Pyroelectric energy harvesting using low–TC (1–x)(Ba0.7Ca0.3)TiO3–xBa(Zr0.2Ti0.8)O3 bulk ceramics

Lead‐free ferroelectric ceramics (1–x)(Ba_0.7Ca_0.3)TiO₃–xBa(Zr_0.2Ti_0.8)O₃ (BCTZ100x) with x = 0.20, 0.30, 0.40, 0.50, 0.60, 0.70, and 0.80 were evaluated for their pyroelectric energy harvesting performance, using the Olsen cycle. As the composition ratio x increased, the crystal phase changed to tetragonal, orthorhombic, rhombohedral, and cubic; the phase boundaries crossed each other in the vicinity of BCTZ70. The crossover phase transition behavior between first‐order and diffuse phase transition changed to only the diffusion phase transition with increasing x. A pinching effect occurred because an increase in dielectric constant was also observed. Energy densities N_D of 229 mJ/cm³ and 256 mJ/cm³ for BCTZ50 and BCTZ80 were obtained, respectively, in temperature of 30°C‐100°C and an electric field of 0‐30 kV/cm. These N_D values are over two times higher than that of soft–Pb(Ti,Zr)O₃ (PZT), which exhibits piezoelectric performance equivalent to BCTZ50 at room temperature. Compared with soft–PZT, BCTZ50 and BCTZ80 exhibited larger N_D values owing to their lower Curie temperatures (T_C ~ 50°C‐110°C). We conclude that low–T_C ferroelectrics are useful for pyroelectric energy conversion based on the Olsen cycle even if they are unsuitable for piezoelectric applications at high temperatures.     

A correlative study between the microstructure and electrical properties of lead-free piezoceramics (1-x)BHT-xYLG driven by the synergistic action of multiple factors

Piezoceramics are important functional materials with broad applications in electronics, communication, medicine, and energy. Increasing environmental protection and human health concerns about the toxicity of lead have impelled the development of lead-free piezoceramics. At this work, in combination with phase field simulation, grain growth mechanism, ferroelectric domain wall model, and defect dipole formation mechanism, (1-x)Ba(Hf_0.02Ti_0.98)O₃-x(Y_0.5Li_0.5)GeO₃ [(1-x)BHT-xYLG, x = 0–0.40 mol%] lead-free piezoceramics were designed and fabricated via the two-step synthesis and solid-state reaction method, which yielded superior comprehensive electrical property. Phase field simulation respectively simulates the distribution of ferroelectric domains of O and T phase under different external electric fields (0–20 kV/mm) and the phase states under different temperatures (?100 to 150 °C). This research finds that their electrical properties are affected by the synergistic action of multiple factors, e.g., orthorhombic-tetragonal phase boundary (O-T PB), grain size (GS), domain-wall (DW), and defect dipole (DD), etc., rather than any single factor.     

High pyroelectric performance in Pb0.99Nb0.02[(Zr0.57Sn0.43)0.94Ti0.06]0.98O3/Al2O3 composites by design

High pyroelectric performance around human body temperature is essential for ultra-sensitive infrared detectors of medical systems. Herein, toward human health monitoring, composite ceramics (1-x)Pb_0.99Nb_0.02[(Zr_0.57Sn_0.43)_0.94Ti_0.06]_0.98O₃/xAl₂O₃ (x = 0, 0.1, and 0.2) were designed. A metastable ferroelectric (FE) phase was induced in the anti-FE matrix by the Al₂O₃ component-induced internal stress, and in turn FE-anti-FE phase boundary was constructed. The ceramics at x = 0.2 exhibit high pyroelectric coefficient with p = 10.9 × 10⁻⁴ C·m⁻²·K⁻¹ and figures of merit with current responsivity F_i = 6.23 × 10⁻¹⁰ m·V⁻¹, voltage responsivity F_v = 12.71 × 10⁻² m²·C⁻¹, and detectivity F_d = 7.03 × 10⁻⁵ Pa^−1/2 around human body temperature. Moreover, the enhanced pyroelectric coefficients exist in a broad operation temperature range with a large full width at half maximums of 18.5°C and peak value of 29.2 × 10⁻⁴ C·m⁻²·K⁻¹ at 48.2°C. The designed composite ceramic is a promising candidate for infrared thermal imaging technology of noncontact human health monitoring system.     

