Leading role of grain boundaries in colossal permittivity of doped and undoped CCTO

The role of grain boundaries in the colossal permittivity ɛ of doped and undoped calcium copper titanate, CaCu₃Ti₄O₁₂ (CCTO), is illustrated by a first correlation – over four orders of magnitude – between ɛ and the capacity of grain boundaries, not that of grains, deduced from the analysis of impedance measurements. The DC resistance of the CCTO sample which is essential to make efficient capacitors for technological applications, as well as the loss factor tan(δ), are found to be correlated with the resistance of the grain boundaries rather than that of the grains. The correlation extends over almost seven orders of magnitude. These findings, consistent with the internal barrier layer capacitance (IBLC) model, indicate the leading role of grain boundaries in the origin of the capacitance of CCTO samples.     

Suppressing resistance degradation in SrTiO3-based colossal permittivity capacitor material

At present, research on colossal permittivity materials is extensive but challenging to achieve simultaneous properties of colossal permittivity, low loss, and high resistivity. Resistance degradation also restricts industrial application of colossal permittivity materials. In this work, a new method has been proposed to improve resistivity of colossal permittivity ceramics by making metal ions diffuse on ceramic grain boundaries, thus inhibiting the diffusion of oxygen vacancies at grain boundaries. Sr_0.99La_0.01TiO₃(SLT10) ceramics were synthesized by traditional solid-state method, and then Bi₂O₃ (35%)-Al₂O₃ (10%)-MgO (20%)-CuO (25%)-SiO₂ (10%) mixed oxidant was selected to percolate into ceramics. The resistivity of SLT10 ceramics improved remarkably (from 2.1×10⁸ Ω cm to 1.23×10¹¹ Ω cm under DC 100 V) with a colossal permittivity (16695 @1 kHz) and a low dielectric loss (0.016 @1 kHz), as well as excellent frequency stability (20 Hz–2 MHz) and temperature stability (-170 °C to 375 °C). The source of high insulation resistivity of the SLT10 ceramic sample was discussed. Subsequent examination uncovered that grain in the SLT10 ceramics percolated with metal ions displayed semiconducting characteristics, wherein insulation grain boundaries significantly influenced the ceramic's resistivity and served as formidable potential barriers constraining long-range movement of charge carriers. Experimental analysis demonstrated that the resistance degradation behavior of the SLT10 ceramics was suppressed, the breakdown voltage was increased, and the service life was extended.     

Effect of ionic radius on colossal permittivity properties of (A,Ta) co-doped TiO2 (A= alkaline-earth ions) ceramics

(A, B) co-doped TiO₂ ceramics attract great interests due to the excellent dielectric properties. In this work, the (A, Ta) co-doped TiO₂ ceramics were prepared by a solid state reaction process. The effect of the acceptors ionic radius on the structure and properties of TiO₂ ceramics was investigated. According to XRD analysis, the main phase is rutile TiO₂ for all samples. Due to the larger ionic radius, it is hard to replace Ti site in TiO₆ octahedron. As a result, the content of the secondary phase increased with increasing ionic radius. The dielectric properties were significantly enhanced by co-doping of alkaline-earth ions and tantalum ions, and the best dielectric constant obtained at 3% (Sr, Ta) co-doped compositions, where ε’ = 2.1 × 10⁵, tanδ = 0.21. Meanwhile, the XPS analysis suggested that the concentration of the defect dipoles exhibit a maximum in Sr-doped TiO₂ ceramics. The larger ionic radius of the acceptors leads to the more stability of the defect structure. However, for Ba ions, the replacement concentration decreased due to the excessive ionic radius, which in turn reduces the defect concentration. This work is meaningful for the further investigations on TiO₂-based colossal permittivity materials.     

The effect of titanium excess and deficiency on the microstructure and dielectric properties of lanthanum doped Ba0.55Sr0.45TiO3 with colossal permittivity

The temperature dependent dielectric properties of (Ba_0.54875Sr_0.44875La_0.0025)Ti_(1+x)O₃ with both an excess and a deficiency of 0.25 mol.% TiO₂ were investigated. The samples were prepared by the mixed oxide method and sintered in a conventional oven at temperatures ranging from 1400 °C to 1475 °C. The cubic perovskite structure was confirmed with XRD at room temperature. The sample with an excess of 0.25 mol.% Ti exhibited reduced grain growth while abnormal grain growth was observed for samples without Ti modification. Samples exhibited colossal permittivity for all modified compositions. With a 0.25 mol.% deficiency of Ti a permittivity over 65,000 and a tan δ under 0.05 were measured over a temperature range of ?68 °C to 150 °C and a frequency range between 50 kHz and 1 MHz. This paper shows that by fine tuning the composition, materials with new, exciting and widely adjustable dielectric properties can be achieved.     

