A-site Nonstoichiometry Research Articles

Effect of A-Site Non-stoichiometry on Structure and Microwave Dielectric Properties of Ca x (Li0.36Nd0.36Bi0.14Na0.14)TiO3 Ceramics

By adding a small amount of calcium to the starting composition, Li0.36Nd0.36Bi0.14Na0.14TiO3, oxygen vacancies are suppressed and, therefore, the microwave properties are enhanced. This study not only obtained a kind of ceramic with excellent microwave dielectric properties, e r ∼ 160, Q × f ∼ 1300 GHz and τ f ∼ 10 ppm/°C, but also gives a way to optimize the compositions with various donor and acceptor dopants for better performance in microwave ceramics.

The Electrochemical Performance of LSM with A-site Non-Stoichiometry under Cathodic Polarization

The activation behavior of a LSM based-cathode in the SOFC is characterized by a rapid reduction in the electrode polarization resistance after cathodic current passage treatment. This activation behavior has been attributed to mechanisms that either enhance the kinetics of oxygen reduction or remove impediments from the oxygen reduction mechanism. Due to the sensitivity of the activation process to the operation history, a comprehensive study was carried out to reveal the relationship between the activation conditions, the steady state cathode performance and the long term stability. This work demonstrates the application of a two electrode configuration half-cell operated for the impedance analysis of LSM-based electrodes. The effects of current density and overpotentials on the activation behavior of LSM based electrode were investigated. The impacts of temperature and oxygen partial pressure to the activation process and final performance are also discussed. Long term degradation test of LSM electrodes were carried out on both half-cells and full cells with various activation processes. Numerical simulation of the LSM-based cathode was conducted with a multi-step oxygen reduction reaction (ORR) mechanism for a better understanding of the nature of the activation process. The results are of great relevance as the electrode performance depends on not only the fabrication but also on the initial break-in procedure.

The Electrochemical Performance of LSM with A-site Non-Stoichiometry under Cathodic Polarization

The electrochemical performance of (La0.8Sr0.2)xMnO3 (x=1.0, 0.95, 0.9) electrodes with YSZ electrolyte has been investigated as a function of the A-site non-stoichiometry. The effect of cathodic bias on the electrode polarization resistance and the mechanism of oxygen reduction reaction have been studied. The passage of cathodic current leads to extensive impact to the LSM/YSZ interface microstructure and significantly enhances the electrochemical activity. LSM with different A-site non-stoichiometry also exhibit distinct activation behavior under cathodic bias. The final steady state performance of LSM electrode is determined by both of the polarization condition and the A-site non-stoichiometry. Under high current density (300mAcm-2), the stoichiometric LSM shows better performance than the A-site non-stoichiometric LSM electrodes, in contrast with the fact that it has the highest polarization resistance under OCV condition.

Large pyroelectric properties at reduced depolarization temperature in A-site nonstoichiometry composition of lead-free 0.94NaxBiyTiO3\u20130.06BazTiO3 ceramics

Nonstoichiometry lead-free 0.94NaxBiyTiO3–0.06BazTiO3 (NxByT–0.06BzT) (from x = y = 0.5, z = 1.00 to x = 0.5, y = 0.534, z = 1.02) ceramic compositions were prepared by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (Td) can be lowered by modifying x, y and z. The pyroelectric coefficient (p) of nonstoichiometry NxByT–0.06BzT compositions is greatly increased, compared with stoichiometry NBT–0.06BT composition, from 3.15 × 10−4 C m−2 °C−1 at room temperature (RT) and 23.9 × 10−4 C m−2 °C−1 at Td, and reaches maxima of 6.99 × 10−4 C m−2 °C−1 at RT and 75.3 × 10−4 C m−2 °C−1 at Td for x = y = 0.52 and z = 1. The figures of merits, Fi, Fv, and FD, also have been improved from 1.12 × 10−10 m v−1 and 0.021 m2 C−1 to 2.50 × 10−10 m v−1, 0.047 m2 C−1 and 16.63 × 10−6 Pa−1/2, respectively, for N0.52B0.52T–0.06BT composition at RT. Furthermore, N0.52B0.52T–0.06BT composition shows a huge enhancement in Fi, Fv and FD to 26.9 × 10−10 m v−1, 0.39 × 10−10 m2 C−1 and 138.7 × 10−6 Pa−1/2, respectively, at Td. The same composition also presents FC values which are ~2.58 and ~2.86 (×10−9 C cm−2 °C−1) at RT at 100 and 1000 (Hz). N0.5B0.534T–0.06BT and N0.5B0.534T–0.06B1.02T compositions show a large p values at a wide temperature range. The enhanced pyroelectric properties make nonstoichiometry N0.52B0.52T–0.06BT composition a promising candidate for pyroelectric and other applications at wide temperatures range.

