Compressive mechanical properties of partially sintered porous alumina of bimodal particle size system

This paper examined theoretically and experimentally packing behavior, sintering behavior and compressive mechanical properties of sintered bodies of the bimodal particle size system of 80 vol% large particles (351 nm diameter)–20 vol% small particles (156 nm diameter). The increased packing density as compared with the mono size system was explained by the packing of small particles in 6-coordinated pore spaces among large particles owing to the similar size relation between 6-coordinated spherical pore and small particle. The sintering between adjacent large particles dominated the whole shrinkage of the powder compact of the bimodal particle size system. However, the bimodal particle size system has a high grain growth rate because of the different curvatures of adjacent small and large particles. The derived theoretical equations for the compressive strengths of both mono size system and bimodal particle size system suggest that the increase in the grain boundary area and relative density by sintering dominate the compressive strength of a sintered porous alumina. The experimental compressive strengths were well explained by the proposed theoretical models. The strength of the bimodal particle size system was high at low sintering temperatures but was low at high sintering temperatures as compared with that of mono size system of large particles. This was explained by mainly the change of grain boundary area with grain growth. The stress–strain relationship of the bimodal particle size system showed an unique pseudo-ductile property. This was well explained by the curved inside stress distribution along the sample height. The inside stress decreases toward the bottom layer. The fracture of one layer of sintered grains over the top surface proceeds continuously with compressive time along the sample height when an applied stress reaches the critical fracture strength.     

The sintering kinetics of ultrafine tungsten carbide powders

The sintering kinetics of nano grained tungsten carbide (n-WC) powders has been analyzed by non isothermal and isothermal sintering. Non isothermal sintering experiments reveal a multi staged sintering process in which at least three major sub-stages can be distinguished. The isothermal shrinkage strain also exhibits an asymptotic behavior with time indicating an end point density phenomenon in most of the temperature ranges. Combined microstructural and kinetic data analyses suggest that differences in the sinterability of inter and intra agglomerate pore phases introduce sub-stages in the sintering process which manifest as stagnant density regions in both the isothermal and non isothermal experiments. Kinetic analysis of the data reveals very low activation energies for sintering suggesting that particle rearrangement and agglomeration at low temperatures may be brought about by surface diffusion leading to neck growth and grain rotation. At higher temperatures rapid grain boundary diffusion by overheating along inter particle boundaries induced by sparking may be a dominant sintering mechanism. Although grain growth and densification in conventional WC powders generally obey an inverse relation to each other, in n-WC powders both can act synergistically to increase the net densification rate. In fact, complete densification cannot be achieved in n-WC powders without grain growth as one abets the other.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

Flash microwave sintering of zirconia by multiple susceptors cascade strategy

In the field of flash sintering, microwave energy represents an interesting way to densify ceramics complex shapes, thanks to a contactless volumetric heating. Attaining a fast and homogeneous heating is a critical parameter and hybrid heating, using silicon carbide susceptors, is generally used. In this study, an original multiple susceptors cascade strategy is developed, using both SiC and 3D-printed ZrO₂ susceptors. This novel configuration follows perfectly the flash heating scheme, even for high heating rates up to 1000 K.min^-1 and leads to a high stability of the “flash” hybrid heating. Flash microwave sintering produced dense (97 % relative density) microstructures within 45 s. Based on comprehensive multiphysics simulations of the overall process, in-situ dilatometry measurements, kinetics method analysis and microstructural characterizations, this work highlights the sintering behavior of zirconia and the temperature distribution during flash microwave sintering.     

Liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing

The effects were investigated of the starting particle size (i.e., nanometer or submicrometer powders), content of Y₃Al₅O₁₂ additives (YAG; in the range 5–20 wt.%), and difference of size scales between the two particle types on the liquid-phase assisted flash sintering of SiC from powder mixtures prepared by aqueous colloidal processing. It was found that flash sintering benefits from the refinement of the particles size, the increase in additive content, and the smaller size scale of the particulate additive. It was also found that under the present flash sintering conditions (i.e., 900 °C furnace temperature, 13 A current, and 50 s in flash state) the resulting ceramics are, despite the formation of liquid phase, porous to a greater or lesser extent, and exhibit decreasing porosity gradients from their surface to the centre. These observations are rationalized to extract guidelines for powder batch design contributing to the pressureless ultrafast sintering of non-oxide advanced ceramics.     

