Early Stages Of Sintering Research Articles

Densification behaviour and sintering mechanisms of Cu- or Co-doped SnO2: A comparative study

Compared with the CuO–SnO2 system, there is a lack of experimental evidence of a eutectic liquid phase in the CoO–SnO2 system. It is therefore believed that the densification behaviour and sintering mechanisms of the two systems are completely different. However, our present study clearly demonstrated that the Co- and Cu-doped SnO2 ceramics exhibited a similar densification behaviour. The 1wt.% Cu- or Co-doped SnO2 ceramics could reach over 95% relative density when sintered to 1250°C with a heating rate of 5°Cmin−1. It was found that both CoO4/3- and CuO-doped SnO2 follow the same mass transport mechanism – viscous flow sintering during early-stage sintering. This was verified with the classic isothermal and constant-heating-rate models, as well as Frenkel’s viscous flow theory. The viscous flow mechanism dominated densification up to ∼73 and ∼78% relative density with the apparent activation energies of 362 and 440kJmol−1 for Co and Cu doping cases, respectively.

Surface phenomena during the early stages of sintering in steels modified with Fe–Mn–Si–C master alloys

The characteristics of the metallic powder surface play a critical role in the development of strong bonds between particles during sintering, especially when introducing elements with a high affinity for oxygen. In this study, Mn and Si have been combined in a Fe–Mn–Si–C master alloy powder in order to reduce their chemical activity and prevent oxidation during the heating stage of the sintering process. However, when this master alloy powder is mixed with an iron base powder, differences in chemical activity between both components can lead to an oxygen transfer from the iron base powder to the surface of the master alloy particles. The present research is focused on studying the evolution of the master alloy particle surface during the early stages of sintering. Surface characterization by X-ray Photoelectron Spectroscopy (XPS) shows that the master alloy powder surface is mostly covered by a thin easily reducible iron oxide layer (~1nm). Mn–Si particulate oxides are found as inclusions in specific areas of the surface. Evolution of oxides during sintering was studied on green compacts containing iron powder, graphite and Fe–Mn–Si–C master alloy powder that were heat treated in vacuum (10−6mbar) at different temperatures (from 400, 600, 800 to 1000°C) and analyzed by means of XPS. Vacuum sintering provides the necessary conditions to remove manganese and silicon oxides from the powder surface in the range of temperatures between 600°C and 1000°C. When sintering in vacuum, since the gaseous products from reduction processes are continuously eliminated, oxidation of master alloy particles due to oxygen transfer through the atmosphere is minimized.

Sintering behavior and microwave dielectric properties of (1−x)Li2TiO3+xLiF ceramics prepared by enhanced sintering

Sintering behavior, structure and microwave dielectric properties of (1−x)Li2TiO3+xLiF (0≤x≤0.7) ceramics prepared by enhanced sintering were studied by thermal dilatometry, X-ray diffraction (XRD), Scanning Electron Microscopy (SEM ), Raman spectra and microwave resonant measurement. It was found that the short range ordering and order–disorder phase transition temperature in this case increased compared with that observed for the specimen prepared from prealloyed powders because of the compositional inhomogeneity. The early stage sintering was enhanced by the chemical potential gradient originated from the compositional gradient between Li2TiO3 and LiF. However unequal diffusion rates between LiF and Li2TiO3 particles left behind large pores in the sintered bodies. Transient liquid phase sintering was confirmed in the x≥0.2 compositions. An optimized microwave dielectric properties with εr of ~22.4, Q×f of ~110,000GHz and τf of ~3.2ppm/°C could be obtained for the x=0.1 composition after sintering at 1100°C/2h.

