Statistical evaluation of the mechanical properties of partially oxygenated YBCO,oxygenated YBCO and oxygenated YBCO(Ag) single grains

It is of great significance to enhance the mechanical properties in single grain YBCO bulk superconductors, as mechanical failure limits the suitability of these materials for high-field applications. The influence of the microstructure on the mechanical properties of YBCO bulk materials was studied in 11 partially oxygenated YBCO, 11 oxygenated YBCO, and 10 oxygenated YBCO(Ag) melt-processed bulk single grains in a statistically significant way using Brazilian testing. Surprisingly, the oxygenation cracks evolving during the tetragonal to orthorhombic phase transition do not influence the average tensile strength of YBCO bulk single grains, with compaction cracks and pores as the main microstructural flaws responsible for mechanical failure. The mechanical properties of bulk YBCO single grains even improve following oxygenation, as indicated by an increase in Weibull modulus. An addition of 10 wt% Ag₂O to YBCO increases the average tensile strength and the Weibull modulus further, making the material mechanically more robust. However, an overlap in the measured tensile strengths of oxygenated YBCO and oxygenated YBCO(Ag) bulk materials reveals that not all silver-containing samples have better mechanical properties compared to oxygenated YBCO bulk samples.     

Electrode Effects on Microstructure Formation During FLASH Sintering of Yttrium‐Stabilized Zirconia

Systematic microstructural statistics for 3 mol% yttria‐stabilized zirconia synthesized by both conventional sintering and flash sintering with AC and DC current were obtained. Within the gage section, flash sintered microstructures were indistinguishable from those synthesized by conventional sintering procedures. With both techniques, full densification was obtained. However, from both AC and DC flash sintered specimens, heterogeneous grain size distributions and residual porosity were observed in the proximity of the electrodes. After DC sintering, an almost 400 times increased average grain size was observed near cathode compared to the gage section, unlike areas close to the anode. Concepts of Joule heating alone were not sufficient to explain the experimental observations. Instead, the activation energy for grain growth close to the cathode is lowered considerably during flash sintering, hence suggesting that electrode effects can cause significant heterogeneities in microstructure evolution during flash sintering. Microstructural characterization further indicated that microfracturing during green‐pressing and variations in contact resistance between the electrodes and the ceramic may also contribute to grain size gradients and hence local variations of physical properties.     

Preparation of Dense MgB2 Bulk Superconductors by Spark Plasma Sintering

Fully dense MgB₂ bulk specimens (∼higher than 99% dense) were prepared using spark plasma sintering (SPS) at 1250°C for 15 min. Microstructure analyses revealed that faceted MgO particles of ∼8% volume fraction were dispersed in the MgB₂ matrix. A sharp superconducting transition with an onset temperature of 38.5 K was confirmed by both magnetization and resistivity measurements.     

Reactive Sintering and Retrograde Densification of Bulk Bismuth-Based Superconductors

Densification was observed when bulk ceramic superconductors with a composition of Bi_1.8Pb_0.4Sr₂Ca₂Cu₃O_10+δ were sintered for short times (<4 h) near the incongruent melting temperature. The initial shrinkage of the powder compacts was correlated with the existence of a transient liquid phase. Retrograde densification, which is a decrease in density, occurred for longer sintering times and was attributed to changes in pore structure. The results provide direction for future processing of bulk bismuth-based superconductors and powder-in-tube tapes for the fabrication of long-length wire.     

On the grain size proportionality constants calculated in M.I. Mendelson's “Average Grain Size in Polycrystalline Ceramics”

In 1969, M.I. Mendelson published a paper in the Journal of the American Ceramic Society that introduced a proportionality constant of 1.558 for estimating the average 3D grain size from the mean 2D lineal intercept under the assumption of lognormally distributed grains. Recent simulations by the authors revealed that the lognormal parameterization in the original work actually calculates the median grain size instead of the mean. The relationship between the mean caliper diameter and mean lineal intercept was found to be 1.60 when using common parameterizations. In addition, it is demonstrated through simulations that the correct proportionality constant can range from 1.776 to below unity depending on a material's grain size dispersion, such that 1.60 should only be used as a crude approximation.     

