Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Large Piezoresponse and Ferroelectric Properties of (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–Bi(Mg0.5Ti0.5)O3 Thin Films Prepared by Chemical Solution Deposition

Bulk ceramic 72.5 mol%(Bi_0.5Na_0.5)TiO₃–22.5 mol%(Bi_0.5K_0.5)TiO₃–5 mol%Bi(Mg_0.5Ti_0.5)O₃ (BNT–BKT–BMgT) has previously been reported to show a large high‐field piezoelectric coefficient (d₃₃* = 570 pm/V). In this work, the same composition was synthesized in thin film embodiments on platinized silicon substrates via chemical solution deposition. Overdoping of volatile cations in the precursor solutions was necessary to achieve phase‐pure perovskite. An annealing temperature of 700°C resulted in good ferroelectric properties (P_max = 52 μC/cm² and P_r = 12 μC/cm²). Quantitative compositional analysis of films annealed at 650°C and 700°C indicated that near ideal atomic ratios were achieved. Compositional fluctuations observed through the film thickness were in good agreement with the existence of voids formed between successive spin‐cast layers, as observed with electron microscopy. Bipolar and unipolar strain measurements were performed via double laser beam interferometry and a high effective piezoelectric coefficient (d_33,f) of approximately 75 pm/V was obtained.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Mg0.5Ti0.5)O3 Lead‐Free Piezoceramics

Lead‐free BNT‐based piezoceramics, (1?x)Bi_0.5Na_0.5TiO₃–xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BNT–xBMT] (0.00 ≤ x ≤ 0.06) binary system, were synthesized using a conventional ceramic fabrication method. Effect of Bi(Mg_0.5Ti_0.5)O₃ (BMT) substitution on room temperature (RT) crystal structure, and temperature dependence of electric properties were investigated. The XRD indicates that a pure perovskite phase is formed. The introduction of BMT decreases E_C of BNT from 7.3 to 4.0 kV/mm, and increases d₃₃ from 58 pC/N to 110 pC/N for the x = 0.05. The system shows a typical ferroelectric (FE) polarization loop P(E) and butterfly bipolar strain‐electric S(E) curve at RT. For the composition of 0.95BNT–0.05BMT antiferroelectric (AFE) phase appears near 80°C, characterized by a constricted P(E) loop and altered bipolar S(E) butterfly, and gradually prevails with increasing temperature. Temperature dependence of dielectric constant shows that T_C increases from 310°C for pure BNT to 352°C for the x = 0.05. The results indicate that the piezoelectric properties of BNT have been improved by means of Bi(Mg_0.5Ti_0.5)O₃ substitution.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Lead‐Free (Ba0.70Sr0.30)TiO3‐Modified Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics with Large Electric Field–Induced Strains

The dielectric, ferroelectric, and electric field–induced strain behavior of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT) ceramics modified with (Ba_0.70Sr_0.30)O₃ (BST) were investigated as a function of composition and temperature. The ceramic samples were synthesized by a solid‐state mixed oxide method and sintered at 1125°C for 2 h. The XRD and Raman spectra showed coexisting rhombohedral and tetragonal phases throughout the entire compositional range with the tetragonal phase becoming dominant at higher BST concentrations. For all compositions, the temperature dependence of the dielectric spectra revealed a frequency dependence that is characteristic of a relaxor mechanism. This suggests that these ceramics lacked long‐range order and it appears that the maximum disorder was observed for the composition with 5 mol% BST (BNKT–0.05BST sample). This was evidenced by the observation of pinched hysteresis loops, even at room temperature, and a significant decrease in the P_r and E_c values which resulted in large electric field–induced strains (S_max) of 0.40% and a normalized strain coefficient ( = S_max/E_max) of 732 pm/V. This significant strain enhancement at the composition of x = 0.05 may be attributed to both a composition‐induced structural phase transition and a field‐induced relaxor to ferroelectric phase transition.     

