Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Temperature-independent permittivity of xBaTiO3–(1−x)(0.5Bi(Mg1/2Ti1/2)O3–0.5BiScO3) ceramics

xBaTiO₃–(1−x)(0.5Bi(Mg_1/2Ti_1/2)O₃-0.5BiScO₃) or xBT–(1−x)(0.5BMT–0.5BS) (x=0.45–0.60) ceramics were prepared by using the conventional mixed oxide method. Perovskite structure with pseudo-cubic symmetry was observed in all the compositions. Dielectric measurement results indicated that all the samples showed dielectric relaxation behavior. As the content BaTiO₃ was decreased from 0.60 to 0.45, temperature coefficient of permittivity (TCε) in the range of 200–400 °C was improved from −706 to −152 ppm/°C, while the permittivity at 400 °C was increased from 1208 to 1613. The temperature stability of permittivity was further improved by using 2 mol% Ba-deficiency. It was found that lattice parameter and grain size of the 2 mol% Ba-deficient ceramics were smaller than those of their corresponding stoichiometric (S) counterparts, with TCε in the range of 200–400 °C to be improved noticeably. For example, TCε of the Ba-deficiency sample with x=0.45 was −75 ppm/°C in the temperature range of 200–400 °C and the permittivity was 1567 at 400 °C. The results obtained in this work indicated that xBT–(1−x)(0.5BMT–0.5BS) ceramics are very promising candidates for high temperature capacitor applications.     

Dielectric properties and microstructures of non-reducible high-temperature stable X9R ceramics

New X9R dielectrics based on the precursors of (1 ? x) BaTiO₃–xLiTaO₃ (BT–xLT) compositions and containing no lead and bismuth constituents were successfully prepared in this study. After sintering at 1150–1250 °C in a reducing atmosphere of 97%N₂–3%H₂, major phases including BaTiO₃ and Ba₂LiTa₅O₁₅ and a minor phase of Li₂TiO₃ were identified in the sintered ceramics. The intensities of the XRD peaks corresponding to the Ba₂LiTa₅O₁₅ phase rose with the increasing x value in the precursors. The grain sizes of the sintered ceramics appeared to fall in the range between 0.3 and 0.5 μm and slightly increased with the x value of the BT–xLT ceramics. The best composition in term of dielectric properties was found in the BT–0.25LT ceramic, which was marked with a dielectric constant of 895, tan δ of 1.01% and 7.1%, TCCs of ?3.04% and ?14.80%, and electrical resistivities of 9.9 × 10¹² and 1.6 × 10¹² Ω cm, respectively at 25 °C and 200 °C.     

Ionic conductivity of Mn2+ doped dense tin pyrophosphate electrolytes synthesized by a new co-precipitation method

Mn²⁺-doped Sn_1−xMn_xP₂O₇ (x = 0–0.2) are synthesized by a new co-precipitation method using tin(II)oxalate as tin(IV) precursor, which gives pure tin pyrophosphate at 300 °C, as all the reaction by-products are vaporizable at <150 °C. The dopant Mn²⁺ acts as a sintering aid and leads to dense Sn_1−xMn_xP₂O₇ samples on sintering at 1100 °C. Though conductivity of Sn_1−xMn_xP₂O₇ samples in the ambient atmosphere is 10⁻⁹–10⁻⁶ S cm⁻¹ in 300–550 °C range, it increases significantly in humidified (water vapor pressure, pH₂O = 0.12 atm) atmosphere and reaches >10⁻³ S cm⁻¹ in 100–200 °C range. The maximum conductivity is shown by Sn_0.88Mn_0.12P₂O₇ with 9.79 × 10⁻⁶ S cm⁻¹ at 550 °C in ambient air and 2.29 × 10⁻³ S cm⁻¹ at 190 °C in humidified air. It is observed that the humidification of Sn_1−xMn_xP₂O₇ samples is a slow process and its rate increases at higher temperature. The stability of Sn_1−xMn_xP₂O₇ samples is analyzed.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.     

