Control of the Grain Boundary Electrical Features of the Semiconducting SrTiO3 Ceramics Synthesized from Surface-Coated Powders

Polycrystalline SrTi_0.99Nb_0.01O₃ (STNO) ceramics were synthesized by hot-press sintering Na-coated semiconducting STNO powders. The chemical and electrical characteristics of the grain boundary of the ceramics were investigated, and their relations were discussed in terms of process parameters. The diffusion coefficients as well as the activation energy of the Na ions near the grain boundary were obtained at particular heat-treatment conditions; it was demonstrated that these values can be used to design specific electrical features of the grain boundary. A systematic variation in the electrical characteristics of the grain boundary with process parameters was observed; it indicates that this synthesis method can be used for fine control over the chemical and electrical properties of the semiconducting ceramics.     

Structural evolution and dopant occupancy preference of yttrium-doped potassium sodium niobate thin films

Sodium potassium niobate (KNN) is the most promising candidate for lead-free piezoelectric material, owing to its high Curie temperature and piezoelectric coefficients among the non-lead piezoelectric. Numerous studies have been carried out to enhance piezoelectric properties of KNN through composition design. This research studied the effects of yttrium concentrations and lattice site occupancy preference in KNN films. For this research, the yttrium-doped KNN thin films (mol% = 0, 0.1, 0.3, 0.5, 0.7 and 0.9) were fabricated using the sol-gel spin coating technique and had revealed the orthorhombic perovskite structures. Based on the replacement of Y³⁺ ions for K⁺/ Na⁺ ions, it was found that the films doped with 0.1 to 0.5 mol% of yttrium had less lattice strain, while films with more than 0.5 mol% of Y³⁺ ions had increased strain due to the tendency of Y³⁺ to occupy the B-site in the perovskite lattice. Furthermore, by analysing the vibrational attributes of octahedron bonding, the dopant occupancy at A-site and B-site lattices could be identified. O-Nb-O bonding was asymmetric and became distorted due to the B-site occupancy of yttrium dopants at high dopant concentrations of >0.5 mol%. Extra conduction electrons had resulted in better resistivity of 2.153× 10⁶ Ω at 0.5 mol%, while higher resistivity was recorded for films prepared with higher concentration of more than 0.5 mol%. The introduction of Y³⁺ improved the grain distribution of KNN structure. Further investigations indicated that yttrium enhances the surface smoothness of the films. However, at high concentrations (0.9 mol%), the yttrium increases the roughness of the surface. Within the studied range of Y³⁺ _, the film with 0.5 mol% Y³⁺ represented a relatively desirable improvement in dielectric loss, tan δ and quality factor, Q_m.     

Effect of zinc oxide as a sintering aid on proton migration across Σ5 (310)/[001] tilt grain boundary of barium zirconate

The effect of zinc oxide as a sintering aid on proton migration across a barium zirconate super cell with a Σ5 (310)/[001] tilt grain boundary was investigated using density functional theory. The proton was trapped by high energy barriers in the range of 1.28–1.49 eV near the zinc ion that was substituted for a zirconium ion at the grain boundary. However, there were available pathways for proton migration across the grain boundary away from the zinc ion with relatively low energy barriers in the range of 0.61–0.78 eV. Therefore, the zinc ion located at the grain boundary did not affect the energy barrier for proton migration, although the number of mobile protons could decrease as a result of being trapped near the zinc ion.     

TEM,XPS and SIMS Analyzes on Grain Boundary of Lanthanum Chromites

The morphological characteristics, chromium valence state, and cation transport in the vicinity of grain boundary in La_1-xCa_xCrO₃ were investigated by using TEM/EDS, XPS and SIMS techniques. The width of grain boundary was around 1 nm where anomalous enrichment of calcium was observed. Higher valence state of chromium such as Cr⁶⁺(d⁰) was detected in the grain boundaries whereas Cr³⁺(d³) and Cr⁴⁺(d²) were dominant in the bulk. Very fast interdiffusion of alkaline earths was observed in the Sr²⁺-La_0.75Ca_0.25CrO₃ system. All observed phenomena were correlated by assuming the A-site vacancy which may be induced by the formation of Cr⁶⁺ at grain boundaries.     