In-situ synchrotron x-ray transmission microscopy of the sintering of multilayers

This letter reports on in-situ characterization of the high temperature sintering of multilayer ceramic capacitors by high-resolution synchrotron x-ray imaging. Microstructural evolution was obtained in real time by a continuous recording of 2-dimensional radiographs. Anisotropic strains were measured for different layers. Quantification of defects was conducted with 3-dimensional nano-computed tomography. These in-situ observations prove that electrode discontinuities occur at the early stage of sintering and originate from initial heterogeneities linked to the particulate nature of the starting powders.

Applied Pressure on Altering the Nano‐Crystallization Behavior of Al86Ni6Y4.5Co2La1.5 Metallic Glass Powder during Spark Plasma Sintering and Its Effect on Powder Consolidation

Metallic glass powder of the composition Al86Ni6Y4.5Co2La1.5 was consolidated into 10 mm diameter samples by spark plasma sintering (SPS) at different temperatures under an applied pressure of 200 MPa or 600 MPa. The heating rate and isothermal holding time were fixed at 40°C/min and 2 min, respectively. Fully dense bulk metallic glasses (BMGs) free of particle‐particle interface oxides and nano‐crystallization were fabricated under 600 MPa. In contrast, residual oxides were detected at particle‐particle interfaces (enriched in both Al and O) when fabricated under a pressure of 200 MPa, indicating the incomplete removal of the oxide surface layers during SPS at a low pressure. Transmission electron microscopy (TEM) revealed noticeable nano‐crystallization of face‐centered cubic (fcc) Al close to such interfaces. Applying a high pressure played a key role in facilitating the removal of the oxide surface layers and therefore full densification of the Al86Ni6Y4.5Co2La1.5 metallic glass powder without nano‐crystallization. It is proposed that applied high pressure, as an external force, assisted in the breakdown of surface oxide layers that enveloped the powder particles in the early stage of sintering. This, together with the electrical discharge during SPS, may have benefitted the viscous flow of metallic glasses during sintering.

Impact of morphological parameters onto simulated light scattering patterns

We have investigated the impact of the variation of various parameters of fractal aggregates on simulated light scattering patterns. Static light scattering is commonly used to measure soot in a flame and such a study could help to improve experimental approaches. Aggregate models, used for our light scattering simulations, are based on real soot structures that can be found under laboratory conditions in a premixed ethane/air flame (McKenna-type burner, equivalence ratio ϕ=2.5). Our work was not focused on modeling and analysis of aggregates that are typically encountered in the atmosphere, therefore the results might be of limited interest to climate scientists. In our study, the variation of all parameters that enter into the standard fractal equation were investigated. Additionally effects when varying the overlap of primary particles, the incident wavelength and the complex refractive index are discussed. For numerical simulations two different codes were used, the T-Matrix (when particles are in point contact) and the DDScat program (which is capable of performing light scattering simulations by overlapping spheres). Comparisons between these two methods show very good agreement. The results demonstrate that the radius of gyration is responsible for the amount of light scattered towards the back direction while the total volume of an aggregate defines the shape of the light scattering patterns. Small changes of the fractal dimension can be neglected (provided that the fractal prefactor is accordingly modified in a suitable way). The overlap level, if the radius of gyration is kept constant, introduces barely visible changes to the light scattering diagrams which suggest that a simple aggregate model, composed of particles being in point contact, can be used instead of a structure in early sintering stage when overlap of primary particles is not so high.

The Effect of Particle Size Distributions on the Microstructural Evolution During Sintering

Microstructural evolution and sintering behavior of powder compacts composed of spherical particles with different particle size distributions (PSDs) were simulated using a kinetic Monte Carlo model of solid‐state sintering. Compacts of monosized particles, normal PSDs with fixed mean particle radii and a range of standard deviations, and log‐normal PSDs with fixed mode and a range of skewness values were studied. Densification rate and final relative density were found to be inversely proportional to initial PSD width. Grain growth was faster during the early stages of sintering for broad PSDs, but the final grain sizes were smaller. These behaviors are explained by the smallest grains in the broader PSDs being consumed very quickly by larger neighboring grains. The elimination of the small grains reduces both the total number of necks and the neck area between particles, which in turn reduces the regions where vacancies can be annihilated, leading to slower densification rates. The loss of neck area causes grain growth by surface diffusion to become the dominant microstructural evolution mechanism, leading to poor densification. Finally, pore size was shown to increase with the width of PSDs, which also contributes to the lower densification rates.

