Microstructure and Optical Property in an Irregular Multilayer Comprising Ferroelectric and Paraelectric Materials

A special sequence of multilayer, consisting of PbZr_0.5Ti_0.5O₃ and SrTiO₃ films, was fabricated using a simple chemical solution deposition. X‐ray diffractometer (XRD) measurement reveals that each film in this multilayer has been crystallized into the single perovskite phase. The high‐angle annular dark‐field scanning transmission electron microscopy (STEM) image shows that the obtained SrTiO₃/PbZr_0.5Ti_0.5O₃ multilayer contains three components with different optical thicknesses: dense and porous PbZr_0.5Ti_0.5O₃ layers, together with dense SrTiO₃ layers. This multilayer system exhibits superior optical performance, with a peak reflectivity of～95% and a bandwidth of ～113 nm, rendering its promising candidate as dielectric mirrors, optical cavities, and selective filters.     

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.     

Kirkendall porosity in barium titanate–strontium titanate diffusion couple

Inter-diffusion between perovskites BaTiO₃ and SrTiO₃ diffusion couple has been studied by determining the crystalline phases and analyzing the microchemistry and microstructure of undoped BaTiO₃–SrTiO₃ stacks sintered at 1250 °C in air. The Kirkendall effect manifested by: (1) an inter-diffusion zone containing (Ba_1−xSr_x)TiO₃ and (Sr_1−xBa_x)TiO₃ solid solutions, (2) the migration of the initial BaTiO₃–SrTiO₃ interface, and (3) the Kirkendall porosity was observed. The inter-diffused regions on both sides of the initial interface contain grains exhibiting the characteristic core–shell structure with distinctive solute contents between core and shell. TiO₂-rich polytitanates, notably Ba₄Ti₁₃O₃₀ and Ba₆Ti₁₇O₄₀ containing a minor amount of Sr from inter-diffusion, have been detected at the BaTiO₃ side near the initial BaTiO₃–SrTiO₃ interface. An analogy between the BaTiO₃–SrTiO₃ diffusion couple and Kirkendall's original α-brass-Cu couple is presented.     

Insight for the construction of R-T phase boundary in KNN piezoceramics from the view of energy band structure and electron density

The crystal structures and associated energy band structures of classic lead-based PZT (PbZr_0.5Ti_0.5O₃) and lead-free KNN (K_0.5Na_0.5NbO₃) piezoelectric ceramics prepared by a solid-state method are systematically studied based on the First-principles calculations. For better understanding relations between energy band structures and the phase structures in the PbZr_0.5Ti_0.5O₃ and KNN, the components PT (PbTiO₃)/PZ (PbZrO₃) and KN (KNbO₃)/NN (NaNbO₃) are also calculated. The results suggest that the vital contribution to R-T morphotropic phase boundary (MPB) in PbZr_0.5Ti_0.5O₃ comes from the hybridization of A-site and B-site ions, which causes stretching and tilting of oxygen octahedron and therefore leads to the formation of R and T phases. The KNN behaves differently. Moreover, it's concluded that the band structure highly inherited from tetragonal PbTiO₃ plays an important role in the construction of the R-T phase boundary according to the comparison of PbZr_0.5Ti_0.5O₃ and KNN. In general, the work inspires an idea from the view of energy band structure to realize R-T phase coexistence as well as improved piezoelectric properties in KNN-based lead-free piezoceramics.     

