Sintering Kinetics Research Articles

Tailoring sintering kinetics and dielectric properties of Li2SiO3 ceramics by CaO–B2O3–SiO2 glass dopant for LTCC substrate applications

Tailoring sintering kinetics and dielectric properties of Li2SiO3 ceramics by CaO–B2O3–SiO2 glass dopant for LTCC substrate applications

Porosity evolution during heating of copper made from powder by friction extrusion

Friction extrusion was used to compact and extrude solid copper rods from feedstock powders. Following extrusion, considerable porosity was observed throughout the extrudate cross section due to the entrained porosity in the feedstock material and the extrusion process. The thermal stability of the extrudate was investigated via a series of heat treatments. Porosity evolution exhibited three distinct stages—an unchanged plateau (0–300 °C) followed by an abrupt increase (400–500 °C) and ultimately a reduction, as the temperature increases (>500 °C). The peak porosity measured was ∼25%. The underlying driving force for pore evolution is described as the competition between the internal pore pressure, material strength, and sintering kinetics, as a function of temperature. The observed porosity evolution and driving force are not expected to be limited to copper. Thus, this manuscript reveals an important consideration regarding the microstructure thermal stability as advanced manufacturing methods involving direct powder extrusion are explored.

Spark Plasma Sintering of Al-=SUB=-2-=/SUB=-O_3-SiC ceramics. Study of the Microstructure and Properties

The features of spark plasma sintering of submicron Al2O3 powders with different contents (0, 0.5, 1.5, 5 vol.%) of β-SiC nanoparticles have been studied. The microstructure and hardness of Al2O3 + 5 vol.% SiC ceramics obtained by sintering Al2O3 powders with β-SiC particles of various types (nanoparticles, submicron particles, fibers) have been studied. Sintering was carried out at heating rates (Vh) from 10 to 700oC/min. The sintering process of Al2O3 + SiC ceramics with low heating rates (V_h=10-50oC/min) has a complex three-stage character, with a flat area in the temperature range of 1200-1300oC. At high heating rates (V_h>250oC/min), the usual three-stage character of sintering is observed. The analysis of temperature dependences of compaction was carried out using the Young-Cutler model; It was found that the kinetics of powder sintering is limited by the intensity of grain boundary diffusion. It is shown that the dependence of the hardness of Al2O3 + SiC ceramics on Vh has a nonmonotonic character, with a maximum. In the case of pure alumina, an increase in Vh leads to a monotonic decrease in hardness Keywords: Alumina, silicon carbide, density, diffusion, hardness.

Высокоскоростное электроимпульсное плазменное спекание мелкозернистых керамик Al-=SUB=-2-=/SUB=-O_3-SiC. Исследование микроструктуры и механических свойств

The features of spark plasma sintering of submicron Al2O3 powders with different contents (0, 0.5, 1.5, 5 vol.%) of -SiC nanoparticles have been studied. The microstructure and hardness of Al2O3 + 5 vol.% SiC ceramics obtained by sintering Al2O3 powders with -SiC particles of various types (nanoparticles, submicron particles, fibers) have been studied. Sintering was carried out at heating rates (Vh) from 10 to 700 °C/min. The sintering process of Al2O3 + SiC ceramics with low heating rates (Vh = 10–50 °C/min) has a complex three-stage character, with a flat area in the temperature range of 1200–1300 °C. At high heating rates (Vh > 250 °C /min), the usual three-stage character of sintering is observed. The analysis of temperature dependences of compaction was carried out using the Young-Cutler model; It was found that the kinetics of powder sintering is limited by the intensity of grain boundary diffusion. It is shown that the dependence of the hardness of Al2O3 + SiC ceramics on Vh has a nonmonotonic character, with a maximum. In the case of pure alumina, an increase in Vh leads to a monotonic decrease in hardness.

