溶胶-凝胶一步烧结法合成巨介电常数材料CaCu3Ti4O12

通过溶胶-凝胶法合成了巨介电常数材料类钙钛矿型钛酸铜钙(CaCu3Ti4O12,CCTO)先驱物,采用一步烧结法得到纯度较高的CCTO材料。利用X射线衍射、扫描电子显微镜、介电性能测试和阻抗谱测试对样品进行了结构、微观形貌、介电-频率、阻抗谱等分析表征,并讨论了CCTO的微观电导机理和晶粒晶界的性质,计算了晶界活化能。结果表明,此法制备的CCTO样品具有较好的结晶质量、较高的纯度以及优良的介电性能。     

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.     

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.     

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

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

Influence of Sb-doping on dielectric properties of NaCu3Ti3TaO12 ceramics and relevant mechanism(s)

A series of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics were prepared by the conventional solid-state reaction technique, and their dielectric properties, crystalline structures, microstructures and complex impedance were investigated systematically. All the ceramics show the main phases of perovskite-related crystallographic structure, and their dielectric properties change significantly with the increasing Sb-doping. All these ceramics exhibit giant dielectric-permittivity properties, and impedance spectroscopy analysis reveals that NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics are electrically heterogeneous and composed of insulating grain boundaries and semiconducting grains. Moreover, CuO secondary phase and Cu²⁺/Cu¹⁺, Ti⁴⁺/Ti³⁺, Sb⁵⁺/Sb³⁺ and Ta⁵⁺/Ta³⁺ aliovalences are found to exist in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics through XRD, EDS and XPS analysis. Therefore, CuO segregation and aliovalences of metal ions were suggested to contribute greatly to the internal barrier layer capacitance effect formation in NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics. Furthermore, Sb-doping could decrease the tanδ of NaCu₃Ti₃Ta_1−xSb_xO₁₂ ceramics at low frequencies, and the reason was discussed.     

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.     

Colossal dielectric permittivity and relevant mechanism of Bi2/3Cu3Ti4O12 ceramics

Bi_2/3Cu₃Ti₄O₁₂ (BCTO) ceramics with pure perovskite phase were successfully prepared by traditional solid-state reaction technique. Uniformly distributed and dense grains with the grain size of 2–3 μm were observed by SEM. A giant low-frequency dielectric permittivity of ~3.3×10⁵ was obtained. The analysis of complex impedance revealed that Bi_2/3Cu₃Ti₄O₁₂ ceramics are electrically heterogeneous. There are three kinds of dielectric response detected in Bi_2/3Cu₃Ti₄O₁₂ ceramics, which existed in the low-frequency range, middle-frequency range, and high-frequency range, respectively. Through the study of dielectric spectrum at different temperatures, the relatively low activation energy of 0.30 eV for middle-frequency dielectric response was calculated, which suggested that this Middle-frequency dielectric response can be ascribed to grain boundaries response. In view of the analysis of dielectric spectrum at low temperatures, the activation energy of 0.07 eV for high frequency dielectric response was found. This value illustrated that dielectric response at high frequencies was associated with grains polarization effect. The comparison of dielectric spectra of Bi_2/3Cu₃Ti₄O₁₂ ceramics with different types of electrodes revealed that giant low-frequency dielectric constant was attributed to the electrode polarization effect.     

