Sintering Activation Energy Research Articles

Sintering behavior and physical properties of ZnO‐modified BaZrO3 ceramics

AbstractBarium zirconate (BaZrO3) ceramics possesses poor sintering properties, in order to improve its densification behavior and reduce the sintering temperature, zinc oxide (ZnO) was selected as a sintering aid in this study. The effects of ZnO content on the sintering kinetic parameters and physical properties of BaZrO3 ceramic samples heated at 1300–1600°C were investigated. As the addition of ZnO increased from 0 to 5 wt.%, the calculated sintering activation energy decreased from 117.3 to 19.22 kJ/mol. The change of activation energy leads to the decrease of the optimum adding amount of ZnO with the increase of heat treatment temperature, showing 1300°C (5 wt.%), 1400°C (3 wt.%), 1500°C (2 wt.%), 1600°C (1 wt.%). The 1500°C sintered sample with 2 wt.% ZnO achieved a relative density of 96.00% and a maximum cold modulus of rupture (CMOR) of 94.5 MPa.

Effects of Ti Containing Cu-Based Alloy on Sintering Mechanism, Element Diffusion Behavior and Physical Properties of Glass-Ceramic Bond for Cubic Boron Nitride Abrasive Tool Materials

Ti containing Cu-based (TC) alloy reinforced glass-ceramic bond was fabricated for cubic boron nitride (CBN) abrasive tool materials, and its crystal composition, phase transformation, sintering activation energy, microstructure, element diffusion mathematical model, physical properties, and the bonding mechanism between the TC alloy reinforced glass-ceramic bond and the CBN grains were systematically investigated. The results showed that the structure, composition and sintering behavior of glass-ceramic were influenced by TC alloy adding. The generated TiO2 affected obviously the precipitation of β-quartz solid solution Li2Al2Si3O10, thus improving the relative crystallinity, mechanical strength and thermal properties. By establishing the mathematical model for element diffusion, the element diffusion coefficients of Ti and Cu were 7.82 and 6.98 × 10-11 cm2/s, respectively, which indicated that Ti diffused better than Cu in glass-ceramic. Thus, Ti4+ formed a strong Ti-N chemical bond on the CBN surface, which contributed to improving the wettability and bonding strength between CBN and glass-ceramic bond. After adding TC alloy, the physical properties of the composite were optimized. The porosity, bulk density, flexural strength, Rockwell hardness, CTE, and thermal conductivity of the composites were 5.8%, 3.16 g/cm3, 175 MPa, 90.5 HRC, 3.74 × 10-6 °C-1, and 5.84 W/(m·k), respectively.

Spark Plasma Sintering of Si3N4 Ceramics with Y2O3–Al2O3 (3%–10% wt.) as Sintering Additive

The ceramic samples fabricated by spark plasma sintering of powder mixtures based on silicon nitride (Si3N4) were investigated. The powder mixtures were made by wet chemical methods from commercial α-Si3N4 powder (the particle size <5 μm) and Y2O3-Al2O3 sintering additive (3% to 10% wt.). Sintering was carried out at the heating rate of 50 °C/min and the load of 70 MPa until the shrinkage end. The powder mixtures and ceramic samples were characterized by scanning electron microscopy and X-ray diffraction. The shrinkage of the powder mixtures during sintering was analyzed, and the activation energy of sintering was calculated according to the Young-Cutler model. The density, microhardness, and fracture toughness of the ceramic samples were also measured. All samples had high relative densities (98%–99%), Vickers microhardness 15.5–17.4 GPa, and Palmquist fracture toughness, 3.8–5.1 MPa∙m1/2. An increase in the amount of sintering additive led to a decrease in the shrinkage temperature of the powder mixtures. The amount of β-Si3N4 in the ceramics decreased monotonically with the increasing amount of sintering additive. The shrinkage rate did not decrease to zero when the maximum compaction was reached at 3% wt. of the sintering additive. On the contrary, it increased sharply due to the beginning of the Si3N4 decomposition.

