Hot‐Pressing Kinetics and Densification Mechanisms of Boron Carbide

The hot‐pressing kinetics of boron carbide at different stages in the hot‐pressing process was investigated. Based general densification equation and pore‐dragged creep model, the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature, sintering pressure and dwell time. Stress exponent of n ≈ 3 at the initial dwell stage suggests the plastic deformation may dominates the densification. The further TEM observations and the calculation based on effective stress and plastic yield stress also indicate that plastic deformation may occur and account for the large increase in density at the initial stage of sintering. Calculated grain size exponent of m ≈ 3 suggests that the grain‐boundary diffusion dominates the densification at the final stage. During the final stage of sintering, grain growth may be determined by evaporation/condensation and grain‐boundary migration.     

Influence of vanadium substitution on sintering behaviour of Pb3(VO4)2(1−x)(PO4)2x ceramics

Intermediate-stage sintering has been investigated in lead orthophosphovanadates Pb₃(VO₄)_2(1−x)(PO₄)_2x. It was found that rich-vanadium compounds such as Pb₃(VO₄)₂ and Pb₃(VO₄)_1.6(PO₄)_0.4 densify rapidly with important grain growth. For these compounds grain growth is controlled by grain boundaries and densification occurs by a mixed mechanism with lattice and grain boundary diffusion. For Pb₃(PO₄)₂, sintering mechanism supports a model of grain-boundary-controlled densification and grain growth is a surface diffusion-controlled pore drag mechanism. Moreover, the presence of phosphorus in compounds' formulae, tends to decrease the grain-boundary mobility, preventing pore-boundary separation. The kinetics analysis highlights the importance of vanadium substitution in modifying the diffusion coefficient of rate-limiting species.     

Kinetics and mechanisms for the densification and grain growth of the α-alumina fibers isothermally sintered at elevated temperatures

Understanding the densification and grain growth processes is essential for preparing dense alumina fibers with nanograins. In this study, the alumina fibers were prepared via isothermal sintering at 1200, 1300, 1400, and 1500 °C for 1–30 min. The phase, microstructure, and density of the sintered fibers were investigated using XRD, SEM, and Archimedes methods. It was found that the phase transformation during the isothermal sintering enhances the densification of Al₂O₃ fibers in the initial stage, while the pores generated during the phase transformation retard the densification in the later period. The kinetics and mechanisms for the densification and grain growth of the fibers were discussed based on the sintering and grain growth models. It was revealed that the densification process of the fibers sintered at 1500 °C is dominated by the lattice diffusion mechanism, while the samples sintered at 1200–1400 °C are dominated by the grain boundary diffusion mechanism. The grain growth of the Al₂O₃ fibers sintered at 1200–1300 °C is governed by surface-diffusion-controlled pore drag, and that sintered at 1400 °C is dominated by lattice-diffusion-controlled pore drag.     

Sintering paths and mechanisms of pure MgAl2O4 conventionally and microwave sintered

In this paper, a comparative study between conventional and microwave sintering of pure spinel MgAl₂O₄ is presented. The goal is to clarify and identify the possible microwave effects on densification and microstructure of the pure spinel. Sintering trajectories obtained for microwave and conventional sintering are similar and converge into a unique trajectory. Therefore, microwave processing does not refine the grain size of pure spinel. The dominant mechanism of initial and intermediate stages of sintering was determined from the shrinkage curves and sintering trajectory. It was shown that densification is mostly controlled by grain boundary diffusion for both processes. Porosity of microwave and conventionally sintered samples was also characterized by mercury porosimetry and BET analysis. The evolution of the open porosity and pore size distribution is the same whatsoever the process used. This work shows that microwave sintering does not differ that much from conventional sintering on pure MgAl₂O₄ material.     

