Abnormal Curie temperature behavior and enhanced strain property by controlling substitution site of Ce ions in BaTiO3 ceramics

Dopants have a great effect on the phase transition behavior and the properties of the ferroelectrics. Here we report an abnormal Curie temperature (T_c) behavior and enhanced strain property by controlling the doping site of Ce ions in the BaTiO₃ ceramics. The Ce doped A-site and B-site BaTiO₃ ceramics (BT-xCe-A and BT-xCe-B, x=2, 4, 6, 8 mol%) were prepared by a conventional solid state reaction method through a different sintering temperature. The Raman test and the XPS results give evidence that Ce is successfully incorporated into Ba-site as Ce³⁺ in the BT-xCe-A samples, and into Ti-site as Ce⁴⁺ in the BT-xCe-B samples. Different doping sites have distinct phase transition behavior. Compared with the BT-xCe-A ceramics, the BT-xCe-B ceramics show higher T_c, and the T_c show abnormal increasing behavior with the increase of the Ce content. In the Ce doped BaTiO₃ system, this phenomenon has not been reported before. The origin of the higher T_c and its increasing behavior is discussed from the viewpoint of the larger local strain field generated by the Ce⁴⁺ ions entering into B-site. Besides, the BT-xCe-B ceramics show a stronger diffuse phase transition behavior. The reason is considered that the Ce substituting B-site leads to a multiphase coexistence, which induces more frustration states for the polarization according to the random defect field theory. Due to such distinct phase transition behavior, the BT-xCe-B ceramics show the enhanced maximum polarization (P_max) and enhanced strain properties compared with the BT-xCe-A ceramics. This work may provide a promising way to design high performance materials by controlling the substituting site of the dopant in other lead-free systems.     

Enhanced piezoelectric response and high-temperature sensitivity by site-selected doping of BiFeO3-BaTiO3 ceramics

Effect of Zn site-selected doping on electrical properties, high-temperature stability and sensitivity of piezoelectric response for BiFeO₃-BaTiO₃ ceramics was investigated. The results revealed that the addition of Zn leaded to an evident modification of the microstructure. The B-site selected doping was a more effective approach in improving piezoelectric properties as well as their thermal stability than those of A-site selected doping. Moreover, the enhanced piezoelectric properties accompanying by excellent high-temperature stability and sensitivity in B-site selected doping ceramics were obtained. The microstructure, domain switching behavior and temperature-dependent piezoelectric response in Zn site-selected doping ceramics were investigated, and their relationships with improving piezoelectric properties and high-temperature stability were explored. These results showed that the B-site selected doping ceramics had excellent piezoelectric properties (d₃₃ = 192pC/N) along with a high-temperature stability (T_d = 450 °C).     

Doping behaviors of dysprosium,yttrium and holmium in BaTiO3 ceramics

The difference in doping behaviors of intermediate rare-earth ions and their effects on the dielectric property and microstructure of BaTiO₃(BT)–MgO–Re₂O₃ (Re = Dy, Ho, Y) system were investigated. Compared to Y and Ho, Dy ions provided BT ceramics with a high rate of densification and much enhanced shell formation due to their high solubility in BT. However, the microstructure of the Dy doped specimen was unstable at high temperatures in terms of grain growth. Until the specimen was densified, the tetragonality of Dy doped specimen was remarkably decreased and the substitution amount of Dy ions for A-site was larger than that of Y and Ho ions. After complete densification, the tetragonality was increased again and the B-site incorporation of Dy ions was increased far more than that of Y and Ho ions resulting in grain growth. This different behavior was considered to result in temperature coefficient of capacitance curves in the Dy doped specimen different from that of typical core–shell grains.     