Исследование влияния добавок LiCl и LiF на кинетику электроимпульсного плазменного спекания мелкозернистого оксида алюминия

The effect of the lithium chloride and lithium fluoride additives (2 wt. %) on the kinetics of Spark Plasma Sintering of industrial fine alumina powder was investigated. The Al2O3 + 2 % LiF powder was obtained by mixing the α-Al2O3 fine powders with the aqueous solution of LiF. The Al2O3 + 2 % LiCl fine powder was obtained by joint grinding of the components in a planetary mill. The sintering of the powders was performed with the heating rates of 10 and 50 °C/min up to the temperature corresponding to the end of the shrinkage. The ceramics sintered with the heating rate of 10 °C/min had the relative density of 97.4 – 98.7 %. The addition of lithium fluoride into the alumina powder was found to allow reducing the temperature of the beginning of the intensive powder shrinkage from 1400 – 1500 °C down to 1255 – 1335 °C. LiCl was shown to evaporate at low heating temperatures and not to affect the compaction intensity of the Al2O3 powder. The presence of the overheated LiF melt (not having enough time to evaporate from the specimen volume completely) in the ceramics leads to the appearance of the residual porosity and to the reduction of the hardness of the ceramics. Using the Young – Cutler equation, the powder compaction mechanisms were determined for pure alumina α-Al2O3 and for the powder compositions with the LiCl and LiF additives in the rapid heating conditions. The sintering activation energy was shown to be close to the one of the grain boundary diffusion. The LiF melt was found to promote the sliding of the Al2O3 fine particles during the low-temperature compaction stage.

Mechanism of thermal gradients in spark plasma sintering of titanium and the resultant graded microstructure: A novel strategy of incorporating master sintering curve into finite element modeling

Spark plasma sintering (SPS) is an efficient consolidation process for metals and ceramics because of its high heating rates, lower sintering temperatures and shorter holding time than other sintering processes. To understand and utilize the thermal gradients that occur in SPS, the present work developed a strategy to incorporate master sintering curve (MSC) into a thermal-electric (TE) finite element modeling (FEM) to enable continuous nodal-specific simulation of the evolution of thermal and electrical properties of the powder compact with respect to both relative density and temperature during SPS. This combined model is proposed as an alternative to the thermal-electric-mechanical fully coupled model that is more computationally expensive and difficult to parameterize experimentally. Pure Ti was selected as the material for experimental verification because of its technological importance for aerospace and defense applications. While lab-scale Ti samples were sintered for construction and validation of MSC, a scaled-up Ti sample with diameter of 70 mm was made via SPS, and its microstructure were characterized to verify the accuracy of the MSC - TE FEM combined model. Both modeling and experimental results showed that the local relative density of the pure Ti decreased as the location moved radially from center to surface. The location-specific relative densities predicted by the combined model were within ±1.5% deviation from the experimental results, which verified the ability and accuracy of the MSC-TE FEM combined model to guide experimental design and parameter selection for control and utilization of gradients in SPS to achieve graded microstructure. • SPS of pure Ti has an apparent sintering activation energy of of 156.5 kJ/mol • Master sintering curve is incorporated into FEM for improved prediction of gradients • The accuracy of MSC-FEM combined model is experimentally validated • Deviation between the experimental and simulated results are <1.5%

Sintering of Ce3+-doped yttria nanoparticles prepared by precipitation method

Cerium doped yttrium oxide nanoparticles with various Ce3+ concentrations between 0.001 and 0.010 at% have been synthesised by precipitation method using ammonium hydroxide as a precipitation agent. The synthesised powders are characterised by a mean particle size of ca. 55 nm. Highly dense specimens, with a relative density> 98.8%, were obtained by sintering the green compacts shaped by pressure filtration, at 1550 °C for 3 h in air. The sintering behaviour of Ce3+ doped Y2O3 was studied by constructing Master Sintering Curves (MSC); the results showed that the apparent activation energy of sintering for Ce3+ doped Y2O3 increases with the increase of cerium concentration. The segregation of larger Ce3+ cations in the grain boundaries is likely to be responsible for the increase in the sintering activation energy.

Study on the Densification of Osmium by Experiment and First Principle Calculations.

The sintering of osmium is critical for the preparation of raw material targets for film coating, which is the main application area of osmium. In order to get a better understanding of the intrinsic mechanism of densification of osmium, a serial study on the sintering behavior of osmium has been made in this study. By the master sintering curve (MSC) and constant heating rate (CHR) method, the sintering activation energy of nanosized osmium is evaluated to be about 340 kJ/mol, which is higher than most other metals. The density-functional theory calculation indicates the higher energy barrier of the surface atom and vacancy migration and lacking migration tunnel of inner point vacancies. For example, the diffusion of osmium atoms on the surface of particles is mainly limited by Os (1010), which has an energy barrier as high as 1.14 eV, that is higher than the W atom on W (110) of 0.99 eV. The vacancy migration energy barrier inside osmium's grains is higher than 3.0 eV, while that of W is only 1.7 eV. This means that it is more difficult for osmium to achieve a high density compared with W, which is consistent with the experimental results. Accordingly, the proposed strategy provides a new opportunity to design a sintering process for target fabrication with excellent properties for various applications.