The effective diffusivities in porous media with and without nonlinear reactions

The question of whether effective diffusivities in porous materials under reactive and nonreactive conditions are equal is addressed. Previous studies had considered the problem with first-order reactions. We study the issue with two nonlinear reactions—a second-order reaction and one governed by the Michaelis-Menten kinetics. Pore network and continuum models of porous media are utilized to estimate the effective diffusivities under reactive and nonreactive conditions. We show that the two effective diffusivities are significantly different. The difference is due to the heterogeneities of the porous material, and the fluctuations that they cause in the spatially varying local concentrations and diffusivities, and can be as large as a few orders of magnitude. Theoretical analysis of diffusion and reactions in porous media is also presented that supports the results of the simulations. In particular, it is shown that the results of pore network simulations cannot be fitted to the classical continuum equation of diffusion and reaction, and that a more complex continuum equation should be used for this purpose.     

Solid-state sintering of core-shell ceramic powders fabricated by particle atomic layer deposition

The properties of technical ceramics are highly dependent on their microstructure, which evolves during sintering. Sintering is the process by which ceramic parts are subjected to high temperatures to activate chemical diffusion and the consumption of porosity. During the initial stage of sintering, interparticle necks between neighboring particles form and subsequently increase in size, consuming porosity as the particle centers move closer together. To experimentally determine how this process depends on particle surface composition, particle atomic layer deposition (ALD) was used to deposit a thin film of amorphous aluminum oxide (Al₂O₃) onto yttria-stabilized tetragonal zirconia (3YSZ) particles, producing core-shell structured powders. The uniformity of the Al₂O₃ film was confirmed with transmission electron microscopy and energy dispersive spectroscopy. Scanning electron microscopy was used to observe microstructural evolution during sintering, and the dihedral angles of Al₂O₃ and 3YSZ grains were measured to determine the ratio of interfacial energies between the 3YSZ|3YSZ, 3YSZ|Al₂O₃, and Al₂O₃|Al₂O₃ interfaces. Analysis of the densification kinetics revealed that the initial stage of densification is dependent on the material at the surface of the particles (ie, the Al₂O₃ film) and is controlled by the diffusion of Al³⁺ cations through Al₂O₃. Once the Al₂O₃ film has coalesced, the sintering behavior is controlled by the densification of the core material (3YSZ). Thus, core-shell powders fabricated by particle ALD sinter by a two-step process where the kinetics are dependent on the material present at interparticle contacts.     

Reactive flash sintering of high-entropy oxide (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7: Microstructural evolution and aqueous durability

Herein, the phase evolution, densification and grain growth process of the high entropy ceramics during flash sintering were systematically characterized and quantified to understand the microstructural evolution for the first time. It was demonstrated that the densification rate of (La_0.2Nd_0.2Sm_0.2Eu_0.2Gd_0.2)₂Zr₂O₇ by flash sintering in this work was generally around 60 times that of conventional sintering at 1600 °C, while the grain growth rate by flash sintering was only around 1.5–6 times that of conventional sintering, indicating that grain growth was suppressed during flash sintering. The grain growth mechanisms by flash sintering and conventional sintering could be both attributed to surface diffusion and volume diffusion. In addition, the flash sintered high-entropy ceramics as promising immobilization materials for high-level radioactive waste (HLW) exhibited excellent aqueous durability with normalized leaching rates of Nd, Gd and Zr approximately 10^?6～10^?7 g m^?2 d^?1 after 42 days, which were much lower than most reported pyrochlore materials.     

A geometrical model for coalescing aerosol particles

Geometrical relationships for two unequal-sized coalescing aerosol particles in the entire coalescing process are obtained by considering the coalescing process at the point where the neck size equals the diameter of the small particle and under the assumption that each particle is in the shape of a spherical cap. Comparisons on the shrinkage length have been made between present model and the Yadha and Helble [(2004). Modeling the coalescence of heterogenous amorphous particles. Journal of Aerosol Science, 35, 665–681] geometrical model. Excess surface area of titania nanoparticles sintering at different temperatures has also been compared between predictions from the present model, the Koch and Friedlander [(1990). The effect of particle coalescence on the surface area of a coagulating aerosol. Journal of Colloid and Interface Science, 140, 419–427] model, and the literature data. The results show that the present model gives reasonable predictions based on an overall solid-state diffusion mechanism at the start and the volume solid-state diffusion mechanism at later stages of the coalescing process.     

