Activation Energy For Densification Research Articles

Simultaneous control of mechanical strength and hierarchical structure in freeze‐casted porous alumina by two‐step sintering

AbstractTo increase mechanical strength of porous ceramics, here, an effective two‐step sintering (TSS) technique capable of producing highly porous alumina with enhanced mechanical strength is suggested. Based on the sintering theories, here, a significantly lower activation energy for densification at low temperature region allowed a beneficial temperature range for the TSS to be deduced. With a specific TSS regime (T1 = 1550°C and T2 = 1400°C), significantly higher compressive strength levels (8.00–15.24 MPa) were measured with an apparent porosity of 56.49% compared to conventional sintering (1.03–1.86 MPa) with similar apparent porosity of 57.84%. Specifically, another TSS regime (T1 = 1550°C and T2 = 1380°C) left submicron‐sized open pores within the lamellar walls, providing a hierarchical porous structure with enhanced mechanical strength. An evaluation of the mechanical stability by a finite element analysis indicated outstanding compressive strength even with small pores in the lamella walls.

Effect of the Europium during spark plasma sintering of yttrium silicate powder

Spark plasma sintering process was used to densify yttrium silicate powder doped with Europium, at temperature of 1673 K. Densification rate was evaluated during spark plasma sintering of yttrium silicate powder doped with 0, 1, 2.5 and 5 Eu weight percent. The activation energy for densification of the powder was determined at early stage of the sintering. After sintering the microstructure was analysed from sintered coupons doped with Europium+3, identifying an yttrium silicate second phase present.

Comparative study of the densification kinetics of the FCC phase Al0.3CoCrFeNi and BCC phase AlCoCrFeNi high-entropy alloys during spark plasma sintering

The densification kinetics of the FCC-matrix Al0.3CoCrFeNi and BCC-matrix AlCoCrFeNi HEAs at sintering temperatures ranging from 950 ℃ to 1050 ℃ are comparatively studied utilizing the steady-state creep model. The densification activation energy of the Al0.3CoCrFeNi and AlCoCrFeNi powder in their low-stress exponent stage is linearly fitted to be 264.91 and 220.99 kJ/mol, respectively. The densification mechanism of both the two HEAs in the low-stress exponent stage is proved to be atomic diffusion. However, the densification activation of the Al0.3CoCrFeNi in the high-stress exponent stage increases to 554.77 kJ/mol and the densification mechanism changes to creep. The densification activation energy of the AlCoCrFeNi in its high-stress exponent stage cannot be obtained by linear fitting because of the abnormal increase of the stress exponent at the sintering temperature of 1050 ℃. The BCC-FCC transformation in the AlCoCrFeNi reduces its creep resistance because of different deformation behavior in BCC (dislocation cell) and FCC phase (cross-glide and twinning). As a result, dislocation density is higher in the FCC phase (4.3 ×1013/m2) compared with the BCC phase (3.8 ×1013/m2), which benefits the densification in the creep-controlled high-stress exponent stage. Furthermore, the stronger electromigration effect induced by a higher heating rate is demonstrated to further reduce the densification activation energy from 124.20 to 72.53 kJ/mol and accelerate densification in the diffusion-controlled heating period.

Densification behavior and properties of Li4SiO4 ceramic breeder with the addition of SiC as a sintering aid

