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.
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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.
In this work, it was shown that the crucial aspect for practical applications of a newly discovered (In + Nb) co-doped TiO
2
material is the temperature stability of its dielectric permittivity (
′). ...Despite an extremely large
′ value of 5.1 × 10
4
and a low loss tangent (tan
δ
0.03) successfully obtained in 10% (In + Nb) co-doped TiO
2
, careful inspection revealed that
′ was largely changed below room temperature (RT) as a result of an ambient-RT dielectric relaxation, giving rise to a large value of the temperature coefficient. However, this result can be effectively improved by decreasing the co-doping concentration. Although this dielectric relaxation also occurred in 2.5% (In + Nb) co-doped TiO
2
, its
′ variation below RT was slight. Notably, very high
′ 1.57 × 10
4
and ultra-low tan
δ
0.006 (at 30 °C and 10
2
Hz) with an excellent temperature coefficient of less than ±7% in the range of −70 to 180 °C were achieved. The giant
′ response over a broad temperature range in (In + Nb) co-doped TiO
2
was primarily due to the polarization of highly localized electrons in large defect-dipole clusters. The additional polarization relaxation near the RT range might be associated with interfacial polarization of delocalized electrons originating from uncorrelated Nb
2
5+
Ti
3+
A
Ti
defect dipoles.
In this work, it was shown that the crucial aspect for practical applications of a newly discovered (In + Nb) co-doped TiO
2
material is the temperature stability of its dielectric permittivity (
′).
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.
Dye wastewater from textile industries is reported to be a major river pollutant. Zinc stannate (ZTO) was grown directly on zinc oxide (ZnO) nanorod-coated polyester fiber membranes and porous ...ceramic substrates by a mild hydrothermal method, where the nanorods supplied zinc ions for ZTO growth. Photocatalytic degradation of a methyl orange aqueous solution under UV-light irradiation was monitored for up to 3 h duration. The higher photocatalytic activity of ZnO/ZTO catalysts on ceramic substrates was attributed to the large surface area of the nanocomposites. 50% methyl orange and ∼95% methyl orange could be degraded within 1 and 3 h of UV-light irradiation, respectively, by using the porous-ceramic-supported catalysts (C-ZnO/10ZTO), because of efficient charge separation. Moreover, the formation of ZTO islands on ZnO nanorods led to an enhancement in the photocatalytic activity in the exposed areas of electron-rich ZnO nanorods.
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.
In3+ and Nb5+ co-doped TiO2 (INTO) ceramics were sintered at 1400 °C for 5 and 10 h. The room temperature dielectric permittivity of the as-fired INTO ceramics significantly decreased from ε′≈ 7824 ...to 4669 (at 1 kHz) as sintering time increased, while the microstructure changed slightly. The corresponding loss tangent (tanδ) values were ≈ 0.040–0.047. The resistance of the insulating part and associated conduction activation energy greatly increased with increasing sintering time, corresponding to an increase in the breakdown voltage (Eb). After removing the outer surface layer, Eb was greatly reduced from 104 to 101 V/cm, corresponding to the large increase in tanδ (≈ 0.3–0.4) and ε′ (≈ 105–106). Very high ε′≈ 50,000 with low tanδ≈ 0.06 can be achieved by annealing the polished samples in air, indicating the important role of the surface layer in controlling the dielectric and electrical properties of INTO ceramics. Characterizations revealed that the formation of an insulating surface layer of the as-fired INTO ceramics may not be associated with the chemical compositional gradients of either the Ti3+ or oxygen vacancies. It was suggested that the inner and outer surface regions of INTO ceramics may consist of the same phase but with a slightly different composition.
The effects of the sintering temperature on microstructures, electrical properties, and dielectric response of 1%Cr3+/Ta5+ co-doped TiO2 (CrTTO) ceramics prepared using a solid-state reaction method ...were studied. The mean grain size increased with an increasing sintering temperature range of 1300–1500 °C. The dielectric permittivity of CrTTO ceramics sintered at 1300 °C was very low (ε′ ∼198). Interestingly, a low loss tangent (tanδ ∼0.03–0.06) and high ε′ (∼1.61–1.9 × 104) with a temperature coefficient less than ≤ ±15% in a temperature range of −60 to 150 °C were obtained. The results demonstrated a higher performance property of the acceptor Cr3+/donor Ta5+ co-doped TiO2 ceramics compared to the Ta5+-doped TiO2 and Cr3+-doped TiO2 ceramics. According to a first-principles study, high-performance giant dielectric properties (HPDPs) did not originate from electron-pinned defect dipoles. By impedance spectroscopy (IS), it was suggested that the giant dielectric response was induced by interfacial polarization at the internal interfaces rather than by the formation of complex defect dipoles. X-ray photoelectron spectroscopy (XPS) results confirmed the existence of Ti3+, resulting in the formation of semiconducting parts in the bulk ceramics. Low tanδ and excellent temperature stability were due to the high resistance of the insulating layers with a very high potential barrier of ∼2.0 eV.
Polyethyleneimine (PEI)-polyacrylonitrile (PAN)-cellulose membrane (PEI-PAN cellulose membrane) was developed by immobilizing PEI onto PAN coated cellulose support via electrostatic force to be used ...for determination of Cu(II) by colorimetric method. The membrane was prepared in two-step process, in which 4% PAN solution was prepared and coated onto the membrane via filtration method, followed by the immersion of the dried PAN-cellulose membrane in the 2% PEI solution. The morphologies of the PEI-PAN membranes were investigated with field emission scanning electron microscope. The results confirmed the presence of a coated layer on the surface. Infrared spectra of before and after coated samples were obtained by Fourier transform infrared (FTIR) spectroscope. The results showed that there were NH
2
functional group of PAN and PEI with their characteristic transmittance peaks. Determination at the parts per million level of Cu(II) (0.5–2.0 mg L
–1
) was achieved by filtration of a sample solution and simultaneous colorimetric analysis using a UV-Vis spectrophotometer (at 650 nm). Cu(II) ion formed the light blue cupramine complex with PEI immobilized on the membrane by filtration of a 50 mL sample solution buffered with 0.01 M dihydrogen phosphate (pH 7). Energy dispersive X-Ray fluorescence spectrum of the Cu(II) detected membrane confirmed the capability of Cu(II) extraction of the PEI-PAN cellulose membrane from the water samples. The detection limit of the Cu(II) determination by this method was found to be 0.27 mg L
–1
. The accuracy and precision of the method were estimated by Cu(II) determination at 0.5 and 2 mg L
–1
of Cu(II) solutions and the results were accurate with above 89% recovery percentage and below 5.5% relative standard deviation. Interference ions were studied and found that Fe(III) and Zn(II) slightly interfered with the Cu(II) determination. The present method was tested for the detection of treated textile wastewater and tap water spiked with 2 mg L
–1
of Cu(II). The test results were acceptable and good to determine wastewater prior to discharge into the environment that could be applied for on-site monitoring of Cu(II) in treated wastewater by filtration detection method.
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