High-resolution FIB-TEM-STEM structural characterization of grain boundaries in the high dielectric constant perovskite CaCu3Ti4O12

In this work, the grain boundaries composition of the polycrystalline CaCu₃Ti₄O₁₂ (CCTO) was investigated. A Focused Ion Beam (FIB)/lift-out technique was used to prepare site-specific thin samples of the grain boundaries interface of CCTO ceramics. Scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectrometry (EDXS) and Electron Energy Loss Spectroscopy (EELS) systems were used to characterize the composition and nanostructure of the grain and grain boundaries region. It is known that during conventional sintering, discontinuous grain growth occurs and a Cu-rich phase appears at grain boundaries. This Cu-rich phase may affect the final dielectric properties of CCTO but its structure and chemical composition remained unknown. For the first time, this high-resolution FIB-TEM-STEM study of CCTO interfacial region highlights the composition of the phases segregated at grain boundaries namely CuO, Cu₂O and the metastable phase Cu₃TiO_4.     

The effects of sintered sample thickness on the dielectric properties of CaCu3Ti4O12 ceramics prepared at 1000–1100 °C in air

In this study, the relationship between dielectric properties and sintered sample thickness (d) (0.41–2.74 mm) of CaCu₃Ti₄O₁₂ (CCTO) ceramics prepared at 1000–1100 °C in air was investigated. Compacted green ceramic bodies in varying weights and sintered at different temperatures were prepared to obtain ceramics with varying thicknesses. The samples were evenly polished on either surface, electroded and measured by a precision LCR meter at 20 Hz–1 MHz. The results showed that the ε′ increased monotonically with increasing sintering temperature and indicated a linear relationship with d for samples sintered at ≥1040 °C, while the ε′ for samples sintered at ≤1030 °C was barely affected. Complex impedance analysis showed that the ρ_gb was nearly constant with the change of d. The inverse effect of ε′ and ρ_gb with d can also be observed for the gradual thickness reduction sample sintered at 1040 °C, confirming that the grain boundary plays a key role in the variation of dielectric properties for CCTO ceramics.     

Effects of strontium/lanthanum co-doping on the dielectric properties of CaCu3Ti4O12 prepared by reactive sintering

The extremely high dielectric constant of the cubic perovskite CaCu₃Ti₄O₁₂ (CCTO) has attracted increasing attention for a variety of capacitive elements in microelectronic device applications. In this research, the influence of Sr and La replacing Ca and Cu, respectively, to simultaneously controlling the intrinsic properties of grain boundaries in a co-doped CCTO ceramic has been investigated. The preparation was done using high purity compounds milled and mixed by mechano-synthesis and further consolidated by reactive sintering without calcination. Characterization by XRD confirmed the formation of single-phase CCTO ceramic and a residual amount CaTiO₃. The microstructure and composition analyzed by SEM/EDX showed a smaller grain size for the co-doped CCTO. Impedance measurements indicated the smallest dielectric loss for the co-doped ceramics compare to pure and single-doped CCTO, while reaching a higher dielectric permittivity than single-doped ceramics. The CCTO-SrLa sample also showed high thermal stability of the dielectric permittivity between 100 and 470?K, and the lowest loss between 200 and 300?K. This behavior was attributed to the lower bulk resistance exhibited by the co-doped sample.     

