Liquid-phase Sintering Model Research Articles

Microwave dielectric properties, low-temperature sintering mechanism, and defects behaviour of temperature stable (1-x)Li2Zn3Ti4O12-xSr3(VO4)2 ceramics

(1-x)Li2Zn3Ti4O12-xSr3(VO4)2 (0.1 ≤ x ≤ 0.4) microwave dielectric ceramics were fabricated by solid-state sintering technology. The impact of SV addition on the microstructure, dielectric properties, sintering process, and defects behaviour was studied. The formation of SrTiO3 and the glass phase were observed via XRD and TEM, and the latter resulted in a decrease in the sintering temperature. The variations in microwave dielectric properties were consistent with the empirical mixture rules calculated by XRD refinement, and a near-zero τf value was obtained. The Li, Zn and V elements of the glass phase and the liquid phase sintering model were deduced via DSC, TEM and Raman spectroscopy. Then, the defect behaviour, such as oxygen vacancies, Ti3+, and V4+, was investigated by XPS and complex impedance spectroscopy. It was found that the generation and migration of defects occurred much more easily in 0.7LZT-0.3 SV than in LZT, resulting in a higher dielectric loss. Finally, the 0.7Li2Zn3Ti4O12-0.3Sr3(VO4)2 ceramic sintered at 900 °C exhibited excellent microwave dielectric properties of εr = 17.8, Q × f = 41,891 GHz, and τf = −4.4 ppm/°C and good compatibility with silver electrode, showing a good potential application for LTCC.

Effects of Concentrations of NaCl on the Sintering and Thermal Conductivity of Forsterite

This paper aims to study the effects of NaCl on the sintering of forsterite as well as the thermal conductivity of the sintering products. In the sintering process, NaCl played a role in the system by mainly providing a liquid phase sintering environment in order to promote grain growth and forsterite sintering. Moreover, the model of liquid phase sintering was established in this paper. In the initial sintering stage, the phenomenon of particle re-arrangement was not significant, and dissolution-precipitation was regarded as the dominant sintering mechanism. With the extension of the sintering time, the middle and later stages of the sintering process were mainly controlled by the diffusion mechanism. A test of the thermal conductivity of samples with 40% and 50% NaCl was carried out, which indicated that the thermal conductivity of the two samples ranged from 0.3 to 0.4 W·m-1·k-1. At the same temperatures, the thermal conductivity of the sample with a salt content of 50% was lower than that of the sample with a salt concentration of 40%. Also, the heat insulation effect of the sample with a salt content of 50% was better than that of the sample of 40% . Hence, it can be concluded that the higher the salt content in the sample, the lower the thermal conductivity of the sample.

Features of Forming Disks of Gas Turbine Engines of Heat-Resistant Nickel Alloys with a Physical Model of Liquid-Phase Sintering of Pellets

The paper outlines the test results for technology which enables to manufacture the prototype jet engine disks out of powder nickel-based alloy EP741NP using a liquid phase sintering, hot isostatic pressing and plastic deforming. The results are based upon the liquid phase sintering model for powder materials. The obtained test results make it possible to manufacture the isothermal rolled disk.

Three-dimensional simulation of isotropic coarsening in liquid phase sintering I: A model

A three-dimensional, Potts model of liquid phase sintering in a system with full solid wetting was introduced in order to investigate the coarsening kinetics and microstructures associated with this process. Kinetic Monte Carlo simulation was used to probe the coarsening dynamics and obtain the properties of solid particles, including the volume of critical nuclei and the distribution of particle sizes as a function of time. It was found that the average particle volume increased linearly with time and that the particle size distributions were consistent with those obtained experimentally, as in the W–Ni–Fe and Sn–Pb systems. In obtaining these results careful consideration was given to the role of the initial microstructural features in the subsequent evolution of the system.

