Dielectric properties and tunability of Ba(ZrxTi1−x)O3 ceramics under high DC electric field

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.25, 0. 3, 0.35, 0.4) ceramics were prepared by the traditional ceramic processing and their structural, surface morphological, dielectric properties, tunable properties as well as the mechanism of their nonlinear dielectric constant under DC electric field were systemically examined. The Zr ions substitution of Ti ions has a strong effect on the dielectric properties and the grain sizes. The results show Ba(Zr_xTi_1−x)O₃ (x = 0.25, 0.3, 0.35) ceramics to be promising candidates for the DC electric field tunable materials for microwave electronics application, because they exhibit high tunability (27.6%, 26.3%, 19.4%, respectively) as the strength of electric field is up to 2 kV/mm, low dielectric loss (0.001–0.002, 0.001–0.002, 0.004–0.005, respectively) at 10 kHz at room temperature and low temperature coefficient of capacitance.     

Preparation and performance of Na-doped La2Ce2O7 electrolytes for protonic ceramic fuel cells

The protonic material La₂Ce₂O₇ exhibits good tolerance to H₂O and CO₂ compared to BaCeO₃-based materials and has become increasingly popular for operation at low-to-intermediate temperatures in protonic ceramic fuel cells. In this work, doping La₂Ce₂O₇ with Na in a series with varying compositions is studied. All of the precursors are prepared by a common citrate-nitrate combustion method. X-ray diffraction images reveal that all of the La_2-xNa_xCe₂O_7-δ samples have a cubic structure. The La_2-xNa_xCe₂O_7-δ pellets are characterized by scanning electron microscopy and are observed to be dense without holes. The effects of Na-doping on the La₂Ce₂O₇ electrical conductivity are carefully investigated in air at 350–800 °C and 5%H₂-95% Ar environments at 350–700 °C. It is found that different levels of Na doping in La₂Ce₂O₇ are conducive to improving the electrical conductivity and sinterability. Among the pellets, La_1.85Na_0.15Ce₂O_7-δ exhibited the highest electrical conductivity in air and 5% H₂-95% Ar atmospheres. Anode-supported half cells with La_1.85Na_0.15Ce₂O_7-δ electrolyte are also fabricated via a dry-pressing process, and the corresponding single cell exhibited a desirable power performance of 501 mW cm⁻² at 700 °C. The results demonstrate that La_1.85Na_0.15Ce₂O_7-δ is a promising proton electrolyte with high conductivity and sufficient sinterability for use in protonic ceramic fuel cells operating at reduced temperatures.     

Influence of Gd2O3 addition on thermophysical properties of La2Ce2O7 ceramics for thermal barrier coatings

In order to develop new candidate ceramic materials for thermal barrier coatings (La_1?xGd_x)₂Ce₂O₇ ceramics were prepared by pressureless-sintering at 1600 °C for 10 h in air. The phase structure, micro-morphology and thermophysical properties of (La_1?xGd_x)₂Ce₂O₇ ceramics were investigated, respectively. XRD results revealed that pure (La_1?xGd_x)₂Ce₂O₇ ceramics with defect fluorite structure were synthesized and SEM showed that their microstructures were dense and no other phases existed among the particles. With the increasing temperature, their thermal expansion coefficients increased, while the thermal conductivities decreased. The thermophysical results indicated that thermal expansion coefficients of these ceramics were higher than that of 8YSZ, and their thermal conductivities were much lower than that of 8YSZ. The lower thermal conductivities of these ceramics were mainly attributed to more oxygen vacancies and substitution atoms. These results imply that the (La_1?xGd_x)₂Ce₂O₇ ceramics can be explored as candidate materials for the ceramic layer in TBC system.     

Abnormal variation of microwave dielectric properties in A/B site co-substituted (Ca1−0.3xLa0.2x)[(Mg1/3Ta2/3)1−xTix]O3 complex perovskite ceramics

A/B site co-substituted (Ca_1?0.3xLa_0.2x)[(Mg_1/3Ta_2/3)_1?xTi_x]O₃ ceramics (0.1 ≤ x ≤ 0.5) were prepared by solid state reaction and the structures, microstructures and dielectric properties were investigated. B site 1:2 cation ordering and oxygen octahedra tilting lead to monoclinic symmetry with space group P2₁/c for x = 0.1. For x above 0.1, the ordering was destroyed and the crystal structure became orthorhombic with space group Pbnm. The B site 1:2 cation ordering tended to be destroyed to form 1:1 ordering by the A site La³⁺ substitution. The dielectric constant increased linearly with increasing content of Ti⁴⁺ as the increasing second Jahn–Teller distortion enhanced the B site cation rattling. The temperature coefficient of resonant frequency and Qf values showed abnormal variations, which were refined to be caused by the increasing A site cation vacancy and diffused distribution of small size ordering domains respectively. Good combination of microwave dielectric properties was obtained at x = 0.5, where ?_r = 48, Qf = 21,000 GHz and τ_f = 2.2 ppm/°C.     

