Early Stages Of Sintering Research Articles

Structural defect and activated alumina of spheric α-Al2O3 improving alumina ceramics density from the industrial coarse gibbsite

The quality of alumina ceramics generally depends on alumina powder. High-density alumina ceramics were prepared from industrial coarse gibbsite. Near-spheric α-Al2O3 with particles smaller than 500 nm was produced via the synergy of B and F from NH4BF4. The near-spheric α-Al2O3 calcined at low temperature generated alumina ceramics with high density and low porosity. Meanwhile, the wet pretreatment yielded smaller particles with structural distortion and high microstrain and aluminum hydroxide, notably increasing the relative density of alumina ceramics. 99.18% relative density, 0.017% porosity, and 27.54 GPa Vickers hardness of alumina ceramics were achieved at 1650°C for 2 h without additives and pressed sintering. In addition, the early stage of sintering of alumina green body followed particle boundary diffusion with reduced activation energy in the sintering process. The close packing of the spheric α-Al2O3, high microstrain of α-Al2O3, structural defects with considerable AlO4 units calcined at low temperature, structural distortion in the alumina, and activated alumina on the α-Al2O3 surface collectively promoted the densification of alumina ceramics, achieving remarkable hardness, high density, and tiny porosity.

Fast pressureless sintering of highly transparent AlON ceramics by reducing Y enrichment with nano-sized granular Y2O3 as additive

Employing nano-sized granular Y2O3 (NGY) as additive, AlON ceramics with transmittance ≥80% across wavelengths at 380–4150 nm were successfully fabricated via pressureless sintering at 1880 °C for 2.5 h, the dwelling duration is much shorter than the ≥7 h reported in literature. The distribution of Y2O3 in starting mixtures, along with phase transformation, microstructure evolution and densification process of the samples during heating were thoroughly studied to reveal the fast densification mechanism. Local enrichment of Y2O3 in the AlON/Y2O3 mixtures was significantly reduced when using NGY instead of micro-sized lamellar Y2O3. The reduction of excessive local Y2O3 concentration resulted in less α-Al2O3 was decomposed from AlON during heating, and further suppressed local aggregation/coarsening at ≤1600 °C. The excellent microstructure obtained at early stage of sintering, together with the improved effective Y2O3 concentration in whole sample led by the decrease in local Y2O3 enrichment accelerate the densification process of AlON.

Phase-field simulation of core-rim structure at the early sintering stage in TiC-WC-Ni cermet

Understanding the formation mechanism of core-rim structure in TiC-based cermets is essential for optimizing their mechanical properties, such as strength and toughness. In this work, the formation process of core-rim structure in TiC-WC-Ni cermet at the early sintering stage was investigated by means of the multiphase and phase-concentration modeling framework coupled with CALPHAD (CALculation of PHAse Diagrams) databases. The effect of W content in the Ni binder phase on the rim thickness was examined. The results reveal that W in the Ni binder phase diffuses into the undissolved TiC particles at the early sintering stage, leading to the formation of (Ti,W)C solid solution phase on the particle surface. This solid solution phase serves as the rim which constitutes the core-rim structure together with TiC particles. Furthermore, there is a positive correlation between the rim thickness and the W content in the Ni binder phase. Phase-field simulation exhibits a good agreement with the scanning electron microscopy (SEM) observations of the annealed WC-Ni/TiC–Ni cermet diffusion couple. This study offers a novel approach for exploring the formation mechanism of core-rim structure in TiC-based cermets.

In Situ Sintering of CdSe/CdS Nanocrystals under Electron Beam Irradiation.

