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.     

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.     

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 Kaolinite additions on sintering behavior and dielectric properties of CaCu3Ti4O12 ceramics

Commercial Kaolinite was employed as sintering aid to reduce the sintering temperature of CaCu₃Ti₄O₁₂ (CCTO) ceramics. The effects of Kaolinite content and sintering temperature on the densification, microstructure and dielectric properties of CCTO ceramics have been investigated. The density characterization results show that the addition of Kaolinite significantly enhanced the relative density of CCTO ceramics to about 92 %. X-ray diffraction results show CCTO ceramics with a low amount of Kaolinite exhibited perovskite-like structure, but 1.0 wt% Kaolinite additions resulted in the formation of a secondary phase, CaO–TiO₂–Al₂O₃–SiO₂ glass phase was formed and improved the dielectric constant of ceramics, which was supported by scanning electron microscopy–energy dispersive X-ray results. CCTO ceramic with 1.0 wt% Kaolinite addition possessed well temperature and frequency stability of dielectric constant. It was found that Kaolinite lowered the dielectric loss of the samples.     

Studies of structural, spectroscopic and electrical properties of sodium molybdate ceramics

Anhydrous polycrystalline sodium molybdate (Na₂MoO₄) ceramics has been prepared by a solid-state reaction technique. The formation of the compound in cubic system is confirmed by a preliminary structural analysis using X-ray diffraction data. Energy dispersive spectrum analysis of Na₂MoO₄ has confirmed its chemical formula and composition. Spectroscopic studies of the compound have been carried by a vibration spectroscopy (Raman/FTIR) in order to understand its molecular structure at microscopic level. The complex impedance spectroscopy technique has been used to study the electrical properties of the material as a function of frequency (10²–10⁶ Hz) at different temperatures (23–450 °C), and also to investigate the fundamental mechanism involved in the material. Impedance analysis also indicates that below 300 °C, the material electrical conduction is related to the grain volume. Above 300 °C, the contribution of grain boundary is clearly evident. The electrical processes in the material are found to be temperature-dependent, and due to relaxation phenomena in it. A frequency dependent maximum of the imaginary electrical impedance is found to obey an Arrhenius law with activation energy of 1.07 eV. The frequency dependence of electrical conductivity spectra does follow the universal power law.     

Sol-gel derived CaCu3Ti4O12 ceramics: Synthesis, characterization and electrical properties

The giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) has been synthesized by sol-gel method, for the first time, using nitrate and alkoxide precursor. The electrical properties of CCTO ceramics, showing an enormously large dielectric constant ? ∼ 60,000 (100 Hz at RT), were investigated in the temperature range from 298 to 358 K at 0, 5, 10, 20, and 40 V dc. The phases, microstructures, and impedance properties of final samples were characterized by X-ray diffraction, scanning electron microscopy, and precision impedance analyzer. The dielectric permittivity of CCTO synthesized by sol-gel method is at least three times of magnitude larger than that synthesized by other low-temperature method and solid-state reaction method. Furthermore, the results support the internal barrier layer capacitor (IBLC) model of Schottky barriers at grain boundaries between semiconducting grains.     

Abnormal dielectric behaviors in Mn-doped CaCu3Ti4O12 ceramics and their response mechanism

Mn-doped CaCu₃Ti₄O₁₂ (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu₃Ti₄O₁₂ by about 2 orders of magnitude (from 10⁴ to 10²). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity.     

Relationship between microstructural evolution and electric properties of B2O3–CaCu3Ti4O12 composite ceramics

Calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) powder was mixed with boric oxide (B₂O₃) glassy phase up to 8% by weight to prepare the composite ceramics. The effects of B₂O₃ incorporation on the microstructures, electric and dielectric properties of CCTO ceramics have been systematically investigated. Adding B₂O₃ results in both the decrease of the CCTO grain size. These microstructural changes conjointly affect the dielectric constant. In addition, further B₂O₃ increase leads to its aggregation at grain boundaries. Meanwhile, the B₂O₃ addition reduces the nonlinear coefficient and influences the breakdown electric field.     