The kinetics of combustion of chars derived from sewage sludge

Experiments have been conducted to determine the combustion characteristics of sewage sludge chars in electrically heated beds of silica sand fluidised by air. The effects of the initial size of the char particles, the temperature of the bed and [O₂] in the fluidising gas were investigated. Also, the temperatures of burning particles were measured with embedded thermocouples. The kinetics of combustion were measured at temperatures low enough for the CO formed by initial reaction between the carbon and oxygen to burn at some distance away from the particle. Accordingly, the particle is only heated by the enthalpy of the reaction C+0.5O₂→CO. The activation energy for the intrinsic kinetics of combustion of the char was estimated to be 130–144 kJ/mol. The former value makes allowance for the fact that the particles are at a temperature in excess of that of the bed (determined by a heat balance on a reacting particle), whilst the latter value assumes that the particles are at the same temperature of the bed. It is probable that the lower value is closer to the actual value, thought to be 135±15 kJ/mol, reflecting the catalytic nature of the ash skeleton on which the carbon is supported. It was possible to obtain good agreement between measured burnout times and those predicted using the grain model of Szekely J, Evans JW, Sohn HY. Gas–solid reactions. New York: Academic Press; 1976, for the case where the kinetics are controlled by a combination of: (i) external mass transfer of oxygen from the particulate phase to the external surface of the burning char particle, (ii) diffusion of oxygen from the external surface into the porous matrix to the surfaces of grains, of which the solid is composed, and (iii) diffusion of oxygen into the microporous grains, where reaction occurs with the carbon. It was found that, for particles with diameters of 2 mm or larger, the initial rates of reaction, for bed temperatures in excess of 750 °C, are dominated by external mass transfer. This explains the dependence of the rate of oxidation of unit mass of char on 1/d_p, and the relatively small influence of temperature on these rates. Particles of char from sewage sludge are so reactive that it is essential to make allowance for a difference in temperature between the particle and the bed. Thus, experimental determinations on particles with d_p∼6.5 mm, suggested a difference in temperature of ∼150 K, in line with calculations using a steady-state heat balance.     

Mathematical simulation of mass transfer processes under heterogeneous reactions in polydisperse porous media

Submitted is a theoretical study of mass transfer processes in polydisperse porous media in the presence of chemical reactions. Kinetic regime of methane pyrolysis in a porous carbon skeleton considering external and internal diffusion resistances for different initial distributions of particles forming the porous medium is investigated. Derived is a general analytical expression describing the influence of the inner reaction surface variation on the degree of the pore filling for an arbitrary initial particle size distribution. Expressions defining the time of pores filling by pyrocarbon based on approximate and exact solutions of the equation for the probability density function (PDF) of particle size distribution are received. Dependence of pore filling time on effective diffusion coefficient and initial particle size distribution using both solutions for PDF-equation is compared. It is shown, that the dominant factor influencing the pore filling time is the dispersion of particle size distribution.     

A microscopic model for catalyst surfaces—II: Supported catalysts

A new “pebbly surface” model of supported metal catalysts is presented which can explain observed multiplicity and oscillatory behaviour of catalyst particles. This model combines the dynamic behavior of the individual metal crystallites on the surface of the support with diffusion and heat transfer processes in the porous particle. The bifurcation of oscillations is analyzed in terms of relaxation oscillations as well as by Hopf's theorem and it is shown how the bifurcation phenomena depend on the system parameters. The parameter study may be used to suggest operating conditions and catalyst design parameters such that sintering effects may be reduced. Qualitative comparisons with data for carbon monoxide oxidation at various temperatures show very good agreement; however, this present simple form of the pebbly surface model seems unable to predict the long period oscillations observed in hydrogen oxidation.     

Spherical porous granules in MgOFe2O3Nb2O5 system: In situ observation of formation behavior using high-temperature confocal laser-scanning microscopy

The pyrolytic reactive granulation process, yielding ceramic spherical porous granules, is simple, consisting of typical ceramic processing methods, viz., wet-ball milling of powders, vacuum drying, granulation via sieving through a screen mesh, and one-step heat treatment for local reactive sintering within each granule. Here, the microstructural development of spherical porous granules was successfully visualized by in situ high-temperature confocal laser-scanning microscopy during the heating up to 1400 °C in air. Based on the result of the in situ observation, a simple but powerful size-controlling process of spherical porous granules, viz., multiscreen sieving after the heating was demonstrated. Nearly monodispersed spherical porous granules composed of pseudobrookite-type MgFeNbO₅ were easily obtained.     