Colloidal semiconductor nanocrystals have attracted widespread attention due to their tremendous electrical and optical properties. Nanoparticles exhibit a strong tendency to aggregate and sinter in a short period of time during processing or use due to their large surface area-to-volume ratio, which may lead to significant changes in their required performance. Therefore, it is of great significance to conduct in-depth research on the sintering process and mechanism of nanoparticles to maintain their stability. Here, the sintering process of CdSe/CdS core/shell nanocrystals under continuous electron beam irradiation was studied using in situ transmission electron microscopy (TEM). In the early stages of sintering, CdSe/CdS nanocrystals approached each other at a distance of approximately 1-2 nm. As the exposure time to the electron beam increased, the movement of surface atoms on the nanocrystals led to contact between them. Subsequently, the atoms on the contact surfaces underwent rapid motion, resulting in the rapid formation of the neck between the particles. The neck formation between adjacent particles provides strong evidence of a sintering mechanism dominated by surface atom diffusion rather than Ostwald ripening. Further research in this area could lead to the development of improved methods to prevent sintering and enhance the stability of nanocrystals, ultimately contributing to the advancement of nanomaterial-based devices and materials with long-lasting performance.

Fast densification mechanism of highly transparent AlON ceramics pressureless sintered by co-doping Y2O3 and MgO additives

By co-doping 0.15 wt% Y2O3 and 0.60 wt% MgO, AlON ceramics with high transmittance in visible and infrared range (the maximum transmittance of 81.8%@3600 nm, 78.1%@400 nm) were successfully fast fabricated via pressureless sintering at 1880 °C for 2.5 h. The phase assemblage, microstructure and densification process of samples doped (Y2O3/MgO single- and co-doped) and undoped additives during heating were monitored to explore the densification mechanism. MgO was found to be an exceptional inhibitor for both AlON decomposition and grain growth. By incorporating MgO as one constituent of additives, the content of α-Al2O3 that decomposed from AlON in samples during heating was significantly reduced and Mg2+ illustrated inhibition to grain boundary migration, resulting in particle aggregation/coarsening were effectively suppressed at ≤ 1600 °C. Based on the obtained microstructure with compacted small particles and negligible particle aggregation/coarsening at early stage of sintering, facilitated by the acceleration of Y3+ on mass transfer, rapid densification of AlON was achieved.

Differential densification in Ho:Y2O3 transparent ceramics

Differential densification has been observed in the sintering of Ho:Y2O3 ceramics at heating rates of ≤3 °C. The temperature gradients in compacts during the heating process were simulated using ANSYS software. At a heating rate of 3 °C/min, the simulation results indicate a small temperature discrepancy of about 2 °C between the surface and the center area of the samples. Compared with those in the near-center (NC) area of the compact, the grain coarsening and rearrangement in the near-surface (NS) area are more pronounced during the initial and intermediate sintering stages. As a consequence, both the pore distribution and microstructure in the NS area were more uniform, which accelerated the elimination of pores in the subsequent sintering stage. This observation suggests that particle rearrangement in the early sintering stage is one of the main reasons for the enhancement in the ultimate densification of the ceramics. The NC area could be fully densified by using hot isostatic pressing (HIP). After HIP, the Ho:Y2O3 ceramics have high in-line transmittance of 80% at the wavelength of 500 nm.

Effect of ammonia injection stages and segregation degrees on the characteristics of iron ore sintering process

To reduce solid fuel consumption and carbon emissions during iron ore sintering, ammonia injection-assisted sintering was proposed, and its feasibility was verified through sinter pot experiments. The effects of ammonia injection stages and segregation degrees on the sintering characteristics were investigated based on the uneven heat distribution in the sintering bed. The results show that ammonia injection improves fuel combustion efficiency, sinter bed temperature and melting quantity index. The sinter ore strength and yield were improved, and the effect of injecting ammonia in the early sintering stage was more significant. When the ammonia injection segregation degree is 0.042 %/min, the sinter ore yield and tumbler index increase from 61.93% and 49.4% to 68.85% and 58.8%, respectively, the average fuel combustion efficiency increases from 89.02% to 92.30%. When the segregation degree increases from 0 to 0.125 %/min, the standard deviation of the uniformity coefficient of the high-temperature index first decreases and then increases, reaching the minimum value when the segregation degree is 0.042 %/min. A segregation degree of 0.042 %/min can achieve the most uniform heat distribution in the sintering bed and the best sinter ore quality, which provides a reference for applying ammonia injection-assisted sintering in industrial production.