High dielectric permittivity and dielectric relaxation behavior in a Y2/3Cu3Ti4O12 ceramic prepared by a modified Sol−Gel route

In this work, Y_2/3Cu₃Ti₄O₁₂ ceramics were fabricated via a modified sol?gel route. Structural, dielectric, and electrical parameters were systematically investigated. The XRD results indicate that a CaCu₃Ti₄O₁₂ phase (JCPDS No. 75–2188) is present in every sintered sample. SEM images of Y_2/3Cu₃Ti₄O₁₂ ceramics disclose a fine-grained ceramic microstructure. Interestingly, high dielectric permittivity, ～6600–7600, with loss tangents of ～0.918–1.086 were achieved in the sintered Y_2/3Cu₃Ti₄O₁₂ samples. Density functional theory (DFT) calculations were used to investigate the most stable structure of the Y_2/3Cu₃Ti₄O₁₂ ceramics. Our DFT results reveal that two calcium vacancies (V_Ca) are isolated from each other. We also determined the lowest energy configuration of an oxygen vacancy (V_O) in the Y_2/3Cu₃Ti₄O₁₂ ceramics occurred during the sintering process. We found that the V_O is trapped close to the Y atom in this structure. Both computational and experimental studies specify that the oxygen vacancy is located close to the Y atom in the Y_2/3Cu₃Ti₄O₁₂ lattice and it might be a bivalent oxygen vacancy. As a result, due to charge balance, charge compensation of the transition ions, i.e., Cu and Ti ions, might take place. The charge compensation mechanisms in the Y_2/3Cu₃Ti₄O₁₂ lattice were verified using an XPS technique. Impedance spectroscopy confirms the presence of an inhomogeneous microstructure consisting of semiconducting grains and insulating grain boundaries in the sintered Y_2/3Cu₃Ti₄O₁₂ ceramics. This electrical result is consistent with the computational analysis, showing that a charge compensation mechanism might be involved in generation of the grains' semiconductive region due to the presence of a V_O. Consequently, high dielectric permittivity in Y_2/3Cu₃Ti₄O₁₂ may have originated from an internal barrier layer capacitor (IBLC) effect.     

Colossal dielectric permittivity and relevant mechanism of Bi2/3Cu3Ti4O12 ceramics

Bi_2/3Cu₃Ti₄O₁₂ (BCTO) ceramics with pure perovskite phase were successfully prepared by traditional solid-state reaction technique. Uniformly distributed and dense grains with the grain size of 2–3 μm were observed by SEM. A giant low-frequency dielectric permittivity of ~3.3×10⁵ was obtained. The analysis of complex impedance revealed that Bi_2/3Cu₃Ti₄O₁₂ ceramics are electrically heterogeneous. There are three kinds of dielectric response detected in Bi_2/3Cu₃Ti₄O₁₂ ceramics, which existed in the low-frequency range, middle-frequency range, and high-frequency range, respectively. Through the study of dielectric spectrum at different temperatures, the relatively low activation energy of 0.30 eV for middle-frequency dielectric response was calculated, which suggested that this Middle-frequency dielectric response can be ascribed to grain boundaries response. In view of the analysis of dielectric spectrum at low temperatures, the activation energy of 0.07 eV for high frequency dielectric response was found. This value illustrated that dielectric response at high frequencies was associated with grains polarization effect. The comparison of dielectric spectra of Bi_2/3Cu₃Ti₄O₁₂ ceramics with different types of electrodes revealed that giant low-frequency dielectric constant was attributed to the electrode polarization effect.     

High‐energy storage density and excellent temperature stability in antiferroelectric/ferroelectric bilayer thin films

The antiferroelectric/ferroelectric (PbZrO₃/PbZr_0.52Ti_0.48O₃) bilayer thin films were fabricated on a Pt(111)/Ti/SiO₂/Si substrate using sol‐gel method. PbZr_0.52Ti_0.48O₃ layer acts as a buffered layer and template for the crystallization of PbZrO₃ layer. The PbZrO₃ layer with improved quality can share the external voltage due to its smaller dielectric constant and thinner thickness, resulting in the enhancements of electric field strength and energy storage density for the PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer thin film. The greatly improved electric breakdown strength value of 2615 kV/cm has been obtained, which is more than twice the value of individual PbZr_0.52Ti_0.48O₃ film. The enhanced energy storage density of 28.2 J/cm³ at 2410 kV/cm has been achieved in PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film at 20°C, which is higher than that of individual PbZr_0.52Ti_0.48O₃ film (15.6 J/cm³). Meanwhile, the energy storage density and efficiency of PbZrO₃/PbZr_0.52Ti_0.48O₃ bilayer film increase slightly with the increasing temperature from 20°C to 120°C. Our results indicate that the design of antiferroelectric/ferroelectric bilayer films may be an effective way for developing high power energy storage density capacitors with high‐temperature stability.     