Influence of Graphene Sheets on Compaction and Sintering Properties of Nano-Zirconia Ceramics.

The use of a nanostructured graphene-zirconia composite will allow the development of new materials with improved performance properties and a high functionality. This work covers a stepwise study related to the creation of a nanostructured composite based on ZrO2 and graphene. A composite was prepared using two suspensions: nano-zirconia obtained by sol-gel synthesis and oxygen-free graphene obtained sonochemically. The morphology of oxygen-free graphene sheets, phase composition and the morphology of a zirconia powder, and the morphology of the synthesized composite were studied. The effect of the graphene sheets on the rheological and sintering properties of a nanostructured zirconia-based composite powder has been studied. It has been found that graphene sheets in a hybrid nanostructure make it difficult to press at the elastic deformation stage, and the composite passes into the plastic region at a lower pressure than a single nano-zirconia. A sintering mechanism was proposed for a composite with a graphene content of 0.635 wt%, in which graphene is an important factor affecting the process mechanism. It has been determined that the activation energy of the composite sintering is more than two times higher than for a single nano-zirconia. Apparently, due to the van der Waals interaction, the graphene sheets partially stabilize the zirconia and prevent the disordering of the surface monolayers of its nanocrystals and premelting prior to the sintering. This leads to an increase in the activation energy of the composite sintering, and its sintering occurs, according to a mixed mechanism, in which the grain boundary diffusion predominates, in contrast to the single nano-zirconia sintering, which occurs through a viscous flow.

Effect of powder size and geometry on consolidation of Mo30%W alloy by spark plasma sintering

Molybdenum alloys are important structural components for a variety of high temperature applications in the aerospace and nuclear industries. Spark plasma sintering has been successful in producing highly dense refractory metals with desirable microstructural characteristics, where the final microstructure and sintering mechanisms depend heavily on the starting powder size and morphology. To understand how powder size and morphology affect consolidation of a molybdenum 30 wt% tungsten alloy (Mo30W), three different powder feedstocks were sintered and the solid compacts characterized. These powders were prepared utilizing chemical reduction, ball-milling, and gas atomization processes. Additionally, to determine the effect of particle size on sintering, two different size distributions of the gas atomized material were also tested. In-situ sintering dilatometry data was analyzed to determine the apparent activation energy for the initial stage sintering of each powder. Powder geometry was found to have the greatest impact on the sintering of Mo30W powders, with the rougher, more angular powders densifying at lower temperatures compared with the smoother spherical powders densifying at higher temperatures. Powder size contributed much less to the densification of the powders using SPS. From these experiments, optimum sintering temperatures can be determined, with 1600 °C being sufficiently high to fully densify the chemical reduction and ball-milled materials and a maximum hold temperature of >1800 °C being required for the spherical atomized powders.

Analysis of sintering behavior of alumina green bodies molded under a strong magnetic field based on master sintering curve theory

Understanding and controlling the anisotropic sintering shrinkage behavior of the green body in the strong magnetic field molding technique bring out the superior functions of ceramics. This study aimed to elucidate the sintering behavior of green bodies molded under a strong magnetic field using the master sintering curve (MSC) theory. Green bodies were prepared to obtain MSCs in the directions perpendicular and parallel to the applied magnetic field. A unique MSC of the alumina green bodies molded with and without a magnetic field was obtained using sintering shrinkage ratio estimated by various heating rates. The apparent activation energy of sintering in the direction perpendicular and parallel to the applied magnetic field was 663 kJ/mol, independent of the measured direction, which is higher than that without an applied magnetic field (562 kJ/mol). The (0001) planes of the sintered body obtained from the green body molded in the magnetic field were oriented perpendicular to the magnetic field, whereas the samples without a magnetic field were randomly oriented. Consequently, we found that the grain boundaries with high consistency in the sample applied to a magnetic field should increase the apparent activation energy because of its grain orientation.

The Importance of Surface Oxygen for Lithiation and Morphology Evolution during Calcination of High‐Nickel NMC Cathodes

AbstractNanoscale morphology has a direct impact on the performance of materials for electrochemical energy storage. Despite this importance, little is known about the evolution of primary particle morphology nor its effect on chemical pathways during synthesis. In this study, operando characterization is combined with atomic‐scale and continuum simulations to clarify the relationship between morphology of cathode primary particles and their lithiation during calcination of LiNi0.8Mn0.1Co0.1O2 (NMC‐811). This combined approach reveals a key role for surface oxygen adsorption in facilitating the lithiation reaction by promoting metal diffusion and oxidation, and simultaneously providing surface sites for lithium insertion. Furthermore, oxygen surface termination is shown to increase the activation energy for sintering, leading to smaller primary particle sizes at intermediate temperatures. Smaller particles provide both shorter diffusion lengths for lithium incorporation and increased surface site density for lithium insertion. These insights provide a foundation for more tailored syntheses of cathode materials with optimized performance characteristics.

