Electrical Conductivities of (CeO2)1−x(Y2O3)x Thin Films

Electrical properties of CeO₂ thin films of different Y₂O₃ dopant concentration as prepared earlier were studied using impedance spectroscopy. The ionic conductivities of the films were found to be dominated by grain boundaries of high conductivity as compared with that of the bulk ceramic of the same dopant concentration sintered at 1500°C. The film grain-boundary conductivities were investigated with regard to grain size, grain-boundary impurity segregation, space charge at grain boundaries, and grain-boundary microstructures. Because of the large grain boundary and surface area in thin films, the impurity concentration is insufficient to form a continuous highly resistive Si-rich glassy phase at grain boundaries, such that the resistivity associated with space-charge layers becomes important. The grain-boundary resistance may originate from oxygen-vacancy-trapping near grain boundaries from space-charge layers. High-resolution transmission electron microscopy coupled with a trans-boundary profile of electron energy loss spectroscopy gives strong credence to the space-charged layers. Since the conductivities of the films were observed to be independent of crystallographic texture, the interface misorientation contribution to the grain-boundary resistance is considered to be negligible with respect to those of the impurity layer and space-charge layers.     

Examination of Grain Boundaries of Mn-Zn Ferrites by AES and TEM

AES analysis of grain boundaries of Mn-Zn ferrites, combined with ion etching, shows that CaO and SiO₂ dopants are enriched in a layer of ∼2 nm; TEM showed no separate phase at the grain boundaries. Using TEM combined with X-ray microanalysis three types of secondary phases, one crystalline and two amorphous, were found at the multiple-grain junctions. Although the absence of a planar second phase has not been proved, these results argue against the presence of a high-resistivity glassy layer and support the hypothesis that the high resistivity is caused by segregation of Ca during cooling, causing oxidation of the grain boundaries.     

Design of Electroceramics for Solid Oxides Fuel Cell Applications: Playing with Ceria

Nanostructured samaria- and gadolinia-doped ceria (SDC and GDC) powders were synthesized at low temperature (400°C) using diamine-assisted direct coprecipitation method. Fast-firing (f.f.) processes, where sintering temperatures are reached in a short time to promote lattice diffusion, were compared with conventional sintering, for the formation of dense microstructures from the nanostructured powders. Highly dense SDC and GDC samples (96%) with reduced grain size (150 nm) were obtained by f.f. even at 1300°–1400°C and, unexpectedly, high electrical conductivity and low blocking effect at grain boundary was obtained. Conventionally sintered samples showed that the grain boundary resistivity decreased with increasing the grain size, in agreement with the increase in geometrical bulk volume/grain boundary area ratio. Conversely, f.f. samples showed grain boundary resistivity smaller for small grain size. The above effect was observed only for high dopant (>10% molar) contents. The combined effect of powder grain size, dopant content, and sintering temperature–time profile, can be exploited to tune ceria microstructures for specific ionic device applications.     

Gd_2O_3掺杂对BaTiO_3陶瓷电性能的影响

采用溶胶-凝胶法制备了掺杂不同量Gd2O3(分别为0 001、0 002、0 003、0 005、0 007mol mol)的BaTiO3陶瓷,并对其电性能进行了研究。结果表明:Gd2O3掺杂使BaTiO3陶瓷的电阻率明显下降,当添加量为0 002mol mol时,电阻率最小,为1 27×105Ω·m;晶粒电阻随着温度的变化,呈现NTC效应,而晶界电阻随着温度的变化,呈现PTC效应,而且晶界电阻远远大于晶粒电阻,说明Gd2O3掺杂BaTiO3陶瓷的PTC效应是一种晶界效应;Gd2O3掺杂使BaTiO3陶瓷的介电常数和介电损耗在低频时明显增大,在高频时又明显降低。     

Electrical properties and thermal expansion of cordierite and cordierite-mullite materials

Commercially available cordierite and mullite powders were used to obtain cordierite and composite materials with mullite content up to 65 wt.% by attrition milling, uniaxial pressing and sintering. The employed cordierite powders were the coarse, medium and fine single granulometric fractions and the binary mixtures of them with 30, 50 and 70 wt.% of the smaller size component. Mullite powder employed in composites was a 7 h-attrition milled one. The dielectric constant (), dielectric loss tangent (tan δ), resistivity (ρ) and thermal expansion coefficient () were measured. The influence of the porosity, mullite and glassy phase contents and grain size in the electrical parameters was analyzed. The thermal expansion coefficient as a function of the composition was studied.     

