Giant dielectric permittivity (ε′) with low loss tangent (tanδ) was reported in (In + Nb) co‐doped TiO2 ceramics. Either of electron‐pinned defect‐dipole or internal barrier layer capacitor model was ...proposed to be the origin of this high dielectric performance. Here, we proposed an effectively alternative route for designing low‐tanδ in co‐doped TiO2 ceramics by creating a resistive outer surface layer. A pure rutile‐TiO2 phase with a dense microstructure and homogeneous dispersion of dopants was achieved in (In + Nb) co‐doped TiO2 ceramics prepared by a simple sol‐gel method. Two giant dielectric responses were observed in low‐ and high‐frequency ranges, corresponding to extremely high ε′≈106‐107 and large ε′≈104‐105, respectively. After annealing in air, a low‐frequency dielectric response disappeared and could be restored by removing the outer surface of the annealed sample, indicating the dominant electrode effect in the initial sample. Annealing can cause improved dielectric properties with a temperature‐ and frequency‐independent ε′ value of ≈1.9 × 104 and cause a decrease in tanδ from 0.1 to 0.035. High dielectric performance in (In0.5Nb0.5)xTi1−xO2 ceramics can be achieved by eliminating the electrode effect and forming a resistive outer surface layer.
Substitution of (Al3+, Nb5+) co–dopants into TiO6 octahedral sites of CaCu3Ti4O12 ceramics, which were prepared by a solid state reaction method and sintered at 1090°C for 18h, can cause a great ...reduction in a low–frequency loss tangent (tanδ≈0.045–0.058) compared to those of Al3+ or Nb5+ single–doped CaCu3Ti4O12. Notably, very high dielectric permittivities of 2.9−4.1×104 with good dielectric–temperature stability are achieved. The room–temperature grain boundary resistance (Rgb≈0.37–1.17×109Ω.cm) and related conduction activation energy (Egb≈0.781–0.817eV), as well as the non–Ohmic properties of the co–doped ceramics are greatly enhanced compared to single–doped ceramics (Rgb≈104–106Ωcm and Egb≈0.353–0.619eV). The results show the importance of grain boundary properties for controlling the nonlinear–electrical and giant–dielectric properties of CaCu3Ti4O12 ceramics, supporting the internal barrier layer capacitor model of Schottky barriers at grain boundaries.
The effects of different BaTiO3 sizes (≈100 nm (nBT) and 0.5–1.0 μm (μBT)) on the dielectric and electrical properties of multiwall carbon nanotube (CNT)-BT/poly(vinylidene fluoride) (PVDF) ...composites are investigated. The fabricated three-phase composites using 20 vol% BT with various CNT volume fractions (fCNT) are systematically characterized. The dielectric permittivity (ϵ′) of the CNT-nBT/PVDF and CNT-μBT/PVDF composites rapidly increases when fCNT > 0.015 and fCNT > 0.017, respectively. The former is accompanied by the dramatic increase in the loss tangent (tan δ) and conductivity (σ), but surprisingly, not for the latter. At 103 Hz, the low tan δ and σ values of the CNT-μBT/PVDF composite are about 0.06 and 6.82 × 10−9 S cm−1, while its ϵ′ value is greatly enhanced (≈154.6). The variation of the dielectric permittivity with fCNT for both composite systems follows the percolation model with percolation thresholds of fc = 0.018 and fc = 0.02, respectively. With further increasing fCNT to 0.02, ϵ′ is greatly increased to 253.8, while tan δ ≤ 0.1. Without μBT particles, the ϵ′ and tan δ values of the CNT/PVDF composite with fCNT = 0.02 are as high as ≈240 and >103, respectively. Greatly enhanced dielectric properties are described in detail.
The effects of sodium hydroxide (NaOH) concentration on setting time, compressive strength and electrical properties at the frequencies of 100Hz–10MHz of high calcium fly ash geopolymer pastes were ...investigated. Five NaOH concentrations (8, 10, 12, 15 and 18molar) were studied. The liquid to ash ratio of 0.4, sodium silicate to sodium hydroxide ratio of 0.67 and low temperature curing at 40°C were selected in making geopolymer pastes. The results showed that NaOH concentration had significant influence on the physical and electrical properties of geopolymer paste. The pastes with high NaOH concentrations showed increased setting time and compressive strength due to a high degree of geopolymerization as a result of the increased leaching of silica and alumina from fly ash. The dielectric constant and conductivity increased with NaOH concentration while tanδ decreased due to an increase in geopolymerization. At the frequency of 103Hz, the dielectric constants of all pastes were approximately 104S/cm and decreased with increased frequency. The relaxation peaks of tanδ reduced with an increase in NaOH concentration and ranged between 2.5 and 4.5. The AC conductivity behavior followed the universal power law and the values were in the range of 3.7×10−3–1.5×10−2 at 105–106 Hz.