A Novel One‐Step Flame Synthesis Method for Tungsten‐Doped CCTO

An efficient synthesis is used for the first time to prepare CaCu₃Ti_4?xW_xO₁₂ (x = 0.01, 0.03, and 0.05) electroceramics for energy storage capacitors. CaCu₃Ti_4?xW_xO₁₂ ceramics are synthesized via flame synthesis of metal nitrates precursors in nonaqueous solution using cheap, stable, and insoluble solid TiO₂ powder. The pathway yielded a CaCu₃Ti₄O₁₂ (CCTO) phase with the traces of CuO and CaTiO₃ sintered at 1050°C for 30 h. The SEM micrograph shows the grains with smooth surfaces associated with cubical appearance and the size range of 1.5–7, 2.0–7.5, and 2.0–8.0 μm for CCTWO01, CCTWO03, and CCTWO05, respectively. The EDX and XPS analyses show the presence of Ca, Cu, Ti, W, and O elements confirming the purity of these ceramics. The complex impedance and modulus (M) spectroscopy show that the dielectric constant (ε_r) values of the W‐doped CCTO were dominantly affected by the electrical properties of the grain boundary, which is also evident from the SEM micrographs. The grain‐boundary resistance decreased with increasing tungsten content. The activation energies for the grain boundaries were calculated from the impedance and modulus data using the slope of the ln τ versus 1/T and were found to be in the range 0.62–0.67 eV.     

Structure,electrical and luminescent properties of the NBT:Er-0.3CT:Pr composite ceramics

A binary composite ceramic Na_0.5Bi_0.5TiO₃:0.002 Er-0.3CaTiO₃:0.003 Pr (NBT:Er-0.3CT:Pr) was fabricated by a conventional solid state reaction route. The effect of sintering temperature on the microstructure and phase structure of the ceramics has been investigated. And the basic electrical properties such as dielectric properties and ferroelectric performances of the ceramics were examined. Macro long-ranged ferroelectric order cannot be induced via electric field in the composite ceramics. And under local electric field using PFM, obvious ferroelectric switching is identified in the ceramics. Furthermore, the up-conversion luminescence, photochromic reaction, and luminescence modulation have been achieved in the ceramics. Additionally, photo-stimulated luminescence and thermal luminescence are obtained. The present study indicates that optical properties are tightly related to the sintering temperature. The prepared ceramic NBT:Er-0.3CT:Pr owns a piezoelectric performance and multi luminescence properties, which can widen the applications of ferroelectric ceramics.     

Thickness dependent dielectric properties of calcium copper titanate ceramics measured in a controlled atmosphere

Phase pure CaCu₃Ti₄O₁₂ CCTO ceramics are prepared by solid-state synthesis route. The effect of measuring atmospheres (air and dry N₂) on the stability and reproducibility of electrical properties of CCTO as a function of sample thickness (as-prepared to thinned down) is investigated. As-sintered CCTO prepared at 1080 °C for 5 h with an initial thickness of 2.31–2.32 mm is reduced subsequently by fine grinding to 1.835 mm and then to 1.65–1.5 mm. Large inconsistency in the impedance spectra is observed when the samples are measured in air despite the thickness variations. Stable and reproducible dielectric properties are obtained in dry N₂. A relatively closer resistivity (∼2 × 10⁸ Ωcm at 23 °C in N₂) regardless of the sample thickness suggests the absence of any barrier layer at the sample surface. Increased space charge accumulation at grain boundaries (GBs) leading to much larger dielectric constant (ε′) was observed in air at 23 °C. A temperature (from 23 °C to 225 °C) and frequency (from 1 Hz to 1 MHz) independent and stable ε′ is observed when samples are tested in N₂. Much lower tan δ values with large ε′ are observed for both as prepared (0.010 ± 0.001 with ε′ of 9,663 ± 4 at 1.4 kHz) and the thinned down (0.015 ± 0.000 with ε′ of 9,352 ± 5 and 4.5 kHz) samples at 23 °C in N₂.     

Temperature stable dielectric properties of Mg2TiO4–MgTiO3–CaTiO3 Ceramics Over a Wide Temperature range