Enhancing piezoelectric properties of Ba0.88Ca0.12Zr0.12Ti0.88O3 lead-free ceramics by doping Co ions

The piezoelectric properties of lead-free Ba_0.88Ca_0.12Zr_0.12Ti_0.88O₃ (BCZT) ceramics were greatly optimized by doping Co ions using a CoO powder. The role of Co²⁺ and Co³⁺ in enhancing the piezoelectric properties and the relationship between the content ratio Co³⁺/Co²⁺ and piezoelectric performance were studied. The X-ray diffraction patterns of all samples indicated that crystalline phases were a BCZT-based single perovskite structure regardless of the Co ion content. The phase transition temperature and lattice distortion degree were related to the Co ion content and the content ratio Co³⁺/Co²⁺ because Co²⁺ resulted in higher oxygen vacancy generation, whereas Co³⁺ induced larger lattice shrinkage. The ceramic containing 0.10 wt% of Co ion showed the best piezoelectric and dielectric performance with the highest piezoelectric constant d₃₃ ~ 490 p.m./V at room temperature and the highest Curie temperature T_c of 110 °C, which increased by 29% and 16%, respectively. In this case, the content ratio Co³⁺/Co²⁺ reached the maximum value of 0.86. The high piezoelectric properties and phase stability of BCZT ceramics by doping Co ions make these ceramics promising piezoelectric materials for practical applications.     

Optimizing the piezoelectric properties of Ba0·85Ca0·15Zr0·10Ti0·90O3·lead-free ceramics via two-step sintering

The two-step sintering of lead-free Ba₀·₈₅Ca₀·₁₅Zr₀·₁Ti_0·9O₃·(BCZT) ceramics was investigated as a way to enhance its piezoelectric properties. The variations in grain size as a function of the calcination and sintering conditions and its effect on performance is discussed. Results indicate that as the calcination and first-step sintering temperatures increased, grain size became large and was independent of the second sintering step. Large grains were responsible for the enhanced piezoelectric properties by causing lattice distortion, larger domains, and easy motion of domain walls. The BCZT ceramic calcined at 1200 °C and sintered at 1540 °C without holding and then cooled to 1400 °C and held at 1400 °C for 4 h exhibited optimal performance with the highest remnant polarization P_r ∼13.5 μC/cm², the largest piezoelectric constant d₃₃ ∼ 529 pC/N at room temperature, and the highest Curie temperature T_c ∼125 °C. Two-step sintering has been turned out to be an effective method to realize high-performance BCZT ceramics by microstructure optimization.     

Effects of sintering temperature and Bi2O3, Y2O3 and MgO co-doping on the dielectric properties of X8R BaTiO3-based ceramics

Bi₂O₃, Y₂O₃ and MgO co-doped BaTiO₃ (BT)-based X8R ceramics were synthesized successfully for the first time. The effects of the sintering temperature and Bi₂O₃, Y₂O₃ and MgO dopants on the dielectric properties were investigated systematically. Bi₂O₃ doping can increase the Curie temperature (T_c), but reduces the overall dielectric permittivity. On the other hand, Y₂O₃ doping is beneficial to the formation of core-shell microstructure and the increase of T_c, whereas MgO can prevent excessive Y₂O₃ from diffusing into grain core, and thereby further contributes to the generation of the core–shell microstructure. The generation of the typical core-shell microstructure was confirmed and investigated in detail by using transmission electron microscopy (TEM). It is argued that the synergistic effects of Bi₂O₃, Y₂O₃ and MgO co-doping in terms of the formation of the core-shell structure and the increase of T_c, can help improve the temperature stability of the dielectric permittivity effectively. Increasing the sintering temperature leads to an increase in the grain size, which in turn leads to an increase in the overall dielectric permittivity due to the grain size effect.     

Composition-independent Curie point (Tc) in ferroelectric (1 − x)PbTiO3–xBi(Li1/2Nb1/2)O3 solid solution

A new ferroelectric solid solution (1 − x)PbTiO₃–xBi(Li_1/2Nb_1/2)O₃ has been explored to develop high-temperature piezoelectric material. An interesting observation has been found regarding its Curie point (T_C) and tetragonal lattice strain (c/a − 1). With increasing composition (x), the Curie point (T_C) decreases up to x = 0.10 and thereafter remains constant. In concurrence with the T_C, the tetragonal lattice strain (c/a − 1) follows a similar trend. Neutron powder diffraction analysis suggests this anomalous behavior is due to the robust off-centering characteristic of the Bi⁺³ ion 6S² lone pair effect at the A-site compared to ions at B-site.     