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3-based lead-free ceramics

The issue of how to achieve an electrocaloric effect (ECE) and pyroelectric effect in a material simultaneously remains to be a challenge for developing practical solid-state cooling devices and RF-detectors. Here, we structure a polymorphic phase transition (PPT) region by doping modification in KNN-based ceramics, which are developed to achieve the ECE. The direct measured ECE and pyroelectric properties are investigated in lead-free (1-x)K_0.5Na_0.5NbO₃-xBi_0.5Na_0.5ZrO₃ (KNN-xBNZ) ceramics. The adiabatic temperature change (∆T) of 0.22 K at 100°C, 0.14 K at 70°C and 0.16 K at 30°C can be obtained under an electric field of 35 kV cm^–1 for x = 0.03, 0.04 and 0.05, respectively. In addition, the temperature dependence of pyroelectric coefficient (p) is established for all compositions via the Byer-Roundy method. A large p of 454.46 × 10^–4 C m^–2 K^–1 is detected at Curie temperature (T_C) in the ceramics with x = 0.03. Achieving electrocaloric effect and pyroelectric performance simultaneously may shed light and provide a feasible design scheme for developing practically useful electrocaloric and pyroelectric materials.     

Piezoelectric Property and Strain Behavior of Pb(Yb0.5Nb0.5)O3–PbHfO3–PbTiO3 Polycrystalline Ceramics

(1?x)Pb(Hf_1?yTi_y)O₃–xPb(Yb_0.5Nb_0.5)O₃ (x = 0.10–0.44, y = 0.55–0.80) ceramics were fabricated. The morphotropic phase boundary (MPB) of the ternary system was determined by X‐ray powder diffraction. The optimum dielectric and piezoelectric properties were achieved in 0.8Pb(Hf_0.4Ti_0.6)O₃–0.2Pb(Yb_0.5Nb_0.5)O₃ ceramics with MPB composition, where the dielectric permittivity ε_r, piezoelectric coefficient d₃₃, planar electromechanical coupling k_p, and Curie temperature T_c were found to be on the order of 1930,480 pC/N, 62%, and 360°C, respectively. The unipolar strain behavior was evaluated as a function of applied electric field up to 50 kV/cm to investigate the strain nonlinearity and domain wall motion under large drive field, where the high field piezoelectric d₃₃* was found to be 620 pm/V for 0.82Pb(Hf_0.4Ti_0.6)O₃–0.18Pb(Yb_0.5Nb_0.5)O₃. In addition, Rayleigh analysis was carried out to study the extrinsic contribution, where the value was found to be in the range 2%–18%.     

Improved sinterability and microwave dielectric properties of [Zn0.5Ti0.5]3+-doped ZnAl2O4 spinel solid solution

Low-permittivity ZnAl_2-x(Zn_0.5Ti_0.5)_xO₄ ceramics were synthesized via conventional solid-state reaction method. A pure ZnAl₂O₄ solid-state solution with an Fd-3m space group was achieved at x ≤ 0.1. Results showed that partial substitution of [Zn_0.5Ti_0.5]³⁺ for Al³⁺ effectively lowered the sintering temperature of the ZnAl₂O₄ ceramics and remarkably increased the quality factor (Q × f) values. Optimum microwave dielectric properties (ε_r = 9.1, Q × f = 115,800 GHz and τ_f = −78 ppm/°C) were obtained in the sample with x = 0.1 sintered at 1400°C in oxygen atmosphere for 10 h. The temperature used for the sample was approximately 250°C lower than the sintering temperature of conventional ZnAl₂O₄ ceramics.     

Dielectric and structural studies of ferroelectric phase evolution in dipole-pair substituted barium titanate ceramics

Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ ceramics with x equal to 0, 0.0025, 0.005, 0.01, 0.025, and 0.05 have been prepared by conventional solid-state reaction. Structural and dielectric characterization have been performed to investigate the effect of dipole-pair substitution concentration on the macroscopic dielectric properties. Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ evolves from a classic ferroelectric to a diffuse phase transition (DPT) as x increases. Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ for x ≥ 0.01 possesses diffuseness parameters comparable to Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PMN-PT) and recently reported (Ba_0.97Pr_0.03)(Ti_0.9425Ce_0.05)O₃ (BPTC), yet it lacks the frequency and temperature dependence of T_m necessary to be a strictly defined relaxor ferroelectric. Additionally, Ba{[Ga_0.05,Ta_0.05]Ti_0.9}O₃ possesses a relative permittivity, ɛ_r, of 700 ± 16% and dissipation factor less than 0.05 at 10 kHz within the temperature range [−75°C, 120°C]. In comparison to BaTiO₃, Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ possesses enhanced electrical resistivity at and above room temperature. In situ XRD, including Rietveld refinement, have been performed to determine the lattice parameter, coefficient of thermal expansion, and phase transition temperature (T_c) of each composition within the temperature range [RT, 1000°C], thus linking the dielectric properties with the material's structure. These studies have been corroborated by temperature-dependent Raman spectroscopy to compare the T_c determined by electrical and structural characterization. The properties of Ba{[Ga_x,Ta_x]Ti_(1−2x)}O₃ are discussed in context with available models that describe donor and acceptor dopants spatially separated in the parent matrix, inter-relating lattice parameter, Curie temperature, and other material properties.     