Comparison of the catalysed decomposition of hydrogen peroxide over superconducting and insulating barium cuprates—kinetics of decomposition over Y2BaCuO5

Catalytic activities of two non-conducting cuprates (Ba₂Cu₃O₅, Y₂BaCuO₅) and a superconducting cuprate YBa₂Cu₃O_7–δ, prepared by two different methods, were compared using H₂O₂ solution. The two non-conducting barium cuprates were found to be about two orders of magnitude more active than the superconducting perovskite-like YBa₂Cu₃O_7–δ. The near coincidence of the kinetic curves for the catalysed decomposition of H₂O₂ over YBa₂Cu₃O_7–δ, prepared either from the perovskite-like Ba₂Cu₃O₅ and Y₂O₃ or from its components, proves that Ba₂Cu₃O₅ is essential in the formation of YBa₂Cu₃O_7–δ. First-order kinetics were observed for the catalysed decomposition of H₂O₂ solution over the non-perovskite Y₂BaCuO₅ at the initial stages of the reaction, similar to that of the superconducting perovskite catalyst. A tentative scheme of the possible catalytic reactions for the decomposition of H₂O₂ solution over the insulator Y₂BaCuO₅ is given.     

Phase formation and dielectric properties of (Pb,Ba)[(Zn1/2W1/2),Ti]O3 ceramics

PbTiO₃ and/or BaTiO₃ were systematically introduced into Pb(Zn_1/2W_1/2)O₃ and resultant phase developments in terms of perovskite formation were investigated. Ceramic powders were prepared via a B-site precursor route to further assist the perovskite formation. Weak-field dielectric properties of the sintered samples were examined. For Pb(Zn_1/2W_1/2)O₃-rich compositions, multiphase ceramics resulted and formation of monophasic perovskite turned out to be not successful even by the B-site precursor method. Values of the perovskite formation yield and the maximum dielectric constant increased with increasing fractions of the substituent species.     

Study of PbZr0.53Ti0.47O3 solid solution formation by interaction of perovskite phases

The formation of Pb(Zr_0.53Ti_0.47)O₃ solid solution around the MPH was studied by mutual interaction of perovskite PZT phases (rhombohedral and tetragonal) and by interaction of PZT phases with PbTiO₃ or PbZrO₃ at sintering temperature. Starting perovskite phases were prepared by the mechanical homogenization of oxidic precursors and calcination of mixtures at 1000 °C. The final Pb(Zr_0.53Ti_0.47)O₃ ceramic systems prepared from perovskite mixtures were monophasic (tetragonal symmetry) in comparison with the biphasic ceramics prepared from calcinate with the same stoichiometry. The magnitude of deviation from equilibrium chemical composition or fluctuation from final stoichiometry in PZT phase in starting powder perovskite mixtures was not crucial for the formation of monophase Pb(Zr_0.53Ti_0.47)O₃ ceramic system prepared using such a method.     

Low temperature sintering and dielectric properties of (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3−xCu2+ ceramics obtained by the sol-gel technique

Lead-free Cu²⁺-modified (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ (BCZT−xCu²⁺) piezoelectric ceramics was synthesized by sol-gel method. The effects of Cu²⁺ additions on sintering characteristics, the phase structure, microstructure, electrical properties and complex impedance characteristic were investigated systematically. The XRD patterns exhibited a pure perovskite structure without impurity phase in all samples. SEM micrographs, temperature dependence of dielectric constant and polarization-electric field (P-E) hysteresis loops indicated that a small amount of Cu²⁺ addition affected the properties obviously. The results revealed that the addition of Cu²⁺ significantly improved the sinterability of BCZT ceramics which resulted in a reduction of sintering temperature from 1440 °C to 1230 °C. The TG-DSC was analyzed to verify the reaction process of BCZT−Cu²⁺ materials. (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃ ceramics with x=0.020 Cu²⁺ exhibited good electrical properties: ε_m=12,112, T_c=360 K, ε_r=2614, tan δ=0.026, K_p=0.47 and d₃₃=382 pC/N. The results indicated that Cu²⁺-modified BCZT ceramics could be a promising candidate for commercial purposes.     

