Kinetic demixing and grain boundary conductivity of yttria-doped zirconia part I - experimental observations

This work is directed towards a comprehensive study on the role of the microstructure and local chemistry of grain boundaries on the ionic conductivity of yttria (9 mol%)-stabilized zirconia and YSZ-alumina composites. It has been performed on samples prepared from two batches of YSZ powders containing ≈1.0 or 1.6 wt% SiO₂. Electrical conductivity measurements show that the grain boundary conductivity (σ_gb) increases with the sintering temperature and the cooling rate at the end of sintering or when the amount of Si in the ceramic decreases. Alumina additions lead to a decrease in σ_gb of the samples containing 1.0 wt% SiO₂, while σ_gb passes through a maximum in the highly silicon contaminated materials. These results coupled with TEM X-ray microanalysis, which have shown important gradients of the concentration ratio Al/Si in the grains, near the second phase, and in the glassy precipitates, suggest a competitive effect between the insulating alumina particles and the strong interaction of Al₂O₃ for SiO₂, removing it from grain boundary localities. On the other hand, XPS analyses show that Si and Y segregate near the interfaces. Analysis of these results suggests a kinetic demixing process and allow us to explain the beneficial effect of a faster cooling rate at the end of sintering by the lower amount of Si rejected in grain-boundary localities. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

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.     

Improved total conductivity of nanometric samaria-doped ceria powders sintered with molten LiNO3 additive

Nanometric 20% molar Sm-doped ceria (SDC20) powders were synthesized by co-precipitation in the presence of N, N, N′, N′ tetramethylethylendiamine (TMEDA). SDC20 powders were sintered using lithium nitrate salt in various concentrations (0.1, 1, 3, and 10 mol% with respect to the SDC20 total moles) as an additive to promote the liquid phase sintering and without additive for comparison. The addition of the Li salt allowed reducing significantly the sintering temperature of SDC. Electrochemical impedance spectroscopy (EIS) measurements were performed to estimate the contribution of grain boundary and bulk to the electrical conductivity in different sintering conditions. Liquid phase sintering allowed to produce dense samples with enhanced ionic conductivity especially at the grain boundary when compared to the samples sintered without additive. The additive liquid phase was evaporated in large part at the high temperatures throughout the sintering process. Residual extra phases were segregated at the grain boundary, generated probably by reaction of the Li salt with impurities, which were removed by a chemical etching.     

The influence of grain structures on the electrical conductivity of a BaZr0.95Y0.05O3 proton conductor

The grain structure and the electrical conductivity of a Y-doped BaZrO₃ perovskite type proton conductor were investigated in terms of preparation method. Powders of Y-doped BaZrO₃ were prepared using two methods and were sintered at various temperatures and durations. In this study, grain structures showed no effect in regard to the two preparation methods. The samples which were sintered for a relatively long period showed broad grain diameter distributions (GDD). It was also revealed that most of grain boundaries were assigned as random and this trend was not affected by sintering condition. The bulk and grain boundary conductivities of the sample sintered for long period were significantly lower than those of other samples by more than three orders of magnitude. However, this was not related to differences of grain structure and grain boundary characteristics.     

Electrical conductivity of yttria-stabilized zirconia with cobalt addition

Yttria-stabilized zirconia is the most developed solid electrolyte for use in high-temperature solid oxide fuel cells. Commercial yttria-stabilized zirconia powders reach high densification at temperatures higher than that of the usual anode materials. Reduction of the sintering temperature of the solid electrolyte could allow for co-firing of both ceramic components, thereby reducing production costs. The main purpose of this work was to study the effect of small cobalt additions on densification and on electrical conductivity of 8 mol% yttria-stabilized zirconia. Linear shrinkage results show that the onset temperature for shrinkage decreases with increasing cobalt content. Impedance spectroscopy measurements reveal that the electrical conductivity depends on the sintering profile. For specimens sintered at 1400 °C for 0.1 h the electrical conductivity of grains and grain boundaries are almost unchanged with that of 8YSZ. In contrast, for specimens sintered at the same temperature but for 0.5 h of soaking time, the electrical conductivity is higher in 0.025 mol% samples and is lower for 1 mol% Co doped 8YSZ. Degradation of the microstructure by increased porosity was obtained for high additive contents.     

