Limitations of the Augis and Bennett Method for Kinetic Analysis of the Crystallization of Glasses and Conditions for Correct Use

The equation proposed by Augis and Bennett for determining the kinetic exponent of the Johnson–Mehl–Avrami (JMA) model is thoroughly analyzed; a new expression, calculated accurately with no assumptions introduced, is proposed. This new method of calculation has been extended to the different kinetic models more commonly used in the literature for describing solid-state reactions. However, determining the JMA exponent from the Augis and Bennett method can lead to an incorrect interpretation of the reaction mechanism unless an additional, independent test is used. A testing method for verifying the applicability of the Augis and Bennett method is proposed. The kinetic analysis of the crystallization of Ge_0.3Sb_1.4S_2.7 has been used for checking this method.     

刚玉莫来石—ZrB_2复合材料的烧结初期动力学研究

研究了刚玉莫来石—ZrB2复合材料烧结初期的动力学关系，认为烧结机理服从界面扩散—体积扩散过程。阐明了ZrB2的引入对刚玉莫来石材料烧结的影响，并给出了复合材料的烧结活化能Q。     

Initial sintering mechanism and additive effect in zirconia ceramics

Y₂O₃-stabilized ZrO₂ (YSZ) ceramics have been used for various engineering applications because of their excellent mechanical properties or oxide-ion conductivity applicable to solid-electrolyte. The performance of YSZ depends on the sintered texture that is directly determined by sinterability of raw powders. A new kinetic analysis method of diffusion mechanism based on an initial sintering theory (grain-boundary or volume diffusion) is theoretically derived, the initial sintering mechanism of hydrolytic YSZ powder is experimentally determined, and the effects of powder characteristics on sinterability are discussed. Furthermore, the additive-enhanced sintering is proposed. A small amount of Al₂O₃ significantly enhances the densification. Using the additive effect, the low-temperature degradation that is the fault of zirconia ceramics can be improved by decreasing the sintering temperature.     

A study of agglomeration of coal-ash in fluidized beds

Experimental data on agglomeration of coal-ash particles in a fluidized bed have been presented. It has been observed that above the “initial sintering temperature”, the ash particles are defluidized at velocities above their minimum fluidization velocity. The expression for minimum fluidization velocity has been modified by including a force-term due to the phenomenon of sintering in addition to those due to drag, gravity and kinetic energy.     

Fabrication of textured alumina by magnetic alignment via gelcasting based on low-toxic system

The controlled development of texture in ceramics allows to improve some electrical, piezoelectric and mechanical properties of advanced ceramics materials by tailoring the microstructure. A highly textured microstructure of undoped dense alumina was attained by applying a novel combination of gelcasting techniques with magnetic alignment followed by sintering. A newly synthesized low-toxic acrylic monomer based on galactose was introduced to the gelcasting procedure. Thermal analysis of the gelcast has been performed to examine decomposition process of the new binder and match appropriate sintering rate. The effectiveness of the proposed procedure has been related to rheological properties of the suspension by clarifying the influence of the powder and the dispersant content. It has been also related to the idle time of polymerization by clarifying the influence of the initiator content. The new method is compared to slip casting in high magnetic field that has been used widely so far.     

Fabrication of embedded fine line inductors by constrained sintering and formulation of photoimageable conductive paste

Abstract

For miniaturisation and precision of electronic device, the new technologies such as photoimageable thick film process were combined with conventional thick film process, and constrained sintering with near zero shrinkage in the x and y direction has been proposed. In this research, photoimageable conductive paste for forming embedded components via constrained sintering by low temperature cofired ceramic (LTCC) technology was formulated. Afterwards, by optimising paste formulation, formation process of fine line and sintering method, miniaturised LTCC components especially embedded fine line inductors were fabricated and their properties such as line resolution, surface morphology and yield were investigated. As a result, embedded fine line inductors formed by constrained sintering with fine line resolution of 20 μm and yield over 90% were acquired.     

