Ferroelectric and pyroelectric properties of (Na0.5Bi0.5)TiO3-BaTiO3 based trilayered thin films

Trilayered Bi_3.25La_0.75Ti₃O₁₂ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Bi_3.25La_0.75Ti₃O₁₂ (25 nm) and Pb(Zr_0.4Ti_0.6)O₃ (25 nm)/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ (300 nm)/Pb(Zr_0.4Ti_0.6)O₃ (25 nm) thin films without undesirable phases have been deposited on Pt/Ti/SiO₂/Si substrates. It was found that the Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃ layers are very effective to inhibit the charge transport in the trilayered films. Much better insulating properties than those of (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ films have been achieved in the trilayered films. The trilayered films show good dielectric, ferroelectric and pyroelectric properties. Remnant polarizations 2P_r of 16 µC/cm² and 34 µC/cm², pyroelectric coefficients of 4.8 × 10^− 4 C m^− 2 K^− 1 and 7.0 × 10^− 4 C m^− 2 K^− 1 have been obtained for the Bi_3.25La_0.75Ti₃O₁₂/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Bi_3.25La_0.75Ti₃O₁₂ and Pb(Zr_0.4Ti_0.6)O₃/(Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃/Pb(Zr_0.4Ti_0.6)O₃ thin films, respectively. The trilayered films are promising candidates for sensor and actuator applications.     

Influence of doping Nb5+ and Mn2+ on the PTCR effects of Ba0.92Ca0.05(Bi0.5Na0.5)0.03TiO3 ceramics

As a positive temperature coefficient of resistivity (PTCR) material, Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics with donor doping of Nb⁵⁺ and acceptor doping of Mn²⁺ were prepared by a conventional mixed oxide method. The influence of contents of Nb⁵⁺ and Mn²⁺ on the microstructure and PTCR characteristics of Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramics sintered at 1,360°C for 2 h was investigated. The result showed that the Curie temperature (T _c) was shifted to a lower temperature with increasing of the content of Nb⁵⁺ and the resistance jump (ρ_max/ρ_min) was enhanced with doping of Mn²⁺. The grain size of ceramic sample decreased with increasing of contents of donor Nb⁵⁺ and acceptor Mn²⁺. The Ba_0.92Ca_0.05(Bi_0.5Na_0.5)_0.03TiO₃ ceramic with 0.4 mol%Nb⁵⁺ and 0.04 mol%Mn²⁺ exhibited a low ρ_RT of 5.0 × 102 Ω cm, a typical PTCR effect of ρ_max/ρ_min > 10³, and a T _c of 158°C.     

The synthesis of lead-free ferroelectric Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 thin films by sol-gel method

(1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ [BNT-BKT-100x] thin films have been successfully deposited on Pt/Ti/SiO₂/Si substrates by a sol-gel process together with rapid thermal annealing. A morphotropic phase boundary (MPB) between Bi_0.5Na_0.5TiO₃ and Bi_0.5K_0.5TiO₃ was determined around x ∼ 0.15. Near the MPB, the film exhibits the largest grain size, the highest ε value (360) and the largest P_r value (13.8 μC/cm²). The BNT-BKT thin film system is expected to be a new and promising candidate for lead-free piezoelectric applications.     

Ferroelectric properties of lithia-doped (Bi0.95Na0.75K0.20)0.5Ba0.05TiO3 ceramics

Lead-free piezoelectric (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics have been prepared by conventional process for different lithium substitutions. The SEM images show that the ceramics are well sintered at 1428 K. Dielectric and ferroelectric measurements have been performed. With the increasing of lithium substitution, the Curie temperature of the (Bi_0.95Na_0.75K_0.20−xLi_x)_0.5Ba_0.05TiO₃ ceramics shifts from 570 K to 620 K, but the maximum value of the dielectric constant decreases from 6700 to 4700 correspondingly. A relatively larger remanent polarization of 36.8 μC/cm² has been found in the x = 0.05 sample. The coercive field decreases as the lithium substitution amount increases. An optimized d₃₃ = 194 × 10^− 12 C/N and a relative dielectric constant ε_r = 1510 have been obtained in (Bi_0.95Na_0.75K_0.15Li_0.05)_0.5Ba_0.05TiO₃.     