Dielectric properties and relaxor behavior of high Curie temperature (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3–Bi(Mg0.5Ti0.5)O3 Lead-free ceramics

The (1?x)(Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃?xBi(Mg_0.5Ti_0.5)O₃ [(1?x)BCZT–xBMT, x=0.1–0.7] lead-free solid solution ceramics were prepared by the conventional mixed oxide method. The phase structure was investigated by X-ray diffraction and results show that a single perovskite phase was obtained in all of these samples, suggesting that the added BMT diffused into BCZT to form a solid solution. Dense ceramics with relative densities >95% were obtained, and a small amount of BMT (≤50 mol%) acted as grain growth promoter, had an evident effect on grain size growth. Further increase of the BMT content inhibited the grain growth of BCZT samples. Temperature dependence of the dielectric properties showed that all the BCZT–BMT solid solution ceramics exhibited relaxor-like characteristics. With increasing BMT content, the Curie temperature was first increased and then decreased, giving a maximum value of 218 °C for the 0.4BCZT–0.6BMT composition. Furthermore, stable dielectric constants and low losses were obtained with 0.5≤x≤0.7 in the wide temperature range, indicating that the system possess potential for high-temperature application.     

Crystal structure and electrical properties of (Li,Ce, Nd)-multidoped CaBi2Nb2O9 high temperature ceramics

(Li, Ce, and Nd)-multidoped CaBi₂Nb₂O₉ (CBN) Aurivillius phase ceramics were prepared via a conventional solid-state sintering route. The crystal structure including bond lengths and bond angles, microstructure, dielectric constant, DC resistivity, and piezoelectric properties were systematically investigated. Rietveld-refinements of X-ray results indicated that small quantity of (Li, Ce, Nd) doping (< 2.5 mol%) increases orthorhombic distortion, because of the smaller ionic radii of doping ions. However, orthorhombic distortion obviously decreased with increasing (Li, Ce, Nd) doping concentration from 5 to 25 mol%. The replacement of asymmetric A-site Bi³⁺ with 6s2 lone pair electrons by symmetric Li⁺, Ce³⁺ and Nd³⁺ ions decreased the orthorhombic distortion. The morphologies and electrical properties of sintered ceramics were tailored by the introducing (Li, Ce, Nd) multi-dopants. The improvement of piezoelectric properties of modified-CBN ceramics were attributed to decreasing grain sizes and morphotropic phase boundary (MPB). Ca_0.85(Li_0.5Ce_0.25Nd_0.25)_0.15Bi₂Nb₂O₉ (CBNLCN-15) ceramics had optimum properties, and d₃₃ and T_c values were found to be ~ 13.1 pC/N and ~ 900 °C, respectively.     

Impact of the A-site rare-earth ions (Ln3+ – Sm3+, Eu3+, Gd3+) on structure and electrical properties of the high entropy LnCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3 perovskites

High entropy perovskites LnCr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2O₃ ceramics were produced by solid-state reactions from oxides. The B-site chemical composition was fixed (Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2) and A-site composition was varied by the rare-earth ions (Ln = Sm³⁺, Eu³⁺ and Gd³⁺). The entropy of B-sublattice mixing was 1.609R J/(mol*K). The dependences of the lattice parameters, microstructure features, and electrical properties were discussed as function of the A-site rare-earth ions. The correlation of the lattice parameters with the nature of the A-site rare earth ions was demonstrated. Impact of the rare-earth ions in A-site on microstructural parameters was observed. Charge conduction mechanisms were discussed in details for a wide range of temperatures.     

Large piezoelectric coefficient with enhanced thermal stability in Nb5+-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics

Chemical doping is an indispensable tool to tailor the properties of the commercial piezoelectric materials. However, a high piezoelectric coefficient with enhanced thermal stability is rarely achieved by one dopant in some high-performance ferroelectrics, e.g., the recently discovered eco-friendly (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT) ceramics. In order to optimize the piezoelectric property in BCZT system by a simple way, we investigated the doping effect of Fe³⁺, Nb⁵⁺ and Bi³⁺ cations in BCZT ceramics respectively. The results indicate that only Nb⁵⁺-doped BCZT ceramics display a combination of large piezoelectric coefficient and enhanced thermal stability, compared with others. Moreover, the established phase diagrams and in-situ transmission electron microscope (TEM) observations reveal that such optimized piezoelectric properties after Nb⁵⁺ doping originates from (i) the low polarization anisotropy near the ambient tetragonal (T)-orthorhombic (O) phase transition and (ii) the easy domain wall motion of persistent miniaturized ferroelectric domains upon heating.     