Tailoring transport properties through nonstoichiometry in BaTiO3–BiScO3 and SrTiO3–Bi(Zn1/2Ti1/2)O3 for capacitor applications

The ceramic perovskite solid solutions BaTiO3–BiScO3 (BT–BS) and SrTiO3–Bi(Zn1/2Ti1/2)O3 (ST–BZT) are promising candidates for high-temperature and high-energy density dielectric applications. A-site cation nonstoichiometry was introduced in these two ceramic systems to investigate their effects on the dielectric and transport properties using temperature- and oxygen partial pressure-dependent AC impedance spectroscopy. For p-type BT–BS ceramics, the addition of excess Bi led to effective donor doping along with a significant improvement in insulation properties. A similar effect was observed on introducing Ba vacancies onto the A-sublattice. However, Bi deficiency registered an opposite effect with effective acceptor doping and a deterioration in the bulk resistivity values. For n-type intrinsic ST–BZT ceramics, the addition of excess Sr onto the A-sublattice resulted in a decrease in resistivity values, as expected. Introduction of Sr vacancies or addition of excess Bi on A-site did not appear to affect the insulation properties in air. These results indicate that minor levels of nonstoichiometry can have an important impact on the material properties, and furthermore it demonstrates the difficulties encountered in trying to establish a general model for the defect chemistry of Bi-containing perovskite systems.

Point defect engineering of high temperature piezoelectric BiScO3-PbTiO3 for enhanced voltage response

BiScO3-PbTiO3 is the most promising system among high sensitivity piezoelectric BiMO3-PbTiO3 perovskite solid solutions with high Curie temperature, which are under extensive investigation for expanding the operation temperature of state of the art Pb(Zr,Ti)O3 (PZT) up to 400°C. The viability of these alternative materials requires the development of specific point defect engineering that allow a range of piezoelectric ceramics comparable to commercial PZTs to be obtained, optimized for the different applications. A distinctive feature of BiMO3-PbTiO3 systems is the simultaneous presence of both Bi3+ and Pb2+ at the A-site of the perovskite. This enables the possibility of introducing charged point defects without incorporating new chemical species, just by defining an A-site non–stoichiometry. In this work, we present a comprehensive study of the effects of Bi substitution for Pb, along with the formulation of Pb vacancies for charge compensation. Results indicate an overall lattice stiffening that yields reduced polarizability and compliance, and dominates over a limited enhancement of the ferroelectric domain wall dynamics, so as a largely enhanced voltage response is obtained. Specifically, BiScO3-PbTiO3 with the A-site non-stoichiometry is shown to be very suitable as the piezoelectric component of magnetoelectric composites for magnetic field sensing.

