Sintering kinetics of zirconium dioxide

Conclusions We investigated the sintering of zirconium dioxide over the temperature range 2000–2350°K as a function of the purity of the material and the defectivity of the oxide crystal lattice. The activation energy of sintering and the vacancy diffusion constant were calculated.The decrease in activation energy resulting from distortion of the crystal lattice is, on the whole, inadequate for activation of sintering.Translated from Ogneupory, No.5, pp. 51–57, May, 1969.     

Sintering kinetics and mechanism of vitreous nanoparticles

The sintering of vitreous nanoparticle doublets is investigated numerically by a volume of fluid method coupled to Hamaker summation and experimentally by a high-temperature sintering flow reactor as well as by doublet shape analysis in the transmission electron microscope. In particular, the characteristic differences between nanoparticulate and bulk sintering are studied. The sintering mechanism of vitreous nanoparticles is determined to be viscous flow with interparticle van der Waals interactions acting as additional driving force. The early stages of the nanoparticle sintering kinetics are inversely proportional to the square of the particle size, instead of an indirect proportionality to the first order of the particle size for the entire bulk process. The transition between nanoparticulate and bulk sintering is localised to primary particle diameters of approx. 200–300 nm.     

Some properties of irregular 3-D particles

This paper discusses some of the properties of irregular particles that are of interest to engineers, including volume, density and surface area. Numerical and statistical information on these properties is essential (a) for a better understanding of particulates, (b) to suggest more efficient ways to utilize particulate materials and (c) to permit the creation of mathematical models that can reduce the need for lengthy real-world testing. While the motivation, examples and applications are from the construction materials industry, the results should be of interest to others. Measurement techniques used included X-ray computed tomography (CT) and multiple projected images, augmented by traditional laboratory techniques. To compare the results of these techniques, a set of 12 rocks were studied of which six were between 19 mm and 12.7 mm (0.75 in. to 0.5 in.) in size, and six were between 12.7 mm and 6.3 mm (0.5 in. to 0.25 in.) in size. Microfine versions of these rocks (< 80 μm equivalent spherical diameter) were also studied and compared. The shapes of the rocks were studied by relating three dimensions to their volume and surface area. These three physical dimensions were defined in two different ways: direct measurement of three unique orthogonal dimensions on the rock surface, and dimensions obtained from the use of absolute first moments of volume and principal second moments of volume. These measurements and calculated moments allowed the development of three-parameter equivalent shape models based on rectangular parallelepipeds and tri-axial ellipsoids. All types of three-parameter equivalent shape models considered provided acceptable accuracy in predicting both volume and surface area, with the box models being generally more physical and realistic than the ellipsoid-based equivalent shape models for the type of rocks considered.     

Drag of non-spherical solid particles of regular and irregular shape

The drag of a non-spherical particle was reviewed and investigated for a variety of shapes (regular and irregular) and particle Reynolds numbers (Re_p). Point-force models for the trajectory-averaged drag were discussed for both the Stokes regime (Re_p ? 1) and Newton regime (Re_p ? 1 and sub-critical with approximately constant drag coefficient) for a particular particle shape. While exact solutions were often available for the Stokes regime, the Newton regime depended on: aspect ratio for spheroidal particles, surface area ratio for other regularly-shaped particles, and min-med-max area for irregularly shaped particles. The combination of the Stokes and Newton regimes were well integrated using a general method by Ganser (developed for isometric shapes and disks). In particular, a modified Clift-Gauvin expression was developed for particles with approximately cylindrical cross-sections relative to the flow, e.g. rods, prolate spheroids, and oblate spheroids with near-unity aspect ratios. However, particles with non-circular cross-sections exhibited a weaker dependence on Reynolds number, which is attributed to the more rapid transition to flow separation and turbulent boundary layer conditions. Their drag coefficient behavior was better represented by a modified Dallavalle drag model, by again integrating the Stokes and Newton regimes. This paper first discusses spherical particle drag and classification of particle shapes, followed by the main body which discusses drag in Stokes and Newton regimes and then combines these results for the intermediate regimes.     

Crystallization kinetics of basalt glass

The crystallization behaviour of basalt glass at elevated temperatures was studied using glass samples prepared by melting the natural basalt rock from the Thrace region of Türkiye. DTA and XRD analysis revealed the crystallization of augite [(Ca Fe Mg) SiO₃ at 800 °C. The kinetics of crystallization of augite were studied by applying the DTA measurements carried out at different heating rates and the activation energies of crystallization and viscous flow were measured as 238 kJ mol⁻¹ and 413 kJ mol⁻¹, respectively. The resultant basalt glass-ceramic revealed very fine and homogeneous microstructure.     

