Sintering Rate Research Articles

Comparing Microstructures of ITO Sputtering Targets Prepared by Tin Doped Indium Oxide Powders and In2O3-SnO2 Mixed Powders

Abstract Indium Tin Oxide sputtering target (ITO) is widely used in the formation of electrically transparent thin films, which adopt direct current magnetron sputtering process for electrodes in flat panel displays, solar cells, gas sensors and so on. In order to improve ITO thin film property, ITO sputtering target requires high purity, homogeneous microstructure, high density and high electrical conductivity. In the present paper, ITO sputtering targets were fabricated using In2O3-SnO2 mixed powders and tin doped indium oxide powders. Meanwhile, their discrepancies in microstructure and sintering mechanism, especially grain size distribution, grain morphology, major element distribution and sintering rate, were studied. The results show that the sintering rate of the mixed powders is faster than that of the doped powders, but it is difficult to obtain homogeneous microstructure for the mixed powders. In contrast, it is easy to obtain the ITO sputtering target with homogeneous microstructure using the doped powders, under the condition that the decomposition of indium oxide is inhibited in process of sintering. These results are helpful to fabricate ITO sputtering target with good microstructure, while decrease nodulation and improve production efficiency.

The mechanical and thermophysical properties of La2(Zr1−xCex)2O7 ceramics

The La2(Zr1−xCex)2O7 (x = 0, 0.3, 0.5, 1.0) powders were synthesized by coprecipitation-calcination method. The XRD results indicated that La2(Zr0.5Ce0.5)2O7 comprised of La2Ce2O7 and La2Zr2O7, whereas single phase La2(Zr0.7Ce0.3)2O7 with pyrochlore structure was synthesized at 1400 °C. The bulk La2(Zr1−xCex)2O7 (x = 0.3, 0.5, 1.0) were sintered at 1600 °C and showed excellent phase stability at 1450 °C. The La2Ce2O7 possessed the highest microhardness both in as-sintered state and after heat treatment at 1450 °C for different times, followed by La2(Zr0.5Ce0.5)2O7 and La2(Zr0.7Ce0.3)2O7. The La2(Zr0.5Ce0.5)2O7 had the highest fracture toughness of 3.8 ± 0.2 MPa m1/2 in as-sintered state, followed by La2(Zr0.7Ce0.3)2O7 (3.5 ± 0.2 MPa m1/2) and La2Ce2O7 (2.6 ± 0.3 MPa m1/2), which are much higher than that of La2Zr2O7 and 8YSZ. The sudden thermal expansion decrease of La2Ce2O7 was fully suppressed in La2(Zr0.7Ce0.3)2O7. The La2(Zr0.7Ce0.3)2O7 also had the lowest sintering rate of 1.13 × 10−7 s−1 at 1400 °C, as well as the lowest thermal conductivity of 1.06 Wm−1K−1 at 900 °C.

Synthesis of BSCF perovskites using a microwave-assisted combustion method

Ba0.5Sr0.5Co0.8Fe0.2O3−Δ (BSCF) perovskite-type oxide was synthesized using a microwave-assisted combustion method. Following about 10min of exposure to microwave, the combustion reaction was self-ignited and it produced a porous powder. For comparison, BSCF perovskite powders were also synthesized by conventional heating. Two different fuel types were used in the two heating methods. The crystalline BSCF powders were characterized using scanning calorimetry and thermogravimetry (DSC/TG), X-ray diffraction (XRD), BET surface area measurement, scanning electron microscopy (SEM), and dilatometric analysis. The microwave heating method resulted in a powder with nanometric crystallites, submicrometric particles, and a specific surface area of 9.93m2/g, which corresponds to a BET diameter of approximately 100nm. The dilatometric analysis showed that the microwave-synthesized BSCF powder has the maximum sintering rate at 970°C, but the sintering process initiates at lower temperatures.

Synthesis of Titanium Dioxide Aerosol Gels in a Buoyancy-Opposed Flame Reactor

Aerosol gels are a novel class of materials with potential to serve in various energy and environmental applications. In this work, we demonstrate the synthesis of titanium dioxide (TiO2) aerosol gels using a methane-oxygen coflow diffusion flame reactor operated in down-fired configuration (fuel flow in the direction opposite to buoyancy forces). Titanium tetraisopropoxide was fed as a precursor to the flame under different operating conditions. Control of the monomer size and crystalline phase of TiO2 gel particles was achieved by adjusting the flame operating conditions, specifically the flame temperature, which was shown to significantly influence the phase transformation and rate of particle growth and sintering. The resulting materials were characterized for their physical and optical properties. Results showed that the TiO2 aerosol gels had effective densities in the range 0.021–0.025 g/cm3, which is 2 orders of magnitude less than the theoretical mass density of TiO2. The monomer size distribution, crystalline phase, and UV-Vis absorbance spectra of the gels showed distinct characteristics as a function of flame temperature.Copyright © 2015 American Association for Aerosol Research

