A new type of Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3−x (Me = Y,Gd) high-entropy ceramics used in thermal barrier coatings

Seeking for new ceramics with excellent thermophysical properties as thermal barrier coatings candidate materials has become a hot research field. In this study, Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x high-entropy ceramic powders were successfully synthesized by the method of solid-state reaction, and the ceramics with single phase were prepared by pressureless sintering at 1600°C. The phase composition, microstructure, element distribution, high-temperature thermal stability, and thermophysical properties of the ceramics were studied. The results showed that Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x ceramics were composed of SrZrO₃ phase and the second phase of AB₂O₄ spinel (i.e., SrY₂O₄ and SrGd₂O₄). The content of the second phase was gradually increased after heat treatment at 1400°C, which significantly improved the thermophysical and mechanical properties of the ceramics. The microhardness and fracture toughness of the ceramics were improved compared with that of SrZrO₃. The thermal conductivities of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 1.30 and 1.28 W m⁻¹ K⁻¹ at 1000°C, which were about 35% and 40% lower than that of SrZrO₃ (1.96 W m⁻¹ K⁻¹) and yttria-stabilized zirconia (2.12 W m⁻¹ K⁻¹), respectively. The thermal expansion coefficients of Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3−x (Me = Y, Gd) ceramics were 12.8 × 10⁻⁶ and 14.1 × 10⁻⁶ K⁻¹ at 1300°C, respectively, which was more closer to the superalloys compared with SrZrO₃ ceramic (11.0 × 10⁻⁶ K⁻¹).     

Stability and electrical conductivity of BaCe0.85Tb0.05M0.1O3−δ (M=Co,Fe, Y,Zr, Mn) high temperature proton conductors

High temperature proton conductors (HTPCs) must possess both high electrical conductivity and good chemical and structural stability for practical applications. In this work, BaCe_0.85Tb_0.05M_0.1O_3−δ (BCTM) high temperature proton conductors doped with M=Co, Fe, Y, Zr, Mn were prepared by the sol-gel combustion method using EDTA and citric acid as the chelating agents. The structural and chemical stability, and electrical conductivity of the BCTM perovskites were measured in different atmospheres by the powder X-ray diffraction (XRD) and four-probe techniques. The results indicate that the perovskite phase formed in the BCTM oxides after calcination at 1000 °C can be preserved well in H₂ and water vapor-containing atmospheres, whereas the perovskites may be segregated into metal oxides and react with CO₂ into carbonates in the CO₂ environment due to the strong alkalinity of the BaCeO₃ composites. The electrical conductivity of the BCTM oxides in hydrogen atmosphere is noticeably higher than in the air atmosphere, confirming the dominant protonic conductivity in the BCTM perovskites. Doping with Mn or Co ions in the BCTb perovskite favors improving the electrical conductivity with lower activation energy. The BCTMn perovskite demonstrates an N-shaped variation trend in electrical conductivity with temperature due to the multi-valence states for Mn ions presented at different temperatures.     

Enhancing the sinterability and electrical properties of BaZr0.1Ce0.7Y0.2O3-δ proton-conducting ceramic electrolyte

A novel strategy was proposed to enhance the sinterability and electrical properties of BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) proton-conducting electrolyte by adding 10 wt.% La_0.9Sr_0.1Ga_0.8Mg_0.2O_3-δ (LSGM) to form a 90 wt.% BZCY–10 wt.% LSGM (BL91) composite electrolyte. XRD patterns showed that no reaction occurred between the BZCY and LSGM electrolytes after sintering at 1400°C, 1450°C, 1500°C, and 1550°C for 10 h. The BL91 composite electrolyte exhibited higher relative densities and Vickers hardness and excellent electrical properties compared with those of the BZCY electrolyte. A combined approach of equivalent circuit model and distribution of relaxation time analysis was used to distinguish the bulk and grain-boundary contributions to the total conductivity and electrode processes. The introduction of 10 wt.% LSGM serves as a grain-boundary pinning phase, which can reduce the mobility of grain boundaries, thereby increasing sintered density and enhancing conductivity in BL91. A solid oxide fuel cell with proton-conducting BL91 and BZCY membranes was tested, in which the former displayed higher power outputs than the latter. Ohmic and interfacial polarization resistances decreased by approximately 20%, thereby revealing the remarkable electrical properties of the BL91 electrolyte. Results demonstrated that BL91 composite is a development prospect proton-conducting electrolyte.     

