Late Stages Of Sintering Research Articles

Microscale viscous sintering model application to the preparation of metal bonded diamond grinding wheels

During sintering, with the interfacial free energy decreasing, particles bond to each other and the dense density increases, which are familiar phenomena in the preparation of metal bond diamond wheels and affect the mechanical properties of the grinding wheel. Herein, a sintering model of metal bond diamond wheels at a microscopic scale was constructed based on the computational fluid dynamics (CFD) and discrete element method (DEM), which was carried out to reveal the flow field, pressure field, and kinetic properties of the metal material in the heating process. Meanwhile, the finite element method (FEM) was used to analyze the thermal stress of diamond particles. The results show that with the increase in temperature, the viscous flow of particles intensifies. In the middle and late stages of sintering, the porosity of the particles changes significantly, and the densification rate is accelerated. Under the same temperature conditions, compared with the dodecahedral diamond, the octahedron produces less thermal stress and has better mechanical properties.

A predictive study on effective thermal conductivity of sintered nickel powder under different thermal processing conditions

Effective Thermal Conductivity (ETC) plays a key role in the thermal performance evaluation of samples with porous structure, which can be characterized by multiple physical properties such as particle sizes, porosity, and surface energy. The thermal conductivity of un-sintered porous material has been highlighted in most previous studies, while the evaluation of sintered thermal conductance is scarcely mentioned before. This study aims to develop an analytical model for the ETC prediction of sintered nickel powders based on the circuit equivalent model and sintering neck formation theory. Here, sixteen groups of sintering samples in three particle sizes are fabricated according to orthogonal design with four levels and four key factors, including sintering temperature (650∼800 °C), compressional stress (10∼25 MPa), heating rate (7∼20 °C/min), and holding time (1∼3 h). A dimensionless factor, formulated by the ratio of shrinkage rate to sintering contact area fraction, is proposed to reflect the effect of pore combination and grain growth at the late stage of sintering. The presented model is experimentally validated with an average deviation of three particle sizes equal to 7.11%, 6.38%, and 3.48%, respectively. Besides, simulated ETC agrees well with measured data for samples with medium-high porosity (0.44∼0.71) compared to those prepared with pore-forming agents.

Densification Mechanism and Microstructure Evolution of Ti‐48Al‐2Cr‐8Nb Alloy by Spark Plasma Sintering

AbstractIn this work, Ti‐48Al‐2Cr‐8Nb alloy is sintered via spark plasma sintering with pre‐alloyed powders prepared by the atomized method. The densification mechanism and microstructure evolution of the spark plasma sintered Ti‐48Al‐2Cr‐8Nb alloy are analyzed. The sintering process is divided into two stages of densification and microstructure transformation. First, the densification mechanism of the powder materials at different sintering temperatures is investigated. At the beginning of the sintering process, the powder particles are closely rearranged under pressure. In the middle of the sintering process, the plastic deformation of the powder particles and the material migration between the adjacent particles promote the densification into the high‐speed stage. In the late stages of sintering, the unique equilibrium phase transition of the TiAl alloy allows the densification to be completed quickly. Additionally, the formation process and phase transition mechanism of the near gamma structure, the duplex structure, the full‐lamellar structure, and spherical structure in Ti‐48Al‐2Cr‐8Nb alloy are systematically studied.

Limited grain growth and chemical ordering during high-temperature sintering of PtNiCo nanoparticle aggregates

High-temperature sintering of ternary PtxNi100−x−yCoy (x = 28–44%, y = 40–54%) nanoparticles of interest in catalysis was studied in situ and in real-time with synchrotron-based x-ray diffraction. For the first time we were able to experimentally capture the early stage of the thermal treatment, and found the nanoparticles to undergo an unusual two-step coalescence process that involves transient growth and restructuring of the nanoparticles. The coalescence process is accompanied by lattice contraction, likely due to composition evolution towards a random alloy. In the late stage of sintering, evidence was found for self-limited grain growth and L10 chemical ordering. The order–disorder transition temperature was found to be around 800 °C in all four ternary alloy compositions studied. Fitting of the experimental data with the model for grain growth with size-dependent impediment leads to an activation energy for mass transport of about 100 kJ mol−1, and may be used as a predictive tool to estimate particle size as a function of heat treatment temperature and duration.