Estimate microstructure development from sintering shrinkage: A kinetic field approach

Methods such as the master sintering curve (MSC) or the kinetics method proposed by Wang and Raj (WR) are well-known techniques for determining the activation energy of sintering. However, a comprehensive sintering model also requires knowledge of porosity-dependent moduli, various constants, and grain growth behaviour. In this work, we detail a new method that allows the step-by-step identification of all the required sintering parameters. It is based on an experimental design that employs the MSC method and does not require additional tests. This work aims to estimate the grain growth behaviour during sintering from the shrinkage observed during the final stage, avoiding the use of a time-consuming interrupted-sintering study and subsequent analysis by microscopy. The grain growth behaviour was estimated by determining the densification behaviour and temperature-dependent parameters at the intermediate stage of sintering followed by determination of grain growth behaviour during the final stage based on the observed reduction in the rate of densification. The experimental MSC was incorporated into a Skorohod–Olevsky-based model, allowing the sintering behaviour to be predicted with high accuracy. This method is superior to the traditional MSC approach as it allows the sintering trajectory to be modelled and incorporates parameters that are compatible with finite element simulation.

Effect of sintering activation energy on Si3N4 composite ceramics

This study investigated the activation energy and kinetic characteristics of silicon nitride (Si3N4) composite ceramics produced using different preparation methods. The effects of the linear shrinkage, expansion ratio, and heating rate on the sintering process were analysed in detail. The obtained results reveal that the finer particle size produced using the urea homogeneous precipitation method obviously enhanced the densification rate and reduced the atomic diffusion distance. Moreover, the densification sintering process was carried out efficiently, and the largest densification rate was achieved in advance. According to the Arrhenius curve, the activation energy of the Si3N4 composite ceramics calculated using the urea homogeneous precipitation method (310.94 kJ/mol) was lower than that achieved using the mechanical ball-milling method (365.11 kJ/mol). Additionally, the flexural strength and hardness of the Si3N4 composite ceramic prepared using the urea homogeneous precipitation method were 740 ± 42 MPa and 16.20 ± 30 GPa, respectively, which is attributed to this composite ceramic's higher diffusion rate and small grain size.

Densification behaviour of pure copper processed through cold pressing and binder jetting under different atmospheres

PurposeBinder jetting is a promising route to produce complex copper components for electronic/thermal applications. This paper aims to lay a framework for determining the effects of sintering parameters on the final microstructure of copper parts fabricated through binder jetting.Design/methodology/approachThe knowledge gained from well-established powder metallurgy processes was leveraged to study the densification behaviour of a fine high-purity copper powder (D50 of 3.4 µm) processed via binder jetting, by performing dilatometry and microstructural characterization. The effects of sintering parameters on densification of samples obtained with a commercial water-based binder were also explored.FindingsSintering started at lower temperature in cold-pressed (∼680 °C) than in binder jetted parts (∼900 °C), because the strain energy introduced by powder compression reduces the sintering activation energy. Vacuum sintering promoted pore closure, resulting in greater and more uniform densification than sintering in argon, as argon pressure stabilizes the residual porosity. About 6.9% residual porosity was obtained with air sintering in the presence of graphite, promoting solid-state diffusion by copper oxide reduction.Originality/valueThis paper reports the first systematic characterization of the thermal events occurring during solid-state sintering of high-purity copper under different atmospheres. The results can be used to optimize the sintering parameters for the manufacturing of complex copper components through binder jetting.

Исследование особенностей высокоскоростного спекания мелкозернистых сверхнизкокобальтовых твердых сплавов на основе карбида вольфрама. Часть III. Влияние графита на кинетику спекания порошков WC – Co

The features of high-speed spark plasma sintering (SPS) of plasma-chemical nanopowders WC – (0.3, 0.6, 1) wt. % Co with the addition of 0.3 and 0.5 wt. % graphite were studied. The structural features of the ultralow-cobalt hard alloys with graphite addition during SPS are due to the simultaneous influence of an increased concentration of oxygen adsorbed on the surface of plasma-chemical WC – Co nanoparticles during mixing with graphite, and the effect of graphite, which leads to a decrease in activation energy of sintering due to a decrease in the intensity of formation of η-phase particles in “oxidized” WC – Co nanopowders, as well as the formation of a fairly uniform fine-grained structure. Samples of fine-grained ultralow-cobalt hard alloys with increased hardness and fracture toughness were obtained (for a WC – 0.6 wt. % Co – 0.3 wt. % C hard alloy with an average grain size of ~ (1 – 1.5) mm, the hardness is Hv = 20.2 – 20.5 GPa with a minimum crack resistance coefficient KIC = 9.2 – 10.4 MPa·m1/2).