Effect of sintering temperature on the dielectric and varistor properties of SnO<Subscript>2</Subscript>–Zn<Subscript>2</Subscript>SnO<Subscript>4</Subscript> composite ceramics

The effect of sintering temperature from 1350 to 1450 °C on the dielectric and varistor properties of SnO₂–Zn₂SnO₄ composite ceramics has been systematically investigated. With the increasing of sintering temperature, the average grain size increased from about 1 to 5 μm and the breakdown electric field decreased from 117 to 3 V/mm. The relative dielectric constant increased with sintering temperature and it achieved the maximum of 1.2 × 10⁴ (40 Hz, 0 °C) at 1425 °C. With excessive increasing of sintering temperature, the relative dielectric constant decreased and the microstructure of the ceramic bulk became porous. In the spectra of imaginary part of the complex modulus, a peak was exhibited and the peak’s position shifted to high frequency with increasing testing or sintering temperature. The activation energy related to the peak was about 0.4 eV and this value was thought to be associated with the oxygen vacancies. Based on the sintering effect, the mechanism of oxygen vacancies in SnO₂–Zn₂SnO₄ composite ceramics was proposed and accordingly, the varistor and giant permittivity properties are well understood based on the grain boundary barrier model.     

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).     

Microstructures and dielectric properties of Ba[(Co0.7Zn0.3)1/3Nb2/3]O3-based ceramics prepared by aqueous gelcasting-assisted solid-state method

The aqueous gelcasting-assisted solid-state method (AGAS) and traditional solid-state reaction method (TSSR) were used to prepare Ba[(Co_0.7Zn_0.3)_1/3Nb_2/3]O₃ (BCZN)-based ceramics. The effects of different powder-preparation methods on the microstructures and dielectric properties of the BCZN-based ceramics were investigated. X-ray diffraction analysis showed completely the same phase compositions regardless of the preparation method adopted. The relative permittivity (ε _r) did not significantly vary between the two methods. However, BCZN with CeO₂-added ceramics prepared by AGAS had higher and more uniform density (6.374 g/cm³) and high quality factor of resonant frequency (Q × f) value (75,843 GHz) than those prepared by TSSR because of the more uniform microstructures, as shown by scanning electron microscopy images. The temperature coefficients of resonant frequency (τ _f  = 7.4 ppm/°C) of the ceramics prepared by AGAS were also closer to zero than those prepared by TSSR.     

Dielectric responses of Na<Subscript>0.65</Subscript>Bi<Subscript>0.45</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics based on the composition design of changing the Na/Bi ratio

Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics were successful prepared by the conventional solid-state reaction technique. Compared to Na_0.50Bi_0.50Cu₃Ti₄O₁₂ (NBCTO), the composition of Na_0.65Bi_0.45Cu₃Ti₄O₁₂ was designed in terms of changing the Na/Bi ratio. Colossal dielectric permittivity of ~1.2 × 10⁴ at 1 kHz was obtained in Na_0.65Bi_0.45Cu₃Ti₄O₁₂ ceramics. Interestingly, three frequency dispersions were observed in the frequency dependence of dielectric constant measured at different temperatures. The investigation of electric modulus displayed that the giant low-frequency dielectric constant was attributed to Maxwell–Wagner polarization at the grain boundaries and the frequency dispersion in middle-frequency range was due to the grain polarization. Except grain response and grain boundaries response reflected by two semicircles in the impedance spectroscopy, another electrical response associated with nonzero high frequency intercept was found. The grain resistance Rg and grain boundaries resistance R _gb was ~600 Ω and 3.9 × 10⁵ Ω, respectively. The large intrinsic permittivity as high as ~700 was obtained. Furthermore, two dielectric anomalies observed in the temperature dependent of dielectric constant were discussed in detail. The results indicated change in the Na/Bi ratio had a significant effect on the electrical properties of NBCTO ceramics.     