Environmental Transmission Electron Microscopy Study of the Origins of Anomalous Particle Size Distributions in Supported Metal Catalysts

In this Environmental Transmission Electron Microscopy (ETEM) study we examined the growth patterns' of uniform distributions of nanoparticles (NPs) using model catalysts. Pt/SiO2 was heated at 550 degrees C in 560 Pa of O-2 while Pd/carbon was heated in vacuum at 500 degrees C and in 300 Pa of 5%H-2 in Argon at temperatures up to 600 degrees C. Individual NPs of Pd were tracked to determine the operative sintering mechanisms. We found anomalous growth Of NPs occurred during the early stages of catalyst sintering wherein some particles started to grow significantly larger than the mean, resulting in a broadening of the particle size distribution (PSD). The abundance of the larger particles did not fit the log normal distribution. We can rule out sample nonuniformity as a cause for the growth of these large particles, since images were recorded prior to heat treatments. The anomalous growth of these particles may help explain PSDs in heterogeneous catalysts which often show particles that are significantly larger than the mean, resulting in a long tail to the right. It has been suggested previously that particle migration and coalescence could be the likely cause for such broad size distributions. We did not detect any random migration of the NPs leading to coalescence. A directed migration process was seen to occur at elevated temperatures for Pd/carbon under H-2. This study shows that anomalous growth of NPs can occur under conditions where Ostwald ripening is the primary sintering mechanism.

Effect of SiC Addition on Sintering Behavior and Properties of Indium Tin Oxide

The sintering behavior of ITO ceramics with the addition of fine and coarse SiC was studied. SiC addition promoted the densification of ITO during early stage sintering compared to pure ITO. On the other hand, the addition of SiC resulted in the development of numerous pores in the microstructures due to the oxidation of SiC during sintering. In particular, swelling of the samples, which lead to an expansion of the sample, was observed when 1.0 wt% of fine SiC was added. Fine SiC particles are believed oxidize faster than coarse SiC particles, and SiC oxidation during sintering resulted in a low density and high porosity regardless of the SiC particle size.

In situ transmission electron microscopic investigations of reduction-oxidation reactions during densification of nickel nanoparticles

Abstract

Microstructure Development and Alloying Elements Diffusion during Sintering of Near-β Titanium Alloys

Two near-β alloys, Ti-5Al-5Mo-5V-2Cr-1Fe and Ti-10V-3Fe-3Al, were produced by the blended element powder metallurgy technique. The use of (i) elemental powders with the Al-V master alloy in the case of Ti-5Al-5Mo-5V-2Cr-1Fe and, (ii) the complex Al-Fe-V master alloy in Ti-10V-3Fe-3Al has highlighted the influence of different alloying elements and their combination on microstructure evolution and chemical homogenisation. While Fe has the fastest diffusivity in Ti and its addition improves the density of both sintered alloys it also results in accelerated rates of grain coarsening. The combination of Al and V in the master alloy powder inhibits the diffusion of V into the Ti matrix. The unexpectedly slow diffusion of Cr at the early sintering stage in Ti-5Al-5Mo-5V-2Cr-1Fe was attributed to the formation of surface oxides on the Cr powders.