The effects of sintering atmospheres on piezoelectric performances of Co-doped Ba0.88Ca0.12Zr0.12Ti0.88O3 ceramics

This work investigates the effects of content ratio Co³⁺/Co²⁺ on piezoelectric properties of lead-free Ba_0.88Ca_0.12Zr_0.12Ti_0.88O₃ (BCZT) ceramics. For this purpose, 0.10 wt% Co ions using a CoO powder doped BCZT ceramics were sintered under various atmospheres (ranging from pure nitrogen to 100 vol% oxygen concentration) to vary the ratio value Co³⁺/Co²⁺. X-ray photoelectron spectroscopy analysis revealed the coexistence of Co³⁺ and Co²⁺ for oxygen concentration below 40 vol% and the single oxidation state of Co³⁺ for oxygen concentration over 50 vol%. Co²⁺ substitution could induce more oxygen vacancy to accelerate densification and grain growth, whereas Co³⁺ substitution usually leads to larger lattice distortion to generate a stable asymmetric structure. When oxygen concentration was 30 vol% and Co³⁺/Co²⁺ near to 1.0, the respective superiority of Co³⁺ and Co²⁺ is brought into full play, and the ceramic showed the highest piezoelectric constant d₃₃^* ∼ 518 pm/V (380 pm/V for undoped BCZT) and the relatively high Curie temperature T_c ∼ 105°C (95°C for undoped BCZT). This study suggests that optimizing sintering atmosphere might be an effective way to enhance the piezoelectric properties for some composition-modified piezoelectric BCZT ceramics.     

B-site-doped BiFeO3-based piezoceramics with enhanced ferro/piezoelectric properties and good temperature stability

Here, B-site doped 0.725BiFe_0.98M_0.02O₃-0.275BaTiO₃ (M = Fe, Sc, Ga, and Al) + 0.8 mol% MnO₂ (abbreviated as BF, BS, BG, and BA) (BFM-BT) ceramics were designed and prepared to modulate octahedral distortions. According to bond-valence calculations based on XRD Rietveld refinement data, B-site doped BFM-BT ceramics tended to have a higher distortion as the radius of the doping ion decreases, and obtained a great improvement of ferroelectric and piezoelectric performances. B-site-doped BFM-BT ceramics significantly inhibited the formation of impurities, leading to better ferroelectric and piezoelectric performances. The BFM-BT ceramics exhibited high Curie temperature of 519-530℃ and good temperature stability for piezoelectric performances. The d₃₃ values of BF, BS, and BA ceramics remained the room temperature value ranging from room temperature to 470℃. Meanwhile the content of impure phases, oxygen vacancies and valence of Fe³⁺ to Fe²⁺ decreased with the decreasing radii of B-site doping ions.     

Chemical Synthesis,Sintering and Piezoelectric Properties of Ba0.85Ca0.15 Zr0.1Ti0.9O3 Lead‐Free Ceramics

The preparation of Ba_0.85Ca_0.15 Zr_0.1Ti_0.9O₃ (BCZT) powders by wet chemical methods has been investigated, and the powders used to explore relationships between the microstructure and piezoelectric properties (d₃₃ coefficient) of sintered BCZT ceramics. Sol–gel synthesis has been shown to be a successful method for the preparation of BCZT nanopowders with a pure tetragonal perovskite phase structure, specific surface area up to 21.8 m²/g and a mean particle size of 48 nm. These powders were suitable for the fabrication of dense BCZT ceramics with fine‐grain microstructures. The ceramics with the highest density of 95% theoretical density (TD) and grain size of 1.3 μm were prepared by uniaxial pressing followed by a two‐step sintering approach which contributed to the refinement of the BCTZ microstructure. A decrease in the grain size to 0.8–0.9 μm was achieved when samples were prepared using cold isostatic pressing. Using various sintering schedules, BCZT ceramics with broad range of grain sizes (0.8–60.5 μm) were prepared. The highest d₃₃ = 410.8 ± 13.2 pC/N was exhibited by ceramics prepared from sol–gel powder sintered at 1425°C, with the relative density of 89.6%TD and grain size of 36 μm.     

