Tunable electrocaloric and energy storage behavior in the Ce,Mn hybrid doped BaTiO3 ceramics

The electrocaloric effect and energy storage property are tuned in the Ba_1-xCe_xTi_0.99Mn_0.01O₃ ceramics prepared by the solid state reaction method. The ceramics with lower Ce content (x?=?0.005, 0.015) show a better ΔT and ΔT/ΔE response. The ceramics with higher Ce content (x?=?0.030, 0.040, 0.045) represent the broader ΔT peaks (50?K–60?K), and the higher energy storage density and efficiency. The largest electrocaloric response (ΔT_max?=?1.22?K, ΔT/ΔE?=?0.41?K mm/kV) is found in the Ba_0.995Ce_0.005Ti_0.99Mn_0.01O₃ ceramics, which is comparable or even higher than that of the most isovalent substituting BaTiO₃-based ceramics reported before. The maximum energy storage density 0.11?J/cm³ (E?=?30?kV/cm) is obtained for the Ba_0.970Ce_0.030Ti_0.99Mn_0.01O₃ ceramics, with high efficiency of 65–88% over a wide temperature range of 72?K. This work may open more opportunities to design high electrocalaric and energy storage performance lead-free systems from the viewpoint of the heterovalent and size mismatch substitution.     

Nonstoichiometry and Li‐ion transport in lithium zirconate: The role of oxygen vacancies

Understanding Li‐ion migration mechanisms and enhancing Li‐ion transport in Li₂ZrO₃ (LZO) is important to its role as solid absorbent for reversible CO₂ capture at elevated temperatures, as ceramic breeder in nuclear reactors, and as electrode coating in high‐voltage lithium‐ion batteries (LIBs). Although defect engineering is an effective way to tune the properties of ceramics, the defect structure of LZO is largely unknown. This study reports the defect structure and electrical properties of undoped LZO and a series of cation‐doped LZOs: (i) depending on their charge states, cation dopants can control the oxygen vacancy concentration in doped LZOs; (ii) the doped LZOs with higher oxygen vacancy concentrations exhibit better Li⁺ conductivity, and consequently faster high‐temperature CO₂ absorption. In fact, the Fe (II)‐doped LZO shows the highest Li‐ion conductivity reported for LZOs, reaching 3.3 mS/cm at ~300°C that is more than 1 order of magnitude higher than that of the undoped LZO.     

Enhanced electrocaloric response and energy storage performance of Li-substituted BaTiO3 ceramics

Recently, ferroelectric and antiferroelectric ceramic materials have gained a lot of interest for the development of environment-friendly highly-efficient electrocaloric refrigeration and energy-storage devices. In this work, lead-free Ba_1−xLi_xTiO₃ ceramics with x = 0, 0.01, 0.02, 0.03, 0.04, and 0.05 were synthesized by the conventional solid-state reaction method, and the effect of Li doping on dielectric, leakage current, ferroelectric, electrocaloric, and energy storage properties of BaTiO₃ ceramics was systematically investigated. The XRD and Raman studies confirmed that the structure of Ba_1−xLi_xTiO₃ remains tetragonal as for BaTiO₃. The Li substitution shifted the phase transition (T_C) of BaTiO₃ slightly towards the lower temperature side. Significant drop in leakage current was observed with an addition of Li content. The maximum values of the electrocaloric effect (ΔT), electrocaloric responsivity, and coefficient of performance were found to be 1.44 K, 0.24 × 10⁻⁶ K m/V, and 5.75, respectively, for x = 0.04 at an applied field of 60 kV/cm near the Curie temperature. The maximal value of energy storage density was found to be 0.42 J/cm³ with an energy storage efficiency of 60% for x = 0.05. Our results suggested that lead-free Ba_1−xLi_xTiO₃ ceramic material is a promising candidate for potential applications in solid-state refrigeration technology and high-efficiency energy storage devices.     

