Thermoelectric properties of non-stoichiometric CaMnO3-δ composites formed by redox-activated exsolution

A novel synthesis route for preparing well-defined composites based on CMO (CaMnO₃) has been established taking advantage of the unique phase relations in the system Ca-Mn-O at reducing- and oxidizing atmosphere, respectively. Samples corresponding to stoichiometric CMO and composites with 5 and 10 vol % of Ruddlesden-Popper (Ca₄Mn₃O₁₀)- and spinel (CaMn₂O₄)-phases, respectively, were prepared with final densities >91 %. The presence of secondary phases significantly enhanced the electrical conductivity compared to stoichiometric CMO. The highest electrical conductivity was observed for CMO with 10 vol % spinel varying between 55 and 75 S/cm at temperatures between 100 and 900 °C. This composition also exhibited the highest figure-of-merit (zT) in this study, reaching 0.083 at 800 °C.     

Phase transition and oxygen permeability of Pr0.6Sr0.4FeO3-δ ceramic membrane at high temperature

Perovskite-type Pr_0.6Sr_0.4FeO_3-δ (PSF) material was prepared by the sol-gel method and systematically evaluated as an oxygen transport membrane (OTM). The material was accompanied by a phase transition with the temperature elevated, and the detailed phase evolution process was accurately detected by the high-temperature in situ X-ray diffraction and the thermogravimetric analysis technologies. The phase transition is related to the oxygen vacancy concentration. The effects of phase transition on structural parameter, oxygen permeability, rate-controlling step, and stability were investigated. The high-temperature cubic phase has a higher thermal expansion coefficient than the orthorhombic phase, which is more favorable for the movement of lattice oxygen. Combining experimental results and oxygen permeation model, demonstrated that the phase transition leads to the rate-controlling step changing from bulk diffusion to oxygen interfacial exchange. Furthermore, the high-temperature cubic phase is beneficial to limit the migration of the Sr ions and ensure higher operation stability.     

Rapid and high sensing response to acetone based on La1-xBaxMnO3+δ nanopowders

Different La_1-xBa_xMnO_3+δ nanopowders (with x = 0–0.20) were prepared using a traditional sol-gel method. Each sample had a size distribution between 50 and 80 nm with a single perovskite-type structure. The La_1-xBa_xMnO_3+δ based sensors showed excellent gas-sensing performance toward acetone (25–500 ppm) at 300 °C. It was also found that the La_0.80Ba_0.20MnO_3+δ sensor exhibited a fast response (≈25 s) and recovery time (≈15 s) in the presence of 100 ppm acetone gas, while the response value of the La_0.80Ba_0.20MnO_3+δ sensor to 500 ppm acetone reached 1350%. Moreover, these sensors also had good reproducibility and sensitivity to acetone. It reveals that Ba²⁺ as doping agent can partially convert Mn³⁺ to Mn⁴⁺, introducing numerous hole carriers into the p-type LaMnO_3+δ semiconductor and increasing the amount of chemisorbed oxygen ions on the surface, which can enhance the response of the sensor toward acetone. Moreover, this doping effect may also increase the hybridization of Mn 3d - O 2p orbits and double exchange interaction existing in the hybrid Mn³⁺/Mn⁴⁺ system, thereby promoting the carriers' conduction rate and gas sensing speed. These results suggest that the La_1-xBa_xMnO_3+δ perovskite-type structures can have large applicability in industrial acetone sensing.     

Effects of 90° domain switching on electric generation properties of PZT ceramic

The influence of domain switching on the electric generation properties of lead zirconate tinatate (PZT) ceramic has been investigated after static and cyclic loadings under various conditions. A PZT ceramic of size ϕ8.0 mm×0.17 mm, consisting of a tetragonal lattice structure, i.e., c/a≠1.014, was used. Domain switching occurred as a result of the applied stress, where three different switching modes were employed: (I) simple 90° switching; (II) 90° switching with 90° rotation; and (III) 90° switching with 180° rotation. The rates of the switching mode were different: the simple switching mode (Mode I) was 20%; and both complicated switching modes (Mode II and III) were 40%. The extent of 90° domain switching was different depending on the grain and where the direction of the tetragonal structure (c-axis direction) was affected, e.g., the closer the parallel between the c-axis and the stress direction, the stronger the domain switching. The electric generation voltage increased with increasing applied cyclic stress; however, that voltage dropped suddenly as the stress value was close to its elastic limit. This is due to the 90° domain switching. Such domain switching (reduction of the electric voltage) occurred in the early cyclic stage.     