Influences of poling condition and sintering temperature on piezoelectric properties of (Na0.5Bi0.5)1 − xBaxTiO3 ceramics

The structure, ferroelectric characteristics and piezoelectric properties of (Na_0.5Bi_0.5)_1 − xBa_xTiO₃ (x = 0.04, 0.06, 0.10) ceramics prepared by conventional solid state method were investigated. The influences of poling condition and sintering temperature on the piezoelectric properties of the ceramics were examined. The piezoelectric properties of the ceramics highly depend on poling field and temperature, while no remarkable effect of poling time on the piezoelectric properties was found in the range of 5-25 min. Compared with (Na_0.5Bi_0.5)_0.96Ba_0.04TiO₃ and (Na_0.5Bi_0.5)_0.90Ba_0.10TiO₃, the piezoelectric properties of (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ are more sensitive to poling temperature due to the relatively low depolarization temperature. Moderate increase of sintering temperature improved the poling process and piezoelectric properties due to the development of microstructural densification and crystal structure. With respect to sintering behavior and piezoelectric properties, a sintering temperature range of 1130-1160 °C was ascertained for (Na_0.5Bi_0.5)_0.90Ba_0.10TiO₃.     

Fabrication of lead-free (Na0.82K0.18)0.5Bi0.5TiO3 piezoelectric nanofiber by electrospinning

(Na_0.82K_0.18)_0.5Bi_0.5TiO₃ nanofibers were synthesized by sol-gel process and electrospinning. Scanning electron microscopy was used to verify that the diameters and lengths are in the range of 150-600 nm and several hundreds of micrometer. Perovskite structure and grain size (20-70 nm) were verified by X-ray diffraction and transmission electron microscopy. The high effective piezoelectric coefficient d₃₃ (96 pm/V) was measured by scanning force microscopy. It may be attributed to easily tilting the polar vector of domain for an electric field and the increase in the number of possible spontaneous polarization direction near the rhombohedral-tetragonal morphotropic phase boundary. The research shows that there are potentional applications for (Na_0.82K_0.18)_0.5Bi_0.5TiO₃ nanofiber in nanoscale lead-free piezoelectric devices.     

Structure and ferroelectric properties in (K0.5Bi0.5)TiO3-BiScO3-PbTiO3 piezoelectric ceramic system

(K_0.5Bi_0.5)TiO₃-BiScO₃-PbTiO₃ ceramics were synthesized by conventional solid-state method. A morphotropic phase boundary (MPB) was confirmed with the aid of structural analysis. Two dielectric anomalous peaks were observed, the one around dielectric maximum temperature (T_m) due to phase transformation from ferroelectric to paraelectric while the second one could be ascribed to space charges. Furthermore, the existence of space charges also resulted in the independence of T_m with frequency at low lead composition. A new high temperature piezoelectric ceramic, 0.30(K_0.5Bi_0.5)TiO₃-0.30BiScO₃-0.40PbTiO₃ close to MPB exhibited excellent electrical properties with T_m of 384 °C, d₃₃ of 247 pC/N, k_p of 38.9%, P_r of 19.41 μC/cm², and E_c of 2.25 kV/mm, indicative of a candidate for high temperature application.     

Structure and electrical properties of (1 − x)Bi0.5Na0.5TiO3-xBi0.5K0.5TiO3 ceramics near morphotropic phase boundary

The binary lead-free piezoelectric ceramics with the composition of (1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ were synthesized by conventional mixed-oxide method. The phase structure transformed from rhombohedral to tetragonal phase in the range of 0.16 ≤ x ≤ 0.20. The grain sizes varied with increasing the Bi_0.5K_0.5TiO₃ content. Electrical properties of ceramics are significantly influenced by the Bi_0.5K_0.5TiO₃ content. Two phase transitions at T_t (the temperature at which the phase transition from rhombohedral to tetragonal occurs) and T_c (the Curie temperature) were observed in all the ceramics. Adding Bi_0.5K_0.5TiO₃ content caused the variations of T_t and T_c. A diffuse character was proved by the linear fitting of the modified Curie-Weiss law. Besides, the ceramics with homogeneous microstructure and excellent electrical properties were obtained at x = 0.18 and sintered at 1170 °C. The piezoelectric constant d₃₃, the electromechanical coupling factor K_p and the dielectric constant ?_r reached 144 pC/N, 0.29 and 893, respectively. The dissipation factor tan δ was 0.037.     