Preparation of the LTCC composite ceramics with low permittivity

In this work, the LTCC composite ceramics containing ??-alumina and quartz based on the binary system BaO?CB₂O₃ were prepared by traditional solid-state preparation process at a sintering temperature of 900 °C. Sintering mechanism and physical properties of the LTCC composite ceramics are investigated and discussed in detail in terms of their mineral phase composition. The results indicate that, by the chemical combination of barium hydroxide octahydrate and an aqueous solution of boric acid, a barium borate phase can be formed form the binary system BaO?CB₂O₃ and consequently supply a liquid sintering aid for the fabrication of LTCC composite ceramics at a sintering temperature of 900 °C. The introduction of ??-alumina to the binary system BaO?CB₂O₃ can improve the sintering behavior whereas the presence of quartz in the composite ceramics is important to achieve low permittivity. By the combination of ??-alumina, quartz and BaO?CB₂O₃ composition, the dense LTCC composite ceramics, which is characterized by excellent dielectric properties (permittivity: 3.56; 4.83; dielectric loss: 3 × 10^?4; 4 × 10^?4), can be fabricated availably.     

Influence of Sr/Ba ratio on the energy storage properties and dielectric relaxation behaviors of strontium barium titanate ceramics

Sr _x Ba_1?x TiO₃ (x = 0.50–0.70) ceramics were prepared by conventional solid-state method. The effects of Sr/Ba ratio on the microstructures, energy storage properties and dielectric relaxation behaviors of ceramics were systematically investigated. Scanning electron microscopy observations revealed that the grain size was inhibited with increasing Sr molar fraction. The Sr_0.6Ba_0.4TiO₃ ceramics obtained the highest energy density of 0.3629 J/cm³ attributed to the increase of average breakdown strength resulting from the decrease of grain size and the optimizing of microstructure. In order to investigate the influence of Sr/Ba ratio on the dielectric relaxation behaviors, the activation energy has been calculated from the relaxation of dielectric loss and the complex impedance spectra by the Arrhenius relationship, respectively. The same results indicated that the decrease of grain size resulting in more grain boundaries, it was difficult for transferring charge and making an orientation under external electric field. Meanwhile, more defects existed at grain boundary and accelerated the thermally activated motions of defects, leading to the increase of activation energy.     

Spark plasma sintering and decomposition of the Y<Subscript>3</Subscript>NbO<Subscript>7</Subscript>:Eu phase

The spark plasma sintering (SPS) of Eu³⁺-doped Y₃NbO₇ niobate phase was investigated to obtain dense ceramics. Although SPS allowed obtaining high-density ceramics, decomposition of the niobate phase occurred at high temperature and was promoted by the SPS process, which limited its use as an optical material. The niobate phase has been prepared by two synthesis methods: a solid-state route and a sol–gel method. The purity, density and microstructure of the dense ceramics were analyzed after spark plasma sintering. Translucent ceramics were only obtained from sol–gel powders after SPS at 1600 °C during 20 min with a heating rate of 5 °C/min. The sintering study of the pure niobate phase showed that during SPS process and especially in the presence of the high electrical field the Y₃NbO₇ phase is metastable at 1600 °C. A decomposition of the niobate compound is clearly demonstrated by luminescence measurements when high heating rates were used.     

Effects of SrO–B2O3–SiO2 glass additive on the microstructure and dielectric properties of CaCu3Ti4O12

The effect of SrO–B₂O₃–SiO₂ glass additive (SBS) on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics was investigated. This SBS–added CCTO ceramics were prepared by the solid state reaction. The undesirable impurity phases Ca₃SiO₅ started appearing in the XRD patterns, suggesting a possible chemical reaction between CaTiO₃ and SiO₂ (the devitrification production of SBS glass). The SBS glass additive promoted the grain growth and densification of CCTO ceramics. Cole–Cole plots of conductance suggested that the resistivity grain boundary decreased with increasing amount of SBS glass (when x = 0–2 wt%), then increased (when x = 2–3 wt%). The addition of SBS glass was desirable to increase the dielectric constants (up to 10⁴) and lowered the dielectric losses of CCTO over the frequency range of 450–40 kHz at the relatively lower sintering temperature for relatively shorter sintering time (1,050 °C, 12 h).     

