Voltage‐Dependent Bulk Resistivity of SrTiO3:Mg Ceramics

Single phase ceramics of composition Sr(Ti_1–xMg_x)O_3–x: 0 ≤ x ≤ 0.01 were prepared by sol–gel synthesis and characterized by X‐ray diffraction, scanning electron microscopy, impedance spectroscopy, and current–voltage measurements. The bulk and grain‐boundary conductivities increase on application of a small dc bias voltage in the range 3–200 V/cm and at temperatures in the range 150°C–800°C. A qualitatively similar increase in conductivity occurs on increasing in the surrounding atmosphere, which shows that conduction is p type. The conductivity increase is reversible on removal of the dc bias or on reducing and is not observed in undoped SrTiO₃. It is an intrinsic property of the bulk material, differs from the voltage‐dependent effects observed with varistors and is attributed to changes in redox equilibria between oxygen species at the surface which cause changes in carrier concentration in the interior. A capacitive model of this low‐field dc bias effect is presented and compared with a memristive model of high field resistance degradation.     

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.     

Oxygen permeation characteristics of sol‐gel derived barium‐substituted strontium ferrite membranes

The pervoskite‐type oxides have received attention due to their potential applications in catalysis, solid oxide fuel cells, gas sensors, and gas separable membranes. In view of their importance in oxygen separation from air, Ba_xSr_1?xFeO_3?δ (0≤x≤1.0) samples have been synthesized by sol‐gel process and investigated with regard to phase(s), oxygen permeation, and electrical conductivity. These compounds possess at room temperature, a perovskite‐type cubic, mixture of rhombohedral and hexagonal, and hexagonal phase(s) depending upon the composition 0≤x≤0.94, x=0.96‐0.98, and x=1.0, respectively. The barium incorporation causes initially enhancement but decrease in electrical conductivity above x=0.94. Above 800°C, all the compositions exhibit a stable cubic phase. The compacts made in the form of discs serve as stable oxygen permeable membranes displaying flux density () of ~2.45‐3.58 mL/cm².min at 1000°C. A good correlation has been demonstrated between the oxygen permeation and the electrical conductivity data. The maximum values of and conductivity correspond to Ba_xSr_1?xFeO_3?δ (x=0.94) with a perovskite‐type cubic structure. Hence, this membrane is quite suitable for oxygen separation technology.     

Oxygen Nonstoichiometry and Thermodynamic Quantities of La2Ni0.95Al0.05O4.025 + δ

The oxygen nonstoichiometry of La₂Ni_0.95Al_0.05O_4.025 + δ (LNAO) is measured as a function of oxygen partial pressure (pO₂) and temperature by coulometric titration method and thermogravimetric analysis (TGA) in 800°C–1000°C temperature range and 10^?16‐1 atm pO₂ range. The partial molar quantities for mixing of oxygen were calculated and results were compared with the literature results on La₂NiO_4 + δ (LNO). The variation in activity coefficient of holes versus oxygen nonstoichiometry illustrated an early positive deviation of the activity coefficient of holes from unity, leading to  ≈  7 at δ  ≈  0.08, which was lower than the literature value of  ≈  14 for La₂NiO_{4  +  δ} at δ  ≈  0.08, indicating lesser deviation of LNAO from ideal solution behavior. The effective mass of holes () was 1.02–1.21 times the rest mass (m_o), which indicated the band‐like conduction and allowed the effect of the small degree of polaron hopping to be ignored. The comparison of oxygen nonstoichiometry and partial molar quantities showed that incorporation of interstitial oxygen is less favorable in LNAO in comparison to LNO.     

The Lowered Dielectric Loss and Grain‐Boundary Effects in La‐doped Y2/3Cu3Ti4O12 Ceramics

The effects of La concentration on the electrical conductivity and electric modulus of Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics (0.00 ≦ x ≦ 0.20) were investigated in detail. Proper amount of La substitution in Y_2/3?xLa_xCu₃Ti₄O₁₂ ceramics made the dielectric loss decreased. When x = 0.10, Y_2/3?0.10La_0.10Cu₃Ti₄O₁₂ ceramics exhibited the highest grain‐boundary resistance (0.893 MΩ) and the lowest dielectric loss (about 0.025 at 1 kHz), meanwhile the samples exhibited a relatively high dielectric constant above 6000 over a wide frequency range from 40 Hz to 1 MHz. The decreased dielectric loss was attributed to the enhanced grain‐boundary resistance. With the increase in La concentration, the dielectric relaxation behaviors correlated with the grain‐boundary effects were significantly enhanced. By La doping, the activation energies for the conduction in grain boundaries were slightly depressed, and the activation energies for the relaxation process in grain boundaries were slightly changed. Based on the activation values, it can be concluded that the doubly ionized oxygen vacancies had substantial contribution to the conduction and relaxation behaviors in grain boundaries.     

