Sintering mechanisms in YBa2Cu3O7−x superconducting ceramics

Mechanism of the liquid-phase sintering for Nd:YAG ceramics

An apparent activation energy for the rearrangement process in the initial stage of sintering was determined in the temperature range from 1400° to 1600°C on vapor phase decomposed alumina.The slopes of log-log plots for the shrinkage of this alumina were unity. From this result it was concluded that the rearrangement process occurred in the early stage of sintering. The apparent activation energy for the process was 45kcal/mol. This value was far lower than that obtained for various diffusion processes.

Pressure sintering by power law creep

16 - Sintering

A phase field model is used to describe the microstructural development during the ceramic sintering. The evolution of the density is governed by Cahn-Hilliard equation, while the long-range order (lro) parameter fields by the time-dependent Ginzburg-Landau equation. In the simulation, green microstructures that consist of circular particles with different particle-size distributions and green densities have been produced by the stochastic growth model. The porosity of 25.6% was considered. The formation and growth of sintering neck, the seal, spheroidization as well as disappearance of pores and growth of grains are observed during simulation. The simulation results show grain boundary diffusion and surface diffusion are the dominate mechanism at the initial sintering stage. The predicted growth exponent of sintering neck and grain is consistent with the existing theoretical analysis.

Densification behaviour and sintering kinetics of ThO 2–4%UO 2 pellet

The microstructure in AO-excess SrTiO3 (A = Sr2+, Ca2+, Ba2+) ceramics is strongly affected by the formation of Ruddlesden-Popper fault–rich (RP fault) lamellae, which are coherently intergrown with the matrix of the perovskite grains. We studied the structure and chemistry of RP faults by applying quantitative high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy analyses. We showed that the Sr2+ and Ca2+ dopant ions form RP faults during the initial stage of sintering. The final microstructure showed preferentially grown RP fault lamellae embedded in the central part of the anisotropic perovskite grains. In contrast, the dopant Ba2+ ions preferably substituted for Sr2+ in the SrTiO3 matrix by forming a BaxSr1−xTiO3 solid solution. The surplus of Sr2+ ions was compensated structurally in the later stages of sintering by the formation of SrO-rich RP faults. The resulting microstructure showed RP fault lamellae located at the surface of equiaxed BaxSr1-xTiO3 perovskite grains.

The geometries of various sintering wire compacts are determined for both surface- and bulk-transport controlled processes. The results indicate very little difference in the geometries during the initial stage of sintering. However, significant differences become apparent when the latter stage of sintering is approached. Under the action of a bulk-transport mechanism, compact shrinkage is expected. The present calculations of shrinkage versus neck size are compared favourably with previously published data. As a result, the relation between shrinkage and neck size is modified. Finally, the specific surface area variation with neck growth is determined for both forms of mass transport, indicating this may be an alternative measure of sintering progression.

Microstructure and mechanical properties of PM Fe–Cu–Sn alloys containing solid lubricants

The hot‐pressing kinetics of boron carbide at different stages in the hot‐pressing process was investigated. Based general densification equation and pore‐dragged creep model, the densification and grain growth kinetics were analyzed as a function of various parameters such as sintering temperature, sintering pressure and dwell time. Stress exponent of n ≈ 3 at the initial dwell stage suggests the plastic deformation may dominates the densification. The further TEM observations and the calculation based on effective stress and plastic yield stress also indicate that plastic deformation may occur and account for the large increase in density at the initial stage of sintering. Calculated grain size exponent of m ≈ 3 suggests that the grain‐boundary diffusion dominates the densification at the final stage. During the final stage of sintering, grain growth may be determined by evaporation/condensation and grain‐boundary migration.

The constant heating rate method employed in sintering studies offers several advantages over the isothermal method, particularly the fact that all the parameters that describe the sintering phenomena can be obtained from a single sample. The purpose of this work is to determine the parameters of sintering kinetics of nanosized Ni-Zn ferrite powders synthesized by combustion reaction. The nonisothermal sintering method was studied using a constant heating rate (CHR). The Ni-Zn ferrite powders, with average particle size varying from 18 nm to 29 nm, were uniaxially pressed and sintered in an horizontal dilatometer at a constant heating rate of 5.0 °C/min from 600 °C up to complete densification, which was reached at 1200 °C. The compacts were characterized by scanning electron microscopy (SEM). Experimental results revealed three different sintering stages, which were identified through the Bannister Theory. The shrinkage and the shrinkage rate analyzed showed a viscous contribution in the initial sintering stage, which was attributed to the mechanism of structural nanoparticle rearrangement.

In an attempt to minimize the acetaldehyde formation at the processing temperatures (280–300°C) and the outer–inner transesterification reactions in the poly (ethylene terephthalate) (PET)–poly(ethylene naphthalate) (PEN) melt‐mixed blends, the hydroxyl chain ends of PET were capped using benzoyl chloride. The thermal characterization of the melt‐mixed PET–PEN blends at 300°C, as well as that of the corresponding homopolymers, was performed. Degradations were carried out under dynamic heating and isothermal conditions in both flowing nitrogen and static air atmosphere. The initial decomposition temperatures ( T i ) were determined to draw useful information about the overall thermal stability of the studied compounds. Also, the glass transition temperature ( T g ) was determined by finding data, indicating that the end‐capped copolymers showed a higher degradation stability compared to the unmodified PET and, when blended with PEN, seemed to be efficient in slowing the kinetic of transesterification leading to, for a finite time, the formation of block copolymers, as determined by 1 H‐NMR analysis. This is strong and direct evidence that the end‐capping of the OH chain ends influences the mechanism and the kinetic of transesterification. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

Anisotropy in activation energy of textured LiCoO2 for the initial stage of sintering

Solid-state sintering is one of the most widely used material processing technologies in modern manufacturing. The preponderance of previous computer simulations focused on free sintering (without externally applied pressures), in which the morphology evolution and densification in the system are driven by surface energy reduction. In this work, we develop an efficient algorithm to simulate solid-state sintering with hot pressing by explicitly considering interfacial diffusion as well as rigid body movement under external pressure. Particle movements including both translations and rotations are taken into account in the model, which are directed by the associated local stress state in the sintering system. A novel “geometric force” is also introduced to stochastically model geometrically necessary plastic deformations of the sintering particles to accommodate the initial fast densification due to the applied pressure. Subsequent evolution of the overall morphology and inter-particle connections are mainly controlled by interfacial energy minimization, while coarsening is considered in the later sintering stages. The utility of our method is illustrated by sintering 2D compacts of polydisperse circular particles and equal-sized elliptical particles. Significant coarsening occurs in the polydisperse particle system, while no significant grain growth is observed in the equal-sized ellipse system.

Principles of crystal growth from the solid state in relation to the preparation of large crystals