Reduction of Ti4+ to Ti3+ in Boron‐Doped BaTiO3 at Very Low Temperature

Reduction of Ti⁴⁺ to Ti³⁺ was found in boron‐doped BaTiO₃ ceramics when we sintered the samples at very low temperature (>850°C) in 5%H₂/Ar. Such reduction did not occur in pristine BaTiO₃ ceramic. The methods such as UV–vis spectroscopy, luminescence spectroscopy, and X‐ray photoelectron spectroscopy confirmed the reduction by showing the presence of Ti³⁺. The results of Ti–K‐edge X‐ray absorption near‐edge structure measurement (XANES) indicated that boron doping changed the geometry of Ti‐oxygen in BaTiO₃ to some extent. It was likely that some boron ions stayed at interstitial sites of BaTiO₃ lattice and acted as donors, which might trigger the reduction. The reduced boron‐doped BaTiO₃ were semiconducting and had very low room‐temperature resistivity (<100 Ω m). However, different from the n‐type rare‐earth‐doped BaTiO₃ ceramics, they did not display positive temperature coefficient resistance (PTCR) behavior.     

High‐Q microwave ceramics of Li2TiO3 co‐doped with magnesium and niobium

In order to solve the problems of acceptor/donor individual doping in Li₂TiO₃ system and clarify the superiority mechanism of co‐doping for improving the Q value, Mg + Nb co‐doped Li₂TiO₃ have been designed and sintered at a medium temperature of 1260°C. The effects of each Mg/Nb ion on structure, morphology, grain‐boundary resistance and microwave dielectric properties are investigated. The substitution of (Mg_1/3Nb_2/3)⁴⁺ inhibits not only the diffusion of Li⁺ and reduction in Ti⁴⁺, but also the formation of microcracks in ceramics, which promotes the enhancement of Q value. The experiments reveal that Q × f value of Li₂TiO₃ ceramics co‐doped with magnesium and niobium is 113 774 GHz (at 8.573 GHz), which is increased by 113% compared with the pure Li₂TiO₃ ceramics. And the co‐doped ceramics have an appropriate dielectric constant of 19.01 and a near‐zero resonance frequency temperature coefficient of 13.38 ppm/°C. These results offer a scientific basis for co‐doping in Li₂TiO₃ system, and the outstanding performance of (Mg + Nb) co‐doped ceramics provides a solid foundation for widespread applications of microwave substrates, resonators, filters and patch antennas in modern wireless communication equipments.     

Preparation and structural characterization of Fe-doped BaTiO3 diluted magnetic ceramics

Fe-doped BaTiO₃ ceramics produced from powders prepared by solid state reaction at various processing parameters (temperature, time) were investigated and discussed. X-ray diffraction analysis confirmed the existence of the tetragonal phase in BaTi_1−xFe_xO_3−x/2 (x=0.005) ceramics. The microstructural characterization reveals that a low calcination temperature leads to a higher densification of the bulk samples at around 96% relative density with average grain sizes ~6 µm. The oxidation state of the metal ions in the sintered samples analysed by means of X-ray photoelectron spectroscopy showed the presence of Ti⁴⁺2p_3/2 and Ti⁴⁺2p_1/2 peaks in Fe-doped BaTiO₃ and the lack of Ti³⁺ ions, which confirms the compositional quality of these ceramics. The electron paramagnetic resonance measurements proved the incorporation of Fe ions as Fe³⁺ in the BaTiO₃ lattice.     

A new series of low-temperature cofirable Li3Ba2La3(1-x)Y3x(MoO4)8 microwave dielectric ceramics

A new series of Li₃Ba₂La_3(1-x)Y_3x(MoO₄)₈ microwave dielectric ceramics were prepared by a conventional solid-state reaction method. The Rietveld refinement results further confirm that Li₃Ba₂La_3(1-x)Y_3x(MoO₄)₈ belongs to a monoclinic system with space group C2/c. Scanning electron microscopy results reveal that Li₃Ba₂La_3(1-x)Y_3x(MoO₄)₈ ceramics can be well sintered at a low sintering temperature. In addition, the permittivity (ε_r) of Li₃Ba₂La_3(1-x)Y_3x(MoO₄)₈ ceramics was found to decrease gradually with increasing substitution content of Y³⁺, while the quality factor (Qxf) and temperature coefficient of resonant frequency (τ_f) increase with x monotonously. The x?=?0.4 ceramic sintered at 700?°C for 4?h possesses optimum microwave dielectric properties of ε_r ～ 14.4, Qxf ～ 14,994?GHz (at 9.0?GHz) and τ_f ～?+?6.9?ppm/°C. Particularly, no chemical reaction between the matrix phase and Ag metal suggests that the Li₃Ba₂La_1.8Y_1.2(MoO₄)₈ ceramic might be a promising candidate for low-temperature co-fired ceramic applications.     