Large Electromechanical Strain in Lead‐Free Binary System K0.5Bi0.5TiO3‐Bi(Mg0.5Ti0.5)O3

Solid solution formation in the lead‐free binary system (1?x)K_0.5Bi_0.5TiO₃?xBi(Mg_0.5Ti_0.5)O₃ has been studied for compositions x ≤ 0.12. X‐ray powder diffraction shows single‐phase perovskite for x < 0.1, and a mixed phase region between tetragonal and pseudocubic phases for compositions 0.04 ≤ x ≤ 0.06. Large electromechanical strains of ~0.3% at fields of 50 kV/cm are recorded in the mixed phase region, with d₃₃* (S_max/E_max) values of ~600 pm/V. The materials sustain polarization at low electric fields with remnant polarization ~18 μC/cm² and coercive field ~20 kV/cm for x = 0.06. Relative permittivity‐temperature plots display relaxor characteristics, with peak temperature ~340°C.     

Dielectric and Piezoelectric Properties of (1−x)K0.5Bi0.5TiO3–xBa(Ti0.8Zr0.2)O3 Ceramics

The properties of relaxor ceramics in the compositional series (1?x)K_0.5Bi_0.5TiO₃–xBa(Ti_0.8Zr_0.2)O₃ have been investigated. Values of T_m, the temperature of maximum relative permittivity, decreased from 380°C at x = 0.0 to below room temperature for x > 0.7. Compositions x = 0.1 and 0.2 were piezoelectric and ferroelectric. The maximum value of d₃₃ piezoelectric charge coefficient, 130 pC/N, and strain, 0.14%, occurred at x = 0.1. Piezoelectric properties of x = 0.1 were retained after thermal cycling from room temperature to 220°C, consistent with results from high‐temperature X‐ray diffraction indicating a transition to single‐phase cubic at ~300°C.     

A Novel BiFeO3–BaTiO3–BaZrO3 Lead‐Free Relaxor Ferroelectric Ceramic with Low‐Hysteresis and Frequency‐Insensitive Large Strains

A novel (0.67?x)BiFeO₃–0.33BaTiO₃–xBaZrO₃ lead‐free relaxor ferroelectric ceramic was developed by a solid‐state reaction method. Measurements of temperature‐dependent dielectric permittivity and the polarization/strain hysteresis loops demonstrated an obvious evolution of dielectric relaxor behavior at room temperature (RT) from nonergodic to ergodic states. A significantly enhanced electrostrain of ~0.37% at 7 kV/mm with a relatively small hysteresis of ~39% and a low‐frequency sensitivity was found at x = 0.04, showing large potential for actuator applications. This was basically attributed to a rapid response of forward and backward switching between ergodic and ferroelectric phases owing to similar free energies and large local random fields.     

Large Strain Response and Fatigue‐Resistant Behavior in Ternary Bi0.5Na0.5TiO3–BaTiO3–Bi(Zn0.5Ti0.5)O3 Solid Solutions

A ternary solid solution (1 ? x)(0.88Bi_0.5Na_0.5TiO₃–0.12BaTiO₃)‐xBi(Zn_0.5Ti_0.5)O₃ (BNBZT, BNBZTx) was designed and fabricated using the traditional solid‐state reaction method. The temperature and composition dependence of dielectric, ferroelectric, piezoelectric, and fatigue properties were systematically investigated and a schematic phase diagram was proposed. The substitution with Bi(Zn_0.5Ti_0.5)O₃ was found to shift the phase transition (ferroelectric tetragonal to relaxor pseudocubic phase) to lower temperatures. At a critical composition x of 0.05, large electric‐field‐induced strain response with normalized strain S_max/E_max as high as 526 pm/V was obtained under a moderate field of 4 kV/mm around room temperature. The strain exhibited good temperature stability within the temperature range of 25°C–120°C. In addition, excellent fatigue‐resistant behavior was observed in the proposed BNBZT solid solution after 10⁶ bipolar cycles. These give the BNBZT system great potential as environmental friendly solid‐state actuator.     

Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics

A new type of (0.7?x)Bi_0.5Na_0.5TiO₃‐0.3Sr_0.7Bi_0.2TiO₃‐xLaTi_0.5Mg_0.5O₃ (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm³ with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 10⁶ fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.     

Enhanced Piezoelectric and Dielectric Responses in 92.5%(Bi0.5Na0.5) TiO3 ‐7.5%BaTiO3 Ceramics

Structure and piezoelectric coefficient (d₃₃) of lead‐free 7.5% mole BaTiO₃‐doped (Bi_0.5Na_0.5) TiO₃ (BNT‐7.5%BT) polycrystalline piezoceramics have been characterized systematically as a function of poling electric (E) field. Dielectric permittivity and loss were also measured as functions of frequency and temperature. The piezoelectric coefficient d₃₃ after poling at E = 35 kV/cm can reach d₃₃~186 pC/N, which is the highest value reported among (1?x) BNT–xBT compositions. A prior poling E field can reduce rhomobherdal lattice distortion, and enhance tetragonal phase and polarization ordering, that contribute significantly to the rapid raise of d₃₃ and lower depolarizing temperature (T_d). The reduced dielectric permittivity for the poled sample is attributed to ordered state and the pinning of field‐induced nanodomain walls by the presence of oxygen vacancies.     

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Perovskite solid solution ceramics of (1 ? x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (BT–BMN) (x = 0.05–0.2) were synthesized by solid‐state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT‐based ceramics. X‐ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor‐like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature‐stable permittivity plateau (from ?50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 10¹³ Ω cm at x = 0.15, and an energy density of about 1.13 J/cm³ is achieved in the sample of x = 0.1.     

Composition‐ and Temperature‐Dependent Large Strain in (1−x)(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)– xNaNbO3 Ceramics

Ternary solid solutions of (1 ? x)(0.8Bi_0.5Na_0.5TiO₃–0.2Bi_0.5K_0.5TiO₃)– xNaNbO₃ (BNKT–xNN) lead‐free piezoceramics were fabricated using a conventional solid‐state reaction method. Pure BNKT composition exhibited an electric‐field‐induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric‐to‐relaxor temperature T_F‐R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic‐to‐ergodic relaxor transition was presented, which featured the pinched‐shape polarization and sprout‐shape strain hysteresis loops. A strain value of ~0.445% (under a driving field of 55 kV/cm) with large normalized strain of ~810 pm/V was obtained for the composition of BNKT–0.04NN, and the large strain was attributed to the reversible electric‐field‐induced transition between ergodic relaxor and ferroelectric phase.     

Temperature‐Insensitive High Strain in Lead‐Free Bi0.5(Na0.84K0.16)0.5TiO3–0.04SrTiO3 Ceramics for Actuator Applications

Lead‐free piezoelectric ceramics, 0.96[{Bi_0.5 (Na_0.84K_0.16)_0.5}_1?xLi_x(Ti_1?yNb_y)O₃]–0.04SrTiO₃ (BNKLiTN–ST) with x, y = 0–0.030, were synthesized by solid‐state reaction method. X‐ray diffraction patterns indicated that Li and Nb successfully diffused into the BNKT–ST lattice and formed a pure perovskite structure with x, y ≤ 0.025. Increasing the Li and Nb contents (x, y = 0.020) induced a phase transformation from the coexistent rhombohedral–tetragonal phases for pure BNKT–ST ceramics to a pseudocubic phase, resulting in degradation of the remnant polarization and coercive field. However, the field‐induced strain was markedly enhanced at x, y = 0.020, giving rise to a giant dynamic piezoelectric constant (d₃₃* = S_max/E_max = 800 pm/V). Furthermore, the temperature dependence of the field‐induced strain response showed temperature‐insensitivity up to 120°C. To explore its potential for device applications, a 10‐layered stack‐type multilayer actuator was fabricated from the optimal composition (x, y = 0.020). This actuator showed a large S_max/E_max of 600 pm/V at a relatively low driving field of 4.5 kV/mm suggesting highly promising results in lead‐free BNT‐based ceramics.     