Effect of Ce3+ doping on phase and microwave dielectric properties of 0.94MgTiO3−0.06(Ca0.8Sr0.2)TiO3 ceramics

In this study, 0.94Mg_(1-3x/2)Ce_xTiO₃−0.06(Ca_0.8Sr_0.2)TiO₃ (MCe_xT−CST, 0≤x≤0.01) composite ceramics were prepared at a low temperature of 1175°C by using the 50-nm-sized powders. The effects of Ce³⁺ doping on crystalline phase, microstructure, and microwave dielectric properties of MCe_xT−CST were studied. A main ilmenite (Mg,Ce)TiO₃ phase and a minor perovskite (Ca_0.8Sr_0.2)TiO₃ phase coexist well with the appearance of impurity MgTi₂O₅ phase in MCe_xT−CST. The dielectric properties of MCe_xT−CST are affected by the molecular polarizability, the impurity phase, and the Ce³⁺ doping. The replacement of Mg²⁺ by high valence Ce³⁺ could effectively inhibit the formation of oxygen vacancy, resulting in the enhancement of Q×f. When x = 0.005, MCe_xT−CST exhibits microwave dielectric properties with a moderate ε_r of 21.5, a high Q×f of 67 000 GHz, and a near-zero τ_f of −0.74 ppm/°C. The results reveal that the Ce³⁺ substitution is a prospective approach to optimize the microwave dielectric properties of MgTiO₃-based ceramics.     

Piezoelectric properties and temperature stabilities of Mn- and Cu-modified BiFeO3–BaTiO3 high temperature ceramics

The structures, microstructures, electrical properties and the thermal stability have been investigated for the MnO₂-doped (1 ? x)BF–xBT system and the MnO₂ and CuO-doped (1 ? x)BF–xBT system, where x ranges from 0.25 to 0.35. The XRD analysis shows that the two systems have a single perovskite phase, and the MnO₂ and CuO-doped (1 ? x)BF–xBT system has a morphotropic phase boundary (MPB) with the coexistence of rhombohedral and pseudo-cubic phases in the system about x = 0.325. The addition of small amount of CuO was quite effective to lower the sintering temperature. The diffusive phase transition characteristics were observed in the MnO₂-doped (1 ? x)BF–xBT system and a normal ferroelectric phase transition characteristics were observed in the MnO₂ and CuO doped (1 ? x)BF–xBT system. Compared with the MnO₂ doped (1 ? x)BF–xBT system, the ?_m, Curie temperature (T_c), depoling temperature (T_d), and piezoelectrical properties were improved evidently with the MnO₂ and CuO doping.     

Suppression of Sr surface segregation in La0.8Sr0.2Co0.2Fe0.8O3-δ via Nb doping in B-site

Sr surface segregation has been one of the main reasons for the cathode performance degradation during the long-term operation of solid oxide fuel cells (SOFC). Investigation on Sr segregation mechanism and proposing strategies on suppressing the Sr surface segregation are significant for SOFC development. In this paper, La_0.8Sr_0.2Co_0.2Fe_0.8-xNb_xO_3-δ (LSCFNb, x = 0, 0.02, 0.04, 0.06, 0.08 and 0.1) are prepared via sol-gel method. The electrochemical performance and long-term stability are tested through electrochemical impedance spectroscopy (EIS) and constant current polarization. The results show that the long-term stability of LSCFNb cathodes are strongly affected by Nb content. Combining the results of ICP and XPS, it's revealed that the Sr surface segregation can be effectively suppressed with the increase of Nb content. The LSCFNb cathodes gain the optimal electrochemical performance when x = 0.04, with minimum polarization resistance of 0.27 Ω cm² at 750 °C and oxygen reduction reaction activation energy of 1.54 eV. After cathodic polarization for 144 h, the polarization resistance and activation energy of LSCFNb4 cathode increase slightly, revealing it a promising cathode material for SOFC research.     

Effects of water atmosphere on chemical degradation of PrBa0.5Sr0.5Co1.5Fe0.5O5+δ electrodes

Protonic ceramic fuel cells (PCFC) based on a state-of-the-art electrolyte and cathode materials with extremely low ohmic resistance and high surface exchange rate in the intermediate temperature range (500–650 °C) have demonstrated exceptional power output in recent studies. However, reliable long-term operation remains a challenging issue in the development of PCFCs for practical applications. In particular, the water generated at the cathode has been reported to accelerate cation segregation and phase destruction of materials, thereby resulting in significant performance degradation. In this study, we investigate the underlying mechanism of the rapid chemical and electrochemical degradation of thin film PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ model electrodes in a water atmosphere_. The electrochemical degradation, concurrent with the formation of Ba- and Sr-enriched surface clusters, was more significant in the water atmosphere than a dry atmosphere. Water adsorption onto the electrode surface was found to substantially alter the chemical states of the electrodes. In particular, the increased oxygen vacancies caused an increase in the electrostatic attraction, in turn, facilitating the cation segregation and phase destruction of the electrodes.     