Processing,microstructure and piezoelectric properties of Li-doped BCZT ceramics

Li₂CO₃ is a promising additive to reduce the sintering temperature for (Ba_0.85Ca_0.15) (Zr_0.1Ti_0.9)O₃ (BCZT) ceramics, however, the solubility of Li₂CO₃ in water and the high volatility of Li₂O at elevated temperatures make the processing and densification of BCZT-Li₂CO₃ ceramics (known as BCZT-L) challenging. In our work, an optimized processing route was developed to obtain dense and flat BCZT-L ceramics made with 0–10 mol% of Li₂CO₃ and involving sintering at 1300 °C–1400 °C. The chemical and structural evolution of BCZT-L ceramics during sintering with and without a BCZT powder bed are comprehensively documented and the distribution of Li in the matrix has been observed through TOF-SIMS to explain the effects of Li doping on the piezoelectric properties. The d₃₃ and k_p of BCZT-L initially increased with Li content, but then decreased with excess Li. The decreased d₃₃ and k_p with excess Li is associated with Li aggregation in the BCZT matrix.     

Relationship between electrochemical behavior and Li/vacancy arrangement in ramsdellite type Li2+xTi3O7

The changes of Li⁺/vacancy arrangement in Li_2+xTi₃O₇ with a ramsdellite-type structure upon topo-electrochemical Li⁺ insertion were investigated by the entropy measurement of reaction combined with the Monte Carlo simulation. The experimental entropy measurement was conducted by potentiometric and calorimetrical methods. The obtained experimental data were in good accordance with simulated results.The results indicated that the ordered Li⁺/vacancy arrangement appeared at the compositions of x ∼ 0.45 and ∼1.20, where the observed entropy of reaction humped. The ordering of Li/vacancy were also indicated at the composition x ∼ 0.24 and 1.16 in Li_2+xTi₃O₇ by the Monte Carlo simulation which considers the most stable Li/vacancy arrangement in terms of Coulombic interaction. This substantial agreement between electrochemical behaviors and computational results confirmed that the formation of superstructure arising from Li/vacancy arrangement during the electrochemical reaction deeply related to the atomic level Coulombic interactions.     

Structural and Magnetoelectric Behavior of Lanthanum Manganite‐Based Multiferroic Heterostructures

The effect of fabrication method on the structure of (100 ? x) wt% BaTiO₃ + x wt% La_0.7Ba_0.3MnO₃ (BT + BLM) and (100 ? x) wt% Na, Bi, Sr‐doped PZT + x wt% La_0.65Pb_0.35MnO₃ (PZTNB‐1 + PLM) magnetoelectric ceramics was studied. Profound interdiffusion of two interacting phases occurs in nearly all cases. The BT + BLM and PZTNB‐1 + PLM ceramics exhibit low piezoelectric parameters even with small manganite contents (10–20 wt%). The increased content of the magnetostrictive phase complicates the polarization process due to the high conductivity of La_0.7Ba_0.3MnO₃ and La_0.65Pb_0.35MnO₃. Doping of BaTiO₃ and PZTNB‐1 with small additions of manganite components affects piezoelectric properties, thereby lowering efficiency of the resulting material.     