Simulated UO2 fuel containing CsI by spark plasma sintering

Herein, an innovative preparation procedure has been deployed enabling, for the first time, the incorporation of volatile fission product simulant into highly dense nuclear fuel pellets. Highly volatile fission products were embedded in a dense UO2 matrix in the form of CsI by simply mixing starting materials and consolidation in a Spark Plasma Sintering step at 1000 °C with a 5 min dwell time. CsI particles were evenly distributed throughout the pellet and were located at the grain boundaries. The sintering rate is dependent on the O/U ratio of the powder. Addition of CsI also acts as a sintering aid, reducing the temperature of maximum densification.

­­Additive Manufacturing for Solid Oxide Cell Electrode Fabrication

Electrode microstructure plays a critical role in determining the electrochemical performance and durability of Solid Oxide Cells. Additive manufacturing can potentially offer a highly-defined electrode microstructure, as well as fast and reproducible electrode fabrication. Selective laser sintering (SLS) is an additive manufacturing technique in which the three-dimensional structures are created by bonding subsequent layers of powder using laser power. Although selective laser sintering can be applied to a wide range of materials, including metals and ceramics, the scientific and technical aspects of the manufacturing parameters and their impact on microstructural evolution during the process are not well understood. In the present study this novel approach for electrode fabrication was evaluated by conducting a proof of concept study. A Ni-patterned fuel electrode was laser sintered on a YSZ substrate. The optimization process of laser parameters (laser sintering rate and laser power) and the electrochemical results of a full cell with laser sintered electrode are presented. The challenges and prospects of using selective laser sintering for SOC fabrication (for example, to create electrodes with unique 3D hierarchical porous structures) are discussed. Figure 1

Aggregate characteristics accounting for the evolving fractal-like structure during coagulation and sintering

Coagulation and partial coalescence (or sintering) frequently results in fractal-like aerosol structures in natural and industrial processes. The asymptotic form of such structures is described reasonably well with the so-called fractal dimension, Df. Little is known, however, for its evolution, from spheres to fractal-like particles and, in particular, its effect on aerosol primary particle and collision diameters that determine the environmental impact or manufactured product performance. So the effect of a variable or constant Df on product crystalline (TiO2) and amorphous (SiO2) aerosol particle characteristics is elucidated over their process synthesis parameter space ( maximum temperature 1600–2000K, cooling rate 103–106K/s and precursor molar fraction 10−4–10−1). Aerosol dynamics by coagulation and sintering are simulated accounting for the evolving fractal-like structure by either a linear interpolation or detailed mesoscale simulations from spherical to asymptotic fractal-like structures. In addition, two sintering rates for SiO2 as well as expressions for the effect of particle structure on sintering rate are compared in terms of product particle characteristics. Neglecting the evolution of Df hardly affects the product primary particle and soft-agglomerate diameters but overestimates the agglomerate collision diameter growth rate during the hard- to soft-agglomerate transition. This underpredicts the hard-agglomerate diameter by 25–30% at high cooling rates (105–106K/s).

Preparation and sintering behaviour of La5.4WO12−δ asymmetric membranes with optimised microstructure for hydrogen separation

La5.4WO12−δ (LaWO) is a promising membrane candidate for a variety of H2-related applications due to its appreciable levels of mixed proton–electron conduction and its stability in moist reducing atmospheres at elevated temperatures. Governed by Wagner theory, the H2 permeation performance of a membrane can be enhanced by reducing its thickness. Therefore, the present work deals with preparing LaWO supported membranes with reduced thickness and optimised microstructure. Combining a dense membrane with a porous supporting layer is associated with mismatched sintering rates, which ultimately lead to bending effects. Therefore, the sintering behaviour of both the dense membrane and the porous substrate must be carefully adjusted to each other. For this purpose, single and co-fired membrane and substrate layers were produced by tape casting. Sintering experiments were carried out with an optical dilatometer. The shrinkage and microstructural evolution of the layers were evaluated in terms of the anisotropic shrinkage forces and the membrane rigidness counteracting the substrate shrinkage. The results were used to develop asymmetric LaWO membranes with optimal microstructure. High membrane density was combined with a substrate porosity of ~30% and minimised bending (40µm). The LaWO membrane–substrate assembly displayed a He leakage of 10−5hPadm³cm−2s−1, which is a value that satisfies further practical demands.