Lithium orthosilicate (Li4SiO4) is regarded as a vital tritium breeder candidate due to its favorable properties, such as high lithium contents, superior tritium release behavior, and irradiation stability. However, the poor crushing load and low thermal conductivity of the Li4SiO4 ceramic pebbles fabricated via existing methods will lead to the blocking of the tritium release channels, giving rise to thermal stress concentration. To overcome the above shortcomings, the SiC was employed as sintering aid to improve Li4SiO4 ceramic pebbles via simple wet method in this paper. The effects of SiC content on the shrinkage, internal structure evolution, and crushing load of the Li4SiO4 ceramic pebbles are comprehensively investigated, and the densification process is analyzed in detail. It was found that the addition of SiC improves the crushing load (54 N) significantly owing to decreasing the densification activation energy. Moreover, the Li4SiO4 ceramic pebbles with excess Li2CO3 further displays excellent properties at 800 °C, such as the relative density of 90.3%, the crushing load of 65.3 N, and the thermal conductivity of 1.97 W/(m·K) respectively, which is much higher than that of traditional methods (at least 1000 °C). The Li4SiO4 ceramic pebbles with homogeneous micro-pore distribution, the porosity of 7.3% were further measured by using the X-ray computer tomography technique. Therefore, this proposed tactic has great prospects for the service performance of tritium breeding materials in the blanket.

In Situ Microstructure Characterization of Potassium Di-Phosphate (KDP) Densification during Cold Sintering

In order for ceramic additive manufacturing (AM) to achieve its full potential, it is increasingly important to develop a more rigorous understanding of fundamental phenomena that govern the kinetics and thermodynamics of ceramic AM processes. In the case of additive build processes, such as direct ink write and ceramic extrusion, methods for densifying the resulting green-body product need to be considered to complement the efficiencies of ceramics AM, itself. One densification route, at least for monolithic components, built layer-by-layer, is offered by the recently developed cold sintering process, whereby high-density final product is achieved through addition of a small amount of liquid solvent and application of modest uniaxial compressive stress at relatively low temperature. In situ small-angle X-ray scattering methods and X-ray diffraction have been applied to characterize and quantify the pore morphology evolution during cold sintering for a model system: potassium di-phosphate, KH2PO4 (KDP). It is shown that both temperature and applied stress affect the densification rate, but stress has a stronger effect on the evolving morphology. A regime with an approximate linear densification rate can be identified, yielding an effective densification activation energy of ≈90 kJ/mol.

Sintering densification behavior of ZnO–SnO2 binary ceramic targets

AbstractIn this work, high‐performance ZnO–SnO2 binary ceramic targets for magnetron sputtering of transparent conductive oxide (TCO) films were prepared by pressureless oxygen atmosphere sintering. The sintering behavior and densification mechanism of the ZnO–SnO2 binary targets were analyzed by systematically studying the oxide powder state, formation process of the solid reaction phase, and evolution of the target microstructure. The data revealed that the ZnO–SnO2 powder treatment improved the sintering activity and the powder dispersion; furthermore, it promoted a mutual reaction between the different components during sintering and the homogeneity of the target composition. The densification of the pure SnO2 ceramic target was difficult to achieve, and the addition of ZnO led to a continuous interaction between the ZnO and SnO2 components. The Zn2SnO4 phase started to form, and a temporary shrinkage of the target occurred above 800°C. After formation of the stable Zn2SnO4 and SnO2 phases, the target shrunk rapidly with increasing temperature, densification occurred during growth, and the two phases started to interact. The sintering temperature provided the driving force for the target densification, with the densification activation energy of the ZnO–SnO2 binary ceramic target estimated to be 580 kJ/mol based on the master sintering curve. A binary ceramic target with a high density (99.78% relative density), a fine grain size, and a homogeneous phase structure was achieved at a temperature of 1600°C. These findings are promising for the further improvement and performance enhancement of SnO2‐based materials.

Densification behaviors of Al2O3 ceramics during flash sintering

AbstractThe densification behaviors of MgO‐doped‐Al2O3 ceramics in the flashing stage and the steady stage were investigated using the classic kinetic model. The results show that the most densification of MgO‐doped Al2O3 was completed during the flashing stage. The densification mechanism transferred from particle rearrangement resulted from Columbic force among particles under the effect of electrical field in the flashing stage to the lattice diffusion in the steady stage. Therefore, the densification rate in the steady stage dramatically decreased. Additionally, the estimated densification activation energy in the steady stage of flash sintering is 396 kJ/mol, much lower than the activation densification of lattice diffusion measured from conventional sintering, likely due to the effect of electric field/current‐induced point defects on the diffusion.

