Internal Barrier Layer Capacitor Model Research Articles

Optimizing sintering conditions and microstructure for enhanced dielectric and humidity sensing properties of Na1/3Ca1/3Tb1/3Cu3Ti4O12 ceramics

This study investigated the impact of sintering temperature and resultant microstructure on the dielectric behavior and humidity sensing performance of Na1/3Ca1/3Tb1/3Cu3Ti4O12 (NCTCTO). The pure NCTCTO phase was obtained. Relative density and grain size tended to increase with the sintering temperature. The porous NCTCTO exhibited remarkable humidity sensing performance with minimal hysteresis error (2 %) and rapid response and recovery times (1.5 min). The dense NCTCTO ceramic with a high relative density of about 99 %, demonstrated significant giant dielectric properties, with ε′∼8500 and low loss tangent (0.08). The dielectric behavior was interpreted using the internal barrier layer capacitor model. The increase in ε′ with humidity was ascribed to the absorption of H2O molecules at the grain boundary, leading to increased interfacial polarization. This work delineates the critical influence of ceramic microstructure on the humidity sensing and giant dielectric properties of NCTCTO ceramics, paving the way for their application in advanced electronic devices.

Giant permittivity response and enhanced nonlinear electrical properties in a novel perovskite-like ceramic with multiple elements

Copper calcium titanate (CaCu3Ti4O12, CCTO) ceramics are receiving a great deal of attention for advanced electronic and energy storage device applications owing to their giant permittivity response and good temperature stability. However, the development and real applications of CCTO ceramics are generally restricted by their large dielectric loss and low breakdown electric field strength. In the present study, a novel type of perovskite-like Na1/3(Ca1/2Sr1/2)1/3(Bi1/2Y1/2)1/3Cu3Ti4O12 (NCSBYCTO) ceramics were designed via multiple elements strategy based on the high-entropy concept. Consequently, all the NCSBYCTO ceramics present a giant permittivity response (103∼104), good stability of temperature and frequency, and remarkable relaxation characteristics, especially the NCSBYCTO ceramic sintered at 1050 °C exhibits a relative balance between the giant permittivity of about 7.0 × 103–9.5 × 103 and the lower dielectric loss of approximately 0.043–0.294 at a frequency range of 104–106 Hz under room temperature. Furthermore, all the NCSBYCTO ceramics present an obvious nonlinear characteristic. More encouragingly, a higher breakdown field strength of around 2552.56 V/cm accompanied by a larger nonlinear coefficient of about 6.48 is achieved in the NCSBYCTO ceramic sintered at 1070 °C. A significant electrical heterogeneity structure and electron defect structure induced by the existence of mixed-valence Cu+/Cu2+ and Ti3+/Ti4+ are also found in the NCSBYCTO ceramics. The giant permittivity response, low dielectric loss and improved nonlinear properties can be considered the synergistic effect of the defect mechanism and the internal barrier layer capacitor model. This finding offers a route for designing giant permittivity ceramics with brilliant dielectric properties and nonlinear behaviors, which is expected to benefit dual-functional device applications of capacitors and varistors.

Microstructure and dielectric properties of Na and Ni co-substituted CaCu3Ti4O12 ceramics with high dielectric constant and low loss

Preparation of Na and Ni co-substituted CaCu3Ti4O12 (CCTO) ceramics through sol-gel method is carried out to enhance the dielectric properties. On the morphology and dielectric properties of CCTO ceramic, the effects of Na and Ni co-substitution are investigated. The results demonstrate that at room temperature, Na and Ni co-substituted Ca0.993Na0.007Cu2.95Ni0.05Ti4O12 exhibits significantly increased grain size and consequently a high dielectric constant exceeding 12000 across a broad frequency range (1–105 Hz). Meanwhile, its dielectric loss is reduced to 0.016 at 1 kHz, a phenomenon that can be ascribed to the raised resistance and activation of grain boundary. Furthermore, Ca0.993Na0.007Cu2.95Ni0.05Ti4O12 also yields a breakdown field strength of 2.3 kV/cm. The giant dielectric response can be explained by the internal barrier layer capacitor model. These excellent comprehensive performances not only contribute to obtain simultaneously low dielectric loss and high breakdown strength for CCTO ceramics with high dielectric constant, but also can facilitate the practical application of CCTO ceramics in advanced capacitors.