Influence of Amorphous Grain Boundary Phases on the Superplastic Behavior of 3-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZP)

Amorphous silicate grain boundary phases of varying chemistry and amounts were added to 3Y-TZP in order to determine their influence on the superplastic behavior between 1200° and 1300°C and on the room-temperature mechanical properties. Strain rate enhancement at high temperatures was observed in 3Y-TZP containing a glassy grain boundary phase, even with as little as 0.1 wt% glass. Strain rate enhancement was greatest in 3Y-TZP with 5 wt% glass, but the room-temperature hardness, elastic modulus, and fracture toughness were degraded. The addition of glassy grain boundary phases did not significantly affect the stress exponent of 3Y-TZP, but did lower the activation energy for superplastic flow. Strain rate enhancement was highest in samples containing the grain boundary phase with the highest solubility for Y₂O₃ and ZrO₂, but the strain rate did not scale inversely with the viscosity of the silicate phases. Grain boundary sliding accommodated by diffusional creep controlled by an interface reaction is proposed as the mechanism for superplastic deformation in 3Y-TZP with and without glassy grain boundary phases.     

Effects of Samarium Oxide Addition on the Phase Composition, Microstructure, and Electrical Resistivity of Aluminum Nitride Ceramics

The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm₂O₃ additive were investigated. The phase composition was approximately consistent with that estimated from the Sm₂O₃–Al₂O₃ phase diagram using the amount of added Sm₂O₃ and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm₂O₃ additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 10¹⁰–10¹²Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.     

Phase transformation and interface segregation behavior in Si3N4 ceramics sintered with La2O3–Lu2O3 mixed additive

Microstructure and mechanical property of silicon nitride (Si₃N₄) ceramic are strongly dependent on the selection of sintering additives. When rare‐earth (RE) oxide is used as the sintering additive, segregation of RE ions at interface between Si₃N₄ grain and intergranular glassy film (IGF) is believed to play a critical role. Although the ionic radius of RE ion is known to be an empirical parameter to modify the mechanical property, the correlation between the segregated ions and their ionic radii is still under controversy. In order to address this issue, (i) rate of α‐β phase transformation and (ii) segregation behavior at the interface were studied for Si₃N₄ ceramics sintered using mixture of La₂O₃ and Lu₂O₃ as additives in this study. Specimens of Lu content 30% and higher exhibited lower activation energies for the α‐β phase transformation as compared with those of Lu content 20% and lower. In terms of the segregation behavior, La was preferably segregated at one site and Lu at the other site along β‐Si₃N₄/IGF interface in the specimens of Lu content 30% and higher. It is understood from these results that Lu segregation site should be more closely related with grain growth.     

多晶快离子导体的晶界性质对其电导的影响

建立了理想的多晶快离子导体的显微结构模型,根据模型推导出了简单的多晶离子导体的电阻率与晶界密度和晶界电导性的关系式。由该模型出,通过对其等效交流阻抗电路的简化和交流阻抗的计算,说明了多晶快离子导体材料的复平面交流阻抗谱形状随测试温度的变化和晶界性质对这种变化的影响。     

PTCR Characteristics and Fabrication of Porous, Sb-Doped BaTiO3 Ceramics

Sb-doped BaTiO₃ ceramics containing corn-starch were prepared by sintering at 1350°C for 1 h in air. In this study, the effect of corn-starch on positive temperature coefficient of resistivity (PTCR) characteristics and microstructures of Sb-doped BaTiO₃ ceramics was investigated. It was found that the porosity and pore size increased and the grain size slightly decreased with increasing corn-starch content. XRD results showed the presence of BaTiO₃ peaks only in the Sb-doped BaTiO₃ ceramics with and without corn-starch. The PTCR jump of the Sb-doped BaTiO₃ ceramics with corn-starch was over 10⁶ and 1–2 orders higher than that of samples without corn-starch. The increase in the room-temperature resistivity with increasing corn-starch content was attributed mainly to the increase in the electrical barrier height of grain boundaries and the porosity as well as the partial decrease in the donor concentration of grains and the grain size. It was also noticed that the grain boundary resistivity contributed largely to the total resistivity of the Sb-doped BaTiO₃—corn-starch ceramics.     

Ionic Conductivity in Calcium Silicate Glasses

Glasses in the system CaO–SiO₂ were prepared with the composition varying from 40 to 55 mole % CaO. The Na₂O impurity content ranged from 0.01 to 1.30 mole %. The electrical resistivity was insensitive to the Na₂O content but decreased with increasing CaO concentration. The activation energy for conduction ranged from 33.54 to 31.23 kcal/mole as the CaO concentration increased. The conduction mechanism was considered in terms of three arbitrary parameters: (1) equivalent oxygen packing density, (2) ionic radius, and (3) ion-oxygen attraction.     