Sintering behavior of the Ti(C,N)‐based cermets with graphite or diamond additives

AbstractCombining DSC/TG–QMS analysis and dilatometry experiments at constant heating rates, the sintering behaviors of the Ti(C,N)‐based cermets with carbon additives were studied. It is found that the additives of diamond or graphite could promote the interactions among raw powders during solid‐state sintering. The outgassing behavior and endothermic effect were enhanced, thereby resulting in the increasing densification activation energy. The negative activation energy during the liquid phase sintering indicates that the dissolution–precipitation process occurs easily and provides a rapid densification path. The dissolution–precipitation process for cermets with .6‐wt% graphite or .6‐wt% diamond additive became more efficient than for cermets without additional carbon additive. The white‐core/gray‐rim grains were clustered together in the cermets with .6‐wt% graphite additive, whereas they were distributed evenly in the cermets with .6‐wt% diamond additive. Moreover, the average sizes of ceramic grains in the cermets without additional carbon additive, and with .6‐wt% graphite or .6‐wt% diamond additives, were .453, and .517, or .525 μm, respectively.

Spark plasma sintering of UO2 nanopowders: Pressure, heating rate and current effects

We analysed with different methods the densification of UO2 nanopowders in SPS under constant heating rate (CHR) and isothermal sintering conditions. The apparent activation energy of densification in SPS (75 kJ/mol with CHR method) is significantly smaller than in conventional sintering. It is shown that this is likely not an effect of the applied current. We also observed a threshold stress at 64 MPa for the transition from pressure-insensitive sintering (stress exponent n≈0) to pressure-assisted sintering, suggesting that the contribution of the capillary stresses in such nanopowders is comparable with the typical stress applied in SPS.

Measurement of apparent sintering activation energy for densification of clays

AbstractClays are raw materials with properties that are necessary for the manufacture of ceramic tiles. The characteristics of clay ceramic raw materials may vary within the same mineral deposit. Clay blending promotes better use of clay reserves, thereby increasing the applicability and life cycle of raw materials. Therefore, it is important to understand the mechanisms controlling the firing of ceramic tiles. In this study, three different clays from a clay deposit were assessed and ten formulations were prepared using the mixture design method. The formulations were analysed using differential thermal and thermogravimetric analyses and dilatometric analysis. Subsequently, the most refractory and fluxing formulations were subjected to thermal tests under various heating rates, similar to the process used for the calculation of apparent sintering activation energy for the densification of clays and for pyroplasticity tests. It is suggested that a mineral deposit can be assessed based on activation energy and thermal kinetics, expanding the alternatives available to the miner through the planning of mixtures with various clays and thus reducing the energy costs of the industrial process.

Thermodynamic and kinetic analyses of sintering in Al‐doped Y2O3 nanoparticles

AbstractThis work investigates the effects of doping on both the thermodynamics and kinetics of sintering in aluminum‐doped yttrium oxide nanoparticles (Al‐doped Y2O3), with the objective of delineating their interdependent effects at different stages of the process. Direct measurements of surface and grain boundary energies using differential scanning calorimetry showed that Al‐doping decreases both interfacial energies, leading to an increase in dihedral angle (from 152.7 ± 5.6° to 165.8 ± 5.5°) and, therefore, sintering stress. Densification and grain growth analyses showed that despite this increase in sintering stress, the onset of sintering is delayed for the Al‐doped samples, demonstrating that a large dihedral angle is a necessary but not sufficient condition for densification. The measurements of activation energies for densification and grain growth point out that Al suppresses grain boundary mobility by increasing the activation energy from 400 to 448 kJ/mol, hindering densification at the intermediate stages of sintering. At temperatures above 1150℃, grain growth is activated in the Al‐doped samples, which rapidly releases the accumulated sintering stress and exhibits a higher densification rate than in undoped Y2O3. This study demonstrates a complex interconnectivity between the thermodynamics and kinetics at different temperature ranges of sintering and reinforces the need for a comprehensive description for proper design of sintering aids.