Relating adatom emission to improved durability of Pt–Pd diesel oxidation catalysts

Sintering of nanoparticles is an important contributor to loss of activity in heterogeneous catalysts, such as those used for controlling harmful emissions from automobiles. But mechanistic details, such as the rates of atom emission or the nature of the mobile species, remain poorly understood. Herein we report a novel approach that allows direct measurement of atom emission from nanoparticles. We use model catalyst samples and a novel reactor that allows the same region of the sample to be observed after short-term heat treatments (seconds) under conditions relevant to diesel oxidation catalysts (DOCs). Monometallic Pd is very stable and does not sinter when heated in air (T⩽800°C). Pt sinters readily in air, and at high temperatures (⩾800°C) mobile Pt species emitted to the vapor phase cause the formation of large, faceted particles. In Pt–Pd nanoparticles, Pd slows the rate of emission of atoms to the vapor phase due to the formation of an alloy. However, the role of Pd in Pt DOCs in air is quite complex: at low temperatures, Pt enhances the rate of Pd sintering (which otherwise would be stable as an oxide), while at higher temperature Pd helps to slow the rate of Pt sintering. DFT calculations show that the barrier for atom emission to the vapor phase is much greater than the barrier for emitting atoms to the support. Hence, vapor-phase transport becomes significant only at high temperatures while diffusion of adatoms on the support dominates at lower temperatures.

Additive Manufacturing for Solid Oxide Cell Electrode Fabrication

Additive manufacturing can potentially offer a highly-defined electrode microstructure, as well as fast and reproducible electrode fabrication. Selective laser sintering is an additive manufacturing technique in which three-dimensional structures are created by bonding subsequent layers of powder using a laser. Although selective laser sintering can be applied to a wide range of materials, including metals and ceramics, the scientific and technical aspects of the manufacturing parameters and their impact on microstructural evolution during the process are not well understood. In the present study, a novel approach for electrode fabrication using selective laser sintering was evaluated by conducting a proof of concept study. A Ni-patterned fuel electrode was laser sintered on an yttria-stabilized zirconia substrate. The optimization process of laser parameters (laser sintering rate and laser power) and the electrochemical results of a full cell with a laser sintered electrode are presented. The challenges and prospects of using selective laser sintering for solid oxide cell fabrication are discussed.

Microwave and conventional sintering of SiC/SiC composites: Flexural properties and microstructures

SiC/SiC composites were fabricated by the PIP process via microwave and conventional heating from 800°C to 1200°C. The flexural properties and microstructures of the as-fabricated SiC/SiC composites were investigated. The results indicated that the flexural strength and toughness have the same changing tendency for both conventional and microwave sintering. However, at the same PIP sintering temperature, the flexural properties of the microwave sintered SiC/SiC are higher than that of the conventional sintered ones. The higher residual strength of SiC fibers properties and more cracks in the composite matrix resulting from higher heating rates of the microwave sintering brought about the better flexural properties.

The role of vanadium additive in the activated sintering and shrinkage rate of tungsten–vanadium alloys

Spark plasma sintering (SPS) is a common and effective way to fabricate tungsten-based materials for preliminary investigations of their application in fusion reactor. The selection of doping materials and their appropriate concentration in tungsten is important for activated consolidation and to improve the shrinkage rate during the sintering process. The impact of vanadium concentration on the shrinkage rate of tungsten–vanadium (W–V) alloys has been studied in this work. Improvement in the shrinkage rate and mechanical strength of W–V alloys has been achieved by increasing the V concentration. The residual porosity was gradually decreased and the activated sintering conditions got better with the increase of V concentration. The saturation of shrinkage rate has been found at 1550°C for W-10 wt.%V.

Fe-doped YSZ electrolyte for the fabrication of metal supported-SOFC by co-sintering

Considering Fe as sintering aid for Yttria Stabilized Zirconia (YSZ), studies have been conducted under conditions necessary for the fabrication of MS-SOFC by co-sintering. Pure and Fe doped YSZ was sintered at 1350°C in air and in argon atmosphere and a comparative study has been performed. Structural characterizations were carried out using X-ray diffraction (XRD) techniques, while ionic conductivity was measured in air using four-probe impedance spectroscopy. It is found that Fe enhances the sintering rate of YSZ in both air and argon atmosphere. All samples sintered in argon atmosphere are characterized by better ‘sinterability’, larger lattice parameter, higher density, larger grain size and lower conductivity, as compared to samples sintered in air. Ionic conductivity is found to decrease with increase in Fe concentration for all sintered samples.