Phase relationships and transport in Ti-, Ce- and Zr-substituted lanthanum silicate systems

The solubility of Ti⁴⁺ in the lattice of apatite-type La_9.83Si_6−xTi_xO_26.75 corresponds to approximately 28% of the Si-site density. The conductivity of La_9.83Si_6−xTi_xO_26.75 (x = 1–2) is predominantly oxygen-ionic and independent of the oxygen partial pressure in the p(O₂) range from 10⁻²⁰ to 0.3 atm. The electron transference numbers determined by the modified faradaic efficiency technique are lower than 0.006 at 900–950 °C in air. The open-circuit voltage of oxygen concentration cells with Ti-doped silicate electrolytes is close to the theoretical Nernst value both under oxygen/air and air/10%H₂–90%N₂ gradients at 700–950 °C, suggesting the stabilization of Ti⁴⁺ in the apatite structure. Titanium addition in La_9.83Si_6−xTi_xO_26.75 (x = 1–2) leads to decreasing ionic conductivity and increasing activation energies from 93 to 137 kJ/mol, and enhanced degradation in reducing atmospheres due to SiO volatilization. At p(O₂) = 10⁻²⁰ atm and 1223 K, the conductivity decrease after 100 h was about 5% for x = 1 and 17% for x = 2. The solubility of Zr⁴⁺ in the La_9.83Si_6−xZr_xO_26.75 system was found to be negligible, while the maximum concentration of Ce⁴⁺ in La_9.4−xCe_xSi₆O_27−δ is approximately 5% with respect to the number of lanthanum sites.     

Microwave dielectric properties of BaxLa4Ti3 + xO12 + 3x (x = 0.0–1.0) ceramics

In the BaO–La₂O₃–TiO₂ system, the Ba_nLa₄Ti_3 + nO_12 + 3n homologous compounds exist on the tie line BaTiO₃–La₄Ti₃O₁₂ besides tungstenbronze-type like Ba_6 − 3xR_8 + 2xTi₁₈O₅₄ (R = rare earth) solid solutions. There are four kinds of compounds in the homologous series: n = 0, La₄Ti₃O₁₂; n = 1, BaLa₄Ti₄O₁₅; n = 2, Ba₂La₄Ti₅O₁₈; n = 4, Ba₄La₄Ti₇O₂₄. These compounds have the layered hexagonal perovskite-like structure, which has a common sub-structure in the crystal structure. These compounds have been investigated in our previous studies. In this study, we have investigated the phase relation and the microwave dielectric properties of Ba_xLa₄Ti_3 + xO_12 + 3x ceramics in the range of x between 0.2 and 1.0. With the increase in x, the dielectric constant ɛ_r locates around 45, the quality factor Q × f shows over 80,000 GHz at x = 0.2 and the minimum value of 30,000 GHz at x = 0.9, and the temperature coefficients of resonant frequency τ_f is improved from −17 to −12 ppm/°C. At x = 0.2, the ceramic composition obtained has dielectric constant ɛ_r = 42, the temperature coefficient of the resonant frequency τ_f  = −17 ppm/°C and a high Q × f of 86,000 GHz.     

Influence of TiO2 doping on thermo-optical properties of pyrochlore Sm2(Zr1–xTix)2O7 (0≤x≤0.15) ceramics

Sm₂(Zr_1–xTi_x)₂O₇ (0≤x≤0.15) ceramics have been fabricated by pressureless-sintering method at 1973 K for 10 h in air. The influence of TiO₂ doping on microstructure and thermo-optical properties of Sm₂(Zr_1–xTi_x)₂O₇ ceramics is investigated by X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy measurements. The partial substitution of Ti⁴⁺ for Zr⁴⁺ results in a significant increase in emissivity at low wavelengths contrasted with undoped Sm₂Zr₂O₇. Sm₂(Zr_0.85Ti_0.15)₂O₇ ceramic exhibits a high emissivity of above 0.70 at 1073 K in a wavelength range of 3–16 µm, where the highest value at this temperature is more than 0.90 especially in the wavelength range of 9–14 µm. FT-IR spectra and optical absorption spectra unveil the mechanisms of enhanced emissivity in Sm₂(Zr_1–xTi_x)₂O₇ (0.05≤x≤0.15) ceramics in the intermediate infrared range, especially at the wavelengths of 3–8 µm, due to Ti⁴⁺ ion substitution for Zr⁴⁺ ion.     

