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.     

Dielectric properties of CaCu3Ti4O12 improved by chromium/lanthanum co-doping

The dielectric properties of Cr + La co-doped CaCu₃Ti₄O₁₂ ceramics prepared by a solid-state reaction method were evaluated and compared to Cr-doped, La-doped, and parent CaCu₃Ti₄O₁₂ (CCTO). Their structure and grain size were evaluated by X-ray diffraction and scanning electron microscopy, respectively. No secondary phase was detected based on the XRD analysis. The results show that, the room temperature dielectric loss of the co-doped samples is reduced to 43% compared to CCTO and their dielectric permittivity is higher than the un-doped, Cr-doped, and La-doped samples at frequencies over 325 kHz, 30 kHz, and 12 Hz, respectively. Furthermore, the temperature stability of the co-doped sample is significantly more convenient than that of CCTO, and its dielectric loss is three times lower. The results also indicated that the co-doping method is effective in reducing the dielectric loss, still maintaining the high dielectric permittivity.     

Colossal dielectric response and relaxation behavior in novel system of Zr4+ and Nb5+ co-substituted CaCu3Ti4O12 ceramics

In this work, we developed a novel system of isovalent Zr⁴⁺ and donor Nb⁵⁺ co-doped CaCu₃Ti₄O₁₂ (CCTO) ceramics to enhance dielectric response. The influences of Zr⁴⁺ and Nb⁵⁺ co-substituting on the colossal dielectric response and relaxation behavior of the CCTO ceramics fabricated by a conventional solid-phase synthesis method were investigated methodically. Co-doping of Zr⁴⁺ and Nb⁵⁺ ions leads to a significant reduction in grain size for the CCTO ceramics sintered at 1060 °C for 10 h. XRD and Raman results of the CaCu₃Ti_3.8-xZr_xNb_0.2O₁₂ (CCTZNO) ceramics show a cubic perovskite structure with space group Im-3. The first principle calculation result exhibits a better thermodynamic stability of the CCTO structure co-doped with Zr⁴⁺ and Nb⁵⁺ ions than that of single-doped with Zr⁴⁺ or Nb⁵⁺ ion. Interestingly, the CCTZNO ceramics exhibit greatly improved dielectric constant (~10⁵) at a frequency range of 10²–10⁵ Hz and at a temperature range of 20–210 °C, indicating a giant dielectric response within broader frequency and temperature ranges. The dielectric properties of CCTZNO ceramics were analyzed from the viewpoints of defect-dipole effect and internal barrier layer capacitance (IBLC) model. Accordingly, the immensely enhanced dielectric response is primarily ascribed to the complex defect dipoles associated with oxygen vacancies by co-doping Zr⁴⁺ and Nb⁵⁺ ions into CCTO structure. In addition, the obvious dielectric relaxation behavior has been found in CCTZNO ceramics, and the relaxation process in middle frequency regions is attributed to the grain boundary response confirmed by complex impedance spectroscopy and electric modulus.     

Effects of multiwall carbon nanotubes on dielectric and mechanical properties of CaCu3Ti4O12 composite

This work reports the synthesis of calcium copper titanate (CaCu₃Ti₄O₁₂)/multiwall carbon nanotubes (MWCNT) composites using ultrasonic technique followed by sintering in a high vacuum furnace. The effect of MWCNT content (0.05, 0.1, 0.15, and 0.2 wt%) on the structural, dielectric, and mechanical properties of CaCu₃Ti₄O₁₂ (abbreviated as CCTO) were investigated by TEM, XRD, FTIR, FESEM-EDAX, dielectric measurement, as well as tensile strength and flexural strength tests. XRD patterns revealed that the MWCNT loading did not affect the phase structure; however, the average crystallite size (D) was reduced from 60.88 nm to 40.79 nm. The samples had porous structures and the porosity reduced from 45.57% to 40.73% with MWCNT loading. The dielectric and mechanical properties of CCTO were enhanced with an increase in MWCNT loading. An important observation was that the CCTO mixed with 0.2 wt% MWCNT exhibited the highest dielectric permittivity (ε_r = 27,768) and the lowest dielectric loss (tan δ = 0.52) at 1 kHz. With the addition of 0.2 wt% MWCNT, the values of load, tensile, and flexural strength increased to 10.38 kN, 101.88 MPa, and 275.07 MPa, respectively, due to improvement in densification. These outcomes have values for the fabrication of CCTO and the optimization of its performance for electronic devices such as capacitors and antennas.     

