A Study of Sialon Synthesis from Kaolin by Carbothermal Reduction and Simultaneous Nitriding

X-ray diffraction, derivatographic analysis, and the BET method are used to study the processes of synthesis of sialons by carbothermal reduction and simultaneous nitriding of kaolin of the Kyshtymskoe deposit. The synthesis is performed by roasting a mixture of kaolin with colloidal graphite S-1 poured freely or pressed into specimens in a discontinuous-flow furnace with a graphite heater. The influence of the synthesis conditions on the phase composition, the specific surface of the powders, the gas permeability, and the pore size in sialon specimens is established. It is shown that when pressings fabricated from kaolin mixed with graphite S-1 are roasted in nitrogen, the resulting material can have a porosity of 60% with pores < 1 μm in size and a gas permeability of 0.02 – 0.08 μm², which can become a basis for developing a technology of membrane materials.     

低成本大孔陶瓷膜支撑体的制备与表征(英文)

以高岭土和白云石为主要原料,通过反应烧结法制备低成本大孔陶瓷膜支撑体,对制备的支撑体进行了结构和性能表征.结果表明:在高岭土中引入质量分数为20%的白云石,可显著抑制高岭土的高温烧结;加入白云石后制备的支撑体在1 150~1 300℃保温1h后,主晶相为莫来石、堇青石和钙长石,平均孔径和抗弯强度随烧成温度升高而增大,而水通量和孔隙率降低;加入20%白云石并在1 250℃保温1 h制各的大孔支撑体的孔隙率和平均孔径分别为44.6%和4.7μm,抗弯强度和纯净水通量分别达到47.6MPa和10.76m3/(m2·h·bar).     

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m³ m⁻² h⁻¹ bar⁻¹, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.     

Technical properties of compounded kaolin sample from Griva (Macedonia, Greece)

The purpose of this paper is to establish the possible industrial applications of Griva kaolin. This kaolin comes from the alteration of gabbroic rocks from the Griva area, Macedonia, Greece. Mineralogically, the kaolin is composed mainly of kaolinite associated with vermiculite and plagioclases, and traces of quartz, amphiboles, halloysite and smectites. The kaolin grain-size analysis shows that the kaolin is mostly finer than <20 μm with 20% of <2 μm. The amounts of Fe, Mg and Ca for the < 20 μm fraction are higher than those of standard commercial kaolins. The kaolinite is disordered with low crystallinity. Kaolinite crystals do not present typical pseudohexagonal stackings, showing irregular morphology, and vermicular aggregates booklets. The Brookfield viscosity of about 450 cp (at 70% solids 12 rpm), the high brightness (83% on < 20 μm fraction) and the dispersability in water suspensions would favour its use as filler in the paper and paint industries. However, the low crystallinity kaolinite precludes its use as filler in rubber industry. The kaolin shows a liquid limit of about 48% and a plastic limit of 34%. The firing characteristics indicate its possible use as a ceramic raw material for stoneware and sanitary ware products.     

Ceramic membranes from reaction-sintered silicon nitride on nitride and oxide substrates

The gas permeability and pore sizes in membranes and porous materials from reaction-sintered silicon nitride (RSSN) are studied. The substrates for the RSSN membranes are highly porous (80% pores) silicon nitride with pores 50–80 μm in size and alumina-based ceramics with pores about 10 μm in diameter. Silicon powder with a specific surface of 1.4 g/cm² is deposited onto the substrates in the form of an aqueous dispersion by various methods including filtering and sintered in nitrogen at 1300–1400°C. The main difficulty of the process is in preventing the formation of defects at the places where the largest channel pores reach the surface of the substrate. Membranes with 0.5 -1 μm pores can be obtained on both types of substrate after a single deposition.     

Sialon based on natural clay raw materials

Conclusions Conditions were worked out for the synthesis of '-sialon on the basis of natural clay raw materials. It was established that the maximum yield of sialon is obtained from nitriding nonbriquetted mixtures of Novoselitsk kaolin and graphite. It was found that the yield of sialon is increased with a rise in the Al₂O₃SiO₂ ratio in the clay material.We studied the structure of the resulting sialon and also the kinetics of milling. It was shown that at 1450°C there forms 'sialon of variable composition: 0.58 (Al₂O₃·AlN)–0.42 Si₃N₄, and moreover a rise in temperature to 1700–1750°C leads to the displacement of the composition into the region adjacent to the AlN corner in the phase diagram for the Si-Al-O-N system and the appearance of the sialon with the structure of the polytype 15RAlN.Translated from Ogneupory, No. 4, pp. 44–48, April, 1982.     