Mass transfer between irregular particles and liquid in packed beds

Dissolution of irregular particles has been studied in packed beds for the systems of alabaster-water, marble-aqueous HCl solutions and mossy zinc-aqueous acidic I₂ solutions, where the sphericity of particles varied in a wide range. The observed values of the Sherwood number have been correlated well by

     

Sintering kinetics of high-alumina ceramics with complex additives

Results of a study of ceramic compaction kinetics under conditions of isothermal and nonisothermal heating are given. Kinetic equations of the compaction process using a structure/energy parameter as the dependent variable are given, and on this basis values of apparent activation energy for two compaction stages are determined. It is demonstrated that modification transitions and formation of unstable phases and solid solutions in the course of ceramic firing have a substantial effect on the mechanism of ceramic compaction. Translated from Steklo i Keramika, No. 4, pp. 9 – 12, April, 1999.     

Sintering of NaCl powder: Mechanisms and first stage kinetics

Isothermal NaCl sintering experiments are conducted between 500 and 790 °C on powders 75, 100, 150 and 400 μm in average diameter. Along with literature data, the results are compared with theoretical predictions for the initial stage of sintering, to draw a general picture of the process over a wide spectrum of temperatures and particle sizes. Where densification is observed, the mechanism is grain boundary diffusion of material from boundary sources. As observed in previous work, there is a transition in dominance between this mechanism and the alternative, non-densifying, mechanism of evaporation–condensation. More detailed analysis shows that, after initial and effectively instantaneous plastic yielding driven by the surface energy, boundary diffusion will always dominate the initial stage of sintering for the conditions of this investigation, leading to some (possibly very small) degree of densification. For particles above about 150 μm at temperatures within 300 °C of the melting point of NaCl this mechanism will rapidly be displaced by evaporation–condensation, halting densification. An approximate equation is derived for the rate of sintering by evaporation–condensation in the presence of a finite-pressure inert atmosphere; the rate of this mechanism is found to be significantly reduced when sintering is carried out at pressures approaching atmospheric.     

Pyrolysis kinetics for oil-shale particles

Experimental and mathematical investigations of the pyrolysis kinetics for single particles (12.7 mm diameter cylinders and spheres) of 22 gal/ton (91.8l/t) oil shale are presented. Machined samples of uniform geometry were suspended in a nitrogen stream and pyrolysed over a range of temperatures (384–520 °C), while weight losses were continuously measured with a Cahn recording thermobalance (TGA). In addition to weight-loss measurements, centreline temperature histories were obtained from several samples that were instrumented with microthermocouples. Other samples were partially pyrolysed, quenched, cross-sectioned and visually examined to aid in the understanding of the pyrolysis mechanism. Photographs of partially pyrolysed samples are included and discussed. Two mathematical models were developed in order to represent the pyrolysis process. The weight-loss data were then compared with the results obtained from these models. A non-isothermal shrinking-core model was developed first, as a direct consequence of having observed a shrinking-core mechanism in partially pyrolysed samples. This model resulted in an apparent pyrolysis activation energy of 110 kJ/mol. Subsequently, a simplified, non-isothermal homogeneous model was developed, tested, and resulted in a pyrolysis activation energy of 148 kJ/mol. It was shown that either model could account for the experimental centreline temperature histories and for the pyrolysis rates. The non-isothermal homogeneous representation is considerably less complex than previously published pyrolysis models, and would be preferred for use as a subprogram in a comprehensive retorting model.     

Sintering of glass matrix composites with small rigid inclusions

We investigated the effect of dispersed crystalline particle volume content Φ on sintering of glass matrix composites (GMC) for low-temperature co-fired ceramics (LTCC) applications. Such composites typically consist of alumo-borosilicate glass and α-Al₂O₃ powders of similar average particle size (D₅₀ ≈ 3 μm). Sintering shrinkage was observed by dilatometry and heating microscopy and was backed up by glass viscosity measurements. Microstructure analysis revealed that α-Al₂O₃ particles do neither show significant dissolution into the liquid phase nor detectable crystallization throughout LTCC firing schedules. Therefore, in this study α-Al₂O₃ particles were treated as small rigid inclusions. It was found that Φ lowers the shrinkage rate of GMC. While the lowering is small for small Φ and at the early stage of densification it progressively increases during sintering, and final shrinkage shifts up to 170 K to higher temperatures for Φ = 0.45. The behaviour observed could be explained assuming that sintering is controlled by the effective viscosity, which progressively increases non-linearly during densification due to the gradually wetting of the surface area of corundum particles. We could demonstrate that Al₂O₃ cluster can cause residual pores and reduce the attainable shrinkage. The reduction of attainable shrinkage is found to depend on Φ³, reaching about 8% at Φ = 0.45.     

