Sintering Phenomena Research Articles

Microwave flash sintering of metal powders: From experimental evidence to multiphysics simulation

Flash sintering phenomena are predominantly associated with ceramics due to thermal runaway of their electric conductivity noticeably represented in materials such as zirconia or silicon carbide. Because of their high electric conductivity, flash sintering of metals is nearly inexistent. In this work, an original metal powder flash sintering method based on a microwave approach is presented. Within the developed approach, an unusually fast (60 s) thermal and sintering runaway of Ti-6Al-4V powder is experimentally revealed under microwave illumination. This phenomenon is simulated based on an electromagnetic-thermal-mechanical (EMTM) model. The developed multiphysics model reveals that the metal powder specimen's runaway does not result from its intrinsic material properties, but results from the resonance phenomenon thermally activated by the surrounding tooling material. The EMTM simulation predicts with a very good accuracy the microwave repartition and the resulting densification and powder specimen's shape distortions observed experimentally. The comparison of the microwave and conventional sintering kinetics indicates an important acceleration of the sintering behavior under microwave heating. The developed sintering approach has a potential of the implementation for time-effective mass production of small metal parts.

Evolution of phase composition and microstructure of commercial Al2O3 gel in different heat treatment condition

Evolutions of phase composition and microstructure of commercial Al2O3 gel in different heat treatment conditions (temperature, atmosphere and additives) were investigated. There was almost no effect of atmosphere and carbon additive on phase evolution of Al2O3 gel during heating, γ-Al2O3 formed at 800 °C, γ-Al2O3 and minor θ-Al2O3 co-existed at 1000 °C, and single phase of ɑ-Al2O3 occurred at heating temperature ≥1200 °C. Atmosphere and carbon had great effects on morphology and crystal size of Al2O3 particles. Crystal size of spherical-shape Al2O3 particles was 10–20 nm after heating at 800–1000 °C in air, afterwards, they rapidly grew into micro or macro scale when temperature was above 1200 °C, and sintering phenomena of worm-like Al2O3 particles were observed. In the presence of carbon, spherical-shape Al2O3 particles grew slightly from 10 to 20 nm to 50–60 nm with the temperature increasing from 800 °C to 1500 °C in reducing atmosphere, carbon inclusions in Al2O3 grain boundaries triggered a steric hindrance of Al2O3 particles growth. Al2O3 gel had a high reactive ability and could react with microsilica to form nano mullite crystals at relatively lower temperature.

Hydrogen Utilization in Green Fuel Synthesis via CO2 Conversion to Methanol over New Cu-Based Catalysts

The use of hydrogen as an energy vector and raw material for “very clean liquid fuels” manufacturing has been assessed by the catalytic conversion of CO2 to methanol over copper based catalysts. A systematic evaluation of copper based catalysts, prepared varying the chemical composition, has been carried out at 0.1–5.0 MPa of total pressure and in the range of 453–513 K by using a semi-automated LAB-microplant, under CO2/H2 reactant mixture (1/3), fed at GHSV of 8.8 NL∙kgcat−1∙h−1. Material’s properties have been investigated by the means of chemical-physical studies. The findings disclose that the addition of structure promoters (i.e., ZrO2/CeO2) strongly improves the textural properties of catalysts, in term of total surface area and exposure of metal surface area (MSA), also reducing the sintering phenomena. The results of the catalytic study clearly prove a structure-activity relationship at low reaction pressure (0.1 MPa), while at higher pressure (3.0–5.0 MPa) the reaction path is insensitive to structure and chemical composition.

Segregation Phenomena in Size-Selected Bimetallic CuNi Nanoparticle Catalysts.

