Incorporation of chemical elements into the A and/or B sites of the ABO3 perovskite structure generates disorder (topological disorder, oxygen vacancies, …) in this structure which gives rise to ...effects more or less important on its chemical and physical properties. In particular, the structure, microstructure and the dielectric properties are affected under the effect of doping. In this regard, we investigate the above mentioned effects on BaTiO3 (BT) compounds doped with different percentages of Y. Indeed, X-Ray Diffraction (XRD) and Raman analyses show that Yttrium (Y) may replace both Ba and Ti atoms, and noticeably reduces the grain size of the sintered samples in conformity with Scanning Electron Microscopy (SEM) images. We show that the generated disorder consecutive to incorporation of Yttrium is also evidenced by the values of Urbach energy. We obtain relatively high values of the permittivity under doping without relaxation effect of this parameter but with the presence of a weak diffuse character of the ferro-to-paraelectric transition. Moreover, we show that resonances may be induced in BT under doping with Yttrium, and that the Positive Temperature Coefficient of Resistivity (PTCR) is present in the undoped (BT) and Y-doped BT samples.
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•Yttrium is shown to enter both Ba and Ti sites under doping in the BaTiO3 matrix.•Y introduced disorder and other imperfections as evidenced by the Urbach energy values.•Relatively high values of the permittivity and a weak PTCR effect were obtained.•Resonances may be observed at lower frequencies than the predicted theoretical ones.
Positive temperature coefficient of resistivity (PTCR) hollow fibers that exhibit self-regulating heating characteristics have potential applications in temperature-swing adsorption systems (TSA), ...such as CO2 recovery and drying of compressed air. La-doped BaTiO3 hollow fibers displaying a PTCR effect were produced by phase inverting a casting solution consisting of N-methly-2-Pyrrolidone, polymethyl methacrylate, polyvinylpyrrolidone, BaTiO3, TiO2, and La2O3 through a spinneret into a coagulating waterbath. This was followed by polymer debinding, high temperature sintering between 1350−1400 °C and annealing in air at 1175 °C to produce hollow fibers of the composition Ba0.9975La0.0025TiO3. Hydrothermal synthesis was implemented to deposit an adsorbent porous zeolite X layer within the hollow fiber lumen, which was confirmed by electron dispersive X-ray spectroscopy and CO2 adsorption at 0 °C. Hence, these materials can be applied to energy efficient TSA gas separation processes. The results are discussed in terms of hollow fiber microstructure, adsorption characteristics and electrical properties.
A series of Lanthanum (La)-doped BaTi0.97Y0.03O3 ceramics were successfully prepared using the sol gel process and their structural, microstructural, optical and dielectric properties investigated. ...The X-ray diffraction (XRD) patterns of all the as-prepared samples showed a tetragonal phase without the presence of any secondary phase which was also confirmed by Rietveld refinement. Raman spectroscopy analysis revealed the appearance of the A1g mode, which can be considered as the signature of the co-doping with La at the Ba-sites and Y at the Ti-sites in the BaTiO3 matrix. Scanning electron microscopy (SEM) analysis revealed a decrease in grain size with the incorporation of La into the BaTi0.97Y0.03O3 matrix. Ultraviolet–Visible (UV–Vis) spectroscopy results showed that La did not significantly influence the band gap energy of the BaTi0.97Y0.03O3 matrix and therefore it preserved its insulating character in contrast to other dopants (Iron, Samarium) as reported in the literature. Dielectric measurements showed that doping with La lowered both the temperature value of the maximum of the permittivity (Tm = 44 °C for x = 5% at 10 kHz) and of the dielectric losses (tan δ < 0.02 for x = 3% at 10 kHz). Moreover, a weak positive temperature coefficient of resistivity (PTCR) effect was observed on the Lanthanum doped samples for the two compositions x = 1% and 3%.
•La3+ occupy both Ba and Ti sites which influences grain size, resistivity and loss.•Lanthanum preserved the insulating character of the BaTi0.97Y0.03O3 matrix.•All La-doped BaTi0.97Y0.03O3 samples approach their resonance frequency.•Weak PTCR effect was observed for the compositions x = 1% and 3%.
High temperature dielectric relaxation behaviors of single phase Mn3O4 polycrystalline ceramics prepared by spark plasma sintering technology have been studied. Two dielectric relaxations were ...observed in the temperature range of 200 K–330 K and in the frequency range of 20 Hz–10 MHz. The lower temperature relaxation is a type of thermally activated relaxation process, which mainly results from the hopping of oxygen vacancies based on the activation energy analysis. There is another abnormal dielectric phenomenon that is different from the conventional thermally activated behavior and is related to a positive temperature coefficient of resistance (PTCR) effect in the temperature region. In line with the impedance analyses, we distinguished the contributions of grains and grain boundaries. A comparison of the frequency-dependent spectra of the imaginary impedance with imaginary electric modulus suggests that both the long range conduction and the localized conduction are responsible for the dielectric relaxations in the Mn3O4 polycrystalline samples.