Semiconductivity in Acceptor‐Doped BaTi1−xHoxO3−x/2−δ/2

Acceptor‐doped BaTiO₃ powders of formula: BaTi_1?xHo_xO_3?x/2?δ/2: x = 0.0001, 0.001, 0.01, 0.03, and 0.07, were prepared by sol‐gel synthesis, fired at 800°C–1500°C and either quenched or slow‐cooled to room temperature. Electrical properties of ceramics depended on firing conditions, Ho content, and cooling rate. Pellets of all x values fired at 800°C–1000°C were insulating and, from the presence of OH bands in the IR spectra, charge balance appeared to involve co‐doping of Ho³⁺ and H⁺ ions without necessity for oxygen vacancy creation. At higher firing temperatures, OH bands were absent. Pellets fired at 1400°C in air and slow cooled were insulating for both low x (0.0001) and high x (0.07) but at intermediate x (0.001 and 0.01) passed through a resistivity minimum of 20–30 Ω cm at room temperature, attributed to the presence of Ti³⁺ ions; it is suggested that, for these dilute Ho contents, each oxygen vacancy is charge compensated by one Ho³⁺ and one Ti³⁺ ion. At higher x, charge compensation is by Ho³⁺ ions and samples are insulating. A second, more general mechanism to generate Ti³⁺ ions, and a modest level of semiconductivity, involves reversible oxygen loss at high temperatures.     

Phase Transition and Large Electrostrain in Lead‐Free Li‐Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

Large piezoelectric effect is achieved in Li‐doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃(BCTZ) ceramics by use of tuning the phase boundaries. Rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) multiphase coexistence is constructed in the ceramics by changing Li contents. The high piezoelectric constant d₃₃ (493 pC/N) and large electrostrain (dS_max/dE_max = 931 pm/V) have been observed in the Li‐doped (Ba, Ca)(Ti, Zr)O₃ ceramics at low sintering temperature (1350°C/2 h). The significant enhancement in materials properties is ascribed to the multiphase region around room temperature induced by Li‐doped effect.     

Role of A-site (Sr), B-site (Y), and A,B sites (Sr,Y) substitution in lead-free BaTiO3 ceramic compounds: Structural,optical, microstructure,mechanical, and thermal conductivity properties

Strontium and Yttrium-doped and co-doped BaTiO₃ (BT) ceramics with the stoichiometric formulas BaTiO₃, B_1-xSr_xTiO₃, Ba_1-xY_xTiO₃, BaTi_1-xY_xO₃, Ba_1-xY_xTi_1-xY_xO₃, and Ba_1-xSr_xTi_1-xY_xO₃ (x = 0.075) noted as BT, BSrT, BYT, BTY, BYTY, and BSrTY have been synthesized through sol-gel method. X-ray diffraction (XRD) patterns of the prepared ceramics, calcined at a slightly low temperature (950 °C/3h), displayed that BT, BSrT, and BYT ceramics possess tetragonal structures and BTY, BYTY, and BSrTY have a cubic structure. The incorporation of the Ba and/or Ti sites by Sr²⁺ and Y³⁺ ions in the lattice of BaTiO₃ ceramic and the behaviors of the crystalline characteristics in terms of the Y and Sr dopant were described in detail. The scanning electron microscopy (SEM) images demonstrated that the densification and grain size were strongly related to Sr and Y elements. UV–visible spectroscopy was used to study the optical behavior of the as-prepared ceramic samples and revealed that Sr and Y dopants reduce the optical band gap energy to 2.74 eV for the BSrTY compound. The outcomes also demonstrated that the levels of Urbach energy are indicative of the created disorder following the inclusion of Yttrium. The measurements of the thermal conductivity indicated the influence of the doping mechanism on the thermal conductivity results of the synthesized samples. Indeed, the thermal conductivity of BaTiO₃ is decreased with Sr and Y dopants and found to be in the range of 085–2.23 W.m^-1. K^?1 at room temperature and decreases slightly with increasing temperature from 2.02 to 0.73-W.m^-1. K^?1. Moreover, the microstructure and grains distribution of the BT, BSrT, BYT, BTY, BYTY, and BSrTY samples impacted the compressive strength, hence; the compressive strength was minimized as the grain size decreased.     