Disappearance and recovery of colossal permittivity in (Nb+Mn) co-doped TiO2

We investigate the effects of doping and annealing on the dielectric properties of metal ions doped TiO₂ ceramics. Colossal permittivity (CP) above 10⁴ was observed in single Nb ion doped TiO₂, which was dominated by electron transport related interfacial polarization. Moreover, the CP can be dropped to 120 when simultaneously introducing Mn ion into the sample. The disappearance of CP behaviors maybe due to the multivalence of Mn which would inhibit the reduction of Ti⁴⁺ to Ti³⁺, and thus reduce delocalized electrons. Interestingly, the CP was recovered for the (Nb+Mn) co-doped TiO₂ after post-sintering heat treatment in N₂ atmosphere. The recovery of CP in the sample after annealing can be ascribed to the semiconducting grain and the insulating grain boundary, according to impedance spectroscopy. We therefore believe that this work can help us understand the mechanism of CP from a new perspective.     

Ionic conduction,colossal permittivity and dielectric relaxation behavior of solid electrolyte Li3xLa2/3-xTiO3 ceramics

Perovskite-type solid electrolyte lanthanum lithium titanate (LLTO), exhibiting high intrinsic ionic conductivity, has been attracting interests because of its potential use in all solid-state lithium-ion batteries. In this work, we prepared LLTO ceramics by solid state reaction method and studied their conductivity and dielectric properties systematically. It is found that the bulk conductivity of LLTO is several orders of magnitude higher than the grain boundary conductivity. In addition, colossal permittivity was observed in LLTO ceramics in wide frequency/temperature ranges. Two non-Debye type relaxation peaks were observed in the imaginary part of permittivity, resulting from Li⁺ ions motion and accumulation near interfaces of grains/grain boundaries/electrodes. It is suggested that colossal permittivity may originate from the lithium ion dipoles inside the samples and the interfacial polarization of lithium ion accumulation near the grain boundaries. These results clarify the relations among colossal permittivity, relaxation behavior and ionic conduction in solid ion conductor ceramics.     

A-site Ca2+ substitution induced polyvalent Cu cations and correlated polaron relaxations in colossal permittivity Li1-xCaxCuNb3O9

LiCuNb₃O₉ has been reported newly a colossal permittivity (CP) perovskite, in which the B-site NbO₆ octahedra play a bridging role in the polaron hopping. However, how the A-site modification affects the origin of the polarons and further the CP behaviours remains unexplored. To this end, A-site Ca²⁺ was incorporated to form Li_1-xCa_xCuNb₃O₉, and the local states, dielectric relaxations and conduction behaviours were comprehensively studied. The substitution induces the polyvalent Cu cations, i.e. Cu⁺/Cu²⁺/Cu³⁺. Bond valence sum calculations imply that Cu²⁺ and Cu³⁺ are underbonded, and Cu⁺ is overbonded, while B-site Nb⁵⁺ shows slightly different with theoretical pentavalence. All the compositions exhibit a similarly room-temperature CP response, but present two dielectric relaxations, i.e. T_R1:170–300 K and T_R2:260–400 K. Comprehensive investigations on universal dielectric response and bulk dc conductivity indicate that the T_R1 follows the variable-range-hopping where the electron hopping between the mixed Cu⁺/Cu²⁺, while T_R2 contributes from the Cu³⁺ nearest neighbor hopping.     

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.     

Low dielectric loss,colossal permittivity,and high breakdown electric field in Al-doped Y2/3Cu3Ti4O12 ceramics

The miniaturization and high capacitance of electronic components are driving the development of high-performance electronic ceramic materials. In this work, we design a new strategy to achieve satisfactory dielectric properties with low loss, colossal permittivity, and a high breakdown electric field (E_b) in Al-doped Y_2/3Cu₃Ti₄O₁₂ (YCTO) ceramics prepared by a solid-phase synthesis method. The dielectric loss decreased with Al doping in the YCTO. The dielectric constant and the E_b were improved upon Al doping. With Al doping levels of 0.03 and 0.05, Y_2/3Al_0.03Cu_2.97Ti₄O₁₂ and Y_2/3Al_0.05Cu_2.95Ti₄O₁₂ ceramics displayed, respectively, a suppressed loss tangent of about 0.028 and 0.031, a high dielectric constant of approximately 9540 and 11792, and an E_b of approximately 4.32 and 4.54 kV/cm. The improved dielectric properties of the produced ceramics were closely linked to enhanced grain boundaries resistance. This study explores the physical mechanism behind the high performance of the YCTO-based ceramics, and also provides theoretical support for the application of devices comprising YCTO and related materials.     