The zero resonant frequency temperature coefficient (τ_f) of microwave dielectric ceramics (MWDCs) at high and low temperature have attracted great attention in the development of microwave communication equipment. In this work, the Mg₂TiO₄–MgTiO₃–CaTiO₃ (MMC) ceramics with meeting the application requirements of 5G communication were prepared by traditional solid-phase sintering after investigating the relationship among phase compositions of xMg₂TiO₄-(0.931-x)MgTiO₃-0.069CaTiO₃ and 0.34Mg₂TiO₄-0.591MgTiO₃-yCaTiO₃, sintering process, and dielectric properties in detail. The results show that the dielectric properties of MMC ceramics are strongly affected by the phase relative contents of MgTiO₃, Mg₂TiO₄ and CaTiO₃. For instance, MMC ceramics with approximate τ_f = 0 is contributed by mutual compensation of Mg₂TiO₄ and MgTiO₃, in which the Mg₂TiO₄ phase plays an important role in decreasing the τ_f value; and the increase of CaTiO₃ will greatly increase the ε_r value for MMC ceramics, while has a negative effect in the Q × f value. After three-phase regulation, the 0.32Mg₂TiO₄-0.611MgTiO₃-0.069CaTiO₃ microwave dielectric ceramic has a better dielectric temperature stability, associated with dielectric properties of ε_r = 19.7, Q × f = 55,400 GHz (at 8.43 GHz), τ_f- = 4.5 ppm/°C (?40 °C–25 °C), and τ_f+ = ?5.1 ppm/°C (25 °C–90 °C).     

Effects of sintering temperature on the internal barrier layer capacitor (IBLC) structure in CaCu3Ti4O12 (CCTO) ceramics

The formation of the internal barrier layer capacitor (IBLC) structure in CaCu₃Ti₄O₁₂ (CCTO) ceramics was found to be facilitated by the ceramic heat treatment. Electrically insulating grain boundary (GB) and semi-conducting grain interior areas were characterized by impedance spectroscopy to monitor the evolution of the IBLC structure with increasing sintering temperature T_S (975–1100 °C). The intrinsic bulk and GB permittivity increased by factors of ≈2 and 300, respectively and the bulk resistivity decreased by a factor of ≈10³. These trends were accompanied by increased Cu segregation from the CCTO ceramics as detected by scanning electron microscopy and quantitative energy dispersive analysis of X-rays. The chemical changes due to possible Cu-loss in CCTO ceramics with increasing T_S are small and beyond the detection limits of X-ray absorption spectroscopy near Cu and Ti K-edges and Raman Spectroscopy.     

Enhanced dielectric energy storage performance of A-site Ca2+-doped Na0.5Bi0.5TiO3–BaTiO3–BiFeO3 Pb-free ceramics

(1-x) (0.98Na_0.5Bi_0.5TiO₃–0.01BaTiO₃–0.01BiFeO₃)–xCaTiO₃ (NBB-xCT) ceramics were produced using traditional solid-state synthesis methods. The surface morphology, domain structure, and electrical properties of the ceramic samples were systematically studied. In addition, the temperature and frequency stabilities of the NBB-15CT sample at 200 kV/cm were tested. Generally, NBB-xCT ceramics exhibit a typical single perovskite phase structure. The results indicate that the NBB-15CT ceramics showed a high energy density of 3.14 J/cm³ at 250 kV/cm. The piezoresponse force microscopy (PFM) results showed that the addition of CT broke the macrodomains of the 0.98Na_0.5Bi_0.5TiO₃-0.01BaTiO₃-0.01BiFeO₃ ceramic and helped to form nanodomains, leading to an improved energy storage performance. The above performance indicates that the specimens possess very good temperature-and frequency-dependent energy storage performances at 30–150 °C and 1–100 Hz. Moreover, the electric energy storage and release in the NBB-15CT ceramic indicated that the power density could reach 55.30 MV/cm³ at 180 kV/cm. Therefore, the NBB-15CT ceramic is a promising material for electrical capacitors.     

Role of Cu and Y in sintering,phase transition,and electrical properties of BCZT lead-free piezoceramics