Improved dielectric properties and grain boundary response of SrTiO3 doped Y2/3Cu3Ti4O12 ceramics fabricated by Sol-gel process for high-energy-density storage applications

A chemical solution processing method based on sol-gel chemistry (SG) was used to synthesize (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ (x = 0, 0.05, 0.1, 0.15, 0.2, 0.25) ceramics successfully. The 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics sintered at 1050 °C for 20 h showed fine-grained microstructure and high dielectric constant (ε′ ≈ 1.7 × 10⁵) at 1 kHz. Furthermore, the 0.85Y_2/3Cu₃Ti₄O₁₂-0.15SrTiO₃ ceramics appeared distinct pseudo-relaxor behavior. Two electrical responses were observed in the combined modulus and impedance plots, indicating the presence of Maxwell-Wagner relaxation. Sr vacancies and additional oxygen vacancies had substantial contribution to the sintering behavior, an increase in grain growth, and relaxation behaviors in grain boundaries. The contributions of semiconducting grains with the nanodomain and insulating grain boundaries (corresponding to high-frequency and low-frequency electrical response, respectively) played important roles in the dielectric properties of (1-x)Y_2/3Cu₃Ti₄O₁₂-xSrTiO₃ ceramics. The occurrence of the polarization mechanism transition from the grain boundary response to the electrode one with temperature change was clearly evidenced in the low frequency range.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Enhanced electrocaloric,pyroelectric and energy storage performance of BaCexTi1−xO3 ceramics

BaCe_xTi_1−xO₃ (BCT) ceramics with compositions x = 0, 0.1, 0.12 and 0.15 were synthesized using conventional solid state reaction route. Systematic exploration of enhancing electrocaloric effect (ECE) in BaTiO₃ by rare earth dopant Ce is presented. BaCe_0.12Ti_0.88O₃ exhibited an electrocaloric strength of ∼0.35 K m/MV at 351 K, which caters the need for a series of high-level ECE material. Further, the temperature dependence of pyroelectric coefficient is established for all compositions. The pyroelectric figure of merits (FOMs) for current responsivity (F_i), voltage responsivity (F_v), detectivity (F_d) and energy harvesting (F_e and F_e^*) are calculated and the results reveal that x = 0.1 could be a technologically superior candidate for pyroelectric devices. Further, BaCe_0.15Ti_0.85O₃ exhibited highest electrical energy storage performance of 115 kJ/m³ compared with 71 kJ/m³ in BaTiO₃. Our findings in this work may provide a better understanding for developing high ECE materials combined with pyroelectric and energy storage performance of Ce substituted BaTiO₃ ceramics.     

Crystal structure and microwave dielectric properties of Zn0.9Ti0.8−xSnxNb2.2O8 ceramics

The crystal structure and microwave dielectric properties of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.00, 0.05, 0.10, 0.15) ceramics sintered at temperatures ranging from 1100 °C to 1140 °C for 6 h were investigated. A single phase with ixiolite structure was obtained. With the increase of Sn content, the dielectric constant decreased attributed to the decrease of dielectric polarizability. The Qf value decreased with the decrease of packing fraction and grain size. The temperature coefficient of resonant frequency (τ_f) increased due to the increase of the bond valence of Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ ceramics. The excellent microwave dielectric properties of ? = 35.05, Qf = 49,100 GHz, τ_f = ?27.6 × 10^?6/°C were obtained for Zn_0.9Ti_0.8?xSn_xNb_2.2O₈ (x = 0.05) specimens sintered at 1120 °C for 6 h.     