Pulse energy-storage performance and temperature stability of Bi2O3-added BaTiO3 based ceramics

The temperature stability and temperature stability range of barium titanate-based pulse energy-storage ceramics were modified by Bi₂O₃ tailoring in (Ba_0.98-xLi_0.02Bi_x) (Mg_0·04Ti_0.96)O₃ (x = 0, 0.025, 0.05, 0.075, 0.1) and (Ba_1.03-1.5xLi_0.02Bi_x) (Mg_0·04Ti_0.96)O₃ (x = 0.125, 0.15, 0.2, 0.25) ceramics. Excellent pulse energy-storage performances of ceramic films are achieved via the new dual priority strategy of establishing cationic vacancies and forming a liquid phase. The dielectric constant plateau appears due to the cubic phase and space charges. Outstanding temperature stability, frequency stability and antifatigue performance are obtained in the ceramics, and their variations are all less than 15%. The comprehensive energy-storage properties with dual priority parameters of energy-storage density and efficiency of 3.13 J/cm³ and 91.71%, accompanied by an excellent pulse discharge energy density of 2.48 J/cm³, current density of 1313.23 A/cm² and power density of 195.26 MW/cm³ are gained at x = 0.1. The perfect pulse energy-storage performances as well as ultrahigh stability are correlated with synergistic effects of multiphase coexistence, cubic phase, liquid-phase sintering, grain size, ceramic resistance, space charges and polar nanoregions. The comprehensive parameters indicate that the (Ba_0·88Li_0·02Bi_0.1) (Mg_0·04Ti_0.96)O₃ ceramics have potential application in high precision fields.     

Structure,electrical and dielectric properties of Ca substituted BaTiO3 ceramics

BaTi_1-xCa_xO_3-x [BTC100x] ceramics were synthesized via solid-state reaction method. Effect of Ca substitution on the structure, electrical and dielectric properties of BTC100x ceramics was systematically investigated. Calcined BTC100x powders were in tetragonal phase when x?≤?0.01, whereas transformed to cubic at x?>?0.01. Additionally, the diffraction peak (200) shifted to lower angles with increasing x, indicating increased unit cell volume. Meanwhile, Ba_0.97Ca_0.03TiO₃ [BC3T] ceramic was prepared and studied, to compare with BaTi_0.97Ca_0.03O_2.97 (BTC3). It was found that pure BaTiO₃ [BT] and BC3T ceramics had the similar structural and dielectric properties, whereas BTC3 ceramic showed much difference，XRD patterns, Raman spectrum, impedance spectra and dielectric-temperature spectra provided strong evidence of Ca²⁺ substitution at Ti site in BT lattice. Finally, BTC100x ceramics were produced and dielectric properties were investigated. With increasing x, the Curie temperature decreased from 128?°C (BT) to 42?°C (BTC5).     

Defects characterization of Dy-doped BaTiO3 ceramics via electron paramagnetic resonance

The point defects and the structural and dielectric properties of Dy-doped BaTiO₃ ceramics prepared at 1400 °C were investigated. The solubility of Dy in the self-compensation mode was determined to be x = 0.07 for (Ba_1−xDy_x)(Ti_1−xDy_x)O₃, and no EPR signals associated with the Dy³⁺ Kramers ion or the Ba and Ti vacancies were detected using the electron paramagnetic resonance (EPR) technique. As x increases, the dielectric behavior changed from a first-order phase transition to a diffuse phase transition to a Y7R dielectric-temperature stability. A strong EPR signal at g = 1.974, which is rare among rare-earth-doped BaTiO₃ ceramics appeared unexpectedly in the single-phase (Ba_1−xDy_x)Ti_1−x/4O₃ ceramics with deliberately designed Ti vacancies. This signal was attributed to ionized Ba vacancy defects. A preference for the self-compensation mode of Dy³⁺ ions is responsible for the appearance of Ba vacancies. The real formula of the nominal (Ba_1−xDy_x)Ti_1−x/4O₃ is expressed as (Ba_1−xDy_3x/4)(Ti_1−x/4Dy_x/4)O₃. In addition, the defect chemistry is discussed.     