Investigation of consolidation mechanisms induced by applied electric/electromagnetic fields during the early stages of spark plasma sintering

This work aims at investigating the role of the electric/electromagnetic fields generated by the pulsed current applied during Spark Plasma Sintering (SPS) process on the sintering mechanisms. The selected model material was a metallic granular medium composed of microsized particles of pre-oxidized copper. Its electrical behavior and correlated microstructural modifications were studied in two complementary enclosures. First, a demonstrator equipped with a modulable pulsed electric current generator has allowed analyzing, at low temperature and without mechanical loading, the effects related to the application of an electric wave, by controlling and modulating its characteristics (i.e. shape, frequency, amplitude). An abrupt electrical transition, named “Branly effect” from an insulating to a conductive state, is observed in the granular copper medium. The increase of pulses frequency is shown to strongly reduce the critical time to generate the electrical transition. Moreover, a commercial SPS device, with specific electrical and thermal instrumentation, was implemented by varying applied stress and using conductive and insulating dies. The analysis of the electrical behavior coupled with post-mortem microstructural observations allowed to highlight that the coupling between pulsed current and mechanical stress promotes specific mechanisms in SPS. Under high stress, interparticle contact area increase and the insulating oxide layer is damaged by microcracking. The coupling with the flow of the pulsed current involves local overheating and dielectric breakdown, favoring ignition of Branly effect. Micro-welds are formed between particles, creating privileged paths of the pulsed current and initiating densification at low temperature (250 °C).

TUDY OF THE INITIAL MAGNETIC PERMEABILITY OF LiTiZnMnFERRITES OBTAINED BY LIQUID-PHASE SINTERING UNDER RADIATION-THERMAL AND THERMAL CONDITIONS

The measuring the temperature dependence of the initial permeability was used to study the features of phase and structural transformations in lithium-titanium ferrites as a function of time, heating and cooling rates, and the temperature of liquid-phase sintering under thermal and radiation-thermal heating. Ferrite was synthesized from powder mixture by solid-phase synthesis. A low-melting additive bismuth dioxide was used to obtain the ferrite ceramics by liquid-phase sintering. RT sintering was carried out by heating the samples with a pulsed (1.5–2.0) MeV electron beam. It was established that the additive leads to a less defective state of sintered ferrites, while the action of radiation enhances this effect in the early stages of sintering. The regularities of the influence of the heating and cooling rates of compacted samples on the change in the initial magnetic permeability of sintered ferrites are established.

An interface nucleation rate limited sintering kinetic model applied to in situ sintering Al2O3-SmAlO3 composites

A nucleation rate limited sintering model was recently developed based on observations of bicrystal sintering. This work validates the applicability of this model for sintering of polycrystalline clusters of Al2O3-SmAlO3 at high temperature in the range of 1130–1610 ℃. The model fits the data well and agrees with trends observed during bicrystal sintering. A temperature dependence to the dominant sintering strain deformation modes is observed from in situ heating experiments performed in a transmission electron microscope (TEM). The observations provide insights into how temperature influences the early stages of sintering by affecting the pore size distribution through local de-sintering. This provides insights into the role heating rate and sintering schedule play in microstructural evolution that influences the grain size versus density trajectory.