Morphology and Crystallinity of Coalescing Nanosilver by Molecular Dynamics

Sintering and its final stage of coalescence of silver nanoparticles with various morphologies have been investigated in vacuo between 400 and 1000 K by molecular dynamics simulations using the embedded atom method (EAM). It was found that the Ag nanoparticle melting temperature increases with increasing particle size and approaches the bulk melting point of Ag for bigger particles (>10 nm) consistent with simulation literature and experimental data. The motion of surface and bulk Ag atoms within Ag nanoparticles is monitored closely during their sintering. Early on, the sintering or coalescence of nanoparticles is dominated by surface diffusion whereas a transition toward plastic flow sintering can be observed near their melting point. The sintering rate of straight nanoparticle chains is much slower than that of more compact structures. The formation of new crystal domains during Ag particle sintering is demonstrated for the first time to the best of our knowledge, and mechanisms leading to formation of...

Sintering Kinetics Across Pore Closure Transition Accounting for Continuous Increase in Grain Coordination with Density

The influence of dihedral angle and particle coordination on the evolution of densification rate is investigated throughout the intermediate and final stages of sintering. From a review of literature data, heuristic expressions are proposed for representing different trajectories of evolution of coordination from the onset of skeleton formation up to full density. The influence of dihedral angle and coordination on the threshold density for pore closure is established by revisiting the Plateau–Rayleigh instability criterion. The intermediate and final stages are modeled using representative volume elements that do not restrict coordination to integer values. Computational results enlighten the interplay between coordination and dihedral angle on the evolution of sintering stress, bulk viscosity, and sintering rate. The results allow quantifying the amplitude of the drop of densification rate at pore closure transition. The monotonous increase of coordination with relative density brings about a monotonous increase of sintering rate at constant particle size. Simulations emphasize the beneficial effect of higher initial density and coordination in the green part.

Theory of Sintering in Presence of Pressure and Torque

Experiments like the sintering of single crystal spheres on a single crystal plate, the sintering of particle rows or 2D arrangements of powder particles as well as recent experiments by in‐situ Synchrotron Computer Tomography show movements of entire particles beyond the particle center approach described by the well‐known two particle model. Several attempts exist to describe these cooperative material transport processes by an analytical model of the sintering of two wires by grain‐boundary diffusion. The described mechanisms are either rotations caused by asymmetric sintering necks or the application of a torque or changes of the rate of sintering by a normal stress in the sintering neck. To develop a more comprehensive model of sintering, volume diffusion was included in addition to grain‐boundary diffusion and all driving forces mentioned above are used. As a result, the influence of normal stress, torque, and ratio of the sintering neck curvatures on the stress along the contact grain boundary, the diffusion current and the rate of removal along the sintering neck are shown. The contribution of volume diffusion and grain‐boundary diffusion to sintering was also investigated, confirming that a large sintering neck and high sintering temperatures result in a dominating influence of volume diffusion. The shrinkage rate and the rate of rotation provided by this model were used in a subsequent numerical simulation. For several stages of sintering, the influence of particle radius and sintering temperature on the rolling angle of sintering particles was analyzed in 2D. Furthermore, the impact of external normal stresses on the rolling angle is shown. The simulation model gives a surprising result: The speed of wire rolling shows the same behavior as sintering kinetics—a high speed for small particles, small sintering neck diameter and high temperature, but the cumulated rolling angle for small particles is low compared to that for larger particles. Finally, a new approach for simulating particle rotation and rolling caused by torques due to grain‐boundary anisotropy in 3D is presented, which is based on an existing Discrete Element Method sintering model. This model is applied to a row of particles confirming previous observations from the 2D sintering simulations.

Effect of HA Content on Microstructure and Compression Properties of Ti-Nb-Sn/HA Composites Fabricated by Plasma Current Activated Sintering

Novel bio-composites were synthesized by plasma current activated sintering from the Ti-35Nb-2.5Sn/HA powders ball-milled for 12 h. The aim of this study was to investigate the effects of HA content (5, 10 and 15 wt%) on sintering properties, microstructure and compression properties of Ti-35Nb-2.5Sn/HA bio-composites. Results indicated that sintering rate decreased slightly with the increase of HA content. The phases of sintered composites were mainly˰ڂ˽̤̹˼˰̘̑˼˰Ca3(PO4)2(TCP), TiO2, CaTiO3 and TixPy. The grain size of sintered composites reduced with the increasing of HA content, and sintered composites with ultra fine grains were fabricated finally. The compression test showed that all the sintered composites had low elastic modulus and high compression strength. The elastic modulus of Ti-35Nb-2.5Sn/15HA sintered composites was 22GPa with a high strength of 877MPa.