Dielectric Relaxation in Zr‐Doped SrTiO3 Ceramics Sintered in N2 with Giant Permittivity and Low Dielectric Loss

SrTiZr_xO₃ (x = 0, 0.002, 0.006, 0.01, and 0.014) ceramics with a weak temperature‐dependent giant permittivity (>10⁴) and a very low dielectric loss (<0.01) were fabricated using the conventional solid‐state reaction method by sintering them in N₂ at 1500°C. With increasing Zr content, the permittivity decreased from approximately 48 000 to 18 000 and the dielectric loss decreased from approximately 0.005 to 0.003. According to the XRD, XPS, and ac conductivity analysis, the dielectric properties of pure SrTiO₃ ceramics sintered in N₂ were due to the existence of the giant defect dipoles generated by the fully ionized oxygen vacancies and Ti³⁺ ions, while the dielectric properties of SrTiZr_xO₃ (x > 0) ceramics were also influenced by the defect dipoles (). The giant permittivity and low dielectric loss phenomenon could be explained by giant defect dipoles related to oxygen vacancies.     

Composition‐induced structural transitions and enhanced strain response in nonstoichiometric NBT‐based ceramics

In this work, the nonstoichiometric 0.99Bi_0.505(Na_0.8K_0.2)_0.5‐xTiO₃‐0.01SrTiO₃ (BNKST(0.5‐x)) ceramics with x=0‐0.03 were synthesized by conventional solid‐state reaction method. The composition‐induced structural transitions were investigated by Raman spectra, dielectric analyses, and electrical measurements. It is found that the relaxor phase can be induced through the modulation of the (Na, K) content. The (Na, K) deficiency in BNKST(0.5‐x) ceramics favors a more disordered local structure and can result in the loss of long‐range ferroelectricity. The x=0.015 critical composition possesses relatively high positive strain S_pos of 0.42% and large signal piezoelectric constant d₃₃* of 479 pm V^?1 at 6 kV mm^?1, along with the good temperature (25‐120°C) and frequency (1‐20 Hz) stability. The recoverable large strain responses in nonstoichiometric ceramics can be attributed to the reversible relaxor‐ferroelectric phase transition, which is closely related to the complex defects (, , and ) and the local random fields. This work may be helpful for the exploration of high‐performance NBT‐based lead‐free materials by means of A‐site compositional modification.     

Stability and Decomposition of Ca‐Substituted Lanthanum Ferrite in Reducing Atmospheres

Calcium‐substituted lanthanum ferrites (La_1?xCa_xFeO_3?δ x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized in air and subsequently decomposed in reducing atmospheres. The partial pressure of oxygen () was controlled by varying the H₂/H₂O ratio by bubbling hydrogen/argon mixtures through water baths at controlled temperatures. Three regions of mass loss were identified as the was reduced, two of which were determined to be associated with decomposition reactions. Calcium was shown to decrease the thermal stability of the perovskite compound, but rather than incrementally increasing the required for decomposition proportional to calcium concentration, all samples partially decomposed at a single . The extent of the partial decomposition was dependent on the amount of calcium substitution and temperature. The perovskite phase remaining after the partial decomposition was found to fully decompose at the same oxygen partial pressure as pure lanthanum ferrite.     

Phase Equilibria in the ZrO2–MgO–MnOx System

Phase equilibria were experimentally investigated in the MgO–MnO_x and the ZrO₂–MgO–MnO_x systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnO_x system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO₂–MgO–MnO_x phase diagram were constructed for air conditions ( = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for = 10^?4 bar. The β‐spinel and halite phases of the MgO–MnO_x system were found to dissolve up to 2 and 5 mol% ZrO₂. A continuous c‐ZrO₂ solid solution forms between the boundary ZrO₂–MnO_x and ZrO₂–MgO systems. It stabilizes in the ZrO₂–MgO–MnO_x system down to at least 1613 K in air and down to 1506 K at = 10^?4 bar.     

Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba0.6Sr0.4TiO3 ceramics

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba_0.6Sr_0.4TiO₃ (0.6‐BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., and . The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, T_m, because T_m was close to room temperature for the 0.6‐BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A‐site vacancy loading. The negatively charged and were mainly compensated by oxygen vacancies and likely partly compensated by holes, h^?, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.     

Multi‐Site and Multi‐Ionization of Sn in the Doping of BaTiO3

This article considers the diverse substitutional effects of the Sn cations in the BaTiO₃ lattice and its impact on the electrical conduction as a function of A/B stoichiometry, oxygen partial pressure, and temperature. High‐density specimens were fabricated in the different oxygen partial pressures to control the valence state of Sn ion. Specifically, the nonstoichiometric materials were sintered in a low pO₂ atmosphere (10^?14 atm at 1320°C) and in a high pO₂ atmosphere (10^?0.21 atm at 1320°C), respectively. It is found that Sn occupying the Ti‐site acts as an acceptor dopant, and the electronic conductivity varies from a n‐type to p‐type transition, with increasing oxygen activity as mostly expected. However, there is an unusual case noted with Sn doping the A‐site where the conductivity, σ, is invariant at high pO₂'s, i.e., σ ~  with m ≈ 0 in the high pO₂ regime. The variation of the conductivity is explained by a valence changing of Sn ion from +2 to +3 to +4 with increasing oxygen partial pressure, and we model this data across all conditions within a self‐consistent defect chemistry model.     

Electrical Properties and Relaxor Phase Evolution of Li‐Modified BNT‐BKT‐BT Lead‐Free Ceramics

By conventional ceramics sintering technique, the lead‐free 0.85Bi_0.5Na_0.5(1?x)Li_0.5xTiO₃‐0.11Bi_0.5K_0.5TiO₃‐0.04BaTiO₃ (x =0–0.15) piezoelectric ceramics were obtained and the effects of Li dopant on the piezoelectric, dielectric, and ferroelectric properties were studied. With increasing Li addition, the temperature‐dependent permittivity exhibited the normal ferroelectric‐to‐ergodic relaxor (FE‐to‐ER) transition temperature (T_FE‐ER, abbreviated as T_F‐R) decreasing down to room temperature. The increasing Li content also enhanced the diffuseness of the FE‐to‐ER transition behavior. For composition with x = 0.15, a large unipolar strain of 0.37% ( = S_max/E_max = 570 pm/V) was achieved under 6.5 kV/mm applied electric field at room temperature. Both unipolar and bipolar strain curves related to the temperature closely, and when the temperature reached the T_F‐R, the normalized strain achieved a maximum value (e.g., for x = 0.10, = 755 pm/V) owing to the electric‐field‐induced ER‐to‐FE state transition.     

Sr(Ga0.5Nb0.5)1−xTixO3 Low‐Loss Microwave Dielectric Ceramics with Medium Dielectric Constant

The microwave dielectric properties of Sr(Ga_0.5Nb_0.5)_1?xTi_xO₃ (x = 0, 0.1, 0.2 and 0.3) ceramics have been investigated together with their microstructures. Single‐phase solid solutions are achieved in this series of ceramics. The ordering features are comprehensively analyzed by transmission electron microscopy and Raman spectroscopy. Local 1:1 ordering in B‐site leads to a double‐cubic structure with space group , while Ti substitution disrupts this 1:1 ordering between Ga and Nb, and the metastable ordering between Ti and (Ga + Nb) is speculated to form due to their large size difference. The dielectric constant and temperature coefficient of resonant frequency increase nonlinearly as x increases, while the Qf value decreases gradually. The variation trend of Qf value is mainly attributed to the intrinsic loss because of the increasing vibrational anharmonicity by Ti substitution. The ordering transition from short coherence, long‐range ordering to short‐range ordering with increasing Ti content has an agreeable and weak effect on the Qf value. The best combination of microwave dielectric properties is achieved for the composition of x = 0.3: ε_r = 46.6, Qf = 42 200 GHz and τ_f = 5.0 ppm/°C.     

