CaCu3Ti4O12 ceramics from different methods: microstructure and dielectric

CaCu₃Ti₄O₁₂ (CCTO) ceramics were synthesized by methods of sol–gel, traditional solid-state reaction, and thermal decomposition of organic solution. The results exhibit that the microstructures and electric characteristics are affected by the methods of synthesis. The X-ray diffraction patterns show that all samples have a perovskite-like CCTO phase. Moreover, CCTO ceramic from traditional solid-state reaction have the phases of TiO₂ and CuO. The scanning electron microscopy images show that CCTO ceramics from different methods have different grain sizes, grain boundaries, and densities. Dielectric properties of the CCTO ceramics were characterized in a broad frequency range (10–10⁷ Hz) at room temperature. The CCTO ceramics from thermal decomposition of organic solution have the dielectric constant of more than 5 × 10⁴ at 10 Hz. The nonlinear relationship between the current density and the electric field strength can be observed in all the three samples.     

Low-temperature synthesis of CaCu3Ti4O12 powders, ceramics and thin films via an organic solution

The giant-dielectric-constant material CaCu₃Ti₄O₁₂ (CCTO) was synthesized via an organic solution containing stoichiometric amounts of the metal cations, which is done at lower temperature and shorter reaction time than the conventional solid-state reaction. A stable solution was prepared by dissolving calcium nitrate, copper nitrate, and tetrabutyl titanate in grain alcohol. CCTO powders, ceramics and thin films were synthesized via the solution. The phases, microstructures, and dielectric properties of samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric spectroscopy. XRD results identify both samples as single phase CCTO. The CCTO ceramics has a low-frequency permittivity of 3.5 × 10⁴. The CCTO thin films has a low-frequency permittivity of 3.1 × 10⁴. Both the CCTO ceramics and CCTO thin films exhibit two dielectric relaxations at room temperature. The low leakage current density of CCTO thin films shows that it is suitable for memory device applications.     

Oleic acid assisted synthesis of CaCu3Ti4O12 powders and ceramics by sol–gel process

The CaCu₃Ti₄O₁₂ (CCTO) powders were prepared via a sol–gel method using oleic acid (OA) as surfactant. The obtained samples were characterized by thermogravimetric/differential scanning calorimeter Fourier transform-infrared spectroscopy, X-ray diffraction (XRD) and scanning electron microscope, the relative density and the dielectric properties of the ceramics were also investigated. The XRD patterns confirm the formation of CCTO crystal phase and no new peaks appeared whether the dispersant was added or not. The dispersibility of the CCTO powders is improved by adding OA as surfactant. The CCTO prepared without surfactant contains particles of various sizes ranging from 200 to 250 nm whereas the CCTO prepared with OA shows uniform particle size about 120–150 nm. The CCTO ceramics prepared with OA has higher dielectric constant (199484) and lower dielectric loss (0.0977) at 25 °C (10 kHz).     

Effects of SrO–B2O3–SiO2 glass additive on densification and energy storage properties of Ba0.4Sr0.6TiO3 ceramics

SrO–B₂O₃–SiO₂ glass powders were prepared and employed as sintering aids to reduce the sintering temperature of Ba_0.4Sr_0.6TiO₃ ceramics. The effects of glass content and sintering temperature on the densification, dielectric properties and energy storage properties of Ba_0.4Sr_0.6TiO₃ ceramics have been investigated. The relative density characterization results indicate that densification of Ba_0.4Sr_0.6TiO₃ ceramics with glass content becomes apparently from sintering temperature of 1,060 °C. XRD results show all Ba_0.4Sr_0.6TiO₃ ceramics exhibit a perovskite structure without the formation of a secondary phase. The dielectric constant and dielectric loss decreased gradually with increasing glass content. The relationship between dielectric constant and breakdown strength was discussed using the thermochemical model. A discharged energy density of 0.44 J/cm³ with an energy efficiency of 67.4% was achieved for Ba_0.4Sr_0.6TiO₃ ceramic with 2.0 wt% glass addition sintered at 1,180 °C.     

