Sinteractivity, proton conductivity and chemical stability of BaZr0.7In0.3O3-δ for solid oxide fuel cells (SOFCs)

In³⁺ was used as dopant for BaZrO₃ proton conductor and 30 at%-doped BaZrO₃ samples (BaZr_0.7In_0.3O_3-δ, BZI) were prepared as electrolyte materials for proton-conducting solid oxide fuel cells (SOFCs). The BZI material showed a much improved sinteractivity compared with the conventional Y-doped BaZrO₃. The BZI pellets reached almost full density after sintering at 1600 °C for 10 h, whereas the Y-doped BaZrO₃ samples still remained porous under the same sintering conditions. The conductivity measurements indicated that BZI pellets showed smaller bulk but improved grain boundary proton conductivity, when compared with Y-doped BaZrO₃ samples. A total proton conductivity of 1.7 × 10⁻³ S cm⁻¹ was obtained for the BZI sample at 700 °C in wet 10% H₂ atmosphere. The BZI electrolyte material also showed adequate chemical stability against CO₂ and H₂O, which is promising for application in fuel cells.     

The relationship between chemical composition distributions and specific grain boundary conductivity in Y-doped BaZrO3 proton conductors

The relationship between chemical composition at a grain boundary and specific grain boundary conductivity is studied in Y-doped BaZrO₃ ceramics sintered at 1800 °C. Y enrichment at the grain boundary as yttrium concentration increased and Ba deficiency in ceramics as an increase of sintering time are observed. At the grain boundary, phase segregation, i.e. existence of secondary phase and amorphous phase which prevent proton migration, is not observed. The results indicate that at the grain boundary, concentration of Ba vacancy and Y substituted into Zr site is changed. Grain boundary conductivity shows significant dependences to Ba deficiency and Y concentration in ceramics. It is due to significant dependences of specific grain boundary conductivity to Ba deficiency and Y concentration in ceramics. The results indicate that proton migration across the grain boundary is sensitive to concentration of those effectively charged defect, i.e. Y substituted into Zr site and Ba vacancy at grain boundary.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of a Y-doped ∑31 grain boundary in alumina

The atomic structures and energetics of clean and Y-doped general grain boundary (GB) ∑31/(0001) models in α-Al₂O₃ are studied by a series of high precision ab initio calculations. A large supercell with 700 atoms and periodic boundary conditions is adopted for undoped and Y-doped GB with different substitution sites and concentrations. It is shown that Y atoms preferably segregate to the central column of the 7-member Al ring. This is explained as more favorable bond formation for Y in this position and lower GB energy. The calculated GB formation energy for the clean and Y-doped cases is respectively 3.99 and 3.67 J/m². On the average, the GB region in ∑31 has a slightly lower charge density than the bulk crystalline region. In addtition, the GB induces a long ranged asymmetric electrostatic potential distribution on each side of the grain boundary.     

Optimised NASICON ceramics for Na+ sensing

NASICON dense ceramics were obtained from solid state reaction between SiO₂, Na₃PO₄·12H₂O and two different types of zirconia: monoclinic ZrO₂ and the yttria-doped tetragonal phase (ZrO₂)_0.97(Y₂O₃)_0.03. Higher temperatures were needed to obtain dense samples of the yttrium free composition (1265 °C). The electrical conductivity, at room temperature, of the yttria-doped samples sintered at 1230 °C (0.20 S/m) is significantly higher than the value obtained with the material prepared from pure ZrO₂. The impedance spectra show that the differences in conductivity are predominantly due to the higher grain boundary resistance of the undoped ceramics, probably due to formation of monoclinic zirconia and glassy phases along the grain boundary. Further improvement of the electrical conductivity could be achieved after optimization of the grain size and density of grain boundaries. A maximum conductivity value of about 0.27 S/m at room temperature was obtained with the yttria-doped samples sintered at 1220 °C for 40 h. Yttria-doped and undoped ceramics were tested as Na⁺ potentiometric sensors. The detection limit of the yttria-doped sample (10⁻⁴ mol/l) was one order of magnitude lower than the obtained with the undoped material. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Sintering and electrical conductivity of gadolinia-doped ceria

Bulk specimens of Ce_0.9Gd_0.1O_2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H₂ mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (～0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.     