Surface segregation, restructuring, and sintering phenomena in size-selected copper-nickel nanoparticles (NPs) supported on silicon dioxide substrates were systematically investigated as a function of temperature, chemical state, and reactive gas environment. Using near-ambient pressure (NAP-XPS) and ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), we showed that nickel tends to segregate to the surface of the NPs at elevated temperatures in oxygen- or hydrogen-containing atmospheres. It was found that the NP pretreatment, gaseous environment, and oxide formation free energy are the main driving forces of the restructuring and segregation trends observed, overshadowing the role of the surface free energy. The depth profile of the elemental composition of the particles was determined under operando CO2 hydrogenation conditions by varying the energy of the X-ray beam. The temperature dependence of the chemical state of the two metals was systematically studied, revealing the high stability of nickel oxides on the NPs and the important role of high valence oxidation states in the segregation behavior. Atomic force microscopy (AFM) studies revealed a remarkable stability of the NPs against sintering at temperatures as high as 700 °C. The results provide new insights into the complex interplay of the various factors which affect alloy formation and segregation phenomena in bimetallic NP systems, often in ways different from those previously known for their bulk counterparts. This leads to new routes for tuning the surface composition of nanocatalysts, for example, through plasma and annealing pretreatments.

Establishing a Novel Methodology to Correlate the Macroscopic and Microscopic Degree of Sintering in Magnetite Pellets during Induration

The quality of product pellets is a result of the physico‐chemical phenomena involved in the induration process. Sintering is the primary phenomenon, and its degree or extent contributes substantially to the evolution of the metallurgical and mechanical properties of a pellet. During the induration of magnetite pellets, sintering proceeds through the oxidized and non‐oxidized magnetite phases. Sintering of these phases has been previously studied on a single pellet at the macroscopic scale using an optical dilatometer. A deeper understanding requires corroboration of these studies through characterization at the microscopic scale. In the present work, the observations recorded at the microscopic scale are quantified using image processing techniques to correlate them to the macroscopic measurements. Distance transformation, which is an image processing principle, is adapted in a novel way to digitize the microstructures and to determine the degree of sintering in a pellet quantitatively. This methodology has potential applications as a generic tool to follow the sintering phenomenon and process kinetics at any stage during induration.

Study on the preparation of molybdenum silicides by the silicothermic reduction of MoS2

In this paper, a novel process about the silicothermic reduction of molybdenum disulfide was proposed to prepare molybdenum silicides. This new method has a short process, which is beneficial for energy saving, environmental protection and cost reduction relative to the traditional methods by using pure molybdenum and silicon as the raw materials. It was found that the phase compositions and microstructures of products were greatly influenced by the molar ratio of Si to MoS2 and temperature. From the experimental results, it was concluded that MoSi2, Mo5Si3 and Mo3Si can be prepared when the temperature was above 1100 °C, 1300 °C and 1500 °C, and the Si/MoS2 molar ratio was 4, 2.6 and 7/3, respectively. When the Si/MoS2 molar ratio was less than 4 and the temperature was below 1100 °C, only one Mo-Si compound (MoSi2) was generated; while above 1200 °C, the excessive MoS2 could react with MoSi2 to generate Mo5Si3, but Mo5Si3 can't react with excessive MoS2 below 1300 °C; until above 1400 °C, Mo5Si3 and MoS2 can react to generate Mo3Si. However, even at 1600 °C, excessive MoS2 couldn't react with Mo3Si but can be decomposed. Besides molybdenum silicides, other products SiS or SiS2 can also be generated. SiS could escape from sample at 1100 °C in the form of gas while SiS2 couldn't. Both the increases of temperature and molar ratio of Si to MoSi2 were beneficial for the decrease of the residual sulfur content in the final products. From the experimental results, it was found that as increasing the temperature or the molar ratio of Si to MoS2 from 7/3 to 4, the grain size of products increased and the sintering phenomenon among grains was more and more obvious.

