Effects of a slurry's particle dispersion and flocculation state on spray-dried granule morphology

In this study, we investigated the relationship between slurry and spray-dried granule properties. Various slurries were prepared using different sizes of Mn-Zn ferrite particles and evaluated using flow curve and adsorbed dispersant amount measurements and sedimentation tests. Spray-dried granules were evaluated using morphology observations, density and strength measurements, and compression tests. The results demonstrated that a slurry with a relatively high particle dispersion could produce a solid granule with a relatively low density during spray-drying. The results further suggested that the change in a slurry's particle dispersion and flocculation state during drying strongly influenced the granule morphology.     

Effects of particle size distribution and sintering temperature on properties of alumina mold material prepared by stereolithography

Alumina mold materials prepared by stereolithography usually have considerable sintering shrinkage, and their properties related to casting have been rarely studied. In this study, alumina molds materials were prepared by stereolithography, and the effects of particle size distribution and sintering temperature on the properties of the materials were investigated. Results show that the viscosity of the slurries decreases as the fraction of fine powder increases, and the particle size distribution affects the curing behaviors slightly. Sintering shrinkage increases as the fraction of fine powder or the sintering temperature increases. Although lower sintering shrinkage can be achieved by sintering at 1350 °C or 1450 °C, the mold materials sintered at lower temperatures would continue to shrink under the service temperature of 1550 °C, and thus 1550 °C is determined as the optimal sintering temperature. As the fraction of fine powder increases, the creep resistance first increases and then decreases, and specimens prepared with 0.1 fraction of fine powder exhibit the best creep resistance with the droop distance of 4.44 ± 0.45 mm. Specimens prepared with 0.1 fraction of fine powder and sintered at 1550 °C exhibit linear shrinkage of 6.36% along the X/Y direction and 11.39% along the Z direction, and have a flexural strength of 78.15 ± 3.50 MPa and porosity of 30.12 ± 0.08%. The resulting material possesses relatively low sintering shrinkage, proper mechanical strength, porosity and high-temperature properties that meet the requirements for casting purposes.     

The effect of alumina-based sintering aid on the microstructure,selected mechanical properties,and coefficient of friction of Cf/SiC composite prepared via spark plasma sintering (SPS) method

C_f-SiC air brake discs are being developed due to their high-temperature oxidation resistance compared to conventional C_f/C discs. The C_f-SiC air brake discs should have a coefficient of friction (COF) close to 0.4, a low wear rate, a density higher than 95% of the theoretical density, and flexural strength of more than 200 MPa. To reach the properties of C_f-SiC composite to the required characteristics of the air brake disc, different amounts of alumina-based sintering aid were used. For this purpose, first silicon carbide nanoparticles, sintering aids Al₂O₃–MgO, MgAl₂O₄, Al₂O₃–Y₂O₃, Al₂O₃–SiO₂–MgO, and carbon fiber (20 wt%) with a 5-mm length were prepared. Next, the final composite bulk was created via the SPS method at 1900 °C under a pressure of 50 MPa. The density of the sample sintered with the Al₂O₃–SiO₂–MgO sintering aid was higher than that of other sintering aids. The density value was obtained at 98% and 100% at 8 wt% and 4 wt% respectively. It was also found that the use of 4 wt% of Al₂O₃–SiO₂–MgO offered better mechanical properties compared to 8 wt%, due to the absence of Al₈Si₄O₂₀ phase at 4 wt%. The examination of mechanical properties showed that the hardness (3564 Vickers) and flexural strength (479 MPa) of the sample with the Al₂O₃–SiO₂–MgO sintering aid were higher than those of other sintering aids. The samples with the Al₂O₃–SiO₂–MgO sintering aid with 4 wt% revealed a COF of 0.41, showing the closest feature to the desired indices of aircraft brake discs.     

Effects of zinc nitrate as a sintering aid on the electrochemical characteristics of Sr0.92Y0.08TiO3–δ and Sr0.92Y0.08Ti0.6Fe0.4O3–δ anodes

Although Sr_0.92Y_0.08TiO_3–δ (SYT) and Sr_0.92Y_0.08Ti_0.6Fe_0.4O_3–δ (SYTF) have been widely considered as promising materials for solid oxide fuel cell anodes, the poor densification restricts their commercial applications. As a sintering aid, zinc nitrate successfully stimulates the sintering process and improves densification. An investigation of linear shrinkage and scanning electron microscopy images indicated that the sinterability of SYT and SYTF materials was effectively improved by impregnating the green body with 5 mol% of Zn. It was found that the modification with zinc lowered the activation energy of the electrical conduction process and significantly improved the electrical conductivities of SYT and SYTF at all atmospheric conditions. The maximum power densities of the SYT and SYTF anodes were improved from 19.7 and 34.1 mW cm^–2 to 56.9 and 92.3 mW cm^–2 in H₂ and from 10.2 and 19.4 mW cm^–2 to 47.5 and 61.9 mW cm^–2 in CH₄, respectively.     