Optimized sintering properties and temperature stability of MgZrTa<Subscript>2</Subscript>O<Subscript>8</Subscript> ceramics with CuO addition for microwave application

Microwave dielectric ceramics CuO–modified MgZrTa₂O₈ were synthesized by the conventional solid-state reaction method. The effects of CuO additives on the sintering characteristics and microwave dielectric properties have been investigated. With CuO addition, the sintering temperature of MgZrTa₂O₈ ceramics can be effectively lowered from 1475 to 1375 °C without decreasing its dielectric properties obviously and the temperature coefficient of the resonant frequency of MgZrTa₂O₈ ceramics have been optimized to near-zero. The crystalline phase exhibited a wolframite crystal structure and no second phase was detected at low addition levels. The grain growth of CuO–modified MgZrTa₂O₈ ceramics was accelerated due to liquid phase effect. The relative dielectric constants (ε_r) were correlated with apparent density and were not significantly different for all levels of CuO concentration. The quality factors (Q?×??) and temperature coefficient of resonant frequency (τ_?), which were strongly dependent on the CuO concentration, were analyzed by the grain size and the dielectric constant respectively. A best Q?×?? value of 116400 GHz and τ_? value of ?6.19 ppm/℃ were obtained for specimen with 0.05 wt% CuO addition at 1375 °C.     

Very low loss tangent and giant dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics prepared by the sol–gel process

The pure phase of CaCu₃Ti₄O₁₂ (CCTO) powder can be successfully synthesized by the sol–gel process. CCTO ceramic samples were synthesized at different sintering temperatures of 1015 and 1050?°C and sintering times of 8 and 10 h. X-ray diffraction results indicated a pure phase for all ceramic samples. Rietveld refinements were adopted for the calculation of lattice constants. Scanning electron microscopy micrographs revealed the effect of sintering conditions on the microstructural evolution of ceramic samples. X-ray absorption near edge spectroscopy was performed to determine the oxidation state of Cu and Ti ions in ceramic samples. The dielectric and non-linear current voltage properties of CCTO ceramic samples were systematically investigated. Interestingly, very low loss tangent (tanδ?<?0.017 at 30?°C and 1 kHz) and giant dielectric constant (ε′?～?10,942) with temperature coefficients less than ±15% in a wide temperature range of ?60 to 125?°C were obtained in the CCTO ceramic sample sintered at 1015?°C for 10 h (CCTO1-10). This suggests a potential use for CCTO1-10 sample in capacitor applications. All CCTO ceramic samples display non-linear characteristic with non-linear coefficient (α) and breakdown field (E _b ) values in the range of 5.69–11.02 and 1415–4294, respectively.     

Sintering behavior and microwave dielectric properties of Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZnO doped (Zr<Subscript>0.8</Subscript>Sn<Subscript>0.2</Subscript>)TiO<Subscript>4</Subscript> ceramics

(Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics were fabricated via conventional solid-state reaction method. Sintering behavior, phase composition, microstructure and microwave dielectric properties of Y₂O₃–ZnO doped ZST ceramics were investigated. Only a single ZST phase was identified by X-ray diffraction patterns. The variation tendencies of dielectric constants as well as Q × f values were in accordance with the bulk densities. The appropriate Y₂O₃ and ZnO additions could not only efficiently lower the sintering temperature to 1240 °C, but also noticeably improve the densification and microwave dielectric properties of ZST ceramics. But excessive additives deteriorated the microstructures and comprehensive properties of samples. A dielectric constant ε _r of 39.73, a Q × f value of 48,545 GHz (at 5.5 GHz), and a τ _f value of ?2.13 ppm/°C were obtained for 1 wt% ZnO doped ZST ceramics with 0.5 wt% Y₂O₃ addition sintered at 1240 °C.     

Improved microwave dielectric properties of Mg4Nb2O9 ceramics with CaO–B2O3–SiO2 glass additions

The effects of CaO–B₂O₃–SiO₂ (CBS) glass addition on the sintering temperature and dielectric properties of Mg₄Nb₂O₉ ceramics have been investigated using X-ray diffraction, Scanning electron microscopy and Differential thermal analysis. The CBS glass can change to liquid phase at about 750 °C and a small amount of CBS glass addition to Mg₄Nb₂O₉ ceramics can greatly decrease the sintering temperature to about 1,125 °C. It is revealed that the reduced sintering temperature is attributed to the formation of liquid phase. The major phases of the sample are Mg₄Nb₂O₉ and MgNb₂O₆. The relationship between τ _f values and the content of glass additions have the reverse change trends. The Mg₄Nb₂O₉ ceramics with 2wt% glass addition sintered 1,125 °C exhibit good microwave dielectric properties: dielectric constant (ε _r ) of 13 and Q·f value of 69,000 GHz.     