(Zn2+ + Cd2+) co-doped CaCu3Ti4O12 ceramics with enhanced dielectric permittivity and reduced dielectric loss tangent

The present study systematically investigated the structure, dielectric properties, and electrical response of Ca1-xCdxCu3-yZnyTi4O12 (x = y = 0, 0.025, 0.05, and 0.10). The XRD analysis indicates the presence of a CaCu3Ti4O12 phase in all the sintered ceramics, with no indications of impurity phases found. Achieving high dielectric permittivity and low loss tangent can be accomplished through codoping with Zn2+/Cd2+. At room temperature and 1 kHz, the Ca0.95Cd0.05Cu2.95Zn0.05Ti4O12 ceramic has a high dielectric constant of ∼1.61 × 105 and a low loss tangent of ∼0.03. In addition, codoping ions can enhance the stability of dielectric permittivity with respect to temperature variations. The utilization of impedance spectroscopy as a technique confirms the heterogeneous microstructure observed in sintered materials. The results of this investigation suggest a potential association between the internal barrier layer capacitor model and the underlying cause of the colossal dielectric characteristics observed in Ca1-xCdxCu3-yZnyTi4O12 materials. The analysis of X-ray photoelectron spectroscopy indicates the presence of Cu+ and Ti3+ species, which could potentially exert a significant impact on the development of n-type semiconducting grains in Ca1-xCdxCu3-yZnyTi4O12 ceramics. This influence is attributed to the existence of oxygen vacancies. Theoretical simulations revealed that a Zn atom is situated in proximity to a Cd atom within the CCTO structure. Furthermore, our findings indicate that the oxygen vacancy does not interact with the dopants. Our electron density analysis suggests that the presence of Cu+ and Ti3+ ions, as observed by XPS measurements, is a consequence of the existing oxygen vacancy.

Structural, dielectric, impedance, and electric modulus characteristics of Y2CaCuO5 high-k dielectric ceramics

With the increasing trends of integrated circuit densification, device miniaturization, and exponential growth of microelectronic components there is a huge responsibility on the part of material researchers to derive, design, and develop new materials. Materials that can overcome the existing limitations in the present compounds available in the market or can expedite the process of material fabrication are of great interest. This feat can be achieved by the fabrication of new materials, and analysis of various characterizations viz. (structural, dielectric, transport behavior, and impedance spectroscopy). With this perspective, in this communication, a high permittivity dielectric material belonging to the Y-Ba-Cu-O ternary systems where barium is replaced by calcium (Y2CaCuO5) is synthesized using solid-state reaction route. X-ray diffraction, scanning electron micrograph, and energy-dispersive X-ray spectroscopic study confirm the phase and microstructure of the sample. The observed higher value of the dielectric constant is understood from Koop’s phenomenological theory and internal barrier layer capacitance model. The dispersive behavior of conductivity is interpreted by Jonscher’s power law. Overlapping large polaron tunneling model for conduction is observed. Impedance spectroscopy analysis reveals non-Debye type of relaxation with variable time constants. According to Nyquist plots; up to 250 °C only grain effect (bulk) is dominantly present; however, at higher temperatures, the contribution from both grain and grain boundary is seen.

Dielectric relaxations without colossal permittivity in Mg and Nb co-doped BaTiO3 ceramics

Magnesium (Mg) and Niobium (Nb) co-doped BaTiO3 ceramics are prepared using a solid-state reaction method under an air atmosphere, and their crystal structures, microstructures, valence states and dielectric properties are studied. The ceramics adopt a cubic perovskite structure, and the lattice parameter increases with doping content owing to the larger ionic radii of dopants. The complex permittivity and electric modulus as a function of frequency and temperature are analyzed. Dielectric relaxations without colossal permittivity are observed. The relaxation processes are thermally activated and their variations with temperature are analyzed. The activation energy values for the relaxation processes are determined by the Arrhenius relationship and increase with doping content. By analyzing the electric modulus, the activation values for high-frequency relaxation are found to be associated with the electrical properties of grain, and the contribution of grain boundary could be neglected based on internal barrier layer capacitor (IBLC) model. The electron hopping between the different Ti valences, which is confirmed by X-ray photoelectron spectroscopy, is supposed to play an important role in high-frequency relaxation. The permittivity plateaus are related to the inhomogeneous structures of grain and grain boundary, and the low-frequency permittivity plateau is attributed to the space-charge polarization at grain boundaries. By comparing with Cu/Nb co-doped BaTiO3, the absence of colossal permittivity is ascribed to high resistivity of the samples. Charge density and migration as well as the insulating grain boundaries are critical for the formation of space charge and a high permittivity.