Development of Conductivity of Acrylic Polymer Using Ionic Liquids Incorporated with Zinc Oxide Nanoparticles

In this study, we synthesized antistatic and ultraviolet-resistant acrylic films with a combination of ionic liquids and ZnO nanoparticles for the prevention of static electricity and ultraviolet instability. ZnO and two different ionic liquids such as 1-ethyl-2,3-dimethylimidazolium ethyl sulfate and methyl-tri-n-butylammonium methylsulfate were preferred to achieve conductive and ultraviolet-resistant acrylic films. To obtain the highest ultraviolet protection factor and the lowest surface resistivity for the acrylic film, the combined effect of ZnO nanoparticles and the ionic liquids was utilized. The surface resistivity, thermal conductivity and effusivity, thermogravimetric analysis, and ultraviolet resistivity of the films were investigated. Surface morphology of the films and distribution of ZnO were also observed by scanning electron microscopy. The acrylic polymer exhibits higher ultraviolet resistance and lower transmission even in the low content of ZnO nanoparticles as compared with the neat polymer. The film consisting of methyl-tri-n-butylammonium methylsulfate ionic liquid showed the highest electrical conductivity performance even after 150 days. Consequently, ZnO nanoparticles are determined to be influential on ultraviolet-resistant properties, whereas ionic liquids are efficient on electrical conductivity performance of the acrylic polymer.     

Secondary Grain Growth of Li2O-A12O3-SiO2-TiO2 Glass-Ceramics

The kinetics of secondary grain growth in a Ti0₂-nucleated β-spodumene solid-solution glass-ceramic was studied. The thermal stability of the grains was excellent. Grain growth followed the cube-root-of-time law. The activation energy of the grain boundary migration was 55 ± 10 kcal/mol. Grain growth inhibition due to Ti0₂ precipitates and the residual glassy phase was closely examined. The excellent thermal stability of the grains is due to grain growth inhibition by the residual glassy phase, not by rutile precipitates. It is suggested that the diffusion of A²⁺, and probably the simultaneous diffusion of Li⁺, through the residual glass is the rate-limiting process for the grain boundary migration.     

Sintering Temperature Dependence of Grain Boundary Resistivity in a Rare-Earth-Doped ZnO Varistor

We present a rare-earth-doped ZnO ceramic with nonohmic electrical properties. Analysis of the microstructure and composition indicates that the ceramic is composed of the main phase of ZnO and the second phase of rare-earth oxides (e.g., Dy₂O₃, Pr₆O₁₁, Pr₂O₃). The average grain size is markedly increased from 3 to 18 μm, with an increase in the sintering temperature from 1150° to 1350°C. The corresponding varistor voltage and nonlinear coefficient decrease from 1014 to 578 V/mm, and from 15.8 to 6.8, respectively. The resistivity of grain and grain boundary evaluated by the complex impedance spectrum indicates that the resistivity of the grain is approximately constant (∼10³Ω), and the resistivity of the grain boundary decreases. The relative dielectric permittivity of the sintered ceramic samples is much larger than that of pure ZnO ceramic, which should be ascribed to the internal boundary layer capacitance effect.     

Effects of Annealing Temperature on Microstructure and Electrical and Optical Properties of Radio-Frequency-Sputtered Tin-Doped Indium Oxide Films

Indium tin oxide (ITO) films (0.3 μm thick), with a doping level of 28 mol% SnO₂, were prepared by a radio frequency magnetron sputtering mehthod. The effects of postannealing on the microstructure and the electrical properties of the ITO films were investigated. The as-sputtered film showed an amorphous structure, whereas the films annealed at 350° and 510°C exhibited crystalline structures with grain sizes of 0.12 and 0.14 μm, respectively. Examination by TEM showed that the postannealing treatment induced SnO₂ precipitates along the grain boundaries. The resistivity increased with increasing postannealing temperatures. The mobility of carriers appears to be responsible for the resistivity increase in these specimens. The mobility change is discussed in connection with the SnO₂ precipitates.     

