezoelectric ceramics 0.98(K_xNa_1?x)NbO₃–0.02BiScO₃ (0.98 K_xN_1?xN–0.02BS) (x = 0.30–0.60) doped with 0.8 mol% Mn were prepared by conventional solid-state sintering. The effects of K/Na ratio on the phase structure and electrical properties of the Mn doped 0.98 K_xN_1?xN–0.02BS (0.98 K_xN_1?xN–0.02BS–Mn) were mainly studied. It is experimentally demonstrated that the electrical properties strongly depend on K/Na ratio in the 0.98 K_xN_1?xN–0.02BS–Mn ceramics and when x = 0.45 the ceramics exhibit optimum electrical properties: d₃₃ ～ 308 pC/N, k_p ～ 0.495, ?_r ～ 1577, tan δ ～ 0.028. These results show that the 0.98 K_xN_1?xN–0.02BS–Mn ceramic with x = 0.45 is a promising lead-free piezoelectric material.     

Structural phase transitions and electrical properties of (KxNa1−x)NbO3-based ceramics modified with Mn

(K_0.5Na_0.5)NbO₃ (KNN) ceramics and KNN containing Li, Ta modified with 2 mol% of manganese have been produced using the mixed-oxide ceramics synthesis route. The structure and properties of these piezoelectric ceramics modified with manganese have been investigated using high resolution X-ray diffraction and electrical characterisation. The structural information about the ceramics was determined by Rietveld refinement with Fullprof. The phase transition temperatures observed with X-ray diffraction compares well with the values from dielectric studies. The addition of Mn slightly reduced the phase transition temperatures and for the sample containing only Li, the phase changed from orthorhombic to monoclinic phase with space group Pm. The dielectric, piezoelectric and ferroelectric properties of the samples decreased with Mn addition due to hard doping effects resulting from oxygen vacancies in the perovskite lattice.     

Dielectric properties and diffuse phase transition behavior of CuO-doped lead-free Ba(ZrxTi1−x)O3 ceramics

CuO-doped lead-free Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared through a solid state processing technique, and the effects of CuO on microstructure, dielectric properties and diffuse phase transition behavior were investigated. The average grain sizes were increased by CuO doping. The temperature and frequency dependences of the dielectric constant revealed that CuO-doped BZT20 ceramics exhibited broad diffuse phase transition behavior. The dielectric constant increased with increasing CuO concentration. The value of T_m and degree of diffusion (γ) changed regularly in the studied compositional ranges. The BZT20 samples with 1.0 mol% CuO doping, sintered at 1310 °C, showed excellent dielectric property and lower diffusivity with ε_m=21,371 and γ=1.87. These results can be explained by the disordered distribution of Cu ions in the B sites and the weakened bonding force with oxygen ions in Cu substituted BZT20 structure.     

Investigation of fracture toughness of modified (KxNa1 − x)NbO3 lead-free piezoelectric ceramics

While most of the previous studies have focused on the processing and electrical properties of KNN-based ceramics, very little research has been carried out to evaluate their mechanical behavior. This work presents for the first time an examination of the fracture toughness, K_IC, of the most widely studied (K_xNa_1 ? x)NbO₃ (KNN)-based lead-free ceramics modified with lithium, tantalum and antimony. The samples were produced through the conventional mixed-oxide route and the K_IC values were measured using the single edge V-notched beam (SEVNB) method under four-point bending. The mean K_IC values were determined to be 0.48 ± 0.18 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)NbO₃, 0.8 ± 0.18 MPa m^1/2 for (K_0.5Na_0.5)(Nb_0.9Ta_0.1)O₃, 0.86 ± 0.04 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.9Ta_0.1)O₃ and 1.06 ± 0.21 MPa m^1/2 for (K_0.48Na_0.48Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ compositions. The microstructure, phase structure and dielectric constant values of the samples have been used to correlate the results of the K_IC values.     