Influence of sintering conditions on the electrical properties of 10% ZrO2–10% Y2O3–CeO2 (mol%)

Yttria–zirconia doped ceria, 10% ZrO₂–10% Y₂O₃–CeO₂ (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.     

Highly dispersed anodes for solid oxide fuel cells using NiO/YSZ/BZY triple-phase composite powders prepared by spray pyrolysis

NiO/Y₂O₃-stabilized ZrO₂ (YSZ)/Y-doped BaZrO₃ (BZY) triple-phase composite powders were prepared by spray pyrolysis and evaluated for Ni/YSZ/BZY cermet anodes, which are considered effective for dry CH₄ operation in solid oxide fuel cells. The structure of the particles in these powders was fine crystal fragments, and the individual material phases were clearly separated and highly dispersed within the particles. The Ni/YSZ/BZY cermet anodes fabricated with these composite powders maintained a fine electrode microstructure equivalent to that in a simple Ni/YSZ cermet anode manufactured using a composite powder prepared by spray pyrolysis. Furthermore, the addition of BZY improved the anode performance in humidified H₂ and dry CH₄ operation.     

Effect of microstructure on chemical stability and electrical properties of BaCe<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3 − <Emphasis Type="Italic">δ</Emphasis></Subscript>

Y-doped barium cerate BaCe_0.9Y_0.1O_3???δ was synthesised by a solid-state reaction method. Materials with different average grain sizes and grain boundary surface areas were obtained. The effect of microstructure on the chemical stability in the CO₂ and H₂O-containing atmosphere and electrical properties was analysed and discussed. To evaluate the chemical stability of BaCe_0.9Y_0.1O_3???δ , the exposure test was performed. Samples were exposed to the carbon dioxide and water vapour-rich atmosphere at 25 °C for 700 h. Thermogravimetry supplied by mass spectrometry was applied to analyse the samples before and after this comprehensive test. The mass loss for samples before and after the test and the amount of BaCO₃ formed during the test were directly treated as the measure of chemical instability of BaCe_0.9Y_0.1O_3???δ in the atmosphere rich in carbon dioxide and water vapour. As it was observed, the BaCe_0.9Y_0.1O_3???δ chemical stability towards CO₂ and H₂O is not affected by the materials’ microstructure. Electrical properties of BaCe_0.9Y_0.1O_3???δ which differs with microstructure were determined using electrochemical impedance spectroscopy (EIS). It was found that the grain interior resistivity and activation energy of grain interior conductivity is microstructure independent. However, the effect on microstructure was seen on the EIS spectra in the range of grain boundary contribution. Therefore, the lowest activation energy and the highest conductivity were observed for a material with the lowest grain boundary surface area.     

Effects of cobalt metal addition on sintering and ionic conductivity of Sm(Y)-doped ceria solid electrolyte for SOFC

The effects of cobalt addition (0.5 and 1 wt.%) on densification and ionic conductivity of Ce_0.9Sm_0.1O_1.95 (10SDC) and Ce_0.9Sm_0.075Y_0.025O_1.95 (2.5Y-SDC) have been studied. X-ray diffraction (XRD) showed that Co had changed to Co₃O₄ and Co₃O₄ + CoO after firing at 900 °C and 1300 °C respectively. The addition of Co promoted densification to occur at lower temperatures with a more uniform grain growth and greatly improved both grain boundary and bulk conductivity for 10SDC. Significant improvement of grain boundary for the 2.5Y-SDC samples was obtained, even at 1300 °C sintering, while bulk conductivity was slightly improved. Rapid grain growth along with improvement of ionic conductivity was observed when the samples were sintered further at higher temperature. Superior ionic conductivity of the 2.5Y-SDC samples with Co addition to that of the bare 10SDC suggested the potential use of Co as the co-dopant in this system to reduce the content of costly rare earth usage.     