A molecular simulation study on the sintering mechanism of TiO2

The sintering process of TiO₂ nanoparticles with different particle sizes and temperatures was studied thoroughly using equilibrium molecular dynamics simulations. The results show that when two nanoparticles contact, the sintering process was initiated by the merge of surface atoms of nanoparticles, and the process subsequently drives the internal atom to merge until two particles being merged homogeneously. There is mutual attraction between atoms and the particles gain kinetic energy to migrate due to the heating at high temperatures, leading to a faster sintering reaction. Moreover, there is a large difference in the sintering speed between 1600 and 1800 K. In the vicinity of the melting point, a small change in temperature makes a great impact on the sintering rate of TiO₂ nanoparticles. Furthermore, by using the Lindemann index, it can be found that the larger particles have higher lattice structure stability compared to the smaller ones. The larger particle has a greater effect on the sintering behaviors when particles with different sizes’ contact. As a consequence, the sintering of two particles with different sizes is mainly initiated by the smaller particles moving toward the larger particle and is ended up with the atoms of smaller particle spreading around the larger particle. Therefore, the large nanoparticle size reduces the overall sintering rate.     

The impact of flash sintering on densification and plasticity of strontium titanate: High heating rates,dislocation nucleation and plastic flow

Flash sintering involves very rapid densification of ceramic powder compacts during a thermal runaway induced by an applied voltage and current. The mechanisms of fast densification are still not well-understood. The present study investigates the impact of high heating rates during flash sintering on densification, dislocation density and plasticity of SrTiO₃. After flash sintering, a high dislocation density of almost 10¹⁴ m⁻² was observed by TEM. Uniaxial compression at 1150 °C revealed very high deformation rates. It is argued that for SrTiO₃, dislocations are generated and migrate during flash sintering. This becomes possible by the very high heating rates, which conserve high driving forces for sintering up to high temperatures. High driving forces of several 10 MPa are preserved up to high temperatures. Thus, the sintering stress can be above the flow stress of SrTiO₃ (5 MPa), and the nucleation of dislocations occurs, paving the path for plastic flow.     

Particle-vapor interaction in deposition systems: influence on deposit morphology

We have here presented methods to study interactions of vapors and particles in systems involving simultaneous deposition of vapors and particles. Besides estimating vapor and particle concentration profiles in the boundary layer adjacent to the deposition surface, their deposition rates are also calculated. In particular, we consider formation of porous preforms by deposition of silica particles and germania particles/vapors during the manufacturing of optical fibers. The process conditions not only dictate the relative rates of germania particle and vapor deposition on the deposition surface, but also controls the fraction of germania crystallinity in the resulting deposit. Moreover, the loss and migration behavior of the deposited germania during the sintering of the porous preform is extremely sensitive to the germania crystalline fraction. Our methods predict the germania weight percent deposited during the deposition process as a function of the deposition conditions, along with the fraction of germania that is crystalline/amorphous. The germania loss and migration behavior during the sintering step is also estimated. In predicting the germania loss and migration behavior, we have developed methods to systematically take into account the simultaneous heating of the preform, sintering of the porous preform, diffusion of gas species through the pores and the gas-solid reaction. Based on the methods developed here for deposition and sintering, processes have been developed which have resulted in sintered glasses with very high germania content (50 weight percent), and, without any glass quality issues of glass seeds, blank splitting or glass crizzling (devitrification).     

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.     

Molecular dynamics computation of gas-phase nanoparticle sintering: a comparison with phenomenological models

The mechanism and kinetics of the growth of silicon nanoparticles via particle–particle interactions has been investigated through the use of classical molecular dynamics (MD) trajectory calculations. Computations over a broad range of temperatures and particle sizes have shown that particle sintering is very dependent on size and temperature when solid-like, and considerably less sensitive when liquid-like. These atomistic computations have been used for the first time to validate previously postulated phenomenological mechanisms/models for both solid and liquid particle coalescence. The results have shown that solid-like particles sinter by a solid-state diffusion mechanism while liquid-like particles sinter by a viscous flow mechanism.     

Electric field-assisted solid-state reaction of BaCO3-TiO2 system

We report phase transition process during the solid-state reaction of BaCO₃-TiO₂ system under the assistance of electric field. Experiments were conducted at a constant heating rate with preset field strength and current limit. Solid-state reaction was completed upon reactive flash sintering taking place at ~1002℃ under 200 V/cm. Hexagonal BaTiO₃ phase, which rarely occurs at such temperatures, was obtained after flash sintering at a current density of 23.5 mA/mm². It is speculated that oxygen deficiency during flash sintering triggered cubic-to-hexagonal transition of BaTiO₃. Furthermore, X-ray diffraction results show that solid-state reaction takes place prior to flash sintering. Electric field could accelerate the reaction but did not alter the sequence of phase evolution.