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.
The Mg2+/Ta5+ codoped rutile TiO2 ceramic with a nominal composition (Mg1/3Ta2/3)0.01Ti0.99O2 was synthesized using a conventional solid-state reaction method and sintered at 1400 °C for 2 h. The ...pure phase of the rutile TiO2 structure with a highly dense microstructure was obtained. A high dielectric permittivity (2.9 × 104 at 103 Hz) with a low loss tangent (<0.025) was achieved in the as-sintered ceramic. After removing the outer surface, the dielectric permittivity of the polished ceramic increased from 2.9 × 104 to 6.0 × 104, while the loss tangent also increased (~0.11). The dielectric permittivity and loss tangent could be recovered to the initial value of the as-sintered ceramic by annealing the polished ceramic in air. Notably, in the temperature range of −60–200 °C, the dielectric permittivity (103 Hz) of the annealed ceramic was slightly dependent (<±4.4%), while the loss tangent was very low (0.015–0.036). The giant dielectric properties were likely contributed by the insulating grain boundaries and insulative surface layer effects.
Abstract
Cost-effective reduced graphene oxide sheets decorated with magnetite (Fe
3
O
4
) nanoparticles (Fe
3
O
4
-rGO) are successfully fabricated via a chemical vapor deposition (CVD) technique ...using iron (III) nitrate as an iron precursor, with glucose and CH
4
as carbon sources, and NaCl as a supporting material. TEM analysis and Raman spectroscopy reveal hierarchical nanostructures of reduced graphene oxide (rGO) decorated with Fe
3
O
4
nanoparticles. Fe
K
-edge x-ray absorption near edge structure (XANES) spectra confirm that the nanoparticles are Fe
3
O
4
with a slight shift of the pre-edge peak position toward higher energy suggesting that the fabricated Fe
3
O
4
nanoparticles have a higher average oxidation state than that of a standard Fe
3
O
4
compound. The hierarchical Fe
3
O
4
-rGO is found to exhibit an excellent catalytic activity toward the reduction of triiodide to iodide in a dye-sensitized solar cell (DSSC) and can deliver a solar cell efficiency of 6.65%, which is superior to a Pt-based DSSC (6.37%).
The effects of the sintering temperature and doping level concentration on the microstructures, dielectric response, and electrical properties of W6+-doped TiO2 (WTO) prepared via a solid-state ...reaction method were investigated. A highly dense microstructure, pure rutile-TiO2, and homogenously dispersed dopant elements were observed in all of the ceramic samples. The mean grain size increased as the doping concentration and sintering temperature increased. The presence of oxygen vacancies was studied. A giant dielectric permittivity (ε′ ~ 4 × 104) and low tanδ (~0.04) were obtained in the WTO ceramic sintered at 1500 °C for 5 h. The ε′ response at a low temperature was improved by increasing the doping level concentration. The giant ε′ response in WTO ceramics can be described by the interfacial polarization at the interface between the semiconducting and insulating parts, which was supported by the impedance spectroscopy.
The dielectric properties of (A3+, Nb5+) co-doped TiO2 (A = Al, Ga and In) ceramics prepared by a solid state reaction method were studied. Sintered (A3+, Nb5+) co-doped TiO2 ceramics with A = Al and ...Ga were found with a dense ceramic microstructure. Besides the internal barrier layer and surface barrier layer capacitor effects, the formation of electron pinned defect dipoles at a specific region may have probably an influence on the dielectric response in these co-doped TiO2 ceramics doping with different A3+ dopants (A = Ga and In). All of the sintered ceramics exhibited giant dielectric permittivity ranging from 103–104 and low loss tangent (tanδ≈0.05 at 1kHz). To understand the cause of large dielectric permittivity in these materials, the theoretical modellings based on density functional theory were carried out. Our calculation results revealed that triangular shape of A2VOTi (VO=oxygen vacancy) was found in (A3+, Nb5+) co–doped TiO2 with A = Ga and In. Moreover, the diamond shape of 2Nb doped TiO2 was observed to be the most preferable configuration. These shapes may generate large defect-dipole clusters in this structure and hence, the dielectric constants are dramatically improved.
