Influence of B2O3 additives on microstructure and electrical properties of ZnO-Bi2O3-Sb2O3-based varistors

B₂O₃-doped ZnO-Bi₂O₃-Sb₂O₃-based varistors were fabricated by conventional ceramic technique. The microstructure and electrical properties were investigated by SEM, XRD and electrical measurements. With the addition of B₂O₃, the liquid-assisted sintering based on Bi₂O₃ was improved, and the Bi₂O₃-B₂O₃ glass and Zn₃(BO₃)₂ phase were formed on the grain boundaries. The doping of B₂O₃ markedly improved the varistor performance of the ZnO-Bi₂O₃-Sb₂O₃-based varistors. The nonlinear coefficient of the sample with 3.5 mol% B₂O₃ sintered at 1100 °C reached 56 and the leakage current was only 0.3 μA.     

Grain size dependent grain boundary defect structure: case of doped zirconia

The electrical properties of 3 mol% Y₂O₃ doped ZrO₂ were measured by impedance spectroscopy as a function of grain size, and the microstructure studied by SEM and HREM. In spite of the very clean grain boundaries, the grain boundary conductivity was still found to be ∼2 orders of magnitude lower than the bulk conductivity, while it increased with decreasing grain size. The low grain boundary conductivity, according to the Schottky barrier model, is due to the depletion of oxygen vacancies in the grain boundary space charge layers. Within this framework, the grain boundary space charge potential and the concentration of oxygen vacancies in the space charge layers were calculated; it was found that the space charge potential decreased, but the oxygen vacancy concentration increased with decreasing grain size. Analyses of literature results for 8.2 mol% Y₂O₃ and 15 mol% CaO doped ZrO₂, respectively, revealed similar phenomena.     

掺杂方法对W-1.0wt.%La2O3合金显微组织和力学性能的影响(英文)

对采用溶液燃烧合成法并结合不同掺杂方法(液-液(WL10)、液-固(WLNO)和固-固(WLO))制备的纳米尺度粉末及采用常压烧结法制备的超细晶W-1.0wt.%La₂O₃合金进行对比研究。与纯钨相比,W-1.0wt.%La₂O₃合金具有超细晶粒和优异的力学性能。烧结后,WLO样品的平均晶粒尺寸大于WL10和WLNO样品的平均晶粒尺寸;WL10和WLNO的显微硬度相差不大,但大于WLO的显微硬度。相对于其他样品,在1500℃烧结后的WL10样品中La₂O₃颗粒呈现最佳分布状态,均匀分布在晶界或晶粒内部,且平均尺寸最小,晶界和晶粒内部的La₂O₃颗粒平均尺寸分别为(57±29.7)和(27±13.1)nm。与传统方法制备的钨材料相比,采用液-液掺杂方法制备的W-1.0wt.%La₂O₃钨合金呈超细的显微组织和优异的性能。     

Structure and Properties of Y2O3-Doped Al2O3-MWCNT Nanocomposites Prepared by Pressureless Sintering and Hot-Pressing

This study describes the combined effects of multi-walled carbon nanotubes (CNTs) additions and Y₂O₃ doping on the microstructures and mechanical properties of Al₂O₃-CNT nanocomposites fabricated by pressureless and hot-press sintering processes. A uniform dispersion of CNTs within the Al₂O₃ matrix was successfully attained via a combined approach using surfactant, sonication, and adequate period of incubation. Small amounts (1 wt.%) of Y₂O₃, as dopants, significantly affected the densification and properties of pressureless sintered monolithic Al₂O₃ and its nanocomposites at low CNT concentrations (<1 wt.%); however, they hardly showed any improvement at higher CNT contents. As opposed to the pressureless sintering, pressures applied during high temperature sintering in combination with the Y₂O₃ doping contributed in generating a homogenous microstructure and improved the densities (7 and 15%) and microhardness (11 and 12%) of Al₂O₃ reinforced with higher CNT contents (2 and 5 wt.%), respectively. Adding on, hot-pressed Y₂O₃-doped Al₂O₃ reinforced with 2 and 5 wt.% CNTs showed higher hardness (19 and 70%), flexural strength (10 and 5%), and fracture toughness (26 and 11%), respectively, compared to similar but CNT-free samples. These results showed that pressure-assisted sintering and Y₂O₃ are promising for the fabrication of CNT-reinforced Al₂O₃ nanocomposites, especially at higher CNT concentrations.     

