Stage Of Sintering Research Articles

The analysis of thermoelectric powder compaction mechanisms within field-activated sintering of skutterudites and Heusler alloys

The analysis of the shrinkage rate of powders, based on the power-law creep model of a porous body, was carried out in this paper to calculate the compaction parameters of CoSb3-based skutterudites and Fe2VAl-based Heusler alloys within field-activated sintering. It was indicated that this method, which had already been used for metal and ceramic powders, is applicable for thermoelectric powders. The values of strain rate sensitivity were obtained, and the corresponding powder compaction mechanisms have been defined. The main creep mechanism for skutterudites was found to be a dislocation climb, that later was replaced by grain boundary sliding, and the last sintering stage was associated with diffusional creep. The main creep mechanism for Heusler alloys was grain boundary sliding, later replaced by diffusional creep. Keywords: field-assisted sintering, numerical simulation, powders compaction, skutterudites, Heusler alloys, thermoelectrics.

Mechanisms and Energetics in the Early Stages of Solvent-Assisted Low-Temperature Sintering of ZnO

Mechanisms and Energetics in the Early Stages of Solvent-Assisted Low-Temperature Sintering of ZnO

Effect of shears between particles of mouldable iron powder on strength of manufactured parts

With a view to increase the strength of the products, two schemes have been developed for forming iron powders. Schemes involve intensive shifts between iron particles. One of the schemes is designed for forming billets, which after their sintering are deformed in dies without heating. The second of the schemes is designed for manufacturing directly at the stage of compacting the powder of conical thin-walled bushings of variable cross-section. As a result of the shifts between the particles, the densities of the molded billets were achieved, which allowed them to be tested for shear strength up to the sintering stage. The dependences between the density in the sections of the molded billet, its hardness and strength after sintering are experimentally determined. The dependence of the shear stress on the work piece on its molding scheme, the effect of magnetic pulse processing (MPP), and the powder composition is studied and described by the regression equation. The dependence of strength distribution in manufactured thin-walled bushings on the place of its measurement, the impact of MPP and powder composition is described similarly.

Thermal Properties of Copper Nanoparticles at Different Sintering Stages Governed by Nanoscale Heat Transfer

Thermal Properties of Copper Nanoparticles at Different Sintering Stages Governed by Nanoscale Heat Transfer

Densification and microstructure evolution of hot-pressed Er:SrF2 transparent ceramics

Erbium doped strontium fluoride (Er:SrF2) ceramics were fabricated by the hot-pressed (HP) sintering method without any sintering aids, and the densification and microstructure evolution of Er:SrF2 ceramics were investigated. The microstructure of Er:SrF2 ceramics was analyzed by SEM, and light scattering defects, including residual pores and un-dissolved ErF3 compound, were identified. The Er:SrF2 ceramic hot-pressed at 900 °C for 2 h exhibited the highest transmittance, and its transmittance at 500 nm and 1200 nm are 90.4% and 91.7%, respectively. In addition, it was clarified that the grain growth kinetic mechanism of Er:SrF2 ceramics at the final stage of sintering is related to the solute drag.

A predictive study on effective thermal conductivity of sintered nickel powder under different thermal processing conditions

Effective Thermal Conductivity (ETC) plays a key role in the thermal performance evaluation of samples with porous structure, which can be characterized by multiple physical properties such as particle sizes, porosity, and surface energy. The thermal conductivity of un-sintered porous material has been highlighted in most previous studies, while the evaluation of sintered thermal conductance is scarcely mentioned before. This study aims to develop an analytical model for the ETC prediction of sintered nickel powders based on the circuit equivalent model and sintering neck formation theory. Here, sixteen groups of sintering samples in three particle sizes are fabricated according to orthogonal design with four levels and four key factors, including sintering temperature (650∼800 °C), compressional stress (10∼25 MPa), heating rate (7∼20 °C/min), and holding time (1∼3 h). A dimensionless factor, formulated by the ratio of shrinkage rate to sintering contact area fraction, is proposed to reflect the effect of pore combination and grain growth at the late stage of sintering. The presented model is experimentally validated with an average deviation of three particle sizes equal to 7.11%, 6.38%, and 3.48%, respectively. Besides, simulated ETC agrees well with measured data for samples with medium-high porosity (0.44∼0.71) compared to those prepared with pore-forming agents.