The influences of sintering temperature on the microstructure and giant dielectric properties of a new co-doped TiO2 system, i.e., V and Ta co-doped TiO2, were investigated. The grain size of ...(V1/2Ta1/2)0.01Ti0.99O2 ceramics was enlarged with increasing sintering temperature. Dense microstructure and homogeneous dispersion of dopants were achieved in the ceramics sintered at 1400–1500 °C for 5 h. The dielectric permittivity in the frequency range 40–106 Hz of the (V1/2Ta1/2)0.01Ti0.99O2 ceramics significantly increased with the mean grain size, while the dielectric loss tangent was reduced to 0.033 at 102 Hz. Furthermore, the high-temperature stability of the dielectric permittivity was improved with increasing mean grain size. The electrically heterogeneous microstructure consisting of semiconducting grains and insulating grain boundaries and/or surface layers was confirmed using impedance spectroscopy. The conduction inside the semiconducting grains was attributed to electron hopping between Ti4+ and Ti3+, which was confirmed by X-ray photoelectron spectroscopy. Very high resistivity with a large conduction activation energy of the insulating parts was suggested as the primary cause of the giant dielectric permittivity with low loss tangent.
•Sintering temperature has effects on microstructure and dielectric properties.•Correlation between grain size and giant dielectric permittivity was found in VTTO.•1% VTTO exhibited very low tanδ value of ≈0.033 with high ε′≈ε′≈7.07 × 103.•Giant−dielectric response was attributed to the interfacial polarization effect.
Giant dielectric behavior and electrical properties of monovalent cation/anion (Li+, F−) co‐doped CaCu3Ti4O12 ceramics prepared by a solid‐state reaction route were systematically investigated. ...Substitution of Li+ and F− led to a significantly enlarged mean grain size. A reduced loss tangent (tanδ ~0.06) with the retainment of an ultra‐high dielectric permittivity (ε′ ~7.7‐8.8 × 104) was achieved in the co‐doped ceramics, while the breakdown electric field and nonlinear coefficient of CaCu3Ti4O12 ceramics were increased by co‐doping with (Li+, F−). The variations in nonlinear electrical properties and giant dielectric response, as well as the dielectric relaxation, were well explained by the Maxwell‐Wagner polarization model for an electrically heterogeneous microstructure, in which a Schottky barrier height at the grain boundaries (GBs) was formed. ε′ was closely correlated to the GB capacitance. Significantly decreased tanδ value and enhanced nonlinear properties were related to a significant increase in the GB resistance, which was attributed to the significantly increased potential barrier height and conduction activation energy at the GBs. The semiconducting nature of the grains was also studied using X‐ray photoelectron spectroscopy and found to originate from the presence of Cu+ and Ti3+ ions.
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•(Al1/2Ta1/2)0.05Ti0.95O2 exhibits a very low tanδ˜0.012 and high ε′˜104.•Excellent dielectric―temperature stability (Δε′˜2.69% at 150 °C) is achieved.•(Al1/2Ta1/2)0.05Ti0.95O2 ...exhibits nonlinear current-voltage properties.•Giant dielectric and electrical properties are originated at internal interfaces.
The nonlinear current-voltage and dielectric properties of (Al3++Ta5+) co-doped TiO2 (ATTO) ceramics are represented. Microstructural analysis reveals that sintered ceramics are highly dense and nonporous. ATTO ceramics exhibit low loss tangent (tanδ<0.1) and large dielectric permittivity (ε′˜103-104). Furthermore, ATTO ceramics exhibited nonlinear current-voltage characteristics. The best dielectric properties are achieved in the (Al1/2Ta1/2)0.05Ti0.95O2 ceramic with a very low tanδ (˜0.012) and high ε′ (˜104) with excellent dielectric-temperature stability with a temperature coefficient at 150 °C as low as 2.69%. The resistivity of insulating phases decreases with increasing Al3+ and Ta5+ concentrations, which is correlated to decreased breakdown voltage. The nonlinear electrical properties and dielectric response are explained based on the electrical response of internal interfaces.