Large electrocaloric effect with ultrawide temperature span in Na1/2Bi1/2TiO3-based lead-free ceramics

Innovative cooling technologies are recognized by many industries as a crucial part of their system design. A large electrocaloric effect (ECE) and extended working temperature are the key issues hindering the realization of electrocaloric refrigeration technology. In this work, large ECE (ΔT = 0.8–0.9°C @ 4 kV/mm) with an ultrawide temperature span from 30 to 120°C is noted for lead-free (Na_1/2Bi_1/2)_0.80Sr_0.20(Zn_1/3Nb_2/3)_xTi_1-xO₃ ceramics. The excellent ECE performance can be ascribed to the evolution of polar nanoregions. Our work suggests that this material is promising for applications in solid-state refrigeration systems with a broad range of operating temperatures.     

Dielectric properties and electrocaloric effect of yttrium-modified BaTiO3 ceramics

The electrocaloric effect (ECE) is a promising candidate to replace the vapor-compression cooling technology, which has reached its end of improving the energy utilization efficiency. In the present work, the Y-modified BaTiO₃ ceramics with nominal compositions of Ba(Ti_1-xY_x)O₃ (abbreviated as BT-100xY, where x = 0.0125, 0.025, 0.0375, 0.050 and 0.0625) have been prepared through the conventional solid-state reaction sintering method. The dielectric properties and electrocaloric effect of BT-100xY ceramics have been investigated in detail. The XRD patterns indicate that all the BT-100xY ceramics possess pure perovskite structure without secondary phases. The temperature dependence of dielectric permittivity reveals that the BT-1.25Y, 2.5Y, 3.75Y and 5.0Y are normal ferroelectrics, and the BT-6.25Y is a relaxor ferroelectric. The ECE is calculated through the indirect equation based on Maxwell relation. The BT-2.5Y exhibits the largest ΔT = 1.26 K and the largest ΔS = 1.68 J/kg · K among all the BT-100xY ceramics, and the BT-2.5Y also exhibits the largest ΔT/ΔE = 0.296 × 10^?6 K · m/V and the largest ΔS/ΔE = 0.394 × 10^?6 J · m/kg · K?V. The ECE in our work is comparable with or even larger than that of BaTiO₃-based ceramics previously reported, which indicates that the BT-100xY ceramics are promising ECE materials.     

Phase transition behavior of Pb(Hf,Sn, Ti,Nb)O3 ceramics at morphotropic phase boundary

The phase transition and dielectric properties of Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics (0 ≤ x ≤ 0.03, correspondingly abbreviated as H1, H2, H3, and H4) at the morphotropic phase boundary were systematically investigated. X-ray diffraction results and P-E hysteresis loops show that the dominate orthorhombic antiferroelectric phase and a small amount of the tetragonal FE phase coexist in Pb_0.988(Hf_0.945Sn_xTi_0.03-xNb_0.025)O₃ ceramics. As the Sn content increases, the antiferroelectricity is significantly enhanced, accompanied with an increased Curie temperature and sharply reduced peak dielectric constant. H1 and H2 experience an irreversible field-induced AFE-FE phase transition at the ambient temperature, and the transition from a metastable FE phase to the original AFE phase is observed in H2 when heated to 60°C. H3 and H4 experience an invertible AFE-FE phase transition, along with an enhanced forward phase switching field E_F. Moreover a decreased backward phase switching field E_A for H4 is detected as the electric field increases due to the AFE/FE coexistence. These results reveal the unique phase transition characteristics of AFE materials near the phase boundary, which is helpful for better understanding of AFE/FE materials.     