Influence of Basicity on High-Chromium Vanadium-Titanium Magnetite Sinter Properties, Productivity, and Mineralogy

The effect of basicity on high-chromium vanadium-titanium magnetite (V-Ti-Cr) sintering was studied via sintering pot tests. The sinter rate, yield, and productivity were calculated before determining sinter strength (TI) and reduction degradation index (RDI). Furthermore, the effect of basicity on V-Ti-Cr sinter mineralogy was clarified using metallographic microscopy, x-ray diffraction, and scanning electron microscopy-energy-dispersive x-ray spectroscopy. The results indicate that increasing basicity quickly increases the sintering rate from 25.4 mm min−1 to 28.9 mm min−1, yield from 75.3% to 87.2%, TI from 55.4% to 64.8%, and productivity from 1.83 t (m2 h)−1 to 1.94 t (m2 h)−1 before experiencing a slight drop. The V-Ti-Cr sinter shows complex mineral composition, with main mineral phases such as magnetite, hematite, silicate (dicalcium silicate, Ca-Fe olivine, glass), calcium and aluminum silico-ferrite (SFCA/SFCAI) and perovskite. Perovskite is notable because it lowers the V-Ti sinter strength and RDI. The well intergrowths between magnetite and SFCA/SFCAI, and the decrease in perovskite and secondary skeletal hematite are the key for improving TI and RDI. Finally, a comprehensive index was calculated, and the optimal V-Ti-Cr sinter basicity also for industrial application was 2.55.

Practical and Theoretical Research on the Sintering Process

The Novolipetsk Metallurgical Combine (NLMK) has collaborated with leading scientific-research institutes and scientists in the area of sinter production for decades. The combine’s sinter plant has been used to conduct many factory tests, and the results of those tests have helped refine and advance the theory of the main processes which take place during the sintering of iron-ore-bearing materials. The theoretical gains have in turn facilitated the introduction of a number of effective technologies. 1. Charging of the mix onto the sintering machine. During the 1980s and 1990s, several foreign sinter plants that sinter coarse-grained charges began to abandon the practice of two-layer charging that had been used on the sintering machines at the NLMK and the West-Siberian, Cherepovets, and Karaganda metallurgical combines. The change from twolayer charging brought about marked increases in sinter production, which led to study of the effect of different factors on the performance of the sintering machines at the NLMK with the use of two-layer and one-layer charging [1, 2]. This was the first such study for sinter plants that operate on iron-ore concentrates. The factory tests established the following [1]: 1) the unit gasdynamic resistance of the bottom third of the lower layer of a two-layer charge is 1.8 times greater than that of the middle third and 4 times greater than that of the top third. Thus, the factor which limits sintering rate is not the boundary layer (although it makes its own negative “contribution”), as was stated in [2]. Instead, the limiting factor in this case is the lowest part of the bottom layer; bulk density in this part is greater and the high-temperature range is broader than in the overlying parts of the layer; 2) the average gas-pressure gradient in the lower half of the bottom layer of the charge is about 70% of the total resistance of the layer; 3) the transition to one-layer charging increases the sintering rate by 10%, which confirms that there is a net improvement in the gasdynamic structure of the layer; and 4) the content of the ‐5-mm fraction in the skip sinter rose by 0.2 abs.%.

Sintering and Microstrusture of Al2O3 and Al2O3-ZrO2 Ceramics

Alumina powder and two commercial 3 mol% yttria-partially stabilized zirconia powders–0.3 wt% Al2O3-doped (Al-Y-PSZ) and without Al2O3 (Y-PSZ)–were used to produce alumina-zirconia (Al2O3-ZrO2) slip cast composites. The influence of the substitution of Al2O3 by 50 vol% of the different zirconia powders on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al2O3 and the different composites at temperatures in the range of 1100-1600°C was studied and related to the sample hardness. An increase in the sintering rate was observed when 50 vol% Y-PSZ was substituted by 50 vol% Al-Y-PSZ. 50 vol% zirconia was effective to reduce the rate of Al2O3 grain growth in the final sintering stage. For 50 vol% Al-Y-PSZ a smaller ZrO2 grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al2O3 grain size of the sintered samples became greater than about 1μm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400°C, since de composites with 50 vol% zirconia reduced the rate of Al2O3 grain growth a decrease in hardness up to 1600°C was not observed.