Effects of A-site nonstoichiometry on oxide ion conduction in 0.94Bi0.5Na0.5TiO3–0.06BaTiO3 ceramics

Lead free 0.94(Bi[Formula: see text]Na[Formula: see text]TiO3–0.06BaTiO3 ceramics were prepared by conventional solid-state mixed oxide route with the A-site stoichiometry modified to incorporate donor-doping (through Bi-excess) and acceptor-doping (through Na-excess). Both stoichiometric and nonstoichiometric ceramics exhibited a single perovskite phase with pseudo-cubic symmetry. A significant improvement in the dielectric properties was observed in Bi-excess compositions and a deterioration in the dielectric properties was observed in Na-excess compositions. Impedance spectroscopy was utilized to analyze the effects of A-site nonstoichiometry on conduction mechanisms. Compositions with Bi-excess resulted in an electrically homogeneous microstructure with an increase in resistivity by [Formula: see text]3–4 orders of magnitude and an associated activation energy of 1.57[Formula: see text]eV which was close to half of the optical bandgap. In contrast, an electrically heterogeneous microstructure was observed in both the stoichiometric and Na-excess compositions. In addition, the Na-excess compositions exhibited low resistivities ([Formula: see text]-cm) with characteristic peaks in the impedance data comparable to the recent observations of oxide ion conduction in (Bi[Formula: see text]Na[Formula: see text]TiO3. Long term annealing studies were also conducted at 800[Formula: see text]C to identify changes in crystal structure and electrical properties. The results of this study demonstrates that the dielectric and electrical properties of 0.94(Bi[Formula: see text]Na[Formula: see text]TiO3–0.06BaTiO3 ceramics are very sensitive to Bi/Na stoichiometry.

Hardening in non-stoichiometric (1 − x)Bi0.5Na0.5TiO3–xBaTiO3 lead-free piezoelectric ceramics

The role of A-site non-stoichiometry was investigated in lead-free piezoelectric ceramics based on compositions in the 1 − x(Bi0.5Na0.5TiO3)–xBaTiO3 system near the morphotropic phase boundary, where x = 0.055, 0.06, and 0.07. Donor doping was introduced through the addition of excess Bi, however, there were no changes in the crystal structure. In contrast, acceptor doping was introduced through the addition of excess Na and was found to promote rhombohedral distortions. A significant improvement of dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (E c) which is an indication of domain wall pinning as found in hard Pb(Zr x Ti1−x )O3. This result was further confirmed via remanent polarization hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Q m) as well as a decrease in piezoelectric coefficient (d 33), dielectric loss (tan δ), remanent polarization (P r), and dielectric permittivity, which are all the typical characteristics of the effects of “hardening.” The mechanism for the observed hardening in A-site acceptor-doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry.

Enhancement of Thermoelectric Performance of Sr0.9Ba0.1Ti0.8Nb0.2O3 Ceramics by A-Site Cation Nonstoichiometry**Supported by the National Basic Research Program of China under Grant No 2013CB632506, the National Natural Science Foundation of China under Grant Nos 51202132, 51002087 and 11374186, and the Independent Innovation Foundation of Shandong University under Grant No IIFSDU 2012TS028.

Sr0.9Ba0.1-x Ti0.8Nb0.2 O3 ceramics (x=0, 0.01, 0.02 and 0.05) are prepared by solid state reaction, whose thermoelectric properties are investigated from 323 K to 1073 K. By introducing A-site nonstoichiometry, the absolute See-beck coefficient is enhanced, while the electrical resistivity is surprisingly reduced due to the significantly enhanced carrier mobility. These results are dramatic in thermoelectric materials, effectively enhancing the power factor. Moreover, the thermal conductivity is reduced, thus the thermoelectric performance of Sr0.9Ba0.1 Ti0.8Nb0.2O3 ceramic is significantly enhanced by A-site nonstoichiometry.

Structure and ferroelectricity of nonstoichiometric (Na0.5Bi0.5)TiO3

Stoichiometric (Na0.5Bi0.5)TiO3 (NBT) adopts the ABO3 perovskite structure with the A-site equally occupied by Na+ and Bi3+ ions. However, non-stoichiometric compositions can be synthesized intentionally or unintentionally. To determine the effect of A-site nonstoichiometry on the crystal structure and ferroelectricity of NBT, the composition of (Na0.5−xBi0.5+x)TiO3+x was varied using x = −0.01, −0.005, 0, 0.005, and 0.01. High resolution synchrotron x-ray diffraction and Rietveld refinement revealed that a shift in either direction from x = 0 results in a decrease in the spontaneous ferroelastic strain. Ferroelectric hysteresis and piezoelectric coefficients were found to be optimum in the stoichiometric composition.