Improved optical, dielectric, and nonlinear properties of CaCu3Ti4O12 thin films by chromium doping

In the present work, the optical and dielectric properties, microstructures, and nonlinear electrical performances were systematically investigated for Ca1-xCrxCu3Ti4O12 (x = 0.00 - 0.40) thin films prepared on Pt/Ti/SiO2/Si substrates using the sol-gel method. X-ray photoelectron spectroscopy (XPS) verified that Cr3+ and Cr6+ were present in the thin films. Additionally, the content of secondary phase of titanium oxide (TiO2) increased, the grain size decreased, and the surface roughness improved with the increase in the doped content of Cr. Due to the reduced grain size and oxygen vacancy concentration, the optical energy band gap increased from 2.92 to 3.54 eV. The change in the dielectric constants (Ɛr) of the thin films followed the internal barrier layer capacitor model. Doping was found to decrease the dielectric loss (tan δ) and enhance the nonlinear properties, which was due to the presence of TiO2 secondary phase, high density, reduced grain size, incremental potential barrier height and grain boundary resistance. The Ca0.8Cr0.2Cu3Ti4O12 thin film had a high Ɛr of around 9340, low tan δ of around 0.012 at 1 kHz, nonlinear coefficient of about 4.64, and breakdown field of ca. 89 kV/mm. These results indicated that the doping of an appropriate amount of Cr could effectively modify the microstructure of CaCu3Ti4O12 thin films and improve their optical and electrical properties.

Tuning electrical heterogeneity in CaCu3Ti4O12-ZnO ceramics for high dielectric and nonlinear properties

Tuning the electrical heterogeneity of CaCu3Ti4O12 (CCTO) ceramic is extremely important to improve its properties. In this work, the CCTO ceramics with enhanced dielectric and nonlinear properties were obtained by the introduction of nano ZnO based on the electrical heterogeneity modulation strategy. As a result, all the CCTO-ZnO samples show the colossal dielectric constant of 104–105 at room temperature. Meanwhile, the dielectric constant of CCTO-ZnO samples exhibits better stability in a wide frequency range from 101 Hz to 105 Hz and over a temperature scope of 30–210 °C. Most importantly, the x = 0.04 sample presents a relatively lower room-temperature dielectric loss of about 0.07 at 103 Hz. Furthermore, the breakdown field strength accompanied by the nonlinear coefficient of CCTO-ZnO ceramics is also improved. The enhancement of dielectric and nonlinear behaviors in these ceramics is mainly ascribed to the tunable electrical heterogeneity as per the internal barrier layer capacitor (IBLC) model.

Giant dielectric properties of Na1/2La1/2Cu3Ti4O12 perovskite ceramic: First-principles and experimental investigations

Na1/2La1/2Cu3Ti4O12 ceramics were successfully fabricated via a sol-gel method. We found that the crystal structure of these sintered ceramics is identical to that of CaCu3Ti4O12, ICSD number 01-075-2188. A small quantity of CuO was observed due to the substitution of Na + at Cu2+ sites of the CaCu3Ti4O12 structure. Additionally, a small number of related Cu phases were discovered at grain junctions in these ceramics. Our Ab initio Molecular Dynamic simulations clearly revealed that oxygen vacancies are created during the sintering process. Additionally, at room temperature, our simulations showed that sites of the oxygen vacancies in the Na1/2La1/2Cu3Ti4O12 lattice are occupied by free oxygen, resulting in a lower oxygen vacancy density. Sintered Na1/2La1/2Cu3Ti4O12 ceramics present excellent dielectric properties, a high dielectric permittivity of 7780–9000 and a rather low dielectric loss tangent (0.041–0.042). From our electron density analysis, it was clearly seen that Cu+ and Ti3+ observed by the XPS measurements originated from the presence of oxygen vacancies in the Na1/2La1/2Cu3Ti4O12 structure. Electrical, DFT, and XPS studies indicated that the internal barrier layer capacitor (IBLC) model is the most appropriate for the colossal dielectric response in these ceramics.