Effects of rare-earth Sm<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on relaxation behavior and electric properties of 0.5PNN-0.5PZT ceramics

Rare-earth Sm₂O₃-doped Pb(Ni_1/3Nb_2/3)_0.5(Zr_0.3Ti_0.7)_0.5O₃ (0.5PNN-0.5PZT + xSm) piezoelectric ceramics were prepared by a two-step solid-state reaction method. The effects of Sm₂O₃ doping on phase structure, electric properties and dielectric relaxation of 0.5PNN-0.5PZT ceramics were investigated. The results showed that Sm³⁺ tended to occupy the B-site of Zr⁴⁺. The XRD patterns showed that all ceramics had a pure perovskite phase structure. With the increasing of Sm₂O₃ addition, Curie temperature moved towards low temperatures obviously and dispersion coefficient γ first increased and then decreased gradually. When x = 0.2 wt%, the γ reaches at maximum (1.998), it was indicated that a nearly complete diffuse phase transition existed and relaxation behaviors enhanced. But the ceramics with x = 0.2 wt% exhibited terrible electric properties: d₃₃ = 605 pC/N, k_p = 0.55, ε_r ~ 5020, tanδ ~ 2.20 %. It showed that the relaxation behaviors have relationships with electric properties to some extent.     

Structure and energy storage properties of Ti vacancies charge compensated Re 2O3-doped SrTiO3 (Re = Pr, Nd, Gd) ceramics

By introducing Ti vacancies in advance for charge compensation, single cubic perveskite rare-earth doped strontium titanate ceramics with the formula Re _0.02Sr_0.98Ti_0.995O₃ (Re–STO, Re = Pr, Nd, Gd) were prepared via solid-state reaction method. All Re–STO ceramics sintered at 1,410 °C in air for 3 h showed a similar dense microstructure with the grain size lower than 10 μm. The room temperature dielectric constant ε _r of Re–STO ceramics was higher than 3,000 (@ 1 kHz) and changed <8 % within the applied bias electric field. Especially, the room temperature dielectric loss tanδ of Re–STO ceramics was lower than 0.02 (@ 1 kHz), and the average breakdown strength E _b surpassed 14 kV/mm, demonstrating that Re–STO ceramics could be very promising for high-voltage capacitor applications. The temperature stability of the ε _r of Re–STO ceramics was evaluated in a temperature range of ?50 to 200 °C. Meanwhile, the energy storage density of Re–STO ceramics was investigated as a function of the applied bias electric field.     

Phase evolutions and electric properties of BaTiO3 ceramics by a low-temperature sintering process

BaTiO₃ (BT) nanoparticles were synthesized by a modified polymeric precursor method in a weak acid solution. The synthesized process of BT precursor with increasing calcination temperature was investigated through thermal analysis (DTA/TG), X-ray diffraction, transmission electron microscope and Fourier-transform infrared spectroscopy. Good dispersive and homogeneous cubic BT nanoparticles were calcined at 800 °C, whereas dense BT ceramics were sintered at ~1,160 °C. The present results showed that the dielectric, piezoelectric and ferroelectric properties of BT ceramics were dependent on the ceramics densification and crystallographic structure. The excellent electric properties (P _r = 10.5 μC/cm², d ₃₃ = 217 pC/N, k _p = 0.32 %) were found at a sintering temperature of 1,160 °C, which was due to the coexistence of tetragonal and orthorhombic phase. The depressed electric properties at higher sintering temperature were associated to oxygen vacancies and impurity phases. In addition, phase evolutions of BT nanoparticles and ceramics were all stated in detail.     

Dielectric properties of Sr3CuNb2O9 perovskite ceramics

The dielectric constant ? and loss tangent tanδ of Sr₃CuNb₂O₉ perovskite ceramics prepared by solid-state reactions have been measured at temperatures from 300 to 900 K and frequencies from 25 to 1 × 10⁶ Hz. The results demonstrate that the samples slowly cooled from the temperature of the final, high-temperature firing (1200°C) have relatively low permittivity (? ? 10) and dielectric losses (tanδ ? 0.005 at 1 kHz) at room temperature, with no strong dielectric dispersion and no prominent maxima in the temperature dependences of their permittivity and dielectric loss. The ceramics quenched from 1300°C exhibit a pronounced Debye-type low-frequency relaxation and strong dielectric dispersion in conjunction with high permittivity ? ? 2000 at low frequencies and/or high temperatures. The observed dielectric anomalies in the Sr₃CuNb₂O₉ ceramics can be understood in terms of Maxwell-Wagner relaxation at dielectric inhomogeneities associated with the quenching-induced difference in oxygen-vacancy concentration between the grain bulk and surface layer.