Structural, microstructural and electrical properties evolution of (K,Na,Li)(Nb,Ta,Sb)O3 lead-free piezoceramics through NiO doping

Excellent piezoelectric properties have been reported in the (K,Na)NbO₃-LiTaO₃-LiSbO₃ system and have been regarded as a new candidate of lead-free piezoelectric material. Nevertheless, there are still some structural and electrical aspects that remain controversial with respect to the role of dopants in this system. NiO doping modifies the (K,Na,Li)(Nb,Ta,Sb)O₃ structure, giving rise to the appearance of the TTB-like secondary phase and to changes on the orthorhombic to tetragonal phase transition temperature. The microstructural characterization reveals that sintering process is assisted by a transient liquid phase. The presence of Ni in the liquid phase indicates that Ni^II ions could act as new nucleus for the secondary phase crystallization. Thus, as higher the amounts of liquid phase, higher the secondary phase appearance. The modifications on the structure and microstructure of the system cause a reduction of the piezoelectric constant, which is accompanied by an increase on the mechanical quality factor.     

Microstructure and piezoelectric properties of CuO added (K, Na, Li)NbO3 lead-free piezoelectric ceramics

Lead-free Na_0.5K_0.5NbO₃ (NKN) and Na_0.475K_0.475Li_0.05NbO₃ (NKLN) ceramics doped with CuO were prepared by the mixed oxide route. The powders were calcined at 850-930 °C and sintered at 850-1100 °C. Small additions of CuO reduced the sintering temperature and increased the density to 96% theoretical. Cu first appears to enter the A site then the B site. In NKLN the orthorhombic-tetragonal and tetragonal-cubic phase transitions are approximately 150 °C lower and 50 °C higher, respectively than in NKN. With increasing addition of Cu to NKN and NKLN the remanent polarization (P_r) increased and coercive field (E_c) decreased. NKLN prepared with 0.4 wt% CuO exhibited a saturation polarization (P_sat) of 30 μC/cm², remanent polarization (P_r) of 27 μC/cm² and coercive field (E_c) of 1.0 kV/mm. CuO caused the NKLN ceramics to harden considerably; the mechanical quality factor (Q_m) increased from 50 to 260, d₃₃ ∼ 285 and piezoelectric coupling factors were >0.4.     

Normal sintering of (K, Na)NbO3-based lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics have received more attention due to the environmental protection of the earth. (K, Na)NbO₃-based ceramics are one of the most promising candidates. Normal sintering of un-doped and Li/Ta co-doped (K, Na)NbO₃ ceramics was investigated to clarify the optimal sintering condition for densification, microstructure and electrical properties. It was found that density increased greatly within a narrow temperature range but turned to decrease when the sintering temperature slightly exceeded the optimal one. Piezoelectric properties also showed similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density. The grain growth and property change as a function of sintering temperature were discussed on basis of the formation of liquid-phase and the composition deviation caused by the volatilization of alkali components during sintering.     

Orthorhombic to tetragonal phase transition due to stress release in (Li,Ta)-doped(K,Na)NbO3 lead-free piezoceramics

XRD and Raman scattering experiments revealed an interesting finding that phase structure changed from orthorhombic to tetragonal symmetry when pulverizing (Li,Ta)-doped (K,Na)NbO₃ lead-free piezoelectric ceramics (sintered body) to powder. Both orthorhombic and tetragonal phases coexist in the Li_0.05(Na_0.51K_0.49)_0.95Nb_0.80Ta_0.20O_3.00 (LKNNT) sintered bulk sample at room temperature, but almost only the tetragonal phase is observed in the ground powder. In addition, annealing experiment enhanced the formation of tetragonal phase and improved the temperature stability of piezoelectricity. It is revealed that the internal stress existing in the LKNNT ceramics favors the formation of orthorhombic phase, which transfers to tetragonal phase when the stress was released.     

Low-temperature sintering of (K,Na)NbO3-based lead-free piezoceramics with addition of LiF

Dense Li/Ta-codoped KNN-based piezoceramics with d₃₃^* up to 375 pm/V were successfully fabricated by conventional sintering at a temperature as low as 900 °C by using LiF as a sintering additive. The reduction of densification temperature up to 200 °C was realized by a transient liquid phase sintering mechanism, consequently no grain boundary phase was observed in the sintered samples. It was found that the addition of LiF could further shift down the tetragonal–orthorhombic transition point (T_T-O), indicating that a small amount of Li⁺ could diffuse into the A-site of KNN matrix. The introduction of LiF enhanced the linearity of strain curves of the ceramics, which is unambiguously in favor for the actuator application. The present work reveals that low-temperature sintered LiF-doped KNN-based piezoceramics demonstrates promising potential in multilayer-structured actuator applications.     