Pressure-less spark plasma sintering effect on non-conventional necking process during the initial stage of sintering of copper and alumina

In the present study, we focus on the characterization of the necking mechanisms during the early stages of pressure-less spark plasma sintering (PL-SPS) compared to conventional sintering (CS) of two different types of powdered materials (Cu and α-Al2O3). SEM observations of the evolution of particle morphology and necks from the as-received powders to sintered ones show the nature of the neck between particles which were either in contact or not. For alumina, no particular necking process (melt or viscous bridge) was observed regardless of the sintering conditions (PL-SPS and CS), even for a very high heating rate 455 °C/min. For copper, this neck morphology is unequivocally not typical of conventional ones, thus, suggesting mass transport by an ejection mechanism. This particular morphology was seen occasionally. In comparison, the conventionally sintered Cu particles presented a smoother surface, with conventional curved necks suggesting the contribution of surface diffusion mechanisms. Based on partial pressure calculations, a direct thermal effect might not explain the observed non-conventional neck for copper. On the other hand, local field enhancement effect and local favourable thermal breakdown voltage conditions are described and discussed in order to support the experimental results.

Decarburization and improvement of ultra fine straight WC–8Co sintered via microwave sintering

Cemented tungsten carbide with ultra fine grains was prepared via microwave sintering. η phase (W3Co3C) was formed on the surface of the samples during the preparation process. Extra carbon black was premixed and the influence of carbon content on mechanical properties was studied. The results show that the maximum value of hardness and transverse rupture strength are HRA 93.2 and 3396 MPa respectively when the carbon black content is 0.45%. The microstructure investigated by SEM show that the WC grains growth mainly occurs during the early stage of microwave sintering by the coalescence of grains.

Effect of calcination temperature on the fabrication of transparent lutetium titanate by spark plasma sintering

Transparent lutetium titanate (Lu2Ti2O7) bodies were fabricated by spark plasma sintering using Lu2O3 and TiO2 powders calcined from 700°C to 1200°C. No solid-state reaction was identified after calcination at 700°C, whereas single-phase Lu2Ti2O7 powder was prepared at 1100 and 1200°C. The calcination at 700°C promoted densification at the early stages of sintering, whereas residual pores at grain boundaries resulted in Lu2Ti2O7 bodies with low transparency. Low-density and opaque Lu2Ti2O7 bodies formed owing to the coarsening of the powder calcined at 1200°C. The Lu2Ti2O7 body sintered using the powder calcined at the moderate temperature of 1100°C had a density of 99.5% with the highest transmittances of 41% and 74% at wavelengths of 550nm and 2000nm, respectively.

The effect of anisotropic dimensional change on the precision of steel parts produced by powder metallurgy

The dimensional characteristics of ring-shaped parts produced with an Fe–Cu–P alloy by a conventional powder metallurgy process were measured on both green and sintered parts. The dimensional change was investigated by dilatometry, and the work highlighted that the anisotropy observed is mainly due to the early sintering stage. In this study, where the alloy is characterised by a slight densification, the anisotropic behaviour has a negligible effect on the precision of the height and the internal diameter, while the effect on the precision of the external diameter may be relevant.The results have been generalised, considering larger volume variations and different anisotropy parameters, and some relations between the anisotropy parameters and the attainable ISO tolerance classes have been determined.

Post Treatment Process and Selective Laser Sintering Mechanism of Polymer-Coated Mo Powder

A novel preparing method of polymer-coated molybdenum powder was presented. A type of multi-component polymer-coated molybdenum powder was chosen for selective laser sintering. The effect of the process parameters on the part's characteristics is investigated. Based on our study for dynamic laser sintering process of polymer-coated molybdenum powder, its laser sintering mechanism was reported as follows: at the early stage of laser sintering, the viscous flow is the major mechanism; during the laser sintering, the melting/solidification is the major mechanism. Furthermore, a model corresponding to the mechanism was discussed schematically, which could be used to explain the material migrating mode during laser sintering process. The laser sintering experiments of polymer-coated Mo powder was conducted in the self-developed laser sintering machine, and optimized parameters have been acquired. At last, the post treatment process of laser sintered parts has been developed and refractory metal parts of Mo/Cu composites are gained. The post treatment process includes debinding, high temperature sintering and melting infiltration, which is sintering framework-Mo by high temperature combining with Cu impregnation method. It is found that the mechanical properties of parts have been greatly improved after the post treatment process.