Microstructure,ferroelectric and piezoelectric properties of Bi0.5K0.5TiO3-modified BiFeO3–BaTiO3 lead-free ceramics with high Curie temperature

The effects of composition, sintering temperature and dwell time on the microstructure and electrical properties of (0.75 ? x)BiFeO₃–0.25BaTiO₃–xBi_0.5K_0.5TiO₃ + 1 mol% MnO₂ ceramics were studied. The ceramics sintered at 1000 °C for 2 h possess a pure perovskite structure and a morphotropic phase boundary of rhombohedral and pseudocubic phases is formed at x = 0.025. The addition of Bi_0.5K_0.5TiO₃ retards the grain growth and induces two dielectric anomalies at high temperatures (T₁ ～ 450–550 °C and T₂ ～ 700 °C, respectively). After the addition of 2.5 mol% Bi_0.5K_0.5TiO₃, the ferroelectric and piezoelectric properties of the ceramics are improved and very high Curie temperature of 708 °C is obtained. Sintering temperature has an important influence on the microstructure and electrical properties of the ceramics. Critical sintering temperature is 970 °C. For the ceramic with x = 0.025 sintered at/above 970 °C, large grains, good densification, high resistivity and enhanced electrical properties are obtained. The weak dependences of microstructure and electrical properties on dwell time are observed for the ceramic with x = 0.025.     

Temperature-stable dielectric properties from −20 °C to 430 °C in the system BaTiO3–Bi(Mg0.5Zr0.5)O3

Ceramics in the solid solution series (1 − x)BaTiO₃–Bi(Mg_0.5Zr_0.5)O₃ are single-phase tetragonal for compositions x ≤ 0.05, and cubic for x ≥ 0.1. Plots of relative permittivity (ɛ_r), versus temperature show double peaks for x = 0.03 and x = 0.05, changing to a single, frequency-dependent peak for compositions, x ≥ 0.1. A progressive decline in ɛ_r max with increasing x leads to near temperature-stable dielectric properties over a wide temperature range. For x = 0.3, ɛ_r = 570 ± 15%, from −20 °C to 430 °C, and tan δ ≤ 0.02 from 30 °C to 420 °C. For x = 0.4, ɛ_r = 600 ± 15% from 25 °C to 420 °C, and tan δ ≤ 0.02 from 55 °C to 280 °C (at 1 kHz). Values of dc resistivity were ∼10⁹ Ω m at 250 °C and ∼10⁶ Ω m at 400 °C. A piezoelectric strain of ∼0.25% (at 40 kV/cm) was recorded for composition x = 0.03.     

(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.     

Comparison between microwave and conventional calcination techniques in regard to reactivity and morphology of co-precipitated BaTiO3 powder,and the electrical and energy storage properties of the sintered samples

Barium titanate (BaTiO₃) nanopowders were synthesized by an aqueous co-precipitation method followed by calcination. Either 2.45 GHz microwaves or conventional heating was used in order to investigate the impact of these techniques on the synthesis time, microstructure, and electrical properties of the materials. The heating temperatures ranged from 620 °C to 810 °C. X-ray diffraction (XRD) revealed pure BaTiO₃ formation by microwave heating in a noticeably shorter time (five minutes) compared to conventional heating (3 h). Field emission scanning electron microscopy (FESEM) results confirmed that the microwave process led to nanocube formation, whereas in the conventional procedure, the particles tended to form spherical shapes. To evaluate the electrical properties, the samples heated at 620 °C were conventionally sintered at 1280 °C, 1330 °C, and 1380 °C. Higher dielectric, piezoelectric, and ferroelectric properties and more energy-saving efficiency (ε_r=1012, tan δ=0.035 d₃₃=85 pC/N, p_r=6.2 µC/cm² and η=48% respectively) were achieved in the microwave-heated BaTiO₃ sintered at 1380 °C compared to the conventionally heated BaTiO₃ (ε_r=824, tan δ=0.030 d₃₃=75 pC/N, p_r=5 µC/cm² and η=27%) demonstrating that microwave calcination substantially affects the final electrical properties.     