Preparation and Electric Properties of Bi0.5Na0.5TiO3–Bi(Al0.5Ga0.5)O3 Lead‐Free Piezoceramics

A new lead‐free BNT‐based piezoelectric ceramics of (1 ? x)Bi_0.5Na_0.5TiO₃–xBi(Al_0.5Ga_0.5)O₃ (x = 0, 0.02, 0.03, 0.04, and 0.05) were synthesized using a conventional ceramic fabrication method. Their structures and electrical properties were investigated. All the samples show a typical ferroelectric P(E) loops and S(E) curves at room temperature. The optimal properties are obtained at the composition of the x = 0.03. The substitution of Bi(Al_0.5Ga_0.5)O₃ enhances piezoelectric constant and increases Curie temperature from 58 pC/N and 310°C of pure BNT to 93 pC/N and 325°C of the x = 0.03. The temperature‐dependent P(E) loops and S(E) curves of 0.97BNT–0.03BAG indicate that phase transition from ferroelectric to antiferroelectric takes place over a very wide temperature region from 80°C to 180°C. The results show that the introduction of BAG improves the electrical properties of BNT.     

Tailoring Strain Properties of (0.94−x)Bi1/2Na1/2TiO3–0.06BaTiO3–xK0.5Na0.5NbO3 Ferroelectric/Relaxor Composites

A remarkable progress in the quest of lead‐free piezoceramics for actuator applications has been made with the development of incipient piezoceramics featured by giant strains. A drawback, however, is the high electric field required to generate this giant strain. A powerful approach to overcoming this drawback lies in relaxor/ferroelectric (FE) composites comprised such giant strain materials (matrix) and a FE or nonergodic relaxor (seed). In this study, we investigate the effect of K_0.5Na_0.5NbO₃ content in the matrix and the volume ratio of seed to matrix using composites of 0.93Bi_1/2Na_1/2TiO₃–0.07BaTiO₃ as a seed and (0.94 ? x)Bi_1/2Na_1/2TiO₃–0.06BaTiO₃–xK_0.5Na_0.5NbO₃ as a matrix. The strain of all matrices, independent of their K_0.5Na_0.5NbO₃ content, was found to be enhanced by adding a certain amount of seed. An optimum strain is achieved for the composite comprised of a matrix with x = 0.02 K_0.5Na_0.5NbO₃ and 10% seed. By means of a differential analysis on the temperature‐dependent dielectric permittivity, it was shown that the seed phase is still present in the composites despite the naturally expected diffusion process during sintering.     

Stability of High‐Temperature Dielectric Properties for (1 − x)Ba0.8Ca0.2TiO3–xBi(Mg0.5Ti0.5)O3 Ceramics

Ceramics in the solid solution system, (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃, were prepared by a conventional mixed oxide route. Single‐phase perovskite‐type X‐ray diffraction patterns were observed for compositions x < 0.6. A change from tetragonal to single‐phase cubic X‐ray patterns occurred at x ≥ 0.1. Dielectric measurements indicated relaxor behavior for x ≥ 0.1. Increasing the Bi(Mg_0.5Ti_0.5)O₃ content improved the temperature sensitivity of relative permittivity ?_r at high temperatures. At x = 0.5, a near‐plateau relative permittivity, 835 ± 40, extended across the temperature range, 65°C–550°C; the permittivity increased at x = 0.6 to 2170 ± 100 for temperatures 160°C–400°C (1 kHz). The corresponding loss tangent, tanδ, was ≤0.025 for temperatures between 100°C and 430°C for composition x = 0.5; at x = 0.6, losses increased sharply at >300°C. Comparisons of dielectric properties with other materials proposed for high‐temperature capacitor applications suggest that (1 ? x)Ba_0.8Ca_0.2TiO₃–xBi(Mg_0.5Ti_0.5)O₃ ceramics are a promising base material for further development.     

