Dielectric properties of CaCu3Ti4O12 ceramics with Zn substitution for Cu

Abstract

CaCu_3–xZn_xTi₄O₁₂ (x is from 0 to 1·0) polycrystalline samples were fabricated via a two-step solid state reaction process. The lattice parameter of the monophasic CaCu₃Ti₄O₁₂ phase increased as Zn content increased. Scanning electron microscopy (SEM) images of the CCTO ceramic show bimodal grain size distribution and the grain size decrease largely with the appearance of Zn₂TiO₄ second phase. The dielectric permittivity of pure CCTO ceramic is ～1·5×10⁴ at f?=?100 Hz. The dielectric constant of the sample largely increased with Zn substitution in the frequency range f<10⁴ Hz. The highest dielectric constant was 6·2×10⁴ at f?=?100 Hz with Zn substitution of x?=?0·8. The improved dielectric properties are believed to be related to the presence of a thin grain boundary barrier layer. The resistivity of the grain boundary decreased largely with Zn substitution as evidenced from the impedance plots.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

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.     

Enhanced non-linear current-voltage response of Te-doped calcium copper titanate ceramics

The influence of partial replacement of Ti⁴⁺ ions by Te⁴⁺ in calcium copper titanate lattice on dielectric and non-linear current- voltage (I–V) characteristics was systematically studied. There was a remarkable increase in the values of the nonlinear coefficient (α) with Te⁴⁺ doping concentration in CaCu₃Ti_4-xTe_xO₁₂ (where, x=0, 0.1, 0.2).For instance, the α values increase from 2.9 (x=0) to 22.7 (x=0.2) for ceramics sintered at 1323 K/8 h. The room temperature value of current density (J) at the electrical field of 250 V/cm for CaCu₃Ti_3.8Te_0.2O₁₂ ceramics is almost 400 times higher than that of the pure CaCu₃Ti₄O₁₂ ceramics sintered at 1323 K. A systematic investigation into I–V behaviour as a function of temperature gave an insight into the conduction mechanisms of undoped and doped ceramics of calcium copper titanate (CCTO). The calculated potential barrier value for doped ceramics (~ 0.21 eV) dropped down to almost one third that of the undoped ceramics (~ 0.63 eV).     

Origin of significantly enhanced dielectric response and nonlinear electrical behavior in Ni2+-doped CaCu3Ti4O12: Influence of DC bias on electrical properties of grain boundary and associated giant dielectric properties

CaCu_3-xNi_xTi₄O₁₂ (x?=?0, 0.05, and 0.10) powders were synthesized using a solid state reaction method. Phase structure and microstructure analyses revealed that all sintered CaCu_3-xNi_xTi₄O₁₂ ceramics were of a pure phase. The CaCu₃Ti₄O₁₂ ceramics had a dense microstructure and grain sizes were enlarged by doping with Ni²⁺. Interestingly, the dielectric permittivity was significantly enhanced, whereas the loss tangent was greatly suppressed to ～0.046–0.034 at 1?kHz. All sintered ceramics exhibited non-Ohmic characteristics. Clarification of the influences of DC bias showed that the dielectric permittivity and loss tangent values were increased by DC bias. The resistance of grain boundaries and the associated conduction activation energy of CaCu_3-xNi_xTi₄O₁₂ ceramics were reduced as the DC bias voltage increased. Therefore, the observed non-Ohmic behavior and significantly enhanced dielectric properties should be closely related to variation in the Schottky barriers at the grain boundaries.     

Ferroelectric and dielectric properties of Nd2−xCexTi2O7 ceramics

The solid solution system Nd_2?xCe_xTi₂O₇ has been investigated. The solubility limit of Ce in Nd_2?xCe_xTi₂O₇ was found to be 0·5–0·75 according to X-ray diffraction and X-ray photoelectron spectroscopy results. Ce substitution increases the b and c axes and the volume of the unit cell due to its larger ionic radius. Nd_2?xCe_xTi₂O₇ (x?=?0·05, 0·25, 0·5, 0·75) textured ceramics were fabricated using spark plasma sintering. The ferroelectric and dielectric properties of the ceramics were studied. Ce substitution decreases the Curie point T_c of Nd_2?xCe_xTi₂O₇ compounds. The results suggest that the T_c of Ce₂Ti₂O₇ is <1445°C.     