Fabrication of textured ferroelectric ceramics by magnetic alignment via gelcasting

Grain-oriented ferroelectric ceramics have attracted more interest recently because they may provide near single crystal properties. In the present study, a novel process combining magnetic alignment and gelcasting was explored to prepare grain-oriented ferroelectric ceramics with different crystal structures. In a strong magnetic field, ceramic particles in slurry were aligned by the magnetic force and then locked in situ by polymerization via a gelcasting technique. This process was found effective for ferroelectric ceramics with a bismuth layer structure (Bi₄Ti₃O₁₂) and tungsten bronze structure (Sr_0.5Ba_0.5Nb₂O₆). The sintered samples show highly anisotropic structure and enhanced physical properties. However for perovskite structured ferroelectric ceramics (BaTiO₃), the green compact shows grain orientation, while after sintering the sample become random again.Thus for certain materials using the conventional ceramic processes, i.e., using conventional starting powders, gelcasting under strong magnetic fields (10 T) and pressure-less sintering, the preparation of dense grain-oriented ceramic materials is possible.     

Microstructure and enhanced seebeck coefficient of textured Sr3Ti2O7 ceramics prepared by RTGG method

Textured Sr₃Ti₂O₇ ceramics (abbreviated as S3T) with grain orientation degree higher than 0.7 were successfully prepared by reactive-templated grain growth (RTGG) technique combined with tape casting process, using plate-like Sr₃Ti₂O₇ particles as templates. The phase transition, microstructure and thermoelectric properties were investigated. As the sintering temperature and soaking time increase, the density and texture fraction of ceramics increase. Textured Sr₃Ti₂O₇ ceramics were sintered in air and annealed in argon + graphene carbon (Ar+C) atmosphere. The results showed that the resistivity of textured Sr₃Ti₂O₇ ceramics annealed in Ar+C environment decreased significantly (seven orders of magnitude). After the annealing process, the texture fraction decreased, and Sr₃Ti₂O₇ phase on the surface of textured ceramics decomposed to SrTiO₃. The Seebeck coefficient was negative, and the absolute value of S increased with increasing temperature. Textured Sr₃Ti₂O₇ ceramics showed the maximum Seebeck coefficient (−399 μV/K) at 1073 K, which is higher than those of non-textured Sr₃Ti₂O₇-based materials.     

Phase evolution and <110>-orientation mechanism in RTGG-processed BaTiO3 ceramics with electrical properties

The <110>-oriented BaTiO₃ ceramics were fabricated using BaCO₃ matrix and H_1.08Ti_1.73O₄.nH₂O (HTO) template particles, and the mechanism of BaTiO₃ phase formation was investigated. The dielectric, ferroelectric, and piezoelectric properties were also investigated. The transformation of the HTO phase into the TiO₂ bronze or TiO₂ (B) phase was observed at 600°C, where the BaTiO₃ nucleation was accompanied by the formation of a Ba₂TiO₄ phase. The TiO₂ phase reacted with the Ba₂TiO₄ phase at 800°C to give a BaTiO₃ phase, whereas its reaction with the BaTiO₃ resulted in the formation of BaTi₂O₅ phase that got decomposed into BaTiO₃ and Ba₆Ti₁₇O₄₀ phase at sintering temperature ≥1300°C. Sintering with samples’ embedding in BaTiO₃ powders prevented the formation of the Ba₆Ti₁₇O₄₀ secondary phase. The crystallographic orientation along the <110> direction (F₁₁₀) was developed by the epitaxial grain growth mechanism. In addition to the contribution of the grain-size increment for enhancing the F₁₁₀, the preservation of the platelike structure was also found to have a significant impact. The ceramics prepared by the embedded sintering (grain size ≈12.4 µm and F₁₁₀ = 83%) exhibited the room-temperature dielectric constant of 1708 and piezoelectric strain constant of 445 pm/V, which are higher than those of the BaTiO₃ ceramics with randomly oriented grains.     