Thermal Stability of Semiconductor Nanocrystal Solids: Understanding Nanocrystal Sintering and Grain Growth

Nanomaterials are naturally metastable with respect to bulk solids. This raises the very important fundamental problem of their morphological stability, especially when nanoscale crystallites are touching or nearly touching each other, such as in thin-film devices. In some cases, nanostructuring must be preserved under operational conditions (e.g., in quantum dot LEDs, lasers, photodetectors, and nanogranular thermoelectric devices). In other cases, we use nanocrystalline particles as precursors to a material with large crystalline grains and aim to sinter them as efficiently as possible (e.g., in polycrystalline thin-film solar cells). We carried out a systematic study of sintering and grain growth in materials composed of various sub-10 nm semiconductor grains. The boundaries between individual semiconductor grains have been chemically engineered using inorganic surface ligands. We found that the early stages of sintering and grain growth of nanocrystalline semiconductors are controlled by the ion mobility at the nanocrystal surfaces, while the late stages of grain growth are controlled by the mobility of the grain boundaries. This appears to be a general phenomenon for semiconductor nanocrystals, and it leads to several interesting and counterintuitive trends. For example, III–V InAs nanocrystals are generally much more resilient against sintering and grain growth compared to II–VI CdSe nanocrystals even though bulk CdSe has significantly higher melting point temperature than InAs (1268 °C vs 942 °C). Grain growth can be dramatically accelerated when coupled to solid−solid phase transitions. These findings expand our toolbox for rational design of nanocrystal materials for different applications.

Shell-like structure to limit further densification formed during flash sintering under alternative current electric field for 8 mol %Y<sub>2</sub>O<sub>3</sub>-doped ZrO<sub>2</sub>

A phenomenon often appears that the final achieved density is limited during conventional flash sintering even under alternative current electric field. By observing the cross-sectional microstructure of the flash sintered 8 mol %Y2O3-doped ZrO2 at this state, we found that a thin surface portion with near full density surrounds a porous inside with lower density. The densified thin surface portion is shell-like structure covering a whole of the sintered compact. The formation of this characteristic shell-like structure in the early stage of flash sintering greatly restricts subsequent densification of porous inside. In order to suppress the formation of the shell that inhibit densification, a technique of controlling a steep power spike at a flash event must be necessary to be appropriately transitioned.

Antimicrobial activity of cuprous oxide-coated and cupric oxide-coated surfaces

Disease can be spread through contact with contaminated surfaces (fomites). For example, fomites have been implicated in the spread of meticillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa. Antimicrobial surface treatments are a potential method of reducing disease transmission from fomites, and broad-spectrum activity is desirable. To test cuprous oxide (Cu2O) and cupric oxide (CuO) coatings for antimicrobial activity against 12 micro-organisms including bacteria and fungi. We fabricated two surface coatings. The Cu2O coating was fabricated in a simple two-step process using polyurethane to bind the active copper oxide particles; CuO was prepared by heat treatment of Cu2O particles in air to produce cupric oxide (CuO) and to cause early-stage sintering to form a continuous coating. The antimicrobial activity was examined with 10μL of microbial suspension droplets followed by counting cells as colony-forming units (cfu). The coatings rapidly killed nine different micro-organisms, including Gram-negative and Gram-positive bacteria, mycobacteria and fungi. For example, the Cu2O/PU coating killed 99.9997% of P.aeruginosa and 99.9993% of S.aureus after 1h. Efficacy was not reduced after weekly cleanings. The antimicrobial activity of the Cu2O coating was unchanged after abrasion treatment, and the coatings were not cytotoxic to human cells. The combination of broad-spectrum antimicrobial activity, abrasion resistance, and low toxicity of the Cu2O coating suggests potential use in healthcare settings.

Investigation of Electrical Transitions in the First Steps of Spark Plasma Sintering: Effects of Pre-Oxidation and Mechanical Loading within Copper Granular Media.

Spark Plasma Sintering (SPS) has become a conventional and promising sintering method for powder consolidation. This study aims to well understand the mechanisms of densification encountered during SPS treatments, especially in the early stages of sintering. The direct current (DC) electrical behavior of copper granular medium is characterized. Their properties are correlated with their microstructural evolutions through post-mortem scanning electron microscope (SEM) observations to allow a thorough understanding of the involved Branly effect that is suspected to occur in SPS. The electrical response is studied by modifying the initial thickness of the oxide layer on particles surfaces and applying various mechanical loads on the granular medium. Without load and at low current, the measured quasi-reversible behavior is connected to the formation of spots at the microcontacts between the particles. By increasing the current, the Branly transition from an insulating to a conductive state suddenly occurs. The insulating oxide layer is destroyed, and micro-bridges are created. The application of a mechanical pressure strongly modifies the DC Branly effect. Increasing low stress leads to a strong decrease in the breakdown field. For high-applied pressure, successive drops in the electric field are detected during the electrical transition. These successive drops are induced by microcracking of the insulating oxide layer.