Effect of spark plasma sintering temperature on the phase equilibria and dielectric properties of BaTi2O5 ceramics

The effect of sintering temperature (ranging from 1055 to 1200 °C) on the phase ingredient and dielectric property of the nominal BaTi₂O₅ ceramics (starting with the Ba/Ti of 1:2) fabricated by a spark plasma sintering method were systematically studied. At the first stage, BaTi₂O₅ component was enhanced in the sintering temperature range of 1055–1120 °C; it turned out to be the dominant phase. For these BaTi₂O₅ phase dominated ceramics, the Curie temperature T _c rised on increasing the sintering temperature and saturated around 440 °C with the maximum dielectric constant of 500. Further increasing the sintering temperature, the decomposition of the obtained BaTi₂O₅ into BaTiO₃ extensively happened; the ceramics turned to be the BaTi₂O₅ and BaTiO₃ coexisting state. These ceramics can be characterized by two dielectric anomalies. One at ~420 °C stood for the phase transition of BaTi₂O₅ while the other at ~150 °C stood for the transition of BaTiO₃, which is exceptionally high as the normal BaTiO₃ ceramics. Further increasing the sintering temperature (until 1200 °C) would dramatically enhance the BaTiO₃ phase; the ceramics showed T _c at 130 °C with the maximum dielectric constant of 1800.     

Effect of crystallizable glass addition on sintering and dielectric behaviors of barium titanate ceramics

A crystallizable glass which can precipitate barium titanate was added to BaTiO₃ ceramics to study its effect on sintering behavior and dielectric properties of the composites. High densification (>95 % theoretical density) was achieved by addition of glass phase and the dielectric constant of composites was enhanced through the crystallization of glass phase. A composite with 90 wt% BaTiO₃ and 10 wt% glass showed a dielectric constant of ~2,300 at room temperature at 1 kHz and a dielectric breakdown strength about 140 kV/cm.     

Effects of SrO–B2O3–SiO2 glass additive on dielectric properties of Ba(Fe0.5Nb0.5)O3 ceramics

SrO–B₂O₃–SiO₂ (SBS) glass powders were prepared and employed as sintering aids to reduce the sintering temperature of Ba(Fe_0.5Nb_0.5)O₃ (BFN) ceramics. The effects of glass content on the dielectric properties and breakdown strength of BFN ceramics have been investigated. The volume density characterization results of (1 ? x) BFN ? x SBS ceramics indicate that the sintering temperature of BFN ceramics decreased by 200–350 °C with SBS glass addition (when x = 0, 0.01, 0.03 and 0.05). The XRD patterns show BFN ceramics indicate cubic crystal structure and without the formation of a secondary phase. The dielectric constant and dielectric loss decreased gradually with increasing glass content, and the dielectric loss decreased by one order of magnitude with SBS glass addition (when x = 0.05). The breakdown strength of (1 ? x) BFN ? x SBS ceramics increase with increasing glass content, in which is about 33.90 kV/cm with SBS glass addition (when x = 0.05). These improvements in the dielectric characteristics of BFN ceramics have great scientific significance for their applications.     

Microstructure and dielectric properties of Ca1–3/2xBixCu3Ti4O12 (x = 0, 0.05, 0.10, 0.15 and 0.20) ceramics

Influence of bismuth substitution on calcium site in CaCu₃Ti₄O₁₂ has been investigated. Compositions of Ca_1-3/2xBi_xCu₃Ti₄O₁₂ (x = 0, 0.05, 0.10, 0.15 and 0.20) were fabricated by solid-state sintering method. Crystal structure is remained cubic. X-ray diffraction indicates the presence of secondary phase of CuO in CCTO ceramics. Bismuth doping restrains the formation of CuO phase apparently. The grain size of CaCu₃Ti₄O₁₂ ceramics was greatly decreased by Bi³⁺ doping, resulting from the ability of bismuth to inhibit the grain growth. The dielectric and electric properties of CCTO ceramics were found to be influenced by bismuth doping. The fitting results of the complex impedance spectra showed an increase of the resistance of grain and grain boundary by bismuth substitution. Ca_0.70Bi_0.20Cu₃Ti₄O₁₂ showed the highest dielectric constant in the low frequency range. A modest composition such as Ca_0.85Bi_0.10Cu₃Ti₄O₁₂ expressed the optimized dielectric properties of higher dielectric constant (1.3 × 10⁴) and lower dielectric loss (0.06) than pure CCTO. The low and high temperature dielectric loss spectra demonstrate the interfacial polarization of the initial and secondary oxygen ionization, relating with the grain and grain boundary (the electrode contact for Ca_0.70Bi_0.20Cu₃Ti₄O₁₂) respectively.     