Temperature dependent phase transition of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric

Phase transformation and electric properties of lead-free piezoceramics (Bi_0.5Na_0.5)_1?xBa_xTiO₃ with x=0.05, 0.06, and 0.07(BNB5T, BNB6T and BNB7T) were investigated using dielectric, piezoelectric and ferroelectric measurements. Electric field induced strain measurement shows “W” shape bipolar strain characteristics for BNB5T with typical ferroelectric P–E curve, while BNB6T and BNB7T, possessing pinch-off P–E, exhibit “V” shape field-induced strain. All the BNBxT specimens exhibit relaxor characteristic, identified by the Debye Law. Dielectric properties measured at elevated temperatures with the frequency variation (10–500 kHz) reveal frequency dispersion below the T_d point, but no dispersion between T_d and T_m, which may be ascribed to an intermediate phase transition. By adding more Ba²⁺ ions, the region of intermediate phase, distinguished by frequency dependence dielectric constant, expands to lower temperature. Moreover, the ferroelectric properties measured at elevated temperature were carried out below and at the depolarization temperature to well investigate the existence of this phase. Much less ε–T profile dispersion were observed during the investigation of BNB6T and BNB7T, leading to possible existence of an intermediate phase in the investigated compositions. The results suggest that the linear field-induced-strain of (Bi_0.5Na_0.5)_1?xBa_xTiO₃ are expected to be attributed to the intermediate phase.     

Investigation of phase boundaries in the system (KxNa1−x)1−yLiy(Nb1−zTaz)O3 using high-throughput experimentation (HTE)

A high-throughput experimental (HTE) approach starting from dry, fine-grained powders was used to synthesize bulk samples in the system (K_xNa_1?x)_1?yLi_y(Nb_1?zTa_z)O₃, a doped variant of the piezoelectric (K_0.5Na_0.5)NbO₃ (KNN). Starting from the system (K_0.5Na_0.5)_1?yLi_y(Nb_1?zTa_z)O₃ known from the works of Saito et al. an effort was made to establish a higher order phase diagram. Special emphasis was put on expanding the known morphotropic phase boundary that constitutes a region of special interest for electroceramic materials as it features maximum piezoelectric properties. Analyses were performed using a HTE-compatible technique, namely automated powder X-ray diffraction (XRD).     

Crystal structure and orthorhombic–tetragonal phase transition of nanoscale (Li0.06Na0.47K0.47)NbO3

Lead-free nano (Li_0.06Na_0.47K_0.47)NbO₃ powders, with pure perovskite structure and various grain sizes between about 30 and 60 nm, have been successfully synthesized by a novel sol–gel method, in which Nb₂O₅ was used as the Nb source. The refining XRD and Raman spectra were used in combination with TEM to investigate the evolution of lattice structure and phase transformation behavior as a function of grain size. The results demonstrated that the growth process of grains has been divided into two stages, and the distortion of unit cell apparently decreases with decreasing grain size. At around 35 nm, the phase structure of (Li_0.06Na_0.47K_0.47)NbO₃ changed from orthorhombic to tetragonal. This phenomenon is related to a grain size-induced structural phase transition. For the well accepted wisdom that niobates get super piezoelectric properties in orthorhombic–tetragonal transition region, our results suggested a critical size for the application of (Li_0.06Na_0.47K_0.47)NbO₃ in nano piezoelectric devices.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Enhanced piezoelectric strain of BiFeO3–Ba(Zr0.02Ti0.98)O3 lead-free ceramics near the phase boundary

The xBiFeO₃-(1-x)Ba(Zr_0.02Ti_0.98)O₃ + 1.0 mol% MnO₂ (xBF-BZT) lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method. The structure, dielectric, and piezoelectric properties were studied. X-ray diffraction (XRD) analysis showed that xBF-BZT ceramics exhibited pure perovskite structure with the coexistence of tetragonal and rhombohedral phases (0.66 ≤ x ≤ 0.74). The Curie temperature T_c, the dielectric constant ε_r (1 kHz), dielectric loss tanδ (1 kHz), piezoelectric constant d₃₃, coercive field E_c (80 kV/cm), and remnant polarization P_r (80 kV/cm) of 0.7BF-0.3BZT-Mn ceramics were 491°C, 633, 0.044, 165 pC/N, 35.6 kV/cm, and 22.6 μC/cm², respectively. The unipolar strain of 0.7BF-0.3BZT reached up to 0.20% under the electric field of 60 kV/cm, which is larger than that (0.15%) of BiFeO₃–BaTiO₃ ceramics. These results indicated that the xBF-BZT ceramics were promising candidates for high-temperature piezoelectric materials.     