(Al3+, Nb5+) co–doped CaCu3Ti4O12: An extended approach for acceptor–donor heteroatomic substitutions to achieve high–performance giant–dielectric permittivity

Substitution of (Al³⁺, Nb⁵⁺) co–dopants into TiO₆ octahedral sites of CaCu₃Ti₄O₁₂ ceramics, which were prepared by a solid state reaction method and sintered at 1090 °C for 18 h, can cause a great reduction in a low–frequency loss tangent (tanδ≈0.045–0.058) compared to those of Al³⁺ or Nb⁵⁺ single–doped CaCu₃Ti₄O₁₂. Notably, very high dielectric permittivities of 2.9 ? 4.1 × 10⁴ with good dielectric–temperature stability are achieved. The room–temperature grain boundary resistance (R_gb≈0.37–1.17 × 10⁹ Ω.cm) and related conduction activation energy (E_gb≈0.781–0.817 eV), as well as the non–Ohmic properties of the co–doped ceramics are greatly enhanced compared to single–doped ceramics (R_gb≈10⁴–10⁶ Ω cm and E_gb≈0.353–0.619 eV). The results show the importance of grain boundary properties for controlling the nonlinear–electrical and giant–dielectric properties of CaCu₃Ti₄O₁₂ ceramics, supporting the internal barrier layer capacitor model of Schottky barriers at grain boundaries.     

Development of non-stoichiometric SrBi4+2xTi4O15+3x (−0·04 ≤x≤ 0·04) ceramics

Abstract

The non-stoichiometric compositions of Bi layered structural SrBi₄Ti₄O₁₅ ferroelectric materials (SrBi_4+2xTi₄O_15+3x, x=?0·04, ?0·02, 0, 0·02 and 0·04) are investigated as the main precursors to find the influence of Bi₂O₃ content on the characteristics of SrBi₄Ti₄O₁₅ ceramics. The effect of compositional variation on the sintering and the dielectric characteristics of SrBi₄Ti₄O₁₅ ceramics are deduced with the aid of X-ray diffraction patterns, scanning electron microscopy observation and dielectric–temperature curves. From the scanning electron microscopy observations, the SrBi_3·92Ti₄O_14·88 and SrBi_3·96Ti₄O_14·94 ceramics reveal two phased grain growth, bar typed and irregularly disc typed grains coexist; the other SrBi_4+2xTi₄O_15+3x ceramics reveal irregularly disc typed grains. From the X-ray patterns, the Bi₂Ti₂O₇ and SrTiO₃ phases are observed in the SrBi_3·92Ti₄O_14·88 and SrBi_3·96Ti₄O_14·94 ceramics. Except the SrBi_3·96Ti₄O_14·94 ceramics, the other SrBi_4+2xTi₄O_15+3x ceramics have revealed a splitting peak in the (119) plane. It is also shown that the compositional variation has apparent influences on the grain morphologies, the bulk densities, the Curie temperatures and the maximum dielectric constants of SrBi_4+2xTi₄O_15+3x ceramics.     

Low temperature preparation of CaCu3Ti4O12 ceramics with high permittivity and low dielectric loss

Low temperature preparation of CaCu₃Ti₄O₁₂ ceramics with large permittivity is of practical interest for cofired multilayer ceramic capacitors. Although CaCu₃Ti₄O₁₂ ceramics have been prepared at low temperatures as previously reported, they have rather low permittivity. This work demonstrates that CaCu₃Ti₄O₁₂ ceramics can not only be prepared at low temperatures, but they also have large permittivity. Herein, CaCu₃Ti₄O₁₂ ceramics were prepared by the solid state reaction method using B₂O₃ as the doping substance. It has been shown that B₂O₃ dopant can considerably lower the calcination and sintering temperatures to 870 °C and 920 °C, respectively. The relative permittivity of the low temperature prepared CaCu₃Ti_4−xB_xO₁₂ ceramics is about 5 times larger than the previously reported results in the literature. Furthermore, the dielectric loss of the CaCu₃Ti_4−xB_xO₁₂ ceramics is found to be as low as 0.03. This work provides a beneficial base for the future commercial applications of CaCu₃Ti₄O₁₂ ceramics with large permittivity for the cofired multilayer ceramic capacitors.     