Processing of highly oriented (K,Na)NbO3 thin films using a tailored metal-alkoxide precursor solution

Highly oriented lead-free K_0.5Na_0.5NbO₃ (KNN) thin films were synthesized by chemical solution deposition. The analysis of the KNN precursor solution revealed that the KNN precursor consists of complex metal alkoxides of potassium and sodium hexaalkoxy niobates with highly symmetric Nb-O octahedra. KNN precursor films were crystallized in a perovskite phase with a (1 0 0) preferred orientation on Pt(1 0 0)/MgO(1 0 0) substrates at 650 °C. The three-dimensional relationship between KNN(1 0 0) and Pt(1 0 0)/MgO(1 0 0) is confirmed by the fourfold symmetry of the pole figure. The fourfold symmetry indicates that the synthesized films are oriented in both the c and a, b directions on the Pt(1 0 0) surface. Although the insulating resistance was not sufficiently high at room temperature, the (1 0 0)-oriented KNN thin films showed potentially larger polarizations compared to the KNN thin films with no preferred orientation at low temperatures. The 2P_r and 2E_c of the oriented KNN films at −190 °C were 41.0 μC/cm² and 90 kV/cm, respectively.     

Application of [001]-textured (K,Na)(Nb,Sb)O3-CaZrO3 thick films to piezoelectric energy harvesters

A (K, Na)NbO₃ (KNN)-based lead-free piezoceramic for a piezoelectric energy harvester (PEH) should exhibit a large k_ij, Q_m, d_ij, and a small ε^T₃₃ to generate a large power output. Texturing was used to enhance the piezoelectricity of KNN-related piezoceramics. In particular, the k_ij and d_ij values of the KNN-based piezoceramics were considerably improved after texturing along the [001] direction, with a small change in the ε^T₃₃/ε₀ value, indicating that the [001]-textured KNN-based thick films are good candidates for use in PEHs. The 0.97(K_0.5Na_0.5)(Nb_0.93Sb_0.07)O₃-0.03CaZrO₃ [KN(N_0.93S_0.07)-CZ] thick film was textured along the [001] direction, and this thick film exhibited a large k_p (0.57) and d₃₃ × g₃₃ (25.7 pm²/N), which is the largest d₃₃ × g₃₃ of the KNN-based piezoceramics reported to date. A cantilever PEH fabricated using the textured KN(N_0.93S_0.07)-CZ thick film exhibited a power density of 21.4 μW/mm³ at the resonance frequency. This is the highest power density observed for PEHs fabricated using lead-free piezoceramics. The PEH also exhibited a power density of 0.023 μW/mm³ at the off-resonance frequency. Therefore, the textured KN(N_0.93S_0.07)–CZ thick film is a good candidate for use in PEHs.     

Improved Li and Sb doped lead-free (Na,K)NbO3 piezoelectric ceramics for energy harvesting applications

Piezoelectric energy harvesting is the most widely investigated technology for renewable energy applications. In this work, (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ piezoelectric ceramics were prepared through conventional mixed oxide fabrication methods with different sintering temperatures. Although the (Na_0.5K_0.5)NbO₃ piezoelectric material is representative among the lead-free ceramics, it is difficult to densify by typical sintering techniques owing to its easy evaporation properties of potassium (K⁺) and sodium ion (Na⁺). Hence, lithium (Li⁺) and antimony ion (Sb⁵⁺) were used for the partial substitution of (Na_0.5K_0.5)NbO₃. With the optimized sintering temperature, Li⁺ and Sb⁵⁺ are expected to be crucial in increasing the density and enhance the piezoelectric and ferroelectric properties. In this study, the phase, microstructure, and dielectric and electrical properties of (1-x)(Na_0.5K_0.5)NbO₃-xLiSbO₃ ceramics depending on the sintering temperature is examined by employing X-ray diffraction, field emission scanning electron microscopy, impedance analyzer, and mechanical force system for energy harvesting.     

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.     