The temperature dependence of the relative grain‐boundary energy of yttria‐doped alumina

Atomic force microscopy was used to measure the dimensions of grain‐boundary thermal grooves on the surfaces of Al₂O₃, 100 ppm Y‐doped Al₂O₃, and 500 ppm Y‐doped Al₂O₃ ceramics heated at temperatures between 1350°C and 1650°C. The measurements were used to estimate the relative grain‐boundary energies as a function of temperature. The relative grain‐boundary energies of Al₂O₃ decrease slightly with increased temperature. When the doped samples were heated, there was an overall increase in the grain‐boundary energy, attributed to a reduction in the grain boundary excess at higher temperature. The overall trend of increasing grain‐boundary energy was interrupted by abrupt reductions in grain‐boundary energy between 1450°C and 1550°C. In the same temperature range, there is an abrupt increase in the grain‐boundary mobility that is associated with a complexion transition. When the 100 ppm Y‐doped sample was cooled, there was a corresponding increase in the relative grain‐boundary energy at the same complexion transition temperature, indicating that the transition is reversible.     

活性磷酸钙微观形貌对SAN悬浮聚合的影响

活性磷酸钙作为悬浮聚合的分散剂,在苯乙烯-丙烯腈共聚物(SAN)聚合体系中对聚合产物形貌粒径有重要影响。对活性磷酸钙(HAP)进行扫描电镜观察,发现不同来源的HAP表面形貌有较大差异。将这几种活性磷酸钙作为分散剂用于苯乙烯-丙烯腈共聚物(SAN)的悬浮聚合。对比聚合产物的粒子形貌及粒径分布,发现当HAP微观形貌为均匀竹叶形或棒状,且堆积松散时得到的聚合物粒子外观规整,粒径分布均匀。     

Texture Development and Microstructure Evolution in Liquid-Phase-Sintered α-Alumina Ceramics Prepared by Templated Grain Growth

Texture development in alumina that contains calcia and silica and has been templated with platelet-shaped α-Al₂O₃ particles has been evaluated. The texture fraction is shown to be related directly to template growth. Texture quality is controlled by the template concentration, decreasing at template concentrations of >10%, as a result of template–template interactions during tape casting. Almost fully textured alumina has been obtained at template concentrations of ≥20%. The growth of template grains is much more rapid in the radial direction and is shown to be inversely related to the thickness of the grain-boundary liquid. The activation energy for growth (376 kJ/mol) and the inverse relation with the grain-boundary thickness indicate that template growth in the radial direction is controlled by Al³⁺ diffusion.     

Microstructure in Silicon Nitride Containing β-Phase Seeding: III, Grain Growth and Coalescence

The mechanical properties of Si₃N₄ materials depend mainly on the microstructure, which originates during the densification process. The microscopic evidence indicates that β-Si₃N₄ seeds incorporated in the starting powders play an important role in microstructural development, especially in the heterogeneous grain growth of β-Si₃N₄ grains during sintering. The growth of β-grains is initiated from the β-seeds, resulting in a core/shell microstructure. The presence of Moiré fringes and dislocations is attributed to misfit strain and compositional differences between the core and the shell. Coalescence can occur at the final stage of sintering.     

Computer Simulation of Final-Stage Sintering: I, Model Kinetics, and Microstructure

A Monte Carlo model for simulating final-stage sintering has been developed. This model incorporates realistic microstructural features (grains and pores), variable surface difusivity, grain-boundary diffusivity, and grain-boundary mobility. A preliminary study of a periodic array of pores has shown that the simulation procedure accurately reproduces theoretically predicted sintering kinetics under the restricted set of assumptions. Studies on more realistic final-stage sintering microstructure show that the evolution observed in the simulation closely resembles microstructures of real sintered materials over a wide range of diffusivity, initial porosity, and initial pore sizes. Pore shrinkage, grain growth, pore breakaway, and reattachment have all been observed. The porosity decreases monotonically with sintering time and scales with the initial porosity and diffusivity along the grain boundary. Deviations from equilibrium pore shapes under slow surface diffusion or fast grain-boundary diffusion conditions yield slower than expected sintering rates.     