Na&Ca removal from Zhundong coal by a novel CO2-water leaching method and the ashing behavior of the leached coal

Zhundong coalfield in China is the largest intact coalfield in the world. However, serious fouling and slagging issues occurred when the Zhundong coal combustion, because of its high Na and Ca contents. In this study, a novel CO2-water leaching method was proposed to remove Na and Ca in Zhundong coal. The removal efficiencies of Na and Ca and the ash characteristics of the leached coal were investigated in detail. The results show that the CO2-water leaching can be easily achieved at room temperature under atmospheric pressure. The removal efficiencies of Na and Ca by the CO2-water leaching were significantly higher than those of water leaching. It is attributed to the effective removal of the organic Na and Ca in the coal. In addition, the CaCO3 in the coal was also effectively removed by CO2-water leaching. The volatility of Na from CO2-water leached coal is only 25% of that of raw coal during the combustion at 900°C. The alkali-acid ratio (B/A) of the CO2-water leached coal ash was significantly lower than those of the raw coal and the water leached coal ashes. The fusion temperature of the CO2-water leached coal ash significantly increased, compared with those of the raw coal and the water leached coal ashes. The deformation temperature of the CO2-water leached coal ash was as high as 1202°C, much higher than those of the raw coal and water leached coal ashes, which were 1098°C and 1142°C, respectively. Besides, the agglomeration and sintering phenomena of coal ash was also inhibited significantly by the CO2-water leaching. Therefore, the CO2-water leaching method was found to be an effective and promising method for AAEM removal from Zhundong coal, and can markedly mitigate the fouling and slagging during Zhundong coal combustion.

Highly active and stable Pt-Ni/CeO2-SiO2 catalysts for ethanol reforming

In this work, the Pt-Ni/CeO2-SiO2 catalyst was prepared by wet impregnation and tested for steam and oxidative steam reforming of ethanol at low-temperature (300–600 °C). The solids were characterized by X-ray fluorescence (XRF) and diffraction (XRD), nitrogen adsorption, Raman spectroscopy, temperature programmed reduction (TPR) and oxidation (TPO). Reforming tests were carried out between 10,000 and 30,000 h−1, recording total conversion at T > 450 °C, independently from the contact time and the feed composition; in addition, hydrogen yield was marginally affected by O2 co-feeding, especially in the mild-temperature range. During stability tests, carried out at 500 °C and 20,000 h−1 for at least 45 h, oxygen addition reduced carbon formation rate (CFR) of almost 30%, assuring a complete conversion for 55 h. Catalyst deposition on high surface area silica improved active species dispersion and metal-support interaction, preventing sintering phenomena and resulting in a much lesser coke selectivity than that observed for the silica-free sample.

Effect of compacting pressure on shape retention during supersolidus liquid phase sintering of Cu base alloys

ABSTRACTThe compacting pressure is one of the key parameters which affects sintering phenomena, such as fragmentation, rearrangement and densification. However, the type of alloy also has a similar significance which should be considered. To demonstrate the effect of the mentioned parameters, two types of copper base alloys, i.e. Cu–9Sn–8Pb and Cu–28Zn, were investigated in the present study. Prealloyed powders were compacted at 100 and 400 MPa, respectively, and sintered in the range of 890–970°C for 20 min. In situ images of the samples were taken at various sintering conditions, and the shapes were recorded. For both materials, in case of compaction at lower pressure, the gravity-induced distortion known as ‘elephant foot’ phenomenon was not observed, and the specimens show shape retention during sintering even at higher temperatures, while samples compacted at 400 MPa showed different responses to the sintering conditions: A-shaped distortion (elephant foot) occurred in the bronze alloy, while in the brass alloy O-shaped distortion was observed.