Effects of electrospraying parameters on deposition of La0.3Sr0.7Fe0.7Cr0.3O3−δ cathode layer on GDC

High performance in intermediate temperature solid oxide fuel cells requires improvements especially in the microstructure of the cathode layer. New cobalt-free cathode materials are used because cobalt-containing cathodes have higher thermal expansion coefficients, poor long-term chemical stability, and lower mechanical stability. Recently cobalt-free cathodes have been proposed to solve these issues by using deposition methods other than electrospray deposition (ESD). In this study, ESD method is used to develop a cobalt-free cathode layer. The electrolyte layer is gadolinium-doped ceria that is deposited with La_0.3Sr_0.7Fe_0.7 Cr_0.3O_3−δ (LSFCr) prepared by 2-butoxyethanol and ethylene glycol solvents as opposed to conventional solvents. Experimental ESD parameters are tested at different levels and combinations by applying statistical experimental design methods to optimize the microstructure. Coating deposited as such demonstrated higher electrochemical performance than similar electrodes fabricated by other methods.     

Influence of sintering temperature on energy storage properties of BaTiO3–(Sr1−1.5xBix) TiO3 ceramics

The effect of sintering temperature on microstructure, dielectric properties and energy storage properties of BaTiO₃–(Sr_1?1.5xBi_x)TiO₃ (x = 0.09) (BT–SBT) ceramics was investigated. The sintering temperature has pronounced influence on the grain size, shrinkage, and dielectric properties of the BT–SBT ceramics. With increasing sintering temperature, the dielectric constant increases largely. However, the increasing tendency of the dielectric breakdown strength (BDS) is less noticeable but become more evident with the consideration of Weibull modulus. For the BT-SBT ceramics, the unreleased energy density decreases and the electric field stability of the energy storage efficiency enhances with the increase of sintering temperature.     

Experimental and theoretical study on enhanced electrical properties of nickel-substituted La0.5Sr0.5CoO3−δ ceramics

The effect of Ni substitution on the thermal behavior, crystal structure, densification, and electrical properties of La_0.5Sr_0.5Co_1–yNi_yO_3-δ (y = 0.00–0.08) (LSCN) ceramics was discussed based on experimental measurements and theoretical calculations to search for a ruthenium–free and lead–free conductive oxide for thick film resistors. Ceramics were synthesized by the solid–state reaction, and calculations were performed with first–principle density functional theory (DFT). Results showed that the replacement of Ni ion to Co ion could help decrease the densification temperature and enhance the densification level and improve the conductivity of LSCN. Theoretical calculations, including the crystal structure, bond population, total energy, and density of states (DOS), supported the experimental results well. The maximum conductivity of 3155 S/cm was achieved as y = 0.04 was sintered at 1200 °C, and the peak temperature coefficient of resistance (TCR) of 2405.7 ppm/°C occurred at y = 0.06.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part II: Influence on oxygen transport and phase stability

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) in its cubic perovskite phase has attracted much interest for potential use as oxygen transport membrane (OTM) due to its very high oxygen permeability at high temperatures. However, performance degradation due to a sluggish phase decomposition occurs when BSCF is operated below 840?°C. Partial B-site substitution of the transition metal cations in BSCF by larger and redox-stable cations has emerged as a potential strategy to improve the structural stability of cubic BSCF. In this study, the influence of yttrium doping (0…10?mol-%) on oxygen transport properties and stability of the cubic BSCF phase is assessed by in situ electrical conductivity relaxation (ECR) and electrical conductivity measurements during long-term thermal annealing both at 700?°C and 800?°C. Detailed phase analysis is performed by scanning electron microscopy (SEM) after long-term annealing of the samples in air at different temperatures.     

Transport properties of BaCe0.85Y0.15O3−δ at closed-cycle refrigerator temperature

The electrical conducting properties of both hydrated and dehydrated BaCe_0.85Y_0.15O_3?δ (barium cerate, BCY) were investigated at low temperature (473–203 K) by an AC impedance analyzer combined with a dielectric interface. For the BCY, the bulk and grain boundary conductivities were separated with the equivalent circuit model, and the bulk conductivity was approximately two orders of magnitude higher than the grain boundary conductivity. At very low temperature (203 K), a single semicircle was obtained in the impedance plot, whereas three distinct semicircles were plotted in modulus plot due to the three different resistance components in the system. The activation energy of bulk conductivity was 0.55 eV and 0.57 eV for the hydrated and dehydrated BCY samples, respectively.     