High-performance ceramics with chemical formula (Ni1/3Ta2/3)xTi1−xO2 with excellent dielectric properties are demonstrated. The dopants of Ni2+ and Ta5+ in TiO2 caused the formation of oxygen ...vacancies and free electrons. The (Ni1/3Ta2/3)xTi1−xO2 exhibited low loss tangent of 0.046 and a high dielectric permittivity of 3.5–4.5 × 104 with a very weak dependence on temperature (−60 to 200 °C). Broadband dielectric spectroscopy shows at least four dominant sources in the dielectric relaxation response in the temperature range of − 253–210 °C. DFT calculations indicate the formation of defect clusters, which are the largest contributors to the dielectric response, and these are found to be dominant even at temperatures down to − 253 °C. Both grain boundary and surface layer mechanisms in the ceramics contribute to the dielectric response at the relatively high temperatures. The sample–electrode contact effect associated with oxygen vacancy diffusion is dominant at high temperatures above 150 °C.
The effects of DC bias on the dielectric and electrical properties of co-doped (In
1/2
Nb
1/2
)
x
Ti
1−
x
O
2
(IN-T), where
x
= 0.05 and 0.1, and single-doped Ti
0.975
Nb
0.025
O
2
ceramics are ...investigated. The low-frequency dielectric permittivity (
′) and loss tangent of IN-T ceramics with
x
= 0.05 and 0.1 are greatly enhanced by applying a DC bias at 40 and 20 V, respectively, whereas the relatively high-frequency
′ remains unchanged. The induced low-frequency Maxwell-Wagner polarization completely vanishes by immediately applying no DC bias. After overload limited measurement, this polarization permanently emerges without DC bias, whereas the primary polarization remains unchanged. Using combined
Z
′′ and
M
′′ spectroscopic plots, it is found that the strongly induced-polarizations are contributed from the combination effects of the sample-electrode contact and resistive outer surface. Very high performance of the colossal permittivity in IN-T ceramics is attributed to the formation of a resistive outer-surface layer and insulating grain boundaries. These results not only provide important insights into the origins of the colossal dielectric response in the IN-T ceramic system, but are also important for deciding the doping conditions of TiO
2
-based materials for practical applications.
The effects of DC bias on the dielectric and electrical properties of co-doped (In
1/2
Nb
1/2
)
x
Ti
1−
x
O
2
(IN-T), where
x
= 0.05 and 0.1, and single-doped Ti
0.975
Nb
0.025
O
2
ceramics are investigated.
The structural and electrical parameters of sintered CaCu
3−
x
Sn
x
Ti
4
O
12
ceramics (
x
= 0, 0.05, and 0.10) were systematically investigated. Single-phase CaCu
3
Ti
4
O
12
was detected in ...all-ceramic samples. The grain size in the CaCu
3−
x
Sn
x
Ti
4
O
12
ceramics decreased as
x
increased. A high dielectric permittivity of ~ 6736–19,992 and a reduced loss tangent of ~ 0.028–0.033 was obtained in the ceramics with
x
= 0.05 and 0.10. In addition, the temperature stability of the dielectric permittivity and loss tangent also improved by doping with Sn ions. The dielectric response of the CaCu
3−
x
Sn
x
Ti
4
O
12
ceramics was closely associated with an internal barrier layer capacitor model. X-ray photoelectron spectroscopy indicated the existence of mixed Cu
+
/Cu
2+
and Ti
3+
/Ti
4+
in all ceramic samples, which promoted the hopping of electrons between Cu
+
↔ Cu
2+
and Ti
3+
↔ Ti
4+
and was the possible origin of semiconducting grains in the samples. The presence of Sn
2+
was detected by X-ray photoelectron spectroscopy indicated a reduction in the oxidation state of the Sn ions due to the charge compensation that occurred for the replacement of Cu host sites.
Microstructure, dielectric, and electrical properties of CaCu
3−
x
Ge
x
Ti
4
O
12
ceramics with
x
= 0–0.10 prepared by a conventional solid-state reaction method are investigated. Single-phase of ...CaCu
3
Ti
4
O
12
was detected in all sintered ceramics. The substitution of Ge
4+
into Cu
2+
sites results in the grain size of CaCu
3−
x
Ge
x
Ti
4
O
12
ceramics to decrease, compared to CaCu
3
Ti
4
O
12
ceramic. Unusually, although grain sizes of CaCu
3−
x
Ge
x
Ti
4
O
12
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 CaCu
3−
x
Ge
x
Ti
4
O
12
ceramics might be caused by intrinsically compensating mechanisms of charge carriers.
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