Lead-free (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃-xwt%CuOywt%Y₂O₃ (BCZT-Cu_xY_y) ceramics with high piezoelectricity were synthesized by the conventional solid-state reaction method. The role of Cu and Y (Cu/Y) in sintering, phase transition, and electrical properties of such ceramics was systematically studied. The results indicated that the sintering temperatures of BCZT-Cu_xY_y decreased by at least 100?°C due to the low melting point of CuO. The promotion effect of Cu/Y on phase transition lied in the improvement of T_C by 5–15?°C and the coexistence of O+T phase near room temperature. The contribution of Cu/Y to electrical properties was mainly ascribed to the grains growth, the formed oxygen vacancies and lattice distortions, and the donor doping effect of Y³⁺. Adding 0.10?wt% Cu²⁺ and 0.06?wt% Y³⁺ into BCZT dramatically improved the electrical properties as following: d₃₃ =?552 pC/N, ε_m =?10175, ε_r =?4546, tanδ =?0.016, T_C =?100?°C, k_p =?0.475, Q_m =?157.2, P_r =?10.82 μC/cm² and E_C =?2.33?kV/cm. A plausible mechanism was obtained to explain the reaction process and the favorable performances of BCZT-Cu_xY_y. Co-doping Cu²⁺ and Y³⁺ into BCZT could be a promising method to improve and balance the sintering, phase transition, and electrical properties for potential practical applications of lead-free piezoceramics.     

Significantly enhanced breakdown field with high grain boundary resistance and dielectric response in 0.1Na0.5Bi0.5TiO3-0.9BaTiO3 doped CaCu3Ti4O12 ceramics

Electrical performances are strongly associated with the electrical heterogeneity of grains and grain boundaries for CaCu₃Ti₄O₁₂ (CCTO) ceramics. In this work, the dielectric ceramics of 0.1Na_0.5Bi_0.5TiO₃-0.9BaTiO₃ (NBT-BT) doped CCTO were fabricated by a conventional solid-state reaction method, and the ceramics were sintered at 1100 °C for 6 h. Relatively homogeneous microstructures are obtained, and the average grain sizes are characterized about 0.9∼1.5 μm. Impressively, a significantly enhanced breakdown field of 13.7 kV/cm and a noteworthy nonlinear coefficient of 19.4 as well as a lower dielectric loss of 0.04 at 1 kHz are achieved in the 0.94CCTO-0.06(NBT-BT) ceramics. It is found that the improved electrical properties are attributed to the increased grain boundary resistance of 3.7 × 10⁹ Ω and the Schottky barrier height of 0.7 eV. This is originated from the NBT-BT compound doping effect. This work demonstrates an effective approach to improve electrical properties of CCTO ceramics by NBT-BT doping.     

Pseudo Phase Diagram and Microwave Dielectric Properties of Li2O–MgO–TiO2 Ternary System

Li₂O–MgO–TiO₂ ternary system is an important microwave dielectric ceramic material with excellent properties and prospect in both scientific research and application. A phase diagram of the Li₂O–MgO–TiO₂ ternary system was established in this article, based on earlier research results and our present work. Microwave dielectric properties with compositions in different regions of the phase diagram have been analyzed. We found that the 0.33 Li₂MgTi₃O₈–0.67 Li₂TiO₃ ceramics sintered at 1200°C exhibited excellent dielectric properties: Q × f value = 80 476 GHz (at 7.681 GHz), ε_r = 24.7, τ_f = +3.2 ppm/°C. We also designed two ceramic systems in the Li‐rich region of the Li₂O–MgO–TiO₂ ternary system, which received little attention in the past decades, because many excellent single‐phase ceramics, such as Li₂MgTiO₄, Li₂MgTi₃O₈ and MgTiO₃, have been found in the Ti‐rich region. The ceramic systems have low sintering temperatures but also relatively poor dielectric properties.     

Nonlinear I–V electrical behaviour of doped CaCu3Ti4O12 ceramics

In this work a comparative study of undoped CaCu₃Ti₄O₁₂ (CCTO) and doped with Fe³⁺(CCTOF) and Nb⁵⁺(CCTON) ceramics, was aimed to modify the electronic transport. XRD patterns, FE-SEM microstructural analysis, impedance spectroscopy and I–V response curves were afforded to correlate the microstructure with the nonlinear I–V behaviour. The appearance of nonlinear behaviour in doped CCTO samples has been correlated with the ceramic microstructure that consists in n-type semiconductor grains, surrounded by a grain boundary phase based on CuO. The presence of this secondary grain boundary phase is the responsible of the assisted liquid phase sintering in CCTO ceramics. Doped samples showed cleaner grain boundaries than CCTO and nonlinearity in the I–V response.     