Interstitial chemistry in sintering of metallic materials – often overlooked but decisive

Sintering is a process by which particulate materials – loose or compacted – are transformed into a body that may be fully dense or may still contain pores, but in any case has structural integrity and load-bearing capacity. The physical mechanisms – especially the transport processes – that are responsible for these changes have been studied since the 1950s. However, the chemical part of sintering also is of decisive importance in particular for metallic systems, especially concerning the interstitial elements O and C. Any metal powder that has ever been exposed to air bears oxygen on the surface, and this oxygen has to be removed in the early stages of sintering to enable the physical transport processes to become effective. In the present work, various chemical reactions involving oxygen and/or carbon are described, and it is shown how the alloying system selected as well as the starting powder grade affect these reactions and the properties of the final products.

Mechanisms and energetics in the early stages of solvent-assisted low-temperature sintering of ZnO

The Hydro/Solvothermal Sintering (HSS) is a technique enabling the low temperature sintering of ceramics and composites, by using a solvent and uniaxial pressure. This energy-efficient technique also opens new perspectives in designing new composites with tailored functional properties. While the solvent plays a key role in reducing the sintering temperature, the modeling and deep understanding of sintering mechanisms remain an open field of investigation. This study unveils the energetics and mechanisms involved in the first stage of HSS. The strategy highlighted in this paper includes: (i) the building of a hydrothermal sintering device equipped with a dilatometer to monitor the shrinkage in situ, (ii) numerical stress calculations at the contact between particles to show the suitability of the two-particle kinetic equation, and (iii) the use of anisothermal and non-conventional stepwise isothermal methodologies for the investigation of mechanisms and energetics. The model material, ZnO, was densified to high relative densities with acetic acid as a solvent, and a pressure and temperature of 320 MPa and 150 °C, respectively. With these experimental conditions, the kinetic analysis obtained from the two methodologies is consistent and implies a dissolution reaction of the material as the rate controlling mechanism with possible coupling to grain boundary sliding. The activation energy of 90 kJ.mol—1 is determined with the different analysis methodologies. The interest of the stepwise isothermal methodology in obtaining accurate activation energy is also discussed.

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

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.

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.

Grain growth in sintering: A discrete element model on large packings

Sintering is a high temperature process used for ceramic or metallic powder consolidation that consists of concurrent densification and grain growth. This work presents a coupled solid-state sintering and grain growth model capable of studying large packings of particles within the Discrete Element Method (DEM) framework. The approach uses a refinement for large particle size ratios of previously established contact laws to model shrinkage. In addition, mass transfer between neighboring particles is implemented to model grain growth by surface diffusion and grain-boundary migration. The model assumptions are valid for initial and intermediate stage sintering. The model is validated on a two-particle system by comparing neck and particle size evolutions with those obtained by phase-field and meshed-based methods. Simulations on large packings (up to 400 000 particles) with particle size distributions originating from experiments are performed. The results of these simulations using physical data from the literature are compared to experimental data with good accordance of the key features of the microstructure evolution (densification kinetics, grain size-density trajectory, evolution of the mean grain size and of the size distribution). The simulations show that even at an early stage of sintering, hardly detectable grain growth actually affects the sintering kinetics to a non-negligible extent and that the realism of DEM simulations of sintering is improved when grain growth is considered. Taking advantage of the possibility to simulate large packings, the model elucidates the influence of the initial particle size distribution on the grain growth kinetics.