Grain-boundary structure and microstructure development mechanism in 2–8 mol% yttria-stabilized zirconia polycrystals

Microstructural developments during sintering in 2 and 3 mol% Y₂O₃-stabilized tetragonal zirconia polycrystals (2Y- and 3Y-TZPs) and 8 mol% Y₂O₃-stabilized cubic zirconia (8Y-CSZ) were systematically investigated in the sintering temperature range of 1100–1500 °C. Above 1200 °C, grain growth in 8Y-CSZ was much faster than that in 2Y- and 3Y-TZPs. In the grain-boundary faces in these specimens, amorphous layers did not exist; however, Y³⁺ ions segregated at the grain boundaries over a width of ∼10 nm. The amount of segregated Y³⁺ ions in 8Y-CSZ was significantly less than in 2Y- and 3Y-TZPs. This indicates that an increase in segregated Y³⁺ ions retards grain growth. Therefore, grain growth behavior during sintering can be reasonably explained by the solute-drag mechanism of Y³⁺ ions segregating along the grain boundary. The segregation of Y³⁺ ions, which directly affects grain growth, is closely related to the driving force for grain-boundary segregation-induced phase transformation (GBSIPT).     

Fabrication and optical properties of a highly transparent Nd：YAG ceramic

A neodymium doped yttrium aluminum garnet (Nd:YAG) transparent ceramic was fabricated by a solid state reaction method using commercial α-Al₂O₃, Y₂O₃, and Nd₂O₃ powders as starting materials and tetraethyl orthosilicate (TEOS) as sintering aid. The morphology and microstructure of the nanopowders and the Nd:YAG transparent ceramic were investigated. The fully dense Nd:YAG ceramic with an average grain size of ∼20 μm was obtained by vacuum sintering at 1720°C for 12 h. Few pores and grain-boundary phases were observed. The in-line transmittance of the ceramic was 81.5% at 1064 nm.     

The influence of the grain boundary phase on the mechanical properties of Si3N4–MoSi2 composites

The microstructure and mechanical properties of Si₃N₄–MoSi₂ composites doped with two different sintering additive systems, Y₂O₃–Al₂O₃ and Lu₂O₃, were investigated. It was found that the composite doped with Y₂O₃–Al₂O₃ had an amorphous grain boundary phase, while the grain boundary phase of the Lu₂O₃-doped composites was completely crystallized. The Si₃N₄–MoSi₂ composite containing Lu₂O₃ had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y₂O₃–Al₂O₃. This is attributed to the crystallization and higher softening temperature of the Lu₂O₃-doped grain boundary phase compared with that doped with Y₂O₃–Al₂O₃. However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi₂ particles in Si₃N₄ can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si₃N₄ and MoSi₂. The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

Microstructure and properties of Y2O3-doped steel-cemented WC prepared by microwave sintering

Steel-cemented WC was prepared by ball milling,cold compacting and microwave sintering with Fe powder as the matrix,WC as the hard phase and the addition of rare earth Y2O3.The results show that the interface of the WC particles and Fe matrix exhibits excellent wettability and liquidity when the microwave sintering temperature reaches 1,280°C.The density and mechanical properties of the steel bonded WC carbides could be greatly improved,the hard phases become finer and more uniform dispersed owing to the addition of Y2O3.With the increase of the Y2O3contents,the grain becomes uniform and fine first,and then gathers and grows up.The relative density,microhardness and bending strength all rise first,reaching the maximum values of 97.29%,HV1024 and 1,267.60 MPa at 0.5%Y2O3,respectively,and then decrease.Moreover,the relative density and mechanical properties of the steel-cemented WC with nano-Y2O3are higher than that with micron-Y2O3,which indicates that the effect of nano-Y2O3is better than that of the micron-Y2O3.     