Grain growth kinetics of textured 0.92Na0.5Bi0.5TiO3—0.08BaTiO3 ceramics by tape casting with Bi2.5Na3.5Nb5O18 templates

Plate-like Bi_2.5Na_3.5Nb₅O₁₈ particles were used as templates to fabricate grain-oriented Na_0.5Bi_0.5TiO₃—BaTiO₃ (NBTBT) ceramics by reactive-templated grain growth. The effects of sintering conditions on the grain orientation and microstructure of the textured NBTBT ceramics were investigated, and the kinetic mechanism of grain growth is discussed. The results show that textured ceramics were successfully obtained with orientation factor more than 0.6. NBTBT specimens are composed of strip-like grains and equiaxed shaped grains. The textured ceramics have a microstructure with strip-like grains aligning in the direction parallel to the casting plane and exhibit an {h00} preferred orientation. The degree of grain orientation increases initially, then decreases with increasing sintering temperature and soaking time. The maximum texture fraction is 0.69 when sintered at 1185 °C for 6 h. The kinetic exponent n and activation energy Q of the two types of grain in textured NBTBT ceramics were calculated. The results show that the grain growth mechanism of oriented grains is controlled not only by grain lattice diffusion, but also by grain boundary diffusion. The grain growth mechanism of matrix grains is mainly controlled by the grain boundary curvature.     

Interface Defect Chemistry and Effective Conductivity in Polycrystalline Cerium Oxide

Polycrystalline cerium oxide exhibits increasing electronic and decreasing ionic conductivity upon reduction of the grain size. In the present study, the origin of this effect was examined. Temperature-programmed reduction (TPR) and oxygen titration measurements on nanocrystalline cerium oxide revealed a large excess oxygen deficiency associated with the surface. Using a two-phase model for the combined system of the bulk phase in equilibrium with a surface layer, this enhanced oxygen deficiency could be explained by a reduced binding energy of surface oxygen ions in agreement with results from atomistic computer simulations. The model also revealed that this segregation of oxygen vacancies is the origin of an intrinsic space charge potential. Translating this effect to polycrystalline cerium oxide and taking into account the segregation of dopants and the accumulation/depletion of charge carriers, it was possible to model the grain size dependence of electrical conductivity and thermopower of polycrystalline cerium oxide. A straightforward 1-dimensional numerical model and a change from Boltzmann to Fermi-Dirac statistics allowed to calculate the conductivity of heavily doped polycrystalline cerium oxide for grain sizes in the range of 5–10,000 nm and acceptor concentrations up to 20%. Using this approximation, the effect of grain size on mixed ionic/electronic conductivity and the electrolytic domain boundary was investigated.     

ZnO Grain Boundaries: Electrical Activity and Diffusion

The operation and performance of electroceramics are commonly dependent on the characteristics of electrically active grain boundaries. To date, our understanding of the role of specified additives and heat treatments on the grain boundary properties remains underdeveloped. We describe efforts directed towards improving our understanding by (a) fabrication and analysis of individual boundaries, (b) improved control and simplification of boundary chemistry (c) systematic investigation of properties (e.g., I-V, DLTS, D_O and D_M) as a function of boundary structure and chemistry and (d) development of appropriate energy band, defect and diffusion models. Following this approach, preliminary results suggest that lattice defects play critical roles in controlling both the electrical and diffusive properties of the boundaries while the additives appear to act in supportive manner by activating the key lattice defects particularly with respect to the electrical activity of the boundaries.     

Study on the electrical and optical properties of ITO and AZO thin film by oxygen gas flow rate

Transparent conductive oxide (TCO) thin films such as tin doped indium oxide (ITO), zinc doped indium oxide (IZO) and Al doped zinc oxide (AZO) have been widely used as transparent electrode for display. ITO and AZO thin films for display was prepared by the facing targets sputtering (FTS) system. The FTS method is called a plasma-free sputter method because the substrate is located apart from plasma. This system can deposit the thin film with low bombardment by high energetic particles in plasma such as γ-electrons, negative ions and reflected Ar atoms. ITO and AZO thin films were deposited on glass substrate at room temperature with oxygen gas flow rate and input power. And the electrical, structural and optical properties of the thin films were investigated. As a result, the resistivity of ITO, AZO thin film is 6?×?10^?4 Ω cm, 1?×?10^?3 Ω cm, respectively. And the optical transmittance of as-deposited thin films is over 80% at visible range.     

Electron-Ion Transport in ZrO2-Y2O3-CeO2 Ceramics

Simultaneous conduction of oxide ions and electrons in solid ceramic systems provides the capability for oxygen transport under a concentration gradient without the need for an externally applied electric field. In the present study, ionic transference numbers have been measured in the ZrO₂-5.8%Y₂O₃-10%CeO₂ system by open circuit Emf measurements involving different metal/metal oxide electrodes. In order to correlate the ionic transference number with grain size, high-density ceramic discs of different grain sizes (50 nm–5 m) were prepared by sintering pressed powders at various temperatures and times. Hydrothermal synthesis was used to prepare nanocrystalline powders of the above material with uniform crystallite size (10 nm) and chemistry. Emf measurements on the samples suggested both ionic and electronic transport, the ionic transference number decreasing with increase in the grain size. This observation was attributed to an increase in the amount of continuous crystalline grain boundary phase in the ceramics as the grain size increased. The presence of crystalline silicate and zirconate phases in the grain boundary region was confirmed by electron microscopic imaging combined with microanalysis. In the large grain (5 m) ceramics, the ionic transference number decreased linearly with temperature. As the grain size decreased, a maximum occurred in the ionic transference number vs. temperature curve. This maximum became more pronounced at smaller grain sizes. Better grain-grain contact and the doping effect of trivalent Ce in the grain boundary core are proposed to explain this observation.     