Spark plasma sintering of B 4C ceramics: The effects of milling medium and TiB 2 addition

Boron carbide (B 4C) ceramics, with a relative density up to 98.4% and limited grain growth, were prepared at 1600–1800 °C by spark plasma sintering (SPS) technique. The effects of powder milling medium (water and 2-propanol) on the powders' surface characteristics and TiB 2 addition on the sintering densification were investigated. The ball milling processing of B 4C powders in water can promote the sintering of B 4C ceramics. A B 2O 3 layer on B 4C particle surface is concluded to promote the densification of the B 4C ceramics at an early sintering stage. This B 2O 3 layer, which normally inhibits the densification process at the final stage of the sintering, can be reduced through reaction with TiB 2 particles, resulting in further densification of the B 4C ceramics.

Kinetics of Initial Coarsening During Sintering of Nanosized Powders

Grain growth during sintering is a critical issue for the manufacture of nanocrystalline bulk materials from nanosized powders. The grain growth process during sintering can be viewed as consisting of two parts: initial coarsening during early and intermediate stages of sintering and latter stage grain growth during the final stage of sintering. The latter stage grain growth is the normal grain growth that has been well studied and reported in the literature. The initial coarsening, which often inevitably causes a material to lose nanoscaled grain size characteristics, however, is not well studied at all. In this investigation, the initial coarsening during sintering of nanosized powders was studied by both nonisothermal and isothermal experimental techniques using tungsten as an example material. The results show that the initial coarsening during the heat-up process of a sintering cycle is sufficient to increase the grain size beyond the nanoscale. The kinetics of initial coarsening is found to be linear rather than polynomial, as predicted by the conventional power law of grain growth. The analysis of activation energies showed that surface diffusion is the primary mechanism for interparticle mass transport during the initial coarsening. The linear kinetic behavior could be attributed to the pinning of grain boundaries by surface grooves and high concentration of defects as the result of the synthesis of nanosized powders.

Cooperative material transport during the early stage of sintering

The complex transport processes contributing to sintering are not yet fully understood, partially because in-situ observations of sintering in three dimensions (3D) are very difficult. Here we report a novel experiment in which monocrystalline copper spheres are first marked with microscopic boreholes drilled using a focused ion beam, after which high-resolution synchrotron X-ray tomography is carried out to measure translational, rolling and intrinsic rotation movements of some hundred spheres during sintering. Unlike in 1D and 2D systems, we show that, in 3D, intrinsic rotations are more pronounced than angular rolling rearrangements of the particle centres and become the dominant mechanism of particle movement. We conclude that in addition to the well-known neck growth and centre approach mechanisms, grain boundary sliding caused by the different crystallographic orientations of the individual spheres occurs.

Microstructure and properties of ultrafine WC-10Co composites with chemically doped VC

Vanadium carbide is the most effective grain growth inhibitor for ultrafine WC-Co composites due to its high solubility and mobility in the cobalt phase at relatively low temperatures; however, there are still some debates over the best way to introduce it into the WC-Co formulation. In this paper, the differences between admixed and chemically doped grain growth inhibitors on the microstructural development and properties of an ultrafine WC-10Co composite are discussed. The densification rate of chemically doped samples is slower in the early stage of sintering and the WC grain sizes of the sintered alloys are finer than those of admixed samples, leading to the increase of hardness and transverse rupture strength of the sintered alloys. The effectiveness of the chemically doped inhibitor is attributed to the formation of vanadium rich layers on the surfaces of tungsten carbide powders during reduction and carbonization, which alters the surface and interface energies of WC grains, impedes the contact with each other of WC grains and contributes to the resistance to W diffusion across the layer during sintering, resulting in the inhibition of nanosized particle coalescence.