Enhanced intrinsic permittivity and bulk response in Y2/3Cu3Ti4+xO12 ceramics

Considering the contribution of the mixed valence structure of Ti³⁺ and Ti⁴⁺ to the semiconductivity of grain, compositions with the formula of Y_2/3Cu₃Ti_4+xO₁₂ were designed and prepared. The dielectric bulk responses of Y_2/3Cu₃Ti_4+xO₁₂ ceramics were explored in detail. Changing Ti stoichiometry gives rise to an increase of the intrinsic permittivity. Y_2/3Cu₃Ti_3.925O₁₂ ceramic exhibits a higher intrinsic permittivity of ~120 at 60 MHz than that of pure Y_2/3Cu₃Ti₄O₁₂ ceramics (87 at 60 MHz). Additionally, the activation energies of bulk responses are significantly enhanced by changing Ti stoichiometry, which is closely linked with the increase of Ti³⁺/Ti⁴⁺.     

Ceramic pigments based on chromium doped alkaline earth titanates

In this paper M(Ti_1?xCr_x)O₃ with M=Mg, Ca, Sr, Ba and x=0.01, 0.03, 0.05, 0.1 compositions have been prepared by solid state reaction in order to analyze their reactivity, structure, limit of solid solution, stability and pigmenting properties in conventional ceramic glazes. Cr–MgTiO₃ crystallizes magnesium ilmenite MgTiO₃ (SG=R-3) with residual MgCr₂O₄ spinel that inhibits the Cr entrance in solid solution and do not show pigmenting properties. Cr–CaTiO₃ crystallizes orthorhombic perovskite (SG=Pbnm) pink solid solutions associated to Cr⁴⁺–Ti⁴⁺ substitutions which above x=0.03 unstabilizes in glazes and produce light green shades. Cr–SrTiO₃ crystallizes ideal cubic perovskite (SG=Pm3m) pink solid solutions that unstabilizes in glazes above x=0.01. Finally Cr–BaTiO₃ crystallizes tetragonal perovskite (SG=P4mn) with residual peaks of BaCrO₄ at x=0.1 showing pink solid solutions until x=0.05 that unstabilizes in glazes above x=0.03. All powders show aggregates (between 2–10 μm of size) of fine particles (between 200–1000 nm).     

Phonon behaviors and dielectric functions in Bi0.5Na0.5TiO3-based ceramics by Raman scattering and optical ellipsometry

The dielectric functions and lattice vibrations of (0.935-x)Bi_0.5Na_0.5TiO₃-0.065BaTiO₃-xSrTiO₃ (BNBSTx) ceramics were investigated by temperature-dependent spectroscopic ellipsometry and Raman scattering. Based on the analysis of dielectric functions with the Tauc-Lorentz dispersion model, it was found that the band gap and the temperature coefficient increased with the addition of SrTiO₃. Moreover, the frequency, intensity and/or full width at half maximum of phonon modes related to Bi and Ti-localized phonon modes exhibited local maximum/minimum around the depolarization temperature and permittivity-maximum temperature, which indicated the ongoing structural transformation. In addition, the Sr introduction affected the TiO₆ tilting and the off-center displacement of Ti⁴⁺. These results seem to be significant for the enhanced response functions with the addition of SrTiO₃ in BNBSTx ceramics.     

