Catalyst Sintering and Reactor Design

The subject of sintering (sometimes termed coarsening) has received considerable attention since the late 1960s, and a review [1] appeared in this journal in 1975. Significant progress has been made since then, in particular, with regard to the mechanisms and kinetics of sintering and measurement techniques. With these advances it is now possible to utilize the understanding gained for the design of reactors undergoing catalyst sintering. In fact, a few attempts have already been made.     

An Arrhenius-Type Viscosity Function to Model Sintering Using the Skorohod–Olevsky Viscous Sintering Model Within a Finite-Element Code

The ease and ability to predict sintering shrinkage and densification with the Skorohod–Olevsky viscous sintering (SOVS) model within a finite-element (FE) code have been improved with the use of an Arrhenius-type viscosity function. The need for a better viscosity function was identified by evaluating SOVS model predictions made using a previously published polynomial viscosity function. Predictions made using the original, polynomial viscosity function do not accurately reflect experimentally observed sintering behavior. To more easily and better predict sintering behavior using FE simulations, a thermally activated viscosity function based on creep theory was used with the SOVS model. In comparison with the polynomial viscosity function, SOVS model predictions made using the Arrhenius-type viscosity function are more representative of experimentally observed viscosity and sintering behavior. Additionally, the effects of changes in heating rate on densification can easily be predicted with the Arrhenius-type viscosity function. Another attribute of the Arrhenius-type viscosity function is that it provides the potential to link different sintering models. For example, the apparent activation energy, Q , for densification used in the construction of the master sintering curve for a low-temperature cofire ceramic dielectric has been used as the apparent activation energy for material flow in the Arrhenius-type viscosity function to predict heating rate-dependent sintering behavior using the SOVS model.     

Sintering of Zinc Oxide

The fundamental processes involved in the sintering of ionic solids are highly complex. Of particular significance are the initial stages of sintering during which surface effects are of prime importance. The sintering of zinc oxide has been studied by the method of changes in surface area at temperatures from 300° to 1000°C. and with varying oxygen pressures (from 10^-5 to 760 mm. of Hg). It has been demonstrated that significant sintering occurs at temperatures far below the Tammann temperature. It also has been shown that under specific conditions a very high defect concentration may exist in the surface of the material and that this is closely related to the phenomenon of sintering. Correlation has been made with an electron-microscope investigation. Detailed X-ray studies on single-crystal specimens of zinc oxide have shown that interstitial zinc constitutes the defects. Activation energies have been deduced for the sintering process which suggest that the rate-controlling step is diffusion in the surface zone in the low-temperature region which changes to lattice or complex diffusion and viscous-flow mechanisms about the Tammann temperature.     

Si3N4—MgAl2O4—Al2O3系材料无压烧结的研究

本文对Ｓｉ３Ｎ４－ＭｇＡｌ２Ｏ４－Ａｌ２Ｏ３系复合材料的无压烧结进行了研究，讨论了Ａｌ２Ｏ３含量对材料性能的影响及烧结工艺对材料性能和显微结构的相互关系。实验表明：两段法烧结可以得到性能良好的Ｓｉ３Ｎ４－ＭｇＡｌ２Ｏ４－Ａｌ２Ｏ３复合材料。     

Microstructural Evolution during the Sintering of Nanostructured Ceramic Oxides

Using small-angle scattering, together with density measurements, X-ray diffraction, nitrogen adsorption, and both transmission and scanning electron microscopy, the microstructural evolution during sintering in nanostructured ceramic oxides has been studied. The availability of samples with sufficient size and uniformity has permitted absolute calibration of the scattering data and made possible determinations of microstructural parameters and surface areas that are statistically representative of the undisturbed nanophase morphologies. A fully quantitative microstructural model has been developed, and the effects of varying processing parameters during powder synthesis and sintering have been assessed. Predictions are made regarding the achievable sintered full-density microstructures.     