Synthesis and microwave absorption properties of Ni–Zn–Mn spinel ferrites

A series of Ni_0·5?xZn_0·3?xMn_0·2+2xFe₂O₄ ferrites was successfully prepared by the sol–gel autocombustion method. The structure and electromagnetic properties of the powders were characterised by X-ray diffraction, SEM and vector network analysis. The pure powders were formed by heating at 1200°C for 3 h in air, and grain sizes increased as the amount of substitution ranged from x?=?0·0 to x?=?0·25. For samples with x?=?0·1, a minimum reflection loss of ?27·57 dB was observed at 11·0 GHz with the less than ?10 dB absorption bandwidth at 8·0 GHz with 3·8 mm thickness. The results indicate that substitution with Mn and Zn ions can greatly improve the microwave absorption properties of NiFe₂O₄ ferrites.     

Enhanced electrical and photoluminescence properties of BiSbO4-doped CaCu3Ti4O12 ceramics by modifying grain boundary response

CaCu₃Ti₄O₁₂-xwt%BiSbO₄ ceramics (CCTO-xwt%BSO, x = 0, 1, 2, 3) were prepared by solid-state reaction method. The microstructure, dielectric properties, varistor properties, photoluminescence properties of CCTO-xwt%BSO ceramics were studied in this work. Results showed that all samples formed CaCu₃Ti₄O₁₂ (CCTO) single phase. Doping BiSbO₄ (BSO) restrained the abnormal grain growth and increased the grain boundary density of ceramics. The introduction of BSO led to the increase of the grain boundary resistance, reducing the dielectric loss and enhancing the temperature stability of dielectric properties. The nonlinear electrical characteristics are enhanced with proper concentration of BSO. And the improved varistor performance with breakdown electric field of ～3.98–34.6 and nonlinear coefficient of ～1.49–2.96 are obtained for CCTO-xwt%BSO samples. In addition, the photoluminescent emission of the samples is enhanced with the addition of appropriate equivalent BSO, showing the potential applications in novel devices with photoluminescent/electrical multifunctional properties.     

Investigation on CuCa3Ti4O12 Modified BiFeO3‐Based Perovskite Ceramics

(BiFeO₃)_1?x–(CuCa₃Ti₄O₁₂)_x (x = 0.00, 0.10 and 0.20) ceramics had been prepared via the mixed route. The crystal structure transformation of BiFeO₃ (BFO) from rhombohedral to pseudotetragonal phase was induced by 20 mol% CuCa₃Ti₄O₁₂ (CCTO). Impedance measurements suggested the increase in grain boundary resistance, and CCTO addition could block the conduction path and stabilize the motion of defects. The dielectric constant and the loss tangent of BFO as a function of frequency were found to be incremental on increasing CCTO contents. The enhancement of magnetization would be attributed to the effective suppression of the spiral spin structure and the structure transformation.     

Microstructural evolution,non‐Ohmic properties,and giant dielectric response in CaCu3Ti4−xGexO12 ceramics

The microstructural evolution, non‐Ohmic properties, and giant dielectric properties of CaCu₃Ti_4?xGe_xO₁₂ ceramics (x=0‐0.10) are systematically investigated. The Rietveld refinement confirms the existence of a pure CaCu₃Ti₄O₁₂ phase in all samples. Significantly enlarged grain sizes of CaCu₃Ti_4?xGe_xO₁₂ ceramics are associated with the liquid phase sintering mechanism. Enhanced dielectric permittivity from 6.90×10⁴ to 1.08×10⁵ can be achieved by increasing Ge⁴⁺ dopant from x=0‐0.10, whereas the loss tangent is remarkably reduced by a factor of ≈10. Non‐Ohmic properties are enhanced by Ge⁴⁺ doping ions. Using impedance and admittance spectroscopies, the underlying mechanisms for the dielectric and nonlinear properties are well described. The improved nonlinear properties and reduced loss tangent are attributed to the enhanced resistance and conduction activation energy of the grain boundaries. The largely enhanced permittivity is closely associated with the enlarged grain sizes and the increase in the Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ ratios, which are calculated from the X‐ray absorption near‐edge structure.     