Cerium-doped LaSi3N5: Computed electronic structure and band gaps

First-principle calculations are performed using VASP to enhance the understanding of the electronic structure of stoichiometric LaSi₃N₅ and Ce-doped LaSi₃N₅. Electronic structure and band gaps are calculated using DFT and the screened Coulomb hybrid functional HSE06. Five doping schemes are investigated: La³⁺/^*Ce³⁺, La³⁺/□ (□ = cation vacancy), La³⁺/Ce³⁺, La³⁺/Ce²⁺ compensated by one framework N³⁻/O²⁻ substitution, and combined La³⁺/Ce³⁺and La³⁺/□ substitutions. The vacancy is electronically balanced by N³⁻/O²⁻ substitutions. Two pseudopotential files for Ce^III are used to investigate the effect of f orbitals on the calculated band gap. There was no change in the calculated value of the band gap using DFT method for the ^*Ce^III doped LaSi₃N₅ compared to the stoichiometric LaSi₃N₅ (3.15 eV). HSE06 provided more precise band gaps. The calculated band gap of Ce-doped LaSi₃N₅ is 4.65 eV with the possible electronic transitions from Ce spin-up 4f to Ce spin-up 4f states, or La 5d states..     

The effects of sintering temperature and content of x on phase formation,microstructure and dielectric properties of (1−x)(Bi0.4871Na0.4871La0.0172TiO3)−x(BaZr0.05Ti0.95O3) ceramics prepared via the combustion technique

(1?x)(Bi_0.4871Na_0.4871La_0.0172TiO₃)?x(BaZr_0.05Ti_0.95O₃) ceramics (abbreviated (1?x)BNLT?xBZT) where 0.1≤x≤0.3 were fabricated by the combustion technique using glycine as fuel. BNLT and BZT powders were calcined at temperatures of 825 °C for 4 h and 925 °C for 6 h, respectively. After that they were mixed with the different compositions. It was found that the optimum sintering temperature of (1?x)BNLT?xBZT ceramic was obtained at 1125 °C for 2 h. This ceramic had the highest density. The structure of the (1?x)BNLT?xBZT ceramics exhibited the co-existence of tetragonal and rhombohedral phases with x≤0.1. The tetragonality increases with the increase of x content. The average grain size, the density and the Curie temperatures decrease with increasing x content. The maximum dielectric constant and the highest P_r were at about 4850 and 12.7 μC/cm², respectively, and were obtained by the 0.85BNLT?0.15BZT sample.     

Ultrasound assisted synthesis of Gd and Nd doped ceria electrolyte for solid oxide fuel cells

In this study, the ceramic powders of Ce_1?xGd_xO_2?x/2 and Ce_1?xNd_xO_2?x/2 (x=0.05, 0.10, 0.15, 0.20 and 0.25) were synthesized by ultrasound assisted co-precipitation method. The ionic conductivity was studied as a function of dopant concentration over the temperature range of 300–800 °C in air, using the impedance spectroscopy. The maximum ionic conductivity, σ_800 °C=4.01×10^?2 Scm^?1 with the activation energy, E_a=0.828 kJmol^?1 and σ_800 °C=3.80×10^?2 Scm^?1 with the activation energy, E_a=0.838 kJmol^?1 were obtained for Ce_0.90Gd_0.10O_1.95 and Ce_0.85Nd_0.15O_1.925 electrolytes, respectively. The average grain size was found to be in the range of 0.3–0.6 μm for gadolinium doped ceria and 0.2–0.4 μm for neodymium doped ceria. The uniformly fine crystallite sizes (average 12–13 nm) of the ultrasound assisted prepared powders enabled sintering of the samples into highly dense (over 95%) ceramic pellets at 1200 °C (5 °C min^?1) for 6 h.     

Effects of structural characteristics on microwave dielectric properties of (1 − x)Ca0.85Nd0.1TiO3–xLnAlO3 (Ln = Sm,Er and Dy) ceramics

Dependence of microwave dielectric properties on the structural characteristics of (1 − x)Ca_0.85Nd_0.1TiO₃–xLnAlO₃ (Ln = Sm, Dy and Er) ceramics were investigated as a function of LnAlO₃ content (0.05 ≤ x ≤ 0.25). For the specimens with SmAlO₃, a single phase with orthorhombic perovskite was obtained through the entire composition, however, Dy₂Ti₂O₇ and Er₂Ti₂O₇ were detected as a secondary phase along with the orthorhombic perovskite phase for the specimens with DyAlO₃ (x = 0.25) and ErAlO₃ (0.10 ≤ x ≤ 0.25), respectively. With an increase of LnAlO₃ content, the dielectric constant decreased due to the smaller ionic polarizability of LnAlO₃ than Ca_0.85Nd_0.1TiO₃. The temperature coefficient of the resonant frequency (TCF) decreased with LnAlO₃ content resulted from an increase of oxygen octahedral distortion.     