氧离子导体La1.9Y0.1Mo2O9细晶粒陶瓷的制备和电学性质研究

采用溶胶凝胶法合成的La_1.9Y_0.1Mo₂O₉纳米晶粉体, 结合微波烧结技术制备出不同晶粒度的La_1.9Y_0.1Mo₂O₉块体样品. 利用X射线衍射仪(XRD)、高分辨透射显微镜(HRTEM)、场扫描显微镜(SEM)对粉体及陶瓷块体的物相、 形貌进行了表征, 利用交流阻抗谱仪测试了样品不同温度下的电导率. 实验结果表明, 掺Y的La_1.9Y_0.1Mo₂O₉能将高温立方β 相稳定到室温; 块体样品致密均匀, 平均晶粒度范围在60 nm–4 μm之间; 致密度高的样品表现出高的电导率, 其中900 ℃烧结样品的电导率600 ℃时高达0.026 S/cm, 比固相反应法制备的La_1.9Y_0.1Mo₂O₉样品高出约1倍. 总结认为样品的致密性对电导率影响较大, 是通过影响晶界电导率来影响总电导率的, 样品的晶粒度(在60 nm–4 μm范围内)对电导率的影响还不能确定. 关键词： 氧离子导体 1.9Y_0.1Mo₂O₉')" href="#">La_1.9Y_0.1Mo₂O₉ 细晶粒陶瓷 微波烧结     

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.     

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.     

Role of the microstructure on the transport properties of Y-doped zirconia and Gd-doped ceria

Transmission electron microscopy characterizations and XPS analyses have allowed us to show the influence of the microstructure and nanochemistry on the transport properties of Y₂O₃-(9 mol%)-stabilized zirconia (YSZ) and Gd₂O₃ (10 mol%)-doped ceria (GDC). The grain boundary electrical conductivity (σ_gb) and oxygen diffusion coefficient (D_o) of conventional YSZ ceramics increase with the grain size, while an opposite behavior was found for GDC samples. This difference was attributed to glassy precipitates present at YSZ grain boundaries. Furthermore, it was shown that kinetic demixing processes take place during cooling, at the end of sintering. This causes important changes in the cationic species distribution at interfaces and plays an important role on the transport properties of these two materials. Paper presented at the 9th EuroConference on Ionics, Ixia, Rhodes, Greece, Sept. 15 – 21, 2002.     

纳晶铜晶粒尺寸对热导率的影响

用热压烧结法制备得到纳晶铜块体. 用激光法测定了不同温度下制备得到的纳晶铜块体的热导率, 并建立卡皮查热阻模型对样品热导率进行模拟. 通过对比, 模拟结果与实验数据基本一致. 随着热压烧结温度的升高, 纳晶铜晶粒尺寸也随之增大. 在900和700 ℃其热导率分别达到了最大和最小值且所对应的热导率分别为200.63和233.37 W·m^-1·K^-1, 各占粗晶铜块体热导率的53.4%和60.6%. 验证了纳晶铜热导率在一定的晶粒尺寸范围内具有尺寸效应, 随着晶粒尺寸的减小, 热导率逐渐减小.     

Heterogeneous ceramics formed by grain boundary engineering

Two examples were selected to emphasize the potential of grain boundary engineering in the performance design of heterogeneous ceramics. Gadolinium-doped ceria-based powders were co-fired with additions of silica, and silica and lanthanum oxide, to test the silica scavenging role of lanthanum. The formation of one ionic conducting secondary phase, instead of an insulating phase, was attempted. The structural, microstructural, and electrical characterization of these samples confirmed the formation of one apatite-type lanthanum silicate-based phase and a significant enhancement of the grain boundary conductivity of these materials. One second approach addressed the formation of one mixed conductor, with electronically conductive grain boundaries, surrounding the grains of one lanthanum gallate-based electrolyte (core-shell type microstructure). Fe-doped grain boundaries were formed by selective Fe-diffusion (thermally assisted) from lanthanum ferrite screen printed layers. Combined microstructural and electrical characterization showed that the adopted solution was also effective.     