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

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

Doping level effects in Nb self-doped Bi3TiNbO9 high-temperature piezoceramics with improved electrical properties

Nb self-doped Bi₃Ti_1-xNb_1+xO₉ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) high-temperature piezoelectric ceramics were fabricated through the conventional solid-state sintering method. The effects of different Nb self-doping levels on the microstructure, piezoelectric activities, and electrical conduction behaviors of these Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics were studied in detail. Large doping level effects on piezoelectric activity and resistivity were confirmed, which might be ascribed to the evolution of the crystal structure and the variations of the oxygen vacancy concentration and the grain anisotropy induced by Nb doping. An optimized piezoelectric coefficient (d₃₃) of 11.6 pC/N was achieved at x = 0.04 with a Curie temperature of 906°C. Additionally, an improved DC resistivity of 6.18 × 10⁵ Ω·cm at 600°C was acquired in this ceramic. Furthermore, the ceramic exhibited excellent thermal stability with the d₃₃ value maintaining 95% of its initial value after being annealed at 850°C for 2 hours. These results showed that Nb self-doped Bi₃Ti_1-xNb_1+xO₉ ceramics might have great potentials for high-temperature piezoelectric applications.     

Structure,dielectric properties of novel Ba(Zr,Ti)O3 based ceramics for energy storage application

(1-x) Ba(Zr_0.2Ti_0.8)O₃-x Na_0.5Bi_0.5TiO₃ (x = 0, 10, 20 30, 40, 50 mol%) (BZTN) ceramics are prepared by the traditional solid phase method. All BZTN ceramics exhibit a pseudo-cubic BZT based perovskite structure. Both the average grain size and the relaxor ferroelectricity of BZTN ceramics gradually increase with increasing NBT content. The W_rec of 3.22 J/cm³ and η of 91.2% is obtained for the BZTN40 ceramic at 241 kV/cm. BZTN40 ceramic also exhibits good temperature stability from room temperature to 150 °C and frequency stability from 1 Hz to 100 Hz. A P_D of 0.621 J/cm³ and a t_0.9 of 82 ns is obtained for the BZTN40 ceramic at 120 kV/cm. BZTN ceramics show application potential in energy storage and pulse power capacitors.     

Non-stoichiometric BZT-BCT ferroelectrics with visible/near-infrared photoresponse for broadband photodetection

The bulk photovoltaic effect in ferroelectric materials exhibits great potential in photoelectric applications. However, their photoresponse is generally limited to the UV region due to the wide bandgap feature of most perovskite ferroelectric materials, while the narrowing of bandgap is often accompanied by the degradation or even extinction of the ferroelectric polarization. Herein, 0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-BCT) with bandgap of 2.4 eV arising from the existence of Ti³⁺ is prepared, and then titanium vacancies are introduced to BZT-BCT ferroelectric solid solutions through B-site off-stoichiometry (0.5Ba(Zr_0.2Ti_0.8)O₃-0.5(Ba_0.7Ca_0.3)Ti_{1 − x%}O₃, BZT-BCT_{1 − x%}) to further tune the bandgap. Lowest bandgap of 2.1 eV with near-infrared (NIR) absorption is obtained for BZT-BCT_0.990 composition. Moreover, both the ferroelectricity and piezoelectricity (P_r = 14.3 μC/cm², d₃₃ = 415 pC/N) are well maintained after mitigating the effect of the increased oxygen vacancies associated with Ti-deficiency on the spontaneous polarization by oxygen-annealing treatment. More specifically, the oxygen-annealing BZT-BCT_0.985 composition, showing at least 2.4-fold photovoltage and 2.2-fold photocurrent of the pristine BZT-BCT ceramic, exhibits excellent transient pyroelectric current and stable photovoltaic current under xenon, NIR, and monochromatic visible lights. The photoelectric device based on the oxygen-annealing BZT-BCT_0.985 ceramic performs wide-spectrum photodetection properties from 405 nm light to NIR light, which makes it a potential candidate for self-powered photodetector applications.     

In situ TEM observation on the ferroelectric-antiferroelectric transition in Pb(Nb,Zr,Sn,Ti)O3/ZnO

Ceramic composites of (1-x)Pb_0.99{Nb_0.02[(Zr_0.57Sn_0.43)_0.937Ti_0.063]_0.98}O₃ (PNZST)/xZnO were recently reported to exhibit exceptionally high pyroelectric coefficients near human body temperature due to the ferroelectric-antiferroelectric transition of the matrix grains. In the present work, a comparative study is conducted on two composites of x = 0.1 and 0.4 with in situ heating transmission electron microscopy (TEM). The results verify the presence of strain field in the PNZST grain adjacent to a ZnO particle and the stabilized ferroelectric phase at room temperature in the composite of x = 0.1. During heating, the ferroelectric matrix grain transforms to the antiferroelectric phase, contributing to the pyroelectric effect. In the composite of x = 0.4, high-angle annular dark-field imaging combined with energy-dispersive X-ray spectroscopy reveal the existence of both ZnO and Zn₂SnO₄. The formation of Zn₂SnO₄ indicates that Sn in the PNZST matrix grain is selectively extracted, and decomposition of the perovskite phase has taken place. The decomposition products in the form of fine particles are observed to facilitate the nucleation of the antiferroelectric phase and restrict the motion of the phase boundary during heating. The larger amount of ZnO and Zn₂SnO₄ and the decomposition of the PNZST perovskite phase are suggested to be responsible for the much lower pyroelectric coefficient in the x = 0.4 composite.     