Two-Step Sintering of Samaria-Doped Ceria

Samaria-doped ceria is a candidate solid electrolyte for applications in intermediate-temperature solid oxide fuel cells. One of the main concerns related to this solid electrolyte is its low sinterability, even when prepared by chemical methods. In this work, an alternative method of sintering was used aiming obtaining high densification with negligible grain growth in the late stage of sintering. Commercial samaria-doped ceria powder was pressed into pellets and sintered following the two-step sintering process. The first (peak) temperature was varied from 1100 to 1500 °C and the dwell temperature between 1050 and 1400 °C with soaking time of 5 h. The sintered ceramics were characterized by several techniques to correlate the processing-property characteristics. Relatively high densification has been obtained. Electrical conductivity data obtained by impedance spectroscopy could account for the observed improved densification.

Transport mechanisms and densification during sintering: I. Viscous flow versus vacancy diffusion

Different materials transport mechanisms lead to distinctly different morphological evolution during the sintering of ceramic particles. These behaviors are analyzed using meso-scale, finite-element models based on rigorous formulations of coupled equations for continuum transport and interfacial phenomena. While such two-particle models are simplistic with respect to a real powder compact, they nevertheless provide important mechanistic understanding of the sintering behavior of different systems. Calculations clearly show how viscous flow mechanisms for glassy particles produce simultaneous shrinkage and neck growth due to the global nature of materials transport. In contrast, results for crystalline systems without grain boundaries show that the more localized nature of diffusive transport leads to neck growth with very little densification until late stages of sintering. Surprisingly, surface vacancy diffusion leads to system elongation before densification occurs. Changes caused by the presence of a grain boundary are discussed in a companion paper [Djohari, H., Derby, J.J., 2009. Transport mechanisms and densification during sintering: II. Grain boundaries. Chem. Eng. Sci., in press doi: 10.1016/j.ces.2009.05.022].

Influence of Agglomeration on the Transparency of Yttria Ceramics

In this work, five yttria powders with slightly different states of agglomeration, inherited from various procedures of dewatering the same precursor, were densified by a combination of vacuum sintering followed by hot isostatic pressing (HIP). In order to relate the densification behavior of each powder to its state of agglomeration, all powders were characterized by tap density measurements, X‐ray diffraction, nitrogen adsorption, and laser scattering, while the microstructures of the corresponding densified samples were studied by optical and scanning electron microscopy. The five yttria powders produced sintered samples that differed remarkably from each other in terms of transparency. These discrepancies were related to the degree of fineness in the powders at two different levels. At the level of primary particles, fine and weakly agglomerated powder was very sinterable, causing abnormal grain growth to occur only in the very late stage of sintering. However, the resulting entrapped pores and reduction due to vacuum sintering were responsible for poor optical properties. At the agglomerate level, a bimodal size distribution was identified for all powders. For powders showing severe agglomeration of the primary particles, increasing the relative content of the smaller size population of agglomerates was found to trigger abnormal grain‐growth earlier during presintering. This was attributed to the density around large agglomerates exceeding a critical threshold in the green bodies. Finally, transparency was achieved in samples for which presintering was stopped before grain growth became abnormal. This confirmed that the key to successfully obtaining transparency was to keep porosity intergranular, which could be removed subsequently by HIP treatment.

Cosintering Simulation and Experimentation: Case Study of Nanocrystalline Zirconia

Laminates consisting of two different nanocrystalline zirconia materials (undoped and 3Y‐TZP doped) were sintered, and their densification and curvature were quantified using optical dilatometry. The experimental results agreed well with a continuum‐mechanical description for the cosintering of laminates with diffusion coefficients and activation energies determined using densification of the free‐sintering materials. The comparison covered various relative layer thicknesses and heating rates, but was developed only for the intermediate and late stages of sintering. Sintering stresses and compatibility stresses also were determined from the model.

Analysis of microwave sintering of ceramics

AbstractThe microwave heating of ceramic materials has been analyzed by solving the equations for grain growth and porosity (Svoboda and Riedel, 1992) during the late stages of sintering, coupled with the heat conduction equation and electric field equations for 1‐D slabs. Microwave power absorption and heating profiles have been calculated for A12O3 and SiC in the absence of sintering, and calculations have been carried out to study the effect of increasing dielectric loss ofAl2O3 as a function of temperature. A comparison of the densification and grain growth for A12O3 during microwave and conventional sintering indicates that within the framework of the present model, there is no difference between the two heating modes during the late stages of sintering.