Evolution of the intergranular phase during sintering of CaCu3Ti4O12 ceramics

CaCu₃Ti₄O₁₂ (CCTO) ceramics have been processed by solid state reaction and sintered at 1100 °C for different times. A clear increase of the dielectric constant of the material up to values of 6 × 10⁴ has been observed with the sintering time. This increase is accompanied by a limited grain growth and intergranular Cu-oxide phase thickness reduction. The disappearance of the Cu-oxide phase is caused by the incorporation of Cu cations into the grains, contributing to the increase of the dielectric constant. Raman spectroscopy shows the decrease of TiO₆ octahedral rotational modes with the sintering time due to the incorporation of Cu cations into the CCTO grains. XANES measurements show that the Cu main oxidation state is Cu²⁺ and does not change with the sintering time. The fitting of the experimental dielectric constant to the Internal Barrier Layer Capacitance (IBLC) model reveals the change of the intergranular phase dielectric constant, caused by a compositional change due to the incorporation of Cu into the CCTO grains.     

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).     

Effect of La-doping on the properties of CaCu3Ti4O12 dielectric ceramics

Nano-size Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ (χ = 0.00, 0.05, 0.10, 0.15 and 0.20) precursor powders were prepared via the sol–gel method and the citrate auto-ignition route and then processed into micro-crystal Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics under heat treatment. Characterization of the as-obtained ceramics with XRD and SEM showed an average grain sizes of ∼1–2 μm, indicating La³⁺ amount to have little impact on grain size. The room-temperature dielectric constant of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics sintered at 1000 °C was of the order of 10³–10⁴ despite the variation of χ values. Compared with CaCu₃Ti₄O₁₂, La³⁺-doped CaCu₃Ti₄O₁₂ showed a flatter dielectric constant curve related to frequency. It was found that the loss tangent of the Ca_1−χLa_2χ/3Cu₃Ti₄O₁₂ ceramics was less than 0.20 in ∼600–10⁵ Hz region, which rapidly decreased to a minimum value of 0.03 by La³⁺doping with χ = 0.05. Our measurement of the ceramics conductivities (σ) also indicated that the appropriate introduction of La³⁺ into CaCu₃Ti₄O₁₂ would distinctly result in its dielectric properties.     

Complex permittivity and relaxation processes in CaCu3Ti(4−x)MnxO12

The effects of Mn-doping on CaCu₃Ti₄O₁₂ (CCTO), i.e. the electrical properties of CaCu₃Ti_(4−x)Mn_xO₁₂ were studied in order to get a deep insight into the origin of the colossal dielectric constant of CCTO. It was found that 1 mol% substitution of Mn for Ti decreases the dielectric constant largely to a factor of 1/100, supporting the point of view that the native defects are responsible for the dielectric response of CCTO.     

(Na,K)NbO3–CaCu3Ti4O12 perovskite composites for supercapacitor based piezoelectric devices

The supercapacitor based piezoelectric material composite (Na,K)NbO₃–CaCu₃Ti₄O₁₂ (NKN–CCTO) is investigated for possible application in piezoelectric devices. (1−x)NKN–xCCTO (0.015≤x≤0.06) with different sintering conditions is researched for supercapacitor based piezoelectric applications. The 0.94NKN–0.06CCTO composite sintered at 975 °C shows the highest dielectric permittivity of 796. Clear SEM images of (1−x)NKN–xCCTO reveal that these compositions have high density well-crystallized structures. The composition and sintering temperature dependence of dielectric permittivities and piezoelectric coefficients, plotted in three dimensions, show that the 0.985NKN–0.015CCTO composite sintered at 1025 °C has a moderate dielectric permittivity of 405 and a piezoelectric constant of 98 pC/N.     