Defect‐driven evolution of piezoelectric and ferroelectric properties in CuSb2O6‐doped K0.5Na0.5NbO3 lead‐free ceramics

Defect greatly affects the microscopic structure and electrical properties of perovskite piezoelectric ceramics, but the microscopic mechanism of defect‐driven macroscopic properties in the materials is not still completely comprehended. In this work, K_0.5Na_0.5NbO₃+x mol CuSb₂O₆ lead‐free piezoelectric ceramics were fabricated by a solid‐state reaction method and the defect‐driven evolution of piezoelectric and ferroelectric properties was studied. The addition of CuSb₂O₆ induces the formation of dimeric (DC1) and trimeric (DC2) defect dipoles. At low doping concentration of CuSb₂O₆ (0.5‐1.0 mol%), DC1 and DC2 coexist in the ceramics and harden the ceramics, inducing a constricted double P‐E loop and high Q_m of 895 at x=0.01. However, DC2 becomes more dominant in the ceramics with high concentration of CuSb₂O₆ (≥1.5 mol%) and thus leads to softening behavior of piezoelectricity and ferroelectricity as compared to the ceramic with x=0.01, giving a single slanted P‐E loop and relatively low Q_m of 206 at x=0.025. All ceramics exhibit relatively high d₃₃ of 106‐126 pC/N. Our study shows that the piezoelectricity and ferroelectricity of K_0.5Na_0.5NbO₃ ceramics can be tailored by controlling defect structure of the materials.     

Ferroelectric,piezoelectric and electrostrictive properties of Sn4+‐modified Ba0.7Ca0.3TiO3 lead‐free electroceramics

Lead‐free Ba_0.7Ca_0.3Ti_1?xSn_xO₃ (x=0.00, 0.025, 0.050, 0.075, and 0.1, abbreviated as BCST) electroceramic system was prepared by the solid‐state reaction method and its ferroelectric, piezoelectric, and electrostrictive properties were investigated. X‐ray diffraction shows that the compositions with x≤0.05 exhibit a tetragonal crystal structure having P4mm symmetry; while the compositions x=0.075 and 0.1 exhibit a mixed P4mm+Amm2 phase coexistence of tetragonal and orthorhombic and P4mm+Pmm pseudo‐cubic lattice symmetries, respectively, at room temperature. The dense microstructure having relative density ~90%‐92% and average grain size in the range ~2.36 μm to 8.56 μm was observed for BCST ceramics. Temperature‐dependent dielectric measurements support the presence of phase coexistence and show the decrease in Curie temperature (T_C) with Sn⁴⁺ substitution. The dielectric loss (tan δ) values in the temperature range (?100°C to 150°C) was observed to be <4%, for all BCST ceramics. The BCST compositions exhibit typical polarization‐electric field (P‐E) hysteresis and electric field induced strain (S‐E) butterfly loop, which confirms the ferroelectric and piezoelectric character. The compositions x=0.025, 0.05 and 0.075 show the peaking behavior of displacement current density () to an applied electric field () (J‐E) which implies the saturation state of polarization. The maximum electrostrictive coefficient (Q₃₃) value of 0.0667 m⁴/C² was observed for x=0.075 and it is higher than some of the significant lead‐based electrostrictive materials. The compositions x=0.05 and 0.075 exhibit the notable electrostrictive properties that may be useful for piezoelectric Ac device applications. The observed results are discussed and correlated with the structure‐property‐composition.     

ZrB2–ZrCxN1−x Eutectic Composites Produced by Melt Solidification

Ceramic eutectics are naturally occurring in‐situ composites and can offer superior mechanical properties. Here, ZrB₂–ZrC_xN_1?x quasi‐binary ceramic eutectic composites were produced by arc‐melting a mixture of ZrB₂, ZrC, and ZrN powders in an N₂ atmosphere. The arc‐melted ZrB₂–ZrC_xN_1?x composites containing 50 mol% of ZrB₂ (irrespective of the ZrC/ZrN ratio) showed rod‐like eutectic structures, where ZrC_xN_1?x single‐crystalline rods were dispersed in the ZrB₂ single‐crystalline matrices. Multiple orientation relationships between the ZrC_xN_1?x rods and the ZrB₂ matrices were observed, and one was determined as ZrB₂ {} //Zr_xN_1?x {111} and ZrB₂ < > //ZrC_xN_1?x < > . The rod‐like eutectic composites had higher hardness than the hypo‐ and hypereutectic composites and the 50ZrB₂–40ZrC–10ZrN (mol%) eutectic composite showed the highest Vickers hardness (H_v) of 19 GPa.     