Improved dielectric properties in A′‐site nickel‐doped CaCu3Ti4O12 ceramics

The improved dielectric properties and voltage‐current nonlinearity of nickel‐doped CaCu₃Ti₄O₁₂ (CCNTO) ceramics prepared by solid‐state reaction were investigated. The approach of A′‐site Ni doping resulted in improved dielectric properties in the CaCu₃Ti₄O₁₂ (CCTO) system, with a dielectric constant ε′≈1.51×10⁵ and dielectric loss tanδ≈0.051 found for the sample with a Ni doping of 20% (CCNTO20) at room temperature and 1 kHz. The X‐ray photoelectron spectroscopy (XPS) analysis of the CCTO and the specimen with a Ni doping of 25% (CCNTO25) verified the co‐existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺. A steady increase in ε′(f) and a slight increase in α observed upon initial Ni doping were ascribed to a more Cu‐rich phase in the intergranular phase caused by the Ni substitution in the grains. The low‐frequency relaxation leading to a distinct enhancement in ε′(f) beginning with CCNTO25 was confirmed to be a Maxwell‐Wagner‐type relaxation strongly affected by the Ni‐related phase with the formation of a core‐shell structure. The decrease of the dielectric loss was associated with the promoted densification of CCNTO and the increase of Cu vacancies, due to Ni doping on the Cu sites. In addition, the Ni dopant had a certain effect on tuning the current‐voltage characteristics of the CCTO ceramics. The present A′‐site Ni doping experiments demonstrate the extrinsic effect underlying the giant dielectric constant and provides a promising approach for developing practical applications.     

Influence of magnesia doping on structure and electrical conductivity of pyrochlore type GdSmZr2O7

Abstract

Polycrystalline ceramic samples of magnesia doped GdSm_1–xMg_xZr₂O_7–x/2 have been prepared by conventional solid state reaction method using high purity oxides. The influence of magnesia dopant content on densification, microstructure and electrical properties of GdSm_1–xMg_xZr₂O_7–x/2 ceramics are investigated. Magnesia doping promotes the sintering densification behaviour of GdSm_1–xMg_xZr₂O_7–x/2 ceramics. GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0, 0·05, 0·10) ceramics have a single phase of the pyrochlore type structure, while GdSm_1–xMg_xZr₂O_7–x/2 (x?=?0·15, 0·20) ceramics consist of the pyrochlore type structure and a small amount of magnesia as the second phase. The total conductivity of GdSm_1–xMg_xZr₂O_7–x/2 ceramics obeys the Arrhenius relation, and gradually increases with increasing temperature from 723 to 1173 K. GdSm_1–xMg_xZr₂O_7–x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1·0×10^–4 to 1·0 atm at each test temperature. The maximum value of the total conductivity is 1·29×10^–2 S cm^–1 at 1173 K for the GdSm_0·85Mg_0·15Zr₂O_6·925 ceramic.     

Thermal stability of CaCu3Ti4O12: Simultaneous thermal analysis and high-temperature mass spectrometric study

Thermal stability of calcium copper titanate was studied by differential scanning calorimetry, thermogravimetry and high-temperature mass spectrometry. Calcium copper titanate (CCTO) had no thermal effects and mass losses caused by thermal dissociation or any phase transitions, besides melting, in the temperature range of 298–1423?K. The melting point of calcium copper titanate is 1398?K. The endothermic effect at 1250?K was associated with the decomposition of copper (II) oxide segregated in the intergrain space of the CaCu₃Ti₄O₁₂-CuO ceramics. In this connection, we proposed a simple method for estimating the content of copper oxide in the CaCu₃Ti₄O₁₂-CuO composite. The processes of evaporation of CaCu₃Ti₄O₁₂ in the temperature range of 1500–2100?K were studied by high-temperature mass spectrometry. In the temperature range of 1500–1750?K, easily volatilized copper oxide was evaporated selectively from the calcium copper titanate. At the temperature of 2100?K, atomic calcium and titanium oxides, TiO and TiO₂, were present in the vapor.     