Optimized dual-function varistor-capacitor ceramics of core-shell structured xBi2/3Cu3Ti4O12/(1-x)CaCu3Ti4O12 composites

xBi_2/3Cu₃Ti₄O₁₂/(1-x)CaCu₃Ti₄O₁₂ composites were prepared by traditional solid-state reaction method. Extremely high nonlinear coefficient of 25 and breakdown field of 18.92 kV·cm⁻¹ were obtained in small current range of 0.1−1 mA·cm^-2. In addition, reduced dielectric loss of 0.055 was achieved with high dielectric constant of 1369. Optimized nonlinear and dielectric properties were integrated to make the composites a promising dual-function varistor-capacitor candidate. Microstructure analysis discovered two areas with various Bi/Ca ratio, designated as Bi-H and Bi-L respectively. It was found that the maximum ratio of Bi-H/Bi-L heterogeneous interface corresponded to optimized nonlinear and dielectric performance, which was associated with elevated potential barrier height and huge grain boundary resistance. Combined with relaxation analysis, a core-shell structure was proposed to elaborate microstructure evolution in xBi_2/3Cu₃Ti₄O₁₂/(1-x)CaCu₃Ti₄O₁₂ composite. According to the core-shell model, variation of heterogeneous interface was illustrated on how to influence nonlinear properties, which was well fitted to experimental results.     

Electrical conductivity of hexagonal Ba(Ti0.94Ga0.06)O2.97 ceramics

Impedance spectroscopy is used to estimate the bulk conductivity, σ_b, of hexagonal (6H)-Ba(Ti_0.94Ga_0.06)O_2.97 ceramics in air, N₂ and 5%H₂/95%N₂ between 400 and 1000 °C. Isothermal plots of log σ_b vs log pO₂ in the temperature range between 700 and 1000 °C show the presence of a p–n transition with slopes of ∼−1/4 and +1/4 in the n- and p-type regions indicating that the conductivity obeys the ‘extrinsic’ model proposed by Smyth and co-workers for undoped and acceptor-doped cubic BaTiO₃-based materials [J. Am. Ceram. Soc. 64 (1981) 556; J. Am. Ceram. Soc. 65 (1982) 167]. The activation energy E_a for oxidation in the p-type region to produce free holes is similar for 6H-Ba(Ti_0.94Ga_0.06)O_2.97 and cubic BaTiO₃-based ceramics with an estimated value from σ_b data of ∼0.8 eV. The band gap for 6H-Ba(Ti_0.94Ga_0.06)O_2.97 ceramics is ∼3.2 eV.     

Ferroelectric properties of Li‐doped BaTiO3 ceramics

We prepared 3 kinds of Li⁺‐doped BaTiO₃ ceramics by the solid‐state reaction method: (i) (Ba_1?xLi_x)TiO_3?x/2 having A‐site Li⁺, (ii) Ba(Ti_1?xLi_x)O_3?3x/2 having B‐site Li⁺, and (iii) x/2 Li₂CO₃+BaTiO₃ mixed one, for which we investigated the stable site of Li. The density of all prepared ceramics is above 95%. The results show that the lattice structure, the grain size, and the electric properties of Li⁺‐doped BaTiO₃ ceramics are dependent on Li⁺ site. According to the increase in Li content, the cell volume of Ba_1?xLi_xTiO_3?x/2 decreases, but that of BaTi_1?xLi_xO_3?3x/2 increases. That of x/2Li₂CO₃+BaTiO₃ decreases by the small addition of Li, but increases by the large addition of Li. All Li⁺‐doped ceramics show antiferroelectric‐like double hysteresis loops. The shape of loops and the dielectric properties are also dependent on the Li site. We suggest that the role of oxygen vacancy accompanied by the Li‐doping is important. By comparison with the results of 3 type ceramics, it is concluded that at x/2Li₂CO₃+BaTiO₃ ceramics, the Li⁺ prefers to favorably substitute Ba²⁺ at A site for the low concentration of Li but its location was changed to Ti⁴⁺ site for the high concentration of Li.