Determination of porosity from shrinkage curves during sintering of granular materials

This paper investigates the implementation of an analytical model to determine the porosity of a granular material during reactive and nonreactive sintering. A graphical interpretation of this model is proposed to calculate the porosity by comparing the experimental shrinkage curve with the ideal one. For the nonreactive sintering, some examples have been taken from the literature to illustrate the application of this method for two granular materials (alumina and zircon). In the case of reactive sintering, we have used our experiments to study the sintering behavior of magnesium hydroxide. The shrinkage curve was determined by dilatometer and the porosity was measured by helium pycnometer. The comparison revealed that the porosity calculated from the model is fully consistent with the porosity measurements in the both cases.     

微波烧结在陶瓷材料中的应用

微波烧结作为一种陶瓷材料烧结的新方法,近年来得到飞速发展。本文介绍了微波烧结的原理,阐述了微波烧结在陶瓷 材料中的应用,并分析了微波烧结存在的问题及其解决途径。     

New methods of determination of kinetic parameters of theoretical models from experimental data

Integral and integro-differential methods have been developed for determining unknown characteristics of a kinetic experiment, including the rates of chemical reactions, the initial concentrations of substances, and Michaelis constants. These methods are applicable to a wide class of reactions and are most efficient when the system of differential equations considered is linear with respect to all, or a group of, unknown parameters. For an open multienzyme system obeying the Michaelis-Menten system of equations that is nonlinear in unknown parameters, it is demonstrated how the initial system of equations can be transformed into a new system that is linear with respect to new variables functionally depending on the initial parameters. A method has been developed for reformulating the initial nonlinear problem of determining the unknown parameters into a linear one. In this method, the initial problem is immersed into the more general problem of identifying the coefficients of a differential equation such that the initial formula is one of the solutions of this equation. This method can also be used to determine kinetic parameters of theoretical models that are formulated in terms of both concentrations and activities. A method of complete identification of chemical kinetic problems has been analyzed for the case of the incomplete observability of some components of the reaction, and it has been concluded that the kinetics of the substrate and intermediates in a multistep consecutive reaction can be qualitatively and quantitatively reconstructed by monitoring its product provided that the product concentration is precisely determined.     

Low-temperature sintering of highly conductive ZnO:Ga:Cl ceramics by means of chemical vapor transport

A new technology for sintering a ZnO + Ga₂O₃ powder via chemical vapor transport based on HCl has been developed. The proposed sintering method has the following advantages: a low sintering temperature of 1000–1100 °C, there is no need to use of expensive dopant nanopowders, the possibility of multiple re-sintering, and the absence of changes in the diameter of the ceramics after sintering. A ZnO:Ga:Cl ceramics with a density of 5.31 g/cm³, a hardness of 2.0 GPa, and a resistivity of 1.46 × 10^–3 Ω?cm has been synthesized. The solubility limit of the Ga₂O₃ dopant has been estimated at about 3 mol %. At a higher doping level, the content of the ZnGa₂O₄ spinel phase becomes significant. In addition, ZnO:Ga:Cl thin films with a resistivity of 2.77 × 10^–4 Ω?cm can be grown by DC magnetron sputtering of the synthesized ceramics.     

粉煤灰提取氧化铝技术研究进展

粉煤灰的高附加值资源化关系经济建设和环境保护两大重要问题.粉煤灰是工业固体废料但也是一种重要的潜在铝资源,从粉煤灰中提取氧化铝备受瞩目.重点分析总结了粉煤灰中氧化铝提取技术进展,根据处理手段的不同,氧化铝提取技术分为烧结法和直接湿法浸出两大类.石灰石烧结法、碱石灰烧结法和纯碱烧结法是烧结法最主要的三种工艺,相关研究众多,但大多仅限于实验室;湿法酸浸潜力巨大,已有盐酸浸出法成功应用于工业,但浸出过程的研究亟待充实和深入;湿法碱浸工艺仅有很少量的实验室研究.因此,湿法酸浸工艺是未来研究的一个重要方向.     