Size effect on the irradiation performance of coated fuel particles

Outer coatings that were as near alike as possible were applied to two different sizes of inert TRISO particles that were larger than those commonly used to fuel HTGR reactors and these particles were then irradiated in a test reactor to observe the influence of particle size on outer coating failures that resulted from irradiation-induced shrinkage of coatings onto the more stable SiC substrates over which they were applied. Outer coatings of plain pyrocarbon and of Si-alloyed pyrocarbon were used to make up two test pairs of particles with diameters of about 1050 and 1300 μm. For a fast-neutron fluence of 5.5 × 10²⁵ n/m² (E > 29 fJ) at an irradiation temperature of 1125K, failure was about twice as high in the larger 1300 μm particle of each test pair as in the smaller 1050 μm particle (16 vs 8%), with each of the coating types having roughly the same behavior. This observed size effect is somewhat greater than predicted by volume-dependent Weibull theory, which estimates failure of the larger size particles at 13% when the smaller size particles fail at a rate of 8%. However, experimental uncertainties are sufficient to account for the difference between observed size effects and those predicted by Weibull theory. The conclusion that the irradiation performance of coated particles is size dependent is reinforced by the fact that failure for regular 850 μm fueled particles in the same irradiation test was essentially zero.     

Preparation of silver powders in micron size from used photographic films via leaching-cementation technique

The results of a leaching study of used photographic films containing silver with nitric acid are presented. Effect of reaction time, reaction temperature and acid concentration on silver dissolution efficiency was determined. The results obtained show that leaching of about 98% of silver is achieved using − 2 mm spent (used) photographic film size at a reaction temperature of 90 °C for 50 min reaction time with 6% nitric acid concentration. Different solid:liquid ratios of 1:10 to 1:50 g/ml were also studied. Cementation study indicates that practically 100% of silver is deposited with addition of Zn metal powder. SEM photomicrographs show that sphere particles of about 1 μm diameter were obtained.     

Natural rectorite mineral: A promising substitute of kaolin for in‐situ synthesis of fluid catalytic cracking catalysts

Fabrication of high‐performance fluid catalytic cracking (FCC) catalysts is suffering from the shortage of high‐quality kaolin that has long been used as matrix or starting material for synthesizing FCC catalysts. This work aimed at exploring the potential of rectorite, a natural aluminosilicate mineral, to substitute kaolin for preparing FCC catalysts through in‐situ synthesis technique. The physicochemical properties of a rectorite mineral, including its chemical composition, structure, thermal behavior, and chemical reactivity, were systemically investigated and compared with those of commercial kaolin. The results showed that the rectorite mineral suitably treated could substitute kaolin for synthesizing FCC catalysts. Moreover, we had shown that a hydrothermally stable ZSM‐5/rectorite composite in which ZSM‐5 crystals of ca. 2 μm in size were overgrown on preformed rectorite substrate could be synthesized using the rectorite mineral calcined at 800°C as raw material. When used as FCC additive, the obtained ZSM‐5/rectorite composite demonstrated enhanced light olefin (ethylene and propylene) yields. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Technological characterization of kaolin: Study of the case of the Borborema–Seridó region (Brazil)

The technological characterization of kaolin from the Borborema–Seridó region has shown that it is predominantly kaolinitic, pseudoplastic and thixotropic; approximately 50% of its particles size is less than 2 μm. By using electronic paramagnetic resonance (EPR) and Mössbauer spectroscopy (ME), it was observed that Fe²⁺ and Fe³⁺ substitute Al³⁺ in octahedral sites of kaolinite. After magnetic separation followed by chemical bleaching, kaolin reached a brightness index of 87.72% ISO and an opacity index of 85.58% ISO, indicating that it can be used mainly as a filler and a coating in the paper industry, paint and pottery, among other industries.     