Role of ceria in the improvement of SO2 resistance of LaxCe1 − xFeO3 catalysts for catalytic reduction of NO with CO

Perovskite-type catalysts with LaFeO₃ and substituted La_xCe_1 − xFeO₃ compositions were prepared by sol–gel method. These catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), CO temperature-programmed reduction (CO-TPR), and SO₂ temperature-programmed desorption (SO₂-TPD). Catalytic reaction for NO reduction with CO in the presence of SO₂ has been investigated in this study. LaFeO₃ exhibited an excellent catalytic activity without SO₂, but decreased sharply when SO₂ gas was added to the CO + NO reaction system. In order to inhibit the effect of SO₂, substitution of Ce in the structure of LaFeO₃ perovskite has been investigated. It was found that La_0.6Ce_0.4FeO₃ showed the maximum SO₂ resistance among a series of La_xCe_1 − xFeO3 composite oxides.     

Multiscale study of ferroelectric–relaxor crossover in BaSnxTi1−xO3 ceramics

A systematic study of BaSn_xTi_1−xO₃ solid solutions (x = 0–0.20) by a combined field-induced dielectric and ferroelectric analysis with Raman and PFM investigations was realized, in order to obtain new insights concerning the composition-induced modification of the structural phase transitions and ferroelectric–relaxor crossover induced by the increase of Sn addition. The ceramics prepared via solid state reaction and sintering at 1400 °C/4 h showed average tetragonal symmetry for x ≤ 0.15 and cubic for x = 0.20. However, the dielectric and Raman analysis demonstrated that x = 0.05 and x = 0.15 are characterized by a coexistence of phases, which enhances their macroscopic properties (polarization for x = 0.05 and permittivity for x = 0.15). The domain structure shows a gradual modification when increasing Sn addition. No detectable domain structure has been found for x ≥ 0.15. All the compositions show local d₃₃(V) hysteresis loops at room temperature.     

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd_2?xCe_xTi₂O₇ has been investigated. The solubility limit of Ce in Nd_2?xCe_xTi₂O₇ was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd_2?xCe_xTi₂O₇ (x?=?0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point T_c of Nd_2?xCe_xTi₂O₇ compounds. The results suggest that the T_c of Ce₂Ti₂O₇ is <1445°C.     

Microwave dielectric properties of BaO–Ta2O5–TiO2 system

Microwave dielectric properties of the BaO–Ta₂O₅–TiO₂ system were investigated by the solid-state reaction method. It was recognized that the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions have the higher Q · f value in comparison with the Ba₈(Ta_4 − xNb_x)Ti₃O₂₄ solid solutions. The limit of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions was approximately x = 0.75; the lattice parameter c of the solid solutions, which is related to the change in the B(1)O₆ octahedron, was significantly increased in the composition range from 0 to 0.75. The Q · f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions are remarkably improved by the Sn substitution for Ti; the highest Q · f value of 59,100 GHz is obtained at x = 0.75. Moreover, the ɛ_r and τ_f values of the Ba₁₀Ta_7.04(Ti_1.2 − xSn_x)O₃₀ solid solutions at x = 0.75 were 25.6 and 30.3 ppm/°C, respectively.     

Dielectric and energy storage properties of Mn-doped Ba0.3Sr0.475 La0.12Ce0.03TiO3 dielectric ceramics

Microstructure, temperature and frequency dependent dielectric and energy storage properties of Ba_0.3Sr_0.475La_0.12Ce_0.03Ti_1−xMn_xO₃(x = 0 − 0.005) have been investigated with X-ray diffractometer, and scanning electron microscope, broad band dielectric spectrometer and ferroelectric analyzer. The doping of Mn substantially decreased the dielectric loss, but made some of Ce³⁺ be oxidized to Ce⁴⁺ entering into B-site, which resulted in the formation secondary phases. For the Mn-doped composition, extremely low dielectric loss (10⁻⁵ order of magnitude at 10 kHz) could be obtained at room temperature. The relaxation mechanism at low temperature is of the dipole type for the undoped composition and that at high temperature (>500 K) is governed by the trap controlled ac conduction, respectively. The energy storage properties were improved by the doping with Mn due to the increase of insulation. Maximum energy density of 0.953 J/cm³ could be obtained for x = 0.003 composition with the BDS of 247 kV/cm and efficiency of 93%.