Improved dielectric and nonlinear properties of CaCu3Ti4O12 ceramics with Cu-rich phase at grain boundary layers

The formation and compositions of grain boundary layers are very important factors to improve the electrical properties of CaCu₃Ti₄O₁₂ (CCTO) ceramics. In present work, the dielectric and nonlinear properties of the CCTO ceramics are enhanced by controlling the Cu-rich phase degree at grain boundary layers. The dense CCTO ceramics were prepared successfully through mixing the nanometer and micrometer powders and using the cold isostatic pressing process. The average grain size of these CCTO ceramics is about 30.71(±11.76) ∼ 62.01(±32.16) μm, and their grain microstructures show the Cu-rich phases at grain boundary layers. The CCTO ceramics with the mass ratios of nanometer and micrometer powders 7:3 display a giant dielectric constant of 5.4 × 10⁴, low dielectric loss of 0.048 at 10³ Hz, enhanced nonlinear coefficients of 11.12, as well as the noteworthy breakdown field of 4466.17 V/cm. The complex impedance spectroscopy results indicate that the giant dielectric behavior is due to the electrically heterogeneous grain/grain boundary characteristics from internal barrier layer capacitance (IBLC) model. The lower dielectric loss and the higher breakdown field are attributed to the high resistance grain boundary layers with the Cu-rich phase. The improved nonlinear properties are related to the increased Schottky barrier height at grain boundary. This work may provide a potential way to design the CCTO ceramics with excellent electrical properties from the viewpoint of controlling the response of the Cu-rich phase grain boundary.     

Improved dielectric,nonlinear and magnetic properties of cobalt-doped CaCu3Ti4O12 ceramics

The dielectric properties and voltage–current nonlinearity of the pure and various cobalt-doped CaCu₃Ti₄O₁₂ (Co-doped CCTO) prepared by solid-state reactions were investigated. The improved dielectric properties in the Co-doped CCTO, with a dielectric constant $\text{ε'}$ ≈7.4?×?10⁴ and dielectric loss $\text{tan}\text{δ}$?≈?0.034, were observed in the sample with a Co doping of 5% (CCTO05) at room temperature and 1?kHz. The related multi-relaxations, RII (?20 to 40?°C) and RIII (100–150?°C), were demonstrated to be a Debye-like relaxation and a Maxwell–Wagner relaxation related to oxygen vacancies. The low dielectric loss of CCTO05 was associated with the high grain boundary resistance and the increase in cation vacancies. The improved nonlinear electrical properties (CCTO05, with nonlinear coefficients $\text{α}$?≈?5.22 and breakdown electric field ${\text{E}}_{\text{b}}$?≈?300.46?V/cm) and the ferromagnetism in Co-doped CCTO were also discussed.     

Enhanced dielectric properties and grain boundary potentials in sulfur-doped CaCu3Ti4O12 thin films

CaCu₃Ti₄O₁₂ (CCTO) has been reported to possess a colossal dielectric constant owing to the intrinsic interfacial polarization via charge accumulations across the grain boundary. Herein, we explore the effects of unusual anion-doping on the dielectric properties of sputter-deposited CCTO thin films using an example of sulfur-doping. A post-annealing process of the films was utilized in a flowing H₂S atmosphere for the sulfur-doping. The incorporation of sulfur into the perovskite structure was evidenced with the changes in chemical states, such as the reduced cations of Cu⁺ and Ti³⁺, the increased concentration of oxygen vacancies, and the formation of S-O bonds. The sulfurized CCTO thin films demonstrated an enhanced relative permittivity of ∼620 at 100 Hz, which is substantially better than that of the unsulfurized film. Direct measurement of the grain-boundary potential using Kelvin probe force microscopy suggests that the enhanced relative permittivity is associated with an increased Schottky barrier height.     

Colossal dielectric properties of Na1/3Ca1/3Sm1/3Cu3Ti4O12 ceramics: Computational and experimental investigations