Dielectric properties of BiFeO3 ceramics obtained from mechanochemically synthesized nanopowders

Dielectric behaviour of BiFeO₃ ceramics, obtained by hot-pressing of nanopowders produced by mechanochemical synthesis from Bi₂O₃ and Fe₂O₃ oxides (weight ratio 2:1), was studied in the temperature range 125?C575?K. The ceramics was found to exhibit step-like dielectric response ??*(T) with high permittivity values, similar to the behaviour of materials with giant dielectric permittivity. Three overlapping relaxation processes contribute to the dielectric response: i) relaxation in the low-temperature range (220?C420?K), characterized by activation energy of 0.4?eV, ii) relaxation in the temperature range 320?C520?K with activation energy of 1.0?eV and iii) broad dielectric anomaly in the vicinity of 420?K, which disappears after 1?h annealing at 775?K. The low-temperature relaxation is ascribed to the carrier hopping process between Fe²⁺ and Fe³⁺ ions. The presence of mixed valence of the Fe ions was proved by X-ray photoelectron spectroscopy. Dielectric relaxation in the middle-temperature range is considered as a result of grain boundary effect and internal barrier layers related to Bi₂₅FeO₄₀ phase as verified by X-ray diffraction. The high-temperature dielectric anomaly we relate to short-range hopping of ordered oxygen vacancies.     

Heterogeneously doped nanocrystalline ceria films by grain boundary diffusion: Impact on transport properties

Heterogeneous doping of nanocrystalline ceria films, by controlled in-diffusion along grain boundaries, is explored as a means for modifying the space charge potential and the inhomogeneous distribution of defects in the space charge layer known to control the electrical properties of nanocrystalline electroceramics. Nanocrystalline cerium oxide thin films were grown by pulsed laser deposition and modified by a novel doping technique. Thin diffusion sources were deposited and cations such as Ni²⁺ and Gd³⁺ were in-diffused at temperatures of 700–800 °C along columnar grain boundaries normal to the surface; the resulting diffusion profiles were examined by Time-of-Flight SIMS. The properties of these modified films were compared with as-deposited samples, with in-diffusion resulting in decreased electrical conductivity. It is proposed that the variation in conductivity results from a redistribution of charge carriers in the space charge layers due to a change in the space charge potential.     

Characteristics of thin film supercapacitor with ruthenium oxide electrode and Ta2O5+x solid oxide thin film electrolyte

We report findings related to a solid-state thin film supercapacitors (TFSCs) fabricated with ruthenium oxide electrodes and hydrogen doped tantalum oxide electrolyte in order to investigate a feasibility for solid oxide thin film electrolyte in all solid state micropower sources with hydrogen conducting electrolyte. The TFSCs in this study has a cell structure of RuO₂/Ta₂O₅/RuO₂/Pt. Radio frequency, off-axis r.f. sputtering deposition of a Ta₂O₅ thin film was performed on the bottom amorphous RuO₂ electrode which was deposited by an on-axis direct current reactive sputtering, within hydrogen atmosphere to enhance the concentration of mobile proton (H⁺) ions. Scanning electron microscopy (SEM) measurements reveal that the RuO₂/Ta₂O₅/RuO₂ hetero-interfaces have no inter-diffusion problems. Room temperature charge–discharge measurements with constant current clearly reveal typical supercapacitor behavior. Also, owing to the fast diffusion of H⁺ ions within the Ta₂O₅ lattice without any structural deterioration, the capacitance per volume of RuO₂/Ta₂O₅/RuO₂/Pt TFSCs was maintained to be constant during above 800 cycles. This result indicated that the solid oxide thin film has possibility as the solid proton conducting electrolyte for all solid state micropower sources.     

Modeling of transport properties of interfacially controlled electroceramics: Application to n-conducting barium titanate

Grain boundary regions in n-conducting barium titanate (BaTiO₃) are re-oxidized during the cooling process after sintering the ceramics in air. The kinetics of this re-oxidation process is determined by rapid transport of oxygen along the grain boundaries and slow (rate-determining) diffusion of cation vacancies from the grain boundaries into the grains until the diffusion process is frozen-in. Based on numerical calculations of frozen-in diffusion profiles of cation vacancies at grain boundary regions for various cooling rates, a modified Schottky-barrier model is introduced in order to calculate the grain boundary resistivity as a function of temperature from the Curie-point up to 900°C. A change of the activation energy at approximately 500°C is predicted owing to an enrichment of holes in the space charge layers at elevated temperatures. The modeling results are compared with experimental data for BaTiO₃-based positive temperature coefficient resistors (PTCRs).     