Effects of LiF addition on microstructure and dielectric properties of CaCu3Ti4O12 ceramics

The effects of small amounts of lithium fluoride sintering aid on the microstructure and dielectric properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics were investigated. CCTO polycrystalline ceramics with 0.5 and 1.0 mol% LiF, and without additive were prepared by solid state synthesis. Good densification (>90% of the theoretical density) was obtained for all prepared materials. Specimens without the sintering aid and sintered at 1090 °C exhibit secondary phases as an outcome of the decomposition reaction. The mean grain size is controlled by the amount of LiF in specimens containing the additive. Impedance spectroscopy measurements on CaCu₃Ti₄O₁₂ ceramics evidence the electrically heterogeneous nature of this material consisting of semiconductor grains along with insulating grain boundaries. The activation energy for grain boundary conduction is lower for specimens prepared with the additive, and the electric permittivity reached 53,000 for 0.5 mol% LiF containing CCTO.     

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.     

Low-cost flame synthesized La2/3Cu3Ti4O12 electro-ceramic and extensive investigation on electrical,impedance, modulus,and optical properties

An environmentally-friendly synthesis route and low-cost starting materials are more appropriate for the production of ceramic materials at the industry level. With this concern we prepared the La_2/3Cu₃Ti₄O₁₂(LCTO), which is isostructural of CaCu₃Ti₄O₁₂ (CCTO), using the low-cost TiO₂ instead of a high-cost of titanium source (titanium isopropoxide or titanium chloride) using a low-cost wet-chemical route. Although, there are lots of synthetic methods reported for LCTO fabrication in terms of duration, cheap reagents, energy consumption, feasibility, etc. The present method is far better than the others. The prepared ceramic samples were sintered at 1050 ^°C/12 h and studied their structural, morphology and impedance, and modulus studies for further confirmation. The prepared LCTO ceramic shows the pure phase with the cubic type of morphology. The homogenous distribution of all the elements was observed through dispersive X-ray analysis. X-ray photoelectron spectroscopy studies revels that La is in +3 oxidation state, Cu is in a +2 oxidation state, and Ti is multiple (+3 and + 4) oxidation state. The LCTO ceramic displayed the very high dielectric constant (∼3852) and dielectric loss (0.322), at 1 kHz and at room temperature. Calculated the activation energy using the impedance and modulus data and it shows the superior to that of CCTO ceramic synthesized by the same method. The prepared samples exhibited Debye-type relaxation, which is evoked from the impedance and modulus studies. The calculated optical energy bandgap of LCTO (2.06 eV) is found to be lesser than that of the well-known structure of perovskites (BaTiO₃ (3.28 eV), PbTiO₃ (3.18 eV), LiNbO₃ (3.78 eV) and BiFeO₃ (2.67 eV) as well as structure of spinel CoCr₂O₄ (3.10 eV) and LuFe₂O₄ (2.18 eV)) materials.     

Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach

Grain boundaries of CaCu₃Ti₄O₁₂ (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al₂O₃ shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58,000 to 81,000 at 1?kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 10²–10³?Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73?eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.     

High permittivity and low dielectric loss of the (Ca0.9Sr0.1)1-xLa2x/3Cu3Ti4O12 ceramics

In this work, we have systematically studied the effects of La³⁺/Sr²⁺ dopants on the crystal structure, microstructure, dielectric response and electrical properties of (Ca_0.9Sr_0.1)_1-xLa_2x/3Cu₃Ti₄O₁₂ (x = 0, 0.025, 0.05 and 0.075) ceramics. XRD results show that the lattice parameter increases with the increase in the La³⁺ content. SEM micrographs illustrate that a small amount added of La³⁺ can reduce the grain size of CCTO during sintering. With increasing La³⁺ content, the grains grow larger. Dielectric measurements indicated that all doped samples synthesized by the solid-state reaction exhibit giant dielectric constants ε'>10⁴ over a large frequency range (10 Hz to 1 MHz) and at any temperature below 600 K. In particular, the ceramic with x = 0.05 exhibits a colossal dielectric permittivity ～5.49 × 10⁴; which increases by about 50% compared to that of the undoped ceramic. In addition, the doped ceramic also presents a low dielectric loss ～ 0.08 at 20 °C and 0.6 kHz. The giant dielectric properties of these samples can be explained by the (IBLC) model.     