Effects of glass additions on the dielectric properties and energy storage performance of Pb0.97La0.02(Zr0.56Sn0.35Ti0.09)O3 antiferroelectric ceramics

In this work, CdO–Bi₂O₃–PbO–ZnO–Al₂O₃–B₂O₃–SiO₂ low softening point glass powders were prepared and employed as sintering aid to improve the dielectric breakdown strength and reduce the sintering temperature of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics. The effects of glass content and sintering temperature on the densification, microstructure, dielectric properties and energy storage performance of Pb_0.97La_0.02(Zr_0.56Sn_0.35Ti_0.09)O₃ antiferroelectric ceramics have been investigated. With inclusion of glass, sintered densities comparable to those obtained by conventional sintering are achieved at only 1,050 °C. The breakdown strength of glass-added samples was notably improved due to the reduction of the grain size. The antiferroelectric to ferroelectric switching field and the ferroelectric to antiferroelectric field both increased with increasing glass content. The dielectric constant and dielectric loss decreased gradually with increasing glass content. As a result, the highest recoverable energy density of 3.3 J/cm³ with an energy efficiency of 80 % was achieved in 4 wt% glass-added sample sintered at 1,130 °C.     

Effects of BaNb2O6 addition on microstructure and dielectric properties of BaTiO3 ceramics

(1 ? x)BaTiO₃–xBaNb₂O₆ [(1 ? x)BT–xBN] ceramics with x = 0, 0.005, 0.008, 0.01, 0.02, 0.03 were prepared by a conventional solid-state reaction route. The effect of BN addition on phase composition, microstructure and dielectric properties of BT-based ceramics were investigated by X-ray diffraction, scanning electron microscope and impedance spectroscopy. The results showed that a systematic structure change from the ferroelectric tetragonal phase to pseudo-cubic phase was observed near x = 0.01 at room temperature. It resulted in a considerable change of density, grain size and dielectric properties of the samples when BN was introduced. Meanwhile, it also lowered the sintering temperature of the ceramics. The dielectric constant peak and the variation rate of capacitance at Curie temperature are markedly depressed and broaden with increasing BN content. Especially, the ceramics with x = 0.008 and x = 0.01 showed good dielectric properties over the measured temperature range. Optimal dielectric properties of ε = 3,851, tanδ = 0.7 % at room temperature and Δε/ε₂₅ ≤ ±6.8 % (?55 to 125 °C) were obtained for the BT-based ceramics doped with 0.8 mol% BN, which was obviously superior to BaTiO₃ and BaNb₂O₆ ceramic, and it met the requirements of EIA X7R specifications.     

The influence of the segregation of Cu-rich phase on the microstructural and impedance characteristics of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The influence of sintering conditions on the microstructural features and impedance characteristics of the giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) was investigated. The microstructure and impedance characteristics were found to be strongly dependent on the sintering conditions. Sintering of the CCTO ceramics at elevated temperatures (>1100 °C) for prolonged durations resulted in the segregation of Cu-rich phase, mostly confined to the surface, which was in concomitance with the appearance of the additional semicircle at the low frequency end in Impedance (Z*) plots. The absence of this additional semicircle in the Cu-deficient CCTO ceramics and the appearance of the same in Cu-rich CCTO ceramics that were deliberately fabricated corroborated the above observations. Also, La_2/3Cu₃Ti₄O₁₂ (LCTO), a low dielectric constant member of CCTO family, which consisted of small grains without the segregation of Cu-rich phase at the grain boundary, did not reveal the presence of additional semicircle in the Z*plots.     

Effects of MnO2 doping on microstructure and microwave dielectric properties of Ba4.2Nd9.2Ti18O54–NdAlO3 ceramics

MnO₂ doped Ba_4.2Nd_9.2Ti₁₈O₅₄–NdAlO₃(13 wt%) (BNT–NA) microwave dielectric ceramics with the near zero τ _f and the wide range of sintering temperature were prepared by conventional solid state method. The effects of Mn⁴⁺ doping on the microstructures and microwave dielectric properties of BNT–NA ceramics were investigated. XRD patterns showed only a single BaNd₂Ti₅O₁₄ phase was identified in all samples and there was no second phase. The sintering temperature decreased from 1,380 to 1,320 °C as MnO₂ content increased from 0.1 to 0.9 wt%. The MnO₂ doped BNT–NA ceramics could be densified at a lower sintering temperature. The MnO₂ additive had a positive effect on lowing sintering temperature of BNT–NA ceramics. The τ _f varied from negative to positive with the increase of MnO₂. Excellent microwave dielectric properties were achieved in Ba_4.2Nd_9.2Ti₁₈O₅₄–NdAlO₃ ceramics doped with 0.3 wt% MnO₂ and sintered at 1,380 °C for 2 h: ε _r  = 66.5, Q × f = 13,948 GHz, τ _f  = 0.4 ppm/°C.     