Study of the sintering behaviour of MgB2 superconductor during hot-pressing

The densification behaviour of MgB 2 was studied when commercial powder was hot-pressed in the temperature range 1070–1190 °C. The mechanisms active during sintering were investigated by continuously recording the shrinkage vs. time and elaborating the data on the basis of Kingery’s model for liquid phase sintering. XRD and SEM analysis on final dense bodies were used to evaluate secondary phases formation during sintering and the effect of magnesium sublimation. AC magnetic susceptibility measurements were also performed to correlate the microstructural and morphological modifications induced by hot-pressing and the superconducting properties.

Grading porous ceramics by centrifugal sintering

Graded porous structures in ceramics have conventionally been synthesized by applying centrifugal acceleration during sintering. A gradual change in porosity is caused by enhanced sintering using centrifugal force, which increases linearly along the radius of rotation. A theoretical understanding of the formation of graded structures has been demonstrated based on the kinetic model of liquid-phase sintering, which is considered to be driven by both capillary and centrifugal forces. The observed structure is in accordance with the theoretical approach which predicts linear shrinkage in the radial direction that is proportional to the power of one fifth of the distance from the surface.

Optimizing press tool shapes by numerical simulation of compaction and sintering—application to a hard metal cutting insert

Designing, manufacturing and commissioning of the tooling often determines the delivery time of a new powder-metallurgical component. To speed-up this process, computer simulations of die compaction and sintering could assist in the development stage. Today, finite element simulations of the compaction process in combination with appropriate material (or constitutive) laws for the powder allow quantitative predictions of the density distribution in the green body as well as the tool loadings. The shape after firing can be predicted by using an appropriate material law for sintering. The sinter distortions are a result of inhomogeneous density distributions in the green body. These distortions can be minimized by optimum press kinematics, and the remainder can be compensated by appropriately shaped punch surfaces. After presenting an overview about the underlying models and simulation methods for compacting and liquid phase sintering (LPS) the simulation procedure together with the optimization technique is demonstrated for a cutting insert made of hard metal powder. Finally, the shape predictions of the LPS model are compared with a simplified model.

Gas Pressure Sintering of Silicon Nitride Powder Coated with Al2O3 and TiO2

Gas pressure sintering kinetics of silicon nitride powder coated with 10 wt% (9:1) Al2O3 and TiO2 have been studied at 1850°C with a pressure schedule of 0.3 MPa in the first stage and 1 MPa in the second stage. The rates have been analyzed with a liquid‐phase sintering model. Diffusion‐controlled intermediate‐stage kinetics have been observed. The role of second‐step pressurization with nitrogen and argon has been determined by monitoring the kinetics. Pressurization at an earlier stage (∼90% relative density) reduces the densification rate but produces a denser material at the final stage. Although final density is greater, a porous surface layer forms on samples sintered with argon pressurization at the second stage. No such porous layer is formed in the case of pressurization with nitrogen. The mechanism of the intermediate‐stage kinetics has been discussed with respect to the nature of the product analyzed by XRD after sintering.

A model of liquid phase sintering by the homogenization

We study the first stage of liquid phase sintering, when the particles rearrangement due to capillary forces is over. We give the boundary value problem satisfied by the displacement field of points of the medium in the phase of elastic compression of solid particles, for given capillary forces acting as a density of external forces, by using the homogenization method and we characterize the mechanical behavior of this constrained medium from the material properties of each elementary components.

A LIQUID PHASE SINTERING MODEL: APPLICATION TO Si3N4 AND WC-Co

A model for liquid phase sintering is examined. The model accounts for solid densification during sintering by particle rearrangement and contact flattening, with large pore filling and grain coarsening in the final stages of sintering. The liquid phase, which develops during heating, aids the solution and diffusion of solid atoms. The model is used to simulate the sintering of Si3N4. The bulk viscosity is calculated for various sintering histories. The bulk viscosity, subjected to a normalisation, is shown to be approximately a unique function of density. A simplified version of the model is developed for the case where rearrangement is small. The model is applied to WC-Co and is modified to account for solution of WC in Co and rearrangement prior to binder melting. The sintering of a WC-Co turning insert is simulated and conclusions are drawn on the importance of the constitutive model in predicting shape distortions in such cases. © 1997 Acta Metallurgica Inc.