Effect of Ba nonstoichiometry on the phase composition,microstructure, chemical stability and electrical conductivity of BaxCe0.7Zr0.1Y0.1Yb0.1O3−δ (0.9≤x≤1.1) proton conductors

The perovskite proton conductors Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x=0.9, 0.94, 0.98, 1.0, 1.03, 1.06, and 1.1) have been successfully prepared by the conventional solid state reaction route. X-ray diffraction (XRD) patterns of the samples indicate that Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x≥1.0) samples possess a single phase orthorhombic structure, but a secondary phase (Y,Ce)O_2−δ exists in Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ (x<1.0) samples. SEM photographs show that the grain size of Ba_xCe_0.7Zr_0.1Y_0.1Yb_0.1O_3−δ increases and the porosity decreases with Ba²⁺ content varying from x=0.9 to 1.1. Because of ZnO addition as sintering aid, the sintering temperature of the samples reduces from 1550 °C to 1250 °C. The chemical stability of the samples against CO₂ decreases with the increase in Ba content from x=0.9 to 1.1. All the samples show a excellent stability against water vapor at 850 °C. The conductivities of the samples increase and the activation energies reduce with the increase in Ba content. The present results suggest that it is very important to control the stoichiometry of cations to obtain desired perovskite type high temperature proton conductors.     

Pechini法合成BaCe0.7Zr0.2Gd0.1O3-δ质子导体

采用Pechini法合成了用Zr部分取代Ce并用Gd掺杂的新型高温质子导体BaCe0.7Zr0.2Gd0.1O3-δ陶瓷前驱体，前驱体在1100℃煅烧得到了钙钛矿结构的陶瓷体。前躯体用TG—DTA分析了煅烧特性，用XRD测定了陶瓷粉体的相组成。陶瓷粉体压制成试样后在1450℃烧结后，经密度测试和SEM微观结构观察，证实得到了致密陶瓷体，其实测烧结密度大于理论密度的92％。     

Thermal and chemical induced expansion of La0.3Sr0.7(Fe,Ga)O3−δ ceramics

The linear thermal expansion coefficients (TECs) of perovskite-type La_0.3Sr_0.7Fe_1−xGa_xO_3−δ (x=0–0.4), determined by dilatometric and high-temperature X-ray diffraction techniques, are in the range (19–41)×10⁻⁶ K⁻¹ at 770–1170 K, decreasing when the oxygen partial pressure or gallium concentration increases. At oxygen pressures from 10⁻⁴ to 1 atm, the isothermal chemically induced expansion of La_0.3Sr_0.7Fe(Ga)O_3−δ ceramics is a linear function of the oxygen nonstoichiometry. The magnitude of changes in δ and, thus, chemical expansion both are reduced by gallium doping. The ratio between isothermal chemical strain and nonstoichiometry variations, (ε_C/Δδ), follows an Arrhenius-type dependence on temperature and varies in the range (1.7–5.9)×10⁻². The drastic increase in the thermal expansion at temperatures above 700 K, typical for ferrite-based ceramics, was shown to be mainly apparent, resulting from the chemically-induced expansion of the lattice due to oxygen losses. The TEC values, corrected for the chemical strain on heating, are close to the TECs at low temperatures and increase with gallium content. The observed correlations between the thermal and chemical expansion and ionic conductivity of La_0.3Sr_0.7Fe_1−xGa_xO_3−δ are discussed in terms of their relationships with the oxygen deficiency and cation composition.     