Dielectric and piezoelectric properties of Y2O3 doped (Bi0.5Na0.5)0.94Ba0.06TiO3 lead-free piezoelectric ceramics

Y₂O₃ doped lead-free piezoelectric ceramics (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (0-0.7 wt%) were synthesized by the conventional solid state reaction method, and the effect of Y₂O₃ addition on the structure and electrical properties was investigated. X-ray diffraction shows that Y₂O₃ diffuses into the lattice of (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ to form a solid solution with a pure perovskite structure. The temperature dependence of dielectric constant of Y₂O₃ doped samples under various frequencies indicates obvious relaxor characteristics different from typical relaxor ferroelectric and the mechanism of the relaxor behavior was discussed. The optimum piezoelectric properties of piezoelectric constant d₃₃ = 137 pC/N and the electromechanical coupling factor k_p = 0.30 are obtained at 0.5% and 0.1% Y₂O₃ addition, respectively.     

Photoluminescence and electrical properties of Er-doped (K0.5Na0.5)NbO3-based transparent ceramics

Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02Ba(Bi_0.5Nb_0.5)O₃ transparent fluorescent ceramics were prepared using traditional solid-phase method. The (K_0.5Na_0.5)NbO₃ (KNN) ceramics were modified by introducing the second group elements Ba(Bi_0.5Nb_0.5)O₃ and rare earth ions Er³⁺. The effects of Er³⁺ on the structure, optical and electrical properties of transparent ceramics (K_0.5Na_0.5)NbO₃-Ba(Bi_0.5Nb_0.5)O₃ were investigated. The ceramics form a single perovskite structure and have a pseudo-cubic phase structure. Nanoscale grain size was obtained for ceramics, and the smallest average grain size is 80 nm. The ceramics have high transmittance. The ceramic 0.1 mol% Er-doped 0.98(K_0.5Na_0.5)NbO₃-0.02 Ba(Bi_0.5Nb_0.5)O₃ achieved the highest transmittance for this system with 62% and 52% at near-infrared light (1000 nm) and visible light (700 nm), respectively. The ceramics have up-conversion luminescence properties and also maintain good electrical properties. Under 980 nm excitation, the samples showed two green emission bands (518–536 nm, 536–557 nm) and one red emission band (646–677 nm). In addition, the ceramics have relaxation ferroelectricity, and a high dielectric constant. These functional ceramics with multiple properties will have greater research significance and application value.

     

Morphotropic phase boundary and piezoelectric properties of (Bi1/2Na1/2)1 − x(Bi1/2K1/2)xTiO3-0.03(Na0.5K0.5)NbO3 ferroelectric ceramics

Lead-free piezoelectric ceramics, (Bi_1/2Na_1/2)_1 − x(Bi_1/2K_1/2)_xTiO₃-0.03(Na_0.5K_0.5)NbO₃ (x = 0.10-0.40) were synthesized by conventional solid-state sintering. A morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases was confirmed. Two dielectric anomalies can be observed, showing diffused phase transition behavior. There is no shift of the dielectric maximum temperature with frequency due to the contribution of space charge at high temperatures, similar to pure (Bi_1/2Na_1/2)TiO₃. The materials near MPB show a strong compositional dependence with the optimal properties of a d₃₃ of 167 pC/N, a k_p of 35.5%, a P_r of 27.6 μC/cm² and a E_c of 27.9 kV/cm, suitable for future application.     

Optical properties of rare-earth doped epitaxial Sr0.5Ba0.5Nb2O6 thin films grown by pulsed laser deposition

Optical quality rare-earth (RE) (Nd³⁺, Eu³⁺, Gd³⁺ and Yb³⁺) doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) epitaxial films of ~ 170 nm thick have been successfully grown on MgO (100) single crystal substrates using pulsed laser deposition technique. Strong residual stress in these films has been revealed by Raman spectroscopic studies. Two kinds of in-plane orientations with respect to the MgO substrate co-exist. All the film samples show high transparency in the visible wavelength. Their band-gap energies appear to be independent of the types of dopant. Photoluminescence (PL) spectra of RE-doped SBN ceramics show a strong and broad emission band at around 600 nm (2.07 eV). The peak position of this emission band changes slightly with different RE-dopants. Thin film samples, however, yield a broad PL band at around 385 nm (3.22 eV). This UV emission shows no observable shift in the peak position for different dopants. Apart from these broad emission bands, conspicuous emission lines from Eu³⁺ and Nd³⁺ ions are also noted. The origins of these PL spectra are discussed.     