Peculiar effects of microwave sintering on ZnO based varistors properties

Non-ohmic properties of doped zinc oxide are widely used in varistors applications. It is well established that final properties of the component are strongly correlated with reactivity of the added phases during sintering process and with final microstructure. In this paper, the specific effects of the hybrid single-mode microwave sintering process on the microstructure and electrical properties of a ZnO-based composition are investigated. Nano-sized ZnO-based powder with a proper amount of Bi₂O₃, Sb₂O₃, CoO and MnO is synthesized by a liquid route and is sintered within a short time (less than 10 min) in a conventional (CV) or by an hybrid single-mode microwave (MW) furnaces. Distinct differences can be seen in the density, reaction kinetics and dopant diffusivity: higher kinetics of MW leads to denser pellet, faster reaction among dopants and faster diffusion of cobalt and manganese into ZnO grains although grain sizes are almost identical between CV and MW. These differences in terms of chemistry and microstructure lead to sharp contrasts in electrical properties.     

Synthesis and characterization of titanium doped sodium beta″-alumina

High dense Na-β″-Al₂O₃ electrolyte materials have been synthesized by solid state reaction with boehmite, magnesia, sodium carbonate and titania as the starting materials. The effects of TiO₂ doping percentage on the properties and microstructures of the prepared samples were investigated by X-ray diffraction (XRD), scan electron microscope (SEM), three point bending and ionic conductivity tests. The results indicated that both the relative densities and the phase purities of the samples could effectively improved after doping with TiO₂. The proper doping amount of TiO₂ was to form the transient liquid phase during the sintering process, which would reduce the steric effect and accelerate the diffusivity. Moreover, the mechanical performance of the obtained sample increased to the value above 280 MPa as the doping amount of TiO₂ was larger than 1 wt%. As to the electric properties, if the doping amount was less than 1 wt%, the grain boundary resistivity reduced as the density increased, so the ionic conductivity of the Na-β″-Al₂O₃ was enhanced obviously. However, when the doping amount was above 1 wt%, the ionic conductivity was deteriorated because of the increased resistivity caused by the broadening grain size.     

Influence of Y^3＋ doping on structure and electrochemical performance of layered Li1.05V3O8

LiOH.H2O,V2O5 and Y(NO3)3 were used as raw materials to synthesize the precursors containing Li,V and Y by liquid-state reaction,then the cathode materials Li1.05YxV3-xO8(x=0,0.002 5,0.005,0.01,0.02,0.1,0.2)for lithium-ion battery were obtained by calcining the precursors.The influence of Y3 doping on structure,conductivity and electrochemical performance of Li1.05V3O8 were investigated by using XRD,cyclic voltammograms,AC impedance,etc.The results show that Li1.05YxV3-xO8 with different doping amounts have well-developed crystal structure of layered Li1.05V3O8 and lengthened interlayer distance of(100) crystal plane.Y3 can insert into crystal lattice completely when the doping amount is small and the impurity phase of YVO4 is found when x≥0.1.There is no change in the process of Li insertion-deinsertion with Y 3 doping.The conductivity is clearly improved due to small amount of Y 3 doping and it tends to increase first and then decrease with increasing doping amount.The initial discharge capacity and plateau potential are both enhanced with proper amount of Y3 doping.When x is 0.005,the first specific discharge capacity reaches 288.9 mA.h/g,4.60%larger than that of undoped sample(276.2 mA.h/g).When x≤0.1,the average discharge plateau potentials are enhanced by about 0.15 V,which makes for higher energy density.     

Development of thermoelectric module based on dense Ca3Co4O9 and Zn0.98Al0.02O legs

Ca₃Co₄O₉ (p-type) and Zn_0.98Al_0.02O (n-type) pellets were prepared by conventional sintering (CS) and Spark Plasma sintering (SPS) starting from the oxides. The best p-type sample was SPS Ca₃Co₄O₉ obtained from pre-sintered pellets, with electrical conductivity σ = 144 S/cm and Seebeck coefficient S = 172 μV/K at 800 °C, while thermal conductivity κ = 2.00 W/m×K and figure of merit ZT = 0.23. The best n-type sample was CS Zn_0.98Al_0.02O showing σ = 83 S/cm and S = ?268 μV/K at 800 °C, while = 5.03 W/m×K and ZT = 0.127. The output power of a module based on SPS Ca₃Co₄O₉ and CS Zn_0.98Al_0.02O legs was 2.26 mW (with T = 500 °C, ΔT = 248 °C).     