Wetting‐induced soil structural changes: The theory of liquid phase sintering

Rapid wetting of structurally unstable soils causes aggregate disintegration and settlement near the soil surface, thereby reducing soil porosity and intake properties. Beneath the slaked surface layer where unsaturated flow conditions develop, structural changes driven mostly by capillary forces take place at slower rates. The collapse of structural voids in the sublayer is due to coalescence of aggregates into larger units. We propose to adopt a model of liquid phase sintering of glass compacts to describe wetting induced densification of aggregated soil. The analogy between these processes is based on the observations that (1) the reduction in soil strength upon wetting is similar to the effect of high temperatures on glass powders and (2) in both systems capillary forces induce viscous flow. A sintering model which assumes a porous medium with tractable geometry was modified to accommodate soil conditions. The model considers the reduction in the water‐coated solid surface areas (i.e., water‐air interfaces) as the energy source for the soil viscous flow. Data from wetting experiments of aggregated silt loam were used to test the plausibility of the theory. The main unknown in the system was the viscosity of wet soil which was estimated to be in the range of 1.37×106 to 1.9×106 Pa s−1. The use of solid matrix with tractable geometry and energy considerations should prove useful for other studies of the dynamics of soil structure.

A model for liquid phase sintering

A quantitative model for liquid phase sintering is developed based on the following ideas. During heating a liquid phase forms, which is easily mobile, wets the solid particles completely, dissolves solid atoms and provides an easy diffusion path for them. The solid density increases by particle rearrangement and by the flattening of particle contacts. Driving (or retarding) forces result from capillary stresses, from applied mechanical stresses, from the pressure of gas entrapped in closed pores and from differences in chemical potential of the dissolved and precipitated matter. At higher densities the driving force may become very small, since the liquid pressure decreases and a negative contribution from the solid-liquid interface energy increases. At this stage grain coarsening plays an important role for the continued filling of larger and larger pores. The model is applied to describe nonisothermal densification curves measured on Si3N4 for various hold temperatures, axial stresses and green densities. Adjusting a moderate number of parameters all having a physical meaning leads to good agreement between theory and experiment.

Densification process of α′-sialon ceramics

The liquid-phase sintering process of α′-sialon ceramics has been investigated by high-temperature dilatometry and microstructural observation. In addition, isothermal shrinkage measurements have been performed to examine the densification kinetic parameter. It has been confirmed that densification kinetic parameters in the solution-reprecipitation stage are much larger than the rate exponent predicted for the classic liquid-phase sintering model, and are slightly smaller than that for the viscous flow process. Rapid shrinkage was observed in the solution-reprecipitation stage from the results of shrinkage rate, and corresponds to pore elimination by particle rearrangement and cooperative flow of particle/liquid mixture. These processes provide the major contribution to shrinkage. In addition, the liquid flow process occurs when the silica content in the raw powder increases, but it is retarded due to the formation of α′-sialon. It is anticipated that particle rearrangement and cooperative flow, as well as liquid-flow processes, take place in the solution-reprecipitation stage of sintering of Si3N4-based materials, and cause a large amount of shrinkage.

Model for the Effect of Powder Packing on the Driving Force for Liquid‐Phase Sintering

A model for liquid‐phase sintering is presented that explicitly considers the effect that the pore size distribution of the sintering compact has on the capillary forces that drive densification. In particular, the effect that liquid redistribution in the pore structure has on the driving force for sintering is considered under the assumption that the liquid can easily move to find a low‐energy configuration in the pore structure. It is shown that, for a powder compact that has a narrow pore size distribution, densification exhibits approximately the same time dependencies as those predicted by the Kingery model for liquid‐phase sintering. However, systematic changes in the absolute densification rate with the volume fraction of liquid, and the mean and breadth of the pore size distribution, are predicted. With more extreme pore size distributions, such as a bimodal distribution, behavior significantly different from that predicted by Kingery is found. In particular, it is predicted that, without there being a change in sintering mechanism, abrupt changes in densification rate may occur if the peaks in the bimodal distribution are well separated. The model provides a rational basis for interpreting how powder packing and processing steps can influence densification by liquid‐phase sintering.