A comparative study of microstructure and electrical properties of (Al, Nb) co-doped TiO2 ceramics by different sintering methods

(Al, Nb) co-doped TiO2 colossal dielectric ceramics were effectively made using conventional sintering (CS), flash sintering (FS) and two-step flash sintering (TSFS), respectively. The structure and electrical characteristics of ceramic samples were thoroughly investigated. FS and TSFS can decrease the processing temperature by 20% and shorten the sintering time by 30 times compared with CS. All sintered samples were pure rutile TiO2 structure. The TSFS samples with finer grain size had higher permittivity than the CS samples. With increasing sintering temperature, the permittivity of the co-doped TiO2 ceramic increased first and then decreased. When the furnace temperature was 1080 °C, the TSFS ceramic sample showed the best overall performance: the dielectric constant (ε’) of 1.4 × 105, the dielectric loss (tanδ) of 0.57 at 1 kHz, and the nonlinear coefficient of 5.8. XPS and IS spectra indicated that the excellent dielectric properties are attributed to the internal barrier layer capacitance model. Therefore, the TSFS method provides an ideal way to prepare co-doped TiO2 colossal dielectric ceramics.

Effect of Sn4+–Isovalent doping concentration on giant dielectric properties of Sn Ta0.025Ti0.975-O2 ceramics

The Sn x Ta 0.025 Ti 0.975- x O 2 ( x %Sn(TTO)) ceramics with x = 2.5–10% were prepared using a standard mixed-oxide method and sintered at 1450 °C for 3 h to achieve a dense microstructure. The effects of the isovalent–Sn 4+ doping concentration on the crystal structure, microstructure, giant dielectric behavior, and electrical properties were systematically investigated. Continuously enlarged lattice parameters and bond lengths with a single rutile–TiO 2 phase were observed as x % increased. The mean grain size was slightly reduced (∼17.3–14.6 μm) due to an increased oxygen vacancy and the solute drag effect. The dielectric permittivity (ε′) decreased with increasing x %, whereas the loss tangent (tanδ) was remarkably reduced. The semiconducting grain resistance of the x %Sn(TTO) ceramics remained unchanged owing to the same Ta 5+ donor concentration. The insulating grain boundary (GB) resistance was extremely increased by more than two orders of magnitude when x % increased from 2.5 to 5.0%, leading to the significantly improved giant dielectric properties. The optimized low tanδ∼0.02 and high ε′∼10 4 with temperature coefficient less than ±15% in the range of -60–210 °C were reasonably described by the internal barrier layer capacitor model. Improved dielectric properties can be obtained by engineering GB by varying the Sn 4+ –isovalent doping concentration. This study provides an important approach for improving the dielectric properties of co–doped TiO 2 without the creation of complex defect clusters inside the grains.

Effects of Sintering Conditions on Giant Dielectric and Nonlinear Current-Voltage Properties of TiO2-Excessive Na1/2Y1/2Cu3Ti4.1O12 Ceramics.

The effects of the sintering conditions on the phase compositions, microstructure, electrical properties, and dielectric responses of TiO2-excessive Na1/2Y1/2Cu3Ti4.1O12 ceramics prepared by a solid-state reaction method were investigated. A pure phase of the Na1/2Y1/2Cu3Ti4.1O12 ceramic was achieved in all sintered ceramics. The mean grain size slightly increased with increasing sintering time (from 1 to 15 h after sintering at 1070 °C) and sintering temperature from 1070 to 1090 °C for 5 h. The primary elements were dispersed in the microstructure. Low dielectric loss tangents (tan δ~0.018–0.022) were obtained. Moreover, the dielectric constant increased from ε′~5396 to 25,565 upon changing the sintering conditions. The lowest tan δ of 0.009 at 1 kHz was obtained. The electrical responses of the semiconducting grain and insulating grain boundary were studied using impedance and admittance spectroscopies. The breakdown voltage and nonlinear coefficient decreased significantly as the sintering temperature and time increased. The presence of Cu+, Cu3+, and Ti3+ was examined using X-ray photoelectron spectroscopy, confirming the formation of semiconducting grains. The dielectric and electrical properties were described using Maxwell–Wagner relaxation, based on the internal barrier layer capacitor model.