A comparative study between the mixed-oxide and high-energy milling planetary method on electrical and microstructural properties for a SnO2-based ceramic system

SnO₂ varistors doped with Co₃O₄, Cr₂O₃, and Sb₂O₅ were prepared separately by two mechanical processes: mixed-oxide and high-energy milling planetary method. A comparison on the electrical and microstructural properties of the samples obtained by both methods was made. The best results on these characteristics were achieved through the milling planetary route, obtaining a nonlinear coefficient of 33 and a breakdown field of 2620 V cm^?1 at a sintering temperature of 1350 °C. The samples synthesized by this last technique showed not only high density values, reaching 90.5% of the theoretical density, but they also exhibited a homogeneous microstructure, which compete with those obtained by chemical methods, with the advantage that the milling planetary method is cheaper, faster and easier than the chemical routes.     

Mechanical behaviors of alumina ceramics doped with rare-earth oxides

The effects of three types of additives Y₂O₃, La₂O₃, and Sm₂O₃ on the sintering and mechanical behaviors of alumina ceramics were investigated. The bending strengths of alumina ceramics with Sm₂O₃ and Y₂O₃ additions were 455 and 439 MPa, respectively, higher than that with La₂O₃ addition. The fracture toughness of the ceramics with Sm₂O₃ and Y₂O₃ were also higher than that with La₂O₃ addition. The fracture mode of rare earth oxides doped alumina ceramics exhibited obvious transgranular fractures as well as intergranular fracture. The results of research show that the improvement of bending strength and fracture toughness of alumina ceramics with rare earth oxides was achieved by refining the grain size and strengthening the grain boundary.     

Investigation of the effect of Al2O3–Y2O3–CaO (AYC) additives on sinterability,microstructure and mechanical properties of SiC matrix composites: A review

Appropriate properties of SiC ceramic such as high hardness, low density, high melting point and high elastic modulus make this material as a favorite candidate for different industrial applications. Although some disadvantages including high sintering temperature, low sinterability, and low fracture toughness have restricted the use of this material, previous studies showed that using Al₂O₃-Y₂O₃ additives plays an effective role in the improvement of sinterability as well as the enhancement of the properties of these composites. Moreover, the addition of CaO results in the acceleration of the formation of molten phase and the improvement of sinterability. In addition, the use of these additives cause the formation of the intermetallic phases of Al₅Y₃O₁₂ (YAG) and CaY₂O₄ and by activating the mechanisms of crack deflection, crack bridging, phase transformation, strengthening the grain boundary and changing the fracture mode from intergranular to transgranular results in improved mechanical properties. This paper attempts to investigate the effect of using Al₂O₃–Y₂O₃–CaO (AYC) additives on sinterability, microstructure, and mechanical properties of SiC matrix composites including the composites reinforced with SiC fibers and SiC matrix nano-composites. Finally, the effect of the post-sintering annealing process under two conditions i.e., with and without applying pressure (pressureless sintering) on microstructure and mechanical properties has been studied.     

Comparison of Oxidation Behavior and Electrical Properties of Doped NiO- and Cr2O3-Forming Alloys for Solid-Oxide, Fuel-Cell Metallic Interconnects

The goal of this paper was to determine if NiO-forming alloys are a viable alternative to Cr₂O₃-forming alloys for solid-oxide fuel-cell (SOFC) metallic interconnects. The oxide-scale growth kinetics and electrical properties of a series of Li- and Y₂O₃-alloyed, NiO-forming Ni-base alloys and La-, Mn-, and Ti-alloyed Fe–18Cr–9W and Fe–25Cr base ferritic Cr₂O₃-forming alloys were evaluated. The addition of Y₂O₃ and Li reduced the NiO scale growth rate and increased its electrical conductivity. The area-specific-resistance (ASR) values were comparable to those of the best (lowest ASR) ferritic alloys examined. Oxidation of the ferritic alloys at 800°C in air and air+10% H₂O (water vapor) indicated that Mn additions resulted in faster oxidation kinetics/thicker oxide scales, but also lower oxide scale ASRs. Relative in-cell performance in model SOFC stacks operated at 850°C indicated a 60–80% reduction in ASR by Ni+Y₂O₃, Ni+Y₂O₃, Li, and Fe–25Cr+La,Mn,Ti interconnects over those made from a baseline, commercial Cr₂O₃-forming alloy. Collectively, these results indicate that NiO-forming alloys show potential for use as metallic interconnects.     