晶界在多晶ZrO2基固体电解质中的作用

在实验的基础上将ZrO₂基固体电解质的晶界分为三类:“清洁”晶界、含非晶相的晶界和含晶态相晶界,并分析了各类晶界的形态与性质,其中根据有效介质理论的计算,晶态晶界相的电导率约为基体的三倍,因而据此提出了一个运用“晶界工程”进一步提高ZrO₂导电性的方法。用理论模型分析了各类晶界对ZrO₂氧传感器输出电势的影响。 关键词：     

Electric conduction properties of boron-doped ceria

In the present study, the effect of the addition of boron on the electrical conduction properties of nanocrystalline cerium oxide (CeO₂) was investigated. Pellets consisting of pure CeO₂ and a mixture of CeO₂ and 10 mol.% of boron oxide (B-CeO₂ samples) were sintered at 800 °C as well as 1100 °C and their electrical conduction properties investigated by impedance spectroscopy at different temperatures and oxygen partial pressures. The nanocrystalline B-CeO₂ samples exhibit a higher electronic grain boundary conductivity and higher activation energy compared to a pure CeO₂ sample (1.41 eV for B-CeO₂ vs. 1.21 eV for pure CeO₂). According to electron energy-loss spectroscopy analysis, (i) boron can be detected only at the grain boundaries and (ii) cerium cations are lightly reduced at the grain boundaries. The results are consistent with both the formation of a space charge layer with a positive space charge potential but also with conduction along a glassy cerium-boron-oxide phase.     

Fine crystal structure of porous corundum ceramics

The microstructure of corundum ceramics based on powders with a varying grain size has been investigated. Both commercial alumina powders and those fabricated by denitration of aluminum salts in a high-frequency discharge plasma were used. An increase in the plasma-chemical Al₂O₃ powder content in the sample was found to change the pore structure of the corundum ceramics from a high-porosity ceramic skeleton with a well-developed system of channel-forming pores to ceramics with isolated pores. The change in the pore structure was observed for 50% porosity and caused an increase in the level of crystal lattice microdistortions. An increase in the sintering temperature from 1200 to 1650°C is shown to be responsible for a two-fold increase in the average crystallite size and for annealing of lattice defects along grain boundaries.     

Ceria based mixed conductors with adjusted electronic conductivity in the bulk and/or along grain boundaries

Ceramic samples of Ce_1 ? xPr_xO_2 ? δ (CPO) and Ce_1 ? xGd_xO_2 ? δ (CGO) were obtained by different sintering schedules, including the use of cobalt as a sintering aid, added by mixing the precursor powders with cobalt nitrate solution; this allowed one to obtain different microstructural features and to change the transport properties, with emphasis on changes in grain boundary behaviour. Cobalt plays a double effect as sintering aid and also to induce important changes of grain boundary properties. Specific changes of grain boundary properties were ascribed by de-convolution of impedance spectra.Relatively high levels of mixed conductivity could be attained by adding a lanthanide species to yield ionic transport, whereas electronic conduction was promoted by the mixed valence character of PrO_x, combined with the additional contribution of Co-rich grain boundaries. These effects can be used to tune preferential electronic conductivity at bulk or grain boundary level. Oxygen permeability and a modified e.m.f. method were used to obtain the overall ionic transport number under oxidising conditions and its dependence on processing conditions. Additions of PrO_x induce bulk electronic conduction which assumes a greater role at lower temperatures. Further enhancement of electronic conductivity is attained by effects of Co-addition. Though Co-rich grain boundaries also yield significant levels of electronic conductivity in CGO, this contribution becomes minor at intermediate temperatures, due to differences between the activation energies for electronic and ionic conduction.     

Improved magnetic properties and fracture strength of NdFeB by dehydrogenation

Effects of the dehydrogenation of the hydrogen decrepitated (HD) powders on the magnetic properties and the fracture strength of sintered NdFeB magnets were studied. It was found that the lattice parameters and the crystal phase of NdFeB changed significantly with the various hydrogen contents of the resultant HD powders due to the different degrees of dehydrogenation. The magnetic properties and fracture strength increased with decreasing hydrogen content, reaching the maximum increases of 200% for both intrinsic coercivity and bending strength, which can be ascribed to the improved microstructure of the sintered NdFeB magnets. The hydrogen remaining in the HD powders diffused out and affected drastically the grain and grain boundaries by the hydrogen out-take channel during the subsequent sintering process.     