Study of the Effect of a Small Addition of ZrO-=SUB=-2-=/SUB=- on the Density and Grain Growth of Alumina

The effect of an additive of 0.5 wt.% of zirconium oxide on the spark plasma sintering (SPS) kinetics of alumina powders has been studied. Ceramics were obtained by mixing alumina powders in a planetary mill with grinding media from stabilized zirconium oxide. The activation energy of SPS was estimated using the Young-Cutler model. It has been shown that the density and average grain size in ceramics obtained from submicron alumina powders are significantly affected by the nonequilibrium state of the interfaces formed as a result of the transformation of the amorphous phase present on the alumina powder particle surface. The grain size and density of ceramics obtained from fine powders are significantly affected by the coalescence of ZrO2 particles. Keywords: Alumina, zirconium oxide, spark plasma sintering, density, diffusion, activation energy.

Исследование влияния малой добавки ZrO-=SUB=-2-=/SUB=- на плотность и рост зерен мелкозернистого оксида алюминия

The effect of an additive of 0.5 wt. % of zirconium oxide on the spark plasma sintering (SPS) kinetics of alumina powders has been studied. Ceramics were obtained by mixing alumina powders in a planetary mill with grinding media from stabilized zirconium oxide. The activation energy of SPS was estimated using the Young-Cutler model. It has been shown that the density and average grain size in ceramics obtained from submicron alumina powders are significantly affected by the nonequilibrium state of the interfaces formed as a result of the transformation of the amorphous phase present on the alumina powder particle surface. The grain size and density of ceramics obtained from fine powders are significantly affected by the coalescence of ZrO2 particles.

Investigation on sintering processes and mechanical properties of Ti–Ta alloys by molecular dynamics simulation.

The aim of this work was to introduce an EAM potential function to describe the sintering kinetics of Ti–Ta alloys by molecular dynamics simulation to understanding the dynamics behavior during the Ti–Ta alloys sintering process. The potential energy and Lindemann index during heating were calculated and the evolution of local crystal structure, during heating was characterized. The results showed that the size of Ti and Ta nanoparticles had a great significant impact on the melting point of Ti–5Ta (wt%) alloys. To further verify the suitability of the EAM potential function in determining Ti–Ta alloys mechanical properties, the elastic modulus of Ti–5Ta (wt%) alloys was determined both experimentally and by molecular dynamics simulation. The experimental measurements and simulation calculations were in good agreement with each other, demonstrating the accuracy of the EAM potential function. This work provides ideas and guidance for further improving the potential applications of Ti–Ta alloys.

Doped Strontium Titanate Anode for Solid Oxide Fuel Cells: Electrical and Sintering Behavior

When doped strontium titanate is used as anode material for solid oxide fuel cells, it demands good electron-ion mixed conductivity and proper sintering kinetics. In this work, La0.5Sr1.5Ti1.5Fe0.5O3-δ, La0.5Sr1.5Ti1.5Co0.5O3-δ, and La0.5Sr1.5Ti1.5Ni0.5O3-δ anodes with double perovskite structure were synthesized. The porosity of the anodes was between 38.5% and 18.7% after sintering at 1200 °C in air. The apparent electrical conductivity reached 500–600 S⋅cm−1 at 700 °C under H2 atmosphere and no pre-reduction process at high temperature was involved. La0.5Sr1.5Ti1.5Co0.5O3-δ anode showed the highest sintering activity and it reached the peak rate of sintering shrinkage at 1209 °C. The doping of Fe, Co and Ni elements into strontium titanate exhibited great potential to adjust the conductivity and sintering kinetics for anode materials.

Visco-elastic sintering kinetics in virgin and aged polymer powders

This work provides a novel discrete element method (DEM) framework for modelling the visco-elastic sintering kinetics in virgin and aged polymer powders. The coalescence of particle pairs, over long times, is described by a combined three-stage model of the sintering process, where each stage is dominated by a different driving force: adhesive contact force, adhesive inter-surface force and surface tension. The proposed framework is implemented in MercuryDPM, an open-source package for discrete particle simulations. To quantitatively calibrate the particle-scale parameters, Bayesian filtering is used. Experimental data on Polystyrene (PS), Polyamide 12 (PA12), and PEEK powders, both virgin and aged, are analysed and confirm over a wide range of times the existence of the three distinct sintering mechanisms. In good agreement with the experimental observations, the estimation of sintering time is achieved with a significant accuracy compared to Frenkel's model. This study provides an efficient and reliable approach for future studies of strength evolution in powder-bed fusion processes.