Decrease in the dielectric loss of CaCu3Ti4O12 at high frequency by Ru doping

CaCu_3−xRu_xTi₄O₁₂ (x=0, 0.03, 0.05 and 0.07) electronic ceramics were fabricated using a conventional solid-state reaction method. The microstructure, grain sizes and dielectric properties as well as the impedance behaviours of the ceramics were carefully investigated. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) results indicate that ruthenium (Ru) dopant inhibits the growth of grains during the sintering process by promoting the formation of high melting point oxides of Ca and Ti. The study on the frequency dependence of dielectric properties suggests that Ru doping shifts the dielectric loss peak of CCTO to a much lower frequency, thereby reducing the dielectric loss of CCTO at high frequency (f>1.0 MHz) accordingly. When doped with proper amount of Ru, the high frequency dielectric loss of CCTO is reduced to a very low value (tanδ<0.05). Our study conclusively suggests that Ru-doped CCTO, with sufficiently low dielectric loss and decent permittivity, presents potential applications at high frequency.     

Conduction of (Na,Ce) doped Bi5Ti3FeO15 ceramics after different annealing processes

Abstract

Abstract

Bi_5?xNa_x/2Ce_x/2Ti₃FeO₁₅ (BNTF-100x, x?=?0, 0·05, 0·10, 0·20, 0·30, 0·40 and 0·60) ceramics were prepared via solid state sintering process. With x increasing, the resistivity increases at first and then decreases. The lower the concentration of oxygen vacancies, caused by more (Na,Ce) dopant adding, the less the deep trap energies in the band gap for the activated electrons to be mobile. For the samples containing high concentration of oxygen vacancies, the conductivities are higher when they are annealed in the air and oxygen than that in the vacuum. On the contrary, for the samples containing low concentration of oxygen vacancies, the conductivities of the samples in different annealing conditions behave quite the same way. Most (Na,Ce) doped Bi₅Ti₃FeO₁₅ ceramics exhibit intrinsic conduction at high temperatures with corresponding activation energies of 1·1–1·6 eV, while the extrinsic conduction has a close relationship with the concentration of oxygen vacancies and the activation ability of electrons.     

Correlation Between Photoluminescence and Structural Defects in Ca1+xCu3−xTi4O12 Systems

Ca_1+xCu_{3 ? x}Ti₄O₁₂ powders were synthesized by a conventional solid‐state reaction. X‐ray diffraction (XRD) was performed to verify the formation of cubic CaCu₃Ti₄O₁₂ (CCTO) and orthorhombic CaTiO₃ (CTO) phases at long range. Rietveld refinements indicate that excess Ca atoms added to the Ca_{1 ? x}Cu_{3 ? x}Ti₄O₁₂ (x  =  1.0) composition segregated in a CaTiO₃ secondary phase suggesting that solubility limit of Ca atoms in the CaCu₃Ti₄O₁₂ lattice was reached for this system. The FE‐SEM images show that the Ca_1+xCu_{3 ? x}Ti₄O₁₂ (0  <  x  <  3) powders are composed of several agglomerated particles with irregular morphology. X‐ray absorption near‐edge structure spectroscopy (XANES) spectra indicated [TiO₅V_o^z]‐[TiO₆] complex clusters in the CaCu₃Ti₄O₁₂ structure which can be associated with oxygen vacancies (V_o^z  =  V_o^x, V_o^?, and V_o^??) whereas in the CaTiO₃ powder, this analysis indicated [TiO₆]–[TiO₆] complex clusters in the structure. Ultraviolet‐visible (UV–vis) spectra and photoluminescence (PL) measurements for the analyzed systems revealed structural defects such as oxygen vacancies, distortions, and/or strains in CaCu₃Ti₄O₁₂ and CaTiO₃ lattices, respectively.