Calcium-doping for enhanced temperature stability of (Ba0.6Sr0.4)1−xCaxTiO3 thin film dielectric properties

Ternary (Ba_0.6Sr_0.4)_1−xCa_xTiO₃ (BSCT) (x = 0, 0.1, 0.2, 0.3 and 0.4) thin films with thickness of around 500 nm were prepared on Pt(111)/TiO₂/SiO₂/Si substrates by sol-gel methods. BSCT forms the complete solid solutions in a single cubic perovskite structure. The lattice constant, dielectric constant, tanδ and tunability of BSCT decrease, whereas the temperature stability of dielectric properties increases with increasing the Ca concentration. From 25 to 100 °C, the decrease of tunability is about 11% for BSCT with 40 at.% of Ca. BSCT thin films exhibit the comparable tunability, low loss and enhanced temperature stability.     

Enhanced multiferroic properties and tunable magnetic behavior in multiferroic BiFeO3-Bi0.5Na0.5TiO3 solid solutions

A series of multiferroic (1−x)BiFeO₃−x(Bi_0.5Na_0.5)TiO₃ (BF-BNT) (x = 0 − 0.6) solid solution ceramics were prepared by a sol-gel method. The XRD results show that increasing BNT content induce a gradual phase transformation from rhombohedral to pseudocubic structure near x = 0.4. Compared with pure BiFeO₃, superior multiferroic properties are obtained for x = 0.3 with remnant polarization P_r = 1.49 μC/cm² and saturated magnetization M_s = 0.51 emu/g. Importantly, the paramagnetic (PM) to ferromagnetic (FM) transition is observed for the solutions, and the Curie temperature (T_C) can be tuned by varying the content of BNT. This observed FM ordering is discussed in terms of the possible existence of the long-range superexchange interaction of Fe³⁺-O-Ti-O-Fe³⁺ in the chemically ordered regions.     

Synthesis and characterization of (Na0.85K0.15)0.5Bi0.5TiO3 ceramics by different methods

The (Na_0.85K_0.15)_0.5Bi_0.5TiO₃ (BNKT) powders were synthesized by solid-state method, sol-gel method and stearic acid method. Microstructure, piezoelectric and dielectric properties of the ceramics were investigated. Attempts had been made to understand the reaction processes by using thermo gravimetric (TG) and differential scanning calorimetry (DSC). The BNKT powders have a perovskite structure with average crystallite sizes of 168 nm, 85 nm and 79 nm, corresponding to the solid-state method, the sol-gel method and the stearic acid method, respectively. The ceramics derived from the powder synthesized by sol-gel method presents the most homogeneous microstructure and largest grain size (5-7 μm). The effects of average crystallite size on microstructures and electric properties of the BNKT ceramics were investigated. Both the piezoelectric properties and dielectric properties were enhanced with the increase of grain size.     

Enhanced temperature stability of modified (K0.5Na0.5)0.94Li0.06NbO3 lead-free piezoelectric ceramics

To improve both the temperature stability and the mechanical quality factor of (K_0.5Na_0.5)_0.94Li_0.06NbO₃ (KNLN6) ceramics, dense (K₄CuNb₈O₂₃, Bi₂O₃)-modified KNLN6 lead-free ceramics were prepared. Results showed that the (K₄CuNb₈O₂₃, Bi₂O₃)-modified ceramics exhibited a flat, temperature-stable behavior over the range of 20–120 °C. K₄CuNb₈O₂₃ (KCN) and Bi₂O₃-codoping changed the KNLN6 to “hard” ceramics with a significant improvement of mechanical quality factor, Q _m, from 82 to 756. Meanwhile, the piezoelectric constant, d ₃₃, and the planar electromechanical coefficient, k _p, still maintained relatively high levels (d ₃₃ ~118 pC/N, k _p ~35.6%). These results indicate that the modified KNLN6 ceramics are promising lead-free piezoelectric candidates for practical applications.     