We investigated the effects of the Sm-dopant content and the cooling rate on the electrical properties and microstructure of Ba1.022−xSmxTiO3 (BST) ceramics, which were sintered at 1200°C for 30min ...in a reducing atmosphere and then reoxidized at 800°C for 1h. The results indicated that the cooling rate affected the electrical properties and the microstructure of the BST samples, whose room-temperature resistivity increased with increasing cooling rate. The semiconducting BST ceramics showed a pronounced positive temperature coefficient of resistivity effect, with a resistance jump greater by 3.16 orders of magnitude, along with a low room-temperature resistivity of 157.4Ωcm at a cooling rate of 4°C/min. The room-temperature resistivity of the specimen was lower after sintering for 30min at 1150°C during cooling.
Perovskite ceramics (Ba0.6Pb0.4)TiO3 modified with PbO-B2O3-Al2O3-WO3 special
glass was prepared with the conventional mixed oxide method. X-ray
diffraction analysis (XRD) of the obtained materials ...confirmed singlephase
and pure tetragonal structure. The Rietveld method was used to determine
unit cell parameters. Uniform deformation of the tetragonal parameter was
observed with addition of the glass. Dielectric measurements revealed the
remarkable influence of special glass admixture on the value of dielectric
permittivity and dielectric losses, as well as the Curie temperature.
However, the most important achievement of the investigations is obtaining a
significant PTCR effect in the sample containing 6 wt.% of special glass
admixture.
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Detailed structural and dielectric properties of Lanthanum-doped barium titanate
Ba
1
−
x
Lax
Ti
(
1
−
x
∕
4
)
O3 ceramic powders BLTx (where
x
=
0
.
0
0
; 0.10; 0.20; 0.30 and 0.40)/BT, BLT10, ...BLT20, BLT30 and BLT40, synthesized by the sol gel process, calcined at 900∘C for 3
h and sintered at 1250∘C for 6
h, have been investigated. The phase formation and crystal structure of the samples were checked by X-ray diffraction (XRD) and Raman spectroscopy. The samples crystallize in the pure perovskite structure that transforms from tetragonal to pseudocubic under doping with La; results that have been confirmed by Rietveld Refinement technique. The estimated average crystallite size of the samples was about 23
nm. Dielectric parameters (dielectric permittivity and losses) were determined in the temperature range room temperature (RT) — 280∘C and in the frequency range 500
Hz–2
MHz. La doping gives rise to a strong decrease of the ferro-to-paraelectric transition temperature, and the frequency dependence of the permittivity shows that the samples with
x
=
0
.
0
0
and
x
=
0
.
1
0
reach their resonance frequency. The frequency dependence of impedance and electric modulus properties were studied over a wide frequency range from 1
kHz to 2
MHz at various temperatures to confirm the contributions from grains and grain-boundaries. The complex impedance analysis data have been presented in the Nyquist plot which is used to identify the corresponding equivalent circuit and fundamental circuit parameters; it was found that the grain boundaries resistance is dominant at room temperature. The frequency dependence of the parameters permittivity, losses and AC conductivity reveals that the relaxation process is of the Maxwell–Wagner type of interfacial polarization.
Ba
1−
x
Sb
x
TiO
3
(
x
= 0.001 and 0.0015) and 0.25 wt% Si
3
N
4
-added Ba
0.999
Sb
0.001
TiO
3
was studied in terms of the correlation between its electrical properties and microstructure. The ...electrical properties were determined by measuring the room temperature resistivity of specimens sintered at 1240 - 1380 °C in air for 1 h, while the microstructure was investigated using an optical microscope and a scanning electron microscope to evaluate the correlation between room temperature resistivity and grain size due to changes in the sintering temperature. In summary, 0.25 wt% Si
3
N
4
-added Ba
0.999
Sb
0.001
TiO
3
underwent semiconductorization at lower sintering temperatures than Ba
0.999
Sb
0.001
TiO
3
, which was due to the discontinuous grain growth and Ba vacancy concentration in grain.
K(1−x)SrxNbO3-based ceramic materials were prepared via the conventional solid-reaction method, and the positive temperature coefficient of resistance (PTCR) effect of the materials was studied. The ...results showed that an excellent PTCR effect with a resistance anomaly (Rmax/Rmin) up to 1.40×105 could be obtained using 10mol% Sr2+-doped KNbO3 ceramic materials. As the donor Sr2+ content increased, the PTCR performances increased, while the transition temperature changed slightly. The complex impedance measurements proved that the PTCR effect in potassium niobate-based ceramic materials was likely the result of the resistance of the grain-boundary.
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