Yttrium doping of Ba0.5Sr0.5Co0.8Fe0.2O3-δ part I: Influence on oxygen permeation,electrical properties,reductive stability,and lattice parameters

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) exhibits a very high oxygen permeability in its cubic perovskite phase, making it a promising candidate for high-temperature energy-related applications such as oxygen-transport membranes. It suffers, however, from a pronounced phase instability at application-relevant temperatures below 840?°C which is presumed to result from a valence change of B-site cobalt. In an attempt to stabilize the cubic BSCF phase, monovalent Y³⁺ was doped in small concentrations (1–10?mol-% yttrium) onto its B-site. The influence of this doping on the physico-chemical properties (electrical conductivity, reductive stability, lattice constant), on the sintering behavior, and on the oxygen permeation of BSCF has been systematically investigated. Despite a slightly adverse effect to permeability (decrease in oxygen permeation by about 20–30%), a doping concentration of 10?mol-% Y is found to completely suppress secondary-phase formation and, hence, stabilize the cubic BSCF system at 800?°C. These findings are extremely promising with regard to a long-term operation of BSCF in atmospheres free of acidic impurity gases.     

High temperature thermoelectric properties of Ca3Co4O9+δ by auto-combustion synthesis and spark plasma sintering

A rapid method for the synthesis of Ca₃Co₄O_9+δ powder is introduced. The procedure is a modification of the conventional citric-nitrate sol–gel method where an auto-combustion process is initiated by a controlled thermal oxidation–reduction reaction. The resulting powders inherit the advantages of a wet chemical synthesis, such as morphological and compositional homogeneity, and fine, well-defined particle sizes coming from the controlled nature of the auto-combustion. Optimized spark plasma sintering (SPS) processing conditions were determined and used to fabricate dense and highly c-axis oriented samples. The microstructure and thermoelectric transport properties were determined both parallel (||) and perpendicular (⊥) to the SPS pressure axis in order to investigate any possible anisotropy variations in the transport properties. At 800 °C, power factors of 506 μW/m K² (⊥) and 147 μW/m K² (||), thermal conductivities values of 2.53 W/m K (⊥) and 1.25 W/m K (||), and resulting figures-of-merit, ZT, of 0.21 (⊥) and 0.13 (||) were observed.     

Influence of Al3+ doping for V5+ on the structural,optical, thermal and electrical properties of V2-xAlxO5-δ (x=0–0.20) ceramics

This paper reports an investigation on the structure-properties correlation of trivalent metal oxide (Al₂O₃)-doped V₂O₅ ceramics synthesized by the melt-quench technique. XRD patterns confirmed a single orthorhombic V₂O₅ phase formation with increasing strain on the doping of Al₂O₃ in place of V₂O₅ in the samples estimated by Williamson-Hall analysis. FTIR and Raman investigations revealed a structural change as [VO₅] polyhedra converts into [VO₄] polyhedra on the doping of Al₂O₃ into V₂O₅. The optical band gap was found in a wide semiconductor range as confirmed by UV–visible spectroscopy analysis. The thermal and conductivity behavior of the prepared samples were studied using thermal gravimetric analysis (TGA) and impedance analyzer, respectively. All the prepared ceramics exhibit good DC conductivity (0.22–0.36 Sm^-1) at 400 ?C. These materials can be considered for intermediate temperature solid oxide fuel cell (IT-SOFC)/battery applications due to their good conductivity and good thermal stability.     

Effects of sintering temperature on properties of Na0·5K0·5NbO3-LiSbO3-BiFeO3 lead free ceramics

Abstract

Abstract

Lead free 0·95K_0·5Na_0·5NbO₃-0·05LiSbO₃ (KNN-LS) ceramics doped with 0·4?mol.-% BiFeO₃ (BF) have been prepared by the conventional mixed oxide method with sintering temperature at 1065-1135°C in this paper. The samples are characterised by X-ray diffraction analysis and scanning electron microscopy. The dielectric and piezoelectric properties are also investigated. The results present that initially the increase in the sintering temperature is very effective in improving the density and electric properties. However, the properties of the samples would be deteriorated as they are sintered over the optimum temperature. The KNN-LS (doped with 0·4%BF) ceramics shows excellent properties with sintering temperature at 1100°C.     

Sr掺杂对CaMnO3+δ基化合物高温电学性能的影响

以硝酸盐为原料,采用溶胶.凝胶法结合常压烧结制备出Ca<,1-x>Sr<,x>MnO<,3>陶瓷块体材料,研究了其物相、微观形貌以及Sr掺杂对其高温电学性能的影响.结果表明,制备出的块体材料为单一物相,为正交晶系钙钛矿结构,结构致密,Sr掺杂可以有效改变其电学性能,Sr取代Ca位的材料Seebeek系数变化不大但电阻率...