Fluid dynamics thermo-mechanical simulation of sintering: Uniformity of temperature and density distributions

One of the main contemporary challenges of sintering simulations is taking into account all the coupled physics present in the sintering process. The model should predict the convection, surface to surface radiation, conduction, specimen sintering phenomena, and also the proportional integral derivative regulation. A fluid dynamics thermo-mechanical model (FDTM) is implemented to predict the degree of uniformity of heating and compaction of a large size gear shape specimens under various process conditions. The FDTM model is validated by a dilatometer test indicating a very good agreement with the experimental data. The complex shape sintering simulation shows that the temperature and densification gradients depend strongly on the sample thickness. A large volume of material can be easily heated if the specimen’s shape allows the hot gas to make its way inside the sample volume. If this condition is not satisfied, it is shown that a large and closed volume of material can generate significant temperature and density gradients.

Coal-Based Reduction and Magnetic Separation Behavior of Low-Grade Vanadium-Titanium Magnetite Pellets

Coal-based reduction and magnetic separation behavior of low-grade vanadium-titanium magnetite pellets were studied in this paper. It is found that the metallization degree increased obviously with an increase in the temperature from 1100 °C to 1400 °C. The phase composition transformation was specifically analyzed with X-ray diffraction (XRD). The microscopic examination was carried out with scanning electron microscopy (SEM), and the element composition and distribution were detected with energy dispersive spectroscopy (EDS). It is observed that the amounts of metallic iron particles obviously increased and the accumulation and growing tendency were gradually facilitated with the increase in the temperature from 1100 °C to 1400 °C. It is also found that the titanium oxides were gradually reduced and separated from ferrum-titanium oxides during reduction. In addition, with increasing the temperature from 1200 °C to 1350 °C, silicate phases, especially calcium silicate phases that were transformed from calcium ferrite at 1100 °C, were observed and gradually aggregated. However, at 1400 °C some silicate phases infiltrated into metallic iron, as it appears that the carbides, especially TiC, could probably contribute to the sintering phenomenon becoming serious. The transformation behavior of valuable elements was as follows: Fe2VO4 → VO → V → VC; FeTiO3 (→ FeTi2O5) → TiO2 → TiC; FeCr2O4 → Cr → CrC; FeTiO3 (→ FeTi2O5) → Fe0.5Mg0.5Ti2O5; (Fe3O4/FeTiO3→) FeO → Mg0.77Fe0.23O. Through the magnetic separation of coal-based reduced products, it is demonstrated that the separation of Cr, V, Ti, and non-magnetic phases can be preliminarily realized.

Phase transformation and morphology evolution of sepiolite fibers during thermal treatment

In this study, the phase transformation and micromorphology evolution of sepiolite fibers were studied along with the increase of temperature from 300 to 1300°C, using raw sepiolite samples as raw materials. Through characterization by scanning electron microscope (SEM), high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), thermo-gravimetric analysis (TGA/DTG) and differential scanning calorimeter (DSC), it was found that the determined transition temperature values for the dehydrations of the hygroscopic water and the zeolitic water were 120°C and 340°C, and the values for the dehydration of the bound water and the dehydroxylation of the hydroxyls were about 500°C and 810°C. Moreover, orthoenstatite (MgSiO3) was recrystallized from dehydroxylated phase at 852°C and then transformed into protoenstatite at 970°C, and enstatite crystals grew along with c-axis of the fiber with the increase of temperature. Amorphous phase as the other phase of sepiolite dehydroxylation recrystallized into cristobalite in the temperature range of 1130–1200°C and the samples began to melt at the above temperature range. In terms of morphology, both sepiolite bundles and laths remained constant after heating up to 1000°C and sintering phenomenon appeared when the temperature was up to 1100°C. The bundles partly melted and adhered each other, which further formed a porous structure after calcination at 1200°C. When the temperature was up to 1300°C, the sample substantially melted except for a small amount of enstatite and cristobalite crystals. The present study reflects the thermal reaction of sepiolite, which could be applied in exploring the reinforcement mechanism of sepiolite reinforced ceramic body composites.