Structural,dielectric, vibrational and magnetic properties of Sm doped BiFeO3 multiferroic ceramics prepared by a rapid liquid phase sintering method

Rare earth Sm substituted Bi_1−xSm_xFeO₃ with x=0, 0.025, 0.05, 0.075 and 0.10 polycrystalline ceramics were synthesized by a rapid liquid phase sintering method. The effect of varying composition of Sm substitution on the structural, dielectric, vibrational, optical and magnetic properties of doped BiFeO₃ (BFO) ceramics have been investigated. X-ray diffraction patterns of the synthesized rare earth substituted multiferroic ceramics showed the pure phase formation with distorted rhombohedral structure with space group R3c. Good agreement between the observed and calculated diffraction patterns of Sm doped BFO ceramics in Rietveld refinement analysis of the X-ray diffraction patterns and Raman spectroscopy also confirmed the distorted rhombohedral perovskite structure with R3c symmetry. Dielectric measurements showed improved dielectric properties and magnetoelectric coupling around Néel temperature in all the doped samples. FTIR analysis establishes O–Fe–O and Fe–O stretching vibrations in BiFeO₃ and Sm-doped BiFeO₃. Photoluminescence (PL) spectra showed visible range emissions in modified BiFeO₃ ceramics. The magnetic hysteresis measurements at room temperature and 5 K showed the increase in the magnetization with the increase in doping concentration of Sm which is due to the structural distortion and partial destruction of spin cycloid caused by Sm doping in BFO ceramics.     

Effects of Mn-Addition on the Microstructure and Ferroelectric Properties of High-Temperature CaBi2Nb2O9 Ceramics

High temperature ferroelectric ceramics CaBi₂Nb₂O₉ + x wt% MnCO₃ (CBNO-x) are prepared by conventional sintering method. The microstructures and ferroelectric properties are studied. The Mn-added CBNO-based ceramics have a single Aurivillius phase structure. Nearly isotropic grains are obtained and the grains become larger with the increase of MnCO₃ addition. The Curie temperature T _c decreases slightly with the increase of MnCO₃ addition. The relative dielectric constant, dielectric loss, and the P–E hysteresis loop measurements indicate that Mn ions creates “soft” and “hard” doping effects simultaneously. Superior polarization performance (2P _r of 3.0 μC/cm² and 2E _c of 40.2 kV/cm) is obtained for the CaBi₂Nb₂O₉-0.375 wt% MnCO₃ ceramics.     

Influence of Ag doping on electrical and magnetic properties of La0.625(Ca0.315Sr0.06)MnO3 ceramics

Polycrystalline Ag-doped [La_0.625(Ca_0.315Sr_0.06)MnO₃]_1-x:Ag_x (LCSMO) ceramics with (x = 0, 0.03, 0.05, 0.10, 0.15, and 0.20) were prepared by sol-gel method, and their structures and properties were characterized. X-ray diffraction results indicated that all bulk samples had single phase with orthorhombic phase (space group of Pbnm) without impurities. With the increase of Ag doping content, the resistivity of the samples decreased, while the remanent magnetization and coercive field increased. The metal to insulator transition temperature (T_p), temperature coefficient of resistance (TCR) and Curie temperature (T_c) for x = 0.20 were determined as 300 K, 9.38% (292.6 K) and 291.86 K respectively. The highest MR value of 28.36% (295.03 K) was obtained at x = 0.15. XPS data revealed that substitution between A-site ions and Ag⁺ could increase the ratio of Mn⁴⁺ ion. Double exchange effect (DE) enhanced by changing Mn–O bond distance, Mn–O–Mn bond angle, and increasing Mn⁴⁺ ion concentration. These features promoted the transfer of itinerant electron between Mn³⁺ and Mn⁴⁺ ions. However, the magnetization obtained at x = 0.20 was less than that at x = 0, as diamagnetic Ag released magnetism of the samples. The results suggested that the LCSMO polycrystalline ceramics could be used as a candidate to prepare room temperature infrared detectors, magnetic sensors or magneto-electric devices, and so on.     