Master Sintering Curve: A Practical Approach to Sintering

One of the ultimate objectives for sintering studies is to be able to predict densiflcation results under different thermal histories for a given processing method. It has been reported that the geometric parameters related to sintering often are functions only of density for a given powder and green-body process, provided that one diffusion mechanism dominates in the sintering process. Based on this report, the concept of a master sintering curve has been developed that characterizes the sintering behavior for a given powder and green-body process regardless of the heating profiles. The formulation and construction of the master sintering curve are given in this paper. A model experiment on sintering of alumina is used and analyzed to demonstrate this new concept. Examples of the master sintering curves obtained from other powder systems (ZnO, nickel, A1₂O₃(5 vol% TiO₂), and A1₂O₃(5 vol% ZrO₂)) are presented. When this new method is used, densification behavior can be predicted under arbitrary temperature-time excursions following a minimal set of preliminary experiments, and these predictions can be used in planning sintering strategies. Moreover, deviations from the assumption of a single mechanism can be observed readily.     

Synthesis of silicon nanoparticles with a narrow size distribution: A theoretical study

This work presents a study of the processes involved in synthesis of narrowly distributed silicon nanoparticles from the thermal decomposition of silane. Two models are proposed, one which simultaneously solves the kinetic mechanism of Swihart & Girshick (1999, Journal of Physical Chemistry B 103, 64–76) while adjusting the sintering parameters; and another which adjusts the kinetic and surface growth mechanisms while neglecting coagulation and sintering. The models are applied to simulate the centreline of the hot-wall reactor and process conditions of Körmer et al. (2010, Journal of Aerosol Science 41, 998–1007). Both models are shown to give good agreement with experimental PSDs at a range of process conditions. However, it is reported that an unphysical sintering process is obtained when attempting to use Swihart & Girshick's kinetic mechanism, while solving for the sintering parameters. The model with adjusted gas-phase and surface growth processes gives better quantitative and qualitative agreement with experimental results. It is therefore recommended that further study into the kinetic and heterogeneous growth mechanisms be conducted in order to better understand the fundamental processes occurring in this hot-wall reactor.     

Non-isothermal sinter-crystallisation of satin glazes: A kinetic model

Many materials of a glass-ceramic nature are obtained by glass particle sinter-crystallisation, whose process kinetics have been studied very little. The present study analyses the physico-chemical transformations that develop during the firing of a complex commercial satin glaze (containing more than five phases), with high frit content, particularly focusing on sinter-crystallisation kinetics. Glaze sintering and phase evolution were studied by hot stage microscopy (HSM) and X-ray diffraction (XRD). The glaze melting and crystallisation ranges and the kinetic parameters of the crystallisation process were determined by differential thermal analysis (DTA). Glaze sinter-crystallisation behaviour and the development of the crystal mass fractions, residual glass compositions, and effective viscosity, η_eff, during heating are discussed, based on Rietveld analysis of the XRD data. A new kinetic model was developed that describes the non-isothermal sinter-crystallisation of materials exhibiting three-stage sintering. A two-step kinetic model is involved: sintering with concurrent surface crystallisation (corresponding to sintering stages I and II) and sintering with simultaneous partial melting of crystalline phases (sintering stage III). The experimental data obtained by hot stage microscopy (HSM) at different heating rates were compared with those estimated by the model. The results obtained by the two methods exhibited very good agreement. The crystallisation kinetic parameters (activation energy, E_cr, and Avrami index, n, obtained by DTA, were consistent with the viscous flow activation energy, Q₂, corresponding to the first sintering step (sintering stages I and II). The model was used to calculate the values of the effective sintering viscosity, η_s, of the glaze melt. These values were compared with the experimental effective viscosity, η_eff, data obtained by hot stage microscopy (HSM). Both sets of results exhibited good agreement. Effective sintering viscosity, η_s, which is readily measurable, helps better understand the role played by the different glaze constituents and by the firing conditions in sintering, enabling more rational design of these materials.     