Contour Fitting of Fused Filaments Cross-Section Images by Lemniscates of Booth: Application to Viscous Sintering Kinetics Modeling.

A method for image analysis was implemented to determine the edge pixels of two biopolymer-based thermoplastic filaments during their hot melt isothermal sintering at 120 °C. Successive inverted ellipses are adjusted to the contour of the sintered filaments and lead to the identification of the parameters of the corresponding lemniscates of Booth. The different steps of the morphological image analysis are detailed, from 8-bit coded acquired images (1 frame/s), to the final fitting of the optimized mathematical functions describing the evolution of the filaments envelope. The complete sequence is composed of an initial pure viscous sintering step during the first minute, followed by viscoelastic swelling combined with melt spreading for a longer time, and then the stabilization of the sintered filaments shape for over 2 min at high temperatures. Using a master curve obtained from Hopper’s abacus, the characteristic viscous sintering time is assessed at tvs = 78 s, confirming the one previously found based on the measurement of the bonding neck length alone. Then, the full description of the evolution of the thermoplastic filaments envelope is assessable by image analysis during sintering trials as a result of its digital modeling as successive lemniscates of Booth, reflecting geometry changes in the molten state.

Sabatier principle of metal-support interaction for design of ultrastable metal nanocatalysts.

The stability of supported nanocatalysts is crucial to meeting environmental and energy challenges and necessitates fundamental theory to relieve trial-and-error experimentation and accelerate lab-to-fab translation. Here, we report a Sabatier principle of metal-support interaction for stabilizing metal nanocatalysts against sintering based on the kinetic simulations of 323 metal-support pairs using scaling relations from 1252 energetics data. Too strong of an interaction is shown to trigger Ostwald ripening, whereas too weak of an interaction stimulates particle migration and coalescence. High-throughput screening of supports enables the sintering resistance of nanocatalysts to reach the Tammann temperature on homogeneous supports and far beyond it on heteroenergetic supports. This theory, which is substantiated by first-principles neural network molecular dynamics simulations and experiments, paves the way for the design of ultrastable nanocatalysts.

Effect of tungsten nanoparticles on sintering of the Sn-Cu-Co-W powder material

The effect of tungsten nanoparticles on the kinetics of sintering of the Sn-Cu-Co-W powder material used as a binder in diamond tools was studied. The W16,5 grade tungsten powder was mechanically activated in the AGO-2U planetary centrifugal mill for 60 minutes at the carrier rotation frequencies of 800 RPM. The mixture of tungsten, tin, copper, and cobalt powders was compacted by static pressing in press dies and then sintered in vacuum at the temperature of 820°C. The morphology and sizes of powder particles, as well as the structure of the sintered samples, were studied by the methods of scanning electronic microscopy. It has been demonstrated that tungsten nanoparticles have a noticeable effect on the process of dissolution-reprecipitation of cobalt in liquid-phase sintering.

Towards pressureless sintering of nanocrystalline tungsten

The challenge of sintering ultrafine-grained tungsten to >99% density and towards nanocrystallinity is hereby addressed by optimized pressureless two-step sintering. It successfully produced tungsten samples with 99.3% theoretical density and 290 nm average grain size, with a uniform grain structure, good grain boundary cohesion, and 7.8 GPa hardness that is the highest in all pressurelessly sintered tungsten. The critical role of initial powders and the resultant green bodies was noted, which greatly affects later-on sintering kinetics and microstructural uniformity. Several key questions concerning two-step sintering of metallic tungsten were addressed, including the selection of the first- and second-step sintering temperatures, the thermodynamically required critical density to start the second-step sintering, and grain growth kinetics during sintering. The lessons learned here should be directly transferable to other refractory metals and their alloys and would help address the ultimate task of pressureless sintering of bulk nanocrystalline refractory metals/alloys.