Dielectric properties of barium titanate–molybdenum composite

The barium titanate–molybdenum composites were prepared through solid state reaction method in argon atmosphere. The microstructure, resistivity, and dielectric properties of the composites were investigated. XRD results indicated that chemical reactions between barium titanate (BaTiO₃:BT) and molybdenum (Mo) have taken place during sintering, resulting in the formation of BaMoO₄ (BM) and BaTi₂O₅ (BT₂). The resistivity decreased with the increasing amount of Mo in the composites. The composites (when x = 5 and 20 wt.%) showed lower dielectric constant than pure BaTiO₃, especially, the dielectric constant (when x = 20 wt.%) reached a minimum value (<10⁴), while composites (when x = 10 and 15 wt.%) showed rather high dielectric constant at temperatures range from 25 °C to 160 °C. The dielectric constant of the composite gradually decreased with increase in frequency at the room temperature. The dielectric constant of composite (when x = 5 wt.%) comes up to 10⁴, and the T_c (Curie temperature) of the composite was relatively higher than that of BT (120 °C).     

Enhanced electrical properties of lead-free BNLT–BZT ceramics by thermal treatment technique

Electrical properties of lead-free solid solution ceramics from the Bi_0.4871Na_0.4871La_0.0172TiO₃ (BNLT) and BaZr_0.05Ti_0.95O₃ (BZT) system have been improved by a thermal treatment technique. A modified two step mixed-oxide method was employed for the preparation of the (1?x)BNLT–xBZT ceramics, where x=0.06, 0.09, 0.12 and 0.20. After sintering at 1125 °C for 4 h, the BNLT–BZT ceramics were annealed at 825, 925 and 1025 °C. The annealing treatment caused an increase in dielectric constant of BNLT–BZT ceramics with x≤0.09 mol% and with x higher than 0.09 mol% the dielectric value dropped considerably. The ferroelectric properties of all annealed ceramic samples tend to decrease with increasing annealing temperature as confirmed by the slimmer P–E loops. The piezoelectric coefficient (d₃₃) increased with annealing temperatures and a maximum value of ～170 pC/N was obtained from the ceramic samples annealed at 1025 °C with x=0.02.     

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.     

Microstructure and electrical properties of BaTiO3/Cu ceramic composite sintered in nitrogen atmosphere

BaTiO₃/xCu composite ceramics with x = 0–30 wt.% were fabricated by the traditional mixing oxide method and their microstructure, relative density, electric conductivity, permittivity and dielectric loss were measured as a function of the Cu mass fraction. The X-ray diffraction (XRD) patterns indicated that the dense composite has no chemical reaction between BaTiO₃ and Cu during sintering, and the relative diffraction intensity of Cu increased with the increase of Cu. The electric properties showed that the percolation threshold of BaTiO₃/Cu composites was x = 0.25 and its conductivity increased as the Cu content increased after that. With increasing Cu content up to 30 wt.%, the permittivity (?_r) markedly increased from ～3000 for monolithic BaTiO₃ to ～8000 at 1 kHz. Additionally, the temperature coefficient of this system was less than 5% in the temperature range of 25–115°C.     

High temperature properties of multiferroic BiFeO3–DyFeO3–BaTiO3 solid solutions

Dielectric and magnetic properties of the xBiFeO₃–yDyFeO₃–zBaTiO₃ solid solution ceramics at high temperature range of RT ∼600 °C have been characterized. For the more detailed understandings of the multiferroic property, the relation between the crystal structure transition, magnetic transition, dielectric transition with increasing temperature have been analyzed. Residual magnetization M_r under the low and high applied magnetic fields (H = 20 Oe, 8 kOe) and the dielectric properties, ɛ_r and tan δ, with varying measuring frequency and temperature have been characterized using the vibrating sample magnetometer and LCR meter, respectively. The neutron diffraction data has been collected at the temperature range of RT ∼800 °C. The low DyFeO₃ concentration samples (y = 0, 0.025) show the magnetic transitions at temperature range of 410–430 °C, while the high DyFeO₃ samples (y ≥ 0.05) show the additional transition at 250–290 °C. The magnetic transition at 410–430 °C corresponds to the crystal structural transition to the tetragonal P4mm from the rhombohedral R-3c, at which the BiFeO₃ and the DyFeO₃ samples lose their antiferromagnetic ordering.     