Citrate gel processing of oxide superconductors

Citrate gel processing of superconducting oxides of both the La-Ba-Sr-Cu-O and Y-Ba-Cu-O types has been investigated because of the need to produce pure, homogeneous powders of a controlled quality. The production and decomposition of the gels has been studied using TGA, SEM and XRD. The sintering of the powders obtained from the gels has been investigated using dilatometry such that the densification kinetics may be related to initial powder particle size, temperature, time, cooling rate and atmosphere used for sintering. Sintered pellets have been characterised structurally using XRD and TEM. AC magnetic susceptibility measurements have been made on the pellets so that the value and width of the superconducting transition may be related to the conditions of sintering. It has been shown that although the products of low temperature decomposition are finer and sinter more readily, they do not make good superconductors.     

利用中碱废丝制造玻璃马赛克

利用烧结法制得了玻纤废丝马赛克.获得了组成,烧结,退火等工艺参数.试验结果表明,试样具有优良的物理化学性质.     

闪烧技术在固体氧化物燃料电池关键材料制备中的应用

闪烧技术是一种新兴的陶瓷烧结技术,因具有烧结温度低、功耗低、时间短等显著特点而受到世界范围内的广泛关注。自2010年闪烧现象被报道以来,研究人员在闪烧实验装置的搭建、不同陶瓷材料的闪烧现象、过程参数的控制及优化等方面开展了大量工作,闪烧技术得到了飞速的发展。对闪烧技术的过程参数及影响规律进行归纳和总结,重点对近年来闪烧技术在固体氧化物燃料电池领域的应用进展进行了详细介绍。     

Sintering Effects on the Porous Characteristics of Functionally Gradient Ceramic Membrane Structures

A method to drain cast porous ceramics has been conceived and established, where samples were shown to have a functionally gradient cross-section with a continuously increasing mean particle size between the two principal surfaces.Ceramic discs approximately 45 mm in diameter, and 3.3 mm thick were cast by sedimentation. These green bodies were dried prior to sintering. Maximum sintering temperature and the length of the sintering soak time were varied for samples made from suspensions of both 5 and 10 volume percent solids. Mercury porosimetry was used to obtain the porosity and pore size distribution in each sample. Additionally, a number of atomic force microscopy (AFM) measurements were made on some samples in order to correlate bulk porous properties with those on the outside surfaces.The results showed that as the sintering temperature increased, the densification of the bodies proceeded more rapidly. In general, the longer the sintering soak time, the denser the samples became as well. For the samples prepared at the lower temperatures however, the porosity showed less of a soak time dependence. The green body had a clustered and asymmetric microstructure, which contributed to differing degrees of localized densification and coarsening effects depending on the sintering temperature. Densification effects were more pronounced with the samples made from the more concentrated suspenisions.There was an inverse correlation between the bulk and surface pore dimensions, attributable to the different size scales of particles in the two regions. The much finer surface layer particles were able to undergo some amount of densification while surface diffusion sintering mechanisms were primarily at work elsewhere in the structure.     

混凝土淤渣玻璃陶瓷的烧结性能及析晶

以混凝土淤渣为主要原料制备了混凝土玻璃陶瓷,研究了混凝土玻璃陶瓷的烧结析晶特征及烧结工艺性能的影响。结果表明,随着烧结温度的提高和烧结时间的延长,材料烧结程度逐渐增大,但当烧结时间到达一定值后,收缩率逐渐趋向不变。烧结温度超过850℃后,材料体积收缩增长的速率降低,材料烧结的趋势将放缓;在低于850℃烧结时,玻璃中析出晶体的量较少,直到在950℃保温时钙铝黄长石相晶体才大量析出。材料的烧结温度和析晶温度相差100℃左右,这有利于控制烧结和析晶过程。同时以引入不同添加剂对混凝土淤渣玻璃陶瓷密度和颜色的影响也进行了探讨。     

Sintering deformation caused by particle orientation in uniaxially and isostatically pressed alumina compacts

Effect of particle orientation on deformation during sintering is reported for model systems; one made with industrial grade low soda alumina, which has an elongated particle shape, and the other a special alumina with a spherical particle shape. To ensure the homogeneous packing density of particles, compacts were made by uniaxial pressing followed by cold isostatic pressing. The particle orientation was examined with a polarized light microscope and was found to be an important cause of sintering deformation. In a green body, for elongated shape of particles, the particle orientation occurred during uniaxial pressing, causing the anisotropic sintering shrinkage during sintering and thus the sintering deformation. No particle orientation nor shrinkage anisotropy was noted in the system made with the powder of spherical particle shape.     