Low‐Temperature Sintering of Bi0.5(Na,K)0.5TiO3 for Multilayer Ceramic Actuators

We investigated the influence of CuO amount (0.5–3.0 mol%), sintering temperature (900°C–1000°C), and sintering time (2–6 h) on the low‐temperature sintering behavior of CuO‐added Bi_0.5(Na_0.78K_0.22)_0.5TiO₃ (BNKT22) ceramics. Normalized strain (S_max/E_max), piezoelectric coefficient (d₃₃), and remanent polarization (P_r) of 1.0 mol% CuO‐added BNKT22 ceramics sintered at 950°C for 4 h was 280 pm/V, 180 pC/N, and 28 μC/cm², respectively. These values are similar to those of pure BNKT22 ceramics sintered at 1150°C. In addition, we investigated the performance of multilayer ceramic actuators made from CuO‐added BNKT22 in acoustic sound speaker devices. A prototype sound speaker device showed similar output sound pressure levels as a Pb(Zr,Ti)O₃‐based device in the frequency range 0.66–20 kHz. This result highlights the feasibility of using low‐cost multilayer ceramic devices made of lead‐free BNKT‐based piezoelectric materials in sound speaker devices.     

Influence of Zn doping on microstructures and dielectric properties in CaCu3Ti4O12 ceramic synthesised by semiwet route

Abstract

A single phase of calcium copper titanate [CaCu₃Ti₄O₁₂ (CCTO)] was produced at lower temperature and shorter calcination time via a novel semiwet route. Undoped CCTO and zinc doped CaCu_3?xZn_xTi₄O₁₂ samples with x?=?0·10, 0·20 and 0·30 were prepared by this method for the first time using solid TiO₂ powder in metal nitrate solutions. The CaCu_3?xZn_xTi₄O₁₂ ceramics were characterised by thermogravimetric/differential thermal analysis, X-ray diffraction, SEM and EDX techniques. The SEM images of the sintered CaCu_3?xZn_xTi₄O₁₂ ceramics showed average grain size in the ranges of 2–6, 8–13, 12–16 and 14–20 μm for x?=?0·00, 0·10, 0·20 and 0·30 respectively. Energy dispersive X-ray spectroscopy studies confirm the purity of parent and Zn doped CCTO ceramics. At room temperature, the dielectric constants of Zn doped CCTO are always higher than pure CCTO. CaCu_3?xZn_xTi₄O₁₂ (x?=?0·20) ceramic has the maximum value of ?_r≈4347 along with the minimum value of tan?δ≈0·14 at 1 kHz.     

Columbite-rich multiphase TiO2 nanoceramic with superior mechanical and dielectric properties

Columbite-rich multiphase TiO₂ nanoceramics with outstanding mechanical and dielectric properties were successfully prepared through high-pressure sintering by using anatase-type TiO₂ as precursor. High-pressure sintering combined with phase transformation assisted consolidation can effectively refine the grain size of the recovered samples. This process is conducive to obtain nanocrystalline columbite-rich TiO₂ ceramics with excellent performance. The highest hardness is approximately 12.76 GPa, which is 2.5 times higher than that of ordinary coarse-grained ceramics. The effect of columbite phase on the hardness of multiphase ceramics is discussed. The columbite-rich TiO₂ ceramic shows a colossal permittivity (～8 × 10³) and low loss (～0.2) at 1 kHz and room temperature, which are superior to that of undoped rutile polycrystalline ceramics. This ceramic shows a steadier frequency-dependent dielectric permittivity and loss than rutile TiO₂ crystal. These results enrich the fundamental knowledge of columbite-rich TiO₂, thereby enabling the exploitation of new applications.     