Abstract
In this study, we achieved significantly enhanced giant dielectric properties (EG-DPs) in Sc
3+
–Ta
5+
co-doped rutile-TiO
2
(STTO) ceramics with a low loss tangent (tanδ ≈ 0.05) and high ...dielectric permittivity (ε′ ≈ 2.4 × 10
4
at 1 kHz). We focused on investigating the influence of insulating surface layers on the nonlinear electrical properties and the giant dielectric response. Our experimental observations revealed that these properties are not directly correlated with the grain size of the ceramics. Furthermore, first-principles calculations indicated the preferred formation of complex defects, specifically 2Ta diamond and 2ScV
o
triangular-shaped complexes, within the rutile structure of STTO; however, these too showed no correlation. Consequently, the non-Ohmic properties and EG-DPs of STTO ceramics cannot be predominantly attributed to the grain boundary barrier layer capacitor model or to electron-pinned defect-dipole effects. We also found that the semiconducting grains in STTO ceramics primarily arise from Ta
5+
, while Sc
3+
plays a crucial role in forming a highly resistive outer surface layer. Notably, a significant impact of grain boundary resistance on the nonlinear electrical properties was observed only at lower co-dopant concentrations in STTO ceramics (1 at%). The combination of low tanδ values and high ε′ in these ceramics is primarily associated with a highly resistive, thin outer-surface layer, which substantially influences their non-Ohmic characteristics.
The giant dielectric behavior of CaCu
3
Ti
4
O
12
(CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (tan
δ
) of this material is too ...large for many applications. A partial substitution of CCTO ceramics with either Al
3+
or Ta
5+
ions generally results in poorer nonlinear properties and an associated increase in tan
δ
(to ~0.29–1.15). However, first-principles calculations showed that self-charge compensation occurs between these two dopant ions when co-doped into Ti
4+
sites, which can improve the electrical properties of the grain boundary (GB). Surprisingly, in this study, a greatly enhanced breakdown electric field (~200–6588 V/cm) and nonlinear coefficient (~4.8–15.2) with a significantly reduced tan
δ
(~0.010–0.036) were obtained by simultaneous partial substitution of CCTO with acceptor-donor (Al
3+
, Ta
5+
) dopants to produce (Al
3+
, Ta
5+
)-CCTO ceramics. The reduced tan
δ
and improved nonlinear properties were attributed to the synergistic effects of the co-dopants in the doped CCTO structure. The significant reduction in the mean grain size of the (Al
3+
, Ta
5+
)-CCTO ceramics compared to pure CCTO was mainly because of the Ta
5+
ions. Accordingly, the increased GB density due to the reduced grain size and the larger Schottky barrier height (
Φ
b
) at the GBs of the co-doped CCTO ceramics were the main reasons for the greatly increased GB resistance, improved nonlinear properties, and reduced tan
δ
values compared to pure and single-doped CCTO. In addition, high dielectric constant values (
ε
′ ≈ (0.52–2.7) × 10
4
) were obtained. A fine-grained microstructure with highly insulating GBs was obtained by Ta
5+
doping, while co-doping with Ta
5+
and Al
3+
resulted in a high
Φ
b
. The obtained results are expected to provide useful guidelines for developing new giant dielectric ceramics with excellent dielectric properties.
(Co, Nb) co-doped rutile TiO2 (CoNTO) nanoparticles with low dopant concentrations were prepared using a wet chemistry method. A pure rutile TiO2 phase with a dense microstructure and homogeneous ...dispersion of the dopants was obtained. By co-doping rutile TiO2 with 0.5 at.% (Co, Nb), a very high dielectric permittivity of ε′ ≈ 36,105 and a low loss tangent of tanδ ≈ 0.04 were achieved. The sample–electrode contact and resistive outer-surface layer (surface barrier layer capacitor) have a significant impact on the dielectric response in the CoNTO ceramics. The density functional theory calculation shows that the 2Co atoms are located near the oxygen vacancy, creating a triangle-shaped 2CoVoTi complex defect. On the other hand, the substitution of TiO2 with Nb atoms can form a diamond-shaped 2Nb2Ti complex defect. These two types of complex defects are far away from each other. Therefore, the electron-pinned defect dipoles cannot be considered the primary origins of the dielectric response in the CoNTO ceramics. Impedance spectroscopy shows that the CoNTO ceramics are electrically heterogeneous, comprised of insulating and semiconducting regions. Thus, the dielectric properties of the CoNTO ceramics are attributed to the interfacial polarization at the internal insulating layers with very high resistivity, giving rise to a low loss tangent.
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