A novel fabrication route for auxetic polyethylene,part 2: Mechanical properties

Auxetic ultra high molecular weight polyethylene (UHMWPE) has been fabricated by omitting the extrusion stage usually required to form the characteristic nodule‐fibril microstructure of this material. This new route consists of compaction followed by multiple sintering treatments, with the best results using two successive sintering treatments. This article examines the mechanical properties of cylindrical compacts subjected to between one and four successive sintering treatments. The indentation resistance of the double‐sintered material was found to be 2.5 times that of conventionally processed UHMWPE and similar to that found in the extruded form of auxetic polyethylene. The flexural strength and strain to failure increase dramatically with the first sintering treatment and then remain almost constant. This processing route has potential for the production of more complex, useful shapes than can currently be produced since it removes the limitations imposed by the geometry of the barrel required for the extrusion stage. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Microstructural surface analysis of Ni/Cu front contact after peel force test to improve contact adhesion

Ni/Cu two-step plating has been applied to solar cells as a metallization technique since it has low contact resistance after nickel sintering process, and is suitable for fabricating narrow fingers. However, a reliable adhesion of the Ni/Cu contact is still one of the remaining challenges. In this experiment, a peel force test was used to investigate the dependence of varied sintering temperatures on the adhesion of the Ni/Cu front contact. The surface of the bus bars was observed by field emission scanning electron microscope (FE-SEM) to analyse the high and low adhesion regions. When the adhesion result was high, silicon chunks from the substrate were often observed on the nickel layer (ribbon side). Also, the influence of oxidation on the nickel surface during sintering was discussed with the oxygen atomic ratio, which was measured by energy dispersive spectroscopy (EDS). Decent adhesion values up to 1.16 N/mm average with 2.47 N/mm maximum have been achieved with good soldering process.     

Zirconium-diboride silicon-carbide composites: A review

Zirconium diboride (ZrB₂) and silicon carbide (SiC) composites have long been of interest since it was observed that ZrB₂ improved the thermal shock resistance of SiC. However, processing of these materials can be difficult due to high and different sintering temperatures and differences in the thermodynamic stability of each material. ZrB₂–SiC composites have been processed in a variety of ways including hot-pressing, spark-plasma sintering, reactive melt infiltration, pack cementation, chemical vapor deposition, chemical vapor infiltration, stereolithography, direct ink writing, selective laser sintering, electron beam melting, and binder jet additive manufacturing. Each manufacturing method has its own pros and cons. This review serves to summarize more than 60 years of research and provide a coherent resource for the variety of methods and advancements in development of ZrB₂–SiC composites.     

Highly dense 0.3CaCeNbWO8-0.7LaMnO3 composite ceramics fabricated by cold sintering process

The cold sintering process (CSP) has been used for fabricating functional ceramics at a low sintering temperature. In this study, highly dense 0.3CaCeNbWO₈-0.7LaMnO₃ composite ceramics have been successfully fabricated by CSP. The phase structure, microstructure, and electrical properties of composite ceramics have been investigated. The composite ceramic is mainly composed of a tetragonal CaCeNbWO₈ phase with scheelite structure and an orthorhombic LaMnO₃ phase with perovskite structure. The relative density of composite ceramic is 94.5%, and is higher than that of single phase ceramic. The resistivity of composite ceramic exhibits negative temperature coefficient characteristics, with an aging coefficient less than 2%. Such a sintering methodology is of great significance, since it provides a feasible idea for preparing composite ceramics.     