Rapid fabrication of lanthanum strontium cobalt ferrite (LSCF) with suppression of LSCF/YSZ chemical side reaction via flash light sintering for SOFCs

The chemical side reaction between a lanthanum strontium cobalt ferrite (LSCF) cathode and an yttria-stabilized zirconia (YSZ) electrolyte is a key problem for solid oxide fuel cell (SOFC) fabrication. Herein, we demonstrate rapid fabrication of an LSCF cathode using a novel flash light sintering (FLS) technique to suppress the chemical side reaction; notably, no Gd-doped ceria interlayer is required. The FLS-fabricated LSCF cathode shows comparable crystalline development and microstructural evolution to a cathode fabricated using the conventional high-temperature sintering process. A clear LSCF/YSZ interface appears, indicating suppression of detrimental chemical side reactions via rapid and surface-dominant sintering kinetics. Detailed interfacial microstructure analysis reveals the absence of Sr segregation and SrZrO3 formation at the FLS-LSCF cathode/YSZ electrolyte interface. Remarkable performance of 0.9 W cm–2 power density at 750 °C is achieved for a cell incorporating the rapidly fabricated LSCF cathode on the interlayer-free YSZ electrolyte owing to suppression of the insulating secondary phases and significantly reduced polarization resistance. Hence, rapid LSCF cathode fabrication via the novel FLS method effectively suppresses secondary phase formation at the LSCF/YSZ interface and greatly simplifies SOFC fabrication by eliminating the doped-ceria-based interlayer. Moreover, this novel method could help overcome interfacial chemical reaction problems in various solid-state devices, while also providing new application opportunities for high-performance electrolyte/electrode materials.

Influence of the Addition of Ni‐Coated Carbon Nanotubes on the Mechanical Properties of Highly Porous Zirconia Cellular Structures

Herein, the effect of the addition of Ni‐coated carbon nanotubes (CNTs) on the microstructure, mechanical properties, and morphology of zirconia porous cellular structures is studied, aiming to increase their mechanical strength. Highly porous structures of monolithic zirconia (ZrO2) and zirconia reinforced with Ni‐doped CNTs (ZrO2 + CNT/Ni) are fabricated through the replica method. Scanning electron microscopy and micro‐computed tomography (microCT) analyses show that ZrO2 + CNT/Ni specimens present lower overall porosity (88.0%) than monolithic ZrO2 (93.8%), indicating improved sintering kinetics. Compressive tests for the ZrO2 + CNT/Ni samples result in a compressive strength of 0.82 ± 0.10 MPa, a threefold value of that of the monolithic sample (0.25 ± 0.07 MPa). The ZrO2 + CNT/Ni samples also present a higher elastic modulus (41.02 ± 4.94 MPa) than the ZrO2 ones (21.92 ± 5.40 MPa). Thus, the addition of Ni‐coated CNTs significantly improves the strength of highly porous zirconia cellular structures.

The Structural Evolution and Densification Mechanisms of Nanophase Separation Sintering

Nanophase separation sintering (NPSS) facilitates low temperature, pressureless sintering through the formation of solid phase necks driven by phase separation. Systems that have been shown to exhibit this phenomenon are W–Cr, Cr–Ni and to a lesser degree Ti–Mg. Initial information on the average rate-limiting sintering kinetics in these systems was obtained using traditional master sintering curve analysis, but it is very clear that multiple processes occur during NPSS, and these should each have their own characteristic kinetics. Here we analyze these three systems in greater kinetic detail using densification rates in a Kissinger-style analysis derived explicitly for densification data. For the W–Cr and Cr–Ni systems two critical temperatures were identified: one at low temperatures for the formation of the secondary phase necks, and a second one at high temperatures corresponding to the onset of rapid densification. The activation energies of these processes are different, and reflective of bulk solute diffusion and interdiffusion, respectively. Combined with microstructural observations, these data show that the onset of rapid densification at high temperatures is facilitated by the presence of the second-phase necks, and occurs at the point where the system can fully interdiffuse, rehomogenizing those necks. These observations help explain why the Ti–Mg system does not densify well, because it does not exhibit redissolution at high temperatures. These results help clarify the conditions needed to achieve NPSS and may support design of new alloys for NPSS behavior.