Analysis of electro-active regions and conductivity of BaMnO3 ceramic by impedance spectroscopy

Polycrystalline BaMnO₃ ceramic powders were prepared using the conventional mixed oxide route accompanied with several milling processes. Single phase formation was verified by recording the X-ray diffraction pattern of the powder as well as sintered pellet at room temperature. Scanning electron micrograph and energy dispersive X-ray spectrum of cross-sectional view have shown that sintered pellet is highly porous and contains only Ba, Mn and O elements, respectively. Analysis of impedance spectroscopy was carried out via the complex impedance and complex modulus formalisms. These results have shown that BaMnO₃ behave as semiconducting material. Furthermore, as a consequence of electrically inhomogeneous nature of the sample, it was observed that the electroactive regions (such as grain, grain boundary and sample-electrode interface) are overlapped in the applied frequency domain with dominant grain boundary effect. An equivalent circuit model (R _g C _g)(R _gb Q _gb)(R _e Q _e) was employed to fit the temperature dependent impedance spectroscopy data. Study of grain and grain boundary conductivities suggest that grains are more conductive than grain boundaries and conduction mechanism followed correlated barrier hopping (CBH) model.     

氧含量对CaCu3Ti4O12巨介电常数和介电过程的影响

本文研究了在真空、空气和氧气中烧结制备的三种 CaCu₃Ti₄O₁₂陶瓷材料的介电特性. 交流阻抗测量结果表明在10—300 K温度范围, 三种样品的介电温谱中均出现三个平台, 其电阻实部和电容虚部在相应温度出现损耗峰, 真空条件烧结的样品具有较高的介电平台和较明显的电阻实部与电容虚部峰值, 表明氧含量和氧空位对CaCu₃Ti₄O₁₂的介电性质具有重要影响, 介电温谱出现的三个平台分别源于晶粒、晶界及氧空位陷阱.温谱分析表明晶粒的激活能与烧结气氛有较大关系,氧空位引起的电子短程跳跃及跳跃产生的极化子是晶粒电导和电容的主要起源.氧空位陷阱的激活能基本与烧结气氛无关,约为0.46 eV. 氧空位对载流子的陷阱作用是CaCu₃Ti₄O₁₂ 低频高介电常数的重要起源.     

Electrical properties of CoO,NiO, CuO and ZnO doped beta″-alumina

The measurements of ionic conductivity of sintered beta″-alumina samples doped with CoO, NiO, CuO and ZnO were caried out. It was found that conductivities of these samples are lower than conductivity of Li₂O stabilized beta″-alumina. For CoO, NiO, CuO as well as Li₂O stabilized beta″-alumina the bending of Arrhenius plots was observed. For samples doped with ZnO the plots were linear in whole 20°C–450°C temperature range. The doping effect on bulk conductivity was stronger than on grain boundary conductivity.     

Cation Segregation in an Oxide Ceramic with Low Solubility: Yttrium Doped α-Alumina

The segregation behaviour of a cation (yttrium) with a low solubility in the polycrystalline oxide host (a-Al₂O₃) has been investigated at temperatures between 1450 and 1650°C using analytical scanning transmission electron microscopy. Three distinct segregation regimes were identified. In the first, the yttrium adsorbs to all grain boundaries with a high partitioning coefficient, and this can be modelled using a simple McLean-Langmuir type absorption isotherm. In the second, a noticeable deviation from this isotherm is observed and the grain boundary excess reaches a maximum of 9 Y-cat/nm² and precipitates of a second phase (yttrium aluminate garnet, YAG) start to form. In the third regime, the grain boundary excess of the cation settles down to a value of 6–7 Y-cat/nm² that is in equilibrium with the YAG precipitates. In a material (accidentally) co-doped with Zr, the Zr seems to behave in a similar way to the Y and the Y + Zr grain boundary excess behaves in the same way as the Y grain boundary excess in the pure Y-doped system. In this latter system, Y-stabilised cubic zirconia is precipitated in addition to YAG at higher Y + Zr concentrations.     