A modified sol?gel technique was used to synthesize a high dielectric ceramic, Na_1/3Ca_1/3Sm_1/3Cu₃Ti₄O₁₂. The crystal structure of this sintered ceramic matches the standard pattern of a body?centered cubic (bcc) system within the Im3 space group (JCPDS No. 75–2188). No impurity phases were observed. Interestingly, a high dielectric permittivity of ～1.14–1.35 × 10⁴ and a low loss tangent of ～0.027–0.039 were achieved in this sintered Na_1/3Ca_1/3Sm_1/3Cu₃Ti₄O₁₂ ceramic. Our DFT calculations disclosed that substitution of Na⁺ ions at Cu²⁺ sites causes an observed excess Cu concentration. As a result, metastable insulating phases were formed at a relatively high sintering temperature. Additionally, our electron density calculations revealed that Na ions lose their electrons to Sm ions, whereas the oxidation states of Cu and Ti are unaltered. Our results show that Cu⁺ and Ti³⁺ were observed after introducing an oxygen vacancy into this lattice. Significantly different values of R_g, R_gb, and E_g, E_gb support an internal barrier layer capacitor as the most likely origin of the giant dielectric properties of this ceramic. XPS results show mixed Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ in all ceramics, suggesting that electron hopping between Cu⁺?Cu²⁺ and Ti³⁺?Ti⁴⁺ is the probable origin of the n?type semiconducting state inside the grains.     

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.     

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.     

Origin of high dielectric performance in fine grain-sized CaCu3Ti4O12 materials

We report on high dielectric constant (8.3 × 10³, 10⁴ Hz), low dielectric loss (0.029, 10⁴ Hz) as well as fine grain size (∼840 nm) achieved in pure CaCu₃Ti₄O₁₂ (CCTO) ceramics through a combination of sol–gel method, spark plasma sintering and annealing process. By adjusting the sintering temperature and annealing conditions, the composition variations, valence states and microstructures of CCTO ceramics are systematically studied, which provide direct clues in understanding the origin of their excellent dielectric response. Through the studies on the dielectric, impedance, modulus and conductivity properties of CCTO ceramics, a modified brick-layer model based on two interfacial polarizations originating from sub-grain boundary and grain boundary barriers is proposed to explain their dielectric behaviors. The high dielectric constant of CCTO ceramics is mainly dominated by the sub-grain contribution; and the reduced dielectric loss is attributed to the decreases of electrical conductivity and relaxation loss.     

Structural and dielectric properties of CuO nanoparticles

The DC conduction and dielectric behaviour of copper oxide nanoparticles prepared by sol-gel method and sintered at 950 °C were studied in the temperature range of 200–526 K. The formation of single phase monoclinic CuO was confirmed by x-ray diffraction. Chemical composition of the CuO ceramic was investigated with X-ray photoelectron spectroscopy (XPS) technique. Although XRD analysis shows the formation of single phase CuO, XPS spectra revealed the presence of Cu³⁺ and Cu²⁺. Deviation from linearity ln (σ_DC) vs. 1/T plot at ~390 K was observed, which indicates that DC conduction in the CuO pellet is dominated by two different conduction mechanisms. The results obtained on AC conductivity indicate that AC conduction mechanism could be well explained by the multihopping model at low frequencies, while high frequency AC conductivity data can be described by small polaron tunnelling model. The dielectric relaxation mechanism in the CuO pellet was studied by impedance spectroscopy. It was found that while dielectric constant is an increasing function of temperature, it decreases with increasing frequency. The obtained impedance spectra indicated that the grain boundary effects and intergranular activities play a crucial role on the dielectric relaxation processes.     

Enhanced dielectric and mechanical properties of CaCu3Ti4O12/Ti3C2Tx MXene/silicone rubber ternary composites

Dielectric polymer composites with conducting fillers would have great potential for diverse applications if their severe leakage loss could be addressed. In this regard, ternary composites using both ceramic and conducting materials as fillers might be an enabler for high dielectric constant and low dielectric loss. Herein, ternary composites with both Ti₃C₂T_x MXene conducting nanosheets and CaCu₃Ti₄O₁₂ (CCTO) dielectric particles embedded in silicone rubber were studied. It was found that a ternary composite with 1.2 wt% (0.40 vol%) Ti₃C₂T_x MXene and 12 wt% (2.58 vol%) CCTO could provide an overall superior performance that include a high dielectric constant of 8.8, low dielectric loss of less than 0.0015, good thermal stability up to 450 °C, and excellent mechanical properties with tensile strength of 569 kPa, elastic module of 523 kPa and elongation at break of 333%. The outstanding performance is attributed to the improved uniform dispersion and good interfacial compatibility of mixed fillers in the polymer matrix, suggesting ternary composites might be a better option over their binary counterparts in preparing high performance dielectric composites.     