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.

Effect of the Ag addition on the compressibility, sintering and properties of Ti6Al4V/xAg composites processed by powder metallurgy

This work investigates the fabrication of Ti6Al4V/xAg composites processed by powder metallurgy. An atomized Ti6Al4V powder was blended with 0, 5, 10, 15 and 20 vol% of Ag particles, compacted in a die, and sintered above Ag melting point. The compressibility of the mixtures was analyzed, and the sintering kinetics was evaluated by dilatometry tests. The characterization of sintered samples by microtomography, SEM and diffraction analysis allowed determining the effect of Ag amount on the sintering process and on microstructural changes arising during sintering. The mechanical properties of sintered materials were investigated by microhardness. The compressibility of Ti6Al4V powder was found to be improved by adding soft Ag particles, which resulted in denser packing and larger plastic deformation. The densification during compaction can be predicted by Heckel model and the rule of mixture. It was next found that sintering was driven by volume diffusion in the solid state for Ag content lower than 10 vol% and involved a liquid phase for Ag content higher than 15 vol%. In the latter case, the liquid fills the interconnected pores under the action of capillary forces as the quantity of liquid increased. The microstructure was composed mainly of α-Ti, Ti2Ag, and Ag, which filled the pores between Ti6Al4V particles. Microhardness data showed a maximum value with 20 vol% of Ag. This value is similar to those reported for highly dense Ti6Al4V. It has thus proved that close to full-dense Ti6Al4V/20Ag composites can be processed by powder metallurgy.

A novel technique for the development of acetabular cup by cold isostatic compaction and sintering of UHMWPE powder with optimized processing parameters

AbstractLife of a metal on ultra‐high molecular weight polyethylene (UHMWPE) total hip replacement is often limited to 10–15 years, due to wear loss and aseptic loosening. Due to its high melt flow index, UHMWPE is typically processed by ram extrusion or compression molding technique, but yet to be processed to the full potential of its mechanical integrity in acetabular shape without any fusion defects or weak bonding. The main objective of the present study is to develop a novel technique to fabricate defect‐free acetabular cups with desired bearing characteristics and surface finish by sintering medical‐grade UHMWPE GUR 1050 powder after its cold isostatic compaction with optimum processing parameters. Sintering kinetics of UHMWPE is studied comprehensively using a thermomechanical analyzer. The influence of compaction pressure, sintering temperature, and sintering duration on sintering kinetics of UHMWPE is explored to realize their optimum. The optimally processed UHMWPE has the relative density of 97% and Vickers hardness of 5.4 with tensile yield strength and elastic modulus of 21.5 and 625 MPa, respectively. The newly developed acetabular cup exhibited inherent plateau‐finished bearing surface with an average surface roughness of <100 nm, having good bearing characteristics and desired dimension.

Improving sintering kinetics and compositional homogeneity of Inconel 625 superalloy open-cell foams made by suspension impregnation method

Open-cell Inconel 625 superalloy foams were prepared by suspension impregnation method which involves replication of pure nickel foam ligaments with a mixed powder suspension and subsequent heat treatments. The objective of the present study is to find a suitable method ensuring complete densification of the superalloy foam ligaments without using low-melting additives and achieving uniform chemical composition at moderate sintering temperatures. Foams prepared using the above methodology were evaluated for the microstructure and compositional variation across the foam ligaments by scanning electron microscopy (SEM) coupled with electron dispersive spectroscopy (EDS). Quasi-static compressive properties of the foams were evaluated. It is shown that if elemental powders of Cr, Mo and Ni are used as additives to the superalloy powder suspension, high ligament density with uniform chemical composition conforming to the Inconel 625 specification is achieved at moderate sintering temperatures. These results are attributed to the formation of low melting eutectics and to acceleration of the diffusion kinetics due to the non-equilibrium thermodynamic conditions created by large compositional gradients in the presence of elemental powders.