Reduction kinetics of magnetite concentrate particles by carbon monoxide

ABSTRACTIn this study, the reaction kinetics of carbon monoxide reduction of magnetite ore concentrate in the temperature range of 973–1173 K (700–900°C) has been investigated using the thermal-gravimetric analysis. Various methods were used to obtain the governing mechanism and reaction rate constant of this reaction. It was observed that the sintering phenomenon affects the concentrate particles’ reduction at temperatures higher than 1173 K (900°C). CO reduction of magnetite concentrate to pure iron or iron carbide occurred as a single-step reaction. The first-order reaction model and diffusion models are two important mechanisms for this reaction in the operating temperature range. The following equation is the result of our research work relevant to carbon monoxide reduction of the magnetite concentrate particles in order to predict the progress of the reduction reaction (with R2 about 0.98):

Microwave roasting with size grading based on the influence of carbon on vanadium extraction from stone coal via microwave roasting-acid leaching

Carbon in stone coal has a double-edged function in the microwave roasting-acid leaching of stone coal; it provides stone coal with good heating characteristics, but it hinders the oxidization of vanadium and causes the sintering phenomenon.

Investigation on multi-layer selective inhibition sintering process using finite element analysis

Recently, Additive Layer Manufacturing (ALM) has attracted a lot of attention to produce complex parts in short time, without the usage of fixtures, molds, dies or additional tools. One of the cost effective ALM technique in utilizing the indigenous polymers is Selective Inhibition Sintering Process (SISP). This article examines the effect on thermal loading to polymers structure with respect to part dimensional stability. The multiple layers of polymers with inhibitors at its periphery are modelled using Finite Element Method. The birth and death finite element behaviour is incorporated to achieve layer by layer sintering phenomenon. Coupled transient thermo - structural analysis is performed through sequential coupling method to evaluate the heat distribution and structural displacement of the three dimensional part. The simulation results reveal that parts with good dimensional accuracy with less residual stress is attainableusing SISP.

Mixed Ba1−xLaxF2+x fluoride materials as catalyst for the gas phase fluorination of 2-chloropyridine by HF

The stability under different atmospheres (dry or wet nitrogen or air, HF, reaction medium) and the catalytic performances of BaF2-LaF3 based catalysts for the fluorination of 2-choropyridine by Cl/F exchange with HF as fluorinating agent were investigated. Thermal decomposition of metal trifluoroacetate and precipitation from nitrate precursor methods were used as synthesis methods. Depending on the synthesis method and the La/La+Ba molar ratio (x), an intimate mixture of BaF2 and LaF3 metal fluorides or a Ba1−xLaxF2+x mixed metal fluoride with high specific surface area were formed. In comparison with BaF2, the mixed metal fluorides led to higher specific surface areas and prevented the sintering phenomenon observed under wet atmosphere which led to a significant decrease of the specific surface area. Moreover, Ba1−xLaxF2+x catalysts had not evolved under the fluorination reaction contrary to BaF2 for which a stable BaF2−xClx material (with x close to 0.8) was formed by reaction with the by-product HCl. The turn over frequency (TOF) was similar for all barium based catalysts for the fluorination of 2-chloropyridine. Nevertheless, among the studied materials, Ba0.5La0.5F2.5 was the most promising catalyst for the transformation of 2-chloropyridine due to high activity (related to its high specific surface area and its moderate strength of Lewis acidity) and high stability.

Microstructure and mechanical properties of nonmagnetic Fe-25Mn-xCu-C steels by super solidus liquid phase sintering

In this work, nonmagnetic steels Fe-25Mn-xCu-C were prepared by high manganese pre-alloyed steel powders through powder metallurgy (PM) technique. Four types of steels specimen were created to investigate the microstructure evolving with sintering process, mechanical properties and magnetic properties. The microstructures, fracture surfaces, phase constitutions and mechanical properties of Fe-25Mn-xCu-C were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), and tensile strength test. The results showed that super solidus liquid-phase sintering (SLPS) phenomenon was conclusively verified, for the first time, in the Fe-Mn pre-alloyed powders: Liquids generated by SLPS process from pre-alloyed powders could improve the binding condition between the particles and enhance the densification. X-ray diffraction (XRD) experiment and physical property measurement system (PPMS) measurement verified the nonmagnetic properties of steels with single austenite phase. It is confirmed that mechanical properties are intensively influenced by the characteristic and quantity of liquids between the matrix particles. The fracture mechanism of the steel is dominated by intergranular decohesion mode. The preliminary study found this kind of new non-magnetic steel exhibits relatively high density. With the efficiency in fabricating and the non-magnetic property, this work foresees good prospects for application in the steel components manufacturing industry.