Sintering behaviour of a glass obtained from MSWI ash

The sintering behaviour of a glass obtained by Municipal Solid Waste Incinerator (MSWI) bottom ash (WG) was investigated and compared with a Na₂O–MgO–CaO–SiO₂ composition (CG). The sintering activation energy, E_sin, and the energy of viscous flow, E_η, were evaluated by dilatomeric measurements at different heating rates. The formation of crystalline phases was evaluated by Differential Thermal Analysis (DTA) and X-Ray Diffraction (XRD), and observed by Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM). In CG, the sintering started at ≈10¹³ dPa s viscosity and E_sin (245 kJ/mol) remains constant in the measured range of shrinkage, up to 9%. In WG the densification started at ≈10¹¹ dPa s, E_sin resulted to be 395 kJ/mol up to 5% shrinkage, 420 kJ/mol at 8% and 485 kJ/mol at 10% shrinkage. The sintering rate decreased due to the beginning of the pyroxene formation and the densification stopped in the temperature range 1073–1123 K after formation of 5 ± 3% and 13 ± 3% crystal phase, at 5 and 20 K/min, respectively. Higher densification and improved mechanical properties were obtained by applying the fast heating rate, i.e. 20 K/min.     

Sintering and reactive crystal growth of diopside-albite glass-ceramics from waste glass

Diopside-albite glass-ceramics were fabricated by sintering the powder mixtures of crystallization promoters and waste glass. Two kinds of promoters were synthesized using kaolin clay, talc and chemical reagents. The crystalline phases were formed by a reactive crystallization between promoters and glass during sintering. The effect of promoter components, additions and sintering temperatures on the crystallizing and densifying behavior, microstructures and mechanical properties of glass-ceramics was investigated. The results showed that the higher densities and better mechanical properties were obtained for the glass-ceramics with 12-15% crystallization promoters sintered at 950 °C for 2 h.     

Sintering and crystallization of plates prepared from coarse glass ceramic frits

The focus of this study was the development of a CAS glass-ceramic (14.0CaO·5.5Al₂O₃.80·5SiO₂) for use in civil engineering as a decorative material. This glass-ceramic system was chosen because it can be used to produce plates with a similar appearance to those of marble and granite materials. Ceramic plates were produced by the Grain Glass Sintering process from frits with a coarse grain size (2.1 mm average particle size). Powder frit samples (5.3-μm average particle size) were prepared and characterized. The results were compared with those obtained with coarse particles. Thermal treatment was performed in one stage, with the heating rate varying from 2 to 25 °C min^?1. Temperatures were defined by differential thermal analysis and heating microscopy. With these results, it was possible to establish correlations among the esthetic effects, the microstructure, and the heat treatment conditions (temperature, heating rate, and holding time). The relative density ranged from 0.92 to 0.96. The formation of wollastonite and pseudowollastonite was observed. An esthetic effect similar to that of natural marble was more apparent in those samples in which the microstructure presented a visual contrast between the bulk and the coarse frit. Lower heating rates intensified the visual contrast and reduced porosity near the surface regions.     

Sintering of nano-sized titania particles and the effect of chlorine impurities

The consolidation behaviour of two titania nanopowders with comparable specific surface area is reported in this study. Surface impurities were identified as a key element influencing the sintering behaviour of titania nanopowders. Raw materials for the synthesis processes of the nanopowders were titanium tetrachloride (TiCl₄) and titanium tetraisopropoxide (TTIP), respectively. Particle properties, densification and sintering behaviour were investigated by several techniques (SEM, TEM, XRD, MIP and dilatometry). Phase transformation and densification of the nanopowder, prepared from TiCl₄, started at lower temperature compared to the powder synthesized from TTIP. However, a drastic grain growth occurred during the final stage of sintering of the TiCl₄ synthesized nanopowder. Chlorine impurities were identified by STA-MS. After purification of this nanopowder, the chlorine was effectively removed from particle surfaces and grain growth was reduced.     

Sintering behavior of poly(methyl methacrylate) particles

The sintering behavior of poly(methyl methacrylate) (PMMA) particles was studied by photographic, means and mechanical testing. Interpretation of the data gives the sequence of events taking place during high temperature sintering (without pre-compaction). Optical observations, show essentially two main sintering regions while mechanical studies also give an intermediate transition zone separating these two regions. It is suggested that the flow mechanisms involved in the sintering process are viscoelastfc flow in the lower temperature range and shorter times and non-Newtonian viscous flow in the higher temperature range and longer times.     

Conversion kinetics of container glass batch melting

Understanding the batch-to-glass conversion process is fundamental to optimizing the performance of glass-melting furnaces and ensuring that furnace modeling can correctly predict the observed outcome when batch materials or furnace conditions change. To investigate the kinetics of silica dissolution, gas evolution, and primary foam formation and collapse, we performed X-ray diffraction, thermal gravimetry, feed expansion tests, and evolved gas analysis of batch samples heated at several constant heating rates. We found that gas evolving reactions, foaming, and silica dissolution depend on the thermal history of the batch in a similar manner: the kinetic parameters of each process were linear functions of the square root of the heating rate. This kinetic similarity reflects the stronger-than-expected interdependence of these processes. On the basis of our results, we suggest that changes in furnace operating conditions, such as firing or boosting, influence the melting rate less than what one would expect without consideration of batch conversion kinetics.