Densification mechanisms of UO2 consolidated by spark plasma sintering

Despite the growing interest in the spark plasma sintering (SPS) of uranium dioxide, its sintering mechanisms have yet to be studied in great detail. Herein we propose a direct method to calculate the apparent activation energy for densification, Q_act, and the stress exponent, n, for SPS of nearly stoichiometric UO₂. A set of experiments performed at different heating rates (CHR) and different pressures levels allowed us to calculate Q_act and n, respectively, though we were limited to a theoretical density between 50% to 75 %. The master sintering curve was employed as a complementary method to compare Q_act. The average values were Q_act =96 kJ/mol (CHR), Q_act = 100 kJ/mol (MSC) and n = 1.4. We have therefore proposed grain boundary diffusion coupled with grain boundary sliding as the densification mechanism. The activation energy in SPS tends to be lower compared with that in other processes like conventional sintering (250?450 kJ/mol), creep (350?550 kJ/mol) and hot pressing (222 kJ/mol and 480 kJ/mol).This decrease could be due to the effect of the electric field combined with the higher heating rates, typical of SPS.     

Reassessing cold sintering in the framework of pressure solution theory

Cold sintering densification and coarsening mechanisms are considered from the perspective of the non-equilibrium chemo-mechanical process known in Earth Sciences as pressure solution creep (or dissolution-precipitation creep). This is an important mechanism of densification and deformation in many geological rock formations in the Earth’s upper crust, and although very slow in nature, it is of direct relevance to the cold sintering process. In cold sintering, we select particulate materials and identify experimental processing parameters to significantly accelerate the kinetics of dissolution-precipitation phenomena, with appropriate consideration of chemistry, applied stress, particle size and temperatures. In the theory of pressure solution, pressure-driven densification is considered to involve the consecutive stages of dissolution at grain contact points, then diffusive transport along the grain boundaries towards open pore surfaces, and then precipitation, all driven by chemical potential gradients. In this study, it is shown that cold sintering of BaTiO₃, ZnO and KH₂PO₄ (KDP) ceramic materials proceeds by the same type of serial process, with the pressure solution creep rate being controlled by the slowest kinetic step. This is demonstrated by the values of activation energy (E_a) for densification, which are in good agreement with the existing literature on dissolution, precipitation, or coarsening. The influence of pressure on the morphology of ZnO grains also supports the pressure solution mechanism. Other characteristics that can be understood qualitatively in terms of pressure solution are observed in the in systems such as BaTiO₃ and KDP. We further consider activation energies for grain growth with respect to the precipitation process, as well as evidence for coalescence and Ostwald ripening during cold sintering. For completeness we also consider materials that show significant plastic deformation under compression. Our findings point the way for new advances in densification, microstructural control, and reductions in cold sintering pressure, via the use of appropriate transient solvents in metals and hybrid organic-inorganic systems, such as the Methylammonium lead bromide (MAPBr) perovskite.     

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.     

Densification and grain growth during solid state sintering of LaPO4

This work is devoted to the kinetic study of densification and grain growth of LaPO₄ ceramics. By sintering at a temperature close to 1500 °C, densification rate can reach up to 98% of the theoretical density and grain growth can be controlled in the range 0.6–4 μm. Isothermal shrinkage measurements carried out by dilatometry revealed that densification occurs by lattice diffusion from the grain boundary to the neck. The activation energy for densification (E_D) is evaluated as 480 ± 4 kJ mol⁻¹. Grain growth is governed by lattice diffusion controlled pore drag and the activation energy (E_G) is found to be 603 ± 2 kJ mol⁻¹. The pore mobility is so low that grain growth only occurs for almost fully dense materials.     

Effect of Pore Distribution on Microstructure Development: I, Matrix Pores

A model has been developed to describe the effect of the matrix (first-generation) pore distribution on microstructure development in the final stages of sintering. A model of simultaneous densification and grain growth was used to predict the effects of the number of pores per grain and the pore size distribution on microstructure evolution. Increasing the number of pores per grain was predicted to increase the densification rate, the grain growth rate, and the relative densification rate/grain growth rate ratio. Narrowing the pore size distribution was predicted to inhibit grain growth initially and to increase the densification rate indirectly. Overall, the pore distribution was predicted to have a strong influence on microstructure development and sintering kinetics.     

Segregation-controlled densification and grain growth in rare earth-doped Y2O3

Cation doping of Y₂O₃ is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La³⁺ and Gd³⁺), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y₂O₃ during field-assisted sintering/spark plasma sintering (FAST/SPS). While Gd³⁺ is showing no significant effect on densification, La³⁺ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La³⁺ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration-corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La³⁺ to the grain boundary, which could not be observed for Gd³⁺. The results indicate that segregated La³⁺ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self-diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation.     