The effect of yttrium oxide on the sintering behavior and hardness of tungsten

In this study, the relative density and hardness of Y₂O₃ dispersed tungsten alloy were investigated as functions of the Y₂O₃ content and sintering temperature. The sintering temperature and the amount of the second phase were varied from 1800 to 2500°C and 0 to 2.0 weight pct, respectively. The relative density of the alloys is higher than that of pure tungsten in the range from 2000 to 2500°C, whereas the density is lower at 1800°C. As the Y₂O₃ content increases, the sintered density increases at a given temperature. The transition temperature (T_tr), where the relative density of the alloys exceeds that of pure tungsten, is reduced with increased Y₂O₃ particle content. In order to examine the effect of the second phase on the mechanical property, the hardness of pure tungsten and the alloys are measured. The hardness is mainly dependent upon the relative density of the alloys, rather than the amount of the second phase and tungsten grain size. The relationship between hardness and density is discussed according to the plasticity theory of porous materials.     

Fabrication of high temperature oxides dispersion strengthened tungsten composites by spark plasma sintering process

The paper describes the fabrication process of high temperature oxides, such as Y₂O₃, HfO₂ and La₂O₃, dispersed tungsten composites by spark plasma sintering. The oxide contents varied from 0 to 5 wt% and sintering was conducted for 3 min at 1700 °C. Among three kinds of oxides, Y₂O₃ is the most efficient element to consolidate W powder. As dispersed up to 5 wt% Y₂O₃ into the matrix, the relative density of the W composite is increased up to nearly 100% of theoretical value. In order to analyze the effect of Y₂O₃ particles on the densification of W powders, the microstructure of W–Y₂O₃ composite is observed using the transmission electron microscopy. By this experiment, it is found that dark phases, which had been known as Y₂O₃ phase, are composed of W, Y and O. Therefore, during sintering, W atoms move through Y₂O₃ phases as well as W grain boundaries, thereby W and Y₂O₃ are soluble, and so sinterability of W is enhanced. The hardness of the composite is increased from 350 to 510 kg/mm² with increasing Y₂O₃ contents since the relative density is increased and the grain size is reduced from 20 to 4 μm. However, in case of HfO₂ and La₂O₃, the hardness of the composites is decreased even though the grain size is reduced because of their lower relative densities.     

Densification and properties of ODS-iron based alloy

Y₂O₃ dispersion strengthened iron-based powders and oxide dispersion strengthened（ODS） alloys were prepared by hydrothermal synthesis and spark plasma sintering（SPS）,respectively.The effects of Y₂O₃,vibratory milling treatment,and Ti element on the microstructure and mechanical properties of the materials were investigated by scanning electron microscope（SEM） and micro-electronic universal tester.The results show that the best mechanical properties are obtained with 1 wt.% Y₂O₃ addition.The size of agglomerated particles can be decreased to a certain degree by vibratory milling treatment.With the addition of Ti element,the tensile strength and hardness of the samples were improved.     

Sol–gel synthesis of Y2O3-doped ZnO thin films varistors and their electrical properties

Y₂O₃-doped ZnO–Bi₂O₃ thin films were fabricated on silicon substrates by sol–gel process and annealed in air at 750 °C for 1 h. Microstructure and electrical properties of ZnO thin films were investigated. XRD analysis shows that all peaks of ZnO thin films are well matched with hexagonal wurtzite structure of ZnO. SEM results present that the ZnO grain size decreases with the increase of dopant concentration, which means that rare earth doped can refine the grain size. The thickness of each layer is uniform and the value of thickness is about 80 nm. The nonlinear V–I characteristics with the leakage current of 0.46 μA, the threshold field of 110 V/mm and the nonlinear coefficient of 3.1 could be achieved when the films contain 0.2% (mole fraction) yttrium ion.