An investigation of (Na0.5K0.5)NbO3–CaTiO3 based lead-free ceramics and surface acoustic wave devices

Lead-free (Na_0.5K_0.5)NbO₃ ceramics doped with CaTiO₃ (0–3 mol%) have been prepared by the conventional mixed oxide method in this paper. All of the CaTiO₃ doped (Na_0.5K_0.5)NbO₃ specimens do not deliquesce as exposed to water for a long time. The samples are characterized by X-ray diffraction analysis, Raman scattering spectra, scanning electron microscopy, and atomic force microscopy. The dielectric, piezoelectric and ferroelectric properties are also investigated. The results show that the addition of CaTiO₃ is very effective in preventing the deliquescence and in improving the electric properties of (Na_0.5K_0.5)NbO₃ ceramics. Finally, surface acoustic wave devices based on lead-free ceramics have been successfully fabricated and their characterization is presented.     

Phase transition,microstructure, and dielectric properties of Li/Ta/Sb co-doped (K,Na)NbO3 lead-free ceramics

Li/Ta/Sb co-doped lead-free (K_0.4425Na_0.52Li_0.0375)(Nb_0.93−xTa_xSb_0.07)O₃ (abbreviated KNLNST_x) piezoelectric ceramics, with Ta-doping ratio of x ranging from 0.0275 to 0.0675, were synthesized using the conventional solid-state reaction method at the sintering temperature of 1130 °C. The effects of Ta content on the microstructure, dielectric properties, and phase transition behavior of the prepared ceramics were systematically investigated. The X-ray diffraction results show that all KNLNST_x ceramics formed a secondary phase, which is assigned to the tetragonal tungsten-bronze type (TTB) structure phase, and showed a phase transition from an orthorhombic symmetry to a tetragonal symmetry across a composition region of 0.0375<x<0.0475. The grain shape and size that correspond to the phase structure transformations can be clearly observed in the scanning electron microscopy images. As x increased to 0.0475, the KNLNST_0.0475 ceramics changed from orthorhombic to tetragonal structure and showed excellent piezoelectric properties of d₃₃=313 pC/N, k_p=47%, and ε_r=1825. By contrast, samples of x=0.0375 with orthorhombic symmetry exhibited poor piezoelectric properties, with d₃₃=200 pC/N and ε_r=1015. These results indicate that phase structure is vital in the piezoelectric properties of KNN lead-free ceramics.     

Structure,microstructure, dielectric and piezoelectric properties of (1-x-y)BiFeO3-xPbFe0.5Nb0.5O3-yPbTiO3 ceramics

Ceramics of seven quasi-binary concentration sections of the ternary solid solution system (1-x-y)BiFeO₃-xPbFe_0.5Nb_0.5O₃-yPbTiO₃ were prepared by the conventional solid-phase reaction method in the range of 0.05 ≤ x ≤ 0.325; 0.05 ≤ y ≤ 0.325. By using x-ray diffraction technique, the phase diagram of the system was constructed which was shown to contain the regions of tetragonal and rhombohedral symmetry and the morphotropic phase boundary between them. Grain morphology, dielectric and piezoelectric properties of selected solid solutions were investigated. The highest piezoelectric coefficient d₃₃ = 50 pC/N was obtained. Dielectric characteristics of ceramics revealed ferroelectric relaxor behavior and region of diffuse phase transition from the paraelectric to ferroelectric phase in the temperature range of 600–800 K.     

Structure,microstructure and electrical properties of Cu2+ doped (K,Na,Li)(Nb,Ta,Sb)O3 piezoelectric ceramics

This work studies the effects of copper doping on the properties of the (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ piezoelectric ceramic material. Cu²⁺ incorporation into the perovskite structure produces a transformation of the crystalline lattice from tetragonal to orthorhombic symmetry together with an increase of the secondary phase. The grain size of the ceramic samples is increased due to the formation of a liquid phase during sintering, which increases with the Cu²⁺ content. EDS analysis reveals that the secondary-phase regions present a Cu and Nb-rich composition, indicating that the Cu-excess accommodates through the formation of this secondary phase. Cu-doping induces a rapid increase of the orthorhombic–tetragonal phase transition temperature, while the tetragonal–cubic phase transition temperature is decreased, the latter becoming more diffuse with the increase of Cu content. The piezoelectric properties of the material are reduced with the copper concentration, whereas the mechanical quality factor increases by a factor of nearly four.     

The phase structure and electric properties of low-temperature sintered (K,Na)NbO3-based piezoceramics modified by CuO

0.25 wt% CuO-doped (Li,K,Na)(Nb,Ta)O₃–AgSbO₃ lead-free piezoceramics with pure perovskite structure were successfully prepared at a sintering temperature below 1000 °C. The sintering temperature of KNN-based piezoceramics was effectively reduced by about 100 °C due to the enhanced densification process induced by the addition of CuO. Besides, the acceptable sintering temperature window was broadened by the addition of CuO. It is found that the CuO-doped samples show slightly higher tetragonal–orthorhombic phase transition point (T_T−O) but a lower Curie point (T_c), compared to undoped ones. The KNN-based piezoceramics became “hard” as CuO was added, supported by an increase of Q_m. Fairly good electrical properties of d₃₃^*=383 pm/V, ε_r=860, Q_m=188 and T_c=215 °C could be obtained in dense CuO-modified KNN-based piezoceramics sintered at 970 °C, demonstrating promising potential in practical applications.     