Microstructure,grain orientation,and properties of ITO ceramics sintered with various heating rates

In this study, the microstructure, grain orientation, electrical resistivity, nanohardness, and fracture strength were systematically investigated in ITO ceramics sintered with different heating rates. It was found that ITO ceramics sintered at 1600°C consist of main phase (In₂O₃) and secondary phase (In₂SnO₅) and the distributions of coarse grains with high indium content and fine grains with high tin content are interval. The higher heating rate was found to refine the grain size and increase the {1000} textured secondary phase. The increase in electrical resistivity of ITO ceramics was due to the decrease in oxygen vacancies caused by the reducing oxide decomposition. Moreover, the nanohardness of fine grains was found to be higher than that of coarse grains due to the reinforcement effect of Sn element. Besides, the increase of heating rate is beneficial to enhance the fracture strength due to the higher resistance of grain boundary to fracture propagation and the predominantly intergranular fracture.     

Microstructure, Microchemistry, and Flexural Strength of Mullite Ceramics

The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575°C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600°C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al₂O₃ contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al₂O₃–SiO₂ phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200°C, and it decreases at and above 1350°C, irrespective of the presence of the glassy phase.     

The microstructures, magnetic properties and impedance analysis of Mn–Zn ferrites doped with B2O3

The relationship between the microstructure, magnetic properties and impedance spectroscopy of Mn–Zn ferrites doped with B₂O₃ (up to 0.5 wt.%) has been further investigated. The ferrites were prepared by using a citrate gel processing route. A uniform microstructure with relatively small grains (9.6±0.7 μm) is observed for undoped ferrites (boron-free), which enables good magnetic properties to be achieved (initial permeability μ_i is 2400, power loss P_L is 26.3 kW/m³ at 100 mT).The results on the samples doped with B₂O₃ show that the doping does not benefit the magnetic properties of these gel-derived ferrites, but it promotes grain growth significantly. Discontinuous grain growth at low doping levels (<0.2 wt.%) results in poor magnetic properties. A maximum value of the initial permeability (μ_i: 2600) and a second minimum value (37.2 kW/m³ at 100 mT) in power loss are obtained at the 0.25 wt.% B₂O₃ doping level when the sample has a relatively uniform microstructure with larger grain size (39.5±3.3 μm). With further increases in B₂O₃ doping (0.5 wt.%), the increased porosity and presence of a B-rich phase result in deteriorated magnetic properties. The results of impedance measurement are closely related to the changes in the microstructure which result from these B₂O₃ additions. By using two models for impedance measurement analysis (The Koops’ model and the simple model), the contributions of B₂O₃ to grain boundary resistivity and bulk resistivity can be separated. It is shown that, whilst B₂O₃ has previously been considered to act as only a grain boundary additive, the impedance analysis indicates that both boundary resistivity and grain (bulk) resistivity are increased, thus implying the possible solution of some B₂O₃ within the ferrite spinel structure or an effective change in composition of grain as result of presence of B₂O₃ at the grain boundaries.     

PET‐ionomers and PETi/MMT nanocomposites: A comparison of the effect of ionic groups on their crystallinities and physical properties

Poly(ethylene terephthalate) (PET) and PET/modified montmorillonite (A10‐MMT) nanocomposites containing up to 3 mol% of dimethyl 5‐sulfoisophthalte sodium salt (DMSi) were prepared by in situ polymerization (PETi and PETNi, respectively). In results from transmission electron microscopy (TEM, ∼ 10 nm), clusters were observed with sizes of 30 nm at an ionic content of 3 mol%. Clusters were not observed in the PETNi samples due to the favorable interaction between the positively charged edges of the clay platelets and the negatively charged ionic groups. On the other hand, the degree of exfoliation was considerably improved by the small ionic content. Remarkable increases in the storage moduli of PETi and PETNi were also observed with increasing DMSi content in the glassy region. However, the storage modulus of PETi decreased rapidly in the rubbery region above the transition temperature as temperature increased. The dynamic viscosity of PETNi decreased with DMSi content, indicating that ion‐pair sites located at the clay edges acted as selective plasticizers in the PET ionic region. Moreover, the introduction of a small ionic content into the PET matrix led to a decrease in crystallinity and slower crystallization rates than in the pure PET. POLYM. COMPOS., 2011. © 2010 Society of Plastics Engineers     

Resistivity-Microstructure Relations in Lithia-Stabilized Polycrystalline β"-Alumina

The sodium ion resistivity of lithia-stabilized polycrystalline β"-alumina was measured as a function of temperature for fine-grained and coarse-grained specimens with a chemical composition of 8.80 Na₂0-0.75 Li₂O-90.45 A1₂O₃ (wt%). A model is presented which explains the dependence of sodium ion resistivity on grain size. Using the model the activation energy was determined for the transport of sodium ions across a grain boundary in this form of sodium β"-alumina.