Double pentavalent (Sb5+, Nb5+) and trivalent (Sm3+, Y3+) co-doped Ti0.9Zr0.1O2 colossal dielectric permittivity multilayer ceramics for the miniaturization of the next-generation electronics

Materials with colossal dielectric permittivity (CP) are in the focus of interest for the development of miniaturization and integration of electronic components. Despite the extensive study of these new classes of co-doped TiO₂ CP materials, the preparation of multilayer ceramics using this kind of CP materials is still challenging work. Here, we synthesize a series of (Sb⁵⁺, Nb⁵⁺) and (Sm³⁺, Y³⁺) co-doped Ti_0.9Zr_0.1O₂ ceramics (SNSYTZO) through the conventional solid-state reaction method. XRD spectrum identifies that ceramics under x = 0.04 show a perfect rutile phase with the tetragonal crystal structure; however, minor brookite orthorhombic crystal structure appears when x > 0.04. FESEM images show the prepared ceramics have excellent densification and low porosity. Dielectric, modulus, and impedance spectrum are systematically explored the underlying CP mechanism and compared with each other to find the optimal materials composition to prepare further multilayer ceramics, which is fabricated by the industrial tape casting method. FESEM, together with surface element mapping, indicates that all doping elements are homogeneously distributed. Also, we investigate the dielectric response without/with DC bias. This work sheds light on a promising feasible route to prepare the miniaturization of the next-generation electronics via a large scale industrial tape casting method.     

Optimize the dielectric properties of CaCu3Ti4O12 ceramics by adjusting the conductivities of grains and grain boundaries

Reduction of dielectric loss for CCTO ceramics is a prerequisite for their applications. Considering internal barrier layer capacitance effect, improving the capacitance and grain boundary resistance is an effective way to reduce dielectric loss. Therefore, more conductive Ti³⁺ and Cu⁺ ions were introduced to grains by adding carbon to ceramic bodies, improving the permittivity of CCTO ceramics. Annealing was performed to increase the grain boundary resistance. The dielectric loss of the CCTO ceramics thus prepared, which maintain a giant permittivity, is significantly reduced. Specifically, the CCTO ceramic with carbon addition, which was sintered at 1080 °C for 8 h and air annealed at 950 °C for 2 h, exhibits a giant permittivity of about 2.50(5)×10⁴ and a low dielectric loss of less than 0.050(2) from below 20 Hz to 50 kHz at room temperature. Meanwhile, its dielectric loss at 1–10 kHz is less than 0.050(2) from below room temperature to about 100 °C.     

Colossal permittivity in Ta-doped SrTiO3 ceramics induced by interface effects and defect structure: An experimental and theoretical study

The lack of systematic research on the phase structure, defect structure, and polarization mechanism hinders the full comprehension of the colossal permittivity (CP) behavior for SrTiO₃-based ceramics. For this purpose, Ta-doped SrTiO₃-based ceramics were synthesized in an N₂ atmosphere with a traditional method. When the appropriate amount of Ta was doped, colossal permittivity (ԑ_r ∼ 62505), low dielectric loss (tanδ ∼ 0.07), as well as excellent temperature stability (−70 °C–180 °C, ΔC/C_25°C ≤ ±15%) were obtained in the Sr_0.996Ta_0.004TiO₃ ceramic. The relationship between Ta doping, polarization mechanism, and dielectric performance was systematically researched according to experimental analysis and theoretical calculations. The first-principle calculations indicate that the Ta⁵⁺ ion prefers to replace the Sr-site. The defect dipoles and oxygen vacancies formed by heterogeneous-ion doping play an active role in regulating the dielectric performance of ceramics. In addition, the interface barrier layer capacitance (IBLC) effect associated with semi-conductive grains and insulating grain boundaries is the primary origin of colossal permittivity for Sr_1-xTa_xTiO₃ ceramics. The polarization mechanism and defect structure proposed in the study can be extended to the research of SrTiO₃ CP ceramics. The results have a good development prospect in colossal permittivity (CP) materials.     