Effect of Nd-doping on structural,thermal and electrochemical properties of LaFe0.5Cr0.5O3 perovskites

The structural, thermal and electrochemical properties of the perovskite-type compound La_1−xNd_xFe_0.5Cr_0.5O₃ (x=0.10, 0.15, 0.20) are investigated by X-ray diffraction, thermal expansion, thermal diffusion, thermal conductivity and impedance spectroscopy measurements. Rietveld refinement shows that the compounds crystallize with orthorhombic symmetry in the space group Pbnm. The average thermal expansion coefficient decreases as the content of Nd increases. The average coefficient of thermal expansion in the temperature range of 30–850 °C is 10.12×10⁻⁶, 9.48×10⁻⁶ and 7.51×10⁻⁶ °C⁻¹ for samples with x=0.1, 0.15 and 0.2, respectively. Thermogravimetric analyses show small weight gain at high temperatures which correspond to filling up of oxygen vacancies as well as the valence change of the transition metals. The electrical conductivity measured by four-probe method shows that the conductivity increases with the content of Nd; the electrical conductivity at 520 °C is about 4.71×10⁻³, 6.59×10⁻³ and 9.62×10⁻³ S cm⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. The thermal diffusivity of the samples decreases monotonically as temperature increases. At 600 °C, the thermal diffusivity is 0.00425, 0.00455 and 0.00485 cm² s⁻¹ for samples with x=0.10, 0.15 and 0.20, respectively. Impedance measurements in symmetrical cell arrangement in air reveal that the polarization resistance decreases from 55 Ω cm⁻² to 22.5 Ω cm⁻² for increasing temperature from 800 °C to 900 °C, respectively.     

The effect of the hydrothermal synthesis variables on barium titanate powders

In this work, the influence of (a) Ba excess in the starting hydrothermal mixture with TiO₂, (b) hydrothermal reaction temperature, and (c) washing cycles on the hydrothermal synthesis of barium titanate (BaTiO₃) were investigated to assess their relative contributions to the final characteristics of the sintered oxide. BaTiO₃ cake was prepared by hydrothermal synthesis at 150 °C and 180 °C using BaOH₂·8H₂O and TiO₂·xH₂O as starting hydrothermal mixture with an excess of Barium (+1 Ba mol% and +2 Ba mol%). The obtained BaTiO₃ cake was washed several times from 0 to 14 (W_n<15) using simple de-ionized water and then sintered at 1120 °C for 3 h. All considered hydrothermal syntheses variables strongly contribute to the final characteristics of the sintered BaTiO₃ powders in terms of Ba²⁺/Ti⁴⁺ molar ratio, crystalline structure and mean particle size. In particular, it is clear from these experiments that the removal of the unfavorable barium salts from BaTiO₃ cake by long washing cycles before final calcination is a critical step in the hydrothermal synthesis of BaTiO₃.     

Low variation in relative permittivity over the temperature range 25–450 °C for ceramics in the system (1 − x)[Ba0.8Ca0.2TiO3]–x[Bi(Zn0.5Ti0.5)O3]

The dielectric and ferroelectric properties of the ceramic system, (1 − x)Ba_0.8Ca_0.2TiO₃–xBi(Zn_0.5Ti_0.5)O₃, were investigated for compositions 0 ≤ x ≤ 0.4. X-ray powder diffraction patterns indicated tetragonal symmetry at x ≤ 0.05, switching to pseudocubic at x ≥ 0.1, with a single-phase solid solution limit at 0.2 < x < 0.3. The x = 0 and 0.05 samples were ferroelectric; a change to relaxor behaviour occurred at x ≥ 0.1, with broad frequency dependent peaks in plots of relative permittivity versus temperature. A significant reduction in the temperature dependence of relative permittivity occurred at x = 0.3, with ɛ_r = 1030 ± 15% over the temperature range ∼25–425 °C, and loss tangent, tan δ ≤ 0.01 from 110 °C to 420 °C. The dc resistivity values for x = 0.3 were ∼10⁹ Ω m at 300 °C and ∼10⁶ Ω m at 450 °C.     