Electrical properties of Co-doped CaCu3Ti4O12

Co-doped CaCu₃Ti₄O₁₂ samples were synthesized by solid-phase reaction. Electrical properties were studied by impedance spectroscopy in wide temperature (25–450 °C) and frequency (10 Hz–10 MHz) intervals. It was shown that the presence of the copper oxide interlayer significantly reduces the value of the dielectric constant. The amount of impurity copper in the CaCu₃Ti_4-хCo_хO_12-δ samples (x = 0.06; 0.12; 0.24) rise with an increase in the cobalt content. The samples are characterized by a granular microstructure, with an average grain size ranging from 2 to 10 μm. The impedance of the samples was simulated at a temperature of 25 °C and in the range of 100–450 °C. It was found that the samples are characterized by low- and high-frequency polarization. The conductivity activation energy varied from 0.94 to 0.87 eV depending on the cobalt content. The CaCu₃Ti_3.94Co_0.06O_12-δ sample are characterized by the best values of the dielectric permittivity and the dielectric loss tangent, ε = 400 and tanδ = 0.2 (at 1 MHz and room temperature), respectively.     

Dielectric Characteristics of UV‐Curable CaCu3Ti4O12 Composite Thick Film Capacitors on Cu Foils

Dielectric composite thick films containing a high dielectric constant CaCu₃Ti₄O₁₂ (CCTO) filler in a UV‐cured polymer matrix were investigated as flexible planar capacitors on Cu foils. Dielectric performance depended on the volume fraction and size of CCTO particles dispersed in the cured polymer matrix. As a result, the thick films containing 33.3 vol.% CCTO with an average particle size of 0.47 μm exhibited a dielectric constant of ~80 and a dielectric loss of ~0.06 at 10 kHz. The effective medium theory model incorporating a morphology fitting parameter has been proven to be most close to the experimental values.     

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.     

Ge4+ doped CaCu2.95Zn0.05Ti4O12 ceramics: Two-step reduction of loss tangent

CaCu₃Ti₄O₁₂ ceramics have been extensively studied for their potential applications as capacitors in recent years; however, these materials exhibit very large dielectric losses. A novel approach to reducing the dielectric loss tangent in two steps, while increasing the dielectric permittivity, is presented herein. Doping CaCu₃Ti₄O₁₂ with a Zn dopant reduces the loss tangent of the ceramic material from 0.227 to 0.074, which is due to the increase in grain boundary (GB) resistance by an order of magnitude (from 6.3× 10³ to 3.93 × 10⁴ Ω cm). Zn-doping slightly changes the microstructure and dielectric permittivity of the CaCu₃Ti₄O₁₂ ceramic, which reveals that the primary role of the Zn dopant is to tune the intrinsic properties of the GBs. Surprisingly, the addition of the Ge⁴⁺ dopant into the Zn²⁺-doped CaCu₃Ti₄O₁₂ ceramic sample led to a further decrease in the loss tangent from 0.074 to 0.014, due to enhanced GB resistance (3.1 × 10⁵ Ω cm). The grain size increased remarkably from 2–3 μm to 85–90 μm, corresponding to a significant increase in the dielectric permittivity (~1–4 × 10⁴). The large increase in GB resistance is due to the intrinsic potential barrier height at the GBs and the segregation of the Cu-rich phase in the GB region. First-principles calculations revealed that Zn and Ge are preferentially located at the Cu sites in the CaCu₃Ti₄O₁₂ structure. The substitution of the Ge dopant does not hinder the role of the Zn dopant in terms of improving the electrical properties at the GBs. These phenomena are effectively explained by the internal barrier layer capacitor model. This study provides a way of improving the dielectric properties of ceramics for their practical use as capacitors.     

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.     

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.