Comparison of Precursor Preparation Routes on Final Density of Y3Fe5O12 Garnets Prepared via Reactive Sintering.

Yttrium iron garnet was obtained using four methods of synthesis. A modified citrate method and a modified citrate method with YIG (yttrium iron garnet, Y3Fe5O12) nucleation were used. In two subsequent methods, YIP (yttrium iron perovskite, YFeO3) and α-Fe2O3 obtained in the first case by the citrate method and in the second by precipitation of precursors with an ammonia solution were used as the input precursors for reaction sintering. Differential scanning calorimetry (DSC) measurements of the output powders obtained by all methods allowed to identify the effects observed during the temperature increase. Dilatometric measurements allowed to determine the changes in linear dimensions at individual stages of reaction sintering. In the case of materials obtained by the citrate method, two effects occur with the increasing temperature, the first of which corresponds to the reaction of the formation of yttrium iron perovskite (YIP), and the second is responsible for the reaction of the garnet (YIG) formation. However, in the case of heat treatment of the mixture of YIP and α-Fe2O3, we observe only the effect responsible for the solid state reaction leading to the formation of yttrium iron garnet. The obtained materials were reaction sintered at temperatures of 1300 and 1400 °C. Only in the case of material obtained from a mixture of perovskite and iron(III) oxide obtained by ammonia precipitation at temperature of 1400 °C were densities achieved higher than 98% of the theoretical density. The use of Hot Isostatic Pressing (HIP) in the case of this material allowed to eliminate the remaining porosity and to obtain full density.

The mineral transformation and molten behaviors of biomass waste ashes in gasification-melting process

Understanding the various ash transformation properties is vital for improving gasification behavior and optimizing industrial operation conditions during the gasification-melting process of biomass wastes. In this work, five typical biomass wastes were selected to investigate the slag property. The ash fusion process, mineral transformation and AAEMs solidification behaviors were performed and predicted by mineral transformation analysis and thermochemical calculation. The results indicated that ash fusion process contained sintering, swelling, bubbling and melting stages, which was related to ash chemical compositions. Besides, the ash flow temperature had a linear relationship with the basic-to-acid ratio of biomass waste ashes. High SiO2 content would lead to strong mineral crystallinity at high temperatures. The mineral transformation was also highly related to ash compositions. The complicated reactions of inorganic matters occurred when SiO2 content lied between 50 wt% and 70 wt%. For AAEMs solidification, the alkali was prone to react simultaneously with both SiO2 and Al2O3 to form eutectoid, and the range of 35–65 wt% for SiO2 + Al2O3 contents was beneficial for alkali solidification. All the obtained data was meaningful to provide a modifying reference for the efficient and clean utilization of various biomass wastes during gasification process.

Sericite instead of feldspar in porcelain stoneware: Effect on sintering and phase evolution

AbstractThe growing demand for fluxes, dictated by the shift toward porcelain stoneware, is urging the ceramic tile industry to find a viable alternative to feldspathic materials. Sericite, a microcrystalline form of muscovite, is present in many ceramic raw materials. It melts during firing and can be a suitable substitute for potassic feldspar. The aim of this study is to understand in depth the effects of replacing feldspar with sericite in porcelain stoneware batches with nearly the same chemical composition. Characterization includes phase composition (X‐ray powder diffraction, XRPD Rietveld), melt chemical composition (calculated), sintering behavior (optical thermo‐dilatometry), and microstructure (scanning electron microscope (SEM). The results indicate a different vitrification path, mostly in the early stage of sintering, related to the different mineralogical composition and particle size distribution of sericite for K‐feldspar. Nevertheless, at temperatures higher than 1150°C, the bodies have similar phase composition, microstructure, melt chemical composition, and physical properties. Densification kinetics is faster in the batch with sericite, while the body with feldspar is able to reach a higher bulk density.