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.

Water‐assisted cold isostatic pressing to enhance sinterability of alumina ceramics

AbstractWater‐Assisted cold isostatic Pressing (WAP) enabled pressure‐less sintering of fine grained (0.6 μm) and nearly full dense (99.3%) weakly translucent alumina ceramics. As a term of comparison, Dry Pressed (DP) samples, prepared under identical processing conditions, maintained an opaque appearance, and their relative density did not exceed 97.7%. The enhanced compaction of WAP allowed to lower the sintering temperature down to 1350°C, which is approximately 100–150°C lower than that of DP powder. WAP samples resulted in an apparent activation energy for sintering (640 kJ/mol) matching literature values, confirming that the enhanced sinterability was attributed to an improved green density rather than a change in sintering mechanisms.

Efficiently sintering of MSWI fly ash at a low temperature enhanced by in-situ pressure assistant: Process performance and product characterization

Municipal solid waste incineration (MSWI) fly ash disposal is an urgent task with some technical bottlenecks. In this study, a novel pressure-assisted sintering method was employed to treat the MSWI fly ash. A series of pressure-assisted sintering experiments were carried out by varying mechanical pressures and sintering temperatures, and their properties of compressive strength, density and heavy metals leaching behavior were determined to screen out the optimal conditions. Instrumental analysis of XRF, SEM, XRD and TEM-EDX and calculation kinetics were conducted to explore the enhancement mechanism of pressure-assisted sintering. With the help of mechanical pressure, a high-strength ceramic product was produced from MSWI fly ash sintered at a low temperature (400 °C), which never occurred in the conventional low-temperature sintering process. Maximum compressive strength of 218.30 ± 4.08 MPa was obtained at 400 °C and 100 MPa, which was much higher than conventional construction materials of brick and cement. In addition, the leaching concentrations of heavy metals obtained from pressure-assisted sintering process were lower than the standard limitation. The SEM and XRD results revealed that the increased mechanical properties and the decreased heavy metals leaching concentration were mainly attributed to the increased density and crystalline degree. The kinetics calculation results indicated that the sintering activation energy was much lower than the sintering process without pressure, suggesting surface diffusion and grain boundary diffusion were main sintering mechanisms in the pressure-assisted sintering process. These findings proved that pressure-assisted sintering could be a promising method to treat fly ash together with producing high-value building materials.

Sintering behavior and mechanism of tungsten powders prepared by solution combustion synthesis combined with hydrogen reduction

Nanosized tungsten powders were fabricated by solution combustion synthesis combined with hydrogen reduction. The powder had a size of 20 nm but possessed a large numbers of lattice defects. The fracture surface images at different temperatures show that the as-synthesized tungsten powder could be sintered via a pressureless process to relative density up to 95.78% at 1773 K. Kinetic analysis suggests that grain-boundary diffusion is one of the primary mechanisms of mass transport during the intermediate stage of sintering. The sintering properties are attributed to the ultrafine grain and the high sintering activation caused by the effect of the solution combustion synthesis method. It reveals in detail that the as-synthesized tungsten powder has a lower sintering activation energy compared to commercial nanosized tungsten powder, with a measured hardness of 633 HV.

Densification of glass powder studied by master sintering curve and master kinetic curve methods

AbstractThe master sintering curve method was applied to analyze the sintering and crystallization behavior of 15Na2O.10CaO.75SiO2 (SLS) and 17Na2O.33CaO.50SiO2 (NC2S3) glasses, in mol %. The densification of samples was successfully analyzed using the master sintering curve (MSC) method; it gives activation energy Q = 310–340 kJ.mol−1, which is in good agreement with the expected activation energy of the viscous flow. While the obtained activation energy enables predicting the sintering behavior of SLS glass, the actual results are different from the estimated data for NC2S3. The activation energy of crystallization of NC2S3, determined by constructing Master Kinetic Curves (MKC) was lower than the sintering activation energy: this impairs the prediction ability of MSC for NC2S3 glass.