Densification mechanism of spark plasma sintered ZrB2 and ZrB2-SiC ceramic composites

Sintering mechanisms are investigated by determining activation energy of densification (in the temperature range of 1650 °C to 1950 °C) during spark plasma sintering of ZrB2 and SiC-reinforced ZrB2 ceramic composite. The fractured surfaces of sintered composites reveal completely dense microstructures above 1800 °C, whereas porous microstructure prevailed in ZrB2 even after sintering at 1950 °C. Activation energy of 80 ± 16 kJ/mol is obtained for ZrB2 with grain boundary diffusion as governing diffusion mechanism in the temperature range of 1650 to 1950 °C. On the other hand, dominant densification mechanism of grain boundary diffusion (activation energy of 89 ± 19 kJ/mol) estimated in the range of 1650 to 1800 °C changed to lattice diffusion (activation energy of 601 ± 10 kJ/mol) with the addition of 20 vol% SiC in ZrB2. The change of prominent densification mechanism in ZrB2 with SiC, in addition to its impact on the mechanical performance with systemic increase of sintering temperature, is unravelled here.

Influence of TiO2 on the densification behaviour of Yb2O3

The effect of temperature and heating rate on the densification of ytterbia (Yb2O3), with and without titania (TiO2) doping was investigated. It is shown that up to a certain doping level, titania doping enhances the densification behaviour of ytterbia. The effect of titania doping on crystal structure confirms that titania is substitutionally incorporated in ytterbia up to the solubility limit, which corresponds well with the densification results. The increased densification rate of titania-doped ytterbia is attributed to the formation of cation vacancy and lattice distortion. Using constant heating rate experiments, the activation energy for densification has been calculated and it is shown that in the intermediate density range (60 % to 85 %), the activation energy is independent of the density. Titania doping increases the activation energy for densification.

Densification, grain growth mechanism and mechanical properties of Mo[sbnd]10Nb refractory targets fabricated by SPS

Refractory Mo10Nb targets were fabricated by spark plasma sintering (SPS) technology at an extensive range of preparation temperatures (900–1800 °C) and holding times (6–15 min) under 30 MPa. The densification and grain growth mechanisms of Mo10Nb targets sintered by SPS were revealed by the power-creep and grain growth models. Different effective stress (n) and grain growth exponents (m) corresponded to various densification and grain growth mechanisms. The densification mainly dominated at temperatures of 900 to 1300 °C, whereas the grain growth predominantly occurred at temperatures of 1400 to 1800 °C. The densification was basically controlled by particle rearrangement when n < 1. Besides, grain boundary diffusion was the major densification mechanism for n = 1–2, and the densification mechanism was the dislocation climbing when n was close to 3. The activation energies for densification were calculated to be 119.30 kJ/mol (n = 1) and 271.79 kJ/mol (n = 2). In addition, the grain boundary diffusion was regarded as the primary grain growth mechanism. The activation energies for grain growth were calculated to be 188.14 kJ/mol (t = 6 min), 172.49 kJ/mol (t = 10 min) and 137.85 kJ/mol (t = 15 min). Besides, compared with the grain size, the porosity was identified to be the cardinal influencing factor on the Vickers hardness of Mo10Nb targets sintered by SPS.