Discussion on magnetic-induced polarization Ampere’s force by in situ observing the special particle growth of alumina during microwave sintering

Internal microstructure evolution during alumina microwave sintering was in situ investigated with synchrotron radiation computed tomography (SR-CT). Two special microstructure evolution phenomena were continuously observed from the experimental images of the sample at different sintering times throughout the entire process of the sintering, which we called “suppressed particle growth” and “particle homogenization”. These two special phenomena were further confirmed by the two curves of “average particle radius” and “particle radius standard deviation” versus sintering time which were directly extracted from the full-field SR-CT results. A polarization Ampere’s force model was proposed to provide a possible explanation for these special phenomena, which introduced the effect of magnetic field on insulating ceramic materials, a topic rarely discussed in previous studies. The polarization Ampere’s force model may explain these two special sintering phenomena observed in the in situ experiment. On one hand, ceramic particles may sustain “Ampere’s force” that pointed toward the particle center according to this model, thereby possibly leading to the special suppressed-particle-growth phenomenon; on the other hand, large particles may sustain a strong force in our model, which may explain the other special phenomenon of particle homogenization. In return, these two special phenomena can also serve as probable experimental evidence of our polarization Ampere’s force model. This study may offer some help for revealing the complex mechanisms during microwave sintering and for preparing materials with expected microstructure and excellent properties.     

转子构型对短纤维分散度及混炼胶力学性能影响的实验研究

通过对三种具有代表性的转子对短纤维在橡胶中分散度及力学性能的影响的实验研究，发现剪切啮合型转子可有效地改善混炼胶中短纤维的分散性，提高胶料的力学性能。     

Experimental and modeling study of the solid state polymerization of poly(ethylene terephthalate) over a wide range of temperatures and particle sizes

The solid state polymerization (SSP) of poly(ethylene terephthalate) was studied experimentally over a wide range of pellet sizes and temperatures. A comprehensive model was developed. It considered polycondensation, degradation and polycondensation of vinyl end groups together with diffusion. The reaction rate constants, diffusivities and the corresponding activation energies were obtained through parameter identification using experimental data. The effects of the reaction temperature and pellet size on the SSP time were also investigated. A decrease in the particle size decreases the concentration of the vinyl end groups and narrows the concentration distribution of end groups inside poly(ethylene terephthalate) particles. A decrease in the size of pellets also favors diffusion. Nevertheless it is preferable that the size of pellets be between 1 and 2 mm because too small pellets bring about difficulties with preparation and handling. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and thermal properties of phase‐change microcapsules incorporated with nano alumina particles in the shell

Novel phase‐change microcapsules with paraffin as core and melamine‐formaldehyde (MF) resin as shell were synthesized through in situ polymerization, in which nano alumina (nano‐Al₂O₃) particles were dispersed in the shell by mixing nano‐Al₂O₃ with MF prepolymer solution using the direct addition method (i.e., adding nano‐Al₂O₃ into the MF prepolymer solution directly) and the predispersed addition method (i.e., predispersing the nano‐Al₂O₃ homogenously in water under the assistance of dispersant and wetting agents before mixing with the MF prepolymer). Scanning electron microscope experiments demonstrated that the predispersed addition method yielded the microcapsules having the better dispersion and less self‐agglomeration of alumina, compared to the direct addition method. Fourier transform infrared spectroscopy, energy dispersive X‐ray spectroscopy, and electron backscatter diffraction imaging confirmed that the nano‐Al₂O₃ particles were successfully incorporated in the shell by the predispersed addition method. The phase change behavior of microcapsules incorporated with different contents (up to 12.7% relative to the microcapsule) of nano‐Al₂O₃ particles in the shell was investigated by differential scanning calorimeter. The results revealed that the encapsulation efficiency for this kind of novel microcapsules was >77% and the incorporation of nano‐Al₂O₃ in the shell affected the phase change temperature. Thermal gravimetric analysis indicated that the addition of nano‐Al₂O₃ improved the thermal stability of microcapsules remarkably. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

间接式移动蓄热器相变材料熔化凝固实验

针对回收工业余热用于分散式用户供热的应用背景,本文设计搭建了间接式移动蓄热器实验系统,选取了赤藻糖醇作为相变材料,对相变材料进行了示差扫描量热法和过冷度的测试分析,获得了相变材料较为准确的蓄热性能参数和过冷度情况。此外,本文还通过对实验数据的整理及分析总结了间接式蓄热器内相变材料在充放热过程中的温度变化情况,在此基础上分析了蓄热材料的熔化和凝固规律。实验结果表明:在相变材料的熔化过程中,蓄热器上部相变材料熔化较快,下部相变材料熔化较慢;在相变材料的凝固过程中,蓄热器下部相变材料凝固较快,上部相变材料凝固较慢。水平方向上材料熔化凝固情况基本一致。通过本文的实验研究,初步掌握了间接式移动蓄热器相变内材料的熔化凝固情况,并由此分析了间接式移动蓄热器优化的方向,为该技术的进一步完善奠定了基础。     