Thermal expansion and thermal conductivity of SmxZr1−xO2−x/2 (0.1 ≤ x ≤ 0.5) ceramics

A study was conducted of the effect of additions of samarium oxide on the thermal expansion and thermal conductivity of zirconium oxide for thermal barrier coatings. Sm_xZr_1?xO_2?x/2 (0.1 ≤ x ≤ 0.5) ceramic powders synthesized with a chemical-coprecipitation and calcination method were sintered at 1873 K for 15 h. Structures of the synthesized powders and sintered ceramics were identified by X-ray diffractometer. The morphologies of ceramic powders were observed by transmission electron microscope. The thermal expansion coefficients and thermal diffusion coefficients of Sm_xZr_1?xO_2?x/2 ceramics were studied with a high-temperature dilatometer and a laser flash diffusivity technique from room temperature to 1673 K. The thermal conductivity was calculated from thermal diffusivity, density and specific heat of bulk ceramics. Sm_0.1Zr_0.9O_1.95 ceramics consists of both monoclinic and tetragonal structures. However, Sm_0.2Zr_0.8O_1.9 and Sm_0.3Zr_0.7O_1.85 ceramics only exhibit a defect fluorite structure. Sm_0.4Zr_0.6O_1.8 and Sm_0.5Zr_0.5O_1.75 ceramics have a pyrochlore-type lattice. With the increase of Sm₂O₃ content, the linear thermal expansion of Sm_xZr_1?xO_2?x/2 ceramics increases except for Sm_0.1Zr_0.9O_1.95. The thermal conductivities of Sm_xZr_1?xO_2?x/2 ceramics ranged from 1.41 at 873 K to 1.86 W m^?1 K^?1 at room temperature in a test temperature range of room temperature to 1673 K, and the results can be explained by phonon scattering mechanism.     

Kinetic demixing/decomposition of Ba0.5Sr0.5CoxFe1−xO3−δ (x = 0.2 and 0.8)

Microstructural changes due to kinetic demixing within sintered BSCF ceramics (Ba_0.5Sr_0.5Co_xFe_1?xO_3?δ, x = 0.2 and 0.8: BSCF5528 and BSCF5582, respectively) have been investigated. When the specimens were subjected to 2 A/cm² at 1000 °C and pO₂ = 10^?5 atm, there was a significant enhancement of grain growth as well as 2nd phase formation observed in BSCF5528. At the anode, cobalt deficient aggregates within the grains; and, at the cathode, cobalt rich 2nd phase particles were observed on the grain surfaces of the microstructure. Such phenomena were not observed in BSCF5582, even under higher current density (7 A/cm²) and longer delay time. These results were explained by the kinetic demixing/decomposition.     

Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and SrCo_0.8Fe_0.2O_3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O₂) = 10⁻⁴ to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O₂) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.     

Photoluminescence and structural characteristics of double tungstates A(M1−X PrX)W2O8 (A = Li,Cs, M = Al,Sc, La)

In this article, photoluminescence of Pr³⁺ ions in the double tungstate A(M_1?X Pr_X)W₂O₈ (A = Li, Cs, M = Al, Sc, La; 0.0 ≤ X ≤ 0.1) are characterised. By varying ion radius in A and M sites the crystal structure was modified and even in crystals with similar structural characteristics three distinctive types of luminescence are observed. When the substitution ions in both A and M sites are relatively small the host lattice exhibits luminescence dominantly. With the small A site ion (Li⁺) and the large M site ion (La³⁺, 1.03 Å) the Pr³⁺ ion exhibits prominent luminescence. With the very large A site ion (Cs⁺, 1.67 Å) and relatively small M site ion (Sc³⁺, 0.75 Å) the Pr³⁺ exhibits both the 4f²–4f5d excitation and the ³P_J manifold excitations in the absorption spectrum. These excitation levels lead to two strong emissions from the Pr³⁺. PL characteristics are discussed with respect to crystal structural criteria.     

Chemical stability and electrical properties of Nb doped BaCe0.9Y0.1O3−δ as a high temperature proton conducting electrolyte for IT-SOFC

BaCe_0.9?xNb_xY_0.1O_3?δ (where x=0, 0.01, 0.03 and 0.05) powders were synthesized by solid-state reaction to investigate the influence of Nb concentration on chemical stability and electrical properties of the sintered samples. The dense electrolyte pellets were formed from the powders after being uniaxially pressed and sintered at 1550 °C. The electrical conductivities determined by impedance measurements in temperature range of 550–750 °C in different atmospheres (dry argon and wet hydrogen) showed a decreasing trend with an increase of Nb content. For all samples higher conductivities were observed in the wet hydrogen than in dry argon atmosphere. The chemical stability was enhanced with increasing of Nb concentration. It was found that BaCe_0.87Nb_0.03Y_0.1O_3?δ is the optimal composition that satisfies the opposite demands for electrical conductivity and chemical stability, reaching 0.8×10^?2 S cm^?1 in wet hydrogen at 650 °C compared to 1.01×10^?2 S cm^?1 for undoped electrolyte.     