Densification mechanisms and microstructural evolution during spark plasma sintering of boron carbide powders

Micron-sized boron carbide (B₄C) powders were subjected to spark plasma sintering (SPS) under temperature ranging from 1700 °C to 2100 °C for a soaking time of 5, 10 and 20 min and their densification kinetics was determined using a creep deformation model. The densification mechanism was interpreted on the basis of the stress exponent n and the apparent activation energy Q_d from Harrenius plots. Results showed that within the temperature range 1700–2000 °C, creep deformation which was controlled by grain-boundary sliding or by interface reaction contributed to the densification mechanism at low effective stress regime (n = 2,Q_d = 459.36 kJ/mol). While at temperature higher than 2000 °C or at high stress regime, the dominant mechanism appears to be the dislocation climb (n = 6.11).     

Dielectric properties of Ba1−xSrxTiO3 ceramics prepared by microwave sintering

A comparative study on the dielectric properties of Ba_1?xSr_xTiO₃ (x=0.1–0.6) ceramics prepared by microwave sintering (MS) and conventional sintering (CS) has been done. It was found that MS samples need lower temperature and much shorter time than CS samples to obtain the same degree of densification. Compared with CS samples, MS samples possessed smaller grain size, better densification and more uniform grain growth. The dielectric properties of the samples were measured as a function of temperature. It was observed that the dielectric constant was higher for MS samples compared with that of CS samples especially in the ferroelectric phase.     

Effect of Pore Distribution on Microstructure Development: III, Model Experiments

Model experiments have been conducted on a series of alumina samples in which the microstructures have been tailored to conform to the classical configuratins depicted in the models of final-stage sintering. Simultaneous measurements of sintered density, grain size, pore number density, and pore size distribution were made as a function of sintering time at constant temperature (1850°C). The data supported a model of grain-boundary-diffusion-controlled densification and surface-diffusion-controlled grain growth. An atom flux equation for grain-boundary diffusion transport was deduced from the data. The kinetics analysis highlights the importance of incorporating the number of pores per grain as an independent variable in mechanistic studies of final-stage sintering. The number of pores per unit volume was identified as a critical factor influencing densification kinetics. The effect of pore distribution on microstructure development was simulated for comparison with the data obtained from the model experiments.     

On the modeling of the co2‐catalyzed sintering of calcium oxide

A comprehensive mathematical model for the CO₂‐catalyzed sintering of CaO is proposed. It takes into account the mechanisms of surface diffusion and grain boundary diffusion, catalyzed by CO₂ chemisorption and dissolution, respectively. In addition, the model proposed here considers the change in pore size distribution during sintering, grain growth, and the densification by lattice diffusion, which is the intrinsic sintering mechanism of the CaO. Model predictions are validated using experimental data on the sintering of two CaO samples, one of them derived from pure CaCO₃ and the other from limestone. It is found that impurities in limestone‐derived CaO do not significantly affect the CO₂ dissolution or chemisorption processes; however, they strongly increase the rate of sintering by lattice diffusion. It is also established that low temperatures and CO₂ partial pressures promote the coarsening by surface diffusion, whereas high temperatures and CO₂ partial pressures favor densification. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3286–3296, 2017     

Effect of silica doping on the densification and grain growth in zinc oxide

The ability of silica (SiO₂) in controlling the densification and grain growth behavior of nano crystalline zinc oxide (ZnO) has been systematically studied. It has been observed that SiO₂ acts as a sintering inhibitor in the ZnO–SiO₂ system up to 4 wt.% limiting value beyond which densification behavior of the system remains almost unchanged, especially above 1100 °C. The addition of SiO₂ to ZnO retards grain growth which in turn results a finer ultimate grain size as compared to the undoped ZnO. However, stabilization in grain size occurs at ≥4 wt.% SiO₂ addition. It has been observed that SiO₂ incorporation changes the grain growth mechanism up to 4 wt.% addition, beyond which no remarkable changes was noticed. The grain growth (n) shows distinctly different slopes as a function of sintering time for the SiO₂ doped ZnO systems than undoped ZnO. The different slopes tend to indicate that different diffusion mechanisms and probably the formation of a secondary phase (Zn–Si–O) at the grain boundary control the densification and grain growth. The thermal expansion coefficient of the system has been found to decrease substantially beyond 4 wt.% SiO₂ addition to ZnO.     