The electrical properties of (Ba,Ca, Ce,Ti, Zr)O3 thermistor for wide-temperature sensor architecture

A series of new negative temperature coefficient (NTC) thermal materials based on (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ (0.00 ≤ x ≤ 0.20) ceramics were synthesized by a solid-state method. X-ray diffraction, scanning electron microscope and X-ray photoelectron spectroscopy were used to demonstrate the crystal structure, morphology, and composition of the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics, which were composed of solid solution based on the BaTiO₃ phase. The average grain size of doped ceramic samples experienced the process of first decreasing and then increasing. The doping of Ce has reduced the sintering temperature. The temperature-dependent resistance analysis revealed that with the change of doping amount x, the thermal constant B_300/1200 (1.21 × 10⁴–1.13 × 10⁴ K) and the activation energy E_a300/1200 (0.9777–1.0471eV) was initially increased to maximum values at x = 0.05, followed by the decreasing when x > 0.05. It has been established that the concentration of oxygen vacancies is affected by the transition between Ce⁴⁺ and Ce³⁺ provided by high levels of Ce doping. (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics exhibited excellent negative temperature characteristics in the range of 300–1200 °C. Moreover, the temperature resistance linearity was improved after samples were aged. Hence, the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics were regarded as a promising material for high-temperature NTC thermistors in a wide temperature range.     

Origin of low dielectric loss and giant dielectric response in (Nb+Al) co‐doped strontium titanate

(Nb+Al) co‐doped SrTiO₃ ceramics with a nominal composition of Sr(Nb_0.5Al_0.5)_xTi_1‐xO₃ (x = 0, 0.02, 0.04, and 0.06) were fabricated using the conventional solid‐state reaction method; giant permittivity (10500) and low dielectric loss (0.03) were obtained at x = 0.06. Dielectric and impedance spectroscopy, X‐ray photoelectron spectroscopy, and Raman spectroscopy, were employed to study why the dielectric property improved. The results indicate that the giant dielectric response occurs because of the combined effects of the off‐center Ti³⁺ reorientation and conduction of electrons with the polar ordering structure Ti³⁺/Ti⁴⁺. In contrast, the low dielectric loss can be attributed to electron localization that occurs because of the defect dipole . These fundamental understandings will benefit the design of doped SrTiO₃ ceramics with desired performance.     

[Bi0.5(Na0.4-xLixK0.1)]0.96Sr0.04Ti0.975Ta0.025O3 lead-free RELAXOR ceramics with the enhanced recoverable energy density

The high recoverable energy density (W_rec) of 1.83 J/cm³ was achieved at 120 kV/cm by simply introducing 0.050 mol. % Li⁺ ions into the A-site of Bi_0.5(Na_0.4K_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ lead-free relaxor ceramics. Bi_0.5(Na_0.4-xLi_xK_0.1)]_0.96Sr_0.04Ti_0.975Ta_0.025O₃ (BNKSTT-100xLi) ceramic samples were prepared by solid-state reaction method. The influence of Li⁺ ions on structure and electrical properties was analyzed in detail. XRD patterns and Raman spectra of these samples were revealed a pure perovskite-type structure. BNKSTT-100xLi ceramics exhibited an obvious relaxor characteristic with the downshift of T_F-R to ambient temperature. J - E loops confirmed the relaxor ferroelectric like nature. Leakage current density was proved to decrease with the increasing Li⁺ ions concentration. At the composition x = 0.050, remnant polarization (P_r) kept steady while maximum polarization (P_max) and breakdown strength (DBS) increased obviously, which was beneficial to the enhancement of recoverable energy density.     

Ultra‐Broad Temperature Stability Obtained with Ce‐Doped BaTiO3‐Based Ceramics

Ce‐doped BaTiO₃‐based ceramics were prepared and studied to satisfy ultra‐broad temperature stability (from ?55°C to 300°C, capacitance variation rate based on C_20°C is within ±15%). The sample with 0.6 mol% CeO₂ succeeds to achieve this performance with a remarkably high ceiling temperature of 300°C. Meanwhile, the sample has good dielectric and electrical properties at room temperature (ε_r = 1667, tanδ = 1.478%, ρ_V = 5.9 × 10¹² Ω·cm). Ce ion can substitute for Ti ion as Ce⁴⁺ or Ba ion as Ce³⁺. The substitution decreases the spontaneous polarization of BaTiO₃, and then weakens the ferroelectricity of BaTiO₃. As a result, the temperature stability of samples is improved obviously. Besides, CeO₂ addition promotes the formation of exaggerated grains, which are consisting of Ba₆Ti₁₇O₄₀.