Three-Dimensional Solar Cell Finite-Element Sintering Simulation

The sintering process is ubiquitous in manufacturing, but the design-oriented modeling of sintering has presented considerable challenges. This type of modeling is necessary to be able to predict deformation and thus design appropriate powder compacts so that after they are sintered, the desired dimensions will be achieved. Currently this is done through a costly and time-consuming trial and error process. In our research, an application of the Skorohod–Olevsky viscous sintering constitutive equation in a finite-element (FE) model is developed and used to model solar cell manufacturing. Experimental measurements are used to determine the properties of the solar cell materials, and these are used to calculate the parameters for the FE model. Simulation results are compared with experimental data and analysis has been made to evaluate the adequacy and usefulness of this approach.     

Peculiarities of the formation of planar micro-optic elements on porous glass substrates under the effect of laser radiation followed by sintering

The theoretical and practical approaches to the formation of micro-optic elements (MOE) for the integrated optical communication on substrates made of different materials, including silicate glasses, have been reviewed. The applicability of high-silica porous glasses (PG) fabricated by a homogeneous acid leaching of two-phase alkali borosilicate glasses as basic matrices for laser formation of MOE of different types (microlenses and microchannels) has been discussed. It has been demonstrated that sintering laser-modified PGs at temperatures ensuring the collapse of the pore can be used to stabilize MOE optical properties. The results of choosing the time-temperature mode for sintering PG substrates with a planar MOE-strip waveguide (SW) providing the SW fixation on a completely sintered substrate were presented.     

新型陶瓷原料--绢英岩在生产中的作用机理

绢英岩作为一种新原料上世纪八十年代在国内部分地区开始使用，它属于花岗变斑岩型，原矿呈泛青的白灰色致密块状，有滑腻感。由于绢英岩成分稳定，有害杂质含量低，以及烧结范围宽、烧后白度高等优点，决定了它在陶瓷工业上具有广泛用途。本文从绢英岩的矿物组成、结构特征出发，探讨其在陶瓷生产中的作用机理，以对该原料的广泛使用奠定一定的理论基础。     

Grain-Growth Transition During Sintering of Colloidally Prepared Alumina Powder Compacts

When the grain size in partially sintered compacts of alumina was measured as a function of density, we found that the grain-growth behavior fell into two distinct regions. In the region where the porosity remained interconnected, grain growth was negligible; when the continuous pore network collapsed into isolated pores, grains grew rapidly. The transition in grain-growth behavior was observed at approximately 90% of theoretical density. A simple phenomenological method for obtaining the transition in grain growth is suggested. It is based on the idea that an abrupt increase in grain size should be accompanied by a significant decrease in the rate of sintering since the sintering rate changes as the third or fourth power of the grain size. The method consists of fitting the sintering data to an exponential function. The transition then appears as a change in the time constant for the exponential. The low rate of grain growth in the region where the pores are interconnected contradicts earlier work in the literature where significant grain growth during intermediate-stage sintering has been reported. This difference is explained in terms of the homogeneity of packing of our powder compacts, which were prepared by colloidal processing.     

Electric Fields Obviate Constrained Sintering

The phenomenon of constrained sintering, where large rigid inclusions of alumina have been shown to significantly reduce the rate of sintering of titania [Bordia and Raj (1988) J. Am. Ceram. Soc. 71, 302–310], is shown to subside nearly completely during flash sintering carried out under modest electrical fields. The result is explained by a different mechanism for volumetric and shear deformation under electric fields. It is proposed that vacancy and interstitials generated within the grains migrate to grain boundaries and pores to produce both volumetric and shear strain at equal rates, since, in this way, the diffusion distance for both modes of deformation becomes the same. In conventional sintering, where transport occurs from one interface to another, the diffusion distance for shear is twice as far as for densification, which retards sintering should it become controlled by shear deformation, as seen in constrained sintering.