Control of particle size and carboxyl group distribution in soap‐free emulsion copolymerization of methyl methacrylate–ethyl acrylate–acrylic acid

Polymer microspheres with narrow size distribution and with carboxyl groups on their surfaces were synthesized by soap‐free emulsion polymerization of methyl methacrylate (MMA), ethyl acrylate (EA), and acrylic acid (AA), and the distribution of –COOH in the latex was determined by conductometric titration. Effects of ingredients on polymerization, latex particle size (D_p), and its distribution, and the distribution of –COOH were investigated. Results showed that monomer conversion and the amount of embedded –COOH (E_a) decreased, and D_p increased with increasing amounts of NH₄HCO₃. The amounts of surface –COOH (S_a) and water –COOH (F_a) and the number of –COOH on each square centimeter of the particles' surface (S_d) increased with increasing amounts of NH₄HCO₃ and AA. With the increase of initiator (APS) and AA, D_p deceased. E_a increased with the increase of AA. F_a increased and then remained constant, and S_d decreased with the increase of initiator. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 433–438, 2004     

燃煤电厂脱硫粉煤灰单颗粒研究及元素组成

研究电厂脱硫粉煤灰以粒径在3～45μm的颗粒为主,数量占80%以上,并多是表面光滑的球形微珠,主要的矿物成分是莫来石和石英,它的排放因子达到了2.45 kg/t。脱硫粉煤灰的元素组成选取了Mn、Zn、Cr、Cu、Pb、Ni、Cd七种元素,其中Mn在脱硫粉煤灰中的含量最高,但Mn元素的相对富集因子则小于Zn元素的相对富集因子,说明元素含量高其相对富集因子不一定大,还需要考虑电厂燃烧的煤种中的元素含量,从而来确定元素的富集行为。     

Particle size and distribution of biodegradable poly‐D,L‐lactide‐co‐poly(ethylene glycol) block polymer nanoparticles prepared by nanoprecipitation

A biodegradable block copolymer, poly‐D ,L ‐lactide (PLA)‐co‐poly(ethylene glycol) (PEG), was prepared by the ring‐opening polymerization of lactide with stannous caprylate [Sn(Oct₂)] as a catalyst; then, the PLA–PEG copolymer was made into nanoparticles by nanoprecipitation under different conditions. The average molecular weight and structure of PLA–PEG were detected by ¹H‐NMR and gel permeation chromatography. The sizes and distributions of the nanoparticles were investigated with a laser particle‐size analyzer. The morphologies of the nanoparticles were examined by transmission electron microscopy. The effects of the solvent–nonsolvent system, operation conditions, and dosage of span‐80 on the sizes and distributions of the nanoparticles are discussed. The results show that acetone–water was a suitable solvent–nonsolvent system and the volume ratio of the nonsolvent phase to the solvent phase (O/W) (v/v), the concentration of PLA–PEG in the solvent phase, and the dosage of span‐80 had important effects on the particle sizes and distributions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1884–1890, 2005     

Effect of particle size on the properties of Mg(OH)2‐filled rubber composites

A novel nanomagnesium hydroxide powder and three kinds of micro‐Mg(OH)₂, with different particle sizes, were chosen as fillers and mixed with ethylene–propylene–diene monomer rubber (EPDM) to form a series of composites by a traditional rubber‐processing technique. The results showed that the mechanical properties of composites improved with decreasing particle size. The nanocomposites were far stronger than the microcomposites, which also supported the view that rubber reinforcement requires nanoreinforcement. The effect of particle size on the fire resistance of composites was investigated by cone calorimetry and limiting oxygen index analysis, which showed that the particle size of powder had an impact on the fire resistance of composites. For the composites filled with untreated powder, the peak value of heat release rate decreased and T_ign increased with decreasing particle size. In conclusion, the fire resistance of nanocomposites was better than that of microcomposites. Surface modification of particles sometimes substantially improved the mechanical properties of nanocomposites, but had no effect on either the mechanical properties of microcomposites or the fire resistance of nanocomposites and flame retardance. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 2341–2346, 2004     

添加剂的粒度及加入方式对RBSN显微结构及性能影响

本文研究了添加剂（Ｙ２Ｏ３、Ｌａ２Ｏ３）的颗粒度和加入方式对反应烧结氮化硅显微结构和性能的影响。结果表明：以细颗粒及液态加入添加剂．改善了显微结构．可显著提高其性能。     

Synthesis of ZSM‐5 by hydrothermal method with pre‐mixing in a stirred‐tank reactor