Co-doping effects of A-site Y3+ and B-site Al3+ on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics

Different doping elements have been used to reduce the dielectric losses of CaCu₃Ti₄O₁₂ ceramics, but their dielectric constants usually are undesirably decreased. This work intends to reduce their dielectric losses and simultaneously enhance their dielectric constants by co-doping Y³⁺ as a donor at A site and Al³⁺ as an acceptor at B site for substituting Ca²⁺ and Ti⁴⁺, respectively. Samples with different doping concentrations x = 0, 0.01, 0.02, 0.03, 0.05 and 0.07 have been prepared. It has been shown that their dielectric losses are generally reduced and their dielectric constants are simultaneously enhanced across the frequency range up to 1 MHz. The doped sample with x = 0.05 exhibits the highest dielectric constant, which is well over 10⁴ for frequency up to 1 MHz and is about 20% higher than the undoped sample. Impedance spectra indicate that the doped samples have much higher grain boundary resistance than the undoped one.     

Improved dielectric and non-ohmic properties of (Zn + Zr) codoped CaCu3Ti4O12 thin films

The good dielectric and non-ohmic properties of CaCu₃Ti₄O₁₂ and CaCu_2.95Zn_0.05Ti_4-xZr_xO₁₂ (x?=?0, 0.05 and 0.10) thin films prepared by a sol-gel method were determined. The enhanced dielectric properties, with a dielectric constant of ${\epsilon }^{\text{'}}$ ≈ 4357 and a dielectric loss of tan δ?≈?0.019, of the CaCu_2.95Zn_0.05Ti_3.95Zr_0.05O₁₂ (ZnZr05) thin film at 1?kHz and room temperature were investigated. The XPS spectrum showed that the ZnZr05 film can produce copper vacancies V_Cu” and mixed valence structures for Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺ inside the crystal. The ZnZr05 film maintained a high dielectric constant due to the large grain sizes and the presence of the mixed valence structures, while its low tan δ was attributed to an increase in the V_Cu” concentration. At the same time, the enhanced nonlinear coefficient (4.2) and low leakage current (193?μA) of the ZnZr05 film were explained in detail.     

Giant dielectric constant,dielectric relaxations,and tunable properties of Sm2/3Cu3Ti4O12 ceramics

The polycrystalline Sm_2/3Cu₃Ti₄O₁₂ (SCTO) ceramics have been prepared by solid-state reaction. The crystallinity of the compound has been investigated by Rietveld refinement which has revealed a cubic structure with space group Im3. It is observed that at low frequencies, SCTO ceramic exhibits tremendously high values of dielectric permittivity ε′, larger than 32,000, at room temperature. Two distinct, thermally triggered, dielectric relaxations have been noted. This mechanism has been confirmed through impedance analysis of the ceramics. The complex impedance plane shows three semicircles, which confirm the existence of two dielectric relaxations in SCTO ceramics. In general, the electrical as well as dielectric behavior of SCTO ceramics are seen to be reasonably analogous to those of CaCu₃Ti₄O₁₂ (CCTO) ceramics. The emergence of the enormous dielectric constant in SCTO ceramic is accredited to the combined effect of polarization both at the sample-electrode interface as well as at the insulating grain boundary interface. The SCTO ceramics are identical to the CCTO ceramics in their structure and composition and hence, as the above results indicate, the IBLC effect mechanism, originally put forward for CCTO ceramics, is furthermore plausible to account for the mammoth values of dielectric constant in SCTO 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.