Nonstoichiometric (La0.95Sr0.05)xGa0.9Mg0.1O3−δ electrolytes and Ce0.8Nd0.2O1.9–(La0.95Sr0.05)xGa0.9Mg0.1O3−δ composite electrolytes for solid oxide fuel cells

A-site nonstoichiometric electrolytes (La0.95Sr0.05)xGa0.9Mg0.1O3−δ (LSGM, x = 0.97, 1.00, 1.03), and their composites Ce0.8Nd0.2O1.9 (NDC)–LSGM, were synthesized and investigated. The nonstoichiometry efficiently enhanced the total conductivity of LSGM electrolyte, and the A-site deficient composition showed the highest total conductivity above 550 °C (σt,LSGM(x=0.97) = 0.880 S m−1 > σt,LSGM(x=1.03) = 0.808 S m−1 > σt,LSGM(x=1.00) = 0.582 S m−1 at 600 °C). The cubic fluorite and perovskite structures were adopted by all the composites. The LSGM additive significantly promoted the grain growth of the composite electrolyte. The grain boundary conductivities of the composite electrolytes were more or less 5–10 times higher than that of NDC electrolyte at 500 °C. The optimum A-site nonstoichiometry was found to be x = 0.97 in composite electrolytes. This study provides a possible route to design high performance single phase or composite electrolytes for SOFCs.

Properties of A-site nonstoichiometry (Pr0.4)xSr0.6Co0.2Fe0.7Nb0.1O3−σ (0.9 ≤ x ≤ 1.1) as symmetrical electrode material for solid oxide fuel cells

In order to solve the carbon deposition and sulfur adsorption problems during operations of solid-oxide fuel cells (SOFCs), (Pr0.4)xSr0.6Co0.2Fe0.7Nb0.1O3−σ (PxSCFN, x = 0.9, 0.95, 1.0, 1.05 and 1.1) oxides are synthesized by the solid state reaction method and investigated as both cathode and anode for SOFCs. For PxSCFN (x = 0.9–1.05) powders, it is found that cubic perovskite phase is formed after sintering at 1050 °C; however, when x increases to 1.1, a single impurity phase is detected for P1.1SCFN sample by X-ray diffraction. The effects of A-site nonstoichiometry on thermal expansion coefficient, electrical conductivity and polarization resistance are investigated. The excess of A-site Pr elements in PxSCFN (x = 1.05 and 1.1) results in a decrease in grain size and the creation of more active sites for oxygen reduction reaction. AC impedance reveals that P1.05SCFN as symmetrical electrodes has the best electrochemical catalytic performance both in O2 and in wet H2 atmosphere. The maximum power densities of a P1.05SCFN/LSGM/P1.05SCFN cell reach as high as 1.13 W cm−2 in wet H2 and 0.67 W cm−2 in wet CH4 at 900 °C. Therefore, P1.05SCFN is a potential symmetrical electrode material for SOFCs.

Remarkable dependence of electrochemical performance of SrCo0.8Fe0.2O3-δ on A-site nonstoichiometry