高岭土原位合成高硅铝比小晶粒NaY分子筛

以不同温度焙烧的苏州高岭土为原料,采用原位晶化法合成高硅铝比小晶粒NaY分子筛,考察晶种胶添加量、陈化温度、晶化温度和m(高土)∶m(偏土)对晶化过程和产物性质的影响。结果表明,m(高土)∶m(偏土)=1时,在晶种胶添加质量分数15%、陈化温度90℃和晶化温度100℃条件下,以普通高岭土为原料原位合成NaY样品的相对结晶度约为80%,骨架硅铝比(SiO2与Al2O3物质的量比)为6.4,平均粒径约500nm。调整原料中m(高土)∶m(偏土)可以控制原位晶化样品中的分子筛含量。以细化高岭土为原料合成的NaY分子筛(平均约445nm)粒径小于普通高岭土合成的样品。骨架硅铝比高于6.0的原位晶化样品的骨架坍塌温度高于950℃,具有很高的结构稳定性。     

Inverting High Spectral Resolution Aerosol Optical Depth to Determine the Size Distribution of Atmospheric Aerosol

Measurements of aerosol optical depth are presented in the spectral interval 0.40–1.10 μm, which includes the most transparent region of the solar spectrum and the near infrared. The measurements were obtained by a grating spectrometer with a resolution ≈ 0.5 nm during the 1994 summer season at a mountain site (about 850 m above sea level) in the South of Italy. Spectral regions free from gas absorption features have been singled out and used to retrieve the aerosol columnar size distribution. Inversions have been performed by using the Phillips-Twomey inversion method along with a χ² criterion which allows one to choose a suitable value for the regularization parameter. The result of the inversions are presented in the particle radius range 0.1 ÷ 3 μm and indicate the presence of a bimodal aerosol with the second mode radius at about 1.0 μm undergoing transformations which are well correlated with relative humidity.     

Cavitation erosion of structural ceramics

Various aluminas, zirconias, silicon carbides, silicon carbide-boron carbide composites, boron carbide and a sialon were eroded for up to 30 h in distilled water at 18°C using an ultrasonic vibratory facility operating at 20 kHz with a peak-to-peak amplitude of 50 μm, and the progress of erosion was followed by using weight loss and microscopical techniques. The measured incubation period (weight loss less than 0·1 mg) varied from 15 min for reaction-bonded silicon carbide to 150 min for 99·9% alumina, and the maximum erosion rate varied from 0·11 mg/h for sialon to 8·25 mg/h for 97·5% alumina. Overall, sialon had the highest resistance to cavitation erosion. Some results depended critically on the surface finish. Microscopical details are presented, and their relationships to the erosion damage are discussed.     

A note on particle size analyses of kaolins and clays

Based on paradigmatic experimental results for one selected type of kaolin the systematic differences in the measurement of particle size distributions of kaolins and clays by the sedimentation method and by the low angle laser light scattering (LALLS) method are studied. A theoretically sound shape transformation procedure for sedimentation results is proposed, based on a physically justified modification of the Stokes law, which takes into account the oblate (plate-like) shape of kaolin and clay particles. It is found that, with realistic estimates of the shape factor (aspect ratio), the corrected sedimentation data come very close to the light scattering data. This indicates at the same time a way to extract shape information from the comparison of two independent size measurements. Scanning electron microscopy of the 2-μm-undersize sedimentation fraction shows kaolin particles with a disc diameter of 3–5 μm and is thus in full agreement with the interpretation of the size measure as an “equivalent disc diameter”.     

La—Ni system porous anode in an alkaline fuel cell

La—Ni system alloys in fine powder form were used to make the hydrogen electrode for an alkaline fuel cell. LaNi₅ electrodes were found to have the highest electrode performance among the La—Ni system electrodes. Using LaNi₅ powder of mean particle size 3.4 μm for the electrode, the limiting current density was 180 mA/cm². However mixing the same size powder (mean particle size 3.4 μm) with powder of mean particle size 40.5 μm in a ratio of 4:1, the limiting current density of the new electrode was 250 mA/cm². The electrode was found to have both relatively fine pores in the range 0.01–0.2 μm and relatively large pores in the ranges 0.5–1 μm and 4–5 μm. Thus, it was found that not only relatively fine pores but relatively large pores had an important effect upon the LaNi₅ electrode characteristics.     

Combustion of Boron-Based Slurries in a Ramburner

In a combustion chamber equipped with windows, boron and boron alloy slurries were burnt. The fraction of the solids in these mixtures was 25%. For the quantitive analysis of the reaction products, three spectrometer systems were used: an optical multichannel analyzer for the range 0.2 μm to 0.9 μm, an acusto-optical tunable IR-filter for the range 1.2 μm - 2.4 μm, and rotating IR-filters covering the range from 2.5 μm to 14.5 μm. The determination of the gas-phase temperature of the exhaust plume was obtained by evaluation of the OH rotation bands. The particle temperature was measured with a fast two-color pyrometer. During the burning process of boron with an average particle size of 2 μm, only a weak background radiation was detected. Mg and AlB₂, produced a greater amount of particles in the exhaust due to their particle size of about 50 μm. The spectra of the exhaust plume show typical bands for the particular metals and the combustion products of the JET Al fuel. Particle size distribution, surface characteristics, and temperature are important parameters for the ignition and combustion of solids. Also, the flow field inside the combustion chamber influences the burning behaviour of the solid particles. Therefore the slurries were injected against the main flow direction to produce high turbulence.     