Influence of Y2O3 contents on sintering and mechanical properties of B4C-Al2O3 multiphase ceramic composites

In this study, B4C-Al2O3 multiphase ceramic composites were prepared by spark plasma sintering (SPS) and Y2O3 was used as a sintering aid. Results show that in the range of 1350–1500°C, nearly full dense B4C-Al2O3 multiphase ceramic composites with a high densification rate (4×10−3/s) can be fabricated. Addition of Y2O3 can lead to reduce activation energy of densification and plastic flow mechanism during sintering of the ceramic composites. At the same time, Y2O3 can react with Al2O3 to form liquid Y3Al5O12, which can effectively reduce the densification temperature of the ceramic composites. With the increase of Y2O3 content, the temperature was lowered from 1500 °C to 1350 °C. When the Y2O3 content was 1.0 vol.%, relative density, hardness, fracture toughness, and flexural strength of the ceramic composites were 98.60%, 23.75 GPa, 4.89 MPa•m1/2, and 464.96 MPa, respectively. The toughening mechanism of the ceramic composites followed particle toughening and micro-crack toughening. The internal grains of the composites were tightly bonded, they were mainly fractured in the transgranular form and the cross section was relatively rough with a high bending strength of 450 MPa.

The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.

Densification kinetics of nano-hematite using microwave assisted dilatometry

The aim of this paper was to study the densification kinetics of nano-hematite using a microwave assisted dilatometry at 2.45 GHz and kinetics models as an approximation to understand the sintering behavior using microwave heating. A horizontal dilatometer coupled to a multimodal microwave oven and conventional horizontal dilatometer were used to compare the results. The hematite was synthesized by the modified sol-gel method in order to ensure the purity, nanometric scale and intermediate particle size distribution in the starting powder. Novel values of apparent activation energy for densification of nano-hematite were obtained for the initial and intermediate stages of sintering using microwave heating at 2.45 GHz. The results showed that the microwave heating needs 10% less energy to start the initial sintering stage and 56% less energy for densification in the intermediate stage both compared with conventional heating sintering. After the final stage of sintering, smaller grain growth was obtained in microwave heating, as well as a reduction in the final grain size considering the increase in the heating rate.

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, Qact, 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 Qact 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 Qact. The average values were Qact =96 kJ/mol (CHR), Qact = 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.

Master sintering curve of binary mutually immiscible molybdenum-copper powders

Abstract In this work, the densification behavior (including shrinkage response and density fluctuation) and the dominant diffusion mechanism of binary mutually immiscible Molybdenum–Copper (Mo–Cu) composite powders are studied by a master sintering curve (MSC) strategy. Mo–Cu powders with an average particle size of 191.6 nm are sintered at temperatures up to 1300 °C with constant heating rates. The apparent densification activation energy is determined by mean residual square (MRS) method. The obtained value for apparent activation energy and the microstructures variation of sintered Mo–Cu indicate that surface diffusion is a dominant densification mechanism during the initial stage of sintering, while volume and (or) grain boundary diffusion dominate(s) at higher-temperature sintering stage. The MSC is constructed and subsequently validated by non-isothermal sintering experiments. Results demonstrate that the MSC is a promising strategy in estimating the densification and microstructure evolution behavior during sintering of Mo–Cu composites and could be conducive to design sintering process reasonably for Mo–Cu composites or other binary powders system.

Impact of high energy ball milling on densification behaviour of magnesium aluminate spinel evaluated by master sintering curve and constant rate of heating approach

The present study investigates the influences of high energy ball-milling on densification of a commercial sub-micron magnesium aluminate spinel powder. High energy milling decreases the crystallite size and modifies the particle size distribution of powder resulting in better compaction of green bodies. Milling, also, increases the number of structural defects.The concept of master sintering curve (MSC) and the constant rate of heating (CRH) approach were employed to evaluate the activation energy of sintering. It was demonstrated that, according to MSC, the milled samples exhibit lower densification activation energy compared to the as-received powder (530 kJ mol−1 vs 750 kJ mol−1). Analysing the densification using CRH approach reveals that the sintering activation energy of milled samples remains constant during the sintering process, while that of as-received samples increases with densification. The estimated activation energy using MSC method is thus an average value of the values measured over the whole process of sintering.