A Novel Method of Predicting Ultrasonic and Elastic Properties of Isotropic Ceramic Materials after Sintering from the Properties of Partially Sintered or Green Compacts

A novel method for predicting the porosity dependence of ultrasonic and elastic properties of isotropic ceramics after sintering based on the properties of green or partially sintered compacts has been proposed. The method is based on the observation that the ratio of ultrasonic shear wave to longitudinal wave velocity is a function of porosity only and varies linearly with the longitudinal wave velocity. It is also shown that the predicted trend of variation of Poisson's ratio with porosity agrees with the predictions of the Mori–Tanaka mean field approach. The method can be used as a quality control tool during the preparation of ceramic materials.     

Kinetics of glycerol effect on biodiesel production for optimal feeding of methanol

The enzymatic production of biodiesel has been considered as an eco-friendly process. Candida antarctica lipase B (CalB) has been studied for its application for biodiesel production because of its high activity and stability. Enzyme deactivation caused by alcohol and effect of glycerol has to be resolved for the industrial application of this process. In traditional kinetic studies of biodiesel production, the effects of alcohol and oil were only considered in the kinetic equation, while the effect by glycerol was neglected. A new kinetic model incorporating glycerol effect is proposed in this paper. The proposed kinetic equation is applied by predicting the supplying rate of methanol in a fed-batch addition of methanol. The conversion rate was improved from 59.7% to 94.6% in a fed-batch by considering glycerol effect.     

铬酸钠氢还原烧结法制备氧化铬绿颜料

提出了一种氧化铬绿颜料的全新制备工艺。通过研究在低温氢还原过程和烧结过程中影响氧化铬颜料性能的关键因素及其作用机理,制备出性能优异的氧化铬绿颜料。结果表明,低的氢还原温度和相应Al添加剂有利于得到高质量的氧化铬绿颜料;氧化铬的烧结机制为蒸发-凝聚;此外,副产物以NaOH形式回收。该工艺基于中国科学院过程工程研究所自主研发的铬盐清洁工艺与集成技术,实现了氧化铬绿颜料的短流程清洁生产。     

Constitutive modeling of the behaviour of cermet compacts during reaction sintering

This study deals with the identification of a constitutive equation describing the mechanical behaviour of a nickel ferrite based cermet during sintering. This constitutive equation considers the material as a continuum and may enable one to predict the densification behaviour of a powder under different thermal treatments and the impact of compact geometry, external loading on strain and stress generation. A classical viscous equation of the Newtonian type that includes a term describing free sintering densification has been chosen. The method used for the identification of the parameters of this equation is the one proposed Gillia et al., which is based on dilatometry measurement. It includes a stairway thermal cycle for the determination of the free sintering term and intermittent loading for estimating the viscosity. This approach has been successfully applied to nickel ferrite cermet. The model has been found to be adequate to model the densification behaviour up to 1250 °C, but experimental and theoretical efforts must be accomplished to describe the behaviour above this temperature, when the material exhibits swelling.     

Investigation of temperature approximation methods during flash sintering of ZnO

The lattice expansion in ZnO, using in-situ X ray diffraction, has been investigated during flash sintering with varying current densities. While current flow through the specimen enhances the kinetics of sintering for ZnO, the temperature is not high enough to claim thermal runaway or localized melting. Unlike the case of yttria stabilized zirconia [1,2], experimental temperature approximations predict comparable specimen temperature to conventional sintering temperature of ZnO. Microstructural analysis supports the findings of the in-situ temperature approximations. In comparison with black body radiation, a gap between theoretical value and measured value was found due to flaws in the theoretical model. In addition, a new type of flash sintering was introduced, with current ramp, to avoid the power spike which has been the source of much debate about the transition from voltage to current control. The advantage of this method is in the controlled sintering kinetics thus avoiding the channeling found in dielectric materials [3].     

Elastic properties of porous polycrystalline thoria—A relook

Elastic constant–porosity relation for polycrystalline thoria reported by previous researchers has been reanalyzed on the basis of the Mori–Tanaka mean field approach and a power law dependence of moduli with porosity. It indicates that the shear modulus dependence on Young's modulus is possibly related to the sintering characteristics of the material rather than pore morphology. A new method has been suggested for predicting variations of elastic properties and porosity with the progress in sintering of thoria based on experimental data at a single porosity only. The predicted values agree with the experimental data quite well.