Fabrication and piezoelectric properties of BaTiO3/BaTiO3-Bi(Mg1/2Ti1/2)O3-BiFeO3 composites

We fabricated xBaTiO₃ (BT)/(1-x)[BaTiO₃-Bi(Mg_1/2Ti_1/2)O₃-BiFeO₃] (BT-BMT-BF)?+?0.1?wt%MnCO₃ composites by spark plasma sintering and investigated the effect of BT content x, BT powder size, and BT-BMT-BF composition on piezoelectric properties. For xBT/(1-x)(0.3BT-0.1BMT-0.6BF) +?0.1?wt%MnCO₃ (x?=?0–0.75) composites with a 0.5-µm BT powder, the dielectric constant was increased with x, and the relative density was decreased at x?=?0.67 and 0.75, creating optimum BT content of x?=?0.50 with a piezoelectric constant d₃₃ of 107?pC/N. When a larger 1.5-µm BT powder was utilized for the composite with x?=?0.50, the d₃₃ value increased to 150?pC/N due to the grain size effect of the BT grains. To compensate for a compositional change from the optimum 0.3BT-0.1BMT-0.6BF due to partial diffusion between the BT and 0.3BT-0.1BMT-0.6BF grains, a 0.5BT/0.5(0.275BT-0.1BMT-0.625BF)?+?0.1?wt%MnCO₃ composite with the 1.5-µm BT powder was fabricated. We obtained an increased d₃₃ value of 166?pC/N. These results provided a useful composite design to enhance the piezoelectric properties.     

Enhancement of the dielectric,piezoelectric, and ferroelectric properties in BiYbO3-modified (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3 lead-free ceramics

Lead-free (1−x)(Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃−xBiYbO₃ [(1−x)BCTZ−xBYO] piezoelectric ceramics in the range of BYO concentrations were prepared by the conventional oxide-mixed method, and the effect of BYO content on their microstructure, crystalline structure, density and electrical properties was investigated. A dense microstructure with large grain was obtained for the ceramics with the addition of BYO. The ceramics with x=0.1% exhibit an optimum electrical behavior of d₃₃~580 pC/N, r~10.9 Ω, k_p~56.4%, and tan δ~1.12% when sintered at a low temperature of ~1350 °C. When the measuring electric field is 40 kV/cm, the well-saturated and square-like P–E loops for the ceramics were observed with P_r~12.2 μC/cm² and E_c~1.83 kV/cm.     

Li2O excess (Na0.51K0.47Li0.02)(Nb0.8Ta0.2)O3 lead free piezoelectric ceramics

1 mol% Li₂O excess (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ ceramics were prepared by the conventional mixed oxide method and sintered from 950 to 1200 °C. Also, Li₂O was employed as a sintering aid for high densification and low temperature sintering process. X-ray diffraction results of 1 mol% Li₂O excess (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ lead free piezoelectric ceramics indicated that the specimens were well crystallized and have tetragonal structure. The specimens which sintered at 1050 °C showed the highest piezoelectric properties compared with others. The measured piezoelectric constant and electromechanical coupling coefficient were 231 pC/N and 38.9%, respectively. Curie temperature of (Na_0.51K_0.47Li_0.02)(Nb_0.8Ta_0.2)O₃ ceramics was 344.32, 344.4 and 344.5 °C at 1, 10 and 100 kHz, respectively.     

Hydrothermal synthesis and piezoelectric property of Ta-doping K0.5Na0.5NbO3 lead-free piezoelectric ceramic

Ta-doping K_0.5Na_0.5Nb_1−xTa_xO₃ (x = 0.1, 0.2, 0.3, 0.4) powder was synthesized by hydrothermal approach and its ceramics were prepared after sintering and polarizing treatment in this work. The K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics near morphotropic phase boundary (MPB), which exhibited optimum piezoelectric properties of d₃₃ = 210 pC/N and good electromechanical coupling factors of K_p = 0.3. The domain structure has been observed from TEM images which indicates that the K_0.5Na_0.5Nb_0.7Ta_0.3O₃ ceramics have good piezoelectric and ferroelectric properties for it is near the MPB.