Rotation-coalescence of confined particles in Ni 1– xO/NiAl 2O 4 composites

The Ni 1– x O/NiAl 2O 4 powders of specified molar ratios (1:9, 32:1, 69:1, designated as N 1S 9, N 32S 1, N 69S 1) were sintered and annealed at 1873 K for 1–80 h and studied by transmission electron microscopy with regard to the orientation change of the intragranular particles. It was found that the N 1S 9 and N 32S 1 specimens contains only intergranular particles; whereas the N 69S 1 specimen contains both inter- and intragranular particles in the late stage of sintering. In N 69S 1 specimen, the intergranular spinel particles, either isolated or slightly coalesced as sausage-like chains, were able to detach from the Ni 1– x O grain boundaries and became parallel-epitaxial with respect to the host Ni 1– x O grains upon annealing. The orientation change can be ascribed to Brownian rotation and coalescence of the particles, and the rotation of the particles mostly proceeds over non-specific contact planes due to the dragging effect in the coalescence process. The epitaxial spinel particles could possibly be preserved in the Ni 1– x O grains when the intergranular particles were coalesced to a larger size to suppress grain boundary separation.

Allowance for gas solubility and the finite strength of vacancy sinks in the sintering of a ceramic

The process of diffusional evolution (shrinkage or growth) of gas-filled pores in a hot-pressed ceramic is analyzed, assuming that the gas is soluble in the matrix. It is additionally assumed that the strength of vacancy sources and sinks is finite. An equation is derived for the porosity of the ceramic at a late stage of sintering.

Structural order in Ba(Zn1/3Ta2/3)O3, Ba(Zn1/3Nb2/3)O3 and Ba(Mg1/3Ta2/3)O3 microwave dielectric ceramics

The process and nature of structural ordering and the factors that influence them have been investigated in the microwave dielectric perovskites, barium zinc tantalate (BZT), barium zinc niobate (BZN), and barium magnesium tantalate (BMT), sintered at various temperatures. The samples were characterized mainly by X-ray powder diffraction and transmission electron microscopy. The results show that short-range 1 : 1 B-site order features strongly in the early stages of ordering in BZT and BZN, but it is extremely rare in BMT, for which most grains commence with 1 : 2 order. As sintering progresses, 1 : 1 order is replaced by 1 : 2 long-range order in BZT and by disorder in BZN. Orientational variants of the ordered domains within grains occur in similar numbers when order is fine-scale, but their distribution is less homogeneous in well-ordered samples. Local inhomogeneities in the degree of order within grains, which will affect dielectric properties, correlate with both residual non-stoichiometry and the presence of dislocations. Incompletely reacted starting materials which may persist to late stages of sintering can also strongly influence order. Anomalously large ordered domains at grain boundaries are attributed to grain-boundary migration accompanied by enhanced diffusion. The results indicate that with starting materials that are well-mixed and homogeneous at the nanoscale, tailoring of physical properties should be possible by controlling the type and degree of order through chemical composition.

Densification and creep in the final stage of sintering

Densification and creep by grain boundary diffusion is modelled for the late stage of sintering. The grain structure is described as a regular array of tetrakaidekahedra with pores at each grain boundary between next-nearest neighbors. The diffusion problem on the surface of the tetrakaidekahedron is solved numerically using the heat-conduction option of the finite element code ANSYS. From the calculated normal stresses on the boundary facets one assembles the macroscopic constitute behavior. Since the assumed grain shape is the Wigner-Seitz cell of the body-centered cubic lattice, the resulting viscosity tensor has cubic symmetry. Isotropic bulk and shear viscosities are obtained by applying the procedures developed for the elasticity theory of polycrystals. The resulting bulk viscosity is well approximated by a closed-form solution developed previously. Due to the pronounced cubic anisotropy of the model, the isotropic shear viscosity cannot be determined unambiguously. The model includes the effect of viscous grain boundary sliding. The influence of surface diffusion on the sintering rate is also explored.

Equilibrium pore surfaces, sintering stresses and constitutive equations for the intermediate and late stages of sintering—II. Diffusional densification and creep

Constitutive equations for the intermediate and late stages of sintering are derived assuming grain boundary diffusion to be the controlling transport mechanism. The surface of the pore space is assumed to be in equilibrium. Numerical results for the shape of equilibrium surfaces are taken from a companion paper [J. Svoboda, H. Riedel and H. Zipse, Acta metall. mater. 42, 435 (1994)]. For open porosity the contacts between grains turn out to be nearly circular in all cases, so that the diffusion problem in the contact areas can be solved readily. The macroscopic response of the powder compact can then be built up of the mechanical and the surface tension forces acting across the contacts. The analysis is carried out for a random, or homogeneous distribution of contacts around each particle, and for simple cubic, body-centered cubic, and face-centered cubic grain arrangements. The cubic viscosities are averaged using various methods, the best of which appears to be the self-consistent method.