Coexistence of Ferroelectric Phases and Phonon Dynamics in Relaxor Ferroelectric Na0.5Bi0.5TiO3 Based Single Crystals

A combination of polarized Raman technique, infrared reflectance spectra, and first‐principles density‐functional theoretical calculations were used to investigate structure transformation and lattice vibrations of Na_0.5Bi_0.5TiO₃, Na_0.5Bi_0.5TiO₃–5%BaTiO₃, and Na_0.5Bi_0.5TiO₃–8%K_0.5Bi_0.5TiO₃ single crystals. It was found that Na_0.5Bi_0.5TiO₃ is of a two‐phase mixture with rhombohedral and monoclinic structures at room temperature. Correspondingly, three Raman‐active phonon modes located at 395, 790, and 868 cm^?1, which were previously assumed as A₁ modes of rhombohedral phase have been reassigned as A^′′, A^′, and A^′ modes of monoclinic phase in the present work. In particular, a strong low‐frequency A^′′ mode at 49 cm^?1 was found and its temperature dependence was revealed. Two deviations from linearity for the abrupt frequency variation in the A^′′ mode and Ti–O bond have been detected at temperatures of ferroelectric to antiferroelectric phase transition T_F–AF and dielectric maximum temperature T_max. The appearance of Na–O vibrations at 150 cm^?1 was found below T_max, indicating the existence of nanosized Na⁺TiO₃ clusters. The observed Raman and infrared active modes belonging to distinct irreducible representations are in good agreement with group‐theory predictions, which suggests 9A₁+9E and 36A^′′+24A^′ modes for the rhombohedral and monoclinic phases of Na_0.5Bi_0.5TiO₃, respectively.     

Impedance Spectroscopy of (Bi1/2Na1/2)TiO3–BaTiO3 Ceramics Modified with (K0.5Na0.5)NbO3

The electrical and dielectric properties of (1 ? x)(0.94Bi_1/2Na_1/2TiO₃–0.06BaTiO₃)–x(K_0.5Na_0.5NbO₃) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n‐type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature‐dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.     

Giant electromechanical strain response in lead‐free SrTiO3‐doped (Bi0.5Na0.5TiO3–BaTiO3)–LiNbO3 piezoelectric ceramics

Lead‐free 0.985[(0.94?x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xSrTiO₃]–0.015LiNbO₃ [(BNT–BT–xST)–LN, x=0‐0.05] piezoelectric ceramics were prepared using a conventional solid‐state reaction method. It was found that the long‐range ferroelectric order in the unmodified (BNT–BT)–LN ceramic was disrupted and transformed into the ergodic relaxor phase with the ST substitution, which was well demonstrated by the dramatic decrease in remnant polarization (P_r), coercive field (E_c), negative strain (S_neg) and piezoelectric coefficient (d₃₃). However, the degradation of the ferroelectric and piezoelectric properties was accompanied by a significant increase in the usable strain response. The critical composition (BNT–BT–0.03ST)–LN exhibited a maximum unipolar strain of ~0.44% and corresponding normalized strain, S_max/E_max of ~880 pm/V under a moderate field of 50 kV/cm at room temperature. This giant strain was associated with the coexistence of the ferroelectric and ergodic relaxor phases, which should be mainly attributed to the reversible electric‐field‐induced transition between the ergodic relaxor and ferroelectric phases. Furthermore, the large field‐induced strain showed relatively good temperature stability; the S_max/E_max was as high as ~490 pm/V even at 120°C. These findings indicated that the (BNT–BT–xST)–LN system would be a suitable environmental‐friendly candidate for actuator applications.