Effect of substitutions on dielectric behavior of La2NiMnO6

The substituted La₂NiMnO₆ (LNMO) double perovskite powder samples are prepared by hydroxide co-precipitation method. The electrical properties such as conductivity, dielectric constant and impedance analysis of the substituted sample, have been studied. The activation energy is determined by dc conductivity plot gives the fair idea of conduction mechanism. The dielectric and impedance analysis suggests the contribution of extrinsic and intrinsic effects in dielectric relaxation due to substitution at A and B site cations. The Nyquist plot reveals the role of grain and grain boundaries in charge conduction mechanism. The large grain and grain boundary resistance value and activation energy ～100 meV of substituted LNMO samples shows the presence of variable range hopping (VRH) conduction mechanism.     

Synthesis of Y-doped BaCeO3 nanopowders by a modified solid-state process and conductivity of dense fine-grained ceramics

Powders of BaY_xCe_1 ? xO_3 ? δ (x = 0, 0.1 and 0.15) with specific surface area of 6–8 m²g^? 1 (BET equivalent particle size of 130–160 nm) were prepared by a modified solid-state route using nanocrystalline BaCO₃ and CeO₂ raw materials. These powders showed excellent densification at relatively low temperatures. Dense (96–97% relative density) ceramics with submicron grain size (0–4–0.6 µm) were obtained after sintering at 1250–1280 °C. Ceramics sintered at 1450 °C revealed only a moderate grain growth (grain size ≤ 2 µm), uniform microstructure and very high density (≥ 98%). The total conductivity of the submicron ceramics at 600 °C was comparable with the reference values reported in the literature, meaning that the high number of grain boundaries was not a limiting factor. On lowering temperature, the contribution of the blocking grain boundaries becomes progressively more important and the conductivity decreases in comparison to coarse-grained ceramics. Microscopic conductivities of grain interior and grain boundary are the same irrespective of grain size meaning that the different macroscopic behaviour is only determined by a geometric factor (a trivial size effect).     

The effect of sintering on the grain boundary conductivity of lithium lanthanum titanates

The Li-ion conductivity is a sensitive function of the composition and the microstructure for the perovskite-type Li-ion conductor. In this study, the effect of sintering temperature and thus microstructure and Li loss on the grain boundary conductivity of Li-conducting La_0.57Li_xTiO₃ (x=0.3 and 0.35) were studied. The grain and the grain boundary conductivity were obtained from the impedance patterns. As the sintering temperature increased from 1100°C to 1350°C, the Li content decreased due to the evaporation of Li during sintering. The samples were nearly fully dense after sintering at 1200°C or above. The grain boundary conductivity increased rapidly with sintering temperature due to the increasing grain size. When the grain conductivity was nearly independent on the sintering condition, the grain boundary conductivity change was mostly explained by the microstructural change. Some possible reasons for the low grain boundary conductivity was discussed.     

高自旋极化氧化物材料的颗粒边界磁电阻效应

颗粒边界磁电阻是高自旋极化氧化物颗粒体系中由于颗粒边界的存在而导致显著的磁电阻效应。本文将这种磁电阻效应定义为颗粒边界磁电阻效应。这里所说的颗粒边界,包括各种自然和人工晶界、粉末颗粒表面、复合材料中的颗粒界面等多种情况;所涉及的材料包括高自旋极化氧化物多晶、压缩粉末和各种复合材料等。对颗粒边界磁电阻效应的研究,不仅有助于人们进一步理解高自旋极化氧化物磁输运性质的基本机制,并为寻求具有高磁电阻效应的新型自旋电子学器件提供理论基础。本文综述了高自旋极化氧化物颗粒边界磁电阻研究的主要背景和发展现状,介绍了该领域中主要的实验发现和理论模型,展望了未来的发展。