Microstructure and microwave dielectric properties of Li2Ti1−x(Zn1/3Nb2/3)xO3 ceramics

Li₂Ti_1?x(Zn_1/3Nb_2/3)_xO₃ (0≤x≤0.5) ceramics were prepared by a solid state ceramic route, and the phase purity, microstructure, and microwave dielectric properties were investigated. The XRD results suggest the formation of solid solutions for all studied compositions (0≤x≤5). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the samples. The Q×f value increases with x up to x=0.2 and then decreases with the further increase of x. The best microwave dielectric properties of ε_r=20.5, Q×f =75,257 GHz, and τ_f =15.4 ppm/°C could be obtained when x=0.2.     

Structures and microwave dielectric properties of Ca(1−x)Nd2x/3TiO3 ceramics

Ca_(1?x)Nd_2x/3TiO₃ microwave dielectric ceramics were prepared by the mixed oxide route; powders were calcined at 1100 °C and sintered at 1450–1500 °C. High density, single phase products were obtained for all compositions. Grain sizes ranged from 1 μm to 100 μm. There was evidence of significant discontinuous grain growth in mid range compositions; all ceramics were characterised by complex domain structures. With increasing Nd content there was a evidence of a transition from an orthorhombic Pbnm structure to a monoclinic C2/m structure. This was accompanied by a decrease in relative permittivity (?_r) from 180 to 78, and decrease in the temperature coefficient of resonant frequency (τ_f) from +770 ppm K^?1 to +200 ppm K^?1. The product of dielectric Q value and resonant frequency (Q × f) varied in a grossly non-systematic way, exhibiting a peak at 13,000 GHz in Ca_0.7Nd_0.2TiO₃.     

Enhanced temperature stability in the O–T phase boundary of (K,Na)NbO3-based ceramics

In order to obtain excellent electrical properties and its temperature stability of KNN-based ceramics to meet the practical applications, a new lead-free material system of (1−x)K_0.5Na_0.5Nb_0.96Sb_0.04O₃-xBi_0.5Na_0.5Zr_0.8Sn_0.2O₃ (KNNS-xBNZS, 0 ≤ x ≤ 0.060) was designed, and the enhanced electrical properties (eg, d₃₃ ~ 465 pC/N, ε_r ~ 3318, S_uni ~ 0.133%) is obtained in the ceramics with x = 0.04. The physical origin of enhanced electric properties should be ascribed to the phase instability of R-T, resulting in a low-energy barrier, which can greatly facilitate the polarization switching. Moreover, the temperature stable of piezoelectric constant (d₃₃ or d₃₃*) is measured by three distinctive methods in different phase boundaries (O, O-T, R-T) at the temperature range 20°C-180°C, revealing that the O-T phase boundary has a relatively good temperature stability. A mode is used to show how to effectively modified the piezoelectric constant (d₃₃ or d₃₃*) and its temperature stability, we believe that such a strategy may further improve the temperature stability of d₃₃ or d₃₃* value in KNN-based ceramics.     

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(Na_xCa_1−x)₃Co₄O₉ (x=0.05−0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a low-temperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 °C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys.     

