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 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.
•High ε′=1.6×104 and very low tanδ=0.015 were obtained in Ca0.925Yb0.05Cu3Ti4O12.•Good temperature stability with Δε′<±15% ranging from −70 to 130°C was achieved.•ε′ was nearly independent of ...frequency (102–106Hz) and dc voltage (0–40V).•Rgb of Ca1−3x/2YbxCu3Ti4O12 ceramics was estimated to be ∼0.7–12.5GΩcm at RT.
Dielectric properties of Ca1−3x/2YbxCu3Ti4O12 ceramics were investigated. Grain growth of CaCu3Ti4O12 ceramics was strongly inhibited by Yb3+ doping ions, which can be ascribed to the effect of solute drag of an aliovalent Yb3+ dopant. High dielectric permittivity (ε′=1.6×104) and very low loss tangent (tanδ=0.015) at 1kHz with good temperature stability of ε′ ranging from −70 to 130°C (Δε′<±15%) were achieved in a Ca0.925Yb0.05Cu3Ti4O12 ceramic prepared by a modified sol–gel method. ε′ was found to be nearly independent of frequency (102–106Hz) and dc voltage (0–40V). Notably, the grain boundary resistances of Ca1−3x/2YbxCu3Ti4O12 ceramics at room temperature were estimated to be ∼0.7–12.5GΩcm. Very high-performance dielectric properties of Ca1−3x/2YbxCu3Ti4O12 ceramics were primary attributed to their improved electrical response at grain boundaries.
Mechanisms contributing to the overall loss tangent (tan
δ
) of CaCu
3
Ti
4
O
12
ceramics were investigated via doping with Yb
3+
ions and annealing in an O
2
atmosphere. A high tan
δ
value for ...CaCu
3
Ti
4
O
12
ceramics in a low-frequency range was primarily caused by direct-current (DC) conduction. It can be greatly suppressed by annealing in an O
2
atmosphere to fill oxygen vacancies at grain boundaries (GBs) and enhance GB resistance. The tan
δ
value in a middle frequency range originated from oscillation of ionic dipole moments. Ceramics which contain a relatively high atomic mass number (Yb
3+
-doped CaCu
3
Ti
4
O
12
) generated more dissipated energy compared with undoped ceramics. A rapid increase in tan
δ
in a high frequency range was attributed to the relaxation of active dipole moments that gave rise to the giant dielectric response. Balancing these factors, while simultaneously retaining high values of dielectric permittivity, is an ideal strategy to effectively improve the dielectric performance of CaCu
3
Ti
4
O
12
-based compounds.
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.
•Low-temperature tanδ of CaCu3Ti4-x(Ta0.5In0.5)xO12 was greatly reduced.•Low-temperature Rgb of CaCu3Ti4-x(Ta0.5In0.5)xO12 was extremely decreased.•Free electrons can be localized in defect clusters, ...e.g., In23+-VO··-Cu2+/Cu+/Ti3+.
The effects of (In3+/Ta5+) co-dopants on the crystalline structure, microstructure, dielectric response, and electrical properties of CaCu3Ti4-x(Ta0.5In0.5)xO12 ceramics are investigated. By increasing co-doping concentration, the lattice parameter is enlarged, while the mean grain size (98.6 → 11.3 μm) and the dielectric permittivity (ε′≈1.0 × 105 → 3.5 × 104) are significantly reduced, while the low-frequency loss tangent greatly increases. The grain boundary (GB) resistance (Rgb) and capacitance (Cgb) are significantly reduced, corresponding to the variations in tanδ and ε′, respectively. The decrease in Rgb is attributed to a decrease in the conduction activation energy of the GBs from 0.705 to 0.537 eV, while the electrical properties of the grains slightly change. Surprisingly, tanδ values in a low-temperature range (e.g., tanδ ~ 0.03 at −60 °C and 40 Hz) of the CaCu3Ti4-x(Ta0.5In0.5)xO12 ceramics are much lower than that of the CaCu3Ti4O12 ceramic (tanδ ~ 0.10) even though the low-temperature Rgb values of the co-doped ceramics are greatly decreased by ~2–4 orders of magnitude. In a low-temperature range, free electrons can be localized in defect clusters, e.g., In23+-VO··-Cu2+/Cu+/Ti3+, resulting in a low tanδ. In a relatively high-temperature range, tanδ is controlled by Rgb.
Abstract
A modified sol-gel method was used to successfully produce Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
ceramics with high dielectric permittivity. The dielectric permittivity of Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
...ceramics reaches values larger than 10
4
at room temperature and 1 kHz. Moreover, these ceramics exhibit two distinct thermally induced dielectric relaxations over a broad temperature range. The loss tangent is indeed small, ~0.032–0.035. At low temperatures, dielectric relaxation was attributed to the oxygen vacancy effect, while at high temperatures, it was attributed to grain boundary and sample-electrode contact effects. Our calculations revealed that Y and Na ions are likely to occupy Ca and Cu sites, respectively. As a result, other Cu related phases, especially CuO, were observed at the grain boundaries. Based on our analysis, there is a charge compensation between Na and Y ions in Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
. Additionally, the Cu
+
and Ti
3+
states observed in our XPS study originate from the presence of an oxygen vacancy in the lattice. Last, the primary cause of the enormous dielectric permittivity of Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
ceramics primarily comes from the internal barrier layer capacitor effect.
CaCu
3
Ti
4
O
12
(CCTO) ceramic powders were successfully prepared by a wet-chemical combustion method. The phase formation, microstructure, giant dielectric response, and nonlinear electrical ...properties of the sintered ceramics were systematically investigated. The main phase in the CCTO powder is clearly indicated in the XRD pattern. A dense and fine-grained microstructure was obtained by sintering the compacted CCTO powders. Dielectric permittivity values are in the range of ~ 10
3
–10
4
with a very low tan
δ
of ~ 0.03–0.11 at 1 kHz. Interestingly, a high breakdown electric field (9741.6 V/cm) and nonlinear coefficient (9.9) are observed in the CCTO ceramic sintered at 1030 °C for 1 h. Impedance spectroscopy analysis revealed that the electrical conductivity in the grains and grain boundaries is completely different. The electrically heterogeneous microstructure clearly indicates that the giant dielectric response and nonlinear electrical properties are correlated with the electrical response of the insulating grain boundaries. The origin of the
n
-type semiconducting grains is evident by considering the oxidation states of the Ti and Cu ions, which are quantitatively and qualitatively analyzed using X-ray absorption spectroscopy.
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