Effect of sintering temperature on the microstructure and electrical properties of zirconium doped barium titanate ceramics

Lead-free Ba(Zr_0.15Ti_0.85)O₃ (BZT15) ceramics were synthesized by adopting the solid-state synthesis method. The effect of increasing sintering temperature (T_s) in the range of 1,350–1,450 °C on the microstructure, dielectric, polarization, and electric field induced strain of the ceramics was studied. Fine grained (~260 nm) BZT15 ceramics displayed single phase perovskite structure with relative densities >94 % of the theoretical density. Both grain size and shape were influenced by the sintering parameters. With increase in T_s, not only the maximum dielectric constant decreased from 11,412 to 8,734 along with an increase in the degree of diffuseness, but also interestingly the Curie temperatures were found to vary within an interval of 61–73 °C. Optimum sintering temperature has been found resulting in high remnant polarisation and strain in these ceramics. The properties observed are attributed to a contribution from all polar vectors present in coexistent phases.     

Study on properties of Ca2Zn4Ti15O36 ceramics with CaO-B2O3-SiO2 glass

The effects of CaO-B₂O₃-SiO₂ (CBS) glass additions on the sintering temperatures and dielectric properties of Ca₂Zn₄Ti₁₅O₃₆ ceramics have been investigated using X-ray diffraction, Scanning electron microscopy. A small amount of CBS glass addition to Ca₂Zn₄Ti₁₅O₃₆ ceramics can greatly decrease the sintering temperature to about 975 °C. It is revealed that the reduced sintering temperature is attributed to the formation of liquid phase. The major phases of the samples are Ca₂Zn₄Ti₁₅O₃₆ and rutile. The more glass added, the more rutile phase formed. The τ_f values shift towards negative first and then toward positive with the increasing glass additions. The Ca₂Zn₄Ti₁₅O₃₆ ceramics with 4 wt% glass addition sintered 1,050 °C for 2 h exhibit good microwave dielectric properties: Q·f values of 31,000 GHz and dielectric constant (ε _r ) of 44.7.     

Low temperature sintering and dielectric properties of Li2ZnTi3O8–TiO2 composite ceramics doped with CaO–B2O3–SiO2 glass

The effects of CaO–B₂O₃–SiO₂ (CBS) glass addition on the sintering temperature and dielectric properties of Li₂ZnTi₃O₈–TiO₂ (LZT) composite ceramics have been investigated. Due to the compensating effect of rutile TiO₂ (τ_f ≈ +450 ppm/ °C), the temperature coefficient of resonant frequency (τ_f) for Li₂ZnTi₃O₈ + 4 wt% TiO₂ with biphasic structure was adjusted to a value near zero. The pure LZT ceramics were usually sintered at high temperature of about 1,160 °C. It was found in our experiment that a small amount of CBS glass additives could effectively lower the sintering temperature of LZT ceramics to 900 °C. With increasing the content of CBS glass, both of dielectric constant (ε_r) and quality factor (Q × f) value decreased. Typically, the 1 wt% CBS glass added Li₂ZnTi₃O₈ + 4 wt% TiO₂ ceramic sintered at 900 °C for 4 h exhibited good microwave dielectric properties of ε_r = 26.9, Q × f = 23,563 GHz and τ_f = ?1.5 ppm/ °C, which made it promising for low temperature co-fired ceramics technology application.     

Influence of different dopant sites by lanthanum on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The effects of La³⁺ doped in calcium copper titanate (CCTO) at Ca²⁺ site and Cu²⁺ site were examined. The doped compositions, La_0.1Ca_0.85Cu₃Ti₄O₁₂ (LCCTO) ceramics and CaLa_0.1Cu_2.85Ti₄O₁₂ (CLCTO) ceramics were prepared by the solid-state method. The microstructure, dielectric properties, complex impedance and nonlinear I–V characteristics were studied. And it was found that La³⁺ doped at Ca²⁺ site achieved lower sintering temperatures than that doped at Cu²⁺ site in CCTO ceramics. The dielectric loss (tan δ) of LCCTO ceramics was about 0.05 at 40 kHz when the sample was sintered at 1080 °C. Dielectric constant (ε′) of LCCTO ceramics was about 3.2 × 10⁴ when the sample was sintered at 1100 °C, which was larger than CLCTO ceramics examined under the same process condition with sintering temperatures vary. The impedance analysis revealed that LCCTO ceramics had an influence of resistance of grain boundaries, which was stronger than that of CLCTO ceramics. Meanwhile, both LCCTO ceramics and CLCTO ceramics had a nonlinear-Ohmic property.