Effect of Y doping on transport properties of La0.8Sr0.2MnO3 polycrystalline ceramics

In this study, La_0.8-xY_xSr_0.2MnO₃ (LYSMO) polycrystalline ceramics were prepared by means of sol-gel technique using methanol as solvent. X-ray diffraction (XRD) showed all samples to possess standard perovskite structure. Scanning electron microscopy (SEM) revealed samples with high compactness and grain size from 27.80 to 29.73 μm. Resistivity–temperature tests indicated sharp metal-insulator transition behavior of all samples accompanied by rapid transformation from ferromagnetism to paramagnetism (FM-PM). As Y³⁺ doping amounts rose, radius of A-site ions decreased, metal-insulator transition temperature (T_p) of polycrystalline samples shifted to lower temperatures, and resistivity increased. Temperature coefficient of resistance (TCR) and magnetoresistance (MR) were affected by introduction of Y³⁺. At x = 0.06, peak TCR and peak MR reached 4.85% K⁻¹ and 52.34%, respectively. Using double exchange (DE) interaction mechanism, electric transport performances of as-prepared ceramics were explained. These findings look promising for future applications of LYSMO materials in magnetic devices and infrared detectors.     

Significantly improved dielectric and piezoelectric properties by defects in PbNb2O6-based piezoceramics

PbNb₂O₆ (PN)-based piezoceramics are considered important materials used in high temperatures. Whereas, the main problem of PN-based ceramic is its low electrical performance such as piezoelectric constant (d₃₃), severely restricting its practical applications. Here, based on the principle of regulating structure and performance via defect engineering, a new way of artificially tailoring point defects by A-site nonstoichiometry is implemented in this study. The results show that the PN-based ceramics with A-site deficiency exhibit significantly enhanced dielectric and piezoelectric properties. Particularly, the relative dielectric permittivity (ε_r) increased by 165% (from 197 to 523) and the d₃₃ increased by 177% (from 30 to 83 pC/N) while maintaining a high Curie temperature (T_C ~ 543 °C), and the resultant superior comprehensive performance is nearly highest in PN-based ceramic system. The X-ray photoelectron spectrometer and Raman spectra reveal that A-site deficiency leads to a decreased level of oxygen vacancies and NbO₆ octahedral distortion as well as disorderly motions of A-site cations, which further causes the markedly decreased domain size exhibited in piezoresponse force microscopy (PFM) images. Consequently, the improved domain structure gives rise to improved dielectric and piezoelectric properties. The results of this work provide a simple and effective strategy to improve the dielectric and piezoelectric properties while keeping a high T_C in the PN-based ceramics system, meeting the urgent demands of high-temperature applications.     

Improved magnetic performance of Co-doped La2NiMnO6 ceramics prepared at low temperature

Co-doped La₂NiMnO₆ (La₂Co_xNi_1-xMnO₆, LC_xNMO, x = 0.1-0.5) ceramics were fabricated at relatively low temperature of 700 °C, to obtain double-perovskite with remarkable magnetic performance. It was found that the LC_xNMO ceramics in P2₁/n and R-3 phases could be well densified, with high relative density (>99 %) and fine grain size (0.5−1 μm). An increasement in cell volume was observed with the doping of Co, and the valence state changed from (Mn³⁺ + Ni³⁺/Co³⁺) to (Mn⁴⁺ + Ni²⁺/Co²⁺), revealing the improvement of B-site cations ordering. Moreover, the smaller grains resulted from the lower sintering temperature also decreased the level of antisite disordering. Consequently, the value of M_S/M_theo was significantly improved from 75.49% to 88.36 % by optimizing Co-doping concentration while the Curie temperature was still rather high (>250 K). The LC_0.5NMO ceramic was found to have the maximum M_S (4.86 μ_B/f.u.), attributing to the large B-site ordering because of higher Mn⁴⁺ and Ni²⁺/Co²⁺ content.