Sr and Zr Co-Doped CaCu3Ti4O12 Ceramics with Improved Dielectric Properties

The dielectric constant of CCTO materials can be as high as 104, which makes it suitable for use in electronic devices but the high dielectric loss limits its application. In this paper, a series of Sr and Zr co-doped CCTO ceramics having the formula Ca0.8Sr0.2Cu3Ti4−xZrxO12 (x = 0.1, 0.2, 0.3, 0.4) were obtained via a solid-state reaction technique. We force the effect of the Zr content on the phase composition, microstructure, cationic valence states, impedance, and dielectric properties of the as-prepared ceramics to reduce dielectric loss. The results demonstrate that Sr and Zr co-doping increases dielectric constant and reduces dielectric loss simultaneously, and the maximum dielectric constant (1.87 × 105, 1 Hz) and minimum dielectric loss (0.43, 102 Hz) are obtained when x = 0.3. Mixed Cu+/Cu2+ and Ti3+/Ti4+ valence states are observed to coexist in the co-doped material lattices, which promote dipole polarization, and thereby increase the dielectric constant of the ceramics. The dielectric properties of the materials are analyzed according to the internal barrier layer capacitance model, which elucidates the contributions of the grains and grain boundaries to dielectric performance. The maximum grain boundary resistance (3.7 × 105 Ω) is obtained for x = 0.3, which contributes toward the minimum dielectric loss (0.43) obtained for this ceramic at a frequency less than 1 kHz. The average grain sizes of the samples decrease with increasing Zr content, which is the primary factor increasing the grain boundary resistance of the co-doped ceramics.

Giant dielectric properties of Mg doped CaCu3Ti4O12 fabricated using a chemical combustion method: Theoretical and experimental approaches

In this work, CaCu3-xMgxTi4O12 (x = 0, 0.05, and 0.10) ceramics were successfully prepared via a chemical combustion method. No impurity phase was detected in these two ceramics. Fine−grained microstructures were formed in all CaCu3-xMgxTi4O12 ceramics. Interestingly, high dielectric permittivities of ∼6.58 × 103 − 2.87 × 104 with low loss tangents of ∼0.011 − 0.025 were obtained in the Mg-doped CaCu3Ti4O12 ceramics. According to the electrical measurements, improved dielectric properties in the doped ceramic samples were a result of an enhanced grain boundary response. To elucidate the electronic structure of CaCu2.90Mg0.10Ti4O12, first-principles calculations were carried out. It was found that two Mg atoms preferentially interact. Moreover, by adding an oxygen vacancy into the CaCu2.90Mg0.10Ti4O12 structure, our results revealed that it was likely to be isolated from the Mg atoms, indicating an oxygen loss suppression during the sintering process. This resulted in an enhanced grain boundary resistance in the CaCu2.90Mg0.10Ti4O12 ceramic. Based on both the experimental and computational studies, the internal barrier layer capacitor (IBLC) model was found to be the primary origin of the colossal dielectric response in all CaCu3-xMgxTi4O12 ceramics examined in the current study.

Enhancing giant dielectric properties of Ta5+‐doped Na1/2Y1/2Cu3Ti4O12 ceramics by engineering grain and grain boundary