Effect of crystalline structure on the cell morphology and mechanical properties of polypropylene foams fabricated by core-back foam injection molding

Foam injection molding (FIM) is an advanced technology for preparing lightweight plastic foams, but its inferior mechanical performance remains a challenge. In this study, microcellular injection-molded β-polypropylene (β-PP) foams with high ductility were successfully prepared by combing the β-nucleating agent with controllable temperature field. Foaming results showed that the microcellular β-PP foams exhibiting a cell size of about 8 μm and cell density over 10⁸ cells/cm³ were prepared with a crystalline diameter approximately 5 μm, while PP foams had a rather large cell size approximately 150 μm and low cell density of 10⁵ cells/cm³ with 30 μm crystalline size. As a result, this significant improvement in cell structure as well as the crystalline size lead to a significant increment of 86% for the ductility of β-PP foams. This work offers a facile strategy to prepare injection-molded foams with desirable mechanical properties for their wide range of applications, such as automotive construction and consumer electronics.     

Optimization of the microstructure of porous composite cathodes in solid oxide fuel cells

A comprehensive micromodel to predict the electrochemical performance of porous composite LSM‐YSZ cathodes in solid oxide fuel cells (SOFCs) is developed. The random packing sphere model is used to estimate the cathode microstructural properties required for the micromodel. The micromodel developed takes into account the complex interdependency among the mass transport, electron and ion transports, and the electrochemical reaction, and can be used for optimization of the microstructure of porous LSM‐YSZ composite cathodes. It is shown that the electrochemical performance of these cathodes depends on the microstructural variables of the cathode porosity, thickness, particle size ratio, and size and volume fraction of LSM particles. The effect of these microstructural variables on the cathode total resistance, as the objective function to achieve the optimum microstructure for the cathode, is studied through computer simulation. The results indicated that for a LSM‐YSZ cathode operated at the average temperature of 1073.15 K, bulk oxygen partial pressure of 0.21 atm, and total current density of 5000 Am^?2, and constrained to the minimum value of 1 μm for the size of LSM particles and 0.25 for the cathode porosity, the optimum microstructure is obtained at the particle size ratio of unity, LSM particle size of 1 μm and volume fraction of 0.413, porosity of 0.25, and thickness of 60 μm. The cathode total resistance corresponding to the cathode optimized is estimated to be 0.291 Ω cm². © 2011 American Institute of Chemical Engineers AIChE J, 2012     

轮胎半焦制备碳化硅晶须的研究

以轮胎半焦和石英砂为原料,采用碳热还原法制备出碳化硅晶须。利用X射线衍射仪(XRD)和扫描电镜(SEM)对制得的产物进行物相组成和形貌分析,探究反应温度(1 300~1 500 ℃)、反应时间(120~300 min)、升温程序以及半焦粒度对合成碳化硅晶须的影响规律。结果表明:温度控制在1 350 ℃左右,且采用先升至1 500 ℃成核再降至1 350 ℃保温生长的加热方式时更有利晶须生长;原料粒度能够同时对碳化硅的生成和晶须的生长产生影响;随着半焦粒度的减小,碳化硅晶须的产量和质量均呈先升高后下降的趋势。在轮胎半焦粒度100~120目(150~120 μm)、反应温度1 350 ℃、反应时间240 min的最佳条件下,碳化硅产率为95.36%,制得的晶须直径为50~120 nm,长度为50~80 μm且分布均匀。     

Extrusion of highly porous silicon nitride ceramics with bimodal pore structure and improved gas permeability

Highly porous Si₃N₄ ceramics with bimodal pore structure were prepared by the extrusion processing with petroleum coke of 30 μm as pore‐maker. The microstructure, mechanical strength, and gas permeability were investigated. The microstructure with petroleum coke contained not only numerous fine pores by interlocking the high aspect ratio β‐Si₃N₄ grains, but also some large pores of 15‐25 μm left by the burnout of petroleum coke. The resultant samples obtained an improved gas permeability of 1.2 × 10^?12 m², which is approximately two times that of samples without petroleum coke addition. Furthermore, the mechanical strength is still superior even at a porosity of 67% in comparison with the other porous ceramics used in the current diesel particulate filter.