Structural and Dielectric Properties in (1−x)BaTiO3–xBi(Mg1/2Ti1/2)O3 Ceramics (0.1 ≤ x ≤ 0.5) and Potential for High‐Voltage Multilayer Capacitors

Structural and dielectric properties of (1?x)BaTiO₃–xBi(Mg_1/2Ti_1/2)O₃ (x = 0.1–0.5) were investigated to understand the binary system and utilize it for high‐voltage, high energy density capacitors. The solubility limit for Bi(Mg_1/2Ti_1/2)O₃ in a BaTiO₃ perovskite was between x = 0.4 and x = 0.5. A phase with pseudocubic symmetry was formed for x = 0.1–0.4; a secondary phase developed at x = 0.5. Dielectric measurements showed highly diffusive and dispersive relaxor‐like characteristics from 10 to 40 mol% of Bi(Mg_1/2Ti_1/2)O₃. These compositions also showed high relative permittivity with low‐temperature coefficients of permittivity over a wide range of temperatures ?100°C–600°C. Relaxation behavior was quantitatively investigated using the Vogel–Fulcher model, which revealed the activation energy of 0.17–0.22 eV. Prototyped multilayer capacitors of 18 mm × 17 mm × 4 mm dimensions with a capacitance of 12.5 nF at 1 kHz were successfully constructed and demonstrated multiple charge–discharge characteristics up to 10 kV.     

Valence state of europium and samarium in Ln2Hf2O7 (Ln = Eu,Sm) based oxygen ion conductors

Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) pyrochlores have been prepared via mechanical activation of oxide mixtures, followed by heat treatment for 4h at 1450 and 1600 °C, respectively. According to the ESR data, the Eu cations on the Hf site in the Hf_1-xEu_xO₆ octahedra in pyrochlore Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) are most readily oxidized and reduced. Oxidation at 840 °C for 24h in air reduces the total conductivity of the Ln₂(Hf_2-xLn_x)O_7-x/2 (Ln = Sm, Eu; x = 0.1) by a factor of 2.5–6, due to the decrease in the concentrations of oxygen vacancies and Ln²⁺ ions as a result of the oxidation. The anomalous low-frequency behavior of the permittivity of the Eu₂(Hf_2-xEu_x)O_7-x/2 (x = 0.1) at ~800 °C can be understood in terms of the changes in the oxygen sublattice of the pyrochlore structure as a result of the oxidation of divalent europium and partial filling of oxygen vacancies at this temperature.     

Tuning of electrical properties of xBi(Zn0.5Ti0.5)O3-(1-x) (Ba0.5Sr0.5)TiO3 ceramics by composition design and bandgap engineering

Herein, the xBi(Zn_0.5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ (x = 0.05, 0.10, 0.15, 0.20) novel negative temperature coefficient (NTC) ceramic materials were fabricated by solid-state method. X-ray diffraction revealed that xBi(Zn_0·5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ successfully formed solid solution. The UV–vis diffuse spectra of the samples indicate that the band gap increases with the increasing Bi(Zn_0·5Ti_0.5)O₃ content. The resistance temperature curve showed that with the increase of Bi(Zn_0·5Ti_0.5)O₃ content, the resistivity ρ of the ceramics at 400 °C increased from 5.96 × 10⁶ to 2.67 × 10⁷ Ω cm, as well as an increase in the B_400/800 from 12374.6 to 13469.1 K. The enhanced resistivity is attributed to the increased band gap and reduced carrier pairs caused by the Bi(Zn_0.5Ti_0.5)O₃ modification. The impedance data indicates that the conduction process is activated by thermal. The ceramic samples exhibit the excellent NTC characteristics over a range of 400 °C–1000 °C. Hence, the xBi(Zn_0.5Ti_0.5)O₃-(1-x) (Ba_0.5Sr_0.5)TiO₃ ceramics have the potential to become high temperature NTC ceramics that can operate in a wide temperature range.     

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.     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.