AbstractVarious strategies to improve the dielectric properties of ACu3Ti4O12 (A = Sr, Ca, Ba, Cd, and Na1/2Bi1/2) ceramics have widely been investigated. However, the reduction in the loss tangent (tanδ) is usually accompanied by the decreased dielectric permittivity (ε′), or vice versa. Herein, we report a route to considerably increase ε′ with a simultaneous reduction in tanδ in Ta5+–doped Na1/2Y1/2Cu3Ti4O12 (NYCTO) ceramics. Dense microstructures with segregation of Cu– and Ta–rich phases along the grain boundaries (GBs) and slightly increased mean grain size were observed. The samples prepared via solid‐state reaction displayed an increase in ε′ by more than a factor of 3, whereas tanδ was significantly reduced by an order of magnitude. The GB–conduction activation energy and resistance raised due to the segregation of Cu/Ta–rich phases along the GBs, resulting in a decreased tanδ. Concurrently, the grain–conduction activation energy and grain resistance of the NYCTO ceramics were reduced by Ta5+ doping ions owing to the increased Cu+/Cu2+, Cu3+/Cu2+, and Ti3+/Ti4+ ratios, resulting in enhanced interfacial polarization and ε′. The effects of Ta5+ dopant on the giant dielectric response and electrical properties of the grain and GBs were described based on the Maxwell–Wagner polarization at the insulating GB interface, following the internal barrier layer capacitor model.

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

CaCu3-xZnxTi4O12 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 CaCu3-xZnxTi4O12 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 CaCu3-xZnxTi4O12 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.

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.

Dielectric property of perovskite La0.86Cu3Ti3.42Al0.58O12 ceramic

A novel perovskite La0.86Cu3Ti3.42Al0.58O12 (LCTAO) ceramic was fabricated by a conventional solid-state method, and the dielectric property was systematically investigated. Pure phase with a body-centered cubic structure was confirmed by X-ray diffraction. High dielectric permittivity was performed in wide temperature and frequency ranges. Impedance spectroscopy analysis demonstrated microstructure was electrically heterogeneous, which resulted in a giant dielectric response based on the internal barrier layer capacitor (IBLC) model. The results of X-ray photoelectron spectroscopy indicated the active electron hopping process of the mixed-valence states charge carriers, i.e. Cu+/Cu2+, Ti3+/Ti4+, may be responsible for the semiconduction of grains.

Controlling microstructure and significantly increased dielectric permittivity with largely reduced dielectric loss in CaCu3−xGexTi4O12 ceramics

Microstructure, dielectric, and electrical properties of CaCu3−xGexTi4O12 ceramics with x = 0–0.10 prepared by a conventional solid-state reaction method are investigated. Single-phase of CaCu3Ti4O12 was detected in all sintered ceramics. The substitution of Ge4+ into Cu2+ sites results in the grain size of CaCu3−xGexTi4O12 ceramics to decrease, compared to CaCu3Ti4O12 ceramic. Unusually, although grain sizes of CaCu3−xGexTi4O12 ceramics decrease, their dielectric permittivity is increased by doping. Enhanced dielectric permittivity ~ 35,000–42,000 with reduced loss tangent ~ 0.037–0.053 was achieved in x = 0.025–0.10. Improved dielectric properties with reduced loss tangent might be originated by enhanced grain boundary response, especially the largely increased grain boundary resistance. The result obtained from an impedance spectroscopy technique indicates the formation of an internal barrier layer capacitor model in all sintered ceramics. The giant dielectric permittivity of CaCu3−xGexTi4O12 ceramics might be caused by intrinsically compensating mechanisms of charge carriers.

Significantly improving the giant dielectric properties of CaCu3Ti4O12 ceramics by co-doping with Sr2+ and F- ions

Improved dielectric properties of CaCu3Ti4O12 ceramics were produced by co-doping with Sr2+/F- cation/anions. The grain sizes of the Ca1-xSrxCu3Ti4O12-xFx ceramics were considerably decreased, producing a large amount of insulating grain boundary (GB) layers. Optimized dielectric properties with a high dielectric permittivity of ε′∼6.44 × 104 and a low loss tangent (tanδ∼0.030) were produced. The non-Ohmic properties also improved. The Schottky barrier height (ΦB) of the GBs increased by co-doping with Sr2+/F- ions. The electrical properties of the semiconducting grains changed slightly. Accordingly, the increase in the total GB resistance (Rgb) significantly decreased the tanδ and increased the breakdown electric field. The increased Rgb resulted from the improved intrinsic properties of the GBs (that is, ΦB increased) and the controlled microstructure (that is, the insulating GB layers increased). The macroscopic giant dielectric and electrical properties were described using an internal barrier layer capacitor model related to the Schottky barrier height at the GBs.