Analyzing coarsening versus densification phenomenon present in partial sintering of Al2O3/TiO2 composites

Alumina–titania (Al2O3/TiO2) composites are well-known for their physical and microstructural properties, which allow several engineering applications. However, these properties change when different sintering temperatures are used, due to the existence of two antagonistic phenomena (grain coarsening versus densification) that occurs during sintering process. The aim of this study was to evaluate the microstructural and physical properties of the alumina–titania composites and to understand these two antagonistic phenomena when different partial sintering temperatures are used. The composites were prepared from alumina and titania powders by uniaxial pressing and sintered at different temperatures (1100–1400 °C). The structural analysis showed that in addition to expected phases (α-Al2O3 and rutile), the alumina–titania composite sintered at 1100 °C presents anatase phase, while those obtained at 1300 and 1400 °C present tialite phase. The morphology analysis revealed a random distribution of grains with irregularly shaped pores. According to the Archimedes-based immersion test, all composites have a relative bulk density ranging 60.43–73.59%, showing that none of them reached the final sintering stage. Statistical analysis revealed that the average grain size of the matrix ranges from 3.62 to 5.24 μm. Moreover, the statistical analysis also showed that the grain coarsening predominates over densification for the composites sintered from 1100 to 1150 °C. The densification predominates over grain coarsening for the composites sintered from 1150 to 1200 °C, and from 1250 to 1400 °C. The two phenomena coexist only for the composites sintered from 1200 to 1250 °C.

Sustainable Production of Powder Metallurgy Aluminum Foams Sintered by Concentrated Solar Energy

Porous aluminum foams were successfully fabricated following the space-holder powder metallurgy method with a solar sintering stage. Al foams with porosities of 50, 60, and 70 vol.% were sintered in a low-cost Fresnel lens. Green parts were prepared using aluminum powder as the main metallic material and saccharose as a soluble space-holder. The dissolution stage was designed for each foam and required longer periods of time, between 8 and 32 h, as the design porosity increased. Brown parts were fully sintered by concentrated solar energy at a lower temperature (500 °C) and for shorter times (12–20 min) than those required by conventional sintering techniques (640 °C, ~9 h). The evaluation of density and the characterization of pore size and distribution in the sintered foams was carried out. All obtained foams were stable and presented a homogeneously distributed porosity, very close to the design porosity, with differences lower than 2.1 vol.%, and with approximately half being characterized as open porosity. Moreover, the solar sintered foams presented a high quality, and similar or even greater mechanical properties (such as compressive strength and impact energy absorption) than those achieved by conventional techniques. Foams with 50 vol.% of porosity exhibited the best mechanical behavior, in terms of impact-energy absorption (24.42 MJ/m3) and compressive strength (27.4 MPa).

Numerical investigation into pressure-assisted sintering using fully coupled mechano-diffusional phase-field model

To investigate the effect of contact stress between particles on the evolution of microstructure in pressure-assisted sintering, an improved fully coupled mechano-diffusional phase-field model (PFM) was presented, in which the design of the interpolation method for elastic modulus ensured that the normalised contact stress distribution was approximated to that in conventional contact problems. The immediate advantage was that the consequent strain energy in the contact and neck area could be directly characterised without additional approximate treatment.The simulation results showed that the sintering neck evolved under the joint action of the mechanical mechanism of elastic contact and the mass diffusion mechanism for pressure-assisted sintering. Elastic contact played a dominant role before significant neck growth in the early stage of sintering. As the neck grew, mass diffusion became the dominant factor, but the normalised contact stress distribution in the contact area always satisfied that of the elastic contact problem. Owing to the additional contribution of strain energy, the greater the pressure, the faster the sintering neck growth. Further study showed that the diffusion-induced stress hindered the growth of the sintering neck according to the presented fully coupled model presented, and that the strain energy stored within the contact area will accelerate the sintering 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.

Analysis of biomedical titanium implant green parts by X-Ray tomography

In this study, X-ray tomography was applied as a non-destructive test to investigate the internal defects of the green part produced with Powder injection molding (PIM). PIM is a material processing technology that combines injection molding with powder metallurgy. The determination of multiple parameters such as rheology, molding, bonding, and sintering stages in the PIM method requires a long time, energy, and high cost. Metal and ceramic injection molded parts are known to be susceptible to defects like cracks, weld lines, bubbles, or gaps. The examination and elimination of these defects are critical to the safety of the products because even a very short crack can significantly reduce the tensile and fracture strength of the parts. The internal structure analysis of the green parts after molding is very difficult. Defects (internal cracks, micropores, jetting, etc.) occurring in the green part will occur in the sintered. Therefore, it is essential to analyze the possible defects on molded parts before applying the ongoing process to save time and costs. Since the medical implants are in complex shape in general, the more internal defects can be seen at molding stage, it is needed to check the internal structure without any destruction, because there is no chance toleave them for recycle in the case of any defect. In experimental studies, it was observed that there were defects of edge, interior, and gaps in X-ray analysis for four different injection parameters. In X-ray analysis, the defects free parts were obtained for green part 4 parameters (temperature of 237 °C, flow rate of 20.11 cm3/s and pressure of 120 MPa). As a result of X-ray analysis, the difference between green part 1 and green part 4 is clearly visible in terms of defect. Injection temperatures, speeds and pressure play important role on originating of defects.