Densification mechanism and microstructure characteristics of nano- and micro- crystalline alumina by high-pressure and low temperature sintering

Fully dense ceramics with retarded grain growth can be attained effectively at relatively low temperatures using a high-pressure sintering method. However, there is a paucity of in-depth research on the densification mechanism, grain growth process, grain boundary characterization, and residual stress. Using a strong, reliable die made from a carbon-fiber-reinforced carbon (C_f/C) composite for spark plasma sintering, two kinds of commercially pure α-Al₂O₃ powders, with average particle sizes of 220 nm and 3 μm, were sintered at relatively low temperatures and under high pressures of up to 200 MPa. The sintering densification temperature and the starting threshold temperature of grain growth (T_sg) were determined by the applied pressure and the surface energy relative to grain size, as they were both observed to increase with grain size and to decrease with applied pressure. Densification with limited grain coarsening occurred under an applied pressure of 200 MPa at 1050 °C for the 220 nm Al₂O₃ powder and 1400 °C for the 3 μm Al₂O₃ powder. The grain boundary energy, residual stress, and dislocation density of the ceramics sintered under high pressure and low temperature were higher than those of the samples sintered without additional pressure. Plastic deformation occurring at the contact area of the adjacent particles was proved to be the dominant mechanism for sintering under high pressure, and a mathematical model based on the plasticity mechanics and close packing of equal spheres was established. Based on the mathematical model, the predicted relative density of an Al₂O₃ compact can reach ～80 % via the plastic deformation mechanism, which fits well with experimental observations. The densification kinetics were investigated from the sintering parameters, i.e., the holding temperature, dwell time, and applied pressure. Diffusion, grain boundary sliding, and dislocation motion were assistant mechanisms in the final stage of sintering, as indicated by the stress exponent and the microstructural evolution. During the sintering of the 220 nm alumina at 1125 °C and 100 MPa, the deformation tends to increase defects and vacancies generation, both of which accelerate lattice diffusion and thus enhance grain growth.     

A case study of the effect of Ni substitution on the sintering behaviours of Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membranes

ABSTRACT

The sintering behaviours of undoped and 8 mol% B-site Ni-doped Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) under various conditions are systematically investigated. Ni doping increases the lattice parameter of BSCF. The grain size of the undoped BSCF increases from 1.52 to 64.8 μm with increasing sintering temperature from 900 at 1150 °C, while the grain size of the Ni-doped counterpart increases from 3.06 to 34.1 μm as the sintering temperature increases from 1000 to 1200 °C. The grain growth kinetics is analysed using the expression Dⁿ = tK ₀exp(-Q/RT). The grain growth exponent n is found to be 3 for the undoped BSCF and 4.7 for the Ni-doped BSCF. The grain growth activation energy (Q) is 650.7±30 kJ/mol for the undoped BSCF and 803.4±37 kJ/mol for the Ni-doped BSCF. This indicates that the Ni dopant has a negative influence on the densification and grain growth of BSCF.     

Sintering and densification of nanocrystalline ceramic oxide powders: a review

Abstract

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth.     

Preparation of tabular corundum aggregate from different sources alumina raw powder: Sintering kinetics and establishment of kinetics model

In this paper, we report the use of four types of commercial alumina raw powders as raw materials for the preparation of tabular corundum aggregates under the same conditions. The influence of the transition phases of alumina raw powder on the sintering kinetics of tabular corundum is discussed, the sintering model of materials with pseudomorphic structure is established, and the mechanisms underlying the different performances of various commercial tabular corundum samples are evaluated. The following conclusions were drawn based on the results of the study. (1) A double tetrakaidecahedron model was established and was shown to satisfactorily describes the sintering mechanism of alumina raw powder with pseudomorphic structure, which accords with the porosity change trend of sintered body and provides a basis for perfecting the sintering theory. (2) Compared with the other transition phases, γ-Al₂O₃ shows the largest phase transformation volume contraction, which provides the driving force for the sintering process via an increase in surface energy and mainly acts in the densification and grain growth stages. Thus, high-quality refractory raw materials are prepared with optimized physical properties and Intracrystalline pores or pore clusters in the crystal structure. The preparation of these high-quality refractory products is of importance for prolonging the life of these materials and also meeting rising energy demands.