Defect engineering on phase structure and temperature stability of KNN‐based ceramics sintered in different atmospheres

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (Abbreviated as KNN‐0.045BNZ) ceramics have been prepared by the conventional solid‐state sintering method in reducing atmosphere ( = 1 × 10^?10 atm) and air. For ceramics sintered in reducing atmosphere, only Mn²⁺ ions exist in ceramics who preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form defects () at x > 0.4. For ceramics sintered in air, mixed Mn²⁺, Mn³⁺, and Mn⁴⁺ ions coexist here. The Mn²⁺ ions preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4 and then Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site at x > 0.4. Meanwhile, the Mn³⁺ ions and Mn⁴⁺ ions substitute for Nb⁵⁺ ions in B‐site to form defects () at x = 0.2‐0.8. The (, , and ) dipolar defects show a positive dipolar defect contribution (DDC) to the , whereas the dipolar defects () show a negative DDC to the . The dipolar defects ( ‐ and ) can help improve the temperature stability of . The 0.4% MnO‐doped KNN‐0.045BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d₃₃ = 300 pC/N and 0.2% MnO‐doped KNN‐0.045BNZ ceramics sintered in air possess optimal piezoelectric constant d₃₃ = 290 pC/N.     

Long‐Range Ordered Structure of Ti–B–C–N in a TiB2–TiCxN1−x Eutectic Composite

A TiB₂–TiC_xN_1?x quasi‐binary eutectic composite, with a rod‐like faceted texture and a long‐range ordered structure of Ti–B–C–N, was prepared by the arc‐melting of TiB₂, TiC, and TiN powders. Hexagonal single‐crystalline TiC_xN_1?x rods were grown in a single‐crystalline TiB₂ matrix with a crystal orientation relationship of TiB₂ //TiC_xN_1?x and TiB₂ [0001]//TiC_xN_1?x [111]. A long‐range ordered structure of Ti–B–C–N was formed by the intermixing of the coherent interplanar spacings of seven TiB₂ (0001) and nine TiC_xN_1?x (111) planes.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 Ceramics

(1 ? x)MgTiO₃–xCa_0.8Sr_0.2TiO₃ (0.04 ≤ x ≤ 0.2, MT‐CST) composite ceramics were prepared by the conventional solid‐state reaction process. The phase composition, microwave dielectric properties, and microwave dielectric loss mechanisms were studied. Ca_0.8Sr_0.2TiO₃ was employed as a τ_f compensator for MgTiO₃, and they coexisted well without forming any secondary phases. Interestingly, significant dielectric relaxations associated with oxygen vacancy defects were observed in all the MT‐CST ceramics through the dielectric‐temperature spectra. Thermally simulated depolarization current was therefore conducted to obtain the defects associated with extrinsic dielectric loss mechanisms. The concentrations of both defect dipole and in‐grain oxygen vacancies increased with the increasing x, which could induce microwave dielectric loss consequently. It demonstrated that the behaviors of Q × f were basically influenced by phase composition and defects here. Temperature‐stable ceramics can be achieved at x = 0.06, where the microwave dielectric properties were ε_r = 21.19, Q × f = 110 900 GHz (f = 9.295 GHz), and τ_f = ?0.9 ppm/°C, respectively.     

Defects and Transport Properties in TiNb2O7

The electrical conductivity of TiNb₂O₇ was characterized as a function of temperature, and . The total conductivity was independent of in the low oxygen partial pressure regime, while a dependency of was observed at higher oxygen partial pressures. The conductivity increased with increasing under oxidizing conditions below 700°C. Mixed electronic and protonic conduction was indicated by H/D isotope exchange and transport number measurements. A defect model based on interstitial type of hydration was established and fitted to the conductivity data allowing for determination of physicochemical parameters of hydration and electron migration.