SrCo(0.8)Fe(0.2)O(3-δ) is a controversial material whether it is used as an oxygen permeable membrane or as a cathode of solid oxide fuel cells. In this paper, carefully synthesized powders of perovskite-type Sr(x)Co(0.8)Fe(0.2)O(3-δ) (x = 0.80-1.20) oxides are utilized to investigate the effect of A-site nonstoichiometry on their electrochemical performance. The electrical conductivity, sintering property and stability in ambient air of Sr(x)Co(0.8)Fe(0.2)O(3-δ) are critically dependent on the A-site nonstoichiometry. Sr(1.00)Co(0.8)Fe(0.2)O(3-δ) has a single-phase cubic perovskite structure, but a cobalt-iron oxide impurity appears in A-site cation deficient samples and Sr(3)(Co, Fe)(2)O(7-δ) appears when there is an A-site cation excess. It was found that the presence of the cobalt-iron oxide improves the electrochemical performance. However, Sr(3)(Co, Fe)(2)O(7-δ) has a significant negative influence on the electrochemical activity for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The peak power densities with a single-layer Sr(1.00)Co(0.8)Fe(0.2)O(3-δ) cathode are 275, 475, 749 and 962 mW cm(-2) at 550, 600, 650 and 700 °C, respectively, values which are slightly lower than those for Sr(0.95)Co(0.8)Fe(0.2)O(3-δ) (e.g. 1025 mW cm(-2) at 700 °C) but much higher than those for Sr(1.05)Co(0.8)Fe(0.2)O(3-δ) (e.g. only 371 mW cm(-2) at 700 °C). This remarkable dependence of electrochemical performance of the Sr(x)Co(0.8)Fe(0.2)O(3-δ) cathode on the A-site nonstoichiometry reveals that lower values of electrochemical activity reported in the literature may be induced by an A-site cation excess. Therefore, to obtain a high performance of Sr(x)Co(0.8)Fe(0.2)O(3-δ) cathode for IT-SOFCs, an A-site cation excess must be avoided.

Effects of A-Site Nonstoichiometry on Dielectric and Piezoelectric Properties of Pb-Free (Na0.53+x K0.47)(Nb0.55Ta0.45)O3 Ceramics

(Na0.53+x K0.47)(Nb0.55Ta0.45)O3 (NKNT) ceramics prepared by a solid state process were investigated at 0 ≤ x ≤ 0.02 to determine the effects of Na content in NKNT ceramics. With Na nonstoichiometry, piezoelectric properties were varied but grains were all cube-like perovskites with no impurity phases. Piezoelectric coefficient (d 33) was sensitive to Na content while polymorphic phase transition (PPT) temperature was not. PPT temperature between orthorhombic and tetragonal phases (T O-T) was around room temperature within the compositional range tested. The highest d 33 obtained was 330 pC/N at Na0.545 (x = 0.015) increased from 310 pC/N at Na0.53 (x = 0), which was due to the morphotropic phase boundary (MPB) effect around (Na0.53K0.47) of NKNT ceramics.

Dielectric and ferroelectric properties of A-site non-stoichiometric Na 0.5Bi 0.5TiO 3-based thin films

The A-site non-stoichiometry of [(Na 0.7K 0.2Li 0.1) 0.45Bi 0.55]TiO 3 + x (NKLBT) films were epitaxially deposited on LaNiO 3(100)/Si substrates using metal organic decomposition. The structural evolution of NKLBT films annealed at different temperatures is studied and a single perovskite phase can be found at the low temperature of 600 °C. Ferroelectric hysteresis measurement shows a higher remanent polarization value of 15.6 μC/cm 2 with a lower coercive field of 89 kV/cm at 450 kV/cm due to the lower concentration of oxygen vacancies by the donor doping effect. The frequency dependence of the capacitance–voltage behavior and the correlation between the capacitance–voltage and polarization–electric field are analyzed. The dielectric constant and dissipation factor are measured to be 690 and 0.04, respectively, at a frequency of 1 MHz.

A-Site and B-Site Non-stoichiometry and Sintering Characteristics of (Sr1−xLax)1−yTi1−zO3 Perovskites

The A-Site and B-Site non-stoichiometry and sintering characteristics of (Sr1−xLax)1−yTi1−zO3 perovskites were studied using samples prepared by solid-state mixing. A-Site or B-Site deficiency was observed for La-doped SrTiO3 perovskites, and this increased with increasing La content. For A-Site deficient (Sr1−xLax)1−yTiO3 perovskites, the cell volumes decreased although the reverse was observed for B-Site deficient (Sr1−xLax)Ti1−zO3 perovskites. This may be related to number of B-Site vacancies in the perovskites through charge compensation. It was found that A-Site and B-Site deficiencies increase the density of (Sr1−xLax)1−yTi1−zO3 perovskites. These characteristics may prove useful in the development of solid oxide fuel cell interconnect materials.