Kinetics of the sintering of superconducting ceramics YBa 2− xSr xCu 3O 7− y

The densification kinetics of ceramics YBa 2− x Sr x Cu 3O 7− y (YBC,YBSC: doped with strontium) was studied at constant heating rate with a dilatometer. The different sintering regimes were clearly detected and analysed tentatively according to the available densification models. The data for the initial and intermediate stages fitted reasonably well with the Johnson and Coble models, and the average activation energies for grain boundary and volume diffusion, were roughly estimated. For YBC compositions, a satisfactory fit with a liquid-phase sintering model was obtained at temperatures higher than about 960°C.

Densification and Sintering Kinetics in Sintered Silicon Nitride

The sintering sequence of Y2O3‐Al2O3‐doped Si3N4 was investigated with respect to the relationship between densification, α→β transformation, and microstructural development. Quenching studies were performed to reveal these interactions during a complete sintering cycle. Isothermal studies were conducted to examine the sintering kinetics and compared to Kingery's liquid‐phase sintering model. The bulk density increased to ≥90% of theoretical density with only minor transformation taking place. Major transformation occurred in a late sintering stage and was accompanied by the development of elongated grains. The kinetic order of the densification process, taking into account an appropriate correction, was larger than any of the rate exponents predicted by the Kingery model, indicating that other single or mixed mechanisms were active.

Particle rearrangement during liquid phase sintering of silicon nitride

The experimental and analytical studies of both particle rearrangement and solution-diffusion-precipitation processes simultaneously acting during liquid phase sintering of Si 3N 4 with addition of MgO are described. By using a liquid-phase sintering model it is possible to distinguish between particle rearrangement and diffusion-controlled densification. The results show that an optimum particle rearrangement cannot be reached (especially for polydisperse powders with a higher content of β-phase) because very rapid α- to β-phase transformation retards particle rearrangement.

The fabrication of O?-sialon ceramics by pressureless sintering

Low-porosity ceramics with O′-sialon as the major crystalline phase have been successfully fabricated by pressureless sintering. Densification at 1400 to 1800° C is interpreted using Kingery's liquid-phase sintering model. Particle rearrangement accounts for a significant proportion of the overall densification and is closely related to the amount and viscosity of the liquid present. The oxidation resistance of fabricated samples is good but, at temperatures above 1300° C, decreases with increasing alumina content in the starting mix. Modulus of rupture values of about 420 MPa at room temperature are high enough to encourage further work.

Reaction hot-pressing of zircon-alumina mixtures

Measurements have been made of the concurrent rates of reaction and of densification during the hot-pressing of mixtures of alumina and zircon powders. The results can be interpreted in terms of a liquid-phase sintering model and evidence is presented for separable stages of particle rearrangement, diffusion-controlled densification and residual reaction subsequent to attainment of zero porosity.

THE INFLUENCE OF A PARTIALLY WETTING SECOND PHASE ON THE SINTERING OF SOLID PARTICLES

AbstractA geometrical model is considered which describes the sintering kinetics between two solid, spherical particles covered by a partially wetting second phase. Conditions selected are such that the grain boundary is not penetrated by the second phase. The model is kinetically similar to the case of solid-state sintering by grain-boundary diffusion and can also be compared with Kingery’s model of liquid-phase sintering. It predicts that the sintering rate of two solid particles remains virtually unchanged when a second phase (solid or liquid) is added. An example is the sintering behaviour of SmCo5. The applicability of the model to carbide-metal and oxide-metal systems is discussed.