Microstructures and properties of 45S5 bioglass® & BCP bioceramic scaffolds fabricated by digital light processing

The porous bioceramic scaffolds require favorable mechanical properties and excellent bioactivity to promote bone repair. In this work, 45S5 bioglass® & biphasic calcium phosphate (BCP) bioceramic scaffolds with a diamond lattice structure (porosity of 70%) were manufactured accurately by digital light processing (DLP). After optimizing the solid loading of bioceramic suspension to 40 vol%, scaffolds with the minimum pore size of 400 µm were fabricated successfully under exposure energy of 12.54 mJ/cm2. In the initial stage of sintering, crystallization occurred during 45S5 bioglass® itself, bioglass also promoted hydroxyapatite (HA) decomposition to tricalcium phosphate (TCP). With extending holding time to 4 h, Si4+ from 45S5 bioglass® diffused into TCP to produce Ca5(PO4)2SiO4, which could enhance the compressive strength of bioceramic scaffolds. Meanwhile, bioglass would assist the growth and roughening of ceramic grains because of the ion-diffusion from bioglass to BCP. However, as holding time extended to 6 h, abnormal grain growth and cracks occurred which would decrease compressive strength. The best value of compressive strength is 1.735 MPa at the holding time of 4 h. 45S5 bioglass® also accelerated ion exchange such as Si4+ and CO32- between bioceramic and SBF solution. This kind of bioceramics may be used as bioactive scaffolds with customizable fine structures and provide potential applications in bone tissue engineering.

Reactive SPS of Nd :YAG transparent ceramics with LiF sintering additive

1 at% Nd 3+ :YAG transparent ceramics were obtained for the first time by reactive spark plasma sintering using LiF sintering additive. The effect of the dwell sintering time on the phase composition, microstructure, and transparency of the material obtained at 1475 °C and an external pressure of 60 MPa was studied. It is shown that positive densification dynamics characterize Nd 3+ :YAG ceramics over the entire sintering (30–60 min). The increase in the isothermal exposure is accompanied by an increase in the average grain size from 8.7 to 12.9 μm and the optical transmission in the visible wavelength range from 48.1 to 71.5%. At the same time, a more balanced structural-phase state of the ceramics is achieved due to the removal of vacancies and a decrease in the contribution of microstresses along the grain boundaries during the evaporation of carbon- and Li-, F-contained components. LiF-doped 1 at% Nd 3+ :YAG ceramics obtained at 1475 °C/60 min/60 MPa are characterized by average grain size of 12.9 μm, Vickers hardness of 1300 HV, linear optical transmittance 71.5% at λ = 1064 nm, and a residual porosity of 0.001 vol% with an average section pore size of 122 nm. • LiF-doped Nd 3+ :YAG TCs were obtained for the first time by the reactive SPS method. • The pre-optimized consolidation parameters (1475 °C/60 MPa) were fixed. • The effect of the dwell sintering time (30–60 min) on properties of TCs was studied. • The positive densification dynamics over the dwell sintering stage was shown. • The completely evaporating of carbon impurities was revealed for dwell times ≥45 min.