One-step thermal processing of BaCe0.8Y0.2O3−δ hydrogen permeable multichannel hollow fiber membrane

BaCe_0.8Y_0.2O_3−δ (BCY) is the most widely studied proton-conducting material and is frequently fabricated as a dense membrane for hydrogen separation. However, the limitation of preparing dense BCY membranes is the extremely high sintering temperature (>1500°C). Herein, the BCY 7-channel hollow fiber membrane was prepared by a one-step thermal processing (OSTP) with Co₂O₃ as a sintering aid. The results showed that the addition of 1 wt.% Co₂O₃ at a reduced temperature of 1350°C was the optimum composition and sintering condition for densification and forming a single perovskite-phase structure. The hydrogen permeation flux of the BCY hollow fiber membrane reached up to 0.34 ml min⁻¹ cm⁻² at 900°C. The long-term stability test was conducted for 300 h. The successful attempt of such a strategy provides a green and straightforward path for the preparation of dense ceramic proton-conducting membranes on a large scale. This promotes its industrial application in high-temperature hydrogen separation.     

Microwave dielectric properties of novel temperature stable high Q Li2Mg1−xZnxTi3O8 and Li2A1−xCaxTi3O8 (A = Mg,Zn) ceramics

The Li₂Mg_1?xZn_xTi₃O₈ (x = 0–1) and Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) ceramics are synthesized by solid-state ceramic route and the microwave dielectric properties are investigated. The Li₂MgTi₃O₈ ceramic shows ?_r = 27.2, Q_u × f = 42,000 GHz, and τ_f = (+)3.2 ppm/°C and Li₂ZnTi₃O₈ has ?_r = 25.6, Q_u × f = 72,000 GHz, and τ_f = (?)11.2 ppm/°C respectively when sintered at 1075 °C/4 h. The Li₂Mg_0.9Zn_0.1Ti₃O₈ dielectric ceramic composition shows the best dielectric properties with ?_r = 27, Q_u × f = 62,000 GHz, and τ_f = (+)1.1 ppm/°C. The effect of Ca substitution on the structure, microstructure and microwave dielectric properties of Li₂A_1?xCa_xTi₃O₈ (A = Mg, Zn and x = 0–0.2) has also been investigated. The materials reported in this paper are excellent in terms of dielectric properties and cost of production compared to commercially available high Q dielectric resonators.     

Microstructures and mechanical properties of Ce1−xCaxO2−y (x = 0.05, 0.1, 0.2) with different sintering temperatures

In this study, the microstructures and mechanical properties of Ca-doped CeO₂ affected by different sintering temperatures and various dopant concentrations have been systematically investigated. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) revealed that a seemingly individual Ca-doped CeO₂ system may consist of multiple types of doped CeO₂ phases. X-ray photoelectron spectroscopy (XPS) analysis demonstrated that the chemical state of the Ce element is less dependent on the sintering temperature, while increasing the sintering temperature may cause segregation of the Ca elements to form CaO phase. Nanoindentation tests showed that Ca-doped CeO₂ has better fracture toughness compared to rare-earth (RE) doped CeO₂; the relationship between the sintering temperature and the mechanical property has been discussed.     