On the swelling of silver powder during sintering

Silver powder compacts may suffer an overall expansion during sintering. A simultaneous analysis of dimensional changes, mass loss and pore closing in the course of sintering showed that volume change results from the competition between classical sintering phenomena inducing densification and material creeping under stresses caused by internal gas pressure, which induces swelling. The internal gas includes air trapped during pressing and oxygen released from particle surface inside the pores that closed during pressing or sintering. This swelling phenomenon is particularly strong with powders with large specific surface area and compacts with high green density. It is less effective during fast sintering, as observed during direct induction heating experiments. Electrical conductivity is strongly affected by swelling. Powder compacts may exhibit a lower conductivity after sintering when interparticle bonding does not balance the density decrease occurring during sintering.

Flash (Ultra-Rapid) Spark-Plasma Sintering of Silicon Carbide.

A new ultra-rapid process of flash spark plasma sintering is developed. The idea of flash spark plasma sintering (or flash hot pressing - FHP) stems from the conducted theoretical analysis of the role of thermal runaway phenomena for material processing by flash sintering. The major purpose of the present study is to theoretically analyze the thermal runaway nature of flash sintering and to experimentally address the challenge of uncontrollable thermal conditions by the stabilization of the flash sintering process through the application of the external pressure. The effectiveness of the developed FHP technique is demonstrated by the few seconds–long consolidation of SiC powder in an industrial spark plasma sintering device. Specially designed sacrificial dies heat the pre-compacted SiC powder specimens to a critical temperature before applying any voltage to the powder volume and allowing the electrode-punches of the SPS device setup to contact the specimens and pass electric current through them under elevated temperatures. The experimental results demonstrate that flash sintering phenomena can be realized using conventional SPS devices. The usage of hybrid heating SPS devices is pointed out as the mainstream direction for the future studies and utilization of the new flash hot pressing (ultra-rapid spark plasma sintering) technique.

Sintering of raw glazes for floor and porcelain tiles: A non-isothermal kinetic model

New fritless glaze compositions have been developed for floor and porcelain tiles from a new boron raw material obtained by calcination. The complexity of this type of composition also translates into complex behaviour during firing in which, concurrently or in partially overlapping form, very different processes take place, such as crystalline phase dissolution, the crystallisation of new phases, and sintering phenomena. This paper analyses the physico-chemical transformations that develop during the firing of these new glaze compositions, focusing in particular on the sintering process and sintering kinetics. A multi-step kinetic model was developed in this study to encompass complex glaze compositions (containing up to nine components). The model appropriately describes the sintering degree of progress of five materials (a boron raw material and four glazes) with temperature, in constant-rate heating experiments, determined using hot stage microscopy (HSM). For the three glazes with the lowest starting amorphous phase content, sintering was assumed to take place in three individual, parallel and partially overlapping, steps. Sintering of the boron raw material was described by a single step, whereas for the fourth glaze, with a starting amorphous phase content intermediate between that of the raw material and the other glazes, two steps were needed. The evolution of crystalline phase content with temperature was used to identify the sintering mechanisms in each step. For each step, the kinetic model, pre-exponential factor, and activation energy were determined and related to the composition of the materials. As a novelty, the Avrami–Erofeev model, which is usually applied in describing the kinetics of chemical reactions and phase transitions, was used to describe each individual step.