Equilibrium pore surfaces, sintering stresses and constitutive equations for the intermediate and late stages of sintering—I. computation of equilibrium surfaces

The intermediate stage of sintering is characterized by equilibrium surfaces of the open pore space between the grains. These doubly curved surfaces are determined numerically for the three cubic arrangements of grains (simple cubic, b.c.c. and f.c.c). The numerical program constructs surfaces of constant mean curvature connected via the dihedral angle to grain boundaries having zero mean curvature. The pore morphologies resulting from the analysis are different from the conventional picture of interconnected pore channels along grain edges. According to the present results, open porosity rather consists of interconnected pillow-shaped gaps between next-nearest grain neighbors. The transition to closed porosity occurs when the connections between these gaps close, a process which leads to relatively large isolated pores on two-grain junctions between former next-nearest neighbors. The numerical results for the contact areas between grains and for the sintering stress are approximated by quadratic functions of the pore volume fraction and of the dihedral angle, and the best-fit coefficients are tabulated. The results are used in a companion paper in which constitutive equations for the mechanical behavior during sintering are derived.

The kinetics of hot-pressing for undoped and donor-doped BaTiO3 ceramics

The kinetics of hot-pressing for undoped and donor-doped BaTiO3 of high purity have been studied at 1050‡ C and at 2.3 to 27.6 MPa in an oxidizing atmosphere. The stress exponent and grain size exponent of the densification rate are indicative of control of densification by a grain boundary diffusion mechanism. The activation enthalpy (∼400 kJ mol−1) for the densification process has been measured over the range of 1050 to 1 200‡ C. No significant influence of the donor is observed on the densification of BaTiO3 until the late stages of sintering.

Multiple small-angle neutron scattering characterization of the densification of ceramics: application to microporous silica

Multiple small-angle neutron scattering was used to characterize the microstructure evolution of porous silica as a function of thermal processing. This new technique offers a statistically significant determination of microstructure morphology in the 0.08–10 μm range, which was previously inaccessible without increasing the resolution of the currently available scattering spectroscopy beamlines. All of the scatterers, which in the present work are pores within ceramic bodies, are measured whether they are open or closed. Earlier mercury porosimetry and nitrogen desorption measurements of the microporous silica system indicated that there are two major populations of pores in this material, differing in pore size by approximately an order of magnitude. In this work, it was found that densification during the intermediate stages of sintering is accompanied by coarsening to radii > 0.22 μm within the population of large pores in microporous silica. In the late stages of sintering, there are no detectable pores with radii > 0.08 μm, and radii of gyration equal to 33 and 28 nm were measured. Finally, Monte Carlo simulations were carried out to complement the neutron-scattering measurements, to predict the influence on the results of the bimodal distribution, and to explore the sensitivity of the multiple-scattering method.

Affect of Powder Characteristics on the Sinterability of Calcium Hydroxyapatite

Influence of powder characteristics such as specific surface area, agglomerate size and shape, on their microstructures and mechanical properties of green and sintered bodies was studied for seven kinds of calcium hydroxyapatites. A heat treatment above 1300°C gave tricalciumphosphate (TCP) for all powders. Before the formation of TCP, decarbonation took place in carbonate-containing powders above 800°C, while carbonate-free powders were stable up to 1200°C. Powders with high specific surface areas (SSA), especially larger than 40m2/g, had good sinterabilities, and gave dense bodies about 99% relative density after a heat treatment at 1200°C. On the contrary, powders with lower SSA, about 10m2/g, had poor sinterabilities and gave porous bodies. Poor sinterability was mainly ascribed to the stability and needle like shape of the crystallites. Furthermore, presence of large voids among the agglomerates prevented densification of the compacts at the late stage of sintering. The high density Ca-HAP ceramics prepared in this report have mechanical properties comparable with those reported in the literatures.

An approximate analytical model for the late-stage sintering of an array of rods by viscous flow

The late stages of sintering of a linear array of rods by viscous flow have been studied theoretically. An approximate model in which the actual interface profile is assumed to be locally planar has been developed. Within the framework of the model, analytic solutions to the steady-state Navier-Stokes equations have been derived and the course of sintering followed by advancing the profile through a sequence of short-duration time intervals. After each step the profile is mathematically redefined and new values for the velocities and pressure calculated. In the later stages a logarithmic plot of the reduced neck radius versus time displays a distinctly nonlinear character. Thus the kinetics cannot be described by a simple exponential function as can the early stages when Kuczynski’s geometric simplifications remain valid. A comparison between the two models reveals the rate of neck growth in the later stages to be significantly less for the locally planar interface model. It is suggested that this reduction is due to a lower stress gradient for the flow or material into the neck as a result of the dependence of stress in the neck region upon the changing curvature everywhere.