A comparative investigation on protonic ceramic fuel cell electrolytes BaZr0.8Y0.2O3-δ and BaZr0.1Ce0.7Y0.2O3-δ with NiO as sintering aid

Solid oxide fuel cells based on proton-conducting ceramic electrolytes, i.e., protonic ceramic fuel cells (PCFCs), are promising in operating at intermediate to low temperature. BaZr_0.8Y_0.2O_3-δ (BZY) and BaZr_0.1Ce_0.7Y_0.2O_3-δ (BZCY) are two typical electrolyte materials for PCFCs. However, there is still a lack of basis for making a choice between the two materials. In this paper, we present a comparison investigation on practical BZY and BZCY electrolytes with NiO of 2 mol.% as sintering aid. Their crystal structure, sinterability, microstructure, and electrical conductivity in humid air and hydrogen (3% H₂O) are measured and analyzed. Anode-supported PCFCs based on the two electrolyte materials are prepared and their electrochemical performances are tested and analyzed in association with an examination on their microstructure. The results show that both materials can be densified after sintered with NiO aid at 1400 °C for 6h. Ni is doped into the interstitial of BZY while it occupies the B site of perovskite lattice of BZCY. The sintered BZY has small grains and many grain boundaries while BZCY has large grains and much fewer grain boundaries, resulting in lower conductivity of BZY than that of BZCY. A PCFC with BZY electrolyte gives a peak power density of 360 mW cm^?2 at 700 °C, while this value for a PCFC with BZCY is 855 mW cm^?2. Although the performances of BZCY seems much better than those of BZY, a stability test in 10% CO₂-containing Ar at 650 °C shows BZY is stable while BZCY reacts with CO₂ to form BaCO₃ and CeO₂.     

Preparation and dielectric properties of (Sr1−xCax)Fe0.5Nb0.5O3; (x=0.0, 0.1, 0.2) ceramics

In the present work, strontium calcium iron niobate ((Sr_1?xCa_x)Fe_0.5Nb_0.5O₃; SCFN) (x=0, 0.1, and 0.2) powders were synthesized for the first time using a molten salt technique. The pure phase perovskite obtained at a relative low calcination temperature of 800 °C was characterized using the X-ray diffraction technique (XRD). SCFN ceramics were fabricated and their properties were investigated. The XRD data of the SCFN ceramics was consistent with an orthorhombic symmetry. However, the solubility of Ca in the SCFN ceramics had an upper limit at x=0.1. All ceramics showed a large dielectric constants. The Ca doping inhibited grain growth, but produced an improvement in dielectric–temperature stability. Furthermore, the doping reduced loss tangent, especially for the x=0.1 sample. These results suggest that the SCFN ceramics prepared from molten salt synthesis exhibit a good dielectric performances, compared to many high dielectric materials that have been prepared using the conventional method.     

Site-selective substitution and resulting magnetism in arc-melted perovskite ATiO3-δ (A = Ca,Sr, Ba)

Magnetic properties in perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba) were investigated before and after arc melting. Crystal structure analysis was conducted by powder synchrotron X-ray diffraction with Rietveld refinements. Quantitative chemical element analysis was carried out by X-ray photoelectron spectroscopy. Magnetic measurements were conducted by vibrating sample magnetometer and X-ray magnetic circular dichroism (XMCD). The magnetic properties are found to be affected by impurities of 3d elements such as Fe, Co, and Ni. Depending on the composition and crystal structure, the occupation of the magnetic ions in perovskite titanates is selectively varied, which is interpreted to be the origin of the different magnetic behaviors in arc-melted perovskite titanates ATiO_3-δ (A = Ca, Sr, Ba). In addition, both formation of oxygen vacancies and the reduction of Ti⁴⁺ to Ti³⁺ during arc-melting also play a role as proven by XMCD. Nevertheless, preferential site occupation of magnetic impurities is dominant in the magnetic properties of arc-melted perovskite ATiO_3-δ (A = Ca, Sr, Ba).     

Oxygen losses and electrical conductivity of SrTi1−yNbyO3+δ materials

Porous samples were used to evaluate the changes in electrical conductivity of SrTi_1−yNb_yO_3−δ materials with the oxygen partial pressure; this is needed to avoid slow responses to changes in the atmosphere. The power law dependence on the oxygen partial pressure corresponds to a predominant n-type conductivity. Deviations in strongly reducing conditions were interpreted on assuming that the charge imbalance caused by niobium additions may change with the working conditions. This defect chemistry model was confirmed by varying the oxygen partial pressure in the atmosphere. A coulometric titration technique was used to evaluate the changes in oxygen stoichiometry.     

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.