Nanocrystalline CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> powder by PVA sol–gel route: synthesis,characterization and its giant dielectric constant

Nanocrystalline CaCu₃Ti₄O₁₂ powders were synthesized by a simple PVA sol–gel route and calcined at 700 and 800°C in air for 8 h. The diameter of the powders ranges from 40–100 nm. The calcined CaCu₃Ti₄O₁₂ powders were characterized by TG-DTA, XRD, FTIR, SEM, and TEM. Sintering of the powders was conducted in air at 1100°C for 16 h. The XRD results indicated that all sintered samples had a typical perovskite CaCu₃Ti₄O₁₂ structure although the sintered samples contained some amount of CaTiO₃. SEM of the sintered CaCu₃Ti₄O₁₂ ceramics showed the average grain sizes of 13–15 μm. The samples exhibit a giant dielectric constant, ε′∼10⁵ at 150 to 200°C with weak temperature dependence below 1 kHz in the sample sintered using the powders calcined at 700°C. The Maxwell–Wagner polarization mechanism is used to explain the high permittivity in these ceramics. It is also found that all sintered samples have the same activation energy of grains, which is ∼0.122 eV.     

Optical and transport properties of Cd0.8Zn0.2S:Cu compounds prepared by modified chemical co-precipitation method

Cd_0.8Zn_0.2S bulk compound doped with different concentrations of copper have been prepared by chemical co-precipitation (CCP-I) and modified CCP method (CCP-II). The structural, optical and electrical properties of Cd_0.8Zn_0.2S:Cu compounds grown by these methods have been investigated by using XRD, SEM, EDAX, UV spectroscope and DC electrical conductivity techniques. X-ray diffraction studies on Cd_0.8Zn_0.2S compounds doped with various mole% of copper revealed that they possessed polycrystalline nature with hexagonal structure. SEM micrographs taken on samples grown by CCP-I showed that the crystallites have needle like shape and no such regular shape was observed on CCP-II grown samples and in both of them the crystallite size varied differently with the variation in the concentration of copper. The energy gap of a sample grown by CCP-II, evaluated from optical absorption studies, are more than that of the sample grown by CCP-I. This may be attributed to the variation observed in the crystallite sizes of the respective sample grown by two methods. Further, the electrical conductivity variations in CCP-I and II samples were explained based on E_g value, crystallite size and grain boundary conduction in the samples. The activation energies of both CCP-I and II samples were explained on the basis of the differences in their preparation methods. Finally, it was concluded that the CCP-II method is more suitable to grow Cd_0.8Zn_0.2S:Cu compounds with uniform average crystallite size and energy gap for low doping concentration of Cu (∼4 mole%).     

Al2O3-Y2O3-ZrO2三相复合陶瓷的介电谱研究

采用传统的固相反应法,在1400–1500 ℃下烧结,制备得到Al₂O₃-Y₂O₃-ZrO₂三相复合陶瓷.样品的结构、形貌和电性能分别用X射线衍射(XRD)、扫描电子显微镜(SEM)及介电谱表征.XRD表明此三相复合体系无其他杂相,加入Y₂O₃及ZrO₂后使得Al₂O₃成瓷温度降低;SEM表明此体系晶粒直径为200–500 nm,并且样品随烧结温度的升高而变得更加致密,晶界更加清晰;介电损耗谱中出现峰值弛豫现象,根据Cole-Cole复阻抗谱得出其为非德拜弛豫. 关键词： 2O₃-Y₂O₃-ZrO₂三相陶瓷')" href="#">Al₂O₃-Y₂O₃-ZrO₂三相陶瓷 介电弛豫 阻抗谱 热导率     

The effect of evolving grain shape and alignment on the coercivity in thermomechanically deformed Nd13.9(Fe0.92Co0.08)80.3B5.3Ga0.5 permanent magnets