Nonuniformity of electrical Characteristics in microstructures of ZnO surge varistors

The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The Al/sub 2/O/sub 3/ dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain.     

CaCO3-SiO2添加对MnZn铁氧体物相及性能的影响

采用氧化物陶瓷工艺制备了Mn0.7Zn0.24Fe2.06O4铁氧体.用X射线衍射仪、扫描电镜、B-H分析仪分别表征了CaCO3-SiO2添加对MnZn铁氧体物相、微结构和磁性能的影响.结果表明,添加于MnZn铁氧体的CaCO3-SiO2主要富集于晶界,且生成另相Ca2ZnSi2O7.随着CaCO3-SiO2含量的增加...     

Microstructure of cobalt oxide doped sintered ceria solid solutions

The sintering of ceria solid solutions, such as Ce_0.9Gd_0.1O_1.95 (CGO10), is strongly promoted by the addition of 1 cat% of cobalt oxide, lowering the maximum sintering temperature by 200^∘C and triplicating the maximum densification rate. This change in sintering behavior results from cobalt ion segregated at the grain boundaries. An average cobalt ion boundary coverage is at maximum 3.0 ± 1.9 at/nm² and is shown to depend on the cooling rate. Coverage by segregated gadolinium is also found and amounts to 13.2 ± 11.4 at/nm² for a slowly cooled sample. From cobalt excess measured at the boundary, an estimated concentration of only 0.06 cat% of cobalt oxide is necessary to promote the sintering effect. The remaining amount of cobalt oxide is found in triple points and as particles in clusters. It is expected that the amount of cobalt oxide necessary for fast densification can be reduced with a doping process that distributes the additives more homogeneously.     

金属氧化物掺杂改善LiFePO4电化学性能

采用氧化物前驱体对磷酸铁锂(LiFePO4)进行少量金属离子掺杂,用X射线衍射、电子扫描显微镜、循环伏安法和恒电流充放电对掺杂的LiFePO4进行了研究.结果表明,少量的掺杂离子在很大程度上提高了LiFePO4的电化学性能,特别是大电流放电性能中1.0%(摩尔分数)的Nb5 掺杂LiFePO4的1 C放电比容量约130 mAh/g.掺杂后的电化学性能与掺杂离子的半径、价态密切相关,半径合适、价态高的离子对提高LiFePO4的电化学性能有利.     

Measurement of electrical characteristics of ZnO microvaristors

A measurement technique is developed to measure the characteristics of zinc oxide (ZnO) microvaristors directly. We used Cu wires, a micromanipulator, and an optical microscope to gain ohmic contact of the measurement apparatus with ZnO particles. With our system, the I−V characteristics and C⁻²−V characteristics were measured for 25 µm ZnO microvaristors. Our measurements show that these microparticles behave as varistors. Further measurement on the ZnO microvaristors and observation of their fine structure may help to understand the varistor behavior. It also can pave the way to the understanding of the electrical characteristics of grain boundaries as well as the double Schottky barrier formed at the grain boundaries. © 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Preparation, microstructure and electrical properties of Li1.4Al0.4Ti1.6(PO4)3 nanoceramics

NASICON-type Li_1.4Al_0.4Ti_1.6(PO₄)₃ solid electrolytes were prepared by various processes, such as crystallization of glasses, spark plasma sintering (SPS) and conventional sintering process from nanosized precursor powders synthesized by a sol–gel route. The experimental results showed that grain size and relative density were the main factors determining the ionic conductivity of the bulk materials. The SPS technique produced ceramics with nearly 100% of the theoretical density. Maximum room temperature conductivities, 1.39?×?10^?3 S cm^?1 and 1.12?×?10^?3 S cm^?1 of grain boundary conductivity and total conductivity, respectively were obtained which were the highest values for Li⁺ inorganic oxide conductors as reported. Crystallization of ceramics from a glass was also certified as a favorable route to fabricate a bulk material with high conductivity.     

Magnetization and Electric Properties of Pr-doped ZnO

Magnetic susceptibility of ZnO-varistors doped with Pr-ion was measured to elucidate the valence state of the Pr-ion and the effect of thermal treatment. Magnetic susceptibility _χ(T) of the Pr-ion obeyed Curie-Weiss law, from which its valence state was estimated. The valence state of the Pr-ion varied with annealing condition of specimens, and it was indicated that the Pr-ion in ZnO ceramics was more reducible in comparison with pure \hboxPrO_x ceramics. Nonlinear current-voltage characteristics was diminished with the reduction of the valence state of Pr ions. According to Auger electron spectra, the Pr-ions segregated at grain boundaries in ZnO ceramics.