The effect of Bi1.5Zn0.5Nb0.5Ti1.5O7 cover layer on the dielectric and tunable properties of Ba0.6Sr0.4TiO3/Bi1.5Zn0.5Nb0.5Ti1.5O7 bilayered thin films

Bi_1.5Zn_0.5Nb_0.5Ti_1.5O₇ (BZNT) thin films with different thicknesses as cover layers were deposited on the Ba_0.6Sr_0.4TiO₃ (BST) thin films on the Pt/Ti/SiO₂/Si substrates by radio frequency magnetron sputtering method. The microstructure, surface morphology, dielectric and tunable properties of BST/BZNT heterogeneous bilayered films were investigated as a function of the thickness of BZNT films and the effect of BZNT films on the asymmetric electrical properties of BST/BZNT bilayered films was discussed. It was found that BZNT cover layer significantly improved the leakage current and the dielectric loss, and the dielectric constant and tunability of BST/BZNT bilayered thin films simultaneously decreased with the increasing thickness of BZNT films. The BST/BZNT bilayered thin film with a 50 nm BZNT cover layer gave the largest figure of merit (FOM) of 33.48 with the upper tunability of 55.38%. The asymmetric electrical behavior of BST/BZNT bilayered films is probably related to an internal electric field caused by built-in voltages at Pt/BST and BZNT/Au interfaces.     

Electrical and luminescence properties of Er-doped Bi0.5Na0.5TiO3 ceramics

The influences of Er content on the dielectric and photoluminescence performances of Bi_0.5Na_0.5TiO₃-xEr (x = 0, 0.005, 0.01, 0.015, 0.02, 0.03) ceramics have been investigated. The results show that Bi_0.5Na_0.5TiO₃-xEr ceramics with x = 0.01 Er have maximum values of photoluminescence and piezoelectric properties. A bright green emission at 550 nm and enhanced piezoelectric response are achieved in the ceramic Bi_0.5Na_0.5TiO₃-0.01Er at room temperature. Furthermore, the photoluminescence performance of the ceramics is significantly enhanced by electric poling.     

Temperature dependence of ferroelectric and dielectric properties of textured 0.98(0.94Na0.5Bi0.5TiO3–0.06BaTiO3)–0.02K0.5Na0.5NbO3 thick film

Textured 0.98(0.94Na_0.5Bi_0.5TiO₃–0.06BaTiO₃)–0.02K_0.5Na_0.5NbO₃ thick film was prepared by reactive templated grain growth (RTGG) method. The effect of temperature on ferroelectric and dielectric behaviors of the thick film was investigated. Its dielectric constant as a function of temperature displayed typical relaxation behavior, which was similar to that of NBT-based bulk ceramics. Remnant polarization, saturation polarization, and coercive field of the thick film all decreased with increasing temperature. Dielectric constant and tunability of the film were also dependent on temperature. Electric field dependence of dielectric constant of the thick film suggested a transition from ferroelectric to antiferroelectric phase at around the depolarization temperature. A strong increase in leakage current density with increasing temperature was observed, which could be related to the enhanced activity of conductivity carriers.     

Structure,ferroelectric, piezoelectric and ferromagnetic properties of BiFeO3–Ba0.6(Bi0.5K0.5)0.4TiO3 lead-free multiferroic ceramics

(1?x)BiFeO₃–xBa_0.6(Bi_0.5K_0.5)_0.4TiO₃ + 1 mol% MnO₂ lead-free multiferroic ceramics were fabricated by a conventional ceramic technique and the effects of Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃ doping and sintering temperature on the microstructure, ferroelectric, piezoelectric and ferromagnetic properties of the ceramics were studied. All the ceramics show good electric insulation with the resistivity values of 1.97 × 10⁹–1.20 × 10¹⁰ Ω cm. After the addition of Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃, two dielectric anomalies are observed at high temperatures (T ₁ ~ 453–710 °C and T ₂ ~ 716–755 °C, respectively). The ceramic with x = 0.275 exhibits the optimum piezoelectricity (d ₃₃ = 48 pC/N and k _p = 13.6 %, respectively). The Ba_0.6(Bi_0.5K_0.5)_0.4TiO₃ doping and the increasing in sintering temperature improve significantly the ferromagnetic properties of the ceramics. The ceramic with x = 0.25 sintered at 1,040 °C gives the optimum remnant magnetization M _r of 0.13 emu/g.