A series of samples of nominally identical composition have been melt-quenched and thermomechanically deformed to varying degrees, resulting in a variation in crystallographic alignment. The effect of the alignment on the coercivity has been studied using measurements of the temperature dependence of coercivity. The analysis was performed using a well-known phenomenological model which relates the coercivity to the anisotropy field, through a parameter α, and the saturation magnetization, through an effective demagnetization coefficient N_eff. It was discovered that the trends in α and N_eff as a function of crystallographic grain alignment are the opposite to what is expected and measured previously in analogous sintered samples of varying crystallographic alignment. An explanation for this behavior is proposed in which the evolving grain shape and grain boundary topography caused by deformation becomes the controlling factor in the coercivity rather than the crystallographic alignment of the grains.     

Preparation and luminescence properties of Tb doped ZrO2 and BaZrO3 phosphors

ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ powders are prepared by combustion synthesis method and the samples were further heated to 500, 700 and 1000 °C to improve the crystallinity of the materials. The structure and morphology of materials have been examined by X-ray diffraction, Raman spectra and scanning electron microscopy. It is remarkable that all the samples of ZrO₂:Tb³⁺ and BaZrO₃:Tb³⁺ have similar morphology. These images exhibited homogeneous aggregates of varying shapes and sizes, which are composed of a large number of small cuboids and broken cuboids. The cuboids and broken cuboids size of all the samples are less than 0.5 μm. Photoluminescence for both materials increases with increase of temperature and found maximum for the samples heated to 1000 °C with 5 mole% doping of Tb³⁺ ions. Luminescence is almost double for the zirconia compared to that of barium-zirconate.     

Atomistic studies of the structure and composition of grain boundaries in Cu3Au and Ni3Al

In this paper we investigate the atomic structure and composition of grain boundaries in Cu₃Au (weakly ordered compound) and Ni₃Al (strongly ordered compound). Computer simulations employing both the molecular statics and Monte Carlo methods were performed and the Finnis-Sinclair type many-body central force potentials used. First, grain boundaries in stoichiometric alloys are studied with the goal to investigate the impact of ordering strength on the grain boundary structure and composition. In Cu₃Au grain boundaries may become compositionally disordered even at room temperature and the compositional disordering is associated with segregation of gold. In contrast, in Ni₃Al grain boundaries remain compositionally ordered up to very high temperatures. Secondly, the structures of grain boundaries and the effect of Ni and Al segregation in non-stoichiometric Ni₃Al are investigated. Nickel segregation leads to compositional disordering at grain boundaries, while aluminum segregation, which is strongly selective, leads to an ordered grain boundary structure with high Al content. The possible relationship between structural and compositional characteristics of grain boundaries and their mechanical properties, in particular the grain boundary brittleness and its alleviation by additional alloying, are then discussed in the light of the results of this study.     

Enhancement of conductivity in La2Mo2O9 ceramics fabricated by a novel three-stage thermal processing method

Pure and dense La₂Mo₂O₉ ceramic electrolytes with grain sizes of 1–3 μm were fabricated from nanocrystalline powders by a novel three-stage, one-cycle, pressureless thermal processing method at temperatures as low as 600 °C. Phase formation, microstructure and grain size of the samples were examined using X-ray diffraction and scanning electron microscopy. Density of the sintered samples was determined as in the range of 94–96% of the theoretical density by weight/geometric measurements. Impedance spectroscopy was used to characterize the electrical properties of the sintered samples. The conductivity of the three-stage sintered samples reaches a value of 0.018 S/cm at 600 °C and 0.05 S/cm at 700 °C, much higher than that of the samples fabricated by conventional solid-state reaction method, but similar to that of the samples sintered at 950 °C for 12 h from the same nanocrystalline powders. The high conductivity of these samples was attributed to the co-operation of the excellent performance of nanocrystalline powders and the advantages of the novel three-stage low-temperature thermal processing.