Prospects for using boron in metallurgy. Report 1

On the basis of literature and our own data, the effect of boron on characteristics of all stages of metallurgical processes (from sintering, smelting of cast iron and ferroalloys, to steel production) and on the properties of the resulting slag and metal was studied. To intensify the pellets hardening at the stage of liquid-phase sintering and to improve their metallurgical properties, it is sufficient to have 0.20 – 0.35 % of boron oxide in them. According to the laboratory studies, the presence of boron oxide in pellets increases their compressive strength by 1.5 – 1.7 times and hot strength by 3 – 4 times. While studying the mechanism and kinetics of sulfur removal, it was shown that the presence of boric anhydride significantly intensifies processes of pellets desulfurization. Their intensive progress goes to the zones of lower temperatures of 1050 – 1100 °С. To increase the sinter strength characteristics, it is possible to add B2O3 to the charge. The introduction of 0.44 % of B2O3 does not affect the sinter abrasion. The content of fines (0 – 5 mm) in comparison with the base sample is reduced by 1.5 times. The use of boron pellets in blast-furnace smelting makes it possible to increase the basicity of the final slag from 1.10 to 1.16. In this regard, the sulfur distribution coefficient increases from 48 to 74. The sulfur content in cast iron decreases by 0.005 %. The possibility of using boron and its compounds to improve the technical and economic indicators of production and the quality of pellets, sinter and cast iron is shown on the base of the presented theoretical, laboratory-experimental and industrial data.

Reverse cation segregation and crack self-healing of Ba0.3Sr0.7Fe0.9Mo0.1O3-δ perovskite four-channel hollow fiber membrane

The cation segregation in a perovskite-type membrane deleteriously affects the membrane performance and durability. Recovery of segregation is a great challenge because of the complexity of segregation products and severe limitations from the stability of perovskite oxide. This work presents a novel approach for reversing the surface cation segregation by hydrogen-induced annealing which further shows the ability of healing defects and cracks in the oxide bulk. Ba0.3Sr0.7Fe0.9Mo0.1O3-δ (BSFM) four-channel hollow fiber membranes were prepared by one-step thermal processing. A low hydrogen concentration (5 vol %) and a mild temperature (900 °C) were used to anneal the as-prepared BSFM hollow fiber membrane and served to trigger outright reverse segregation of massive complex oxide precipitations at grain boundaries and a self-healing of cracks formed during the sintering stage. The reverse segregation significantly alters the composition and structure which directly affects the performance of BSFM membrane, such as mechanical strength, electrical conductivity, oxygen permeability, and stability. A maximum breaking load of 31.6 N is reached for the hydrogen-induced membrane which is approximately 6 times of the as-prepared BSFM membrane. Hydrogen-induced membranes, having high oxygen permeation flux and low activation energy, can operate stably in partial oxidation of methane catalytic membrane reactor for more than 120 h. Our results demonstrate the feasibility of hydrogen-induced reverse segregation and crack self-healing in the perovskite-type dense membrane, suggesting new directions to design the high-performance materials based on perovskite oxides for application in catalysis, electrocatalysis, and separations in the future.

Assessment of the Role of Speciation during Cold Sintering of ZnO Using Chelates.

Cold sintering (CS) is a chemically driven densification technique enabling a substantial decrease in the sintering temperature of oxides, by several hundreds of degrees Celsius. Although the densification process in CS is known to be mainly driven by pressure solution creep, additional fundamental aspects driving the interfacial chemistry reactions are still a subject of debate. Herein, we focus on the aspect of speciation in the densification process. The densification of zinc oxide (ZnO) by CS using zinc acetylacetonate hydrate (Zn(acac)2·xH2O), a versatile ligand often used as a precursor for ZnO synthesis in wet chemistry, is reported. The successful densification of ZnO using H2O and Zn(acac)2·xH2O confirms the importance of speciation in CS, as ZnO has a very low solubility in pure H2O. The evolution of the system at different stages of sintering and the role of the Zn(acac)2·xH2O species were evaluated.

Influence of additives on the crystallization and thermal conductivity of container glass cullet for foamed glass preparation

Foamed glass preparation is a complex combination of reactions, greatly influenced by the composition of the used glass and often hindered by simultaneous crystallization. The crystallization phenomenon is undesirable in foamed glass production since it decreases the quality of the final product. In this work the influence of different types of additives (foaming agents, flux agents, crystallization inhibitors and nucleating agent) on the crystallization of waste container glass and properties of the sintered samples (density and thermal conductivity) was studied. Results of our study confirmed partial crystallization during sintering stage. We found that waste container glass manifests complex crystallization with the formation of four main crystalline phases, which can be inhibited with the addition of fluxing agents (B2O3 and borax). Moreover, here we show that prevention of the crystallization can lead to a significant decrease of the thermal conductivity.