Oxygen permeability of A-site nonstoichiometric Ba xCo 0.7Fe 0.2Nb 0.1O 3 − δ perovskite oxides

Perovskite oxides Ba x Co 0.7Fe 0.2Nb 0.1O 3 − δ (B x CFN, x = 0.95–1.04) are synthesized by solid-state reaction method. The effect of A-site nonstoichiometry on the phase development, sinterability, electrical conductivity and oxygen permeability of sintered B x CFN are investigated. A-site deficiency reduces the cubic phase formation temperature and promotes the sintering process significantly. The electrical conductivity of B x CFN decreases but oxygen vacancy concentration increases with increasing A-site deficient level, suggesting that the charge compensation is completed via the creation of oxygen vacancies rather than the valence increase of B-site cations. A moderate A-site deficiency increases remarkably the oxygen permeation flux, while excessive A-site deficiency deteriorates the oxygen permeation process due to the occurrence of defect association between oxygen vacancies and other opposite charged defects.

Influence of A-site nonstoichiometry on sintering, microstructure and electrical properties of (Bi 0.5Na 0.5)TiO 3 ceramics

Lead-free (Bi 0.5Na 0.5) 1+ x TiO 3 ceramics ( x = −0.02, −0.01, −0.005, 0, 0.005 and 0.01) were prepared by ordinary sintering. The effect of A-site stoichiometry on the densification, microstructure, dielectric properties, high-temperature impedances, and piezoelectric properties was explored. It was found that the high conductivity of (Bi 0.5Na 0.5)TiO 3 (BNT) ceramics should be mainly attributed to the formation of A-site cation vacancies during sintering. Improved physical and electrical properties can be achieved in the sample with A-site cation excess. The control of the stoichiometry proves to be an effective way to improve BNT ceramics for possible application.

Synthesis and Performance of ( La0.75Sr0.25 ) 1 − x ( Cr0.5Mn0.5 ) O3 Cathode Powders of Solid Oxide Fuel Cells by Gel-Casting Technique

The synthesis and performance of (LSCM) cathode powders of solid oxide fuel cells by gel-casting and solid-state reaction methods were investigated. Differential thermal analysis and X-ray diffraction analysis indicate that the phase formation temperature of LSCM powders synthesized by the gel-casting technique is 50–100°C lower than that by the solid-state reaction method. The reason is related to the homogeneously distributed and immobilized precursor particles in a polymeric network, achieved during the cross-linking and gel formation steps. The LSCM cathode prepared by the gel-casting powder is shown to have a much higher electrochemical activity for the reduction than that by the solid-state reaction powder. However, significant A-site nonstoichiometry (0.1 or higher) should be avoided to prevent the formation of a secondary ( spinel phase.

The electrochemical performance of LSM/zirconia–yttria interface as a function of a-site non-stoichiometry and cathodic current treatment

The adhesion and the electrochemical performance of Sr doped LaMnO 3 (LSM) electrode with zirconia–yttria electrolyte has been investigated as a function of the A-site non-stoichiometry. In addition the effect of cathodic current treatment on the electrode resistance and the mechanism of oxygen reduction have been carefully studied. The LSM/electrolyte interface region was examined by SEM/EDS and XRD. The adhesion of porous LSM electrode coating is critically dependent on free La activity near the interface region and the formation of a pyrochlore phase, La 2Zr 2O 7. A slight A-site deficiency (∼0.1) is effective in inhibiting the formation of such a pyrochlore phase and it greatly improves the adhesion to the electrolyte. The cathodic current treatment significantly enhances the electrochemical activity of LSM and also modifies the reaction mechanism for oxygen reduction. This effect appears to be associated with the surface composition of LSM, near the electrode/electrolyte interface, which changes with prevailing oxygen partial pressure near the three-phase boundary region under the condition of current flow.