This work proposed a synthesis route of ZSM‐5 via the hydrothermal method with premixing in a stirred tank reactor (STR). Effects of various operating conditions, including pre‐mixing time, molar ratio of SiO₂/Al₂O₃, TPAOH (organic template agents) concentration, NaCl (alkali metal cations) concentration, crystallization temperature, and crystallization reaction time, on the average particle size (PS) and particle size distribution (PSD) were investigated. It was found that the pre‐mixing time in the STR significantly affect the formation of proto‐nuclei in premixing process and crystal growth in hydrothermal reaction process, and consequently influence the PS and PSD of the prepared ZSM‐5. ZSM‐5 with good thermal stability, a PS of 380 nm, PSD of 0.17–0.9 µm, pore diameter of 2.31 nm, pore volume of 0.19 cm³ · g^?1 and specific surface area of 337.25 m² · g^?1 were obtained under the optimal conditions of a crystallization reaction time of 24 h, a crystallization temperature of 130 °C, a molar ratio of SiO₂/Al₂O₃ of 200, a TPAOH concentration of 3.5 mol · L^?1, NaCl concentration of 0.3 mol · L^?1, and a pre‐mixing time of 5 h. This work indicated that the operating conditions including premixing time have a significant effect on its PS and PSD.     

Electrospray modeling of highly viscous and non‐Newtonian liquids

The electrohydrodynamic spraying of highly viscous and non‐Newtonian aqueous solutions of sodium alginate were experimentally modeled with high direct‐current electric fields. A prototype electrospray setup comprising a nozzle connected to a high‐voltage counter electrode connected to earth and a curing facility to solidify the droplets was used. The main aim was initially set to extend the knowledge of the electrospray to highly viscous liquids, where shear thinning was the main rheological feature of fluid flow through the nozzle of the spray system. To model the process, the effects on the size of beads of the electric field strength, nozzle diameter, flow rate, and the material properties of density, viscosity, surface tension, and electrical conductivity were characterized. The size distribution of the beads was obtained after the droplets were cured in a calcium chloride solution with an image analyzer system. The rheological study, carried out on different concentrations of alginate solution (i.e., 1–3 w/v %), showed a significant reduction in the viscosity as a function of the shear rate. Considering the shear‐thinning behavior of the solutions, in the modeling we applied the viscosity at the operational shear rate in the nozzle. Four dimensionless groups were introduced to obtain the relationship between the dimensionless group representing diameter and the other groups in the dripping and jet modes with statistical analysis of the experimental data. The proposed equations correlated the size of beads within ±10% deviations as compared to the experimental results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis of Fine-Grained Polycrystalline Diamond Compact and Its Microstructure

Sintering of a fine-grained polycrystalline diamond compact with grains less than 1 μm in size was successfully carried out by making a laminate on a WC/Co powder compact under sintering conditions of 5.8 GPa and 1430° to 1480°C for 30 min, in which small amounts of poly(ethylene glycol) and fine powder of cubic boron nitride were added to the starting diamond powder. The former played the role of preventing agglomeration of the diamond powder and the latter of suppressing abnormal grain growth during sintering. Microstructural observation of the polycrystalline diamond showed that in the regions near the WC/Co layer a comparatively large amount of Co metal was present between diamond grains, but in other regions the amount of Co decreased, and the diamond grains were seen to be bonded strongly. The Vickers hardness of the polycrystalline diamond was 55 ± 5 GPa with 19.6 N load.     

Highly thermally conductive room‐temperature‐vulcanized silicone rubber and silicone grease

In this study, the effects of thermally conductive filler type (α‐Al₂O₃, SiC), volume fraction of the filler, and filler particle size distribution on the thermal conductivity and viscosity of room‐temperature‐vulcanized (RTV) silicone rubber and silicone grease were investigated. We were interested to find that silicone grease (or the RTV silicone rubber) had a maximum thermal conductivity (～1.48 W/mK) and a minimum viscosity (～3.4 × 10⁴ mPa s), with a definite total volume fraction of the filler (0.55) when the distribution of filler sizes (the number ratio of two different particles sizes, i.e., 0.8 and 6 μm) was 600–700. We were able to increase the thermal conductivity of the RTV silicone rubber and silicone grease beyond 2 W/mK by increasing the total volume fraction of the filler with adequate filler size distributions. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2397–2399, 2003     

连续本体法聚合ABS树脂的研究进展

概述了丙烯腈-丁二烯一苯乙烯（ABS）的技术概况及ABS本体聚合工艺特点和关键技术,重点阐述了连续本体ABS的研究进展,并对国内本体ABS的发展进行了展望。