Strong photoluminescence and good electrical properties in Eu-modified SrBi2Nb2O9 multifunctional ceramics

Bismuth layer-structured ferroelectric (BLSFs) ceramics of Sr_1−xEu_x Bi₂Nb₂O₉ (SBT-xEu, x=0.000, 0.002, 0.004, 0.006) were prepared by a conventional solid-state reaction method. All the samples have a bismuth oxide layered structure with a dense microstructure. The ferroelectric, piezoelectric, dielectric and optical properties of the ceramics were investigated. After Eu³⁺ doping, samples show a bright red photoluminescence upon blue light excitation of the 400–500 nm. Upon the excitation of 465 nm light, the materials have two intense emission bands peaking around 593 nm (yellow) and 616 nm (red). Meanwhile, good electrical properties with large piezoelectric constant d₃₃ of 14 pC/N and large remnant polarization 2P_r of 11.97 μC/cm² are obtained at x=0.006. Moreover, this material has a high Curie temperature (T_c=429 °C) and high resistivity, which makes the material resistant to thermal depolarization up to its Curie temperature. This feature indicates that the SBN-xEu ceramics have a latent use in high temperature applications.     

Annealing effect on temperature coefficient of resistivity in La1−xSrxMnO3 ceramics

We investigated annealing effects of La_1?xSr_xMnO₃ (x = 0–0.6) on electrical resistivity and the temperature coefficient of resistivity (TCR). The annealed samples’ resistivity was lower than those of non-annealed samples. For example, annealing changed the resistivity of x = 0.3 at 25 °C from 4.50 × 10^?5 to 3.71 × 10^?5 Ω m. Remarkable difference in TCR was observed after annealing, for x = 0.3, 0.45, and 0.5. For x = 0.3, the TCR after annealing was 4000 ppm/°C, which was 1250 ppm/°C greater than that before annealing. We investigated (1) crystal phase, (2) Mn average valence, (3) Mott insulator–metal transition temperature, and (4) microstructure. The microstructure was remarkably varied for annealed x = 0.3 and 0.5. The average grain size of the x = 0.3 increased from 1.60 up to 2.38 μm. Results show that annealing affects resistivity and TCR because of grain growth during annealing.     

Microstructure and electrical properties of relaxor (1 − x)[(K0.5Na0.5)0.95Li0.05](Nb0.95Sb0.05)O3–xBaTiO3 piezoelectric ceramics

In order to solve the low temperature stability of electrical properties in KNN-based ceramics, (1 ? x)[(K_0.5Na_0.5)_0.95Li_0.05](Nb_0.95Sb_0.05)O₃–xBaTiO₃ [(1 ? x)KNLNS–xBT] lead-free piezoelectric ceramics were prepared by the conventional solid-state sintering method. The introduction of BT stabilizes the tetragonal phase of KNLNS ceramics at room temperature, results in a typical ferroelectric relaxor behavior, and shifts the polymorphic phase transition to below room temperature. Moreover, there is a strong BaTiO₃ concentration dependence of relaxor behavior and electrical properties, and the ceramic with x = 0.005 exhibits optimum electrical properties and typical relaxor behavior (d₃₃ = 269 pC/N, k_p = 0.50, ?_r = 1371, tan δ = 0.03, T_C ～ 349 °C and γ = 1.88024). These results indicate that the BT is an effective way to improve the temperature stability as well as the electrical properties of KNN-based ceramics.