Effect of grain boundary state and grain size on the microstructure and mechanical properties of alumina obtained by SPS: A case of the amorphous layer on particle surface

The work was aimed at the investigation of kinetics of Spark Plasma Sintering (SPS) of the α-Al₂O₃ particles with amorphous surface layers and investigation of the effect of the amorphous layers on the grain growth and on the mechanical properties of alumina. The objects of investigations comprised:(i) submicron α-Al₂O₃ powder, (ii) submicron α-Al₂O₃ powder with the amorphous layers on the particles' surfaces, and (iii) the fine-grained α-Al₂O₃ powder. The submicron powders (i) and (ii) were used to analyze the effect of the amorphous layers on the sintering kinetics. Powders (i) and (iii) were used to analyze the effect of the initial particle sizes on the shrinkage kinetics. The effect of the temperature regime and of the rate (V_h) on the shrinkage kinetics of the submicron and fine alumina powders has been studied. The shrinkage curves were analyzed using the Young–Cutler and Coble models. The sintering kinetics was shown to be determined by the intensity of grain boundary diffusion for the submicron powders and by simultaneous lattice diffusion and grain boundary one for the fine powders. The amorphous layers on the surfaces of the submicron α-Al₂O₃ particles were found to affect the grain boundary migration rate and the Coble equation parameters at the final stages of SPS. The abnormal characteristics of the alumina ceramics sintered from the submicron powder with the amorphous layers on the particles’ surfaces were suggested to originate from the increased concentration of the defects and of the excess free volume at the grain boundaries formed during crystallization of the amorphous layers.     

Effect of the annulus aeration on annulus leakage and particle circulation in a three-phase spout-fluid bed with a draft tube

The distribution of flow between the annulus and the draft tube sections of a liquid spout-fluid bed with a draft tube was analyzed in the presence of air flow through the annulus using a tracer response technique.The responses of the tracer injections prove that part of the flow originating from the annulus liquid inlet leaks into the draft tube when any amount of annulus liquid inlet flow is present and this annulus leakage increases when air flow is introduced into the annulus. This finding extends the recently acquired knowledge about the annulus leakage and cross flow in the entrance region below the draft tube in liquid spout-fluid beds to three phase spout-fluid beds with a draft tube where the annulus is aerated.The experimental system studied consisted of a semi-circular column of 80 mm diameter equipped with a semi-circular draft tube of 20 mm diameter. Liquid and air flows coexisted in the annulus. The liquid velocities were well above the minimum fluidization velocity and the particles were glass beads of 3 mm diameter.Experimentally determined values of the superficial annulus and draft tube liquid velocities from the tracer response analyses together with the experimental draft tube pressure gradients were used as input into Grbavcic et al.'s [Z.B. Grbavcic, R.V. Garic, D.V. Vukovic, Dz.E. Hadzismajlovic, H. Littman III, M.H. Morgan, S.Dj. Jovanovic, Hydrodynamic modeling of vertical liquid-solids flow. Powder Technol. 72, (1992)183-191] variational model for vertical non-accelerating liquid-solids flow to calculate the draft tube voidage and the particle mass flux assuming an air free draft tube. Results indicate that annulus aeration lowers the particle mass flux in this particular column geometry.     

钛系聚酯的性能、可纺性及纤维加工中色相变化研究

采用自制的钛系催化剂合成聚酯,以同一聚合釜合成的常规锑系聚酯为对比样,系统考察钛系聚酯的切片质量指标、纺丝性能、后加工性能、纤维色相方面的变化等。结果表明,相同装置上合成的钛系聚酯的纺丝、后加工性能与常规锑系聚酯未见差异,完全可以满足纤维生产要求;非锑聚酯应用于生产纤维时纤维的成型加工（纺丝、牵伸）等可以改善钛系聚酯切...     

Effect of HCl concentration and reaction time on the change in the crystalline state of TiO2 prepared from aqueous TiCl4 solution by precipitation

TiO₂ powder with 50% by volume or more of brookite phase was obtained by heating aqueous TiCl₄ solution, whose final concentration of HCl after reaction was kept between 4.9 and 6.4 M, at 80 °C for 15 h. Rutile-type TiO₂ was obtained at 5 h of reaction time, a mixture of predominant brookite and rutile at 10–15 h while rutile phase was formed at 25 h. Brookite phase was transformed directly to rutile phase with increase of reaction time but to rutile via anatase phase through heat treatment.