Giant dielectric response and grain heterogeneity of Y2/3Cu3Ti4O12–TiO2 composite ceramics fabricated by hydrothermal process

Due to the boom of the electronic information industry, dielectric ceramic materials are widely applied in passive components such as multilayer ceramic capacitors. In this article, Y_2/3Cu₃Ti₄O₁₂–TiO₂ composite ceramics with outstanding dielectric properties (ε_r = 2.29 × 10⁶ at 20 Hz, ε_r = 4.49×10⁵ at 1 kHz) were successfully prepared by hydrothermal and in situ solid-state methods. The minimum dielectric loss value (at 1 kHz) of the obtained composite ceramics is about 0.57. The experimental results show that Y_2/3Cu₃Ti₄O₁₂ has a composite perovskite structure with cation vacancies, which causes internal electron-pinned defect-dipole, and the easy entry of excessive titanium ions to produce donor defects. Meanwhile, the heterogeneous grains with a pomegranate-like microstructure were observed in the prepared composite ceramics, and interfacial polarization between subgrain boundaries was substantiated by impedance analysis. The abundant weakly trapped electrons and more polarized interfaces formed by grain and subgrain boundaries are the primary sources of the excellent dielectric properties of composite ceramics.     

The influence of TiO2 content on the properties of glass ceramics: Crystallization,microstructure and hardness

The effects of compositional variation, crystallization behavior, crystalline phases and microstructure formed in the SiO₂3Al₂O₃3CaO (SAC) glass system using various amounts of TiO₂ as nucleating agent were investigated by Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and Fourier transform infrared spectroscopy (FTIR) techniques. The crystallization kinetics and mechanical properties of SAC glass ceramics were studied using crystallization peak temperature (T_p) of three different glasses as obtained from DTA, the activation energy (E) and Avrami exponent (n) were also determined. The crystallization peak temperature (T_p) and activation energy (E) were found to increase with the increase in TiO₂ content. The major crystalline phases were anorthite and wollastonite along with gehlenite and titanite as the minor crystalline phases present in the glass ceramic system. The studies showed that the three dimensional crystalline structure and the microhardness increased with the increase of TiO₂ content in the glass ceramics system.     

Effects of TiO2 additive on ultra-low-loss MgO–LiF microwave dielectric ceramics

MgO ceramics have good microwave dielectric properties, but the high sintering temperatures limit its application. The effects of TiO₂ additive on the phase composition and microwave dielectric properties of MgO ceramics with 4mol%LiF were investigated by solid state reaction method. TiO₂ and MgO form Mg₂TiO₄ in a magnesium-rich environment with 4mol%LiF at about 900 °C, which as a solid solution or second phase had a huge impact on MgO ceramic with 4mol % LiF. When the content of TiO₂ less than 2mol %, Mg₂TiO₄ as a solid solution in MgO ceramics, which made the grain of MgO larger. When the content of TiO₂ more than 2mol %, Mg₂TiO₄ as a second phase in MgO ceramics, which made the microwave dielectric properties of MgO ceramics bad. Typically, the MgO-4mol%LiF-0.5mol%TiO₂ ceramic sintered at 1075 °C for 6 h acquired the best dielectric properties: ε_r = 9.7, Qf = 617,000 GHz and τ_f = −59.49 ppm/°C.     

Optimize the dielectric properties of CaCu3Ti4O12 ceramics by adjusting the conductivities of grains and grain boundaries

Reduction of dielectric loss for CCTO ceramics is a prerequisite for their applications. Considering internal barrier layer capacitance effect, improving the capacitance and grain boundary resistance is an effective way to reduce dielectric loss. Therefore, more conductive Ti³⁺ and Cu⁺ ions were introduced to grains by adding carbon to ceramic bodies, improving the permittivity of CCTO ceramics. Annealing was performed to increase the grain boundary resistance. The dielectric loss of the CCTO ceramics thus prepared, which maintain a giant permittivity, is significantly reduced. Specifically, the CCTO ceramic with carbon addition, which was sintered at 1080 °C for 8 h and air annealed at 950 °C for 2 h, exhibits a giant permittivity of about 2.50(5)×10⁴ and a low dielectric loss of less than 0.050(2) from below 20 Hz to 50 kHz at room temperature. Meanwhile, its dielectric loss at 1–10 kHz is less than 0.050(2) from below room temperature to about 100 °C.