Varistor and electrical properties of MgO.(Fe2O3)1−x(Bi2O3)x ceramics

In this study, MgO.(Fe₂O₃)_1−x(Bi₂O₃)_x (x = 0.01, 0.02, 0.04, 0.08) samples were prepared by the conventional ceramic process. Microstructure studies revealed that the samples contain MgFe₂O₄ grains surrounded by insulating Bi₂O₃-rich phases. DC electrical resistivity of the samples was increased by Bi₂O₃ content up to 1.1 MΩ.cm for the sample with x = 0.08. Current density- electric field investigations suggested that the samples with x = 0.01, 0.02 and 0.04 exhibited varistor properties. The sample with x = 0.01 showed excellent varistor properties with a non-linear coefficient of 45 and a threshold electric field value of 160 V/cm. Variation of D.C electrical conductivity versus temperature indicated that the activation energy values for the conduction were increased by Bi₂O₃ content from 0.334(5) eV to 0.857(5) eV. A.C electrical conductivity spectra of the samples obey the universal power law and the charge transport mechanism is based on electron hopping via sudden translational motion between the ferric and ferrous ions.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

X-ray diffraction,dielectric and Raman studies of the Ba1−xNaxTi1−x(Nb1−ySby)xO3 ceramics

Ba_1−xNa_xTi_1−x(Nb_1−ySb_y)_xO₃ (BNTNSO) ceramics with compositions (x=0.05; y=0; 0.10; 0.20 and 0.30) have been prepared by conventional solid-state method and sintered in the temperature range 1350–1400 °C. Phase purity and structure are investigated using X-ray diffraction (XRD) data. The structural study showed that our synthesized compounds are single phase and crystallize in the tetragonal system with P4mm space group at room temperature. Based on a phenomenological model, dielectric and Raman properties of BNTNSO compounds have been explored. Referring to this model, dielectric properties of ceramics have been investigated in broad ranges of temperature (100–500 K) and frequency 1–10³ kHz). The dielectric permittivity evolution as a function of temperature and frequency has exhibited a classical ferroelectric character for 0≤y≤0.20 and a relaxor type behavior for y=0.30. The investigation of Raman spectra as a function of temperatures and compositions, confirmed the dielectric behavior. These results indicate that the y=0.20 composition is of extreme significance as far as its technological and industrial applications are concerned, which refers basically to its interesting physical properties and environmentally friendly characters, especially as its transition temperature is equal to the room temperature. The used samples show that the substitution of Nb by Sb favors and maintains the relaxor characters without changing the transition temperature.     

Investigation on the phase transition behavior and electrical properties of textured Lix(K0.48Na0.52)1−xNbO3 piezoelectric ceramics

Lead-free Li_x(K_0.48Na_0.52)_1−xNbO₃ (KNN–xLi) ceramics were successfully prepared by the tape casting method and solid-state reaction. The effects of Li content on phase transition, microstructure and electrical properties were investigated. XRD results show strong preferred orientation, indicating the presence of textured structure in the samples. With increasing Li content, the Currie Temperature shifts to higher temperature while the phase transition temperature between orthorhombic and tetragonal phases shifts to lower temperature. The sample with x=0.05 is determined to contain two perovskite phases with orthorhombic and tetragonal structures at room temperature, respectively, and exhibits the maximum values of piezoelectric coefficient of 286 pC/N and planar electromechanical coupling factor of 0.45.     

Temperature stable and high-Q microwave dielectric ceramics in the Li2Mg3−xCaxTiO6 system (x=0.00–0.18)

Microwave dielectric properties of Li₂Mg_3−xCa_xTiO₆ (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500 °C, the Li₂TiO₃ phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600 °C to 900 °C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li₂TiO₃ phases, which eventually formed the Li₂Mg₃TiO₆ phase. The results indicated the Li₂Mg₃TiO₆and CaTiO₃ co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO₃. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713 GHz to 79,845 GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8 ppm/°C due to the increased amount of CaTiO₃. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280 °C for 6 h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246 GHz and τf=−0.7 ppm/°C.     

p-Type/n-type behaviour and functional properties of KxNa(1-x)NbO3 (0.49 ≤ x ≤ 0.51) sintered in air and N2

Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N₂ using a conventional mixed oxide route. Oxygen vacancies (V_O^??) played a major role in the semi-conduction mechanism in low p(O₂) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O₂) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N₂-sintered samples with d₃₃?=?125?pC/N and k_p?=?0.38 obtained for K_0.51Na_0.49NbO₃.