Fine-tuning of negative permittivity behavior in amorphous carbon/alumina metacomposites

In this work, amorphous carbon/alumina (C/Al₂O₃) metacomposites with tunable negative permittivity were synthesized by a facile impregnation method. We analyzed the influence of carbon content and the carbonization temperature on the microstructure and electrical properties of the metacomposites. It was found that the conductive carbon networks were formed in the composites with the higher carbon contents and carbonization temperatures, which lead to the appearance of a metal-like conduction and negative permittivity behavior. Low-frequency plasmonic state provided by plenty of free electrons in the carbon networks resulted in the negative permittivity, and its numerical values could be effectively adjusted by controlling the carbon content and carbonization temperature, which could be well explained by Drude model. This work provides a feasible way to prepare metacomposites with tunable negative permittivity, which will expand their potential applications in shielding, absorbing and novel capacitor fields.     

Colossal permittivity in BaTiO3-0.5wt%Na0.5Ba0.5TiO3 ceramics with high insulation resistivity induced by reducing atmosphere

The donor-acceptor co-doping grain-fine BaTiO₃-0.5wt%Na_0.5Ba_0.5TiO₃ (BT-0.5wt%NBT) ceramics are obtained by conventional solid-state reaction method and sintered in different atmospheres. The dielectric properties are sensitively influenced by the sintering atmosphere, which the colossal permittivity can be activated and increased by enhanced atmospheric reducibility with the increase of H₂ content. However, the excessive H₂ content can make a significant deterioration in dielectric loss and insulation resistivity. The impedance spectra, XPS and EPR measurements indicate that sintering atmosphere can effectively regulate the concentration and distribution of charge carriers (delocalized electrons, oxygen vacancies and defects), which induce the interfacial polarization and hopping polarization. When sintered in 0.1% H₂/N₂, the sample possesses a relatively good comprehensive performance with colossal permittivity (ε_γ>4×10⁴), ultra-low dielectric loss (tanδ=0.0218) and high insulation resistivity (ρ_v>10¹¹Ω·cm), which related to the moderate values of resistance and conduction activation energy for the grain boundaries.     

Colossal permittivity in TiO2 co‐doped by donor Nb and isovalent Zr

Effect of isovalent Zr dopant on the colossal permittivity (CP) properties was investigated in (Zr + Nb) co‐doped rutile TiO₂ ceramics, i.e., Nb_0.5%Zr_xTi_1?xO₂. Compared with those of single Nb‐doped TiO₂, the CP properties of co‐doped samples showed better frequency‐stability with lower dielectric losses. Especially, a CP up to 6.4 × 10⁴ and a relatively low dielectric loss (0.029) of x = 2% sample were obtained at 1 kHz and room temperature. Moreover, both dielectric permittivity and loss were nearly independent of direct current bias, and measuring temperature from room temperature to around 100°C. Based on X‐ray photoelectron spectroscopy, the formation of oxygen vacancies was suppressed due to the incorporation of Zrions. Furthermore, it induced the enhancement of the conduction activation energy according to the impedance spectroscopy. The results will provide a new routine to achieve a low dielectric loss in the CP materials.     

Influence of core-shell structure on the cure depth in photopolymerizable alumina dispersion

The heterogeneous nucleation-and-growth processing was used to obtain core-shell particles based on alpha alumina core with silica layer. Presence of silica shell was confirmed by transmission electron microscopy (TEM) and zeta potential measurement. The coverage of aluminum oxide surface by SiO₂ improved a cure of the depth of photopolymerizable ceramic dispersion around 20%. The proposed surface modification enables the production of thicker cured layer which is favorable for additive manufacturing methods such as stereolithography. Thus, the number of layers that have to be printed to form the 3D object might significantly decrease, thereby reducing time and costs of fabrication.     

采用核/壳技术制备P(VAc—BA)乳液的研究

采用核/壳技术制备了P(VAc—BA)系乳液,探讨了核/壳组成变化对乳液性能的影响。研究表明,核/壳比相同时,壳中丙烯酸丁酯(BA)含量对乳液T_g值、成膜性能及膜的力学性能均有影响;当核层和壳层中单体组成相同时,壳层的厚度越大,MFT值则越低;在总组成和核/壳比一定时,只改变核层和壳层单体的组成,也可以在一定范围内使乳液的特性发生变化。     

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.